Synthesis and Characterization of Late Transition Metal Complexes as Models for the Active Site of Hydrogenases
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Researchers have turned to biological systems that utilized hydrogen in order to develop new catalysts that do not require platinum. In nature, hydrogen is oxidized or reduced with certain efficiency by using metalloenzymes called hydrogenases. Currently there are three types of hydrogenases: [NiFe], [FeFe], and [Fe]-hydrogenase. These hydrogenases have shown unusual structures, such as having carbonyl and cyanide ligands bound to the active site. The synthesis of the potentially tridentate ligands, bis-(2-thiophenyl)phenylphosphine (H2PS2) and bis-(2-thiobenzyl)benzylamine (H2NS2), has been optimized. These ligands contain the rich sulfur environment that is found in the metalloenzyme and have shown prior success in making stable metal complexes in various oxidation states. Metal complexes of ruthenium and osmium containing these ligands were successfully synthesized and characterized. The discovery of [Fe(CN)x(CO)y] units in hydrogenase enzymes has prompted the study of iron-cyanide-carbonyl compounds. Recently, compounds of the general structure [FeII,III(CN)4L2]2-,1-, where L = DMSO, CO, pyridine, were synthesized for the first time. This prompted studies of related compounds of the congener elements of iron, specifically using ruthenium and osmium. These studies have produced the first compounds of ruthenium with the general structure, [RuII(CN)4L2]2- where L = CO and pyridine. Iron carbonyl complexes with the H2PS2 ligand have been previously used to mimic the iron centers in hydrogenase enzymes. To expand on these studies, ruthenium was used to replace iron in the general structure [MII(CO)3(PS2)]. Studies show that the monomer loses carbonyls producing a trimer, [RuII3(CO)5(PS2')3]. Various compounds were also synthesized using Li2NS2 in place of Li2PS2. The synthesis of analogs for the binuclear [NiFe] and [FeFe] hydrogenase active sites has been sought for some time. Only a few such compounds had been previously characterized. Using [FeII(CN)2(CO)3I]- as a starting compound, the synthesis of a binuclear compound, [FeII(CN)2(CO)2NiII(S4)] (S4 is 3,7-dithianoane-1,9-dithiolate). The X-ray structure of this compound reproduces certain aspect of the active site of [NiFe] hydrogenase enzymes. Characterization of synthesized compounds was done using single crystal X-ray crystallography, infrared spectroscopy, NMR and electrochemistry.