Photocatalytic Oxidation of 2-Mercaptoethanol to Disulfide using Sb(V)-, P(V)-, and Ge(IV)-porphyrin Complexes

Abstract

Visible-light irradiation of MeCN solution containing di(hydroxo)metallo(tetraphenyl)porphyrin complex <TEX>$(tppM(OH)_2$</TEX>: 1a; <TEX>$M=Sb(V)^+Br^-$</TEX>, 1b; <TEX>$M=P(V)^+Cl^-$</TEX>, 1c; M=Ge(IV)) and 2-mercaptoethanol (2-ME) as a substrate under aerated condition gave bis(2-hydroxyethyl)disulfide (2-HEDS) as an oxidative product of 2-ME. It is indicated that the oxidation of 2-ME should proceed with a photocatalytic process by 1, because the turn over number (TON) for the formation of 2-HEDS was over unit. The TON was determined to be 642 as a maximum value when 1a was used as a sensitizer. The formation of 2-HDES was extremely slow under argon atmosphere. The fluorescence of 1 was not quenched by 2-ME at all, and the free energy change (<TEX>${\Delta}G$</TEX>) with electron transfer (ET) from 2-ME to excited triplet state of <TEX>$1(^31^*)$</TEX> was estimated as a negative value. The quenching rate constant (<TEX>$k_r$</TEX>) of <TEX>$^31^*$</TEX> by 2-ME, obtained by the kinetics for the formation of 2-HEDS, strongly depends on <TEX>${\Delta}G$</TEX>. These findings indicate that 1-sensitized oxidation was initiated by photoinduced ET from 2-ME to <TEX>$^31^*$</TEX> to generate both radical cation of 2-ME (<TEX>$2-ME^{+\bulle}$</TEX>) and porphyrin radical anion (<TEX>$1^{-\bulle}$</TEX>), resulting that the formation of 2-HEDS can be proceeded by the dimerization of <TEX>$2-ME^{+\bulle}$</TEX>, and through a catalytic cycle due to returning to 1 by the ET from <TEX>$1^{-\bulle}$</TEX> to molecular oxygen.