Coordination complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base macrocycles “Old Complexes Offer New Attractions”

Accepted Manuscript Title: Coordination Complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base Macrocycles “Old Complexes Offer New Attractions” Author: Emma L. Gavey Melanie Pilkington PII: DOI: Reference:

S0010-8545(15)00114-9 http://dx.doi.org/doi:10.1016/j.ccr.2015.03.017 CCR 112050

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

19-1-2015 23-2-2015 6-3-2015

Please cite this article as: E.L. Gavey, M. Pilkington, Coordination Complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base Macrocycles “Old Complexes Offer New Attractions”, Coordination Chemistry Reviews (2015), http://dx.doi.org/10.1016/j.ccr.2015.03.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Coordination Complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base Macrocycles “Old Complexes Offer New Attractions”

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Emma L. Gaveya and Melanie Pilkingtona* * Corresponding author: Melanie Pilkington,

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Department of Chemistry, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, Canada. L2S 3A1.

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Email: [email protected]

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Tel +1 (905) 688-5550 Ext 3403

Abstract

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Two classes of complexes of 15-membered Schiff base macrocycles with a pentadentate (N3X2) cavity (where X = O, N, S) prepared via a metal templated condensation reaction between suitably functionalized 2,6-diacetylpyridine and polyamines are surveyed. The structures and

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properties of transition metal and lanthanide complexes are presented. Particular emphasis is

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placed on the unique structural changes accompanying the thermal and light induced spin crossover transition of a [Fe(N3O2)(CN)2] complex as well as a detailed exploration of Mn(II)

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complexes that in addition to their magnetic properties have been investigated for potential applications in biomedical diagnostics, as enzyme mimics for superoxide dismutase as well as for their antimicrobial activity. The remarkable versatility of this family of complexes as building blocks for the self-assembly of discrete and polymeric systems which include clusters, chains and photoswitchable molecule-based magnets is also reviewed. Keywords: Schiff base macrocycles, pendentate macrocyles, polynuclear, spin crossover, cyanometalates, metal templation

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Table of Contents

1.2

Schiff base macrocycles................................................................................................ 4

1.3

Historical Perspective .................................................................................................. 5

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Scope............................................................................................................................. 4

Mononuclear complexes......................................................................................................6 Early studies (1967-1980) ............................................................................................ 6

2.1.1

Structural reports of transition metal complexes of Class I macrocycles.................... 6

2.1.2

Fe(II) and (III) complexes: preliminary magnetic studies ........................................... 8

2.2

Later studies (1995-present)....................................................................................... 11

2.2.1

Fe(II) complexes: full elucidation of SCO behaviour ................................................ 11

2.2.2

Mn(II) complexes: detailed exploration ..................................................................... 14

2.2.3

Additional transition metal complexes ....................................................................... 18

2.2.4

Class II - Macrocycles with hindered backbones....................................................... 19

2.2.5

Macrocycles with covalently appended axial ligands ................................................ 20

2.2.6

Rare earth complexes ................................................................................................. 21

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2.1

Polynuclear and Polymeric Complexes ..............................................................................23

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3.

1.1

3.1

Linear chains .............................................................................................................. 23

3.1.1

3d Paramagnetic polycyanometalate linkers ............................................................. 23

3.1.2 3.1.3 3.1.3 3.1.4 3.2

3d Paramagnetic cyanometalate linkers with ‘blocking’ ligands .............................. 27 Diamagnetic polycyanometalate linkers .................................................................... 30 4d and 5d Polycyanometalate linkers......................................................................... 31 Non-polycyanometalate linkers .................................................................................. 34 Higher order magnetic structures: 2- and 3-D .......................................................... 36

3.2.1 3.2.2 3.3

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Introduction .........................................................................................................................4

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Polycyanometalate linkers with 4d metal ions ........................................................... 36 Polycyanometalate linkers with 3d metal ions ........................................................... 39 Summary of magnetic properties................................................................................ 40

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Dual property materials ..................................................................................................42

5.

Concluding remarks........................................................................................................43

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Acknowledgements .......................................................................................................47

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References......................................................................................................................48

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1.

Introduction

1.1

Scope This review focuses on one particular family of 15-membered N3X2 Schiff base

macrocycles whose coordination chemistry spans almost 50 years. Particular attention is paid to

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the synthesis, structures and magnetic properties of polynuclear complexes assembled from these macrocycles which have not been reviewed to date. For an excellent broader introduction into

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this field of chemistry the reader is referred to the recent review on Schiff base and non-Schiff base macrocyclic ligands by Rezaeivala and Keypour [1], as well as the earlier review of

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Radeka-Paryzek et al., highlighting the first lanthanide complexes prepared from pentadentate

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aza and oxaaza macrocycles [2].

Schiff base macrocycles

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Schiff bases containing an imine or azomethine group (-R2C=N-R; where R = an aryl or alkyl group) formed via the condensation of a primary amine with an active carbonyl are named after the Italian chemist Hugo Schiff who first discovered them [3]. The reaction to prepare a

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Schiff base is reversible, occurring via a carbinolamine intermediate which typically requires the yields, Scheme 1.

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removal of water to push the equilibrium to the right in order to obtain the compounds in good

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[PLACE SCHEME 1 HERE]

Scheme 1. Imine Schiff base formation

Schiff bases coordinate to both transition metals and lanthanide ions and have played a fundamental role in the development of the field of coordination chemistry over the past five decades and they currently play a prominent role in the fields of bioinorganic and supramolecular chemistry as well as molecular magnetism. In the 1960s, synthetic methods for the preparation of a range of polyaza, polythia and polyazathia macrocycles were developed largely employing template strategies that lead to a burgeoning of interest in the field resulting in Schiff bases becoming one of the cornerstones of the field of macrocyclic chemistry. In this context, the 15membered N5 complex L1 (Fig. 1) is the second reported example of a synthetic macrocyclic 4 Page 4 of 106

ligand prepared employing a metal templation strategy. The role of the metal ion in the cyclization reaction is to pre-organize the molecular precursors via the formation of a metal complex, thus the size and charge of the metal ions employed are important considerations. The pyridyl head groups serve to increase the rigidity of the ligand which is kinetically favourable for the templation reaction. Over the years, macrocycles incorporating pyridyl fragments have lent

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themselves to a range of biological applications which include host-guest compounds [4], biomimetics [5], antibiotics [6], as well as for metal chelation and extraction [7]. In addition to

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these more commonly reviewed properties, they have also been extensively studied for their magnetic properties with a view to developing new families of probes for medical diagnostics

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and for potential information storage applications in the form of spin crossover compounds,

1.3

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single molecule magnets (SMMs) and single chain magnets (SCMs), vide infra.

Historical Perspective

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In the 1970s, Nelson and Drew published a series of papers describing the coordination complexes of a family of 15-membered (N3X2) Schiff base macrocycles, where X = O, N or S [8]. These complexes remained unexplored for 20 years until a [Mn(N5)(H2O)2]2+ macrocycle,

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(1) was exploited as a building block for the preparation of a magnetic 2-D network by the late

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Olivier Kahn [9]. Two years later, the spin crossover properties of the [Fe(N3O2)(CN)2] complex (2), were revisited by Sato et al. [10] The structural and magnetic properties of these two

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compounds initiated a resurgence of research concerning their applications as suitable building blocks for the preparation of molecular magnets. Although there are several reviews describing the broader class of Schiff base macrocyclic complexes [1-2, 11], renewed interest in this specific class of macrocycles warrants a review of both their discovery together with more recent studies, as well as an overview of the structural and magnetic properties of the polynuclear compounds assembled from these complexes. The focus of this review is therefore placed on the coordination chemistry of a family of 15-membered (N3X2) Schiff base macrocycles (L1-L11) divided into two classes (rigid I, and flexible II) whose structural formulae are presented in Figure 1. It should be noted that several larger N3O2 [12], N5 [13] and N3S2 [14] macrocycles are also reported in the literature but are beyond the scope of this study. In section 3 the polynuclear complexes of class I are organized according to their structural topologies and the nature of their secondary building blocks. The magnetic properties of these complexes are summarised at the end of section 3. 5 Page 5 of 106

[PLACE FIGURE 1 HERE] Fig. 1. Structural formulae of the two classes of pentadentate (N3X2) Schiff base macrocycles

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(L1-L11) presented in this review.

Mononuclear complexes

2.1

Early studies (1967-1980)

2.1.1

Structural reports of transition metal complexes of Class I macrocycles

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The N5 macrocycle (L1) was first reported in 1967, when Hawkinson described the crystal

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structure of an [Fe(L1)(NCS)2]ClO4 complex (3), and noted the unusual seven coordinate

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pentagonal bipyramidal geometry of the central Fe(III) ion, Fig. 2 [15].

[PLACE FIGURE 2 HERE]

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Fig. 2. The molecular structure of the first N5 macrocyclic complex [Fe(L2)(NCS)2]ClO4 (3) [15-

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16]. The ClO4- counter ion and H-atoms are omitted for clarity.

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Ten years later, Nelson et al. described the preparation of a series of coordination complexes of (L1) via the 1:1 Schiff base condensation of 2,6-diacetylpyridine with triethylenetetramine, templated by a series of metal cations, where M(II) = Mn, Fe, Co, Ni, Cu, Zn, as well as Mg(II) and Fe(III), Scheme 2, left [20]. Interestingly, when X = O and M = Ag(I) or Pb(II), the 30membered [2+2] macrocycle formed in high yield (Scheme 2, right), indicating that both the size of the cation and its affinity for the X substituents on the diamine precursor direct the preferred course of the condensation reaction [13, 14b].

[PLACE SCHEME 2 HERE] Scheme 2. Left, General synthesis of the Schiff base N3X2 macrocycle via metal templation around a metal chloride salt. Right, molecular structure of the 30-membered [2+2] macrocycle.

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Following this strategy, the molecular structures of four complexes with stoichiometries [Fe(L1)(NCS)2]ClO4 (3) [17], [Fe(L1)(H2O)2]Cl(ClO4) (4) [18], [Zn(L1)(NCS)2](Cl2)(C2H4) (5) [16], and [Mg(L1)(H2O)Cl]Cl (6) [19], were determined by X-ray crystallography. In all four cases, the metal ions adopt a pseudo-D5h pentagonal bipyramidal geometry, with the five donor atoms of the macrocycle coordinating in the equatorial plane, and two monodentate ligands

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occupying the axial positions. Intrigued by the unusual coordination environment of the metal cations, Nelson prepared and characterized families of (L1) and (L2) complexes, as part of a

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series of papers describing quinquedentate macrocyclic ligands [8, 17, 20].

Following the synthesis of the [Mg(L1)(H2O)Cl]Cl complex (6), the Mg(II) cation was

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exploited as a template for the synthesis of the chloride, perchlorate, tetraphenylborate and thiocyanate derivatives of (L1) and (L2) [20]. Although no single crystal X-ray diffraction data were collected for this series of complexes [other than (6)], macrocycle formation was confirmed

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by IR spectroscopy, with each complex exhibiting a C=N str at 1650 cm-1, consistent with formation of the Schiff base bis-imine bond. The templating role of the Mg(II) cation was

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demonstrated by the failure of the macrocycle to form in the absence of a metal cation. For these reactions, in the absence of any metal ion, an oily product or a mixture of products was obtained

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in which the carbonyl groups of the diketone starting material were observed in the IR spectra.

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The IR spectra also indicated the presence of bound water molecules for each of the complexes with the exception of the thiocyanate derivative. Consistent with this, the electronic spectra

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indicated that the chloride, perchlorate and tetraphenylborate derivatives were salts, with noncoordinating counterions balancing the charge of the bis-aqua complexes; while the thiocyanate derivative was a neutral complex, with two coordinating counterions. Mass spectra of the complexes did not show parent ion peaks, but the spectra of the chloride derivatives did contain fragment ions corresponding to the loss of a single chloride ion from complexes of (L1) and (L2) respectively. 1H and 13C NMR data for selected complexes were in agreement with the suggested macrocyclic structures. Given the data, a seven-coordinate pentagonal-bipyramidal geometry was assigned to all of the complexes [20]. A second series of studies were then carried out where the counterion and axial ligands were kept constant, while the nature of the templating metal ion was varied. Following this strategy, the thiocyanate derivatives of the Mn(II), Fe(II), Zn(II) and Cd(II) complexes of (L1) and (L2) were prepared [8]. This family of complexes were similarly characterized by IR and electronic spectroscopy, elemental analysis and mass spectrometry. The crystal structure of 7 Page 7 of 106

[Mn(L2)(NCS)2] (7) was determined and was the first reported structure of a macrocyclic complex of (L2), Fig. 3. This complex, like other members of the series, has pentagonal bipyramidal geometry with axial thiocyanate ligands, similar to the [Fe(L1)(NCS)2]ClO4 complex (3) presented in Fig. 2. Given that the IR spectra of the Fe(II), Zn(II), and Cd(II) to all four complexes [8].

