Phosphanes with bulky oligosilyl substituents

By reaction of dichloroheptasilane [(SiMe3)2MeSi]2SiCl2 with lithiumphosphanides LiPHR, the silylphosphanes [(SiMe3)2MeSi]2SiClPHR with R = 2, 4, 6-tri-tert-butylphenyl ( = supermesityl, Mes∗) (1) and Si(SiMe3)3 ( = hypersilyl, Hyp) (2) were prepared. Both compounds were characterized with X-ray diffraction, multinuclear NMR spectroscopy and elemental analysis. Compound 1 did not react with n-BuLi, but only with a large excess of tert-BuLi. Phosphasilene [(SiMe3)2MeSi]2Sidouble bond; length as m-dashPMes∗ could be identified by a 31P NMR signal at +346 ppm. All attempts to separate it from the reaction mixture failed due to many by-products which had formed through SiSi and SiP bond cleavage. Lithiation of 2 was possible with 4.2 equiv. of tert-BuLi, and crystals of the lithiumphosphanide [(SiMe3)2MeSi]2SiClPLiHyp (3) could be obtained from THF, albeit in a quality not sufficient for X-ray diffraction. All attempts to achieve LiCl elimination and formation of the phosphasilene [(SiMe3)2MeSi]2Sidouble bond; length as m-dashPSi(SiMe3)3 failed due to the unusual stability of the lithiumphosphanide. Prolongued refluxing in toluene (110 °C) only led to complete loss of coordinated THF, and 31P7Li spin spin coupling could be observed in the 31P NMR spectrum (1JPLi = 84 Hz). Reaction of potassium phosphanide [(SiMe3)3Si]SiMe3PK with SiCl4 led to the formation of [(SiMe3)3Si](SiMe3)P(SiCl3) (4), which could be successfully characterized with X-ray diffraction and multinuclear NMR spectroscopy. SiP bond lengths vary between 218 pm (SiCl3) and 230 pm (hypersilyl). Despite these differences, 31P29Si coupling constants are nearly identical (92.4 Hz and 85.5 Hz, respectively).