Cavity-Controlled Coordination of Square Planar Metal Complexes and Substrate Selectivity by NHC-Capped Cyclodextrins (ICyDs)

Encapsulation of a metal center inside the cavity of a cyclodextrin (CD) allows the control of the possible coordination modes for the metal depending on the nature and size of the cavity. We show here that the formation of encapsulated square planar Au-III and Pd-II complexes with CD-N-heterocyclic carbene (NHC) ligands, is only possible in the largest beta- and gamma-CD cavities. In the case of alpha-CD-derived ligands and Pd-II, an unexpected reversal of the NHC ligand was observed. These extraverted or introverted conformations of metal complexes were carefully studied by NMR and their different behaviors depending on the cavity could be revealed. The size of the CD cavity was also found to have a significant effect on the ability of Au-I to be oxidized into Au-III. Furthermore, in the Pd-II-catalyzed nucleophilic allylation of aldehydes, the beta-CD-derived ligand was able to discriminate different substrates from a mixture according to their size, leading to a significantly favored reaction with the smallest substrate.