Remarkable accelerating and decelerating effects of the bases on CO2 reduction using a ruthenium NADH model complex

An NAD+-type earth-abundant metal complex enabling photo-driven alcohol oxidation

For the first time, an NAD⁺-type earth-abundant metal complex [Zn(pbn)₂(H₂O)](ClO₄)₂ (1) was found to exhibit photo-induced oxidizing ability to convert various primary and secondary alcohols to the corresponding aldehyde and ketone compounds. In addition, a two-electron-reduced Zn(II) complex [Zn(pbnH-pbnH)(ClO₄)₂] (1red) comprising the novel C-C coupling ligand, obtained by the photo-induced oxidation of alcohols by 1, was successfully isolated and completely characterized. We clarified that the photochemical oxidation of alcohols by 1 to produce 1red proceeds via an electron transfer followed by proton transfer mechanism as elucidated by kinetic analysis on the basis of absorption spectroscopic measurements.

More Than Just a Reagent: The Rise of Renewable Organohydrides for Catalytic Reduction of Carbon Dioxide

Stoichiometric carbon dioxide reduction to highly reduced C1 molecules, such as formic acid (2e^- ), formaldehyde (4e^- ), methanol (6e^- ) or even most-reduced methane (8e^- ), has been successfully achieved by using organosilanes, organoboranes, and frustrated Lewis Pairs (FLPs) in the presence of suitable catalyst. The development of renewable organohydride compounds could be the best alternative in this regard as they have shown promise for the transfer of hydride directly to CO₂ . Reduction of CO₂ by two electrons and two protons to afford formic acid by using renewable organohydride molecules has recently been investigated by various groups. However, catalytic CO₂ reduction to ≥2e^- -reduced products by using renewable organohydride-based molecules has rarely been explored. This Minireview summarizes important findings in this regard, encompassing both stoichiometric and catalytic CO₂ reduction.

Fused 1,5-Naphthyridines: Synthetic Tools and Applications

Heterocyclic nitrogen compounds, including fused 1,5-naphthyridines, have versatile applications in the fields of synthetic organic chemistry and play an important role in the field of medicinal chemistry, as many of them have a wide range of biological activities. In this review, a wide range of synthetic protocols for the construction of this scaffold are presented. For example, Friedländer, Skraup, Semmlere-Wolff, and hetero-Diels-Alder, among others, are well known classical synthetic protocols used for the construction of the main 1,5-naphthyridine scaffold. These syntheses are classified according to the nature of the cycle fused to the 1,5-naphthyridine ring: carbocycles, nitrogen heterocycles, oxygen heterocycles, and sulphur heterocycles. In addition, taking into account the aforementioned versatility of these heterocycles, their reactivity is presented as well as their use as a ligand for metal complexes formation. Finally, those fused 1,5-naphthyridines that present biological activity and optical applications, among others, are indicated.

A Novel Photo-Driven Hydrogenation Reaction of an NAD+-Type Complex Toward Artificial Photosynthesis

The photocatalytic reduction of carbon dioxide (CO₂) to value-added chemicals is an attractive strategy to utilize CO₂ as a feedstock for storing renewable energy, such as solar energy, in chemical bonds. Inspired by the biological function of the nicotinamide adenine dinucleotide redox couple (NAD⁺/NADH), we have been developing transition-metal complexes containing NAD⁺/NADH-functionalized ligands to create electro- and/or photochemically renewable hydride donors for the conversion of CO₂ into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO₂, however, further examples, as well as investigation, of these photo-driven NAD⁺/NADH-type hydrogenation and organic hydride transfer reactions are required not only to explore the mechanism in detail but also to develop a highly efficient catalyst for artificial photosynthesis. In this paper, we report the synthesis, characterization, and photo-induced NAD⁺/NADH conversion properties of a new ruthenium(II) complex, [Ru(bpy)₂(Me-pn)](PF₆)₂ (1), which contains a new NAD⁺-type ligand, Me-pn (2-methyl-6-(pyridin-2-yl)-1,5-naphthyridine). In addition, we have succeeded in the isolation of the corresponding two-electron reduced ruthenium(II) complex containing the NADH-type ligand Me-pnHH (2-methyl-6-(pyridin-2-yl)-1,4-dihydro-1,5-naphthyridine), i.e., [Ru(bpy)₂(Me-pnHH)](PF₆)₂ (1HH), by the photo-induced hydrogenation reaction of 1. Thus, in this study, a new photo-driven NAD⁺/NADH-type hydrogenation reaction for possible CO₂ reduction using the NAD⁺/NADH redox function has been constructed.

Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate

We report a novel metal-free chemical reduction of CO₂ by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble-metal-based hydrides such as [Ru(tpy)(bpy)H]⁺ and [Pt(depe)₂H]⁺. Chemical CO₂ reduction to the formate anion (HCOO^-) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. ¹³CO₂ experiments confirmed the formation of H¹³COO^- by CO₂ reduction with the formate product characterized by ¹H NMR and ¹³C NMR spectroscopy and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium toward the ionic products. After CO₂ reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO₂ reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO₂ by sustainable, recyclable, and metal-free organo-hydrides.