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[PLACE FIGURE 3 HERE]

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dithiocyanate complexes were almost identical, a pentagonal bipyramidal geometry was assigned

Fig. 3. Molecular structure of [Mn(L2)(NCS)2], (7) at 293 K [8, 16]. H-atoms are omitted for

2.1.2

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clarity. Fe(II) and (III) complexes: preliminary magnetic studies

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Following the structural reports by Nelson and Drew described above, attention then shifted towards the magnetic properties of Fe(II) and Fe(III) complexes of (L1) and (L2). A

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family of complexes with the formula [Fe(L1)X2]ClO4 (where X = Cl-, Br-, I- and NCS-) were thoroughly studied by various means in order to investigate their zero field splitting (ZFS) parameters (D), with a view to probing the anisotropy associated with D5h symmetry. Electron

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spin resonance (EPR) studies were performed by Cotton et al., from which D values ranging

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from 0.3 to > 1 cm-1 were extracted [21]. The magnitude of the D values was found to increase in the order Cl- < NCS- < Br- < I-. Mössbauer measurements were carried out by Nelson et al.,

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which indicated that the D term was positive for complexes of Cl-, Br-, and I-, and negative for the NCS- complex of [22]. Reiff and Scoville gathered temperature-dependent magnetic susceptibility data on powder samples of the I- and NCS- complexes [23]. From this data, the Kotani expression was used to calculate D values of 7.98 and 10.3 cm-1 for the NCS- and Icomplexes respectively – significantly higher than the values elucidated from the earlier EPR studies, which the authors call into question [23]. Electrochemical studies on aqueous solutions of Fe(II) and Fe(III) complexes of (L1) were carried out by Chen and Bard, with a view to the use of such compounds in photoelectrochemical cells [24]. Cations of the form [FeII(L1)]2+ (8) and {[FeIII(L1)]2O}3+ (9) were studied by cyclic voltammetry and spectrophotometry in solutions of varying pH, and the standard potentials for reactions between various redox couples in each system were determined. Complexes based on (8) and (9) were determined to be promising with regards to photoelectrochemical applications, 8 Page 8 of 106

due to their high water solubility and ability to cycle repeatedly between Fe(II) and Fe(III) over a wide pH range (5-11) [24]. Magnetic susceptibility and Mössbauer studies of several d5 complexes of (L1) were carried out by Nelson and Drew. Complexes [Fe(L1)(NCS)2]ClO4 (3) and [Fe(L1)Cl2]ClO4 (10) were high spin (HS), with magnetic moments in good agreement with the theoretical value of

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5.92 BM at room temperature [8]. The isomer shifts and quadrupole splitting observed in the Mössbauer spectrum of the related [Fe(L2)(NCS)2] complex (11) confirmed that at room

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temperature the metal ion adopts an S = 2, HS state. Mössbauer studies also showed that the N5 macrocyclic ligand field of (L1) is stronger than that of the N3O2 macrocycle (L2) [8]. Cyclic

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voltammetry studies indicated that the harder oxygen atoms in (L2) were more effective at stabilising the lower oxidation states of the metal than (L1): while Fe(II) complexes of (L1) were oxidised in minutes, complexes of (L2) were stable in solution [8].

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In the 1980s, the magnetic properties of the Fe(II) bis-cyanide complexes of (L1) and (L2) were examined, with very interesting results [25]. Magnetic susceptibility and Mössbauer

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spectroscopy studies were carried out on both [Fe(L1)(CN)2]∙H2O (12), and [Fe(L2)(CN)2]∙H2O (2) between 80 and 300 K. The N5 system (12) adopts a low spin (LS) ground state at ambient temperatures [25a]. The presence of the strong field cyanide ligands leads to a large splitting of

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the d-orbitals, stabilizing a low spin configuration for the Fe(II) centre. However, the splitting of

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the d-orbitals under 7-coordinate, pentagonal bipyramidal geometry does not allow for a ‘closed shell’ electron configuration, with all electrons paired (Fig. 4, left). To maximize the crystal field

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stabilization, the macrocycle alternatively acts as a tetradentate ligand, with one of the amine groups remaining uncoordinated [25a]. The geometry of the metal centre is then 6-coordinate pseudo-octahedral, allowing for a closed shell ground state (Fig. 4, right). The added crystal field stabilisation compensates for the increase in macrocyclic strain [25a].

[PLACE FIGURE 4 HERE]

Fig. 4. The d-orbital splitting diagrams for low spin Fe(II) ion in a 7-coordinate, pentagonal bipyramidal environment (left), and a 6-coordinate, octahedral environment (right). In contrast, the magnetic properties of the N3O2 system (2), which offers a slightly weaker ligand field, varied with temperature. At low temperatures (< 150 K), the system adopts a low 9 Page 9 of 106

spin (LS), S = 0 state. The geometry was assumed to be 6-coordinate, for the reasons described above, with one oxygen atom uncoordinated. At temperatures between 150 and 200 K, Mössbauer measurements indicated that the complex existed in a 1:1 ratio of high spin (HS) to LS. At high temperatures (> 230 K), the complex was present solely in the HS S = 2 state [25a]. Given the lack of X-ray quality crystals, the coordination geometry of the HS and LS states

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could not be definitively assigned, yet these data clearly established the presence of a thermallycontrolled stepwise spin crossover (SCO) transition. Given these observations, more detailed

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studies were carried out to probe the nature of the thermal SCO transition [25b]. Slow cooling of the complex showed a transition from the HS state to a 1:1 mixture of HS/LS states at 207 K.

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This transition was reversible, with Tc↑ = 222 K, giving a hysteresis loop with a width 15 K, Fig. 5a [25b]. When the sample was cooled more quickly (10 – 20 K/min) from 300 K to 150 K, no hysteresis effects were observed. The sample remained in the HS form for several minutes before

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rapidly and irreversibly undergoing a transition to the pure LS state, Fig. 5b [25b]. Fast cooling of the sample from ambient temperatures to 77 K ‘froze in’ the HS state; if the sample was

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subsequently warmed above 110 K, an irreversible transition to the LS state occurred [25b].

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[PLACE FIGURE 5 HERE]

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Fig. 5. Schematic diagrams of the temperature dependence of tHS/ttot for a) slowly decreasing and increasing temperature where Tc↓ = 207 K and Tc↑ = 222 K; b) faster decrease of temperature

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from 250 K to 134.8 K and subsequent increase of temperature where Tc = 207 K. Adapted from reference [25b].

The thermodynamics and kinetics of this transition were also studied. After comparing the hysteresis loop and X-ray diffraction patterns with those of similar macrocyclic complexes, the system was determined to be thermodynamically first order. The rate constant varied between k = 6.77 x 10-3 and 2.31 x 10-3 s-1, with an activation energy of 7.3 kcal.mol-1 [25b]. Variable temperature X-ray powder diffraction measurements on a sample of the complex revealed that the transition from HS to LS was accompanied by a change in diffraction pattern, indicating that the HS, HS/LS and LS states have different unit cells [25b]. A crystallographic phase change accompanying the SCO transition supported the proposition that the coordination geometry of the HS and LS states were different. Unfortunately, no crystal structures of the two phases were 10 Page 10 of 106

available to confirm this hypothesis and such a change in coordination geometry accompanying a thermal spin crossover transition had not been previously observed for SCO complexes [25b].

Later studies (1995-present)

2.2.1

Fe(II) complexes: full elucidation of SCO behaviour

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2.2

Following this work, investigation into the unusual SCO behaviour of (2) lay largely dormant for twenty years until it was revisited by Sato et al. in 2001 [10]. X-ray quality single

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crystals of [Fe(L2)(CN)2]∙H2O (2), were obtained and the structure of the HS complex was determined at room temperature. The complex crystallizes in the monoclinic space group Cc and

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is 7-coordinate, as postulated earlier by Nelson, with the macrocycle acting as an essentially planar pentadentate ligand, and the two cyanide ligands occupying the axial positions [10].

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Unfortunately on cooling, the crystals shattered; hence no low temperature structure of the complex in the HS state could be measured. The magnetic properties of the single crystals were investigated, Fig. 6. Upon cooling from room temperature to 130 K, the expected transition from

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HS to LS state was observed, with Tc(1)↓ = 159 K for the HS/LS to LS transition. On warming, the crystals again reversed the transition, with Tc(1)↑ = 172 K, giving a hysteresis loop of width

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13 K. When the crystals were heated further from 176 to 207 K, the magnetic susceptibility

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decreased, as the single crystals fragmented into powder. A second cooling and heating cycle gave relaxation temperatures of Tc(2)↓ = 225 K and Tc(2)↑ = 198 K between the intermediate

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HS/LS phase and a pure HS geometry with a hysteresis loop of 27 K, consistent with Nelson’s observations [10]. Subsequent thermal cycles did not alter the hysteresis loop further. The reason as to why the powder sample does not afford 100% LS complex on cooling remains unresolved to-date.

[PLACE FIGURE 6 HERE]

Fig. 6. Plot of χMT versus T for [Fe(L2)(CN)2]∙H2O, (2). Adapted from reference [10]. Sato and co-workers also studied the photomagnetism of this complex. Light-induced excited spin-state trapping, or the LIESST effect, was first reported by Decurtins et al. in 1984 [26]. It involves a photoinduced LS to HS transition, in which the compounds are quantitatively trapped in the metastable HS state at sufficiently low temperatures. The discovery of the LIESST effect suggests that SCO compounds may be used as optical switching devices [27]. However, 11 Page 11 of 106

until 2001, this effect had only been observed at temperatures below 80 K [28]. In a comprehensive review on this subject [29], Letard discusses the temperature parameters surrounding the LIESST effect, and notes that the development of compounds which display LIESST properties at higher temperatures is important if these materials are to be employed in practical applications.

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In order to investigate the possibility of a photoinduced SCO transition, a sample of (2) was cooled to 5 K and irradiated with green light from a Hg-Xe lamp (550 nm, 1.5 mW.cm-2).

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Magnetic measurements showed a persistent change in susceptibility, and the conversion was confirmed by IR spectroscopy after the sample was illuminated at 15 K. Mössbauer studies were

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carried out to determine the degree of LS → HS conversion at 15 K and the transition proved to be complete [10]. Following illumination, the magnetic susceptibility of the sample showed a temperature-dependent two-step relaxation from pure HS to pure LS with a relaxation

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temperature of Tc(LIESST) = 130 K for the initial decrease in χT, followed by complete conversion back to the LS state at 180 K. To make sure that the metastable HS state was not present simply

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because it was ‘frozen in’ due to rapid cooling, the LIESST effect was reversed by irradiating the sample with a diode laser (850 nm, 0.3 mW.cm-2) [10]. The observed Tc(LIESST) of 130 K is currently one of the highest relaxation temperature reported for any mononuclear Fe(II) complex,

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and represents an important step towards the application of such compounds in molecular

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devices. Following this study, Letard et al. investigated the change in the dielectric constant of the [Fe(L2)(CN)2]∙H2O macrocycle (2) accompanying the light-induced spin transition [30]. The

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HS dielectric constant ԑ ’HS was 2.93 while ԑ ’LS was 2.74 [30], reflecting a 6% change between the HS and LS states. DFT methods were then used to calculate the change in the mean electronic polarizability associated with the HS → LS transition, which was determined to be 12.08 a.u [30]. Letard points out that the electrical detection of a light-induced spin transition suggests that SCO compounds may be used as photoswitchable capacitors for electronic data storage [30].

The structural transformation of (2) was conclusively proven when both the HS and LS molecular structures of the [Fe(L2)(CN)2]∙H2O macrocycle were reported in 2007 [31]. This later study focussed on the complete thermal conversion of the HS state to the LS state, although different levels of transition can be achieved by varying the cooling rate and other experimental conditions. Sato’s earlier structural determination of the HS state in the monoclinic Cc space group had several anomalies, such as an unexpectedly long Fe-O bond length, so the HS crystal 12 Page 12 of 106

structure was recollected at 293 K. In contrast to the earlier report, the room temperature HS complex crystallises in the monoclinic space group C2/c [32]. The Fe(II) lies on a two-fold axis of symmetry, with two identical Fe-O bonds [31-32]. On cooling the two-fold axis of symmetry is lost, as one of the Fe-O bonds is broken to give the low temperature LS complex (P21/c space group). The Fe(II) ion is now 6-coordinate, in a distorted octahedral environment [31]. This Fig. 7

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[PLACE FIGURE 7 HERE]

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structural conversion is particularly interesting since it is completely reversible in the solid state,

Fig. 7. Molecular structures of [Fe(L2)(CN)2]∙H2O, (2) in the HS (left) and LS (right) states showing the reversible transition from 7-coordinate to 6-coordinate geometry, H-atoms are

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omitted for clarity.

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This structurally characterized, reversible change in coordination geometry accompanying the thermal SCO transition is unprecedented in the chemical literature. Importantly, the marked change in coordination geometry associated with the transition drastically affects the crystal

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packing of the complexes. A huge decrease in coordination volume from 17.0(1) to 10.0(1) Å3

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was observed corresponding to a change of around 40% [31]. Letard states that this kind of reversible change in coordination volume is necessary for industrial SCO applications [31].

In

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more recent work Letard et al. studied the effect of dehydration and rehydration on the SCO properties of Fe(L2)(CN)2]∙H2O (2) [33]. The number and nature of solvent molecules present in a crystal lattice affects the parameters surrounding a spin transition. The progress of the loss and gain of lattice water was followed by thermogravimetric analysis and X-ray powder diffraction. A phase change corresponding to the loss of lattice water molecules was observed around 380 K, accompanied by a decrease in unit cell volume and a loss of symmetry (triclinic unit cell). Upon rehydration, similar peaks to the original compound were observed, but broadened; and the crystallinity was not recovered. The thermally-induced excited spin-state trapping (T(LIESST)) effect in the original, dehydrated and rehydrated complexes was investigated by warming after quench-cooling each of the samples. Each of the three samples displayed different behaviour, with the rehydrated sample exhibiting a T(TIESST) of 156 K (compared to 135 K for the original

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sample, and 69 K for the dehydrated sample) – the highest value recorded for a mononuclear Fe(II) complex to date [33]. A thorough investigation of the photomagnetic behaviour of the Fe(II) complex of (L1), [Fe(L1)(CN)2].H2O (12), was carried out by Letard et al. in 2007 [34]. In keeping with previous observations [25a], the complex was diamagnetic in the temperature range 5 – 300 K, with χMT <

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0.25 cm3.K.mol-1. The LIESST effect was studied in the manner established by Letard, involving cooling of the sample to 10 K, followed by irradiation with light from a Krypton Laser (λ =

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530.9 nm, 5 mW cm-1). A significant increase in χMT was observed, with a value of 2.4 cm3.K.mol-1 being reached after 1 hour [34]. The maximum value of 2.86 cm3.K.mol-1, attained

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around 40 K, indicated that the photoinduced conversion was nearly quantitative. The T(LIESST) was determined to be 105 K. An examination of the decay kinetics following irradiation indicated that the metastable HS state is extremely long-lived. The authors postulate

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that the unusual lifetime is primarily affected by the inner coordination sphere of the complex, and most significantly by the coordination number of the ligand [34].

