Electrochemical & Photochemical Hydrogen Production Catalyzed By Bioinspired [2Fe2S] Model Complexes

Inspired by high efficiency of Fe-Fe hydrogenases for catalyzing the reduction of protons to hydrogen, many chemists are engaged in creating electrochemical and photochemical hydrogen production catalyst systems based on the [2Fe2S] model complexes of the Fe-Fe hydrogenase active site. Concerning the electrochemical hydrogen production, studies on electrochemical properties of [2Fe2S] model complexes in organic solvents were dominant. Since the Fe-Fe hydrogenase active site is surrounded by an unknown number of water molecules and works in a water-containing medium at relatively low reduction potentials, water solubility of [2Fe2S] model complexes should be improved to explore the effect of water on proton reduction. As for photocatalytic systems, Fe-based complex has not been used as catalyst for proton reduction because of the unsuccessful electron transfer from Ru-photosensitizers to [2Fe2S] model complexes. On the basis of this context, diiron model complexes were synthesized by introduction of the water soluble phosphine ligand 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA). The electrochemical properties of the model complexes were studied in water-containing and pure water solution. With an intermolecular electron transfer system, photoinduced electron transfer from Ru-photosensitizers to [2Fe2S] model complexes and visible light driven hydrogen generation catalyzed by diiron model complexes were accomplished.In this dissertation, three diiron dithiolate complexes with DAPTA ligand(s), (μ-pdt)[Fe(CO)₂(DAPTA)][Fe(CO)₂L] (L = CO, 2; DAPTA, 3; PTA, 4), were prepared with complex [(μ-pdt)Fe₂(CO)₆] (1, pdt = 1,3-propanedithiolato) as a precursor. Complexes 2-4 were characterized by HRMS, IR, ¹H,¹³C & ³¹P NMR spectroscopy and elemental analysis. With complexes 2-4 as electrocatalysts, significant decrease in reduction potentials for the Fe¹Fe^I to Fe⁰Fe^I process was observed in CH₃CN/H₂O mixtures. The introduction of DAPTA ligand(s) to the diiron dithiolate model complexes indeed made the water solubility of 3 and 4 sufficient for electrochemical studies in pure water, which showed that the proton reduction from acetic acid in pure water was electrochemically catalyzed by 3 and 4. Complex 4 was protonated by acetic acid in water, resulting in an anodic shift of the potential for proton reduction electrocatalyzed by complex 4.To avoid the intramolecular reductive quenching of the excited Ru-photosensitizer by [2Fe2S] model complexes, an intermolecular electron transfer system was adopted. With N,N-diethyldithiocarbamate anion as electron donor, photoinduced electron transfer was observed from a reduced photosensitizer Ru(bpy)₃⁺ to bio-inspired [2Fe2S] model complexes 1 and 5, generating the one electron reduced species of complexes 1 and 5. The electron transfer was verified by transient absorption spectroscopy and kinetic traces obtained from laser flash photolysis.With ascorbic acid as as both electron and proton donor in CH3CN/H2O, electron transfer from Ru(bpy)₃⁺ to [2Fe2S] model complexes 1 and 5 was verified by laser flash photolysis. Hydrogen generation initiated by photo-excited ^*Ru(bpy)₃²⁺ was observed from the three component system containing Ru(bpy)₃²⁺, ascorbic acid and complexes 5.Complexes [{(μ-SCH₂)₂NCH₂C₆H₅}{Fe(CO)₂(P(Pyr)₃)}{Fe(CO)₂L}] (L = CO, 6; L = P(Pyr)₃, 7) were prepared to improve the photo-stalility of diiron catalysts, which have the lowest reduction potentials for the single and double CO-displaced diiron complexes reported so far. The electron transfer from photo-generated Ru(bpy)₃⁺ to 6 or 7 was detected by laser flash photolysis. Visible light driven hydrogen generation catalyzed by Fe-based catalysts was successfully observed from a three-component system, consisting of a ruthenium polypyridine complex, the [2Fe2S] model complex 6 or 7, and ascorbic acid as both electron and proton donor in CH₃CN/H₂O. Under optimal conditions, the total turnover number for hydrogen evolution was 4 based on 6 and 86 based on Ru(bpy)₃²⁺. A plausible proton reduction mechanism was proposed.