Periodic Reporting for period 4 - CALCEAM (Cooperative Acceptor Ligands for Catalysis with Earth-Abundant Metals)
Periodo di rendicontazione: 2022-02-01 al 2022-07-31
This project aimed at the development of new organic molecules (ligands) that bind transition metal atoms and modify their chemical properties so that they can serve as catalysts, that is facilitate effective chemical transformations for the synthesis of chemicals.
Currently, both research labs and industrial production plants make extensive use of noble metals (Palladium, platinum, Rhodium, Iridium,...) for catalysis. These are efficient, but associated with a high monetary and environmental cost as their extraction is energy intensive and their concentration in waste streams is potentially detrimental to the environment. It would be advantageous to be able to use the more abundant, less expensive, sometimes less toxic metals of the first transition series (Iron, Cobalt, Nickel) instead. However, the reactivity of latter have proven more difficult to control, yielding often inefficient or unselective processes.
To tackle this problem, we synthesized new organic molecules that can cooperate with such metal centers to activate organic molecules and achieve catalytic transformations. Specifically, we used ligands that incorporate multiple bonds (C=E, where E is N, O, C, and B=N) as a reactive cooperative site. Because the interaction between the multiple bond and the metal is labile, our ligands incorporate two "arms" terminated with a strongly-binding phosphorus atom that anchor the reactive site to the metal. For comparison, an analogous ligand containing a third phosphorus atom instead of a multiple bond was included in the study. We investigated the binding of these "pincer" ligands to earth-abundant metals and the reactivity of the formed complexes with small molecules such as Hydrogen, Silanes, aromatic compounds. We evaluated the produced molecules in industrially relevant catalytic reactions including C–C cross coupling and hydrogenation and characterized their mode of action in details using both experimental and theoretical methods. These findings will inform further development of new catalysts, both in our own labs and in the broader scientific community.
Currently, both research labs and industrial production plants make extensive use of noble metals (Palladium, platinum, Rhodium, Iridium,...) for catalysis. These are efficient, but associated with a high monetary and environmental cost as their extraction is energy intensive and their concentration in waste streams is potentially detrimental to the environment. It would be advantageous to be able to use the more abundant, less expensive, sometimes less toxic metals of the first transition series (Iron, Cobalt, Nickel) instead. However, the reactivity of latter have proven more difficult to control, yielding often inefficient or unselective processes.
To tackle this problem, we synthesized new organic molecules that can cooperate with such metal centers to activate organic molecules and achieve catalytic transformations. Specifically, we used ligands that incorporate multiple bonds (C=E, where E is N, O, C, and B=N) as a reactive cooperative site. Because the interaction between the multiple bond and the metal is labile, our ligands incorporate two "arms" terminated with a strongly-binding phosphorus atom that anchor the reactive site to the metal. For comparison, an analogous ligand containing a third phosphorus atom instead of a multiple bond was included in the study. We investigated the binding of these "pincer" ligands to earth-abundant metals and the reactivity of the formed complexes with small molecules such as Hydrogen, Silanes, aromatic compounds. We evaluated the produced molecules in industrially relevant catalytic reactions including C–C cross coupling and hydrogenation and characterized their mode of action in details using both experimental and theoretical methods. These findings will inform further development of new catalysts, both in our own labs and in the broader scientific community.
New synthetic routes toward a number of new ligands incorporating C=C and C=N, and B=N bonds have been developed, giving access to those in multigram quantities. The coordination of these ligands to Nickel and, to a lesser extent, to Cobalt and Iron, has been investigated, and they have been found to bind in the desired mode. The reactivity patterns of the obtained complexes has been studied.
Of particular interest is the discovery that a very polar B=N double bond can also bind side-on to a metal. The latter acts as a masking unit for a reactive Nickel center, which is in turn able to activate molecular hydrogen and catalyze hydrogenation reactions.
