Georgios Karotsis

A calix[4]arene 3d/4f magnetic cooler.

The success with which coordination chemists have produced (often aesthetically pleasing) molecules with fascinating physical properties is derived from the systematic exploration of the effects of ligand design, metal identity, and heating regime upon cluster symmetry, topology, and nuclearity. The design of molecular nanomagnets—model systems with which to investigate the possible implementation of spinbased solid-state qubits and molecular spintronics—has been the subject of much interest in recent years because their molecular nature and inherent physical properties allow the crossover between classical and quantum physics to be observed. The synthesis of new types of molecular nanomagnets therefore remains an exciting challenge, but the range of organic ligands employed thus far is surprisingly restricted. Undoubtedly the most successful route has been to employ small, flexible polydentate ligands in self-assembly. An alternative approach would be to entirely encapsulate the magnetic skeleton within a large rigid organic or inorganic sheath whose dual role could also include the introduction of redox activity, surface compatibility, or simply the removal/control of dipolar interactions. Calix[4]arenes (C4s) are typically bowl-shaped molecules which have been exploited in the formation of various nanometer-scale supramolecular architectures. Their rigid conformations can be utilized in self-assembly, or combined with functionalization at the upper rim to present binding sites for assembly-directing metal centers. 10] The polyphenolic nature of these molecules therefore renders them good ligand candidates for the isolation of paramagnetic cluster compounds. In this regard only one cluster compound, having greater than four transition metals, has been shown to form with methylene-bridged para-tert-butylcalix[4]arene 1 (Figure 1a). Thiacalix[4]arenes and their oxidized derivatives 2–4 (Figure 1b) possess additional donor atoms, and these have been used in the formation of a number of polynuclear transition-metal or Ln clusters. The additional donor atoms (relative to 1) around the molecular skeleton play a key role in supporting complex formation by taking part in the bonding within the metal-cluster framework. For our purposes, readily accessible methylene-bridged C4s present the potential to 1) form novel cluster compounds at the lower rim of the bowl-shaped macrocycles, and 2) easily alter the upper-rim properties to access a vast library of new metal clusters containing supramolecular building blocks. These features may therefore allow control of the interactions between clusters (thereby modulating their orientation in the solid state), or variation of the degree of cluster isolation (or encapsulation) through alteration of the upper rim of the calix[4]arene. We have recently reported the formation of the first Mn cluster and the first single-molecule magnet (SMM) to be isolated using any methylene-bridged C4 (Figure 1c). The mixed-valent Mn2Mn II 2 complex is housed between two Figure 1. a) para-tert-Butylcalix[4]arene sused in transition-metal cluster formation. b) Thiacalix[4]arenes used in transitionand lanthanidemetal cluster formation. c) Mn2Mn II 2 SMM formed with 1. [14] Hydrogen atoms omitted for clarity.

