Developing models for the active sites of hydrogenase enzymes

Model complexes for the Ni-Fe and Fe-only hydrogenase enzymes were synthesized and characterized. For the Ni-Fe-hydrogenase, the monomeric NiTRENTK+ and FeTRENTK+ models were synthesized and characterized by X-ray crystallography. In addition, the reactivity of the NiTRENTK+ model toward N₃− and SCN− was shown including the crystal structure of NiTRENTK(SCN) where the SCN− is shown to bind through the nitrogen end.; The preliminary Fe-only hydrogenase model Fe₂(μ-S ₂C₃H₆)(CO)₆ (1) was synthesized and characterized. Reduction of 1 in the presence of a proton donor leads to the formation of {lcub}Fe₂(μ-CO)(μ-S₂C ₃H₆)(CO)₅H{rcub}− monoanion, a molecule bearing both the bridging carbon monoxide ligand and the terminal hydride proposed to be involved in part of the catalytic cycle of Fe-only hydrogenase.; Compound 1 was reacted with two equivalents of Et₄ NCN to make (Et₄N)₂[Fe(μ-S₂C ₃H₆)(CO)₄(CN)₂] (2) or two equivalents of CNtBu to generate Fe₂(μ-S ₂C₃H₆)(CO)₄(CNtBu) ₂ (3). Whereas protonation or oxidation of 2 leads to an insoluble precipitate, such reactions with compound 3 lead to soluble materials. Triflic acid protonates 3 across the Fe-Fe bond leading to [Fe₂(μ-H)(μ-S₂C₃H ₆)(CO)₄(CNtBu)₂](OTf) ( 3-H+) which exists as four distinct isomers. In addition, 3-H+ reacts with trimethylamine-N-oxide to liberate CO leaving behind open sites for potential substrate binding. Irradiation of 3-H+ with visible light under 98 psi D ₂ gas leads to the formation of HD. Complex 3-H+ is the first Fe-hydrogenase model bearing two cyanide analogs demonstrating reactivity with hydrogen similar to that of the enzyme.