coordination spheres in spin crossover compounds

Coordination Chemistry Reviews 407 (2020) 213148

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Unusual metal centres/coordination spheres in spin crossover compounds Juan Olguín Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, Col. San Pedro Zacatenco, Avenida IPN 2508, Ciudad de México 07360, Mexico

a r t i c l e

i n f o

Article history: Received 1 August 2019 Accepted 5 December 2019

Keywords: Spin crossover Manganese Cobalt Chromium Bistable complexes HS and LS complexes

a b s t r a c t The most widely studied spin crossover (SCO) systems are those involving hexacoordinate iron(II) complexes immerse in the ‘‘magic” N6 coordination sphere. In this review the properties of SCO-active metal complexes containing unusual metal centres and/or coordination spheres are analysed, except for Fe3+ complexes that have been recently reviewed. The SCO-properties of the less studied SCO metal centres Cr2+, Mn2+, Mn3+ and Co2+ are presented, showing great advances in the development of bistable materials containing first row transition metals other than Fe. Finally, an analysis of the changes in the structural parameters, bond lengths and angles, during the spin conversion is presented for those complexes structurally characterised by X-ray crystallography in both high- and low-spin states. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chromium(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Manganese(II) and (III) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Manganese(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Manganese(III) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2.1. Hexadentate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2.2. Tridentate ligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Cobalt(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Five-coordinate cobalt(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.2. Six-coordinate cobalt(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2.1. N6 coordination sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2.2. N4O2 coordination spheres. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Abbreviations: VT, Variable temperature; SCO, Spin crossover; ST, Spin transition; HS, High spin; LS, Low spin; IS, Intermediate spin; Cp, Cyclopentadienyl; Cp*, Pentamethylcyclopentadienyl; S.O, Spin only; T½ Transition temperature, the temperature where the HS and LS states coexist in a 1:1 ratio; RT, Room temperature; DT½, Hysteresis loop width; T½;, Transition temperature in cooling mode; T½", Transition temperature in warming mode; JT, Jahn Teller; Av, Average value. E-mail address: [email protected] https://doi.org/10.1016/j.ccr.2019.213148 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Octahedral first-row transition metal complexes possessing d4 to d7 electronic configurations can exist in either the high-spin (HS) or low spin (LS) state. The HS-state will contain the maximum number of unpaired electrons, whereas, the LS-state will have the maximum number of paired electrons, for each electron configura-

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 1.1. Octahedral crystal field splitting and electron filling diagram for the HS and LS-states for d4 to d7 electron configurations metal complexes. The electronic configurations that show strong JT-distortions are framed in a box.

tion. From a crystal field theory view point, strong field ligands provoke a large energy gap (Do) between the two types of symmetry related d-orbitals, the low-lying energy t2g orbitals, and the highlying energy eg orbitals, which, in turn is larger than the spin pairing energy (P), thus the resulting complexes are stabilised in the LSstate (Do > P), maximising the number of paired electrons. However, in the case of weak field ligands the Do value is smaller than P, resulting in complexes with the maximum number of unpaired electrons, that is the HS-state. In some cases, the ligands can provide such an energy field that the value for both the spin pairing (P) and the ligand field splitting (Do) energies are around the same magnitude (crossover point), thus the influence of an external stimulus can reversibly switch between the HS and LS-states [1]. The most common and widely use external trigger is temperature, probably since most of the techniques utilise for the characterisation of SCO-active complexes are readily adapted for varying it. However, there are examples of other external stimuli capable of switching the spin state of SCO-active metal complexes such as: pressure, external magnetic field [2], light [3,4] and light driven isomerisation of ligands [5,6], and solvation/desolvation [7]. The spin state switching not only results in a change in the number of electrons populating the frontier orbitals separated by Do, and thus the number of paired and unpaired electrons which in turn changes the magnetic properties of the complexes, but other physicochemical properties change as well, such as: colour, metal to ligand bond lengths and angles, dielectric constant and electrical resistance [8]. SCO-active complexes that show abrupt and reversible interconversion (spin transition, ST) between the two possible spin states, HS and LS-states or [0]/off and [1]/on states, can be considered as molecular switches. The metal ions known to show SCO properties are Cr2+, Mn2+/3+, Fe2+/3+ and Co2+/3+, in Fig. 1.1 the possible electronic configurations in octahedral geometries (crystal field) for both the HS and LS-states for these metal ions are shown. SCO-active compounds can be characterised by a wide range of techniques due to the change of its physicochemical properties during the spin conversion. Some examples of commonly used techniques are described as follows. One of the most widely used techniques is magnetometry, as there is a dramatic change in the magnetic properties, where the amount of HS- and LS-state is proportional to the magnetic susceptibility (vM), vM T ¼ cHS vHS þ ð1  cHS ÞvLS [2]. Modern equipment can vary temperature, pressure and magnetic field during the measurements, thus permitting to characterise thermal-, pressure- and magnetic field-induced SCO, additionally, light irradiation can be applied to the samples to measure any light-induced SCO. Another commonly used technique is

differential scanning calorimetry [9] (DSC) that permits to obtain thermodynamic data such as DH and DS for the spin conversion. Electronic paramagnetic resonance (EPR) can indeed be utilised to demonstrate the coexistence of both spin states in the same sample, however, quantitative analysis is often difficult [2]. Due to the change in electron population during a spin conversion and change in colour, electronic spectroscopy is a useful technique for the characterisation of SCO-active compounds in solution and solid state [10]. Vibrational and rotational-vibrational spectroscopies can be used as well for determining and quantifying spin conversions [11], in the solid and solution states, however, in most of the cases the complexes that can be characterised by these techniques are those containing ligands and/or co-ligands possessing diagnostic functional groups, e.g. C@O, N@C, N„CAE, etc. Recently, photoluminescence spectroscopy has been used for determining SCO when a chromophore is located in the ligand and its photoluminescence properties change depending on the spin state of the metal centre [12–14]. Nuclear magnetic resonance (NMR) can be used for the magnetic characterisation of the samples in solution, particularly by the Evans method, from which it is possible to obtain the magnetic susceptibility in solution, and thus to determine the spin state [15,16]. Finally, X-ray crystallography is a powerful tool to characterise thermally-induced SCO, due to the change in bond angles and lengths during the spin conversion, commonly, the HS-state possesses longer metal to ligand bond lengths than the LS-state, and the bond angles are more distorted in the HS than the LS-state (see below). From all these techniques it is possible to obtain the HS, or LS, fraction in the sample, for the construction of spin transition curves, which are plots of the HS-fraction (or LS) vs. external stimulus. Each SCO-active complex can be characterised by its transition temperature (T½), which is defined as the temperature where the HS and LS-states coexist in a 1:1 ratio [2]. In some cases, the SCO-active complexes can show hysteresis, when the cooling transition temperature (T½;) is different from the warming transition temperature (T½"), Fig. 1.2, resulting in a hysteresis loop (DT½) conferring the system with memory effect (bistability) [17]. Abrupt and hysteretic ST are favoured by crystallographic phase changes [18] and/or communication between the metal centres (cooperativity) [19], that can be achieved through intra-and/or intermolecular interactions, e.g. bridging ligands, for the former, and hydrogen bonding, anion/cation-p, p-p, Van der Waals interactions, etc., [20] for the latter. This type of bistable molecular switches can be used for a wide range of applications, including nanotechnology [21], namely display and memory devices, sensors, MRI contrast agents, etc. [22–27].

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 1.2. Example of a hysteretic and abrupt spin transition possessing two different transition temperatures in the cooling and warming mode, T½; and T½" respectively.

The most widely studied SCO systems are those involving hexacoordinate iron(II) complexes immerse in the ‘‘magic” N6 coordination sphere. In many of such complexes, the six N-donors surrounding the metal ion come from five- and/or six-member heterocycles, imines and/or amines. The latter combination of functional groups provides better odds to observe the desired spin crossover properties in iron(II) complexes. Nonetheless, it has been shown that other donor atoms could be utilised to produce octahedral SCO compounds, and yet more other coordination numbers and metal centres. The lack of more examples of other potentially SCO-active metal centres could be due to the fact that some of them are air and moisture sensitive (Cr2+, Mn2+ and Co2+) and, in addition, that the crossover profiles for these metal centres are, in general, gradual and incomplete. Another difference is that both the HS and LS-states are paramagnetic, which may result in states admixing, and/or (anti)ferromagnetic exchange behaviour, complicating the analysis of the magnetic properties. However, the fact that both spin states possess unpaired electrons could confer other properties to the SCO complexes, such as single molecule/chain magnetism, spin canting, spin frustration in polynuclear systems, etc. In this review the properties of SCO-active metal complexes containing unusual metal centres and/or coordination spheres are analysed, except for complexes based on Fe3+, as a recent review by Harding and co-workers [28] has been published covering this type of complexes. The SCO-properties of the less studied SCO metal centres Cr2+, Mn2+, Mn3+ and Co2+ have been covered in a 2004 review by Garcia and Gütlich [29], thus in this work the progress since 2004 in the synthesis and characterisation of novel SCO-active complexes containing non-iron first-row transition metals is presented, showing great advances in the development of bistable materials.

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The aim of this review is to describe and analyse the magnetic and structural properties of recent SCO-active metal complexes of Cr2+, Mn2+, Mn3+ and Co2+ published since 2004, however, some older examples are utilised as a context for the recent discoveries. Regarding the magnetic properties, the spin only magnetic moment, lS.O = [n(n + 2)]½ (n = number of unpaired electrons), will be utilised as a reference value to calculate the HS (or LS) percentage of the compounds described in this manuscript. It is important to mention that lS.O is only an approximation, since it considers g = 2 and negligible spin-orbit coupling. In many cases lS.O values are very close to the experimental magnetic moment values of first row transition metal complexes, except for HS-cobalt(II) complexes and complexes possessing high anisotropy. Thus, the reader should be cautious when HS or LS percentages are stated throughout the manuscript as they are approximations and not accurate values, however, in most cases, the available structural data obtained by X-ray crystallography agrees with such approximation.

2. Chromium(II) complexes Only three types of chromium(II) (d4) complexes have shown SCO-properties, and can switch from the HS (S = 2) to the LS (S = 1) state by decreasing temperature, the chemistry of the first two types of SCO-active systems have been previously reviewed [29,30], see Fig. 2.1. One of the reasons for the lack of examples for this particular ion could be that d4 systems show the smallest entropy gain due to spin transition among the d4-d7 octahedral complexes, which is relevant from the Gibbs free energy point of view, as a large spin-entropy change would favour more the spin transition than a small one [31]. The other possibility for the low number of examples for Cr2+ could be its highly air sensitivity, resulting in chromium complexes with no possibility for showing SCO after oxidation, unless strict inert conditions are maintained. However, there are more examples by far of Mn3+ SCO-active complexes, which also has a d4 configuration, than Cr2+, probably since Mn3+ is more air stable (see Section 3.2). The first SCO-active Cr2+ complex was reported in 1989, containing a diphosphine and two iodido ligands in an octahedral geometry, trans-[CrII(depe)2I2] (depe = 1,2-bis(diethylphosphino) ethane) [32,33], that is in a P4I2 coordination sphere, where the diphosphino ligands coordinate in the equatorial positions, whereas the two iodido ligands occupy the apical sites, trans to each other. This compound shows a thermochromic effect changing from purple brown at room temperature (HS) to violet at low temperature (LS). VT-magnetic measurements revealed that this complex undergoes an abrupt crossover (spin transition, ST, will be use in this manuscript to describe abrupt spin crossover) with T½ = 171 K, where the magnetic moment at room temperature is 4.84 mB and 2.82 mB at 90 K, in good agreement with mS.O. values of 4.90 and 2.83 mB for the HS and LS states, respectively. The second type of Cr2+ SCO-active complex corresponds to the dark green organometallic dinuclear complex [CrII2(l2:g5-P5)(g5Cp*)2]X, where X = PF6 or SbF6 [34], (Fig. 2.2). In this complex each

Fig. 2.1. Scheme of the three types of SCO-active Cr2+ complexes and numbering scheme for the indenyl ligand system.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 2.2. vMT vs. T for the complex [CrII2(l2:g5-P5)(g5-Cp*)2](SbF6), inset: same plot for the PF6 analogue. Figure reproduced, with permission, from Ref. [34]. Copyright RSC 1994.

chromium centre is coordinated to a terminal Cp* ligand and bridged by a P 5 heterocycle. Both complexes undergo an abrupt and hysteretic ST (PF6 T½  33 and DT½ = 2 K; SbF6 T½  23 and DT½ = 2 K) below which they are essentially diamagnetic, Fig. 2.2. In a detail structural study by VT-X-ray crystallography on the SbF6 analogue [35], it was shown that upon cooling (290 to 170 K) the Cr  Cr distance, in the one crystallographically independent complex, decreases by 0.034 Å. Below 160 K there is a phase change from Fddd (room temperature) to I2/a, and the presence of two symmetry independent complex cations. In the temperature range 150–25 K, one of the cations suffers considerable geometry changes involving the Cr-P bonds, Fig. 2.3. At 25 K the metal-metal distance for this cation is reduced by 0.31 Å, whereas for the second cation this distance only decreases by 0.08 Å. At 25 K both complex cations had suffered a further decrease in the Cr  Cr distance to 2.782(2) and 2.798(2) Å (c.f. 3.1928(5) Å at room temperature). The above-mentioned structural changes fit nicely with the experimental magnetic measurements where at room temperature meff = 4.1 mB, decreasing to 2.8 mB at 25 K, followed by ST at 23 K. The authors suggested that the changes in the magnetic properties at 160 and 23 K could be due to changes in the antiferromagnetic coupling between the Cr2+ ions mediated by the P-donors or a two-step ST of the type [LS-LS] ? [HS-HS]:[LS-LS] ? [HS-HS].

The third type of SCO-active Cr2+ complexes was reported by Hanusa, Yee and collaborators, corresponding to mononuclear organometallic complexes containing substituted indenyl ligands of the type [CrII(g5-R-indenyl)2] [36,37]. In this type of compounds both indenyl ligands coordinate to the metal centre via the cyclopentadienyl unit in a g5 fashion, Fig. 2.1. Additionally, these compounds show thermochromic effect; in the HS-state are purple and green in the LS-state. The parent compound containing unsubstituted indenyl ligand is a diamagnetic dinuclear complex [CrII(indenyl)2]2; introduction of substituents in the indenyl skeleton hampers the formation of this type of dimers. It is important to note that the spin state of the resulting complexes is somehow dependent on the configuration adopted by the indenyl ligands, that is a staggered geometry will stabilise the HS-state, whereas a gauche or eclipsed one stabilises more the LS-state. In the former type, a centrosymmetric system is obtained, in which the indenyl p-orbitals cannot interact with the metal d orbitals [38]. When the 1,3-substituents are more sterically demanding, the most stable configuration is gauche, thus reducing symmetry with concomitant mixing of the indenyl p-orbitals and Cr d orbitals, increasing the HOMO-LUMO gap, stabilising the LS-state [39]. For instance, the complex containing 2-methylindenyl ligand, [CrII(2Me-indenyl)2] (see Fig. 2.1 for numbering scheme) is stabilised in the HS-state, as demonstrated by magnetic measurements in solid and solution, meff = 4.3–4.4 mB from 25 to 300 K. Single crystal X-ray crystallography showed that this complex is stabilised in the staggered conformation [36]. Interestingly, the nature and positioning of the substituents in the indenyl ligands play an important role on the spin state of the resulting complex. By introducing a methyl group at the 1-position, instead of the 2-position, the compound containing 1-methylindenyl ligand, [CrII(1Me-indenyl)2], undergoes a gradual SCO centred at ~130 K, instead of being locked in the HS-state as its 2-methyl analogue (see above). At room temperature the magnetic measurements showed meff = 4.1 mB, slightly smaller than the expected mS.O. = 4.5 mB, indicating an incomplete transition to the HS-state, upon cooling a meff value of 2.87 mB at 20 K was found, in good agreement for mS.O. = 2.83 mB. VT-single crystal X-ray crystallography was used to characterise the SCO event, at 105, 173 and 298 K, Table 2.1. At all temperatures two 1Me-indenyl ligands sandwich the metal centre in an eclipsed meso configuration. At 105 K, the average Cr-C bond length of 2.179 (9) Å is in good agreement for the LS-state (ranging from 2.18 to 2.22 Å, for this type of complexes) [36]. The methyl group attached to the cyclopentadienyl moiety is only slightly displaced from the

Fig. 2.3. Perspective view of the cation complex [CrII2(l2:g5-P5)(g5-Cp*)2]+ at different temperatures showing the decrease of the Cr-P disorder on decreasing temperature and phase transition at 150 K from Fddd to I2/a [35]. Anion and H-atoms are not shown for the sake of clarity.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148 Table 2.1 Spin state and selected bond lengths and angles for complexes containing substituted indenyl ligands of the type [CrII(R-indenyl)2] CompoundRef.

T [K]

[CrII(2Me-indenyl)2] [36] [CrII(1Me-indenyl)2] [36]

[CrII(1,3-Si-indenyl)2]¼(C6H14) [39] [CrII(4,7-Me-indenyl)2] [37]

173 105 173 298 173 173

[CrII(1,2,3-Me-indenyl)2] [37] [CrII(2,4,7-Me-indenyl)2] [37]

173 173 293

[CrII(2,4,5,6,7-Me-indenyl)2] [37] Notes:

a

Temperature of X-ray acquisition.

