Magnetic properties of calixarene-supported metal coordination clusters

Coordination Chemistry Reviews 402 (2020) 213066

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

Review

Magnetic properties of calixarene-supported metal coordination clusters Rebecca O. Fuller a,b,c,⇑, George A. Koutsantonis b, Mark I. Ogden a a

School of Molecular and Life Sciences, Curtin University, Bentley, WA 6102, Australia Chemistry, School of Molecular Sciences M310, The University of Western Australia, Crawley, WA 6009, Australia c School of Natural Sciences-Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia b

a r t i c l e

i n f o

Article history: Received 11 April 2019 Accepted 11 September 2019

Keywords: Calixarenes Magnetism Supramolecular chemistry Cluster compounds

a b s t r a c t Multinuclear metal complexes have shown a unique capacity to provide interesting magnetic, chemical and electronic properties by virtue of the remarkably diverse range of structural types that they exhibit. Calix[n]arenes are now a mature synthetic platfom that provide for a diverse spatial arrangement of binding groups, making them highly suitable to use for the formation of multinuclear metal ion complexes. Increasing interest has been shown in the development and properties of magnetic materials based on calixarene macrocycles. Here, we review the magnetic properties of calixarene-supported metal clusters with emphasis on those examples exhibiting slow magnetic relaxation. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1. Single molecule magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Impact of calixarene structure on cluster formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Magnetic properties of calixarene-supported clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. TM clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1.1. Homometallic mixed valence manganese complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1.2. Homometallic transition metal clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Ln clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Heterometallic 3d-4f clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction For the last two decades calixarenes have been shown to be excellent metal ‘‘cluster keepers” [1]. Investigation of these materials has predominantly focussed on their synthesis and structural characterisation, but as the area matures more attention is being directed to the application of the useful properties of these clusters. While there are many properties that might be exploited for ⇑ Corresponding author at: School of Natural Sciences-Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia. E-mail address: [email protected] (R.O. Fuller). https://doi.org/10.1016/j.ccr.2019.213066 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

applications, our focus here is on the potentially useful magnetic properties of calixarene-supported metal clusters. Calix[n]arenes consist of [n] phenol groups, most often connected by methylene linkers to make a cyclic structure (Scheme 1) [2]. The smaller members of the series (n = 4,5) can assume a nonplanar vase-like shape, making them interesting host molecules that can accommodate guests in the cavity defined by the aromatic rings [3]. The array of phenol O atoms also allows the calixarenes to act as ligands for a wide range of metals. The calixarenes can be readily synthesised in a number of ring sizes on multigram scales, with n = 4, 5, 6 and 8 being the most commonly studied [1,4]. Functionalisation is also well established, with modifications

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Scheme 1. The calix[n]arenes, where n is the number of phenol groups (n = 3 + y) 4–8, R is often a p-alkyl substituent e.g. a tert-butyl group and X the linker between phenol moiety. The abbreviations have been given for the most studied oligomer, n = 4. The abbreviation for the calixarene ligands in this review are of the form RCnAX-Hy. The final term Hy is used to show degree of protonation (y = 1–5), it is omitted if the ligand is fully deprotonated.

Fig. 1. (left) Molecular structure of complex 9, [Cu4(tBuC4AS)2(CH2Cl2)2] viewed through the Cu4 plane with their thermal ellipsoids set at 30% probability. Hydrogen atoms and solvent molecules omitted for clarity. Cyan = Cu, red = O, yellow = S, grey = C. (right) Representation of metal array in complex 9. Acute Cu. . .O. . .Cu < 99° involve O* while more obtuse bonds are labelled O.

easily achieved at the phenol O atom (so-called lower rim), the para position (the upper rim), and the ortho linking atoms. These characteristics have resulted in calixarenes being one of the most extensively studied families of macrocycles [5,6]. Calixarenes bind metal cations most often at the lower rim via the phenolic groups, with a concomitant loss of protons. The tendency of the phenolate oxygen atoms to bridge metal centres enables the formation of polymetallic clusters, encapsulated by the calixarene ligands (Fig. 1). It has been noted that the factors that dictate the nuclearity of the clusters was the calix[n]arene ring size, the phenoxide donors and the introduction of coordinating capacity at the phenol linker sites [1]. The phenoxo bridges also provide pathways for superexchange, allowing magnetic coupling between the neighbouring metal centres. Modification of the ligand by replacement of the methylene linkers with sulfur groups, to give the family of thiacalixarenes, provides additional sites for metal binding [7]. The upper and lower rims of the calixarene can also be readily modified to provide additional donor atoms and to alter solubility [8–10]. The tendency of calixarenes to form polymetallic clusters, of a range of different d, mixed d-f and f block metal cations combined with their versatile functionalisation, suggests that they may be useful supporting ligands for clusters that exhibit a range of magnetic behaviours. 1.1. Single molecule magnets Magnetic molecules are being developed as the future of high density data storage in quantum computers [11,12]. In conventional computers, data occurs in a distinct state (either a 1

or a 0) and the hard drive density is limited by the thermal instability which arises from the onset of superparamagnetic behaviour in magnetic alloys presently used as storage materials. In contrast, data storage based on materials where the magnetism is molecular in origin have increased stability and density. This arises from each molecule being able to act as the smallest building block of magnetic memory. These molecules can exhibit quantum tunnelling, a process whereby the occurrence of quantum states means information is not only stored as a zero or one, but also as a coherent superposition of states, namely it can be in both states at once. Quantum tunnelling can be switched on and off through application of a small field, giving rise to the potential of these molecules to be used as quantum bits (Qubits) [13]. Qubit components can act as both the memory and processor through an entanglement of states. The parallel processing power mean these computers are likely to perform tasks that are not possible in conventional computers. Molecular spin systems have been proposed as credible qubit materials. For a magnetic molecule to be suitable as a qubit, certain inherent properties need to be exhibited. In the absence of a magnetic field, these molecules need to retain magnetisation and exhibit hysteresis below a certain temperature. The term Single Molecular Magnet (SMM) has been coined to describe such molecules. The origin of the magnetic hysteresis is not from long range magnetic ordering as seen in classical magnetic materials. Rather, it is intrinsic to the molecular features of SMMs. When these molecules are cooled below a certain temperature, TB, the blocking temperature, an anisotropic energy barrier, Ueff, blocks the magnetisation with a specific orientation [14]. As a result, the magnetic relaxation time, s, becomes very long, giving rise to so called ‘‘slow relaxation”. The larger the barrier to magnetic reorientation, the longer the observed relaxation. Ueff can be a simple means to determine the effectiveness of a molecule as an SMM i.e. the value of Ueff should be large for a higher blocking temperature to reversal to be observed. There is a rich diversity of molecules that exhibit SMM behaviour. Each system has its own unique strengths and limitations for achieving slow relaxation. SMM coordination complexes can be crudely grouped into three classes based on the types of metals in the complexes: 3d, heterometallic 3d-4f or 4f compounds. Calixarenes are well known to form all of these complexes. To behave as an SMM, specific structural features are required in each class. For transition metal complexes, the desired molecules have multiple exchange coupled metal centres in a low symmetry arrangement. This ensures that the molecule has both an overall large ground spin state and a high negative zero field splitting, D, to increase relaxation barrier height [14,15]. In contrast to 3d metal ions, lanthanoid centres have intrinsically large anisotropy in addition to a large magnetic moment, making them an ideal choice for

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

improving SMM properties. The focus is no longer on maximising the total spin, rather the aim is to design an appropriate ligand field to enhance a single LnIII ion anisotropy ensuring a large Ueff [16,17]. Heterometallic complexes typically combine the most anisotropic 3d ion with an appropriate 4f ion. The lanthanoid elements not only provide additional spin, but the introduction of a 4f ion with anisotropic character (TbIII, DyIII, HoIII and ErIII ions) to these systems can result in better dynamic properties [18,19]. Synthesis and design of a complex which satisfies the need for practical outcomes is what drives much of the research in this area. To date, the SMM field has been primarily focused on increasing the temperatures at which the molecules function from the very low (liquid helium) values at which they currently operate. Despite the recent success on higher operating temperatures displayed by a dysprosium metallocene complex (see Fig. 10), that exhibits magnetic hysteresis around liquid nitrogen temperatures, [20–22] the air and moisture sensitivity and thermal instability of the metallocene will ensure work continues on new SMM examples. In addition, since good molecular design has not always resulted in magnetic hysteresis, the focus of research in this area is also shifting to providing a more detailed understanding of relaxation mechanisms, as ultimately it is this which determines molecular behaviour [23]. Calixarenes provide abundant examples of potential SMM systems, making them ideal candidates to explore. Supported metal clusters based on calixarenes, are formed by solvothermal, hydrothermal or direct crystallisation methods. These straightforward techniques have resulted in the generation of a large number of complexes, including both homo and heterometallic complexes that are based on alkali, alkaline earth, transition, lanthanoid and actinide metals, see Scheme 1 [5]. Despite the synthetic chemistry involving calixarenes being well established, the study of their material properties is still developing. In the last decade, there has been a steady increase in detailed magnetic studies made on calixarene-based materials. Reports involving magnetic studies are often scattered within literature that is predominantly focussed on synthesis [5,16]. Given the potential of calixarenes to provide a wide variety of readily accessible materials, we feel it is important to summarise their properties and how these relate to structural features. Much of calixarene cluster formation has lacked systemisation providing only an ad hoc approach to synthesis. There are many difficulties associated with the development of rational synthetic routes to new calixarene complexes [24,25]. However, as synthetic control is improved, the design of specific structural motifs should lead to materials with useful magnetic properties. We hope to highlight what features are of importance by presenting a review of all calixarene metal complexes reported by end of 2018 with detailed temperature dependent magnetic susceptibility and hysteresis measurements, paying special attention to those compounds with unique magnetic properties. This review will focus on the magnetochemistry of these molecules after a brief overview of how the calixarene structure impacts on cluster formation.

