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

Abstract

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 (COF bpyMn ) were found to exhibit higher selectivity and activity toward electrochemical CO 2 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 (COF PTMn ), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO 2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COF PTMn exhibited a remarkable enhancement in the intrinsic catalytic CO 2 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 COF PTMn , 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 COF bpyMn 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 COF bpyMn . 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.