Syntheses And Functional Study Of The Mo(W)-S(O) Clusters Containing Organic Polycarboxylate Ligands

Biological nitrogen fixation has already contributed about half of the totalnitrogen input to global agriculture, and the rest is principally from nitrogenousfertilizer by chemical production from the Haber-Bosch synthesis of ammonia.However, it would consume about the 1% of the world's total annual energysupply in order to produce the hydrogen gas together with the high temperaturesand pressures needed for this chemical process. On the contrary, a similarchemical process is carried out by nitrogen-fixing bacteria at ambient temperatureand pressure. As a result, many scientists have devoted their whole life to theinvestigation of the nitrogenases in many fields, such as biochemistry, structurechemistry, coordinate chemistry, crystal chemistry, etc.. Nitrogenases could binddinitrogen and other substrates, reduce these bound substrates by the electronsfrom related other metal clusters, allow the protonation of reduced N atoms,produce H2 as a reaction byproduct, and finally release ammonia as the fixednitrogen. The mechanistic details of this catalytic process remain unclear.Inorganic chemists focus on synthesis of model complexes in order to help todescribe the structure and function of metal sites in metalloenzymes.Dinitrogen as an inert gas is widely used to protect against oxygenation and soon. First, it is a nonpolar chelator with tightly bound σ and π electrons. Secondly,the triple bond in dinitrogen is extremely strong (944 KJ mol-1), therefore, N-Nbond cleavage is extremely difficult. Finally, the large HOMO-LUMO gap meansN2 is reluctant to accept or lose electrons, making redox reactions with dinitrogendifficult although nitrogen displays a range of stable oxidation states in itscompounds. This chemical inertia decides that catalysts are needed in the processof industrial ammonia production. The iron-molybdenum cofactor (FeMo-co) isthe active site where nitrogenases reduce N2, small nitriles, isocyanides, andalkynes. Very recent X-ray study (1.16 ?) confirmed the presence of a centralatom N, sitting inside the center of the FeMo-co by connecting with six "belt"ironatoms. It provides fresh challenges for synthetic chemists and theoreticians. Inorder to further mimic the structure of the FeMo-cofactor of nitrogenases, andfurther understand the coordination environment of the Mo atom inMoFe-cofactor, it is important to investigate the compounds containing Mo, Watoms with polycarboxylate ligands, such as glycine, benzilic acid EDTA etc..Here, we reported the structure, synthesis and functional property of severalcompounds with glycine, benzilic acid, EDTA etc..1. Mo-S compound containing glycine ligandCompound 1 was obtained from the reaction between (NH4)2MoS4 andglycine in mixed solvents (ethanol and water) at ambient temperature. The pHvalue is crucial during the formation of compound 1. It is a netural molecule withthree glycines. One glycine ligand is coordinated to the two molybdenum atomsthrough two carboxylato-oxygens as a bridge. The rest two glycines arecoordinated to the two molybdenum atoms by its α-carboxylate oxygen and thenitrogen atoms, respectively. Together with two μ-S, a terminal oxygen, completethe coordination geometry of the molybdenum as a distorted octahedron.2. Mo(W)-O mononuclear compounds containing benzilic acid ligandCompound 2 and 3 were obtained from the reactions between (NH4)2MS4(Mo, W) and benzilic acid in ethanol and water as solvents at ambient temperature.Two mononuclear compounds have been formed. In each compound, however,two anion complexes are connected by the coordination between the potassiumions and the oxygen atoms from the benzilate, terminal or water. In eachmonomeric cis-dioxo molybdate (tungstate) anion, the central metal atom lies in adistorted octahedral center, consisting of two terminal oxygen atoms and fouroxygen atoms from two bezilate ligands.3. Mo (W)-O multinuclear compounds containing amino acid ligandsCompound 4 was obtained from the reactions betweem Na2MoO4 andglycine in water at 65?C. Compound 5 was obtained from the reaction between(NH4)2MoS4 and ethlenediamine tetraacetate in ethanol and water as mixedsolvents at ambient temperature. 4 is a multinuclear metal oxygen clusters[Mo8O26]4-with amino acid ligands. 5 is a binuclear complex, in which thetungsten atom is coordinated to three unshared oxygen atoms, adopting facstereochemistry, while the remaining fac positions are occupied by three atomsfrom the ligands.Besides, the above complexes have been characterized by elememt analysis,IR, UV-vis, EPR, XPS, XRD, TG-DTA, molar conductance, etc..4. Catalysis activity of Mo-Fe-S clusters in reducing C2H2 to C2H4We have studied the catalytic activity of Mo(W)-S(O) complexescontaining Mo-Fe-S or Fe-S clusters. The results have revealed that[Mo2O2S2(HGly)(Gly)2] 1 exhibited better catalytic activity than othercompounds.5. Investigation of photochromic behaviorThe complexes 2-5 exhibited considerable photochromism when they wereexposed to ultraviolet light for about 10 minutes. Colorless compounds werefound to turn different colors. The defection of electron trapping is supposed tocause this phenomenon when the crystals were induced by irradiation. Thecoloration can gradually fade after standing for some time. The color productshave been investigated by XPS and EPR.