Recent advances in bidentate bis(N-heterocyclic carbene) transition metal complexes and their applications in metal-mediated reactions

An SN1-Approach to Cross-Coupling: Deoxygenative Arylation Facilitated by the β-Silicon Effect

We report a dual metal-catalyzed method for the cross-coupling of unprotected alcohols by exploiting the β-Si effect. This deoxygenative Suzuki-Miyaura reaction tolerates a range of primary, secondary, and tertiary alcohol substrates along with diverse functional groups and heterocycles. Mechanistic experiments including KIE, VTNA, and Eyring analyses suggest the existence of a carbocation intermediate on the reaction pathway, consistent with a rare SN1 pathway for the activation of an electrophile in cross-coupling reactions. A novel bis-imidazolium N-heterocyclic carbene (NHC) ligand was found to be optimal for reactivity, and nickel(0)-, nickel(I)- and nickel(II)- complexes of this ligand were isolated and characterized. In contrast to more well-established shorter chain ligands, these long-chain NHCs are found to have characteristically large bite angles, which may be critical for enabling the deoxygenative arylation of aliphatic alcohols.

Deciphering the influence of PdII and PdIV oxidation states on non-standard chemical bonds within bis(jN-heterocyclic carbene) complexes: insights from DFT

Bis-N-heterocyclic carbene ligands (bis(NHC)) have introduced a new approach to designing homogeneous and heterogeneous catalysts, demonstrating the versatility of ligand concepts in catalysis. This study presents a computational analysis of palladium (+ii and +iv) complexes containing either a normally (bis(nNHC)) or an abnormally (bis(aNHC)) bound CH2-bridged bis-N-heterocyclic carbene ligand; in addition, ancillary ligands are permuted from chlorides (X = Cl) to bromides (X = Br). Density functional theory at the B3PW91/6-31G(d)/Lanl2DZ level in the gas phase was used to investigate the electronic structure and bonding properties of bis(NHC)PdIIX2 and bis(NHC)PdIVX4 for bis(NHC) palladium(ii) dihalide and palladium(iv) tetrachloride complexes, respectively. Results indicate that all of the palladium complex structures prefer a flexible boat-type conformation with an average C 2v symmetry, according to bond property (Ccarbene-Pd and Pd-Cl[Br]) analysis. The strength of these bonds depends on coordinating halide ions (Cl- and Br-), the type of ligand (bis(nNHC) and bis(aNHC)), and the palladium oxidation state (+ii and +iv). Analysis of thermodynamic parameters (ΔH 0, ΔG 0, and ΔE bind) shows an increase in values from an abnormal to normal chelating mode in tetrahalides, whereas the opposite is observed for dihalide complexes. The lower π-backbonding ability of the metal, which is influenced by the quantity and size of halide ions involved, could be one possible explanation for this deficiency.

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

An isolable, chelating bis[cyclic (alkyl)(amino)carbene] stabilizes a strongly bent, dicoordinate Ni(0) complex

Chelating ligands have had a tremendous impact in coordination chemistry and catalysis. Notwithstanding their success as strongly σ-donating and π-accepting ligands, to date no chelating bis[cyclic (alkyl)(amino)carbenes] have been reported. Herein, we describe a chelating, C2-symmetric bis[cyclic (alkyl)(amino)carbene] ligand, which was isolated as a racemic mixture. The isolation and structural characterization of its isostructural, pseudotetrahedral complexes with iron, cobalt, nickel, and zinc dihalides featuring eight-membered metallacycles demonstrates the binding ability of the bis(carbene). Reduction of the nickel(II) dibromide with potassium graphite produces a dicoordinate nickel(0) complex that features one of the narrowest angles measured in any unsupported dicoordinate transition metal complexes.

Application of phosphorus-bridged rigid, bent bis(NHCs) as dipodal ligands in main group and transition metal chemistry

Phosphorus-bridged rigid, bent bis(N-heterocyclic) carbenes have not been reported, so far, despite having structural features that could make them interesting ligands in coordination and main group element chemistry. In previous reports, we had demonstrated that tuning of σ3- and σ4-phosphorus environments in planarised bis(NHCs) affects electronic properties and can provide additional coordination sites. Herein, we report on first examples of synthesis and conversion of 1,4-diphosphabarrelene-related compounds into rigid bent, doubly P-bridged bis(NHCs). The formation of main group element adducts with substrates from group 13, 14 and 15 illustrates opportunities to access novel scaffolds and to create nonplanar branching points. DFT calculations reveal the new bis(NHCs) to be good candidates as novel soft/hard ligands with up to four coordination sites. The synthesis of a dinuclear Fe(CO)4 complex is demonstrated. The thermal retro-[4 + 2] cycloaddition was theoretically and experimentally explored for a variety of ionic and zwitterionic 1,4-diphosphabarrelenes, and the generation and trapping of a dinuclear Fe(0) bis(NHC) complex with a tricyclic 1σ2,4 σ2-diphosphinine scaffold is presented.

