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

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posted on 2012-05-30, 00:00 authored by LiLi Zhao, Fang Huang, Gang Lu, Zhi-Xiang Wang, Paul von Ragué Schleyer
Despite their formal relationship to alkynes, Ar′GeGeAr′, Ar′SnSnAr′, and Ar*SnSnAr* [Ar′ = 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′GeGeAr′ can react with 1–3 equiv of H2 to give mixtures of Ar′HGeGeHAr′, Ar′H2GeGeH2Ar′, and Ar′GeH3. In contrast, Ar′SnSnAr′ and Ar*SnSnAr* react with only 1 equiv of H2 but give different types of products, Ar′Sn­(μ-H)2SnAr′ 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′GeGeAr′ reacts with H2 via three sequential steps: H2 addition to Ar′GeGeAr′ to give singly H-bridged Ar′Ge­(μ-H)­GeHAr′; isomerization of the latter to the more reactive Ge­(II) hydride Ar′GeGeH2Ar′; and finally, addition of another H2 to the hydride, either at a single Ge site, giving Ar′H2GeGeH2Ar′, or at a Ge–Ge joint site, affording Ar′GeH3 + Ar′HGe:. Alternatively, Ar′Ge­(μ-H)­GeHAr′ also can isomerize into the kinetically stable Ar′HGeGeHAr′, which cannot react with H2 directly but can be transformed to the reactive Ar′GeGeH2Ar′. The activation of H2 by Ar′SnSnAr′ is similar to that by Ar′GeGeAr′. The resulting singly H-bridged Ar′Sn­(μ-H)­SnHAr′ then isomerizes into Ar′HSnSnHAr′. The subsequent facile dissociation of the latter gives two Ar′HSn: species, which then reassemble into the experimental product Ar′Sn­(μ-H)2SnAr′. The reaction of Ar*SnSnAr* with H2 forms in the kinetically and thermodynamically more stable Ar*SnSnH2Ar* product rather than Ar*Sn­(μ-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′EEAr′ (E = Ge, Sn) involve both E atoms, and the products with H2 are the singly H-bridged Ar′E­(μ-H)­EHAr′ species rather than Ar′HEEHAr′ or Ar′EEH2Ar′. (II) The heavier alkene congeners Ar′HEEHAr′ (E = Ge, Sn) cannot activate H2 directly. Instead, Ar′HGeGeHAr′ must first isomerize into the more reactive Ar′GeGeH2Ar′. Interestingly, the subsequent H2 activation by Ar′GeGeH2Ar′ can take place on either a single Ge site or a joint Ge–Ge site, but Ar′SnSnH2Ar′ is not reactive toward H2. The higher reactivity of Ar′GeGeH2Ar′ in comparison with Ar′SnSnH2Ar′ 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′EEAr′ (E = Ge, Sn) with H2. Similarly, the carbene-like Ar′HGe: is more reactive toward H2 than is Ar′HSn:. (III) The doubly H-bridged Ar′E­(μ-H)2EAr′ (E = Ge, Sn) species are not reactive toward H2.

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