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

We report herein a new molecular catalyst for efficient water splitting, aluminum porphyrins (tetra-methylpyridiniumylporphyrinatealuminum: AlTMPyP), containing earth's most abundant metal as the central ion. One-electron oxidation of the aluminum porphyrin initiates the two-electron oxidation of water to form hydrogen peroxide as the primary reaction product with the lowest known overpotential (97 mV). The aluminum-peroxo complex was detected by a cold-spray ionization mass-spectrometry in high-resolution MS (HRMS) mode and the structure of the intermediate species was further confirmed using laser Raman spectroscopy, indicating the hydroperoxy complex of AlTMPyP to be the key intermediate in the reaction. The two-electron oxidation of water to form hydrogen peroxide was essentially quantitative, with a Faradaic efficiency of 99 %. The catalytic reaction was found to be highly efficient, with a turnover frequency up to ∼2×104  s-1 . A reaction mechanism is proposed involving oxygen-oxygen bond formation by the attack of a hydroxide ion on the oxyl-radical-like axial ligand oxygen atom in the one-electron-oxidized form of AlTMPyP(O- )2 , followed by a second electron transfer to the electrode.

Synthesis of water-soluble silicon-porphyrin: protolytic behaviour of axially coordinated hydroxy groups.

A new water-soluble silicon(IV)-tetra(4-carboxyphenyl)porphyrin (SiTCPP) with silicon(iv), the second most abundant element on Earth, in the center of porphyrin was synthesized. Fundamental properties including protolytic behaviour of axially coordinating hydroxy groups, and electrochemical behaviour were characterized. The properties were compared with those of silicon(IV)-tetra(2,4,6-trimethylphenyl)porphyrin (SiTMP) and silicon(IV)-tetra(4-trifluoromethylphenyl)porphyrin (SiTFMPP) and discussed in respect to the electron donating/withdrawing effect of the substituents. Two axially coordinating hydroxy groups of SiTCPP exhibit a four-step protolytic behaviour under the acidic conditions along with a single step protolysis of peripheral carboxyl groups. Though SiTCPP and SiTFMPP did not show any reactivity in the photochemical oxygenation of a substrate with K2PtCl6 as a sacrificial electron acceptor, the first oxidation wave in the electrochemical process of SiTCPP and SiTFMPP showed catalytic behaviour in aqueous acetonitrile solution at any pH condition, in contrast to SiTMP which has only a reversible oxidation wave under neutral and weakly acidic conditions. The criteria for the electrochemical oxidative activation of water and the photooxygenation of the substrate were obtained. The higher oxidation wave of Si-porphyrins than ∼0.86 volt vs. SHE is required for the electrochemical oxidation of water, while suitable protecting groups such as a methyl substituent is a requisite for the photochemical oxygenation with K2PtCl6 as a sacrificial electron acceptor.

Hydrogen evolution coupled with the photochemical oxygenation of cyclohexene with water sensitized by tin(IV) porphyrins by visible light.

Hydrogen evolution coupled with the photochemical oxygenation of cyclohexene with water was observed in the system sensitized by Sn(IV)-porphyrin adsorbed on Pt loaded TiO2 nano-particles in aqueous acetonitrile solution upon visible light irradiation.