The Computational Modeling of Organometallic Compounds and Reactions Inspired By Hydrogenases
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Hydrogenases are enzymes capable of catalyzing, reversibly, coupling of protons and electrons into dihydrogen. The active sites of both [NiFe]- and [FeFe]-hydrogenases feature a M₂S₂ core, where two first-row transition metals are tightly held together by two bridging thiolates. In this manner, two “one-electron” metals, in the aspect of redox activity, cooperate to facilitate the “two-electron” H₂ production. Such a delicate apparatus from Nature inspired molecular models composed of two base metals and a dithiolate chelating ligand. Using 1,3-propanedithiolate (pdt) or N,Nbis(2-mercaptoethyl)-1,5-diazacyclohexane/octane (N₂S₂) to hinge metal fragments, these models share a common formula M-(μ-S)₂-M’ and a variety of reactions can be initiated on them. Computational chemistry studies of geometries, electronic structures, reaction energetics and spectral simulations were used to investigate the mechanisms of the following reactions: H2 production. A Lewis acid-base pair is generated on the electro-catalysts M(N₂S₂)-M’ (M = Ni²⁺/Fe(NO)²⁺, M’ = Fe(CO)Cp+ /Fe(NO)²⁺) by reductively dissociating the S-M’ bond during the catalytic cycle. The pair holds a proton and a hydride before their coupling into H₂. The tri-nitrosyl complex Fe(NO)-(N₂S₂)-Fe(NO)₂⁺ is special with multiple electron-buffering Fe(NO)x units such that it can generate two hydrides on irons, which reductively eliminate into H₂. ₂ ²⁺ CO₂ reduction. Ni(₂S₂) metalloligand replaces the redox-active bipyridine of the proven electro-catalyst (bpy)Re(CO)₃Cl to create Ni(N₂S₂)Re(CO)₃Cl. In addition to the electron buffer role in the mechanism, the Ni also stabilizes the up-taken carbon dioxide by establishing a dative O-Ni bond. C-H bond activation. (CO)₃Fe(μ-Me₂-pdt)Fe(CO)(P₂N₂) undergoes intramolecular C-H bond activation under oxidation conditions. The strategically placed amine on the second coordination sphere cleaves the proton from the C-H bond and serves as a proton shuttle, reproducing the role of the pendant amine of [FeFe]-hydrogenase. Ligand isomerization. The trimetallic complex Cp(CO)₂Fe-NC-Fe(CO)₂(μ- pdt)Fe(CO)₃ and derivatives were used to simulate the linkage cyanide isomerization processes during the maturation of [FeFe]-hydrogenase. The energetics of cyanide linkage isomerization is controlled by the oxidation and spin states of the cyanide-bound metals. The computational studies herein confirm the versatility of complexes containing the M₂S₂ core and suggest the ligands and the metals to be of equal importance in contributing to the activity of these organometallic compounds.
Ding, Shengda (2017). The Computational Modeling of Organometallic Compounds and Reactions Inspired By Hydrogenases. Doctoral dissertation, Texas A & M University. Available electronically from