Heptaphosphanortricyclenes with Oligosilyl Substituents: Syntheses and Reactions



Three new nortricyclic P7R3 derivatives with R = (SiMe3)2MeSi– (2), (SiMe3)2PhSi– (3), and cyclo-Si6Me11– (4) were synthesized from red phosphorus, sodium/potassium alloy, and a chlorooligosilane, and their structures were elucidated with X-ray diffraction. Reactions of 2 and 3 and of tri(hypersilyl)heptaphosphane 1 [hypersilyl = (SiMe3)3Si–] with KOtBu and LiOtBu were performed, which led to different results depending on the size of the substituent. With KOtBu, Si–P bonds were cleaved in 2 and 3, and the mono- and dianions [P7R2] [R = (SiMe3)2MeSi: 2a; R = (SiMe3)2PhSi: 3a] and [P7R]2– [R = (SiMe3)2MeSi: 2b] formed. Crystals of the [K+.18-crown-6]+ salt of 2b suitable for X-ray diffraction could be grown successfully. Compound 1 reacted in an unprecedented way with KOtBu at –60 °C. An Si–Si bond was cleaved, and a transient silyl anion formed, which immediately rearranged into a transient heptaphosphanide anion [Hyp2P7] (1a) under expulsion of bis(trimethylsilyl)silylene. The silylene immediately inserted into a P–P bond of the basal P3 ring of 1a, thereby forming two new isomeric anions {Hyp2P7[Si(SiMe3)2]}, 1b and 1c. The structure of 1c could be elucidated by X-ray crystallography, whereas 1b was characterized by 31P NMR spectroscopy. The proposed reaction mechanism could be further confirmed by performing the reaction between 1 and KOtBu in the presence of a large excess amount of dimethylbutadiene, which trapped the silylene. Just the formation of anion 1a was observed in that case. Anion 1a was also obtained quantitatively when KOtBu was replaced by LiOtBu. Compound 1a reacted with trifluoroacetic acid to give Hyp2P7H (1d), which was successfully crystallized and examined by X-ray diffraction. It is the first structural example of a stable and neutral nortricyclic P7 derivative, R2P7H. All compounds were chracterized further by 31P and 29Si NMR spectroscopy and elemental analyses.