Hydrogen bonding in anion recognition: A family of versatile, nonpreorganized neutral and acyclic receptors

The diamides and disulfonamides m-C6H4(CONHAr)(2) (Ar = Ph, 1; p-n-BuC6H4, 2, 2,4,6-Me3C6H2, 3), m-C6H4(SO2NHPh)(2), 4, and 2,6-C6H3N(CONHPh)(2), 5, r eadily synthesized on a multigram scale, bind strongly to halides and aceta te in organic solvents with K-a's as high as 6.1 x 10(4) (NMR spectroscopy) . The binding stoichiometry is 1:1 in solution for all cases except for the 4.F- and 4.OAc- complexes, where both 1:1 and 1:2 binding stoichiometries were found. The association constants in CD(2)C(l)2 (H-1 NMR) follow the tr end Cl- > Br- > I- for all the receptors. F- and OAc- binding may be strong er or weaker than Cl- depending on the nature of the receptor. The presence of the pyridine nitrogen in 5 and of the more rigid amide in 1-3 and 5 vs the less rigid sulfonamide structure in 4 increases selectivity for smaller anions. The enthalpy and entropy of formation for 2.Cl- were Delta H = -31 kJ/mol; Delta S = -23 J/(mol.K) (VT-NMR). The X-ray structure of [PPh4](2) [1. Br][Br]. CH2Cl2, shows 1:1 complexation of Br- via two N-H ... Br- hydr ogen bonds and a syn-syn nonplanar binding conformation for 1. Solution hyd rogen bonding was confirmed by FT-IR and NMR spectroscopy. The receptor con formation changes on complexation. Trends in structure/binding relationship s show receptor flexibility is an important factor in anion recognition.