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In 2014, Letard et al. re-visited (12) with regards to the LIESST effect [24]. The magnetic susceptibility behaviour of (12), as well as several related complexes outside the scope of this review, were studied between 4 and 500 K. No new observations regarding (12) were made, but

d

in the context of the family being studied, the authors observed that the nature of the macrocyle

te

did not appear to affect the stability of the photoinduced states, but did affect the level of photoexcitation. The authors re-iterate the ‘urgent demand’ for the development of compounds

Ac ce p

displaying a LIESST effect at higher temperatures, due to their significant technological potential [24]. 2.2.2

Mn(II) complexes: detailed exploration In recent years, Mn(II) complexes of (L1) and (L2) have been targeted for thorough

exploration by a number of chemical, physical and biological means including magnetic susceptibility and EPR studies, aqueous NMR proton relaxation measurements, and biological testing as enzyme mimics and bacteria inhibitors. A unique dinuclear derivative of macrocycle (L2) was synthesized by Pilkington et al. in 2007, comprising two [Mn(L2)Cl(OH)2]+ complexes tethered via a covalent linkage between the pyridine head-units, Fig. 8 [35]. The dimeric species {[Mn(L2)]Cl(OH2)}2Cl2 (13), was synthesized via a metal-templated Schiff base condensation, with 2,2′,6,6′-tetraacetyl-4,4′-bipyridine as the tetra-ketone precursor. The X-ray crystal structure 14 Page 14 of 106

of this complex shows that both Mn(II) ions adopt a pentagonal bipyramidal environment, with a macrocyclic ring in the equatorial plane, and a chloride and water molecule axially bound to each metal centre. The two macrocyclic rings are offset with respect to one another, with an angle of

[PLACE FIGURE 8 HERE]

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41º between the planes of the two pyridine rings.

cr

Fig. 8. Crystal structure of {[MnL2]Cl(OH2)}22Cl (13); H-atoms and Cl- counterions omitted for

us

clarity [35].

Magnetic susceptibility studies showed the presence of weak antiferromagnetic interactions below 40 K. This work demonstrates that the organic framework of the macrocycle can be

an

synthetically modified to covalently attach a second magnetic unit, providing a strategy for the realization and study of new magnetically interesting dimeric N3O2 macrocyclic building blocks.

M

The monomeric complex [Mn(L1)(H2O)2]Cl2 (1) was comprehensively analyzed by X-ray crystallography, variable-temperature EPR as well as solid- and solution-state UV-vis spectroscopy [36]. The crystal structure of (1) showed the expected 7-coordinate, distorted

d

pentagonal bipyramidal geometry, with two water molecules occupying the axial positions. The

te

maximum deviation from planarity of the macrocyclic ring was observed to be 0.19 Å. A shift in the UV-vis peak from 400 nm in methanol solution to 445 in the solid state suggests a slight

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change in geometry around the Mn(II) center between the two states. The EPR data revealed a dependence of the crystal field on temperature, with the field becoming larger and increasing in rhombicity with decreasing temperature, likely due to small structural changes [36]. NMR Proton Relaxation

Several Mn(II) complexes of (L1) and its derivatives have been characterized by aqueous NMR proton relaxation measurements. This property has potential applications in the field of biomedical diagnostics, as the presence of a paramagnetic metal ion alters the proton relaxation time of water molecules in biological tissues, enhancing image contrast. The relaxivities (r1 and r2, in s-1.mM-1) describe the effect of a compound on longitudinal and transverse proton relaxation times (T1 and T2). Jackels et al. studied two series of Mn(II) macrocyclic complexes based on [Mn(L1)]2+, in which the size and saturation of the macrocyclic cavity were varied 15 Page 15 of 106

respectively [37]. The correlation between the structure of each ligand and complex, and the NMR proton relaxation rate, was probed using a field cycling relaxometer, at three different temperatures. Within the family of complexes with increasing macroyclic cavity size, relaxivity was found to have a first order relationship to the number of coordinated water molecules; and relaxivity per coordinated water molecule increases with decreasing saturation [37]. In contrast

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to the complexes with axial positions available for water binding, Jackels et al. also reported a coordinately saturated derivative of [Mn(L5)]2+, in which the macrocyclic ligand contained

cr

pendant aminoethyl arms which occupy the axial binding positions of the Mn(II) ion [38].

us

[PLACE COMPOUND (14) HERE]

The complex [Mn(L5)]Cl2 (14) was synthesized from the initially isolated [Mn(L5)](PF6)2 complex (15) by counterion exchange. UV-vis and EPR measurements indicated that the pendant

an

arms remain coordinated in solution. NMR proton relaxation measurements gave a very low relaxivity value of 0.92 s-1.mM-1 (24 MHz, 25ºC), suggesting that relaxation was occurring via an

M

‘outer sphere’ mechanism rather than via coordinated ‘inner sphere’ water molecules, not

Dismutase activity

d

surprising since the Mn(II) ion in this complex is coordinatively saturated [38].

te

[Mn(L1)(H2O)2]Cl2 (1) has been thoroughly examined as a synthetic mimic for superoxide dismutase (SOD), the naturally occuring enzyme which catalyzes the disproportionation of the

Ac ce p

harmful superoxide radical O2.-. SOD mimics have important therapeutic applicability for the treatment of conditions involving the presence of excess superoxide, such as inflammatory and vascular diseases [39]. Complex (1) was first studied as a SOD mimic by Riley and Weiss in 1994, as one of twenty macrocyclic Mn(II) complexes being screened in a stop-flow assay to determine SOD activity [39]. However, unlike a number of the amine-only macrocyclic complexes, (1) did not display any such activity. The complex was re-visited by Ivanovic´Burmazovic et al. as part of a mechanistic study concerning water exchange in seven-coordinate Mn(II) complexes which act as SOD mimetics [39]. Complex (1) was studied as the inactive derivative of its non-imine counterparts. Various parameters were measured in aqueous solution, including the water exchange rate constant using

17

O NMR techniques, in an effort to relate

ligand structure to lability. The inactivity of (1) was hypothesized to be due to the decreased flexibility of the macrocyclic ring, hindering the formation of a pseudo-octahedral intermediate 16 Page 16 of 106

[39]. The same authors also examined (1) as a mimic for NO dismutase [40]. As NO is a biologically significant molecule, it is important that SOD mimics be selective for superoxide over NO. SOD-inactive (1) and its SOD-active non-imine analogue were studied to determine whether they also differed in terms of reactivity towards NO, as well as to probe the mechanism of their NO dismutase activity. It was found that both complexes catalyse NO disproportionation

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via an Mn(II)/Mn(III) redox cycle [40]. These results called into question the selectivity of Mn(II) pentaazamacrocyclic SOD mimics.

cr

Ivanovic´-Burmazovic et al. synthesized the dimeric derivative of (1), {[Mn(L1)Cl2]}2 (16), as well as its dimeric non-imine analogue, in order to investigate the change in water

us

exchange and SOD mimetic behaviour in comparison to the monomeric species [41]. The complexes were characterized in both the solid state and aqueous solutions of various pH by an arsenal of methods, including X-ray crystallography, potentiometric and electrochemical studies, 17

O NMR spectroscopy. The two macrocyclic units of (16) were

an

magnetic measurements and

twisted with respect to one another with a dihedral angle of 28.4º, smaller than that observed for

M

the related Mn(II) dimer of (L2) (13), discussed previously [35]. Unlike (13), all four axial positions of (16) in the solid state were occupied by chloride ions (Fig. 9), all of which are replaced by water molecules in aqueous solution. In acidic solution, hydrolysis of the imine

d

bonds prevented the observation of a stable protonated form, while in basic solution, a two-step

te

base-catalyzed addition of water to the imine bonds was recorded [41]. Dc susceptibilty measurements in an applied field of 1 T indicated that the two Mn(II) centres were essentially

Ac ce p

non-interacting, with an exchange coupling constant J of less than 0.1 cm-1 and very weak antiferromagnetic interactions observed at low temperatures. Like its monomeric counterpart, (16) displayed no SOD activity [41].

[PLACE FIGURE 9 HERE]

Fig. 9. Crystal structure of {[Mn(L2)Cl2}2 (16); H-atoms omitted for clarity [41]. Antimicrobial activity A family of complexes with the formula [Mn(L)](ClO4)2, where (L) = symmetric and asymmetric derivatives (L5 to L7) with 15-membered backbones (Fig. 10) were synthesized by Khanmohammadi et al. [42]. 17 Page 17 of 106

[PLACE FIGURE 10 HERE] Fig. 10. The 15-membered macrocycles tested by Khanmohammdi et al. [42].

ip t

Only one of the complexes was previously unreported: the complex based on the asymmetric ligand (L7) with formula [Mn(L7)](ClO4)2 (17). DFT calculations using GAUSSIAN 98 were

cr

performed in order to compare the predicted gas phase structure of (17) with the experimentally determined solid state structure, and the data sets were consistent [42]. A more detailed structural

us

discussion of complexes resulting from macrocycles with pendant arms is presented in section 2.2.5. The complexes were investigated in vitro for their antimicrobial activity against Staphylococcus aureus, E. coli and Candida albicans, in comparison to three standard drugs. The

an

antibacterial activity of the complexes increases with the size of the macrocyclic cavity. The asymmetric complex (17) displayed lower activity than the symmetric complexes of the same

2.2.3

M

cavity size, possibly due to the steric effect of the methyl substituent [42]. Additional transition metal complexes

d

In the 1990s, the N5 derivative (L3) was studied by Khan et al. [43]. A series of

te

macrocyclic complexes of stoichiometry [M(L3)]2+ (18), where M = Mn(II), Co(II), Ni(II) and Zn(II) were prepared. The complexes were characterized by elemental analysis, molar

Ac ce p

conductance as well as IR and UV spectroscopy, but their magnetic properties were not reported. An interesting comparison may also be made between the complexes of (L1) and its thioaza derivative, the N3S2 macrocycle (L4). The first and only synthesis of (L4) was reported by Baker et al. in 2003 [44]. The objectives of this study were to study the metal ion selectivity of several [N3S2] ligand systems. During the course of this work, the Ag(I) complex [Ag(L4)]ClO4] (19), was prepared via a Schiff base condensation reaction analogous to that demonstrated in Scheme 1 [44]. Subsequent transmetallation afforded the copper(II) complex [Cu(L4)](ClO4)2 (20). The crystal structure of this complex revealed that the Cu(II) ion was 5-coordinate, with the Cu(II) ion bound to the N3S2 donor atoms of the macrocyclic ring. The metal ion lies in the N3 plane, with the two sulphur atoms lying above and below the N3 plane, Fig. 11 [44]. The geometry of the complex is distorted trigonal bipyramidal, in contrast to the 7-coordinate pentagonal bipyramidal and 6-coordinate distorted octahedral geometries observed for the 18 Page 18 of 106

transition metal complexes of (L1) and (L2). The lack of expanded coordination geometry for this complex may arize due to the smaller size of the Cu(II) ion, its softer nature, and propensity to undergo a Jahn-Teller distortion. [PLACE FIGURE 11 HERE]

ip t

Fig. 11. Molecular structure of the [Cu(L4)]+ cation, (20) [44]. H-atoms are omitted for clarity.

cr

In 2014 Wang and co-workers investigated the magnetic properties of the [Co(L1)(H2O)]Cl2 complex as part of a larger study on Co(II) complexes with D5h symmetry and

us

found it exhibited field induced slow magnetic relaxation. Interestingly, the energy barrier for the complex is 20.7 cm-1 which is much lower than the energy gap between the Ms = 1/2 and Ms =

an

3/2 doublets hence the authors conclude that the magnetic relaxation processes are likely to be of Orbach or Raman type [45]. These studies show that field-induced slow relaxation is a common phenomenon for Kramer’s ions with an easy-plane magnetic anisotropy, once again

Class II - Macrocycles with hindered backbones

d

2.2.4

M

revealing the unique magnetic properties associated with pentagonal bipyramidal geometry.

Macrocycles (L1 - L4) are structurally flexible, with ethylene linkers between four of their

te

donor atoms. In contrast, the dibenzo derivatives (L8), (L9) and (L10), were studied in order to

Ac ce p

investigate the effect of intermediate rigidity on the formation of the macrocyclic complexes, Fig. 12 [46].

[PLACE FIGURE 12 HERE]

Fig. 12. Molecular structures of the dibenzo derivatives (L8 to L10). The Mn(II) complex of (L9), the Mn(II), Zn(II) and Fe(II) complexes of (L8), and the Mn(II), Zn(II) and Ni(II) complexes of (L10) were synthesized bearing nitrate, perchlorate or water ligands [46].

Elemental analysis and IR spectroscopy confirmed the formation of the

macrocycles. X-ray quality crystals were grown of the [Mn(L8)(ClO4)2] (21) and [Zn(L10) (ClO4)2] (22), derivatives, confirming they are both 7-coordinate, with pseudo D5h geometries and axially coordinated perchlorate ligands, similar to their more flexible counterparts (L1), (L2) and (L4) (but notably different to the copper complex described above which comprises discrete 19 Page 19 of 106

cations and anions) [46]. When a propylene rather than an ethylene chain was used to link the donor atoms, the resulting macrocycles adopt a chair-like structure, indicating the existence of a relationship between cavity size and macrocycle planarity [46]. The hindered macrocycle (L11), with two cyclohexyl substituents, was reported by Zhang et al. [47]. The macrocycle was initially synthesized by templation around MnCl2 to give a

ip t

complex with the formula [Mn(L11)Cl2], (23) and variation in the solvent(s) used for crystallization led to exchange of one axial chloride for methoxide (from methanol/1,2-

cr

dichloroethane [47a]) and ethoxide (from ethanol/dichloromethane [47b]) ions respectively. Xray crystallography confirmed that the Mn(II) centre in each complex was 7-coordinate, with the

us

five donor nitrogen atoms forming the basal plane. Zhang et al. report purely structural details [47], but noted that Mn(II) complexes of this chloro-substituted polyamine ligand are of interest because of their potential catalytic activity in phosphate ester hydrolysis [48], and as superoxide

an

dismutase mimics [49]. Complexes of hindered macrocycles coordinated to the rare earth metals

M

are discussed in Section 2.2.2.

[PLACE COMPOUND (L11) HERE] Macrocycles with covalently appended axial ligands

d

2.2.5

te

Several derivatives of (L5) have been reported by Keypour et al. with pendant arms attached to the two aliphatic amines of the macrocyclic backbone [50]. Macrocycles with 2-

Ac ce p

aminoethyl (depicted in Section 2.3.2) and 2-pyridylmethyl substituents have been synthesized, in which the pendant arms may act as covalently bound axial ligands via nitrogen donor atoms.

