Tetra-phosphonate Calix[4]pyrrole Cavitands as Multitopic Receptors for the Recognition of Ion Pairs

The
synthesis, structural characterization, and binding properties
of two unprecedented multitopic receptors for ion-pair recognition
are described. We isolated two of the six possible diastereomeric
deep cavitand receptors resulting from the installation of four phosphonate
groups at the upper rim of a calix[4]­pyrrole–resorcin[4]­arene
hybrid scaffold. The isolated tetra-phosphonate receptors display
either three (iooo) or four (oooo) of their PO groups oriented
away from the deep and functionalized aromatic cavity. In contrast
to analogous tetra-phosphonate resorcin[4]­arene cavitands, the 14-membered
macrocyclic rings that contain the PO groups in the tetra-phosphonate
calix[4]­pyrrole cavitands are conformationally flexible, always adopting
a conformation locating the phenyl substituents in equatorial position.
The tetra-phosphonate calix[4]­pyrroles exhibited larger affinity constants
than the previously reported bis-phosphonate calix[4]­pyrrole counterparts
in the complexation of both tetramethylphosphonium and octylammonium
chloride salts in nonpolar solvents. We demonstrated that the iooo
diastereoisomer was able to function as a multitopic receptor for
organic chloride salts by switching the geometry of the 1:1 ion-paired
complex from receptor-separated to close-contact depending on the
quaternary or primary nature of the cobound organic cation. The ion-paired
1:1 complexes formed between the diastereomeric receptors and organic
chloride salts were studied and thermodynamically characterized in
solution. The determined stability constant values were compared to
those obtained for the bis-phosphonate counterparts. The structure
of the TMPCl⊂7iooo complex was determined by X-ray
structure, and its formation was also evidenced in the gas phase.