Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO₂ Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

A series of five ruthenium (II) complexes containing a tridentate CNC pincer ligand, a bidentate 2,2′-bipyridine (bpy) ligand, and a monodentate ligand (chloride, bromide, or acetonitrile) were synthesized. The CNC pincer ligands used imidazole or benzimidazole-derived N-heterocyclic carbenes (NHCs) as the C donors and a 4-methoxy-substituted central pyridyl ring as the N donor. All of the complexes were characterized by analytical, spectroscopic, and single-crystal X-ray diffraction methods. These complexes were used as catalysts for visible-light-driven CO2 reduction in the presence and absence of an external photosensitizer (PS). Notably, complex 4C with a benzimidazole-derived CNC pincer ligand and bromide as the monodentate ligand was the most active catalyst tested for both sensitized and self-sensitized CO2 reduction. Thus, this catalyst was the subject of further mechanistic studies using transient absorption spectroscopy (TAS), absorption spectroelectrochemistry (SEC), and computational studies. A mechanism has been proposed for self-sensitized CO2 reduction involving (1) light excitation of the catalyst, (2) reduction by sacrificial donors, (3) halide loss, and (4) CO2 binding to form [RuCO2]+ as the catalyst resting state. The timeline for these steps and the structures of key intermediates are all supported by experimental observations (including TAS and SEC) and supporting computational studies. Subsequent steps in the cycle past [RuCO2]+ were not experimentally observable, but they are supported by computations. Experiments were also used to explain the differences observed for sensitized catalysis. Catalyst 4C is an unusually active catalyst for both sensitized and self-sensitized CO2 reduction, and thus being able to understand how it functions and which steps are turnover-limiting is an important development facilitating the design of commercially viable catalysts for solar fuel formation

Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO₂ Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

A series of five ruthenium (II) complexes containing a tridentate CNC pincer ligand, a bidentate 2,2′-bipyridine (bpy) ligand, and a monodentate ligand (chloride, bromide, or acetonitrile) were synthesized. The CNC pincer ligands used imidazole or benzimidazole-derived N-heterocyclic carbenes (NHCs) as the C donors and a 4-methoxy-substituted central pyridyl ring as the N donor. All of the complexes were characterized by analytical, spectroscopic, and single-crystal X-ray diffraction methods. These complexes were used as catalysts for visible-light-driven CO2 reduction in the presence and absence of an external photosensitizer (PS). Notably, complex 4C with a benzimidazole-derived CNC pincer ligand and bromide as the monodentate ligand was the most active catalyst tested for both sensitized and self-sensitized CO2 reduction. Thus, this catalyst was the subject of further mechanistic studies using transient absorption spectroscopy (TAS), absorption spectroelectrochemistry (SEC), and computational studies. A mechanism has been proposed for self-sensitized CO2 reduction involving (1) light excitation of the catalyst, (2) reduction by sacrificial donors, (3) halide loss, and (4) CO2 binding to form [RuCO2]+ as the catalyst resting state. The timeline for these steps and the structures of key intermediates are all supported by experimental observations (including TAS and SEC) and supporting computational studies. Subsequent steps in the cycle past [RuCO2]+ were not experimentally observable, but they are supported by computations. Experiments were also used to explain the differences observed for sensitized catalysis. Catalyst 4C is an unusually active catalyst for both sensitized and self-sensitized CO2 reduction, and thus being able to understand how it functions and which steps are turnover-limiting is an important development facilitating the design of commercially viable catalysts for solar fuel formation

Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO₂ Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

A series of five ruthenium (II) complexes containing a tridentate CNC pincer ligand, a bidentate 2,2′-bipyridine (bpy) ligand, and a monodentate ligand (chloride, bromide, or acetonitrile) were synthesized. The CNC pincer ligands used imidazole or benzimidazole-derived N-heterocyclic carbenes (NHCs) as the C donors and a 4-methoxy-substituted central pyridyl ring as the N donor. All of the complexes were characterized by analytical, spectroscopic, and single-crystal X-ray diffraction methods. These complexes were used as catalysts for visible-light-driven CO2 reduction in the presence and absence of an external photosensitizer (PS). Notably, complex 4C with a benzimidazole-derived CNC pincer ligand and bromide as the monodentate ligand was the most active catalyst tested for both sensitized and self-sensitized CO2 reduction. Thus, this catalyst was the subject of further mechanistic studies using transient absorption spectroscopy (TAS), absorption spectroelectrochemistry (SEC), and computational studies. A mechanism has been proposed for self-sensitized CO2 reduction involving (1) light excitation of the catalyst, (2) reduction by sacrificial donors, (3) halide loss, and (4) CO2 binding to form [RuCO2]+ as the catalyst resting state. The timeline for these steps and the structures of key intermediates are all supported by experimental observations (including TAS and SEC) and supporting computational studies. Subsequent steps in the cycle past [RuCO2]+ were not experimentally observable, but they are supported by computations. Experiments were also used to explain the differences observed for sensitized catalysis. Catalyst 4C is an unusually active catalyst for both sensitized and self-sensitized CO2 reduction, and thus being able to understand how it functions and which steps are turnover-limiting is an important development facilitating the design of commercially viable catalysts for solar fuel formation

Sensitized and Self-Sensitized Photocatalytic CO₂ Reduction to HCO₂^– and CO under Visible Light with Ni(II) CNC-Pincer Catalysts

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole-derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized by using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2– was analyzed using all of the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TONs) for formate (HCO2–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4Me with a benzimidazole-derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via NiI or Ni0 catalytic cycles. The two active species (NiI and Ni0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs formate as the product

Sensitized and Self-Sensitized Photocatalytic CO₂ Reduction to HCO₂^– and CO under Visible Light with Ni(II) CNC-Pincer Catalysts

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole-derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized by using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2– was analyzed using all of the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TONs) for formate (HCO2–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4Me with a benzimidazole-derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via NiI or Ni0 catalytic cycles. The two active species (NiI and Ni0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs formate as the product

Sensitized and Self-Sensitized Photocatalytic CO₂ Reduction to HCO₂^– and CO under Visible Light with Ni(II) CNC-Pincer Catalysts

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole-derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized by using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2– was analyzed using all of the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TONs) for formate (HCO2–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4Me with a benzimidazole-derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via NiI or Ni0 catalytic cycles. The two active species (NiI and Ni0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs formate as the product