NHC Complexes Research Articles

Acceptorless Dehydrogenation of Primary and Secondary Alcohols Catalysed by Phosphine‐free C, C Chelated Ir (III) NHC Complexes

AbstractHerein, an efficient, atom‐economic, environmentally benign catalytic protocol has been developed for synthesising ketones and aldehydes by acceptorless dehydrogenation of alcohols. The protocol employs C, C chelated Ir (III) NHC complexes to promote acceptorless dehydrogenation of both secondary and primary alcohols. Notably, oxidation of primary alcohol selectively yields the corresponding aldehyde where forming ester byproducts is possible. Using C, C chelated Ir (III) NHC complexes (0.1 mol%), and a catalytic amount of base tBuONa (5 mol%), several aliphatic and aromatic aldehydes and ketones including more challenging carbonyl compounds bearing heterocyclic rings were obtained in moderate to high yields. Moreover, the protocol is also efficient for synthesising industrially important molecules like heliotropin and 3,4,5‐trimethoxy acetophenone in moderate yields. Remarkably, the catalytic system allows the straightforward synthesis of the potentially bio‐active compound cholest‐4‐en‐3‐one through acceptorless dehydrogenation followed by double bond isomerisation under the reaction conditions. Low catalyst loading, mild reaction conditions, high selectivity, short reaction time, and broad‐substrate scopes are a few interesting characteristics of the catalytic protocol described herein.

Zwitterionic Cobalt(I)–NHC Complexes with Tetraphenylborate Ligation: Synthesis, Characterization, and Reactivity

Zwitterionic Cobalt(I)–NHC Complexes with Tetraphenylborate Ligation: Synthesis, Characterization, and Reactivity

Intramolecular Energy Transfer Competing with Light-Driven Intermolecular Proton Transfer in an Iron(II)-NHC Complex? A Query into the Role of Photobasic Ligands and MLCT States.

Inorganic photoacids and photobases comprising of photoactive transition metal complexes (TMCs) offer the ability to modulate proton transfer reactions through light irradiation, while utilizing the excellent optical properties of the latter. This provides a powerful tool for precise control over chemical reactions and processes, with implications for both fundamental science and practical applications. In this contribution, we present a novel molecular architecture amending an Fe-NHC complex with a pendant quinoline, as a prototypical photobase, as a representative earth-abundant TMC based inorganic photobase. We characterize the excited-state properties and proton-transfer dynamics using steady-state absorption and emission spectroscopy as well as pump wavelength dependent transient absorption spectroscopy in various protic solvents. The kinetics and thermodynamics of proton transfer in the quinoline moiety are influenced by both the presence of the metal center and the choice of the solvent. Furthermore, we see indications of intramolecular energy transfer from the quinoline to the MLCT state as a limiting factor for panchromatic photobasicity of the complex.

Synthesis, Molecular Docking, Anti-cholinesterase Activity, Theoretical Investigation, and Catalytic Effect of New Encumbered N-benzyladamantyl Substituted Imidazolidin-2-ylidene Carbene Pd-PEPPSI Complexes

Abstract: This study aimed to describe the preparation of novel PEPPSI type Pd(II)-NHC complexes bearing N-benzyladamantyl substituted imidazolidin-2-ylidene group. All synthesized compounds were characterized by using 1H-NMR and 13C-NMR spectroscopies, FTIR, and elemental analysis techniques. One of the objectives of this study was the synthesis of Pd-NHC complexes with AChE/BChE inhibition activities. Among all the tested compounds, complexes 4b and 4c were found to have the most high potential AChE and BChE inhibitory activities with IC50 values of 21.57 ± 0.23 Mm and 15.78 ± 0.39 Mm, respectively. Conducting molecular docking studies helped us in gathering crucial information about the main binding interactions of inhibitors and enzymes, and the results were in agreement with the biological evaluation. The synthesized Pd-NHC complexes were employed for catalyzing the direct C2- and C5-arylation reaction between aryl (hetero) halide and a variety of heterocyclic systems. In both cases (C2 and C5-arylation), Pd-NHC complexes catalysts provided access to the arylated heterocycles in good to high yields in the presence of 1 mol% catalyst loading at 150°C. The DFT theoretical investigation showed that the Pd-NHC complexes were of ML2X2 type, where the the Pd(II) cation had a square planar geometry. The interaction energies obtained by energy decomposition analysis (EDA) demonstrated that the 4d and 4e complexes were more stable in the presence of more methyl substituents. The chemical indicators demonstrated that the less stable 4c complex was more reactive in regard to the chemical hardness, chemical potential, and electrophilicity values.

