title: Synthesis of water-soluble silicon-porphyrin: protolytic behaviour of axially coordinated hydroxy groups.
abstract: 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.
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Sebastian Nybin
authors.initials: SN
authors.lastname: Remello
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Fazalurahman
authors.initials: F
authors.lastname: Kuttassery
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Takehiro
authors.initials: T
authors.lastname: Hirano
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Yu
authors.initials: Y
authors.lastname: Nabetani
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Yamamoto
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Satomi
authors.initials: S
authors.lastname: Onuki
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Tachibana
authors.affiliation: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. inoue-haruo@tmu.ac.jp.
authors.forename: Haruo
authors.initials: H
authors.lastname: Inoue
mesh_terms: D002384:Catalysis; D053138:Cyclohexenes / Q000737:chemistry; D055664:Electrochemical Techniques; D008958:Models, Molecular; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D011166:Porphyrins / Q000737:chemistry*; D012825:Silicon / Q000737:chemistry*; D012995:Solubility; D014867:Water / Q000737:chemistry*
chemical_list: D053138:Cyclohexenes; D016877:Oxidants; D011166:Porphyrins; D014867:Water; C052568:cyclohexene; D012825:Silicon