173 b

a

Cr-C [Å]

b

2.308(7) d 2.179(9) e 2.205(7) e 2.262(10) e 2.20(2)f 2.18(1) e; 2.17(1) e 2.239(11) d 2.172(4)d; 2.168(5)e 2.227(6) d; 2.187(9) 2.17(1) d

Average value.

c

Inter-C5 ring plane angle [°] 0.0 d 6.8 e 5.5 e 1.3 e 8.92f 6.0 e; 6.7 e 0.0. d 0.0 d; 9.7 e 0.0 d; 7.6e 4.3 d

Hinge anglec [°] 6.9 d 2.2b, e 3.0b, e 5.5b, e 3.1b,f 1.5b,e; 2.1b,

e

5.3 d 1.6 d; 1.8b,e 3.8 d; 2.7b, e 2.0 d d

Fold anglec [°]

Spin state

T½ [K]

1.6 d 3.7b, e 3.0b, e 1.0b, e 8.5b,f 2.5b, e; 4.4b, e 4.0 d 2.6 d; 4.3b,e 3.3 d; 3.2b, e 2.9 d

HS (purple blocks) LS (green blocks) LS HS LS (dark green plates) LS (dark green plates)

Not SCO ~130 K gradual

Mixed (purple needles) LS d,e (dark brown blocks)

>350 K gradual ~250 K, gradual incomplete 150 K >350 K gradual

Mainly LS d; LSe LS (green blocks)

>350 K gradual

For definition of these angles see ref. [40]. Staggered molecule.

C5 plane (average 1.9°). At 173 K, around T½, the Cr-C bond lengths showed an increase of 0.026 Å, compared to the structure collected at 105 K, whereas the methyl group is now closer to the C5 plane, 1.2°. The structure at 298 K shows a significant elongation of the Cr-C bond lengths, 2.262(10) Å (3.8% change) compared to the structure at 105 K, and the methyl group is just 1.1° away from the C5 plane. Interestingly, in toluene d8 solution, the complex is locked in the HS state in the 183–298 K temperature range, confirmed by Evans method (meff > 4.6 mB), as in solution is very likely that the conformation adopted is staggered and not eclipsed as in the solid state (see above). The complex [CrII(1,3-Si-indenyl)2]¼(C6H14) (1,3-Si-indenyl = 1,3-Bis(trimethylsilyl)indene) [39], containing bulkier and more donating groups at the 1,3-positions of the cyclopentadienyl moiety (Fig. 2.1), crystallises in the P21/c space group as dark green hexagonal plates. At 173(2) K the asymmetric unit contains two complex molecules, identical within uncertainty, where the metal centre is sandwiched by two indenyl ligands in a gauche arrangement with twist angles of 86.3 and 83.8°, Fig. 2.4. The average Cr-C distance of 2.20(2) Å indicates that this complex is stabilised in the LS-state, Table 2.1. The average distance between the Si(Me)3 groups and the C5 ring plane is 0.31 Å. VT-magnetic measurements show that this complex is stabilised in the LS-state from 2 to 300 K.

e

f

Eclipsed molecule. Gauche molecule.

An average meff value of 2.8 mB was found for this complex in the 10–150 K range which is consistent with the expected spin only value. Above 250 K there is evidence of a gradual SCO, reaching meff = 3.3 mB at 350 K, which clearly indicates the transition is not complete at this temperature. In another study performed by the same group, the effect of the methyl substituents in either the cyclopentadienyl or arene moieties of the indenyl ligand was investigated [37]. A family of four SCO-active complexes were synthesised and characterised namely: [CrII(4,7-Me-indenyl)2] (4,7-dimethylindenyl), [CrII(1,2,3-Me-indenyl)2] (1,2,3trimethylindenyl), [CrII(2,4,7-Me-indenyl)2] (2,4,7-trimethylindenyl) and [CrII(2,4,5,6,7-Me-indenyl)2] (2,4,5,6,7-pentamethylindenyl), Figs. 2.1 and 2.4. The only compound that showed a complete and relatively abrupt spin crossover with T½  150 K, was [CrII(1,2,3-Me-indenyl)2] (Fig. 2.5). The other compounds showed gradual and incomplete SCO: [CrII(4,7-Me-indenyl)2] a gradual and incomplete transition at c.a. 250 K, whereas, [CrII(2,4,7-Meindenyl)2] and [CrII(2,4,5,6,7-Me-indenyl)2] gradual SCO beginning at 250 K. Interestingly, [CrII(4,7-Me-indenyl)2], [CrII(2,4,5,6,7-MeII indenyl)2] and [Cr (1,2,3-Me-indenyl)2] do not show SCO in toluene d8 solutions but rather, the latter is stabilised in the HS state, and the former two are stabilised in a mixture of HS and LS states, as proven by Evans method. In contrast, [CrII(2,4,7-Meindenyl)2] shows SCO in the same solvent, the beginning of the

Fig. 2.4. Perspective view of three examples of [CrII(g5-R-indenyl)2] SCO-active complexes, showing the three possible conformations: staggered (left), eclipsed (middle), gauche (right). H-atoms are not shown for the sake of clarity.

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Fig. 2.5. VT-magnetic measurements for the complex [CrII(1,2,3-Me-indenyl)2], showing SCO at ~150 K. Figure reproduced, with permission, from Ref. [37]. Copyright ACS 2008.

transition can be observed above 188 K, from that point the meff increases linearly with temperature (0.5 mB every 10 K), until meff reaches a plateau above 288 K (meff = 3.8–3.9 mB in the 288–378 K range). Although the transition in solution is still incomplete the conversion to the HS state is higher than in solid state. The four complexes were characterised by X-ray crystallography at 173 K, except for [CrII(2,4,7-Me-indenyl)2] that was collected at 293 K as well, however utilising a different crystal. In all the structures the two indenyl ligands sandwich the metal centre, in either staggered or eclipsed conformation. A summary of spin state and selected bond lengths and angles are shown in Table 2.1. From these results is clear that the amount of methyl groups in the ligand is relevant in the stabilisation of the LS-state. The ligands containing a higher number of methyl groups stabilise the LS state. The fully methylated ligand, 1,2,3,4,5,6,7-heptamethy lindenyl, produces a LS-locked Cr(II) complex [40], whereas the monomethylated 2Me-Indenyl ligand only stabilises the HS state. The ligands lying in between these two extremes, including 1Meindenyl ligand, produce SCO-active complexes, Fig. 2.6. It seems that the positioning of the methyl group in the indenyl ligand is important as well, for instance the trimethylated ligands 1,2,3Me-indenyl and 2,4,7-Me-indenyl should stabilise in the same amount the LS state, however, the ligand containing more substituents in the six-member ring stabilise better the LS-state. In summary, there are three families of chromium(II) SCOactive complexes. The first one corresponds to one coordination complex in a P4I2 coordination sphere containing a diphosphino and two iodido ligands, trans-[CrII(depe)2I2], showing an abrupt ST due to the presence of highly polarizable I ligand. Other compounds containing similar diphosphino ligands and halides are

only stabilised in the LS-state. However, there is plenty of room for expanding more Cr2+ systems possessing P4X2 coordination sphere, by utilising other anionic co-ligands and maintaining the 1,2-bis(diethylphosphino)ethane moiety, for instance pseudohalogens such as ECN (E = S, Se, BH3), N3, etc. The second class of compounds are organometallic complexes, one of them is a dinuclear complex containing two terminal Cp* II 2 5 5 and one P 5 bridging ligand, [Cr2 (l :g -P5)(g -Cp*)2]X, where  X = PF6 or SbF6. These compounds show a very interesting and hysteretic SCO behaviour, which seems to proceed in two steps. However, the possibilities to expand this family is a challenge for organometallic chemists. The other organometallic family corresponds to neutral sandwich Cr2+ complexes containing substituted indenyl ligands. So far, the substituents in the ligands correspond to alkyl or silyl groups, demonstrating the influence of steric demand and positioning of the substituents in the indenyl group on the spin state of the complex. This system is very promising in terms of developing novel SCO complexes with improved properties, such as hysteretic behaviour. Thus, there is a myriad of opportunities to substitute the indenyl moiety to try to increase cooperativity, for instance polar groups capable of forming H-bonds (NH2, OH, C@O, etc.), deactivating groups (F, NO2, etc), aromatic substituents to produce p-p stacking, etc. Therefore, the SCO chemistry for Cr2+ is very promising, further studies in the SCO-properties of these materials are necessary, such as: VT-X-ray crystallography on the same crystal of the complex to characterise both spin states (particularly for the indenyl and diphosphino ligand), further VT-calorimetric experiments to characterise the ST are needed. 3. Manganese(II) and (III) complexes Manganese can be found in a range of oxidation states (I to VII) in coordination and organometallic compounds, from which only the oxidation states I, II and III could present SCO activity in octahedral geometry; nevertheless, SCO-active complexes have only been described for the II and III oxidation states. In the former, a d5 configuration, the spin transition in octahedral complexes can be observed from S = 5/2 (HS) to S = ½ (LS) state, and recently it has been demonstrated that pentacoordinate systems can undergo ST from the HS-state to the IS-state (S = 3/2), see below, similarly to iron(III) SCO complexes. It is important to note that most of Mn2+ complexes are air sensitive, thus handling of the compounds must be done under inert atmosphere conditions. On the other hand, Mn3+ SCO-complexes, possessing d4 configuration, can switch between S = 2 (HS) to S = 1 (LS) states in octahedral geometries, however, DFT calculations performed by Deeth and Amabilino [41] suggest the possibility of observing S = 2 (HS) to S = 0 (LS) transitions in complexes possessing pentagonal bipyramidal geometry. Complexes of Mn3+ are more air stable than its Mn2+analogues, nevertheless oxidation to Mn4+ could still occur. Thus, there

Fig. 2.6. Numbering scheme and LS-state stabilisation capability of the indenyl ligands for [CrII(R-indenyl)2] complexes, based on the number of methyl groups.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

are more examples of SCO compounds for manganese(III) than for isoelectronic chromium(II) and manganese(II), possibly on the account of its air stability. The SCO-properties of complexes containing manganese(II) and (III) have been reviewed in 2004 [29]. Back then only two types of discrete Mn2+ complexes were known: one of them correspond to the organometallic sandwich complexes containing substituted Cp rings [42–44] and a complex containing a dianionic N4 macrocycle, dibenzotetramethyltetraaza[14]annulene dianion, and a NO axial ligands [45]. Whereas, the known Mn3+ SCO-active complexes were those containing: a pyrrolyl Schiff base ligand, tris(1-(2-azo lyl)-2-azabuten-4-yl)amine [46]; and a complex containing tetramesitylporphyrin radical [47]. In this section, recent advances in the synthesis and characterisation of SCO-active complexes of Mn2+ and Mn3+ are presented. In the case of manganese(II) complexes, no novel systems have been discovered since 2004; however, there is recent work on [MnII (CpR)2] systems, including a dinuclear family of complexes. Gratifyingly, in the case of Mn3+ there are novel families of complexes, including prolific work on complexes containing salen-type ligands producing SCO-active complexes immerse in a N4O2 coordination sphere. 3.1. Manganese(II) complexes In a systematic study, Andersen and co-workers [48] analysed the effect of bulky substituents in the magnetic properties of substituted cyclopentadienyl manganocenes. The systems studied contained one, two or three tert-butyl or trimethylsilyl substituents on the Cp ring. The compounds were characterised by single crystal X-ray crystallography, VT-magnetic measurements and by extended X-ray absorption fine-structure (EXAFS) measurements. Some of the mono- and di-substituted systems were found to be SCO-active. In Table 3.1 relevant bond lengths and magnetic properties for the SCO-active complexes are presented, which agree for the observed magnetic properties (see below). VT-magnetic measurements for the silyl containing family, [MnII(Me3SiCp)2], [MnII(1,3-(Me3Si)2Cp)2] and [MnII(1,2,4(Me3Si)3Cp)2], showed that only the monosubstituted compound is SCO-active (Table 3.1), the di- and tri-substituted analogues are stabilised in the HS-state, meff = 5.9 mB. In the case of [MnII(Me3SiCp)2], shows a meff value of c.a. 5.9 mB in the 300–150 K temperature range, below 150 K the magnetic moment value gradually decreases, reaching ~5.3 mB indicating a spin conversion to the LS-state of approximately 10% of the metal centres. On the other hand, two of the tert-butyl analogues present SCO behaviour. The only complex locked in the HS-state is [MnII(1,2,4(tBu)3Cp)2], whilst, the mono and di-substituted systems show SCO

properties. Complex [MnII(tBuCp)2] shows a relatively abrupt SCO centred around 200 K. Disubstituted [MnII(1,3-(tBu)2Cp)2] shows a much more complex magnetic behaviour. In the temperature range 5–200 K the magnetic moment corresponds to LS-state (~2.0 mB), above 200 K an increment of the magnetic moment is observed. An abrupt SCO takes place around 327 K, above this temperature the magnetic moment reaches a plateau, with a value of 5.5 mB. Interestingly, upon cooling, the shape and T½ are different form the warming mode. The transition becomes gradual with T½; = 314 K, with an apparent hysteresis loop of 13 K, however, this behaviour is irreversible. The authors suggest that the irreversible process could be due to a phase change during the SCO, which was somewhat confirmed by VT-EXAFS experiments. The mixed substituted complex [MnII(1-(tBu)-3-(Me3Si)Cp)2] undergoes a gradual and incomplete spin conversion centred around 174 K [48]. At room temperature the magnetic moment of 5.88 mB indicates the stabilisation of the HS-state. Upon cooling, the magnetic moment gradually decreases reaching a value of 4.21 mB below 100 K, and remains constant down to 5 K. The low temperature value suggests that 30% of the metal centres undergo spin conversion. The presence of both spin states at low temperature was confirmed by EPR spectroscopy. A sample collected at 4 K in frozen solution of methylcyclohexane shows the signals corresponding to the HS (g = 5.95) and LS states (g|| = 2.88 and g\ = 1.95), confirming an incomplete conversion to the LS-state. From these results the authors inferred that electron withdrawing substituents (SiMe3) stabilise better the HS state, whereas electron donating groups, tert-butyl, stabilise the LS-state or promote spin equilibrium. In a computational study performed by Cirera and Ruiz [52] the electronic and steric effects on a series of substituted manganocenes were studied. The authors calculated the energy difference between the HS- and LS-states and the T½ for a series of homo-substituted manganocenes varying the number of methyl, iso-propyl and tert-butyl moieties. A total of 12 structures were evaluated, including the H-substituted parent manganocene, the mono-, 1,3-di- and 1,3,4-tribustituted cyclopentadienyl systems for the three types of substituents and the tetra-substituted systems only for the methyl and isopropyl systems. The authors observed small structural differences between the parent compound [MnII(Cp)2] and the methyl-substituted family, only a 0.53% elongation in the M-L bond lengths. However, on increasing the size of the substituent a significant M-L elongation was observed, being up to 3.2% longer in the iso-propyl family and 4.2% longer in the tert-butyl system. A comparison of the calculated geometries with the available experimental crystal structures show a 1.6% deviation of the M-L bond lengths. Thus, the methyl group has no significant steric influence on the resulting complex,

Table 3.1 Some magnetic properties and bond lengths for [MnII(R-Cp)2] systems. CompoundRef II

[Mn (Cp)2] [49] [MnII(Cp*)2] [50] [MnII(tBuCp)2] [48] [MnII(Me3SiCp)2] [51] [MnII(1,3-(tBu)2Cp)2] [48]

[MnII(1-(tBu)–3-(Me3Si)Cp)2] [48] a b

Temperature of X-ray acquisition. Average values. NA = not available.

T½ [K]

T [K]a

Mn-C [Å]

b

Not SCO Not SCO 211 K, relatively abrupt ~125 K, incomplete and gradual (90% HS) T½" = 327 K (abrupt), T½; = 314 K (gradual), irreversible hysteretic SCO 174 K. Incomplete and gradual (70% HS)

NA RT 151

2.41 2.11 2.14

HS LS LS

238

2.38

Mostly HS

138

2.13

LS

NA

NA

NA

Spin state

8

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rather the observed magnetic differences should rise from electronic factors, whilst for the bigger substituents the steric factors are important and should have an impact on the magnetic properties. An analysis of the molecular orbitals calculated for the parent compound [MnII(Cp)2] demonstrates that the d-orbitals are clearly separated into two sets of orbitals, the lower-lying non-bonding orbitals, x2-y2, xy and z2, and the higher-lying antibonding orbitals, xz and yz, separated by an energy gap, D. Thus, the variation on the magnitude of D, and therefore on the SCO properties, will depend on the interaction of the cyclopentadienyl frontier orbital with the antibonding orbitals. In the case of the complexes containing methyl substituents it is possible to observe that increasing the number of substituents in the ring does not significantly affect the structural parameters, but an increase in the calculated energy gaps of the non-bonding and antibonding orbitals (D) is detected, due to the electron donor nature of the methyl groups. Additionally, an increase in the calculated T½ is observed, that is the LS-state is more stabilised in systems containing more methyl groups on the Cp moiety. However, in the case of bulkier iso-propyl and tert-butyl moieties, a competition between the steric and electronic factors is active. Increasing the number of this type of moieties in the ring produces a decrease in D, due to steric hindrance, even though these substituents are electron donor as well. In fact, the trisubstituted complex [MnII(1,2,4-(tBu)3Cp)2] is locked in the HS-state, as experimentally observed, see above. Scheer, Layfield and co-workers showed that reaction of [MnII(Cp)2] with Li[E(SiMe3)2] (E = P or As) produces the dinuclear complexes [CpMnII{m-E(SiMe3)2}]2 (E = P or As) [53]. Both complexes were characterised by single crystal X-ray crystallography and VT-magnetic measurements. The compound containing P donors, [CpMnII{m-P(SiMe3)2}]2 crystallises in the P1 space group. The asymmetric unit at 243 K consists of two half complex molecules, the other half is generated by independent inversion centres, Fig. 3.1. Each manganese centre is coordinated to a terminal Cp ligand and to two bridging P donors coming from the P(SiMe3)2 moieties, which bridge to the second metal atom. Thus, each metal centre is found in a five-coordinate sphere. The two independent metal complexes found in the asymmetric unit show similar structural parameters, Table 3.2. The crystal structure for the As analogue, [CpMnII{m-As(SiMe3)2}]2, was obtained at 123 K; the asymmetric unit in this case only contained one half complex molecule, the other half is generated by an inversion centre. The coordination geometry of the metal centre is identical to the one observed for the P-analogue, except for the bridging

Table 3.2 Some structural parameters for complexes [CpMnII{m-E(SiMe3)2}]2 (E = P or As).

