N-heterocyclic Carbene Moieties Research Articles

Recyclable N-Heterocyclic Carbene Porous Coordination Polymers with Two Distinct Metal Sites for Transformation of CO2 to Cyclic Carbonates.

Single-component catalysts with integrated multiple reactive centers could work in concert to achieve enhanced activity tailored for specific catalytic reactions, but they remain underdeveloped. Herein, we report the construction of heterogeneous bimetallic porous coordination polymers (PCPs) containing both porphyrin and N-heterocyclic carbene (NHC) metal sites via the coordinative assembly of the NHC functionalities. Three heterobimetallic PCPs (TIPP-Zn-Pd, TIPP-Cu-Pd and TIPP-Ni-Pd) have been prepared to verify this facile synthetic strategy for the first time. In order to establish a cooperative action toward the catalytic CO2 cycloaddition with epoxides, an additional tetraalkylammonium bromide functionality has also been incorporated into these polymeric structures through the N-substituent of the NHC moieties. The resulting heterogeneous bimetallic catalyst TIPP-Zn-Pd exhibits the best catalytic performance in CO2 cycloaddition with styrene oxide (SO) under solvent-free conditions at atmospheric pressure and is applicable to a wide range of epoxides. More importantly, TIPP-Zn-Pd works smoothly and is recyclable in the absence of a cocatalyst under 1.0 MPa of CO2 at 60 °C. This indicates that TIPP-Zn-Pd is quite competitive with the reported heterogeneous catalysts, which typically require a high reaction temperature above 100 °C under cocatalyst-free conditions. Thus, this work provides a new approach to design heterogeneous bimetallic PCP catalysts for high-performance CO2 fixation under mild reaction conditions.

A novel chelating bidentate NHC ligand based on the cationic (η5-cyclopentadienyl)(η6-arene)iron(II) motif

We here present the synthesis of a novel chelating N-heterocyclic carbene (NHC) ligand with the two NHC moieties bound to a cationic (η5-cyclopentadienyl)(η6-arene)iron(II) center and its coordination to palladium(II). Spectroscopic and spectrometric characterization, together with X-ray structural analyses, proves the molecular structure of these new organometallic compounds.

Selective alkane hydroxylation and alkene epoxidation using H2O2 and Fe(II) catalysts electrostatically attached to a fluorinated surface.

Fe(II) complexes with pentadentate ligands, including N-heterocyclic carbene moieties, were prepared and electrostatically attached onto the perfluorinated surface of a mesoporous aluminosilicate. The heterogeneous catalysts were applied to the catalytic oxidation of cyclohexane and cyclohexene using H2O2 as an oxidant in CH3CN, demonstrating high performance and selectivity in alkane hydroxylation and cyclohexene epoxidation.

Ring opening polymerization of lactide promoted by Zinc and Magnesium complexes with a N-heterocyclic carbene-phenoxy-imine hybrid non-innocent ligand

Zinc and magnesium complexes of N-heterocyclic carbene (NHC)-phenoxy-imine ligands derived from chiral (S,S)-trans-1,2-diaminocyclohexane have been synthetized. Either monomeric or dimeric species have been obtained depending on the reaction conditions. All complexes were tested as catalysts for the ring opening polymerization (ROP) of lactide showing moderate activity and control degree. MALDI-ToF-MS spectra of PLA highlighted that magnesium complex promoted the formation of cyclic PLA, whereas zinc analogous and its monomeric species gave linear PLA, suggesting that different polymerization mechanisms occur depending on the metal center. Moreover, according to MALDI-ToF-MS spectra, in the absence of additional initiators, polymers arising from zinc catalysts would bear the NHC-phenoxy-imine hybrid ligand as terminal unit. Computational studies were performed in order to shed light on catalyst behaviors and on the mechanisms involved in the ROP of lactide. Free energy profiles of the initiation step suggest that microstructural differences lie in the ligand role. Indeed, in the absence of additional initiators, the ligand participates to the initiation involving its NHC moiety when wrapped around magnesium and its phenoxy-imine moiety when bound to zinc.

