Proton Responsive and Hydrogen Bonding Ligands in Organometallic Chemistry

Papish, Elizabeth T.; Das, Sanjit; Silprakob, Weerachai; Boudreaux, Chance M.; Manafe, Sonya

doi:10.1016/b978-0-12-820206-7.00080-9

Cited by 8 publications

(6 citation statements)

References 390 publications

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“…NHCs and multidentate ligands containing NHCs are emerging as a powerful class of ligands in organometallics and homogeneous catalysis due to a strong σ donor ability, straightforward synthesis, and several possible modifications. − For example, NHC ligand donor strengths can be altered by saturation of the heterocycle, benzannulation, varying the N -substituents, or introducing a third heteroatom in the heterocyclic ring. − The combination of pyridine rings and NHCs have been used to make bidentate, tridentate, and tetradentate ligated metal complexes, which have found application in catalytic CO 2 reduction. ,,,− Unlike NHCs, pyridine is a weaker σ donor but can serve as a π acceptor, and substitutions on the pyridyl ring can provide another handle to further tune the electronics at the metal center. Tridentate CNC pincers consisting of a pyridyl ring and two NHC rings have often been used along with bipyridine in ruthenium chemistry (Figure ).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2023

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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 [RuCO 2 ] + 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 [RuCO 2 ] + 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.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2023

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“…15 We aimed to evaluate the electronic effects on nickel CNC-pincer catalysts by using different groups (O − , OH, H) para to N on the central pyridine ring. 16 Thus, we synthesized Ni complex 1 (Figure 1) and compared its activity as a photocatalyst for CO 2 reduction with Ni complex 2 made by Sun's group. 17 Catalyst 2 was reported to be an electrocatalyst for CO 2 reduction but not a photocatalyst.…”

Section: ■ Introductionmentioning

confidence: 99%

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

Manafe,

Le,

Lambert

et al. 2024

ACS Catal.

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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 4 Me 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.

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“…Catalysts with OH groups near the metal center (e.g., in 6,6′-dihydroxy-2,2′-bipyridine = 6,6′-dhbp in 2 6OH , Chart ) can serve as metal–ligand bifunctional catalysts in substrate hydrogenation reactions by hydride transfer from the metal combined with H + transfer from the ligand. ,– Examples of metal–ligand bifunctional catalysts bearing protic OH/NH groups proximate to the metal center have included work by Noyori, , Shvo, – and Himeda – and others. – Furthermore, metal complexes of 6,6′-dhbp undergo changes in electronic properties as a function of pH (Scheme ). ,– For example, [Cp*IrOH 2 (6,6′-dhbp)](OTf) 2 ( 2 6OH ) undergoes double deprotonation with an apparent p K a value of 4.1 . Deprotonation of the 6,6′-dhbp ligand results in a more electron rich metal center that can serve to accelerate hydrogenation reactions yielding more a more active metal catalyst .…”

Section: Introductionmentioning

confidence: 99%

Iridium Dihydroxybipyridine Complexes are Effective Catalysts for Hydrodeoxygenation of Vanillyl Alcohol in Water

Yao,

Buell,

Kuppravalli

et al. 2023

Organometallics

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The selective reduction and deoxygenation of ligninderived organic compounds are of interest for modeling a key reaction in the utilization of biomass. Toward this goal, vanillyl alcohol is used as a lignin monomer surrogate herein, and we study its reduction to form creosol in an aqueous solution. Four water-soluble iridium catalysts of the type [Cp*Ir(OH 2 )(bpy R2 )](OTf) 2 (2 R , where Cp* = η 5 -pentamethylcyclopentadienyl anion and bpy R2 = n,n′-R 2 -2,2′-bipyridine with n = 4 or 6) with different R substituents (R = H, OH, Me) in different positions on the bipyridine ligands were studied for this hydrodeoxygenation (HDO) reaction on vanillyl alcohol. Modification of the bipyridine ligands demonstrated that a more electron-rich bpy-derived ligand (R = OH) gives a more efficient HDO reaction. The addition of base serves to further enhance the HDO reaction by deprotonating the protic OH groups (OH groups on n,n′-dihydroxybipyridine where n = 4 in 2 4OH or 6 in 2 6OH ) resulting in a more electron-rich catalyst. Proximal OH groups in 2 6OH produce our most active catalyst, and we can suggest that a metal−ligand bifunctional mechanism of H 2 activation and/or transfer to the substrate may be responsible for the greater efficiency of 2 6OH vs 2 4OH . The catalyst loading could be reduced to 5 × 10 −5 mol % of 2 6OH with 0.5 mol % Na 2 CO 3 and 997,000 turnovers (TON) could be achieved in 20 h at 100 °C. Furthermore, the same catalyst at 1 × 10 −4 mol % produces 836,000 TON under similar but base-free conditions. Such catalytic efficiency in a dilute aqueous solution is noteworthy for potential applications.

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Proton Responsive and Hydrogen Bonding Ligands in Organometallic Chemistry

Cited by 8 publications

References 390 publications

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

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

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

Iridium Dihydroxybipyridine Complexes are Effective Catalysts for Hydrodeoxygenation of Vanillyl Alcohol in Water

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Proton Responsive and Hydrogen Bonding Ligands in Organometallic Chemistry

Cited by 8 publications

References 390 publications

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

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

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

Iridium Dihydroxybipyridine Complexes are Effective Catalysts for Hydrodeoxygenation of Vanillyl Alcohol in Water

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

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

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