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A four-armed tetra-l-imidazolyl ligand (Lig) equipped with the concavc framework of diphenylglycoluril (tetrahydro-3a,6a-diphenylimidazo[4,5-<7] imidazole-2,5(l//,3//)-dione) has been designed to construct a host that contains a potentially catalytically active metal center in or close to a cavity. Reaction of Lig (arm = CH2(OCH2CH2)2Im) with Pd(CH3CN)2Cl2 results in the formation of a complex with the general formula [Pd(Lig)]Cl2. This complex has a cage structure that is unstable and collapses. The collapsed structure has either a left or a right twisted conformation. These conformations intcrconvert rapidly, the activation free energy of the process being 30 ± 2 kJ-mol"1. Ligands without oxygen atoms or only one oxygen atom per arm react with Pd(CH3CN)2Cl2 to afford cage complexes with the molecular formula [Pd(Lig)Cl]Cl. The cages of these complexes do not collapse. The imidazolyl groups and the chloride ions are involved in a scrambling process in such a way that at any moment the Pd2+ ion is surrounded by three imidazolyl groups and one chloride ion. Data are presented suggesting that intramolecular H bonding is a driving force for cage collapse.

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