Periodic Reporting for period 4 - GREENLIGHT_REDCAT (Towards a Greener Reduction Chemistry by Using Cobalt Coordination Complexes as Catalysts and Light-driven Water Reduction as a Source of Reductive Equivalents)
Okres sprawozdawczy: 2020-01-01 do 2020-12-31
Sustainability of the society will be only possible if alternative greener synthetic methods are developed. Modern societies need for efficient production methods of chemicals for drugs, materials, and fuels without environmental harm. Among different opportunities, the use of freely accessible and widespread sunlight to drive endergonic reactions is an ambitious but challenging approach. In this regard, the project explored merging concepts at the edge of the fields of water splitting, solar fuels and photo-redox catalysis by studying the capacity of coordination complexes initially developed for water reduction to perform light-driven catalytic transformations of organic substrates. To this end, we have developed bio-inspired catalytic systems that by some means mimic natural photosynthesis. The development of these effective catalysts required a detailed understanding of the reaction mechanisms involved.
The final goal of the project was the development of new sustainable synthetic methods for greener selective catalytic reductions of organic substrates, without causing damage to the environment by chasing the use of visible light and water as sources of electrons and protons as reducing equivalents. We also aim for expanding the reactivity to a new methodology for the activation of inert molecules and use them as electrophile-electrophile umpolung couplings by rational modification of the catalytic system from basic understating of the reactions.
The final goal of the project was the development of new sustainable synthetic methods for greener selective catalytic reductions of organic substrates, without causing damage to the environment by chasing the use of visible light and water as sources of electrons and protons as reducing equivalents. We also aim for expanding the reactivity to a new methodology for the activation of inert molecules and use them as electrophile-electrophile umpolung couplings by rational modification of the catalytic system from basic understating of the reactions.
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.
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.
"Chemical reactivity on artificial photosynthetic schemes is a ""dream technology"". This technology has the potential to be disruptive and open newer and greener reductive chemical processes that only use water, CO2 and light. In the framework of the project, we have established efficient molecular catalysts based on earth-abundant-elements for reductive transformations using light as a source of energy and water/electron-donor as a source of reductive equivalents. These systems are excellent catalysts for H2O and CO2 reduction, but also to study how those reactions operate. These catalysts have also provided unprecedented light-driven-reductive protocols for the synthesis of organic substrates.
Insight into the mechanism, by combination of experimental and theoretical studies, triggered the development of new and more efficient catalysts, photosensitizers, synthetic protocols and more importantly, let us to find unexpected selectivity patterns in chemical reactivity. To remark is the obtained selectivity for ketone reduction against aldehyde, which overwrites their natural two electron chemistry by a single electron transfer process, as well as the ketones olefins selectivity. Likewise, we have developed a robust and efficient visible-light bimetallic photocatalytic system based on earth-abundant metals that is able to perform the challenging cleavage of unactivated Csp3-Cl bonds at mild reaction conditions. Further electrophile- electrophile reactivity needs to be addressed to access to the full potential of artificial photosynthesis for fine chemistry. To control the selectivity is essential to achieve added value chemicals, and ligand design and supramolecular interactions in water are essential elements to employed for the pursuit. However, still fundamental studies are needed to reach these goals, and to better understand how to integrate these transformations within artificial photosynthetic schemes. In this regard, better characterization of the key reactive intermediates and full elucidation of the catalytic cycles will be pursuit. Finally, new catalytic systems based of reticular materials such as COFs and MOFs are under development. We envision that modified electrodes with well-defined molecular materials could bring them the capacity to perform selective catalytic transformation on target compounds. Moreover, the excellent catalytic activity toward CO2 reduction to CO in pure water, will encourage to explore them towards H+-E umpolung-coupling reactions using water as a source of electrons and protons. In the long term and beyond the project, it is envisioned that the incorporation of artificial photosynthetic schemes for the synthesis of chemicals and fuels will incite a change of the current industrial, society and energy paradigm."
Insight into the mechanism, by combination of experimental and theoretical studies, triggered the development of new and more efficient catalysts, photosensitizers, synthetic protocols and more importantly, let us to find unexpected selectivity patterns in chemical reactivity. To remark is the obtained selectivity for ketone reduction against aldehyde, which overwrites their natural two electron chemistry by a single electron transfer process, as well as the ketones olefins selectivity. Likewise, we have developed a robust and efficient visible-light bimetallic photocatalytic system based on earth-abundant metals that is able to perform the challenging cleavage of unactivated Csp3-Cl bonds at mild reaction conditions. Further electrophile- electrophile reactivity needs to be addressed to access to the full potential of artificial photosynthesis for fine chemistry. To control the selectivity is essential to achieve added value chemicals, and ligand design and supramolecular interactions in water are essential elements to employed for the pursuit. However, still fundamental studies are needed to reach these goals, and to better understand how to integrate these transformations within artificial photosynthetic schemes. In this regard, better characterization of the key reactive intermediates and full elucidation of the catalytic cycles will be pursuit. Finally, new catalytic systems based of reticular materials such as COFs and MOFs are under development. We envision that modified electrodes with well-defined molecular materials could bring them the capacity to perform selective catalytic transformation on target compounds. Moreover, the excellent catalytic activity toward CO2 reduction to CO in pure water, will encourage to explore them towards H+-E umpolung-coupling reactions using water as a source of electrons and protons. In the long term and beyond the project, it is envisioned that the incorporation of artificial photosynthetic schemes for the synthesis of chemicals and fuels will incite a change of the current industrial, society and energy paradigm."