Synthesis of Proline-Based N-Heterocyclic Carbene Ligands

Polymer Backbone Chemistry Shapes the Alkaline Stability of Metallopolymer Anion-Exchange Membranes

Anion-exchange membrane fuel cells and water electrolyzers have garnered significant attention in past years due to their potential role in sustainable and affordable energy conversion and storage. However, the chemical stability of the polymeric anion-exchange membranes (AEMs), the key component in these devices, currently limits their lifespan. Recently, metallopolymers have been proposed as chemically stable alternatives to organic cations, using metal centers as ion transporters. In metallopolymer AEMs, various properties such as alkaline stability, water uptake, flexibility, and performance, are determined by both the metal complex and polymer backbone. Herein we present a systematic study investigating the influence of the polymer backbone chemistry on some of these properties, focusing on the alkaline stability of low-oxophilicity gold metallopolymers. Despite the use of a common N-heterocyclic carbene ligand, upon gold metalation using the same reaction conditions, different polymer backbones end up forming different gold complexes. These findings suggest that polymer chemistry affects the metalation reaction in addition to the other properties relevant to AEM performance.

A Mild Silica Gel Promoted Synthesis and Initial Functional Study of Tetrapyridyl Tetrahydropyrrolopyrrolones

A one-pot strategy with yields up to 82% was reported to generate 2-(pyridin-2-yl)-2-(3,3a,6-tris(5-pyridin-2-yl)-5-oxohexahydropyrrolo[3,2-b] pyrrol-2(1H)-ylidene)acetonitrile 1a and its derivatives 1b-d. Silica gel promoted quantitative conversion from stable intermediate to 1a within 30 min at room temperature. Finally, four chemical σ bonds and two chiral carbons with high diastereoselectivity were achieved. Compound 1a can act as a novel high selective UV-vis and fluorescence "turn-on" probe for Zn²⁺ and Cd²⁺, respond to proton, and show dual-state emission (DSE) characteristics.

Alkaline Stability of Low Oxophilicity Metallopolymer Anion-Exchange Membranes

Anion-exchange membrane fuel cells (AEMFCs) are promising energy conversion devices due to their high efficiency and relatively low cost. Nonetheless, AEMFC operation time is currently limited by the low chemical stability of their polymeric anion-exchange membranes. In recent years, metallopolymers, where the metal centers assume the ion transport function, have been proposed as a chemically stable alternative. Here we present a systematic study using a polymer backbone with side-chain N-heterocyclic carbene (NHC) ligands complexed to various metals with low oxophilicity, such as copper, zinc, nickel, and gold. The golden metallopolymer, using the metal with the lowest oxophilicity, demonstrates exceptional alkaline stability, far superior to state-of-the-art quaternary ammonium cations, as well as good in-situ AEMFC results. These results demonstrate that judiciously designed metallopolymers may be superior to purely organic membranes and provides a scientific base for further developments in the field.

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts*

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the ⁷⁷ Se liquid state NMR of Se-NHC adducts. We demonstrate that ⁷⁷ Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via ⁷⁷ Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state ⁷⁷ Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the ¹³ C isotropic chemical shift from the liquid state NMR and the ¹³ C tensor components are also discussed, and compared with their ⁷⁷ Se counterparts. ⁷⁷ Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state ⁷⁷ Se NMR can provide an in-depth outlook on the properties of NHC ligands.

Synthesis, characterization, and coordination chemistry of a phenanthridine-containing N-heterocyclic carbene ligand

An N-heterocyclic carbene ligand precursor bearing a π-extended phenanthridine (3,4-benzoquinoline) unit is presented. The proligand was isolated as the imidazolium salt of chloride (1•HCl), bromide (1•HBr), or hexafluorophosphate (1•HPF6) counterions. These salts can be deprotonated and the carbene installed on silver centres using Ag2O as both a base and a source of metal ion. The resulting Ag(I) complex (1)AgCl can be used in a transmetalation reaction to generate a Pd(II) coordination complex (1)Pd(CH3CN)Cl2. The characterization and photophysical properties of these complexes is presented, along with a demonstration of the utility of (1)Pd(CH3CN)Cl2 in mediating a catalytic C-N cross-coupling reaction for the preparation of the pharmaceutical Piribedil.

Highly Active Manganese-Based CO2 Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO₂ to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity and required very negative potentials. We report herein that complex fac-[Mn^I(bis-^MesNHC)(CO)₃Br] [1; bis-^MesNHC = 3,3-bis(2,4,6-trimethylphenyl)-(1,1'-diimidazolin-2,2'-diylidene)methane] could catalyze the electrochemical reduction of CO₂ to CO with high activity (TOF_max = 3180 ± 6 s^-1) at a less negative potential. Due to the introduction of the bulky Mes groups, a one-electron-reduced intermediate {[Mn⁰(bis-^MesNHC)(CO)₃]⁰ (2^•)} was isolated as a packed "dimer" and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct reaction between doubly reduced intermediate fac-[Mn(bis-^MesNHC)(CO)₃]^- and CO₂; the tetracarbonyl Mn complex [Mn⁺(bis-^MesNHC)(CO)₄]⁺ ([2-CO]⁺) was captured, and its further reduction proposed as the rate-limiting step.

On the interaction of N-heterocyclic carbene Ir+I complexes with His and Cys containing peptides

In the interaction of an [Ir(+i)(COD)(NHC)Cl] complex with model peptides a chelating motif with a particularly interesting bimetallic peptide-bridged Ir(+iii)–NHC motif was identified with loss of the COD and Cl ligands and oxidation of the metal.

Carbohydrate-based N-heterocyclic carbenes for enantioselective catalysis

Versatile syntheses of C2-linked and C₂-symmetric carbohydrate-based NHC·HCls from functionalised amino-carbohydrate derivatives are reported. The corresponding Rh complexes were evaluated in asymmetric hydrosilylation of ketones.