[PLACE COMPOUND L5 HERE]

The Mg(II), Mn(II), Zn(II) and Cd(II) complexes of (L5 where R= (CH2)2NH2), and the Mn(II) complex of (L5, where R = CH2py, where py = pyridine) were synthesized by templation around the metal chloride [for Mn(II) and Zn(II)] or nitrate [for Mg(II) and Cd(II)], followed by anion exchange to give the perchlorate or hexafluorophosphate derivatives via addition of the appropriate sodium salt [50]. In each case, the metal ion adopts an approximately pentagonal bipyramidal environment, with the pendant arms forming 5-membered chelate rings above and 20 Page 20 of 106

below the macrocyclic plane, Fig. 13 [50]. The authors hypothesize that the pendant arms aid templation, given that metal ions with radii up to 1.03 Å may successfully template the formation of these systems [50d].

ip t

[PLACE FIGURE 13 HERE] Fig. 13. Molecular structure of the [Cd(L5)]2+ cation (24) [50d]. H-atoms and perchlorate

2.2.6

cr

counterions are omitted for clarity. Rare earth complexes

us

For more than a decade, the Radecka-Paryzeck group has been working towards incorporating rare earth elements into (L2) and other Schiff base macrocyclic systems. Initially,

an

the Y(III) chloride and perchlorate complexes of (L2) were formed via metal templation [13a]. Y(III) is a good model for studying the binding sites of proteins which bind Ca(II) ions [51]. A

M

7-coordinate geometry was assigned to the complexes with two water molecules filling the axial positions and the chloride and perchlorate anions acting as counterions. Seven coordinate complexes of Y(III) are quite rare [52], and this was the first reported example of a Y(III) ion

d

acting as the templating agent for the synthesis of a pentadentate Schiff base macrocycle. Soon

te

after these studies, the metal-templated synthesis of several lanthanide complexes of (L2) were reported [53]. Lanthanide complexes are of interest as contrast agents in magnetic resonance

Ac ce p

imaging [54] and other diagnostic tools [55]. The heavier lanthanides [Dy(III), Tm(III), Lu(III) and Er(III)] template the formation of (L1) [53]. Characterization by IR, mass spectrometry and elemental analysis suggests that the complexes have 7-coordinate pentagonal bipyramidal geometry with the five donor atoms of the macrocyclic ring bound to the lanthanide ion, along with two water molecules in the axial positions. The chloride and perchlorate anions were concluded to be present as non-coordinating counter ions, although there is no structural evidence available to corroborate this. The lighter lanthanide ions [La(III), Sm(III) and Eu(III)] proved to be less effective as templating agents, yielding acyclic products [53]. These results confirmed the dependence of metal templation on the ratio of metal ion size to potential macrocyclic cavity with the larger early lanthanides unable to template macrocycle formation. A similar study was performed by Bastida et al. with regards to the hindered macrocycle (L9) and its formyl derivative (L13) [56]. Attempts were made to template the formation of each 21 Page 21 of 106

macrocycle around the trivalent lanthanide series La(III)-Er(III) with nitrate and perchlorate counterions. Although no X-ray quality crystals could be isolated, the resulting products were characterized by elemental analysis, conductance, infrared spectroscopy, FAB mass spectrometry, and magnetic moment. Elemental analysis gave varying degrees of fit for complexes with the formulae [Ln(L)]X3.xH2O.yEtOH, where (L) = (L9) or (L14) and X = NO3- or

ip t

ClO4-. FAB mass spectrometry proved uninformative, with no clear molecular ion peaks apparent. IR spectroscopy confirmed macrocycle formation, and indicated that in complexes of

cr

both (L9) and (L14), bound perchlorate ions were present, due to the splitting of the peak around 1114 cm-1. However, no firm coordination assignments could be make due to the lack of X-ray

us

structural data [56].

More recently, Pilkington et al. investigated whether a Dy(III) complex of (L1) could act as a mononuclear single molecule magnet (SMM) [57]. SMMs are discrete molecules which are

an

magnetized in the presence of a magnetic field, and display slow relaxation of magnetization when the field is removed, due to the large barrier (anisotropy barrier, Ueff) between the spin up

M

and spin down states [58]. The development of SMMs has attracted increasing interest over the last two decades due to the potential applications associated with magnetic behaviour on the nanoscale including data storage [59], molecular spintronics [60] and magnetic refrigeration

d

[61]. For transition metal-based SMMs, the size of the anisotropy barrier is thought to depend on

te

the magnetic anisotropy (large negative D term), and the size of the ground state spin (large S). For a lanthanide-based SMM, the anisotropy barrier arises from the splitting of the mJ

Ac ce p

microstates, due to spin-orbit coupling; the coordination geometry surrounding each metal ion; and the bulk crystal structure. Mononuclear SMMs containing a single lanthanide ion are of special interest in terms of investigating the relationship between structure and magnetism [62], as a consequence of their comparative structural simplicity. The formation of (L1) was templated by DyCl3, resulting in a complex with the molecular formula [Dy(L1)Cl3(H2O)4] (25) which was characterized by elemental analysis, IR, FT-IR, near IR and Raman as well as UV-vis spectroscopy, thermogravimetric analysis and conductance measurements [57]. Ac magnetic measurements in a range of static dc fields from 0 to 5000 Oe confirmed that the complex displayed SMM behaviour, exhibiting the frequency-dependent outof-phase susceptibility signal characteristic of slow relaxation, Fig. 14. This complex represents the first example of the exploitation of (L1) for the formation of a mononuclear SMM, and is one of the few examples of an equatorially-bound ligand enhancing anisotropy in such a system [57]. 22 Page 22 of 106

[PLACE FIGURE 14 HERE] Fig. 14. M vs. temperature for (25) at different frequencies in the region 2.0 – 15 K in zero dc field (bottom) and 5000 Oe dc field (top), showing the frequency-dependent relaxation

cr

Polynuclear and Polymeric Complexes

us

3.

ip t

characteristic of an SMM. Adapted from reference [57].

As discussed above, mononuclear Fe(II) complexes of (L2) exhibit intriguing SCO behaviour. Since these findings, mononuclear complexes of (L1) and (L2) have been increasingly

an

exploited as building blocks for the development of more structurally complex magnetic materials which will be reviewed in this section. The compounds are divided into 1-D linear

M

chains (Section 3.1), and 2- and 3-D higher order structures (Section 3.2). Within each division, compounds are grouped according to the nature of the secondary building block employed. The

d

magnetic properties of the complexes are summarized in Table 1.

Linear chains

3.1.1

3d Paramagnetic polycyanometalate linkers

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te

3.1

Decurtins et al. were the first to use a mononuclear complex of (L2) as a building block for the synthesis of higher order structures. The Mn(II) complex [Mn(L2)(H2O)2]Cl2 (26) was reacted with various tetra- and hexacyanometalate salts to afford new cyano-bridged compounds [63]. The reaction of [Mn(L2)]2+ with [Fe(CN)6]4- afforded a trinuclear complex of stoichiometry [(Mn(L2)(H2O))2(Fe(μ-CN)2(CN)4)]∙2MeOH∙10H2O (27) (Fig. 15), which behaves as a paramagnet with isolated spins until below 4.5 K, where very weak ferromagnetic exchange interactions between Mn(II) ions are observed [63].

[PLACE FIGURE 15 HERE] Fig. 15. Molecular structure of the trinuclear [Mn(II)(L2)-CN-Fe(II)-CN-Mn(II)(L2)] unit (27). H-atoms are omitted for clarity. 23 Page 23 of 106

Following the same synthetic strategy, three anionic linear chains of formula [(Mn(L2)(M(μCN)2(CN)4)]nn- were prepared via the reaction of [Mn(L2)]2+ with paramagnetic [M(CN)6)]3salts where M = Cr(III) (28) and Fe(III) (29), Fig. 16 [63]. These anionic chains were charge balanced by either trinuclear cations of formula {Mn(L2)(H2O)}2{M(μ-CN)2(CN)4)}+ or K+ if

ip t

K3[M(CN)6)] was used in excess. Incorporation of the paramagnetic [M(CN)6]3- ion provides the potential for superexchange between metal ions via the CN bridge giving rise to complex

us

[PLACE FIGURE 16 HERE]

cr

magnetic properties as a result of both magnetically coupled chain and trimer units.

Fig. 16. Left. molecular structure of the linear chain [(MnL2)(Fe(μ-CN)2(CN)4)]n- (29) (bottom),

an

charge balanced with the cationic trimer [{MnL2(H2O)}2{Fe(μ-CN)2(CN)4)}]+ (top) [63]. H-

M

atoms are omitted for clarity.

Although complexes (28) and (29) are isostructural their magnetic properties are reported to be quite different. For the Cr(III) derivative (28), above 20 K the magnetic susceptibility data is

d

consistent with antiferromagnetic interactions between neighbouring Cr(III) and Mn(II) ions

Ac ce p

te

through the -cyanide ligands (Fig 17).

[PLACE FIGURE 17 HERE]

Fig. 17. Plot of MT vs T for the polycrystalline compound (28), measured under a 1 kG magnetic field (black circles). Plot of 1/M vs T (red squares). Below 20 K, MT starts to increase reaching a value of 22.9 emu Kmol-1 at 2 K that is clearly an indication of long range ferromagnetic interactions between cyanide bridged Mn(II)-Cr(III) units within both the trimer units and the chains. In contrast, for the Fe(III) analogue (29), the magnetic susceptibility data is reported to be consistent with the presence of ferromagnetic exchange interactions although no experimental data is presented either in the manuscript or the ESI, hence we are unable to comment further [63].

24 Page 24 of 106

For all of these complexes, the Mn(II) ions once again adopt a pentagonal bipyramidal geometry consistent with the D5h geometry of the original mononuclear complex. These studies demonstrate the variety of structural topologies that can be obtained from one macrocyclic building block by varying the cyanometalate linker(s) and/or the stoichiometry of the reactants. Following the same strategy, Journaux et al. synthesized cyano-bridged trinuclear complexes by

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reacting [Mn(L2)]2+ building blocks with hexacyanometalate salts [M(CN)6)]3- where M(III) = Cr and Fe. In this study, reaction of [Mn(L2)(H2O)2]Cl2 (26) with [Cr(CN)6)]4- afforded a trinuclear

cr

complex of stiochiometry [(Mn(L2)(H2O))2(Cr(μ-CN)2(CN)4)]3Cr(CN)6∙10H2O (30) [64]. The trinuclear complex is a monocation whose charge is balanced by a hexametalate anion. The

us

thiocyanate building block [Mn(L2)(NCS)2] (7) afforded a neutral trimeric species of stoichiometry [(Mn(L2)(H2O))(Cr(μ-CN)2(CN)4)(Mn(L2)(NCS)2]∙2H2O, (31) upon reaction with [Cr(CN)6)]4- [64]. Reaction of [Mn(L2)(NCS)2] (7) with [Fe(CN)6)]4- gave a complex of the form

an

[(Mn(L2)(H2O))2(Fe(μ-CN)2(CN)4)]3NCS∙1.5H2O (32) [64]. The crystal structure comprises centrosymmetric [Mn(II)-Fe(III)-Mn(II)] trimers with π–π stacking interactions between the

M

pyridyl rings of adjacent macrocycles. Hydrogen bonding interactions between the thiocyanate

d

counterions and lattice water molecules connect the layers into a 3-D network, Fig. 18.

te

[PLACE FIGURE 18 HERE] Fig. 18. Crystal packing of the linear Mn(II)-Fe(III)-Mn(II) trimers of (32) showing a section of

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the H-bonded 3-D network; H-bonding and π-π interactions are shown as blue dashed lines. The magnetic properties of complexes (30-32) were characterized. Complexes (30) and (31) display antiferromagnetic interactions at low temperatures while the Mn-Fe-Mn trimer (32) is essentially paramagnetic until 20 K, where T increases rapidly to 30.38 cm3 mol-1 K at 2 K, consistent

with

weak

ferromagnetic

interactions

[65].

The

bridging properties

of

hexacyanometalates were further exploited by Sato et al. when the [Fe(L2)]2+ building block was reacted

with

a

[Cr(CN)6]3-

salt

to

afford

the

trinuclear

complex,

[(Fe(L2)(H2O))2Cr(CN)6]ClO4∙3H2O (33) displaying SMM behaviour (Fig. 19) [66].

[PLACE FIGURE 19 HERE]

25 Page 25 of 106

Fig. 19. Molecular structure of the two independent [Fe(L2)-CN-Cr(III)-CN-Fe(L2)] trinuclear cations of (33) [66]. Until this point, Fe(II)-based SMMs had not been thoroughly explored, because linkage isomerism tended to lead to the conversion of HS Fe(II) to diamagnetic LS Fe(II). However,

ip t

earlier results [67] indicated that (L1) might be effective in preventing linkage isomerism. The dc susceptibility data for (33) gives a MT value of 9.34 cm3mol-1K slightly higher than the spin

cr

only value of 7.875 cm3mol-1K for one Cr(III) and two HS Fe(II) ions which the authors attribute to a spin orbit coupling contribution from the two Fe(II) ions. The data above 50 K obey

us

Curie-Weiss law with a  value of +16.8 K consistent with ferromagnetic exchange between the -CN bridged Cr(III) and Fe(II) ions. Upon cooling MT increases sharply to reach a value of

an

22.2 cm3mol-1K at 8 K before decreasing sharply due to zero field splitting and a field saturation effect (Fig. 20, left). The data above 50 K obey the Curie–Weiss law with a Weiss constant  of +16.8 K consistent with ferromagnetic exchange between the cyanide bridged

M

Cr(III) and Fe(II) ions. Complex (33) did indeed demonstrate slow relaxation of magnetization above 2 K (Fig. 20, right), displaying frequency dependence in both the in- and out-of-phase

d

components of the ac susceptibility, (Fig. 20, right). The relaxation time follows the Arrhenius

te

law with an energy barrier of Ueff = 44 K and a relaxation time, 0= 1.4 x 10-9 s. [66]. This is the

Ac ce p

first example of an Fe(II)-based cyano-bridged SMM.

[PLACE COMPOUND (20) HERE]

Fig. 20. Frequency dependence of the imaginary susceptibility (χ″M) versus temperature for (33) in zero static field (left) and an applied field of 3 Oe (right). Adapted from reference [66]. Following on from the earlier work of Decurtins, Andruh et al. employed the hexacyanometalate linker [Fe(CN)6]3- to form discrete pentanuclear chains [68]. K3[Fe(CN)6] was reacted with [Mn(L1)(H2O)]Cl2.4H2O

to .

give

a

complex

with

the

formula

.

