My research primarily focuses on biomimetics of the active sites of the [FeFe]- and [Fe]-hydrogenases (H2ase) and is classified into three parts. Part A: The one-electron oxidation of asymmetrically disubstituted FeIFeI models of the active site of the [FeFe]-H2ase, (mu-pdt)[Fe(CO)2PMe3][Fe(CO)2NHC] (pdt = 1,3- propanedithiolate, NHC = N-heterocyclic carbene) generates mixed valent FeIIFeI models of the Hox state of [FeFe]-hydrogenase. The spectroscopic properties, structures, reactivities and relative stabilities of the one-electron oxidized mixed valent complexes, (mu-pdt)(mu-CO)[FeII(CO)2PMe3][FeI(CO)NHC]+ are discussed in the context of experimental and theoretical data and biological relevance. Part B: DFT computations find the Fe-Fe bond in the FeIFeI diiron models ((mu- pdt)[Fe(CO)2L][Fe(CO)2L'] ( L, L' = CO, PPh3, or PMe3) is thermodynamically favored to produce the mu-oxo or oxidative addition product, FeII-O-FeII, nevertheless the sulfurbased HOMO-1 accounts for the experimentally observed mono- and bis-O-atom adducts at sulfur. The FeII(mu-H)FeII diiron model, (mu-pdt)(mu-H)[Fe(CO)2PMe3]2 (IV-5), for which the HOMO is largely of sulfur character, exclusively yields S-oxygenation. Deoxygenation with reclamation of the mu-pdt parent complexes occurs in a proton/electron coupled process. The possible biological relevance of oxygenation and deoxygenation studies is discussed. Comprehensive investigations of intramolecular CO site change and intermolecular CO/L (L = PMe3 or CN-) exchange of (mu-pst)[Fe(CO)3]2 (IV-1-O), (mu-pdt)[Fe(CO)3]2 (V-1), and their mono-CN-/PMe3 substituted derivatives indicated that the factors influencing the rate of the CO/L exchange reaction of such diiron carbonyls are intramolecular structural rearrangement (or fluxionality) and nucleophilic attack by the incoming ligand. Part C: X-ray diffraction and spectroscopic studies of a series of mono- and disubstituted complexes, FeI2(CO)xL4-x, x = 2 or 3, showed them to be rudimentary structural models of the [Fe]-H2ase active site in native (FeII(CO)2) or CO-inhibited (FeII(CO)3) states. Full characterization of the advanced model complexes ((NS)FeI(CO)2P, NS = 2-amidophenothiolate; P = phosphine) including x-ray diffraction, DFT computations, and Mossbauer studies revealed the interesting "noninnocent" character of these complexes due to the NS ligand. Ligand-based protonation with a strong acid, HBF4Et2O, interrupted the pi-delocalization over Fe and ligand of complex VII-1 and switched on CO uptake (1 bar) and 12CO/ 13CO exchange of VII-1. The intermediate, VII-1-H+, capable of CO uptake, was defined by DFT calculations.

