| dc.description.abstract | 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.
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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. | en |
