Electrochemical Coupling Of Aromatic Halide Catalyzed By Nickel Complex, Mechanism And Kinetics

Nickel complexes with bipyridine (bpy) as ligand have been studied as a sample of organometallic electrochemistry in this dissertation. Electrochemical character and kinetics of NiCl2(bpy)3 and NiCl2bpy have been investigated by the methods of cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronocoulometry (CC). The mechanisms of electrochemical coupling of aromatic halide with NiCl2bpy as catalyst have been presumed. In addition, ionic liquids were applied to this system as a kind of supporting electrolyte.The electrochemical experiments were performed by using a glassy carbon (GC) working electrode, a saturated calomel electrode (SCE) and a Pt wire auxiliary electrode. CV and DPV showed that Ni(+2)Cl2(bpy)3 was reduced to Ni(0)(bpy)3. And then, Ni(0)(bpy)3 was reduced to Ni(-1) complex.The reduction of Ni(+2)Cl2(bpy)3 at GC was a quasi-reversible process and controlled by diffusion. Diffusion coefficients of Ni(+2)Cl2(bpy)3 in DMF have been obtained at different temperature employing CC. And diffusion coefficients increased with the increase of temperature. The standard rate constant k0 obtained at 29 ℃ is 2.31 x 10-5ms-1 according to rate constants kf at different electrode potentials, which also indicated the reduction of Ni(+2)Cl2(bpy)3 was quasi-reversible. Standard rate constants at different temperature increased with the increase of temperature. According to Arrhenius equation, electrochemical activation energy was obtained on the base of different standard rate constants.The coupling electrolytes were carried out in one-counterpart cell with stainless steel as cathode, a piece of Zn as anode and SCE as reference. Aromatic halide was coupled to the coupling product, biphenyl. The effects of potential, charge, temperature, catalyst and current density on yield have been studied. The yield of potentiostatic electrolyte enhanced to 85.3% under the optimized condition. The maximal yield under constant current electrolyte is 83.9%.For the sake of the mechanism of electrochemical coupling, the electrochemical character of Ni(+2)Cl2bpy was investigated. According to the data of CV andelectrolyte, we found that the reaction followed different mechanisms under different potential. When the electrolytes were performed at three different range of potential, there were three kinds of catalytic cycles.Ionic liquid, BmimBF4, was investigated as a kind of electrolyte in this coupling reaction. Firstly, we studied the conductivity of BmimBF4 and its solution in DMF. Its conductivity increased with the increase of temperature. BmimBF4 showed the properties familiar with strong electrolyte and its limited molar conductivity in DMF is 113.1 cm2smol-1. The electrochemical window of DMF containing O.lmolL-1 BmimBF4 is 3.7 V, cathodal limited potential -2.1 V and anodic limited potential 1.6 V. The electrolytes undergoing in this solution had a maximal yield 63.8%.The four points listed below reflect the innovation of this dissertation.(1) This electrochemical means preparing biphenyl is a "green" method, compared with the traditional organic method needing sharp condition and industrial method demanding high temperature. There is no request for powder of Fe or Zn, and will not result in bulk waste of Fe or Zn.(2) The cylindrical stainless steel electrode is easier to be prepared and cheaper than nickel foam electrode, frequently used in electrochemical coupling before.(3) We found that the reaction followed different mechanisms under different conditions, and conclude a comprehensive mechanism about electrochemical coupling of aromatic halide with NiCl2bpy as catalyst. We also obtained some useful kinetics data concerned with nickel complex at glassy carbon.(4) Ionic liquids, as a kind of supporting electrolyte, were applied to electrochemical research. Ionic liquids are easier to be produced and purified than traditional supporting electrolytes, and can be easily functionalized according to different demands. This is a new concept in electrochemical application of ionic liquids.