A NEUTRAL PARAQUAT RECEPTOR THAT USES ORIENTED DIPOLES PRODUCED BY DATIVE B-N BONDS

The association of paraquat dication (methylviologen) with simple neutral pi-donor molecules is generally quite weak (K-assn < 16 M(-1)) in polar, aprotic solvents. A new type of neutral paraquat receptor is described whose structure incorporates two dative B-N dipoles fixed in convergent orientations that are complementary with the two cationic centers of paraquat. The receptor binds paraquat bis(hexafluorophosphate) as a 1:1 complex with an association constant of 320 M(-1) in acetone as determined by H-1 NMR titration experiments. The complex is held together by a combination of intermolecular forces: (i) ion-dipole stabilization due to the negative ends of the two dative B-N dipoles pointing directly towards the two cationic centers in paraquat, (ii) Attractive pi-pi stacking of the electron-rich and electron-poor biphenyls, and (iii) C-H ... O hydrogen bonding between the relatively acidic paraquat methyl groups and are the basic boronate oxygens. The structurally related diquat bis(hexafluorophosphate) was bound 0.6 kcal/mol more weakly, emphasizing the directional nature of the oriented B-N dipoles.