Ferrocenyl PNNP Ligands-Controlled Chromium Complex-Catalyzed Photocatalytic Reduction of CO to Formic Acid.

3d-transition metal complexes have been gaining much attention as promising candidates for photocatalytic carbon dioxide (CO) reduction systems. In contrast to the group 7-12 elements, Cr in group 6 has not yet been investigated as the catalyst of CO photoreduction because of its intrinsic disadvantages. Cr has a weak reducing ability due to an insufficient number of d electrons and high Lewis acidity which may deactivate the catalyst by strong coordination with a product formate. To overcome these drawbacks, we rationally designed molecular Cr complexes bearing ferrocenyl PNNP tetradentate ligands (, , , and ). These Cr complexes selectively converted CO into formic acid (HCOH) under photocatalytic conditions and, to our knowledge, represent the first molecular Cr catalysts for CO photoreduction. The best catalyst achieved a turnover number of 1180 for HCOH formation with 86% selectivity after 48 h of light irradiation, with a combined use of an organic photosensitizer. Electrochemical and continuous UV-vis absorption analyses clarified the sequential reaction pathways involving multielectron reduction and protonation of a Cr complex. Moreover, through detailed computational studies, photoinduced electron transfer mediated by ferrocenyl groups and intramolecular proton transfer attributed to hemilabile phosphine ligands would be key to the efficient catalysis that overwhelms the inherent disadvantages of Cr.