An Atypical Dinuclear Copper(II) 1,2,4-Triazolyl Complex as a Catalyst for Palladium-Free Csp-Csp Homocoupling of Phenylacetylene

An unusual dinuclear copper(II) complex, labeled as [L2Cu2(solv)2]•2(solv) (where solv = CH3CN or H2O and L = 3,5-bis(trifluoroacetamido)-1,2,4-triazo-1-yl), with L acting as an O,N,N′,O′-donor chelating ligand, was prepared by reaction with Cu2O. The single crystal XRD analysis and the susceptibility measurements revealed a pentacoordinated dinuclear Cu(II) complex with the two Cu atoms in a square pyramidal environment, doubly bridged by the 3,5-substituted triazolate ligands and one coordinated acetonitrile in the apical position, completing the coordination sphere. Systematic voids in the solid-state packing accommodate two additional acetonitrile molecules; all of the acetonitrile molecules can be readily replaced by water or other solvents. The new copper(II) complex was investigated as a catalyst in the Csp-Csp homocoupling reaction of phenylacetylene (Glaser-type reaction), yielding satisfactory results in terms of both isolated yields and procedural simplicity. For a better understanding of the catalytic results, spontaneous aggregation in solution of this copper(II) complex was investigated. The dilution experiments in polar solvents, followed by UV–visible spectroscopy, highlighted the self-aggregation behaviors, which were further corroborated by ESI-MS analyses. The dissociative equilibrium constants, Kd, were determined in ethanol, Kd = 3.89 × 10–3 M, and in CH3CN, Kd = 8.10 × 10–2 M, evidencing the role of the solvent in the association process.