Switching from Separated to Contact Ion-Pair Binding Modes with Diastereomeric Calix[4]pyrrole Bis-phosphonate Receptors

Abstract

We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetrol scaffold. The diastereoisomers differ in the relative spatial orientation of the P═O groups installed at their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. (1)H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P═O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the most thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride, the most effective receptor is the diastereoisomer 4ii with the two P═O groups converging on top of the aromatic cavity. In the nonpolar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. The 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution, the magnitudes of the binding affinities are reduced significantly and the size of the cation does not play a role. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.

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

“…In the bis-phosphonate series 2, we observed that PO groups directed away from the cavity had phosphorus atoms resonating at higher field, 13.2 ppm for 2oo and 14.2 ppm for 2ii. 16 The δ measured for the phosphorus atom of the first isolated tetra-phosphonate isomer (δ = 13.7 ppm) was exactly in the middle of this range.…”

“…The X-ray structures previously described for the three stereoisomeric bisphosphonate 2 testified the exclusive existence, in the solid state, of phosphocine rings having equatorial phenyl groups, that is, a boat-chair conformation for PO(i) and a boattwistboat conformation for PO(o) bridges. 16 Single crystals from the first eluted stereoisomer suitable for X-ray diffraction grew from acetonitrile solution. The analysis of the diffraction data revealed that the isolated tetraphosphonate was the 7oooo stereoisomer ( Figure 4; see the Supporting Information for the computed energies of the acetonitrile inclusion complexes of the two stereoisomers with C 4 symmetry CH 3 CN⊂7iiii and CH 3 CN⊂7oooo).…”

“…Previous studies with bis-phosphonate calix [4]pyrroles 2 had shown that the inward orientation of one PO group produced an energetic disadvantage to a similar extent. 16 We hypothesized that the existence of repulsive electrostatic interactions between the negative end of the dipole moment of the inwardly directed PO group and the included chloride was responsible for the reduction in binding affinity. The calix [4]pyrrole cavitand 8 (Scheme 2) was used as a model system to quantify the electrostatic effect provided by the four phosphonate groups in the 9⊂7oooo complex.…”

Pyrrole, imidazole, and triazole derivatives as ion-pair recognition receptors

“…Because of the reduction of conformational flexibility imposed by the bridging phosphonates, these cavitands possess a permanent deep aromatic cavity closed at one end by the calix [4]pyrrole core. 13 Bis-phosphonate cavitands 12 can act as ion-pair receptors for alkylammonium/phosphonium (primary and quaternary) chloride salts in dichloromethane solution and in the solid state. They feature a single coordination site for the anion, the calix[4]pyrrole core, but two different binding sites for the organic cation.…”

SPECTROSCOPIC STUDY OF EFFECTS OF TETRAALKYLAMMONIUM CATIONS ON F−-SENSING PROPERTIES OF CALIX[4]PYRROLE BORADIAZAINDACENE DYE

“…The hydrogen bonding interaction and affinity of 3 for F − would increase. [27][28][29] Therefore, TMA + , as the smallest quaternary ammonium cation among those tested, was found to induce substantial spectroscopic responses in the F − recognition process of dye 3.…”