Stabilizing Metal–Organic Polyhedra (MOP): Issues and Strategies

Metal–organic polyhedra (MOPs) are discrete, metal–organic molecular entities composed of edge-sharing molecular polygons or connected molecular vertices. Unlike the infinite metal–organic coordination networks popularized by metal–organic frameworks (MOFs), spherical MOPs, also known as nanocages, nanospheres, nanocapsules, or nanoballs, are obtained through the self-organization of metal–carboxylate or metal–pyridine/pyrimidine links to afford cage-like nanoarchitectures. MOPs offer much promise as porous materials owing to their well-defined structures and solution processability. However, these advantages become moot if their poor aqueous stability and/or guest-removal-induced aggregation handicaps remain unaddressed. The concise premise of this contribution limits our discussion to the design principles in action behind recent developments in stable carboxylate MOPs. To highlight the structure–property relationships between the structural and compositional features of these metal carboxylate polyhedra, related scientific challenges and state-of-the-art research directions for further exploration are presented in brief.