This dissertation consists of two independent topics: (1) a molecular loop with interstitial channels in a chiral environment; (2) study of formation of metal-metal bonds in dinickel, dipalladium and dititanium complexes On the first topic, a study of the reaction products of the interaction of cis- Mo2(DAniF)2(CH3CN)4 2+ corner pieces with ortho-, meta- and para- isomers of enatiomerically pure –O2CCH(CH3)C6H4CH(CH3)CO2 – dicarboxylate was performed. First, an enantiomerically pure molecular loop based on two dimolybdenum units and two para-dic arboxylate linkers was synthesized and structurally characterized. Similar reactions with isomeric ortho- and meta- dicarboxylate linkers, as well as with some nonchiral ligands, showed that the structure of the obtained products depends on the geometry of the ligand. Meta- dicarboxylate linker favors the formation of the chelated product and ortho- dicarboxylate linker produces the mixture of chelated molecules and loops. On the second topic, an investigation of the formation of metal-metal bonds was performed. Study of the one-electron bond obtained upon oxidation of Ni2 4+ and Pd2 4+ to Ni2 5+ and Pd2 5+, respectively, was made. The compounds synthesized were studied with various physical methods, such as X-ray crystallography, UV-visible spectroscopy and EPR spectroscopy. The nature of oxidized species as well as the dependence of metalmetal interactions on electron-donating abilities of bridging ligands was studied. It was shown that oxidation takes place on a metal center. The formation of one-electron bond in oxidized species is proposed. Finally formation of Ti2 6+ single bonded compounds by the reduction of two Ti4+ monomers to Ti2 6+ dimer was studied. The nature of the species obtained in solution and in solid state is discussed. The crystal structure shows the presence of two types of hpp ligands – chelating and bridging. NMR study of this compound in solution proposes the rearrangement of this structure to a paddlewheel.

No description available.

A series of dimolybdenum compounds having a Mo2 4+ core coordinated by various ligands, including formamidinate (e.g. DAniF = N, NN-di-p-ansisylformamidinate ), acetate and/or acetonitrile molecules, have been synthesized as building blocks for the construction of Mo2-containing supramolecular arrays. Compound Mo2(DAniF)3(O2CCH3) was specifically designed for the preparation of dimolybdenum pairs, whereas the others meet the needs of Mo2 4+ units for different geometry settings. Compounds described by a general formula [Mo2]L[Mo2], where [Mo2] = [Mo2(DAniF)3]+, have two dimetal units electronically coupled by the central unit L , which consequently engender significant impact on the redox property and electronic structure of the molecule. It is found that in the weakly coupled complex system, [Mo2]M(OCH3)4[Mo2] (M = Zn and Co), the mixed-valence complexes present asymmetric molecular structures with two distinct [Mo2] units corresponding to be a bond order 4.0 (F2B4*2) and 3.5 (F2B4*1), respectively. EPR and magnetic susceptibility measurements for the doubly oxidized species show that there is no significant antifferromagnetic spin coupling. Electron delocalization occurs in the complex system where a N, N'-dimethyloxamidate binds two [Mo2] units within two fused six-membered rings. In this case, the mixed-valence complex has a symmetric molecular structure, implying that the odd electron is fully delocalized over two [Mo2]units. Strong metal-metal interaction is also evidenced by intervalence charge transfer of the mixed-valence species and the diamanetism of the doubly oxidized complex. Remarkably, two isomers varying in linkage conformation, namely, alpha and beta, have been isolated as diaryloxamidate ligands are used as the linker. Studies on the neutral and the oxidized compounds of the two isomers by employing various techniques consistently show that in the alpha form intramolecular electron transfer is blocked , while in the beta form, the electrons are delocalized over the two [Mo2] units. Thus, the mixed-valence complexes of the two isomers are appropriately described by alpha-[Mo2]0(oxamidate)[Mo2]1+ and beta- [Mo2]0.5+(oxamidate)[Mo2]0.5+ respectively.

