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

摘要

Protonation and alkylation of (Idipp)Si=Si-(Idipp) (1) afforded the mixed-valent disilicon(Ⅰ)-borates [(Idipp)(R)Si~Ⅱ=Si~0(Idipp)][B(Ar~F)_4] (1R[B(Ar~F)_4]; R = H, Me, Et; Ar~F = C_6H_3-3,5-(CF_3)_2; Idipp = C[N(C_6H_3-2,6-iPr_2)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-(Ar~F)_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(Ar~F)_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(Ar~F)_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[BAr_4] (M = Li, Ar = C_6F_5; M = Na, Ar = Ar~F) as brown, air-sensitive solids. Single-crystal X-ray diffraction analyses and NMR spectroscopic studies of 1M[BAr_4] 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.