Copper-NHC complexes in catalysis

Although the chemistry of copper has a long history [1a-d], the relatively recent discovery of N-heterocyclic carbene (NHC) as transition-metal supporting ligands has permitted novel vistas to be explored in copper reactivity and catalysis [1e,f]. Shortly after the seminal discovery of Arduengo, Raubenheimer reported a neutral copper carbene complex [1] and [2]. However, the field remained dormant for almost ten years. In the early 2000s, new breakthroughs were achieved: first, the synthesis of NHC–copper using Cu₂O was reported by Danopoulos and followed by the first application in catalysis by Woodward [3] and [4]. The work by Buchwald and Sadighi appeared next, where the first catalysis using a well-defined complex was described [5].

The first reports in this field were based on systems used to mimic their phosphine relatives. NHCs have become ligands of significant interest due to their steric and electronic properties [6], [7] and [8]. Combining the NHC ligand family and copper became, for some, an obvious and productive area [6]. Over the last decade alone, numerous systems have been developed. Copper–NHC complexes can be divided into two major classes: neutral mono-NHC and cationic bis-NHC derivatives: [Cu(X)(NHC)] [9] (X = halide, acetate, hydroxide, hydride, etc.) and [Cu(NHC)(L)][Y] (L = NHC or PR₃; Y = PF₆, BF₄) [10].

The neutral-halide-bearing complexes have been widely used in catalysis, mainly due to their ease of synthesis [9]. In addition to halide-bearing complexes, notable important related compounds have been reported: Nolan and co-workers disclosed the first hydroxide derivative [Cu(OH)(IPr)] (IPr = N,N’-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene) and Sadighi published alkoxides, hydrides and borate species, which permitted novel reactivity to be explored [9g-i]. With respect to cationic derivatives, homoleptic and heteroleptic bis-NHC complexes have been reported and have been efficiently used in catalysis allowing important improvements [10]. In this review, an overview of the two classes and their respective catalytic performance will be presented.