Acta Crystallographica Section B

Structural Science

Volume 64, Part 1 (February 2008)

research papers

Acta Cryst. (2008). B64, 108-119 [ doi:10.1107/S0108768107067079 ]

Effects of fluoro substitution on 4-bromodiphenyl ether (PBDE 3)

J. Klösener, D. C. Swenson, L. W. Robertson and G. Luthe

Abstract: It is our hypothesis that fluoro substitution provides a powerful tool to modulate the desired characteristics and to increase the specificity of studies of structure-activity relationships. 4-Bromodiphenyl ether (PBDE 3) and its five corresponding monofluorinated analogues (F-PBDEs 3) have been synthesized and fully characterized (using ¹H, ¹³C and ¹⁹F NMR spectroscopy, and mass spectrometry). The accurate structure from X-ray crystal analysis was compared with iterative calculations using semi-empirical self-consistent field molecular-orbital (SCF-MO) models. The compounds studied were 4-bromodiphenyl ether (PBDE 3), the ¹³C₆-isotopically labeled PBDE 3 (¹³C₆-PBDE 3) and 2-fluoro-4-bromodiphenyl ether (3-2F), 2'-fluoro-4-bromodiphenyl ether (3-2'F), 3-fluoro-4-bromodiphenyl ether (3-3F), 3'-fluoro-4-bromodiphenyl ether (3-3'F), and 4'-fluoro-4-bromodiphenyl ether (3-4'F). Solid-state intermolecular interactions for PBDE 3 and the F-PBDEs 3 isomers are dominated by weak C-H(F,Br) [pi] and C-HF interactions. The C-F bond lengths varied between 1.347 (2) and 1.362 (2) Å, and the C4-Br bond length between 1.880 (3) and 1.904 (2) Å. These bond lengths are correlated with electron-density differences, as determined by ¹³C shifts, but not with the strength of the C-F couplings. The interior ring angles of ipso-fluoro substitution increased (121.9-124.0°) as a result of hyperconjugation, a phenomenon also predicted by the calculation models. An attraction between the vicinal fluoro and halo substituents (observed in fluoro substituted chlorobiphenyls) was not observed for the bromo substituted F-PBDEs. The influence of a fluoro substituent on the conformation was only observable in PBDEs with di-ortho substitution. Calculated and observed torsion angles showed a positive correlation with increasing van der Waals radii and/or the degree of substitution for mono- to tetra-fluoro, chloro, bromo and methyl substitutions in the ortho positions of diphenyl ether. These findings utilizing F-tagged analogues presented here may prove fundamental to the interpretation of the biological effects and toxicities of these persistent environmental pollutants.

Keywords: fluoro substitution; structure-activity relationships; molecular-orbital models; solid-state intermolecular interactions.