Important reactions are catalyzed by enzymes employing polynuclear cofactors, often characterized by weak-field ligands and transition metal ions within the sum of the van der Waals radii. While the overall stoichiometries and, in many cases, the structures, of the cofactors are known, the roles of the individual metal ions remain uncertain. Our approach is to investigate model clusters stabilized by a hexadentate, trinucleating ligand. The hexaamine ligand \((MeC (CH_2NHC_6H_4-o-NHPh)_3) (^{Ph}LH_6)\) allows facile synthesis of the clusters \((^{Ph}L)Fe_3(thf)_3\), \((^{Ph}L)Fe_3 (py)_3\), and \((^{Ph}L)Fe_3(PMe_2Ph)_3\) (thf = tetrahydrofuran, py = pyridine). The phenyl substituents on the ligand sterically prevent strong M–M bonding, but permit weaker M–M orbital interactions, with Fe–Fe distances near those found in Fe metal. The complex \((^{Ph}L)Fe_3(thf)_3\) exhibits a well-isolated S = 5 or S = 6 ground state over 5 - 300 K, as evidenced by magnetic susceptibility and reduced magnetization data. However, in the stronger-field pyridine and phosphine complexes, temperature dependent susceptibility is observed which is best modeled as a spin state transition from S = 2 to S = 4. Variable-temperature crystallography and Mössbauer spectroscopy reveal a whole-molecule, rather than site-isolated, spin transition. The all-ferrous cluster \((^{Ph}L)Fe_3(thf)_3\) can be oxidized with triphenylmethyl halides or iodine to give singly-oxidized clusters of the form \((^{Ph}L)Fe_3X(L)\) and \([(^{Ph}L)Fe_3(\mu-X)]_2 (X = Cl, Br, I; L = thf, py)\), in which one Fe–Fe distance contracts to 2.30 Å and the others lengthen to 2.6-2.7 Å. The halide and solvent ligands coordinate a unique Fe, but Mössbauer spectroscopy shows that the diiron pair bears the oxidation. Magnetic data can be modeled by considering a high-spin ferrous ion ferromagnetically coupled to an \(S = 3/2 [Fe_2]^{5+}\) unit. When \([(^{Ph}L)Fe_3(\mu-Cl)]_2\) is reacted with two or five equivalents of \(CoCl_2\) in tetrahydrofuran, the fully-substituted complexes \((^{Ph}L)Fe_2CoCl(acn)\) and \((^{Ph}L)FeCo_2Cl(acn)\) (acn = acetonitrile) can be isolated. \(^1H\) nuclear magnetic resonance shows that they are distinct species, not a mixture, and the elemental ratios are confirmed by X-ray fluorescence spectroscopy. Mössbauer spectroscopy shows that the Co preferentially substitutes into the \([M_2]^{5+}\) unit, as the ferrous site doublet is completely absent in \((^{Ph}L)FeCo_2Cl(acn)\). / Chemistry and Chemical Biology

This dissertation details the use of click chemistry to prepare linear and nonlinear polymers containing metal-metal bonds. The incorporation of metal-metal bonds into the polymer simplfies the degradation mechanism, allowing fundamental mechanistic studies of polymer degradation. Click chemistry offered a brand new route to explore the preparation of these useful but intricate metal-metal bond-containing polymers. Chapter I discusses the utility of these types of polymers for mechanistic studies, the preparation of metal dimers with reactive functionalities, and the previous polymerization methods which have been explored. The need for a new polymerization strategy, such as click chemistry, is described. Chapter II explains the preparation of a new metal dimer click synthon, [(η5-C5H4(CH2)3OC(O)(CH2)2C≡CH)Mo(CO)3]2, and the necessary conditions needed to polymerize the synthon using click chemistry. A high molecular weight linear polymer was prepared, suggesting click chemistry is a viable route to nonlinear polymers. Chapter III presents a second novel metal dimer click synthon, [(η5-C5H4(CH2)3N3Mo(CO)3]2, and attempts to use click chemistry to prepare a star polymer containing metal-metal bonds. A small amount of nonlinear polymer was prepared but several reactivity problems were also discovered and addressed. Due to these problems with click chemistry, Chapter IV details a brand new method for preparing asymmetric metal dimers. CpMo(CO)3-Mo(CO)3Cp(CH2)3CH=CH2 is the first reported example of an