The Crystal and Molecular Structure of Benzyl (2-Methoxyphenyl)-Diphenylphosphonium Bromide

The crystal structure of benzyl(2-methoxyphenyl)diphenylphosphonium bromide has been determined from three dimensional X-ray data collected on a three circle diffractometer using MoKα radiation. The salt crystallized in the orthorhombic space group Pbca with cell constants α=15.528 (7), b=16.892 (7), and c=17.921 (8) A. The observed and calculated densities for 8 molecules per unit cell are 1.308 (5) and 1.309 g · cm−3 respectively. The structure was refined by full matrix least squares to final residuals of R=0.089 and R w0.120 for the 1757 independent reflections in the range 2° > 2θ> 42° whose intensities were above background. The cation has near tetrahedral geometry with an average P-C bond distance of 1.819 (14) A and the configuration of the ion is such that the oxygen atom of the methoxy group is trans to the alkyl carbon atom of the benzyl group. The phosphorus-oxygen distance of 2.878 (12) Å suggests a weak bonding interaction and qualitative support for this view is obtained from energy minimization calculations in which the total steric energy made up primarily of bond stretching energies and van der Waals non-bonded interactions is treated. The results are in accord with nmr data previously reported and from which it was concluded that a weak bonding interaction between the methoxy oxygen and the phosphorus atoms causes an upfield shift of the methyl protons of the benzyl group. The molecular structure of the phosphonium cation depicted herein makes it highly improbable that the observed chemical shift is attributable to a magnetic anisotropic effect.