Coordination Chemistry of Ir with Chelating Ligands Containing a Xanthine‐Derived, Protic N‐Heterocyclic Carbene (NHC) Moiety

AbstractA series of protic N‐heterocyclic carbene ligand precursors based on theophylline or adenine has been prepared, and the reactivity and coordination chemistry of these proligands towards iridium(III) centers has been investigated. Complex formation appears to be highly dependent on the proligand structure. In one case, a particularly stable complex is formed that enables fully reversible protonation/deprotonation of the coordinated NHC moiety. The acidity constant of the protonated complex was consequently determined.

Using N-Heterocyclic Carbenes as Weak Equatorial Ligands to Design Single-Molecule Magnets: Zero-Field Slow Relaxation in Two Octahedral Dysprosium(III) Complexes.

We report the synthesis, structures, and magnetic investigations of two new octahedral dysprosium complexes, based on the original N-heterocyclic carbene (NHC) tridentate bis(phenoxide) ligand, of the respective formulas mer-[DyL(THF)2Cl] (1) and mer-[DyL(THF)3][BPh4] (2), where L = 1,3-bis(3,5-di-tert-butyl-2-oxidophenyl)-5,5-dimethyl-3,4,5,6-tetrahydropyrimidin-1-ium chloride and THF = tetrahydrofuran. The short Dy-O distances in the axial direction in association with the weak donor ability of the NHC moiety provide a suitable environment for slow relaxation of magnetization, overcoming the previous single-molecule magnets based on NHC ligands.

Visible light-driven CO2 reduction with a Ru polypyridyl complex bearing an N-heterocyclic carbene moiety.

A novel Ru polypyridyl complex with an N-heterocyclic carbene ligand was successfully synthesised and characterised. The complex exhibited an intense absorption band in the visible-light region derived from the strong electron-donating character of the carbene ligand, and efficiently catalysed the visible light-driven CO2 reduction with the reaction rate of 36.7 h-1.

ProPhenol Derived Ligands to Simultaneously Coordinate a Main-Group Metal and a Transition Metal: Application to a Zn-Cu Catalyzed Reaction.

A new bifunctional ligand bearing chiral N-heterocyclic carbene (NHC) and prolinol moieties is presented. Utilizing the designed ligand, an in situ formed Cu/Zn hetero-bimetallic complex unlocks the asymmetric allylic alkylation reactions of allyl phosphates with zinc keto-homoenolates, leading to the formation of various γ-vinyl ketones with good regio- and enantio-selectivity. DF sT calculation supports that the chelation of allyl phosphates with catalyst promotes the SN 2' addition and the ligand-substrate steric interactions account for the stereoselective outcome.

Anthracenyl Functionalization of Half-Sandwich Carbene Complexes: In Vitro Anticancer Activity and Reactions with Biomolecules.

N-Heterocyclic carbene (NHC) ligands are widely investigated in medicinal inorganic chemistry. Here, we report the preparation and characterization of a series of half-sandwich [M(L)(NHC)Cl2] (M = Ru, Os, Rh, Ir; L = cym/Cp*) complexes with a N-flanking anthracenyl moiety attached to imidazole- and benzimidazole-derived NHC ligands. The anticancer activity of the complexes was investigated in cell culture studies where, in comparison to a Rh derivative with an all-carbon-donor-atom-based ligand (5a), they were found to be cytotoxic with IC50 values in the low micromolar range. The Ru derivative 1a was chosen as a representative for stability studies as well as for biomolecule interaction experiments. It underwent partial chlorido/aqua ligand exchange in DMSO-d6/D2O to rapidly form an equilibrium in aqueous media. The reactions of 1a with biomolecules proceeded quickly and resulted in the formation of adducts with amino acids, DNA, and protein. Hen egg white lysozyme crystals were soaked with 1a, and the crystallographic analysis revealed an interaction with an l-aspartic acid residue (Asp119), resulting in the cleavage of the p-cymene ligand but the retention of the NHC moiety. Cell morphology studies for the Rh analog 3a suggested that the cytotoxicity is exerted via mechanisms different from that of cisplatin.