Sythesis, characterisation, anticancer and antimicrobial activity of Ag-N-heterocyclic carbene complexes containing benzimidazole derivatives

To prepare a novel series of silver(I) complexes, the interaction of benzimidazolium salts having their two nitrogen atoms substituted by bulky groups with Ag2O in DCM has been carried out. Their structures were characterized by elemental analyses, 1H NMR, 13C NMR, elemental analysis, and IR spectroscopy techniques. Further, the antimicrobial and antifungal activities properties of both the salts and their silver(I)–NHC complexes were tested against positive and negative bacteria using the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST) respectively. The results show that silver complexes are effective against E. coli, P. aeruginosa and S. aureus bacterial species with moderate to high activity, and their minimum inhibitory concentrations ranging from 25 to 400 µg/Ml. It was also observed that 3b compound, which has the best antifungal properties among the synthesized compounds, showed high antimicrobial activity when all bacterial species were taken into account. In addition, the salts and complexes were then evaluated for their ability to inhibit the proliferation of four different cancer cell lines (A549, MCF-7, HCT116, SH-SY5Y and BEAS-2B cells lines). AlamarBlue assay demonstrated that the salts and complexes exhibited potent cytotoxicity towards all cancer lines. 2c compound showed the highest cytotoxic activity against BEAS-2B, A549 and MCF-7 cell lines. Except for 2c, compounds 3d, 3e and 3f showed cytotoxic activity at the lowest concentrations against MCF7 cell line.The study highlights the importance of N-heterocyclic carbene ligands in the development of silver-based drugs with anticancer properties.

Continuous Flow Synthesis of Group 11 Metal‐NHC Complexes [M(IPr)(OC(H)(CF3)2)] (with M=Cu, Ag and Au)

AbstractThe weak base method in continuous flow is currently one of the most sustainable and attractive approaches towards the synthesis of transition metal N‐heterocyclic carbene (NHC) complexes. Herein we broaden the scope of this method by using it to produce [M(IPr)(OC(H)(CF3)2)] complexes, including copper, silver, and gold. These compounds bear the alkoxide ligand within the coordination sphere of the catalysts, which can open up new ways to benefit from the unique properties offered by HFIP (1,1,1,3,3,3‐hexafluoroisopropanol) in catalysis, while minimizing the needed amount.

Synthesis and Anticancer Activities of some Ag(I) Complexes Involving N‐Heterocyclic Carbene Ligands

AbstractA new study focused on the synthesis of silver(I)‐N‐heterocyclic carbene complexes. The mono‐Ag(I)‐NHC complexes were successfully synthesized in high yield using a direct complexation method, which involved an excess of Ag2O and imidazolium salts (R= butyl, methylacetate) in dichloromethane. The bis‐Ag(I)‐NHC complexes, on the other hand, were synthesized using a 1 : 2 ratio of Ag2O and imidazolium salts. The ligands and resulting complexes were thoroughly characterized using analytical techniques such as FT‐IR, NMR spectroscopy, LC–MS and elemental analysis. Subsequently, the synthesized complexes were tested against MCF7 breast cancer cells. The results of the study highlighted the remarkable anticancer potential exhibited by the Ag(I)‐NHC complexes against breast cancer cells. Interestingly, it was observed that the mono Ag(I) NHC complexes demonstrated greater potency, with IC50 values ranging from 5 to 13.5 μg/mL, when compared to their bis‐Ag(I)‐NHC counterparts.

Unlocking the potential of water-soluble gold(i)-NHC complexes: unveiling the role of carboxylic acid in cycloisomerization of alkynyl amino acid derivatives.

Hydrocarboxylation of alkyne-containing amino acid derivatives using water-soluble gold(i)-NHC complexes in an aqueous biphasic system at room temperature is described. Addition of silver salts is not required as the carboxylic acid group of the substrate is responsible for the activation of the gold catalyst at room temperature. Our results confirm that the steric bulk of the N-heterocyclic carbene ligands is an important factor in both the stability and the catalytic activity of gold(i) complexes in aqueous medium, and consequently in the recycling (at least 15 times without any loss of activity). The catalytic activity of our most active water-soluble gold(i)-NHC complex has been demonstrated in the cycloisomerization of amino acids derivatives with terminal and internal alkynes in aqueous media at room temperature.

