The road to Au(I)-NHC complexes: straightforward, scalable and efficient synthetic conditions

In this recent report, we describe the multigram syntheses of some of the most common N-heterocyclic carbene gold(I) complexes. This is achieved through straightforward and practical approaches using easily obtained starting materials.

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N-Heterocyclic Carbenes (NHCs) have been a 20-year (and counting) adventure for us. Over the years and with the help of highly-talented scientists, we have seen these entities evolve from lab curiosities, where they were initially considered as tertiary phosphine mimics, into a ubiquitous family of ligands in organometallic chemistry and catalysis. Nowadays, NHCs are truly everywhere and readily available to all chemists in a multitude of flavors and uses.

In spite of a plethora of advances in this field, one aspect that still needs improving is their synthetic access, or at the very least the detailed experimental description of the latter. Easier synthetic access is a must in the evolution of the field and will most certainly lead to further democratization of the use of NHC-based systems in their numerous incarnations.

With this contribution focusing on gold(I)-NHC complexes, we highlight sustainable, user-friendly and scalable access routes that can be performed in air, in reagent grade and greener solvents.

In that context, we describe in details the multigram synthesis of the most common mononuclear N-heterocyclic carbene gold(I) chloride complexes bearing the N,N′-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N′-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (IPr*) ligands. This straightforward methodology is conducted under air and possesses considerable advantages over alternative routes, such as the use of a sustainable reaction solvent, minimal amounts of a mild base and commercially available or easily obtained starting materials.

>200g of [AuCl(IPr)] after drying in one of our favorite baking dishes.

Additionally, we describe the synthesis of the mononuclear gold(I) hydroxide complex bearing the IPr ligand, using the state-of-the-art method. Finally, the improved synthesis of the dinuclear gold(I) hydroxide complex [{Au(IPr)}2(μ-OH)][BF4] is also described.

All through these protocols, we kept the main focus on helping and advising the operator to navigate most of the main problems that accompanies these synthetic conditions. For that reason, numerous troubleshooting and crucial tips and tricks are described in details in hopes of allowing these protocols to be performed by researchers with standard training, while still affording high yields of micro-analytically pure bench-stable materials.

Fady Nahra

Professor, Ghent University

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