NMR studies of proton transfer in 1∶1 tris(trimethoxyphenyl)phosphine oxide–phenol complexes

Abstract

The hydrogen transfer process in adducts between tris(trimethoxyphenyl)phosphine oxide (TMPPO) and eight different substituted phenols has been studied in both the solution and solid states. For this purpose, ¹H and ³¹P NMR solution-state spectra of these complexes have been recorded. The ¹H chemical shift of the hydrogen-bonded proton and the ³¹P chemical shift of the phosphine oxide are largely influenced by the substituents attached to the phenol. Thus, the chemical shift of the hydroxylic hydrogen for complexes in solution varies from lower frequencies (8.2 ppm) for phenol derivatives of high pK_a (e.g. 10.2) to higher frequencies (11.9 ppm) as the pK_a of the phenol decreases. However, for highly acid phenols such as picric acid (pK_a = 0.38), the signal moves to lower frequencies again as a result of the shielding produced by the oxygen atom of the TMPPO residue. On the other hand, the ³¹P chemical shift of the complexes in solution varies with the same trend: as the pK_a of the substituted phenol decreases, the phosphorus signal moves to higher frequencies. The eight complexes have also been studied in the solid state by means of high-resolution CPMAS ¹³C and ³¹P NMR experiments. There is also evidence of the hydrogen transfer process in the solid state which causes changes in the ³¹P shielding tensor, and in the ¹³C chemical shifts of the phenolic C–O (C1) and para (C4) carbons. In spite of crystallographic packaging effects that might occur in the solid phase, the results parallel those obtained for complexes in solution, since the two sets of NMR data follow almost the same pattern.