Simple synthesis of [Au(NHC)X] complexes utilizing aqueous ammonia: revisiting the weak base route mechanism

Aqueous ammonia has been examined as a new weak base for the synthesis of [Au(NHC)Cl] complexes, as well as for the activation of C-H, S-H, and N-H bonds. Its low cost and mild operational conditions (in air and using technical grade solvents) make it an attractive alternative for producing gold-NHC complexes. Synthetic pathways have been investigated in silico, assessing the role of the deprotonation and metalation steps within the reaction mechanisms.

Theoretical Study on the Mechanism of Ru(II)-Catalyzed Intermolecular [3 + 2] Annulation between o-Toluic Acid and 3,5-Bis(trifluoromethyl)benzaldehyde: Octahedral vs Trigonal Bipyramidal

Density functional theory was utilized to investigate the mechanism of Ru(II)-catalyzed aromatic C-H activation and addition of aromatic aldehydes. The proposed catalytic cycle consists of C-H bond activation, aldehyde carbonyl insertion for C-C coupling, lactonization for the formation of the final product, product separation, and catalyst recovery. Our calculations suggest that Ru(OAc)2(PCy3) (referred to as CAT) is the most favorable active catalyst, facilitating the C-H bond activation to form a five-membered ring cycloruthenium intermediate (INT2). Subsequently, the aromatic aldehyde reactant 2a enters the Ru coordination sphere, accelerating the C-C coupling and lactonization for the formation of the final product. The involvement of acetate assists in the final product separation, while INT1 re-enters the Ru coordination sphere to initiate a new catalytic cycle. Utilizing the energetic span model, the apparent activation free energy barrier was computed to be 34.3 kcal mol-1 at 443 K. Furthermore, exploration of the reaction mechanism in the absence of phosphine ligands identified Ru(OAc)2(p-cymene) as the most favorable active catalyst. The derived apparent activation free energy barrier offers a comprehensive explanation for the experimentally observed yields. Additionally, we have examined the disparities between the octahedral and trigonal bipyramidal structures of the catalysts concerning their effects on the reaction mechanisms and apparent activation free energy barriers.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

Palladacyclic architectures have been shown as versatile motifs in cross-coupling reactions. NHC-ligated palladacycles possessing unique electronic and steric properties have helped to stabilize the catalytically active species and provide additional control over reaction selectivity. Here, we report on a synthetic protocol leading to palladacycle complexes using a mild base and an environmentally desirable solvent, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene ligands. The readily accessible complexes exhibit high catalytic activity in the Buchwald–Hartwig amination. This is achieved using low catalyst loading and mild reaction conditions in a green solvent.

Base-free synthesis of benchtop stable Ru(III)-NHC complexes from RuCl3·3H2O and their use as precursors for Ru(II)-NHC complexes

A series of Ru(III)-NHC complexes, identified as [Ru^III(PyNHC^R)(Cl)₃(H₂O)] (1a-c), have been prepared, starting from RuCl₃·3H₂O following a base-free route. The Lewis acidic Ru(III) centre operates via a halide-assisted, electrophilic C-H activation for carbene generation. The best results were obtained with azolium salts having the I^- anion, while ligand precursors with Cl^-, BF₄^-, and PF₆^- gave no complex formation and those with Br^- gave a product with mixed halides. The structurally simple, air and moisture-stable complexes represent rare examples of paramagnetic Ru(III)-NHC complexes. Furthermore, these benchtop stable Ru(III)-NHC complexes were shown to be excellent metal precursors for the synthesis of new [Ru^II(PyNHC^R)(Cl)₂(PPh₃)₂] (2a-c) and [Ru^II(PyNHC^R)(CNC^Me)I]PF₆ (3a-c) complexes. All the complexes have been characterised using spectroscopic methods, and the structures of 1a, 1b, 2c, and 3a have been determined using the single-crystal X-ray diffraction technique. This work allows easy access to new Ru-NHC complexes for the study of new properties and novel applications.

A green route to platinum N-heterocyclic carbene complexes: mechanism and expanded scope

A sustainable and facile weak-base synthetic route to platinum N-heterocyclic carbene (NHC) complexes is disclosed. The mechanism of this reaction is also elucidated via experimental and computational investigations. This straightforward protocol is then used for the synthesis of novel Pt(II)-NHC complexes and its utility is further explored to access key Pt(0)-NHC precatalysts.

Electronically tuneable orthometalated RuII–NHC complexes as efficient catalysts for C–C and C–N bond formations via borrowing hydrogen strategy

A series of simple and electronically tuneable cyclometalated RuII–NHC complexes have been explored as efficient catalysts for various C–C/N bond forming reactions via a BH methodology.

Simple synthesis of [Ru(CO3)(NHC)(p-cymene)] complexes and their use in transfer hydrogenation catalysis

A novel, efficient and facile protocol for the synthesis of a series of [Ru(NHC)(CO₃)(p-cymene)] complexes is reported. This family of Ru-NHC complexes was obtained from imidazol(in)ium tetrafluoroborate or imidazolium hydrogen carbonate salts in moderate to excellent yields, employing sustainable weak base. The ruthenium complexes were successfully utilized in the transfer hydrogenation of ketones as highly active multifunctional catalysts.

Recent advances in the synthesis and derivatization of N-heterocyclic carbene metal complexes

N-heterocyclic carbene (NHC) metal complexes have gained an incredible amount of attention in the course of the last two decades and have become indispensable as an intricate part of a plethora of applications. The areas of their synthesis and derivatization are constantly evolving and bring new, more sustainable, cost-effective and simpler approaches to the design of existing and next generation catalysts and materials. This article provides an overview of the latest developments, focusing on those which have appeared during the last two years.

The “weak base route” leading to transition metal–N-heterocyclic carbene complexes

N-Heterocyclic carbenes (NHCs) are nowadays ubiquitous in organometallic chemistry and catalysis. A simple synthetic route to these is presented.