Monomeric Copper(I), Silver(I), and Gold(I) Alkyne Complexes and the Coinage Metal Family Group Trends

A series of thermally stable, easily isolable, monomeric, and isoleptic coinage metal alkyne complexes have been reported. Treatment of [N{(C3F7)C(Dipp)N}2]Li (the lithium salt of the 1,3,5-triazapentadiene [N{(C3F7)C(Dipp)N}2]H) with AuCl, CF3SO3Ag or CF3SO3Cu in the presence of 3-hexyne led to the corresponding coinage metal alkyne complex [N{(C3F7)C(Dipp)N}2]M(EtCCEt) in good yield (M = Au, Ag, Cu; Dipp = 2,6-diisopropylphenyl). X-ray crystal structures of the three coinage metal alkynes are remarkably similar, and show the presence of trigonal-planar metal sites with η2-bonded 3-hexyne. The M−C and M−N bond distances vary in the order Cu < Au < Ag. The bending of the C−CC bond angle is largest for the gold, followed by Cu and Ag adducts. The gold adduct also shows the largest decrease in CC stretching frequency in Raman, while the Ag adduct shows the smallest change compared to that of the uncoordinated alkyne. DFT calculations on [N{(CF3)C(Ph)N}2]M(EtCCEt) and the related ClM(EtCCEt) predict that the M−alkyne bond energy varies in the order Ag < Cu < Au. The gold adducts are also predicted to have the longest CC, largest deviation of C−CC bond angle from linearity, and smallest CC stretching frequency, followed by the Cu and Ag adducts. In these triazapentadienyl coinage metal adducts, the σ-donation from alkyne → M dominates over the M → alkyne π-back-donation.