T [K] Mn-E [Å] Mn-C [Å] Mn  Mn [Å] E-Mn-E [°] Mn-E-Mn [°]

[CpMnII{m-P(SiMe3)2}]2

[CpMnII{m-As(SiMe3)2}]2

243 2.5075(5)–2.5123(5) 2.326(19)–2.47(2) 3.429(2) 93.83(2) 86.1782)

123 2.5877(7)–2.5980(8) 2.29(3)–2.47(2) 3.641(1) 90.81(2) 89.20(2)

ligands now being occupied by As(SiMe3)2 bridging moieties, Fig. 3.1. Some structural parameters for this complex are presented in Table 3.2. Based on the Mn-C bond lengths it seems that both complexes are stabilised in the same spin state (see below), and as expected, the Mn-E bond lengths are longer for the As-based complex than for the P-analogue. VT-Magnetic measurements on polycrystalline samples of both complexes were obtained in the 2–300 K temperature range, Fig. 3.2. At room temperature complex [CpMnII{m-P(SiMe3)2}]2 shows a meff = 6.76 mB, which is about 20% smaller value than the expected for two isolated Mn2+ (S = 5/2). Upon cooling the magnetic value gradually decreases reaching a value of 0.46 mB, indicating that at low temperature the complex is diamagnetic due to antiferromagnetic exchange. The magnetic profile for this compound indicated strongly antiferromagnetic interactions, however, only the data above 210 K was successfully fitted using the Heisenberg-Dirac-Van-Vleck model, from which an exchange constant J = 13.5 cm1 (H = 2J(SMnASMnB)) was obtained. The values below this temperature were overestimated by this model, suggesting a possible SCO event, however the strong antiferromagnetic exchange precludes any quantitative conclusion. On the other hand, the arsenide based complex magnetic profile is quite unusual. At 300 K meff = 8.10 mB, slightly smaller than the 8.36 mB value expected for two non-interacting Mn2+ centres. This value decreases to 7.83 mB at 105 K, below this temperature an abrupt decrease is observed, reaching 7.14 mB at 96 K. The magnetic moment continues decreasing upon cooling in a steadily manner, but below 75 K the decrease becomes more rapid, reaching 0.98 mB at 2 K. The value at low temperature suggests an essentially diamagnetic ground state. Upon warming, a small hysteresis loop is observed between 96 and 105 K, which is reversible during several subsequent cooling-warming cycles. With the data above 105 K, it was possible to calculate the antiferromagnetic exchange interaction, as for the P-analogue (see above), obtaining a value of J = 1.5 cm1. Similarly, the data below 105 K was overestimated by the model; thus, considering the

Fig. 3.1. Perspective view of the X-ray crystal structure for complex [CpMnII{m-P(SiMe3)2}]2 (left) and [CpMnII{m-As(SiMe3)2}]2 (right). H-atoms are omitted for the sake of clarity.

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

9

Although there are no recent ligand systems that provide the right ligand field for manganese(II) complexes to undergo SCO, the discovery of dinuclear Mn2+ complexes that show a two-step SCO to IS state is an exciting result. Thus, is probable that the use of Cp and related ligands in combination with other coligands, may produce more SCO-active systems. 3.2. Manganese(III) complexes

Fig. 3.2. vMT vs T plots for complexes [CpMnII{m-E(SiMe3)2}]2 (E = P or As). The continue red lines are a theoretical fit of the experimental data. The inset shows the small hysteresis loop for As-based complex. Figure reproduced, slightly modified, with permission from Ref. [53]. Copyright RSC 2012.

abrupt change in the magnetic moment along with the presence of the hysteresis loop, the authors suggested that a SCO-event had taken place. The magnetic value at 96 K is approximately 80% of the data observed at RT, suggesting the presence of a mixed spin state complex, that is, one of the metal centres is stabilised in the HS-state (S = 5/2), whereas the second metal centre is stabilised in the IS-state (S = 3/2). The authors proposed that the abrupt ST observed at 96 K should occur from [HS-HS] to [HSIS]-state, Fig. 3.3. The diamagnetic ground state at 2 K indicates that at this temperature both Mn centres should be stabilised in the same spin state, therefore, a second SCO event should occur below 96 K, from [HS-IS] to [IS-IS]-state, Fig. 3.3. Based on these results, the authors propose that the magnetic profile for the Pbased complex should follow a similar transformation, although with a stronger antiferromagnetic exchange. These results are very unusual for Mn2+ SCO-complexes. Most of them are six-coordinate organometallic complexes that show gradual and incomplete spin conversions (see above). In the case of [CpMnII{m-As(SiMe3)2}]2, which is also an organometallic complex but in a five-coordinate sphere, shows a two-step, abrupt and, for the first time, hysteretic ST coupled with antiferromagnetic exchange at both ends. Additionally, the transformation observed for this dinuclear complex involves the stabilisation of the IS-state, which is not unusual for five-coordinate iron(III) complexes.

Fig. 3.3. Proposed change in orbital population during the two-step SCO-event for [CpMnII{m-As(SiMe3)2}]2.

The complexes presented in this section correspond to Mn3+ compounds in a N4O2 coordination sphere containing iminebased ligands. The first kind involves hexadentate salen-type ligands of substituted salicylaldehyde units and tetraamines, resulting in diimines containing two phenolate moieties, two imine and two secondary amine nitrogen donors in the same ligand strand (Fig. 3.4). The second type of ligands corresponds to tridentate monoanionic ligands, containing two sp2 N-donors and one phenolate or alkoxylate donor, Fig. 3.4. 3.2.1. Hexadentate ligands The first example of this type of complexes based on a diimine ligand derived from two 3-methoxysalicylaldehyde units and a tetraamine moiety was reported by Morgan and co-workers in 2006 [54], Fig. 3.4. In this pioneer work, two diimine hexadentate ligands were synthesised varying on the length of the tetraamine connector, either utilising triethylenetetramine (222, see below) or N,N0 -bis(3-aminopropyl)ethylenediamine (323), to form their respective Mn complexes: black [MnIII(3-MeO-Sal-323)]NO3 and dark brown [MnIII(3-MeO-Sal-222)]NO3. VT-magnetic measurements showed that [MnIII(3-MeO-Sal-323)]NO3 undergoes a gradual and incomplete SCO around 220 K. At 80 K the magnetic moment value of 2.8 lB is in perfect agreement for the lS.O. expected value for the LS-state (S = 1), however upon heating to 300 K, the leff < 4.4 lB is smaller than lS.O. = 4.90 lB, indicating an incomplete conversion to the HS-state (S = 2). On the other hand, [MnIII(3-MeO-Sal-222)]NO3 is stabilised in the HS state in the 80–300 K range. X-ray crystallography demonstrated that the ligands coordinate in a hexadentate form in both complexes, resulting in a distorted octahedral coordination sphere. Nonetheless, in the case of [MnIII(3-MeO-Sal-323)]NO3, which contains two propylene terminal linkers in the tetraamine unit, the phenolates coordinate in a trans-fashion. In contrast, for the more constrained ethyleneconnected analogue, involving a triethylenetetraamine unit, [MnIII(3-MeO-Sal-222)]NO3, the phenolates are found in cis positions. The authors suggested that the difference in the spatial location of the N and O-donors around the metal centre could be responsible for the disparity in magnetic properties. The SCOactive complex was characterised below and above T½, 100 and 300 K, Fig. 3.5. At 100 K, LS-state, the average Mn-N bond lengths are 0.098 Å shorter in average, than the structure collected at 300 K (HS-state), but the Mn-O bond lengths remained practically unchanged, Table 3.3. This structural change indicates that upon conversion to the HS-state, the promoted electron occupies the dx2 y2 antibonding orbital producing an equatorial elongation, Fig. 3.6, instead of an axial elongation when the dz2 antibonding orbital is occupied. The equatorial elongation is reflected in the larger octahedral distortion parameter ROh observed for the HS, Table 3.3. Since the discovery of [MnIII(3-MeO-Sal-323)]NO3 SCO-active complex there has been an intensive and productive research based on this type of complexes, by varying the substituents of the aromatic ring of the ligand, the counterion and, fortuitously, the solvent content in the crystal lattice of such complexes. A summary of the SCO properties and structural parameters of Mn3+

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 3.4. Schematic representation and numbering scheme for anionic hexadentate and tridentate ligands utilised to produce SCO-active Mn3+ complexes.

Fig. 3.5. Perspective view of the X-ray structure of complex [MnIII(3-MeO-Sal-323)] NO3 collected at 100 K (left) and 300 K (right), showing the changes on the Mn-O and Mn-N bond lengths during the SCO event [54].

complexes based on R-Sal-323 ligands is presented in Table 3.3. In this table the compounds have been organised by ligand type, i.e. ligand substituent. In each ligand-type subsection the complexes have been sorted from lower to higher T½; complexes locked in the HS-state are located on top of each subsection, whereas LSlocked complexes are listed at the end of the list, both types of locked complexes are found in italic font to distinguish them from SCO-active complexes. Thus, in this section an analysis of the three possible factors that affect the SCO-properties for this type of complexes is presented. Firstly, a general panorama on the magnetic and structural behaviour for the SCO complexes is presented, followed by a subsection for each one of the three factors that can affect the SCO-properties: the substituents attached to the aromatic ring, followed by the anion and, finally, the solvent content/nature. 3.2.1.1. Magnetic properties. It is possible to observe from Table 3.3 that from 36 complexes analysed in this work, only one complex is

Fig. 3.6. Change in orbital population during the SCO-event for [MnIII(3-MeO-Sal323)]NO3 [54].

found locked in the LS-state in the experimental temperature range, [MnIII(3,5-Br-Sal-323)]PF6½ MeOH [55], which is surprising as most of the complexes based on this type of ligands that are not SCOactive are locked in the HS-state. Additionally, eight metal complexes are found locked in the HS-state, namely [MnIII(Sal-323)] SbF6 [56], [MnIII(3-MeO-Sal-323)]BPh4 [57], [MnIII(3-MeO-Sal-323)] PF6 [57], [MnIII(3-EtO-Sal-323)]BPh4 [57], [MnIII(3-EtO-Sal-323)] NO30.6EtOH [57], [MnIII(5-Br-Sal-323)]NO3 [58], [MnIII(5-Br-Sal323)]PF6 [58] and [MnIII(5-Br-Sal-323)]TCNQ1.52MeCN [59], which contain three different types of ligands, the unsubstituted Sal-323 ligand, and 3-alkoxy or 5-Br substituted ligands, additionally most of them have bulky anions, except for two containing a nitrate anion. The rest of the complexes show gradual and incomplete SCOprofiles. Nonetheless, there are four metal complexes showing abrupt and hysteretic behaviour, that contain either the unsubstituted salicylaldehyde moiety or Cl-substituted ligand, with one or two Cl: [MnIII(Sal-323)]PF6 [60], [MnIII(Sal-323)]AsF6 [56], [MnIII(5-Cl-Sal-323)]TCNQ1.52MeCN [59] and [MnIII(3,5-Cl-Sal323)]NTf2 [61]. In the case of the complexes showing a gradual and incomplete SCO-profile the magnetic moment at room temperature is lower

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Table 3.3 SCO properties and relevant bond lengths and angles for Mn3+ complexes based on diimine hexadentate ligand of the type [MnIII(R-Sal-323)]A (A = anion). HS- and LS-locked complexes are shown in italic font. CompoundRef.

T½ [K]

T [K]a Mn-O [Å]

Mn-N [Å]

ROh [°]b Spin state

Sal-323 [MnIII(Sal-323)]SbF6 [56]

HS-locked

100 295 100 126" 126; 142 110; 146; 164" 295 100 295 100 295

1.8614(10) 1.855(2) 1.8773(14)–1.8836(14) 1.8763(15)–1.8783(16) 1.8702(12)–1.8723(12) 1.8745(19)–1.873(2) 1.880(3)–1.878(3) 1.873(3)–1.878(3) 1.870(5)–1.873(4) 1.854(5)–1.864(5) 1.8811(8)–1.8849(8) 1.876(2)–1.877(2) 1.877(2) 1.8712(19)

2.1139(12)–2.2350(13) 2.107(3)–2.236(3) 1.9899(17)–2.0677(17) 1.9971(18)–2.0827(19) 2.0834(15)–2.2135(17) 2.086(2)–2.210(3) 1.992(4)–2.068(4) 2.006(3)–2.085(4) 2.007(5)–2.123(6) 2.098(6)–2.240(7) 2.0231(10)–2.1570(11) 2.049(3)–2.204(3) 1.985(3)–2.049(3) 2.022(3)–2.097(3)

69.7 71.8 28.9 32.0 54.0 54.5 31.4 36.6 42.1 59.0 44.2 54.1 35.6 46.5

HS-locked HS-locked LS Mainly LS Mainly HS HS LS Mainly LS Mainly LS HS LS HS LS HS

100 – 100 293 100 293 100 300 100 293

1.8516(18)–1.8829(18) – 1.8769(10)–1.8896(10) 1.875(4)–1.879(4) 1.8755(11)–1.8866(11) 1.8710(14)–1.8755(15) 1.8719(9) 1.8679(13) 1.8784(11)–1.8847(11) 1.8775(15)–1.8742(15)

2.065(2)–2.256(2)– – 1.9961(13)–2.0704(13) 2.064(5)–2.190(5) 1.9890(14)–2.0589(14) 2.0538(18)–2.173(2) 1.9883(11)–2.0501(11) 2.0723(19)–2.161(2) 1.9913(13)–2.0638(14) 2.0430(19)–2.166(2)

78.1 – 32.8 49.1 32.8 48.1 45.0 70.7 31.9 44.9

HS-locked – LS HS LS HS LS HS LS HS

HS-locked HS-locked 196. Gradual and incomplete 197. Gradual and incomplete 277. Gradual and incomplete

100 – 100 293 100 293 100

1.8687(10)–1.8586(10) – 1.8706(14)–1.8835(13) 1.8662(13)–1.8810(12) 1.8767(15)–1.8825(14) 1.8704(15)–1.8816(14) 1.8629(14)–1.8726(13)

2.0950(11)–2.2479(12) – 1.9940(17)–2.0775(17) 2.0483(16)–2.1861(19) 1.9913(18)–2.094(2) 2.0435(19)–2.193(2) 2.0668(17)–2.2440(17)

70.2 – 33.9 51.3 34.4 51.2 67.0

HS-locked – LS HS LS HS HS

~100. Gradual

173

1.850(7)–1.902(6)

2.034(8)–2.275(6)

HS

~150. Gradual and incomplete

173

1.873(2)–1.879(2)

2.045(3)–2.145(3)

62.9, 71.8 54.5

>250. Gradual and incomplete ~ 295. Gradual, incomplete and irreversible

– –

– –

– –

– –

– –

HS-locked 80–300 K HS-locked 80–300 K HS-locked ~ 175. Gradual [HS][HS] to [HS][LS] (1:1)

293

1.870(2)–1.875(2)

2.078(3)–2.243(3)

64.4-

HS-locked

293

1.866(3)–1.878(3)

2.091(3)–2.252(4)

74.8

HS-locked

100 100

2.095(5)–2.175(5) 2.110–2.200c, 1.999–2.060d 2.086(4)–2.184(4)–

67.0 65.5c, 58.7d 65.5

HS-locked Mainly HS

294

1.871(4)–1.886(4) 1.867c, 1.881d 1.865(3)–1.869(3)

T½; = 73, T½" = 123, DT½ = 50. Hysteretic and abrupt ~200. Gradual and incomplete

100 220

1.872(2)–1.884(2) 1.871(2)–1.881(2)

2.089(3)–2.222(3) 2.095(2)–2.220(2)

65.58 66.81

HS HS

100

1.8690(12)–1.8737(12)c 1.8726(12)–1.8767(12)e

64.7c 59.5e

HS

293

1.876(2)c 1.878(2)–1.879(2)e 1.8608(13)c 1.8787(13)d

65.0c 64.3e 66.0c 33.4d

HS

24.1 36.8 36.8 62.1

III

[Mn (Sal-323)]PF6 [60]

