The Quest for Optimal 3 d Orbital Splitting in Tetrahedral Cobalt Single-Molecule Magnets Featuring Colossal Anisotropy and Hysteresis

Abstract

Only a few four-coordinated Co2+-complexes show single-molecule magnet (SMM) properties without an applied dc field. Common for those is a large magnetic anisotropy generated by the close proximity of the (Formula presented.) and (Formula presented.) orbitals. This type of magnetic anisotropy is strongly correlated with the extent of structural distortion from ideal tetrahedral towards linear coordination. Quantification of the governing magneto-structural correlations is hence crucial for the development of better SMMs. For this purpose, we synthesized and analyzed four significantly distorted tetrahedral cobalt complexes that exhibit extremely large magnetic anisotropies and slow relaxation of magnetization in zero field. The N−Co−N bite angles vary from 70.8 to 72.7°, and magnetic measurements show strong anisotropy with D-values in the range from (Formula presented.) 75 to (Formula presented.) 114 cm−1. Ab initio calculations supported the experimental results and highlighted a significant increase of D up to (Formula presented.) 141 cm−1, correlated with the reduction of the energy difference between the (Formula presented.) and (Formula presented.) orbitals. Based on these magneto-structural correlations and in contrast to the current assumption that the smaller bite angle is the better, we predict that with an ideal bite angle in the range from 76–78° in distorted Co(N2R)2 complexes, the energy difference between the two important d-orbitals is at a minimum and hence the magnetic anisotropy is maximized.