Ethylene and alkyne carbon-carbon bond cleavage across tungsten-tungsten multiple bonds

摘要

Treatment of [(silox)(2)WH](2) (1) with RCdropCR'(R = R'= H, CH3; R = H, R'= Ph) afforded thermally unstable [(silox)(2)W](2)(mu:eta(2),eta(2) -RCCR')(mu-H)(2) (R = R'= H, 2a; CH3,2b; R = H, R'= Ph, 2c), which lose H-2 and convert to [(silox)(2)W](2)(mu-CR)(mu-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) Angstrom. Thermal degradation of 1(silox)(2)W(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)(2)C]WdropWCl(silox)(2) (3) in the presence of excess 2-butyne afforded [(silox)(2)W](2)(mu:eta(2),eta(2)-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)(2)W](2)(mu-CH)(mu-CH2)(H) (9, -78 degreesC) followed by H-2 loss; ethylene cleavage does not proceed via 2a or 8. Related cleavage chemistry was not observed for [(silox)(2)TaH2](2) (10) and excess ethylene, which formed (SilOX)2HTaEt]2 (11) and, ultimately, [(silox)(2)HTaEt](2) (mu-CHCH2)(mu-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 mu-CR bridges-are discussed. [References: 49]