Molecular hydrogen has been proposed as an energy store to help meet the world's ever increasing demand for clean energy because the oxidation product (produced by either combustion or in a fuel cell) results in the formation of water. To realize this goal, energy efficient catalysts comprised of earth abundant elements must be used. The work in this dissertation describes investigations of diiron dichalcogen catalysts used for proton reduction. These complexes are inspired by the active site of the [FeFe]-hydrogenase enzyme. Catalysts were extensively studied with cyclic voltammetry in conjunction with photoelectron spectroscopy and density functional theory calculations in order to determine the effects that bridging ligands and 2Fe2E (E = S or Se) core substitutions have on the electronic structure and catalytic ability of these complexes. The complex µ-(pyrazine-2,3-dithiolato)diironhexacarbonyl (pyrazine-cat) was prepared and found to catalyze proton reduction at a -0.49 V overpotential, which represents a 16% decrease over the previously studied complex µ-(benzene-1,2-dithiolato)diironhexacarbonyl (benz-cat). Electrochemical investigations in conjunction with DFT calculations indicated the possibility of two mechanisms for proton reduction, both of the ECEC type. The first mechanism is Fe-based and analogous to the mechanism reported for benz-cat. The second is a nitrogen-based mechanism which occurs at more negative potentials than the Fe-based mechanism. Overall, pyrazine-cat maintained the ability to mediate successive redox states similar to benz-cat and the electron withdrawing nature of the pyrazine caused the initial reduction to occur at a lower potential than benz-cat. Ultimately this results in the decreased overpotential for catalytic proton reduction by pyrazine-cat. Investigations of the electronic structure and catalytic ability of complexes of the type (µ-ECH₂XCH₂E-µ)Fe₂(CO)₆ where E = S or Se and X = CH₂, S or Se were also carried out. All complexes were found to catalyze H₂ production from acetic acid in acetonitrile. DFT calculations indicate that when X = S or Se the HOMO changes character from predominatly metal based (X = CH2) to containing significant chalcogen lone pair character. The presence of the chalcogen lone pair character helps to facilitate a rotated structure in either the oxidized or reduced forms of these complexes. Through computations it was found that oxidation of the X = S or Se complexes results in a CO ligand rotating into a semi-bridging position, which opens a vacant site on one of the Fe-centers. The bridgehead bends toward this vacant site donating electron density greatly stabilizing the cation and more interestingly forming a structure which strongly resembles the active site of the [FeFe]-hydorgenase. Complexes which contain a chalcogen in the bridgehead undergo potential inversion, leading to a two-electron initial reduction. This is in part due to electron-electron repulsion between chalcogen lone pair electrons and the reduced Fe-centers, which leads to the formation of a rotated dianion.Complexes with the general structure (µ-E (CH₂)nE-µ)Fe₂ (CO)₆ where E = S or Se and n = 3, 4, or 5 were investigated using cyclic voltammetry, photoelectron spectroscopy, and DFT calculations. Substitution of Se in the 2Fe2E core for S resulted in a lengthening of the FeFe bond. As the linker length increased from n =3 to 5, one of the apical CO's is pushed down due to a steric interaction creating a more obtuse Fe-Fe-C angle. Larger effects of the linker length were seen in the oxidation and reduction chemistry. CV and UPS show that linker length has little effect on the oxidation potential or onset ionization energy. Computations predict that the oxidized structure is rotated, and as the linker length increases there is an agostic interaction which forms between a methylene proton and the vacant site on the rotated Fe-center. Reduction potentials for these complexes are found to decrease with increasing linker length, which was attributed to the steric interaction between the alkane linker and the apical CO helping to facilitate rotation of the anion. Interestingly catalytic potentials were found to depend almost entirely on chalcogen character in the 2Fe2E core, with S-containing catalysts having a lower catalytic potential than Se-containing catalysts. The long known complex [η⁵-CpFe(CO)SMe]₂ was investigated as both a proton reduction and H₂ oxidation catalyst. Reduction of [η⁵-CpFe(CO)SMe]₂ revealed that the complex undergoes a two electron irreversible reduction and the reduced species precipitates onto the glassy carbon electrode surface. The new species on the electrode surface facilitates proton reduction at a -0.3 V overpotential, which is significantly lower (0.9 V) than the most similar complex Fp₂. Unlike previous catalysts of this type, [η⁵-CpFe(CO)SMe]₂ catalytic current does not decrease as overpotential decreases. [η⁵-CpFe(CO)SMe]₂ was also shown to undergo two one-electron oxidations, and in the presence of H₂ and the dication, appears to oxidize H₂. The ability of [η⁵-CpFe(CO)SMe]₂ to both oxidize H₂ and reduce protons to H₂ addresses a known deficiency for catalysts mimicking the function of the active site of the [FeFe]-hydrogenase.

