Monodentate N-heterocyclic Carbene Ligands Research Articles

Nitron-derivative-based palladium carbene complexes: structural characterization, theoretical calculations, and catalytic applications in the Mizoroki-Heck coupling reaction.

New palladium(0) and palladium(ii) complexes with N-heterocyclic carbene (NHC) ligands derived from nitron and its derivatives were synthesized. The structures of most of these complexes were established by single-crystal X-ray diffraction studies. Among the new complexes, the palladium complex with a monodentate NHC ligand derived from nitron demonstrated the highest efficacy as a catalyst precursor in the Mizoroki-Heck coupling reaction of aryl chlorides with alkenes. Theoretical calculations provide valuable insights into the electronic parameters of both the ligands and the palladium complexes, highlighting the significance of a robust Pd-C bond and the π-accepting property of the NHC ligand in achieving enhanced catalytic activity. Notably, catalyst activation occurred much more rapidly with the preformed palladium(0) complex compared to its palladium(ii) counterpart.

Designing successful monodentate N-heterocyclic carbene ligands for asymmetric metal catalysis.

Chiral ligands are of particular importance in asymmetric transition-metal catalysis. Although the development of effective chiral monodentate N-heterocyclic carbenes (NHCs) has been slow, a growing amount of papers have been published in recent years showing their unique efficiency as chiral ancillary ligands. Herein we provide an overview of NHC structures that accomplish high levels of enantioselectivity (≥90% ee) and give guidelines to their use and thoughts on the future of this field.

Amphiphilic polymeric nanoreactors containing Rh(i)–NHC complexes for the aqueous biphasic hydrogenation of alkenes

A rhodium(i) complex bearing a monodentate N-heterocyclic carbene ligand has been confined into the core of amphiphilic core-crosslinked micelles (CCMs).

Raman spectral characterization of silver metal-based complexes of different benzimidazolium ligands.

In this study, Raman spectroscopy has been employed for the characterization of two structurally different monodentate N-heterocyclic carbene ligands (ligand-1 and ligand-2) and their respective complexes (complex-1 and complex-2). The Raman spectral features are found helpful for the confirmation of formation of complexes. The significant Raman spectral features are identified for benzimidazole ring with higher intensities in carbene complexes having more polarizability as compared to their ligands, providing the evidence for the formation of coordinate covalent bond. The successful complexation is further supported by using multivariate data analysis technique, Principal Component Analysis (PCA), which is found very helpful to highlight the variability of Raman spectral data of both ligands and their respective metal complexes from each other. Moreover, the coordination of carbene with Ag(I) is confirmed from the dominant spectral markers of higher intensities at 359cm-1 in complex-1 and 338cm-1 in complex-2. The effective and reliable characterization and confirmation of metal complexes indicates the potential of Raman spectroscopy for its use for the characterization of the organometallic complexes and other chemical products.

C-X (X = N, O) Cross-Coupling Reactions Catalyzed by Copper-Pincer Bis(N-Heterocyclic Carbene) Complexes.

Over the last two decades, N-heterocyclic carbene (NHC)–copper catalysts have received considerable attention in organic synthesis. Despite the popularity of copper complexes containing monodentate NHC ligands and recent development of poly(NHC) platforms, their application in C–C and C–heteroatom cross-coupling reactions has been limited. Recently, we reported an air-assisted Sonogashira-type cross-coupling catalyzed by well-defined cationic copper-pincer bis(NHC) complexes. Herein, we report the application of these complexes in Ullmann-type C–X (X = N, O) coupling of azoles and phenols with aryl halides in a relatively short reaction time. In contrast to other well-defined copper(I) catalysts that require an inert atmosphere for an efficient C–X coupling, the employed Cu(I)-pincer bis(NHC) complexes provide good to excellent yields in air. The air-assisted reactivity, unlike that in the Sonogashira reaction, is also affected by the base employed and the reaction time. With Cs2CO3 and K2CO3, the oxygen-generated catalyst is more reactive than the catalyst formed under argon in a short reaction time (12 h). However, the difference in reactivity is compromised after a 24 h reaction with K2CO3. The efficient pincer Cu-NHC/O2/Cs2CO3 system provides great to excellent cross-coupling yields for electronically diverse aryl iodides and imidazole derivatives. The catalyst scope is controlled by a balance between nucleophilicity, coordinating ability, and the steric hindrance of aryl halides and N-/O-nucleophiles.