When bound to nickel, a new ligands incorporating a C=C bond can activate the H–H bond of hydrogen in a mechanistically distinct way. The H–H molecule first binds to Nickel to form an observable intermediate complex, and then transfer one of its hydrogen atoms to the C=C bond directly, without prior cleavage of the H–H bond. This new mode of activation allows for efficient hydrogenation of certain C–C triple bonds to C=C double bonds without overhydrogenation.
In addition, incorporating a C=C bond in the ligand design allowed to stabilize a reactive intermediate (metallacyclobutane) en route to the formation of 3-membered cycles (cyclopropanes). Detailed reactivity studies showed that this reactive intermediate could be channeled to a different reaction that forms new C=C bonds (olefin metathesis) by changing the chemical composition of the medium. This provides a lead towards the use of Nickel for olefin metathesis, a field currently dominated by heavier elements Ruthenium and Molybdenum.
The new ligands incorporating C=N bonds display unusual reactivity due to the ability of the electron pair located on the N-atom to interact with subtrate molecules. The ligand is able to sample different geometries between two extremes: side-on coordination of bothe C and N, and coordination of C only. This flexibility allows the produced compounds to catalyze the electrochemical reduction of protons to dihydrogen.
Another catalytic reaction under scrutiny is nickel catalyzed C–C coupling. We had anticipated that labile C=E bonds would facilitate shuttling between different coordination modes during a catalytic cycle. In the course of our investigations, we found that the whole cycle can be more efficiently supported by a strongly tridentate (PPP) ligand, proceeding through novel 5-coordinate intermediates. This system gave us a unique opportunity to characterize the role of the different oxidation states and coordination numbers throughout C–C cross coupling catalysis.
Overall, the project provided showed that incoporating various multiple bonds in a "pincer" ligand design opens up new reaction pathways for inexpensive and abundant transition metals of the first row, especially nickel. Some of these results have already been disclosed in three open-access publications and a book chapter. Eight manuscripts based on the project results are in preparation for open-access publication.
Of particular interest is the discovery that a very polar B=N double bond can also bind side-on to a metal. The latter acts as a masking unit for a reactive Nickel center, which is in turn able to activate molecular hydrogen and catalyze hydrogenation reactions.
When bound to nickel, a new ligands incorporating a C=C bond can activate the H–H bond of hydrogen in a mechanistically distinct way. The H–H molecule first binds to Nickel to form an observable intermediate complex, and then transfer one of its hydrogen atoms to the C=C bond directly, without prior cleavage of the H–H bond. This new mode of activation allows for efficient hydrogenation of certain C–C triple bonds to C=C double bonds without overhydrogenation.
In addition, incorporating a C=C bond in the ligand design allowed to stabilize a reactive intermediate (metallacyclobutane) en route to the formation of 3-membered cycles (cyclopropanes). Detailed reactivity studies showed that this reactive intermediate could be channeled to a different reaction that forms new C=C bonds (olefin metathesis) by changing the chemical composition of the medium. This provides a lead towards the use of Nickel for olefin metathesis, a field currently dominated by heavier elements Ruthenium and Molybdenum.
The new ligands incorporating C=N bonds display unusual reactivity due to the ability of the electron pair located on the N-atom to interact with subtrate molecules. The ligand is able to sample different geometries between two extremes: side-on coordination of bothe C and N, and coordination of C only. This flexibility allows the produced compounds to catalyze the electrochemical reduction of protons to dihydrogen.
Another catalytic reaction under scrutiny is nickel catalyzed C–C coupling. We had anticipated that labile C=E bonds would facilitate shuttling between different coordination modes during a catalytic cycle. In the course of our investigations, we found that the whole cycle can be more efficiently supported by a strongly tridentate (PPP) ligand, proceeding through novel 5-coordinate intermediates. This system gave us a unique opportunity to characterize the role of the different oxidation states and coordination numbers throughout C–C cross coupling catalysis.
Overall, the project provided showed that incoporating various multiple bonds in a "pincer" ligand design opens up new reaction pathways for inexpensive and abundant transition metals of the first row, especially nickel. Some of these results have already been disclosed in three open-access publications and a book chapter. Eight manuscripts based on the project results are in preparation for open-access publication.