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

The reactions of the yttrium metallocene allyl complex [(C5Me4)SiMe2(CH2CHCH2)]2Y(C3H5), 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 [(C5Me4)SiMe2(CH2CHCH2)]2YCl, which can be directly prepared free of solvent and alkali metal halide adducts from YCl3(THF) x and [(C5Me4)SiMe2(CH2CHCH2)]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, (C5Me5)2Y(C3H5), 2, the ansa analogue, [Me2Si(C5Me4)2]Y(C3H5), 3, and the lutetium complex, (C5Me5)2Lu(C3H5), 4. Although 1 shows no evidence for insertion of the tethered olefins into the Y−(C3H5) 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 Al2Me6 to generate the allyl-free, tetramethylaluminate-bridged dimer {[(C5Me4)SiMe2(CH2CHCH2)]2Y[(μ-Me)2AlMe2]}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 H2 in diethyl ether to form the expected hydride decomposition product, the ethoxide, {[(C5Me4)SiMe2(CH2CH2CH3)]2Y(OEt)} x , 6. Reaction of 1 with hydrogen in C6D6 and with deuterium in C6H6 shows that hydrogenation of the tethered olefin occurs before D/H exchange with arenes can occur in this system.