[{Mn3(L1)3(H2O)2}{Fe(CN)6}2] 6H2O 2CH3OH (34). A fit of the magnetic susceptibility data for the [Mn(II)-Fe(III)-Mn(II)-Fe(III)-Mn(II)] complex to a spin Hamiltonian expression involving five parameters (J1-4 and g) indicated that one ferromagnetic and three antiferromagnetic interactions were present between the Mn(II) and Fe(III) ions, leading to an S = 5/2 ground state.

26 Page 26 of 106

However, due to the large number of parameters, the authors caution that the extracted information may be unreliable [68]. Pilkington et al. were the first to report a 1D coordination polymer formed from a dimeric derivative of (L2) [69]. The dinuclear complex {[Co(L2)]Cl(OH2)}2Cl2 (35) was slowly diffused with the hexacyanometalate salt, K4[Fe(CN)6] to give a complex of stoichiometry [{Co(L2)]

ip t

(OH2)}2{Fe(CN)6}] (36). Each Co(II) centre within the dimeric unit is connected in a trans conformation to [Fe(CN)6]4- linkers, while each [Fe(CN)6]4- unit is linked to two

cr

{[Co(L2)]Cl(OH2)}2 dimeric macrocycles through the axial CN ligands, giving rise to a zig-zag topology (Fig. 21). Magnetic susceptibility data for both the dinuclear complex (35) and the

us

chain structure (36) revealed the two compounds display very similar magnetic behaviour above 50 K, suggesting that ZFS effects dominate in this temperature region. Below 50 K, the magnetism differs, with complex (36) displaying an upswing in χMT indicative of ferromagnetic

an

interactions mediated either by the cyanometalate linker or through-space dipolar exchange [69].

M

[PLACE FIGURE 21 HERE]

Fig. 21. Crystal packing of the 1D chains of (36), showing the zig-zag topology. H atoms and

3.1.2

te

d

solvent molecules omitted for clarity [69].

3d Paramagnetic cyanometalate linkers with ‘blocking’ ligands

Ac ce p

Tetra- and hexacyanometalate linkers have proven exceedingly useful for the formation of a wide variety of polynuclear compounds, however the multiple binding modes of the polycyanometalates can make it difficult to predict and control the topology of the resulting structures. Using a transition metal complex as a linker with two axial cyanide ligands but all other coordination positions blocked by an alternate ligand(s) has been increasingly exploited as a means of controlling, to some extent, the assembly of multi-dimensional structures [70]. A cyanide-bridged linear chain with LIESST properties was prepared by Sato et al. using these principles of rational design. [Fe(L2)(CN)2]∙H2O (2) was selected as a photomagnetic building block containing two available cyanide ligands for bridging. To create the desired –NC-Fe(II)CN-Mn(II)-NC- bridged topology, this building block was reacted with a [Mn(hfac)2] secondary building block to afford the 1-D chain [Fe(L2)(CN)2)][Mn(hfac)2]n (37) (where hfac = hexafluoroacetylacetone) , Fig. 22a [67]. As intended, the equatorial macrocyclic ligand (L2) and 27 Page 27 of 106

the bidentate hfac ligands directed the linear organization of the product. Unfortunately suitable single crystals of the chain could not be grown to fully elucidate its molecular structure by X-ray crystallography, but its structure was studied by XANES spectroscopy. The variation in magnetic moment of this material with temperature before and after illumination was studied, Fig. 22b. At ambient temperatures, the magnetic susceptibility of 7.40 cm3Kmol-1 corresponded to both

ip t

Fe(II) and Mn(II) in the HS state. Upon cooling, a conversion to 1:1 HS/LS for Fe(II) was observed, while the Mn(II) ions remained in a HS state; Tc↓ = 110 K. At low temperatures,

cr

complete conversion to LS Fe(II) was observed [67]. The complex was then illuminated with green light from a Hg-Xe lamp at 5 K causing an increase in magnetic moment which persisted

us

for hours after the light source was removed. This indicated the formation of the metastable HS Fe(II) state via the LIESST effect. Following illumination, the sample was heated from 5 K back to ambient temperature. An initial increase in magnetic moment was observed, followed by a

an

decrease from 49 to 69 K, as the metastable HS state relaxed. Upon further heating, the thermal SCO transition of LS Fe(II) was again observed [67]. This compound was the first polynuclear

M

LIESST species with magnetic coupling between metal centres reported in the chemical

d

literature.

te

[PLACE FIGURE 22 HERE] Fig. 22. a) Schematic representation of the linear structure of (37); b) χMT versus T plot for (37)

Ac ce p

before and after illumination. Adapted from reference [67]. Applying the same methodology, a series of peripheral organic ligands have been employed for the rational synthesis of 1-D chains. Five 1-D Fe(III)-Mn(II) chains were synthesized from Mn(II) complexes of (L1) and (L2), and dicyano Fe(II) building blocks trans-[Fe(bpb)(CN)2](38), trans-[Fe(bpClb)(CN)2]- (39) and trans-[Fe(bdBrb)(CN)2]- (40) containing bulky equatorial ligands, Fig. 23 [71].

[PLACE FIGURE 23 HERE] Fig. 23. Bulky ligands used for the rational design of 1-D cyanide-bridged chains [71].

28 Page 28 of 106

The five complexes synthesized have stoichiometries [Mn(L1)(Fe(μ-CN)2(bpb)]ClO4∙0.5H2O (41), [Mn(L2)(Fe(μ-CN)2(bpb)]ClO4∙0.5H2O (42), [Mn(L1)(Fe(μ-CN)2(bpClb)]ClO4∙0.5H2O (43), [Mn(L2)(Fe(μ-CN)2(bpClb)]ClO4∙H2O (44) and [Mn(L2)(Fe(μ-CN)2(bpBrb)]ClO4∙H2O, (45) where bpb, bpClb and bpBrb are the ligands shown in Fig. 23. Studies of the variation in magnetic moment with temperature indicated that each complex displays antiferromagnetic

ip t

coupling between neighbouring Fe(III) and Mn(II) centers [71], Table 1. These compounds are excellent examples of both successful rational design, and the relatively rare cyano-bridged

us

[PLACE FIGURE 24 HERE]

cr

Fe(III)-CN-Mn(II) single chain structure, Fig. 24.

Fig. 24. Crystal structure of the 1-D cationic chain [Mn(L2)(Fe(μ-CN)2(bpb)]ClO4∙0.5H2O (42).

an

Counter ions, solvent and H-atoms omitted for clarity [71].

M

More recently, Clerac ́ et al. utilized this strategy to form the first photomagnetic chain comprising SMM units linked by SCO complexes [72]. The bis-cyanide complex [Fe(L1)(CN2)] (13)

was

reacted

with

[Mn(saltmen)(H2O)]+

(saltmen

=

N,N-(1,1,2,2-

d

tetramethylethylene)bis(salicylideneiminate) (46) to afford two distinct products, determined by

te

the order of addition of the reagents. Addition of (46) to (12) gave the discrete trinuclear species [{Mn(saltmen)}2Fe(L1)(CN)2](ClO4)2·0.5CH3OH (47), while the reverse addition yielded the

Ac ce p

linear chain [{Mn(saltmen)}2Fe(L1)CN)2](ClO4)2·0.5C4H10O·0.5H2O (48) [72]. The Mn(III) centres of (48) are hexacoordinated, unlike the heptacoordinated Mn(III) ions of (47), affording a wave-like topology comprised of trinuclear Mn(III)-NC-Fe(II) subunits, Fig. 25.

[PLACE FIGURE 25 HERE]

Fig. 25. a) Representation of H2saltmen, and the asymmetric unit of (48); b) a representation of the coordination chain, [Mn2(saltmen)2Fe(L1)(CN)2]2+ in (48) with H- atoms, perchlorate anions, and interstitial solvent molecules are omitted for clarity ) [72]. Dc and ac magnetic measurements were performed on both complexes. In the dc susceptibility, complex (47) exhibited a high temperature χMT value of 9.0 cm3.K.mol-1, consistent with the 29 Page 29 of 106

presence of two HS Mn(III) ions and one HS Fe(II) ion. A rapid decrease in χMT below 50 K is consistent with the presence of cyanide-mediated antiferromagnetic interactions between the metal centres. Complex (48) exhibited a χMT value of 5.9 cm3.K.mol-1, indicating that the Fe(II) ion in each trinuclear subunit is LS (S = 0). The complex comprises two crystallographically distinct {Mn2(saltmen)2} units, each dominated by a different exchange interaction: one

ip t

ferromagnetic, one antiferromagnetic [72]. Ac magnetic measurements reveal slow relaxation of magnetization for (48) arising from the {Mn2(saltmen)2} units, with an energy barrier of 13.9 K.

cr

Photomagnetic studies show that the LS [Fe(L1)(CN)2] unit in (48) can be reversibly converted to the HS state, allowing for a photoswitchable magnetic interaction between the HS Fe(II) and

us

Mn(III) ions [72].

Jian et al. recently synthesized four Fe(III)-Mn(II) cyanide-bridged complexes using the hindered complex K[Fe(salen)(CN)2].CH3OH (49) as a linker [73]. Units of [Mn(L)]2+ (where L two

isostructural

linear

an

= (bipy)2, (phen)2, L1 and L2) were combined with (49) to give two trinuclear complexes, and chains

respectively.

The

linear

chains

had

the

formula

M

{[Fe(salen)(CN)2][Mn(L)]}ClO4·CH3OH where (L) = (L1) (50) and (L2) (51) respectively. Complexes (50) and (51) yielded similar magnetic susceptibility data, with room temperature χMT values ≈4.9 emu.K.mol-1, in fairly good agreement with the expected value for non-

d

interacting Mn(II) and LS Fe(III) centres. For both complexes, χMT decreases rapidly below 50

te

K, before increasing again below 10 K, indicating that antiferromagnetic coupling occurs between neighbouring Mn(II) and Fe(III) ions. A fit of the data to a model comprising alternating

Ac ce p

identical Fe(III)Mn(II) dimers yielded coupling constants 2J1 = -6.50(2) and 2J2 = -1.57(1) cm-1 for complex (50) and 2J1 = -5.35(2) and 2J2 = -0.93(1) cm-1 for complex (51) [73]. 3.1.3

Diamagnetic polycyanometalate linkers Polycyanometalate linkers of the form [M(CN)x]y- containing diamagnetic metal ions have

also been exploited for the synthesis of chains. However, the resulting 2-D structures exhibit very weak or non-existent exchange interactions between the metal centres. The use of linear dicyanometalate building blocks [M(CN)2]- where M = Ag(I) and Au(I) have been explored for the rational synthesis of linear chains [74]. The Mn(II) complexes of (L1) and (L2) were reacted with the aforementioned dicyanometalate salts to afford two classes of isostructural

1-D

chains

[Mn(L1)(Ag(μ-CN)2][Ag(CN)2],

(52)

and

[Mn(L1)(Au(μ-

30 Page 30 of 106

CN)2][Au(CN)2], (53) as well as [Mn(L2)(Ag(μ-CN)2][Ag(CN)2], (54) and [Mn(L2)(Au(μCN)2][Au(CN)2] (55), Fig. 26 [74].

[PLACE FIGURE 26 HERE]

ip t

Fig. 26. Representation of the 1-D [Mn(L2)(Au(μ-CN)2][Au(CN)2] chain (55) [74]. H-atoms are

cr

omitted for clarity.

Magnetic studies reveal that all of the complexes display very weak antiferromagnetic coupling

us

between Mn(II) centers separated by diamagnetic [M(CN)2]- ions [74]. These compounds provide yet another example of the predictable formation of 1-D chains by rational design, although the diamagnetic linkers do not afford magnetically interesting complexes.

an

In his initial work referenced earlier, Decurtins discovered that the reaction of the [Mn(L2)]2+ macrocycle with [M(CN)4)]2- where M = Ni(II), Pd(II) and Pt(II) afforded three

M

isostructural, neutral linear chains of stoichiometry [(Mn(L2))(M(μ-CN)2(CN)2)]n (56) [63]. Since the Mn(II) ions in these chains are isolated due the diamagnetic nature of the bridging tetracyanometalate units, they all display paramagnetic behaviour. Along with the paramagnetic

d

hexacyanometalates described earlier, Journaux et al. combined the [Mn(L2)]+ building block

te

with the diamagnetic [Co(CN)6)]4-, to yield a Mn-Co-Mn trimer of the formula [(Mn(L2)(H2O))2(Co(μ-CN)2(CN)4)]3NCS∙1.5H2O (57), isostructural to the Mn-Fe-Mn trimer

Ac ce p

(34). No interesting magnetic interactions were observed for the Mn-Co-Mn trimer (57) because of the diamagnetic nature of the LS Co(III) ions [64]. 3.1.3

4d and 5d Polycyanometalate linkers Cyanide-bridged heteronuclear complexes incorporating 4d transition metal ions have also

been prepared and characterized. The 4d and 5d metal ions have more radially expanded dorbitals, allowing for increased magnetic interactions; as well as greater spin-orbit coupling (dependent on Z4), which may lead to larger single-ion anisotropy [75]. Sutter et al. were among the first to utilize heavier metals for the synthesis of a cyanide-bridged linear chain [76]. The octocyanometalate [Nb(CN)8]4- building block was reacted together with [Mn(L1)]2+ to afford a 1-D chain of stoichiometry [Mn(L1)]2[Nb(CN)8]H2O (58) [76]. Each Nb(IV) ion is eight coordinate, bound to three Mn(II) units via cyanide bridges, with five terminal cyanide ligands. 31 Page 31 of 106

Two of the three Mn(II) ions are bound to further Nb(IV) units, while the third Mn(II) ion is bound to a water molecule affording the 1-D polymer, Fig. 27 [76]. The magnetic properties of (58) reveal the presence of antiferromagnetic interactions between the neighbouring Mn(II) and

[PLACE FIGURE 27 HERE]

ip t

Nb(IV) centers [76].

cr

Fig. 27. Ball and stick representation of the partial structure of [Mn(L1)]2[Nb(CN)8]H2O (58)

us

showing the propagation of the 1-D polymer [76]. H-atoms are omitted for clarity.

