Supramolecular potentials and embraces for fluorous aromatic molecules

Abstract

In order to understand better the supramolecular chemistry of fluorous molecules, and particularly coordination complexes and organometallic molecules with perfluoroaromatic substituents, the relevant intermolecular potentials have been parametrised. It is shown that density functional calculations using the Perdew–Wang PWC functional and numerical basis sets adequately describe weak intermolecular attractive energies [e.g. (C₆H₆)₂], but that gradient corrected BP or BLYP functionals do not. Due to a dearth of experimental data on intermolecular energies for perfluoroaromatic systems, these density functional calculations of the supramolecular isomers of (C₆F₆)₂ are used to parametrise the Lennard-Jones plus electrostatic potential for fluoroaromatic and derivative molecules. Intermolecular energies for local interactions in the crystal structures of C₆F₆, C₆F₆·C₆D₆, and octafluoronaphthalene are reported. Strong directionality is not observed in the supramolecular forces between fluoroaromatics and the electrostatic components of the intermolecular energies are small, <15% of the total; the polarisation of the C–F bond is also less than expected from electronegativity differences. Perfluorinated molecules and ions such as [B(C₆F₅)₄]⁻ engage in multiple phenyl embrace motifs, comparable to those of phenylated molecules, despite the reversed polarity of aromatic C–F relative to C–H bonds (and the opposite quadrupole moments of C₆F₆ and C₆H₆). Examples of the sixfold perfluorophenyl embrace (6P^FE), the hexagonal array of sixfold perfluorophenyl embraces (HA6P^FE), and fourfold perfluorophenyl embraces (4P^FE) are described. The intermolecular attractive energies of some of these motifs comprised of concerted edge-to-face and offset-face-to-face interactions are in the range −5.5 to −11.7 kcal mol⁻¹. It is concluded that there are no major differences between the supramolecular motifs and energies of perfluorinated- and hydroaromatics.