Synthesis of Copper(I) Complexes of N‐Heterocyclic Carbene–Phenoxyimine/amine Ligands: Structures of Mononuclear Copper(II), Mixed‐Valence Copper(I)/(II), and Copper(II) Cluster Complexes

Abstract

AbstractCopper(I) bromide complexes (2a and 2b) of NHC–phenolimine ligand precursors {3‐[(1R,2R)‐2‐{[1‐(3,5‐di‐tert‐butyl‐2‐hydroxyphenyl)meth‐(E)‐ylidene]amino}cyclohexyl]‐1‐isopropyl‐4‐phenyl‐3H‐imidazol‐1‐ium bromide (1a) and 3‐[(1R,2R)‐2‐{[1‐(2‐hydroxyphenyl)meth‐(E)‐ylidene]amino}cyclohexyl]‐1‐isopropyl‐4‐phenyl‐3H‐imidazol‐1‐ium bromide (1b), respectively} have been prepared. Complexes 2a and 2b exhibit copper coordination only through the carbene carbon atom (C) and do not spontaneously eliminate HBr to give additional phenoxyimine (NO) bonds, which is attributed to intramolecular hydrogen bonding. Crystallisation of 2a and 2b gives 2a′ and 2b′, respectively, that contain (C) copper(I) bromide and (NO)2 copper(II) coordination. Complex 2b′ also exhibits intermolecular CuIBr interactions giving a Cu2Br2 bridge that links two molecules of 2b′ resulting in an ellipse motif. Reduction of the ligand precursor imine group of 1a allows synthesis of silver(I) and copper(I) NHC–phenolamine complexes 6 and 7, respectively, that also retain the phenol hydrogen atom. Attempts to selectively prepare 2a′ gave a copper(II) complex 9 that exhibits an (NO)2CuII structure with pendant imidazolium salt groups. Reaction between the silver(I) bromide derivative of 1a and CuCl2·2H2O gives a complex derived from a Cu6(O)(OH)4Cl3 core and two (NO) and one (CNO) ligands, respectively. The use of 2a and 7 as precatalysts for 1,4‐conjugate addition to enones and aziridination of alkenes was studied, showing that, whilst both catalysts are active, enantioselectivities are low, which is attributed to the lack of Cu‐(NO) coordination. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)