Electronic properties, of NHC ligand

Fig. 1.10 Schematic illustration of the structural points whose modification can be used to tune the electronic properties of NHC ligands...

Note There are no significant differences in the electronic properties of NHC ligands deriving from N-substitution or indeed unsaturation versus saturation issues [19]. 

Pd-catalyzed C-C-coupling reactions of the Heck type, e. g., Suzuki [48c] and Stille [49c] cross-coupling, are excellent tools for the preparation of biaryls (see also Section 3.1.9). The activity and stability of the catalyst is highly dependent on the steric and electronic properties of the ligands. Sterically demanding basic alkylphosphines e. g., tri(/-butyl)phosphine, have proven to be very effective ligands in the Heck reaction [50] as well as in the Suzuki cross-coupling [51]. NHCs resemble those basic phosphines (see above) and therefore were tested... 

Metal complexes of NHCs are extremely important as catalysts in reactions of importance to the field of organic synthesis. Section 10-1-1 has already addressed the electronic and steric properties of NHC ligands that enhance the catalytic activity of NHC-metal complexes, and section 10-2-3 discussed methods of synthesis. Very little chemistry, however, occurs at Ccarbene. Instead, NHCs act mainly as supporting ligands, so that useful chemistry can take place at other positions on the catalyst complex. We will see applications of the use of NHC-metal complexes for synthesis in Chapters 11 and 12. 

The first efficient catalytic applications of N-heterocyclic carbene ligands with late transition metals were reported for rhodium. Since the mid 1990s, NHC-rhodium chemistry has been extensively studied and may be comparable to palladium or nickel in terms of the number of catalytic applications. Outside the context of catalysis, NHC-Rh complexes have featured prominently in studies probing the electronic properties of NHCs, usually quantified by comparison of the pco frequencies in the IR spectra of [LRh(CO)2Cl] complexes. ... 

Enyne cycloisomerization can be considered as a benchmark reaction to evaluate the catal)Aic activity of new gold catalysts. Furstner and co-workers reported that the electronic and steric properties of NHC ligands on Au complexes can dramatically change the chemo- and regioselectivity of such a type... 

These results suggest that imidazolidin- and imidazol-based skeletons transfer similar amounts of electron density to the metal. The conclusion that changes in the bridge of the NHC skeleton have such a small effect on the electronic properties of the NHC is quite surprising, considering that SIMes- and IMes-based catalysts often show remarkably different catalytic behaviour. It is still unclear if these small changes in the electronic properties of the NHC ligand confer such different catalytic behaviours, or other effects (steric, flexibility, etc.) should be invoked. 

Bielawski et al. described the synthesis and properties of NHC 15 introducing a novel carbene architecture incorporating a l,l -disubstituted ferrocene moiety as backbone [71 ]. In its metal complexes [71,72] this carbene ligand supported a long-range electronic communication between the iron center and the carbene coordinated transition metal. 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

NHC are strong a-donor ligands and are even more electron-rich than the already electron-rich trialkylphosphines. Whereas in the early 1990s n-acceptor properties of NHC were disputed, it became obvious that NHC can act as n-acceptors and that the degree of n-acccptance depends on the nature of the metal bound and the NHC itself (Sanderson et al. 2006). 

The exceptionally strong basicity of the NHC is reflected in their equally great nucle-ophihcity. A convenient way to measure the electronic properties of ligands is through the Aj carbonyl stretching frequencies of the corresponding transition metal carbonyl complexes [113], The standard is known as the Tolnian Electronic Parameter (TEP) and is derived from the [Ni(CO)3L] complex of the ligand. Many different transition metal carbonyl complexes have been used and conversion tables as well as theoretical methods for the computation of TEP values are available [113,114]. 

The cyclooctadiene ligand is easily replaced by two molecules of CO. The vCO stretching frequencies of the carbonyl ligands can be used to estimate the electronic properties of the l,3,7,9-tetramethylxanthine-8-ylidene ligand. The electron donor ability is found to be less than for pyrimidine based carbenes [99] or imidazol-2-ylidenes [100]. It is also one of the strongest NHC rr-acceptor ligands known [98]. 

The selective activation and functionalization of C H bonds has attracted much attention in recent years. In spite of the common proposal, which postulates that NHC ligands coordinated to TM are relatively inert, several examples of C-H activation processes involving NHCs have been reported. Because of the high electron-donating property of NHCs, some TM NHC complexes have been found to undergo facile intramolecular C-H bond activation. 

Postpolymerization modification is yet another method of tailoring polymer properties and can be considered a modular attribute of a macromolecule [37]. Given that the metals in the bis(NHC) polymers are coordinatively unsaturated, the use of an exogenous ligand added postpolymerization was expected to bind to the metal thus altering the physical and electronic properties of the polymer. Addition of PPhj or PCyj to a suspension of polymer 53a in THF quickly affected complete dissolution of the phosphine-bound polymer. Ligation was confirmed by H and P NMR spectroscopy. 

Theoretical studies have been carried out on various ruthenium indenyhdene and benzyhdene complexes in order to estimate the impact of these ligands on olefin metathesis catalytic activity. A comparative study of the key steps associated with the activation of complexes bearing NHCs revealed only minor differences between the benzyhdene and the indenyhdene complexes [77]. It was recently shown that, due to the different steric and electronic properties of these hgands, not all ruthenium indenyhdene complexes foUow the same initiation mechanism... 

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