Matrix‐isolation and electronic structure of vinyl‐substituted silylenes and their complexes with bases [1]

Abstract

Vinyl-substituted silylenes, 2,5-bis(methylene)-l -silacyclopentane-l ,l -diyl (1) and 2-methybne-1 -silacyclopentane-1 ,I -diyl (2), generated photochemically from the corresponding trisilanes in 3-methylpentane (3-MP) at 77 K, showed broad bands at 505 INTROD UCTICINGeneration, structure, and reactions of divalent silicon species, silylenes, have been extensively studied as important reactive intermediates both experimentally and theoretically [2]. The electronic structure of silylenes has been discussed more Dedicated to Prof. Adrian Gibbs Brook on the occasion of his "To whom correspondence should be addressed. seventieth birthday.in-depth since the first matrix isolation of dimethylsilylene by Drahnak et al. in 1979 [3a]; they observed that dimethylsilylene produced by the irradiation of dodecamethylcyclohexasilane in an argon matrix at 10 K or in a 3-methylpentane (3-MP) glass matrix at 77 K showed a characteristic absorption maximum at ca. 450 nm [3a]. Although there was a controversy concerning the absorption spectra of dimethylsilylene [4], recent extensive spectroscopic studies by using matrix isolation [3,51, laser flash photolysis [6], and theoretical calculations [7,8] have confirmed that the 450 nm absorption is ascribed to the n(Si)-3p(Si) transition of singlet dimethylsilylene. Intriguing experimental targets in silylene chemistry involve (1) generation and characterization of silylenes having a triplet ground state and (2) synthesis and isolation of stable singlet silylenes as pure substances. Systematic studies of the electronic and steric effects of substituents on the structure and reactivity of silylene would be required in order to achieve these goals. There have been very limited studies of the substituent effects reported [9] because of difficulty in the generation and matrix isolation of desired silylenes.We report herein the isolation of vinyl-substituted silylenes, 1 and 2, in 3-MP and 2-methyltetrahydrofuran (2-MeTHF) matrices at 77 K [lo]. In disagreement with a recent theoretical prediction 171, the n(Si)-3p(Si) absorption maxima for the vinylsilylenes in 3-MP is shifted to a significantly longer wavelength than that for dimethylsilylene. The origin of the red shift is discussed in terms of the substantial conjugation between the vacant vinyl n-" and 3p(Si) orbitals.A number of experimental

Thermal and Photochemical [2+2] Cycloreversion of a 1,2-Disilacyclobutane and a 1,2-Digermacyclobutane

“…These observations are consistent with the interpretation that the violet solution contains a silylene(s) absorbing at around 540 nm, which upon addition of THF form(s) a silylene−THF complex. The formation of the complex results in a bathochromic shift of the absorption (unfortunately, we could not identify the exact location of the absorption peak within the broad absorption range of 550−650 nm), as has been previously observed for other silylenes . The identity of the species absorbing at 538 nm (denoted as X) could not be determined but we can exclude several possibilities.…”

Silylated Group 14 Ylenes: An Emerging Class of Reactive Compounds

“…Compounds 38 and 39 are interesting because they represent higher analogues of the elusive vinylcarbenes,49–51 which are known to rearrange to cyclopropenes. Although vinylsilylenes had been studied before by Barton52,53 and later by Sakurai,54,55 no examples of vinylated germylenes had previously been reported. Removal of PMe 3 both from 38 and from 39 could be achieved either by application of vacuum or by treatment with B(C 6 F 5 ) 3 (Scheme ).…”

Bis(diisopropylamino)silylene and Its Dimer

“…Apparently, 2-MeTHF stabilizes silylene 1 more than the dimer. The origin of the solvent effects cannot be attributed, however, to the well-established complexation of silylenes with Lewis bases because the complexation should cause significant blue shift of the n → p transition; in reality, the absorption maximum of 1 and its temperature dependence in both 3-MP/2-MeTHF (95/5) and 2-MeTHF solutions were the same as those in 3-MP. Intermolecular complexation of 1 with 2-MeTHF would be hampered because of the low Lewis acidity of 1 due to the effective intramolecular electron donation by the amino substituents.…”