Study On The Synthesis And Catalytic Hydrogen Evolution Properties Of The Iron Sulfur Cluster Complexes Of Fe-Hydrogenase Active Site

Since the first crystal structure of the Fe-only hydrogenase was published in 1998, intensive efforts have been spent on the structural and functional models of Fe-only hydrogenase due to its great capability of catalyzing the reduction of protons to molecular hydrogen at extraordinarily high rates. Presently, the focus was mainly directed at the mimic of monomer structure of 2Fe2S complexes, and the ligand exchange was also focused on the monodentate phosphine ligands. In this synthesis, a series of diiron disulfide complexes were synthesized as the structural and functional models of the Fe-only hydrogenase active site.Two amino-substituted diiron azadithiolate complexes [(μ-SCH₂)₂N(CH₂CH₂NHTs) Fez(CO)₆] (8) and [{(μ-SCH₂)₂NCH₂CH₂N(μ-SCH₂)₂Fe₂(CO)₆}Fe₂(CO)₆] (9) have been synthesized. Protonation of complexes 8 and 9 occurred on the bridged-N and no evidence for the formation of Fe-Feμ-H species was observed. Complexes 8 and 9 showed a better capability of catalyzing electrochemical reduction of protons to molecular hydrogen. Compared with aromatic nitrogen atom bridged 2Fe2S complexes, aliphatic nitrogen bridged complexes 8 and 9 were easily protonated. And complex 9 exhibited a good capability for electrochemically catalyzing protons into hydrogen with the turnover number of 60. Based on complex 9, a dimer structure complex 10 and tetramer structure complex 11 were prepared. The crystal structures show that complex 10 is a macrocyclic structure and complex 11 is a linear molecule. Electrochemical hydrogen evolution catalyzed by these two complexes with ca 80-90 single-run turnovers is observed, indicating good potential as catalysts for future application.The reaction of the Fe₂S₂ complexes 13 and 14 with the bisphosphines dppe and dppm has been studied, which has been found to give three types of complexes, depending on reaction conditions and structure of the substrates. With dppe, under fairly mild reaction conditions, both 13 and 14 gave structures that contain two terminal butterfly Fe₂S₂ cluster cores coordinated to a bisphosphine ligand (15, 16). In the case of 14 also ca. 20% of complex 17 was formed, where one phosphine has been converted to a phosphine sulfide. By contrast, using the ligand dppm, the bisphosphine was only monodentate, giving complex 18 probably because the combined bulk of two phenyl groups and the coordinated complex decrease the reactivity of the second phosphorus for steric reasons. However, under particular conditions, the reactions with dppm give complexes 19 and 21, in which the diphosphine ligand bridges the two Fe of the same molecule. Finally, it was found that complex 16 is a good catalyst for electrochemical production of hydrogen with relatively low reduction potential in the presence of an acid.Complexes 24, 26, 27 and 28 were synthesized in excellent yields by the combined use of Me₃NO and light. The single crystal structure of complex 24 shows that with larger tertiary phosphine ligands, a ba/ba or an ap/ba configuration should be more sterically crowded than an ap/ap configuration. A plausible mechanism was provided for the formation of phosphine sulfide. Two novel photosensitizer-2Fe2S connected trimetallic clusters of Ru-Fe and Re-Fe have been synthesized. The electrontransfer from rhenium excited state to diiron part was identified by the quenching of fluorescence in acid condition.All complexes were characterized by MS, NMR, IR spectra and elemental analysis. The structures of 8-11, 15-22, 24, 28, 39 and 40. were determined by X-ray crystal diffraction.