How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron N-Heterocyclic Complexes: A Comparative Study

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic FeIII-hexa-NHC [Fe(mbmi)3](PF6)3 and heteroleptic FeII-tetra-NHC [Fe(mbmi)2(bpy)](PF6)2 (bpy = 2,2'-bipyridine) complexes. They are compared to the reported FeIII-hexa-NHC [Fe(btz)3](PF6)3 and FeII-tetra-NHC [Fe(btz)2(bpy)](PF6)2 complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)3](PF6)3 and [Fe(mbmi)2(bpy)](PF6)2 are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The FeII/FeIII redox couples of [Fe(mbmi)3](PF6)3 (-0.38 V) and [Fe(mbmi)2(bpy)](PF6)2 (-0.057 V, both vs Fc+/0) are less reducing than [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)3](PF6)3 at 502 nm (ligand-to-metal charge transfer, 2LMCT) and [Fe(mbmi)2(bpy)](PF6)2 at 410 and 616 nm (metal-to-ligand charge transfer, 3MLCT). Lifetimes of 57.3 ps (2LMCT) for [Fe(mbmi)3](PF6)3 and 7.6 ps (3MLCT) for [Fe(mbmi)2(bpy)](PF6)2 were probed and are somewhat shorter than those for [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. [Fe(mbmi)3](PF6)3 exhibits photoluminescence at 686 nm (2LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10-4, compared to (3 ± 0.5) × 10-4 for [Fe(btz)3](PF6)3.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes in Catalytic N-Formylation/N-Methylation of Amines Using Carbon Dioxide and Phenylsilane

A series of six Mn(I) complexes with general formula [MnBr(bisNHC)(CO)3 ], having a bidentate bis(N-heterocyclic carbene) ligand (bisNHC), has been developed by varying the bridging group between the NHC donors, the nitrogen wingtip substituents and the heterocyclic ring. The synthesis of the complexes has been accomplished by in situ transmetalation of the bisNHC from the corresponding silver(I) complexes. Removal of the bromide anion affords the corresponding solvento complexes [Mn(bisNHC)(CO)3 (CH3 CN)](BF4 ). The influence of the bisNHC structure on its electron donor ability has been evaluated by FTIR and 13 C NMR spectroscopy, both in the neutral and cationic complexes. Finally, the isolated Mn(I)-bisNHC complexes have been employed as homogeneous catalysts in the reductive N-formylation and N-methylation of amines with CO2 as C1 source and phenylsilane as reducing agent, showing a high selectivity for the N-methylated product. Preliminary mechanistic investigations suggest that, in the adopted reaction conditions, the formylated product can be formed via different reaction pathways, either metal-catalyzed or not, while the methylation reaction requires the use of the Mn(I) catalyst.

Synthesis and Ligand Properties of Chelating Bis(N-heterocyclic carbene)-Stabilized Bis(phosphinidenes)

NHC-phosphinidene (NHCP) adducts are an emerging class of ligands with proven binding ability for main group and transition metal elements. They possess electron-rich P atoms with two lone pairs (LPs) of electrons, making them interesting platforms for the formation of multimetallic complexes. We describe herein a modular, high-yielding synthesis of bis(NHCP)s, starting from alkylidene-bridged bis(NHC)s ((IMe)2CnH2n; n = 1,3) and triphosphirane (PDip)3 (Dip = 2,6-iPr2C6H3) as phosphinidene transfer reagent. The coordination chemistry of [{DipP(IMe)}2CH2], 1, was studied in detail, and complexes [1·FeBr2] and [1·Rh(cod)]Cl were prepared, showing that the ligand has a flexible bite angle. The dicarbonyl complex [1·Rh(CO)2]Cl, with an average value for the CO stretching frequency of 2029 cm-1, indicates a strongly donating ligand when compared to related complexes. The binding ability of the remaining two phosphorus LPs was demonstrated with AuCl(SMe2), giving the heterotrimetallic complex [1·(AuCl)2·Rh(cod)]Cl. Moreover, [1·Rh(cod)]X (X- = Cl, B(3,5-(CF3)2-C6H3)4) was tested in the catalytic hydrogenation of methyl-Z-α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe2), revealing that the chloride complex was inactive, while the BArF complex demonstrated moderate activity. Additionally, [1·Rh(cod)]Cl was shown to be moderately air- and moisture-stable, slowly decomposing to the corresponding NHC-stabilized bis-dioxophosphorane, which was independently synthesized by treating the free ligand with dry O2.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt3 adducts. By applying complexes 3 with KHBEt3 as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt3 as cocatalyst.

