Tuning the π-Accepting Properties of Mesoionic Carbenes: A Combined Computational and Experimental Study

Mesoionic imidazolylidenes are recognized as excellent electron‐donating ligands in organometallic and main group chemistry. However, these carbene ligands typically show poor π‐accepting properties. A computational analysis of 71 mesoionic imidazolylidenes that bear different aryl or heteroaryl substituents in C2 position was performed. The study has revealed that a diphenyltriazinyl (Dpt) substituent renders the corresponding carbene particularly π‐acidic. The computational results could be corroborated experimentally. A mesoionic imidazolylidene with a Dpt substituent was found to be a better σ‐donor and a better π‐acceptor compared to an Arduengo‐type N‐heterocyclic carbene. To demonstrate the utility of the new carbene, the ligand was used to stabilize a low‐valent paramagnetic tin compound.

Ring-opening copolymerization of ε-caprolactone and δ-valerolactone by a titanium-based metal–organic framework

Copolymerization of ε-caprolactone and δ-valerolactone without any co-catalyst in a solvent-free medium under eco-friendly conditions using earth abundant Ti-metal based MOF, MIL-125.

Stable abnormal N-heterocyclic carbenes and their applications

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 (CO₂), nitrous oxide (N₂O), 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 CO₂ and the catalytic reduction of carbon dioxide, including atmospheric CO₂, 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.

Reactivity of the carbodiphosphorane, (Ph3P)2C, towards main group metal alkyl compounds: coordination and cyclometalation

The carbodiphosphorane, (Ph3P)2C, reacts with Me3Al and Me3Ga to afford the adducts, [(Ph3P)2C]AlMe3 and [(Ph3P)2C]GaMe3, which have been structurally characterized by X-ray diffraction. (Ph3P)2C also reacts with Me2Zn and Me2Cd to generate an adduct but the formation is reversible on the NMR time scale. At elevated temperatures, however, elimination of methane and cyclometalation occurs to afford [κ2-Ph3PC{PPh2(C6H4)}]ZnMe and [κ2-Ph3PC{PPh2(C6H4)}]CdMe. Analogous cyclometalated products, [κ2-Ph3P{CPPh2(C6H4)}]ZnN(SiMe3)2 and [κ2-Ph3P{CPPh2(C6H4)}]CdN(SiMe3)2, are also obtained upon reaction of (Ph3P)2C with Zn[N(SiMe3)2]2 and Cd[N(SiMe3)2]2. The magnesium compounds, Me2Mg and {Mg[N(SiMe3)2]2}2, likewise react with (Ph3P)2C to afford cyclometalated derivatives, namely [κ2-Ph3PC{PPh2(C6H4)}]MgN(SiMe3)2 and {[κ2-Ph3PC{PPh2(C6H4)}]MgMe}2. While this reactivity is similar to the zinc system, the magnesium methyl complex is a dimer with bridging methyl groups, whereas the zinc complex is a monomer. The greater tendency of the methyl groups to bridge magnesium centers rather than zinc centers is supported by density functional theory calculations.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Polar organometallic strategies for regioselective C–H metallation of N-heterocyclic carbenes

This Feature Article focuses on the recent emergence of s-block metal-mediated N-heterocyclic carbene metallations and the new opportunities this methodology offers to access unique anionic NHC fragments.

NormalandabnormalNHC coordination in cationic hydride iodide complexes of aluminium

Sterically demanding NHC aluminium hydride iodide complexes react with one equivalent of NHC to cationic mixednormal–abnormalNHC Al^IIIcomplexes.

Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)₂ ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

The ring-opening polymerization of ε-caprolactone and l-lactide using aluminum complexes bearing benzothiazole ligands as catalysts

A series of aluminum complexes bearing benzothiazole ligands was synthesized and the ring-opening polymerization of ε-caprolactone (CL) and l-lactide (LA) using these aluminum complexes as catalysts was studied.

Cationic aluminum hydride complexes: reactions of carbene–alane adducts with trityl-borate

Reaction of (Idipp)AlH₃ with [Ph₃C][B(C₆F₅)₄] in toluene affords the dimeric aluminum dication [((Idipp)AlH(μ-H))₂][B(C₆F₅)₄]₂ with the corresponding reaction of (IBn)AlH₃ gives [(IBn)₂AlH][B(C₆F₅)₄]₂.

Comparing l-lactide and ε-caprolactone polymerization by using aluminum complexes bearing ketiminate ligands: steric, electronic, and chelating effects

Herein, the l-lactide polymerization rate of a series of Al complexes bearing ketimine ligands was investigated, and the polymerization characteristics between l-lactide and ε-caprolactone were compared.