Solution phase and single crystal diffraction X-ray analyses of diprotonated porphyrin isomers - Etioporphyrin, etioporphycene, and etiocorrphycene bishydroperchlorate salts

The diprotonated, bishydroperchlorate forms of three isomeric beta-octaalkyl-substituted tetrapyrrolic macrocycles, namely, etioporphyrin II (1), etioporphycene (2), and etiocorrphycene (3), have been characterized both in chloroform solution, by UV/visible spectroscopy and H-1 and proton-correlated 2D N-15 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 N-4 plane by N-H ... O hydrogen bonds in all three cases. In solution, H-1 and proton-correlated 2D N-15 NMR experiments reveal molecular ions of relatively high symmetry (D-2h, D-2h, and C-2v 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 H-1 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 (lambda(max) = 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 (lambda(max) = 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 approximate to 3.7 and 3.9, respectively, and diprotonated at pH less than or equal to 0.8 and 1.3, respectively.