Silicon(i) chemistry: the NHC-stabilised silicon(i) halides Si(2)X(2)(Idipp)(2) (X = Br, I) and the disilicon(i)-iodido cation [Si(2)(I)(Idipp)(2)](+)

An efficient method for the synthesis of the NHC-stabilised Si(i) halides Si(2)X(2)(Idipp)(2) (2-X, X = Cl, Br, I; Idipp = C[N(C(6)H(3)-2,6-iPr(2))CH](2)) was developed, which involves the oxidation of Si(2)(Idipp)(2) (1) with 1,2-dihaloethanes. Halogenation of 1 is a diastereoselective reaction leading exclusively to a racemic mixture of the RR and SS enantiomers of 2-X. Compounds 2-Br and 2-I were characterised by single-crystal X-ray crystallography and multinuclear NMR spectroscopy, and their electronic structures were analysed by quantum chemical methods. Dynamic NMR spectroscopy unraveled a fluxional process of 2-Br and 2-I in solution, which involved a hindered rotation of the NHC groups about the Si–C(NHC) bonds. Iodide abstraction from 2-I by [Li(Et(2)O)(2.5)][B(C(6)F(5))(4)] selectively afforded the disilicon(i) salt [Si(2)(I)(Idipp)(2)][B(C(6)F(5))(4)] (3). X-ray crystallography and variable-temperature NMR spectroscopy of 3 in combination with quantum chemical calculations shed light on the ground-state geometric and electronic structure of the [Si(2)(I)(Idipp)(2)](+) ion, which features a Si[double bond, length as m-dash]Si bond between a trigonal planar coordinated Si(II) atom with a Si–I bond and a two-coordinate Si(0) center carrying a lone pair of electrons. The dynamics of the [Si(2)(I)(Idipp)(2)](+) ion were studied in solution by variable-temperature NMR spectroscopy and they involve a topomerisation, which proceeds according to quantum theory via a disilaiodonium intermediate (“π-bonded” isomer) and exchanges the two heterotopic Si sites.