How is the water molecule activated on metalloporphyrins? Oxygenation of substrates induced through one-photon/two-electron conversion in artificial photosynthesis by visible light

Abstract

The reaction mechanism of the highly efficient (phi = 0.60), selective photochemical epoxidation of alkenes sensitized by CO-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) (Ru(II)TMP(CO)), with water acting both as an electron and oxygen atom donor, was investigated. The steady-state light irradiation of the reaction mixture indicated the formation of the Ru(II)TMP (CO) cation radical under neutral conditions, which was effectively trapped by an hydroxide ion to regenerate the starting sensitizer. By means of a laser flash photolysis experiment, the formation of the cation radical as the primary process from the triplet excited state of Ru(II)TMP(CO) was clearly observed. Four kinds of transients were detected in completely different ranges of the delay time: the excited triplet state of Ru(II)TMP(CO) [delay time region <20 micros], the cation radical of Ru(II)TMP(CO)(CH3CN) [20-50 micros], the hydroxyl-coordinated Intermediate [I] [50-200 micros], and the cyclohexane-attached Intermediate [II] [200 micros-8 ms]. A reaction mechanism was revealed that involves RuTMP(CO) cation radical formation from the triplet excited state of the sensitizer, followed by attack of an hydroxide ion to form an hydroxyl-coordinated Ru-porphyrin (Intermediate [I]) and subsequent reaction with cyclohexene to form Intermediate [II]. The kinetics for each step of the successive processes was carefully analyzed and their rate constants were determined. The two-electron oxidation of water by one-photon irradiation, as revealed in the photochemical epoxidation, is proposed to be one of the more promising candidates to get through the bottleneck of water oxidation in artificial photosynthesis.

Efficient Singlet–Singlet Energy Transfer in a Novel Host–Guest Assembly Composed of an Organic Cavitand, Aromatic Molecules, and a Clay Nanosheet

“…Importantly, we believe that the studies presented here using a novel supramolecular assembly have expanded the types of molecules that could serve as candidates for efficient energy-transfer systems, such as in an artificial lightharvesting system. This light-harvesting system, when combined with the photochemical reactions where water molecules serve as electron and oxygen donors, 38 will open a new door to realizing the construction of new types of artificial photosynthetic systems.…”

New Functionalized Polycycles Obtained by Photocatalytic Oxygenation Using Mn(III) Porphyrins in Basic Media

“…Both free-base ligands and metalloporphyrins proved to be efficient for such reactions. In earlier and more recent works, cycloalkenes were effectively functionalized in such reactions, using porphyrin complexes of various transition metal ions [1][2][3][4][5][6]. Also, some aromatic (and aliphatic) alcohols and aldehydes were oxidized to carboxylic acids by solar irradiation of a Sb(V)-porphyrin [7].…”

One Electron‐Initiated Two‐Electron Oxidation of Water by Aluminum Porphyrins with Earth's Most Abundant Metal

“…For the most promising molecular catalyst for the one-electron oxidation-initiated water oxidation on the basis of our previous work, we prepared the water-soluble aluminump orphyrin,c ationic tetra-methylpyridiniumylporphyrinatealuminum (AlTM-PyP). [10,11] The structure is shown in Scheme 1. AlTMPyP has two water molecules as the axial ligands in water (Figure S1 ehi nt he Supporting Information) and exhibits protolytic equilibria among five speciesw ith different protonated or deprotonated axial ligands [11] (Figure S1 a-d).…”

“…In these context, we have been studying the two-electron oxidativea ctivation of water initiated by ap hotochemical one-electrono xidation of ac atalyst in the presence of an alkene to form oxygenated products such as epoxides. [10] Recently,w ealso found that earth-abundant main group elements such as Sn, Al, and Si also induced the photooxygenation of alkenes with water as both electron donor and oxygen-atom donor. [11] On the basis of those studies, we developed as tudy about electrochemical water splitting catalyzed by main group metal complexes and have now succeeded in generating hydrogen peroxide very efficiently with at urnover frequency up to~2 10 4 s À1 by aone-electron oxidation-initiated two-electron oxidationo fw ater catalyzed by aluminum porphyrins incorporating earth's most abundant metal as the central ionwith the lowest knownoverpotential (97 mV).…”