Calix[4]arene-based single-molecule magnets

Calix[4]arene Based Single-Molecule Magnets** Georgios Karotsis, Simon J. Teat, Wolfgang Wernsdorfer, Stergios Piligkos, Scott J. Dalgarno* and Euan K. Brechin* Mr. G. Karotsis, Dr. E. K. Brechin, School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK. Fax: (+44)-131-650-6453 E-mail: ebrechin@staffmail.ed.ac.uk Dr. S. J. Dalgarno, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS. Fax: (+44)-131-451-3180 E-mail: S.J.Dalgarno@hw.ac.uk Prof. Dr. W. Wernsdorfer, Institut Neel, CNRS, Grenoble Cedex 9, France. Dr. S. Piligkos, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Denmark. Dr. S. J. Teat, Advanced Light Source, Berkeley Laboratory, 1 Cyclotron Road, MS6R2100, Berkeley, CA 94720, USA. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. Single-molecule magnets (SMMs) [1] have been the subject of much interest in recent years because their molecular nature and inherent physical properties allow the crossover between classical and quantum physics to be observed. [2] The macroscopic observation of quantum phenomena - tunneling between different spin states, [3] quantum interference between tunnel paths [4] - not only allows scientists to study quantum mechanical laws in great detail, but also provides model systems with which to investigate the possible implementation of spin- based solid state qubits [5] and molecular spintronics. [6] The isolation of small, simple SMMs is therefore an exciting prospect. To date almost all SMMs have been made via the self-assembly of 3d metal ions in the presence of bridging/chelating organic ligands. [7] However, very recently an exciting new class of SMMs, based on 3d metal clusters (or single lanthanide ions) housed within polyoxometalates, [8] has appeared. These types of molecule, in which the SMM is completely encapsulated within (or shrouded by) a “protective” organic or inorganic sheath have much potential for design and manipulation: for example, for the removal of unwanted dipolar interactions, the introduction of redox activity, or to simply aid functionalisation for surface grafting. [9] Calix[4]arenes are cyclic (typically bowl-shaped) polyphenols that have been used extensively in the formation of versatile self-assembled supramolecular structures. [10] Although many have been reported, p- t But-calix[4]arene and calix[4]arene (TBC4 and C4 respectively, Figure 1A) are frequently encountered due to a) synthetic accessibility, and b) vast potential for alteration at either the upper or lower rim of the macrocyclic framework. [11] Within the field of supramolecular chemistry, TBC4 is well known for interesting polymorphic behavior and phase transformations within anti-parallel bi-layer arrays, while C4 often forms self-included trimers. [12] The polyphenolic nature of calix[n]arenes (where n = 4 – 8) also suggests they should be excellent candidates as ligands for the isolation of molecular magnets, but to date their use in the isolation of paramagnetic cluster compounds is rather limited. [13] Herein we present the first Mn cluster and the first SMM to be isolated using any methylene bridged calix[n]arene - a ferromagnetically coupled mixed-valence [Mn III2 Mn II2 ] complex housed between either two TBC4s or two C4s. Reaction of MnBr 2 with TBC4 and NEt 3 in a solvent mixture of MeOH/DMF results in the formation of the complex [Mn III2 Mn II2 (OH) 2 (TBC4) 2 (DMF) 6 ] (1) which crystallises as purple blocks that are in the monoclinic space group P2 1 /c. The cluster (Figure 1B) comprises a planar diamond or butterfly-like [Mn III2 Mn II2 (OH) 2 ] core in which the wing tip Mn ions (Mn1) are in the 3+ oxidation state and the body Mn ions (Mn2) in the 2+ oxidation state. This is a common structural type in Mn SMM chemistry, [14] but the oxidation state distribution here is highly unusual, being “reversed” from the norm in which the body Mn ions are almost always 3+. Indeed the “reversed” core has been seen only once before, in the cluster [Mn III2 Mn II2 (teaH) 2 (acac) 4 (MeOH) 2 ] 2+ (2) (teaH 3 = triethanolamine) and its analogues. [15] The Mn 3+ ions are in distorted octahedral geometries with the Jahn-Teller axes defined by O5(DMF)-Mn1-O6(OH). The four equatorial sites are occupied by the oxygen atoms (O1-O4) of the TBC4, two of which bridge in a µ 2 -fashion to the central Mn 2+ ions (Mn1-O4-Mn2, 103.5°; Mn1-O1-Mn2, 105.4°). These are connected to each other (Mn2-O6-Mn2’, 94.7°) and to the Mn 3+ ions (Mn1-O6-Mn2, 100.4°; Mn1- O6-Mn2’, 98.8°) via two µ 3 -bridging OH - ions, with the two remaining equatorial sites (completing the distorted octahedral geometry on Mn2) filled by terminal DMF molecules. There are no inter-molecular H-bonds between symmetry equivalents of 1, with the closest

A Family of Calix{[}4]arene-Supported {[}(Mn2Mn2II)-Mn-III] Clusters

In the cone conformation calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition-metal centres. Reaction of these molecules with manganese salts in the presence of an appropriate base (and in some cases co-ligand) results in the formation of a family of calixarene-supported [Mn(III)(2)Mn(II)(2)] clusters that behave as single-molecule magnets (SMMs). Variation in the alkyl groups present at the upper-rim of the cone allows for the expression of a degree of control over the self-assembly of these SMM building blocks, whilst retaining the general magnetic properties. The presence of various different ligands around the periphery of the magnetic core has some effect over the extended self-assembly of these SMMs.

Enhancing SMM properties via axial distortion of Mn-3(III) clusters

Replacement of carboxylate and solvent with facially capping tripodal ligands enhances the single-molecule magnet (SMM) properties of [Mn(III)3] triangles.

