| In nature, nitrogenases can catalyze the conversion of dinitrogen to bioavailable form--ammonia in biological nitrogen cycles. This catalytic process is very complicated, and little is known about the exact mechanism at present. The Mo-dependent nitrogenases are composed of two metalloproteins--- Fe protein and MoFe protein, where FeMo-co is thought to be the active site of the substrate binding and reduction. In the FeMo-co, there are two cuboidal subunits [4Fe-3S] and [3Fe-Mo-3S] linked by threeμ2-sulfide bridges. A recent high-resolution 1.16? crystallographic analysis of MoFe- protein revealed that the central cavity of the FeMo-co, previously thought to be unoccupied, contains an interstitial atom (most likely nitrogen atom). This new discovery leads the support of the notion that the Mo atom, which is surrounded by three sulfur atoms, a nitrogen atom from histidine and a pair of oxygen atoms from homocitrate in the center of the cluster, is directly involved in the nitrogen reduction. Interestingly, in vitro biosyntheses of FeMo-co with alternative polycarboxylates (i.e., citrate, malate, citramalate, cis-aconitate) resulted in the lower nitrogen fixing ability than that with homocitrate, however, it remained capable of reduction from C2H2 to C2H4. Thus, it is of great importance to understand the interaction between the metal and homocitrate, when and how homocitrate and Mo atom are inserted into the FeMo-co, and what is the role of homocitrate in the process of the substrate reduction.The successful preparation of a series of single and double cubanes Mo/Fe/S with Mo-bound oxalate, citrate, citramalate and malate, which exhibited good catalytic activity in the reduction of N2H4 to NH3 (or NH4+), stimulated our interest in exploring the reactivity of molybdenum polycarboxylato complexes. In this thesis, the syntheses, crystal structures, spectroscopic properties of several bi(mono)nuclear molybdenum complexes containing nitrilotriacetate, oxalate, benzilate ligands with sulfur(oxo) bridges have been prepared, and their catalytic activity from C2H2 to C2H4 were also investigated.1. Binuclear molybdenum complexes containing nitrilotriacetate ligand with sulfur bridgesThe first binuclear Mo-S-nta complex 1 was obtained by the reaction between (NH4)2MoS4 and nitrilotriacetic acid in mixed solvents (ethanol and water) at ambient temperature. The pH value is crucial for the formation of compound 1. X-ray single crystal structural analysis exhibited that compound 1 is composed of one [Mo2O2S2(nta)2]4-, one [Mo2O2S)2(ntaH))2]2- anion, six potassium ions and four water molecules. In an asymmetric unit, two anions are connected through the strong interactions between the potassiums and the different kinds of oxygen atoms. Each molybdenum atom is surrounded by a terminal oxygen atom, two bridging sulfur atoms, the twoβ-carboxylate oxygens and a nitrogen atom from the nitrilotriacetate ligand. To each nitrilotriacetate, there is one uncoordinated carboxylate group. Two kinds of uncoordinated groups were observed. To the [Mo2O2S2(nta)2]4- anion, the carboxylate is deprotonated, which C-O bonds length are quite close, whereas protonated carboxylate in [Mo2O2S2(ntaH)2]2-. This part of work has been submitted.2. Binuclear molybdenum complexes containing oxalate ligand with sulfur bridgesCompound 2 and 3 were obtained by the reactions between (NH4)2MoS4 and oxalic acid in ethanol / water mixed solvents at ambient temperature. Two compounds have been formed in slightly different pH value. Our experiments has further confirmed that the pH value is crucial for the formation of compounds with different structure. X-ray single crystal structural analysis revealed that complex 2 is composed of one [Mo2O2(μ-S)2(C2O4)2(S2O3)2]6- anion, six potassium ions and two water molecules. Each molybdenum atom is surrounded by a terminal oxygen atom, two bridging sulfur atoms, two carboxylate oxygens from an oxalate as well as a sufur atom from thiosulfate ligand. So, the molybdenum atom adopted distorted octahedron. Actually no thiosulfate salt has been added into the reactant system. According to the reference about the production process of oxalic acid, sulfate is the one of the main impurity. The reaction between the H2S and sulfate occurred to afford the thiosulfate. In 3, the anion is in fact a dimer. Two coordination modes for the oxalates in this compound, one is coordinated to a molybdenum atom by its two carboxylate oxygens to form a five-member ring, the other as a bridge to connect the two complex anions by its four carboxylate oxygens. So, each molybdenum atom is surrounded by a termincal oxygen atom, two bridging sulfur atoms, two carboxylate oxygens from an oxalate as well as an oxygen atom from the bridging oxalte ligand. The 3-D network is formed through the coordination between the potassium ions and the oxygen atoms from the oxalate, terminal or water.3. Bi(mono)nuclear molybdenum-oxo complexes containing oxalate or benzilate ligandsRespectively take (NH4)2MoS4 or MoO2 (acac) 2, the oxalic acid or benzilic acid as the raw material, obtained Compound 4,5 and 6 in ethanol and water mixed solvents at ambient temperature. X-ray single crystal structural analysis revealed that complex 4 is composed of one [MoO2. 