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 tetrafluorosi lane. Reaction of I with two molar equivalents of (t)BuLi yields the d ilithiated 3, which when brought into reaction with SiF4, gives the te trafluoro-substituted four-membered (SiNSiN) ring species RNSiF(2)NRSi F(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-(silylam ino)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-b utylsilylamines. The X-ray crystal structure of 4 reveals the shortest trans-annular Si...Si ring distance (237.6 pm) observed in cyclodisil azanes. Molecular orbital ab initio calculations on the model cyclodis ilazanes 8a-8d were performed at the HF/6-31G level of theory in orde r to account for short observed Si...Si distance in 4 and the analogou s species F(2)SiNR'SiF(2)NR', R' = (t)Bu(2)MeSi, 7. The calculation re veals a very large effect of the fluorine and silyl substituents on th e Si...Si separation. They also show that the (SiNSiN) ring is extreme ly flexible in respect of bending of the endocyclic N-Si-N bond angles . Steric interactions associated with the bulky substituents or lattic e 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.