COPPER(I)-INDUCED ACTIVATION OF MONOSUBSTITUTED ACETYLENES - PERTURBATION OF LIGAND GEOMETRY RESULTING FROM PI-COMPLEXATION

Three complexes between monosubstituted acetylenes, viz. phenylacetylene, propargyl chloride and propargyl alcohol, and copper(I) chloride have been prepared by direct reaction between copper(I) chloride and the relevant acetylenes. Changes in ligand geometry ensuing from pi complexation to copper(I) have been investigated by infrared spectroscopy and crystal structure determinations. In all three compounds copper(I) exhibits trigonal pyramidal coordination geometry with a long apical Cu-Cl bond, the trigonal plane being composed of the midpoint of pi-acetylenic linkage and two chloride ligands. The Cu-C bond lengths are 1.999(4) and 2.066(3) angstrom in [CuCl(HC=CC6H5)] (1), 2.004(3) and 2.004(4) angstrom in [CuCl(HC-=CCH2Cl)] (2) and 2.006(10) and 2.018(9) angstrom in [CuCl(HC=CCH2OH)] (3). The C=C bond is lengthened only slightly on coordination, although nu(C=C) is lowered by 133, 144 and 154 cm-1 for 1, 2 and 3, respectively, and there is moderate bending back of the H-C=C and R-C=C angles, indicating a relatively weak copper(I)-alkyne interaction. nu(C(sp)-H) is lowered by 120, 83 and 110 cm-1, relative to the values for the free ligands, in 1-3, respectively. Although this suggests activation of the C(sp)-H bonds, this is not supported by unusual structural features involving the acetylenic hydrogen atoms.