[MnIII(Sal-323)]AsF6 [56]

132–134, DT½ = 8. Abrupt and hysteretic. Particle size dependent T½; = 146, T½" = 164, DT½ = 18. Hysteretic, relatively abrupt

[MnIII(Sal-323)]NO3EtOH [56]

~ 290. Gradual and incomplete

[MnIII(Sal-323)]Cl [56]

~ 315

3-MeO-Sal-323 [MnIII(3-MeO-Sal-323)]BPh4 [57] [MnIII(3-MeO-Sal-323)]PF6 [57] [MnIII(3-MeO-Sal-323)]BF4H2O [57] [MnIII(3-MeO-Sal-323)]ClO4H2O [57]

HS-locked HS-locked 181. Gradual and incomplete 192. Gradual and incomplete

[MnIII(3-MeO-Sal-323)]NO3 [54]

~ 220. Gradual

III

[Mn (3-MeO-Sal-323)]PF6H2O [57]

3-EtO-Sal-323 [MnIII(3-EtO-Sal-323)]BPh4 [57] [MnIII(3-EtO-Sal-323)]NO30.6EtOH [57] [MnIII(3-EtO-Sal-323)]ClO4 [57] [MnIII(3-EtO-Sal-323)]BF40.4H2O [57] [MnIII(3-EtO-Sal-323)]NO3EtOH [57] C6H13O-Sal-323 [MnIII(4-C6H13O-Sal-323)]NO30.75Acetone [64] [MnIII(6-C6H13O-Sal-323)]NO3H2O [64] C18H37O-Sal-323 [MnIII(4-C18H37O-Sal-323)]NO3 [64] [MnIII(6-C18H37O-Sal-323)]NO3 [64] 5-Br-Sal-323 [MnIII(5-Br-Sal-323)]NO3 [58] [MnIII(5-Br-Sal-323)]PF6 [58] [MnIII(5-Br-Sal-323)]TCNQ1.52MeCN [59] [MnIII(5-Br-Sal-323)]ClO4 [58]

5-Cl-Sal-323 [MnIII(5-Cl-Sal-323)]TCNQ1.52MeCN [59]

[MnIII(5-Cl-Sal-323)]NO3[65]

[MnIII(5-Cl-Sal-323)]ClO4 [65]

3,5-Br-Sal-323 [MnIII(3,5-Br-Sal-323)]ClO4½MeCN [55] [MnIII(3,5-Br-Sal-323)]BF4EtOH [55]

~200. Gradual and incomplete. Solvate-dependent

>281. Gradual and incomplete

75. Gradual and incomplete ~175. Gradual and incomplete

260

1.8641(16)–1.8641(16)c 1.8726(15)e

2.0695(14)–2.2613(14)c 2.0628(15)–2.1929 (15)e 2.091(2)–2.244(3)c 2.100(3)–2.206(3)e 2.1071(16)–2.2201(17)c 1.9826(17)–2.0484 (18)d 2.1055(19)–2.219(2)c 2.072(2)–2.163(2)e

100 200 100 293

1.873(2)–1.877(2) 1.869(2)–1.874(2) 1.879(3)–1.887(3) 1.869(4)–1.881(4)

1.984(3)–2.054(3) 2.024(3)–2.114(3) 1.997(5)–2.066(4) 2.053(6)–2.179(5)

100

66.2c 52.9e

Mainly HS

HS

Localised [HS]:[LS] 1:1 HS

LS Mainly LS LS Mainly HS

(continued on next page)

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Table 3.3 (continued) T½ [K]

T [K]a Mn-O [Å]

Mn-N [Å]

ROh [°]b Spin state

III

~ 200. Gradual and incomplete

III

[Mn (3,5-Br-Sal-323)]ClO4EtOH [55]

>200. Gradual and incomplete

[MnIII(3,5-Br-Sal-323)]NO3EtOH [55] [MnIII(3,5-Br-Sal-323)]PF6 ½ MeOH [55] 3,5-Cl-Sal-323 [MnIII(3,5-Cl-Sal-323)]NTf2 [61]

>250. Gradual and incomplete LS-locked

100 293 100 293 100 100

1.8881(10)–1.8934(10) 1.881(2)–1.880(3) 1.870(4)–1.8888(4) 1.868(3)–1.882(3) 1.8820(16)–1.8831(16) 1.861(2)–1.879(2)

1.9863(12)–2.0569(12) 2.053(3)–2.166(4) 2.0009(5)–2.098(5) 2.063(5)–2.200(4) 1.982(2)–2.045(2) 1.980(3)–2.058(3)

29.8 59.5 47.2 64.8 27.4 25.4-

LS Mainly HS Mainly LS HS LS LS-locked

120 185

1.993(3)–2.065(4) 2.081(2)–2.189(2)c, 2.001(2)–2.0738(19)d

35.5 58.1c, 37.8d

210

1.888(3) 1.8801(17)–1.8818 (17)c, 1.8833(17)–1.8906(17)d 1.885(2)–1.887(2)

2.037(3)–2.132(3)

48.7

LS Localised [HS]:[LS] 1:1 Delocalised [HS]:[LS] 1:1 Mainly HS LS Mixture [HS][LS] LS

CompoundRef. [Mn (3,5-Br-Sal-323)]CF3SO3EtOH [55]

a b c d e

Two-step ST: T1½ = 218, T2½; = 157, T2½" = 172, DT2½ = 15

[MnIII(3,5-Cl-Sal-323)]NO3EtOH [65]

>300. Gradual and incomplete

260 100 293

1.881(3)–1.884(3) 1.8709(14)–1.8815(14) 1.879(19)–1.887(18)

2.065(4)–2.168(4) 1.9852(17)–2.0448(17) 2.013(2)–2.104(2)

55.8 36.1 48.9

[MnIII(3,5-Cl-Sal-323)]ClO4½MeOH [65]

>300. Gradual and incomplete

174

1.872(2)–1.877(2)

1.980(3)–2.058(3)

27.3

Temperature of crystallographic study. ROh = sum of the deviation from 90° of the 12 cis angles in the octahedral coordination sphere [66,67]. HS unit. LS unit. Mostly HS.

than the expected lS.O. = 4.89 lB, clearly indicating a mixture of HS and LS states. Upon cooling most of them show a leff value larger than the expected lS.O. = 2.82 lB, indicating incomplete transitions at both ends. The exceptions for the magnetic behaviour above described are four complexes showing abrupt and hysteretic transitions and a long-alkyl chain-based complex that shows irreversible SCO upon heating, that will be discussed in more depth as follows. The complex [MnIII(Sal-323)]PF6 shows an abrupt, reversible and hysteretic ST with parameters: T½ varying 134–132 K (depending on the particle size) and DT½ = 8 K, Fig. 3.7 [60]. The ST did not show fatigue after three cooling-heating cycles, and was confirmed by DSC measurements, observing an 8 K hysteresis loop as well. The cooperative character of the SCO was corroborated by the parameters: DH = 2.72(1) KJ mol1 and DS = 20.9(1) J mol1 K1, which are higher than the values observed for the pyrrolyl-based system that shows a smaller hysteresis loop. The hysteretic behaviour was proved by VT Raman spectroscopy and X-ray crystallography (see below). Complex [MnIII(Sal-323)]AsF6, shows a relatively abrupt hysteretic ST with parameters: T½; = 146 K, T½" = 164 K and DT½ = 18K. By changing the anion size (c.f. SCO-active PF6 analogue) the transition temperature and hysteresis loop have increased, albeit decreasing the transition abruptness. The size of the counteranion seems to have an important role on the magnetic properties, see below, for instance the SbF6 analogue is locked in the HS-state. Introduction of bis(trifluoromethane)sulfonimidate (NTf2) as counterion to obtain [MnIII(3,5-Cl-Sal-323)]NTf2, results in a SCOactive complex, that shows a two-step ST accompanied by a hysteresis loop for the second step upon cooling (Fig. 3.8) [61]. At RT, the complex is stabilised mostly in the HS-state (meff = 4.6 mB, around 75% in the HS-state). On cooling (5 K min1) a gradual decrease in the magnetic moment is observed, reaching a plateau between 160 and 210 K temperature range where meff = 3.6–3.9 mB. The latter value suggests a spin conversion to the HS:LS ratio close to 1:1, with T½ = 218 K. Further cooling causes an abrupt drop in the magnetic moment to c.a. 2.8 mB, in perfect agreement for a complete transition to the LS-state, T½; = 157 K. When the sample is now heated at 5 K min1 an abrupt transition to the HS:LS state

is observed, however at a different transition temperature, T½" = 172 K, that is a 15 K hysteresis loop. On further warming, the second step to mainly HS-state is once again gradual, and T½ = 218 K is maintained. It is well known that hysteresis loop width is, in many occasions, scan rate dependant [17,62]. In this case, experiments performed at 10.0, 5.0, 2.0, 1.0, 0.5 and 0.1 K min1 showed the hysteresis loop for the second step is scan rate dependant. At the fastest scans (10.0, 5.0 and 2.0 K min1) there is no variation on T½; and T½", however, by reducing the scan rate there is a decreased in the hysteresis width (Fig. 3.8). Interestingly, only the cooling branch is affected, as previously observed for a dinucler iron(II) complex [63]. The hysteretic behaviour and scan rate dependence of DT½ was confirmed by DSC experiments VT-and X-ray crystallography (see below). From the former it was possible to obtain the total energy changes for the transition: DHTot = 6.9 kJ mol1 and DSTot = 37.6 J mol1 K1. The last hysteretic SCO-active complex has been recently reported by Kazakova, Yagubskii, Vasiliev and co-workers [59], namely [MnIII(5-Cl-Sal-323)]TCNQ1.52MeCN, containing a radical anion 7,7,8,8-tetracyanoquinodimethane (TCNQ) as counterion. It is important to note that the Br-based analogue, [MnIII(5-BrSal-323)]TCNQ1.52MeCN, is locked in the HS state indicating the importance of the substituent to tune the ligand field, which will be analysed in a different subsection (see below). The complex [MnIII(5-Cl-Sal-323)]TCNQ1.52MeCN undergoes and abrupt, hysteretic and reproducible ST with parameters T½; = 73 , T½" = 123 and DT½ = 50, which is the largest hysteresis loop found for this type of complexes thus far. It is worth to note that the radical anion TCNQ does not contribute to the magnetic behaviour due to formation of antiferromagnetic coupled dimers, confirmed by X-ray crystallography (see the original work for more information [59]). In addition to the SCO properties, this complex possesses semiconducting behaviour, showing conductivity at room temperature with a value of 1  104 ohm1 cm1 and an activation energy barrier of 0.20 eV, due to the presence of TCNQ anion. In an attempt to increase cooperativity and to form stable Langmuir-films at the air–water interface Albrecht [64] and collab-

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

13

results is clear that incrementing the size of the alkyl chains stabilises the LS-state. The self-assembly ability of this type of compounds was tested by forming Langmuir-Blodgett films in the air–water interface. The complex containing the hexyloxy substituent did not form stable films. However, the longer C18H37O substituent did form stable films. Transfer of the Langmuir films onto solid supports was not very successful, as there was desorption of the compounds during downstrokes, producing multi-layered structures with defects.

Fig. 3.7. VT-magnetic characterisation of compound [MnIII(Sal-323)]PF6 showing an abrupt and hysteretic behaviour (solid line is only a guide). Inset shows an expansion of the hysteresis loop area. This graph was constructed utilising the data found in [60].

orators synthesised four metal complexes containing ligands with long alkoxy chain functional groups. The long alkoxy chains, OC6H13 or OC18H37 were placed in two different positions, ortho or meta to the phenol moiety. Four purple complexes [MnIII(RSal-323)]NO3 (R = 4-C6H13O, 6-C6H13O, 4-C18H37O and 6-C18H37O) were obtained. VT-magnetic measurements showed that the complexes containing C6H13O substituent undergo a gradual and incomplete conversion to the LS-state upon cooling. For the complex [MnIII(4-C6H13O-Sal-323)]NO30.75acetone only one third of the metal centres undergo spin change, with T½ = 100 K, whereas for complex [MnIII(4-C18H37O-Sal-323)]NO3H2O two thirds of the metal centres undergo conversion to the LS-state at T½  150 K. For the longer alkyl chain-based complexes [MnIII(4-C18H37OSal-323)]NO3 and [MnIII(6-C18H37O-Sal-323)]NO3 the magnetic measurements indicate that both complexes are SCO-active, however, with different magnetic profiles. Complex [MnIII(4-C18H37OSal-323)]NO3 shows a gradual transition centred around 250 K, whereas a T½ = 295 K is observed for [MnIII(6-C18H37O-Sal-323)] NO3. Interestingly, when the latter compound is heated up to 350 K the population to the HS is almost complete, nonetheless, cooling the already heated sample, showed that the SCO is deactivated, as only a fraction of the metal centres undergoes spin conversion. The authors proposed that heating the crystalline sample modifies the packing of the complexes along the crystal lattice due to the presence of the long alkyl chains. This hypothesis was supported by the identical magnetic behaviour observed for an amorphous sample of [MnIII(6-C18H37O-Sal-323)]NO3. From these

3.2.1.2. X-ray crystallography. The family of hexadentate ligand RSal-323 coordinates to the metal centre in the same fashion, regardless of the substituent. As mentioned above, the equatorial positions are occupied by two amine and two imine donors, whereas the apical sites are occupied by the phenolate donors. As mentioned in the introduction, the HS-state for octahedral d4 metal complexes shows strong JT-distortions. For this family of complexes, the JT-distortion is reflected in an elongation of the equatorial bonds, upon spin conversion to the LS the equatorial bonds shorten, however, the apical positions do not show significant changes. Thus, in general, the coordination sphere for the HS-state is more distorted than for the LS-state, and therefore the ROh values are larger for the HS than for the LS-state. A fair number of complexes have been characterised by single crystal X-ray crystallography at both the HS- and LS-state. An analysis of the supramolecular interactions present in the crystal lattice shows that, in general, upon conversion to the HS-state, that is increasing temperature, the hydrogen bond interactions formed between the NH-groups of the ligand and the anions are debilitated, explaining the gradual and incomplete magnetic profile for most of the complexes analysed. It has been demonstrated, particularly for FeII complexes, that supramolecular interactions permit communication among spin carriers, increasing cooperativity, and as a result, abrupt and in some occasions hysteretic ST are observed. Although, most complexes behave as described above, there are some exceptions that are worth to analyse in more depth, such as a compound stabilised in a 1:1 HS- and LS-state ratio, and hysteretic and abrupt SCO-complexes. The black compound containing 5-Br substituted ligand, [MnIII(5-Br-Sal-323)]ClO4, showed a gradual and incomplete SCO, centred around 175 K (Table 3.3) [58]. X-ray crystallography was performed on the same crystal at two different temperatures to characterise the spin conversion, at 100 and 294 K. Upon cooling, the cell volume decreases from 2653.7 Å3 at RT to 2581.2 Å3 at 100 K. The asymmetric unit at 100 K consists of two half complex cations and one anion, however, the structural parameters are now different for each cation. One of them is stabilised in the HS-state as the bond lengths and angles are similar

Fig. 3.8. VT-Magnetic moment measurements for complex [MnIII(3,5-Cl-Sal-323)]NTf2 showing the two-step SCO event (left). Hysteresis loop scan rate dependence (right). These graphs were constructed utilising the data found in [61].

