A Trapped Dinuclear-Cu Intermediate of the CuAAC “Click” Reaction that Behaves as a Mechanical Crypt for Copper

Since the metal templated synthesis of catenanes by Sauvage, many mechanically chelating ligands, in which the donor atoms are provided by more than one interlocked covalent component, have been reported. However, despite the importance of controlling metal ion properties in a range of applications from catalysis to medicine, and the unusual properties of metal ions bound in the cavity of a catenane being reported as early as 1985, very few studies focus on the effect of the mechanical bond on the metal complex; typically, it is simply an intermediate en route to a metal-free synthetic target or used to control mechanical motion in prototypical molecular machines. Here we report perhaps the starkest demonstration of the ability of the mechanical bond to control metal ion properties, the unexpected isolation of an interlocked di-nuclear Cu complex in which these typically kinetically labile metal ions are sequestered such that it takes days to remove them from the ligand even under relatively harsh conditions. DFT modelling suggests that the complex is formed as a trapped intermediate in the Cu-mediated alkyne-azide cycloaddition reaction which represents the first unambiguous observation of such a species in this important “click” reaction.