X-Ray diffraction and phosphorus-31 NMR studies of the dynamically disordered 3:2 phenol–triphenylphosphine oxide complex

Abstract

The 3:2 phenol–triphenylphosphine oxide (phenol–TPPO) adduct was studied by means of X-ray diffraction together with high-resolution solid-state ³¹P NMR spectroscopy. The X-ray results showed that the crystalline structure, which belongs to the triclinic P space group, involves disorder. There are two molecules of TPPO and three phenol molecules per unit cell. One of the latter is disordered across an apparent inversion centre. Analysis of NMR data in conjunction with the crystal structure allowed the origin of such disorder to be established, showing that it is dynamic in nature. Variable-temperature NMR experiments were performed and a coalescence temperature was found at 247 K. Spectra recorded below this temperature showed two phosphorus signals. The kinetics for the phenol residue exchanging between two different TPPO moieties (two-site exchange with equal populations) were determined. Thermodynamic parameters for the motion were calculated from Eyring plots. For temperatures ranging from 262.9 to 221.5 K, the bandshape analysis technique was used to derive the required data. For lower temperatures, the selective polarisation inversion experiment (SPI) was performed, whilst high temperature values were derived from variable-temperature T_1ρ studies. The activation enthalpy (ΔH^‡), calculated using the results obtained by bandshape analysis, T_1ρ and SPI, was determined as 38 kJ mol⁻¹, while the activation entropy (ΔS^‡) was found to be −23 J mol⁻¹ K⁻¹ (assuming the transmission coefficient is ½). Phosphorus-31 shielding tensor anisotropies have been derived for this system by spinning sideband analysis at both fast and slow-exchange limits and it has been shown that the tensor is axially symmetric. Single-crystal experiments show that the symmetry axis of the tensor is along the PO bond (within experimental error).