Characterization of the interactions between neptunyl and plutonyl cations and expanded porphyrins

The synthesis of a new hexapyrrolic expanded porphyrin, hexaphyrin (1.0.1.0.0.0) (1), that was found to form unique complexes of the actinide cations neptunyl (NpO2 +) and uranyl (UO2 2+) was recently reported (Sessler et al., Angew. Chem., Int. Ed. Engl. 40 (2001) 591). The present study was undertaken in an effort to generalize these findings and involves an analysis of the neptunyl (NpO2 +) and plutonyl, (PuO2 +) coordination chemistry of several other expanded porphyrin systems, including those previously reported as having formed uranyl (UO2 2+) cation complexes, in particular amethyrin ([24]hexaphyrin(1.0.0.1.0.0), 2), pentaphyrin, ([22]pentaphyrin(1.1.1.1.1), 4), and the Schiff base macrocycle, alaskaphyrin (5). This study was broadened to include the Schiff base ligand 6 (grandephyrin) and its previously unreported uranyl (UO2 2+) complex. Within moments of addition of a neptunium(VI) source to methanol solutions of the expanded porphyrins 1, 2, 4, 5 and 6, dramatic changes in the UV–Vis spectra are observed that are consistent with formation of a neptunyl complex and, in several cases, oxidation of the macrocycle. Similar changes were also observed when hexaphyrin (1), pentaphyrin (4), alaskaphyrin (5), and grandephyrin (6) were exposed to PuO2 2+. Corroborative changes are also observed in the associated 1H NMR spectra and in those portions of the electronic spectrum associated with the actinide cations. Although the formation of the proposed neptunyl and plutonyl complexes could not be substantiated by X-ray diffraction methods, the solid-state structure of the uranyl complex formed from 6 was elucidated and is presented in this report.