Identifying Underexplored and Untapped Regions in the Chemical Space of Transition Metal Complexes

We survey over 230,000 crystallized mononuclear transition metal complexes (TMCs) to identify trends in preferred geometric structure and metal coordination. While we observe d-filling to influence coordination preference, with late TMCs preferring lower coordination number, we also note exceptions. We also observe that 4d and 5d transition metals and 3p-coordinating ligands are systematically undersampled. For the roughly one third of the set of mononuclear TMCs that are octahedral, analysis of the 67 symmetry classes of their ligand environments reveals that complexes most commonly contain monodentate ligands that may likely be removable to leave an open site amenable to catalysis. Due to their frequent use in transition metal catalysts, we analyze trends in coordination by tetradentate ligands in terms of the capacity to support multiple metals and the variability of coordination geometry. We identify promising tetradentate ligands that co-occur in crystallized complexes with labile monodentate ligands, indicating their ability to generate reactive sites. Literature mining suggests that many of these tetradentate ligands are untapped as ligands in catalytic complexes, motivating proposal of a promising octa-functionalized porphyrin in this set as a candidate ligand for catalysis.