Enhanced Carboxylate Binding Using Urea and Amide-Based Receptors with Internal Lewis Acid Coordination: A Cooperative Polarization Effect

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Abstract
A structural design strategy is described that greatly improves the acetate binding ability of neutral urea and amide-based receptors. The enhanced binding is due to a cooperative polarization effect which is induced by intramolecular coordination of the urea or amide carbonyl to a Lewis acidic boronate group. A series of boronate-ureas, 3, and a related bis(boronate-amide), 23, were prepared in two steps from 2-(aminophenyl)boronic acid and their structures elucidated using X-ray crystallography and other spectrometric methods. The abilities of the receptors to associate with tetrabutylammonium acetate in dimethyl sulfoxide solution were determined by 1H NMR titration experiments. Association constants were calculated using nonlinear curve-fitting methods. The boronate-ureas 3 strongly bind to acetate in dimethyl sulfoxide solution with association constants as high as 6 × 104 M-1. This is more than 150 times greater than the association constants for control urea receptors that lacked an appropriate boron substituent. Thermodynamic studies indicate that the enhanced association is due to a favorable enthalpic change. Additional NMR studies eliminated the possibility of proton transfer to the acetate during complex formation. Molecular modeling indicates that the boronate-ureas exhibit improved acetate binding because the intramolecular coordination (i) induces a larger host dipole moment which strengthens the guest/host ion−dipole interaction, and (ii) increases the positive surface potential at the urea NH residues which strengthens short range Coulombic interactions with the anionic acetate. The observed association constants correlate better with calculated host dipole moments, suggesting that for the boronate-ureas described here this is the more influential factor controlling association.