Cobalt Complexes with Aminophenolate Ligands: Spin Crossover and Ligand Versatility

A search for switchable molecules has afforded a family of cobalt complexes featuring derivatives of 2-aminophenol: 4,6-di-tert-butyl aminophenol (H₂L_H) and 2-anilino-4,6-di-tert-butyl aminophenol (H₂L_Ph). The heteroleptic cobalt complexes incorporate a Metpa ligand (tpa = tris(2-pyridylmethyl)amine; n = 0-3), which involves the methylation of the 6-position of the pyridine ring). Eight members of this family have been synthesized and characterized: [Co^II(HL_Ph^AP)^-(tpa)](BPh₄) (1), [Co^II(HL_Ph^AP)^-(Me₂tpa)](BPh₄) (2a), [Co^II(L_Ph^ISQ)(Me₂tpa)](BPh₄) (2b), [Co^II(HL_Ph^AP)^-(Me₃tpa)](BPh₄) (3a), [Co^II(L_Ph^ISQ)(Me₃tpa)](BPh₄) (3b), [Co^III(HL_H^AP)^-(tpa)] (BPh₄)(ClO₄) (4a), [Co^III(L_H^ISQ)(tpa)](BPh₄)(ClO₄) (4b) and [Co^II(HL_H^AP)^-(Me₂tpa)](BPh₄) (5), where the aminophenol-derived ligands are monoanionic in either the open shell radical iminosemiquinonate (L^ISQ) or the closed shell protonated aminophenolate (HL^AP)^-. We isolate high-spin Co(II) and low-spin Co(III) compounds with no thermal interconversions evident in the solid state. In contrast, variable temperature electronic spectra for solutions of 3b suggest Co(II) spin crossover. Density functional theory calculations employing the M06L-D4/def2-TZVPP and TD-CAM-B3LYP/def2-TZVPP levels of theory support the experimental findings. This work presents the first example of a cobalt-iminosemiquinonate complex to exhibit spin crossover. The electronic structure versatility of complexes that integrate redox-active metal ions with both low- and high-spin states, alongside ligands that can exist in various redox and protonation states, holds significant promise for applications in molecular materials and chemical reactivity.