Redox Activity and Two-Step Valence Tautomerism in a Family of Dinuclear Cobalt Complexes with a Spiroconjugated Bis(dioxolene) Ligand

A family of dinuclear cobalt complexes with bridging bis(dioxolene) ligands derived from 3,3,3′,3′-tetramethyl-1,1′-spirobis(indane-5,5′,6,6′-tetrol) (spiroH4) and ancillary ligands based on tris(2-pyridylmethyl)amine (tpa) has been synthesized and characterized. The bis(dioxolene) bridging ligand is redox-active and accessible in the (spirocat–cat)4–, (spiroSQ–cat)3–, and (spiroSQ–SQ)2– forms, (cat = catecholate, SQ = semiquinonate). Variation of the ancillary ligand (Mentpa; n = 0–3) by successive methylation of the 6-position of the pyridine rings influences the redox state of the complex, governing the distribution of electrons between the cobalt centers and the bridging ligands. Pure samples of salts of the complexes [Co2(spiro)(tpa)2]2+ (1), [Co2(spiro)(Metpa)2]2+ (2), [Co2(spiro)(Me2tpa)2]2+ (3), [Co2(spiro)(Me3tpa)2]2+ (4), [Co2(spiro)(tpa)2]3+ (5), and [Co2(spiro)(tpa)2]4+ (6) have been isolated, and 1, 4, and 6 have been characterized by single crystal X-ray diffraction. Studies in the solid and solution states using multiple techniques reveal temperature invariant redox states for 1, 2, and 4–6 and provide clear evidence for four different charge distributions: 1 and 2 are CoIII-(spirocat–cat)-CoIII, 4 is CoII-(spiroSQ–SQ)-CoII, 5 is CoIII-(spiroSQ–cat)-CoIII, and 6 is CoIII-(spiroSQ–SQ)-CoIII. Of the six complexes, only 3 shows evidence of temperature dependence of the charge distribution, displaying a rare thermally induced two-step valence tautomeric transition from the CoIII-(spirocat–cat)-CoIII form to CoII-(spiroSQ–cat)-CoIII and then to CoII-(spiroSQ–SQ)-CoII in both solid and solution states. This is the first time a two-step valence tautomeric (VT) transition has been observed in solution. Partial photoinduction of the VT transition is also possible in the solid. Magnetic and spectroscopic studies of 5 and 6 reveal that spiroconjugation of the bis(dioxolene) ligand allows electronic interaction across the spiro bridge, suggesting that thermally activated vibronic coupling between the two cobalt-dioxolene moieties plays a key role in the two-step transition evident for 3.