Rare earth metal NHC complexes

Early transition and rare earth metal NHC complexes tested in the polymerization of isoprene are shown in Figure 6.15. 

Early transition and rare earth metal NHC complexes were also employed for the polymerization of D- and L-lactide. These systems are particularly noteworthy as they exploit the utility of the NHC to function not only as a... 

Rare earth metal NHC complexes have primarily been applied in polymerization reactions, addition of terminal alkynes and amines to carbodiimides, as well as catalytic C-N coupling. Although a relatively limited amount of research has been carried out in this area, these preliminary investigations have proven the utility of REM NHC complexes as catalysts. 

Danopoulos described the preparation of indenyl- and fluorenyl-fimctiona-lized NHC ETM complexes 29 and 30 through a salt-elimination reaction (Figure 6.3). By a dilferent synthetic method Wang et al. reported elosely related complexes [(NHC)M(CH2SiMe3)2] 31-33 bearing Y, Lu or Sc, which represented the first examples of functionalized NHC covalently bonding to rare earth metal alkyl complexes." ... 

The first rare earth metal NHC compounds were simple monodentate NHC adducts. These eomplexes can be prepared either by the substitution of weakly coordinating donor molecules by the free NHC (isolated or in situ generated) or by simple ligand assoeiation, eommonly used especially for metal complexes with low eleetron eounts where stability benefits from additional electron density. 

Figure 6.3 Indenyl- and fluorenyl-bridged NHC early transition and rare earth metal complexes (Dipp = 2,6-diisopropylphenyl).

Searching for Backbonding in NHC Rare Earth Metal Complexes... 

Summarizing, metal-to-carbene Ti-interactions are generally not observed for early transition and rare earth NHC complexes, the only example so far being uranium compound 53. This type of backbonding, however, seems restricted to highly electron rich rare earth metal centres such as the uranium(III) ion. 

Scheme 6.2 Saturated backbone alkoxy-functionalized NHC complexes of rare earth metals.

Scheme 6.4 Synthesis of rare earth metal complexes with amido-phenoxo-functionalized NHCs.

Figure 6.6 Rare earth metal complexes with indenyl- and fluorenyl-functionalized NHCs.

The list of rare earth-element multiple bonds is still missing terminal metal carbenes. However, CaveU (Apama et al., 2000), Le Hoch (Cantat et al., 2005), and Liddle (Mills et al., 2010 Chart 4) synthesized formal rare earth carbene complexes by incorporating the carbene in the framework of a chelating bis(iminophosphorano)methylene (N2C ) or bis (diphenylthiophosphinoyl)methylene (S2C ) ligand. Also, P. Arnold reported numerous examples of interesting reactivity of f-block complexes supported by A -heterocyclic carbenes (NHCs Chart 4 Arnold and Casely, 2009 Liddle and Arnold, 2005). 

In contrast to unsaturated alkoxy-functionalized NHCs, a deprotonation of corresponding imidazolium salts of saturated analogues did not result in alkali metal adducts but in bicyclic products HL (R = Dipp, Mes, i-Pr) 19. Arnold et al reported a series of lanthanide NHC complexes (Scheme 6.2) based on these ligand precursors by using a protonolysis reaction with internal bases such as rare earth tris[bis(trimethylsilyl)amido] or alkyl compounds. ... 

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. ... 

Cui reported an unusual synthesis of rare earth CCC-pincer bis(NHC) complexes, which are formally aryl-metallated compounds (46). The corresponding bis (imidazolium) dibromide was deprotonated with n-BuLi in situ in the presence of MCI3 [eqn (6.3)]. Unfortunately, the mechanism of the exchange of chlorides by bromides is unknown. ... 

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