Salts bisimidazolium

Addition of butyl lithium to a suspension of palladium(II) diiodide and methylene bridged bisimidazolium salts leads to the in situ formation and complexation of the NHC resulting in the cationic [(chelate)2Pd]l2 in low yield. Higher yields... 

The most widely used method for the preparation of free NHCs is the deprotonation of an azohum salt with NaH or KOBuf [10,14,37]. In the case of N,N -methylene-bridgcd bisimidazolium salts, the preparation of the free dicarbenes is only possible by the use of potassium hexamethyldisilazide (KHMDS) in toluene [14,41]. Other strong bases deprotonate the methylene bridge breaking the bisazol unit [42],... 

Sometimes the reaction products depend on the deprotonation method used. For example, RajanBabu and coworkers have obtained chiral N-hetero-cyclic biscarbenes of Pd and Ni from a binaphtyl bisimidazolium salt. The in situ deprotonation of the bisimidazolium salt with KOfBu prior to the addition of Pd(OAc)2 gives exclusively the trans isomer, 24, while direct reaction of the salt with the metal complex in hot DMSO (see next section) gives a mixture of the cis-(25) and trans-isomers (Scheme 17) [81]. 

Triethylamine has also been used to obtain chelate bis-NHC complexes of Rh, Ru, and Ir [90,92-97]. For example, the reaction of bisimidazolium salts with [RhCl(cod)]2 [90] and [(p-cymcnc)RuCl2h [97] afforded mono-and bis-NHC complexes, depending on the steric hindrance provided by the N-alkyl group, as shown in Scheme 22. 

The C2 - H oxidative addition of imidazolium salts to metal complexes was recently proved for metals other than low valent group 10. The reaction of a ferrocenyl-bisimidazolium salt to [IrCl(cod)]2 in the presence of NEt3 provided the first evidence of the preparation of a stable NHC-Ir(III)-H complex by direct oxidative addition of the imidazolium salt [96]. It was proposed that the ferrocenyl fragment may be sterically protecting the M - H from further reductive elimination, but later it was shown that this fragment was not necessary in order to obtain the desired NHC-Ir(III)-H complexes (Scheme 43) [159]. The role of the weak base (NEt3 in this case) had to be reconsidered in order to explain the overall metallation process, and it was proposed that a mechanism as that shown in Scheme 44 may better explain the process. The oxidative addition of the C2 - H bond of the imida-... 

Substitution of Other Ligands. A variety of other ligands were found to be easily displaced by NHC ligands. We report here some significant examples. Substitution of pyridine in [Ir(COD)Py2] by SIMes gave [(SIMes)Ir(COD)Py]. The steric bulk of SIMes allowed for substitution of only one pyridine. Reaction of biscationic complex [Ir(Cp )(CH3CN)3] +(101) with bisimidazolium salt (102) in the presence of triethylamine led to the displacement of two acetonitrile molecules and formation of cationic complex (103) (equation 14). "... 

The first pincer ligand bearing NHC moieties 1 was presented by Peris and Crabtree in 2001 [9] (Figure 9.1). The bisimidazolium salt precursor of this neutral CNC ligand with a pyridine backbone 1 can be synthesized by a nucleophilic ipso substitution of 2,6-dichloro- or dibromopyridine. With this strategy, also... 

Pyridine-bridged bisimidazolium salts also afforded the preparation of dimetallic Rh(l) species by transmetallation from a silver(l)-NHC complex and deprotonation with a weak base. A pincer pyridine-bis-NHC Rh(lll) complex 330 was obtained from the reaction of the corresponding bisimidazolium salt and [Rh(/r-Cl)(cod)]2 in the presence of... 

The 3 and 3 positions of l,r-mBlhylene bis(imidazole) 14 can be readily substituted with different bromoester 23 conq>ounds to form ester functionalized bisimidazolium salts 24. The carboxylic fimctionality of tiiese salts can be potentially used for condensations with small peptides known to have binding affinities to overexpressed receptor sites of certain tumor cells. Other targeting groups such as 2-nitroimidazole could also be incorporated in a similar manner. The hexafluorophosphate salt of the ethyl ester con )ound reacts with AgaO in DMSO to form the silver carbene conq)lex 25. 

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