Cyclometallated NHCs substituents

On the other hand, the study of a heteroaryl-NHC-p-diketone combination in a cyclometallated NHC-Pt complex " (Figure 10.11) showed that the observed emission was mainly based on the NHC ligand, with a significant metal eontribution. Then, the tuning of photophysical properties can be obtained by a simple ehange in the nature and size of substituent groups as well as the size of the n-system. 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

In this system, the cyclometallation may be favored by the presence of the methyl substituents on the NHC backbone, increasing the a-donor character of the NHC ligand and enhancing the C H activation process. These reactions have been successfully apphed to introduce other ligands onto the metal center, such as alkoxy or amino groups that can be used as proton sponge. 

Alongside these findings, Choudhury and co-workers have demonstrated a wingtip-dictated chemoselective cyclometalation of an NHC framework at electrophilic Ir and Ru metal centres. Under competitive conditions, two different N-substituents (phenyl versus pyridyl) show preferential C-H bond activation which is guided by the electronic nature of the C-H bond involved and the ring size of the resultant metallacycle. Treatment of l-(4-pyridyl)-3-phenylimidazolium chloride with Ag20 followed by [IrCp Cl2]2 in dichloromethane provided exclusive formation of the pyridyl C-H bond activated product in 64% yield (Fig. 37A). 

The most common process involving the N-substituents of NHCs is C-H activation to give cyclometalated species. Such processes may not necessarily be deleterious for catalysis, as they can be reversed by the addition of H2. Indeed, in some cases the cyclometalated species were directly involved in mediating H2 transfer. Other reactions involving C-C and C-N activation of the N-substituents tend to have more drastic implications for catalyst integrity. However, in some cases these reactions involve novel processes that are of interest in their own right. A number of studies considered the decomposition reactions of Grubbs alkene metathesis catalysts and these will be considered in more detail in Section 2.4. 

Although a variety of A-alkyl substituted NHCs were susceptible to C-H activation, as illustrated by Herrmann, " huge differences in reactivity were observed depending on the substituent. For example, treatment of the A-ethyl or A-isopropyl complexes 15 and 16 with base generated cyclometalated iridium hydrides 17 and 18 (Equation (3.5)). Interestingly, the related A-propyl and A-butyl carbene complexes did not yield such cyclometalated complexes. Yamaguchi proposed that the accessibility of the p-hydrogens to the metal was responsible for this remarkable difference. ... 

There are several reports of rare earth NHC complexes with additional stabilization by cyclometallation of one of the ligand side chains. In most cases a C-H activation to form a metal-alkyl complex is required. For example, at the same time as Yb-NHC adduct 13 was reported, Takats observed the cyclometallation of N-Me substituent of IMe as a result of the reaction of Yb(CH2SiMe3)(THF)] with an excess of free carbene to form [(Tp " )( IMe) Yb(CH2 MeiMeXTHF)] 43. ... 

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