[FeFe]-H2ase Enzyme Active Site Biomimetics for Proton Reduction

October 31, 2014

Prof. Marcetta Y. Darensbourg

Photo: Seminar:
The structurally unique diiron catalytic unit that exists in the active site of the [FeFe]-H2ase is of special interest to biomimetic/synthetic chemists as its construction exploits diatomic CO and CN− ligands, perhaps deriving from primordial iron/sulfur chemistry, rather than typical donors covalently bound to a peptide chain. These features, along with the ease of modifying a simple precursor, (µ-S(CH2)3S)[Fe(CO)3]2,  that has core features of the [FeFe]-H2ase enzyme active site (eas), and the possibility to develop base metal catalysts for fuel cell use, have attracted a new coterie of chemists to biomimetic synthesis. This lecture will describe approaches to stabilize and isolate the unusual « rotated » structure shown in the graphic, and insight gained into Nature’s choice of the diiron construct.  Also described is our study of a non-carbonyl diiron complex with an obvious irondithiolate as  bidentate ligand to a second iron, a dinitrosyl iron moiety.  Both the N2S2Fe(NO) metallo-ligand and the Fe(NO)2 unit can exist in two redox levels, observable by reversible waves in the cyclic voltammogram assignable to distinct components, each of which can show electrocatalytic response to acid.   Molecular structures of the diiron trinitrosyl compound in oxidized and one-electron reduced forms demonstrate the accommodation of redox level changes in such bimetallic molecules that involves the electronically versatile NO ligand.  Comparisons to the familiar [FeFe]- and [NiFe]-Hydrogenase biomimetics based on iron carbonyls expand the possible base-metal electrocatalysts into the non-innocence of iron-nitrosyl chemistry.