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

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 C_sp-C_sp 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, K _d, were determined in ethanol, K _d = 3.89 × 10^-3 M, and in CH₃CN, K _d = 8.10 × 10^-2 M, evidencing the role of the solvent in the association process.