Hydrogen Bonding in Anion Recognition:  A Family of Versatile, Nonpreorganized Neutral and Acyclic Receptors

The diamides and disulfonamides m-C(6)H(4)(CONHAr)(2) (Ar = Ph, 1; p-n-BuC(6)H(4), 2, 2,4,6-Me(3)C(6)H(2), 3), m-C(6)H(4)(SO(2)NHPh)(2), 4, and 2,6-C(6)H(3)N(CONHPh)(2), 5, readily synthesized on a multigram scale, bind strongly to halides and acetate 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)Cl(2) ((1)H NMR) follow the trend Cl(-) > Br(-) > I(-) for all the receptors. F(-) and OAc(-) binding may be stronger 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 DeltaH = -31 kJ/mol; DeltaS = -23 J/(mol.K) (VT-NMR). The X-ray structure of PPh(4)(2)1.BrBr.CH(2)Cl(2), shows 1:1 complexation of Br(-) via two N-H.Br(-) hydrogen bonds and a syn-syn nonplanar binding conformation for 1. Solution hydrogen bonding was confirmed by FT-IR and NMR spectroscopy. The receptor conformation changes on complexation. Trends in structure/binding relationships show receptor flexibility is an important factor in anion recognition.