Molecular Devices For Photoinduced Hydrogen Production Based On Non-noble Metal Catalysts

Hydrogen is known as a clean and efficient new-style energy, and it has been considered as a promising alternative to fossil fuels. Conversion of solar energy into molecular hydrogen has attracted more and more attention in recent years. Designing and synthesizing cheap and efficient functional molecular devices for photocatalytic hydrogen production through chemical method is an effective way. Two proton-reduction catalysts based on inexpensive metals are taking into account. One is [2Fe2S] model complex of the Fe-Fe Hydrogenase active site which can electrochemically catalyze the proton reduction. The other one is cobaloximes which has a similar structure of the cobalt complex in vitamin B₁₂. It can mimic the function of the active site of H₂ase. Up to now, no report on the photochemical hydrogen production mediated by the supermolecule consisted of Ru(bpy)₃²⁺-[2Fe2S] was found. Additionly, in the architecture of the molecular device for photocatalytic hydrogen production base on Ru(bpy)₃²⁺-Cobaloximes ([Co^â…¡(dmgBF₂)₂(OH)₂], dmgBF₂ = (difluoroboryl)dimethyl glyoximato), the bridging ligands between the photosensitizer and the cobalt center are all fully conjugated, no literature was found to investigate the influence of the bridge on the functional molecular devices.In this dissertation, a new [2Fe2S] model complex [(μ-S-C₆H₄-4-NH₂)₂Fe₂(CO)₆](1) was prepared. Complex 1 was characterized by IR, ¹H NMR, MS spectroscopy, and X-ray analysis. A [2Fe2S] model complex containing two Ru-photosensitizers [{μ-S-4-((bpy)₂Ru(bpy- 4-CH₃,4'-CONH))C₆H₄}₂Fe₂(CO)₆](PF₆)₄(2) with complex 1 as catalytic center was also prepared. Two electrons are needed when a molecular hydrogen is produced, the purpose of designing complex 2 is to enhance the efficiency of photocatalytic hydrogen production. Complex 2 was characterized by IR, ¹H NMR and HR-MS spectroscopy. Laser flash photolysis proved that the excited photosensitizer was reductively quenched by the [2Fe2S] moiety in complex 2 after light irradiation. The electron transferred from the Fe^â… Fe^â… to the ^*Ru²⁺ center and the important intermediate Fe^â… Fe⁰ species for hydrogen production could not be generated. Therefore complex 2 is not able to act as a photocatalyst for hydrogen evolution.Two heterobinuclear complexes [(bpy)₂Ru(bpy-4-CH3,4'-CONH(4-py))Co(dmgBF₂) (OH)₂)](PF₆) (5) and [(bpy)₂Ru(bpy-4-CH₃,4'-CONHCH₂(4-py))Co(dmgBF₂)₂(OH)₂](PF₆)₂ (6) were prepared, in which the polypyridyl ruthenium photosensitizer and the cobaloxime catalyst are connected either by a conjugated bridge 5 or by an unconjugated one 6. Complexes 5 and 6 were used as photocatalysts for hydrogen generation. Under optimal conditions, the turnover numbers (TON) for hydrogen evolution were 38 for 5 and 48 for 6 in the presence of 300 equiv of both Et₃N and [Et₃NH][BF₄] in the acetone solution during an eight-hour irradiation of visible light (λ＞ca. 400 nm). The molecular device 6 with an unconjugated bridge proved to be more efficient for photochemical hydrogen generation than the molecular device 5 with a conjugated bridge under the same reaction condition. A plausible proton reduction mechanism was proposed.