An unusually large value of (1)J(P-31,P-31) for a solid triphenylphosphine phosphadiazonium cationic complex: Determination of the sign of J from 2D spin-echo experiments

Phosphorus-31 NMR spectra of a solid triphenylphosphine phosphadiazonium salt, [Mes*NP-PPh(3)][SO3CF3], have been acquired at 4.7 and 9.4 T. Analysis of the spectra obtained with magic angle spinning indicates that the two phosphorus nuclei are strongly spin-spin coupled, /(1)J((PP)-P-31-P-31)/=405(5) Hz, despite the unusually long P-P separation, r(p,p)=2.625 Angstrom. Two-dimensional spin-echo spectra provide convincing evidence that (1)J(P-31,P-31) is negative. Semi-empirical molecular orbital calculations at the INDO level support the negative sign for (1)J(P-31, P-31). A large span, 576 ppm, is observed ior the chemical shift tensor of the two-coordinate phosphorus centre (delta(11)=307 ppm, delta(22)=174 ppm, delta(33)=-269 ppm), which is very similar to the value previously reported for the non-coordinated phosphorus centre in the free Lewis acid, [Mes*NP][AlCl4]. The principal components and orientations of the phosphorus shielding tensors of these compounds are compared with those calculated for [HNP]* and ifs phosphine adduct using the ab initio Gauge-including Atomic Orbitals method. The phosphorus chemical shift tensor of the triphenylphosphine moiety has a relatively small span of 33 ppm.