2. Impact of calixarene structure on cluster formation While the structural variation of the calixarenes shown in Scheme 1 are relatively subtle, altering the moieties that link the phenol groups, and changes in the para-substituent, can have substantial impacts on the nature of the coordination compounds formed [1]. In addition, as is the case for most ligand systems, the nature of the complex formed can also depend strongly on the reaction conditions, such as the strength of base used to deprotonate the ligand, and the solvent mixture used. For example, the reaction of t BuC4A-H4 with a lanthanoid salt and triethylamine in DMF solvent results in the crystallisation of a dinuclear complex, [26] whereas

3

an alcohol/DMF solvent mixture under otherwise identical conditions results in the formation of hexanuclear Ln clusters that are discussed below [27]. Here, we will briefly consider some exemplars that demonstrate the impact of altering the calixarene structure on the nature of the coordination clusters formed. Calixarene complexes of manganese have been intensively studied, in large part because of the magnetic properties of Mn clusters that are discussed in Section 3.1.1. When made to react with tBuC4A-H4, manganese forms mixed valence MnIII/MnII tetranuclear clusters with a ‘‘butterfly” like structure (Fig. 3) [28– 31]. The MnIII metal centres are effectively encapsulated by a calixarene ligand, bound to all four phenolate O atoms. In contrast, replacement of the methylene linkers with thia, sulfinyl, or sulfonyl groups leads to the formation of MnII tetranuclear ‘‘sandwich” complexes similar to that shown in Fig. 1, where the metals are situated between the two phenol O4 planes, and the linking groups are included in the primary coordination sphere of the metal ions. Subtle but important differences between these sandwich complexes were observed. For example, the Mn4 complexes of tBuC4AS-H4 and tBuC4SO-H4 are charge neutral, [32] whereas those formed with tBuC4SO2-H4 are anionic due to a l4hydroxide (or fluoride) anion incorporated into the complex [30,33,34]. These anionic complexes are found to have interesting luminescent properties, whereas the neutral complexes do not. Thus, changing the linking group of the calixarene in these complexes can result in substantial changes in the cluster structure, but also quite fine control of the complex properties. Altering the para-substituent of the calixarene from a t-butyl group to a sulfonate group (as in O3SC4A-H4 for example) has a profound impact on the coordination chemistry. The sulfonated macrocycle is water soluble, and in this competitive solvent, the complexes isolated often have the calixarene positioned in the second coordination sphere of the metal, [35] or coordinated through a sulfonate O atom [36,37]. Indeed, a search of the Cambridge Structural Database [38] shows there are no structurally characterised examples of a complex of O3SC4A-H4 where a transition metal ion is coordinated to a phenol O atom, with examples limited to Sc, [39] Eu, [40,41] and Tb, [40], where a single phenol O atom is bound to the metal centre. The addition of potential ligating groups in place of the methylene carbon atoms in O3SC4AS-H4 again gives rise to different metal-binding behaviour. Cobalt, [42,43] copper, [44–46] and zinc [43] complexes of O3SC4AS-H4 have been reported where the metal is coordinated in a tridentate manner to two adjacent phenol O atoms and the bridging S atom (Fig. 2(a)). The S atoms play a different role in M4Ln (M = Ag or Cd) complexes of O3SC4AS-H4 where the M-S interactions support a sandwich-like complex where the Ln atom is coordinated to the eight phenol O atoms (Fig. 2(b)) [47–50]. Copper and zinc complexes of p-sulfonatosulfonylcalix[4]arene, also have the linking sulfonato-O atoms incorporated in the coordination sphere, [51] whereas lanthanoid complexes of this ligand were found to be an exception, and are coordinated only to the four phenol O atoms of the calixarene [52]. This brief overview can only give a flavour of the structural chemistry of calixarene metal complexes. For more details on the structural chemistry of calixarene metal complexes, in addition to the references cited in the introduction, the reader is directed to one of the many reviews available, covering topics such as complexes of methylene linked calixarenes, [53] thiacalixarenes, [54,55] sulfonated calixarenes, [37] and larger calixarenes, [56]. In the context of magnetic properties, it is important to understand the structure and composition of the metal cluster, and the nature of any bridging atoms. As these are the factors that impact on the magnetic properties, the subsequent discussion is structured around the nature of the metal cluster core, rather than the specific calixarene that is supporting the cluster.

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Fig. 2. (a) A dimeric copper complex of O3SC4AS-H3 emphasising the coordination mode of the Cu ions to the calixarene, and (b) a [Ag4Nd(O3SC4AS)2]9 complex anion [47].

3. Magnetic properties of calixarene-supported clusters Two decades have now past since the first magnetic susceptibility measurements were reported for a calixarene complex. The square planar tetranuclear CuII metal core sandwiched between two p-tert-butylthiacalix[4]arenes, [Cu4(tBuC4AS)2(CH2Cl2)2], [57] (Fig. 1) showed appreciable antiferromagnetic coupling at low temperatures. With the exception of the antiferromagnetically coupled [Co3(tBuC4AS-H)2(CH2Cl2)2]
3CH2Cl2, [58] and Li(THF)2] (l-Cl)2[UIIIUIV[(CH2)5]4]C4-pyrrole] complexes, [59] several years passed before detailed magnetic measurements on other complexes became more routine. The following discussion separates the calixarene complexes based on the metal ion(s) involved (transition (TM), lanthanide (Ln), heterometallic (3d-4f)), with further subdivision related to structural motif and coordination number. 3.1. TM clusters Correlation between magnetic properties and structure of TM coordination complexes has been of interest for many years [60,61]. Magnetism provides an avenue for understanding how properties like coordination number, ligand, geometry, and symmetry alter the electronic structure of a molecule. Since the discovery of a TM coordination compound that can act as a magnet [62], the focus for synthetic chemists has shifted. Researchers are no longer simply trying to understand the electronic nature of the complexes prepared, rather they are trying to develop preparative routes for complexes with tailor-made magnetic properties. Although calixarenes have been shown to form an enormous variety of complexes with transition metals [1,5,24], few reports of magnetic studies on TM calixarenes complexes were reported until recently when the field has expanded rapidly. In terms of slow magnetic relaxation observed in TM calixarene clusters, manganese has shown the most promise and will be the focus. A discussion on the magnetic properties of other TM calixarene clusters has also been provided. 3.1.1. Homometallic mixed valence manganese complexes Mixed valence polyoxomanganese clusters are perhaps the most well-known molecules behaving as SMMs. In addition to having a variety of oxidation states that can lead to high ground spin states, MnIII complexes are well known to have Jahn-Teller distortion and are capable of supporting anisotropy that is essential to SMM behaviour. The first reported molecule shown to act as a magnet was a dodecanuclear manganese acetate complex [14,62]. The Mn12 cluster has an unusually large ground spin state (S = 10) and the hysteresis observed was the direct result of the anisotropic character of the molecule. For [Mn12O12(O2CMe)16(H2O)4], the measured

II t Fig. 3. Molecular structure of 1, [MnIII 2 Mn2 (OH)2( BuC4A)2(DMF)6] viewed through a, with thermal ellipsoids set at 30% probability. Hydrogen atoms omitted for clarity. Cyan = Mn, red = O, purple = N, grey = C.

barrier height, Ueff 60 K and corresponding blocking temperature, TB3 K are relatively modest [62] in comparison to the current leader in the field, a dysprosium metallocene with a magnetic blocking temperature of 80 K (Ueff 2200 K) [20]. To increase the temperature at which SMMs operate, an interplay of ligand field, metal geometry and exchange interaction in the molecule is key to further development. Early work towards the synthesis of new SMMs focused on clusters of manganese and other 3d-metals. These molecules have large numbers of metal centres with both large ground spin states and a high negative zero field splitting, D, to increase relaxation barrier height. One of the main challenges for 3d-based SMMs is that the metal ion’s anisotropy needs to be additive, otherwise they cancel i.e. even if the complex has a high total ground spin state from a large number of metal centres, the lack of anisotropy in a symmetric complex decreases Ueff precluding slow relaxation [63]. The first isolated SMM based on a calixarene was a mixed valence tetranuclear MnIIIMnII complex, 1 (Fig. 3) [28]. Table 1 lists all the mixed valence manganese complexes of calixarenes.

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R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066 Table 1 Homometallic manganese calixarene complexes. Formula

Metal geometry

Ueff/kB (K)

J(cm1)

h (K)

g

Ref

II t [MnIII 2 Mn2 (OH)2( BuC4A)2(DMF)6] (1)

Planar butterfly

[28,29]

– – –

1.91 2.0 2.0

[64,65] [66] [67,68]

Distorted fused butterfly units

–

J = 4.93 J0 = 7.77 J = 1.03J0 = 3.02 J = 0.38 JMnII-MnIII = 0.92 JMnIIMnII = -4.48 JMnIII-MnIII = -1.52 –

2.0

Planar butterfly ½ butterfly Distorted fused butterfly units

16.5 (3.5) 29.4 – –

0.76

[MnII2MnIII 2 (OH)2(HC4A)2(MeOH)4(C5H5N)2] (2) [MnIIIMnII (tBuC4A)(dcb)(DMSO)4] (3) II t [MnIII 4 Mn4 (bis- BuC4A)2( 3-O)2( -OH)-( -Cl)4(H2O)(MeOH)

–

–

[68]

Central butterfly unit linked to other moiety Partial cubane Non sandwich

–

J < 10

–

–

[69]

42 (3.5) –

J1 = 1.49J2 = 3.84 J = 9.81

1.64 0.27

2.0 1.99

[70] [71]

l

l

l

(DMF)4] (4) II t [MnIII 6 Mn4 (bis- BuC4A)2(l3-O)2(l3-OH)2-(lCH3O)4(H2O)4(DMF)8(DMF)4] (5) t III II [MnIV 2 Mn10Mn8 (bis- BuC4A)2(C4H11NO2)6(l4-O)4(l3O)6(DMF)10(Cl)6(H2O)2] (6) II t [MnIII 3 Mn2 (OH)2( BuC4A)2(hmp)2(DMF)6] (7) [MnIVMnIII(tBuC8A-H4)(Ph-sao)(l-OCH3)(DMF)2] (8)

Lattice solvent molecules are not listed; Ueff values are from zero field measurements unless field value is given in parentheses; tBuC4A = p-tert-butylcalix[4]arene; dcb = 3,5dichlorobenzoate; DMSO = dimethyl sulfoxide; DMF = dimethylformamide; MeOH = methanol; bis-tBuC4A = bis-p-tert-butylcalix[4]arene; hmpH = 2-(hydroxy-methyl)pyridine; tBuC8A = p-tert-butylcalix[8]arene; Ph-saoH2 = phenyl salicylaldoxime;

Fig. 4. The temperature dependent vMT for complex 1. The solid line is a fit of the experiment to the Hamiltonian. The inset shows the exchange couplings through metal array. Reprinted with permission from [28].

