Copper(II) Hydroxide Complexes of N-Heterocyclic Carbenes and Catalytic Oxidative Amination of Arylboronic Acids

Reactions of the imidazolium salts or their corresponding silver carbene complexes with copper powder afforded unusual copper(II) hydroxide complexes of N-heterocyclic carbenes: [Cu2(μ-OH)(L1)](PF6)2 (2; L1 = 3,5-bis(N-picolylimidazolylidenylmethyl)pyrazolate), [Cu4(μ3-OH)2(L2)2](PF6)4 (3; L2 = 3,5-bis(N-pyridylimidazolylidenylmethyl)pyrazolate), and [Cu4(μ3-OH)2(L3)2](PF6)4 (4; L3 = 3,5-bis(N-pyrimidylimidazolylidenylmethyl)pyrazolate). The same reaction of 3,5-bis(N-thiophenylimidazoliumylmethyl)pyrazole yielded a dinuclear Cu(II)−NHC complex, [Cu2(L4)2](PF6)2 (5; L4 = 3,5-bis(N-thiophenylimidazolylidenylmethyl)pyrazolate). The complexes have been fully characterized by X-ray diffraction analysis, elemental analysis, and IR and ESI-MS spectra. A catalytic study shows that complex 4 is highly active in the N-arylation reactions of imidazoles and aromatic amines with arylboronic acids in methanol at room temperature under base-free conditions.