Synthesis Of Transition-Metal Complexes Of N-Heterocyclic Carbenes From Metals

Transition metal complexes of N-heterocyclic carbenes (NHCs) have been attracting great attention over the past decades because of their wide applications in materials, medicines, especially in catalysis. Development of transition-metal catalyzed organic transformations based on the first row transition metals such as Fe, Co, Ni, and Cu etc is of importance for their relatively low price and toxicity compared with precious metals. The catalytic processes based on NHC complexes of none noble metals have not been well studied because the synthesis of these complexes is not convenient. This dissertation affords new methods of metals replacement and electrolysis into the organometallic synthesis. It mainly includes three parts:In part one, we have prepared iron, cobalt, nickel, and copper NHC complexes by direct reactions of metal powders with silver-NHC complexes or imidazolium salts. The imidazolium salts reaction with Ag2O afforded silver-NHC complexes, and further metal replacement reaction of these silver complexes with metallic powder yielded M-NHC complexes respectively in good yields. These imidazolium salts can also react with the metallic powder under air condition to obtain corresponding M-NHC complexes. The N-heterocyclic carbene complexes of divalent iron, cobalt and nickel are mononuclear, as the Cu-NHCs are dinuclear or trinuclear complexes including +1 or +1.5 valent copper ions. These approaches open up a practical and convenient way to the preparation of metal-NHC complexes especially the non-noble metals, and could provide wide application in synthsis of metal NHC complexes.In part two, we report a facile and efficient electrochemical synthetic procedure for NHC complexes of iron, nickel and copper using imidazoilum salts or Ag-NHC complexes with commercial available metal plates. In the electrochemical process, imidazolium salts or Ag-NHC complexes act as the reactants and the electrolytes. The reacting metals are used as sacrificial anodes, which lose electron to yield metal ions. The same metal plate or Pt plate is taken as auxiliary electrode, where the Ag-NHCs or imidazoilum salts accept electron to form free carbenes. The metal ions combine with the free carbenes to get M-NHC complexes. The electrolysis procedure is simple and highly efficient without using any additional reagents. The electrochemical method offers a short cut to metal NHC complexes.In part three, a series of polynuclear transition metal complexes supported by pyridyl-functionalized N-heterocyclic carbenes have been synthesized and characterized. The Ag(I)-NHC complexes containing tetrametallic cores are rhombic or line form, and the hexanuclear Ag(I)-NHC cluster has three couples of Ag22+. There are metal-metal interactions between the Ag…Ag in these silver clusters. The dinuclear Au(I)-NHC cluster which has aurophilic interactions is prepared by transmetallation from a Ag(I)-NHC complex. These tetranuclear silver complexes are intensely luminescent in their solid states.