N-heterocyclic Carbene Groups Research Articles

Ligand redistribution hinders preparation of a nickel(II)-hydroxo

The synthesis is demonstrated for a heteroleptic Ni(II) complex, with a tridentate ligand bearing N-heterocyclic carbene (NHC), aryloxide, and picolyl donor groups. The compound was reacted with a variety of hydroxide sources, several of which resulted in complex mixtures. Reaction with [NH4]OH affords a bis-ligated nickel compound, in which the ligands are each bound in a bidentate fashion through the NHC and aryloxide donor groups. A desired nickel(II)-hydroxo product was not observed. DFT studies suggest that ligand redistribution is thermodynamically preferred over stabilization of a Ni-hydroxo in either a monomeric or dimeric form.

N-Heterocyclic Carbene Organic Dyes Derived from 2,4,5,6-tetra(9H-Carbazol-9-yl)isophthalonitrile (4CzIPN) Bonded to TiO2 Surfaces

A high-yield route to N-heterocyclic carbene (NHC) derivatives of 2,4,5,6-tetra(9H-carbazol-9-yl)benzene-1,3-dicarbonitrile (4CzIPN) is reported. The NHC-dye was studied in solution by NMR, infrared- and UV–vis spectroscopy, cyclic voltammetry, and photoluminescence. The NHC group bonds to the surface of TiO2 forming a semiconductor/chromophore system that was characterized by cyclic voltammetry, XPS, infrared and UV–vis spectroscopy as well as photoluminescence. The bonding between the NHC group and TiO2 is quite stable towards the photooxidation of sacrificial electron donors under alkaline conditions without applying any protective layers.

Nonradiative Decay and Stability of N-Heterocyclic Carbene Iridium(III) Complexes

Devices based on deep-blue emitting iridium(III) complexes with N-heterocyclic carbene (NHC) ligands have recently been shown to give excellent performance as phosphorescent organic light-emitting diodes (PHOLEDs). To facilitate the design of even better deep-blue phosphorescent emitters, we carried out density functional theory (DFT) calculations of the lowest triplet (T1) potential-energy surfaces upon lengthening the iridium-ligand (Ir-C) bonds. Relativistic time dependent DFT calculations demonstrate that this changes the nature of T1 from a highly emissive metal-to-ligand charge transfer (3MLCT) state to a metal centered (3MC) state where the radiative decay rate is orders of magnitude slower than that of the 3MLCT state. We identify the elongation of an Ir-C bond on the NHC group as the pathway with the lowest energy barrier between the 3MLCT and 3MC states for all complexes studied and show that the barrier height is correlated with the experimentally measured nonradiative decay rate. This suggests that the thermal population of 3MC states is the dominant nonradiative decay mechanism at room temperature. We show that the 3MLCT → 3MC transition is reversible, in marked contrast to deep-blue phosphors containing coordinating nitrogen atoms, where the population of 3MC states breaks Ir-N bonds. This suggests that, as well as improved efficiency, blue PHOLEDs containing phosphors where the metal is only coordinated by carbon atoms will have improved device lifetimes.

Activity and Recyclability Improvement Through Adjusting the Tethering Strategy for Pd-Catalyzed Suzuki–Miyaura Coupling Reaction of Aryl Chlorides

Four mesoporous SBA-15-immobilized N-heterocyclic carbene (NHC) palladium catalysts with different nitrogen ligands were synthesized. The activity and recyclability of the catalysts were investigated in Suzuki–Miyaura cross-coupling reaction of typical aryl chlorides and arylboronic acids. Bulky NHC moiety was essential for efficiency, while tethering of nitrogen ligands on both support and NHC group could dramatically improve recyclability of the catalysts. The strategy offers an alternative strategy for designing highly efficient and recyclable immobilized catalysts. Mesoporous silica-immobilized palladium catalysts with the bulky N-heterocyclic carbene and tethered “throw-away”nitrogen ligands were found to be highly efficient and recyclable for Suzuki–Miyaura coupling reaction of inactive aryl chlorides.

N-Heterocyclic Carbene-Based Conducting Polymer–Gold Nanoparticle Hybrids and Their Catalytic Application

Hybrid nanocomposites of N-heterocyclic carbene (NHC)-functionalized conducting polymers (CPs) with gold nanoparticles (AuNPs) were prepared by concurrent disproportionation and oxidative coupling. The formation of hybrid nanocomposites, NHC-CP/AuNPs, in the simultaneous process was confirmed by transmission electron microscopy, powder X-ray diffraction, cyclic voltammetry, and 13C solid-state NMR analyses. More importantly, the NHC group played a pivotal role in the dispersion of AuNPs. Further, NHC-CP/AuNPs exhibited good catalytic activity for the reduction of 4-nitrophenol.

Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere

A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2 (PPh3 )3 ] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC-pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal-ligand cooperation.

