Molecular Recognition of Phosphate Esters: A Balance of Hydrogen Bonding and Proton Transfer Interactions

The interaction of different phosphate esters with a series of hydrogen bonding receptors has been studied using UV‐visible and NMR spectroscopies and X‐ray crystallography. Critical for binding is a combination of an acidic proton and the potential for bidentate hydrogen bonding either between charged or uncharged components. Phosphotriesters show no binding to the receptors. Phosphodiesters bind to both bis‐(2, 6–diacylaminopyridine) and mono‐(2, 6–diacylaminopyridine) receptors in chlorocarbon solvent via proton transfer to form the pyridinium phosphate ion pair and bidentate hydrogen bonding between the anion and the cation. Titration experiments as well as Job's analyses show that for cyclic and acyclic bis‐(2, 6–diacylaminopyridine) receptors 2: 1 complexes can be formed. X‐ray crystal structures demonstrate that in the solid state two different binding arrangements are present; either a direct bidentate interaction or intramolecular hydrogen bonding with self‐assembly of an oligomeric structure. Phosphomonoesters bind to mono‐(2, 6–diacylaminopyridines) in a similar way to the diesters, via proton transfer and bidentate hydrogen bonding. In this, as in the diester case, only a single acid‐base interaction is possible and proton transfer is preferred. However, in the interaction of phosphomonoesters with bis‐(2, 6–diacylaminopyridine) derivatives two acid‐base interactions are possible between the two pyridines and two POH groups, and little proton transfer is seen. Strong binding (K_a = 1.0 × 10⁵M⁻¹) occurs via tetrahydrogen bond formation. Thus, there appears to be a balance between the occurrence of proton transfer and the number of hydrogen bonds formed between receptor and substrate.