Slow magnetic relaxation in the tetrahedral cobalt(II) complexes [Co(EPh)4]2− (E[dbnd]O, S, Se)

Abstract

Salts of the homoleptic cobalt(II) complexes [Co(EPh)4]2− (E=O, S, Se) were isolated as (Ph4P)2[Co(OPh)4]·(CH3CN) (1), K(Ph4P)[Co(OPh)4] (2), (Ph4P)2[Co(SPh)4] (3), and (Ph4P)2[Co(SePh)4] (4) from reactions of CoCl2 or CoI2 with excess amounts of K(EPh) and (Ph4P)Br in acetonitrile. Single-crystal X-ray structural studies show all four compounds to contain mononuclear [Co(EPh)4]2− complexes with structures conforming to or approaching D2d symmetry. Magnetic susceptibility data for 1–4 indicate anisotropic S=3/2 spin ground states, with axial zero-field splitting parameters ranging from D=−11.1(3)cm−1 in 1 to −83(1) cm−1 in 4. Ac susceptibility measurements reveal slow magnetic relaxation at zero dc field for 2–4, while a bias dc field is required to see this effect in 1. Arrhenius plots of the data indicate spin reversal barriers of Ueff=21(1), 21(1), and 19(1)cm−1 for 1, 3, and 4, respectively, while the plot for 2 shows substantial curvature, indicative of strong intermolecular interactions. For 2, dilution with [Zn(OPh)4]2− was necessary to observe thermally-activated magnetic relaxation, with Ueff=34.0(5)cm−1. The trend in Ueff for 1–4 does not follow the trend in D values, possibly indicating that magnetic relaxation in 2–4 is not fully thermally activated under the conditions probed. Solid-state diffuse-reflectance spectra display d–d excitations that follow the general trend of D values. An analysis of 1–4 within the framework of ligand field theory shows that the increase in |D| occurs in concert with an decrease in the Racah B parameter, highlighting the importance of soft donor ligands in the pursuit of systems with a large magnetic anisotropy.