(NHC)SiCN–R: A Two-Coordinated Si⁰‑Isocyanide Compound as Si(NHC) Transfer Reagent

Experimental and theoretical studies are reported of the first two-coordinated Si0-isocyanide compound (SIDipp)­SiCN–ArMes (1: SIDipp (NHC) = C­[N­(Dipp)­CH2]2, ArMes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr­(CNArMes)­(SIDipp)]­[B­(ArF)4] (2[B­(ArF)4], ArF = B­(C6H3-3,5-(CF3)2)4) with KC8. 2[B­(ArF)4] was obtained from SiBr2(SIDipp) after bromide abstraction with an equimolar mixture of Na­[B­(ArF)4] and ArMesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B­(ArF)4] reacts with SiBr2(SIDipp) via isocyanide exchange to afford the disilicon­(II) salt [Si2Br3(SIDipp)2)]­[B­(ArF)4] (3[B­(ArF)4]), the reaction leading to an equilibrium that favors 3[B­(ArF)4] (Keq(298 K) = 10.6, ΔH° = −10.6 kJ mol–1; ΔS° = −16.0 J mol–1 K–1). 3[B­(ArF)4] was obtained selectively from the 2:1 reaction of SiBr2(SIDipp) with Na­[B­(ArF)4] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar CNHC–Si–C–N skeleton with a V-shaped geometry at the dicoordinated Si0 center, a slightly bent SiCN core, a CNHC–Si–CCNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and ArMes substituents leading to axial chirality and the presence of two enantiomers, (Ra)-1 and (Sa)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si­(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)­(Cl)­SiGeArMes (4) from 1 and Ge­(ArMes)­Cl.