RESUMO
Bis(meta-phenylene)-32-crown-10-based cryptands have been proved to complex diquat much more strongly than bis(meta-phenylene)-32-crown-10 itself; in fact, one containing a pyridyl moiety has one of the highest Ka values yet reported.
Assuntos
Diquat/química , Éteres Cíclicos/química , Bases de Schiff/química , Cristalografia por Raios X , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Estrutura Molecular , Espectrometria de Massas por Ionização por ElectrosprayRESUMO
Strong complexation between a pyridine-containing cryptand and paraquat can be reversibly switched off (and back on) by adding acid (and then base).
RESUMO
Four new bis(m-phenylene)-32-crown-10-based cryptands with different third bridges were prepared. Their complexes with paraquat derivatives were studied by proton NMR spectroscopy, mass spectrometry, and X-ray analysis. It was found that these cryptands bind paraquat derivatives very strongly. Specifically, a diester cryptand with a pyridyl nitrogen atom located at a site occupied by either water or a PF(6) anion in analogous complexes exhibited the highest association constant K(a) = 5.0 x 10(6) M(-1) in acetone with paraquat, 9000 times greater than the crown ether system. X-ray structures of this and analogous complexes demonstrate that improved complexation with this host is a consequence of preorganization, adequate ring size for occupation by the guest, and the proper location of the pyridyl N-atom for binding to the beta-pyridinium hydrogens of the paraquat guests. This readily accessible cryptand is one of the most powerful hosts reported for paraquats.