Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom: Synthesis, Structure, and Properties

Seventeen peralkylated four- and five-membered heteroatom-containing silacycles [R1R2Si]nX [R1 = iPr and/or R2 = tBuCH2; n = 3; X = CH2 (1a, 1b), NR3 (R3 = C6H11) (3a−d), O (5a−c), and n = 4; X = CH2 (2a, 2b), NR3 (R3 = C6H11 or Pr) (4a−d), and O (6a, 6b)] have been synthesized and characterized. The structural features of twelve compounds − 1b, 3a−d, and 5b (Si3X) and 2a, 4a−c, 6a, and 6b (Si4X) − determined by X-ray analysis have been investigated and geometrical differences between the silacycles bearing neopentyl groups and those with isopropyl groups as bulky substituents on silicon atoms have been identified. The ring strain energies for the compounds [R2Si]nX [n = 3, 4; R = Me, iPr; X = CH2, N(iPr), NMe, O, SiH2], estimated by theoretical calculations at the PM3 level, are discussed in relation to experimentally ascertained structural features of the silacycles [R2Si]nX (n = 3, 4; R = iPr; X = CH2, NC6H11, O, SiR2). 29Si NMR spectra for 1−6 showed that the resonances for the α-silicon atoms attached to the heteroatoms X in the two ring-size series were shifted downfield by a deshielding effect due to an electronegative atom X; larger electronegativity (En) of X caused this effect at lower field. The chemical shifts of the Siα atoms showed a good linear correlation with the electronegativities of the heteroatoms X in the [R2Si]nX cycles. In the UV spectra for 1−6, little difference in the longest-wavelength absorption (LWA) maxima was observed for [(iPr)2Si]nX and [(tBuCH2)2Si]nX of the same ring size. The LWA bands for 1−6 and related compounds could be roughly classified into four groups: (a) [R2Si]3X (X = SiR2, GeR′2) λ = 286−300 nm; (b) [R2Si]3X (X = CH2, NR3, O) (1, 3, 5), λ = 230−250 nm; (c) [R2Si]4X (X = SiR2, GeR′2) λ = 274−280 nm; (d) [R2Si]4X (X = CH2, NR3, O) (2, 4, 6), λ = 250−260 nm. With the aid of the ionization potentials and the transition energies (ET), the HOMO and LUMO levels for the series of compounds and the related ones were experimentally evaluated. The HOMO and LUMO levels thus obtained for [R2Si]3X and [R2Si]4X (R = iPr and tBuCH2; X = SiR2, GeR′2, CH2, NC6H11, O) are discussed in terms of the calculated values and frontier orbitals on the basis of PM3 levels. The theoretical prediction for bond scission in the silacycles [(iPr)2Si]nX (n = 3, 4; X = CH2, NMe, O) in which the HOMOs and LUMOs have Si−Si bonding and antibonding character, respectively, was corroborated by the good agreement with experimental results from photolysis of the silacycles [R2Si]nX (n = 3, 4; R = iPr and tBuCH2; X = CH2, NC6H11, O), with bond scission occurring only at the Si−Si bonds, and not at the Si−X bonds. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)