MIL-53 and its Isoreticular Analogues: a Review of the Chemistry and Structure of a Prototypical Flexible Metal-Organic Framework

We present a review of the structural chemistry of metal-organic framework materials with the MIL-53 type structure. This family of materials is well-known for its structural flexibility, and also the wide variety of metal cations and functional ligands that can be included. This gives rise to a set of multivariate materials and isoreticular analogues, to which isoreticular functionalisation can also be applied. Starting from the parent structure, containing infinite chains of octahedral trivalent metal cations cross-linked by benzene-1,4-dicarboxylate, we illustrate the compositional variety possible: this includes materials that contain tetravalent and divalent cations, mixtures of metal cations, functionalised ligands, expanded structures from extended linkers and inclusion of pendant ligands. We emphasise the crystal chemistry of this group of materials, but will give some examples of properties, particularly those associated with the structural breathing effect of relevance to molecular sieving, and also discuss emerging practical applications.