II Complex 1 has a planar butterfly-like MnIII 2 Mn2 (OH)2 array sandwiched between two p-tert-butylcalix[4]arenes (Fig. 3). The inset to Fig. 4 highlights the butterfly topology, with the wing tip metal centres (Mn1 and Mn10 ) are MnIII and the body ions (Mn2 and Mn20 ) are MnII. This array is a common SMM manganese geometry, but the oxidation states of the ions are usually reversed [72]. The

MnIII ions have a Jahn-Teller distorted octahedral environment. The phenolic calixarene oxygens are coordinated through the equatorial sites, with two of these oxo groups forming l2-O bridges with the MnII centres. The distorted oxo bonds are from a coordinated DMF and l3-O(H-) bridging group shared with both MnII ions. The remaining octahedral sites for the Jahn-Teller distorted MnII centres are filled with DMF. The coordinated calixarenes are found to pack in a similar manner to the free ligand. At 300 K, vMT is larger II (15.5 cm3 mol1 K) than expected for an uncoupled MnIII 2 Mn2 (S = 2 3 1 and S = 5/2 respectively) system (14.75 cm mol K). As the temperature is lowered, vMT increases with a maximum of 25 cm3 mol1 K at 5 K (Fig. 4). The behaviour suggests dominant, but weak, ferromagnetic coupling is occurring and can be modelled by a two term isotropic Hamiltonian. The magnitude of both exchange coupling terms is small (|<2.5 cm1|) only differing in sign. For MnII-O-MnII, J is negative, so there is antiferromagnetic coupling. From this the total spin ground state, S is 7 and it is found that a number of excited states also lie just above this in energy. The exchange interactions are likely to be smaller than the zero-field splitting of the MnIII centres so these are probably present in the total ground state. The population of low level states is supported by the slow increase in magnetisation, M with applied field, H at low temperatures. Slow magnetic relaxation is supported by the out of phase signals in the frequency dependent measurements. Since no maximum is observed in the out of phase signal (Fig. 5a), the blocking temperature to reversal is less than 1.8 K, a temperature which represents the limits of the magnetometer used for these measurements. To confirm the slow magnetic

Fig. 5. (a) Plot of the out of phase component (vM”) of the frequency dependent AC susceptibility measurements for complex 1; (b) hysteresis loops for a single crystal measured at milli Kelvin and several scan rates. Reprinted with permission from [28].

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R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

relaxation was the result of a well isolated SMM, temperature and sweep rate dependent hysteresis measurements were performed on single crystal samples. Using a micro-SQuID (Super Quantum Interference Device) set up at milli-Kelvin temperatures, the easy axis of the magnetisation is aligned parallel to the applied field. Loops consisted (Fig. 5b) of quantum steps confirming the molecules do not have any significant intercluster interactions. An extension of this II work with seven such [MnIII 2 Mn2 ] calixarene-based complexes has also been carried out by this group [29]. Magnetic measurements were only carried out on one of the members of the family and results are similar to the original complex. The Arrhenius plot to the single crystal relaxation data found Ueff = 16.5 K with s0 = 6  1010 s. A number of other mixed valence manganese representatives of the calixarene family have been prepared. The butterfly topology of the metal array is central to a number of these. In complex 2 [64,65], the MnII2MnIII 2 (OH)2 motif has been modified through the addition of alternate monodentate ligands and the use of calix[4] arene. The subtle structural change in 2 (c.f. 1) results in a maximum being observed in the out of phase component of the frequency dependent AC susceptibility measurements (Fig. 6) which corresponds to an energy barrier of Ueff = 29 K with s0 = 4.26  108 s. For the half butterfly complex 3, based on MnIIIMnII(tBuC4A) unit no maximum in the dynamic magnetisation is noted [66]. Bis-ptert-butylcalix[4]arene has been used to generate larger structures comprised of multiple butterfly units. Complex 4 [67,68], has two II t fused distorted units a MnIII 4 Mn4 (bis- BuC4A)2 arrangement. The even larger 5, consists of an expanded core with a central butterfly II t core, MnIII 6 Mn4 (bis- BuC4A)2 [68]. The larger cores have not been found to significantly alter the magnetic properties relative to the earlier complexes. With a further increase in the complexity of the III II manganese core MnIV 2 Mn10Mn8 (bis-C4A)2 (complex 6) the frequency dependency of the AC susceptibility signal is lost altogether [69]. Two mixed valence manganese clusters with different alternate structural motifs from the butterfly have also been synthesised. Complex molecule 7 comprises of five manganese atoms arranged II t in two vertex sharing triangles, MnIII 3 Mn2 ( BuC4A)2 (Fig. 7) [70]. Due to the introduction of a hydroxymethylpyridine (hmp) co-ligand, the metal array is no longer simply sandwiched between two calixarenes, rather each of the MnIII ions is at the apex of the deprotonated phenolic calixarene rim. The MnIII ions are bridged by the remaining II metal ions. Like the [MnIII 2 Mn2 ] complexes, vMT vs T is suggestive of weak ferromagnetic interactions. An out-of-phase frequency dependent component to the AC susceptibility with a maximum suggests slow magnetic relaxation. An Arrhenius plot from the out-of-phase component gives Ueff = 42 K with s0 = 1.125  1010 s. Complex 8 consists of a MnIVMnIII dimer, supported by a calix[8]arene. The MnIVMnIII(tBuC8A-H4) molecule is reported to have weak ferromagnetism and frequency dependency in the out of phase behaviour. However, no maxima in the frequency dependent susceptibility measurements is evident in the signal, as a result the barrier height to reversal could not be obtained for this system [71]. The manganese complexes of calixarenes highlight the importance of anisotropy in the generation of 3d SMMs. Large numbers of metal centres are not the only important component to creating systems with slow magnetic relaxation. Low symmetry is key to ensuring that the ground energy is well separated from the excited states so thermal processes do not result rapid relaxation. Increasing the complexity of the metal array was not always advantageous as the molecules increased in symmetry. Anisotropy is better introduced by the addition of complimentary ligands or use of the larger calixarenes, so there is a deviation of the structure away from the sandwich type complex. 3.1.2. Homometallic transition metal clusters Table 2 lists all of the other homometallic TM calixarene complexes reported with magnetic measurements. For these materials,

Fig. 6. A maximum in the out of phase component to of the frequency dependent susceptibility measurements is evident for complex 2. Reprinted with permission from [64].

II t Fig. 7. (a) Molecular structure of complex 7, [MnIII 3 Mn2 (OH)2( BuC4A)2(hmp)2(DMF)6] viewed through the MnII plane with the thermal ellipsoids set at 30% probability. Hydrogen atoms omitted for clarity. Cyan = Mn, red = O, purple = N, grey = C. (b) Metal oxygen core of complex 7.

magnetic studies have been performed to understand the electronic nature of the TM ion, hence dynamic measurements have not been made. Copper(II) complexes with a number of calixarene ligands including: p-tert-butylthiacalix[4]arene, tBuC4AS-H4 [57,73,75] p-tert-butylsulfinylcalix[4]arene, tBuC4ASO-H4 [74] and p-sulfonatothiacalix[4]arene, O3SC4AS-H4 [46] have been reported with magnetic studies. The phenoxo bridging between CuII atoms is similar to the coordination environment of the CuII atoms (Fig. 1) in oxo bridged dicopper(II) compounds. It is well known that the geometry plays an important role in the magnetic properties of

7

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066 Table 2 TM Calixarene complexes with temperature dependent magnetic studies. Metal geometry

J(cm1)

h (K)

g

Ref

J1 = 231.9(4) J1 = 43(1) J2 = 17(2) – – –

0.9 139.42 +7.9 +3.93

1.98 2.0 – – –

[57,73] [74] [75] [76] [46]

[Ni4(tBuC4ASO2)(AcO)4(l4-OH)] (14)

Square planar Square planar Square planar Linked penta units Chain through upper and lower complexation Square planar

–

–

[77]

[Ni2(l- H2O)(formyl-tetra-O-C4A)2(Py)4] (15) [(C2H5)3NH]4{Ni14 (tBuC4AS)6(C5HN2O4)2(H2O)3(DMF)Cl2] (16) [Fe4(tBuC4AS)2] (17) [Fe10(tBuC4AS)4Cl4] (18) [Fe3O(tBuC4AS)2] (19)

Extended chain Linked square/trigonal groups Square planar Bridged pentanuclear Trigonal planar

– 45.83 – –

2.0 – 2.02 – 2.0

[78] [79] [80] [80] [73]

[Fe2(tBuC4A-diamine)] (20) [Fe4(l-O)2(H2O)10(O3SC4A)(phen)4](O3SC4)] (21) [Co2(tBuC8A)2] (22) [Co3(tBuC4AS-H)2(DCM)2] (23) [Co4(tBuC4ASO2)(AcO)4(l4-OH)] (24) [Co8(tBuC4AS)2(N3)2(N6H2)2(CH3COO)4(MeOH)4] (25) [Co8(tBuC4ASO2)2(CAM)2(l4- H2O)2Cl4] (26) [Co8(tBuC4AS)2(CAM)3(l4-Cl)2(MeOH)2(DMA)2] (27) [Co10(tBuC4AS)4(N3)4] (28) [Co12(tBuC4ASO2)4(5-NH2-tz)8(MeOH)6] (29) [Co14(OH)2-(tBuC4ASO2)4(5-ph-tz)2(MeOH)2] (30) [Co24(MoO4)8Cl6(tBuC4AS)6] (31) [Co24(WO4)8Cl6(tBuC4AS)6] (32) [Co24(tBuC4AS)6(MoO4)8Cl6][HPM12O40] (33) [Co24(tBuC4AS)6(WO4)8Cl6][HPM12O40] (34) t [CoII24CoIII 8 (l3-O)24(H2O)24( BuC4AS)6] (35) [Co2(l- H2O)(formyl-tetra-O-C4A)2(Py)4] (36) [Co3(H2O)(tBuC4ASO2)(HCOO)2]2(4,40 -bipy) (37) [Co4Cl(tBuC4AS)(HCOO)3]2(4,40 -bipy)2 (38) [Co4(tBuC4AS)(N3)4(N6H2)(MeOH)] (39) [Co4(p-phenyl-C4AS)(CAM)(l-HCOO)(l4- Cl)(H2O) (MeOH)1.7(DMF)1.3] (40) [Co4(l4-SO4)(O3SC4AS)(4,40 -Hbpy)2] (41)

– Linked dimers Anionic/no bridging atoms Trigonal planar Square planar Octahedral Stacked square planar Stacked square planar Bridged pentanuclear Wheel structure Wheel structure Cubic assemblies Cubic assemblies Cubic assemblies Cubic assemblies CoII core encapsulated CoIII cube Extended chain Linked trigonal units Linked zigzag square units Chain Linked square planar

– – – 30 – 24.80 95.24 –32.92 47.52 31.33 54.07 71.81 76.98 77.54 114.21 70.98 – – 38.41 54.45 36.41

2.0 2.01 – – – – – – 2.0 2.0 – – – – – 2.3 – 2.0 – –

[81] [82] [83] [58] [77] [84] [85] [85] [84] [86] [86] [87] [87] [87] [87] [88] [78] [89] [89] [84] [85]

+1.66

2.4

[90,91]

[Co(H2O)5(MeOH)]  [Co4(l4-SO4)(O3SC4AS)(H2O)6] (42) [Mn4(tBuC4ASO2)2(OH)] (43)

Network linked trapezoids Square planar

21.8 –

2.37 2.096

[90,91] [33]

[Mn4(tBuC4ASO2)2(F)] (44)

Square planar

–

2.084

[33]

