An official website of the United States government
Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd( ii ), Cu( i ), Ni( ii ), Fe(0), Zn( ii ), Ag( i ), and Au( i / iii ) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K( i ), Al( iii ), Zn( ii ), Sn( ii ), Ge( ii ), and Si( ii / iv ). Among them, K( i ), Al( iii ), and Zn( ii ) complexes were used for the polymerization of caprolactone and rac -lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO 2 ), nitrous oxide (N 2 O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO 2 and the catalytic reduction of carbon dioxide, including atmospheric CO 2 , into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.
more »
« less
This content will become publicly available on January 30, 2026
Electron-rich Bis(imino)acenaphthene ligands enable bench-stability on zero-valent Nickel
Low-valent metal complexes, particularly those based on earth-abundant metals, offer a sustainable and cost-effective alternative to precious metal catalysts. However, their intrinsic reactivity often limits their practical applications. Herein, we disclose the synthesis and characterization of two novel air- and moisture-stable nickel(0) complexes supported by electron-rich BIAN ligands. These complexes exhibit remarkable stability and catalytic activity in various industrially relevant C-C bond-forming reactions. Our results demonstrate the potential of these complexes as robust and versatile catalysts for a wide range of synthetic transformations.
more »
« less
- Award ID(s):
- 2216471
- PAR ID:
- 10657594
- Publisher / Repository:
- ChemRxiv
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Transition metal‐catalyzed, non‐enzymatic nitrene transfer (NT) reactions to selectively transform C−H and C=C bonds to new C−N bonds are a powerful strategy to streamline the preparation of valuable amine building blocks. However, many catalysts for these reactions use environmentally unfriendly solvents that include dichloromethane, chloroform, 1,2‐dichloroethane and benzene. We developed a high‐throughput experimentation (HTE) protocol for heterogeneous NT reaction mixtures to enable rapid screening of a broad range of solvents for this chemistry. Coupled with the American Chemical Society Pharmaceutical Roundtable (ACSPR) solvent tool, we identified several attractive replacements for chlorinated solvents. Selected catalysts for NT were compared and contrasted using our HTE protocol, including silver supported byN‐dentate ligands, dinuclear Rh complexes and Fe/Mn phthalocyanine catalysts.more » « less
-
null (Ed.)Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [( o -Ph 2 P(C 6 H 4 )Carb)Au(tht)] 2+ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb + = 9- N -methylacridinium and Carb + = [C(Ar N ) 2 ] + (Ar N = p -(C 6 H 4 )NMe 2 ) are active as catalysts for the cycloisomerization of N -propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [( o -Ph 2 P(C 6 H 4 )C(Ar N ) 2 )Au(tht)] 2+ , which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [( o -Ph 2 P(C 6 H 4 )Carb)] + . While cyclization into phosphonium species is observed for Carb + = [C(Ar N ) 2 ] + , [C(Ph)(Ar N )] + , and 9-xanthylium, [( o -Ph 2 P(C 6 H 4 )-9- N -methylacridinium)] + can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties.more » « less
-
Gerard Parking (Ed.)The oxygen evolution reaction (OER) of water splitting is essential to electrochemical energy storage applications. While nickel electrodes are widely available heterogeneous OER catalysts, homogeneous nickel catalysts for OER are underexplored. Here we report two carbene-ligated nickel(II) complexes that are exceptionally robust and efficient homogeneous water oxidation catalysts. Remarkably, these novel nickel complexes can assemble a stable thin film onto a metal electrode through poly-imidazole bridges, making them supported heterogeneous electrochemical catalysts that are resilient to leaching and stripping. Unlike molecular catalysts and nanoparticle catalysts, such electrode-supported metal-complex catalysts for OER are rare and have the potential to inspire new designs. The electrochemical OER with our nickel-carbene catalysts exhibits excellent current densities with high efficiency, low Tafel slope, and useful longevity for a base metal catalyst. Our data show that imidazole carbene ligands stay bonded to the nickel(II) centers throughout the catalysis, which allows the facile oxygen evolution.more » « less
-
Fluorine is an essential component in many highly effective pharmaceutical drugs, however the selective fluorination of organic molecules poses a challenge. A common route to installing fluorine involves C-F bond cleavage, which is often accomplished using second- or third-row transition metals. Base metal catalysts such as nickel may provide a facile, sustainable, and cheaper alternative for C-F activation. Monodentate N-heterocyclic carbene (NHC) nickel complexes have been reported to undergo C-F activation, however bis-bidentate NHC (RNHC2R1; R, R1 = alkyl or aryl) analogs remain underexplored. This work reports a series of RNHC2R1 nickel(0) complexes with various R1 linkers to determine the effect of the linker on the C-F activation of hexafluorobenzene. Comparisons include a reference nickel(0) complex with two monodentate NHC ligands, and results show that low-valent nickel NHC complexes readily break the C-F bond in C6F6 via oxidative addition. Crystallographic and NMR characterization demonstrate that ligand design and denticity affect the cis versus trans orientation of the final product, with the possibility for additional ligand C-H activation.more » « less