Click-derived multifunctional metal complexes for diverse applications

The Click reaction that involves Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) serves as the most potent and highly dependable tool for the development of many complex architectures. It has paved the way for the synthesis of numerous drug molecules with enhanced synthetic flexibility, reliability, specificity and modularity. It is all about bringing two different molecular entities together to achieve the required molecular properties. The utilization of Click chemistry has been well demonstrated in organic synthesis, particularly in reactions that involve biocompatible precursors. In pharmaceutical research, Click chemistry is extensively utilized for drug delivery applications. The exhibited bio-compatibility and dormancy towards other biological components under cellular environments makes Click chemistry an identified boon in bio-medical research. In this review, various click-derived transition metal complexes are discussed in terms of their applications and uniqueness. The scope of this chemistry towards other streams of applied sciences is also discussed.

How To Enhance the Efficiency of Breslow Intermediates for SET Catalysis

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI^-s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI^-s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI^-s derived from carbenes 1 to 10, it appears that, apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI^-s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a nonpolar solvent is used; large and weakly coordinating cations such as Cs⁺ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis.

Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η2-olefin)(NHC)(NHC’) have been synthesized by a stepwise reaction of [Rh(μ-Cl)(η2-olefin)2]2 with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η2-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η2,η2-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.

Theoretical Study of N-H σ-Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

N-H σ-bond activation of alkylamine by Ni(PCy₃) was investigated using density functional theory (DFT) calculations. When simple alkylamine NHMe₂ is a reactant, both concerted oxidative addition in Ni(PCy₃)(NHMe₂) and ligand-to-ligand H transfer reaction in Ni(PCy₃)(C₂H₄)(NHMe₂) are endergonic and need a high activation energy. When NH(Me)(Bs) (Bs = SO₂Ph, a model of tosyl group used in experiments) is a reactant, both reactions are exergonic and occur easily with a much smaller activation energy. The much larger reactivity of NH(Me)(Bs) than that of NHMe₂ results from the stronger Ni-N(Me)(Bs) bond than the Ni-NMe₂ bond and the presence of the Ni-O bonding interaction between the Bs group and the Ni atom in the product. N-Heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), is computationally predicted to be better than PCy₃ because the Ni-NMe₂ and Ni-N(Me)(Bs) bonds in the IPr complex are stronger, respectively, than those of the PCy₃ complex. The introduction of the electron-withdrawing Bs group to the N atom of amine and the use of IPr as a ligand are recommended for the N-H σ-bond activation. The C-H σ-bond activations of benzene via the oxidative addition and the ligand-to-ligand H transfer reaction were also investigated here for comparison with the N-H σ-bond activation. The differences between the C-H σ-bond activation of benzene and the N-H σ-bond activation of these amines are discussed in terms of the N-H, C-H, Ni-Ph, and Ni-NMe₂, and Ni-N(Me)(Bs) bond energies and back-donation to benzene from the Ni atom.

Cationic Charge-Appended Abnormal Carbenes: Synthesis and Study of Electronically Modified Abnormal N-Heterocyclic Carbenes

A range of palladium complexes featuring electronically modified, imidazole-based abnormal N-heterocyclic carbene (aNHC) ligands have been prepared in the hopes of accessing a new class of cationic aNHC ligands electronically distinct from normal NHCs and aNHCs. These palladium complexes represent the first examples of transition metal-ligated aNHC complexes featuring a cationic moiety adjacent to the abnormal carbene center. It was anticipated that these design principles could facilitate electron transfer between the imidazolinylidene and the cationic heterocycle, thus reducing the electron density at the abnormal carbene center. However, this case study suggests that greater conformational restrictions that allow for heterocycle coplanarity are necessary to achieve significant electron transfer and enable access to a new class of cationic charge-appended aNHCs with unique electronic properties.

