Tuning valence tautomerism in a family of dinuclear cobalt complexes incorporating a conjugated bridging bis(dioxolene) ligand with weak communication.

Valence tautomerism (VT) involves the stimulated reversible intramolecular electron transfer between a redox-active metal and ligand. Dinuclear cobalt complexes bridged by bis(dioxolene) ligands can undergo thermally-induced VT with access to {Co-cat-cat-Co}, {Co-cat-SQ-Co} and {Co-SQ-SQ-Co} states (cat = catecholate, SQ˙ = semiquinonate, Co refers to low spin Co, Co refers to high spin Co). The resulting potential for two-step VT interconversions offers increased functionality over mononuclear examples. In this study, the bis(dioxolene) ligand 3,3',4,4'-tetrahydroxy-5,5'-dimethoxy-benzaldazine (thMH) was paired with Metpa (tpa = tris(2-pyridylmethyl)amine, = 0-3 corresponds to methylation at 6-position of the pyridine rings) to afford [{Co(Metpa)}(thM)](PF) (1a, = 0; 2a, = 2; 3a, = 3). Structural, magnetic susceptibility and spectroscopic data show that 1a and 3a remain in the temperature invariant {Co-cat-cat-Co} and {Co-SQ-SQ-Co} forms in the solid state, respectively. In contrast, 2a exhibits incomplete thermally-induced VT between these two tautomeric forms the mixed {Co-cat-SQ-Co} tautomer. In solution, room temperature electronic absorption spectra are consistent with the assignments from the solid-state, with VT observed only for 2a. From electrochemistry, the proximity of the two 1e-processes for the thM ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thM containing complexes. Comparison of the redox potentials of the Co and thM processes suggests that only 2a has these processes in sufficient proximity to afford the thermally-induced VT observed experimentally. Density functional theory calculations are consistent with the prerequisite energy ordering for a two-step transition for 2a, and temperature invariant {Co-cat-cat-Co} and {Co-SQ-SQ-Co} states for 1a and 3a, respectively. This work presents the third example, and the first formally conjugated example, of a bridging bis(dioxolene) ligand that can afford two-step VT in a Co complex, suggesting new possibilities towards applications based on multistep switching.