Venkatakrishnan et al. used the same [Nb(CN)8]4- building block in conjunction with the [Fe(L1)]2+ macrocycle to synthesize a helical species with single chain magnet (SCM) properties.

an

While SMMs are zero-dimensional molecules, SCMs are 1-D chains. As well as the requirements listed earlier for SMMs, SCMs also require the presence of weak inter- and strong

M

intra-chain interactions [77]. The resulting polymeric complex had the stoichiometry [(H2O)Fe(L1)(Nb(CN)8)Fe(L1)]n, (59) and was comprised of both left-handed and right-handed synthesized.

d

helical chains, Fig. 28 [78]. The isostructural Mo(IV) and W(IV) analogues were also

te

[PLACE FIGURE 28 HERE]

Ac ce p

Fig. 28. Left, the asymmetric unit of [(H2O)Fe(L1)(Nb(CN)8)Fe(L1)]n, (59) ; Right, the rightand left-handed helical chains of (59) running down the b-axis of unit cell (L1 is omitted for clarity) [78]. For both figures the H-atoms are omitted for clarity. As intended, the complex demonstrated SCM properties with a magnetic hysteresis loop observed between 2 and 10 K. Slow relaxation of the magnetization was observed due to the high energy barrier for magnetic reversal resulting from the strong exchange interactions between the Fe(II) and Nb(IV) centers, and the structural anisotropy associated with the incorporation of the low-symmetry heptacoordinate [Fe(L1)]2+ complex [78]. These significant and predicted SCM properties highlight the potential of the [Fe(L1)]2+ building block for the selfassembly of molecule-based magnets.

32 Page 32 of 106

Kahn et al. synthesized a linear Mn(II)-Mo(IV) chain displaying unique photomagnetic properties [79]. A complex with the formula [Mn2(L1)2(H2O)][MoIV(CN)8].5H2O (60) was formed via the reaction of [Mn(L1)(OH2)2]Cl2 with K4[MoIV(CN)8].2H2O. The resulting complex has a waved 1D structure, consisting of alternating [Mn(L1)]2+ and [Mo(CN)8]4- units, with a second [Mn(L1)]2+ attached to each [Mo(CN)8]4- unit (Fig. 29). The magnetic properties of the

ip t

complex were studied both before and after irradiation with UV light [79]. Prior to irradiation, the room temperature χMT value was determined to be 8.74 cm3.K.mol-1, consistent with two

cr

non-interacting Mn(II) centres. A decrease in χMT at low temperatures indicates the presence of weak antiferromagnetic interactions, likely between Mn(II) ions mediated by the NC-MoIV-NC

us

bridges. When the complex was cooled to 10 K and irradiated with UV light, the magnetism increased quickly before slowly reaching saturation. After 10 hours, the light source was removed and the photoinduced magnetization was observed to be stable. Following irradiation,

an

the χMT vs. T data was recorded between 2-260 K, in warming mode. At low temperatures, χMT increased to a value of 21.2 cm3.K.mol-1 by 4 K, then decreased to a value of 8.74 cm3.K.mol-1 by

M

95 K, followed by a continuous increases to a value of 9.14 cm3.K.mol-1 by 260 K. The minimum in the post-irradiation χMT plot indicates the formation of ferromagnetic chains, with interactions

d

between Mn(II) and the photo-oxidized Mo(V) ions [79].

te

[PLACE FIGURE 29 HERE]

Ac ce p

Fig. 29. A view of the wave-like chains of (60) down the a-axis [79]. H atoms and atoms of (L1) are omitted for clarity.

Dunbar et al. very recently reported three trinuclear Mn(II)-Mo(III)-Mn(II) complexes formed from the reactions of K4[Mo(CN)7]·2H2O and [Mn(L)Cl2], where (L) = (L2) and two acyclic derivatives (beyond the scope of this review) [80]. The complex based on (L2), with formula [Mn(L2)(H2O)]2[Mo(CN)7]·7H2O (61), with the two [Mn(L2)]2+ units bound to the central [Mo(CN)7]4- unit via two equatorial cyanide linkers (Figure 30). Dc magnetic susceptibility measurements over the range 1.8-300 K showed a decrease in MT at low temperatures, consistent with antiferromagnetic interactions between the Mn(II) and Mo(III) centres [80]. Although ac susceptibility measurements showed no SMM behaviour for complex (61), one of the acyclic derivatives was the first heptacyanomolybdate-based SMM, suggesting that the 33 Page 33 of 106

binding mode of the cyanide linkers (axial vs. equatorial) plays an important role in the origin of this slow magnetic relaxation [80].

[PLACE FIGURE 30 HERE]

3.1.4

cr

[80]. H atoms and solvent molecules are omitted for clarity.

ip t

Fig. 30. The trinuclear unit of (61), showing the equatorially-bound nature of the Mo-CN linkers

Non-polycyanometalate linkers

us

Non-polycyanometalate linkers between mononuclear complexes of (L1) include azido bridges, dicyanamide linkers, and the [TCNQ].- radical. A polymeric species formed from the Mn(II) complexes of (L1) bridged by azide linkers was reported by Kahn et al. in 2000 [81]. The

an

linear chain [Mn(L1)(N3)PF6]n (62) was formed via the reaction of [Mn(L1)Cl2] with NaN3 and KPF6. Complex (62) is the first example of a linear chain with adjacent Mn(II) ions coordinated

M

in a trans fashion by a single azide linker, Fig. 31 [81]. An azide linker may give rise to both ferromagnetic and antiferromagnetic interactions, depending on its mode of coordination (end-on vs end-to-end) [82]. Dc magnetic measurements indicated the presence of intra-chain

d

antiferromagnetic interactions between Mn(II) ions, as expected from the end-to-end linkage,

te

with χT decreasing rapidly below 90 K [81]. X-ray crystallographic measurements showed an inter-chain distance of > 8.6 Å, suggesting that adjacent chains were well separated and unlikely

Ac ce p

to interact magnetically. This was confirmed by both EPR measurements and the modelling of the magnetic data via Fisher’s expression for susceptibility [81, 83]. The low J value of -4.8 cm-1 (g = 2) derived from the modelling is due to the large Mn-N-N bond angles, which decrease antiferromagnetic interactions.

[PLACE FIGURE 31 HERE]

Fig. 31. View of the polymeric structure of (62) down the c-axis. H-atoms and [PF6]- counterions are omitted for clarity. The use of the dicyanamide anion [N(CN)2]- as a ligand in coordination chemistry is relatively recent. In a similar fashion to the azido anion, the binding mode of the anion dictates the nature 34 Page 34 of 106

of the magnetic interactions between bridged metal centres. Andruh et al. reported the synthesis of a 1D coordination polymer with the formula [Mn(L1){μ1,5-N(CN)2}]PF6 (63) from the 1:1:1 reaction of [Mn(L1)(H2O)2]Cl2.H2O, Na[N(CN)2] and PF6 [84]. The complex forms infinite chains (Fig. 32), with the [PF6]- counterions dispersed in between. The dicyanamide ion acts as a bridge between Mn(II) centres via its 1,5-coordination mode. Magnetic susceptibility

ip t

measurements gave a room temperature χMT values of 4.48 cm3.mol-1.K, as expected for isolated Mn(II) centres (S = 5/2). A marked decrease in χMT is observed at low temperatures, down to a

cr

value of 4.48 cm3.mol-1.K at 2 K. A fit of the magnetic data to Fisher’s infinite chain model indicates the presence of weak antiferromagnetic interactions (J = -0.49 cm-1) between the

us

Mn(II) centres, mediated by the dicyanamido bridges [84].

an

[PLACE FIGURE 32 HERE]

Fig. 32. View of the polymeric structure of (63) down the b-axis. H-atoms and [PF6]- counterions

M

are omitted for clarity.

d

The [TCNQ].- radical has been exploited as a building block for the self-assembly of moleculebased magnetic materials since the 1960s [85]. It was first utilized as a linker between Mn(II)

te

complexes of (L1) in 2003 [86]. The neutral complex [Mn(L1)(TCNQ)2] (64), Fig. 33, was formed via ligand displacement, from the reaction of [Mn(L1)Cl2] with Li[TCNQ]. X-ray

Ac ce p

diffraction measurements revealed π-π stacking interactions between axial [TCNQ].- ligands bound to different Mn(II) ions, separated by a distance of 3.2 Å. The packing diagram showed the mononuclear complexes arranged in an infinite ‘weave-like’ chain along the c-axis, Fig. 33, with an intra-chain Mn(II)-Mn(II) distance of 13.03 Å [86]. Magnetic measurements on (64) gave a room temperature χT value of 4.2 cm3mol-1K, corresponding to the value expected for a single Mn(II) ion, which indicates that the π-stacked [TCNQ].- radicals are behaving as diamagnetic [TCNQ]2- dimers. Both magnetic susceptibility data and EPR data confirm that the [TCNQ].- radicals are mediating weak exchange interactions between adjacent Mn(II) centres within the chain. A fit of the magnetic susceptibility data to Fisher’s expression [83] gives J = 0.18 cm-1, while EPR measurements gave J = -0.15 cm-1 [86]. The low J value makes sense given the sizable distance between the weakly interacting Mn(II) centres.

35 Page 35 of 106

[PLACE FIGURE 33 HERE] Fig. 33. Left, molecular structure of [Mn(L1)(TCNQ)2] (64); Right, packing diagram showing

3.2

Higher order magnetic structures: 2- and 3-D

3.2.1

Polycyanometalate linkers with 4d metal ions

ip t

the formation of 1-D π-stacked chains [86]. H-atoms are omitted for clarity.

cr

In addition to 1-D chains, macrocyclic complexes of (L1) and (L2) have also been exploited for the formation of 2- and 3-D topologies. As previously mentioned, Kahn et al. were the first

us

to use a macrocycle from this family to assemble a higher order structure. Although hexacyanometalate precursors have been well utilized for the synthesis of cyanide-bridged

an

materials [87], they do have the disadvantage of crystallizing as extended networks in a facecentered cubic symmetry, preventing magnetic anisotropy. To overcome this problem, Kahn et al. used a heptacyanometalate [MoIII(CN)7]4- building block, to eliminate the potential for cubic

M

symmetry [9]. The use of the [Mn(L2)(H2O)2]2+ building block was intended to further encourage the formation of a low-symmetry species. A mixed-valence compound of stoichiometry

d

[Mn(L2)][MoIII(CN)7][MoIV(CN)8]2, (65) was obtained [9]. Since the reaction was carried out

te

under oxygen-free conditions, the Mo(IV) species likely formed as a result of disproportionation. The product assembled as a 2-D network of heart-shaped 48-membered rings, (Fig. 34). Each

Ac ce p

heptacoordinate Mo(III) ion has two cyano-Mn(II) linkages and five terminal cyanide groups, while each octacoordinate Mo(IV) ion has four cyano-Mn(II) linkages and four terminal cyanide groups, (Fig 34) [9].

[PLACE FIGURE 34 HERE]

Fig. 34. Structure of a layer of (65), comprised of edge-sharing heart-shaped 48-membered rings; green = Mo (Mo1 = Mo(III) and Mo2 = Mo(IV)), purple = Mn(II) (L2 is omitted for clarity) [9]. Magnetic susceptibility measurements revealed that the Mo(III) and Mo(IV) ions are in the LS state with one unpaired electron (S = 1/2), and no unpaired electrons respectively. Two out of every three Mn(II) ions were LS with a spin of S = ½, with the remaining Mn(II) ions are in the HS state with S = 5/2 [9]. Thus the complex is not only a mixed-valence system, but also mixed36 Page 36 of 106

spin. Furthermore, the compound orders ferromagnetically below 3 K, with interactions occurring via both the cyanide linkers, and (very weakly) the NC-MoIV-CN bridges [9]. Aside from Kahn’s early work, complexes based on the [Mo(CN)7]4- heptacyanometalate building block are quite rare. In more recent years, Dunbar et al. utilized this low symmetry high spin building block to synthesize a magnetic cluster with SMM properties [88]. The anaerobic

ip t

reaction of [Mo(CN)7]4- with [Mn(L1)(H2O)2]2+ afforded the docosanuclear Mo8Mn12 cluster, (66), Fig. 35 [88]. Not only is this compound the first discrete complex based on a [Mo(CN)7]4-

cr

building block, but it also has the most paramagnetic centers together with one of the largest ground-state spin values (S = 31) reported for a cyanide cluster to date, again emphasizing the

us

enormous potential of the pentadentate macrocyclic building block as a precursor to magnetically interesting topologies.

an

[PLACE FIGURE 35 HERE]

Fig. 35 Ball and stick representation of complete (left) and skeleton (right) depiction of the

M

backbone of the magnetic Mo8Mn12 cluster (66) [88].

Detailed magnetic studies of (66) between 2 and 300 K revealed antiferromagnetic interactions

d

between spin carriers, slow relaxation of magnetization and a hysteresis loop at 1.8 K. The broad

te

maxima in the ac susceptibility data is consistent with glass-like behaviour [88]. Octacyanometalate building blocks have also been utilized for the self-assembly of

Ac ce p

magnetic networks and clusters [89]. Following the discovery of the Fe(II)-Nb(IV) SCM system (59), Venkatakrishnan et al. studied the self-assembly of the analogous Fe(III) complex [90]. In this respect, reaction of the [Fe(L1)]3+ macrocycle with [M(CN)8]4- (where M = Mo(IV), W(IV)) afforded cyclic structure of the formula [Fe(L1)3(M(CN)8)2]Cl∙xH2O [90]. The molecular structure of the W(IV) analogue, (67) was characterized by X-ray crystallography, Fig. 36. Magnetic susceptibility studies of the compounds indicated the presence of weak antiferromagnetic interactions between adjacent metal centers through the CN linkages [90]. These studies demonstrate that the structure of the self-assembled product is partly directed by the charge on the [M(L1)]n+ metal species.

[PLACE FIGURE 36 HERE]

37 Page 37 of 106

Fig. 36. Representation of the molecular structure of [Fe(L1)3(W(CN)8)2]Cl∙7H2O (67) [90]. H atoms and solvent molecules omitted for clarity. Very recently, Dunbar et al. reported three multidimensional compounds resulting from the layering of [Mn(L2)(H2O)2]Cl2 (26) and K4[Mo(CN)7]·2H2O under different conditions [91]. The solvent

volume

(3

to

8

mL)

were

varied.

