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 AuIII and PdII complexes with CD‐N‐heterocyclic carbene (NHC) ligands, is only possible in the largest β‐ and γ‐CD cavities. In the case of α‐CD‐derived ligands and PdII, 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 AuI to be oxidized into AuIII. Furthermore, in the PdII‐catalyzed nucleophilic allylation of aldehydes, the β‐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. Carbohydrate chemistry: The formation of encapsulated square planar AuIII and PdII complexes is only possible in the largest cyclodextrin cavities. An unexpected reversal of the ligand was observed with smaller cyclodextrin. The size of the cavity was also found to control the AuI→AuIII oxidation. Finally, these ligands could discriminate between different substrates in a catalytic reaction.