Phosphorus-31 solid-state NMR study of iminophosphines: influence of electronic structure and configuration of the double bond on phosphorus shielding

Abstract

The principal elements of the 31P nuclear magnetic shielding tensors of iminophosphines of the type R-P=N-aryl (aryl = C6H2tBu3; R = tBu, Ph3Sn(tBu)N, aryl-N=P-(tBu)N, aryl-NH, aryl-N=P-(adamantyl)N, Ph2N, carbazolyl, tBu2C=N, iminofluorenyl, Cl, Br, 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 (δ33) is approximately perpendicular to the RPN plane and the low-field component (δ11) is in this plane and close to the P-R bond. Introduction of π-donor substituents (R'2N) at phosphorus produces large increases in δ11, which can be related to corresponding changes in the n-π* 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 tensor component at high field, δ33. Furthermore, a change of the orientation of the principal axes system in the molecular frame occurs, with δ33 being aligned close to the P,N bond axis and the intermediate component, δ22, 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 tensor of the iodo compdound (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 (Δδ 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.