[Mn4(tBuC4ASO2)(AcO)4(l4-OH)] (45) [Mn4(tBuC4AS)2] (46) [Mn4(tBuC4ASO2)2] (47) [Mn5(tBuC8A-H4)(OH)2(C3H6NO2)(DMF)5(CH3O)1.5(HCO2) (C2H3O2)0.5] (48) [Mn4(tBuC4AS)(tBuC4AS-H)(acac)] (49) [Mn14(tBuC4ASO2)3(tBuPO3)6(l4-OH)3Cl(H2O)(MeOH)] (50) [Mn16(tBuC4ASO2)3(PhPO3)7(HPO4)(l4-OH)3Cl(H2O)(MeOH)4] (51) t [VIIIVIV 5 O6(OCH3)8( BuC4A)(CH3OH)] (52)

Square planar Square planar Square planar Tetragonal pyramid

– – – –

– 1.94 1.94 1.96

[77] [32] [32] [92]

26 83.72 85.96 –

2.04 2.0 2.0 2.0

[93] [94] [94] [95]

Formula t

[Cu4( BuC4AS)2(DCM)2] (9) [Cu4(tBuC4ASO)(OAc)3(l-OH)] (10) [Cu4Cl3(HCO2)(tBuC4AS)(MeOH)2(H2O)] (11) [Cu10(tBuC6AS)2(l3- O)2(l3-OH)3(l-AcO)] (12) [Cu2(O3SC4AS)(bpno)
3H2O] (13)

Linked trapezoid units

Square planar Drum core Diamond core Lindqvist structure

J1 = 24.9 J2 = 43.9 J1 = 1.0 – J1 = 4 – J1 = 7.54(4) J2 = 55.8(3) J = 16.4 J = 127(1) – – – – – – – – – – – – – – – – – – – J1 = 3.07(2) J4 = 1.36(6) J = 4.48(7) J1 = 2.24(1) J2 = 1.87 (4) J1 = 2.31(2) J2 = 2.17 (8) J1 = 3.0J2 = 4.4 J1 = 5.57 J1 = 5.57 J1 = 2.1 J2 = 2.7 – – – J1 = 2.0(1) J2 = 17.6(2) J3 = 67.6(2) J4 = 5.5(3) J5 = 0.5(6)

Lattice solvent molecules are not listed; tBuC4AS = p-tert-butylthiacalix[4]arene; DCM = dichloromethane; tBuC4ASO = p-tert-butylsulfinylcalix[4]arene; OAc = acetate; MeOH = methanol; O3SC4AS = p-sulfonatothiacalix[4]arene bpno = 2,20 -bipyridine-1,10 -dioxide; formyl-tetra-O-C4A = tetrapropoxy-p-carboxylatocalix[4]arene
pyridine; Py = pyridine; tBuC4ASO2 = p-tert-butylsulfonylcalix[4]arene; DMF = dimethylformamide; 4,40 -bipy = 4,40 -bipyridine; 5-NH2-tz = 5-amino-1H-tetrazole; 5-ph-tzH = 5-phenyl-1H-tetrazole; tBuC8A = p-tert-butylcalix[8]arene; H2CAM=(1R,3S)-(+)-camphoric acid; DMA = N,N’-dimethylacetamide; O3SC4 = p-sulfonatocalix[4]arene; phen = 1,10phenanthroline; p-phenyl-C4AS = p-phenylthiacalixarene; acac = acetylacetonate;

the molecule. For complexes with Cu-O-Cu bond angles greater than 99°, antiferromagnetic coupling is observed, while those containing more acute bond angles, around 90°, display ferromagnetic coupling (Fig. 8) [96]. The magnetism associated with CuII calixarene complexes is reminiscent of this. The square planar array in complexes 9 [57,73] and 10 [74], have two exchange pathways as a result of the complex having both acute and obtuse Cu-O-Cu metal pairs. In the p-tert-butylsulfinylcalix[4]arene complex 10, both antiferromagnetic and ferromagnetic interactions are supported by the fit of two exchange terms to the Hamiltonian.

However, for the p-tert-butylthiacalix[4]arene example 9, only one exchange term can be fitted as Jahn-Teller distortion has resulted in overlap between only one oxygen p orbital and a Cu centre, hence antiferromagnetic rather than ferromagnetic coupling is likely in both pathways. The square metal array of complex 11 [75] is bound to the lower rim of a single p-tert-butylthiacalix [4]arene. All Cu–O-Cu phenoxobridges are obtuse, and the antiferromagnetic coupling is supported by a negative Weiss constant. The use of a larger thiacalix[6]arene, in complex 12, resulted in the metal core being made up of two square pyramidal subunits

8

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

Fig. 8. The molecular orbital interpretation for different magnetic interactions resulting from the Cu. . .O. . .Cu angle. The acute Cu. . .O. . .Cu (left) results in ferromagnetic interaction while the more obtuse angle (right) leads to an antiferromagnetic interaction.

where the CuII atoms have a distorted square pyramidal coordination enviroment, Cu10(tBuC6AS)2 leading in the dx2  y2 being in the plane [76]. Even though there are some antiferromagnetic pathways, the complex is ferromagnetic overall. An extended system CuII system, 13, has been reported that comprises of Cu2(O3SC4AS) [46] units in chain-like arrangements of the hydrophobic and hydrophilic layers. The susceptibility slowly increases with decreasing temperature, the positive Weiss constant suggesting that the complex is slightly ferromagnetic. Oxo-bridged complexes of NiII, are well known to undergo magnetic exchange. Like CuII, it is known that the Ni-O-Ni bridging angle affects the magnetic interactions [97]. A more acute angle, <100°, results in two spins being located in the eg orbitals so ferromagnetic coupling occurs. As the angle becomes more obtuse, orbital orthogonality leads to an antiferromagnetic interaction. Characterisation of the magnetic properties of NiII complexes is routine, yet very few reports of magnetic measurements have been made on calixarene-based systems. This could likely be the result of the remarkably complicated chemistry that seems to be associated with the formation of these molecules, making them difficult candidates to reproduce effectively and successfully characterise [24]. Complex 14 comprises of a half sandwich Ni4(tBuC4ASO2) cluster, with the square planar array with a structure similar to the aforementioned CuII complex 10 [77]. All donor oxygen atoms of the sulfonyl groups are axial to the plane. The metal ions are bound through four phenoxo and four sulfonyl oxygen atoms with an additional hydroxo bridge shared between all M centres. vMT decreases with decreasing temperature in a similar manner to 10, and at very low temperatures it plateaus. Bond angles observed in the complex suggest that two different types of exchange are possible. The phenolic oxo bridges have angles of <100° while the l4-OH pathways are much more obtuse with angles much greater than 100°. A Hamiltonian with two exchange terms J1 and J2 for different bridging groups was used to model the results. Two extended structures of NiII have also been prepared. In 15, the [Ni2(l-H2O)(formyl-tetra-O-C4A)2(Py)4] subunit [78] involves a metal array is no longer encapsulated between calixarenes, rather it occurs as a bridging unit. Despite the Ni-O-Ni angles being greater than 100° the complex is noted to be slightly ferromagnetic. Complex 16, is comprised of a smaller square planar Ni4 sandwich and a trigonal Ni3 half sandwich with p-tert-butylthiacalix[4]arene, antiferromagnetic magnetic coupling is noted [79]. As a result of spin correlation, iron cations have a number of ground states. Hence, depending on the strength of the ligand field, they can form high and low spin nuclear oxo- or hydroxo-bridged coordination complexes in a number of oxidation states, making

them one of the most studied molecular magnetic systems [98]. A few examples of iron calixarene complexes have been reported, including two high spin FeII (S = 2) complexes of thiacalixarene, [Fe4(tBuC4AS])2] (17) and [Fe10(tBuC4AS])4Cl4] (18) [80]. Complex 17 is of the usual form, six coordinate metal ions in a square planer metal array sandwiched between two thiacalixarene macrocycles. Complex 18 is a dimer of pentanuclear chloro-bridged complexes. Each [Fe5(tBuC4AS)2Cl2] unit has the metal array encapsulated by two calices. The magnetic susceptibility was modelled successfully using a single exchange-coupling constant for each of the FeII pairs in complex 17. Some discrepancies between the fit and the experimental data were found. The poorness of the fit at low temperatures is probably a result of the zero-field splitting terms not being taken into consideration. Namely, for antiferromagnetically coupled ions, the isotropic interaction is larger than the zero field splitting. At low temperatures the magnitude of the zero field splitting within the excited magnetic states is no longer smaller than the isotropic interaction. Complex 18 is noted to display weaker antiferromagnetic coupling, with zero field splitting at low temperatures. Complex 19 [73] has the classic oxo-bridged trinuclear high spin FeIII core between two tBuC4AS [99], with antiferromagnetic coupling from two inequivalent superexchange pathways. Antiferromagnetic coupling is also noted in complex 20, a dinuclear FeIII (S = 5/2, S = 3/2) Schiff base derivative of tBuC4A [81] and a O3SC4AS-linked dinuclear high spin FeIII dimer (21) [82]. Cobalt magnetism is remarkably complex to interpret. This is to be expected, if we consider that for a d7 electronic configuration, low and high spin CoII and high spin CoIII electronic states are possible. It is well known that high spin (S = 3/2) CoII atoms in an axially distorted octahedral environment have spin-orbit interactions that make the interpretation of vMT difficult [96]. The Hamiltonian used to model magnetic exchange does not take into account the spin orbit interaction and hence will not be effective in modelling susceptibility results. Indeed, more complicated theoretical models are required to take into account this type of interaction [100]. Spin-cross over is prevalent in both high spin CoIII and low spin CoII complexes. Hence, as a result of the crystal field, CoIII can have three different spin configurations (low, intermediate and high). The oxygen bridging environment plays a key role in determining the configuration. For high spin CoIII (S = 2) partial filling of the t2g orbitals means Jahn-Teller distortions can occur, hence an increase in the crystal field energy can lead to a stabilisation of the low spin state (S = 0) which is diamagnetic. The intermediate spin state (S = 1) is thought to be induced from spin orbit coupling and a temperature dependent (unstable) Jahn-Teller distortion. Spin transitions from low spin to high spin CoII are also known in the literature [101]. Magnetic studies involving cobalt are the most numerous for calixarene TM clusters. The magnetism of these calixarenes is generally plagued by the complexities of crystal field and Jahn-Teller distortion, like many other cobalt-containing molecules. Complexes formed with numerous members of the calixarene family including Co3(tBuC4AS-H)2 (23), [58] Co4(tBuC4ASO2) (24) [77], Co8(tBuC4AS)2 (25) [84], Co8(tBuC4ASO2)2 (26) [85], Co8(tBuC4AS)2 (27) [85], Co10(tBuC4AS)4 (28) [84], Co12(tBuC4ASO2)4 (29) [86], Co14(OH)2(tBuC4ASO2) (30) [86], Co24(MoO4)8Cl6(tBuC4AS)6 (31) [87], Co24(WO4)8Cl6(tBuC4AS)6 (32) [87], Co24(tBuC4AS)6 (33) [87], and Co24(tBuC4AS)6 (34) [87], are no exception. vMT is dominated by the orbital contributions of the distorted octahedral CoII environment dominate spin–orbit interactions. The negative Weiss constants support antiferromagnetic coupling between the metal centres and or spin–orbit coupling effect of CoII in the complexes. This is also found in the extended calixarene structures formed with cobalt. The subunits for complex 36, Co2(l-H2O)(formyl-tetra-OC4A)2, [78] 37, Co3(H2O)(tBuC4ASO2), [89] 38, Co4Cl(tBuC4AS) [89] 39, Co4(tBuC4AS), [84] 40, Co4(p-phenyl-C4AS), [85] contain a

