Cyclometallated NHCs

Cyclometallated-NHC complexes constitute an important class of donor-fimctionalized NHCs. The fundamental aspects of the cyclometallation reaction... 

Cyclometalated-NHC complexes of Pd 19 and 20 (Figure 9.4) were tested in Heck coupling of -butyl acrylate with several aryl bromides and chlorides. At... 

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. 

On the other hand, Thomas highlighted a C-H activation mechanism (experimental, spectroscopic and computational studies) leading to active NHC-Pt hydride complexes. Conejero and co-workers studied the reactivity of cationic bis(NHC)-Pt complexes, bearing a cyclometallated NHC ligand, while De Jesus and co-workers described water-soluble [(NHC)Pt] complexes substituted with sulfonatoalkyl moities. ... 

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. ... 

Major achievements in cyclometallation processes involving NHC ligands have been achieved by Nolan and coworkers. It was observed that the solvent could play a crucial role in the formation of rhodium- and iridium-based complexes. As shown in Scheme 39, the reaction performed in pentane between [M(COE)2Cl]2 (M = Rh or Ir) and four equivalents of FBu led only to COE substitution by the NHC ligand affording rhodium-based product (243) (the iridium complexing proving difficult to isolate). On the other hand, the same reaction carried out in hexanes gave, via C H activation, the hydride complexes (244) and (245). Finally, in benzene, a unique double cyclometallation process occurred to yield the coordinatively unsaturated 16-electron... 

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. 

Rollover Cyclometalation Pathway in Rhodium Catalysis Dramatic NHC Effects in the C-H Bond Functionalization... 

More recently, Nolan and co-workers have reported an example of C-H activation of the same ligand at Rh upon complexation with the free NHC. Treatment of [Rh(COE)2Cl]2 (COE = cyclooctene) with IMes at room temperature led to the total displacement of the COE ligand with simultaneous splitting of the dimer, affording a bis(NHC) coordinated product [Rh(IMes)(IMes) HCl] ((IMes) = cyclometalated l,3-bis(2,4,6-tri-methylphenyl)imidazol-2-ylidene) with a single C-H functionalised orthomethyl group (Fig. 32). 

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 first example of Ru-mediated activation of C(sp )-H bonds of indenyl-functionalised NHCs was reported in 2009. ° Reaction between free carbene IndCH2CH2lmR (Ind = indenyl, Im = imidazol-2-ylidene, R = Mes/Dipp) and Ru3(CO)i2 in refluxing benzene led to four distinguishable multinuclear Ru-NHC C-H activated products. Each product was characterised using NMR and IR spectroscopy, combustion analysis and ultimately X-ray crystallography. The authors propose that the C-H activated products form via a cyclometalated triruthenium intermediate, in which an indenyl proton (Ha) is susceptible to further activation to deliver the dinuclear r] -coordinated indenyl complex, followed by elimination of H2, CO and a Ru(CO)4 unit (Fig. 43A). Alternatively, a side-chain proton (Hb) and vinylic proton (Ho) may be activated to afford the trinuclear r], ri -coordinated complex, with concomitant loss of H2 (Fig. 43B). 

New applications of chiral palladacycle catalysts were carried out by Liu and Shi [67], They synthesized axially chiral cyclometalated bidentate NHC-Pd(II) complexes derived from binaphthyl-2,2 -diamine and used them for the asymmetric addition of indole to N-tosylaryliniines. 

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