Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The diprotonated, bishydroperchlorate forms of three isomeric β-octaalkyl-substituted tetrapyrrolic macro-cycles, namely, etioporphyrin II (1), etio porphycene (2), and etiocorrphycene (3), have been characterized both in chloroform solution, by UV/visible spectroscopy and 1H and proton-correlated 2D 15N NMR methods, and in the solid state, by single-crystal X-ray diffraction analyses. In the solid state, in marked contradistinction to what is observed for the corresponding free-base forms, the macrocyclic portion of these salts were found to be distorted significantly from planarity with the two perchlorate counteranions being held above and below the average N4 plane by N-H … O hydrogen bonds in all three cases. In solution, 1H and proton-correlated 2D 15N NMR experiments reveal molecular ions of relatively high symmetry (D2h, D2h, and C2v in the case of 1·(HClO4)2, 2·(HClO4)2, and 3·(HClO4)2, respectively) as would be anticipated on the basis of the solid-state results. These same NMR analyses, while revealing slight differences between the three salts in the NH and meso 1H NMR spectral regions, also serve to confirm the generalized congeneric nature of 1·(HClO4)2, 2·(HClO4)2, and 3·(HClO4)2 and support the assignment of the latter two species as being porphyrin-like salts. UV/vis analyses further support this conclusion; in all three instances, strong Soret- and Q-like transitions are observed in dichloromethane that are both distinct from each other (λmax=404, 549, 570, 593; 388, 409, 599, 666; and 419, 559, 604 for 1·(HClO4)2, 2·(HClO4)2, and 3·(HClO4)2, respectively) and from those of the corresponding free-base forms (λmax=396, 496, 530, 565, 619; 382, 570, 617, 657; and 410, 509, 539, 574, 628 for 1, 2, and 3 respectively). Protonation experiments were carried out by exposing dichloromethane solutions of the isomers to aqueous perchlorate/perchloric acid solutions of differing pH. These studies reveal that while porphycene 2 adds two protons readily and concurrently, becoming 50% diprotonated when exposed to perchlorate/perchloric solutions with a pH of around 3.6, porphyrin 1 and corrphycene 3 are protonated in a stepwise manner; they become 50% monoprotonated when exposed to perchlorate/perchloric solutions of pH≍3.7 and 3.9, respectively, and diprotonated at pH≤0.8 and 1.3, respectively.
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