Synthesis and crystal structure of cyclosilazanes with unusual conformations

The 1,3-disilazanes, (Me(3)C)(2)SiNH2-NHSiFR(2), R = Me (1), CHMe(2) (2), and the 1,3,5-trisilazane, (Me(3)C)(2)Si(NHSiFMe(2))(2) (3), are formed in the reaction of lithiated diamino-di-tert-butylsilane with F(2)SiMe(2) and F2Si(CHMe(2))(2). 1 and 2 react with BuLi to give the cyclodisilazanes 4 and 5, (Me(3)C)(2)Si(NH)(2)SiR(2), R = Me (4), CHMe(2) (5). and the cyclotetrasilazane 6, [(Me(3)C)(2)SiNH-SiMe(2)-NH](2). In the reaction of fluorosilanes with the lithium derivatives of 4 and 5 the silylsubstituted cyclodisilazanes 7, 8, 10, and 11 are obtained: 7, (Me(3)C)(2)SiNH(NSiFMe(2))SiMe(2); 8, (Me(3)C)(2)Si(NSiFMe(2))SiMe(2); 10, (Me(3)C)(2)SiNH(NSiF(2)CMe(3))Si(CHMe(2))(2); 11, (Me(3)C)(2)Si(NSiF2C6H5)(NSiF(2)CMe(3))Si(CHMe(2))(2). The reaction of dilithiated diamino-tert-butylmethylsilane with dichloro-tert-butylmethylsilane leads to the formation of the cyclotetrasilazane 9, (Me(3)CMeSi-NH)(4). The crystal structures of 4, 6, and 11 have been determined, 4 forms monoclinic crystals, space group P2(1)/m, Z = 2. The four-membered ring is planar. The NSiN angles are smaller than 90 degrees, and the SiNSi angles are bigger than 90 degrees, 6 forms triclinic crystals, space group <P(1)over bar>, Z = 2. The ring has a saddle conformation, with the Si atoms approximately coplanar and the N atoms alternatively above and below this plane, The N atoms have an almost planar environment (sum of angles 357.4-359.0 degrees) with SiNSi angles between 139.8-142.8 degrees. 11 forms monoclinic crystals, space group C2/c, Z = 8. The angle between the lines of the atoms N1-N2/N2-Si4 is 166.5 degrees and between N2-N1/N1-Si3 is 176.4 degrees. The N2 atom has a pyramidal environment (sum of angles 355.1 degrees), a very rare example of a nitrogen atom bonded to three silicon atoms. The exocyclic silyl groups have cis position, The electron-withdrawing effect of the fluorine atoms leads to a shortening of the exocyclic Si-N bonds.