Unraveling σ and π Effects on Magnetic Anisotropy in cis-NiA4 B2 Complexes: Magnetization, HF-HFEPR Studies, First-Principles Calculations, and Orbital Modeling.

By using complementary experimental techniques and first-principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C2v , has been investigated. Four complexes have the general formula [Ni(bpy)X2 ]n+ (bpy=2,2'-bipyridine; X2 =bpy (1), (NCS- )2 (2), C2 O42- (3), NO3- (4)). In the fifth complex, [Ni(HIM2 -py)2 (NO3 )]+ (5; HIM2 -py=2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM2 -py. Analysis of the high-field, high-frequency electronic paramagnetic resonance (HF-HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from -1 to -10 cm-1 . First-principles SO-CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3, on one hand, and 4 and 5, on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and gi ) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the eg -like orbitals and is due to the difference in the σ-donor strength of NO3- and bpy or HIM2 -py, whereas the difference in anisotropy between the two compounds is due to splitting of the t2g -like orbitals; and 2) the anisotropy of complexes 1-3 arises from the small splitting of the t2g -like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.

Origin of the magnetic anisotropy in heptacoordinate Ni(II) and Co(II) complexes.

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2 H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

Pentanuclear cyanide-bridged complexes based on highly anisotropic Co(II) seven-coordinate building blocks: synthesis, structure, and magnetic behavior.

Pentagonal-bipyramidal complexes [Co(DABPH)X(H(2)O)]X [X = NO(3) (1), Br (2), I (3)] were synthesized, and their magnetic behavior was investigated. Simulation of the magnetization versus temperature data revealed the complexes to be highly anisotropic (D ≈ +30 cm(-1)) and the magnitude of the anisotropy to be independent of the nature of the axial ligands. The reaction of 1 with K(3)[M(CN)(6)] (M = Cr, Fe) produces the pentametallic clusters [{Co(DABPH)}(3){M(CN)(6)}(2)(H(2)O)(2)] [M = Cr (4), Fe (5)]. Both clusters consist of three {Co(DABPH)} moieties separated by two {M(CN)(6)} fragments. In 4, the central and terminal Co(II) ions are bound to cyanide groups cis to one another on the bridging {Cr(CN)(6)}, whereas in 5, the connections are via trans cyanide ligands, resulting in the zigzag and linear structures observed, respectively. Magnetic investigation revealed ferromagnetic intramolecular interactions; however, the ground states were poorly isolated because of the large positive local anisotropies of the Co(II) ions. The effects of the local anisotropies appeared to dominate the behavior in 5, where the magnetic axes of the Co(II) ions were approximately colinear, compared to 4, where they were closer to orthogonal.

Tripodal thiols as ligands for molecular magnets: very strong antiferromagnetic exchange interactions in vanadium(III) clusters.

The synthesis, structural and magnetic characterisation of [V(III)(3)O(tmme)(2)(diimine)(2)Cl] [diimine=2,2'-bipyridine (1) or 1,10-phenanthroline (2)] and (HNEt(3))(2)[V(III)(4)O(tmme)(4)] (3) is reported, in which H(3)tmme is tris(mercaptomethyl)ethane, MeC(CH(2)SH)(3), the thiol analogue of the famous tripodal alcohol ligands typified by H(3)thme [tris(hydroxymethyl)ethane, MeC(CH(2)OH)(3)]. Complexes 1 and 3 have "T-shaped" and square topologies, respectively, and the latter is centred on a rare example of a square-planar oxide. The tri-thiolate ligands bind the periphery of the clusters and provide such strong antiferromagnetic exchange pathways that in both cases only a single total spin state is occupied up to room temperature, in the absence of metal-metal bonding. Magnetic data, electronic structure calculations and electrochemical data are reported.

Chemical tuning of the magnetic relaxation in dysprosium(iii) mononuclear complexes.

A dysprosium(iii) complex, exhibiting slow relaxation of magnetization, was prepared. Crystallographic studies showed a perturbation of local symmetry upon deprotonation of the ligand, with concomitant faster relaxation of magnetization. This was attributed to a large shift in the direction of the main magnetic axis, as indicated by ab initio calculations.

Chromium(III) stars and butterflies: synthesis, structural and magnetic studies of tetrametallic clusters.

We report the synthesis, structures and magnetic properties of a series of chromium(III) metal-centered triangle (or "star") clusters, [Cr(4){RC(CH(2)O)(3)}(2)(4,4'-R'(2)-bipy)(3)Cl(6)] [R = Et, R' = H (2); R = HOCH(2), R' = H (3); R = Et, R' = (t)Bu (4)], prepared by two-step solvothermal reactions starting from [CrCl(3)(thf)(3)]. The product of the first stage of this reaction is the salt [Cr(bipy)(2)Cl(2)](2)[Cr(2)Cl(8)(MeCN)(2)] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr(4){EtC(CH(2)O)(3)}(2){NH(C(R)NH)(2)}(2)Cl(6)] (R = Me (5), Et (6), Ph (7)] where the chelating N-acetimidoylacetamidine NH(C(R)=NH)(2) ligands are formed in situ via condensation of the nitrile solvents (RCN) under solvothermal conditions. Magnetic measurements show the chromium stars to have an isolated S = 3 ground state, arising from antiferromagnetic coupling between the central and peripheral metal ions, analogous to the well-known Fe(III) stars. Bulk antiferromagnetic ordering is observed at 0.6 K. The butterfly complexes have a singlet ground state, with a low-lying S = 1 first excited state, due to dominant wing-body antiferromagnetic coupling.

Tri-, tetra- and octa-metallic vanadium(III) clusters from new, simple starting materials: interplay of exchange and anisotropy effects.

The crystal structure of the monomeric vanadium(III) species mer-[V(bipy)Cl(3)(MeCN)] (1; bipy = 2,2'-bipyridine) is reported. The solvothermal reaction of [V(bipy)Cl(3)(MeCN)]with Na(O(2)CPh) yields the T-shaped cluster [V(3)(O)Cl(3)(O(2)CPh)(2)(bipy)(2)(OEt)(2)], magnetic studies of which show strong intramolecular antiferromagnetic coupling giving a well isolated S = 1 ground state. Solvothermal treatment of 1 with triols yields a series of polymetallic clusters [V(4)Cl(6)(thme)(2)(bipy)(3)], [V(3)Cl(4)(Hcht)(2)(bipy)(2)]Cl and [V(8)(OH)(2)Cl(4)(cht)(4)(O(2)CPh)(6)(bipy)(2)], structurally related to previously reported {M(4)} centred triangles. Magnetic studies of this series reveal very weak intramolecular antiferromagnetic exchange and very strong local zero-field splitting effects.