CO Release from N,C,S-Pincer Iron(III) Carbonyl Complexes Induced by Visible-to-NIR Light Irradiation: Mechanistic Insight into Effects of Axial Phosphorus Ligands

Light-induced CO release from newly synthesized N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands- trans-[Fe(L-κ³ N,C,S)(CO)(PR₂R')₂]PF₆ ([1]PF₆, R = Me, R' = Ph; [2]PF₆, R = R' = Me; [3]PF₆, R = R' = OEt)-were investigated. All the iron(III) carbonyl complexes were stable in solution and showed light-inducible CO release under ambient conditions. Studies on the wavelength dependence of photoreaction revealed that the phosphite complex [3]PF₆ exhibited the most extended photosensitivity including all visible and a part of near-IR light (390-800 nm wavelengths). The phosphine complexes [1]PF₆ and [2]PF₆ showed sensitivity to only the higher-energy region of visible light (390-450 nm). Quantum-chemical calculations and spectroscopic data suggested that all complexes [1]PF₆-[3]PF₆ have dπ-dπ excitation modes to depopulate Fe-C(carbonyl) bonding and potentially induce the CO release by irradiation of light in the near-IR region, although moderately weakened Fe-C(carbonyl) bonding due to stronger π-backbonding by the phosphite ligand rendered the excitation effective on the CO release exclusively in [3]PF₆.