Terminal and Internal Alkyne Complexes and Azide-Alkyne Cycloaddition Chemistry of Copper(I) Supported by a Fluorinated Bis(pyrazolyl)borate.

Copper plays an important role in alkyne coordination chemistry and transformations. This report describes the isolation and full characterization of a thermally stable, copper(I) acetylene complex using a highly fluorinated bis(pyrazolyl)borate ligand support. Details of the related copper(I) complex of HC≡CSiMe are also reported. They are three-coordinate copper complexes featuring η-bound alkynes. Raman data show significant red-shifts in C≡C stretch of [HB(3,5-(CF)Pz)]Cu(HC≡CH) and [HB(3,5-(CF)Pz)]Cu(HC≡CSiMe) relative to those of the corresponding alkynes. Computational analysis using DFT indicates that the Cu(I) alkyne interaction in these molecules is primarily of the electrostatic character. The π-backbonding is the larger component of the orbital contribution to the interaction. The dinuclear complexes such as Cu(μ-[3,5-(CF)Pz])(HC≡CH) display similar Cu-alkyne bonding features. The mononuclear [HB(3,5-(CF)Pz)]Cu(NCMe) complex catalyzes [3 + 2] cycloadditions between tolyl azide and a variety of alkynes including acetylene. It is comparatively less effective than the related trinuclear copper catalyst {μ-[3,5-(CF)Pz]Cu} involving bridging pyrazolates.