Crouthers, Danielle Jerine (2015-08). Synthetic Analogues of the Diiron Hydrogenase Enzyme Active Site and Possible Primordial H2-Producing Catalysts. Doctoral Dissertation. Thesis uri icon

abstract

  • Nature has developed enzymes that utilize the base metals, iron and nickel, in sulfur-rich environments, capable of performing the reversible interconversion of protons and electrons into hydrogen under mild conditions. Recent studies of hybrid enzymes, in which the apo-[FeFe]-hydrogenase protein was loaded with synthetic diiron units, found that the synthetic active site with an azadithioate bridge was fully active at a rate comparable to the native enzyme. In order to compare the carbon to nitrogen-based bridgehead derivatives, dynamic NMR (13C and 1H) studies of a series of simple diiron models, (u-SCH2XCH2S)[Fe(CO)3]2 complexes, X = CR2 or NR, were utilized to examine the fluxional processes that are important in the [FeFe]-hydrogenase active site models. Energy barriers for the CO site exchange in Fe(CO)3 rotors were determined to interrogate the effects of fluxional mobility on the electrochemical response to added acid. The nitrogen-based bridgehead derivatives displayed a 2-fold increase in catalytic activity indicating the proton-directing effect of the pendent base. Also to investigate the effects of an azadithiolate linker (-SCH2NHCH2S-), a series of ligand substituted azadithiolate complexes were synthesized and characterized so that the Bengali laboratory at TAMU-Qatar could explore the photochemical reactions. In another study water-soluble functionalities were introduced into [FeFe]-H2ase active site models by virtue of a sulfonate group incorporated into the -SCH2NRCH2S- dithiolate unit, in order to possibly replace platinum in hydrogen fuel cells that operate in an aqueous media. Advantages of the aryl sulfonate approach include entry into a variety of water-soluble derivatives that are stable in O2-free aqueous solutions. The water solubility of a series of phosphine substituted diiron sulfonate complexes were analyzed leading to one of the first systems where the electrochemisty was studied in water without any organic co-solvents. Finally to help better understand the origin of the [FeFe]-hydrogenase, possible intermediates in the biosynthetic pathway were examined. Evidence that metal sulfide/carbonyl clusters were formed through modifications of a mineral surface were investigated through reactivity studies of various discrete polymetallic compounds. It was demonstrated that the polymetallic compounds were capable of conversion into azadithiolate derivatives through the release of the exogenous metal in the presence of a strong acid.

publication date

  • August 2015