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

to the RT structure (Table 3.3). The second molecule, however, showed smaller Mn-N bond lengths, an average difference of 0.09 Å, indicating that this molecule is stabilised in the LS-state. As previously observed, the Mn-O bond lengths are very similar in the HS-state and the LS-state. Thus, these results indicate that the complex is stabilised in a localised 1:1 mixture of HS and LSstate at 100 K. As expected, the octahedral distortion parameter, ROh, is larger for the HS state (Table 3.3). At 100 K, the molecule stabilised in the HS-state has identical ROh as the one observed at RT, whereas the second molecule has a smaller value, confirming the stabilisation of the LS-state for the latter. The authors propose that the stabilisation of different spin states at 100 K could be due to a difference in the supramolecular interactions along the crystal lattice. The complex cations are organised in chains, where the same spin state units are arranged in a zig-zag manner. The perchlorate anions form hydrogen bonds with the ligand skeleton, interconnecting the chains. For the complex cation locked in the HS-state the H-bonds interactions are denser. Additionally, for this HS complex cation there is p-p stacking interactions within neighbouring molecules. All these interactions produce a much more rigid environment for the molecules locked in the HS-state, blocking the conversion to the LS-state. The hysteretic SCO-complex [MnIII(Sal-323)]PF6 [60] was characterised by. X-ray crystallography, the data set was collected at 100, 126 (warming " and cooling ;) and 142 K, see Table 3.3. The compound crystallises in the P212121 space group, and the asymmetric unit consists on one complex cation and one anion, at all temperatures. The supramolecular interactions present in the crystal lattice was also investigated, showing that at low temperature the NH groups of the ligand are H-bond with the F atoms of the PF 6 , thus one anion bridges two neighbouring cation complexes forming a 1D supra-polymer. Upon warming the well-behaved anion suffers disorder over two positions, which in turns affects its ability to bridge neighbouring cations, resulting in localised ion pairs along the crystal lattice. This decrease in the number and intensity of H-bonding interactions has been seen in other related systems which show gradual and incomplete spin conversions, however, for this system the reduction in supramolecular interactions, and an increase in anion disorder, provoke an abrupt change in spin state. A similar trend is observed for the bulkier SbF6 analogue, [MnIII(Sal-323)]SbF6 [56]. For this complex the Xray crystallography data sets were collected at 110, 146, 164 and 295 K. From which it is possible to observe that the anion plays an important role in the cooperativity of the ST, similarly to the PF6 analogue, upon warming the disorder of the anions increases, thus disrupting the NHF H-bond network during the transition to the HS-state. In the case of complex [MnIII(3,5-Cl-Sal-323)]NTf2 (NTf2 = bis(tri fluoromethane)sulfonimidate) X-ray crystallography data sets were acquired at 260, 210, 185 and 120 K (Table 3.3), along with measurements of cell parameters in the 300–100 K temperature range at 0.2 K min1 scan rate, Fig. 3.9. Confirming the two-step conversion and hysteretic behaviour on the second step. Moreover, it was possible to determine a symmetry breaking occurring after the first transition on cooling (see below), by looking at the appearance of a super structure below 210 K, that reaches a maximum at 185 K, and decreases with temperature until it disappears below 168 K, Fig. 3.9. The asymmetric unit for the structure collected at 260, 210 and 120 K comprises only one complex cation and one disordered anion, but the structure collected at 185 K shows two complex cations and two disordered anions. Single crystal X-ray crystallography at 260 K (Table 3.3) shows bond lengths parameters in agreement for a metal centre stabilised mainly in the HS-state, and the presence of JT-equatorial elongation distortion expected for this type of systems. At this temperature the NTf 2 is disordered

over two positions. Upon cooling to 210 K, the average Mn-L bond length value decreases by 0.02 Å, according to VT-magnetic measurements (see above, Fig. 3.8) at this temperature there is a mixture of [HS][LS] states near to 1:1 ratio, thus the mixed spin state is delocalised. On further cooling to 185 K there are two crystallographic independent complex cations, and the volume cell has doubled. One of the complex cations is clearly stabilised in the HS-state whereas the second one is stabilised in the LS-state according to Mn-N bond lengths and ROh (Table 3.3), indicating that the mixed spin state is now localised. At this temperature the anions are highly disordered which interferes with a proper refinement of the structure (see original publication for more information) [61]. Finally, at 120 K the complex is clearly stabilised in the LS-state, Table 3.3. The authors suggested that coupling a highly rotational disordered anion with Mn3+ SCO active complex resulted in the magnetic behaviour observed for [MnIII(3,5-Cl-Sal-323)]NTf2, probably due to a synergistic coupling between the structural changes produced by JT-distortion, present in the HS-state but not the LS-state, and the rotational disorder provided by the anion, which increases by lowering the temperature, the latter is unusual as, in general, increasing the temperature results in more disorder.

3.2.1.3. Substituent effect. The Mn-based complexes analysed in this section contain 11 different ligands varying on the positioning and nature of the substituents attached to the phenyl ring of the Sal323 ligand, Table 3.3. However, for most of them only one or two SCO-active complexes have been obtained, and/or the counter anion is different from the other complexes, hampering a comparison for substituent analysis. In order to evaluate a general effect of the substituents, it is necessary to maintain the other two possible factors that affect the magnetic properties constant, or at least the most similar as possible, that is the anion and solvent content. Therefore, three groups of complexes containing five different ligands comply these two requirements: 3-MeO-Sal-323, 3-EtO-Sal-323, 5-Cl-Sal-323, 3,5Br-Sal-323 and 3,5-Cl-Sal-323. The selected complexes for comparison also contain three anions in common, BF4, ClO4 and NO3, and most of them contain solvent molecules similar in nature, methanol, ethanol and water, that are capable of hydrogen bonding, except for four anhydrous complexes included in this comparison. In Fig. 3.10, a bar plot of the transition temperature for each of the compared complexes is presented, arranged by anion and ligand substituent, the anhydrous complexes are shown as grey bars. By looking at the T½ for these families of complexes it is possible to observe that the complexes showing the highest T½, and therefore the better stabilization of the LS-state, are those containing the ligand 3,5-Cl-Sal-323, followed by complexes containing two ligand types that have relatively the same T½ values, but smaller than the latter ligand, which are 3-EtO-Sal-323  3,5-Br-Sal-323, and finally the ligand 3-MeO-Sal-323. In the case of ligand 5-ClSal-323 a direct comparison is difficult since the two complexes based on this ligand are isolated as anhydrous solids, and a big difference in the T½ values are observed for both complexes. Among the complexes presented in Table 3.3 it is possible to observe that most of them contain nitrate as the counter anion. In Fig. 3.11 a bar plot of T½ values for the ten nitrate-based complexes is presented. Based on this plot it is possible to confirm that the ligand 3,5-Cl-Sal-323 is the best stabilising the LS-state, and thus the T½ value is the highest for this series as well, followed by 6-C18 long tail alkoxy chain, unsubstituted and 3-ethoxybased ligands. The next ligand type in this series are those containing 3-methoxy and 5-chloro substituents, both with similar T½ values, and at the end of the series the 4- and 6-hexyloxy substituted ligands are found.

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

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It is quite interesting to find that the moderate ortho/para activators 3-alkoxy substituents, do not produce SCO-active complexes with the highest T½ value, but are found below the ligand containing ortho/para deactivators possessing two chloride substituents at the 3- and 5-positions. Another interestingly finding is that the positioning of the substituent is quite important in order to obtain SCO-active complexes with high T½ values. The latter can be deduced from the alkoxy substituent series, as these moieties have been introduced in three possible sites, 3-, 4- and 6-positions. Comparing 3-methoxy and -ethoxy moieties vs. 4- and 6-hexyloxy, we can conclude that complexes based on ligands with substituents at the 3-position show higher T½, probably because at this position the substituents activate the O-phenolate donor, found ortho to the substituents, better than the meta-substituted analogues at the 4- and 6-positions, which could not activate such donor. An exception to this is the complex containing the long alkyl chain C18H13O at the 6-position, and in this case the presence of the long alkyl chain produces supramolecular contacts among them, resulting in an increase in cooperativity and thus in higher T½ values. On the other hand, the 3,5-Cl-substituents, although deactivating, positively induce the meta- and para-positions, which result to be the O-phenolate group again. The presence of both Cl moieties is beneficial as the ligand containing only one Cl is found at the bottom of the series, that is the resulting complexes show lower T½. Additionally, the nature of the halogen is important as well, the T½. values for 3,5-Br-ligand based complexes are found below the dichloride analogue, and for the 5-Br-Sal-323 complexes, only one complex is SCO-active, the other three complexes are locked in the HS-state, Table 3.3. Based on this analysis, it will be interesting to evaluate activating alkyl substituents at the 3-, 5- or both positions, moderate activating alkoxy moieties at 3,5-positions and deactivating 3,5-F based ligands, to evaluate if this type of ligands can produce complexes with T½. close to room temperature, which is important for the construction of nanodevices.

Fig. 3.9. Temperature dependence of the volume cell for complex [MnIII(3,5-Cl-Sal323)]NTf2 (top), average intensity of the superstructure Bragg peaks (middle) and general Bragg peaks (bottom). Figure reproduced, with permission, from Ref. [61]. Copyright RSC 2015.

3.2.1.4. Anion effect. Another factor that can substantially affect the SCO-properties is the size, symmetry and charge of the counterion. In the case of [MnIII(R-Sal-323)]+ a total of 10 different monoanions have been used for the synthesis of SCO-active complexes, see Table 3.3. In Fig. 3.12 a bar plot of the T½ values for five families of metal complexes is shown, each family varies on the ligand substituent, such as 3-MeO, 3,5-Br, 3-EtO, 5-Cl and H-substituents. The

Fig. 3.10. Bar plot of the T½ values for the three sets of [MnIII(R-Sal-323)]+ complexes containing different anions. Anhydrous complexes are shown as grey bars.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 3.11. Bar plot of the T½ values for the ten [MnIII(R-Sal-323)]NO3 complexes containing different ligands. Anhydrous complexes are shown as grey bars.

metal complexes have been arranged in ascending order of T½ value for each family, and a T½ = 0 has been given to HS-locked complexes. It is possible to observe that the two metal complexes containing bulky BPh4 are locked in the HS-state, whereas other anions have been utilised once, therefore only one metal complex exists for Cl, AsF6, TCNQ, NTf2 and TfO anions. In the case of PF6based complexes only two SCO-active complexes have been obtained, whereas, three, five and six complexes for BF4, NO3 and ClO4, respectively, have been obtained. Therefore, for those anions present in more than one metal complex, it is possible to observe the following trend on the T½ value: PF6 < BF4  ClO4 < NO3. In the case of the Cl based complex containng the unsubstituted ligand, the T½ value is the highest of all complexes reported so far, unfortunately only one complex containing chloride has been obtained, so it is difficult to ensure that this type of anion will permit to synthesise complexes with high T½. However, as expected, the size and symmetry of the anion is relevant. It seems that highly symmetric and small anions, such as nitrate and chloride, permit to stabilise better the LS-state and to increase T½, whilst bulkier octahedral anions, such as EF6 (E = P, Sb, As), show lower T½ values, or even turn off the SCO properties, as in the case of AsF6, Table 3.3. Midsize tetrahedral BF4 and ClO4 anions are found in the middle of the series, as expected, due to its size. Thus far, four metal complexes have shown hysteretic and abrupt SCO, two of them contain octahedral bulky PF6 or SbF6 anions in combination with the unsubstituted ligand, whereas the other two complexes contain either 5-Cl or 3,5-Cl substituted ligand and TCNQ radical anion or NTf2 anion, respectively. In three of the complexes, PF6 or SbF6 and NTf2, an anticooperative effect promotes the observation of hysteretic ST. For the former two complexes, the anion disorder increases with temperature, disrupting hydrogen bonding networks upon conversion to the HS-state. For the NTf2 the disorder of the anion increases when the temperature is decreased, observing an interesting two-step transition and symmetry breaking accompanying the ST, that promotes changes in the H-bond interactions. This indicates that H-bond interactions are necessary to observe ST, even in the case of an anticooperative effect, thus is not surprising that BPh4 anion has not produce SCOactive complexes. The last complex that shows hysteretic ST is the

TCNQ-based complex, in which the metal centres can communicate not only through H-bond interactions but by p-p interactions promoted by the radical dimers. Further work should involve the use of simpler halides and pseudohalides, as in the case of Cl, in order to confirm that small anions can increase T½, and the fact that this type of anions are Hbond acceptors. In addition, aromatic anions capable of forming Hbonds should be attempted as well, such as arylsulfonate, to increase communication by p-p interactions as well as H-bonds.

3.2.1.5. Solvent content effect. The last important factor that can fine tune the SCO-properties is the amount and nature of solvent in the crystal lattice. Unfortunately, this factor is very hard to control and predict. For instance, polymorphs and solvatomorphs can be obtained from the same solvent used for crystallisation, depending on many factors such as temperature, water-content, impurities, etc. So, even though the same solvent has been used for the crystallisation of a series of compounds, it is very likely that the amount, or even the nature (water is ubiquitous in the polar solvents use for crystallisation), of the solvent in the crystal lattice might be different. In the case of complexes [MnIII(R-Sal-323)]A (A = anion) the effect of the solvent content depends on the type of substituent attached to the phenyl ring of the ligand. For example, anhydrous complexes of the unsubstituted Sal-323 ligand seem to be abundant, and almost all of them are SCO-active, and even two complexes show hysteretic ST, Table 3.3. Whereas for the other substituted complexes loss of solvent content can deactivate the SCO properties. Complex [MnIII(3-MeO-Sal-323)]PF6H2O is SCOactive, whereas the anhydrous compound is locked in the HSstate, something similar is observed for freshly synthesised [MnIII(3-EtO-Sal-323)]NO3EtOH which is SCO-active, upon time some solvent is loss and complex [MnIII(3-EtO-Sal-323)] NO30.6EtOH is locked in the HS-state. In the case of the nature of the solvent, it seems that H-bond donor and acceptor molecules are important. Most of the complexes shown in Table 3.3 are isolated as methanol, ethanol or water solvates and all are SCO-active, except for [MnIII(3,5-BrSal-323)]PF6 ½ MeOH that is locked in the LS-state. However, there

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

17

Fig. 3.12. Bar plot of the T½ values for five families of [MnIII(R-Sal-323)]+ complexes, varying on the ligand substituents, containing different anions. The complexes locked in the HS-state have been assigned a T½ = 0. Anhydrous complexes are shown as grey bars.

are two examples containing H-bond acceptor MeCN that are SCOactive as well. 3.2.2. Tridentate ligands The first system synthesised from a tridentate ligand was factrans-[MnIII(Py2O)2]Cl3H2O [68] (Fig. 3.4). The ligand Py2O was synthesised in situ, by reacting in one-pot: MnCl24H2O, the imine derived from 2-formylpyridine and 2-aminopyridine, in MeOH/ H2O mixture in air, Fig. 3.13. Under these conditions, the metal is oxidised to Mn3+, additionally, the originally intended imine ligand is attacked by a water molecule, resulting in the addition product Py2O, in which the imine a-carbon suffers a nucleophilic addition, producing the N2O tridentate ligand (Fig. 3.13). When the synthesis is carried out under argon the Mn2+ analogue complex fac-trans[MnII(Py2O)2] is obtained, thus indicating that O2 is required for metal oxidation. VT-magnetic measurements showed that fac-trans[MnIII(Py2O)2]Clsolvents is SCO-active. This complex undergoes an incomplete spin conversion with T½ ~ 250 K. As expected at low temperature, 77 K, the magnetic moment of 2.8 mB is in good agreement for a LS d4 system. On warming, the magnetic moment gradually increases reaching a value of c.a. 4.3 mB at 300 K, indicating that 76% of the metal centres underwent spin conversion [68]. Single crystal X-ray crystallography at 150 K shows that factrans-[MnIII(Py2O)2]Cl3H2O crystallises in the Cc space group, the asymmetric unit consists of one complex cation, one chloride atom and three water molecules. The metal centre is coordinated to two Py2O ligand strands, coordinated in a facial mode. The alkoxylate groups are trans to each other occupying the axial sites, whereas the equatorial positions are occupied by the nitrogen donors. The Mn-N and Mn-O bond lengths values range 2.145(2)–2.186(2) Å and 1.857(2)–1.860(2) Å, respectively, whereas ROh = 61.1°. According to other Mn SCO active complexes in a N4O2 sphere (see above), these values indicate that the complex is stabilised in the HS-state at this temperature, possessing an equatorial elongated JT-distortion, which contrasts to the magnetic measurements results. At 150 K the magnetic moment is around 2.8 mB, indicative of the LS-state. The inconsistency of the magnetic and X-ray measurements in assigning the spin state of the complex

could be a result of utilising different type of samples for each technique. The formula fac-trans-[MnIII(Py2O)2]Cl3H2O was obtained from X-ray studies, indicating that this complex is stabilised in the HS-state at 150 K, thus suggesting that this solvatomorph is not SCO-active. Moreover, the authors did not indicate the type of sample used for magnetic measurements, that is: powder, microcrystalline, crystalline, etc; or even obtained elemental analysis on this sample to determine the content of solvent molecules. It is very likely that the complex formula obtained from the X-ray structure is different from the complex formula used for the magnetic measurements, thus the solvent nature and content of the SCO-active complex is unknown. The fist example of a hydrazone-based Mn3+ complex is black mer-[MnIII(3,5-tBuNPy)]ClO4H2O [69] (Figs. 3.4 and 3.14). VTmagnetic measurements demonstrated that the complex undergoes a gradual and incomplete SCO, centred around 80 K. At 300 K meff = 4.81 mB indicates that most of the metal centres are stabilised in the HS state, on cooling at 77 K the value decreases to 2.84 mB, in good agreement for the LS-state. Single crystal X-ray crystallography at 150 K showed that the complex crystallises in the P1 space group. The asymmetric unit consists on one complex cation, one disordered perchlorate anion, and one disordered water molecule. The metal centre coordinates to two 3,5-tBuNPy ligand strands that are found in a meridional fashion, due to the rigidity of the ligand. The phenolate units are coordinated cis to each other (Fig. 3.14), which is rare for Mn3+ SCO-active complexes (see above). The Mn-N bond lengths range 2.031(3)–2.120(3) Å, and the Mn-O bond lengths 1.875(2)–1.893(2) Å, both values clearly indicate the stabilisation of the HS-state. It has been shown that upon conversion to the HS-state the complexes in trans-N4O2 spheres show an elongation of the equatorial bond lengths, as HS d4 complexes are JT-active; however, in this case there is a clear distortion of the octahedron (ROh = 64.3°), but it is not possible to observe either an elongation or shortening of two trans bonds. The authors suggested that in this case the highly rigid mixed donor ligand presumably lessens the tetragonal distortion [69]. The last examples of Mn3+ SCO-active complex containing a tridentate ligand are black-brown [MnIII(5-Cl-qsal)2]TfO and [MnIII(5Br-qsal)2]TfO [70] (Fig. 3.4). This type of ligands are quite known to

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 3.13. In situ synthesis of ligand Py2O and complex fac-trans-[MnIII(Py2O)2]Cl3H2O (left) and perspective view of the X-ray structure, H-atoms, chloride and water molecules are not shown for the sake of clarity (right).