N-Heterocyclic Carbene Functionalized Covalent Organic Framework for Transesterification of Glycerol with Dialkyl Carbonates

The development of a heterogeneous catalyst through the combination of novel carrier and powerful catalytic active sites is of particular interest. Herein, the successful integration of an N-Heterocyclic carbene (NHC) moiety into a covalent organic framework (COF) was achieved by coupling 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) and equimolar 4,7-bis(4-formylphenyl)-1-methyl-1H-benzimidazole (IM) and 2′3′5′6′-tetrafluoro-[1,1′:4′,1′′-terphenyl]-4,4′-dicarbaldehyde (4F) followed by ionization with 1-bromobutane (C4H9Br) and then deprotonation upon addition of a base. The resulting material exhibited promising heterogeneous catalytic activity towards transesterification reaction of glycerol with dialkyl carbonate. Moreover, good recyclability granted no substantial loss of activity upon five cycles. Combination of COFs and NHCs might synergize their characteristics, thus providing more possibilities for creating new patterns of catalytic reactivity.

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand.

The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimcaC5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh3)3Cl2]. In contrast, Pd(OAc)2 reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle.

Immobilized Pd on a NHC-functionalized metal-organic FrameworkMIL-101(Cr): An efficient heterogeneous catalyst in the heck and copper-free Sonogashira coupling reactions

A heterogeneous palladium catalyst system based on immobilization of palladium moieties on a N-heterocyclic carbene (NHC) modified metal organic framework (MOF) was developed for the Heck and copper-free Sonogashira coupling reactions. In order to prepare this catalyst system, first, MIL-101(Cr) was functionalized with NHC moieties through a post synthetic modification (PSM) approach, and then Pd metal was stabilized on the prepered MIL-101(Cr)-NHC substrate. This material was characterized using various microscopic and spectroscopic techniques and then was used as an efficient heterogeneous Pd catalyst system in the Heck and copper-free Sonogashira reactions. Results of the heterogeneity tests showed that the Pd-NHC-MIL-101(Cr) catalyst can efficiently catalyzed these coupling reactions heterogeneously and no remarkable changes observed in the morphology and structure of MIL-101(Cr) template during the reaction progress. Also, existence of palladium nanoparticles immobilized on the MOF structure affirmed by the TEM and XPS analysis confirmed the oxidation state of Pd. A variety of alkene and alkyne derivatives were synthesized in good to excellent yields using this heterogeneous Pd catalyst system under normal conditions. More importantly Pd-NHC-MIL-101(Cr) catalyst was simply recovered from the reaction medium without remarkable decreasing in its catalytic activities after five times of reusability. The ICP analysis showed the very low Pd and Cr metals leaching, representing high stability and applicability of this catalyst in Pd coupling reactions.

Fluorinated-NHC Transition Metal Complexes: Leading Characters as Potential Anticancer Metallodrugs.

In the last 20 years, N-Heterocyclic Carbene (NHC) ligands have been ubiquitous in biological and medicinal chemistry. Part of their success lies in the tremendous number of topologies that can be synthesized and thus finely tuned that have been described so far. This is particularly true in the case of those derivatives, including fluorine or fluorinated fragments on their NHC moieties, gaining much attention due to their enhanced biological properties and turning them into excellent candidates for the development of novel metallodrugs. Thus, this review summarizes the development that fluorinated-NHC transition metal complexes have had and their impact on cancer treatment.

Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene.

Oxyallyl derivatives are typically elusive compounds. Even recently reported "stabilized" 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.

Central N-heterocyclic carbene moieties in protic pincer-type bis(pyrazole) ligands: Perturbation on steric and electronic properties of ruthenium center

Chelation-assisted C–H bond cleavage of an N,N’-bis(pyrazol-3-ylmethyl)imidazolium salt with RuCl2(PPh3)3 and subsequent anion exchange afforded the cationic pincer-type ruthenium complex [RuCl(PPh3)2(5NCN-H2)]PF6 (3) bearing a five-membered N-heterocyclic carbene (NHC) furnished with two protic pyrazole arms (5NCN-H2). On the other hand, the uncharged dichlorido complex [RuCl2(PPh3)(6NCN-H2)] (4) with a six-membered carbene (6NCN-H2) was obtained by a similar treatment of an N,N′-bis(pyrazol-3-ylmethyl)perimidinium cation. X-ray analysis of 3 and 4 suggested that the different outcome is ascribed to the larger twist of the 6NCN-H2 pincer ligand. Cyclic voltammetry showed that these NHC-centered pincer complexes are oxidized more easily than the pyridine-centered analogue in line with the stronger electron-donating property of the central NHC unit in the pincer ligands. In contrast, the CO stretching frequency of the carbonyl complex [Ru(CO)(PPh3)2(5NCN)] (5) derived from the five-membered NHC complex 3 is unexpectedly higher than that of the pyridine analogue. Computational studies revealed that effective π-back donation to the carbonyl ligand in the NHC-centered pincer complex 5 is retarded by the twist of the pincer plane with six-membered chelation, which allows the d electrons of the metal to spread over the pyrazole π-orbitals.

