Coordination Induced Spin State Transition Switches the Reactivity of Nickel (II) Porphyrin in Hydrogen Evolution Reaction

Abstract Electron spin plays a critical role in chemical processes, particularly in reactions involving metal complexes with unpaired electrons. However, more definitive state‐to‐state experiments are needed to better elucidate the role of electronic spin. Herein, we chose nickel (II) 5,10,15,20‐tetrakis(pentafluorophenyl) porphyrin 1 as a catalyst, which allows switching from a low spin to a high spin state of Ni (II) center through an axial pyridine coordination, for electrocatalytic hydrogen evolution reaction (HER). When pyridine is present, we observed β ‐hydrogenation of porphyrin through electron transfer followed by proton transfer. In contrast, hydrogen evolution mainly occurs via the concerted proton‐coupling electron transfer without pyridine coordination. Similar distinct spin‐dependent selectivity was also observed in chemical reduction of 1 by CoCp 2 with subsequent addition of pyridinium p ‐toluenesulfonate. Computational calculations using density functional theory demonstrated that the transition from low spin to high spin state enriches the ligand's electron density after one‐electron reduction, leading to preferential protonation of β ‐periphery rather than mes o‐position or metal center.