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

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.

Key reaction intermediates of the photochemical oxygenation of alkene sensitized by Ru(II)-porphyrin with water by visible light.

Two key reaction intermediates in the photochemical oxygenation of alkene sensitized by carbonyl-coordinated ruthenium(ii)-porphyrin complex, with water acting both as an electron and oxygen atom donor, are postulated. Under the low concentration of hydroxide ion (<2 x 10(-3) M) added to the reaction mixture of tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(ii) (Ru(II)TMP(CO)), K(4)PtCl(6) as a sacrificial electron acceptor, and cyclohexene as a substrate in aqueous acetonitrile, the major reaction product was cyclohexaneoxide ("Epoxide"), while it drastically decreased along with an increase of 2-cyclohexenol ("Alcohol") by increasing the amount of hydroxide ion (>2 x 10(-3) M). The tendency was more obvious in the case of tetrasodium tetra(4-sulfonate)phenylporphyrinatoruthenium(ii) (Ru(II)TSPP(CO)) in aqueous solution. The "Alcohol" was exclusively formed in the higher concentration region of OH(-), strongly suggesting the presence of acid-base equilibrium among two reaction intermediates. Theoretical DFT calculation indicates that the hydroxyl-coordinated one-electron oxidized Ru-porphyrin (Intermediate (I)), which is formed by the axial ligation of hydroxide ion to the cation radical of Ru-porphyrin generated through electron transfer from the excited triplet state of the sensitizer porphyrins, suffers deprotonation of its axial hydroxide group to lead to an oxo-type complex (Intermediate (II)) formation. The DFT calculation also indicates that the electron spin on the Intermediate (I) is shared by the axial oxygen atom and the central Ru metal, while it is mostly localized on the axial oxygen atom to behave as an oxygen radical in the case of the Intermediate (II). These are very strong indications towards understanding how OH(-) (water molecule) is oxidatively activated on the Ru center: the water molecule is serving as an electron donor ion in the redox cycles. Theoretical calculation predicts that Intermediate (I) allows the epoxidation of alkene and Intermediate (II) can proceed through hydrogen abstraction from the substrate and is rebound to form hydroxylated compound, "Alcohol."

Highly efficient and selective epoxidation of alkenes by photochemical oxygenation sensitized by a ruthenium(II) porphyrin with water as both electron and oxygen donor.

Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) (Ru(II)TMP(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electron acceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene with high quantum yield (Phi = 0.6, selectivity = 94.4% for cyclohexene and Phi = 0.4, selectivity = 99.7% for norbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from a water molecule by an experiment with H(2)(18)O. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide, without forming trans-stilbeneoxide. trans-Stilbene, however, did not exhibit any reactivity. Under neutral conditions, an efficient buildup of the cation radical of Ru(II)TMP(CO) was observed at the early stage of the photoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promote the reaction with reversion to the starting Ru(II)TMP(CO). A possible involvement of a higher oxidized state of Ru such as Ru(IV), Ru(V), Ru(VI) through a dismutation of the Ru(III) species was excluded by an experiment with Ru(VI)TMP(O)(2). Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanism involving an electron transfer from the excited triplet state of Ru(II)TMP(CO) to hexachloroplatinate(IV) and subsequent formation of OH(-)-coordinated Ru(III) species, leading to an oxo-ruthenium complex as the key intermediate of the photochemical epoxidation, was postulated.