Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

Activation and reactivity of (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films. Self-assembled monolayers (SAMs) have shown tremendous number of applications but can suffer from low stability. Here, the authors report air and bench stable carbene precursors allowing facile SAM formation, and furthermore demonstrate an application in biosensing

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Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

Carbon disulfide (CS2) and carbonyl sulfide (OCS) are indirect greenhouse gases that can be effectively trapped by classical, abnormal and remote nitrogen heterocyclic carbenes (NHCs), according to high level ab initio calculations. The process is described through a reaction profile involving two minima, a non-covalent complex and a covalently bound product, connected by a single transition state. Both CS2 and OCS react towards NHCs in a similar way, forming a new C-C bond and leading to very stable products with feasible barriers in many cases, although they vary significantly depending on the NHC structure. The barriers are larger than those reported for CO2, oscillating from barrierless processes up to a maximum of 57.9 kJ mol-1, whereas the products are more stabilized than those incorporating CO2. The lowest barriers for the CS2 + NHC reactions correspond to the largest C-C distances in the products, unlike the CO2 case. Remarkably, the most favored reactions, which are those involving a remote NHC, do not exhibit the highest interaction energies at the TS, but low distortion energy values of the OCS/CS2 moieties. The decomposition of the interaction energy allowed to confirmed that in fact the remote carbene is the less favored one in terms of the electrostatic and exchange terms. Substitution in CO2 of O by a more polarizable atom such as S have a great influence on the balance between the steric plus orbital interaction and the kinetic energy terms, thus making the products between NHCs and OCS/CS2 more stable. Both OCS and CS2 become better charge acceptors than CO2 on going from the starting complexes to the products.

Binding indirect greenhouse gases OCS and CS2 by nitrogen heterocyclic carbenes (NHCs)

The energies of 62 minima and 6 transition states have been calculated with the M06-2x functional and the aug-cc-pVDZ basis set They correspond to carbenes and neutral molecules related to the products obtained by addition of N-heterocyclic carbenes (NHCs) to CO2, CS2, and COS The NHCs belong to two “classical” NHCs (imidazoles and benzimidazoles) and one “remote” NHC (pyrazole) Several of these structures in the same series are related by prototropic tautomerism To compare these calculated energies with experimental X-ray structures, solvation effects by water, calculated with a continuous model (PCM), as well as some dihydrates and dimers, have also been calculated

Prototropic tautomerism of the addition products of N-heterocyclic carbenes to CO2, CS2, and COS

DFT calculations have been carried out to study the detailed mechanisms of the carboxylative cyclization reaction between propargylic alcohols and CO2 catalyzed by N-Heterocyclic Olefins (NHO), as well as the molecular orbital theory. Results indicated that this type of reaction prefers a three steps mechanism controlled by free NHO rather than to be catalyzed by the NHO–CO2 adducts. For the first step, CO2 promotes the hydrogen transfer from alkynol to NHO to form the carboxylate, in which propargylic alcohols was deprotonated by the free NHO acted as the catalyst precursor to form the alkynol anion; meanwhile, alkynol anion captures carbon dioxide to form the carboxylate. We found this CO2 promoting Hydrogen abstraction mechanism would decrease the reaction energy barrier and increase releasing heat of this reaction. Secondly, a five-membered-ring intermediate is easily formed to generate carboxylate via an intramolecular ring-closing reaction. Finally, the production generated through a protonating process.

DFT study of N-Heterocyclic Olefins-catalyzed carboxylative cyclization of CO2 with alkynol: A CO2-promoted hydrogen abstraction mechanism

A series of cationic N-heterocyclic carbene (NHC) precursors that can be utilized as fluorescent chemosensors for carbon dioxide capture were investigated by density functional theory (DFT) calculations. Activation energy barriers for the reactions of the cationic NHC precursors and hydrogen carbonate (HCO3(-)) based on intrinsic reaction coordinate (IRC) profiles as well as proton affinity of the precursors were compared. The calculated proton affinity of 1-ethyl-3-methylimidazol-2-yliene was in good agreement with experimental one within the margin of error. We clarified main factors to lower the activation energy barrier based on the correlation among the number of N-heterocyclic functional group, aromatic ring size, and structural characteristics for the candidate compounds. On the basis of the results, it was verified that some of our model systems spontaneously generate NHCs without any specific catalyst.

Systematic strategy for designing immidazolium containing precursors to produce N-heterocyclic carbenes: a DFT study.

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Systematic strategy for designing immidazolium containing precursors to produce N-heterocyclic carbenes: a DFT study.