Insights into the binding properties of a cuprous ion embedded in the tren cap of a calix[6]arene and supramolecular trapping of an intermediate

Abstract

Coordination of Cu(I) to a tren unit that is covalently linked to a calix[6]arene has been explored. The resulting complex revealed itself very stable in solution under an inert atmosphere, but extremely sensitive to O₂ in solution as well as in the solid state. Therefore, its binding properties towards non-redox ligands have been studied in detail. The electron-rich metal center displays moderate affinity for nitrilo ligands compared to the calix[6]tris-pyridine ligand. Indeed, the binding enthalpy with acetonitrile is only −30 kJ mol⁻¹, whereas it is −72 kJ mol⁻¹ with the tris-pyridine system. In contrast, CO binding is relatively strong due to important π-back donation from the metal center, as evidenced by the CO stretch, which was found to be less energetic (2075 cm⁻¹) than that measured for ligands based on aromatic donors such as imidazole or pyridine. The conformational and dynamic properties of this calix-system have also been studied in detail. With an empty cavity or with the very small CO guest-ligand, the calix-core undergoes partial self-inclusion leading to dissymmetrical conformations. In contrast, nitrilo ligands act as “shoe-trees” that maintain the calix-core in a C_3v symmetrical cone conformation. Very interestingly, the variable T study relative to the ligand exchange process highlighted a two-step dissociative pathway, where Cu–N bond cleavage/formation is differentiated from the nitrilo guest expulsion/inclusion from/into the calixarene cavity.