Synergistic Interplay between Two Iron Centers and a Doubly N-Confused Hexaphyrin Ligand for Highly Efficient Photocatalytic Water Oxidation

High Resolution Image Download MS PowerPoint Slide Transition-metal complexes featuring multiple redox-active centers have the ability to cooperatively accumulate positive charges, making them promising candidates for water oxidation catalysis (WOCs). Herein, we report the high water oxidation activity and detailed reaction mechanism of a dinuclear iron complex bearing a doubly N-confused hexaphyrin ligand ([Fe 2 (DNCH)]) in a photocatalytic system employing tris(2,2′-bipyridine)ruthenium(II) ([Ru II (bpy) 3 ] 2+ ) as a photosensitizer and peroxydisulfate ion (S 2 O 8 2 − ) as a sacrificial oxidant. Furthermore, we synthesized and investigated the catalytic performance of the heterobimetallic analogue [FeZn(DNCH)], in which one of the Fe III ions of [Fe 2 (DNCH)] is substituted with the redox-inactive Zn II . Among the molecular WOCs previously studied under similar photocatalytic conditions, [Fe 2 (DNCH)] showed a high turnover frequency (TOF = 8.7 s −1 ) and turnover number (TON = 2730). Notably, [Fe 2 (DNCH)] displayed a sixfold enhancement in TOF and a fifteenfold increase in TON relative to [FeZn(DNCH)] (TOF = 1.4 s −1, TON = 180). Kinetic analyses of water oxidation reactions catalyzed by [Fe 2 (DNCH)] and [FeZn(DNCH)] support the hypothesis that O−O bond formation proceeds via an atom-proton-transfer (APT) mechanism. Electrochemical measurements revealed that both [Fe 2 (DNCH)] and [FeZn(DNCH)] promote water oxidation at potentials up to approximately 1.2 V, comparable to the Ru II /Ru III redox couple of [Ru II (bpy) 3 ] 2+ . Water oxidation by [Fe 2 (DNCH)] and [FeZn(DNCH)] involves the removal of four and three electrons, respectively, suggesting that the [Fe 2 (DNCH)] system generates a more electrophilic oxidized species. Density functional theory (DFT) calculations further reveal that the DNCH ligand acts as an effective electron-accepting scaffold, stabilizing key transition states during the O−O bond-forming steps. Moreover, the Fe IV =O moiety plays a pivotal role in reducing the overall energy barrier, thereby facilitating a markedly faster water-oxidation process relative to its FeZn counterpart. Our study revealed that the high TOF and TON values observed for [Fe 2 (DNCH)] originate from the synergistic interplay between the dinuclear iron centers and the π-conjugated ligand.