The “Chemistree” of Porous Coordination Networks: Taxonomic Classification of Porous Solids to Guide Crystal Engineering Studies

New approaches to gas/vapor storage and purification are urgently needed to address the large energy footprint, cost, and/or risk associated with existing technologies. In this context, new classes of porous physisorbents, exemplified by porous coordination networks (PCNs), have emerged. There are now >100 000 entries in the Cambridge Structural Database (CSD) metal-organic framework (MOF) subset and the rate of publication, >5000 per year, grows unabatedly. The number of PCNs makes it infeasible to test all of them for sorption performance and it is therefore timely to introduce a classification approach based upon taxonomy to supplement topological classification of PCNs. This taxonomic approach complements existing databases such as the CSD and enable the design (crystal engineering) of new families of PCNs. It also categorizes existing PCNs in a manner useful to crystal engineers. The internal consistency of the taxonomic approach is verified by case studies of several well-known PCNs whereas its utility is demonstrated upon understudied topologies and hard-to-rationalize infinite rod building blocks. Overall, taxonomic classification enables a traffic light system to direct crystal engineers towards finding a “needle in haystack,” that is, a family (platform) of PCNs that is amenable to crystal engineering and systematic structure/property studies.