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Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

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journal contribution
posted on 02.06.2020, 23:05 by Gemma K. Gransbury, Brooke N. Livesay, Jett T. Janetzki, Moya A. Hay, Robert W. Gable, Matthew P. Shores, Alyona Starikova, Colette Boskovic
Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis­(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII}⇌{CoII-SQ-SQ-CoII} (cat2– = catecholate, SQ•– = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3′,3′-tetramethyl-1,1′-spirobi­(indan)-5,5′,6,6′-tetraol (spiroH4) or 3,3,3′,3′-tetramethyl-1,1′-spirobi­(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris­(2-pyridylmethyl)­amine, n = 0–3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII} states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {ZnII-cat-SQ-ZnII} analog have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis­(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis­(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.