Why the Mechanisms of Digermyne and Distannyne Reactions with H2 Differ So Greatly

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• 作者：LiLi Zhao ; Fang Huang ; Gang Lu ; Zhi-Xiang Wang ; Paul von Ragu茅 Schleyer
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：May 30, 2012
• 年：2012
• 卷：134
• 期：21
• 页码：8856-8868
• 全文大小：715K
• 年卷期：v.134,no.21(May 30, 2012)
• ISSN：1520-5126

文摘

Despite their formal relationship to alkynes, Ar鈥睪eGeAr鈥? Ar鈥睸nSnAr鈥? and Ar*SnSnAr* [Ar鈥?= 2,6-(2,6-iPr₂C₆H₃)₂C₆H₃; Ar* = 2,6-(2,4,6-iPr₃C₆H₂)₂-3,5-iPr₂C₆H] exhibit high reactivity toward H₂, quite unlike acetylenes. Remarkably, the products are totally different. Ar鈥睪eGeAr鈥?can react with 1鈥? equiv of H₂ to give mixtures of Ar鈥睭GeGeHAr鈥? Ar鈥睭₂GeGeH₂Ar鈥? and Ar鈥睪eH₃. In contrast, Ar鈥睸nSnAr鈥?and Ar*SnSnAr* react with only 1 equiv of H₂ but give different types of products, Ar鈥睸n(渭-H)₂SnAr鈥?and Ar*SnSnH₂Ar*, respectively. In this work, this disparate behavior toward H₂ has been elucidated by TPSSTPSS DFT computations of the detailed reaction mechanisms, which provide insight into the different pathways involved. Ar鈥睪eGeAr鈥?reacts with H₂ via three sequential steps: H₂ addition to Ar鈥睪eGeAr鈥?to give singly H-bridged Ar鈥睪e(渭-H)GeHAr鈥? isomerization of the latter to the more reactive Ge(II) hydride Ar鈥睪eGeH₂Ar鈥? and finally, addition of another H₂ to the hydride, either at a single Ge site, giving Ar鈥睭₂GeGeH₂Ar鈥? or at a Ge鈥揋e joint site, affording Ar鈥睪eH₃ + Ar鈥睭Ge:. Alternatively, Ar鈥睪e(渭-H)GeHAr鈥?also can isomerize into the kinetically stable Ar鈥睭GeGeHAr鈥? which cannot react with H₂ directly but can be transformed to the reactive Ar鈥睪eGeH₂Ar鈥? The activation of H₂ by Ar鈥睸nSnAr鈥?is similar to that by Ar鈥睪eGeAr鈥? The resulting singly H-bridged Ar鈥睸n(渭-H)SnHAr鈥?then isomerizes into Ar鈥睭SnSnHAr鈥? The subsequent facile dissociation of the latter gives two Ar鈥睭Sn: species, which then reassemble into the experimental product Ar鈥睸n(渭-H)₂SnAr鈥? The reaction of Ar*SnSnAr* with H₂ forms in the kinetically and thermodynamically more stable Ar*SnSnH₂Ar* product rather than Ar*Sn(渭-H)₂SnAr*. The computed mechanisms successfully rationalize all of the known experimental differences among these reactions and yield the following insights into the behavior of the Ge and Sn species: (I) The active sites of Ar鈥睧EAr鈥?(E = Ge, Sn) involve both E atoms, and the products with H₂ are the singly H-bridged Ar鈥睧(渭-H)EHAr鈥?species rather than Ar鈥睭EEHAr鈥?or Ar鈥睧EH₂Ar鈥? (II) The heavier alkene congeners Ar鈥睭EEHAr鈥?(E = Ge, Sn) cannot activate H₂ directly. Instead, Ar鈥睭GeGeHAr鈥?must first isomerize into the more reactive Ar鈥睪eGeH₂Ar鈥? Interestingly, the subsequent H₂ activation by Ar鈥睪eGeH₂Ar鈥?can take place on either a single Ge site or a joint Ge鈥揋e site, but Ar鈥睸nSnH₂Ar鈥?is not reactive toward H₂. The higher reactivity of Ar鈥睪eGeH₂Ar鈥?in comparison with Ar鈥睸nSnH₂Ar鈥?is due to the tendency of group 14 elements lower in the periodic table to have more stable lone pairs (i.e., the inert pair effect) and is responsible for the differences between the reactions of Ar鈥睧EAr鈥?(E = Ge, Sn) with H₂. Similarly, the carbene-like Ar鈥睭Ge: is more reactive toward H₂ than is Ar鈥睭Sn:. (III) The doubly H-bridged Ar鈥睧(渭-H)₂EAr鈥?(E = Ge, Sn) species are not reactive toward H₂.