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.