Tuning Valence Tautomerism in Dinuclear Cobalt Complexes by Modulating Communication in the Bridging Ligand

The capability of bis(dioxolene) ligands to access multiple redox states makes them ideal candidates to tune the electronic properties of metal complexes, for example to achieve valence tautomerism (VT). In this study, a family of dinuclear cobalt complexes have been isolated with the bridging bis(dioxolene) theaⁿ⁻ ligand in the cat²⁻-cat²⁻, cat²⁻-SQ⋅⁻ and SQ⋅⁻-SQ⋅⁻ states (cat²⁻=catecholate, SQ⋅⁻=semiquinonate): [{Co^III(tpa)}₂(thea^cat−cat)](PF₆)₂ (1), [{Co^III(tpa)}₂(thea^cat−SQ)](PF₆)₃ (2), and [{Co^III(tpa)}₂(thea^SQ−SQ)](PF₆)₄ (3) (theaH₄=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene, tpa=tris(2-pyridylmethyl)amine). Multi-technique analysis confirms that 1, 2 and 3 adopt low spin-Co^III containing {Co^III-cat-cat-Co^III}, {Co^III-cat-SQ-Co^III} and {Co^III-SQ-SQ-Co^III} states, respectively. Compound 1 undergoes thermally-induced {Co^III-cat-cat-Co^III} ⇌ {Co^III-cat-SQ-Co^II} VT in the solid- and solution-states above 300 K, involving high spin-Co^II. This interconversion is contrary to expectations, as tpa typically stabilizes low spin-Co^III-catecholate. Compound 2 is mixed-valence class II/II–III, indicating a localized electronic structure, with electron transfer faster than the EPR and solvent rearrangement timescales, but slower than the infrared timescale. Compound 3 exhibits strong antiferromagnetic exchange. The overlap between the dioxolene π-orbitals in theaⁿ⁻ increases the accessibility of the cat²⁻-SQ⋅⁻ state, resulting in VT for 1. This study demonstrates that theaⁿ⁻ can be isolated in multiple oxidation states in metal complexes, which is promising for applications in magnetic materials.