Enhancements in catalytic reactivity and selectivity of homobimetallic complexes containing heteroditopic ligands.

Rh(i) and Ir(i) homobimetallic complexes were synthesised using a heteroditopic ligand system on a xanthene scaffold containing a monodentate N-heterocyclic carbene ligand and a bidentate bis(pyrazol-1-yl)methane ligand. The complexes were tested as catalysts for the two-step dihydroalkoxylation and two-step hydroamination/hydrosilylation reactions. This is the first known report of an organometallic group 9 complex, Ir(i) bimetallic complex, 13, to selectively favour the opposite spirocyclisation product from that reported in the literature, 14cvs.14b. The Ir(i) homobimetallic complex catalyses the intramolecular hydroamination reaction of alkynamines efficiently and proved to be a highly active catalyst for promoting the subsequent hydrosilylation of the pyrrolines; completing the hydrosilylation reactions in less than 40 seconds. A chloro-bridged bimetallic species was observed in the solid state, revealing that the COD co-ligands present underwent an oxidation.

ChemInform Abstract: Atmospheric Hydrogenation of Esters Catalyzed by PNP‐Ruthenium Complexes with an N‐Heterocyclic Carbene Ligand.

New pincer ruthenium complexes bearing a monodentate N-heterocyclic carbene ligand were synthesized and demonstrated as powerful hydrogenation catalysts. With an atmospheric pressure of hydrogen gas, aromatic, heteroaromatic, and aliphatic esters as well as lactones were converted into the corresponding alcohols at 50 °C. This reaction protocol offers reliable access to alcohols using an easy operational setup.

Atmospheric Hydrogenation of Esters Catalyzed by PNP-Ruthenium Complexes with an N-Heterocyclic Carbene Ligand.

New pincer ruthenium complexes bearing a monodentate N-heterocyclic carbene ligand were synthesized and demonstrated as powerful hydrogenation catalysts. With an atmospheric pressure of hydrogen gas, aromatic, heteroaromatic, and aliphatic esters as well as lactones were converted into the corresponding alcohols at 50 °C. This reaction protocol offers reliable access to alcohols using an easy operational setup.

Phosphorescence or Thermally Activated Delayed Fluorescence? Intersystem Crossing and Radiative Rate Constants of a Three-Coordinate Copper(I) Complex Determined by Quantum-Chemical Methods.

The photophysical properties of a cationic three-coordinate copper(I) complex with a monodentate N-heterocyclic carbene ligand and a bidentate phenanthroline ligand have been investigated by employing computational chemistry methods. The absorption spectrum, calculated with the combined density functional theory and multireference configuration interaction method, matches experimentally available data perfectly, thus corroborating the validity of our applied theoretical approach. On the basis of our calculated singlet-triplet gap of 650 cm(-1) and the (reverse) intersystem crossing rates that are both larger than the fluorescence and phosphorescence rates at room temperature, we conclude that thermally activated delayed fluorescence should be observable for this complex in addition to phosphorescence. Torsion of the ligands has only a small impact on the singlet-triplet gap. However, the electronic coupling between the S1 and T1 states-and hence the probability for (reverse) intersystem crossing-is seen to increase substantially when moving from a coplanar to a perpendicular arrangement of the ligands.

