A series of isolable silanoic thio-, seleno-, and telluroesters (LSi(=X)OR) with donor-supported Si=X double bonds (L = beta-diketiminate; X = S, Se, Te).

The first silicon analogues of carbonic (carboxylic) esters, the silanoic thio-, seleno-, and tellurosilylesters 3 (Si=S), 4 (Si=Se), and 5 (Si=Te), were prepared and isolated in crystalline form in high yield. These thermally robust compounds are easily accessible by direct reaction of the stable siloxysilylene L(Si:)OSi(H)L' 2 (L = HC(CMe)2[N(aryl)2], L' = CH[(C=CH2)-CMe][N(aryl)]2; aryl = 2,6-iPr2C6H3) with the respective elemental chalcogen. The novel compounds were fully characterized by methods including multinuclear NMR spectroscopy and single-crystal X-ray diffraction analysis. Owing to intramolecular N-->Si donor-acceptor support of the Si=X moieties (X=S, Se, Te), these compounds have a classical valence-bond N(+)-Si-X(-) resonance betaine structure. At the same time, they also display a relatively strong nonclassical Si=X pi-bonding interaction between the chalcogen lone-pair electrons (n(pi) donor orbitals) and two antibonding Si-N orbitals (sigma*(pi) acceptor orbitals mainly located at silicon), which was shown by IR and UV/Vis spectroscopy. Accordingly, the Si=X bonds in the chalcogenoesters are 7.4 (3), 6.7 (4), and 6.9 % (5) shorter than the corresponding Si-X single bonds and, thus, only a little longer than those in electronically less disturbed Si=X systems ("heavier" ketones).