Pincer ligands incorporating heavy group 16 elements as chalcogen-bond donors: Towards the design of new secondary coordination spheres for CO2 reduction

Abstract: Secondary bonding interactions (SBI) such as hydrogen-bonding are widely recognized as critical components of many catalytic active sights. This is especially true in transformations that require coordination of multiple substrate binding events. Chalcogen-bonding (ChB) is a type of SBI that is mediated by heavier Group 16 elements. Despite its recognized utility in molecular recognition and analyte sensing, ChB has only just begun to be exploited in catalysis. The ability to integrate ChB in catalyst design is an intriguing strategy as the formation and dissociation of chalcogen-bonds is often a low-barrier process. Moreover, the nature of ChB SBIs is well suited to the cooperative binding and bond forming steps necessary to reductively couple carbon dioxide. We have examined the incorporation of chalcogen-bonding units in the form of benzotellurazole and isochalcogenazole (E = Se, Te) moieties into NNN chelating frameworks. The synthesis of these ligands has been accomplished in a straightforward manner and their coordination chemistry examined with a variety of transition metals. Structural studies of the coordination complexes demonstrate the presence of intermolecular chalcogen-bonding in several instances thereby validating the ligand design concept. Preliminary findings have also demonstrated that cobalt complexes of the isochalcogenazole ligands are active for reductive functionalization catalysis. The reactivity of these complexes will be discussed in the context of leveraging ChB interactions for CO2 reduction.