Poly-NHCs metal complexes

Cross-coupling reactions were the first reactions in which poly-NHC metal complexes (and NHC metal complexes altogether) were successfitUy employed as catalysts. Considering also the popularity gained by such reactions over the last 20 years, further corroborated by the award of the Nobel Prize in Chemistry for their discovery in 2010, it cannot be deemed surprising that cross-coupling reactions have become the favorite tool to probe the catalytic efficiency of poly-NHC metal complexes, especially palladium(II) complexes. However, in spite of the great number of complexes of this kind that have been evaluated, litde advance has been made in the discovery of more efficient catalytic systems, as poly-NHC complexes... 

The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane hgands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often hmited to the reaction of aryl iodide or electron-poor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T> 150 °C with tetra-alkylammonium salts as promoters). " An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68." In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive. " Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions. 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

Bridging bis-NHCs allow the coordination of two metals and may facilitate the preparation of dimetallic discrete molecules or polymeric systems. While some poly-NHCs have been designed to prevent chelation and therefore favor the formation of discrete polymetallic complexes, some others rely on the synthetic... 

Poly-NHC Complexes of Transition Metals Recent Applications and New Trends... 

Research on chiral NHCs has flourished in the course of the last 20 years parallehng the success of NHCs as hgands for catalyticaUy active metal complexes and as organocatalysts in their own right. Several chiral poly-NHCs have been also proposed and prepared, but success in their apphcation as Hgands for enantioselective metal catalysts has been up to now moderate, although in the course of the last few years remarkable degrees of asymmetric induction have been achieved in a few instances. The topic has been reviewed by Shi. ... 

Figure 3 Formation of bimetallic complexes from poly-NHC metal complexes bearing free NH moieties.

In the course of the last 5 years, an enormous number of new metal complexes of poly-NHCs has been disclosed in the hterature, which sum... 

Figure 10 Examples of poly-NHC complexes with the metal center in unusual oxidation state.

Poly-NHC metal complexes with the metal in low oxidation state are also of interest. Obviously, the presence of several NHC ligands destabilizes these oxidation states in the absence of other Jt-acceptor ligands (e.g., CO) ... 

Several prominent examples of polynuclear poly-NHC metal complexes with original architectures were produced by the group of Hahn. To name but a few, in 2011, the group reported on the preparation of highly unusual di-NHC-based molecular squares (structure 58, Fig. 12) tetra-nuclear Ir(III) complexes with bridging Janus-type dicarbene Hgands were obtained from a tetranuclear complex precursor upon cycHzation of coordinated diisocyanides. More recently, the same group presented... 

Investigations on the chemistry of poly-NHC metal complexes have of course not been hmited to the preparation of complexes with novel features and properties, as extensively outlined above, but have also heavily involved their apphcations. Catalysis has long been the main field in which the use of these complexes was envisaged, but in more recent years, other applications of poly-NHC metal complexes have been identified and pursued with a notable degree of success. Besides catalysis, today there are at least two other main appHcation areas of these complexes, namely their use as emissive moieties in the development of photo- or electroluminescent devices, and as bioactive compounds, typically with antimicrobial or anti-tumoral properties the following sections will extensively cover the recent advances in these fields. 

Finally, an asymmetric variant of the Suzuki reaction was developed using as catalyst a poly-NHC metal complex with hgand 2 (see also Section 2.1). The complex exhibited a rather low catalytic activity toward aryl iodides and bromides as substrates. However, chiral binaphthyls were prepared with ees up to 70%. ... 

Reports on poly-NHC complexes as catalysts for the Kumada coupling (Eq. 4) have been quite rare, despite the successful results often achieved with mono-NHC metal catalysts for this reaction ... 

C—H activation/functionalization reactions represent one of the classes of chemical transformations that have been most intensively studied in recent years. Poly-NHC metal complexes have a long-standing tradition of involvement as catalysts in these processes, which can be tracked back to their stability under the relatively harsh reaction conditions that are often needed to drive such reactions to completion in a reasonable time. For example, the group of Strassner reported already in 2002 that paUadium(II) complexes with chelating dicarbene ligands can catalyze the... 

---