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

distorted octahedral CoII. Complex 35 is a mixed valence II t CoII24CoIII 8 cluster made up of six Co4 ( BuC4AS) units around a cube of CoIII ions bridged by l -oxygen atoms [88]. Despite the high nuclearity 8 3 of the complex, no strong anisotropy is noted. The Co centres have an octahedral coordination environment and are coupled antiferromagnetically. Despite the difficulties associated with cobalt magnetism, the susceptibility results have been successfully interpreted in two extended structures 41 and 42 made up of tetranuclear CoII, units Co4(l4-SO4)(O3SC4AS) [90,91]. Assuming isotropic exchange between some centres reduces the four J Hamiltonian to two exchange parameters allowing the interpretation of vMT. The total spin states were obtained using the Kambe vector approach, [102] and these were subsequently substituted into a Van Vleck equation. Two weak antiferromagnetic interactions associated with phenoxo bridges (J1) and the l4-SO4 bridges (J4) are present, in agreement with the negative Weiss constant. Calix[8]arene has been used in a novel manner to create dinuclear CoII complex that exhibits strong magnetic anisotropy [83]. Unlike the other CoII complexes discussed thus far, complex 22 is anionic with two tBuC8A ligands bound to two distorted tetrahedral CoII ions (Fig. 9). There are no bridging phenoxo atoms between the two CoII ions. Instead the CoO4 moieties are hydrogen bonded. The strong oxygen interaction with neighbouring carbon and hydrogen lowers the symmetry of the CoII ligand field leading to the strong anisotropy of the antiferromagnetically coupled CoII ions. Complexes of manganese are some of the most extensively studied magnetic molecules. Like iron, manganese has a variety of oxidation states that can lead to alternative ground spin states

9

depending on the ligand field. A number of tetranuclear MnII complexes of calixarenes have been synthesised: Mn4(tBuC4ASO2)2 (43 and 44), [33] Mn4(tBuC4ASO2) (45), [77] Mn4(tBuC4AS)2 (46), [32] Mn4(tBuC4ASO2)2 (47), [32] and Mn4(tBuC4AS)(tBuC4AS-H) (49) [93]. The complexes have a similar motif to other tetranuclear calixarenes, with the four six-coordinate metal ions encapsulated between two calixarene macrocycles. These oxo-bridged MnII complexes are antiferromagnetic i.e. vMT decreases with decreasing temperature. The superexchange within Mn-O-Mn, is a result of overlap of only one of the oxygen p electrons with the MnII d orbital (c.f. Fig. 1). The magnetic susceptibility is interpreted using a single exchange-coupling constant between each of the MnII ions. Similar results have been seen in 48, a pentanuclear MnII complex where the metal centes are six-coordinate in a tetragonal pyramid encircled by the tBuC8A [92]. Larger structures of manganese have also been prepared. Complex 50 Mn14(tBuC4ASO2)3 and 51, Mn16(tBuC4ASO2)3, [94] also comprise of octahedral metal centres, with antiferromagnetic coupling. The complexity of the structures precludes modelling of the data. Oxovanadium complexes are a well known class of magnetic materials. Like many of the other first row transition metals it occurs in a wide number of oxidation states; d1 VIV S = 1/2; d2 VIII S = 0 or S = 1; d3 VII S = 1/2 or S = 3/2. A series of mixed valence polyoxovanadate clusters (52) with calixarenes have been synthesised t [95]. All complexes have the same anion [VIIIVIV 5 O6(OCH3)8( BuC4A) (CH3OH)]-, but cocrystallise with a different conjugate acid of the base (Et4NOH, NH4OH, pyridine, Et3N) used to deprotonate the calixarene in the synthesis. These complexes have a capping calixarene macrocycle coordinated to VIII. The vanadyl ions are arranged in an octahedron with eight methoxo and four phenoxo l2–O bridges as well as a central l6-O atom which links all metal centres. This metal array environment is different to other hexavanadates [103]. DFT calculations found the VIV-VIV undergo antiferromagnetic interactions and the VIII-VIV undergo weak ferromagnetic exchange couplings results in agreement with the measured data. Orbital orthogonality (c.f. Fig. 1) can be used to explain the difference in exchange. 3.2. Ln clusters Lanthanoid complexes are also being developed as molecular magnets [16,17]. In contrast to 3d metal ions, Ln centres have intrinsically large anisotropy in addition to a large magnetic moment, making them an ideal choice for improving SMM properties. Indeed, the promise of lanthanoid-based SMMs is highlighted by the recently developed dysprosium metallocene (Fig. 10), which

Fig. 9. (a) Molecular structure of complex 22, [Co2(tBuC8A)2] viewed through the annulus with thermal ellipsoids set at 30% probability. Hydrogen atoms, t-butyl groups and counterions have been omitted for clarity. cyan = Co, red = O, grey = C. Coordination mode is highlighted in (b). To provide clarity, only a number of atoms have been omitted.

Fig. 10. Molecular structure of the dysprosium metallocene, [(g5-Cp*)Dy(g5-CpiPr5)][B(C6F5)4] [20]. Thermal ellipsoids are set at 30% probability. The hydrogen atoms and B(C6F5)4 counter ion have been omitted for clarity. Green = Dy, and grey = C.

10

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

has exceeding high Ueff [20]. For Ln SMMs, the focus is no longer on maximising the total spin, rather it is on designing an appropriate ligand field to enhance a single LnIII ion anisotropy ensuring a large Ueff. This has fuelled the development of monometallic SMMs, which are no longer bound by common symmetry constraints of transition-metal complexes [104]. The suitability of lanthanoid metals as SMMs arises from their ground states consisting of multiple degenerate states that are bistable, with the additional requirement of anisotropy. Most lanthanoid SMMs are based on single metal centres of either TbIII, DyIII, HoIII, or ErIII, with complexes comprising dysprosium and terbium being the most common. This is a result of the electron density of the 4f orbitals being strongly shaped in a spatial plane. Ligand symmetry can be used to enhance the anisotropy of these centres making them ideal SMMs. However, it is not an essential requirement. Complexes without specific symmetries or ligands involving other metal cations e.g. UIII can still display SMM properties [105]. Many multimetallic lanthanoid complexes have also been reported, the majority having SMM properties weaker than their single ion counterparts [106]. Complex properties arise, in these lanthanoid systems, which tend to display quantum tunnelling in addition to exchange interactions. In the absence of substantial magnetic coupling, the systems only undergo relaxation via quantum tunnelling, a shortcut that bypasses the thermal relaxation barrier. For a system with quantum tunnelling, slow relaxation is not observed without the application of a field (to lift the degeneracy of states), which prevents a loss of magnetisation. Table 3 lists all the f-block calixarene complexes for which magnetic properties have been reported. It is apparent that for many of the complexes, magnetic measurements have not been modelled.

In part this is a result of the complexities associated with the lanthanoid magnetism, where a simple exchange model cannot be used to interpret the magnetic susceptibility measurements reported. It is noted that Ueff is only calculated for a few of the Ln calixarene complexes in Table 3. The inability to determine the barrier energy is the result of many of the Ln-calixarene complexes not having an out of phase component to the AC susceptibility and for those with an out of phase signal, there is no observable maximum due to the limitations of the instrument being reached. For completeness in this review, all Ln containing calixarene complexes with magnetic measurements have been included. A number of molecules have had electronic structure investigated through magnetic measurements, including complexes 53 (Nd10 (tBuC8A)2), [107] 54 (Pr10(tBuC8A)2), [107] 55 (Eu2(tBuC4AS-H)2), [73] 56 (Eu2(bis[quin]-C4AH3)2), [108] 57 (Gd5(OtBuC4A)2), [109] 58 (Gd6(tBuC4A)2), [110] 59 (Gd6(tBuC4ASO2)4), [111] and 60 (Gd6(tBuC4A)2) [27]. These materials are outside the main scope the review and will not be discussed further. For complexes involving Dy, Tb and Ho, the inability to calculate the expected barrier to reversal for a number of complexes in Table 3 generally arises from insufficient anisotropy in the system to prevent relaxation via quantum tunnelling instead of the thermal relaxation barrier at observable temperatures using a standard magnetometer. The absence of directional coordination between the metal centre and the calixarene or an increase in the elaborate nature of the structure (i.e. extended structures, large number of centres etc) and weak interactions between centres either precludes slow relaxation altogether or slow relaxation occurs only at very low temperatures. Kramers ion DyIII, ensures a bistable ground state in complexes 70 (Dy4(tBuC4AS)2) [114],

Table 3 Lanthanoid complexes of calixarenes. Formula

Metal geometry

Ueff/kB (K)

J(cm1)

h (K)

g

Ref

[Nd10(tBuC8A)2(PhPO3)4(OH)2(HCO3)(HCOO)(DMF)14] (53) [Pr10(tBuC8A)2(PhPO3)4(OH)2(HCO3)(HCOO)(DMF)14] (54) [(l-H2O)Eu2(tBuC4AS-H)2(DMF)4] (55) [Eu2(bis[quin]-tBuC4A-H3)2(MeCN)2] (56) [Gd5(OtBuC4A)2(NO3)3(l-CH3O)(l4-O)(l3-OH)(C3H7NO)7(H2O)] (57) [Gd6(l4-O)2(tBuC4A)2(NO3)2(HCOO)2(CH3O)2(DMF)4(MeOH)4] (58) [Gd6(C12H8N2)6(tBuC4ASO2)4(H2O)] (59) [Gd6(tBuC4A)2O2(OH)3.32Cl0.68(HCO2)2(DMF)8(H2O)0.5] (60) [Tb(tBuC4A-H2)(KtL)] (61) [Tb(tBuC4AS-OMe2)(KtL)] (62) [Tb4(tBuC4AS)2(l4-OH)Cl3(CH3OH)2(H2O)3] (63) [Tb4(p-phenyl-C4AS])2(l4-OH)Cl3(MeOH)2(H2O)3] (64) [Tb5(OtBuC4A)2(NO3)3(l-CH3O)(l4-O)(l3-OH)(DMF)7(H2O)] (65) [Tb6(l4-O)2(tBuC4A)2(NO3)2(HCOO)2(CH3O)2(DMF)4(MeOH)4] (66) [Tb6(tBuC4A)2O2(OH)3.32Cl0.68(HCO2)2(DMF)8(H2O)0.5] (67) [Dy(tBuC4A-H2)(KtL)] (68) [Dy(tBuC4AS-OMe2)(KtL)] (69)

