鈴木 航

Abstract Interactions between gold‐based materials and dioxygen (O₂) have motivated researchers to understand reaction mechanisms for O₂ activation by homo‐ and heterogeneous gold catalysts. In this work, gold(I) porphyrin dinuclear complexes were synthesized with a saddle‐distorted porphyrin ligand. The gold(I) porphyrin complexes showed unprecedented O₂ activation in the presence of protic solvents to form gold(III) tetradentate porphyrin complexes. Mechanistic insights into the O₂ activation by the gold(I) center were elucidated by spectroscopic measurements and theoretical calculations, revealing that dissociation of halides on the gold(I) center by alcohol solvents and hydrogen bonding of an N−H proton in the distorted porphyrin with dioxygen played important roles in establishing the unique reactivities of gold(I) complexes.

We have successfully constructed hydrogen-bonded supramolecular assemblies based on a diprotonated saddle-distorted porphyrin (H4DPP2+) and redox-active molecules bearing a carboxylate group, such as a Ru-II polypyridyl complex ((RuCOO-)-C-II) that can act as an electron donor and a benzyl viologen derivative (BV2+COO-) that can act as an electron acceptor. Formation of supramolecular assemblies of H4DPP2+ with (RuCOO-)-C-II(HH4DPP2+((RuCOO-)-C-II)(2) and H4DPP2+((RuCOO-)-C-II)(Cl-)) and BV2+COO- (H4DPP2+(BV2+COO-)(2)) was confirmed in acetone by H-1 NMR measurements, CSI-TOF-MS, and X-ray crystallographic analysis. The photodynamics of H4DPP2+((RuCOO-)-C-II)(2) and H4DPP2+(BV2+COO-)(2) was elucidated by femto-, pico-, and nanosecond time-resolved transient absorption spectroscopy. In H4DPP2+((RuCOO-)-C-II)(2), intrasupramolecular photoinduced electron transfer (ET) occurred from the Ru-II center to the singlet excited state of H4DPP2+ to afford an ET state involving one-electron-reduced H4DPP center dot+ and the corresponding one-electron-oxidized Ru-III complex with the lifetime of 150 ps. On the other hand, in the presence of an external electron donor, intermolecular photoinduced ET occurred from an electron donor to the triplet excited state of H4DPP2+(BV2+COO-)(2) to afford H4DPP center dot+, following intrasupramolecular thermal ET proceeded from H4DPP center dot+ to BV2+COO- to form H4DPP2+(BV center dot+COO-)(BV2+COO-) with the lifetime of 1.1 ms. Thus, H4DPP2+ can act as an electron mediator in hydrogen-bonded supramolecular assemblies. This is the first example of porphyrins as a photosensitizer and an electron mediator in hydrogen-bonded ET systems.

<p>Photocatalytic O₂ reduction reactions proceeded efficiently to produce H₂O₂ using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst.</p>

The two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na2S2O4 gave a corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph. Further reduction of Iph proceeded to form an unprecedented four-electron-reduced porphyrin (IphH(2)), which was fully characterized by spectroscopic and X-ray crystallographic analysis. IphH(2), with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton-coupled four-electron reversible redox system.

<p>The thermodynamic stability of diprotonated saddle-distorted dodecaphenylporphyrin was controlled by the hydrogen-bonding strength of conjugate bases of strong acids or acids having positively charged moieties.</p>