Copper(ii)–hydroxide facilitated C–C bond formation: the carboxamido pyridine system versus the methylimino pyridine system

Abstract

A copper(II)–hydroxide-induced carbon–carbon bond formation reaction is explored with the synthesis of an asymmetric carboxamido–methylimino pyridine Cu(I) complex of [Cu^I(py(N–CO)(NC–C)ph₂^Me2)₂]⁻ (12). Two imine-methyl groups are coupled to form a bridged C–C bond (NC–C–C–CN) at the methyl positions with the reduction of two Cu²⁺ center ions to Cu⁺. The reaction is checked with three dicarboxamido pyridine [Cu^II–OH] complexes, with which dinuclear Cu(I) complexes of [Cu₂(py(N–CO)₂ph₂^R2)₂]²⁻ (R = methyl (3), methyl and allyl (6)) and trinuclear [Cu^II–Cu^I–Cu^II] complex of [Cu₃(⊂₂₀-py(N–CO)₂ph₂dien^Me3)₂]⁺ (9) are obtained. The reactivities of the [Cu^II–L] (L = DMF, OH⁻) complexes in dicarboxamido pyridine, carboxamido–methylimino pyridine and dimethylimino pyridine systems are discussed in terms of the electron delocalization properties of ligands. A cooperative metal–ligand (Cu²⁺ and enamide ligand) interaction is proposed based on the characterization of ligated Cu(II) intermediates with the techniques of X-ray crystallography, UV-vis spectroscopy, cyclic voltammogram, EPR spectroscopy, and DFT calculations.