N-Heterocyclic Carbene (NHC)-Stabilized Silanechalcogenones: NHC -> Si(R-2)=E (E = O, S, Se, Te)

A series of N-heterocyclic carbene-stabilized silanechalcogenones 2a,b (Si=O), 3a,b (Si=S), 4a,b (Si=Se), and 5a,b (Si=Te) are described. The silanone complexes 2a,b were prepared by facile oxygenation of the carbene-silylene adducts 1a,b with N2O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1a,b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2b, 3a, 4a, 4b, and 5b have been confirmed by single-crystal X-ray crystallography. Due to the NHC -> Si donor-acceptor electronic interaction, the Si=E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide-like resonance structures. Nevertheless, these species also exhibit considerable Si=E double-bond character, presumably through a nonclassical Si=E T-bonding interaction between the chalcogen lone-pair electrons and two antibonding Si-N sigma* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si-E distances (between 5.4 and 6.3%) compared with the corresponding Si-E single-bond lengths.