Bridged tetranuclear units Bridged tetranuclear units

– – – – – – – – – – – – – – – 73.7 (9 0 0) 27.9 28.5 – – – 7.6 22.9 – – – – – – – 25.4 –

– – – – J1 = 0.060 – – J = 0.046 – – – – – – – – –

–23.45 27.27 – – – 14.11 – – – – 7.86 4.66 – 17.19 – – –

– – – 2.0 – – 2.0 – – – – – – – – –

[107] [107] [73] [108] [109] [110] [111] [27] [112] [113] [114] [114] [109] [110] [27] [112] [113]

– – – – – – – – – – – – – –

11.04 5.38 – +1.06 2.06 – – – – – 5.73 – – –

– – – – – – – – – – – – – –

[114] [114] [109] [110] [115] [27] [106] [106] [112] [113] [115] [27] [113] [59]

[Dy4(tBuC4AS)2(l4-OH)Cl3(MeOH)2(H2O)3] (70) [Dy4(p-phenyl-C4AS)2(l4-OH)Cl3(CH3OH)2(H2O)3] (71) Dy5(OtBuC4A)2(NO3)3(l-CH3O)(l4-O)(l3-OH)(DMF)7(H2O)] (72) [Dy6(l4-O)2(tBuC4A)2(NO3)2(HCOO)2(CH3O)2(DMF)4(MeOH)4] (73) [Dy4(OH)4(tBuC4ASO2)2(H2O)4(MeOH)4] (74) [Dy6(tBuC4A)2O2(OH)3.32Cl0.68(HCO2)2(DMF)8(H2O)0.5] (75) [Dy12(tBuC4A-tet2-H3)3(tBuC4A-tet2-H2)3(PhCO2)5(OH)16(H2O)21] (76) [Dy19(tBuC4A-tet2-H3)(tBuC4A-tet2-H2)11(CH3CO2)6(OH)26(H2O)30] (77) [Ho(tBuC4A-H2)(KtL)] (78) [Ho(tBuC4AS-OMe2)(KtL)] (79) [Ho4(OH)4(tBuC4ASO2)2(H2O)4(MeOH)4] (80) [Ho6(tBuC4A)2O2(OH)3.32Cl0.68(HCO2)2(DMF)8(H2O)0.5] (81) [Er(tBuC4AS-OMe2)(KtL)] (82) [Li(THF)2](l-Cl)2[UIIIUIV[(CH2)5]4C4-pyrrole] (83)

Dodecahedral Distorted square pyramid Octahedral Linked linear chains Octahedron 7 coordinate 7 coordinate Square planar Square planar Distorted square pyramid Octahedron Octahedron 7 coordinate 7 coordinate Square planar Square planar Distorted square pyramid Octahedron Cubane (disordered) Octahedron Linked Trigonal bipyramids Linked Trigonal bipyramids 7 coordinate 7 coordinate Cubane (disordered) Octahedron 7 coordinate

Lattice solvent molecules are not listed; Ueff values are from zero field measurements unless field value is given in parentheses; tBuC8A = p-tert-butylcalix[8]arene; PhPO3H2 = phenylphosphonic acid; DMF = dimethylformamide; bis[quin] = bis-8-hydroxyquinoline-carbaldehyde-hydrazone-carbonylmethoxy; tBuC4A = p-tert-butylcalix [4]]arene; MeCN = acetylnitrile; MeOH = methanol; KtL = Klaüi’s tripodal ligand = sodium(g5- cyclopentadienyl)tris(diethylphosphito-p)cobaltate(III)); tBuC4AS-OMe2 = ptert-butylthiacalix[4]arene-dimethoxy; p-phenyl-C4AS = p-phenylthiacalix[4]arene; tBuC4AS = p-tert-butylthiacalix[4]arene; tBuC4ASO2 = p-tert-butylsulfonylcalix[4]arene; t BuC4A-tet2 = p-tert-butylthiacalix[4]arene-ditetrazole functionalised C4A; OtBuC4A = p-tert-butyldihomocalix[4]]arene; [(CH2)5]4C4-pyrrole = [(CH2)5]calix[4]tetrapyrrole;

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

71 (Dy4(p-phenyl-C4AS)2) [114] 76 (Dy12(tBuC4A-tet2-H3)3(tBuC4A-tet2-H2)3) [106], and 77 (Dy19(tBuC4A-tet2-H3)(tBuC4Atet2-H2)11), [106]), but there are no maxima observed in the out of phase signal down to temperatures  2 K (the temperature limit of conventional magnetometer). For complexes 63 (Tb4(tBuC4AS)2) [114], 64 (Tb4(p-phenyl-C4AS)2) [114], 65 (Tb5(OtBuC4A)2) [109], 72 (Dy5(OtBuC4A)2) [109], 67 (Tb6(tBuC4A)2) [27], 75 (Dy6(tBuC4A)2) [27], and 81 (Ho6(tBuC4A)2) [27] there is no frequency dependent out of phase susceptibility at all. For complexes 72 and 65 which involve square pyramidal metal arrays [109], the result is perhaps unexpected as the complexes are structurally similar to a Dy5 molecule that has a large barrier to reversal with slow relaxation observed at 40 K and a barrier to reversal of 530 K [116]. Research into lanthanoid-based SMM materials is dominated by monometallic complexes [16]. Calixarenes have multiple coordination sites, making them ideal candidates in the preparation of polynuclear complexes. Increasing metal array size does not preclude materials from having slow relaxation. Rather it is linked in many instances to increasing the symmetry of a molecule, (reducing the anisotropy) where multiple lanthanoid centres have weak coupling. Complex 73 involves an octahedral dysprosium metal core encapsulated between two calixarenes [110]. The molecule exhibited slow magnetic relaxation, with an observable peak in the frequency dependent AC susceptibility measurements (Ueff = 7.6 K with s0 = 1.1x10-6 s). Complex 74 (Dy4(tBuC4ASO2)2) [115] is another polynuclear calixarene complex with SMM

Fig. 11. (a) Molecular structure of complex 74, [Dy4(OH)4(tBuC4ASO2)2(H2O)4(MeOH)4] viewed through the DyIII plane with thermal ellipsoids set at 20% probability. Hydrogen atoms and solvent molecules omitted for clarity. Green = Dy, red = O, yellow = S, grey = C. Coordination is mode is highlighted in (b). To provide clarity, a number of atoms have been omitted.

11

properties (Fig. 11). The molecule consists of calixarenes bridging the distorted cubane core. It was the first reported disordered SMM with the energy barrier to reversal calculated at Ueff = 22.9 K with s0 = 1.1  108 s. Additional coligands that prevent total metal encapsulation by calixarenes is one solution to ensure the formation of alternative metal coordination environments. Kläui’s tripodal ligand has been used successfully in a number of examples to form single lanthanoid calixarene complexes with slow magnetic relaxation. Complex 68, shown in Fig. 12 is comprised of a seven coordinate Dy ion encapsulated between the phenolic lower rim of a calixarene and diethylphosphito groups of Kläui’s tripodal ligand (Dy(tBuC4A-H2)(KtL)) [112]. Typical slow magnetic relaxation is observed for the complex. A bias field (900 Oe) to suppress quantum tunnelling is applied and a Ueff = 73.7 K with s0 = 9.1x10-9 s is then calculated for 68. The thiacalixarene derivative complex 69 (Dy(tBuC4AS-OMe2)(KtL)) has also been synthesised [113]. The structurally related complex has different dynamic properties, in that a double relaxation Ueff = 27.9 K with s0 = 6.5x10-8 s and 28.5 K with s0 = 5.0  10-7 s process occurs (c.f complex 68 which has a single relaxation process under an applied bias field). The 4f coordination environment is well known to influence the dynamic magnetic behaviour. While complex 68 has a close to ideal capped octahedron, the introduction of sulfur atoms and OMe in complex 69 changes the ligand field. The dual relaxation in 69 has been attributed to weak intermolecular interactions. The erbium analogue, complex 82 (Er(tBuC4AS-OMe2)(KtL) was also found to have dynamic properties with Ueff = 25.4 K with s0 = 1.8x10-7 s. The dynamic properties were typical for a Kramers ion, with this being the first erbium SMM supported by a macrocyclic multidentate calixarene. Additional mononuclear structures based on Kläui’s tripodal ligand with calix[4]arene and Tb (61) or Ho (78) did not exhibit slow magnetic relaxation [112]. Unlike dysprosium, terbium and holmium are non-Kramers ions i.e. they have an even number of electrons. Relaxation of the nondegenerate sub-states via quantum tunnelling is common, thus complexes based on these non-Kramers ions need to have specific ligand fields

Fig. 12. Molecular structure of complex 68, [Dy(tBuC4A-H2)(KtL)] with thermal ellipsoids set at 30% probability. Hydrogen atoms and solvent molecules omitted for clarity. Green = Dy, red = O, magenta = P, cyan = Co, grey = C.

