Artificial photosynthesis via two-electron conversion: Photochemical oxygenation sensitized by ruthenium porphyrins with water as both electron and oxygen atom donor

Abstract

Recent progress in the field of artificial photosynthesis is reviewed. Among various approaches to oxidizing the water molecule, attention has been focused on the two-electron chemical conversion processes mediated by metallo-porphyrins upon visible light irradiation. Photochemical oxygenation reactions such as the epoxidation of alkenes sensitized by ruthenium(II) porphyrins with water as both electron and oxygen atom donor have been found. Ru porphyrin induces highly efficient two-electron oxidation of water with a quantum yield of 60 % to form an epoxide from the alkene with high selectivity. The water molecule serves as both an efficient electron and oxygen atom donor in the reaction, in which the oxygen atom of water is incorporated in a useful product, i.e., epoxide.

Photochemical oxygenation of cyclohexene with water sensitized by aluminium(III) porphyrins with visible light

“…They are relatively low compared to those obtained with Ru(II)TMP [14,15]. When other substrates such as norbornene (E 1/2 = 2.17 Volt vs SHE), styrene (E 1/2 = 1.92 Volt vs SHE), and 1-hexene (E 1/2 = 2.71 Volt vs SHE) were examined for the photochemical oxygenation sensitized by Al(III)TMP, very interestingly, almost no reaction was observed, indicating the sensitizer Al(III)TMP exhibited a high selectivity for cyclohexene (E 1/ 2 = 2.08 Volt vs SHE).…”

“…Since cyclohexene was the most reactive substrate in the case of Ru(II) TMP, the lower oxidation ability of Al(III)TMP might not sensitize the oxygenation of other less reactive substrates. In the case of styrene with similar oxidation potential with cyclohexene, steric factor of substrate might be also operating as observed in the case of RuTMP(CO) [14,15].…”

“…(Table 2) The photochemical oxygenation proceeds under either Q-band (560 nm) or Soret-band (420 or 430 nm) excitation. As in the case of Ru(II)TMP, a weakly alkaline condition ([OH À ] <1.5 mM) favoured the epoxide (1) formation [14][15][16][17], while the alcohol (3) was selectively formed under a stronger alkaline condition ([OH À ] = 5 mM). (Table 2) The selectivity of the "epoxide" formation increased with decreasing [OH À ] ( Table 2).…”

“…Among the problems to be resolved for the realization of artificial photosynthesis, the way in which water molecules could be incorporated at the oxidation terminus of the system is one of the most crucial issues [1][2][3][4][5][6][7]. Among various approaches for the oxidation of water upon light irradiation, i.e., by (1) a one-electron process, (2) a two-electron one, (3) or a four-electron one, we have recently focused our attention on the two-electron oxidation of water by one-photon excitation to form oxygenated products of various substrates sensitized by metalloporphyrins [7][8][9][10][11][12][13][14][15][16][17]. Eq.…”

“…For example, we have found that the visible light irradiation of a reaction mixture of CO-coordinated tetra(2,4,6-trimethyl) phenylporphyrinatoruthenium(II) (Ru II TMP(CO)) as a sensitizer, hexachloroplatinate (IV) as an electron acceptor, and an alkene as the substrate in alkaline aqueous acetonitrile induces the highly selective epoxidation of the alkene, with a high quantum yield ($60%) under deaerated conditions [14,15]. Detailed mechanistic study mainly by means of laser flash photolysis has revealed that the water molecule is activated through axial ligation to the one-electron-oxidized Ru-porphyrin to induce an oxygen atom transfer to the substrate with two-electron conversion followed by the second one-electron transfer to the electron acceptor [16,17].…”