New Chromium(II) Bidentate Phosphine Complexes:  Synthesis, Characterization, and Behavior in the Polymerization of 1,3-Butadiene

New chromium(II) bidentate phosphine complexes were synthesized by reacting CrCl2(thf) with various bidentate phosphines [bis(dimethylphosphino)methane (dmpm); 1,2-bis(diethylphosphino)ethane (depe); 1,3-bis(diethylphosphino)propane (depp); 1,2-bis(dicyclohexylphpsphino)ethane (dcpe)] in toluene or diethyl ether as solvent. Single crystals of the complex CrCl2(depe)2 were obtained by recrystallization from toluene at low temperature; the structure of the complex was determined by X-ray diffraction and compared with that of the analogous methyl derivative CrCl2(dmpe)2 previously reported by Girolami et al. (dmpe = 1,2-bis(dimethylphosphino)ethane). All the complexes prepared were then used in combination with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene. They were found to be very active catalysts, giving essentially 1,2-polybutadienes having a syndiotactic or isotactic structure depending on the type of phosphine bonded to the chromium atom. The results obtained also have some mechanistic implications and have been interpreted on the basis of the diene polymerization mechanism previously proposed.