Synthesis and structural characterization of iridium(I) complexes of 20-membered macrocyclic rings bearing chelating bis(N-heterocyclic carbene) ligands.

The reactivities of two 20-membered macrocyclic ligands, each containing two N-heterocyclic carbene (NHC) and two amine groups, towards [IrCl(COD)]2 (COD is cycloocta-1,5-diene) were investigated. Macrocycles containing imidazolin-2-ylidene groups formed the monometallic complex [(1,2,5,6-η)-cycloocta-1,5-diene](5,16-dibenzyl-1,5,9,12,16,20-hexaazatricyclo[18.2.1.19,12]tetracosa-10,21-dien-21,22-diylidene)iridium(I) bromide dichloromethane monosolvate, [Ir(C8H12)(C32H42N6)]Br·CH2Cl2, 2a. The structure of iridium complex 2a at 100 K has triclinic P-1 symmetry. The ligand in 2a coordinates to the Ir center through the NHC moieties in a cis fashion. Additionally, the ligand adopts an umbrella-like structure that appears to envelope the Ir center. The structure displays C-H...Br interactions. Macrocycles containing benzimidazolin-2-ylidene groups formed the bimetallic complex [μ-5,20-dibenzyl-1,5,9,16,20,24-hexaazapentacyclo[22.6.1.19,16.010,15.025,30]dotriaconta-10(15),11,13,25(30),26,28-hexaene-31,32-diylidene]bis{bromido[(1,2,5,6-η)-cycloocta-1,5-diene]iridium(I)}, [Ir2Br2(C8H12)2(C40H46N6)], 2b. The structure of complex 2b at 100 K has orthorhombic Pbca symmetry. Each NHC moiety in 2b coordinates in a monodentate fashion to an Ir(COD) fragment. The structure exhibits disorder of the main molecule. This disorder is found in the portion of the macrocycle containing an amine group. This structure also displays C-H...Br interactions. Finally, the structure of the hexafluorophosphate salt of the imidazolin-2-ylidene-containing macrocycle, namely 5,16-dibenzyl-1λ5,5,9,12λ5,16,20-hexaazatricyclo[18.2.1.19,12]tetracosa-1(23),10,12(24),21-tetraene-1,12-diium bis(hexafluorophosphate), C32H44N62+·2PF6-, 1c, was determined. The structure of macrocycle 1c at 100 K has triclinic P-1 symmetry and was found to contain C-H...F interactions.

Synthesis and structural characterization of nickel(II) complexes of 20-membered macrocyclic rings bearing chelating bis(N-heterocyclic carbene) ligands

20-Membered macrocyclic rings (Lbenz and Limid) containing two amine moieties and two N-heterocyclic carbene (NHC) moieties coordinate in a κ2 coordination mode to Ni(II) through the NHC moieties. Complexes 3a and 3b were synthesized by generating free carbenes in situ from bis-benzimidazolium bromide salts and subsequent addition of one equivalent of [Ni(DME)X2] (X = Cl, Br) at 25 °C. Free carbenes of 20-membered macrocyclic ligands Lbenz and Limid can be generated in situ in THF-d8 and characterized by 1H and 13C NMR spectroscopy. 3b can also be synthesized by treatment of silver(I)-bis(NHC) complex 2a with one equivalent of [NiBr2(DME)] in refluxing methylene chloride for 20 h affording yields of up to70%. Complexes 3a and 3b were characterized by X-ray diffraction. [NiI2(Lbenz)] 5c was prepared by halogen exchange of 3a or 3b with NaI at 25 °C. Ni complexes of Limid can be prepared and characterized in situ but cannot be isolated cleanly from the reaction mixtures. The cis and trans isomers of [NiCl2(Limid)] (4a and 4b) and the bromide derivative 4c were characterized by X-ray crystallography by judicious selection of individual, high-quality crystals obtained from crude reaction mixtures.

