STRUCTURAL CHARACTERIZATION OF THE ACETYLIDE BRIDGE IN DINUCLEAR (MU-C-EQUIVALENT-TO-CSIME3)-AL/AL AND (MU-C-EQUIVALENT-TO-CSIME3)-AL/ZR COMPLEXES

Two rare examples of complexes that contain an acetylide ligand bridging between a main group metal (aluminum) and a transition metal (zirconium) are described. Such complexes are prepared by treatment of dimethylzirconocene with 2 molar equiv of (alkynyl)dimethylaluminum [(RC=C)AlMe2]2 (R = -SiMe3, -cyclohexyl, -CMe3, -CH3; 2a-d). In this reaction an (eta2-alkyne)zirconocene complex is probably formed in situ which is then trapped by the excess (alkynyl)dimethylaluminum to yield the complexes Cp2Zr(mu-eta1:eta2-RCCMe)(mu-C = CR)AlMe2 (6a-d). The complexes 6a (R = SiMe3) and 6b (R = cyclohexyl) were characterized by X-ray diffraction. For comparison, we have also carried out the X-ray crystal structure analysis of the [(trimethylsilyl)ethynyl]dimethylaluminum dimer 2a. The complexes 6 contain a dimetallabicyclic framework. One of the bridgehead positions is a carbon center which is planar-tetracoordinate. In these complexes the -C = CR bridge between zirconium and aluminum can be described as being mainly of mu-(sigma-acetylide) character.