Synthesis and Structure of a CuI3S Cluster Unsupported by Other Bridging Ligands

The facile synthesis of a {LCuI}3(μ3-S) cluster supported by monodentate N-heterocyclic carbene ligands has been accomplished through an approach in which two-coordinate (NHC)Cu+ centers are installed sequentially on a sulfido ligand. The reaction between (IPr)CuCl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and S(SiMe3)2 yields (IPr)Cu(SSiMe3) (2). Treatment of 2 with [(IPr)Cu(NCMe)][BF4] produces the dicopper cluster [{(IPr)Cu}2(μ-SSiMe3)][BF4] (3[BF4]), which undergoes subsequent reaction with (IPr)CuF to afford [{(IPr)Cu}3(μ3-S)][BF4] (1[BF4]) in high yield. The X-ray crystal structure of 1[BF4] establishes it as a rare example of a CuIn(μn-S)Lm cluster, and the first that is not also supported by other bridging ligands.

High-Spin Iron(II) Alkynyl Complexes with N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Reactivity Study

High-spin iron(II) alkynyl complexes [(IPr2Me2)2Fe(C≡CBut)2] (1) and [(IPr2Me2)2Fe(C≡CR)(NHMes)] (R = But 2, SiMe3 3) bearing a monodentate N-heterocyclic carbene ligand IPr2Me2 (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) have been prepared by salt metathesis and/or amine elimination methods and characterized by various spectroscopic methods. Complex 1 reacts with PMe3 (4 equiv) and ButNC (4 equiv) to form trans-[(PMe3)4Fe(C≡CBut)2] (4) and [η3-{(ButC≡C)(But)CC(IPr2Me2)C(NBut)}Fe(NCBut)3] (5), respectively. In contrast, the reactions of 1 with 4-Pri-C6H4NCO and PriNCNPri lead to the formation of the zwitterionic salts 4-Pri-C6H4NC(O)(IPr2Me2) and (PriN)2C(IPr2Me2), respectively. The interaction of 1 with I2 gives ButC≡CC≡CBut and (IPr2Me2)2FeI2. The C(sp)–C(sp3) cross-coupling products n-C8H17C≡CBut and c-C6H11C≡CBut are formed in high yields when 1 is treated with the corresponding alkyl halides n-C8H17X and c-C6H11X (X = Br, Cl). The formation of the ring-opening product 7,7-dimethyloct-1-en-5-yne ...

Palladium–N‐Heterocyclic Carbene (NHC)‐Catalyzed Asymmetric Synthesis of Indolines through Regiodivergent C(sp3)H Activation: Scope and DFT Study

Two bulky, chiral, monodentate N-heterocyclic carbene ligands were applied to palladium-catalyzed asymmetric C-H arylation to incorporate C(sp(3))-H bond activation. Racemic mixtures of the carbamate starting materials underwent regiodivergent reactions to afford different trans-2,3-substituted indolines. Although this CAr-Calkyl coupling requires high temperatures (140-160 °C), chiral induction is high. This regiodivergent reaction, when carried out with enantiopure starting materials, can lead to single structurally different enantiopure products, depending on the catalyst chirality. The C-H activation at a tertiary center was realized only in the case of a cyclopropyl group. No C-H activation takes place alpha to a tertiary center. A detailed DFT study is included and analyses of methyl versus methylene versus methine C-H activation is used to rationalize experimentally observed regio- and enantioselectivities.

Variations on an NHC Theme: Which Features Enhance Catalytic Transfer Hydrogenation with Ruthenium Complexes?

N-heterocyclic carbene (NHC) based ruthenium complexes were studied as catalysts for the transfer hydrogenation of ketones. Variations in the catalyst structure were investigated for their impact on hydrogenation and catalyst stability. Catalyst attributes included bis- or mono-NHC ligands, pendant ether groups in some cases, and arene ligands of varied bulk and donor strength. Ruthenium complexes were synthesized and fully characterized, including complexes with a monodentate NHC ligand containing a tethered ether N substituent (ImEt,CH2CH2OEtRuCl2(η6-arene); arene = benzene (4), p-cymene (5), hexamethylbenzene (6)), a complex with a monodentate NHC ligand with solely alkyl N substituents (ImEt,PentylRuCl2(η6-p-cymene) (8)), and a complex with a bis-NHC ligand ([RuCl(methylenebis(ImEt)2)(η6-p-cymene)]PF6 (7)) (Im = imidazole-derived NHC; superscripts indicate N substituents). X-ray crystal structures were obtained for 4, 5, 7, and 8. All of the ruthenium complexes were tested and found to be active trans...

