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

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 [Co III (3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [Co II (3,5-DTBSQ) 2 (phen)] valence-tautomeric interconversion, where 3,5-DTBSQ - and 3,5-DTBCat 2 - 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 Δ H = 2238 cm - 1 and Δ S = 118.1 J mol - 1 K - 1 for the ls -Co III ⇌ hs -Co II 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 exchange interaction between hs -Co II ion ( S = 3 / 2 ) and each of the two coordinated semiquinonate ( S = 1 / 2 ) ligands in the hs -Co II tautomer. The calculations indicated that the ls -Co III tautomer state is the most stable with an energy separation of Δ E = 4428 cm - 1 (0.55 eV) between this ls -Co III state and the S = 1 / 2 component of the hs -Co II spin ladder. This Δ E value compares favorably with the Δ H value evaluated from variable-temperature susceptibility data.The calculations indicate that, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the ls -Co III and hs -Co II tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. Finally, the results of the calculations were used to assign the electronic transitions seen for the ls -Co III and hs -Co II tautomers.