A Zwitterionic Silylene as Reactive Intermediate and Its Unusual Dimerization to an N-Heterobicyclic Disilane

A way to synthesize the transient zwitterionic silylene L'Si: 8 {L'= CH[(C=CH2)CMe(N(tBu))(2)]} and achieve its facile dimerization to the remarkable N-heterobicyclic disilane 8(2) is described. At first, employing the beta-diketiminate ligand L [L=CH(CMeN(tBu))(2)], both starting materials LH (2) and its N-lithium salt LLi (3) can react with SiBr4 to yield the silylene precursor L'SiBr2 (4) by silicon-induced C-H activation at an exocyclic methyl group on the backbone of the ligand. Compound 4 reacts with SiBr4 above room temperature to afford the unexpected terminal CH(SiBr3)-substituted dibromosilane 6 along with the unique tricyclic trisilane 7. Reduction of 4 with KC8 at 0 degrees C furnishes the novel N-heterobicyclic disilane 8(2), which is a formal dimer of the desired zwitterionic silylene L'Si: (8). It has been reasoned that compound 8(2) may results from [4+1] cycloaddition of two molecules of 8 to give the transient dimer 8(2)', which subsequently undergoes hydrogen transfer from a terminal methyl group on the backbone of the C3N2Si ligand to the low-coordinate Si atom. The latter dimerization can be rationalized by the intrinsic zwitterionic character of 8 and insufficient steric protection through the tBu groups at the nitrogen atoms. The novel compounds 3, 2 have been characterized 4, 6, 7, and 8(2) by H-1, C-13 and Si-29 NMR spectroscopy, mass spectrometry, and elemental analysis. Additionally, the structures of 3, 6, 7, and 8(2) were also established by single-crystal X-ray diffraction analyses.