Chelating bis-NHCs

Polydentate NHC ligands have provided numerous new compounds [19]. Chelate-N-heterocyclic ligands also allow the fine-tuning of topological properties such as steric hindrance, bite angles, chirahty and fluxional behavior. In the case of Ir—chelate—NHC compounds, most of the data that have been reported were obtained as an extension of investigations into their rhodium analogues [19, 20]. 

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Early work by Strassner and co-workers showed that the chelating bis-NHC Pd complexes 32a and 32c were capable of promoting the oxidation of methane, whilst the iodo-analogues 32b and 32d were inactive under the same reaction condition [45], Indeed, in a mixture of TEA and TFAA, in the presence of potassium peroxodisulfate under 20-30 bar of methane, trifluoroactic acid methyl ester is produced, using 32a or 32c as catalyst (Scheme 10.14). hi a more recent work, the authors disclosed the use of pyrimidine-NHC Pd complexes for the same reaction. A shghtly better catalytic activity was obtained with the unexpected cationic complex 34 [46],... 

Triethylamine has also been used to obtain chelate bis-NHC complexes of Rh, Ru, and Ir [90,92-97]. For example, the reaction of bisimidazolium salts with [RhCl(cod)]2 [90] and [(p-cymcnc)RuCl2h [97] afforded mono-and bis-NHC complexes, depending on the steric hindrance provided by the N-alkyl group, as shown in Scheme 22. 

Note It is possible to prepare stable cis-dimethyl compounds ofnickel(II) and palladiumf II) with chelating bis-NHC ligands [138,139]. 

Figure 3,74 Chelating bis-NHC ligands and their transition metal complexes.

M. L. Trudell and Cheng synthesized a chelating bis-NHC ligand (L15) that was optimized specifically for the synthesis of A -heteroaryl-7-azabicyclo[2.2.1]heptanes.50 They demonstrated that this ligand could aminate various pyridyl bromides and chlorides to give the desired product in low to moderate yields. 

The structural features and catalytic applications of rhodium and iridium complexes with chelating bis(NHC) ligands were comprehensively reviewed by our group in 2007 [46] and then in 2009 [47]. Since then, some other interesting examples have been reported in the literature. 

Herrmann and coworkers reported Rh(I) complexes bearing chelating bis-NHC ligands based on 1,2,4-triazoles, benzimidazoles, and imidazoles with different wingtips and alkylidene linkers [48]. Compound 5, depicted in... 

In an example that shows the strong donor character of NHCs, the tripodal polydentate NHC ligand in 11.26 stabilizes Fe(V) as an organo-metallic nitride. Not all potentially chelating bis-NHC ligands in fact form chelates, however, even when the chelate would be thermodynamically favored. Each NHC often binds to a separate metal in a 2 1 complex as kinetic product because, unlike M-PR3, M-NHC bond formation is not reversible, so the error cannot be remedied by reversible dissociation (Eq. 11.42). 

Some chiral complexes were prepared from chelate bis-NHC complexes. For example, axially chiral Rh NHC complexes were prepared from axially dissymmetric l,r-binaphthalenyl-2,2 -diamine, providing a new Rh(lll)-NHC chiral complex 329. ... 

Ir(l) and Ir(lll) complexes containing a chelating bis(NHC) derived from both imidazole 557 and triazole have been prepared and used as efficient catalysts for transfer hydrogenation of ketones,aldehydes,including enolizable ones, and imines. Remarkably, in the case of ketones, a reverse activity (Ir > Rh) was found for these complexes. 

Munz D, Strassner T. Propane activation by palladium complexes with chelating bis(NHC) ligands and aerobic cooxidation. Angew Chem Int Ed Engl. 2014 53 2485-2488. 

The same authors developed an interesting system for aerobic oxidation of propene using the palladium(ii) catalysts with chelating bis(NHC) ligands and NaVOs as co-catalyst. The palladium(ii) catalyst was responsible for C-H functionalization, while the vanadium co-catalyst mediated the O2 activation. Both catalytic cycles were connected by a bromide-bromine redox pair, which mediated the C-H activation by oxygen. Unfortunately, a similar catalytic system for the oxidation of methane was extremely slow, probably due to decomposition of the catalyst. 

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