Ansa-Zirconocenes with Aluminum or Gallium in Bridging Positions

Abstract

Bis(cyclopentadienyl)aluminum and -gallium species of the type (Pytsi)ECp2 [E = Al (2a), Ga (2b); Pytsi = C(SiMe3)2SiMe2(2-C5H4N)] have been prepared from NaCp and respective dichlorides (Pytsi)ECl2 in isolated yields of 65% (2a) and 83% (2b). In addition to applying standard methods of characterization (NMR spectroscopy, CHN analysis, and MS), the molecular structures of both compounds were solved by single-crystal X-ray crystallography. Reactions between 2a or 2b with Zr(NMe2)4 in benzene, followed by addition of an excess of ClSiMe3, gave the first ansa-zirconocene dichlorides with aluminum (4a) or gallium (4b) in bridging position. The gallium compound 4b was isolated in a yield of 40% and characterized by NMR spectroscopy, CHN analysis, and mass spectrometry. The aluminum species 4a could not be obtained in an analytically pure form; it was characterized by NMR spectroscopy, and its molecular structure was determined by single-crystal X-ray analysis. The angle between the two planes of the Cp ligands in 4a was found to be 57.17(11)°, which is larger than in Cp2ZrCl2 (α = 53.5°) and smaller than in silicon-bridged ansa-zirconocene dichlorides (e.g., Me2Si(C5H4)2ZrCl2: α = 60.1°). Crystallization attempts of 4a from DCCl3 solutions gave crystals of the new species (Pytsi)AlCl[(C5H4)ZrCp] (7), which was characterized by NMR spectroscopy and single-crystal X-ray analysis. In comparison to the Pytsi-containing species 2a and 2b, the less sterically protected compound [2-(Me2NCH2)C6H4]AlCp2 (6), which was prepared from NaCp and [2-(Me2NCH2)C6H4]AlCl2 in yields of 94% and characterized by standard methods (NMR, CHN, and MS), reacted with Zr(NMe2)4 to give the known Cp2Zr(NMe2)2. The targeted ansa-species was not formed, as judged by 1H NMR spectroscopy. The gallium-bridged compound 4b and Cp2ZrCl2, respectively, had been tested for polymerizations of ethylene utilizing MAO as an activator (300 equiv of MAO; 5 or 10 μmol of precatalyst; 15 or 70 psi), resulting in activities of 637 to 1080 for 4b and of 640 to 2601 kg PE (mol Zr)−1 h−1 atm−1 for Cp2ZrCl2.