Solution and solid state proton transfer from phenols to triphenylphosphine oxide studied by 1H, 13C and 31P NMR spectroscopy

Abstract

Several complexes between substituted phenols and triphenylphosphine oxide (TPPO) were examined both in solution and in the solid state by NMR spectroscopy. The degree of proton transfer from the phenol to TPPO in solution was studied by the ¹H chemical shift of the phenolic OH proton and by the ¹³C chemical shifts of the phenol C–O (C1) and para(C4) carbons. As the pK_a of the acid decreases, the ¹H signal moves towards higher frequencies as a consequence of the deshielding produced by the proximity of the second oxygen. However, when the hydrogen is largely transferred to the oxide moiety, e.g. in the picric acid complex, the ¹H resonance shifts to lower frequencies again. In turn, the ¹³C chemical shift of C1 is displaced to high frequencies, whereas C4 shifts in the reverse direction. The solid phase was studied through the changes in the ³¹P shielding tensor of the TPPO residue and by the C1 and C4 ¹³C chemical shifts of the phenols. The proton transfer process follows the same pattern observed in solution. The values of the principal components of the ³¹P shielding tensor (σ_ii) move towards those corresponding to symmetric tetrahedral phosphorus environments as the pK_a of the phenol decreases.