asymmetric dimer, and (CH3)3CSi(CH3)2O(CH2)3CpMo(CO)3-Mo(CO)3Cp(CH2)3OC(CH3)2OCH3 is the first example of a bifunctional asymmetric dimer. Chapter V describes the synthesis of a different type of metal dimer, (CH3)2Si[(C5H5)Mo(CO)3]2, which is polymerized by thermal ring opening polymerization. The dimer did not polymerize as expected and yielded an interesting polymer which has both Mo-Mo single bonds and Mo≡Mo triple bonds. Finally, Chapter VI provides a summary of the work as well as an honest perspective of using click chemistry to prepare metal-metal bond-containing polymers. This dissertation includes previously published and unpublished co-authored material. / 10000-01-01

This thesis describes the synthesis, structure, equilibriaand other properties of novel thallium(III) monomeric andplatinum-bonded complexes with nitrogen donor ligandsethylenediamine, diethylenetriamine, triethylenetetramine,porphyrin, 2,2'-bipyridine and 1,10-phenanthroline in solutionand in solid. The existence of three complexes withthe general formula[Tl(en)n]3+(n = 1-3) and their overall stability constantshave been established in pyridine. All three complexes wereidentified by their205Tl and1H NMR chemical shifts and205Tl-1H coupling constants. The formation process of thecomplexes was followed by1H NMR spectroscopy. The crystal structure of[Tl(en)3](ClO4)3was determined; the thallium(III) ion isN-coordinated in a distorted octahedral geometry. Two [Tl(dien)n]3+(n = 1-2) complexes were proved to exist insolution and the structure of the bis-complex [Tl(dien)2]2+inu-facialisomers was determined in solid. In addition,crystal structures of [Tl(en)2CN](ClO4)2with cyanide bridging between two Tl(en)2units forming an infinite chain structure and of[Tl(tren)2(CN)2](ClO4) with a distorted pseudo-octahedral coordinationaround thallium were determined. Thallium(III) complexes with2,2'-bipyridine and 1,10-phenanthroline have been studied inDMSO using205Tl,13C and1H NMR spectroscopy. In addition, aseven-coordinated thallium was found in the crystal structureof [Tl(bipy)3(dmso)](ClO4)3, and six-coordinated thallium in pseudo-octahedralgeometry in [Tl(phen)2Cl2](ClO4). The solvated complex [Tl(dmso)6]3+has been prepared using concentrated aqueoussolution of Tl(ClO4)3by a solvent replacement reaction in DMSO, and thewater-free solid compound [Tl(dmso)6](ClO4)3was crystallized from DMSO. The structure of thecomplex [Tl(dmso)6]3+is a regular octahedron with the Tl-O bonddistance 2.224(3) Å. It represents an easy and secure wayto introduce water-free Tl(III) into organic phase withoutreduction. Through several reactions, novel heteronuclear Pt-Tlcomplexes with the composition [(NC)5Pt-Tl(tpp)]2-, [(NC)5Pt-Tl(thpp)]2-, [(NC)5Pt-Tl(bipy)n](n = 1-2), [(NC)5Pt-Tl(en)n-1](n = 1-3) and [(NC)5Pt-Tl(phen)n](n = 1-2), have been synthesized in solution.Multinuclear NMR (195Pt,205Tl,13C and1H), Raman spectroscopy and X-ray diffraction dataare fully compatible with formation of unsupported Pt-Tl bondedcomplexes both in solution and in solid. The huge1J(195Pt-205Tl) spin-spin coupling constants (48-66 kHz) wereobserved by both195Pt and205Tl NMR spectroscopy in solution and they providea strong evidence of formation of the Pt-Tl bond in solution.In all six determined crystal structures of the Pt-Tl compoundsa very short Pt-Tl bond is found with distances2.6117(5)-2.6375(5) Å. The calculated values of Pt-Tlforce constants (1.38-1.91 N/cm) are characteristic for asingle metal-metal bond. In the Pt-Tl compounds, the oxidation state of the metalions is intermediate between the stable states PtII/PtIVand TlIII/TlI, respectively, and this is reflected by their195Pt and205Tl chemical shifts. It turns out that N-donorligands can really stabilize the Pt-Tl bond both in solutionand in solid. The character of the metal-metal bond anditstheoretical basis are discussed. <b>Keywords:</b>Thallium, Platinum, Cyanide, N-donor ligand,Metal-metal bond, Multinuclear NMR, Raman spectroscopy, X-raydiffraction, Equilibrium, Spin-spin coupling.

