Anna Collins

Mixed-Valent Mn Supertetrahedra and Planar Discs as Enhanced Magnetic Coolers

The syntheses and structures of two decametallic mixed-valent Mn supertetrahedra using 2-amino-2-methyl-1,3-propanediol (ampH2), two decametallic mixed-valent Mn planar discs using 2-amino-2-methyl-1,3-propanediol (ampH2) and 2-amino-2-ethyl-1,3-propanediol (aepH2), and a tetradecametallic mixed-valent Mn planar disc using pentaerythritol (H4peol) are reported. The decametallic complexes display dominant ferromagnetic exchange and spin ground states of S = 22, and the tetradecametallic complex displays dominant antiferromagnetic exchange and a spin ground state of S = 7 +/- 1. All display large (the former) and enormous (the latter) magnetocaloric effect--the former as a result of negligible zero-field splitting of the ground state, and the latter as a result of possessing a high spin-degeneracy at finite low temperatures--making them the very best cooling refrigerants for low-temperature applications.

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.

New structural types and different oxidation levels in the family of Mn6-oxime single-molecule magnets.

The synthesis and magnetic properties of three new members of a family of salicyaldoxime based [Mn6] single-molecule magnets possessing new structural types, core topologies and Mn oxidation state distributions are reported. The isostructural complexes [MnIII6O2(R-sao)6(X)2(EtOH)6] (R = Et, X = Br (1); R = Me, X = I (2)) exhibit single-molecule magnet behaviour with spin Hamiltonian parameters S = 12, g = 1.98 and D = -0.36 cm(-1) in both cases. The hexametallic cluster [MnIII4MnIV2O2(OMe)(4-)(Et-sao)6(MeOH)2].MeOH (3.MeOH) possesses a planar rod-like topology and a mixed valent [MnIV4MnIII2] core, which is unprecedented in this family of [Mn6] SMMs.

Using small molecule crystal structure data to obtain information about sulfonamide conformation.

Understanding the conformations adopted by the sulfonamide group is essential to the understanding of the way that sulfa drugs act upon the body. The relative energies of these conformations in the solid state are estimated from the Cambridge Structural Database (CSD) using cluster analysis, and are used to confirm earlier findings that many high-level ab initio calculations do not reproduce the observed solid-state structure. These conformational studies have been extended to the adjacent torsion angles, and it has been shown that the sulfonamide group significantly affects the form adopted. The relative energies of the observed forms in the solid state have been estimated using data available in the CSD.

Ferromagnetic Ni(II) discs.

A family of planar disc-like hexa-, octa- and decametallic Ni(II) complexes exhibit dominant ferromagnetic exchange. The deca- and octametallic clusters [Ni(II) (10)(tmp)(2)(N(3))(8)(acac)(6)(MeOH)(6)] (1, H(3)tmp=1,1,1-tris(hydroxymethyl)propane; acac=acetylacetonate) and [Ni(II) (8)(thme)(2)(O(2)CPh)(4)(Cl)(6)(MeCN)(6)(H(2)O)(2)] (2, H(3)thme=1,1,1-tris(hydroxymethyl)ethane) represent rare examples of Ni(II)-based single-molecule magnets, and [Ni(II) (10)] (1) possesses the largest barrier to magnetisation reversal of any Ni(II) single-molecule magnet to date.

Structure matching : measures of similarity and pseudosymmetry

A sizeable proportion of structures with Z′ = 2 are thought to exhibit pseudosymmetry, but establishing the extent of the deviation from true symmetry is problematic. By considering both the conformational similarity between the independent molecules and the way in which they are related in space, assessment of the pseudosymmetry of a structure becomes possible. A method of matching two groups of atoms where both these factors are quantified using CRYSTALS [Betteridge, Carruthers, Cooper, Prout & Watkin (2003). J. Appl. Cryst. 36, 1487] is described.

Oxovanadium(IV) cyclam and bicyclam complexes: potential CXCR4 receptor antagonists.

