Synthesis and characterization of a novel series of bis-linked diaza-18-crown-6 porphyrins

Abstract

The synthesis and physicochemical characterization of a novel family of ‘crowned-porphyrins’ is presented. The compounds uniquely possess a 5,15-di(2-alkylamidophenyl)etioporphyrin bis-linked to the nitrogens on opposite sides of a 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane moiety. The size of the cavity in between the porphyrin and the diaza-18-crown-6 ring has been regulated by varying the length of the alkylamido linking units. The structures of 5 and 7 bear diacetamido linkers whereas 6 is linked via dipropionamido groups. The synthetic pathway presented is generalized such that a wide variety of useful porphyrin-based bis-macrocycles can be prepared. Products have been fully characterized by elemental analysis, mass spectrometry, spectrophotometry, ¹H NMR, ¹³C NMR and ¹H–¹³C heteronuclear correlation NMR spectroscopies. X-Ray structural data are presented for the free-base porphyrin 6 and the Zn^II porphyrin 7. The structural data confirm that the porphyrins and their diaza-18-crown-6 substituents adopt cofacial orientations. The size of the cavity between the macrocycles, while demonstrated to accommodate small molecules as shown by the binding of H₂O as an axial ligand to the central Zn^II ion in the crystal structure of 7, will probably not accommodate large ligands or substrates. Moreover, these compounds should allow sequential metallations to yield heterobimetallic species. Therefore, they are ideally suited for study as model systems for biologically important heme-dependent phenomena such as xenon-129 binding to myoglobin or the mimicking of processes related to the structure and function of cytochrome c oxidase.