Comprehensive enhancement in electrocatalytic oxygen evolution performance of nickel and cobalt complexes derived from π-conjugated N-heterocyclic carbene ligands through carbon composite strategy

Tailored NHC ligands featuring π-conjugated appendants along with the establishment of durable π–π stacking interactions between the NHC complexes (Ni/Co) and graphitic carbon substrates to boost the oxygen evolution performance are studied.

SYNTHESIS, CHARACTERIZATION OF NEW ASYMMETRICALLY IMIDAZOLIUM SALTS WITH THEIR Ag(I) AND Pd(II)-N-HETEROCYCLIC CARBENE COMPLEXES

New substituted asymmetrically imidazolium salts with Ag(I) and Pd(II) N-heterocyclic carbene complexes were synthesized and characterized. In order to produce asymmetrically substituted salts, imidazolium salts have been produced using a variety of substituents, including aliphatic chain and phenyl acetamide chloride (5,6,7). Using the carbene site deprotonation process, silver(I)-NHC complexes (8,9,10) were produced from the reaction of asymmetrically substituted imidazolium salts with Ag2O. The appropriate Pd(II)-NHC (11,12,13) were then synthesized using the transmetallation process using Ag(I)-complexes. NHC as transfer reagents. The complexes were identified using spectroscopy methods such as 1H-, 13C-NMR and FT-IR spectroscopy. The recorded spectra of the prepared complexes and their salts agree with theoretical ones.

Not so inert mer-tris-chelate cobalt(III) complex of a hydroxy-pyridine functionalized NHC ligand for cyclic carbonate synthesis.

The homoleptic hydroxy-pyridine functionalized Co(III)-NHC complex (2) demonstrates extraordinary catalytic activity towards the CO2 cycloaddition under mild conditions. Using this catalyst and TBAB, the highest TON (666 667) and TOF (52 713 h-1) were achieved compared to previously reported cobalt catalysts.

Unravelling the kinetics of electro- and photochemical S → O linkage isomerization in Ru(II)-NHC-DMSO complexes utilised for photoinduced substitution reactions.

Our recently reported Ru(III)-NHC complexes 1a and 1b were utilized as suitable precursors to prepare new Ru(II)-NHC-(DMSO)2 complexes 2a and 2b. Complexes 2a and 2b reacted with 2,2'-bipyridine to give complexes 3a and 3b, respectively, with substitution of only one of the DMSO ligands. All new complexes were characterized using various spectroscopic techniques and the molecular structures of 2a and 3a were determined using single-crystal X-ray diffraction technique. Complexes 2a, 2b, 3a, and 3b showed the S → O linkage isomerization of the DMSO ligand upon oxidation of the Ru centre from +II to +III, as confirmed by the thermodynamic and kinetic data obtained from cyclic voltammetry experiments. It was observed that in the bisdimethylsulfoxide complexes 2a and 2b, only one DMSO ligand isomerized, which was further corroborated by the computational studies performed to optimize the geometry of the possible linkage isomers of complexes 2a and 3a in +2 and +3 oxidation states, whereas complexes 3a and 3b showed a high preference for the O-bound isomer in the Ru(III) redox state. The role of NHC in stabilizing the mixed isomer in complexes 2a and 2b and preventing the isomerization of both DMSO ligands coordinated to the Ru centre was studied; moreover, NHC provided good solvent compatibility for photochemical S → O isomerization in all the complexes. Taking advantages of the photoinduced linkage isomerization in 2a and 2b, the synthesis of 3a and 3b was revisited and performed using 2a and 2b, respectively, following a photoinduced substitution reaction in the presence of 2,2'-bipyridine. The kinetics of the reversion from the O-bound to S-bound isomer was found to follow the DMSO-assisted intermolecular S → O isomerization pathway.

Selective synthesis of the missing tiara-like Ni10, Ni5 and Ni6 thiolates by the C-S bond cleavage of bis(thioether) molecules with a DFT study.

A series of new bis(phenylthioether) and bis(benzylthioether) compounds (L1-L5) having hexahydropyrimidine, imidazolidine and dihydroperimidine backbones were synthesized. Instead of giving NHC complexes, these compounds undergo facile C-S bond cleavages upon treatment with Ni(II) salts to selectively give new toroidal Ni(II) thiolates: [Ni10(SPh)20] (1) and [Ni5(SCH2Ph)10] (5), and the known [Ni6(SCH2Ph)12] (8), as confirmed by single crystal X-ray diffraction studies. By-products such as RSSR (R = Ph and CH2Ph) and partially C-S bond cleaved starting compounds were isolated or detected as well from these reactions. The C-S bond cleavage does not take place with L5 having the dihydroperimidine backbone and a plausible mechanism is proposed based on by-products isolated or detected. DFT calculations give insights into the electronic structures of these ring systems and the nature of bonding with which a dichloromethane is present inside the cavity of 1. Experimentally observed absorption spectra of 1, 5 and 8 match with the calculated spectra.

