| Abstract
| - The results of self-consistent field (SCF) nonlocaldensity functional molecular orbital calculations arepresentedfor the various spin states and tautomeric forms of a cobalt complexwith two o-quinone-derived ligands. Inaddition, new variable-temperature solution magnetic susceptibility,EPR, and electronic absorption data arepresented to characterize the low-spin[CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] tohigh-spin[CoII(3,5-DTBSQ)2(phen)] valence-tautomeric interconversion, where3,5-DTBSQ- and 3,5-DTBCat2- arethe semiquinonate andcatecholate forms of3,5-di-tert-butyl-o-benzoquinone, respectively,and phen is 1,10-phenanthroline. The solutionmagnetic susceptibility data were fitted to give ΔH =2238 cm-1 and ΔS = 118.1 Jmol-1 K-1 for thels-CoIII⇌ hs-CoII equilibrium. Appreciablechanges are seen in the electronic absorption spectrum as the complexchangesbetween the two tautomeric forms. Unrestricted SCF calculationsgave J = −594 cm-1 for theparametercharacterizing the antiferromagnetic exchange interaction betweenhs-CoII ion (S =3/2) and each of the twocoordinated semiquinonate (S =1/2) ligands in thehs-CoII tautomer. The calculationsindicated that the ls-CoIIItautomer state is the most stable with an energy separation ofΔE = 4428 cm-1 (0.55 eV)between this ls-CoIIIstate and the S = 1/2 component ofthe hs-CoII spin ladder. ThisΔE value compares favorably with the ΔHvalueevaluated from variable-temperature susceptibility data.Thecalculations indicate that, while there are stilllocalizedelectronic structural features reflecting the different metal andligand oxidation states in the ls-CoIII andhs-CoIItautomeric forms, appreciable covalent interactions exist between thecobalt ion and the ligands. Finally, theresults of the calculations were used to assign the electronictransitions seen for the ls-CoIII andhs-CoII tautomers.
- The results of density functional molecular orbitalcalculations are presented to characterize the valence-tautomericconversion of low-spin[CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] tohigh-spin [CoII(3,5-DTBSQ)2(phen)].The ligands 3,5-DTBSQ- and3,5-DTBCat2- are the semiquinonate and catecholate formsof 3,5-di-tert-butyl-o-benzoquinone,respectively, and phen is 1,10-phenanthroline. The energydifference between these two forms of the complex is calculated, as isthe exchange parameter that characterizes the Co−SQ-magnetic exchange in the hs-CoIIcomplex.
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