Visible-light-induced photocatalysis of poly(pyridine-2,5-diyl). Photoreduction of water, carbonyl compounds and alkenes with triethylamine

Poly(pyridine-2,5-diyl)(PPy) exhibits heterogeneous and visible-light-induced photocatalysis toward the reduction of water, carbonyl compounds and alkenes in the presence of triethylamine (TEA) as a sacrifical electron donor. The water reductive H2 evolution is ehnanced by colloidal noble metals concurrently photoformed from noble metal halides, and colloidal Ru gives the highest apparent quantum yield of H2, 0.21 at 450 nm. With regard to the reduction of carbonyl compounds and electron-deficient alkenes, noble-metal-free PPy leads to the efficient and selective reduction to the corresponding alcohols and dihydro compounds, respectively. Carbonyl compounds whose reduction potentials are as negative as –1.83 V vs. SCE in acetonitrile are photoreducible. Deuterium incorporation experiments with dimethyl maleate and/or dimethyl fumarate in [O-2H]methanol have revealed that the photocatalysis of PPy should proceed through sequential two-electron transfer reduction, affording dideuteriated dimethyl succinate. Sequential electron transfer and hydride transfer mechanisms are proposed.