Ces travaux décrivent la synthèse et l'étude de la réactivité de complexes binucléaires à fer, développés dans le but de mimer l'activité catalytique d'enzymes binucléaires à fer telles que la méthane monooxygénase soluble. Nous avons synthétisé et caractérisé plusieurs complexes binucléaires à fer(III), possédant différents types de groupements (électro-donneurs, électro-attracteurs, donneurs de liaison hydrogène) dans leur seconde sphère de coordination, de façon symétrique ou dissymétrique. Nous avons étudié l'influence de la seconde sphère de coordination de ces différents complexes sur les différents intermédiaires formés (notamment l'intermédiaire µ-peroxo-FeIIIFeIII) lorsque que ces complexes sont exposés au peroxyde d'hydrogène. Nous avons également étudié la capacité de ces différents complexes à catalyser les réactions d'oxydation de différents substrats (sulfures, alcènes et alcanes) par le peroxyde d'hydrogène, en absence et en présence d'eau dans le milieu réactionnel. Une modification intéressante de la chimiosélectivité de la réaction d'oxydation du cyclooctène par le peroxyde d'hydrogène, en présence d'un complexe binucléaire à fer dissymétrique et d'eau dans le milieu réactionnel a été observée. Différentes études d'aspect mécanistique ont été réalisées afin de déterminer l'origine des différents phénomènes observés en catalyse d'oxydation. / This work describes the synthesis and the study of the reactivity of diiron complexes, developed in order to mimic the catalytic activity of diiron enzymes such as the soluble methane monooxygenase. We synthesized and characterized several diiron(III) complexes, bearing different types of groups (electron-donating, electron-withdrawing, hydrogen bond donating) in their second coordination sphere, in a symmetrical or non-symmetrical way. We studied the influence of the second coordination sphere of these different complexes over the different intermediates (particularly the µ-peroxo-FeIIIFeIII intermediate) formed by exposing them to hydrogen peroxide. We also studied the ability of these complexes to catalyze the oxidation of various substrates (sulfurs, alkenes, alkanes) by hydrogen peroxide, in absence or in presence of water in the reaction mixture. An interesting modification of chemoselectivity was observed in the case of oxidation of cyclooctene by hydrogen peroxyde, catalyzed by a non-symmetrical diiron complex, in presence of water in the reaction mixture. Several mechanistic studies were performed in order to investigate on the origin of the phenomenons we observed during oxidation catalysis studies.

<p>Diiron carboxylate proteins constitute an important class of metall-containing enzymes. These proteins perform a multitude of reactions in biological systems that normally involve activation of molecular oxygen at the diiron site.</p><p>During activation and functioning of these proteins their diiron sites undergo redox changes in a rather wide range: from diferrous (FeII-FeII) to high potential intermediate Q(FeIV-FeIV). Two of these redox states are paramagnetic: (FeIV-FeIII), called high potential intermediate X, and (FeII-FeIII), called mixed-valent state of the diiron carboxylate proteins. In the present work it has been shown that these redox states are of functional relevance in two proteins with different functions.</p><p>Ribonucleotide reductase (RNR) from the human parasite<i> Chlamydia trachomatis</i> is a class I RNR. It is typical for class I RNR to initiate the enzymatic reaction on its large subunit, protein R1, by activation from a stable tyrosyl free radical in its small subunit, protein R2. This radical, in its turn, is formed through oxygen activation by the diiron center. In C. trachomatis the tyrosine residue is replaced by phenylalanine, which cannot form a radical. We have shown in the present work, that active <i>C. trachomatis</i> RNR uses the FeIII-FeIV state of the diiron carboxylate cluster in R2 instead of a tyrosyl radical to initiate the catalytic reaction.</p><p>The alternative oxidase (AOX) is a ubiquinol oxidase found in the mitochondrial respiratory chain of plants. The existence of the diiron carboxylate center in this protein was predicted on the basis of a conserved sequence motif consisting of the proposed iron ligands, four glutamate and two histidine residues. In experiments modeling the conditions of the enzyme catalytic cycle, i.e. reduction and reoxygenation of the overexpressed AOX in <i>Escherichia coli</i> membranes we were able to generate an EPR signal characteristic of a mixed-valent Fe(II)/Fe(III) binuclear iron center. The alternative oxidase is the first membrane protein where the existence of the diiron carboxylate center has been shown experimentally.</p>

