Inclusion complexation of diquat and paraquat by the hosts cucurbit [7] uril and cucurbit [8] uril

Abstract

The binding interactions in aqueous solution between the dicationic guest diquat (DQ2+) and the cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) hosts were investigated by 1H NMR, UV/Vis, and fluorescence spectroscopy: mass spectrometry; single-crystal X-ray diffraction; and electrochemical techniques. The binding data were compared with previously reported results for the related paraquat guest (PQ2+). DQ 2+ was found to bind poorly (K=350 M-1) inside CB7 and more effectively (K = 4.8×104M-1) inside CB8. One-electron reduction led to increased binding affinity with both hosts (K r = 1 × 104 M-1 with CB7 and K r = 6×105 M-1 for CB8). While 1H NMR spectroscopic data revealed that DQ2+ is not fully included by CB7, the crystal structure of the CB8·DQ2+ complex - obtained from single-crystal X-ray diffraction - clearly establishes its inclusion nature. Overall, both diquat and its one-electron reduced radical cation are bound more effectively by CB8 than by CB7. In contrast to this, paraquat exhibits selectivity for CB7, but its radical cation forms a highly stable dimer inside CB8. These differences highlight the pronounced sensitivity of cucurbit[n]uril hosts to guest features such as charge, charge distribution and shape. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.