We have further developed and studied catalysts based on earth-abundant elements (cobalt and nickel coordination complexes) as well as artificial metalloenzymes. Among these coordination complexes we have identified highly efficient catalysts that operate at fast reaction rates in several photocatalytic chemical transformations. For instance, we studied catalysts that are efficient electrocatalysts for the reduction of water to hydrogen and how electronic effects impacts into the mechanism. These studies are relevant for further understanding of the critical parameters that govern the reactivity of metal complexes. Besides, the same series of coordination complexes are also active and selective for the reduction of CO2-to-CO under both photo- and electrochemical conditions.
Furthermore, catalysts developed are particularly active in additional photocatalytic transformations, such as the reduction of ketones, aldehydes and olefins. To remark is the control of the selectivity archived thanks to the understanding of the mechanism. The studies pointed out that combining the right photosensitizer and metal complex as key to obtained selectivity. On those bases, we have developed unprecedented selectivity for the reduction of acetophenone in the presence of aliphatic aldehydes, the reduction of ketones vs olefins, and vice versa. Also, we have extended the reactivity to the activation of inert bonds (Csp2-X and Csp3-X; X = Br, Cl and F), currently developing the intramolecular cross-coupling approach, to develop new methodologies for the synthesis of molecules with biological activity.
During the project, we have studied the mechanisms by combining experimental and theoretical studies and unraveled them, which will serve as a guide for future developments of more efficient catalysts and transformations. For instance, we found that merging photoredox catalysts with coordination complexes implies that both will be redox linked. For cobalt complexes, the essential redox equilibrium comprises Co(II) and Co(I) species. The formation and concentration of the low valent and highly reactive Co(I) species is essential since triggering the reactivity and influence the selectivity.
Consequently, the redox potentials of both the cobalt complex and the photoredox catalysts determine the catalytic outcome; tuning both are required to archive high reactivity and selectivity. On the other hand, the protonation of the low valent cobalt intermediates is the TOF-determining transition state (TDTS) of the catalytic cycles in hydrogen evolution, the reduction of ketones, aldehydes and olefins reactions. This has important implications in the selectivity control since aromatic ketones can be single electron reduced by the photoredox catalyst and then trapped by the cobalt hydride species, which dramatically improve the selectivity versus H2 evolution. We have further advanced broadening the reactivity of developed complexes, such as the CO2-to-CO reduction. The identified of the key intermediates of the catalytic cycle, such as the cobalt carbonyl complexes, which is a thermodynamic sink in the catalytic process, guide us to find a solution—the use of light to stabilize them increased the catalytic activity by 2.5 fold. The combination of light and electrochemistry is a powerful approach for the future.
Finally, we performed to first test of a goal beyond the current proposal, the development of selective catalytic reductions of organic substrates using water as sources of electrons and protons as reducing equivalents. To this end, we are developing new materials such as covalent organic frameworks (COF) as a suitable catalyst supports to modify electrodes. We found an excellent catalytic activity of CO2 reduction to CO in pure water. First test indicate that are also promising for other transformation such as the reduction of ketones. Spectroelectrochemical studies of those materials reveal the nature of the catalytic cycle, which has significant differences regarding the molecular counterpart.
