Palladium–benzimidazolium salt catalyst systems for Suzuki coupling: development of a practical and highly active palladium catalyst system for coupling of aromatic halides with arylboronic acids

Iridium(III) Blue Phosphors with Heteroleptic Carbene Cyclometalates: Isomerization, Emission Tuning, and OLED Fabrications

Ir(III) complexes are particularly noted for their excellent photophysical properties in giving blue OLED phosphors. In this study, two distinctive carbene pro-chelates LAH2 + and LBH2 + (or LCH2 +) were employed in preparation of heteroleptic Ir(III) complexes, to which LAH2 + bears a cyano substituted benzoimidazolium along with N-mesityl appendage, while LBH2 + (or LCH2 +) carries the symmetrical benzoimidazolium entity. Notably, the reversible equilibration at high temperature was observed for m, f-ct14 and m, f-ct15 with a single LA chelate. In contrast, only the mer-substituted m-ct16 was obtained upon employing two LA chelates. All Ir(III) complexes exhibited blue photoluminescence (ΦPL ≥ ${\ge }$ 78 %) with short radiative lifetimes (τrad ≤ ${\le }$ 1.05 μs) in solution. The Ph OLED device with m-ct16 afforded an external quantum efficiency (EQE) of 22.8 % at 5000 cd ⋅ m-2. Moreover, the hyper-OLED based on m-ct16 and v-DABNA exhibited EQE1000 of 32.1 % (EQE recorded at 1000 cd ⋅ m-2) and J90 of 15.0 mA cm-2 (current density at 90 % of max. EQE). Its suppressed efficiency roll-off (EQE of 32.1 % and 27.7 % at 1000 cd ⋅ m-2 and 10000 cd ⋅ m-2) demonstrated a milestone in fabrication of blue OLED devices.

Phosphazene-Based Covalent Organic Framework as an Efficient Catalyst (COF-1) for the Dehydration of Fructose to 5-HMF

5-Hydroxymethylfurfural (HMF) is a promising organic platform for producing value-added chemicals. In this work, we focused on using a covalent organic framework (COF-1) as a heterogeneous catalyst for the dehydration of fructose to 5-HMF. The unique phosphazene unit-functionalized pores of COF-1 are essential active sites for catalytic performance. The results show that under the optimized reaction conditions, a maximum yield of 90% was obtained within 1.5 h at 120 °C. Furthermore, the effects of the catalyst load, reaction temperature, and usage of solvents for the improvement of reaction yield were investigated. The catalyst recyclability results showed that the yield of HMF did not change appreciably (90-82%) over five consecutive recycling runs. This work provides a viable strategy by applying phosphazene-based COF-1 for the efficient synthesis of HMF from renewable biomass. The synthesized HMF was further used for the synthesis of the biopolymer monomer furan-2,5-dimethylcarboxylate (FDMC) through N-heterocyclic carbene (NHC)-catalyzed oxidative esterification.

Removal of residual functionalized ionic liquids from water by ultrasound-assisted zero-valent iron/activated carbon

Numerous applications of ionic liquids (ILs) are often accompanied by the generation of aqueous wastes. Due to the high toxicity and poor biodegradability of ILs, effective chemical treatment is of great importance for their removal from aqueous solution. In this work, an ultrasound-assisted zero-valent iron/activated carbon (US-ZVI/AC) micro-electrolysis technique was used to degrade residual functionalized ILs, 1-butyl-3-methyl benzimidazolium bromide ([BMBIM]Br) and 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) in aqueous solution, and the degradation degree, degradation kinetics and possible degradation pathways were investigated. It was shown that the degradation of these functionalized ILs was highly efficient in the US-ZVI/AC system, and the degradation degree was as high as 96.1% and 92.9% in 110 min for [BMBIM]Br and [AMIM]Cl, respectively. The degradation of [BMBIM]Br could be described by the second-order kinetics model, and [BMBIM]⁺ was decomposed in two ways: (i) sequential cleavage of N-alkyl side chain of the cation produced three intermediates; (ii) the 2-positioned H atoms of the benzimidazolium ring were first oxidized, and then the imidazolium ring was opened. The degradation of [AMIM]Cl followed the first-order kinetics rule, and the 2,4,5-positioned H atoms of the imidazolium ring were oxidized to induce ring opening. In addition, the removal of total organic carbon was found to be >87%, which indicates that most of the ILs was mineralized in the degradation process. These results suggest that ultrasound-assisted ZVI/AC micro-electrolysis is highly effective for the removal of residual functionalized ILs from aqueous environment.

A facile synthetic route to benzimidazolium salts bearing bulky aromatic N-substituents

An atom-economic synthetic route to benzimidazolium salts is presented. The annulated polycyclic systems: 1,3-bis(2,4,6-trimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (1-Cl), 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium chloride (2-Cl), 1,3-diphenyl-1H-benzo[d]imidazol-3-ium chloride (3-Cl), and 1,3-di(pyridin-2-yl)-1H-benzo[d]imidazol-3-ium chloride (4-Cl) were prepared in a two-step synthesis avoiding chromatographic work-up. In the key step triethyl orthoformate is reacted with the corresponding N¹,N²-diarylbenzene-1,2-diamines and then further transformed in situ, by alkoxy abstraction using trimethylsilyl chloride (TMSCl), and concomitant imidazole ring closure.

Organosilica-metallic sandwich materials as precursors for palladium and platinum nanoparticle synthesis

Synthesis of organosilica-metallic sandwiches materials thanks to the immobilization of metallic species in the interlayer space of lamellar bilayer nanostructure.

Nanocrystals: why do silver and gold N-heterocyclic carbene precursors behave differently?

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal-N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal-NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (L(1)), 1,3-bis(mesityl)imidazol-2-ylidene (L(2)), and 1,3-bis(2,6-(i)Pr2C6H3)imidazol-2-ylidene (L(3)), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine-boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag-NHCs in the presence of thiols whereas Au-NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (L(3)). This demonstrates that a well-defined route for a given metal cannot be extended to another metal.

Synthesis, characterization and microwave-promoted catalytic activity of novel N-phenylbenzimidazolium salts in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions under mild conditions

A number of novel benzimidazolium salts having aryl substituents such as N-phenyl, 4-chlorophenyl and various alkyl substituents were synthesized. Their microwave-assisted catalytic activities were evaluated in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions using a catalytic system consisting of Pd(OAc)₂/K₂CO₃ in DMF/H₂O under mild reaction conditions with consistent high yields, except those of 2-bromopyridine.

The synthesis of some perhydrobenzimidazolinium salts and their application in pd-carbene catalyzed heck and suzuki reactions

Novel 1,3-dialkylperhydrobenzimidazolinium chloride salts were prepared as precursors of N-heterocyclic carbenes 3a-e by reacting N,N'-dialkylcyclohexandiamine, triethyl orthoformate and ammonium chloride. The salts were characterized spectroscopically and the complexes formed in situ from Pd(OAc)2 and 3 have been tested as catalysts in homogenous Heck and Suzuki reactions.

Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions—A Synthetic Chemist's Perspective

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.