<p>This thesis describes the synthesis, structure, equilibriaand other properties of novel thallium(III) monomeric andplatinum-bonded complexes with nitrogen donor ligandsethylenediamine, diethylenetriamine, triethylenetetramine,porphyrin, 2,2'-bipyridine and 1,10-phenanthroline in solutionand in solid.</p><p>The existence of three complexes withthe general formula[Tl(en)_n]³⁺(n = 1-3) and their overall stability constantshave been established in pyridine. All three complexes wereidentified by their²⁰⁵Tl and¹H NMR chemical shifts and²⁰⁵Tl-¹H coupling constants. The formation process of thecomplexes was followed by¹H NMR spectroscopy. The crystal structure of[Tl(en)₃](ClO₄)₃was determined; the thallium(III) ion isN-coordinated in a distorted octahedral geometry. Two [Tl(dien)_n]³⁺(n = 1-2) complexes were proved to exist insolution and the structure of the bis-complex [Tl(dien)₂]²⁺in<i>u-facial</i>isomers was determined in solid. In addition,crystal structures of [Tl(en)₂CN](ClO₄)₂with cyanide bridging between two Tl(en)₂units forming an infinite chain structure and of[Tl(tren)₂(CN)₂](ClO₄) with a distorted pseudo-octahedral coordinationaround thallium were determined. Thallium(III) complexes with2,2'-bipyridine and 1,10-phenanthroline have been studied inDMSO using²⁰⁵Tl,¹³C and¹H NMR spectroscopy. In addition, aseven-coordinated thallium was found in the crystal structureof [Tl(bipy)₃(dmso)](ClO₄)₃, and six-coordinated thallium in pseudo-octahedralgeometry in [Tl(phen)₂Cl₂](ClO₄).</p><p>The solvated complex [Tl(dmso)₆]³⁺has been prepared using concentrated aqueoussolution of Tl(ClO₄)₃by a solvent replacement reaction in DMSO, and thewater-free solid compound [Tl(dmso)₆](ClO₄)₃was crystallized from DMSO. The structure of thecomplex [Tl(dmso)₆]³⁺is a regular octahedron with the Tl-O bonddistance 2.224(3) Å. It represents an easy and secure wayto introduce water-free Tl(III) into organic phase withoutreduction.</p><p>Through several reactions, novel heteronuclear Pt-Tlcomplexes with the composition [(NC)₅Pt-Tl(tpp)]^2-, [(NC)₅Pt-Tl(thpp)]^2-, [(NC)₅Pt-Tl(bipy)_n](n = 1-2), [(NC)₅Pt-Tl(en)_n-1](n = 1-3) and [(NC)₅Pt-Tl(phen)_n](n = 1-2), have been synthesized in solution.Multinuclear NMR (¹⁹⁵Pt,²⁰⁵Tl,¹³C and¹H), Raman spectroscopy and X-ray diffraction dataare fully compatible with formation of unsupported Pt-Tl bondedcomplexes both in solution and in solid. The huge¹J(¹⁹⁵Pt-²⁰⁵Tl) spin-spin coupling constants (48-66 kHz) wereobserved by both¹⁹⁵Pt and²⁰⁵Tl NMR spectroscopy in solution and they providea strong evidence of formation of the Pt-Tl bond in solution.In all six determined crystal structures of the Pt-Tl compoundsa very short Pt-Tl bond is found with distances2.6117(5)-2.6375(5) Å. The calculated values of Pt-Tlforce constants (1.38-1.91 N/cm) are characteristic for asingle metal-metal bond.</p><p>In the Pt-Tl compounds, the oxidation state of the metalions is intermediate between the stable states Pt^II/Pt^IVand Tl^III/Tl^I, respectively, and this is reflected by their¹⁹⁵Pt and²⁰⁵Tl chemical shifts. It turns out that N-donorligands can really stabilize the Pt-Tl bond both in solutionand in solid. The character of the metal-metal bond anditstheoretical basis are discussed.</p><p><b>Keywords:</b>Thallium, Platinum, Cyanide, N-donor ligand,Metal-metal bond, Multinuclear NMR, Raman spectroscopy, X-raydiffraction, Equilibrium, Spin-spin coupling.</p>

The chemistry of metal–metal (M–M) multiply bonded compounds has fascinated inorganic chemists for a period spanning more than five decades. Since the elucidation of the quadruple bond by Cotton in 1964, thousands of compounds featuring M–M bonds have been isolated and studied. Of these, dinuclear units supported by four bidentate ligands forming a ‘paddlewheel’ motif represent a class of compounds that present unique molecular and electronic structures, and useful electrochemical and magnetic properties.<div><br></div><div>Over the last two and a half decades, our laboratory has focused on studying diruthenium paddlewheel complexes for their easeof preparation, rich electrochemical properties,and remarkable stability. We have isolated a vast number of diverse diruthenium alkynyls in multiple oxidation states, bearing different paddlewheel (equatorial) ligand systems and studied their molecular and electronic structures. Taking advantage of the extended conjugation that exists between the Ru2core and the poly-alkynyl ligand motif, we have also found applications for them in prototypical flash-memory devices. At this juncture, we sought to expand the organometallic chemistry of Ru2to complexes featuring Ru–aryl linkages.