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N-HETEROCYCLIC Carbene Complexes

Lai-Hon Chung, Chun-Yuen Wong
While the fascinating chemistry demonstrated by metalated N-heterocyclic carbene complexes highlights the significance of metalated heterocyclic chemistry, the development of other metalated heterocycles is falling behind, presumably because of the sparseness of general synthetic methodologies. In this Concept article, the strategy to prepare metalated heterocyclic complexes via metal-induced cycloisomerization of heteroatom-functionalized alkynes is presented. The isolation of and calculations on novel ruthenium complexes bearing chromene, chromone, indole, indoline, indolizine and indolizinone moieties prepared from reactions between alkynes and ruthenium complexes are discussed, with emphasis on their mechanistic insights into the ruthenium-induced alkyne transformations and applications in material design and drug discovery...
October 12, 2018: Chemistry: a European Journal
Peter J Rayner, Philip Norcott, Kate M Appleby, Wissam Iali, Richard O John, Sam J Hart, Adrian C Whitwood, Simon B Duckett
Iridium N-heterocyclic carbene (NHC) complexes catalyse the para-hydrogen-induced hyperpolarization process, Signal Amplification by Reversible Exchange (SABRE). This process transfers the latent magnetism of para-hydrogen into a substrate, without changing its chemical identity, to dramatically improve its nuclear magnetic resonance (NMR) detectability. By synthesizing and examining over 30 NHC containing complexes, here we rationalize the key characteristics of efficient SABRE catalysis prior to using appropriate catalyst-substrate combinations to quantify the substrate's NMR detectability...
October 12, 2018: Nature Communications
Peter V Simpson, Marco Falasca, Massimiliano Massi
Re(i) complexes bound to π-conjugated bidentate N-heterocyclic carbene ligands with formulation Re(CO)3(N^NHC)L (where N^NHC represents an imidazole or benzimidazole carbene ligand conjugated to a N-based heterocycle such as pyridine, pyrimidine, quinoline or quinoxaline) are a relatively new class of complexes belonging to the archetypal family of well known luminescent Re(CO)3(diim)X species (where diim is a conjugated diimine ligand and X is a halogen anion). The complexes Re(CO)3(N^NHC)L are characterised by blue-shifted emission compared to Re(CO)3(diim)X, but with shorter excited state lifetime decays and lower quantum yields, in contrast to trends expected by the energy gap law...
October 10, 2018: Chemical Communications: Chem Comm
Evgeniy G Gordeev, Valentine P Ananikov
A well-established oxidative addition of organic halides (R-X) to N-heterocyclic carbene (NHC) complexes of palladium(0) leads to formation of (NHC)(R)PdII (X)L species, the key intermediates in a large variety of synthetically useful cross-coupling reactions. Typical consideration of the cross-coupling catalytic cycle is based on the assumption of intrinsic stability of these species, where the subsequent steps involve coordination of the second reacting partner. Thus, high stability of the intermediate (NHC)(R)PdII (X)L species is usually taken for granted...
October 3, 2018: Journal of Computational Chemistry
Yali Han, Xicheng Liu, Zhenzhen Tian, Xingxing Ge, Juanjuan Li, Min Gao, Yanru Li, Yi Liu, Zhe Liu
Series of half-sandwich Iridium(III) benzimidazole-appended imidazolium-based N-heterocyclic carbene (NHC) antitumor complexes [(η5-Cpx)Ir(C^N)Cl]Cl, where Cpx is pentamethylcyclopentadienyl (Cp*) or its biphenyl derivatives (Cpxbiph) and C^N is a NHC chelating ligand, were successfully synthesized and characterized. IrIII complexes showed potential antitumor activity against A549 cells, at most three times more potent than cis-platin under the same conditions. Complexes could bind to BSA by a static quenching mode, catalyzing the change of NADH to NAD+ and inducing the production of reactive oxygen species (maximum turnover number, 9...
October 2, 2018: Chemistry, An Asian Journal
Violeta Sicilia, Lorenzo Arnal, Sara Fuertes, Antonio Martín, Miguel Baya
The X-ray study on a single crystal of the butterfly-like complex [{Pt(C^C*)(μ-pz)}2] (1), containing a cyclometalated N-heterocyclic carbene ligand as wings (HC^C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene), showed three molecules in the asymmetric unit with intermetallic separations (Å) of 3.2294(4) (1A), 3.2834(4) (1B), 3.1208(6) (1C). From the reaction of 1 with excess of CHI3 in the air and the sunlight, complex [{Pt(C^C*)(μ-pz)I)}2] (2) (dPt-Pt = 2.6079(2) Å) was obtained as the major product, while [IPt(C^C*)(μ-pz)2Pt(C^C*)CHI2] (3) (dPt-Pt = 2...