Renewable Hydride Donors for the Catalytic Reduction of CO2: A Thermodynamic and Kinetic Study

Increasing atmospheric CO₂ concentration and dwindling fossil fuel supply necessitate the search for efficient methods for CO₂ conversion to fuels. Assorted studies have shown pyridine and its derivatives capable of (photo)electrochemically reducing CO₂ to methanol, and some mechanistic interpretations have been proposed. Here, we analyze the thermodynamic and kinetic aspects of the efficacy of pyridines as hydride-donating catalytic reagents that transfer hydrides via their dihydropyridinic form. We investigate both the effects of functionalizing pyridinic derivatives with electron-donating and electron-withdrawing groups on hydride-transfer catalyst strength, assessed via their hydricity (thermodynamic ability) and nucleophilicity (kinetic ability), and catalyst recyclability, assessed via their reduction potential. We find that pyridines substituted with electron-donating groups have stronger hydride-donating ability (having lower hydricity and larger nucleophilicity values), but are less efficiently recycled (having more negative reduction potentials). In contrast, pyridines substituted with electron-withdrawing groups are more efficiently recycled, but are weaker hydride donors. Functional group modification favorably tunes hydride strength or efficiency, but not both. We attribute this problematic coupling between the strength and recyclability of pyridinic hydrides to their aromatic nature and suggest several avenues for overcoming this difficulty.

Novel synthesis of a four-electron-reduced ruthenium(ii) NADH-type complex under water-gas-shift reaction conditions

A four-electron-reduced ruthenium(ii) NADH-type complex, [Ru(bbnpH₄)(CO)₂Cl](PF₆) (bbnpH₄ = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(5,10-dihydrobenzo[b][1,5]naphthyridine)), has been successfully synthesized by mixing an NAD⁺-type ligand, bbnp (bbnp = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(benzo[b][1,5]naphthyridine)), and [Ru(CO)₂Cl₂] under moderate water-gas-shift reaction conditions, which has been fully characterized by single-crystal X-ray structure analysis, ESI-TOF mass spectrometry, and NMR spectroscopy.

Thermodynamic and kinetic hydricities of metal-free hydrides

Thermodynamic and kinetic hydricities provide useful guidelines for the design of hydride donors with desirable properties for catalytic chemical reductions.

Base assisted C–C coupling between carbonyl and polypyridyl ligands in a Ru-NADH-type carbonyl complex

Strong organic base assists quantitative conversion of ruthenium(ii)-NADH-type complex to new metallacycle which was further oxidized to Ru-OCO-bridge complex upon reaction with aq. NH₄PF₆ under air.

[2-(2,2′-Bipyridin-6-yl-κ2N1,N1′)benzo[b][1,5]naphthyridine-κN1]dichloridozinc

The coordination environment of the zinc(II) ion in the title complex, [ZnCl2(C22H14N4)], is distorted trigonal–bipyramidal comprised by three N atoms from the 2-([2,2′-bipyridin]-6-yl)benzo[b][1,5]naphthyridine ligand and two Cl−ions. In the crystal, neighbouring molecules are connected by π–π stacking interactions along thea-axis direction.

Four-Electron Reduction of a New Ruthenium Dicarbonyl Complex Having Two NAD Model Ligands through Decarboxylation in Water

Three Ru-CO complexes, [Ru(pbn)₂(CO)₂]²⁺, [Ru(pbn)₂(CO)(COOH)]⁺, [Ru(pbn)₂(CO)(COO)]⁰ [pbn = 2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine], exist as equilibrium mixtures in aqueous solutions. Thermal decarboxylation of [Ru(pbn)₂(CO)(COOH)]⁺ and/or [Ru(pbn)₂(CO)(COO)]⁰ induces a two-electron reduction of pbn to form [Ru(pbn)(pbnHH)(CO)(OH₂)]²⁺ [pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine] in H₂O.

Chloridobis[2-(pyridin-2-yl-κN)benzo[b][1,5]naphthyridine-κN 1]copper(II) perchlorate acetonitrile disolvate

The copper(II) ion in the title complex, [CuCl(C17H11N3)2]ClO4·2CH3CN, is coordinated by four N atoms from two pbn ligands and one Cl− ion in a distorted trigonal–bipyramidal geometry (τ = 0.84). The asymmetric unit comprises half of the cationic complex molecule, and complete molecules are generated by twofold rotation symmetry with the corresponding axis running through the Cu atom and the coordinating Cl atom. The perchlorate anion is also located on a twofold rotation axis (passing through the Cl atom). In the crystal, there are π–π stacking interactions between the benzonaphthyridine rings of the pbn ligand of neighbouring cations.

Dichlorido[2-(pyridin-2-yl-κN)benzo[b][1,5]naphthyridine-κN1]zinc

The coordination environment of the ZnIIatom in the title complex, [ZnCl2(C17H11N3)], is distorted tetrahedral. The NAD+/NADH-analogous ligand is twisted and chelates through one pyridine N atom and one N atom of the benzonaphthyridine ring system. In the crystal, molecules are stacked along theaaxis and are held together through π–π interactions.