Synthesis and structural characterization of a monomeric di-copper-substituted silicotungstate [γ-H2SiW10O36Cu2(μ-1,1-N3)2]4- and the catalysis of oxidative homocoupling of alkynes

The di-copper-substituted gamma-Keggin silicotungstate with bis-mu-1,1-azido ligands TBA(4)[gamma-H2SiW10O36Cu2(mu-1,1-N-3)(2)] (1, TBA = tetra-n-butylammonium) was synthesized in an aqueous medium. The crystal structure of the anion part of 1 was a monomer of the basal-basal end-on diazido-bridged di-copper-substituted gamma-Keggin silicotungstate. The NMR and CSI-MS spectra of 1 in organic solvents, such as acetonitrile, benzonitrile, and 1,2-dichloroethane, showed that complex 1 was present as a monomer of the di-copper-substituted gamma-Keggin silicotungstate. Complex 1 could act as on effective homogeneous catalyst for the oxidative homocoupling of various types of alkynes, including aromatic, aliphatic, and heteroatom-containing ones. The reaction possibly proceeds as follows: First, the ligand exchange proceeds between the azido groups in 1 and alkynyl groups to form the corresponding diyne with the reduced copper(I) species via the di-copper(II)-alkynyl intermediate, then the reduced species is reoxidized by molecular oxygen, and the oxidized species reacts with an alkyne to regenerate the alkynyl intermediate. (C) 2008 Elsevier Inc. All rights reserved.