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) dibromideNHC adduct SiBr2(SIdipp) (2-Br) in good yields. Heating 2-Br with Li[CpCr(CO)(3)] afforded the complex [Cp(CO)(2)Cr=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)(2)Cr=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 (IMe(2)iPr(2)) to give the complex [Cp(CO)(2)CrSiBr(IMe(2)iPr(2))(2)] (4-Br). Compound 4-Br features a distorted-tetrahedral four-coordinate silicon center. Bromide abstraction occurs readily from 4-Br with Li[B(C6F5)(4)] to give the putative silylidene complex salt [Cp(CO)(2)Cr=Si(IMe(2)iPr(2))(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{(mu-CO)Si(IMe(2)iPr(2))(2)}(2)- Cr(CO)Cp][B(C6F5)(4)](2) (5). All compounds were fully characterized, and the molecular structures of 2-Br5-Br were determined by single-crystal X-ray diffraction. DFT calculations of 3-Br and 3-Cl and their carbene dissociation products [Cp(CO)(2)Cr=Si=X] (X=Cl, Br) were carried out, and the electronic structures of 3-Br, 3-Cl and [Cp(CO)(2)Cr=Si=X] were analyzed by the natural bond orbital method in combination with natural resonance theory.