Synthesis, Characterization Of Organometallic Complexes Containing Triazene Ligands And Electrocatalysis For Hydrogen

Economic and environmental concerns over the world’s ever increasing population explosion and energy consumption have led to considerable interest in new energy development and utilization technologies. Hydrogen is one of most attractive substitute due to its high-energy contents and water as the byproduct of combustion. The conversion between non-carbon energy sources especially water and hydrogen will play a vital role in future systems for storage and delivery of energy.The hydrogen evolution reaction(her) is apparently a very simple which involve two protons and two electrons. One-electron oxidation of hydrogen is highly endergonic because hydrogen is very weakly acidic, so removal of either an electron or a proton is rather difficult without a catalyst. In nature, production of hydrogen by reduction of protons, as well as the oxidation of hydrogen, is carried out relying on hydrogenase enzymes in microbial organisms. Structural studies revealed that hydrogenase enzymes have first-row metals Fe or Ni in their active site, bound to ligands that include CO, CN and propane or nitrogen/oxygen mixed propane. The remarkable discovery that natural enzymes included organometallic complexes as a key component led to intense interest from organometallic/inorganic chemists along with scientists focused on biological and structural aspects of hydrogenase enzymes. A large number of studies showed that the secondary amine in the nitrogen mixed propane may play an important role in the catalysis which attracted chemists to synthesize lots of organometallic complexes that include nitrogen mixed ligands. However, the poor solution in water or high overpotential for hydrogen production limited their utilization. We have devoted to the research on developing new catalysts in order to reduce the overpotential and increase the efficiency of catalysts for hydrogen production. The major results of my thesis are follows:1. Six novel molecular catalysts were designed and synthesized through triazenes and CuCl, CuBr2/CuCl2, NiCl2·6H2O or [Pd(CH3CN)4]Cl2and the structures were analyzed by X-ray crystallography. In addition to complex [Cu3(L1)3](1) with fan impeller type configuration beyond, all metal-centre atoms in other complexes are bridged by four triazenido units that result in oar wheel configurations. Especially, complexes [Cu2(L3)4](5) and [Pd2(L3)4](6) both have a typical tetragonal paddlewheel-type structure. The electrochemical properties were studied by cyclic voltammetry (CV) and the electrocatalytic performances for acetic acid or benzoic acid reduction in acetonitrile were carried out. The results show that complex [Cu3(L1)3](1),[Cu2(L2)4](3) and [Ni2(L2)4](4) are more active than Pt electrode in the same experimental condition. They also show that the asymmetrical catalysts are more active.2. The electrocatalytic performances for proton reduction in acetonitrile-buffer solution were carried out by CV which showed that the her is related to the concentration of complexes and acidity of the solution. Under similar conditions, complex [Cu3(Ll)3](1) and [Pd2(L3)4](6) both have the lowest overpotenitals (OP,566mV) in buffer solution (pH=4.0). Complex [Cu2(L1)4](2) and [Cu2(L2)4](3) have the lowest OP in buffer solution (pH=6.0), which are568and668mV, respectively. While complex [Ni2(L2)4](4) and [Cu2(L3)4](5) both have the lowest OP (609mV) in buffer solution (pH=7.0).3. Under the same condition, controlled potential electrolysis of solution which includes different complexes shows that complex [Cu2(L2)4](3) has the highest turnover frequency (TOF,44) in acetonitrile solution. Complex [Cu3(L1)3](1),[Cu2(L1)4](2),[Cu2(L2)4](3),[Cu2(L3)4](5) all have the highest TOF in weak acid buffer solution (pH=6.0), resulting in180,265,143and195, respectively. While complex [Pd2(L3)4](6) has the highest TOF (225) in acid buffer solution (pH=4.0) and complex [Ni2(L2)4](4) has the highest TOF (125) in buffer solution (pH=4.0) or (pH=8.0). The results further show that TOF of hydrogen production is related to the structure of catalyst and acidity of solution.4. At last, three mechanisms were proposed according to the structures of the six complexes and their electrocatalytic characters for proton reduction. Conclusions and outlooks were drawn based on the whole experiments and theoretical analyses. What’s more, the improvement experiment program was proposed.