Azaparacyclophanes with metal-binding sites. Part 1. Preparation, characterization, and molecular recognition behaviour of hexapus azaparacyclophane and its iron(III) complex

Abstract

All the catechol moieties of a hexapus triaza[3.3.3]paracyclophane having three catechol moieties and six hydrocarbon chains (1) undergo coordination interactions with the iron(III) ion in a 1 : 1 molar ratio of 1 to iron(III) above pH 6 in an aqueous medium. Two different iron(III) complexes of 1, A and B in a 1 : 1 molar ratio, are in the high spin iron(III) state as characterized by electronic, EPR and Mössbauer spectroscopy: A is a hexacoordinated mononuclear complex with half-protonated catecholato groups, subjected to considerable structural distortion; B is a hexacoordinated mononuclear complex with complete dissociation of all the catechol protons, retaining high structural symmetry. The molecular recognition behaviour of 1 and Fe-complex A toward hydrophobic guest molecules has been investigated in aqueous HEPES and acetate buffers at 30.0 °C and µ 0.10 mol dm^–3. The hexapus cyclophane 1 recognizes organic guests through hydrophobic and electrostatic interactions; anionic and non-ionic guests, 7-hydroxy-8-phenylazonaphthalene-1,3-disulfonate (OG) and 2-hydroxy-1-(2-pyridylazo)naphthalene (PAN), respectively, are incorporated into host 1 with binding constants of the order of 10⁵ dm³ mol^–1, while a cationic guest, 2-(p-dimethylaminostyryl)-1-ethylquinolinium (QR), is not included in the host. The guest-binding behaviour of Fe-complex A toward OG and PAN is much enhanced relative to that of 1 as effected by hydrophobic, electrostatic and coordination interactions under conditions identical to those applied to 1; the binding constant for OG is nearly pH-independent in a pH range of 3–8, while that for PAN is pH-dependent because the coordination interaction with the host is subject to change by pH.