Solvent-mediated single-crystal-to-single-crystal transformation of metal–organic cage self-assembly

Metal–organic polyhedra (MOPs) are intrinsically porous cage-like structures; however, they frequently display low porosity due to their agglomeration caused by solvent exchange and removal, which results in the blockage of pore windows. This study presents the unprecedented single-crystal-to-single-crystal transformation of a cuboctahedral MOP-OH ([Cu₂₄L₂₄], H₂L = 5-hydroxybenzene-1,3-dicarboxylic acid) using a simple solvent exchange from coordinating N,N-diethylformamide to weakly coordinating chloroform. Following the solvent exchange, the interaction between the cages in the resultant MOP self-assembly is strengthened while the interaction between the cages and solvents is decreased. The N₂ uptake and the surface areas of the MOPs are highly correlated with the cage arrangements: the tightly packed MOPs in chloroform are the most porous among the prepared samples, whereas the loosely packed MOPs are less porous. Scanning electron microscopy reveals that the tight packing of MOPs prevents the crystals from being deteriorating during solvent removal.