An unusually large value of1J(31P,31P) for a solid triphenylphosphine phosphadiazonium cationic complex: determination of the sign ofJfrom 2D spin-echo experiments

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Abstract
Phosphorus-31 NMR spectra of a solid triphenylphosphine phosphadiazonium salt, [Mes*NP-PPh3][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, , despite the unusually long P—P separation, rP,P = 2.625 Å. Two-dimensional spin-echo spectra provide convincing evidence that 1J(31P,31P) is negative. Semi-empirical molecular orbital calculations at the INDO level support the negative sign for 1J(31P,31P). A large span, 576 ppm, is observed for the chemical shift tensor of the two-coordinate phosphorus centre (δ11 = 307 ppm, δ22 = 174 ppm, δ33 = −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 its 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. Key words: spin–spin coupling constants, solid-state NMR, 31P NMR, MO calculations, phosphadiazonium cation, P—P bonds.
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