12

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

to ensure slow magnetic relaxation can be observed. In contrast, Kramers ions such as dysprosium(III) will always have a bistable ground state due to the electronic nature of the ion itself i.e. a large energy gap between ground and first excited state. From theory [117], a concentration of ligand electrons above and below the xy plane is known to maximise anisotropy for all of the oblate lanthanoid ions. Hence for Tb-based SMMs, axial symmetry is required in the ligand field for the occurrence of a bistable ground state. The [Tb(tBuC4A-H2)(KtL)] complex seems likely to lack the appropriate crystal field for the occurrence of a bistable ground state. Another example highlighting an insufficient ligand field involves the terbium analogue complex 66 of the aforementioned octahedron Dy6-SMM calix[4]arene complex (73) [110]. Similar results are seen for holmium analogues of dysprosium calixarene SMMs. Namely, to overcome quantum tunnelling in Ho-SMMs complexes requires strong ligation across the axial positions of the Ho(III) [118]. This was not achieved in either complex 79, [112] or 80 [115]. 3.3. Heterometallic 3d-4f clusters In addition to pure 3d and 4f systems, 3d-4f coordination complexes have been a focus for SMM studies [19]. The influence of a 3d metal on the magnetic behaviour of a 4f ion in a molecular complex was first investigated in the 1980s [119], but it was only recently the potential of these hybrids as SMMs received more interest. Ideally, for SMM-based applications, complexes combine the most anisotropic 3d ion (MnIII, CoII and NiII) with an appropriate 4f ion. The lanthanoid not only provides additional spin, but the introduction of a 4f ion with anisotropic character (TbIII, DyIII, HoIII and ErIII ions) to these systems can result in better dynamic properties. A number of other 3d-4f systems exist, the interested reader is directed to a review on the subject [19]. Three main features have driven this research: (1) unlike the weak interactions that arise in polynuclear lanthanoid complexes, the 3d metal ions can promote strong exchange interactions; (2) 3d-4f complexes tend to exhibit ferromagnetic interactions, which can lead to a high spin ground state; (3) strong spin–orbit coupling is intrinsic to the LnIII ions [18]. There is still much to be done in the development of these molecules. Indeed, the design principles for ligands that impart strong 3d-4f coupling not yet been fully developed. To produce a 3d-4f complex the ligand needs to have distinct coordination sites for the binding of transition metals and lanthanoid ions [120]. Ligands containing both oxygen and nitrogen donors are ideal. Lanthanoids, being hard acids, are strongly oxophilic, whereas transition metals are able to coordinate to both O and N donors. Calixarenes are highly functionalisable, ideal for the design of new magnetic materials based on mixed 3d-4f metal complexes [121]. Table 4 provides a list of the complexes prepared to date, with tabulation based on the 3d component of the molecule. Complex 88 was the first example of a 3d-4f calixarene-based material [123]. The dimanganese gadolinium metal array was found to exhibit ferromagnetic coupling. Despite the positive Weiss constant, no frequency dependent measurements were made. Since that report, an abundance of 3d-4f calixarene complexes with detailed magnetic studies have been published. To better understand the magnetic properties of complexes, different LnIII analogues are often prepared so the effect of the Ln centre can be investigated by studying a whole family of complexes. The contribution of the 3d centre can be confirmed by using a diamagnetic centre such as YIII or isotropic GdIII. From Table 4 it is evident that very few of the complexes have calculated magnetic parameters. Indeed for molecules 91 (Mn2Pr2), [125] 92 (Mn2Eu2), [125] 102 (Co2Eu2), [125] 105 (Nd2Ni2) and 106 (Er2Nd2), [131] no frequency dependent susceptibility measurements were made, with only exchange coupling interpreted from the temperature dependent

susceptibility measurements. However, for other examples the undetermined Ueff and J arises from the complexity associated with systems. Unlike the mononuclear 4f molecules, the crystal field in 3d-4f complexes is more complicated. Ensuring axial ligation for the oblate ions is not necessarily a simple solution to providing significant anisotropy [19]. Furthermore, for slow magnetic relaxation to be observed, quantum tunnelling in these complexes needs to be minimised. Strong ferromagnetic coupling between centres tends to ensure complexes have a well-defined ground states, with no mixing of low lying excited states. However, magnetic coupling between TM and Ln centres tends to be weak in many cases [19], with the occurrence of weak antiferromagnetic coupling in 3d-4f complexes common. Hexanuclear Mn2Ln4(tBuC4ASO)4 (Ln = Gd, Eu) complexes 89 and 90, [124] and larger cage structures (107– 111) comprised of two Na2Ni12Ln2(tBuC4AS)3](Ln = Dy, Tb) units linked through a variety of bridging ligands [132] provide good examples of weak coupling. Although slow magnetic relaxation is observed in these complexes, it is likely the result of thermal depopulation of Stark levels as the complexes have antiferromagnetic coupling. Since no maximum is observed in the out of phase AC susceptibility, no estimate of Ueff is given. Four complexes (84–89) based on a Ln6Cr(tBuC8A)2 (Ln = Gd, Tb, Dy or Tm) motif further highlight the complexities associated with 3d-4f magnetism [122]. The structure of each analogue consists of two tBuC8A encapsulating the metal array. The phenolic oxygens of a tBuC8A bridge a trigonal Ln3 subunits, with a Cr also bound to phenol O atoms at the waist of one of the Ln3 groups (Fig. 13). For the Tb, Dy and Tm analogues, an out of phase signal in the AC susceptibility is confirmed. No maximum is observed in zero field for Dy and Tm, hence Arrhenius fitting cannot be used to obtained Ueff. Debye modelling is used for the Dy analogue and the barrier to reversal is approximated as Ueff = 7.56 K with s0 = 1.36x10-6 s. A maximum is noted in the frequency dependent measurements of the Tb analogue and Ueff = 18.13 K with s0 = 7.5x10-8 s calculated from the fit. Another family of 3d-4f molecules based on HC4 as the ligand, III involves a MnIII 4 Ln4 core with lanthanoids, Ln = Gd (95), Tb (96) and Dy (97) [127,128]. These molecules all consist of a planar metal array, with a [LnII4(OH)4(NO3)2] unit bound to four [MnIII(HC4A) (dmf)] through shared l3-OH ions (Fig. 14). The HC4A are fully deprotonated, with two oxygens bound terminally to the MnIII and two form bridging groups between MnIII and LnII ions. The octahedral coordination environment of the MnIII ions is Jahn-Teller distorted through the coordinated DMF and OH. The distortion of the MnIII ions is a similar butterfly motif to that in the MnIIIMnII unit in complex 1 [28]. Temperature dependent magnetic susceptibility measurements (Fig. 15) for 95 were suggestive of only weak coupling and several populated spin states, with the maximum value of vMT, being much less than the expected for an isolated ferromagnetically coupled S = 22 ground state. The field dependent magnetisation measurements (Fig. 15) made at different temperatures (2– 7 K) show the magnetisation does not quickly reach saturation. The slow increase in magnetisation is indicative of the depopulation of the low-lying states which have smaller moments. Anisotropy often leads to the spin orientation being frozen at very low temperatures (T < 4 K). However, for complexes that have negligible anisotropy, spin orientation is not frozen. The application of a field in these systems leads to a change in the magnetic entropy, the so called magnetocaloric effect (MCE) [134]. In the case of this complex, the magnetic anisotropy arises from the perpendicular alignment of the Jahn-Teller distorted axes of the MnIII ions. The replacement of the isotropic GdIII by anisotropic TbIII or DyIII ions is of great interest as these complexes behave as SMMs. The complexes of TbIII (96) or DyIII (97) are structural analogues of the GdIII complex but crystallise in a different space group with a different number of solvent molecules. As temperature decreases the spins eventually become

13

R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066 Table 4 3d-4f complexes of calixarenes. Formula

Ueff/kB (K)

J(cm1)

h (K)

g

Ref

trigonal

–

–

–

–

[122]

trigonal

18.13

–

–

–

[122]

trigonal

–

–

–

[122]

trigonal

7.56 (debye) –

–

–

–

[122]

+2.38 3.92

– 2.005

[123] [124]

– – – –

– – – 2.0

[124] [125] [125] [126]

–

2.0

[126]

–

–

[127,128]

Metal geometry

[(Gd6Cr)(tBuC8A)2(O)2(OH)(DMF)5(MeOH)2(H2O)4] (84)

[Mn2(Gd (MeOH))2(OH)( BuC4AS)2] (88) [Mn2Gd4(tBuC4ASO)4(H2O)2] (89)

Sandwich with stacked units Sandwich with stacked units Sandwich with stacked subunits Sandwich with stacked units Square planar Stacked trigonal planar

units

– –

[Mn2Eu4(tBuC4ASO)4(H2O)2] (90) [Mn2Pr2(l4-O)(tBuC4AS)2(DMF)2(MeOH) H2O] (91) [Mn2Eu2(l4-O)(tBuC4AS)2(DMF)0.5(MeOH)2.5 H2O] (92) II III t [MnIII 2 Mn Gd (OH)2( BuC4A)2(NO3)(DMF)4(iPrOH)2] (93)

Stacked trigonal planar units Square planar Square planar Butterfly core

– – – –

III t [MnIII 2 Gd2 (OH)2( BuC4A)2(DMSO)8] (94)

Butterfly core

–

III [MnIII 4 Gd4 (OH)4(HC4A)4(NO3)2(DMF)6(H2O)6] (95)

Square planar unit encasing sqaure planar unit Square planar unit encasing sqaure planar unit Square planar unit encasing sqaure planar unit Distorted fused butterfly units

–

– J1 = 0.263 J2 = 0.091 – – – JMnIIIGdIII = 0.075 JMnIIGdIII = 0.164 JMnIIIMnII = 0.893 JMnIIIGdIII = 0.075 JGdIIIGdIII = 0.006 –

3.0

–

–

–

[128]

5.0

–

–

–

[128]

–

JMnIII= 0.062 JMnIIGdIII = 0.066 JGdIIIGdIII = 0.061 –

–

2.0

[67,68]

–

–

[68]

JMn-Gd = 0.075 JGdIIIGdIII = 0.006 – – – – – – – – – – – –

–

–

[129]

– – – – 0.85 6.32 9.62 – – – – –

– – – – 2.20 2.20 – – – – – –

[129] [125] [130] [130] [131] [131] [132] [132] [132] [132] [132] [129]

–

1.98

[133]

– – –

– – –

[133] [133] [133]

t

[(Tb6Cr)( BuC8A)2(O)2(OH)(DMF)5(MeOH)2(H2O)4] (85) [(Dy6Cr)(tBuC8A)2(O)2(OH)(DMF)5(MeOH)2(H2O)4] (86) [(Tm6Cr)(tBuC8A)2(O)2(OH)(DMF)5(MeOH)2(H2O)4] (87) t

III [MnIII 4 Tb4 (OH)4(HC4A)4(NO3)2(DMF)6(H2O)6] (96) III [MnIII 4 Dy4 (OH)4(HC4A)4(NO3)2(DMF)6(H2O)6] (97)

l

II III t [MnIII 4 Mn2 Gd2 (bis- BuC4A)2(Cl)2( 3-OH)4(MeOH)2(DMF)8]

(98)

GdIII

II III t [MnIII 6 Mn2 Gd2 (bis- BuC4A)2(l4-O)2(l3-OH)2(l-OCH3)2(lOH)2(MeOH)4(DMF)8] (99) III t [MnIII 4 Gd4 (bis- BuC4A)2(l3-OH)4(l-CO3)2(DMF)8(H2O)4] (100)

