Contradistinct Thermoresponsive Behavior of Isostructural MIL-53 Type Metal–Organic Frameworks by Modifying the Framework Inorganic Anion

The influence of simple framework inorganic anions on the thermoresponsive behavior of the isostructural MIL-53 type metal–organic frameworks [AlF(bdc)] and [Al(OH)(bdc)] has been determined using a combination of diffraction and computational techniques. [AlF(bdc)] has an orthorhombic large pore structure from 500 to ∼175 K at which point it undergoes a subtle distortion to form a monoclinic large pore structure that remains stable to 11 K. The orthorhombic large pore form of [AlF(bdc)] exhibits negative thermal expansion from 175–500 K. [Al(OH)(bdc)] has an orthorhombic large pore structure from 500 to 125 K at which point it undergoes a displacive phase transition, a breathing effect, to form a nonporous monoclinic structure. The orthorhombic large pore form of [Al(OH)(bdc)] exhibits positive thermal expansion from 150 to 500 K. The presence of a breathing effect in [Al(OH)(bdc)], and not [AlF(bdc)], is related to the additional contributions to attractive interactions across the shortest dimension of the pore provided by the presence of the hydroxide groups. The display of positive or negative thermal expansion of the orthorhombic large pore structure of either material is related to the rigidity of the constituent corner-sharing chain of AlO 4 X 2 octahedra with the more rigid AlO 4 F 2 octahedra favoring one type of static or dynamic displacement and the less rigid AlO 4 (OH) 2 octahedra favoring a different type of static displacement. Formation of metal–organic frameworks with controlled expansion and displacive phase transition properties, or simultaneously containing mixed thermoresponsive properties, is predicted through control of the identity and amount of the simple inorganic anions in this family of material. The work indicates the importance of considering the simplest species when designing the thermo-mechanical properties of metal–organic frameworks.