Modeling the active sites of [nickel-iron]- and [iron-iron]-hydrogenases

The hydrogenases are metalloenzymes that act to catalytically interconvert dihydrogen with protons and electrons. Three classes of hydrogenase are known and are classified based upon their metal content. The first hydrogenase to be evolved was the [NiFe]-hydrogenase and features a heterobimetallic NiFe active site. The [FeFe]-hydrogenase was evolved later and features a dinuclear iron-containing active site. A third enzyme, named the [Fe]-hydrogenase, occurs in a small number of methanogenic organisms and is not the focus of this research.;Elucidation of the crystal structure of the [NiFe]-hydrogenase active site in 1995 revealed an unusual heterobimetallic NiFe complex. In this complex, an octahedrally-coordinated ferrous ion is coordinated by one terminal carbonyl and two terminal cyanide ligands and is bridged to a Ni(II) ion by two cysteine thiolates. The Ni(II) ion is further coordinated by a pair of terminal cysteine thiolates. A third bridging position is either vacant or coordinated by a hydride ligand depending upon the catalytic state of the enzyme.;Similarly, the report of the crystal structure of the [FeFe]-hydrogenase revealed another unusual metal-containing active site. This hydrogenase contains an active site that is comprised of two Fe atoms that are each coordinated by one terminal cyanide and one terminal carbonyl. The Fe atoms are bridged by a non-protein dithiolate ligand that has been proposed to be an azadithiolate derived from bis(mercaptomethyl)amine and a bridging carbonyl. One Fe atom, the proximal Fe, is further coordinated by a cysteine thiolate that directly links the active site to a [4Fe4S]-cluster. The other Fe atom, the distal Fe, is the site of catalytic activity and contains either a vacant site or a hydride depending upon the state of the enzyme.;With respect to [NiFe]-hydrogenase active site modeling, the Fe subunit of the active site was modeled synthetically as a first attempt at building accurate structural mimics. The substitution chemistry of an electrophilic Fe-cyanide-hydride complex, [HFe(CN)2(CO)3]- was examined, and complexes of monophosphorus donor ligands were prepared. These complexes feature markedly decreased acidity at the hydride and lability of the phosphorus donor ligand. These properties allowed for the synthesis of [HFe(CN)3(CO)2]2-, a new member of a class of Fe-hydride-cyanide-carbonyl complexes. The use of the bidentate phosphine dppv, cis-1,2-bis(diphenylphosphino)ethylene, allowed for the preparation of [HFe(CN)2(CO)(dppv)]- ---a good structural model of the Fe subunit of the [NiFe]-hydrogenase active site.;Utilizing a recently developed class of mononuclear ferrous dithiolate complexes, heterobimetallic NiFe complexes were synthesized. The first generation of these complexes, prepared via the reaction of Fe(S2C3H 6)(CO)2(dppe) and NiCl2(dppe), featured a bridging chloride ligand rather than the biologically relevant hydride. An intermediate in the preparation of the bridging chloride complexes, [(dppe)(CO)2Fe(S 2C3H6)Ni(dppe)]2+, allowed for the formation of the first NiFe-hydride containing [NiFe]-hydrogenase model complex upon reaction with BH4-. Although this complex could not be isolated from its reaction mixture, it was confirmed by independent synthesis via a secondary method.;While previous hydride-containing [FeFe]-hydrogenase models featured only bridging hydrides, we sought to prepare complexes of terminal hydrides that would be more relevant to mimicking this hydrogenase. Di-subferrous precursors, such as Fe2(S2C2H4)(CO)4 (PMe3)2, have been known for decades---well before the elucidation of the structure of the active site. Two-electron oxidation of Fe2(S2C2H4)(CO)4(PMe 3)2 in the presence of excess PMe3 in MeCN resulted in the formation of the diferrous [Fe2(S2C2H 4)(mu-CO)(NCMe)(CO)(PMe3)4](PF 6)2. This complex, when treated with hydride sources such as BH4- or AlH4- at low temperature, generated the first terminal hydride-containing [FeFe]-hydrogenase model complex, [HFe2(S2C2H4)(mu-CO)(CO)(PMe 3)4]PF6. This complex readily protonates with strong acids in MeCN to evolve H2 and regenerate the precursor MeCN-complex. The terminal hydride complex is thermally unstable, isomerizing to give an inert bridging hydride, even in the solid state.