Interaction of phosphorylated molecules with protic solvents and cations. Discussion of the electronic perturbation at the phosphoryl bond of triphenylphosphine oxide

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Abstract
Longitudinal relaxation times T1 of the 31P nucleus of triphenylphosphine oxide (TPPO) in the complexes Mg(TPPO)4(CF3SO3)2 and Cd(TPPO)4(BF4)2 were measured in nitromethane solutions at three different frequencies, leading to the determination of the chemical shift anisotropy (CSA) contribution to the total relaxation rate. The 31P shielding tensor anisotropy Δσ was measured from NMR static spectra recorded on solid samples, allowing the deduction of correlation times τr corresponding to the reorientation of the species in solution. From the line width of the 17O NMR signal, the 17O quadrupolar coupling constant χ was calculated, the value changing significantly from the free ligand (χ = 4.7 MHz) to bound molecules in complexes (χ = 6.7–7.2 MHz). This was explained by a modification of the electronic distribution around the oxygen atom, mainly due to less π back‐donation from O to P; π‐electron transfer was quantified from χ values using the Townes‐Dailey model. The nature of the interaction with these divalent cations is also discussed. The interaction of TPPO with protic solvents such as water, methanol, trifluoroacetic acid and sulphuric acid was studied using a‐procedure similar to that used for complexes, except for the determination of τr, which was reached from 13C relaxation time measurements on the para‐carbons of the phenyl groups. Variations of Δσ and χ are discussed in terms of hydrogen bonding and interaction with hydrogen ions.

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