Diiron carboxylate proteins constitute an important class of metall-containing enzymes. These proteins perform a multitude of reactions in biological systems that normally involve activation of molecular oxygen at the diiron site. During activation and functioning of these proteins their diiron sites undergo redox changes in a rather wide range: from diferrous (FeII-FeII) to high potential intermediate Q(FeIV-FeIV). Two of these redox states are paramagnetic: (FeIV-FeIII), called high potential intermediate X, and (FeII-FeIII), called mixed-valent state of the diiron carboxylate proteins. In the present work it has been shown that these redox states are of functional relevance in two proteins with different functions. Ribonucleotide reductase (RNR) from the human parasite Chlamydia trachomatis is a class I RNR. It is typical for class I RNR to initiate the enzymatic reaction on its large subunit, protein R1, by activation from a stable tyrosyl free radical in its small subunit, protein R2. This radical, in its turn, is formed through oxygen activation by the diiron center. In C. trachomatis the tyrosine residue is replaced by phenylalanine, which cannot form a radical. We have shown in the present work, that active C. trachomatis RNR uses the FeIII-FeIV state of the diiron carboxylate cluster in R2 instead of a tyrosyl radical to initiate the catalytic reaction. The alternative oxidase (AOX) is a ubiquinol oxidase found in the mitochondrial respiratory chain of plants. The existence of the diiron carboxylate center in this protein was predicted on the basis of a conserved sequence motif consisting of the proposed iron ligands, four glutamate and two histidine residues. In experiments modeling the conditions of the enzyme catalytic cycle, i.e. reduction and reoxygenation of the overexpressed AOX in Escherichia coli membranes we were able to generate an EPR signal characteristic of a mixed-valent Fe(II)/Fe(III) binuclear iron center. The alternative oxidase is the first membrane protein where the existence of the diiron carboxylate center has been shown experimentally.

[FeFe]-Hydrogenases (H2ases) are metalloenzymes that can catalyze the reversible reduction of protons to molecular hydrogen as part of the metabolism of certain cyanobacteria and green algae. Due to the low availability of the enzyme, synthetic complexes that mimic the natural active site in structure, function and activity are highly sought after. In this thesis, a number of [FeFe]-H2ases active site model complexes were synthesized to answer open questions of the active site and to develop unprecedented bio-inspired proton reduction catalysts. The first part describes the synthesis and the protonation properties of a [Fe2(μ-adt)(CO)4(PMe3)2] (adt = azadithiolate) complex which contains two basic sites that are similar to those found in the enzyme active site. Unusual kinetic factors give rise to four discrete protonation states. The twofold protonated state is the first model complex that simultaneously carries a proton at the azadithiolate nitrogen and a bridging hydride at the Fe-Fe bond. In the second part, a model complex with an unprecedented amine ligand was synthesized and studied. In analogy to the enzyme active site, the labile amine ligand is expelled after electrochemical reduction. The third part describes a series of model complexes with electronically different aromatic dithiolate ligands. It is demonstrated in one case that the tuning of the ligand by electron-withdrawing substituents results in proton reduction catalysis at an overpotential that is lower than that required by the non-substituted parent compound. The design and the synthetic work towards a new ruthenium-diiron dyad for light-driven hydrogen production are presented in the fourth part. In the final part, differently isotope-labelled mixed valent Fe(I)-Fe(II) model complexes were synthesized, in particular the unprecedented 15N labelled analogue, with the aim to provide EPR-spectroscopic references that will allow the elucidation of the nature of the central atom in the dithiolate bridge of the [FeFe] hydrogenase active site.