<br></div><div><br></div><div>The ‘aryl anion’ is, based on pKa, twenty orders of magnitude more basic than the corresponding acetylide. Arguably, this difference should result in a more electron-rich dinuclear core with new electronic structures waiting to be explored. Although kinetically more reactive than metal–alkynyls, metal–aryls are still more stable than the corresponding metal–alkyls. However, for second-row transition metals like ruthenium, kinetic instability issues are somewhat more suppressed than for their first-row counterparts.<br></div><div><br></div><div>Armed with the knowledge that it was reasonable to expect somewhat stable metal–aryl complexes, the synthesis and characterization, and analyses of molecular and electronic structures of diruthenium aryls were attempted. By employing relatively simple lithium-halogen exchange reactions, both mono and bis-aryl complexes of diruthenium have been isolated. Additionally, two different oxidation states of diruthenium have beenaccessed, namely Ru2(II,III)and Ru2(III,III),by judiciously modifying the paddlewheel ligands. Following this, preliminary reactivity studies of Ru2(II,III) monoaryls of the form Ru2(ap)4Ar were performed, which yielded surprising results. This work led to the conclusion that the diruthenium–aryl interaction is an example of a metal–metal–ligand interaction that can bring reactivity to the distal metal site. Moreover, it was found that even minor changes in axial ligands can bring about major upheavals in electronic structure.<br></div><div><br></div><div>Computational investigations into the electronic structure of the above-mentioned compounds have faced many a barrier because of the complexity of the system. The deep mixing of the metal–metal and metal–ligand valence manifolds is more easily isolated into its constituent parts in the case of relatively simple structures such as the monoaryls, Ru2II,IIIL4Ar. However, electronic structure calculations are fraught with difficulties in the case of heavily distorted axially disubstituted mono and bis-aryls, (X)Ru2III,IIIL4Ar and Ru2III,IIIL4Ar2, respectively. Ru2III,IIIL4Ar2complexes present an interesting case of second order Jahn-Teller distortion (SOJT), which has been adequately modeled. However, the more heavily distorted case of XRu2(ap)4Ar (X = CCH, CN, CO, etc.) pose greater computational challenges, such as low-lying excited states, spin-admixed ground states and difficulties in isolating metal and ligand contributions to the valence manifold. <br></div><div><br></div><div>Our investigations into diruthenium aryls began as a mere curiosity that arose out of a serendipitous discovery. Two years later, our continued efforts in this direction have yielded rather fruitful results. The unusual structures and associated complex bonding motifs in these systems have taught us about the importance of metal–metal–ligand interactions as more than just a sum of metal–metal and metal–ligand parts.<br></div>

Le mercure est présent dans l'environnement à cause de phénomènes naturels (volcans) ou des activités humaines (combustion de combustibles fossiles). Le mercure existe sous forme de mercure élémentaire (Hg0), ionique (HgII) ou organique tel le méthylmercure (MeHg). Ces diverses formes sont en flux constant les uns avec les autres dans le cycle biogéochimique naturel. De par leur grande hydrophobicité et leur capacité à pénétrer les membranes biologiques, les composés organomercuriels contituent la forme la plus toxique de mercure retrouvée dans l’environnement Des niveaux élevés de MeHg ont d’ailleurs été détectés dans la chaire de poissons de nombreuses régions du monde. Conséquemment, une consommation de produits de la mer contaminés représente un grave danger pour la santé humaine. Certaines bactéries isolées à partir d'environnements contaminés par le mercure ont évolué vers un système qui leur permet de convertir efficacement les composés mercuriels présents autant sous forme ionique qu’organique en un mercure élémentaire moins toxique. Cette résistance au mercure s’explique par l'acquisition d'un élément génétique connu sous le nom d’opéron mer. L’opéron mer code entre autre pour deux enzymes importants : la lyase organomercurielle MerB et la réductase mercurielle MerA. MerA catalyse la réduction du HgII conduisant à la formation du mercure élémentaire Hg0 qui est un composé volatile et moins toxique. MerB, quant à elle, catalyse la protonolyse de la liaison carbone-mercure de composés organomercuriels pour produire un composé réduit de carbone et du mercure ionique (HgII). Au vu des effets des organomercuriels et de la réduction de HgII, MerA et MerB sont considérés comme des enzymes clés pouvant servir à la biorestauration des cours d'eau contaminés par les organomercuriels. Une compréhension claire des détails mécanistiques de la façon dont MerA et MerB fonctionnent ensemble au