Toward the Understanding of the Structure-Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO2 Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn-0-Mn-0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn-0-Mn-0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications.