This dissertation describes computational and experimental studies of synthetic complexes that model the active site of the iron-iron hydrogenase [FeFe]H2ase enzyme. Simple dinuclear iron dithiolate complexes act as functional models of the ironiron hydrogenase enzyme by catalyzing isotopic exchange in D2/H2O mixtures. Density Functional Theory (DFT) calculations and new experiments have been performed that suggest reasonable mechanistic explanations for this reactivity. Evidence for the existence of an acetone derivative of the di-iron complex, as suggested by theory, is presented. Bis-phosphine substituted dinuclear iron dithiolate complexes react with the electrophilic species, H+ and Et+ (Et+ = CH3CH2 +) with differing regioselectivity; H+ reacts to form a 3c-2eâ Fe-H-Fe bond, while Et+ reacts to form a new C-S bond. The instability of a bridging ethyl complex is attributed to the inability of the ethyl group, in contrast to a hydride, to form a stable 3c-2eâ bond with the two iron centers. Gas-phase density functional theory calculations are used to predict the solutionphase infrared spectra for a series of CO and CN-containing dinuclear iron complexes dithiolate. It is shown that simple linear scaling of the computed C-O and C-N stretching frequencies yields accurate predictions of the experimentally determined ν(CO) and ν(CN) values. An N-heterocyclic carbene containing [FeFe]H2ase model complex, whose X-ray structure displays an apical carbene, is shown to undergo an unexpected simultaneous two-electron reduction. DFT shows, in addition to a one-electron Fe-Fe reduction, that the aryl-substituted N-heterocyclic carbene can accept a second electron more readily than the Fe-Fe manifold. The juxtaposition of these two one-electron reductions resembles the [FeFe]H2ase active site with an FeFe di-iron unit joined to the electroactive 4Fe4S cluster. Simple synthetic di-iron dithiolate complexes synthesized to date fail to reproduce the precise orientation of the diatomic ligands about the iron centers that is observed in the molecular structure of the reduced form of the enzyme active site. Herein, DFT computations are used for the rational design of synthetic complexes as accurate structural models of the reduced form of the enzyme active site.

The synthesis of diiron thiolate complexes was carried out using two ligands that were expected to furnish improved catalytic activity, solubility in water, and stability to the metal complexes. The water-soluble phosphine 1,3,5-triaza-7- phosphaadamantane, PTA, coordinates to the Fe centers forming the disubstituted complex (m-pdt)[Fe(CO)2PTA]2, which presents one PTA in each iron in a transoid arrangement. Substitution of one CO ligand in the (m-pdt)[Fe(CO)3]2 parent complex forms the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2PTA]. Enhanced water solubility was achieved through reactions with electrophiles, H+ and CH3 +, which reacted with the N on the PTA ligand forming the protonated and methylated derivatives, respectively. The 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IMes, was reacted with (m-pdt)[Fe(CO)3]2 yielding the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2IMes], an electron rich, air stable complex that does not show reactivity with H+. Electrocatalytic production of hydrogen was studied for the all-CO, bis-PMe3, mono- and di-PTA FeIFeI complexes, as well as the PTA-protonated and -methylated derivatives. The all-CO species produce H2, in the presence of the weak HOAc, at their second reduction event, FeIFe0 ?? Fe0Fe0, that occurs at ca. ??1.9 V, through an EECC mechanism. The mono- and di-substituted phosphine complexes present electrocatalytic production of H2 from the Fe0FeI redox state; this reduction takes place at ??1.54 V for (m-pdt)[Fe(CO)3][Fe(CO)2PTA], and at ca. ??1.8 for the disubstituted PMe3 and PTA derivatives. A positive charge on the starting complex does not have an effect on the production of H2. It was found that the protonated and methylated derivatives are not the catalytic species for H2 production. At their first reduction event the neutral precursor forms, and catalysis occurs from the FeIFeI complex in all cases. The possibility of enhanced catalytic activity in the presence of H2 O was explored by conducting electrochemical experiments in the mixed CH3CN:H2O solvent system for the PTA-substituted complexes. The reduction potential of the catalytic peak is shifted to more positive values by the presence of H2 O. The cyclic voltammogram of {(m-pdt)[Fe(CO)2(PTA?? H)]2}2+ in CH3CN:H2O 3:1 shows the reduction of a more easily reduced species in the return scan. This curve-crossing event provides evidence for the (h2-H2)FeII intermediate proposed in the ECCE mechanism.

[NiFe] hydrogenases are metalloenzymes that act as highly efficient molecular electrocatalysts for the interconversion of protons and molecular hydrogen. Unlike any other known molecular electrocatalyst, the members of a subgroup of respiratory membrane-bound [NiFe] hydrogenases are able to maintain H₂ catalysis in the sustained presence of O₂. This O₂-tolerance depends on the ability to respond to oxidative inactivation by O₂ by exclusively forming rapidly reactivated active site states, thus implying a catalytic cycle in which O₂ acts as a competing substrate to H₂. Using isotope ratio mass spectrometry it is proven that the O2-tolerant Escherichia coli Hydrogenase 1 responds to O₂ attack by acting as a four-electron oxidoreductase, catalysing the reaction 2 H₂ + O₂ → 2 H₂O, equivalent to hydrogen combustion. Special features of the enzyme’s electron relay system enable delivery of the required electrons. A small fraction of the H₂O produced arises from side reactions proceeding via reactive oxygen species, an unavoidable consequence of the presence of low-potential relay centres that release electrons from H₂ oxidation. While the ability to fully reduce O₂ to harmless H₂O at the active site to generate the rapidly reactivated state Ni-B, determines if a hydrogenase is O₂-tolerant, the ratio of oxidative inactivation to reductive reactivation rates determines how tolerant the enzyme is. It is shown by protein film electrochemistry that the (αβ)₂ dimeric assembly of Hyd-1 plays an important role in O₂-tolerance by aiding reactivation of one catalytic unit through electron transfer from the other. The teamwork between two redundant partners implicates a new role for dimerisation and represents a new example of cooperativity in biology. Finally, the non-natural amino acid p-azido-L-phenylalanine was synthesised and incorporated into Hyd-1, testing the possibility of introducing labels at specific sites.