Metalloporphyrins and Metallocorroles with Designed Second-Sphere Structures for Catalytic Small Molecule Activation Reactions

Conspectus Small-molecule activation reactions, including the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO2RR), are involved in natural and artificial energy conversion processes. In enzymes, the second-sphere environment of catalytic sites can promote substrate activation, stabilize intermediates, and facilitate proton transfer. Synthetic chemists have endeavored to develop molecular catalysts with functionalized second sphere environments. However, installing functional groups at metal second-sphere sites and understanding their actual roles in promoting reactions are proven to be challenging. This Account describes our recent works in developing metalloporphyrins and related metallocorroles with designed second-sphere structures for ORR, HER, and CO2RR. Metalloporphyrins are ideal platforms for installing second-sphere groups and investigating their roles in catalysis. We designed and synthesized a series of metalloporphyrins by installing different substituents to construct molecular pockets, grab second metal ions, and introduce positively charged or hydrogen-bonding interactions. Importantly, we focus on discussing the roles of these second-sphere structures in improving the catalysis. We used molecular pockets to stabilize active ORR intermediates. By using the αααα atropisomer of Co tetra(2-pivalamidophenyl)porphyrin, we synthesized CoIII-superoxo, CoIII-hydrosuperoxo, and CoIII-hydroperoxo, and more importantly, studied their spectroscopic and reaction natures. By modifying the pocket structure, we revealed thermodynamic and kinetic control effects of the molecular pocket on O2 binding and provided new understanding of steric effects on catalytic ORR. We appended a second metal ion above the porphyrin macrocycle to promote O2 binding at metalloporphyrin sites by stabilizing the resulted O2 adducts. By appending a strong Lewis acidic but redox-inactive Sc3+ ion on CoIII corrole, we realized O2 activation at high-valent CoIII site, while by appending a redox-active CoII ion on CoIII corrole, we could activate O2 by using two electrons to generate bridging peroxo. Moreover, the appended metal ion can assist O–O bond cleavage by accepting the leaving hydroxyl group. We introduced second-sphere groups to promote substrate binding and proton transfer through electrostatic and/or hydrogen-bonding interactions. Appended cationic substituents improve the binding of O2 and CO2 via electrostatic interactions. Importantly, we demonstrated role-specialized cooperation between two appended cationic groups in the CO2RR. In addition to stabilizing CO2 adducts, the two cationic groups can function differently but cooperatively to facilitate C–O bond cleavage. Appended crown ether can construct water networks for efficient proton transfer. Taken together, by modifying second-sphere environments, a series of metalloporphyrins and metallocorroles with high activity and selectivity for these small-molecule activation reactions were developed. With the benefits of directly synthesizing reaction intermediates, we can investigate the reaction natures of these intermediates and, importantly, study the kinetics of key elementary reaction steps. As a result, we demonstrated the critical roles of second-sphere structures not only in improving the whole catalysis but also in regulating key reaction steps. Therefore, this Account is significant to present a metalloporphyrin platform to study second-sphere environment effects on catalysis and highlight the paradigm to understand structure-function relationships with an emphasis placed on key intermediates and corresponding elementary reaction steps.