Metal Ion Effects on Fullerene/Porphyrin Cocrystallization

This investigation presents a systematic examination of the late transition metal octaethylporphyrins M^II(OEP) with M = Co, Ni, Cu, and Zn as cocrystallization agents for C₆₀. In each case, the fullerene was dissolved in benzene and then layered over an equimolar dichloromethane solution of the porphyrin. A striking new columnar structural type for these systems is found for Co^II(OEP) and Zn^II(OEP) with stoichiometric ratios 6:5 for porphyrin/fullerene. In these cocrystals, one fullerene is surrounded by two porphyrins, one is adjacent to only one porphyrin, while one makes no contact with a porphyrin; yet, all three fullerenes are ordered at 90 K. A more typical, also new, 1:1 structure with Cu^II(OEP) and C₆₀ shows back-to-back porphyrins with all eight ethyl groups embracing the fullerene. In these three structures only dichloromethane is found as the solvate. The Ni^II(OEP) structure is related to the known structure of Ni^II(OEP)·C₆₀·2C₆H₆ but has incorporated ca. 5% dichloromethane. In these Ni^II(OEP)/C₆₀ cocrystals, four of the ethyl groups embrace the fullerene, while the other four are displaced to the opposite side of the porphyrin plane where they do not make contact with a fullerene. The porphyrin core conformations of the different metal derivatives also differ, and their possible impact on cocrystal formation is discussed.