Metal-organic frameworks (MOFs) have attracted increasing attention in recent years due to their versatile and highly tuneable structures, comprising metal-containing secondary building units (SBUs) linked by organic ligands. This effectively infinite set of possible combinations yields multidimensional extended frameworks that can be tailored to particular applications, notably gas storage and separation, catalysis and drug delivery. MOFs constructed using trivalent metal cations, in particular Al 3+ , Fe 3+ and Cr 3+ , have been widely studied owing to the inherent flexibility that many of these frameworks exhibit. When combined with linear dicarboxylate linkers, they often adopt MIL-88-type structures (MIL = Materiaux de l'Institut Lavoisier), whereby the material constitutes trimeric [M3O(RCO2)6(H2O)2(X)] (M = metal, X = monoanion) SBUs bridged by dicarboxylate linkers into the acs topology with hexagonal channels running along the crystallographic c axis. This isoreticular class of materials has displayed remarkable breathing properties, swelling or contracting in response to external stimuli such as solvent immersion, pressure and temperature. For example, Férey et al. reported that MIL-88A(Fe), an iron MOF constructed using fumarate linkers with formula [Fe3O(C4H2O4)3(H2O)2(X)], exhibits an 85% increase in the unit cell volume between the dried, contracted form and the open, hydrated structure (Figure ). Other frameworks in this isoreticular series also exhibit these dynamic properties, for example MIL-88D(Cr), where the linker is biphenyl-4,4'-dicarboxylate (bpdc), shows a 235% increase in unit cell volume when the dried solid is immersed in pyridine. Figure . a) Expansion of MIL-88A(Fe) from the dried to hydrated form visualised down the crystallographic c axis. Structure models redrawn from ref [9]; Fe, orange polyhedra; C, grey; O, red; H atoms removed for clarity. Not to scale. b) Schematic of postsynthetic bromination of the fumarate linker in MIL-88A(Sc) showing the potential products, of which the meso compound would be expected.