Abstract
Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exemplified by most metal-organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These "soft" or "stimuli-responsive" CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from first principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These "switching CNs" are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure-function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.
Highlights
Coordination networks (CNs)[1] are a class of covalent network solids comprised of metal or metal clusters connected in two or more directions by organic, inorganic (e.g. Prussian blue and itsPaper analogues, PBAs4,5) or combinations of organic and inorganic linker ligands (Fig. 1)
CNs are a subset of coordination polymers (CPs)[7,8] and represent an extraordinarily diverse and growing class of metal–organic materials (MOMs).[9,10]
Porosity is a direct outcome of the “node-and-linker” approach to the design of CNs that can be credited to Robson and Hoskins, who introduced the concept as a design principle for the generation of porous solids over 30 years ago.[13,14]
Summary
PBAs4,5) or combinations of organic and inorganic (e.g. hybrid ultramicroporous materials, HUMs6) linker ligands (Fig. 1). Porosity is a direct outcome of the “node-and-linker” approach to the design of CNs that can be credited to Robson and Hoskins, who introduced the concept as a design principle for the generation of porous solids over 30 years ago.[13,14] The existence of porosity deservedly attracts attention but is not in itself the only thing that makes CNs of special interest Rather, it is the inherent modularity of CNs that brings with it an ability to design pore size, shape and chemistry. When exible CNs retain porosity a er activation, a type I-like pro le will be followed by the second step at a threshold pressure that coincides with a structural transformation from a less open phase to a more open phase This transformation could be gradual (type F-I, e.g. the “breathing” effect in MIL-53(Cr)69) or sudden Switching has been used for other on/off events in materials chemistry such as thermal expansion/shrinkage, spin crossover, redox, photochromism, photoisomerization and valence tautomerism.[42,43,75,76,77] we analyse and discuss reversible switching in the context of guest sorption by presenting case studies of switching CNs with particular emphasis upon structure–property relationships and the in uence of variables such as metal node, linker ligand, and adsorbate
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