Quantitative Estimation of Ising-Type Magnetic Anisotropy in a Family of C3-Symmetric CoII Complexes

In this paper, the influence of the structural and chemical effects on the Ising-type magnetic anisotropy of pentacoordinate Co-II complexes has been investigated by using a combined experimental and theoretical approach. For this, a deliberate design and synthesis of four pentacoordinate Co-II complexes [Co(tpa)Cl]ClO4 (1), [Co(tpa)Br]ClO4 (2), [Co(tbta)Cl](ClO4)(MeCN)(2)(H2O) (3) and [Co(tbta)Br]ClO4 (4) by using the tripodal ligands tris(2-methylpyridyl)amine (tpa) and tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine) (tbta) have been carried out. Detailed dc and ac measurements show the existence of field-induced slow magnetic relaxation behavior of Co-II centers with Ising-type magnetic anisotropy. A quantitative estimation of the zero-field splitting (ZFS) parameters has been effectively achieved by using detailed ab initio theory calculations. Computational studies reveal that the wavefunction of all the studied complexes has a very strong multiconfigurational character that stabilizes the largest m(s)=+/- 3/2 components of the quartet state and hence produce a large negative contribution to the ZFS parameters. The difference in the magnitudes of the Ising-type anisotropy can be explained through ligand field theory considerations, that is, D is larger and negative in the case of weak equatorial sigma-donating and strong apical -donating ligands. To elucidate the role of intermolecular interactions in the magnetic relaxation behavior between adjacent Co-II centers, a diamagnetic isostructural Zn-II analog (5) was synthesized and the magnetic dilution experiment was performed.