Cooperative Reversible Assembly in Triply Interlocked Al6L4 and Ga6L4 Cages

Abstract

Understanding and controlling the assembly of mechanically interlocked molecules remains a significant challenge. Formation of mechanically interlocked metal–organic cages has, to date, relied exclusively on transition metals due to their predictable coordination geometries and robust bonding. Here, we report, for the first time, the reversible assembly of mechanically interlocked cages based on main-group metals, Al₆L₄ and Ga₆L₄. Structural and computational analyses reveal helical [2]catenane quadruple-decker cage topologies stabilized by six metal–ligand nodes, bridging μ-OH groups, extensive π-stacking, and directional CH···O interactions. Remarkably, simple acid–base cycling triggers fully reversible cage unlocking–recatenation processes in water at room temperature. Unlike transition-metal–mediated cage interlocking, they assemble instantaneously and selectively via an unprecedented cooperative main-group interlocking pathway, without detectable monomeric cage intermediates. Thermodynamic analyses reveal metal-dependent switching, involving entropy-driven dissasembly coupled to strongly enthalpy-driven reassembly, with the Ga₆L₄ cage ~500-fold more stable than Al₆L₄. These findings provide fundamental understanding of new assembly dynamics beyond conventional transition metals.