Fig. 3.14. Perspective view of the crystal structure of complex mer-[MnIII(3,5-tBuNPy)]ClO4H2O at 150 K, H-atoms, perchlorate anion and water molecule are not shown for the sake of clarity (left) and coordination octahedron highlighting the Mn-L bond lengths (right).

provide the right crystal field for SCO iron(III) complexes [28]. VTmagnetic measurements for powder samples of both complexes show an incomplete and gradual SCO. The magnetic moment value for [MnIII(5-Cl-qsal)2]TfO at 350 K is 4.7 mB, lower than the expected mS.O., which corresponds to a 90% of metal centres in the HS state. The magnetic moment gradually decreases with lowering the temperature, reaching 3.3 mB at 25 K, indicating that 80% of the metal centres underwent spin conversion. In contrast [MnIII(5-Br-qsal)2]TfO shows meff = 4.9 mB at 350 K, mostly HS, which gradually decreases to 4.0 mB, indicating that only approximately half of the metal centres underwent spin conversion, to a [HS]:[LS] of c.a. 1:1 mixture. These complexes were not characterised by X-ray crystallography, however a crystal structure of their Mn2+ analogues were collected. Crystal structure of complexes [MnII(5-X-qsal)2] (X = Cl or Br) shows that two ligand strands coordinate to the metal centre in a meridional fashion, where the phenolate units coordinate in cis-mode. Thus, is highly possible that the Mn3+ complexes possess the same coordination sphere, indicating that not only the trans-phenolate/alkoxylate mode is SCO-active. In conclusion, it seems that the N4O2 coordination sphere is suitable for the production of SCO-active complexes of manganese(III). Pioneer work conducted by Morgan and collaborators utilising an hexadentate N4O2 diimine ligand (R-Sal-323), indicated that the oxygen donors (phenolates) should be in trans positions to produce spin conversion. However, the discovery of N2O tridentate ligands (Table 3.4) that form SCO-active complexes with O-donor in cis-positions, X-qsal and 3,5-tBuNPy (see above), indicates that in more flexible ligand scaffolds the cis isomer is SCO active as well, very likely due to the reduced ligand strain that permits the neces-

sary distortion of bond lengths and angles that accompanies the spin conversion.

4. Cobalt(II) complexes The most usual SCO-active coordination sphere for cobalt(II) is N6, similarly to iron(II) SCO chemistry, however, for cobalt(II) systems the nitrogen donors generally correspond to six-member nitrogen heterocycles, particularly terpyridine-like (terpy) ligands, which generally stabilise the LS-state for iron(II) complexes [71]. However, there are other type of ligands, such as imine-based and macrocyclic ligands, that confer the right ligand field for the spin equilibrium in cobalt(II) complexes to occur, for more details see reference [72]. Octahedral d7 cobalt(II) complexes can undergo spin transition from the HS state, S = 3/2, to the LS state, S = ½, that is both states are paramagnetic. Commonly, the spin conversion in cobalt(II) complexes are gradual and incomplete, and few systems show hysteresis [62]. Interestingly, there are examples of cobalt(II) SCO complexes possessing four, five and six coordination numbers [72]. The spin conversion can be detected by X-ray crystallography, among other techniques, as the bond lengths for both spin states are different. The bond lengths for the LS-state are smaller than those observed for the HS state, however, the difference is not as pronounced as for iron(II) systems, c.f. ~0.10 to ~0.20 Å respectively [72]. In addition, the LS-state is JT-active, as it possesses a (t2g)6(eg)1 configuration, being the equatorial shortening a common feature in LS octahedral cobalt(II), instead of axial elongation ubiquitous in JT-active copper(II) d9 complexes.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148 Table 3.4 SCO properties and relevant bond lengths and angles for Mn3+ complexes based on tridentate ligands. CompoundRef.

T½ [K]

III

fac-trans-[Mn (Py2O)2]Cl3H2O [68] mer-cis-[MnIII(3,5-tBuNPy)]ClO4H2O [69] a b c

c

~ 250 K. Incomplete ~ 80 K. Gradual and incompolete

T [K]a

Mn-O [Å]

Mn-N [Å]

ROh [°]b

Spin state

150

1.857(2)–1.860(2)

2.145(2)–2.186(2)

61.1

150

1.875(2)–1.893(2)

2.031(3)–2.120(3)

64.3°

HS,c JT-active HS

Temperature of crystallographic study. ROh = sum of the deviation from 90° of the 12 cis angles in the octahedral coordination sphere [66,67]. Crystal solvatomorph is different from the sample utilised for magnetic measurements.

There are several reviews dealing with SCO-properties of cobalt (II) complexes, the most recent is from 2014 [62,72–76]. And a very recent review (2018) comprising the SCO and valence tautomerism properties of cobalt(II) polymeric species [77]. Therefore, in this section the SCO-properties of recent discrete octahedral cobalt(II) SCO-active complexes containing ligands not related to terpy will be analysed, along with cobalt(II) complexes in five-coordinate environments. 4.1. Five-coordinate cobalt(II) complexes In 2009 Power and co-workers [78] synthesised the air sensitive, deep purple organometallic cobalt(II) complex [CoII(Ar) (NHAr0 )] containing an anionic C-bound bulky aryl ligand (Ar = C6H3-2,6-(C6H3-2,6-iPr2)2) and a bulky anionic amide (NHAr0 = NHC6H3-2,6-(C6H2-2,4,6-Me3)2, namely [CoII(Ar)(NHAr0 )], Fig. 4.1. VT-magnetic measurements indicate that this complex undergoes an abrupt and hysteretic SCO from the HS state S = 3/2 to the LS-state S = ½ upon cooling, with parameters T½ = 229 K and DT½ = 8 K (Fig. 4.2). The meff value at 300 K and at low temperature of 4.62 and 1.77 mB are in good agreement for the HS and LS-state, respectively. In order to confirm the ST, VT-X-ray crystallography experiments of a single crystal of the complex were performed. At 90 K, the data set showed that the cobalt centre is C-bound to the anionic phenyl unit of the Ar ligand and N-bound to the amide moiety of the NHAr0 ligand, additionally, one of the mesitylene wingtip groups of the latter ligand is p-coordinated to the metal centre in a g6 fashion (centroid-Co distance of 1.636(3) Å), resulting in a pseudo-five coordinate environment. Upon warming to 240 K, there is structural disorder in the crystal lattice, containing a 5% of a complex similar to the one observed at 90 K (LS-state), and 95% of another complex differing in the coordination mode of the metal centre (HS-state). In the predominant complex at 240 K, it is possible to observe a change in the coordination mode of the metal centre to a pseudo-three coordinate environment; the g6-p-coordination is loss, and only the ipso carbon of the mesitylene group is now interacting to metal centre, ipso-C-Co 2.393(2) Å at 240 K vs. 2.077(3) Å at 90 K. Nonetheless, the Ar and amide

ligands remain coordinated to the metal centre alike the low temperature structure, albeit a slightly elongation of the bond lengths upon warming is observed. The CAr-Co and HNAr-Co bond lengths at 90 K are 1.977(1) and 1.875(3) Å, whereas at 240 K are 1.992 (2) and 1.880(2) Å, respectively. These results confirm the existence of the ST, however, the transition from the LS-state to the HS-state occurs through a coordination sphere rearrangement. Yuan, Song and co-workers [79] reported the VT magnetic and structural characterisation of the complex [CoII(3,4-lut)4Br]Br (3,4-lut = 3,4-dimethylpyridine, Fig. 4.3). VT-magnetic measurements show a relatively gradual spin conversion with a T½  210 K. At 310 K the magnetic moment value of 4.84 mB is slightly higher than the spin only value, however in good agreement for cobalt (II) in the HS-state (S = 3/2) with a spin-orbit coupling contribution, whereas at 1.8 K the value drops to 1.95 mB, indicating the switching to the LS-state (S = ½). The authors mentioned the presence of a small hysteresis loop, but its magnitude or the experiment scan rate were not stated. EPR spectra confirm the stabilisation of the LS at low temperature, a microcrystalline sample at 4 K shows the parameters g1 = 2.3986(5), g2 = 2.3857(5) and g3 = 2.0063(5), corresponding to a S = ½ system. Interestingly, AC magnetic measurements showed that this complex behaves as a single ion magnet as well. VT single crystal X-ray crystallography revealed the structural changes during the spin conversion. At 123 and 296 K the complex [CoII(3,4-lut)4Br]Br crystallises in the C2/c space group, Fig. 4.3. The asymmetric unit is comprised by half of the complex cation and half-occupancy of bromide counterion. The metal centre is coordinated to four pyridine donors in the equatorial positions and one bromido ligand in the axial site, producing a N4Br coordination sphere. At 296 K the complex is stabilised in the HS-state according to the CoAN and CoABr bond lengths, 2.110(4)–2.136(4) and 2.5145(13) Å, respectively. Upon cooling to 123 K, the CoAN bond lengths are shorter than those observed at 296 K, 1.9686(18)– 1.9677(18) Å, in contrast, the CoABr bond length is elongated to 2.6345(5) Å. Continuous shape measurement analysis showed a value of 0.281 and 0.784 at 296 and 123 K, respectively, relative to an ideal square pyramidal, thus indicating that this complex is found in a distorted square pyramid and that the LS-state is more

Fig. 4.1. Schematic representation of the change in coordination sphere during the ST of complex [CoII(Ar)(NHAr0 )].

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

mon, octahedral cobalt(II) complexes based on acyclic ligands (Table 4.1). Whilst, for N4O2 only octahedral dispositions have been observed, in which the oxygen donors come from phenolate/ alkoxylate-type units or N-oxide moieties (Table 4.2).

Fig. 4.2. VT-magnetic behaviour for complex [CoII(Ar)(NHAr0 )] showing the abrupt and hysteretic ST. Upper inset: inverse molar magnetic susceptibility. Lower inset: Hysteresis loop. Figure reproduced, with permission, from Ref. [78].Copyright ACS 2009.

distorted than the HS-state. The N-Co-N angles slightly increase with the spin conversion to the LS, whereas the Br-Co-N decreases. The metal centre is 0.3670 Å out of the N4 plane at 296 K and decreases to 0.2248 at 123 K. 4.2. Six-coordinate cobalt(II) complexes There are two types of coordination spheres that produce SCOactive cobalt(II) complexes: N6 and N4O2 donors. In the case of all nitrogen donors it is possible to identify two geometries around the metal centre. The first corresponds to a family of trigonal prismatic cobalt(II) metal complexes based on cage-type ligands (Table 4.1). The second type of metal complexes is the most com-

4.2.1. N6 coordination sphere In 2010 Voloshin and co-workers [80] reported a series of substituted tris-dioximate cobalt(II) clathrochelates. The electronic and steric demands of the macrocyclic ligands were tuned by varying both the dioximate (R) and apical boron-capped (R0 ) substituents, Fig. 4.4. The complex containing electron withdrawing moieties in both possible sites, chloro-substituent in the dioximate skeleton and fluoride in the boron site, [CoII(ClLF)], stabilises the cobalt centre in the HS state, as observed by VT-magnetic measurements in the 4–300 K temperature range. By maintaining the chloro substituent and replacing the F unit by electron donating phenyl or n-butyl substituents at the boron site, the complexes [CoII(ClLPh)] and [CoII(ClLBu)] show SCO properties. The phenyl substituted complex [CoII(ClLPh)] shows a magnetic moment of ~3.6 mB at 500 K, which is lower than the expected spin only value for a d7 ion in the HS state, upon cooling a relatively gradual crossover to the LS-state is observed around 60 K, reaching 1.89 mB at 4 K, in good agreement for the LS state. This indicates that [CoII(ClLPh)] undergoes an incomplete spin conversion upon warming of approximately 90%, even when heated up to 500 K. The butyl containing complex [CoII(ClLBu)] shows a similar behaviour. At 500 K the magnetic moment of ~3.25 mB indicates a spin conversion of c.a. 80%, upon cooling the magnetic moment gradually decreases to reach 1.78 mB at 4 K, with a T½ ~ 250 K. The latter is higher than the one observed for the phenyl analogue, which is expected as the n-butyl substituent is a much better electron donating group, thus increasing the ligand field. By replacing the Cl substituents at the dioximate moiety by thioether groups, two different metal complexes were obtained, namely [CoII(SPhLBu)] and [CoII(SBuLPh)], Fig. 4.4. Both complexes are stabilised in the LS state in the 4–200 K temperature range, showing a magnetic moment of 1.77 and 1.89 mB for [CoII(SBuLPh)] and [CoII(SPhLBu)] at 200 K, respectively. However, at higher temperatures the magnetic moment gradually increases to reach 1.98 and 2.32 mB at 300 K for [CoII(SBuLPh)] and [CoII(SPhLBu)], respectively, indicating a very gradual spin conversion with a transition temperature above room temperature. 0 Single crystal X-ray crystallography of complexes [CoII(RLR )] revealed the expected coordination and trapping of the metal centre within the cage ligand, possessing a N6 trigonal prismatic coordination environment. In the case of complexes [CoII(ClLF)] and

Fig. 4.3. Perspective view of complex [CoII(3,4-lut)4Br]Br at 123 (left) and 296 K (right), showing the stabilisation of different spin states. H-atoms and Br counterion are not shown for the sake of clarity.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148 Table 4.1 SCO properties and relevant bond lengths and angles for Co2+ complexes in N6 coordination spheres. CompoundRef. II Cl Bu

[Co ( L )] [80]

–

30

1.8850(13)–2.1472(12)

–c

>300. Gradual and incomplete ~265,d incomplete. ~220 e incomplete. ~175f almost complete

140

1.881(2)–2.042(2)

–c

300

1.939(2)–2.080(3)

–c

30

1.8965(8)–2.1435(8)

–c

[CoII(dpzca)2] [83]

T½; = 168, T½" = 179, DT½ = 11 K, Abrupt and hysteretic

RT 90

2.049(3)–2.145(3) 1.91(1)–2.20(1) Å

110.8 76.1°

[CoII(LNO2 hdz )2](ClO4)2MeOH [84]

>200. Gradual and incomplete >300. Gradual and incomplete

298 123 120

2.015(7)–2.123(5) 1.883(9)–2.252(6) 1.870(4)–2.080; 1.902(4)–2.097(4); 1.834(4)–2.124(4); 1.862(5)–2.100(5)

114.1 134.7 87.6; 84.9; 83.6; 82.4

[CoII4(L2hdz)4]

c d e f g h

c

1.9533(17)–2.0440(18)

[CoII(ClLMe)] [82]

a

~ 250. Gradual and incomplete

ROh [°]b

Mn-N [Å]

RT

[CoII(SBuLPh)] [80]

b

T [K]a

T½ [K]

[85]

Spin state Mainly HS. Delocalised JT-distortion due to LS fraction LS localised JT-distortionf LS localised JT-distortionf Mainly HS. Delocalised JT-distortion due to LS fraction LS-state. Localised JT-distortiong HS LS localised JT-distortiong Mainly HS LS-stateh Mainly [LS]4h

Temperature of crystallographic study. ROh = sum of the deviation from 90° of the 12 cis angles in the octahedral coordination sphere [66,67]. The coordination sphere is trigonal prismatic. From SC-XRD experiments (ADP values). From VT-magnetic studies of crystalline material. From VT-magnetic studies of grinded sample. cis-elongated JT-distortion. equatorial contraction JT-distortion.

Table 4.2 SCO properties and relevant bond lengths and angles for Co2+ complexes based in N4O2 coordination spheres. CompoundRef. [CoII(LNtBu 4 )(dbsq)](B(p-PhCl)4) [86] [CoII(L)2](NO3)2 [86]

II



T½ [K]

T [K]a

>200. Gradual and incomplete >218. Gradual and incomplete, and ferromagnetic coupling

[Co (L )2](B(C6F5)4)2CH2Cl2 [89]

>200. Gradual and incomplete, anti- and ferromagnetic coupling

[CoII(papl)2] [90]

~150. Gradual

a

c d

CoAN [Å]

ROh

Spin state

[°]b

100 400 123

1.888(1)–1.891(2) 1.931(2)–2.368(2) 1.964(3)–1.974(2) 1.986(4)–2.371(3) 2.113(3) 1.959(3)–1.986(3)

115.3 96.90 22.8

273 353 123

2.117(2) 2.040(2)–2.056(2) 2.107(3) 2.056(3)–2.073(4) 2.117(2)–2.126(2) 1.977(2)–1.992(2)

36.28 41.2 26.3

273 147

2.078(3)–2.086(3) 2.078(4)–2.093(3) 1.998(1)c–2.122 1.865(2)c–2.048 (2)d (2)d

44.9 85.05

325

b

CoAO [Å]

2.042(1)c–2.079 (2)d

1.909(2)c–2.042 (1)d

93.98

Mainly HS LS LS equatorial contraction JT-distortion HS/LS mixture Mainly HS LS e quatorial contraction JT-distortion Mainly HS LS equatorial contraction JT-distortion HS

Temperature of crystallographic study. ROh = sum of the deviation from 90° of the 12 cis angles in the octahedral coordination sphere [66,67]. Equatorial coordination. Axial coordination.

