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 γ-Keggin silicotungstate with bis-μ-1,1-azido ligands TBA4[γ-H2SiW10O36Cu2(μ-1,1-N3)2] (1, TBA=tetraTBA=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 γ-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 γ-Keggin silicotungstate. Complex 1 could act as an 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.