In Situ Generation of an N-Heterocyclic Carbene Functionalized Metal-Organic Framework by Postsynthetic Ligand Exchange: Efficient and Selective Hydrosilylation of CO2.

The reported metal-organic framework (MOF) catalyst realizes CO2 to methanol transformation under ambient conditions. The MOF is one rare example containing metal-free N-heterocyclic carbene (NHC) moieties, which are installed using an in situ generation strategy involving the incorporation of an imidazolium bromide based linker into the MOF by postsynthetic ligand exchange. Importantly, the resultant NHC-functionalized MOF is the first catalyst capable of performing quantitative hydrogen transfer from silanes to CO2 , thus achieving quantitative (>99 %) methanol yield. Density-functional theory calculations indicate the high catalytic activity of the NHC sites in MOFs are attributed to the decreased reaction barrier of a reaction route involving the formation of an NHC-silane adduct. In addition, the MOF-immobilized NHC catalyst shows enhanced stability for up to eight cycles without base activation, as well as high selectivity towards the desired silyl methoxide product.

Synthesis and structural characterization of 20-membered macrocyclic rings bearing trans-chelating bis(N-heterocyclic carbene) ligands and the catalytic activity of their palladium(ii) complexes.

Macrocycles consisting of a 20-membered ring containing two imidazolium salt functionalities and of the formula [PhCH2N(CH2CH2CH2)Im(CH2CH2CH2)2][Br]2 (Im = imidazole = 3a, benzimidazole = 3b) were synthesized in 70-75% yields. These salts serve as precursors to macrocycles containing two N-heterocyclic carbene (NHC) moieties. Reaction of the macrocyclic salts 3a and 3b with silver oxide afforded macrocyclic-bis(NHC)silver(i) complexes 4a and 4b. Single-crystal X-ray diffraction studies of macrocyclic-bis(NHC)silver(i) complex 4a revealed a tetranuclear silver core with a short Ag-Ag distance (2.9328 Å). Complexes 4a and 4b serve as carbene transfer reagents to Pd. The treatment of macrocyclic-bis(NHC)silver(i) complexes 4a and 4b with one equivalent of PdCl2(MeCN)2 in methylene chloride afforded square-planar trans-macrocyclic-bis(NHC)Pd(ii)X2 complexes 5a and 5b. Preliminary screening of these palladium complexes showed they are competent precatalysts for Heck and Suzuki coupling reactions.

Reactions between N-Heterocyclic Carbene and Lutetium–Metallofullerenes: High Regioselectivity Directed by Electronic Effect in Addition to Steric Hindrance

The Lewis acid-base pairing reaction between strained N-heterocyclic carbene (NHC) and endohedral metallofullerenes (EMFs) is an efficient strategy to obtain stable derivatives in a highly regioselective manner. Herein, we report an in-depth study on the reactions between 3-dimesityl-1 H-imidazol-3-ium-2-ide (1) and three different EMFs, namely, Lu3N@ Ih(7)-C80, Lu2@ C3 v(8)-C82, and Lu2@ C2 v(9)-C82, respectively. Only one monoadduct is obtained for each EMF under certain conditions, demonstrating surprisingly high regioselectivity and exclusive formation of monoadducts. X-ray results of the derivatives of Lu3N@C80 reveal that an epoxide adduct (2a) with a specific [6,6,6]-carbon atom of the C80 cage singly bonded to the normal carbene center (C2) of the NHC is obtained under ambient condition, whereas a pure argon atmosphere gives 2b with an abnormal C5-bonding structure. In contrast, the derivatives of Lu2@C82 (3 and 4) are both normal C2-bonding [5,6,6]-adducts without oxygen addition, even though air is involved in the reaction. Our theoretical results confirm that the remarkably high regioselectivity and the quantitative formation of monoadducts are direct result from the distributions of molecular orbital and electrostatic potential on the cage surfaces in addition to the previously assumed steric hindrance between the fullerene cage and the NHC moiety.