The complexation of the diquat dication by dibenzo-3n-crown-n ethers
Spectrophotometric investigations of equimolar mixtures of diquat bis(hexafluorophosphate)(2) and a range of dibenzo-3n-crown-n ethers [n=6–12; (10)–(16)] in acetonitrile reveal the existence of charge-transfer absorption bands at ca.λmax. 400 nm. These absorptions are attributable to intermolecular π–π charge transfer between the electron-rich catechol units of the dibenzo-crown ethers and the electron-deficient bipyridinium ring system of the diquat dication. The qualitative conclusion from these experiments, that the most stable 1 : 1 complex is formed between dibenzo-30-crown-10 (14) and diquat bis(hexafluorophosphate)(2). led to the isolation from dichloromethane methanol–n-heptane of red crystals of [diquat.dibenzo-30-crown-10][PF6]2 suitable for X-ray crystallography. Although the crystal structure analysis revealed that there are two independent sets of 1 : 1 complexes (I and II) in the unit cell, the gross structural features of the two complexes are very similar. In addition to the parallel alignment of their three aromatic rings to accommodate the stabilising intermolecular π-π charge-transfer interaction, there is probably some further host–guest stabilisation to be gained on account of favourable electrostatic interactions between the phenolic oxygen atoms in the host and the nitrogen atoms in the pyridinium rings of the guest. Moreover, there is some evidence for weak C–H ⋯ O hydrogen bonding involving principally H-6 and H-6′ on the bipyridinium ring system of the guest and certain –CH2OCH2– oxygen atoms in the host. As evidenced by 1H n.m.r. spectroscopy in CD3COCD3, these non-covalent bonding interactions are probably responsible for the formation of stable and ordered 1 : 1 complexes with similar gross structural features in solution, at least in the cases where dibenzo-30- crown-10 (14). dibenzo-33-crown-11 (15), and dibenzo-36-crown-12 (16) are the hosts. Further evidence for the 1 : 1 stoicheiometry of these solution complexes, as well as for the complex involving dibenzo-27-crown-9 (13), has come from equilibrium constant measurements for the association between the dibenzo-3n-crown-n(n=9–12) hosts (13)–(16) and diquat bis(hexafluorophosphate)(2) in acetone. A quantitative treatment of the charge-transfer absorption bands at 400 nm, which affords Kavalues of 410, 17 500, 10 800, and 2 000 M–1 for n=9, 10, 11, and 12, respectively, provides convincing quantitative evidence for (a) 1 : 1 stoicheiometry and (b) the relative stabilities of the 1 : 1 complexes in solution. In the case of dibenzo-24-crown-8 (12), a complex of 2:l (guest:host) stoicheiometry is believed to be formed in acetone with a Kavalue of 385 000 M–2, as shown by a successful quantitative treatment of the charge-transfer absorption data by an independent method.