om020037n_si_002.pdf (861.46 kB)
Ethylene and Alkyne Carbon−Carbon Bond Cleavage across Tungsten−Tungsten Multiple Bonds
journal contributionposted on 2002-05-24, 00:00 authored by Rebecca L. M. Chamberlin, Devon C. Rosenfeld, Peter T. Wolczanski, Emil B. Lobkovsky
Treatment of [(silox)2WH]2 (1) with RC⋮CR‘ (R = R‘ = H, CH3; R = H, R‘ = Ph) afforded thermally unstable [(silox)2W]2(μ:η2,η2-RCCR‘)(μ-H)2 (R = R‘ = H, 2a; CH3, 2b; R = H, R‘ = Ph, 2c), which lose H2 and convert to [(silox)2W]2(μ-CR)(μ-CR‘) (R = R‘ = H, 4a; CH3, 4b; R = H, R‘ = Ph, 4c). An X-ray structural study of 4b revealed a nearly square W2C2 core and a d(WW) of 2.720(2) Å. Thermal degradation of [(silox)2W(CH2CH3)]2 (5) also produced 4b, and with 2 equiv of C2H4, its formation is nearly quantitative with 2 equiv of EtH as a byproduct. Na/Hg reduction of (silox)2ClW⋮WCl(silox)2 (3) in the presence of excess 2-butyne afforded [(silox)2W]2(μ:η2,η2-MeC2Me) (8), which could be treated with H2 to give 2b or thermolyzed to 4b. A similar reduction of 3 with excess ethylene present afforded 4a via [(silox)2W]2(μ-CH)(μ-CH2)(H) (9, −78 °C) followed by H2 loss; ethylene cleavage does not proceed via 2a or 8. Related cleavage chemistry was not observed for [(silox)2TaH2]2 (10) and excess ethylene, which formed [(silox)2HTaEt]2 (11) and, ultimately, [(silox)2EtTa](μ-CHCH2)(μ-H)2[Ta(silox)2] (12) and EtH. An X-ray structural study of 12 confirmed its configuration. Spectroscopic features of the molecules are addressed, and plausible mechanisms of carbon−carbon bond cleavagewhose thermodynamic impetus is the formation of μ-CR bridgesare discussed.