Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

Finding photostable, first-row transition metal-based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)(3)](2+) as photosensitizer and [S2O8](2-) as sacrificial electron acceptor. Electron-poorer Ni-porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron-poorer porphyrins afforded 6-fold higher turnover frequencies (TOFs; ca. 0.65 min(-1)) than electron-richer analogues. However, the electron-poorest Ni-porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O-2 evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)(3)](3+). Critically, these Ni-porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1-3 % less O-2 than during the first photocatalytic run.

Automated summary

Finding photostable first-row transition metal-based molecular systems for photocatalytic water oxidation is a key step towards sustainable solar fuel production. Nickel(II) hydrophilic porphyrins have been discovered as molecular catalysts for this purpose, showing high turnover frequencies but with issues in the catalytic activity of the electron-poorest Ni-porphyrin.