Non-C2-Symmetric Bis-Benzimidazolium Salt Applied in the Synthesis of Sterically Hindered Biaryls

A novel non-C2-symmetric bis-benzimidazolium salt derived from (±)-valinol has been prepared by a simple and straightforward process in good yield. The structure of bis-benzimidazolium salt provided a bulky steric group on the ethylene bridge; which facilitates the catalytic efficacy in the C(sp2)-C(sp2) formation. Its catalytic activity in Suzuki-Miyaura cross-coupling reaction of unactivated aryl chlorides has been found to have high efficacy in 1 mol% Pd loading. This protocol demonstrated the potential on the synthesis of sterically hindered biaryls.

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC ^C Cyclometalated Ligands

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp² -H or Csp³ -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20^th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based C_NHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

N-Heterocyclic carbene-carbodiimide (NHC-CDI) betaine adducts: synthesis, characterization, properties, and applications

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC-CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

Recent advances in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications

This review summarizes the recent developments in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications.

Continuous flow synthesis and antimicrobial evaluation of NHC* silver carboxylate derivatives of SBC3 in vitro and in vivo

N-heterocyclic silver carbene compounds have been extensively studied and shown to be active agents against a host of pathogenic bacteria and fungi. By incorporating hypothesized virulence targeting substituents into NHC-silver systems via salt metathesis, an atom-efficient complexation process can be used to develop new complexes to target the passive and active systems of a microbial cell. The incorporation of fatty acids and an FtsZ inhibitor have been achieved, and creation of both the intermediate salt and subsequent silver complex has been streamlined into a continuous flow process. Biological evaluation was conducted with in vitro toxicology assays showing these novel complexes had excellent inhibition against Gram-negative strains E. coli, P. aeruginosa, and K. pneumoniae; further studies also confirmed the ability to inhibit biofilm formation in methicillin-resistant Staphylococcus aureus (MRSA) and C. Parapsilosis. In vivo testing using a murine thigh infection model showed promising inhibition of MRSA for the lead compound SBC3, which is derived from 1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene (NHC*).

Pyridine-Chelated Imidazo[1,5-a]Pyridine N-Heterocyclic Carbene Nickel(II) Complexes for Acrylate Synthesis from Ethylene and CO2

Nickel(II) dichloride complexes with a pyridine-chelated imidazo[1,5-a]pyridin-3-ylidene py-ImPy ligand were developed as novel catalyst precursors for acrylate synthesis reaction from ethylene and carbon dioxide (CO2), a highly promising sustainable process in terms of carbon capture and utilization (CCU). Two types of ImPy salts were prepared as new C,N-bidentate ligand precursors; py-ImPy salts (3, 4a–4e) having a pyridine group at C(5) on ImPy and a N-picolyl-ImPy salt (10) having a picolyl group at N atom on ImPy. Nickel(II) complexes such as py-ImPyNi(II)Cl2 (7, 8a–8e) and N-picolyl-ImPyNi(II)Cl2 (12) were synthesized via transmetalation protocol from silver(I) complexes, py-ImPyAgCl (5, 6a–6e) and N-picolyl-ImPyAgCl (11). X-ray diffraction analysis of nickel(II) complexes (7, 8b, 12) showed a monomeric distorted tetrahedral geometry and a six-membered chelate ring structure. py-ImPy ligands formed a more planar six-membered chelate with the nickel center than did N-picolyl-ImPy ligand. py-ImPyNi(II)Cl2 complexes (8a–8e) with tert-butyl substituents exhibited noticeable catalytic activity in acrylate synthesis from ethylene and CO2 (up to 108% acrylate). Interestingly, the use of additional additives including monodentate phosphines increased catalytic activity up to 845% acrylate (TON 8).

