Tuning the flexibility of MIL-88A(Sc) through synthetic optimisation and postsynthetic halogenation

Breathing in metal-organic frameworks (MOFs), the distinctive transformation between a porous phase and a less (or non) porous phase, often controls the uptake of guest molecules, endowing flexible MOFs with highly selective gas adsorptive properties. In highly flexible topologies, breathing behaviour can be tuned by linker modification, which is typically achieved pre-synthetically by the use of functionalised linkers. Herein, we show that MIL-88A(Sc) exhibits the characteristic flexibility of its topology, which can be tuned by (i) modifying synthetic conditions to yield a formate-buttressed analogue that is rigid and porous, and (ii) postsynthetic bromination across the alkene functionality of the fumarate ligand, generating a product that is rigid but non-porous. As well as providing different methodologies for tuning the flexibility and breathing behaviour of this archetypal MOF, we show that bromination of the formate-bridged analogue results in an identical material, representing a rare example of two different MOFs being postsynthetically converted to the same end product.