The local structures of the iron atoms for a series of strongly coupled Fe 2 (TIED)L 4 complexes (TIED = tetraiminethylenedimacrocycle, L = axail ligand) have been investigated by K-edge X-ray Absorption Spectroscopy (XAS). These complexes include not only the well characterized iso-valence CH 3 CN complex, mixed-valence CH 3 CN and Cl − complexes, and previously reported iso-valence CO complex but also the new isolated solids of iso-valence Fe 2 TIED complexes with Cl − , Br − , imidazole, pyridine, histidine, N,N-dimethyformamide (DMF), SCN − , and CN − as axial ligands and mixed-valence complexes with Br − and imidazole as axial ligands. The average Fe-N distances for the first coordination sphere of the iron atom obtained by EXAFS analysis are 1.94, 1.94, 1.95, 1.96, 1.94, 1.93, 1.96, 1.96, 1.96, and 1.96 Å for the iso- and mixed-valence CH 3 CN and imidazole complexes and iso-valence complexes with SCN − , CN − , CO, pyridine, histidine, and DMF as axial ligands, respectively. Two-shell fitting analyses of the complexes gave average iron to the four planar coordinated nitrogen distance of 1.90, 1.91, 1.91, 1.92, and 1.92 Å for the Fe 2 (TIED)L 4 with L = DMF, pyridine, Cl − , Br − , imidazole, and histidine complexes, respectively. The average distances from the center iron to: N(DMF), 2.05; N(pyridine), 2.05; Cl − , 2.33; Br − , 2.45; N(imidazole), 2.08; and N(histidine), 2.07 Å. They are all comparable to related bond distances in the literature. The above data indicate that there is no significant difference in the average Fe-N distances between each of the iso- and mixed-valence pairs. Also different axial ligands do not cause significant impact on the average Fe-N distances from the iron atom to the four coordinated N in the TIED ligand. The threshold edge positions shift about +1 eV from the iso-valence CH 3 CN, Cl − , and Br − complexes to their corresponding mixed-valence complexes. The relatively small shift compared with the normal +2 [special characters omitted] +3 eV edge shift from Fe 2+ to Fe 3+ reflects the oxidation state change of iron from Fe 2+ to Fe 2.5+ . The edge energy of the isovalence diiron complexes with different axial ligands increases in the order of the spectrochemical series of the axial Iigands from strong to weak field Iigands as follows [58, 59]: [special characters omitted] All the complexes studied here have a weak dipole-forbidden 1s → 3d pre-edge transition. The low intensity indicates only a small distortion of the octahedral coordination geometry of the central iron atom.

Arsenic contamination of groundwater associated with mining operations is a widespread problem across the globe. The release of arsenic (As) into the environment occurs naturally by oxidation of exposed sulfide minerals. In the case of gold ores, the mining and beneficiation may also produce As-bearing wastes and this can accelerate the natural mechanisms of As mobilization. The Bakyrchik is the largest gold deposit in Kazakhstan and one of the largest in the world. Gold (Au) is dispersed in pyrite and arsenopyrite in the form of microscopic inclusions. Despite the fact that only 10% of gold ore has been mined to date, it has left behind a dangerous As-containing legacy. Speciation of As has been determined for samples from Bakyrchik to understand the post-processing environment and the mobility of arsenic in the mining-influenced area. As(III) and As(V) have been detected in water samples using HPLC-ICP-MS. The variability of As species across the narrow pH and Eh range indicates that biogeochemical processes can play a role in the speciation of As in water at the study site. In order to understand processes controlling As mobilization in water, the solid phase speciation of As in Bakyrchik sediments, soil, and metallurgical processing products has been investigated using XRD and EPMA. This revealed arsenopyrite, As-bearing pyrite, and their alteration products containing up to 25% As, iron oxides and oxyhydroxides (with up to 2.5% As), haidingerite, and calcium arsenate in studied samples. Sequential extraction demonstrated that in soil and sediment samples As is associated mainly with Al and amorphous Fe oxyhydroxides. Results suggest that the main mechanisms controlling As mobility in Bakyrchik are dilution with regional waters, adsorption onto iron and aluminium oxyhydroxides, and co-precipitation of dissolved As with alteration products of sulfide minerals. Assessment of As bioaccessibility through inhalation demonstrates its strong dependence on the mineralogy rather than on total As content of the solid samples. Calculated cancer and non-cancer risks of inhalation exposure imply that all samples are highly hazardous for human health. With the depletion of the oxide lode ore deposits, gold extraction is moving towards the mining of technologically difficult ores, such as those found at Bakyrchik. A new method of Au recovery from double refractory Au-As-bearing concentrates has been developed, allowing recovery of 97% of Au, and the conversion of up to 95% As into iron-arsenic alloy. Fe-As alloy can contain up to 40% As, and do not require further solidification/stabilisation prior to disposal. The method has been published as a patent with the Patent Office of the Republic of Kazakhstan. Quantitative phase composition of the Fe-As alloy, has been determined by EPMA, QEMSCAN, X-ray and neutron diffraction. Toxicity and solubility of Fe-As alloy in aqueous solutions have been characterised. High-temperature structural behaviour of Fe-As alloy and Fe$_{2}$As in inert atmosphere has been determined by $\textit{in situ}$ synchrotron XRD. Results of the study support the development of the new method as an efficient alternative for processing double refractory Au-As-bearing concentrates.