Very Strong Binding for a Neutral Calix[4]pyrrole Receptor Displaying Positive Allosteric Binding

The
dual-analyte responsive behavior of tetraTTF-calix[4]­pyrrole
receptor 1 has been shown to complex electron-deficient
planar guests in a 2:1 fashion by adopting a so-called 1,3-alternate
conformation. However, stronger 1:1 complexes have been demonstrated
with tetraalkylammonium halide salts that defer receptor 1 to its cone conformation. Herein, we report the complexation of
an electron-deficient planar guest, 1,4,5,8-naphthalenetetracarboxylic
dianhydride (NTCDA, 2) that champions the complexation
with 1, resulting in a high association constant Ka = 3 × 1010 M–2. The tetrathiafulvalene (TTF) subunits in the tetraTTF-calix[4]­pyrrole
receptor 1 present a near perfect shape and electronic
complementarity to the NTCDA guest, which was confirmed by X-ray crystal
structure analysis, DFT calculations, and electron density surface
mapping. Moreover, the complexation of these species results in the
formation of a charge transfer complex (22⊂1) as visualized by a readily apparent color
change from yellow to brown.