These experimental findings agreed with the molecular modelling performed, which suggests 8 possible H-bonds for 1 with H2PO4 -(1.76-2.13 Å, Figures and). In contrast, the distance between the bisurea and calix [4]pyrrole binding motifs of 1 is too large for efficient cooperative binding of Cl -, as indicated by only 4 H-bonds to Cl -of 2.48-2.58 Å in the lowest energy conformation of 1•Cl -, similar to 4 H-bonds of 2.32-2.42 Å in each of the reference complexes 2•Cl -and 3•Cl -(Figure ). Table . Association constants for receptor 1 and control compounds 2 and 3 as determined from titrations with tetrabutylammonium salts of Cl -and H2PO4 -in DMSO with 0.5% H2O at 25 °C.