Internal Catalytic Effect of Bulky NHC Ligands in Suzuki–Miyaura Cross-Coupling Reaction

The Suzuki–Miyaura reaction is one of the most interesting coupling schemes and is widely used in industrial applications. The catalytic cycle of Suzuki–Miyaura coupling has been investigated using N-heterocyclic carbene (NHC) ligands. Detailed analysis of the effect of the NHC ligand points out that bulky groups can promote the reaction in two different ways. First, the bulkiness of the NHC groups can cause steric repulsion in the precursor Pd complexes, which triggers the reaction toward the active Pd(0) compound. Second, in the oxidative addition step, favorable intramolecular π–π and C–H/π interactions between the aryl-chloride reagent and the bulky groups of NHC can decrease the rate determining activation barrier, which can explain previous surprising experimental results. On the basis of our results, we show how appropriate substituents can be designed to promote the key step of catalytic reactions.

Nickel(II) and Palladium(II) Complexes with Chelating N-Heterocyclic Carbene Amidate Ligands: Interplay between Normal and Abnormal Coordination Modes

A series of six Ni(II) and Pd(II) complexes of two bidentate and two tetradentate N-heterocyclic carbene (NHC)/amidate ligands have been prepared. The complexes are uncharged, with square-planar coordination geometries, and the ligands are bound via the NHC groups and the deprotonated amide nitrogen atoms. Pd(II) complexes were prepared for the bidentate ligands, and in each case, two chelating bidentate ligands were bound to the metal center, yielding cis/trans geometric isomeric forms. The Pd(II) complexes of the tetradentate ligands were obtained as a series of constitutional isomeric forms that were separable by fractional crystallization. The constitutional isomers differed in the coordination mode of the NHC groups, which were bound as either “normal” (nNHC) or “abnormal” (aNHC) carbenes. Density functional theory (DFT) studies show that the energies of the isomeric forms increase in the order nNHC/nNHC < nNHC/aNHC < aNHC/aNHC and suggest that the “abnormal” NHC coordination mode occurs in kinetic r...

Force Field Parametrization and Molecular Dynamics Simulation of Flexible POSS-Linked (NHC; Phosphine) Ru Catalytic Complexes

In recent years, N-heterocyclic carbene (NHC) or phospine groups have been put forward as candidate catalysts ligands for olefin metathesis reactions to be performed using multistep methods. Some of these proposed ligands contain polyhedral oligomeric silsesquioxane (POSS) structures linked to NHC rings by means of alkyl chains. Some important properties for the prediction of catalytic activity, such as the theoretically defined buried volume, are related to the conformational characteristics of these complex ligands that can be studied through molecular dynamics simulations. However, the chemical structure of resulting catalytic complexes usually contains atoms or groups that are not included in the common forcefields used in simulations. In this work we focus on complexes formed by a catalytic metal center (Ru) with both phospine and POSS-linked NHC groups. The central part of the complexes contain atoms and groups that have bonds, bond angles, and torsional angles whose parameters have not been previously evaluated and included in existing force fields. We have performed basic ab initio quantum mechanical calculations based on the density functional theory to obtain energies for this central section. The force field parameters for bonds, bond angles, and torsional angles are then calculated from an analysis of energies calculated for the equilibrium and different locally deformed structures. Nonbonded interactions are also conveniently evaluated. From subsequent molecular dynamics simulations, we have obtained results that illustrate the conformational characteristics most closely connected with the catalytic activity.

Ruthenium-Based Complexes Bearing Saturated Chiral N-Heterocyclic Carbene Ligands: Dynamic Behavior and Catalysis

The synthesis of Ru-based catalysts, presenting saturated chiral C2 symmetric (3, 5) and C1 symmetric (4) N-heterocyclic carbene (NHC) ligands bearing N-(S)-phenylethyl groups, was carried out. Variable-temperature NMR studies were conducted to investigate the interconversion of atropisomers in solution. The complex behaviors were rationalized evaluating the rotation barrier of alkylidene and NHC groups around the C−Ru bonds, by DFT calculations. Comparison between NMR data and DFT calculations suggested that interconversion between different atropisomers, which occurs at room temperature, is due to the free rotation of the benzylidene group around the Ru═C bond. The activity and stereoselectivity of 3−5 were investigated in ring-closing metathesis (RCM), asymmetric ring-closing metathesis (ARCM), cross-metathesis (CM), and ring-opening metathesis polymerization (ROMP). 4 showed the highest activity in all reactions and gave a significantly low E:Z ratio in the CM reaction. Modest enantioselectivity in th...

Titanium(IV) Alkoxy-N-heterocyclic Carbenes: Structural Preferences of Alkoxide and Bromide Adducts

A series of titanium(IV) adducts of an amido-tethered N-heterocyclic carbene (NHC) of the form Ti(L)(OiPr)n(Br)3-n (n = 1−3, L = tBuNHCH2CH2[C{NtBu(CHCH)N}]) have been synthesized and characterized. Structural characterization of the n = 2 complex shows marked bending of the two ligands cis (and perpendicular) to the plane of the NHC group in the direction of the carbene, suggestive of a pseudo-back-bonding interaction between adjacent ligands and the carbene p orbital, consistent with a bonding model recently proposed for d0 metal-NHC complexes. However, inspection of a space-filling model and the calculation of bond order using DFT methods suggest that the bending is due to repulsions between lone pairs on the adjacent π-donor ligands in the complexes and not any specific interaction between the NHC and cis π-donor ligands.