Multidentate NHCs

Poyatos et al. investigated the activity of the N-C-chelating oxazole-NHC containing Pt complex 142 with various substrates (Figure 13.16). From phenylacetylene, the p-( )-isomer and the a-isomer were the only two reaction products (60 40 respectively). Styrene hydrosilylation led to the formation of the linear product with 85% selectivity (70% conv.). The cationic Pt" complex, 143, of multidentate NHC, was found to be an efficient catalyst precursor for hydrosilylation of alkynes, and only a- and p-( ) regioisomers were produced (Figure 13.16). The water-soluble Pt complex 144, bearing sulfonated NHC, was an active and recyclable catalyst for the hydrosilylation of phenylacetylene and other alkynes at room temperature in water. ... 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

Figure 3.20 Architectures of complexes of multidentate, pyridine functionalised NHC ligands.

The many synthetic methods available for preparing NHCs have allowed for their ready incorporation into an increasingly diverse set of multidentate ligands. The topic of chelating ligands containing NHCs has been covered in several reviews.Heteroleptic variations have included phosphines, amines, imines, pyridines, oxazolines, amides, alkoxides, aryloxides, thioethers, and other donors. 

Several of the factors that appear to be important in preventing reductive elimination include the use of multidentate ligands, the employment of ancillary ligands that restrict the bite angle and the steric size of the substituents on the NHC and the electronics of these substituents. 

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