THz-EPR-based Magneto-Structural Correlations for Cobalt(II) Single-Ion Magnets With Bis-Chelate Coordination.

New cobalt(II)-based complexes with [NO] coordination formed by two bis-chelate ligands were synthesized and characterized by a multi-technique approach. The complexes possess an easy-axis anisotropy (D<0) and magnetic measurements show a field-induced slow relaxation of magnetization. The spin-reversal barriers, i. e., the splitting of the two lowest Kramers doublets (U), have been measured by THz-EPR spectroscopy, which allows to distinguish the two crystallographically independent species present in one of the complexes. Based on these experimental U energies together with those for related complexes reported in literature, it was possible to establish magneto-structural correlations. U linearly depends on the elongation parameter ϵ of the (pseudo-)tetrahedral coordination, which is given by the ratio between the average obtuse and acute angles at the cobalt(II) ion, while U was found to be virtually independent of the twist angle of the chelate planes. With increasing deviation from the orthogonality of the latter, the rhombicity (|E/D|) increases.