Molecular complexes and main-chain organometallic polymers based on Janus bis(carbenes) fused to metalloporphyrins

Janus bis(N-heterocyclic carbenes) composed of a porphyrin core with two N-heterocyclic carbene (NHC) heads fused to opposite pyrroles were used as bridging ligands for the preparation of metal complexes. We first focused our attention on the synthesis of gold(i) chloride complexes [(NHC)AuCl] and investigated the substitution of the chloride ligand by acetylides to obtain the corresponding [(NHC)AuC[triple bond, length as m-dash]CR] complexes. Polyacetylides were then used to obtain molecular multiporphyrinic systems with porphyrins fused to only one NHC ligand, while main-chain organometallic polymers (MCOPs) were obtained when using Janus porphyrin bis(NHCs). Interestingly, MCOPs incorporating zinc(ii) porphyrins proved to be efficient as heterogeneous photocatalysts for the generation of singlet oxygen upon visible light irradiation.

Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

N-heterocyclic carbene-functionalized metal-organic frameworks for the chemical fixation of CO2

N-heterocyclic carbenes (NHCs) are a class of molecules with a lone pair of carbene electrons and thus, they have the ability to activate CO2 to form imidazolium carboxylates. The incorporation of activated, metal-free NHC moieties into metal-organic frameworks (MOFs) without the decomposition of metal-carboxylate coordination motifs is highly desired owing to the high CO2 affinity and versatile chemical functionalities in MOFs. Herein, we have summarized the recent in situ generation approaches to form metal-free NHC-functionalized MOFs, which are a unique class of CO2-conversion catalysts with high catalytic activity, selectivity and stability, superior to those of homogenous and other heterogeneous NHC analogues. The NHC-functionalized MOFs for catalytic CO2 reduction include reactions such as the hydroboration of CO2, hydrosilylation of CO2, N-methylation using CO2 and hydrogenation of CO2 to formic acid. Overall, the synthetic strategy of metal-free NHC-functionalized MOFs, the unique catalytic pathways of NHC-functionalized MOFs, and potentially new research directions of NHC-functionalized MOFs are discussed, which will guide researchers to attempt to design new NHC-MOFs and extend their catalytic applications in the chemical fixation of CO2.

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.

Synthesis, Structure, and Anticancer Activity of Symmetrical and Non-symmetrical Silver(I)-N-Heterocyclic Carbene Complexes

Synthesis and anticancer studies of three symmetrically and non-symmetrically substituted silver(I)-N-Heterocyclic carbene complexes of type [(NHC)₂-Ag]PF₆ (7-9) and their respective (ligands) benzimidazolium salts (4-6) are described herein. Compound 5 and Ag-NHC-complex 7 were characterized by the single crystal X-ray diffraction technique. Structural studies for 7 showed that the silver(I) center has linear C-Ag-C coordination geometry (180.00(10)^o). Other azolium and Ag-NHC analogues were confirmed by H¹ and C¹³-NMR spectroscopy. The synthesized analogues were biologically characterized for in vitro anticancer activity against three cancer cell lines including human colorectal cancer (HCT 116), breast cancer (MCF-7), and erythromyeloblastoid leukemia (K-562) cell lines and in terms of in vivo acute oral toxicity (IAOT) in view of agility and body weight of female rats. In vitro anticancer activity showed the values of IC₅₀ in range 0.31-17.9 μM in case of K-562 and HCT-116 cancer cell lines and 15.1-35.2 μM in case of MCF-7 while taking commercially known anticancer agents 5-fluorouracil, tamoxifen, and betulinic acid which have IC₅₀ values 5.2, 5.5, and 17.0 μM, respectively. In vivo study revealed vigor and agility of all test animals which explores the biocompatibility and non-toxicity of the test analogues.

Air and Moisture Tolerant Synthesis of a Chelated bis(NHC) Methylpalladium(II) Complex Relevant to Alkyl Migration Processes in Catalysis

An air- and moisture-tolerant alternate synthetic pathway to the preparation of a cationic chelated bis(NHC) methylpalladium(ii) complex, [{(MesIm)2CH2}Pd(Me)(NCMe)][PF6], is described. The pathway involves the isolation of a bis(NHC) AgI complex, [{(MesIm)2CH2}2Ag2][PF6]2, via metallation of the corresponding diimidazolium salt with Ag2O followed by carbene transfer to [(COD)PdBrMe]. This new method avoids a previously reported unstable intermediate that displayed rapid decomposition at room temperature, attaining the targeted cationic methylpalladium(ii) complex in high yield. CO/ethylene copolymerisation catalysis trials are reported showing solvent dependent catalyst lifetime and copolymer yields. Preliminary ethylene insertion studies are also outlined revealing possible pathways leading towards catalyst deactivation.