niveau atomique est donc cruciale dans la mise en œuvre de biotechnologies implicant l’opéron mer dans les efforts de bioremédiation. Dans cette étude, nous avons utilisé la résonance magnétique nucléaire (RMN)et la cristallographie aux rayons X pour caractériser la structure et le mécanisme enzymatique de MerB de E. coli. Sur la base d’études structurales précédentes de MerB de E. coli, trois résidus (Cys96, Asp99 et Cys159) ont été identifiés comme constituant la triade catalytique nécessaire au clivage de la liaison carbone-Hg. En guise de suivi aux études antérieures, mon projet consiste d’abord à utiliser la cristallographie aux rayons X afin de définir les rôles de Cys96, Asp99 et Cys159 dans la liaison du substrat et dans le clivage. Deux approches ont été mises en œuvre pour atteindre cet objectif. Tout d'abord, les mutants MerB ont été testés pour définir le rôle des résidus catalytiques. Deuxièmement, les inhibiteurs de MerB et d'autres substrats non organicomercuriels potentiels ont été utilisés pour explorer le site actif de MerB. Une sérine se retrouve à la position de Asp99 dans quatre variants de MerB répertoriés chez les bactéries. Pour mieux comprendre le rôle de Asp99, nous avons comparé la sérine présente dans le variants MerB de Bacillus megaterium (MerB2) et introduit un variant D99S à la protéine MerB du type sauvage d’E. coli (MerB D99S). Nous avons pu constater que la forme purifiée de MerB D99S se caractérisait par une couleur rose après avoir visualisé sa structure cristalline aux rayons X, révélant la présence d'un métal lié au niveau de son site actif. Les analyses par spectrométrie de masse à plasma à couplage inductif (ICP-MS) et par fluorescence des rayons X indiquèrent que MerB D99S se liait au cuivre au niveau du site actif. En outre, les analyses par résonance paramagnétique électronique (EPR) et des études de RMN ont identifié la forme CuII du cuivre. L'addition de substrats organomercuriels a pu déplacer le CuII entrainant ainsi une diminution de l’activité catalytique de MerB D99S. En revanche, MerB2 n'a pu être co-purifié avec le cuivre, bien que la structure aux rayons X du complexe MerB2-Hg soit pratiquement identique à la structure du complexe MerB D99S-Hg. Ceci suggère que le résidu Asp99 est essentiel au clivage des liaisons carbone-Hg de composés organiques du mercure et dirige la spécificité de la liaison au métal. De plus, la liaison cuivre-MerB D99S propose un lien possible entre l'évolution de MerB et son homologue structural, la protéine NosL. Dans la seconde approche, nous nous sommes intéressés au site actif de MerB en testant sa liaison à des composés organostanniques et à des composés organoplombiques avec un inhibiteur de MerB connu sous le nom de triéthylétain (TET) qui se lie au résidu Asp99 sans s’associer aux cystéines du site actif. Une liaison similaire a été observée avec un autre inhibiteur à savoir le triméthylplomb (TML). Quant au diméthylétain (DMT), il inhibe MerB à l'aide d'un mécanisme alternatif en se liant d'abord à Asp99 puis à Cys96 conduisant à un changement critique dans le site actif perturbant ainsi l’interaction π-cation entre Trp95 et Arg155. D’autres inhibiteurs comme le diéthylétain (DET) et le diéthylplomb (DEL) ont été caractérisés comme étant un substrat de MerB où les deux groupes éthyle ont été clivés pour donner les produits ioniques SnIV PbIV qui se lient au site actif de manière similaire à HgII. DMT, DET et DEL présentent une affinité pour la liaison à MerB supérieure à celle de son substrat initial MeHg. Ces résultats suggèrent que les composés organomercuriels ne sont pas les seuls substrats pour MerB et Asp99 est le premier résidu à se lier aux composés organométalliques suivis de la liaison à Cys96 et Cys159. Ces observations suggèrent un agrandissement de l’éventail d'applications possibles pour MerB dans la bioremédiation de certains sites contaminés par des composés organométalliques tels les organoplombiques et organostanniques. Mot-clé: Organomercuriallyase, Merb, organoplombiques. Organostanniques, protéine de liaison cuivre, carbone liaison métallique clivage, méthylmercure, Organomercuriels, biorestauration, résonance magnétique nucléaire, la cristallographie aux rayons X. / Mercury is introduced into the environment from either natural occurrences (volcanoes) or from human activities (combustion of fossil fuels). Mercury exists as elemental mercury (Hg0), ionic mercury (HgII) or organic mercury like methylmercury (MeHg) and these forms are in constant flux with each other as part of the natural biogeochemical cycle. Organomercurial compounds like MeHg are the most toxic form because of their hydrophobicity and their ability to efficiently permeate membranes and bioaccumulate in organisms. High levels