Distorted fused butterfly units

–

Distorted fused butterfly units

–

III t [FeIII 5 Gd4 (bis- BuC4A)2(l4-O)2(l3-O)2(l3-NO3)2(DMF)8(H2O)6] (101) [Co2Eu2(l4-O)(tBuC4AS)2(DMF)0.5(MeOH)1.5(H2O)2] (102) [(Dy4Co4)(tBuC8A)2(O)2(DEF)8(H2O)4] (103) [(Er4Co4)(tBuC8A)2(O)2(DEF)8(H2O)4] (104) [Nd2Ni2(O3SC4ASO2)2(H2O)6(phen)4]
[Ni(phen)3] (105) [Er2Ni2(O3SC4ASO2)2(H2O)6(phen)4]
[Ni(phen)3] (106) [Na4Ni24Dy4(tBuC4AS)6(BDC)3(CO3)6(OH)8(Cl)4(H2O)10(dma)8] (107) [Na4Ni24Dy4(tBuC4AS)6(NDC)3(CO3)6(OH)8(Cl)4(H2O)10(dma)8] (108) [Na4Ni24Dy4(tBuC4AS)6(BPDC)3(CO3)6(OH)8(Cl)4(H2O)10(dma)8] (109) [Na4Ni24Dy4(tBuC4AS)6(BIPY)3(CO3)6(OH)8(Cl)4(H2O)10(dma)8] (110) [Na4Ni24Tb4(tBuC4AS)6 (BDC)3(CO3)6(OH)8(Cl)4(H2O)10(dma)8] (111) t [CuII4TbIII 5 (bis- BuC4A)2(l3-OCH3)(l-OCH3)(l3-OH)(l4-NO3)(l5-NO3) (MeOH)(DMF)6(H2O)4] (112) [ZnGd3(l4-OH)(tBuC4AS)2(OAc)2(MeOH)(H2O)(DMA)2] (113)

Distorted fused butterfly units Square planar Square antiprism cluster Square antiprism cluster

Cage-trigonal prismatic Cage-trigonal prismatic Cage-trigonal prismatic Cage-trigonal prismatic Cage-trigonal prismatic Distorted fused butterfly units

– – 10.38 – – – – – – – – –

Planar kite

–

[ZnTb3(l4-OH)(tBuC4AS)2(OAc)2(MeOH)(H2O)(DMA)2] (114) [ZnDy3(l4-OH)(tBuC4AS)2(OAc)2(MeOH)(H2O)(DMA)2] (115) [ZnHo3(l4-OH)(tBuC4AS)2(OAc)2(MeOH)(H2O)(DMA)2] (116)

Planar kite Planar kite Planar kite

– – –

J1 = 0.026 J2 = 0.157 – – –

Lattice solvent molecules are not listed; Ueff values are from zero field measurements unless field value is given in parentheses; tBuC8A = p-tert-butylcalix[8]arene; DMF = dimethylformamide; MeOH = methanol; tBuC4A = p-tert-butylthiacalix[4]arene; tBuC4ASO = p-tert-butylsulfinylcalix[4]arene; tBuC4A = p-tert-butylcalix[4]arene; iPrOH = isopropanol; DMSO = dimethyl sulfoxide; HC4A = calix[4]arene; bis-tBuC4A = bis-p-tert-butylcalix[4]arene; DEF = diethyl formamide; OAc = acetate; O3SC4ASO2 = psulfonylcalix[4]arenetetrasulfonate; tBuC4ASO2 = p-tert-butylsulfonylcalix[4]arene phen = 1,10-phenanthroline; BDC = 1,4- benzenedicarboxylic acid; DMA = N,N’dimethylacetamide; NDC = 2,6-naphthalenedicarboxylic acid; BPDC = 4,40 - biphenyldicarboxylic acid; BIPY = 2,20 -bipyridine-5,50 -dicarboxylic acid.

blocked in a particular state, reducing the MCE. The large anisotropy and a high net spin ensure slow relaxation for Tb and Dy complexes below a particular temperature. This is analogous to the superparamagnetic behaviour seen in nanoparticles. AC susceptibility measurements are complex at low temperatures, with a number of

pathways noted. The a cusp of the out of phase was fitted and for Tb, Ueff = 3.0 K with s0 = 1x10-7 s and for Dy, Ueff = 5.0 K with s0 = 3  108 s. The Jahn-Teller distortion of the octahedral MnIII environment II has been key in design of the 3d-SMMs [MnIII 2 Mn2 ] and 3d-4f

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R.O. Fuller et al. / Coordination Chemistry Reviews 402 (2020) 213066

III Fig. 15. Temperature dependent vMT for complex 95 [MnIII 4 Gd4 (OH)4(HCA)4(NO3)2 (DMF)6(H2O)6]. The inset contains the field dependent magnetisation at a number of temperatures. Reprinted with permission from [127].

Fig. 13. (a) Molecular structure of complex 84, [(Gd6Cr)(tBuC8A)2(O)2(OH)(DMF)5 (MeOH)2(H2O)4]. Solvent molecules, t-butyl groups, hydrogen and disordered atoms are omitted for clarity, thermal ellipsoids set at 30% probability. (b) contains a Ln3 tripod unit with Cr bound to a single tBuC8. Green = Gd, cyan = Cr, red = oxygen, grey = C.

III Fig. 14. Metal array in complex 95, [MnIII 4 Ln4 (OH)4(HC4A)4(NO3)2(DMF)6(H2O)6]. Green = Gd, cyan = Mn, purple = N, red = oxygen.

III [MnIII 4 Ln4 ]. The butterfly unit is a useful arrangement for ensuring II this distortion. Two 3d-4f analogues of the [MnIII 2 Mn2 ] moitif were II III prepared, where complex 93 (MnIII Mn Gd ) [126], and 94 (MnIII 2 2 II III GdIII 2 ) have stepwise substitution of Mn with Gd [126]. The substitution of MnII with GdIII results in an apparent decrease in the magnetic anisotropy, which is likely to be the result of the large Gd total spin, with some of the ferromagnetic interactions in [MnIII 2MnII2] i.e. JMn(III)-Mn(II) replaced by weak anitferromagnetic couplings i.e. JMn(II)-Gd(III) and JGd(III)-Gd(III). Larger complexes of bis-C4A have also II III been prepared, inlcuding a series of complexes 98 (MnIII 4 Mn2 Gd2 ), 99 II III III III II (MnIII Mn Gd ) [67,68] and 100 (Mn Gd ), [129] where the Mn of 6 2 2 4 4 III the butterfly unit has been replaced with Gd ion. In addition,

complexes with alternative metal ions and the bis-tBuC4 ligand III II III 101 (FeIII 5 Gd4 ) and 112 (Cu4 Tb5 ), [129] have also been synthesised. Despite the linked distorted butterfly metal arrangement, slow magnetic relaxation has not been reported for any of these complexes. This family of molecules highlights the importance of the exchange coupling for the 3d-4f complexes. Perhaps an unlikely example of slow magnetic relaxation is found in (Ln4Co4)(tBuC8A)2 motif where Ln = Dy (103), and Er (104) [130]. The square-antiprismatic metal array is encapsulated by two tBuC8A (Fig. 16). Generally, with increasing complexity in the molecules, we have seen a decrease in exchange and anisotropy. However for complex 103, a maximum is observed in the frequency dependent out of phase susceptibility, Ueff = 10.38 K with s0 = 9.8  106 s. Slow magnetic relaxation (with no maximum) is also observed in complex 104. A yttrium analogue was also prepared, but no out phase component of the AC susceptibility was found. It seems likely that slow magnetic relaxation is not the result of significant exchange in these complexes, but rather is the result of the thermal depopulation of the low-lying states and the intrinsic anisotropy of the LnIII. A series of tetranuclear complexes of the form ZnLn3(l4-OH)(tBuC4AS) where Ln = Gd (113), Tb (114), Dy (115) or Ho (116) have been prepared. The metal array in each is an essentially planar kite shape and is encapsulated by two thiacalixarenes, with the sandwich narrowed around the zinc ion (Fig. 17) [133]. All metal atoms have distorted coordination environments. At room temperature, vMT for all complexes was close to the expected uncoupled spin system. For Gd, Tb and Dy, vMT is relatively constant (until 50 K) and then drops rapidly as temperature is lowered further. The drop is likely the result of significant anisotropy in the Gd complex and antiferromagnetic exchange in the Tb and Dy complexes. vMT for the Ho complex is seen to slowly decrease with decreasing temperature before rapidly decreasing at temperatures below 50 K. It is likely that the decrease is due to the thermal depopulation of the Stark levels. The out of phase susceptibility measurements for the DyIII (116) complex were frequency dependent below 10 K. No peak maximum was observed at temperatures down to 2 K suggesting that while the molecule may have slow magnetic relaxation, the barrier to reversal is very small. The complexes involving Gd, Tb and Ho did not display any frequency dependent behaviour.

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Fig. 16. (left) Molecular structure of complex 103, [(Dy4Co4)(tBuC8A)2(O)2(DEF)8(H2O)4] with thermal ellipsoids set at 30% probability. t-butyl groups, DEF molecules and hydrogens omitted for clarity. Green = Dy, red = O, cyan = Co, grey = C. (right) Representation of the metal array in complex 103.

Fig. 17. (left) Molecular structure of complex 113, [ZnLn3(l4OH)(tBuC4A)2(OAc)2(MeOH)(H2O)(DMA)2] with thermal ellipsoids set at 30% probability. Hydrogen atoms and solvent molecules are omitted for clarity. Green = Ln, red = O, yellow = S, cyan = Zn, grey = C. (b) (right) Representation of the metal array in complex 113; coordinated solvent molecules on Gd atoms have been omitted.

examples in the broader field, it has been challenging to ensure the strong magnetic coupling between the centres which is required for large energy barriers to reversal. SMMs that operate at liquid nitrogen temperature are now known [20]. Future research in the SMM field is likely to focus on developing air and water stable examples. In addition to providing a detailed understanding of relaxation processes to develop the a structural criteria for materials to be used in applications. Although much promise has been seen in the materials developed to date, significant work is required before calixarene-based magnetic molecules reach their full potential. The most promising direction for improved calixarene SMM materials is likely to be with lanthanoid complexes of dysprosium. Ensuring alternative coordination environments with significant anisotropy for a single metal centre is key to this development. Complexes involving a calixarene with an additional coligand have shown promise. Continuing to explore Tb and Er complexes with appropriate geometries would be well worthwhile as they can display very large barriers. The fact that calixarenes can be readily and systematically varied in structures makes them a useful ligand system for developing these materials, and we expect significant developments in this area to be reported in the future.

4. Conclusions

Declaration of Competing Interest

Calixarenes as a ligand scaffold have provided a variety of metal clusters from transition and lanthanoid metals including heterometallic 3d-4f centres with magnetic properties. Early work involving transition metals used susceptibility to provide electronic information about the complexes. As the size of the complexes has expanded, complexity in the interpretation of measurements meant only qualitative details could be inferred. The introduction of anisotropy to these complexes was key to further development of magnetic properties in these materials. Slow magnetic relaxation can be observed in calixarenes with appropriate structural elements, eg. butterfly-like topology for a metal array in a mixed valence manganese cluster was the first reported calixarene complex with slow magnetic relaxation. Work on calixarene-based SMMs has been extended through the introduction of lanthanoid centres. Numerous examples are now known, with dysprosium containing complexes being the most numerous. As with the broader molecular magnet field, it is noted for the nonKramers ions of Tb and Ho that it becomes difficult to introduce the correct crystal field for slow magnetic relaxation to be observed. For calixarene-based 3d-4f complexes, like many of the TM-Ln

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