PUB - Publikationen an der Universität Bielefeld

The principal elements of the P-31 nuclear magnetic shielding tensors of iminophosphines of the type R-P double bond N-aryl (aryl = C(6)H(2)tBu(3); R = tBU, Ph(3)Sn(tBu)N, aryl-N double bond P-(tBu)N, aryl-NH, aryl-N double bond P-(adamantyl)N, Ph(2)N, carbazolyl, tBu(2)C double bond N, iminofluorenyl, Cl, Pr, I; 5-16) are determined from high-resolution solid-state MAS (magic angle spinning) NMR spectra. Comparison of the data with the results of IGLO calculations on shielding tensors of model compounds is used to assign the orientation of the principal axes system in the molecular frame. The observed shieldings can be correlated with structural features of the compounds. In (E)-configured alkyl- and amino-iminophosphines, the component at high field (delta(33)) is approximately perpendicular to the RPN plane and the low-field component (delta(11)) is in this plane and close to the P-R bond. Introduction of a-donor substituents (R'N-2) at phosphorus produces large increases in delta(11), which can be related to corresponding changes in the n-pi* electronic transition energies. A similar effect is found for a change of the double-bond geometry from (E)- to (Z)-configuration. Electronegative halogen substituents (Cl, Br) give rise to a further substantial increase in the tenser component at high field, delta(33). Furthermore, a change of the orientation of the principal axes system in the molecular frame occurs, with delta(33) being aligned close to the P,N bond axis and the intermediate component, delta(22), perpendicular to the RPN plane, A discussion of these effects is given in terms of partial triple-bond character for the P,N bond. The shape of the shielding tenser of the iodo compound (16) suggests that the electronic environment of the P,N moiety is similar to other systems containing a genuine triple bond, The unique difference between the isotropic chemical shifts in the solid state and in solution (Delta delta 118 ppm) in this case indicates that this phenomenon is confined to the solid state and can be attributed to the substantial polarization of the P,I bond, which is a consequence of the observed intermolecular association.