October 1, 2018: Chemistry: a European Journal
Yali Han, Zhenzhen Tian, Shumiao Zhang, Xicheng Liu, Juanjuan Li, Yanru Li, Yi Liu, Min Gao, Zhe Liu
Series of half-sandwich IrIII N-heterocyclic carbene (NHC) antitumor complexes [(η5 -Cp*)Ir(C^C)Cl] have been synthesized and characterized (Cp* is pentamethyl cyclopentadienyl, and C^C are four NHC chelating ligands containing phenyl rings at different positions). IrIII complexes showed potent antitumor activity with IC50 values ranged from 3.9 to 11.8 μM against A549 cells by the MTT assay. Complexes can catalyze the conversion of the coenzyme NADH to NAD+ and induce the production of reactive oxygen species (ROS), and bonding to BSA by static quenching mode...
September 20, 2018: Journal of Inorganic Biochemistry
Nikolaos Tsoureas, Jennifer C Green, F Geoffrey N Cloke
The reaction of the bis(pentalene)dititanium complex Ti2(μ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) (1) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of an isopropyl substituent to form a tucked-in hydride (3). Whilst pyridine will also effect this cyclometallation reaction to form (5), the pyridine analogue of (3), the bases 1,2,4,5-tetramethyl-imidazole, 2,6-lutidine, DABCO or trimethylphosphine are ineffective. The reaction of (1) with 2,6-dichloro-pyridine affords crystallographically characterised (6) which is the product of oxidative addition of one of the C-Cl bonds in 2,6-dichloro-pyridine across the Ti-Ti double bond in (1)...
September 25, 2018: Dalton Transactions: An International Journal of Inorganic Chemistry
Cheng Chen, Yang Miao, Kimmy De Winter, Hua-Jing Wang, Patrick Demeyere, Ye Yuan, Francis Verpoort
Transition-metal-catalyzed amide-bond formation from alcohols and amines is an atom-economic and eco-friendly route. Herein, we identified a highly active in situ N -heterocyclic carbene (NHC)/ruthenium (Ru) catalytic system for this amide synthesis. Various substrates, including sterically hindered ones, could be directly transformed into the corresponding amides with the catalyst loading as low as 0.25 mol.%. In this system, we replaced the p -cymene ligand of the Ru source with a relatively labile cyclooctadiene (cod) ligand so as to more efficiently obtain the corresponding poly-carbene Ru species...
September 20, 2018: Molecules: a Journal of Synthetic Chemistry and Natural Product Chemistry
Shicheng Shi, Steven P Nolan, Michal Szostak
Transition-metal-catalyzed cross-coupling reactions represent a most powerful tool for the rapid construction of C-C and C-X bonds available to synthetic chemists. Recently, tremendous progress has been made in the burgeoning area of cross-coupling reactions of amides and esters enabled by regio- and chemoselective acyl C-X (X = N, O) cleavage using well-defined Pd(II)-NHC complexes. The use of N-heterocyclic carbenes as ligands in palladium-catalyzed cross-couplings permits reactions of amides and esters that were previously impossible using palladium or could be achieved only under harsh conditions...
September 21, 2018: Accounts of Chemical Research
Yujiao Zhang, Shumiao Zhang, Zhenzhen Tian, Juanjuan Li, Zhishan Xu, Shanshan Li, Zhe Liu
Twelve novel half-sandwich IrIII-NHC complexes [(η5-Cpx)Ir(C^O)Cl] were synthesized and characterized. These complexes showed higher cytotoxic activity toward A549 cells and HeLa cells than cisplatin. An increase in the number of contained phenyl groups was related to better anticancer activity. The reaction of complexes with nucleobases 9-MeA, nucleobases 9-EtG, plasmid DNA and CT-DNA showed no significant effects. These complexes captured hydrogen from NADH and converted it to NAD+, which produced the reactive oxygen species (ROS)...
September 21, 2018: Dalton Transactions: An International Journal of Inorganic Chemistry
Michał Smoleń, Wioletta Kośnik, Roman Gajda, Krzysztof Woźniak, Aleksandra Skoczeń, Anna Kajetanowicz, Karol Grela
Three mono-N-heterocyclic carbene (NHC) ruthenium 2-isopropoxybenzylidene (10 a-c) and one bis(NHC) indenylidene complex (8) bearing an unsymmetrical N-heterocyclic carbene ligand were synthesized and structurally characterized by single-crystal X-ray diffraction. The catalytic activity of the newly obtained complexes were evaluated in ring-closing metathesis (RCM) and ene-yne (RCEYM) reactions in toluene and environmentally friendly 2-MeTHF under air. The results confirmed that although all tested reactions can be successfully mediated by catalysts 10 a-c, their general reactivity is lower than the benchmark all-purpose Ru catalysts with symmetrical NHC ligands...
September 19, 2018: Chemistry: a European Journal
Júlia R Diniz, Tatiani B de Lima, Renan Galaverna, Aline L de Oliveira, Davi A C Ferreira, Fabio C Gozzo, Marcos N Eberlin, Jairton Dupont, Brenno A D Neto
We describe the synthesis of two tetrachloroindate ionic liquids used as probes to study the involvement of NHCs (N-heterocyclic carbenes) in the distillation of imidazolium derivatives. Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS), electrospray ionization mass spectrometry (ESI-MS), atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS) and laser-induced acoustic desorption (LIAD) were used to depict the possibility of the involvement of NHCs during the distillation process...