Chan−Evans−Lam coupling for the synthesis of N–aryl derivatives catalyzed by copper(I) chloride and sterically varied imidazolium salts at mild reaction conditions

First-row transition–metal–catalyzed organic transformations have received colossal interest in the modern synthetic organic chemistry due to their step– and atom–economic approaches. As a consequence, direct arylation of azoles through C–H or N–H activations acquired a broad spectrum of applications for accessing valuable small organic molecules that are of pharmaceutical interest. In this connection, sterically varied coumarin–functionalized imidazolium salts bearing bromide (5–7) and hexafluorophosphate anions (8–10) have been prepared and used as NHC precursors. The salts have been characterized by both, spectroscopic and analytical techniques. A hexafluorophosphate salt 9 was also studied for its structure using the single-crystal XRD technique. The salts that indeed, generate carbene that trap Cu(I) to form Cu(I)–NHC complexes in situ for the Chan−Evans−Lam C–N coupling of 1H–azoles/anilines and phenyl/thiopheneboronic acids, resulting in the formation of N–aryl derivatives in moderate to excellent yields. The structure-activity relationship studies revealed that the steric bulk around the metal center would help increase the catalytic Chan−Evans−Lam C–N coupling potential of the catalysts.

Formation of CNHC∧Calkyl and CNHC∧Caryl κ2-Chelate Complexes through Competitive sp3- and sp2-CH Activations: An Experimental and Computational Study.

Depending on the chelate ring present, cyclometalated complexes are useful catalysts for various reactions. The reactivity of IrIII and RhIII NHC complexes bearing aliphatic or aromatic N,N'-substituents and thus featuring various metalation sites toward cyclometalation has been investigated. The RhIII complex bearing an N-mesityl-N'-benzyl-NHC does not participate in any cyclometalation, while the IrIII complex reacts under metalation of an ortho-methyl group of the Mes substituent to give complex [3] with a six-membered chelate ring. The RhIII and IrIII complexes bearing an N-o-tolyl,N-benzyl-NHC undergo sp2-CH activation to yield the cyclometalated complexes [4] and [5] featuring a five-membered CNHC∧C chelate ring. Density functional theory (DFT) studies corroborated the experimental findings.

Synthesis methods, characterizations and usage areas of medicinal compounds from THP, and their Ag(I)-NHC complexes, and their antimicrobial efficiencies

Natural or synthetic substances can be used to create smart medications, which can enhance cognitive performance in healthy individuals. They are frequently used to boost memory, concentration, creativity, intelligence, and motivation in the fiercely competitive world of today.To achieve optimum results, the drug should be applied to the target site at the appropriate concentration, with as few or no adverse effects as possible due to off-target actions. Recent studies have shown that such drugs, which are often used in chemotherapy, can reduce the level of discomfort cancer patients endure. In this study, metal complexes that can carry silver nanoparticles with electrical and optical properties were formed. This article examines the potential of man-made silver N-heterocyclic complexes as smart drugs. Following the synthesis of new carbene species from the Xthantine compound, metal complexes were produced for this use. The chemical structures of these carbenes and metal complexes were investigated using a variety of methods, including melting point studies, conductivity, 1H-NMR and 13C-NMR, LC-Mass, FT-IR, TGA, and UV-vis spectrophotometry. These metal complexes differ mostly due to their solubility in water. Using the disk diffusion method, the antimicrobial and antibacterial properties of silver(I)-NHC complexes were examined against Gram-positive and Gram-negative bacteria as well as fungi. It has been observed that the antimicrobial activity of 7,9-bis(2-cyanoethyl)-1,3-dimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purine-9-ium silver(I)bromide compound is high. These data suggest that this compound has strong antibacterial properties.

