Theoretical Study of the Structures of 4-(2,3,5,6-Tetrafluoropyridyl)Diphenylphosphine Oxide and Tris(Pentafluorophenyl)Phosphine Oxide: Why Does the Crystal Structure of (Tetrafluoropyridyl)Diphenylphosphine Oxide Have Two Different P=O Bond Lengths?

Joseph R Lane,Graham C Saunders

doi:10.3390/molecules25122778

Abstract

The crystal structure of 4-(2,3,5,6-tetrafluoropyridyl)diphenylphosphine oxide (1) contains two independent molecules in the asymmetric unit. Although the molecules are virtually identical in all other aspects, the P=O bond distances differ by ca. 0.02 Å. In contrast, although tris(pentafluorophenyl)phosphine oxide (2) has a similar crystal structure, the P=O bond distances of the two independent molecules are identical. To investigate the reason for the difference, a density functional theory study was undertaken. Both structures comprise chains of molecules. The attraction between molecules of 1, which comprises lone pair–π, weak hydrogen bonding and C–H∙∙∙arene interactions, has energies of 70 and 71 kJ mol−1. The attraction between molecules of 2 comprises two lone pair–π interactions, and has energies of 99 and 100 kJ mol−1. There is weak hydrogen bonding between molecules of adjacent chains involving the oxygen atom of 1. For one molecule, this interaction is with a symmetry independent molecule, whereas for the other, it also occurs with a symmetry related molecule. This provides a reason for the difference in P=O distance. This interaction is not possible for 2, and so there is no difference between the P=O distances of 2.

Highlights

The crystal structure of 4-(2,3,5,6-tetrafluoropyridyl)diphenylphosphine oxide, 1 (CSD ref. code: TACWIE) [1] crystallized in the space group P21 /c with two symmetry independent molecules in the asymmetric unit (Z0 = 2, R1 = 0.0517)
The two independent molecules possess virtually identical conformations (Figure 1), with very similar geometric parameters (Table 1), but the P=O bond lengths differ by ca. 0.02 Å
This is not an example of bond-stretch isomerism, whereby molecules differ only in the length of one or more bonds [2]; the structures of the two molecules, virtually identical, do exhibit slight differences in some of the bond and torsion angles (Table 1). Nor is it likely to be a case of an apparent change of bond length caused by the presence of an isomorphous impurity [3]; the major axes of the displacement ellipsoids of the oxygen atoms of 1 are perpendicular to the P=O

Summary

Introduction

Code: DPFPPO) [17]) possesses a structure similar to that of 1, crystallizing in the P21 /c space group with Z0 = 2, but the P=O bond lengths were reported to be different, 1.469 and 1.479 Å, the data are of insufficient quality to determine whether this difference is genuine or not. The uncertainties for these distances were not given, but based on those of the unit cell lengths, which are four times greater than those for 1, the reported P=O bond lengths are identical within experimental error.

Results and Discussion

The NCI isosurface a dimerthe of molecules and

Materials and Methods

24: Cocrystals involving triphenylphosphine oxide