Syntheses, spectroscopy, structures, and conformations of .lambda.3-cyclotriphosphazanes: role of negative hyperconjugation

The reactions of As-chlorocyclotriphosphazane [EtNPCl], with phenols or trifluoroethanol yield the respective\ud aryloxy- or trifluoroethoxy-containingX 3-cyclotriphosphazanes [EtNP(OR)]3 (R = C6H4Br-4 (2),C 6H5 (3C,6 H3-Mez-3,5 (4), C6H3Mez-2,6 (5), CH2CF3 (6)) as their cis-transisomericmixtures. The products have beencharacterized by IRand NMRspectroscopy. Thecrystalstructuresofboth thecis (2a) and trans(2b) isomer_softhep-bromophenoxy derivative have been determined by X-ray diffraction. Crystal data for 2a: triclinic, P1, a = 9.872(4) A, b =\ud 13.438(6) A, c = 13.548(8) A, CY = 117.02(5)', 0 = 96.00(6)', y = 105.38(4)O, Z = 2, final R = 0.080. Crystal\ud data for 2b: monoclinic, P21/n, a = 12.721(6) A, b = 13.468(7) A, c = 17.882(5) A, /3 = 101.62(3)O, Z = 4, final\ud R = 0.066. The cis isomer exhibits a chair-triaxial conformation and the trans isomer a boat-triaxial conformation.\ud Conformational preferences of X3-cyclotriphosphazanes have been probed by both MNDO and ab initio calculations\ud on model systems [HNPXIp (X = H, F). In addition to vicinal lone pair repulsions, negative hyperconjugative\ud interactions involving the nitrogen lone pairs and adjacent P-X Q* orbitals are found to be important (especially\ud when X is an electronegative substituent) in determining the conformational preferences of X3-cyclotriphosphazanes.\ud The calculations also show that the axial - equatorial conversion at phosphorus has a large activation barrier in\ud these system