Spin Crossover in an Iron(II) Complex with a Radical Iminosemiquinonate Ligand

Metal complexes that exhibit spin crossover (SCO) or valence tautomerism (VT) are promising candidates for molecular materials. In an exploration of the interplay between SCO and VT in iron chemistry, we combined redox-active aminophenol derivatives (4,6-di-tert-butylphenol (H2LH) or 2-anilino-4,6-di-tert-butylphenol (H2LPh)) with iron to prepare: [FeII(HLPhAP)-(tpa)]BPh4 (1), [FeII(LPhISQ)•-(tpa)]BPh4 (2), [FeII(HLHAP)-(tpa)]BPh4 (3), and [FeIII(LHISQ)•-(tpa)](BPh4)2 (4) (tpa = tris(2-pyridylmethyl)amine). The aminophenol derivatives were found to adopt either closed-shell monoanionic aminophenolate (HLAP)- or open-shell monoanionic radical iminosemiquinonate (LISQ)•- forms. Multitechnique analysis indicates 1 and 3 exist in the HS-FeII-(HLAP)- state, while 4 adopts the LS-FeIII-(LISQ)•- form. The FeII-(LPhISQ)•- tautomer is evident for 2, rather than FeIII-(LPhAP)2-, and incomplete SCO occurs in the solid state, from a mixture of LS-FeII-(LISQ)•- and HS-FeII-(LISQ)•- at low temperature to fully HS-FeII-(LISQ)•- at room temperature. In solution, Evans NMR measurements on 2 are indicative of HS-FeII-(LISQ)•- at room temperature, with low-temperature, frozen-solution EPR spectroscopy suggesting the dominant presence of the LS-FeII-(LISQ)•- species, consistent with SCO in solution as well as in the solid state. This family of complexes demonstrates oxidation- and protonation-state versatility, with the aminophenol ligands modulating the iron oxidation state while adopting distinct redox or protonation forms. Together, these results confirm one of the few known FeII-radical SCO systems in an octahedral environment and demonstrate that redox-active aminophenol-based ligands can facilitate the development of new molecular switches.