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

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 CoCp₂ 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 meso-position or metal center.