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.