Dissertation

Development of Pincer Bis(N-heterocyclic Carbene)−copper Complexes as Catalysts for Cross−coupling Reactions

Domyati, Doaa Mustafa

2018

Overview

The first isolation of thermally stable carbene and the first catalytic application of Pd-NHC complexes as a catalyst in Heck coupling reaction blossomed tremendous growth in the field of organometallic synthesis. While a large number of NHC complexes have been prepared using precious metals as catalysts, there is currently a growing interest to develop the complexes of cost-effective non-noble metals, specifically copper. After the first application of coper-NHC complexes in catalysis by Woodward in 2001, numerous CuI-NHC systems have been developed mainly with mono-NHC ligands. In contrast to mono-NHC and despite the enhanced stability of complexes with bis- or poly-NHC ligands due to chelating effect, the examples of structurally characterized Cu-bis(NHC) systems of lower nuclearity are very limited. Herein, we describe the synthesis of seven new Cu(I) complexes with pincer N-heterocyclic carbene ligands of the type 2,6-bis(3-alkylimidazole-2-ylidene)pyridine, I-(R)CNC, and 2,6-bis(3-alkylimidazole-2-ylidene)methylpyridine, I-(R)C^N^C, where R = Me, Et, iPr, tBu groups. All complexes, namely, [Cu2-I(Me)2CNC](PF6)2, 1; [Cu2-I(Et)2CNC](PF6)2, 2; [Cu2-I(iPr)2CNC](PF6)2, 3; [Cu-I(IMe)C^N^C](PF6), 4; [Cu-I(IMe)C^N^C](PF6), 5; [Cu-I(IMe)C^N^C](PF6), 6; [Cu-I(IMe) C^N^C](PF6), 7, have been characterized by 1H and 13C NMR spectroscopies, elemental analysis, solution conductivity, electrochemical studies and their structures were confirmed by X-ray crystallography except for complex 1. The crystallographic data reveal a binuclear structure containing two Cu atoms at close distance for all the complexes except 5 and 7 showed as a unique mononuclear structure. Solution-state conductivity data reveal a dominant 1:2 electrolyte behavior for complexes 1−3 but 1:1 electrolyte for 4−6, consistent with the sustainable structure in solution of CuI-I(R)CNC complexes. Cyclic and differential pulse voltammetry studies demonstrate quasi-reversible and two irreversible peaks for the one-electron oxidation of solvent-bound and solvent-free binuclear and mononuclear Cu-NHC species in complexes 1−3. In contrast, the cyclic voltammetry of the 4−6 complexes showed a unique reversible CuII/CuI redox couples.The application of the synthesized pincer Cu-NHC (4−7) complexes was examined as catalysts in C−C (Sonogashira−like) and C−N (Ullmann−type) cross-coupling reactions. In the case of Sonogashira-like coupling, broad range of electronically and sterically diverse substrates have been examined which offered moderate to excellent yields. Unlike other copper catalysts that require an inert atmosphere to prevent alkyne homocoupling, the Cu-pincer bis(NHC) complexes provided good to excellent cross-coupling yields in air . Some mechanistic studies to verify the structure and reactivity of Cu-oxygen adducts were also examined. In addition, The C−N coupling of imidazole is also opening up the application to facilitate the synthesis of diverse imidazole as building blocks of pincer bis-imidazolium slats.

• Publisher: ProQuest Dissertations & Theses
• Subject: Inorganic chemistry
• ISBN: 1085670325, 9781085670326