New sugar-incorporated N-heterocyclic carbene precursors and their Ag(I) and Pd(II) complexes: Synthesis, characterization, and cytotoxicity studies

In the present work, a new series of sugar-incorporated N-heterocyclic carbene precursors namely 1-(3,4,5-trimethoxybenzyl)-3-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)-ethyl]imidazolium bromide (7a), 4-methyl-1-(3,4,5-trimethoxybenzyl)-3-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)-ethyl]imidazolium bromide (7b), 4,5-diphenyl-1-(3,4,5-trimethoxybenzyl)-3-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)-ethyl]imidazolium bromide (7c), and 1-(3,4,5-trimethoxybenzyl)-3-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)-ethyl]benzimidazolium bromide (8) were synthesized by the direct reaction of the N-alkylated benzimidazoles with 2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)-ethyl bromide. Ag(I)–NHC complexes (9a-c, 10) of the abovementioned precursors were synthesized by in-situ reaction with Ag2O in light-excluded conditions. The transmetallation reaction technique was used to synthesize Pd(II)–NHC complexes (11a-c, 12) by the reaction of [PdCl2(MeCN)2] with corresponding Ag(I)–NHC complexes. All NHC precursors and their corresponding Ag(I) and Pd(II) complexes were spectrally characterized using 1H &13C NMR, and mass spectrometry. Further, Pd(II) complexes were evaluated for their anticancer activity against eight diverse cancer cell lines, namely; Pancreatic Adenocarcinoma (Capan-1), Colorectal Carcinoma (HCT-116), Glioblastoma (LN229), Lung Carcinoma (NCI-H1975), Acute Lymphoblastic Leukemia (DND-41), Acute Myeloid Leukemia (HL-60), Chronic Myeloid Leukemia (K562), and non-Hodgkin Lymphoma (Z138). Standard antineoplastic drugs docetaxel and etoposide were included as reference compounds. Among the tested complexes, complex 11c displayed broad activity against all tested cancer types, albeit at higher concentrations when compared to the reference drugs.

Insights on the Anion Effect in N-heterocyclic Carbene Based Dinuclear Gold(I) Catalysts.

Dinuclear bisNHC (bis(N-heterocyclic carbene)) gold(I) complexes 3 a and 4 a of general formula [Au2 Br2 (bisNHC)] were tested as catalysts in the cycloisomerization of N-(prop-2-yn-1-yl)benzamide and in the hydromethoxylation of 3-hexyne in the presence of silver(I) activators bearing different counteranions. The catalytic performance of mononuclear NHC complexes (1 a, 2 a) in the same reactions was also studied. The results highlighted the fundamental role of both NHC ligand and counterion in the catalytic cycles and activation process: dinuclear catalysts exhibit higher initial activity even under milder conditions but suffer in terms of stability with respect to mono NHCs. Furthermore, a new dinuclear bisNHC gold(I) complex 4 b of general formula [Au2 (OTs)2 (bisNHC)] (OTs=p-toluenesulfonate) was successfully synthesized and characterized by means of NMR and ESI-MS analyses.

Preparation of Mono and Bis‐Hydrazino‐Substituted N‐Heterocyclic Carbene Boranes

AbstractWe report the formation of NHC complexes of mono‐ and bis‐hydrazino boranes with N−N−B and N−N−B−N−N chains via the reaction of NHC‐boranes with electrophilic azo reagents. The influence of steric hindrance on the carbene and on the azo reagent has been shown to be crucial for the reactivity. The hydroboration of dissymmetric azo reagents is regioselective only when the latter is sufficiently electronically twisted. Bis‐hydrazino borane complexes with two different hydrazine arms could be obtained via sequential addition.

Heteroditopic NHC Ligand Supported Manganese(I) Complexes: Synthesis, Characterization, and Activity as Non-bifunctional Phosphine-Free Catalyst for the α-Alkylation of Nitriles.

In the present work, several manganese(I) complexes of chelating heteroditopic ligands Mn1-3, featuring ImNHC (imidazol-2-ylidene) connected to a 1,2,3-triazole-N or tzNHC (1,2,3-triazol-5-ylidene) donors via a methylene spacer, with possible modifications at the triazole backbone have been synthesized and completely characterized. Notably, the CO stretching frequencies, electrochemical analysis, and frontier orbital analysis certainly suggest that the chelating ImNHC-tzNHC ligands have stronger donation capabilities than the related ImNHC-Ntz ligand in the synthesized complexes. Moreover, these well-defined phosphine-free Mn(I)-NHC complexes have been found to be effective non-bifunctional catalysts for the α-alkylation of nitriles using alcohols and importantly, the catalyst Mn1 containing ImNHC connected to a weaker triazole-N donor displayed higher activity compared to Mn2/Mn3 containing an unsymmetrical bis-carbene donors (ImNHC and tzNHC). A wide range of aryl nitriles were coupled with diverse (hetero)aromatic as well as aliphatic alcohols to get the corresponding products in good to excellent yields (32 examples, up to 95 % yield). The detailed mechanistic studies including deuterium labelling experiments reveal that the reaction follows a Borrowing Hydrogen pathway.