Phosphanes with bulky oligosilyl substituents

Abstract

By reaction of dichloroheptasilane (SiMe 3 ) 2 MeSi] 2 SiCl 2 with lithiumphosphanides LiPHR, the silylphosphanes [(SiMe 3 ) 2 MeSi] 2 SiClPHR with R=2, 4, 6-tri-tert-butylphenyl (=supermesityl, Mes ) (1) and Si(SiMe 3 ) 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 [(SiMe 3 ) 2 MeSi] 2 Si=PMes* could be identified by a 31 P NMR signal at +346 ppm. All attempts to separate it from the reaction mixture failed due to many byproducts 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 [(SiMe 3 ) 2 MeSi) 2 SiClPLiHyp (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 [(SiMe 3 ) 2 MeSi] 2 Si=PSi(SiMe 3 ) 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 31 P 7 Li spin spin coupling could be observed in the 31 P NMR spectrum ( 1 J PLi = 84 Hz). Reaction of potassium phosphanide [(SiMe 3 ) 3 Si)SiMe 3 PK with SiCl 4 led to the formation of [(SiMe 3 ) 3- Si](SiMe 3 )P(SiCl 3 ) (4), which could be successfully characterized with X-ray diffraction and multinuclear NMR spectroscopy. SiP bond lengths vary between 218 pm (SiCl 3 ) and 230 pm (hypersilyl). Despite these differences, 31 P 29 Si coupling constants are nearly identical (92.4 Hz and 85.5 Hz, respectively).