NiN2S2 Metallodithiolato Ligands in Bioinspired Proton Reduction Catalysts: Synthetic and Kinetic Studies
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Date
2021-06-29
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Abstract
Hydrogenases are a diverse group of metalloenzymes widespread in nature; they occur in archaea, bacteria and some eukarya and can be classified according to the metal ion composition of the active site in [NiFe], [FeFe] and [Fe] hydrogenases. Hydrogenase (H2ase) enzymes catalyze one of the simplest molecular reactions, the conversion of dihydrogen into protons and electrons and the reverse reaction, the generation of dihydrogen. The cleavage of dihydrogen occurs at the metal active site that has the capability to increase the acidity of the H2 molecule and include heterolytic splitting. This process is strongly accelerated by the presence of a nearby base. Generation of dihydrogen involves the coupling of a proton and a hydride; the heterolytic nature of this process has been proven by H/D isotope exchange experiments. The [NiFe]- and [FeFe]-H2ase active sites are composed of sulfur-bridged bimetallic centers. The iron atoms are ligated by small inorganic ligands such as CO and CN- and an open coordination site exists on one metal center.
Over the past two decades DuBois and co-workers, have made several functional mimics of H2ase active sites that are mononuclear using abundant and inexpensive first row transition metals such as nickel, iron and cobalt. These mononuclear complexes use diphosphine ligands and emphasize the role of the first and second coordination spheres about the metal. The design criteria for catalysts that are able to produce and oxidize dihydrogen are based on properties of H2ase enzymes. These properties include the presence of an open coordination site at the metal center, placement of a base in close proximity to the metal center, and the proton acceptor capability of the base to avoid high energy intermediates that can be generated in the catalytic process. These properties also maintain the reversibility of the catalyst
The efforts highlighted in this dissertation have been prominent in development of catalytic systems that engage NiN2S2 as a synthon for the H2ase active site mimics. The versatility of the metallodithiolates ligands, as surrogates of conventional phosphines and carbenes, was shown in their monodentate binding capabilities with [FeIFeI], [FeI[Fe(NO)]II] and [(ยต-H)FeIIFeII] systems, as [FeFe]-H2ase bioinspired trimetallics. Furthermore, fundamental kinetic studies have been conducted to understand the nature of the Fe-S bond strength and the possible ligand substitution mechanism which will be valuable in catalyst development. The final chapter mainly focuses on the NO ligand exchange phenomena in hemi-labile bridging thiolates that are capable of hydrogen production. Preliminary studies in [NiFe]-H2ase biomimetics that contains Fe(NO)N2S2 ligand have demonstrated the possibility of a potential NO scrambling scheme. Therefore, kinetics studies will be focused on understanding inter/intra-NO scrambling process which will further our understanding in such systems.
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[FeFe]-Hydrogenase, Metallodithiolato Ligands, Pendant Bases