Tailoring the Multi-Protolytic Equilibrium of Tin Porphyrins as Promising Molecular Catalyst for the Two-Electron Oxidation of Water in Artificial Photosynthesis

Abstract

Two-electron oxidation of water to form hydrogen peroxide is an alternative approach to overcome the photon-flux density problem as the bottleneck subject in artificial photosynthesis associated with molecular systems for photochemical water splitting. Tin-porphyrin complexes show promising activity and selectivity for the two-electron water-oxidation pathway. A key aspect of the reaction mechanism of these systems is the attack of OH⁻/H₂O to the activated axial oxygen atom as the reaction center of the one-electron oxidized species to form O−O bond, which is feasible when an odd electron is localized on axial oxygen. DFT calculations show that the spin of the odd electron is mostly localized on the axial oxygen atoms when they are in deprotonated states; hence, their pKa determine the applicability of these systems under the pH conditions adopted. The milder pH conditions are most preferable. To explore the conditions the effect of various porphyrin ring substituents on the pKa of the protonation–deprotonation process in A4 and A3B tin-porphyrin complexes have been studied here by spectrophotometric titration. Tetra-fluorophenyl (Sn−F4), tetra-chlorophenyl (Sn−Cl4), and tetra-methoxycarbonylphenyl (SnTCPPMester)-substituted Sn−A4 porphyrins have pKa1 values of 10.1 (Sn−F4), 9.8 (Sn−Cl4), and 6.7 (SnTCPPMester), which are in the increasing order of the electron-withdrawing effect, -F (σ_p=0.06)<-Cl (0.23)<-COOMe (0.45). For A3B (A=fluorophenyl or chlorophenyl, B=methoxycarbonylphenyl) Sn-porphyrins, pKa1 shifts to the smaller value (Sn−F3Mester: 9.4, Sn−Cl3Mester: 6.6), improving the pH range of the active species from basic to ambient pH. This study shows how the pH range of the active species can be effectively tailored by choosing the meso-substituents.