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

Three complexes between monosubstituted acetylenes, viz. phenylacetyle ne, propargyl chloride and propargyl alcohol, and copper(I) chloride h ave been prepared by direct reaction between copper(I) chloride and th e relevant acetylenes. Changes in ligand geometry ensuing from pi comp lexation 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-ac etylenic 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) a ngstrom in [CuCl(HC=CCH2OH)] (3). The C=C bond is lengthened only slig htly 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, re lative to the values for the free ligands, in 1-3, respectively. Altho ugh this suggests activation of the C(sp)-H bonds, this is not support ed by unusual structural features involving the acetylenic hydrogen at oms.