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

The reactions of lambda3-chlorocyclotriphosphazane [EtNPCl]3 with phenols or trifluoroethanol yield the respective aryloxy- or trifluoroethoxy-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 IR and NMR spectroscopy. The crystal structures of both the cis (2a) and trans (2b) isomers of the p-bromophenoxy derivative have been determined by X-ray diffraction. Crystal data for 2a: triclinic, P1, a = 9.872(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)-degrees, 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 exhibits a chair-triaxial conformation and the trans isomer a boat-triaxial conformation. Conformational preferences of lambda3-cyclotriphosphazanes have been probed by both MNDO and ab initio calculations on model systems [HNPX]3 (X = H, F). In addition to vicinal lone pair repulsions, negative hyperconjugative interactions involving the nitrogen lone pairs and adjacent P-X sigma* orbitals are found to be important (especially when X is an electronegative substituent) in determining the conformational preferences of lambda3-CyClotriphosphazanes. The calculations also show that the axial --> equatorial conversion at phosphorus has a large activation barrier in these systems.