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

Abstract

The synthesis and characterization of the monocationic cobalt(III) catecholate complex [Co(L-N₄ ^t Bu₂ )(Cl₂ cat)]⁺ (L-N₄ ^t Bu₂ =N,N'-Di-tert.-butyl-2,11-diaza[3.3](2,6)pyridinophane, Cl₂ cat^2- =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-N₄ ^t Bu₂ )(Cl₂ cat)]⁺ 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.

Keywords: catecholate ligand; cobalt; semiquinonate ligand; tetraazamacrocyclic ligand; valence tautomerism.