Metallocene Allyl Reactivity in the Presence of Alkenes Tethered to Cyclopentadienyl Ligands

The reactions of the yttrium metallocene allyl complex [(C 5 Me 4 )SiMe 2 (CH 2 CH CH 2 )] 2 Y(C 3 H 5 ), 1, with ethylene, trimethylaluminum, and hydrogen have been examined to determine how the olefins tethered to the cyclopentadienyl ligands interact with these species that are typically present in olefin polymerizations. Complex 1 was prepared from allylmagnesium chloride and [(C 5 Me 4 )SiMe 2 (CH 2 CH CH 2 )] 2 YCl, which can be directly prepared free of solvent and alkali metal halide adducts from YCl 3 (THF) x and [(C 5 Me 4 )SiMe 2 (CH 2 CH CH 2 )]K in THF. 1 was characterized by NMR spectroscopy and X-ray crystallography, and the data were compared to those of three related complexes: the pentamethyl analogue, (C 5 Me 5 ) 2 Y(C 3 H 5 ), 2, the ansa analogue, [Me 2 Si(C 5 Me 4 ) 2 ]Y(C 3 H 5 ), 3, and the lutetium complex, (C 5 Me 5 ) 2 Lu(C 3 H 5 ), 4 . Although 1 shows no evidence for insertion of the tethered olefins into the Y−(C 3 H 5 ) bond, comparisons of the synthesis and variable-temperature NMR spectra of 1 with 2 − 4 suggest metal−alkene interaction. Complex 1 polymerizes ethylene and reacts with excess Al 2 Me 6 to generate the allyl-free, tetramethylaluminate-bridged dimer {[(C 5 Me 4 )SiMe 2 (CH 2 CH CH 2 )] 2 Y[(μ-Me) 2 AlMe 2 ]} 2, 5 . No involvement of the tethered olefins was observed in connection with this Y−C(allyl) bond breaking and Y−C(Me) bond making. The tethered olefins do become involved in the reaction chemistry of 1 in the presence of hydrogen: the tethered olefins of 1 are hydrogenated. A hydride derivative of 1 was not isolated, but 1 reacts with H 2 in diethyl ether to form the expected hydride decomposition product, the ethoxide, {[(C 5 Me 4 )SiMe 2 (CH 2 CH 2 CH 3 )] 2 Y(OEt)} x, 6 . Reaction of 1 with hydrogen in C 6 D 6 and with deuterium in C 6 H 6 shows that hydrogenation of the tethered olefin occurs before D/H exchange with arenes can occur in this system.