Ethylene and Alkyne Carbon−Carbon Bond Cleavage across Tungsten−Tungsten Multiple Bonds

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 cleavagewhose thermodynamic impetus is the formation of μ-CR bridgesare discussed.