Pentafluorophenylphosphonic Acid as a New Building Block for Molecular Crystal Fabrication

Molecular crystals have been prepared
from pentafluorophenylphosphonic
acid (4). These include 2C6F5PO3H2·H2O (4·4·H2O), [(C6F5PO3H–)­(H3O+)]·C6F5PO3H2 (4·4–·H3O+), [(C6F5PO3H–)­(NH4+)]·C6F5PO3H2 (4·4–·NH4+), [(C6F5PO3H–)­(Me2NH2+)] (4–·DMA+), [(C6F5PO3H–)­(Me2NH2+)]·H2O (4–·DMA+·H2O), [(C6F5PO3H–)­(Me2NH2+)]·C6F5PO3H2 (4·4–·DMA+), [(C6F5PO3H–)­(Me2NH2+)0.5(NH4+)0.5] (4–·DMA+·NH4+), and [(C6F5PO3H–)­(+H3NCH2CO2H)] (4–·Gly+), where DMA+ = dimethylammonium and Gly+ = glycinium. All of
the assemblies incorporate an ammonium cation, a water molecule, or
a hydronium ion in their structure, and these included species act
as adhesive agents. They interact via O/N–H···O
hydrogen bonds and C–H···O contacts with PO3H2 or PO3H– moieties
located in polar sheets, forcing the building blocks to assemble with
a 2D layered arrangement. The robustness of this arrangement is reminiscent
of that observed in metal arylphosphonates and guanidinium sulfonates,
yet the architectures described herein differ significantly from the
second family of compounds in two aspects: the lack of puckering of
the layers and the absence of void spaces to accommodate solvent molecules.
Interestingly, however, just as in guanidinium sulfonates, two structural
types have been recognized depending on the size of the included species:
a single-layer stacking motif and a bilayer stacking motif. Even though
hydrogen bonding is the prevailing interaction in these systems, the
use of perfluoroaryl groups is also central, as these moieties bring
about weak C–F···π, π···π,
C–F···F–C, C–F···H–C,
and O/N–H···F–C interactions that help
to increase the cohesion of the nonpolar regions. As a result, new
architectures are created that significantly differ in some cases
from those prepared from phenylphosphonic acid. However, the weakness
of perfluoroaryl-based interactions is also their strength, as these
interactions are sufficiently flexible to allow changes in the organization
of the aromatic groups in response to changes (ionization) occurring
in the polar regions, as observed in the structures of 4·4·H2O and 4·4–·H3O+. This situation is
quite unusual and may be regarded as some kind of acidity-modulated
polymorphism. The work presented here broadens the knowledge on molecular
crystal formation with arylphosphonic acids, a research area largely
dominated by carboxylic and sulfonic acids. Also, the computational
evaluation of 4·4·H2O and 4·4–·H3O+ described in this report suggests that
this family of fluorinated compounds may show interesting prospects
as molecular semiconductors, a research area that is currently receiving
increased attention.