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: [Cu(2)(mu-OH)(L1)](PF(6))(2) (2; L1 = 3,5-bis(N-pyridylimidazolylidenylmethyl)pyrazolate), [Cu(4)(mu(3)-OH)(2)(L2)(2)](PF(6))(4) (3; L2 = 3,5-bis(N-pyrimidylimidazolylidenylmethyl)pyrazolate), and [Cu(4)(mu(3)-OH)(2)(L3)(2)](PF(6))(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, [Cu(2)(L4)(2)](PF(6))(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.