Porphyrins complexes with ionic ligands

Up to an ionic radius of at least 0.95 A (e.g. Tlm) the porphyrin ligands form stable complexes with metal ions by the indicated donation of a electrons from the central nitrogens. With ions larger than one angstrom the bond will become more ionic and the porphyrin ligand may become bent (e.g. Pb2+). 

The idea that expanded porphyrins, as "bigger porphyrins", could serve as ligands for cations that are too large to be accommodated as 1 1 in-plane adducts by the tetrapyrrolic porphyrins is, of course, not new. Indeed, it has its antecedents in the early work of Marks and Day and in even earlier studies carried out by Woodward and coworkers. In the case of the latter group, for instance, it became apparent sometime between 1966, the lime of the initial disclosure of sapphyrin 3 (a disclosure, incidentally, that represented the first documented report of an expanded porphyrin), and 1983, the time of the follow-up full paper, that the pentapyrrolic sapphyrins, with an inner core of ca. 5.5 A diameter, should be well-suited for the complexation of uranyl ion, U02 (in-plane ionic radius 0.81-0.86 A depending on coordination number ). 

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