[CoII(SBuLPh)] the data set clearly show the stabilisation of the HS and LS-state, respectively. Two different solvatomorphs were obtained for the [CoII(ClLF)] complex, namely [CoII(ClLF)]3C6H6 and [CoII(ClLF)]3C7H8, both complexes were characterised by single crystal X-ray crystallography at 110 and 153 K, respectively, showing the stabilisation of the HS-state by the CoAN bond lengths of 2.0326(10) and 2.0311(16) Å, respectively. Whereas, CoAN bond lengths in complex [CoII(SBuLPh)] at 140 K, ranging from 1.881(2) to 2.042(2) Å (average 1.971 Å), demonstrate the stabilisation of the LS-state, in good agreement with the magnetic measurements.

Additionally, LS cobalt(II) complexes are JT-active, which is possible to observe in the X-ray structure of [CoII(SBuLPh)], possessing two pairs of shorter bond lengths of 1.881(2) and 1.990(2) Å and one pair of longer bond lengths, 2.042(2) Å, however the latter are found cis, instead or trans. VT single crystal X-ray crystallography was performed on [CoII(ClLBu)], Fig. 4.6. This complex crystallises in the C2/c space group at room temperature down to 60 K. Below 60 K there is a phase change to P21/c (see below). At room temperature the asymmetric unit consists of half of the complex molecule, the other half

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 4.4. Structure of tris-dioximate cobalt(II) clathrochelates showing SCO-properties.

of the complex molecule is generated by a C2 axis that sits on the metal centre and is perpendicular to the ligand strand. The CoAN bond lengths of 1.9533(17), 2.0101(17) and 2.0440(18) Å, clearly indicate a large proportion of the HS state of the metal centre at this temperature. According to VT-magnetic data, complex [CoII(ClLBu)] is stabilised as a mixture of HS and LS-states at room temperature, which is reflected in the anisotropic displacement parameters (ADP) of the cobalt centre. In the 295–60 K temperature range the ADP values for the cobalt centre display significant anisotropy, with the maximal displacement aligned perpendicularly to the C2 axis, thus the ‘‘long” CoAN bond lengths correspond to the superposition of two JT-distorted structures. Below 60 K the anisotropy disappears, concomitant to the phase change to P21/c space group, that is an ordering of the JT-axis is produced, due to the conversion to the LS state. Interestingly, the average CoAN bond lengths do not change significantly, however the cell volume decreases by lowering the temperature, Fig. 4.5. Moreover, the D parameter, the mean deviation of the coordinating nitrogen atoms from C-Co-C plane [81], increases by lowering the temperature (Fig. 4.5), indicating that upon conversion to the LS-state the metal centre is no longer found in the centre of the polyhedron formed by the six N-donors. At 30 K the complex [CoII(ClLBu)] crystallises in the P21/c space group, the asymmetric unit is comprised by the entire complex molecule. The CoAN bond lengths, with a range of 1.8850(13)– 1.9501(13) Å for the shorter bonds, and the two longer bonds ranging from 2.0950(12) to 2.1472(12) Å, indicate the LS-state of the metal centre, Fig. 4.6. The authors proposed that the crystal phase change below 60 K could be the result of a rearrangement of the supramolecular interactions in the crystal lattice. Below 60 K there are Cl  O interactions between the chloro substituent of the oximate skeleton and the oxygen of the capping unit of neighbouring molecules, forming polymeric supramolecular structures, which are not present above 60 K. In a different study [82] the methyl-boron capped complex [CoII(ClLMe)] was synthesised showing SCO behaviour and anticooperativity induced by weak intermolecular interactions. The authors analysed the spin transition curves of different type of samples, namely polycrystalline, single crystal, solution and frozen solution and by different techniques, Fig. 4.7. VT X-ray crystallography of a single crystal shows the expected coordination of the cobalt centre in a trigonal prismatic coordination cage. The complex crystallises in the P21/c space group

Fig. 4.5. Variation of the D parameter (top) and cell volume (bottom) of complex [CoII(ClLBu)] with temperature, obtained from single crystal X-ray crystallography. These graphs were constructed utilising the data found in [80].

containing an entire complex molecule in the asymmetric unit, in the 30–300 K temperature range. Upon spin conversion, from the HS to the LS, two pairs of CoAN bond lengths decrease, at 300 K the values range from 1.939(2) to 1.971(2) Å (average 1.955 Å), upon cooling to 30 K the values range from 1.8965(8) to 1.9248 (7) (average 1.9108 Å), this is an average 0.044 Å reduction in the LS-state. Moreover, there is a pair of bond lengths that increase upon conversion to the LS-state, at 300 K the values ranging from 2.080(3) to 2.078(2) Å (average 2.079 Å) and increase to 2.1435 (8)–2.1412(8) Å (average 2.1424 Å), an average increase of 0.063 Å in the LS form. This behaviour is explained in terms of the JT-distortion of the LS cobalt(II) centre, however in this case a cis elongation is observed due to the trigonal prismatic cage environment, as a trans elongation would cause a large amount of strain in the system. As mentioned above, the ADP values in this type of systems correlate to the amount of LS-state present in the crystal lattice. Below 100 K the ADP values are smaller, however upon warming these values increase up to 0.044 Å2 at 300 K, indicating the presence of both the LS and HS at temperatures above 100 K, in good agreement with the magnetic data (see below). A careful analysis of the

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

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Fig. 4.7. Spin transition curves of complex [CoII(ClLMe)] obtained from different type of samples and techniques. Single crystal (cyan ., obtained by ADP analysis); fine crystalline sample (blue ▲) and extremely fine-crystalline sample (red d) obtained by measuring the magnetic susceptibility; Solution sample obtained by Evans method (pink ◄) and frozen solution obtained by integrating the EPR signal of the LS complex in toluene glass (obtained from EPR j). Figure reproduced, with permission, from Ref. [82].Copyright ACS 2014.

Fig. 4.6. Perspective view of the complex [CoII(ClLBu)] at RT (top) and 30 K (bottom). H-atoms and butyl group are not shown for the sake of clarity.

ADP values at different temperatures provided a ST curve (Fig. 4.7), showing a gradual spin conversion upon cooling, T½ ~265 K. VT-magnetic studies of a crystalline material show a gradual and incomplete in the upper limit spin conversion, T½ ~ 220 K. At 300 K the complex is stabilised as a HS:LS mixture, being the HS state in an 85%, upon cooling to 5 K the complex has completely converted to the LS-state. Interestingly, the magnetic profile for [CoII(ClLMe)] of a finely grinded sample is different from the starting crystalline material, it is more abrupt and almost complete in the upper limit; at 300 K the HS state is present in around 90% (Fig. 4.7), and the T½ is around 175 K, which is lower than the one found for the starting crystalline sample and single crystal experiments. Surprisingly, the magnetic behaviour in solution of [CoII(ClLMe)] is much more abrupt. Evans method shows that a toluene solution of complex [CoII(ClLMe)], in the 200–350 K temperature range, the metal centre is completely stabilised in the HS state. Below 200 K the magnetic properties of the toluene solution were measured by EPR, by integrating the signals for the LS cobalt(II) centres, observing a much more abrupt and complete conversion to the LSstate. Thus, in this case, solution behaviour is abrupt compared to the solid-state profile, which is very rare in SCO systems as commonly spin equilibrium in solution is gradual [15]. In most cases the abruptness of a transition is related to the cooperativity among metal centres. The latter is known to depend on the amount and type of supramolecular interactions among the spin carriers, which are diluted or even absent in solution. Therefore, in this system, the

supramolecular interactions present in the solid state deter abruptness, however, when removed, the transition becomes more abrupt as observed in the grinded sample (smaller crystallites) and in solution or frozen solution. The most common type of SCO-active cobalt(II) complexes are found in N6 octahedral environments, particularly containing ligands related to terpy. Recently, new ligand types that promote spin equilibrium in cobalt(II) complexes have been synthesised. Brooker and co-workers [83] synthesised the neutral cobalt(II) complex [CoII(dpzca)2] containing an anionic terdentate ligand, dpzca = di(pyrazine-2-carbonyl)imide anion. This complex was characterised by single crystal X-ray crystallography (Fig. 4.8). At room temperature the complex crystallises in the I41/a space group, the asymmetric unit comprises one quarter of the complex molecule. As expected, two ligand strands coordinate to the metal centre in a meridional fashion through the N-donors of the pyrazine groups (2.145(3) Å) and the central imide nitrogen (2.049(3) Å). Based on the CoAN bond lengths and ROh = 110.8°, it is possible to stablish that the cobalt(II) centre is stabilised in the HS-state at this temperature. Upon cooling the same crystal to 90 K, a phase change is observed to P21/c, the asymmetric unit now consists of the entire metal complex. The equatorial bond lengths, consisting of two imide and two pyrazine donors, which are shorter (with a range of 1.91(1)–1.99(1)Å) compared to the room temperature structure and to the apical bond lengths at 90 K, 2.19(1) and 2.20 (1) Å, indicating an equatorial contraction JT-distortion, moreover, a reduction in the ROh value is also observed to 76.1°, indicating the stabilisation of the LS-state at low temperature. Generally, SCO-active cobalt(II) complexes show gradual and incomplete spin conversion [62], however, VT-magnetic measurements of a crystalline sample of complex [CoII(dpzca)2] shows an unusual abrupt and hysteretic spin transition, with parameters T½; = 168 K, T½" = 179 K and DT½ = 11 K. This behaviour was reproducible over three cycles for crystalline and powder samples, which suggests that the magnetic profile observed could be due to the phase transition detected by X-ray diffraction. Moreover, the LS-state could be reached at room temperature by applying pressure, as observed by Raman spectroscopy using a diamond anvil cell. Additionally, this complex shows a reversible and repro-

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 4.8. Perspective view of complex [CoII(dpzca)2] at different temperature, room temperature stabilised in the HS-state (left) and 90 K stabilised in the LS-state (right).

ducible oxidation process to a cobalt(III) complex stabilised in the LS-state. Wu and co-workers [84] synthesised a family of SCO-active cobalt(II) complexes based on terdentate hydrazone ligands containing both a pyrazine and a pyridine ring, varying on the substituents attached to the pyridine ring (Fig. 4.9). The synthesised II NO2 complexes were [CoII(LH hdz)2](ClO4)2, [Co (Lhdz )2](ClO4)2MeOH II NO2 and [Co (Lhdz )2](BF4)2½MeOH. VT-magnetic measurements in the 5–400 K temperature range, showed a gradual and incomplete conversion to the HS-state for the three complexes. Additionally, the complexes show reversible electrochemical behaviour in acetonitrile solution. In the case of [CoII(LH hdz)2](ClO4)2 two reversible waves were observed for the CoIII/II and CoII/I processes. However, II/I in the case of [CoII(LNO2 process was hdz )2](ClO4)2 only the Co observed. VT X-ray crystallography for the complex [CoII(LNO2 hdz )2](ClO4)2MeOH was performed at 150 and 298 K. At both temperatures the asymmetric unit consists of one complex dication, two disordered perchlorate and one methanol solvent molecule. The metal centre is coordinated to two ligand strands in a meridional fashion. At room temperature the equatorial and axial CoAN bond lengths range from 2.015(7) to 2.046(7) Å (av. 2.033 Å) and from 2.118(6) to 2.123(5) Å (av. 2.121 Å), respectively, whilst ROh = 114.1°. The CoAN bond lengths are slightly shorter compared to other CoN6 SCO-active complex stabilised in the pure HS-state (see above), nonetheless, it is clear that at this temperature the complex is mainly stabilised in the HS-state, and a small percentage of LSstate is still present, as observed by VT-magnetic measurements, corroborating the incomplete conversion to the HS-state. At 123 K the equatorial bond lengths have decreased, ranging from 1.883(9) to 1.965(8) Å (av. 1.930 Å), but the axial bond lengths have slightly increase, ranging from 2.206(8) to 2.252(6) Å (av. 2.229 Å), indicating the stabilisation of the LS-state. Surprisingly, the ROh value for the LS-state increased to 134.7°, one of the highest observed for a cobalt(II) centre in the LS-state, indicating that the LS-state is much more distorted than the HS-state, which is very uncommon, as generally, the HS-state shows a more distorted geometry than the LS-state and thus higher ROh values. Wu, Sato and co-workers [85] synthesised a tetranuclear gridtype cobalt(II) complex, [CoII4(L2hdz)4], based on a bis-terdentate hydrazone ligand containing a central pyrazine unit, capable of bridging two metal centres, and two hydrazone-pyridyl pendant arms, Fig. 4.10. VT magnetic measurements shows a gradual and incomplete SCO in the 5–400 K temperature range. X-ray crystallography of the complex was acquired at 120 K, Fig. 4.11. The complex crystallises in the P1 space group and the asymmetric unit comprises the entire tetranuclear complex and eight perchlorate anions. At this temperature all the metal centres have similar CoAN bond lengths.

Fig. 4.9. Structure of terdentate hydrazone ligands used for the synthesis of CoII SCO-active complexes.

The equatorial bond lengths for the four different cobalt(II) centres range from 1.870(4)–1.975(4) Å (av. 1.928 Å), 1.902(4)–1.984(3) Å (av. 1.943 Å) 1.834(4)–1.972(4) Å (av. 1.923 Å), and 1.862(5)–2.013 (4) Å (av. 1.937 Å), for Co1 to Co4 respectively, Fig. 4.11. Whereas the axial bond lengths range from 2.057(4)–2.080(5) Å (av. 2.067 Å), 2.062(4)–2.097(4) Å (av. 2.080 Å), 2.046(5)–2.124(4) Å (av. 2.085 Å) and 2.072(4)–2.100(5) Å (av. 2.086 Å), for Co1 to Co4 respectively. Additionally, the ROh values for the metal centres are 87.6, 84.9, 83.6, 82.4°, all of which indicate the stabilisation of the LS-state with a slight contribution of the HS-state, which is corroborated by VT magnetic measurements.

4.2.2. N4O2 coordination spheres Krüger and co-workers [86] demonstrated for the first time that cobalt(II) complexes containing semiquinonate ligands can be SCOactive as well as showing valence tautomerism [77,87], the latter being the most common phenomenon for this type of ligands. tBu The complex [CoII(LNtBu = N,N0 4 )(dbsq)](B(p-PhCl)4), where LN4 di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane, dbsq = 3,5-ditert-butylsemiquinonate and B(p-PhCl)4 = tetra(p-chlorophenyl) borate, was characterised by single crystal X-ray diffraction (Fig. 4.12) and VT-magnetic measurements. Single crystal X-ray diffraction for complex [CoII(LNtBu 4 )(dbsq)](B (p-PhCl)4) was acquired at 100 and 400 K, Fig. 4.12. At both temperatures the space group is P21/n and the asymmetric unit consists on the entire complex cation and a (B(p-PhCl)4) anion. At 100 K the average CoANpy, CoANamine and CoAO bond lengths of 1.932, 2.338 and 1.890 Å, along with the ROh = 96.90°, indicate the stabilisation of the LS-state. Moreover, the average C-O bond length of 1.313 Å, along with CAC bond lengths found in the dbsq ligand indicate the semiquinonate nature, two shorter CAC bond lengths C(25)–C(26) and C(28)–C(27), 1.379(3) and 1.385(3) Å, respectively, and a longer CAC bond length for C(24)–C(23), 1.433(3) Å.

Fig. 4.10. Structure of dinucleating hydrazone-based ligand use for the generation of tetranuclear cobalt(II) SCO-active complexes.

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 4.11. Perspective view of SCO-active tetranuclear complex [CoII4(L2hdz)4] at 120 K. H-atoms and BF4 anions have been removed for the sake of clarity.

