NHC-containing metal complexes

The content of this chapter is based largely on the excellent third chapter of the first edition of this book, written by Jeremy Praetorius and Cathleen Crudden at Queens University in Ontario, Canada. However, the use of NHCs and their complexes in synthesis and catalysis has become more widespread, even over the course of just five years, and so this updated chapter features an up-to-date look at the synthesis, activation, and decomposition of NHC-containing metal complexes. 

When the counteranion of the intidazolinm salt was a chloride, the neutral [(NHC)PdCl2(PPh3)] complex was obtained, the chloride triggering PPhs displacement from the metal coordination. This route proved successful also for chiral NHC-containing palladium complexes (147) and (148). The cationic nickel analog (149) has been... 

Over time, NHC-Pt eomplexes with inereasingly sophistieated molecular scaffolds have been reported, such as poly-NHC-Pt complexes, mixed imi-noboryl-NHC-Pt complex, NHC-Pt moleeular square, bis(NHC)cyclo-phane-Pt complex, polynuelear complexes, as well as NHC-mixed metal complexes containing PtM2-metallocyclopropane units or original dou-ble-NHC-Pt complex salt (Figure 10.12). 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Porphyrins and their metal complexes, respectively, occur as active sites in a number of enzymes. A-confiised porphyrins such as 75 have been discussed regarding their analogy to NHCs and their potential importance in porphyrin chemistry. Isolation of the first metal complexes containing palladium. 

Metal complexes with M-heterocyclic carbene ligands were known long before the first stable NHCs were isolated. Wanzlick [5] and Ofele [6] demonstrated as early as 1968 that NHC complexes can be obtained by in situ deprotonation of azolium salts in the presence of a suitable metal complex without prior isolation of the free NHC ligand (Fig. 1). In these cases a ligand of the metal complex precursor (acetate or hydride) acted as a base for the deprotonation of the imidazolium cation. This method has been successfully transferred to other metal precursors containing basic ligands like [Pd(OAc)2] [97] and [(cod)lr(p-OR)2lr(cod)] [98, 99]. Alternatively, an external base such as NaOAc, KOf-Bu or MHMDS (M = Li, Na, K) can be added for the deprotonation of the azolium salt [100]. In general, the in situ deprotonation of azolium salts appears as the most attractive method for the preparation of NHC complexes as it does not require the isolation of the reactive free carbene or its enetetramine dimer. 

The method can be used for certain transition metal complexes containing halides or other labile ligands. For example, this method has been used for the preparation of a bis-NHC - Au(I) complex, starting from AuCl(SMe2), AuCN [162], AuCl(THT) [163], (THT = tetrahydrothio-phene) [162], AuCl(PPh3) [162], and [MCl(CO)5]- (M = Cr, W) [164]. 

Only one example of an NHC-containing olefin metathesis catalyst containing a transition metal other than ruthenium has been reported in the literature. The NHC-osmium complexes 53a and 53b (Scheme 2) are synthesized from the dichloro(i]6-p-cymene)osmium dimer by addition of the NHC prepared in situ and abstraction of the chloride, followed by introduction of the ben-zylidene moiety with phenyl diazomethane. 

Atypical synthesis of naAed NHC-containing complexes has been reported by Cloke and coworkers. As shown in equation (7), the co-condensation of nickel, paUadium, and platinum vapor with the free NHC, TBu, provided two-coordinated homoleptic complexes of Ni(0), Pd(0), and Pt(0) (43). Unfortunately, in addition to the poor yield of the isolated product, this original methodology required a metal vapor synthesis apparatus. 

Oxidative addition of imidazolium salts to NHC-containing complexes of Ni and Pt has also been reported. IDM HI reacted with [(NHC)Pt(DMFU)2] to give the biscarbene hydride complex [(NHC)(IDM)PtHI]. Reaction of IDM HBF4 salt with the homoleptic [(IMes)2M] (143) (M = Ni or Pd) afforded the triscarbene hydride complex [(IMes)2(IDM)MIl] (144) (equation 20). This metal hydride exhibited a remarkable stability both in solution and in solid state. This stability is certainly due to the steric bulk of NHCs that shields and protects the hydride function. ... 

The transmetallation process was found competent for several metals - for instance, Crabtree and coworkers reported the transmetallation of [Ag(NHC)2](AgCl2) to Rh(I) and Ir(i) 155 Owing to its mild reaction conditions, this procedure for the synthesis of NHC-containing complexes has found... 

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