ChemInform Abstract: New Carbon‐ and Sulfur‐Based Ligands in Catalysis

AbstractReview: 71 refs.

New Carbon- and Sulfur-based Ligands in Catalysis

Homogeneous catalysis is a field of research that has gained central importance in both organic and inorganic chemistry and the use of well-defined ligand systems in the synthesis of transition metal complexes has had an enormous impact on the development of such catalysts. Neutral, two-electron donor ligands based on phosphorous and nitrogen have been tremendously successful as ancillary entities for late-transition metal (LTM) catalysts, whereas ligands based on anionic nitrogen, oxygen and the cyclopentadienyl motif (Cp(-)) have propelled early-transition metal (ETM) catalysis forward. We believe that expanding the ligand families capable of acting as successful entities in metal-mediated reactivity and catalysis is crucial for future discoveries in this field. Research in our group therefore tries to identify new non-chiral and chiral ligands for late-transition metal chemistry that are based on neutral, two-electron carbon and sulfur donor atoms. In particular, we have until now focused on the development of modular, monodentate N-heterocyclic carbene ligands (NHCs) that can serve as a basis for the development of chiral ligand frameworks for the application to asymmetric catalytic transformations. In the second major research project developed over the last six years, we have started an investigation on the use of chelating sulfoxide-based ligands in asymmetric late transition-metal based catalysis.

Nickel N‐heterocyclic carbene complexes in the vinyl polymerization of norbornene

AbstractVinyl polymerized norbornene has some useful properties such as good mechanical strength, optical transparency and heat resistance. Several transition metal complexes have been described in the literature as active catalysts for the vinyl polymerization of norbornene. We now report the use of three types of nickel(II) complexes with N‐heterocyclic carbene (NHC) ligands in the catalytic vinyl polymerization of norbornene under a range of conditions. Specifically, two nickel complexes bearing a chelating bis(NHC) ligand, two nickel complexes bearing two chelating anionic N‐donor functionalized NHC ligands as well as one diiodidonickel(II) complex with two monodentate NHC ligands were tested. The solid‐state structure of bis(1,3‐dimethylimidazol‐2‐ylidene)diiodidonickel(II), as determined by X‐ray crystallography, is presented. The highest polymerization activity of 2.6 × 107 g (mol cat)−1 h−1 was observed using the latter nickel complex as catalyst, activated by methylaluminoxane. The norbornene polymers thus obtained are of high molecular weight but with rather low polydispersity. Copyright © 2010 John Wiley & Sons, Ltd.

Chemoenzymatic Dynamic Kinetic Resolution of Axially Chiral Allenes

At the palladium: Dimeric palladium bromide complexes bearing monodentate N-heterocyclic carbene ligands have been identified as efficient catalysts for the chemoselective racemization of axially chiral allenyl alcohols. In combination with porcine pancreatic lipase as biocatalyst, a dynamic kinetic resolution has been developed, giving access to optically active allenes in good yield and high enantiomeric purity (see scheme).

Matching the Chirality of Monodentate N-Heterocyclic Carbene Ligands: A Case Study on Well-Defined Palladium Complexes for the Asymmetric α-Arylation of Amides

N-Heterocyclic carbene ligands derived from C(2)-symmetric diamines with naphthyl side chains are introduced as chiral monodentate ligands, and their palladium complexes (NHC)Pd(cin)Cl are prepared. These compounds exist as a mixture of diastereomers, and the palladium complexes can be successfully separated and their absolute stereochemistry assigned. When used in the asymmetric intramolecular alpha-arylation of amides, oxindoles with quaternary carbon centers can be obtained in high yield and selectivity when correctly matching the chirality of the NHC complexes.