Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN⁰ (L⁰), monoanionic radical Ar-BIAN^•- (L^•-), and dianionic Ar-BIAN^2- (L^2-) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN)₃] complexes (Ar = Ph, n = 2 (1), 1 (1), 0 (1); Ar = 3,5-CF₃-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (3), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-Co^II-L⁰ containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-Co^III-(L^•-)₃ ⇌ HS-Co^II-(L^•-)₂(L⁰) VT interconversion. Experimentally, structural and magnetic data reveal a HS-Co^II-L⁰ containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-Co^II-L⁰-containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-Co^II-(L^•-)₂(L⁰) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-Co^III-(L^•-)₃ ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.