Mixed-valence linear CoIII2Co^II complexes with easy-axis magnetic anisotropy: Experimental and theoretical investigation

An <i>in situ</i> generated ligand formed by the Schiff base condensation of 3,5-di-<i>tert</i>-butylsalicylaldehyde and <i>R</i>(-) or <i>S</i>(+) 2-phenylglycinol was reacted with cobalt(II) acetate tetrahydrate, affording mixed-valence enantiomerically pure [Co^IICoIII2(<i>R</i> or <i>S</i>-L¹)₂ (L²)₂(CO₂CH₃)₂] complexes 1-R and 1-S {where L¹ = (<i>R</i>) or (<i>S</i>)-2,4-di-<i>tert</i>-butyl-6-(4-hydroxo-3-phenylbut-1-en-1-yl)phenolate and L² = (<i>R</i>) or (<i>S</i>) 2-phenylglycinolate} with linear trinuclear metal cores. X-ray structural and BVS analysis revealed the linear arrangement of metal cores with the central cobalt ion in a +2 oxidation state, while the terminal cobalt ions are in a +3 oxidation state. The fitting of the DC magnetic data revealed easy-axis magnetic anisotropy with <i>D</i> = -58.6 cm^-1 (<i>E</i>/<i>D</i> = 0.257) for the Co(II) ion in 1-R. Ac magnetic susceptibility measurements confirm the field-induced dynamic magnetic behavior of 1-R with the relaxation of magnetization, which occurs through a combination of Raman and direct processes. Further CASSCF-based <i>ab initio</i> and NEVPT2 theoretical investigations on the X-ray structure of 1-R (or 1-S) yielded <i>D</i> values of -60.0 cm^-1 (<i>E</i>/<i>D</i> = 0.276) and -57.4 cm^-1 (<i>E</i>/<i>D</i> = 0.296), respectively, and helped rationalize our experimental findings.