Density Functional Study of the Valence-Tautomeric Interconversion Low-Spin [CoIII(SQ)(Cat)(phen)] ⇌ High-Spin [CoII(SQ)2(phen)

Abstract

The results of self-consistent field (SCF) nonlocal density functional molecular orbital calculations are presented for the various spin states and tautomeric forms of a cobalt complex with two o-quinone-derived ligands. In addition, new variable-temperature solution magnetic susceptibility, EPR, and electronic absorption data are presented to characterize the low-spin [CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [CoII(3,5-DTBSQ)2(phen)] valence-tautomeric interconversion, where 3,5-DTBSQ- and 3,5-DTBCat2- are the semiquinonate and catecholate forms of 3,5-di-tert-butyl-o-benzoquinone, respectively, and phen is 1,10-phenanthroline. The solution magnetic susceptibility data were fitted to give ΔH = 2238 cm-1 and ΔS = 118.1 J mol-1 K-1 for the ls-CoIII ⇌ hs-CoII equilibrium. Appreciable changes are seen in the electronic absorption spectrum as the complex changes between the two tautomeric forms. Unrestricted SCF calculations gave J = −594 cm-1 for the parameter characterizing the antiferromagnetic exc...