Spin Crossover and Valence Tautomerism in Neutral Homoleptic Iron Complexes of Bis(pyridylimino)isoindolines

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe₂) at the 4‐positions of the pyridine moieties have been prepared and studied with regard to temperature‐dependent spin and redox states by a combination of ⁵⁷Fe Mössbauer spectroscopy, SQUID magnetometry, single‐crystal X‐ray diffraction analysis, X‐band EPR, and ¹H NMR spectroscopy. While the H‐, methyl‐, and ethyl‐substituted complexes remain in a pure high‐spin state irrespective of the temperature, the 4‐tert‐butyl‐substituted derivative shows spin‐crossover behavior. The methoxy‐ and dimethylamino‐substituted compounds were found to easily undergo oxidation. In the crystalline state, valence tautomeric behavior was observed for the methoxy derivative as a thermally activated charge‐transfer transition, accompanied by a spin crossover above 200 K. The valence tautomerism leads to a chelate with one of the bpi ligands as a dianion radical L^2?. and with an effective spin of S=2.