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

Abstract

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^III-cat-cat-Co^III}, {Co^III-cat-SQ-Co^II} and {Co^II-SQ-SQ-Co^II} states (cat²⁻ = catecholate, SQ˙⁻ = semiquinonate, Co^III refers to low spin Co^III, Co^II refers to high spin Co^II). 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 (thMH₄) was paired with Me_ntpa (tpa = tris(2-pyridylmethyl)amine, n = 0–3 corresponds to methylation at 6-position of the pyridine rings) to afford [{Co(Me_ntpa)}₂(thM)](PF₆)₂ (1a, n = 0; 2a, n = 2; 3a, n = 3). Structural, magnetic susceptibility and spectroscopic data show that 1a and 3a remain in the temperature invariant {Co^III-cat-cat-Co^III} and {Co^II-SQ-SQ-Co^II} forms in the solid state, respectively. In contrast, 2a exhibits incomplete thermally-induced VT between these two tautomeric forms via the mixed {Co^III-cat-SQ-Co^II} 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 thMⁿ⁻ ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thMⁿ⁻ containing complexes. Comparison of the redox potentials of the Co and thMⁿ⁻ 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^III-cat-cat-Co^III} and {Co^II-SQ-SQ-Co^II} 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.