Synthesis of iron(III)-carbonyl complex with variable wavelength range for CO release depending on protonation and deprotonation of axial phosphorous ligands

Abstract

N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands—trans-[Fe(L-κ3N,C,S)(CO)(PR3)2]PF6 ([1]PF6, R = Me; PF6, R = OEt) are light-inducible CO releasers under ambient conditions. Optical wavelength ranges corresponding to such a CO release depend on the phosphorous ligands. Complex [1]PF6 responds to light with wavelengths shorter than 500 nm, while the photosensitivity of [2]PF6 is extended to wavelengths up to 800 nm, reaching the so-called phototherapeutic window. Theoretical and experimental studies suggest that the electron-donating ability of phosphorous ligands is essential to determine the wavelength range corresponding to photo-CO dissociation, although the steric difference in the phosphorous ligands for [1]PF6 and [2]PF6 hampers the exclusive evaluation of the electronic effect for CO dissociation from such complexes. A newly synthesized N,C,S-pincer iron(III) carbonyl complex with 1,3,5-triaza-7-phosphaadamantane (PTA) as axial ligands ([3]PF6) has shown variable photosensitivity for the dissociation of CO in response to protonation of the PTA ligand. Based on this finding, in the present study, [3]PF6 is used to confirm the photo-CO dissociation mechanism proposed for [1]PF6 and [2]PF6 and provide clues to molecular design for adjusting the ranges of optical wavelengths required for CO dissociation from trans-[Fe(PyBPT-κ3N,C,S)(CO)(PR3)2]PF6.