A New Type of Valence Tautomerism in Cobalt Dioxolene Complexes – Temperature‐Induced Transition from a Cobalt(III) Catecholate to a Low‐Spin Cobalt(II) Semiquinonate State

The synthesis and characterization of the monocationic cobalt(III) catecholate complex [Co(L‐N4tBu2)(Cl2cat)]+ (L‐N4tBu2=N,N’‐Di‐tert.‐butyl‐2,11‐diaza[3.3](2,6)pyridinophane, Cl2cat2−=4,5‐dichlorocatecholate) are presented. The complex exhibits valence tautomeric properties in solution; but, in contrast to the usually observed conversion from a cobalt(III) catecholate to a high‐spin cobalt(II) semiquinonate state, valence tautomerism of [Co(L‐N4tBu2)(Cl2cat)]+ leads to the formation of a low‐spin cobalt(II) semiquinonate complex upon raising the temperature. This new type of valence tautomerism for a cobalt dioxolene complex has been unambiguously established by a detailed spectroscopic investigation using variable‐temperature NMR, IR and UV‐Vis‐NIR spectroscopy. Determination of the enthalpies and entropies characterizing the valence tautomeric equilibria in various solutions shows that the influence of the solvent is almost exclusively entropic. A novel twist in temperature‐induced switching of electronic states in cobalt dioxolene complexes: Structural, magnetic, and spectroscopic results demonstrate that valence tautomerism can occur by converting a cobalt(III) catecholate into a low‐spin cobalt(II) semiquinonate complex instead of the usual high‐spin complex.