October 3, 2018: Physical Chemistry Chemical Physics: PCCP
Christopher A Dodds, Alan R Kennedy
The crystal structure of bis-[μ-(1,3-dimesityl-1 H -imidazol-3-ium-2-yl)methano-lato-κ2 O : O ]bis-[di-chlorido-copper(II)], [Cu2 Cl4 (C22 H26 N2 O)2 ], is reported. The complex is assumed to have formed via the insertion of formaldehyde into the copper-carbon bond in an N-heterocyclic carbene complex of copper(I) chloride. The structure of the binuclear mol-ecule possesses a crystallographic-ally centrosymmetric Cu2 O2 central core with the O atoms bridging between the CuII atoms and thus Z ' = 0.5. The copper centres are further ligated by two chloride ligands, resulting in the CuII atoms residing in a distorted square-planar environment...
September 1, 2018: Acta Crystallographica. Section E, Crystallographic Communications
Johannes Soellner, Thomas Strassner
The synthesis and characterization of the first bidentate C^C* cyclometalated platinum(II) complexes based on abnormal N-heterocyclic carbenes (aNHC) is presented. The aNHC ligand precursors are prepared from benzonitriles and anilines to form 1,2,3-trisubstituted imidazolium salts. The title compounds were synthesized by in situ generation of the silver carbene complex, followed by transmetalation to platinum and subsequent introduction of the β-diketonate ligand. Structural characterization by 2D NMR experiments, as well as solid-state structures unequivocally prove the abnormal binding mode of the aNHC ligands...
September 14, 2018: Chemistry: a European Journal
Ibrahim Abdellah, Pauline Kasongo, Axel Labattut, Régis Guillot, Emmanuelle Schulz, Cyril Martini, Vincent Huc
Benzyloxycalix[8]arene supported catalysts bearing N-heterocyclic carbene palladium complexes on each subunit were readily synthesized. Intermediates and catalysts were fully characterized, allowing for a fine control of their structure. X-ray diffraction analysis confirmed the formation of a calix[8]arene bearing eight well-defined NHC palladium complexes. The macrocyclic structure of calix[8]arenes allowed for a scalable and chromatography-free catalyst synthesis under homogeneous conditions, while the catalytic reaction proceeded under heterogeneous conditions, just by changing the nature of the solvent...
October 9, 2018: Dalton Transactions: An International Journal of Inorganic Chemistry
Justin F Binder, Ala'aeddeen Swidan, Charles L B Macdonald
Reported are general synthetic approaches for the syntheses of asymmetrically substituted phosphorus(I) cations by P+ transfer from [dppeP]+ (dppe = 1,2-bis(diphenylphosphino)ethane). The first method grants access to acyclic derivatives and is accomplished by the sequential substitution of dppe using first a sterically encumbered ligand which cannot form a stable homoleptic complex, followed by a second equivalent of a less sterically demanding ligand. The second method grants access to cyclic derivatives and utilizes asymmetric hybrid phosphine/N-heterocyclic carbene ligands...
September 17, 2018: Inorganic Chemistry
Andreas A Danopoulos, Alexandre Massard, Gilles Frison, Pierre Braunstein
Metallotropism of the M[N(SiMe3 )2 ]2 metal fragment in the tautomeric system IAR ⇌ACR involving imidazolium salts/N-heterocyclic carbenes with remote aminide/amine substituents, respectively, is manifested by its CNHC carbophilicity (R=tBu, M=Co, Fe) or NRaminido nitrogenophilicity (R=Cy, M=Co, Fe; R=Mes, M=Fe) and has been rationalized on the basis of steric and electronic effects. The thermolysis products of the [M{N(SiMe3 )2 }2 ]/ IAR ⇌ACR system are also substituent-dependent, leading to a rearranged aminide-functionalized aNHC Co2 complex of an unprecedented type or to ring-opened metallaketenimines; they are postulated to originate from different metalloisomers...
September 4, 2018: Angewandte Chemie
David J Nelson, Feliu Maseras
Computational investigations of the reactions of [Ni(NHC)2] complexes with aryl halides show that the outcomes are governed by the steric impact of the ligand (NHC = N-heterocyclic carbene). Small ligands permit facile oxidative addition, leading to NiII complexes, while larger NHC ligands prevent coordination of the aryl halide and favour the halide abstraction to form NiI.
September 18, 2018: Chemical Communications: Chem Comm
Sharmistha Urinda, Goutam Das, Anup Pramanik, Pranab Sarkar
Tuning photoluminescence properties is of prime importance for designing efficient light emitting diode (LED) materials. Here, we perform a computational study on the effect of normal N-heterocyclic carbene (NHC) and abnormal mesoionic carbene (MIC) ligands on the photoluminescence properties of some Ir(III) complexes, which are very promising LED materials. We find MIC as the privileged ligand in designing triplet emitters. The strong σ-donating and moderate π-accepting properties of MIC render a lower access to the nonemissive triplet metal-centered state (3 MC), resulting in lowering the nonradiative rate constant ( knr ) and correspondingly achieving higher quantum efficiency...
September 27, 2018: Journal of Physical Chemistry. A
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