Finding photostable,first-row transition metal-based molecular systems for photocatalytic water oxidation is as tep 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 [S 2 O 8 ] 2À as sacrificial electron acceptor.E lectron-poorer Niporphyrins bearing 8f luorine or 4methylpyridinium substituents as electron-poorer porphyrins afforded 6-fold higher turnover frequencies (TOFs;c a. 0.65 min À1 )t han electronricher analogues.H owever,t he 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 Vv s. NHE) to be driven by the photochemically generated [Ru(bpy) 3 ] 3+ .C ritically,these Ni-porphyrin catalysts showed excellent stability in photocatalytic conditions,a sas econd photocatalytic run replenished with an ew dose of photosensitizer,a fforded only 1-3 %less O 2 than during the first photocatalytic run.[*] C. Liu, D. van den Bos, B. den Hartog, D. van der Meij, A

Finding photostable,first-row transition metal-based molecular systems for photocatalytic water oxidation is as tep 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 [S 2 O 8 ] 2À as sacrificial electron acceptor.E lectron-poorer Niporphyrins bearing 8f luorine or 4methylpyridinium substituents as electron-poorer porphyrins afforded 6-fold higher turnover frequencies (TOFs;c a. 0.65 min À1 )t han electronricher analogues.H owever,t he 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 Vv s. NHE) to be driven by the photochemically generated [Ru(bpy) 3 ] 3+ .C ritically,these Ni-porphyrin catalysts showed excellent stability in photocatalytic conditions,a sas econd photocatalytic run replenished with an ew dose of photosensitizer,a fforded only 1-3 %less O 2 than during the first photocatalytic run.[*] C. Liu, D. van den Bos, B. den Hartog, D. van der Meij, A