CYCLIC SILYLHYDRAZINES AND THEIR BORANE ADDUCTS

A series of five-, six-, and seven-membered cyclic silylhydrazines hav e been prepared from the reactions of 1,2-bis(bromosilyl)ethane and 1, 3-bis(bromosilyl)propane with 1,1-dimethylhydrazine [leading to 1-(dim ethylamino)-1-aza-2,5-disilacyclopentane (3) and 1-(dimethylamino)-1-a za-2,6-disilacyclohexane (4)] and 1,2-dimethylhydrazine [leading to 1, 2-dimethyl-1,2-diaza-3,6-disilacyclohexane (7) and 1,2-dimethyl-1,2-di aza-2,7-disilacycloheptane (8)] in the presence of triethylamine, resp ectively. The compounds with endocyclic Si-N-Si units (3, 4) are found to be stable for long periods of time, while those with Si-N-N-Si uni ts (7, 8) decompose within a few days at ambient temperature. Compound s 3 and 4 have been reacted with the Lewis acid BH3 to give the dimeth ylamine-borane adducts 5 and 6. All compounds have been fully characte rized by spectroscopic data [IR, MS, NMR (H-1, C-13, N-15, Si-29)]. Si ngle crystals of 5 [6] grown from the melt and studied by low-temperat ure X-ray diffraction analyses are orthorhombic, space group Pbca (No. 61), with a = 11.385(1) [13.300 (1)] Angstrom, b = 9.938(1) [9.837(1) ] Angstrom, c = 17.156(1) [16.364(1)] Angstrom, d(calc) = 1.096 (1.081 ) g cm(-3), and Z = 8 [8]. In both compounds, the BH3 unit is bound to the nonsilylated nitrogen atom, indicating the reduction of the basic ity of nitrogen by Si substitution. The silylated nitrogen atoms show planar coordination, while the borylated amine unit is tetrahedrally c oordinated. From a comparison of the ring geometries of 5 and 6 with k nown open-chain structures, it appears that the C2Si2N ring system of 5 is clearly more strained than that of 6 (C3Si2N) This argument also offers an explanation for the preferred formation of the compounds 1,6 -diaza-2,5,7,10-tetrasila-[4.4.0]bicyclodecane (1) and bi(1-aza-2,6-di silacyclohexyl) (2) as compared to their isomers with different ring s izes. The relative stabilities of these isomers in question have been quantified by ab initio (MP2(fc)/6-31G) calculations of geometries an d energies of the systems [(CH2)(n)(SiH2)]N-N[(SiH2)(CH2)(n)] versus t he annelated molecules (CH2)(n)(SiH2)N-2(CH2)(n)(SiH2) with n = 1-3. T hese results show the annelated isomers (ring enlarged) to be lower in energy for n = 1 and 2, while for n = 3 the N-N bridged nonannelated isomer is preferred.