Metal complexation can have a major influence on the antiviral and coreceptor binding properties of cyclam and bicyclam macrocycles. We report the synthesis of the vanadyl cyclam complexes [V((IV))O(cyclam)SO(4)] (1) and [V((IV))O(cyclam)Cl]Cl (2) and the analogous xylylbicyclam sulfato (3) and chlorido (4) complexes. The X-ray crystal structures of 1.1.33CH(3)OH and 2.CH(3)OH.1.5H(2)O show short V=O bonds (1.6093(19) and 1.599(3) A, respectively) with monodentate sulfate H-bonded to ring NH groups for 1, but a long V-Cl bond (2.650(12) A) for 2. The solid-state structures of 3 and 4 were compared to those of 1 and 2 using vanadium K-edge extended X-ray absorption fine structure (EXAFS) data. These suggested that complex 4 was oligomeric and contained bridging chlorido ligands. Electron paramagnetic resonance (EPR) studies suggested that the SO(4)(2-) (from 1) and Cl(-) (from 2) ligands are readily substituted by water in solution, whereas these remain partially bound for the V(IV) xylylbicyclam complexes 3 and 4. The vanadyl xylylbicyclam complexes were highly active against HIV-1 (III(B)) and HIV-2 (ROD) strains with IC(50) values in the range 1-5 microM for 3 and 0.1-0.3 microM for 4; in contrast the vanadyl cyclam complexes 1 and 2 were inactive. The factors that contribute to the activity of these complexes are discussed. Studies of vanadyl cyclam docked into a model of the human CXCR4 coreceptor revealed that the coordination of vanadium to the carboxylate of Asp171 may be accompanied by H-bonding to the macrocycle and an attractive V=O...H interaction involving the backbone Trp195 alpha-carbon proton of CXCR4. In addition, hydrophobic interactions with Trp195 are present. Both ring configuration and the xylyl linker may play roles in determining the higher activity of the bicyclam complexes.

Building Fe(III) clusters with derivatised salicylaldoximes

The syntheses, structures and magnetic properties of nine new iron complexes containing salicylaldoxime (saoH(2)) or derivatised salicylaldoximes (R-saoH(2)), [Fe(3)O(OMe)(Ph-sao)(2)Cl(2)(py)(3)].2MeOH (1.2MeOH), [Fe(3)O(OMe)(Ph-sao)(2)Br(2)(py)(3)].Et(2)O (2.Et(2)O), [Fe(4)(Ph-sao)(4)F(4)(py)(4)].1.5MeOH (3.1.5MeOH), [Fe(6)O(2)(OH)(2)(Et-sao)(2)(Et-saoH)(2)(O(2)CPh)(6)] (4), [HNEt(3)](2)[Fe(6)O(2)(OH)(2)(Et-sao)(4)(O(2)CPh(Me)(2))(6)].2MeCN (5.2MeCN), [Fe(6)O(2)(O(2)CPh)(10)(3-(t)But-5-NO(2)-sao)(2)(H(2)O)(2)].2MeCN (6.2MeCN), [Fe(6)O(2)(O(2)CCH(2)Ph)(10)(3-(t)But-sao)(2)(H(2)O)(2)].5MeCN (7.5MeCN), {[Fe(6)Na(3)O(OH)(4)(Me-sao)(6)(OMe)(3)(H(2)O)(3)(MeOH)(6)].MeOH}n (8.MeOH) and [HNEt(3)](2)[Fe(12)Na(4)O(2)(OH)(8)(sao)(12)(OMe)(6)(MeOH)(10)] (9) are discussed. The predominant building block appears to be the triangular [Fe(3)O(R-sao)(3)](+) species which can self-assemble into more elaborate arrays depending on reaction conditions. An interesting observation is that the R-saoH(-)/R-sao(2-) ligand system tends to adopt coordination modes similar to carboxylates. The most unusual molecule is the [Fe(4)F(4)] molecular square, 3. While Cl(-) and Br(-) appear to act only as terminal ligands, the F(-) ions bridge making a telling impact on molecular structure and topology.

Assembling molecular triangles into discrete and infinite architectures

Having established that molecules with general formulae [MnIII6O2(R-sao)6(O2CR)2(L)4–6] ([Mn6]) and [MnIII3O(R-sao)3(X)(L)3] ([Mn3]) (saoH2 = salicylaldoxime; R = H, Me, Et etc; X = RCO2−, ClO4−; L = solvent), with the latter being the analogous “half” molecules of the former, exhibit the phenomenon of single-molecule magnetism, we have exploited them as building blocks to construct supramolecular architectures by means of host–guest interactions and coordination driven self-assembly. A number of discrete and infinite architectures, namely [MnIII3O(Ph-sao)3(4Cl-sbz)3(MeOH)3]2(OH)(ClO4)·2MeOH (1·2MeOH), [MnIII3O(Ph-sao)3(4Me-sbz)3(EtOH)3]2(OH)(NO3) (2), {[MnIII3O(Et-sao)3(4,4′-bpy)2(MeOH)] ClO4·1.5MeOH·Et2O}n (3·1.5MeOH·Et2O), {[MnIII3O(sao)3(4,4′-bpe)1.5]ClO4·3MeOH}n (4·3MeOH) and [{MnIII3O(Et-sao)3(O2CPh)(EtOH)}2{4,4′-bpe}2] (5), based on the molecular triangle [Mn3] and various pyridyl-type ligands (4Cl-sbz = 4-chlorostilbazole, 4Me-sbz = 4-methylstilbazole, 4,4′-bpy = 4,4′-bipyridine and 4,4′-bpe = trans-1,2-bis(4-pyridyl)ethylene) were obtained and structurally and magnetically characterized.

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.