Design, Synthesis, and Modulation of Valence Tautomeric Binuclear Cobalt Compounds with Redox-Active Bridged Ligands

Valence tautomeric (VT) compounds are a kind of bistable materials characterized by intramolecular electron transfer (ET). This study focuses on the controlled synthesis and performance modulation of compounds exhibiting VT behavior. Through the design of redox-active ligands and the adoption of auxiliary ligands and counter-anions, three compounds are successfully prepared, that is, [{Co(tpa)}₂(L^cat−cat)](PF₆)₂·8H₂O·4CH₃OH (1), [{Co(tpa)}₂(L^cat−cat)](BPh₄)₂·2CH₂Cl₂ (2), and [{Co(dpa^OMe)}₂(L^cat−cat)](BPh₄)₂·6CH₃CN (3), respectively, where L^cat−cat = 6,6′-((1 E,1′E)-hydrazine-1,2-diylidenebis(methaneylylidene)) bis(3,5-di-tert-butylbenzene-1,2-diol), tpa = tris(2-pyridinylmethyl) amine, and dpa^OMe = 2-methoxy-N,N-bis(pyridine-2-ylmethyl)aniline. Structural, magnetic, and spectroscopic analyzes reveal that 1 and 2 remain in the {Co^III-cat-cat-Co^III} configuration, whereas 3 undergoes VT transition near 350 K. Comparative studies highlight that the modification of auxiliary ligand from tpa to dpa^OMe significantly enhances the intramolecular ET capability. This finding provides valuable insights into how ligand engineering regulates VT behavior, offering a foundation for the design of advanced functional materials.