ChromiumSilicon Multiple Bonds: The Chemistry of Terminal N-Heterocyclic-Carbene-Stabilized Halosilylidyne Ligands

An efficient method for the synthesis of the first N‐heterocyclic carbene (NHC)‐stabilized halosilylidyne complexes is reported that starts from SiBr4. In the first step, SiBr4 was treated with one equivalent of the N‐heterocyclic carbene 1,3‐bis[2,6‐bis(isopropyl)phenyl]imidazolidin‐2‐ylidene (SIdipp) to give the 4,5‐dihydroimidazolium salt [SiBr3(SIdipp)]Br (1‐Br), which then was reduced with potassium graphite to afford the silicon(II) dibromide–NHC adduct SiBr2(SIdipp) (2‐Br) in good yields. Heating 2‐Br with Li[CpCr(CO)3] afforded the complex [Cp(CO)2CrSiBr(SIdipp)] (3‐Br) upon elimination of CO. Complex 3‐Br features a trigonal‐planar‐coordinated silicon center and a very short CrSi double bond. Similarly, the reaction of SiCl2(SIdipp) (2‐Cl) with Li[CpCr(CO)3] gave the analogous chloro derivative [Cp(CO)2CrSiCl(SIdipp)] (3‐Cl). Complex 3‐Br undergoes an NHC exchange with 1,3‐dihydro‐4,5‐dimethyl‐1,3‐bis(isopropyl)‐2H‐imidazol‐2‐ylidene (IMe2iPr2) to give the complex [Cp(CO)2CrSiBr(IMe2iPr2)2] (4‐Br). Compound 4‐Br features a distorted‐tetrahedral four‐coordinate silicon center. Bromide ion occurs readily from 4‐Br with Li[B(C6F5)4] to give the putative silylidene complex salt [Cp(CO)2CrSi(IMe2iPr2)2][B(C6F5)4], which irreversibly dimerizes by means of an Si‐promoted electrophilic activation of one carbonyl oxygen atom to yield the dinuclear siloxycarbyne complex [Cp(CO)Cr{(μ‐CO)Si(IMe2iPr2)2}2‐ Cr(CO)Cp][B(C6F5)4]2 (5). All compounds were fully characterized, and the molecular structures of 2‐Br–5‐Br were determined by single‐crystal X‐ray diffraction. DFT calculations of 3‐Br and 3‐Cl and their carbene dissociation products [Cp(CO)2CrSiX] (X=Cl, Br) were carried out, and the electronic structures of 3‐Br, 3‐Cl and [Cp(CO)2CrSiX] were analyzed by the natural bond orbital method in combination with natural resonance theory. Fleeting observations: The chemistry of the short‐lived halosilylidynes SiX (X=F to I) has not yet been explored. Coordination to a transition‐metal center and stabilization using an N‐heterocyclic carbene (NHC) provides entry to the presently unknown classes B and C of halosilylidyne complexes (see scheme).