Metastable Macrocyclic Bis-Meisenheimer Adduct.

An electron-deficient tetranitroazacalixarene is shown to undergo reversible cyanide capture via nucleophilic aromatic substitution, yielding an unprecedented class of metastable dianionic macrocycle incorporating two Meisenheimer units, fully characterized by single-crystal X-ray diffraction, NMR, and electronic absorption spectroscopies. Acting as a chemical fuel, cyanide transiently drives the formation of this adduct, which can spontaneously regenerate the parent macrocycle under mild conditions, representing a rare demonstration of metastability in a Meisenheimer complex. The dynamic behavior of this system, reminiscent of out-of-equilibrium assemblies, is finely tunable through macrocycle concentration, counterion nature, solvent, and temperature. Detailed crystallographic, spectroscopic, and computational analyses reveal that intramolecular hydrogen bonding plays a key role in stabilizing the adduct. Comparative studies with simpler analogues further highlight the importance of macrocyclic preorganization and non-covalent interactions in governing this reversible reactivity.