DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes

The family of complexes of general formula [Co­(Mentpa)­(Xdiox)]+ (tpa = tris­(2-pyridylmethyl)­amine, n = 0–3 corresponds to successive methylation of
the 6-position of the pyridine rings; X = Br4, Cl4, H4, 3,5-Me2, 3,5-tBu2; diox = dioxolene) was investigated by density functional
theory (DFT) calculations to predict the likelihood of valence tautomerism
(VT). The OPBE functional with relativistic and solvent corrections
allowed accurate reproduction of trends in spin-state energetics,
affording the prediction of VT in complex [Co­(Me3tpa)­(Br4diox)]+ (1+). One-electron oxidation of neutral precursor [CoII(Me3tpa)­(Br4cat)] (1) enabled isolation
of target compounds 1­(PF6) and 1­(BPh4). Solution variable-temperature UV–vis absorption and Evans
method magnetic susceptibility data confirm DFT predictions that 1+ exists in a temperature-dependent
valence tautomeric equilibrium between low-spin Co­(III)-catecholate
and high-spin Co­(II)-semiquinonate forms. The solution VT transition
temperature of 1+ is solvent-tunable
with critical temperatures in the range of 291–359 K for the
solvents measured. Solid-state magnetic susceptibility measurements
of 1­(PF6) and 1­(BPh4) reveal the onset
of VT transitions above room temperature.