Thermal- and light-induced valence tautomerism with a concerted spin transition in an iron tris(diimine) complex

The switching phenomena of spin crossover (SCO) and valence tautomerism (VT) are respectively dominated by iron(ii) and cobalt-dioxolene systems. To explore new possibilities for SCO or VT, the redox-active α-diimine ligand bis((phenyl)imino)acenaphthene (Ph-BIAN), which can adopt neutral (L⁰), monoanionic (L˙^-), and dianionic (L^2-) states, was paired with zinc, cobalt, manganese and iron to give [M(Ph-BIAN)₃](BPh₄)₂ (M = Zn (1), Co (2), Mn (3), Fe (4)). Compounds 1, 2 and 3 adopt a temperature invariant M^II-(L⁰)₃ state, (2 and 3 are high spin (HS)) in the solid- and solution-states. Electrochemical measurements show the metal controls the degree of electronic communication between the Ph-BIAN ligands. In stark contrast to 1, 2 and 3, compound 4 adopts the LS-Fe^III-(L˙^-)(L⁰)₂ (LS = low spin) tautomeric form as the ground state in both the solid-state and in solution. Combined variable temperature solid- and solution-state structural, Mössbauer and electronic spectroscopy, and magnetic measurements, show that 4 undergoes the thermally-induced VT process LS-Fe^III-(L˙^-)(L⁰)₂ ⇌ HS-Fe^II-(L⁰)₃, the only example of VT accompanied by a concerted spin transition in an iron complex with a redox-active ligand. Solid-state photomagnetic measurements suggest that the VT interconversion is also induced by light. Light-induced VT has not been previously observed for complexes other than cobalt-dioxolene, and is potentially afforded here by the unique spin-state change that results in large differences in the Fe-N bond lengths for the two valence tautomers. This study introduces a new example of VT, and suggests that optically-induced VT can be displayed by iron systems, opening alternate pathways toward molecular switches that can be controlled with multiple stimuli.