5(C2O4)(H2O)]- anion and one potassium ion. In this molecular structure unit, molybdenum atom is surrounded by three terminal oxygen atoms, two carboxylate oxygens from an oxalate as well as a oxygen atom from water molecule. So, the molybdenum atom adopted distorted octahedron.Complex 5 is composed of one [MoO3(C2O4)]2- anion,one water molecule and two potassium ions. Each molybdenum atom is surrounded by two terminal oxygen atoms, two bridging oxygen atoms, and two carboxylate oxygens from an oxalate ligand. So, the molybdenum atom adopted distorted octahedron. In 5,molybdenum oxygen bridge bond has charactered the single or duplet bond in turn .It formed 1-D chain shape structure through"-Mo-O=Mo-O-".4. Characterization of compounds 1-5The above complexes have been characterized by elememtal analysis, IR, UV-vis spectra, TG-DTA and molar conductance, etc.. Herein, the IR spectra of compounds 2-3 will be discussed as an example. As we know, different coordination modes for the oxalate ligands for the two compounds were observed in their Infrared spectra. Two very strong absorption peaks 1681,1646 cm-1 were found in compound 2, which are assigned to the symmetric stretching vibration of carboxylate , and the corresponding asymmetric stretching vibration at 1435 cm-1. To 3, symmetric stretching vibrations of oxalate carboxylates at 1714,1691,1603 cm-1 were found while the symmetric stretching vibration of carboxylate at 1394,1324 cm-1. Besides, Mo=O and Mo-O the stretching vibrations were also exhibited slight difference.5. Catalytic activity of Mo-Fe-S clusters in reducing C2H2 to C2H4Nitrogenases not only catalyze the reduction of dinitrogen to ammonia, but can accomplish the reduction of a number of alternative substrates, such as acetylene (C2H2) to ethylene (C2H4), hydrazine to ammonia etc.. Acetylene reduction is a common method for assessing nitrogen fixation, which experimentally simplified the study of N2 fixation. Herein, we use this method to investigate the catalytic activity of the different molybdenum (iron)–sulfur compounds without any enzymatic assistance. Several kinds of Mo/Fe-S compounds,including the binuclear Mo(V)–S–polycarboxylate compounds, such as Mo-S-Homocitrate [Mo2O2S2(C7H5O7)2]6– and Mo-S-citrate [Mo2O2S2 (C6H4O7 )2]6– compounds, a single cubane (Bu4N)2[Fe4S4(SPh)4] and a chainlike compound (Et4N)4[Mo2Fe2S10]·2CH3OH, were chosen as the catalysts, and ethylene was analyzed by gas chromatography. Several features of the catalytic properties could be shown as followed: first, the remarkable specific activity of molybdenum is apparent in our established system. Second, the coordination environment of the molybdenum atom seems to have a great influence on the catalytic activity of the complexes. Among the four binuclear Mo–S complexes with homocitrate, citrate, glycine and nitrilotriacetate ligands, Mo-S-glycine complex gave high activity and relatively good selectivity. In contrast, Mo–S–homocitrate and Mo–S–citrate complexes gave lower activity. It is supposed that amino acids (glycine and nitrilotriacetic acid) exhibit reductive property, which results in less stability and higher catalytic activity of their molybdenum complexes than those of stable Mo–homocitrate/citrate complexes. Third, the combination systems with binuclear Mo–S–polycarboxylate complexes displayed relatively higher catalytic activity, indicating that FeCl3 and the binuclear complexes seemed to react and yielded some kind of Mo-Fe-S cluster with relatively high catalytic activity. Fourth, Mo/Fe = 1:6 1: 7 in the combination systems was chosen, in accordance with the Mo/Fe ratio of FeMo–cofactor.The systems composed of (Et4N)4[Mo2Fe2S10]·2CH3OH and the polycarboxylate ligands, displayed lower activity since it reacted with the ligands to reduce its activity. Thus, catalytic activity of combination systems decreased with increasing amounts of chelating ligands.
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