Addition of Small Electrophiles to N‑Heterocyclic-Carbene-Stabilized Disilicon(0): A Revisit of the Isolobal Concept in Low-Valent Silicon Chemistry

Protonation and alkylation of (Idipp)­SiSi­(Idipp) (1) afforded the mixed-valent disilicon­(I)-borates [(Idipp)­(R)­SiIISi0(Idipp)]­[B­(ArF)4] (1R[B­(ArF)4]; R = H, Me, Et; ArF = C6H3-3,5-(CF3)2; Idipp = C­[N­(C6H3-2,6-iPr2)­CH]2) as red to orange colored, highly air-sensitive solids, which were characterized by single-crystal X-ray diffraction, IR spectroscopy and multinuclear NMR spectroscopy. Dynamic NMR studies in solution revealed a degenerate isomerization (topomerization) of the “σ-bonded” tautomers of 1H[B­(ArF)4], which proceeds according to quantum chemical calculations via a NHC-stabilized (NHC = N-heterocyclic carbene) disilahydronium ion (“π-bonded” isomer) and is reminiscent of the degenerate rearrangement of carbenium ions formed upon protonation of olefins. The topomerization of 1H[B­(ArF)4] provides the first example of a reversible 1,2-H migration along a SiSi bond observed in a molecular system. In contrast, 1Me[B­(ArF)4] adopts a “rigid” structure in solution due to the higher energy required for the interconversion of the “σ-bonded” isomer into a putative NHC-stabilized disilamethonium ion. Addition of alkali metal borates to 1 afforded the alkali metal disilicon(0) borates 1M[BAr4] (M = Li, Ar = C6F5; M = Na, Ar = ArF) as brown, air-sensitive solids. Single-crystal X-ray diffraction analyses and NMR spectroscopic studies of 1M[BAr4] suggest in concert with quantum chemical calculations that encapsulation of the alkali metal cations in the cavity of 1 predominantly occurs via electrostatic cation−π interactions with the SiSi π-bond and the peripheral NHC aryl rings. Displacement of the [Si­(NHC)] fragments by the isolobal fragments [PR] and [SiR]− interrelates the cations [(NHC)­(R)­SiSi­(NHC)]+ to a series of familiar, multiply bonded Si and P compounds as verified by analyses of their electronic structures.(Idipp)­Si＝Si­(Idipp)（简称1）的质子化与烷基化反应，得到混合价态二硅(I)硼酸盐[(Idipp)­(R)­Si(II)＝Si(0)(Idipp)]­[B­(ArF)4]（记为1R[B­(ArF)4]；其中R = H、Me、Et；ArF = 3,5-双(三氟甲基)苯基（C6H3-3,5-(CF3)2）；Idipp = 1,3-双(2,6-二异丙基苯基)咪唑-2-碳烯（C[N(C6H3-2,6-iPr2)CH]2）），该类化合物为红色至橙色的高空气敏感性固体，通过单晶X射线衍射、红外光谱及多核核磁共振波谱完成了表征。对溶液中1H[B(ArF)4]的动态核磁共振研究显示，其“σ键合”互变异构体存在简并异构化（拓扑异构化）过程；量子化学计算表明，该过程经由N-杂环卡宾（N-heterocyclic carbene，NHC）稳定的二硅氢正离子（“π键合”异构体）进行，这一过程与烯烃质子化所生成的碳正离子的简并重排现象具有相似性。1H[B(ArF)4]的拓扑异构化是分子体系中首次观测到的沿Si=Si键发生的可逆1,2-氢迁移反应。与之不同，1Me[B(ArF)4]在溶液中呈现“刚性”结构，这是因为其“σ键合”异构体转化为假定的NHC稳定的二硅甲鎓离子所需的活化能较高。向1中加入碱金属硼酸盐，得到碱金属二硅(0)硼酸盐1M[BAr4]（M = Li时，Ar = 五氟苯基（C6F5）；M = Na时，Ar = ArF），该类化合物为棕色的空气敏感性固体。针对1M[BAr4]的单晶X射线衍射分析与核磁共振波谱研究，结合量子化学计算结果证实，碱金属阳离子主要通过与Si=Si π键及外围NHC芳环之间的静电阳离子-π相互作用，被包裹于1的空腔内。利用等瓣相似片段[PR]与[SiR]−取代[Si(NHC)]片段，可将阳离子[(NHC)­(R)­Si＝Si­(NHC)]+与一系列经典的多键合硅、磷化合物建立关联，这一结论通过其电子结构分析得到了验证。