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

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.

A series of tetrahedral cobalt(II) complexes, [Co(EPh)4]2− (EO, S, Se), were prepared and characterized as mononuclear single-molecule magnets. The magnetic anisotropies and dynamic magnetic properties of the complexes were characterized to probe the influence of the ligand donor atoms on the magnetic behaviors.Figure optionsDownload as PowerPoint slide