Type Of N-heterocyclic Carbene Research Articles

Synthesis of Stable Pentacoordinate Silicon(IV)–NHC Adducts: An Entry to Anionic N-Heterocyclic Carbene Ligands

This work features the previously undescribed interactions of Martin’s spirosilane with different types of N-heterocyclic carbenes (NHCs). The level of interaction proved to be strongly dependent on the size of the Lewis base and could vary from the formation of isolable classical Lewis adducts to abnormal Lewis adducts, as evidenced by X-ray diffraction structure analyses and NMR studies. It has been found that abnormal adducts could be used as precursors for the synthesis of anionic NHCs bearing a weakly coordinating siliconate component. Complexation of these new types of carbenes with gold(I) and copper(I) has been efficiently accomplished. DFT calculations performed on the siliconate-based anionic NHC ligands revealed a high-lying HOMO and therefore a strong σ-donor character.

Quantum Mechanical Study of N-Heterocyclic Carbene Adsorption on Au Surfaces.

There is increasing interest in using N-heterocyclic carbenes (NHCs) as surface ligands to stabilize transition-metal nanoparticles (NPs) and to replace thiols for the preparation of self-assembled monolayers (SAMs) on gold surfaces. This type of surface decoration is advantageous because it leads to improved catalytic activity of NPs and increased stability of SAM, as shown by recent experiments. In this work, we used quantum mechanics combined with periodic surface models to study the adsorption of NHCs on the Au(111) surface. We found that NHCs prefer to bind to the top site with adsorption energies (ΔEs) varying from 1.69 to 2.34 eV, depending on the type of NHC, and the inclusion of solvents in the calculations leads to insignificant variation in the calculated ΔEs. Three types of NHCs were found to bind to Au(111) more tightly and therefore should be better stabilizers than those commonly used. Importantly, by analyzing electronic structures using the Bader charge and energy decomposition analysis, we find that during adsorption NHC acts as an electron donor, transferring its electron density from the lone pair orbital at the carbene center to the empty d orbital of Au with negligible π-back-donation. This binding pattern is very different from that of CO, a ligand commonly used in organometallics, where both interactions are equally important. This leads to the identification of the protonation energies of NHCs as a descriptor for predicting ΔEs, providing a convenient method for computational high-throughput screening for better NHC-type surface ligands.

Quantifying N-heterocyclic carbenes as umpolung catalysts in the benzoin reaction: balance between nucleophilicity and electrophilicity

Different types of N-heterocyclic carbenes, i.e. normal, reduced heteroatom stabilized, remote and abnormal carbenes, are computationally assessed (THF-CPCM-B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)) in the benzoin reaction to study their characteristics as umpolung catalysts.

N‐Heterocyclic Carbene Complexes of Rhodium and Iridium: Steric Effects on Molecular Conformation

AbstractA series of new RhI triazole‐based N‐heterocyclic carbene (NHC) complexes [(cod)Rh(NHC)Cl] 2a–c were obtained by transmetallation from AgI complexes using n‐butyl, benzyl and neopentyl NHC wing tip substituents. The corresponding new RhI ionic complexes (3a‐c) were synthesized bytreating compounds 2a–c with triphenylphosphane and AgBF4. These complexes were characterized by nmr spectroscopy and the structures of 2c, 3a, 3b, and 3c were determined by single‐crystal X‐ray diffraction. Compound 3b formed two polymorphs, the first reported incidence of polymorphism in this type of NHC. Ir analogues, 4, of complexes 3a–c were also synthesized; two complexes are isomorphous with the Rh analogues 4b and 4c while 4a has a different structure. Differences in syn and anti molecular conformation of the NHC ligands is rationalized in terms of stabilizing intramolecular C–H···π interactions and steric crowding of the PPh3 ligand.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)