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

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.

价互变异构（Valence tautomerism, VT）指发生于氧化还原活性配体与氧化还原活性金属之间的可逆受激分子内电子转移过程。双(二氧杂环戊烯)桥联双核钴配合物为实现可控的两步价互变异构转变提供了可行途径，其互变形式为{Co³⁺-儿茶酚根-儿茶酚根-Co³⁺} ⇌ {Co³⁺-儿茶酚根-半醌根-Co²⁺}⇌{Co²⁺-半醌根-半醌根-Co²⁺}（其中cat²⁻=儿茶酚根（catecholate），SQ•⁻=半醌根（semiquinonate））。双核价互变异构配合物的设计灵活性使其相较于两步自旋交叉配合物，在未来器件或材料应用中具备显著优势。本研究中的四种双核钴配合物，分别由去质子化的3,3,3′,3′-四甲基-1,1′-螺双(茚满)-5,5′,6,6′-四醇（spiroH₄）或3,3,3′,3′-四甲基-1,1′-螺双(茚满)-4,4′,7,7′-四溴-5,5′,6,6′-四醇（Br₄spiroH₄）作为桥联配体，并辅以Mentpa辅助配体（tpa=三(2-吡啶甲基)胺（tris(2-pyridylmethyl)amine），n=0~3对应吡啶环6位的甲基化程度）。配套的结构、磁学、光谱学及密度泛函理论（density functional theory, DFT）计算研究显示，四种钴配合物展现出不同的电子结构与价互变异构行为：分别观测到单电子单步部分价互变异构、两步价互变异构、不完全价互变异构，以及温度不变的{Co³⁺-儿茶酚根-儿茶酚根-Co³⁺}状态。通过电化学测试、密度泛函理论计算，以及对混合价态{Zn²⁺-儿茶酚根-半醌根-Zn²⁺}类似物的研究，得以阐明调控单步与两步价互变异构行为的热力学参数。价互变异构转变行为可通过两点进行合理解释：(1) 双(二氧杂环戊烯)配体内部的电子耦合程度；(2) 钴中心与二氧杂环戊烯配体的氧化还原电势匹配性。本工作通过引入具有足够弱分子内耦合作用的混合价II类及II-III类双(二氧杂环戊烯)桥联配体，为开发下一代两步价互变异构配合物指明了明确路径。