The combination of magnetometry and multi-frequency electronic paramagnetic resonance spectroscopy reveal large positive axial zero-field splitings, D’s, for 8 and 9 (24 cm⁻¹ and 32 cm⁻¹), and in-depth ligand field analyses on top of wave function based ab initio calculations demonstrated that the Co(II) out-of-equatorial plane distance, δ, determines the sign and magnitude of D values of five-coordinate trigonal bipyramidal high spin Co(II) complexes.

Abstract

Here, we combined magnetometry, multi-frequency electronic paramagnetic resonance, and wave function based ab initio calculations to investigate magnetic properties of two high spin Co(II) complexes Co(BDP^RP) (BDP^RP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R= ^t Bu for 9). Complexes 8 and 9 featuring effective D_3h symmetry were found to possess D=24.0 and 32.0 cm⁻¹, respectively, in their S=3/2 ground states of . Ligand field analyses revealed that the low-lying d-d excited states make either positive or vanishing contributions to D. Hence, total positive D values were measured for 8 and 9, as well as related D_3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C_3v congeners. In-depth analyses suggested that lowering symmetry from D_3h to C_3v induces orbital mixing between and and admixes excited state into the ground state. Both factors turn the total D value progressively negative with the increasing distance (δ) of the Co(II) center out of the equatorial plane. Therefore, δ determines the sign and magnitude of final D values of five-coordinate trigonal bipyramidal S=3/2 Co(II) complexes as measured for a series of such species with varying δ.