Calixarene supported enneanuclear Cu(II) clusters

New pseudo-trigonal planar supramolecular building blocks housing tri-capped trigonal prismatic [Cu(9)] clusters have been isolated from the facile reaction of Cu(II) salts with p-(t)But-calix[4]arene.

Planar [Ni7] discs as double-bowl, pseudo metallacalix[6]arene host cavities

We report three heptanuclear [Ni7] complexes with planar disc-like cores, akin to double-bowl metallocalix[6]arenes, which form molecular H-bonded host cavities.

A family of double-bowl pseudo metallocalix[6]arene discs

We report the synthesis and magnetic characterisation of a series of planar [M₇] (M= Ni(II), Zn(II)) disc complexes [Ni₇(OH)₆(L₁)₆](NO₃)₂ (1), [Ni₇(OH)₆(L₁)₆](NO₃)₂·2MeOH (2), [Ni₇(OH)₆(L₁)₆](NO₃)₂·3MeNO₂ (3), [Ni₇(OH)₆(L₂)₆](NO₃)₂·2MeCN (4), [Zn₇(OH)₆(L₁)₆](NO₃)₂·2MeOH·H₂O (5) and [Zn₇(OH)₆(L₁)₆](NO₃)₂·3MeNO₂ (6) (where HL₁ = 2-iminomethyl-6-methoxy-phenol, HL₂ = 2-iminomethyl-4-bromo-6-methoxy-phenol). Each member exhibits a double-bowl pseudo metallocalix[6]arene topology whereby the individual [M₇] units form molecular host cavities which are able to accommodate various guest molecules (MeCN, MeNO₂ and MeOH). Magnetic susceptibility measurements carried out on complexes 1 and 4 indicate weak exchange between the Ni(II) centres.

Rare tetranuclear mixed-valent [MnII2MnIV2] clusters as building blocks for extended networks

We report the synthesis of a series of mixed valence Mn(II/IV) tetranuclear clusters [Mn(II)2Mn(IV)2O2(heed)2(EtOH)6Br2]Br2 (1), [Mn(II)2Mn(IV)2O2(heed)2(H2O)2Cl4].2EtOH.H2O (2.2EtOH.H2O), [Mn(II)2Mn(IV)2O2(heed)2(heedH2)2](ClO4)4 (3), [Mn(II)2Mn(IV)2O2(heed)2(MeCN)2(H2O)2(bpy)2](ClO4)4 (4) and [Mn(II)2Mn(IV)2O2(heed)2(bpy)2Br4].2MeOH (5.2MeOH). Clusters 1-5 are constructed from the tripodal ligand N,N-bis(2-hydroxyethyl)ethylene diamine (heedH2) and represent rare examples of tetranuclear Mn clusters possessing the linear trans zig-zag topology, being the first Mn(II/IV) mixed-valent clusters of this type. The molecular clusters can then be used as building blocks in tandem with the (linear) linker dicyanamide ([N(CN)2]-, dca-) for the formation of a novel extended network {[Mn(II)2Mn(IV)2O2(heed)2(H2O)2(MeOH)2(dca)2]Br2}n (6), which exhibits a rare form of the 2D herring bone topology.

Molecular and supramolecular Ni(II) wheels from α-benzoin oxime

The use of alpha-benzoin oxime in Ni(II) chemistry leads to the formation of a family of unusual molecular and supramolecular wheels.

High temperature orbital order melting in KCrF3 perovskite

The effect of high temperatures on the structural properties of the ternary transition metal fluoride KCrF3 has been investigated by high-resolution synchrotron X-ray powder diffraction. The tetragonal room temperature structure (a>c/√2) exhibits large cooperative Jahn–Teller distortions of the CrF6 octahedra which are driven by the antiferrodistortive orbital ordering of the 3d3x2−r2 and 3d3y2−r2 orbitals in the ab plane. At TJT ≈ 973 K, KCrF3 undergoes a first-order structural phase transition to an orbitally disordered cubic phase comprising equivalent Cr–F bond lengths and regular CrF6 octahedra. The onset of the orbitally disordered state is accompanied by an abrupt volume collapse, δV/V = 0.44%, which arises from the more efficient packing of the CrF6 octahedra in the orbital liquid state.