The effects of different substituents and basic agents on the crystal structure of the uranyl complexes of fully deprotonated p-R-hexahomotrioxacalix[3]arene have been investigated. The trigonal equatorial coordination previously observed with R = tert-butyl and NEt3 or DABCO as deprotonating agents, and also with H2NBu as reported herein, appears to be changed to distorted tetragonal coordination in the cases R = methyl and NEt3 or R = tert-butyl and NPr3 or to distorted pentagonal coordination for R = tert-butyl and 4-methylpiperidine, passing through an intermediate case with R = tert-butyl and HNBu2. The rare trigonal uranyl coordination observed in these compounds is likely the result of a subtle equilibrium between electronic and steric factors, the macrocyclic
effect probably playing a major role, and it can be easily modified by any perturbation of these factors. Stable complexes could be investigated by 1H NMR on dissolving the crystals obtained using secondary and tertiary amines in halogenated solvents. Ammonium ion inclusion in the aromatic cavity of an apparently C3v symmetric complex was observed. Although less symmetric forms could not be frozen out at low temperature, the equilibration between two different types of structures was investigated.
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