of MeHg have been found in fish in many areas around the world, and therefore human consumption of contaminated seafood represents a serious danger for human health. Bacteria isolated from mercury-contaminated environments have evolved a system that allows them to efficiently convert both ionic and organic mercury compounds to the less toxic elemental mercury. The mercury resistance is due to the acquisition of a transferable genetic element known as the mer operon. The mer operon encodes for several proteins including two enzymes, the organomercurial lyase MerB and the mercuric ion reductase MerA. MerB catalyzes the protonolysis of the carbon-mercury bond of organomercurial compounds to produce a reduced-carbon compound and inorganic ionic mercury HgII. MerA catalyzes the reduction of HgII to elemental mercury Hg0, which is volatile and less toxic. Due to their ability to cleave MeHg and reduce the resulting HgII product, MerB and MerA are considered crucial to bioremediation efforts to clean up MeHg from contaminated waterways. A clear understanding of the mechanistic details of how MerB and MerA function together at the atomic level is crucial for appropriate utilization of the mer system in bioremediation efforts. We have been using nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography to structurally and mechanistically characterize E. coli MerB. Based on previous structural studies of E. coli MerB, three residues (Cys96, Asp99 and Cys159) have been identified as a catalytic triad which is required for carbon-Hg bond cleavage. As a follow up to the earlier studies, my project involves using X-ray crystallography to define the roles of Cys96, Asp99 and Cys159 in substrate binding and cleavage. Two different approaches were implemented to fulfill this goal. Firstly, MerB mutants were tested to define the role for the catalytic residues. Secondly, MerB inhibitors and other potential non-organomercurial substrates were used to probe MerB active site. The Cys,-Asp-Cys catalytic triad found in E.coli MerB is conserved in all MerB variants except four variants where aspartic acid is replaced by a serine. To understand the role of Asp99, we compared a serine-containing MerB variant (Bacillus megaterium MerB2) and an E. coli MerB mutant (MerB D99S) to wild type E. coli MerB. Interestingly, the purified MerB D99S protein was found to contain a pink color. X-ray crystal structure indicated the presence of a bound metal in the active site of MerB D99S. Analysis by inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence indicated that MerB D99S binds copper in the active site. Further, electron paramagnetic resonance (EPR) and NMR studies identified the copper as CuII. Addition of organomercurial substrate displaces bound CuII but MerB D99S shows diminished catalytic activity. In contrast, MerB2 did not co-purify with copper although the X-ray structure of MerB2-Hg complex is virtually identical to the structure of the MerB D99S-Hg. This suggests that the aspartic acid residue is crucial for the cleavage of carbon-Hg bonds of organomercurials as well as metal-binding specificity. Furthermore, the binding of copper to the MerB D99S protein suggests a possible evolutionary link between MerB and its structural homolog, the copper-binding protein NosL. In the second approach, we probed the active site of MerB through testing its binding to organotin and organolead compounds. The known MerB inhibitor triethyltin (TET) binds to Asp99 without binding to any of the active site cysteines. A similar binding has been observed with trimethylead (TML). Dimethyltin (DMT) inhibits MerB using an alternative mechanism. It first binds to Asp99 then Cys96, which induces a dramatic change in the active site by disrupting a cation-π interaction between Try95 and Arg155. In contrast, diethyltin (DET) and diethylead (DEL) were found to be substrates for MerB, where both ethyl groups were cleaved and the SnIV and PbIV products bound to the active site in a similar manner to HgII. DMT, DET and DEL show higher binding affinity to MerB than its initial substrate MeHg. These results suggest that organomercurials may not be the only substrates for MerB and Asp99 is the first residue to bind to organometals followed by subsequent binding to Cys96 and Cys159. In addition, these observations suggest that there are other possible applications for employing MerB in bioremediation of organolead and organotin contaminated sites while other organometals may have implications when using MerB in bioremediation systems. Keyword: Organomercuriallyase, MerB, Organolead. Organotin, Copper binding protein, Carbon metal bond cleavage, Methylmercury, Organomercuriels, Bioremédiation, Nuclear magnetic resonance, X ray crystallography.

