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

The 3:2 phenol-triphenylphosphine oxide (phenol-TPPO) adduct was studied by means of X-ray diffraction together with high-resolution solid-state P-31 NMR spectroscopy. The X-ray results showed that the crystalline structure, which belongs to the triclinic P (1) over bar space group, involves disorde r. There are two molecules of TPPO and three phenol molecules per unit cell . One of the latter is disordered across an apparent inversion centre. Anal ysis of NMR data in conjunction with the crystal structure allowed the orig in of such disorder to be established, showing that it is dynamic in nature . Variable-temperature NMR experiments were performed and a coalescence tem perature was found at 247 K. Spectra recorded below this temperature showed two phosphorus signals. The kinetics for the phenol residue exchanging bet ween two different TPPO moieties (two-site exchange with equal populations) were determined. Thermodynamic parameters for the motion were calculated f rom Eyring plots. For temperatures ranging from 262.9 to 221.5 K, the bands hape analysis technique was used to derive the required data. For lower tem peratures, the selective polarisation inversion experiment (SPI) was perfor med, whilst high temperature values were derived from variable-temperature T-1 rho studies. The activation enthalpy (Delta H double dagger(;)), calcul ated using the results obtained by bandshape analysis, T-1 rho and SPI, was determined as 38 kJ mol(-1), while the activation entropy (Delta S double dagger) was found to be -23 J mol(-1) K-1 (assuming the transmission coeffi cient is 1/2). Phosphorus-31 shielding tensor anisotropies have been derive d for this system by spinning sideband analysis at both fast and slow-excha nge limits and it has been shown that the tensor is axially symmetric. Sing le-crystal experiments show that the symmetry axis of the tensor is along t he P=O bond (within experimental error).