At 400 K, the average CoANamine show a small increase, compared to 100 K, to 2.345 Å. In the case of the average CoANpy and CoAO bond lengths there is an increase to 1.990 and 1.969 Å, being the largest elongation for the CoAO bond. However, the CoANpy for fully HS-cobalt(II) complexes containing ligands akin to LNtBu is expected to be in the 2.082–2.096 Å range. Regard4 ing the ligand dbsq, it is possible to observe the same pattern as in 100 K, one longer and two shorter CAC bond lengths, indicating the radical nature of the ligand. In addition to these changes and the ROh value of 115.3°at 400 K, it is possible to conclude that the complex has a large proportion of HS-state at this temperature, however, the spin conversion is not complete even at 400 K. VT-magnetic measurements show that in the 40–200 K temperature range the magnetic moment of 3.05 lB for compound [CoII(LNtBu 4 )(dbsq)](B(p-PhCl)4) is maintained almost constant, suggesting the stabilisation of a species with a total spin state of S = 1. Thus, indicating that the cobalt(II) centre is stabilised in the LS-state (S = ½) and the ligand is found as a semiquinonate radical (S = ½). The authors postulate that both spin carriers are strongly ferromagnetically coupled, based on analogous copper(II) complexes. Below 40 K the magnetic moment decreases to

25

approximately 1.10 lB, the authors explained this behaviour as either zero field splitting and/or weak intermolecular antiferromagnetic coupling. Nonetheless, above 200 K it is possible to observe a gradual increase of the magnetic moment, reaching 3.98 lB at 400 K. This behaviour was attributed by the authors to a gradual and incomplete spin conversion of the cobalt centre to the HS-state (S = 3/2). Related HS cobalt(II) complexes containing semiquinonate radical ligand (S = ½) show moderate antiferromagnetic coupling, thus resulting in S = 1 ground state [86]. The magnetic behaviour is in good agreement with the single crystal X-ray crystallography shown above. Murray and co-workers, utilised the tridentate neutral nitroxide radical based ligand, 4-dimethyl-2,2-di(2-pyridyl)oxazolidine-Noxide, (L, Fig. 4.13), to form cobalt(II) complexes of the type [CoII(L)2]2+ [88,89]. The complex [CoII(L)2](NO3)2 was characterised by VT-X-ray crystallography and magnetic measurements. Single crystal X-ray crystallography collected at 123, 273 and 353 K, revealed that all temperatures the complex crystallises in the P21/c space group and the asymmetric unit comprises half of the complex dication and one nitrate anion, the rest of the dication and the second nitrate anion are generated by an inversion centre sitting on the metal centre, Fig. 4.14. The metal centre is coordinated to two L ligand strands, the equatorial positions are occupied by the pyridine donors whereas the oxygen donors coordinate in the apical sites, resulting in a N4O2 coordination sphere. At 123 K the equatorial CoAN bond lengths, 1.959(3) and 1.986 (3) Å, are shorter than the apical CoAO bond length of 2.114(3) Å, and a ROh = 22.8°. Upon heating the CoAN bond lengths increase: 2.040(2) and 2.057(2) Å at 273 K and 2.056(3) and 2.074(4) Å at 353 K. Whereas, the CoAO bond length values are maintained relatively the same, 2.117(2) Å at 273 K and 2.108(3) Å at 353 K, and the ROh has increased to 36.28° and 41.2°, respectively. These results indicate that at low temperature, the cobalt centre is stabilised in the JT-active LS-state, observing an equatorial contraction at 123 K. Upon spin conversion, the equatorial bonds increase in length as the HS-state populates. Additionally, the NO bond lengths of 1.294(4), 1.285(2) and 1.285(4) Å, at 123, 273 and 353 K, respectively, indicate the presence of the neutral radical ligand. Based on these results the authors suggested that the change in bond lengths are due to a spin crossover, instead of a possible metal to ligand electron transfer. VT-magnetic measurements of a polycrystalline sample of [CoII(L)2](NO3)2 shows a quite interesting magnetic profile, Fig. 4.15. At 300 K the magnetic moment has a value of 3.68 lB which decreases upon cooling to 3.53 lB at 218 K. Below this temperature the magnetic moment rapidly increases reaching 3.93 lB at 40 K, followed by an abrupt decrease below 40 K, reaching a final

Fig. 4.12. Perspective view of complex [CoII(LNtBu 4 )(dbsq)](B(p-PhCl)4) at 100 (left) and 400 K (right), showing the structural changes in the Co-donor upon spin conversion. Hatoms, anion and tert-butyl groups are not shown for the sake of clarity.

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J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

Fig. 4.13. Structure of ligand 4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide, (L).

value of 3.31 lB at 2 K. The latter abrupt decrease was assigned by the authors as a combination of Zeeman and zero-field splitting effects. The leff value at 40 K is slightly higher than the expected lSO = 3.87 lB, indicating a ferromagnetic exchange between the two radical ligands (S = ½) and the metal centre (S = ½), indicating a S = 3/2 ground state. The data obtained in the 2–119 K temperature range were used for the calculation of the best fitted parameters: J1 = 63.8 cm1, J2 = 63.9 cm1 and g = 2.10, using a trimer ˆ = 2J1(Sˆ1Sˆ2 + Sˆ2Sˆ3)  2 J2(Sˆ1Sˆ3), where J1 describes model with H the interaction between the radicals (Sˆ1 and Sˆ3) and the cobalt(II) centre (Sˆ2) and J2 describes the interaction between the radical ligands mediated by the cobalt(II) centre. Above 218 K an increase in the magnetic moment is observed reaching 4.13 lB at 400 K, suggesting a 60 to 80% gradual and incomplete spin conversion to the HS-state upon heating. In a different work, Murray and collaborators [89] reported the synthesis and characterisation of two cobalt(II) solvatomorphs, namely [CoII(L)2](B(C6F5)4)2solvent, where solvent = 2Et2O or CH2Cl2. The diethyl ether solvatomorph is locked in the HS-state, according to VT-magnetic measurements and X-ray crystallography. However, [CoII(L)2](B(C6F5)4)2CH2Cl2 shows a spin conversion starting above 200 K, which is incomplete even at 350 K. In the 130–28.8 K temperature range it is possible to fit the data using ˆ = 2J1(Sˆ1Sˆ2 + Sˆ2Sˆ3)  2 J2(Sˆ1Sˆ3), where J1 describes the interacH tion between the radicals (Sˆ1 and Sˆ3) and the cobalt(II) centre (Sˆ2) and J2 describes the interaction between the radical ligands mediated by the cobalt(II) centre, obtaining the parameters J1 = 0.51 2 cm1, J2 = 138.7 cm1 and g = 2.03, which is different from com-

Fig. 4.15. VT-Magnetic profile for complex [CoII(L)2](NO3)2 in the 2–300 K temperature range, and in the 5–400 K (inset), in an applied magnetic field of 1 T. Figure reproduced, slightly modified, with permission, from Ref. [88].Copyright ACS 2013.

plex [CoII(L)2](NO3)2, where both coupling constants were found to be ferromagnetic. Similarly to [CoII(L)2](NO3)2, VT X-ray crystallography on [CoII(L)2](B(C6F5)4)2CH2Cl2 shows that at low temperature (123 K) the complex is stabilised in the LS-state, showing an equatorial compressed JT-distortion, CoAN bond lengths varying 1.977 (2)–1.992(2) Å, with an average of 1.984 Å, whereas the CoAO bond lengths are 2.117(2) and 2.126(2) Å, with an average of 2.121 Å, and a ROh value of 26.3°. Upon heating, 273 K, the JTdistortion disappears, and the CoAN bond lengths increase, ranging from 2.078(4) to 2.093(3) Å, with an average of 2.085 Å, and the CoAO bond lengths are maintained relatively the same, ranging from 2.078(3) to 2.086(3) Å, with an average of 2.082 Å and a ROh value of 44.9°. Lemaire and co-workers [90] demonstrated by VT-magnetic measurements, that the purple octahedral cobalt(II) complex [CoII(papl)2], papl = 1-(2-pyridylazo)–2-phenanthrolate, possessing a N4O2 coordination sphere, Fig. 4.16, shows a complete, gradual and reversible spin conversion, which was confirmed by VTsingle crystal X-ray crystallography on the same crystal at 147 and 325 K. At both temperatures the complex crystallises in the C2/c space group and the asymmetric unit consists of one complex molecule. At 147 K, badly disordered solvent in the crystal lattice

Fig. 4.14. Perspective view of complex [CoII(L)2](NO3)2 at 123 (left), 273 (middle) and 353 K (right) showing the change in Co-donor bond lengths upon heating. Anion and Hatoms are not shown for the sake of clarity.

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

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Fig. 4.16. Perspective view of the complex [CoII(papl)2] at 147 (left) and 325 K (right), showing the change in Co-donor bond lengths upon heating. H-atoms are not shown for the sake of clarity.

could not be refined, thus SQUEEZE methodology was used, the authors assigned the unrefined solvent as half (or less) occupancy CH2Cl2 molecule. At both temperatures, the metal centre is coordinated to two ligand strands in a meridional fashion, where the oxygen donors are found in cis positions. At low temperature the ROh = 85.05° and a the presence of a JT-distortion equatorial contraction, where the equatorial CoAN bond lengths range from 1.865(2) to 1.968(2) Å, average 1.915 Å, and the equatorial CoAO bond length is 1.998(1) Å, whereas the axial CoAO and CoAN bond lengths are 2.122(2) and 2.048(2), respectively, Fig. 4.16, indicate that the complex is stabilised in the LS-state. Upon warming the equatorial CoAN bond lengths increase, with a range of 1.909(2)– 2.011(2) Å, average of 1.947, and the CoAO increases to 2.042(1) Å, whereas the axial CoAO and CoAN bond lengths slightly decrease to 2.079(2) and 2.042(3), respectively, finally the ROh value increases to 93.98°, indicating the stabilisation of the HSstate. 5. Concluding remarks A large variety of SCO-active compounds can be obtained utilising first row transition metals other than ubiquitous Fe. In recent years there has been great advances in the synthesis of d4 metal centres, Cr2+ and Mn3+. In the case of Cr2+ a triple-decker dinuclear complex showing an abrupt and two-step ST, and a novel mononuclear sandwich indenyl-containing family of complexes have been obtained. The latter clearly shows that other arene-type of ligands can be used for the synthesis of Cr2+ SCO-active materials. In the case of Mn3+ prolific work by Morgan and collaborators have demonstrated that abrupt and hysteretic ST can be achieved, and in addition with other researchers, a common coordination sphere for increasing the odds of observing SCO for this metal centre has been stablished, N4O2, which could be considered as the ‘‘magic” coordination sphere for Mn3+ SCO-active complexes. In the case of d5 Mn2+ SCO-active complexes a novel dinuclear complex containing Cp ligand and bridged by [E(SiMe3)2] (E = P or As) units have been synthesised. The As-containing complex shows and abrupt and hysteretic SCO which is unique for this type of metal complexes, thus, a long and hard work for organometallic researchers is upon, in order to develop more bistable systems of Mn2+. Meanwhile, for complexes containing the d7 Co2+ ion, it has been shown that not only octahedral metal complexes are SCOactive, but trigonal prismatic six-coordinate and five-coordinate spheres can be SCO-active as well. For the latter an organometallic system containing both an anionic C-bound aryl and an amide

coordinated ligands showed an abrupt and hysteretic ST, due to a change in the coordination sphere around the metal centre going from pseudo-five-coordinate in the LS-state to pseudo-threecoordinate in the HS-state. Recently, a cobalt(II) centre in a N4Br shows a gradual SCO, additionally, the LS-state behaves as a single ion magnet. In the case of six-coordinate complexes, Voloshin and co-workers have obtained a family of clathrochelates containing cobalt(II) centre in a trigonal prismatic geometry, instead of the most common octahedral environment, possessing SCO properties. In the case of octahedral metal complexes, N6 and N4O2 coordination spheres offer the right ligand field for SCO. Brooker and coworkers synthesised a N6 octahedral cobalt(II) complex, which shows an abrupt and hysteretic ST promoted by a crystallographic phase change. Recently, Wu, Sato and co-workers have synthesised a tetranuclear cobalt(II) complex, showing a gradual and incomplete transition from the [HS]4- to the [LS]4-state, thus leading the research towards multinuclear and multistep SCO-active cobalt(II) complexes. Most of the complexes described in this review have been characterised in the solid state by single crystal X-ray crystallography, thus in some cases both the HS and LS-state have been characterised, observing that, in most of the cases, the M-L bond lengths decrease in the LS-state. However, there are some particularities for each electronic configuration. For instance, HS Mn2+ and LS Co2+ are JT-active, facilitating the structural assignment of the spin state in the solid state. In Table 5.1 the bond lengths for sixcoordinate metal complexes analysed in this work in different spin states is shown. In the case of d4 Cr2+ three different coordination spheres could promote SCO, P4I2, (l2:g5-P5)(g5-C5Me5)2 and (R-indenyl)2, although, only one example for the former two types exists. In the P4I2 coordination sphere, a reduction in both Cr-P and Cr-I bond lengths is observed when switching from the HS to the LS-state, an average reduction of 0.14 and 0.347 Å, respectively (Table 5.1). Whereas for the (l2:g5-P5)(g5-C5Me5)2 system the Cr-P bond length slightly decreases by 0.038 Å, but the Cr-C bond lengths slightly increase by 0.013 Å (Table 5.1). For the (R-indenyl)2 containing complex, the average Cr-C bond lengths decreases by 0.082 Å. The other d4 SCO-active metal centre, Mn3+, undergoes SCO when surrounded by the ‘‘magic” coordination sphere N4O2. In this case, the average Mn-O bond lengths barely increase when converting to the LS-state (0.009 Å), however the average Mn-N bond lengths decreases by 0.102 Å, Table 5.1, this differing behaviour in the donor to metal bond length changes can be explained in terms of the JT-distortion which is active in the HS-state as an equatorial elongation. In most of the analysed structures the oxy-

28

Table 5.1 Distribution of metal to ligand donor bond lengths for the HS- and LS-states for six-coordinate SCO-active complexes of Cr2+, Mn2+, Mn3+ and Co2+. M-L Bond lengths (Å) for each SCO-active coordination sphere (l2:g5-P5)(g5-C5Me5)2

P4I2 M-I

M-P

M-C

M-C

M-N

M-O

M-N

Cr2+ (d4)

HSa

2.52b

3.068b

–

2.7127(4)c

–

–

–

HSa

–

2.190(4)–2.351(3) Av. 2.262(10)b 2.168(5)–2.20(2) Av. 2.18 –

–

2.3439(6)–2.4066(7)c Av. 2.38 –

2.196(3)–2.211(3) Av. 2.204 2.200(5)–2.234(5) Av. 2.217 –

–

LS

2.37(2)–2.44(2) Av. 2.41 2.298(2)–2.455(2) Av. 2.372 –

–

–

–

–

–

HS

–

–

–

–

–

LS

–

–

–

–

–

–

–

HSd

–

–

–

–

2.331(2)–2.424(3 ) Av. 2.375 2.065(5) 2.187(2) Av. 2.133 –

1.850(7)–1.902(6) Av. 1.872 1.870(1)–1.893(1) Av. 1.881 –

–

LS

2.022(3)–2.275(6) Av. 2.132 1.980(3)–2.157(1) Av. 2.030 –

–

–

a,d

LS

–

–

–

–

–

–

–

HSe

–

–

–

–

–

LSa,e

–

–

–

–

–

1.909(2)–2.368(2) Av. 2.069 1.865(2)–2.371(3) Av. 2.023

1.888(1)–2.107(3) Av. 2.024 1.964(3)–2.126(2) Av. 2.059

1.939(2)–2.080(3) Av. 2.004d 1.881(2)–2.147(2) Av. 1.999d 2.015(7)–2.145(3) Av. 2.083e 1.834(4)–2.252(6) Av. 2.01e

Mn

5

(d )

Co2+ (d7)

e

JT-active. Only one example of a SCO compound. Data obtained from complexes locked in the LS-state. Trigonal prismatic coordination sphere. Octahedral coordination sphere.

–

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

c d

N6

M-P

2+

a

N4O2

Spin state

Mn3+ (d4)

b

(arene)2

Metal (dn)

J. Olguín / Coordination Chemistry Reviews 407 (2020) 213148

gen donors coordinate in the apical positions, trans to each other, and the equatorial positions are occupied by the N-donors, in which the JT-elongation is reflected. The change in the average Mn-C bond length for d5 Mn2+ Cpcontaining SCO-active complexes when crossing from the HS- to the LS-state is a significant reduction of 0.242 Å, thus a straightforward assignment of both spin states can be carried out by X-ray crystallography. Finally, for d7 Co2+ SCO-active complexes, two possible coordination spheres can promote spin conversions, N6 and N4O2. The complexes containing a N6 set of donors can be found in trigonal prismatic and octahedral geometries. In the former, the geometry might be dictated by the ligand scaffold, boron-capped trisdioximate clathrochelates. There is a slight reduction of the average CoAN bond lengths in the LS-state of 0.005 Å for this type of complexes, Table 5.1. However, the Co2+ LS-state is JT-active; in this family of complexes it is possible to observe two different types of CoAN bond lengths in the LS-state, one pair of shorter and two pairs of longer CoAN bond lengths arranged in a cis manner, that is a cis-elongation of the JT-axis is observed. For the octahedral N6 SCO-active complexes, containing ligands not related to terpy, the transition to the LS-state is accompanied by a significant reduction of the average CoAN bond lengths of 0.073 Å, moreover, the JT-distortion for the LS-state is observed as an equatorial contraction, and not significant change in the axial bond lengths is observed. In the case of N4O2 coordination spheres, the spin conversion to the LS-state is reflected in a reduction of the average CoAN of 0.046 Å, but an increase in the average CoAO bond lengths of 0.035 Å. However, the analysis utilising the average CoAL bond lengths can be misleading, as the LS-state in this type of complexes shows an equatorial elongation, and both cis and trans isomers are SCO-active. Nonetheless, it is possible to identify both spin states by X-ray diffraction analysis for cobalt(II) complexes in N6 and N4O2 coordination spheres. Although, great advances in the synthesis and characterisation of non-FeII SCO-active compounds have been achieved, including the synthesis of novel bistable molecules based on Mn2+, Mn3+ and Co2+, however, there is still much more research to perform on these ‘‘other” SCO-active metal centres, for example: thermodynamic data obtained from DSC experiments, the introduction of photo-active moieties in the ligand scaffolds to generate LigandDriven Light Induced Spin Change (LD-LISC) [5,6], experiments to determine other external inputs, e.g. pressure, magnetic field, solvation/desolvation and the construction of polynuclear metal complexes to evaluate the coupling of other magnetic phenomena and to access more states that could permit the execution of more complex logical operators. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been financially supported by SEP-Conacyt (CB 2016/286346) and SEP-Cinvestav (Proyecto 270). References [1] A. Hauser, Ligand field theoretical considerations, in: P. Gütlich, H.A. Goodwin (Eds.), Spin Crossover in Transition Metal Compounds I, Springer Berlin Heidelberg, Berlin, Heidelberg, 2004, pp. 49–58.

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