SYNTHESES, SPECTROSCOPY, STRUCTURES, AND CONFORMATIONS OF LAMBDA-3-CYCLOTRIPHOSPHAZANES - ROLE OF NEGATIVE HYPERCONJUGATION

The reactions of lambda3-chlorocyclotriphosphazane [EtNPCl]3 with phen ols or trifluoroethanol yield the respective aryloxy- or trifluoroetho xy-containing lambda3-cyclotriphosphazanes [EtNP(OR)]3 (R = C6H4Br-4 ( 2), C6H5 (3), C6H3-Me2-3,5 (4), C6H3Me2-2,6 (5), CH2CF3 (6)) as their cis-trans isomeric mixtures. The products have been characterized by I R and NMR spectroscopy. The crystal structures of both the cis (2a) an d trans (2b) isomers of the p-bromophenoxy derivative have been determ ined by X-ray diffraction. Crystal data for 2a: triclinic, P1, a = 9.8 72(4) angstrom, b = 13.438(6) angstrom, c = 13.548(8) angstrom, alpha = 117.02(5)-degrees, beta = 96.00(6)-degrees, gamma = 105.38(4)-degree s, Z = 2, final R = 0.080. Crystal data for 2b: monoclinic, P2(1)/n, a = 12.721(6) angstrom, b = 13.468(7) angstrom, c = 17.882(5) angstrom, beta = 101.62(3)-degrees, Z = 4, final R = 0.066. The cis isomer exhi bits a chair-triaxial conformation and the trans isomer a boat-triaxia l conformation. Conformational preferences of lambda3-cyclotriphosphaz anes have been probed by both MNDO and ab initio calculations on model systems [HNPX]3 (X = H, F). In addition to vicinal lone pair repulsio ns, negative hyperconjugative interactions involving the nitrogen lone pairs and adjacent P-X sigma orbitals are found to be important (esp ecially when X is an electronegative substituent) in determining the c onformational preferences of lambda3-CyClotriphosphazanes. The calcula tions also show that the axial --> equatorial conversion at phosphorus has a large activation barrier in these systems.