III

(68),

with

the

formulae

{[(L2)Mn(H2O)][MoIII(CN)7]

cr

{[Mn(L2)]4[(L2)Mn(H2O)]2[Mo (CN)7]3·27H2O}n

Complexes

ip t

type of tube (single, U-tube, H-tube), solvent mixture (MeOH/H2O and DMSO/H2O), and

[Mn(L2)]3[MoIV(CN)8]·29H2O}n (69), {[(L2)MnII(H2O)]2[MoIV(CN)8]2[MnII(L2)]4[MoIII(CN)7]·

us

12H2O}n (65), and [Mn(L2)Mn(H2O)]2[MoIV(CN)8]·9H2O (70) were isolated in crystalline form. Complex (68) packs in a 2D layered architecture, with three different Mo(IV) ions present: one connected to two Mn(II) ions, and two connected to four Mn(II) ions. Complex (69) forms a

an

ladder-like array (Fig 37), with alternating Mo(III) and Mo(IV) units in a 1:1 ratio. The ladders form a 3D structure via hydrogen bonds between the monocoordinated cyanide units and

M

interstitial water molecules. Complex (65) was reported earlier by Kahn et al. (see above, [9]). Complex (70) crystallizes as a neutral trinuclear species which packs into a 3D motif via CNH2O hydrogen bonds. Magnetic susceptibility studies were performed on the four complexes.

d

The high temperature χMT value for (68) was observed to be 26.2 cm3.K.mol-1, slightly lower

te

than expected for six non-interacting Mn(II) ions and three LS Mo(III) ions. χMT increased dramatically between 95 K and 3 K to a maximum value of 189.95 cm3.K.mol-1, followed by a

Ac ce p

decrease until 2 K. A Weiss constant of -30.0 K indicates antiferromagnetic interactions between Mn(II) and Mo(III) centres. The high temperature χMT value of 18.06 cm3.K.mol-1 was observed for (69), in good agreement with the expected value. Antiferromagnetic interactions were again evidences by the negative Weiss constant (θ = -26.7 K). In contrast to the ferromagnetic coupling cyanide-bridged Mn(II) and Mo(III) centres reported earlier for complex (65) [9], Dunbar et al. observed antiferromagnetic interactions between Mn(II) and Mo(III) centres. The dc susceptibility data for complex (70) indicated that any interaction between neighbouring Mn(II) centres was negligible (J = -0.0146 cm-1). Ac studies (1.8-5 K, 5.0 Oe ac field, zero applied dc field) on complex (69) reveal a frequency dependence in the out-of-phase susceptibility, consistent with SMM behaviour. The energy barrier to relaxation was estimated to be 8.0 K [91].

[PLACE FIGURE 37 HERE] 38 Page 38 of 106

Fig. 37 Crystal packing of complex (69) showing the ladder-like arrangement of alternating Mo(IV) and Mo(III) fragments. H atoms and water molecules are omitted for clarity [91]. 3.2.2

Polycyanometalate linkers with 3d metal ions

ip t

As part of his study described earlier [63], Decurtins combined [Mn(L2)]+ and [Fe(CN)6)]3to afford a complex of stoichiometry [(Mn(L2))3(Fe(μ-CN)4(CN)2)(Fe(μ-CN)2(CN)4)]n∙2nMeOH

cr

(71) which crystallizes in a 2-D layer structure, Fig. 18 [63]. This compound exhibits

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ferromagnetic ordering below 6.5 K [63].

[PLACE FIGURE 38 HERE]

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Fig. 38. Molecular structure of the 2-D network, (71) [63]. H-atoms are omitted for clarity.

M

A unique 3-D structure was reported by Sato et al. in 2010 [92]. When [Co(L1)(H2O)2](ClO4)2 and [Cr(CN)6]3- were mixed, a nanotubular structure spontaneously resolved, Fig. 39. The homochiral compound is comprised of interlocked left-handed single helices and right-handed double

d

helical strands, and has the formula [(Co(L1)(H2O)2Cr(CN)6]ClO4∙8H2O, (72) [92]. The unique

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structural topology of this compound is due to the tetra-coordinated nature of the

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hexacyanochromate species (Fig. 38b) [92].

[PLACE FIGURE 39 HERE]

Fig. 39. a) Structure of the asymmetric unit of (72) (H-atoms are omitted for clarity) and b) partial structure of (72) showing the tetra-coordinated linkage mode of the hexacyanochromate species [92].

Variable-temperature magnetic susceptibility studies of (72) indicate the presence of ferromagnetic coupling between adjacent Co(II) and Cr(III) ions above 70 K, and long-range ferromagnetic ordering below 12 K. The compound displays magnetic hysteresis at 1.8 K [92]. The presence of chirality and ferromagnetism suggest that the compound has the potential to be

39 Page 39 of 106

used for the self-assembly of multifunctional materials; however the isolated species proved to be racemic, so further investigation is warranted [92]. Andruh et al. report two bidimensional coordination polymers formed from the reaction of [M(L)]2+ and [M′(CN)6]3- moieties [93]. When (L) = (L1), M = Fe(II) and M′ = Co(III), a complex with the formula [{Fe(L1)}3{Co(CN)6}2].2CH3OH.13H2O (73) results. Complex (73)

ip t

forms a ‘brick wall’ arrangement of 2D layers, with three [Fe(L1)]2+ units surrounding each [Co(CN)6]3- linker in a mer conformation. When (L) = (L2), M = Co(II) and M′ = Fe(III), a

cr

complex with the formula [{Co(L2)}3{Fe(CN)6}2].6CH3OH.6H2O (74) self-assembles. Complex (74) packs in a honeycomb arrangement (Fig. 40), with three [Co(L2)]2+ units surrounding each

us

[Fe(CN)6]3- linker in a fac conformation [93].

an

[PLACE FIGURE 40 HERE]

Fig. 40. Crystal packing of complex (74) viewed down the c-axis, showing the honeycomb

M

arrangement [93]. H atoms, counterions and solvent molecules removed for clarity. Magnetic studies were performed on the two complexes. The high temperature χMT value of

d

10.9 cm-3.mol-1.K for (73) corresponds to the three HS Fe(II) ions in each formula unit, with

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Fe(II)-Fe(II) interactions being extremely weak or absent due to the long interatomic distances (> 7.5 Å). A fit of the magnetic data to a spin Hamiltonian allowed the extraction of the ZF term

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D/kB = -5.5 K. The high temperature χMT value of 10.8 cm-3.mol-1.K for (74) indicates the presence of three HS Co(II) ions and two non-interacting LS Fe(III) ions. At low temperatures, an increase in χMT associated with ferromagnetic interactions between Co(II) and Fe(III) ions mediated by the cyanide bridges is observed [93].

3.3

Summary of magnetic properties The majority of the compounds described in Section 3 are heterometallic materials

assembled via the reaction of (L1) or (L2), with a secondary polycyanometalate building block. Mn(II) complexes of either (L1) or (L2) are by far the most commonly exploited; Fe(II) complexes of (L1) were also utilized along three examples of higher order structures assembled from Co(II) macrocycles of (L2). In each case the macroyclic bound metal centre possesses pentagonal bipyramidal geometry and the polycyanometalate building blocks include di-, tetra-, 40 Page 40 of 106

or hexa-cyano 3d ions, as well as hepta- or octacyano 4d and 5d linkers. Additionally, cyanometalate precursors with blocking ligands, as well as non-polycyanometalate linkers such as azide, dicyanamido and a TCNQ radical have also been employed. The magnetic properties of the 1-, 2- and 3-D structural topologies reviewed in section 3 are summarized in Table 1. The most common metal-metal interaction within this class of and the M′ of the secondary [M′(CN)x]m-

ip t

compound is the M-NC-M′ superexchange interaction, between the M of the [M(N3O2)]n+ unit building block. The strength and sign of these

cr

interactions are dictated by the nature of the orbital overlap between the bridging ligand, and both the geometry and dn configurations of the two metal ions [94]. During the 1990’s a great

us

deal of emphasis was placed on the theoretical and magnetic understanding of cyanide-bridged transition metal species based on Prussian Blue, [95]. Due to the linear nature of the bonding, and the near-perfect octahedral geometries of the metal ions in these complexes, the type and

an

even strength of bonding can, in many cases, be predicted with great accuracy [95a]. Magnetic orbitals which are mutually orthogonal lead to a parallel, ferromagnetic, alignment of spins;

M

while magnetic orbitals which are of the same symmetry and overlap lead to an anti-parallel, antiferromagnetic, spin alignment between the two spin carriers (M and M′). For such Prussian Blue-like materials constructed from octahedral ions, the mutually orthogonal nature of the  and

d

-manifolds of the CN- ligand means that ferromagnetic (FM) interactions are expected for

te

systems where M contains t2gneg0 (0 < n < 6) configurations and Mʹ ions comprise t2g6egn (n < 4),

Ac ce p

or vice versa. For other ion combinations in which there is a combination of ferro- and antiferromagnetic exchange pathways, typically |JAF| > |JF| and a net antiferromagnetic interaction results [95a]. The detailed understanding of the magnetic exchange in the Prussian Blue family ultimately afforded the first room temperature molecule-based magnet [96].

[PLACE FIGURE 41 HERE]

Fig. 41. Frontier orbitals of [ Mn(L1)(CN)2] calculated at the UB3LYP/LACV3P* level. The compounds presented in this review differ in a key way from the well-studied Prussian Blue structures in that the macrocyclic bound transition metal ions are coordinated in pentagonal bipyramidal geometry, leading to a different splitting of the d-orbitals (Fig. 41). Clearly the a1 orbital (dz2) and e1 (dxz and dyz) are of  and -symmetry with respect to the axial cyanide 41 Page 41 of 106

whereas the e2 orbitals are non-bonding (Fig. 41). For the extensively studied heptacoordinate M(II) ions (M(II) = Mn, Fe, Co) the electronic configurations are e12e22a11, e13e22a11 and e14e22a11 respectively in which there is successive population of the e1orbitals which are non-bonding with respect to the axial CN ligands. Firstly we note that exchange via diamagnetic M(CN)xn- ions such as [Ag(CN)2]- , [Au(CN)2]-, [M(CN)4]2- (M = Ni, Pd, Pt) and low spin [Fe(CN)6]4- (52 - 56

ip t

and 27) is extremely weak (|J| < 0.5 cm-1) consistent with prior work on Prussian Blue derivatives, reflecting poor magnetic exchange via the 5-atom bridge. An analysis of Table 1

cr

reveals that magnetic exchange in such systems appears somewhat more sensitive than the Prussian Blue systems. For example, an examination of structures which comprise 7-coordinate

us

Mn(II) exchange coupled via a CN- bridge to low spin Fe(III) reveals eight examples of antiferromagnetic exchange (41 – 45, 49, 51 and 71), but also two examples of ferromagnetic exchange (29 and 32) and one exhibiting a combination of both ferro- and antiferromagnetic

an

exchange! These observations would indicate some sensitivity to the local coordination geometry and that the sign of the exchange interactions are not merely dictated by the geometry and

M

electronic configuration of the ions involved. In this context we note that the M-NC geometries often deviate significantly from linearity (147 - 167o) which will markedly affect the degree of orbital orthogonality. While the antiferromagnetically-coupled examples reveal Mn-NC angles

d

which span the entire range 147 – 167o, the ferromagnetically coupled systems fall within a very

te

similar range 159 – 165o. Thus in contrast to the Prussian Blues, we conclude that the factors that contribute to the size and magnitude of the exchange interactions in these systems arise as a

Ac ce p

result of a subtle interplay between several geometric parameters in which a single dominating parameter such as the magnitude of the M-NC angle is not obvious. In this respect more studies are required if these systems are going to lend themselves to targeted strategies where the exchange between neighboring spins or the alignment of anisotropy axes can be synthetically manipulated and controlled.

4.

Dual property materials Currently much research effort in the field of molecular magnetism is focused on the

formation of ‘dual property materials,’ combining two or more useful properties such as magnetism, optical activity, and/or conductivity within a single molecule [97]. With this aim in mind, the next logical step would be the incorporation of properties such as conductivity or chirality into magnetic materials assembled from this ligand family. As a first step to preparing 42 Page 42 of 106

chiral magnets Pilkington et al. have reported the first chiral derivative of (L2) [Fe(dpN3O2)(CN)2] (75), (where dp = diphenyl) which displays both thermal and LIESST properties (Fig. 42) [98]. In contrast to the parent complex (2), the chiral complex (75) undergoes an incomplete thermal spin crossover transition in the solid state that is accompanied by only one mechanism for this macrocycle to undergo a SCO transition [98].

cr

[PLACE FIGURE 42HERE]

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subtle changes in the coordination geometry suggesting for the first time that there is more than

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Fig. 42. Temperature dependence of the MT product for the R,R’-enantiomer (75). The solid squares represent a bulk measurement in the dark and the open triangles represent

Concluding remarks

M

5.

an

the photoexcitation of the sample for 10 hours with green light (= 10 K ) at 10 K [97].

This review presents the synthesis, characterization and properties of coordination

d

complexes prepared from a family of 15-membered penta-aza, -oxoaza and -thiaaza macrocycles. Fe(II) complexes of the N3O2 macrocycle (L2) exhibit a thermally-induced SCO

te

transition accompanied by a rare change in coordination geometry, as well as a photoinduced transition with the highest LIESST temperatures reported for a mononuclear Fe(II) complex to

Ac ce p

date. Although many of the mononuclear transition metal complexes were prepared fifty years ago their axial ligands have been exploited over the past decade for the self-assembly of higher order magnetic structures particularly incorporating cyanometalate [M(CN)x]y- bridging units. In this review we have demonstrated how these macrocycles can be used synthetically as building blocks for the self-assembly of a wide variety of structurally diverse and topologically interesting molecule-based magnets. Preliminary results have already shown the formation of SMM and SCM behaviour for discrete clusters and 1-D chains respectively; whereas 2-D networks have demonstrated long range ordered phases. Whilst poor magnetic communication between metal ions is observed employing diamagnetic cyanometalate secondary building blocks, the careful choice of paramagnetic bridging units can facilitate strong communication. From the molecular magnetism perspective one of the remaining challenges is further the understanding and ultimately gain control over the magnetic exchange interactions in these complexes. It has also 43 Page 43 of 106

been demonstrated that lanthanide ions can also template the formation of these macrocycles, conferring unusual pseudo D5h geometry on magnetically interesting lanthanide ions such as Dy(III) that normally prefer to adopt 8- or 9-coordinate geometries. Due to the crystal field, pentagonal bipyramidal complexes of Dy(III) ions inherently exhibit axial anisotropy which gives rise to slow relaxation of their magnetization and thus SMM properties. One of the major

ip t

challenges in the field of molecular magnetism is to enhance the energy barrier of SMMs and the labile axial ligands of these complexes could offer a rational methodology for the realization of

cr

mixed lanthanide/transition metal SMMs with large energy barriers.

