Lunsford, Allen Michael (2016-12). Bridging Thiolates in Metalloenzymes: From Model Complexes to Catalytic Small Molecule Activation. Doctoral Dissertation.
Thesis
The discovery of cyano-iron carbonyls in the hydrogenases has inspired synthetic efforts to reproduce the active sites of these efficient H2-producing or H2-oxidation enzymes. Recent demonstration of the possibility of loading apo-HydA ([FeFe]-H2ase enzyme) and apo-HydF (maturase protein) with synthetic analogues of the active site provides evidence that small molecular models are indeed the essential catalyst. Interrogation by EPR spectroscopy of the biohybrids indicate a role for the cyanide in attaching the organometallic unit to a carrier protein; in fact, the cyanide has flipped from its origin in the synthetic diiron unit and inserts into the maturation protein, HydF, resulting in a 4Fe4S docking of the diiron model complex, but placing the cyanide nitrogen next to the 2Fe site. In order to probe the requirements for such cyanide isomerization, we have prepared cyanide-bridged constructs of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. The orientation of the CN bridge is determined by the precursors; no cyanide flipping was observed. Density function theory computations find a high barrier accounts for the kinetically controlled products. In another study, complexes of formulation (u-SCH2XCH2S)[Fe(CO)3]2, with X=CH2, CMe2, CEt2, NMe, NtBu, and NPh were determined to be photocatalysts for release of H2 gas from H3B<-NHMe2. The thermal displacement of H3B<-Net3 from photochemically generated (u-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(u-H)(BH2-Net3)] by P(OEt)3 was monitored by time-resolved FTIR spectroscopy. Kinetic data reveals an associative mechanism for X=CH2 and dissociative mechanism for the alkylated and nitrogen bridgehead species. This allows us to rank the catalysts in terms of their B-H unit binding ability, a key step in the dehydrogenation process. The rate of H2 production from the initially formed (u-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(u-H)(BH2-NHMe2)] complexes was inversely correlated with the lifetime of the analogous (u-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(u-H)(BH2-Net3)] adducts. Finally, a new series of complexes featuring MN2S2 metallodithiolates bound to Mn/Re(CO)3X were synthesized and characterized as electrocatalysts for C)2 reduction. Butterfly type structures resulted from binding of the lone pairs of the sulfur atoms to the Re/Mn(CO)3X unit. Under a CO2 atmosphere, there was a current enhancement in the cyclic voltammogram which is indicative of CO2 reduction to CO. Detailed electrochemical experiments are presented along with an appropriate characterization of catalytic processes using cyclic voltammetry and bulk electrolysis. Two complexes will be used as a case study.
