Interconnected bis-silylenes: a new dimension in organosilicon chemistry

The past two decades have brought remarkable advances in organosilicon chemistry with the isolation of stable silylenes, persila-allene, and disilynes. The extension of this list gives an impression that it will continue to flourish. The judicous employment of sterically appropriate ligands has enabled the synthesis and isolation of compounds with low-valent silicon. Recently, for example, interconnected bis-silylenes were isolated where the two Si atoms are connected by a σ-bond and each Si atom is possessing a lone pair of electrons. The formal oxidation state of each Si atom in the interconnected bis-silylene is +1, so bis-silylenes can be considered as the valence isomers of disilynes. In this Account, we describe the synthesis of interconnected bis-silylenes and assess their potential as a new building block in organosilicon chemistry. In 2009, we reported the isolation of a bis-silylene ((PhC(NtBu)(2))(2)Si(2)) stabilized by a sterically bulky benz-amidinato ligand with tBu substituents on the nitrogen atoms. Prior to our work, Robinson and co-workers described the synthesis of a N-heterocyclic carbene stabilized bis-silylene. In following years, just two more interconnected bis-silylenes have been reported. Density functional theory calculations to establish the geometric and electronic structures of the reported bis-silylenes have shown that the Wiberg bond index (WBI) for all the reported bis-silylenes is ~1. The synthesis of stable (PhC(NtBu)(2))(2)Si(2) prompted explorations of its reactivity. An important facet of silylene chemistry involves oxidative addition at the Si(II) center with unsaturated substrates, a reaction also available for bis-silylenes. Due to the three reaction sites (two lone pairs of electrons and a labile Si(I)-Si(I) single bond) in the interconnected bis-silylenes, we expect novel product formation. A labile Si-Si bond facilitates the reactions of (PhC(NtBu)(2))(2)Si(2) with diphenyl alkyne or adamantyl phosphaalkyne which afforded 1,4- disilabenzene and 1,3-disilacarbaphosphide (CSi(2)P) derivatives, respectively. The former is a noteworthy addition to the silicon analogues of benzene, and the latter serves as a heavy cyclobutadiene. With white phosphorus, a cyclic Si(2)P(2) derivative, an analogue of cyclobutadiene was obtained. The most predominant structural feature of these heavy cyclobutadienes is the presence of two-coordinate P atoms.