Manipulating solvent and solubility in the synthesis, activation, and modification of permanently porous coordination cages

Abstract

Among all porous materials, permanently porous metal–organic architectures offer unparalleled structural diversity and tunability that has led to their extensive development over the past two decades. Most of this work has focused on two- and three-dimensional metal–organic frameworks (MOFs), while the chemistry of zero-dimensional (discrete) porous coordination cages (PCCs) has been comparatively neglected. Molecular coordination cages have largely been explored for solution-phase applications, with interest in exploring their permanent porosity in the solid-state growing only very recently. For all metal–organic materials, solvent plays a critical role in controlling the formation and activation of a suitable phase for a targeted application. Here, the potential solubility of molecular PCCs distinguishes their chemistry in several respects, where control over material solubility depends heavily on ligand functionalization, which can be incorporated either prior to cage assembly or post-synthetically. This review outlines syntheses, phase-control, activation, and modification of permanently porous metal–organic cages, especially with respect to the role of solvent in these processes and the manipulation of cage solubility. Throughout the review, the chemistry of cage-based materials is compared with more exhaustive studies of MOFs, which highlights certain advantages of molecular phases as well as opportunities to further develop their chemistry.