TETRAFLUOROCYCLODISILAZANES - SYNTHESIS, CRYSTAL-STRUCTURE, AND MOLECULAR-ORBITAL AB-INITIO CALCULATIONS

The bis(silylamino)difluorosilane (RHN)(2)SiF2, 1 and the (silylamino)-trifluorosilane (RHN)SiF3 with R = (t)Bu(2)PhSi, 2, were prepared by reaction of lithiated di-tert-butylphenylsilylamine with tetrafluorosilane. Reaction of I with two molar equivalents of (t)BuLi yields the dilithiated 3, which when brought into reaction with SiF4, gives the tetrafluoro-substituted four-membered (SiNSiN) ring species RNSiF(2)NRSiF(2) 4. Reaction of 1 with PF3 yields the cyclic four-membered ring 1,3-diaza-2-phospha-4-sila-cyclobutane, RNSiF(2)NRPF 5. The bis-(silylamino)difluorosilanes 1, and the related R'NHSiF(2)NHR', R'= (t)Bu(2)Me-Si, 6, are also synthesized by treatment of 2 with lithiated di-tert-butylsilylamines. The X-ray crystal structure of 4 reveals the shortest trans-annular Si...Si ring distance (237.6 pm) observed in cyclodisilazanes. Molecular orbital ab initio calculations on the model cyclodisilazanes 8a-8d were performed at the HF/6-31G* level of theory in order to account for short observed Si...Si distance in 4 and the analogous species F(2)SiNR'SiF(2)NR', R' = (t)Bu(2)MeSi, 7. The calculation reveals a very large effect of the fluorine and silyl substituents on the Si...Si separation. They also show that the (SiNSiN) ring is extremely flexible in respect of bending of the endocyclic N-Si-N bond angles. Steric interactions associated with the bulky substituents or lattice effects in the crystal may therefore strongly influence the geometry of the (SiNSiN) cycle. The calculations demonstrate that despite the very short Si Si separation in 4 and 7 there is no bonding interaction between the ring silicon atoms.