Origin of the Magnetic Anisotropy in Heptacoordinate Ni^II and Co^II Complexes

Abstract

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.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…The (117). Adapted with permission from [212], [227], and [226]. Copyright 2013 American Chemical Society.…”

“…Detailed ab initio calculations showed that the high-spin d 7 metal ions with this geometry were expected to have a large and positive D, whereas d 8 metal ions in these coordination environments were predicted to have large and negative D values. It was demonstrated that the main perturbative contributions to D came from the triplet excited states to D and E, but with an essential contribution from the first triplet state [212].…”

“…Detailed magnetic investigations were performed of the hepta-coordinated high-spin Ni II [212]. Detailed ab initio calculations showed that the high-spin d 7 metal ions with this geometry were expected to have a large and positive D, whereas d 8 metal ions in these coordination environments were predicted to have large and negative D values.…”

“…A recent emerging trend is that significantly high magnetic anisotropy can be induced in transition metal complexes with a higher coordination number and with high axial symmetry [88,[212][213][214]. The d orbital splitting diagram for a hepta-coordinated transitionmetal complex with pentagonal bipyramidal (PBP) geometry is depicted in Scheme 10.…”

Tools for Predicting the Nature and Magnitude of Magnetic Anisotropy in Transition Metal Complexes: Application to Co(II) Complexes

“…A lowering of the symmetry is therefore necessary to observe the property. Molecules with the same symmetry but with different electronic configurations of the magnetic ion (different metal ions or different oxidation states) lead to completely different magnetic anisotropies (easy axis or easy plane and with or without rhombic contribution) so that the ways to improve SMMs are far from being intuitive [16][17][18][19][20].…”