Metalloporphyrin-sensitized photoredox reactions: mechanistic studies on the role of axial ligands on photoreactivity

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An investigation of the tin(IV) porphyrin-sensitized photoredox reactions between triphenylphosphine (Ph 3P) as an electron donor and methyl viologen (MV 2+) as an electron acceptor is reported. The reaction is induced only in the presence of water molecules as axial ligands. A number of different axially ligated porphyrin complexes sensitize this reaction with quantum efficiencies that depend strongly on the ligand, pH and the concentrations of the various reagents involved. The reactivities of the sensitizers are in the order SnTPP(OH) 2SnTPP(OPPh 3) 2SnTPP(H 2O) 2SnTPP(H 2O)(Ph 3P) ⪢ SnTPP(Ph 3P) 2 (TPP  tetraphenylporphyrin). The limiting quantum yield of MV + production with SnTPP(OH) 2 is 0.3, while that with SnTPP(Ph 3P) 2 is almost zero. Kinetic and flash photolysis investigations indicate that an oxidative quenching of the triplet tin porphyrin by MV 2+ initiates the photoredox reactions. The key to obtaining net redox conversion in each case is the dark oxidation of a partially (one-electron) oxidized Ph 3P—water (or Ph 3P—hydroxide adduct to Ph 3PO by MV 2+. The photoreactivity is strongly affected by the rates of coordination of potential ligands in the reaction system to an unstable metalloporphyrin with a vacant coordination site which is generated in the course of the reaction. Hydroxide ion can reproduce the reactive SnTPP(OH) 2 by effective coordination to the vacant species which maintains a constant build-up of MV + .