Biomimetic Copper(I)-CO complexes: A structural and dynamic study of a calix[6]arene-based supramolecular system

Four novel calix[6]arene-based cuprous complexes are described. They presen t a biomimetic tris(imidazole) coordination core associated with a hydropho bic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) an d on the imidazore arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical a nd dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformat ions. The preferred conformation was dictated only by the size of the OR1 g roup. When R-1 was an ethyl group, the complex preferentially adopted a fla ttened C-3-symmetrical structure. The corresponding helical enantiomers wer e in conformational equilibrium, which, however, was slow on the H-1 NMR ti me scale at -80 degreesC. When R-1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wo bbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control wa s promoted by the strong conformational coupling between the metal center a nd the host structure. Interestingly, this was not only the result of a cov alent preorganization, but also stemmed from weak interactions within the h ydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper pr oteins in which allosteric effects are common.