Small molecule mimics of hydrogenase enzymes: Synthesis, protonation, and electrocatalysis.

摘要

In nature, H2 is processed by enzymes called hydrogenases, which catalyze the reduction of protons to dihydrogen, as well as the reverse reaction. The active sites of the two most prevalent hydrogenases contain NiFe or FeFe cores, bound to thiolates, cyanide, and carbon monoxide ligands. These enzymes are also rich in Fe-S clusters to allow the necessary redox chemistry of hydrogen oxidation and production. Both enzymes operate at rates and overpotentials comparable with the best synthetic Pt catalysts. Due to growing concern over the climate effects of burning fossil fuels, there is a push to replace these fuels with carbon free alternatives, one option being H2. However, this would require catalysts for H2 production that are based on cheap, easily accessible metals. This problem inspired extensive research on the development of functional small molecule mimics of the hydrogenase enzymes. The work presented herein is motivated by the goal of understanding the mechanism of hydrogenase enzymes, in order to design better catalysts for hydrogen production. Chapter 1 presents an overview of current methods for the production of H2, including methods used in industry, as well as heterogeneous and homogeneous metal catalysts.;Chapter 2 describes the protonation of complexes of the type Fe2(xdt)(CO)4(dppv)2 (xdt= pdt, 1,2-propanedithiolate, or adtNH= azadithiolate; dppv= cis-1,2-bis(diphenylphosphino)ethylene, which form terminal hydrides that are stable at 0 °C for ~ 30 minutes and then isomerize to the corresponding bridging hydrides. Fe2(adtNH)(CO)4(dppv)2 undergoes protonation with weak acids, whereas, the pdt analogue requires strong acid; the difference being attributed to the presence of a pendant base in Fe2(adtNH)(CO)4(dppv)2, which is initially protonated and then relays the proton to the Fe center.;Chapter 3 describes the redox and catalytic properties of the terminal and bridging hydrides of Fe2(xdt)(CO)2(dppv)2. For both the adtNH and the pdt derivatives, the terminal hydride species are reduced at ~150 mV more mild potentials than the corresponding bridging hydrides. The voltammetry of [t-H Fe2(adtNH)(CO)2(dppv)2]+ is strongly affected by relatively weak acids and proton reduction catalysis proceeds at 5000 s-1 with an overpotential (the deviation from the thermodynamic reduction potential of the acid) of 0.7 V.;Chapter 4 describes the development of a new synthesis of complexes of the type (diphosphine)Ni(xdt)(CO)3, in which Fe(CO)4I2 condensed with Ni(xdt)(diphosphine), forming a NiIIFeII ?-iodide intermediate, which is then reduced to form the neutral NiFe complex. With this new synthetic method in hand, we synthesized of new derivatives, varying in the identity of the dithiolate, the diphosphine, and the ligands on the Fe center.;Chapter 5 describes the synthesis of new ferrous dicarbonyl dithiolato diphosphine complexes containing chelating diphosphine and dithiolate ligands. A new building block method for the synthesis of substituted diiron complexes of the type Fe2(xdt)(CO)4(diphosphine), by reaction of Fe(xdt)(CO)2(diphosphine) complexes with an Fe(0) tricarbonyl source, is described.;Chapter 6 describes the synthesis of bimetallic CpCo complexes of the type, (C5H5)Co(xdt)Co(C5H5) (xdt= pdt, 1,2-propanedithiolate; edt, 1,2 ethanedithiolate, and tdt= 3,4-toluenedithiolate), in an effort to synthesize more electron rich model complexes, by replacing the Fe(CO)3 unit with CpCo. These complexes undergo protonation to form bridging hydride species, which catalyze the reduction of protons, albeit at modest rates and fairly high overpotentials. (Abstract shortened by UMI.).