It is clear from our own investigations as well as reviewing the chemistry of these 15-

us

membered Schiff base macrocycles that since their discovery, each successive investigations into this family uncovers yet more to be explored. In this context it has recently come to our attention that Wang and co-workers have successfully prepared single crystals of a 1-D chain (76) from

an

Nelson’s [Fe(L1)CN2] building block via reaction with NaBF4[99]. X-ray crystallography shows that the complex crystallizes as a 1-D chain with one Fe(II) centre in the cavity of the L1 ligand,

M

one unique CN- linker and one BF4- counter ion in the asymmetric unit. Detailed magnetic measurements reveal the rare co-existence of spin-canting, antiferromagnetic ordering, fieldinduced metamagnetism and single-chain behaviour. It is therefore with great interest that we

d

look forward to witnessing the ways in which these macrocycles will continue to inspire future

te

generations of coordination chemists leading to new discoveries as these “Old Complexes”

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continue to “Offer New Attractions” for decades yet to come.

44 Page 44 of 106

6.

Acknowledgements

The authors are grateful to NSERC (DG), the Canada Research Chair’s program (Tier II) and Brock University for financial support. We would also like to thank our coworkers Dr. Qiang contributions

to

this

research

program.

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te

d

M

an

us

cr

their

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Wang, Shari Venneri, Niloofar Zarrabi, Peter Fenlon, Jeffrey Regier and Dr. Roland Acha for

47 Page 45 of 106

7.

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[95] a) K. Hashimoto, S. Ohkoshi, in: P. Day, A. E. Underhill (Eds.), Metal-Organic and Organic Molecular Magnets, Royal Society of Chemistry, 1999, pp. 123-149; b) S. Ferlay, T. Mallah, R.

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Ouahes, P. Veillet, M. Verdaguer, Nature 378 (1995) 701-703; [96] J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, J.S. Miller, Science 252 (1991) 1415-1417.

[97] a) C. Train, M. Gruselle, M. Verdaguer, Chem. Soc. Rev. 40 (2011) 3297-3312; b) E. Coronado, J.R. Galán-Mascarós, A. Murcia-Martínez, F.M. Romero, A. Tarazon, L. Ouahab, E. Yagubskii, in: Organic Conductors, Superconductors and Magnets: From Synthesis to Molecular Electronics, Springer, Netherlands, 2004, pp. 127-142. [98] Q. Wang, S. Venneri, N. Zarrabi, H. Wang, C. Desplanches, J.-F. Letard, T. Seda and M. Pilkington, Dalton Trans. 2015, submitted. [99] D. Shao, S.-L. Zhang, x.-H. Zhao and X.-Y. Wang, Chem. Commun., 2015, DOI: 10.1039/c4cc10003d.

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A family of 15-membered N3X2 Schiff base macrocycles are reviewed, where = N, O or S. A historical review of the discovery of mononuclear N3X2 complexes is presented. The spin crossover properties of [Fe(N3X2)CN2] complexes are reviewed.

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The structural and magnetic properties of polynuclear N3X2 complexes are summarized.

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Dimensionality

Linker

Repeating Unit

Dicyanometalates

Metal Centre / Geometry

 M-NC

cr

n+

[ML]

††

(°)

us

Cmpd

ip t

Table(s)

Exchange

-1

Interactions

|J| (cm )

Magnetic Behaviour

Ref.

II

[Ag(CN)2]

2+

2+

[Ag(CN)2]

2+

[Au(CN)2]

[Mn(L1)]

(53)

[Mn(L1)]

(54)

[Mn(L2)]

(55)

[Mn(L2)]

1D – infinite

[Au(CN)2]

-

1D – infinite

Mn PB-7 I Au L-2

-

1D – infinite

Mn PB-7 I Ag L-2

-

1D – infinite

Mn PB-7 I Au L-2

II

0.065(2)

-

[73]

II

II

0.23(3)

-

[73]

II

II

0.15(6)

-

[73]

AF Mn -Mn

II

II

0.13(5)

-

[73]

151 - 154

P

n/a

-

[62]

156

F Mn -Mn

0.045

-

[62]

7.5(7)

-

[62]

not modelled

-

[62]

AF Mn -Mn

166

AF Mn -Mn

165

AF Mn -Mn

164

II

II

2-

[M(CN)4)]

2+

1D – infinite

ce pt

[Mn(L2)]

II

166

II

ed

Tetracyanometalates (56)

Mn PB-7 I Ag L-2

-

M an

2+

(52)

II

Mn PB-7 II Ni/Pd/Pt SP-4

Hexacyanometalates 2+

(27)

[Mn(L2)]

(28)

[Mn(L2)]

(29)

[Mn(L2)]

(30)

[Mn(L2)]

(31)

[Mn(L2)]

3-

[Cr(CN)6)]

Ac

2+

4-

[Fe(CN)6]

2+

[Fe(CN)6)]

3-

2+

[Cr(CN)6)]

2+

[Cr(CN)6)]

1D – trinuclear

1D – infinite and trinuclear

II

Mn PB-7. II LS Fe OC-6 II

Mn PB-7 III Cr OC-6

II

II

156 - 159 II

III

F/AF Mn -Cr

II

1D – infinite and trinuclear

Mn PB-7 III LS Fe OC-6

1D – trinuclear

Mn PB-7 II Cr OC-6

II

III

159 - 160

F Mn -Fe

145 - 157

AF Mn -Cr

162

AF Mn -Cr

II

4-

II

II

9.8

-

[63]

II

III

6.52

-

[63]

II

3-

1D – trinuclear

Mn PB-7 III Cr OC-6

Page 102 of 106

ip t cr

II

[Mn(L2)]

(33)

[Fe(L2)]

(34)

[Mn(L1)]

3-

1D – trinuclear

Mn PB-7. III LS Fe OC-6

3-

1D – trinuclear

HS Fe PB-7 III Cr OC-6

3-

1D – pentanuclear

Mn PB-7 III LS Fe OC-6

4-

1D – infinite

Co PB-7 II LS Fe OC-6

1D – trimer

Mn PB-7 III Co OC-6

[Fe(CN)6)]

165

us

2+

(32)

II

III

F Mn -Fe

not given

-

[63]

3.76(6)

SMM Ueff = 44.3 K

[65]

0..3, 1.5, 1.6, 2.6

-

[67]

[Cr(CN)6]

M an

II

2+

II

III

155

F Fe -Cr

144 - 155

3 AF, 1 F Mn -Fe

161

F Co -Co

5.4

-

[68]

166

P

n/a

-

[63]

160 -161

AF Mn -Fe1 II III AF Mn -Fe2

J1 = 2.1 J2 = 1.6

FI ordering Tc = 6.4 K

[62]

II

II

2+

[Fe(CN)6]

II

III

II

(36)

2+

{[Co(L2)] }2

[Fe(CN)6]

II

II

II

[Co(CN)6)]

+

[Fe(CN)6)]

(71)

[Mn(L2)]

(72)

[Co(L1)]

(73)

[Fe(L1)]

(74)

[Co(L2)]

2+

2+

3-

2D – lattice

II

Mn PB-7 III LS Fe OC-6

II

III

3-

3D – nanotubular

HS Co PB-7 III LS Cr OC-6

II

155

F Co -Cr

1.2

F ordering < 12 K

[91]

3-

2D – brickwall

HS Fe PB-7 III LS Co OC-6

II

154 - 165

P

n/a

-

[92]

II

3-

2D – honeycomb

HS Co PB-7 III LS Fe OC-6

150 - 159

not given

F ordering < 3K

[92]

1D – trimer

Mn PB-7 III Mo PB-7

not given

-

[79]

not given ([9]) 6.9 ([90])

F ordering † < 3K ([9])

[9], [90]

[Cr(CN)6]

[Co(CN)6]

Ac

2+

4-

ce pt

[Mn(L2)]

ed

2+

(57)

[Fe(CN)6]

II

III

III

F Co -Fe

Heptacyanometalates

II

(61)

2+

[Mn(L2)]

4-

[Mo(CN)7]

AF Mn -Mo

143 - 151

F ([9]), AF ([90]) II III Mn -Mo

II

(65)

2+

[Mn(L2)]

4-

[Mo(CN)7]

2D - network

Mn PB-7 II Mn PB-7 III Mo PB-7

II

146 - 150

III

Page 103 of 106

ip t

0D- cluster

Mn BP-7 III Mo PB-7

4-

2D – layers

us

II

4-

2+

[Mo(CN)7]

2+

[Mo(CN)7]

(66)

[Mn(L1)]

(68)

[Mn(L2)]

cr

IV

Mo SA-8

140 -160

AF overall

not given

glassy behaviour

[87]

not given

F ordering <3K

[90]

not given

SMM Ueff = 8.1 K

[90]

-

[90]

II

Mn PB-7 III Mo PB-7

134 - 151

II

III

II

III

AF Mn -Mo

II

(70)

2D – ladder

4-

1D - trinuclear

[Mo(CN)7]

2+

[Mo(CN)7]

[Mn(L2)] [Mn(L2)]

Mn PB-7 III Mo CTP-7. IV Mo SA- 8 . II Mn PB-7 IV Mo SQ-8

M an

(69)

4-

2+

Octacyanometalates

II

[Mn(L1)]

(59)

[Fe(L1)]

2+

[Mn(L1)]

(67)

[Fe(L1)]

Mn PB-7 IV Nb SQ-8

4-

1D – infinite

Fe PB-7 IV Nb – SQ-8

[Nb(CN)8]

3+

1D – infinite

4-

[Mo(CN)8]

Ac

(60)

1D – infinite

ce pt

2+

4-

[Nb(CN)8]

ed

2+

(58)

145

151- 155

AF Mn -Mo

II

II

AF Mn -Mn

1.46 × 10

−2

AF

not given

-

[75]

153 - 160

AF

20

SCM Ueff = 7.4 K

[77]

155 151

before illumination: II II AF Mn -Mn after illumination: II V AF Mn -Mo

not given

after illumination/ photooxidation: ferromagnetic chain

[78]

161 158

AF Fe -Fe

not given

-

[89]

no single crystal data

AF HS Fe -Mn (after illumination)

0.10

LIESST effect

[66]

162, 155

AF Mn -Fe

|J1| = 1.16(2), |J2|

-

[70]

II

II

Mn – PB-7 IV V Mo → Mo SQ8

III

[W(CN)8]

2D – honeycomb

HS Fe PB-7 IV W DD-8

[Mn(hfac)2]

1D – infinite

LS/HS Fe PB-7 II HS Mn OC-6

4-

143 - 156

III

III

‘Blocked’ complexes

II

(37)

*[Fe(L2)](CN)2

II

II

II

(41)

2+

[Mn(L1)]

-

[Fe(bpb)(CN)2]

1D – infinite

Mn PB-7 III LS Fe OC-6

II

III

Page 104 of 106

ip t Mn PB-7 III LS Fe OC-6

-

1D – infinite

Mn PB-7 III LS Fe OC-6

-

1D – infinite

Mn PB-7 III LS Fe OC-6

[Fe(bpClb)(CN)2]

2+

[Fe(bpBrb)(CN)2]

[Mn(L2)]

*[Fe(L1)(CN2)]

*[Fe(L1)(CN2)]

2+

[Mn(L1)]

(51)

[Mn(L2)]

[Mn(saltmen)(H2 + O)]

1D – trinuclear

1D – infinite

II

HS Fe PB-7 III HS Mn OC-6

1D – infinite

Mn PB-7 III LS Fe OC-6

1D – infinite

Mn PB-7

1D – infinite

Mn PB-7

N3

2+

[N(CN)2]

III

II

III

II

III

-

[70]

-

[70]

-

[70]

-

[70]

[71]

AF Mn -Fe

148, 167

AF Mn -Fe

167, 153

AF Mn -Fe

174, 176

AF Fe -Mn

0.66(9)

-

|J1| = 0.76(4), |J2| = 0.02(8)

(when Fe is LS)

168,175

dominant AF, very weak F III III Mn -Mn II (when Fe is LS)

II

III

II

+

2+

II

= 0.10(3) |J1| = 3.10(1), |J2| = 0.25(1) |J1| = 1.10(1), |J2| = 0.139(8) |J1| = 1.99(6), |J2| = 0.57(1) |J1| = 1.23(1), |J2| = 0.13(5)

163, 155

II

LS Fe OC-6 → HS II Fe – PB-7 III HS Mn – OC-6

Mn PB-7 III LS Fe OC-6

[Fe(salen)(CN)2]

III

II

1D – infinite

2+

II

AF Mn -Fe

II

+

[Fe(salen)(CN)2]

Ac

(50)

[Mn(saltmen)(H2 + O)]

ed

(45)

ce pt

[Mn(L2)]

159, 147

II

M an

2+

(44)

cr

1D – infinite

[Fe(bpClb)(CN)2]

[Mn(L1)]

-

us

-

2+

(43)

(48)

Mn PB-7 III LS Fe OC-6

[Fe(bpb)(CN)2]

[Mn(L2)]

(47)

1D – infinite

2+

(42)

II

II

III

II

III

II

II

II

II

152, 149

AF Mn -Fe

154

AF Mn -Fe

II

n/a

AF Mn -Mn

II

n/a

AF Mn -Mn

II

SMM

|J1| = 3.25(1) |J2| = 0.78(5) |J1| = 2.67(6) |J2| = 0.46(5)

II

Ueff = 13.9 K

[71]

photoswitchabl e -

[72]

-

[72]

4.8

-

[80]

0.49

-

[83]

Other (62)

[Mn(L1)]

(63)

[Mn(L1)]

-

-

Page 105 of 106

-

1D- infinite

II

Fe PB-7

II

II

n/a

AF Mn -Mn

158.35

AF Fe -Fe canted 6.3 °

II

0.18 (theoretical) 0.15 (EPR)

-

[85]

J = -4.37

AF; spin canting; metamagnet SCM

[98]

II

ed

M an

CN

II

Mn PB-7

cr

ip t 2+

[Fe(L1)]

1D – infinite

us

.-

TCNQ

ce pt

(76)

2+

[Mn(L1)]

Ac

(64)

Page 106 of 106