Periodic Reporting for period 1 - NiCO2Cat (Cooperative Nickel Catalysts for CO2 Hydrogenation to Value-Added Products)
Okres sprawozdawczy: 2023-09-01 do 2025-08-31
Catalytic hydrogenation of CO2 using H2 produced with renewable energy is a promising approach for the sustainable production of value-added chemicals such as formic acid or methanol. Moreover, the related catalytic hydrogenation of carbonyl compounds (compounds that contain a C=O double bond) is a ubiquitous transformation in the synthesis of fine chemicals including medicines, flavors, agrochemicals. Molecular catalysts based on transition metals have demonstrated potential for selective CO2 and carbonyl hydrogenation reactions. However, the most efficient catalysts are often based on scarce metals such as ruthenium.
The proposed research aimed at the development of an efficient, cost-effective, potentially scalable nickel-based catalytic system for CO2 and carbonyl reduction with molecular hydrogen. If focused on the development of so-called nickel/olefin pincer complexes, which had recently been shown by the host group to activate H2 via a new cooperative mechanism. Objectives included 1) the synthesis of new catalysts, 2) a study of their reactivity with H2 3) Characterisation of their catalytic activity for hydrogenation, and 4) a study of the mechanism of action of the new catalysts by computational and experimental methods.
The proposed research aimed at the development of an efficient, cost-effective, potentially scalable nickel-based catalytic system for CO2 and carbonyl reduction with molecular hydrogen. If focused on the development of so-called nickel/olefin pincer complexes, which had recently been shown by the host group to activate H2 via a new cooperative mechanism. Objectives included 1) the synthesis of new catalysts, 2) a study of their reactivity with H2 3) Characterisation of their catalytic activity for hydrogenation, and 4) a study of the mechanism of action of the new catalysts by computational and experimental methods.
OBJ1: Objective 1 described in the project proposal has successfully been achieved. A series of olefin-based pincer ligands were designed and synthesized. Their coordination to nickel precursor afforded the intended complexes, which were characterized using NMR spectroscopy and complementary analytical methods. The successful syntheses of these complexes provided the foundation for subsequent reactivity studies.
OBJ2: Objective 2 described in the project proposal has successfully been achieved. Detailed investigations demonstrated that the newly synthesized nickel(0) complex is capable of activating molecular hydrogen (H2). The process was monitored using NMR spectroscopy, including isotopic labeling experiments with D2, which allowed the identification of hydride species and insights into the underlying mechanism. These findings confirmed the potential of nickel pincer complexes to engage in cooperative H-H bond activation.
OBJ3: Initial experiments showed that catalytic hydrogenation of CO2 under accessible conditions was not proceeding. Therefore, the focus was shifted towards catalytic hydrogenation of carbonyl compounds. The results demonstrated efficient hydrogenation activity, thereby validating the ability of the nickel complexes to activate H2 and transfer hydrides to polar organic substrates.
OBJ4: Objective 4 has successfully been achieved in which mechanistic aspects of hydrogen activation were further studied using NMR spectroscopy, deuterium labelling, kinetic measurements and computational modeling (DFT). The results supported a ligand-to-ligand hydride transfer (LLHT) pathway, consistent with preliminary findings of the host group. These combined experimental and computational insights provide a solid conceptual framework for extending this cooperative activation strategy to other transformations.
OBJ2: Objective 2 described in the project proposal has successfully been achieved. Detailed investigations demonstrated that the newly synthesized nickel(0) complex is capable of activating molecular hydrogen (H2). The process was monitored using NMR spectroscopy, including isotopic labeling experiments with D2, which allowed the identification of hydride species and insights into the underlying mechanism. These findings confirmed the potential of nickel pincer complexes to engage in cooperative H-H bond activation.
OBJ3: Initial experiments showed that catalytic hydrogenation of CO2 under accessible conditions was not proceeding. Therefore, the focus was shifted towards catalytic hydrogenation of carbonyl compounds. The results demonstrated efficient hydrogenation activity, thereby validating the ability of the nickel complexes to activate H2 and transfer hydrides to polar organic substrates.
OBJ4: Objective 4 has successfully been achieved in which mechanistic aspects of hydrogen activation were further studied using NMR spectroscopy, deuterium labelling, kinetic measurements and computational modeling (DFT). The results supported a ligand-to-ligand hydride transfer (LLHT) pathway, consistent with preliminary findings of the host group. These combined experimental and computational insights provide a solid conceptual framework for extending this cooperative activation strategy to other transformations.
While their heterogeneous counterparts (e.g. Raney Nickel) are long known, homogeneous Ni-based hydrogenation catalysts are underdeveloped, in part due to difficult handling and sensitivity towards air/water. However, very recent findings had suggested the high potential of homogeneous nickel complexes as hydrogenation catalysts, for example for the semihydrogenation of C≡C triple bonds to C=C double bonds. Nickel complexes had also been developed for CO2 reduction using expensive reductants such as silanes and boranes, which restricts their use as a viable option for large scale implementations. The small number of Ni-catalyzed hydrogenations of polar (C=O, C=N) bonds required harsh conditions (> 150 °C, > 4 atm H2).
Against this backdrop, the catalyst developed in this project operates under comparatively mild conditions (80 °C, 1 atm H2), which represent a significant advance. Furthermore, mechanistic investigations demonstrate a new operating mechanism for this catalyst, wherein the H–H bond is cleaved cooperatively over the Ni/olefin fragment via a so-called ligand-to-ligand hydrogen transfer step. Such a mechanism had been previously observed for the Ni-catalyzed hydrogenation of C≡C triple bonds, but not for C=O bonds. Together, these results open a new path towards efficient hydrogenation catalysts using the earth-abundant metal nickel.
Against this backdrop, the catalyst developed in this project operates under comparatively mild conditions (80 °C, 1 atm H2), which represent a significant advance. Furthermore, mechanistic investigations demonstrate a new operating mechanism for this catalyst, wherein the H–H bond is cleaved cooperatively over the Ni/olefin fragment via a so-called ligand-to-ligand hydrogen transfer step. Such a mechanism had been previously observed for the Ni-catalyzed hydrogenation of C≡C triple bonds, but not for C=O bonds. Together, these results open a new path towards efficient hydrogenation catalysts using the earth-abundant metal nickel.