The kinetic and mechanistic features of a new series ofplatinum-thallium cyano compounds containing a direct andunsupported by ligands metal-metal bond have been studied insolution, using standard mix–and–measurespectrophotometric technique and stopped–flow method.These reactions are interpreted as oxidative addition of the cspecies to the square planar Pt(CN)42-complex. Each of these processes was found to befirst-order in Pt(CN)42-, the corresponding TIIIIcomplex and a cyanide ion donating species whichacts as a catalyst. Both di- and trinuclear complexes werestudied, and the kinetically significant thallium complexes intheir formation and the catalytically active cyanide sourcesare as follows: [(CN)5PtTl(CN)3]3-: Tl(CN)4–(alkaline region), Tl(CN)3(slightly acidic region) and CN–; [(CN)5Pt–Tl(CN)]–: Tl(CN)2+and Tl(CN)2+; [(CN)5Pt–Tl–Pt(CN)5]3-: [(CN)5Pt–Tl(CN)]–and HCN. Appropriatemechanisms were postulated for the overall reactions in allcases, which include i) metal–metal bond formation stepand ii) coordination of an axial cyanide ion to the platinumcenter. Two experimentally indistinguishable kinetic modelswere proposed for the formation of the dinuclear complexeswhich are different in the sequence of the two steps. In thecase of the trinuclear complex, experimental evidence isavailable to exclude one of the alternative reaction paths, andit was proven that the metal–metal bond formation precedesthe axial cyanide coordination. The cyanide ligands coordinated to TIIIIin the Pt–Tl complexes could be replacedsuccessfully with aminopolycarboxylates e.g.: mimda2-, nta3-, edta4-. The [(CN)5Pt–Tl(edta)]4-complex, with a direct metal–metal bond hasbeen prepared in solution by two different reactions: a)dissolution of [(CN)5Pt–Tl](s) in an aqueous solution of edta, b)directly from Pt(CN)42-and Tl(edta)(CN)2-. The decomposition reaction is greatlyaccelerated by cyanide and significantly inhibited by edta. Itproceeds through the [(CN)5Pt–Tl(CN)3]3-intermediate. The formation of [(CN)5Pt–Tl(edta)]4-can proceed via two different pathways dependingon the ratio of the cyanide to the edta ligand concentrations.The’direct path’at excess of edta means theformation of intermediate[(CN)4Pt···Tl(CN)(edta)]4-, followed by a release of the cyanide from theTl–centre followed by coordination of a cyanide from thebulk to the Pt–centre of the intermediate. The’indirect path’dominates in the absence of extraedta and the formation of the Pt–Tl bond occours betweenPt(CN)42-and Tl(CN)4–. Homoligand MTl(CN)4(M = TlI, K, Na) and, for the first time, Tl(CN)3species have been synthesized in the solid stateand their structures solved by single crystal X–raydiffraction method. Interesting redox processes have been foundbetween TIIIIand CN–in non–aqueous solution and in Tl2O3-CN–aqueous suspension. In the crystal structureof Tl(CN)3·H2O, the thallium(III) ion has a trigonal bypiramidalcoordination geometry with three cyanides in the trigonalplane, while an oxygen atom of the water molecule and anitrogen atom from a cyanide ligand attached to a neighboringthallium complex, form a linear O–Tl–N fragment.Cyanide ligand bridges thallium units forming an infinitezigzag chain structure. Among the thallium(III) tetracyanocompounds, the isostructural M[Tl(CN)4](M = Tl and K) and Na[Tl(CN)4]·3H2O crystallize in different crystal systems, but thethallium(III) ion has in all cases the same tetrahedralgeometry in the [Tl(CN)4]–unit. Three adducts of mercury(II) (isoelectronic with TIIII) (K2PtHg(CN)6·2H2O, Na2PdHg(CN)6·2H2O and K2NiHg(CN)6·2H2O) have been prepared from Hg(CN)2and square planar transition metal cyanides MII(CN)42-and their structure have been studied by singlecrystal X–ray diffraction, XPS and Raman spectroscopy inthe solid state. The structure of (K2PtHg(CN)6·2H2O consists of strictly linear one dimensional wireswith PtIIand HgIIcenters located alternately, dHg–Pt= 3.460 Å. The structure of Na2PdHg(CN)6·2H2O and K2NiHg(CN)6·2H2O can be considered as double salts, the lack ofhetero–metallophilic interaction between both the HgIIand PdIIatoms, dHg–Pd= 4.92 Å, and HgIIand NiIIatoms, dNi–Pd= 4.60 Å, seems obvious. Electronbinding energy values of the metallic centers measured by XPSshow that there is no electron transfer between the metal ionsin all three adducts. In solution, experimental findingsclearly indicate the lack of metal–metal bond formation inall studied HgII–CN-–MII(CN)42-systems (M = Pt, Pd and Ni). It is in contrary tothe platinum–thallium bonded cyanides. KEYWORDS:metal–metal bond, platinum, thallium,kinetics, mechanism, stopped flow, oxidative addition, cyanocomplexes, edta, redox reaction, metal cyanides, X–raydiffraction, Raman, NMR, mercury, palladium, nickel, onedimensional wire