Alkyne Adducts of [W2(OCH2tBu)8]. Cases of Perpendicular and Skewed Bridges in Equilibrium with Terminal Bonded Isomers
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- 作者：Malcolm H. Chisholm ; Damon R. Click ; Judith C. Gallucci ; Christopher M. Hadad ; and Paul J. Wilson
- 刊名：Organometallics
- 出版年：2003
- 出版时间：November 10, 2003
- 年：2003
- 卷：22
- 期：23
- 页码：4725 - 4733
- 全文大小：368K
- 年卷期：v.22,no.23(November 10, 2003)
- ISSN：1520-6041
文摘
Alkynes react with a suspension of [W2(OCH2tBu)8](M=M) in hydrocarbon solvents to givealkyne adducts W2(
-RCCR')(OCH2tBu)8, where R = H and R' = Ph, Me, Me3Si and R = Meand R' = Me and Ph. For PhC
CH, there is a -perpendicular mode of alkyne bonding, 
hars/theta.gif" BORDER=0 > =84
, but for MeCCH and MeCCMe, the bridging alkyne is distinctly skewed with respectto the W-W axis, hars/theta.gif" BORDER=0 > = 64 and 67, respectively. In contrast, the PhCCMe adduct has thealkyne bonded to only one tungsten, hars/eta.gif" BORDER=0 >2-PhCCMe, with the alkyne CC axis being 30from the W-W axis. In all of these structures, there are alkoxide bridges, and the dinuclearunit may be viewed as a confacial bioctahedron where either one alkyne and two alkoxidesor three alkoxides occupy the bridging face. In toluene-d8, NMR spectroscopic studies indicatethat certain of these alkyne adducts (MeCCH, MeCCMe, PhCCMe, and Me3SiCCH)exist as a mixture of alkyne-bridged and hars/eta.gif" BORDER=0 >2-alkyne-bonded isomers. The isomers are fluxionalon the NMR time scale by rapid exchange of terminal and bridging alkoxide groups. However,-hars/eta.gif" BORDER=0 >2 alkyne exchange is slow on the NMR time scale. The structural data are comparedwith electronic structure calculations employing density functional theory on modelcompounds of formula W2(alkyne)(OMe)8. The calculations suggest that a -parallel modeof bonding is energetically favorable relative to -perpendicular and, further, that the energydifference between bridge and terminal alkyne adducts is, in all cases, less than 5 kcal mol-1.
-RCCR')(OCH2tBu)8, where R = H and R' = Ph, Me, Me3Si and R = Meand R' = Me and Ph. For PhCCH, there is a -perpendicular mode of alkyne bonding, hars/theta.gif" BORDER=0 > =84, but for MeCCH and MeCCMe, the bridging alkyne is distinctly skewed with respectto the W-W axis, hars/theta.gif" BORDER=0 > = 64 and 67, respectively. In contrast, the PhCCMe adduct has thealkyne bonded to only one tungsten, hars/eta.gif" BORDER=0 >2-PhCCMe, with the alkyne CC axis being 30from the W-W axis. In all of these structures, there are alkoxide bridges, and the dinuclearunit may be viewed as a confacial bioctahedron where either one alkyne and two alkoxidesor three alkoxides occupy the bridging face. In toluene-d8, NMR spectroscopic studies indicatethat certain of these alkyne adducts (MeCCH, MeCCMe, PhCCMe, and Me3SiCCH)exist as a mixture of alkyne-bridged and hars/eta.gif" BORDER=0 >2-alkyne-bonded isomers. The isomers are fluxionalon the NMR time scale by rapid exchange of terminal and bridging alkoxide groups. However,-hars/eta.gif" BORDER=0 >2 alkyne exchange is slow on the NMR time scale. The structural data are comparedwith electronic structure calculations employing density functional theory on modelcompounds of formula W2(alkyne)(OMe)8. The calculations suggest that a -parallel modeof bonding is energetically favorable relative to -perpendicular and, further, that the energydifference between bridge and terminal alkyne adducts is, in all cases, less than 5 kcal mol-1.