Imidazolium salts pincer

Figure 3.149 Synthesis of a C,N,C pincer imidazolium salt and its palladium(ll) complexes.

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

Carbene transfer to palladium yields the palladium complex [Pd(pincer)Me]Cl that undergoes a 1,2-methyl shift to form a pendant imidazolium salt at 150 °C in DMSO. This makes the pincer carbene ligand still relatively thermally stable with regard to reductive elimination. This is probably due to the difficulty encountered by the NHC unit in the pincer carbene ligand to orientate itself perpendicular to the pyridyl plane, the most favourable orientation for the reaction with the cix-methyl group. 

Examples come from Hahn et al. [491] who synthesised the pincer imidazolium salt from a,a -dibromolutidine and A-alkyl-benzimidazole using a standard protocol. Synthesis of the palladium(ll) complex was then achieved with palladium(ll) acetate in DMSO. The structure is similar to that of the palladium(II) pincer carbene complexes with nonannulated pincer ligands. Temperature dependant H-NMR spectroscopy... 

Formation of the palladium(n) complex does not require the silver(I) complex, but can be achieved by reaction of the pincer imidazolium salt with [PdjCdbalj] [136]. As already described for the corresponding CT4,C pincer complex, the C,C,C palladium(ll) complex displays a conformational flexibility with 8// of 51.6 kJ/mol for the C,N,C pincer complex and 74.5 kJ/mol for the C,C,C pincer complex. This difference in enthalpies is unexpected and was explained later by the same authors [503] as stemming from a counter anionic... 

Hollis and coworkers reported also on a transmetalla-tion process from zirconium to rhodium. As illustrated in Scheme 33, imidazolium salt (209) reacted instantaneously with Zr(NMe2)4 to generate the biscarbene pincer (210) with orthometallation of the phenyl group. The biscarbene pincer was not isolated, but reacted with [Rh(COD)Cl]2. The Rh(III) complex (211) was isolated in equilibrium with the dimer (212). 

Directed activation of a C(sp )-H bond by an iron complex was much less investigated than the reaction of C(sp )-H bonds. Li reported the phosphine-directed C (sp )-H to form an iron pincer complex (e.g., [80]). Ohki and Tatsumi reported that Cp Fe complexes bearing imidazolium salts undergo cyclometalation through C-H activation or can cleave the C-H bond of a heteroarene [81]. 

All other reported syntheses of pincer group 9 complexes bearing two NHC donor moieties involve the oxidation of the metal center. The first Rh pincer complex with two NHC donor moieties was reported by Peris and coworkers [30]. They reacted the imidazolium salt l 2HBr [9] with [Rh(COD)(p-Cl)]2 (COD, cyclooc-tadiene) and KBr in presence of triethylamine at 80 °C to obtain the Rh(III) tri-bromido complex 32. Lower temperatures led to the dimetallic species 31, which could be converted to the pincer complex 32 by heating and addition of KBr [30] (Scheme 9.3). 

In a manner similar to platinum ones, rhodium—carbene complexes have been recently tested in the hydrosilylation of alkenes and alkynes (3). Especially, rhodium complexes with N-heterocyclic carbene (NHC) ligands have attracted considerable attention. Their performance is comparable with that of phosphine complexes. The following exemplary ligands were used 1,3-imidazoylidene chelate bis(imidazolinum-carbene) phosphine-functionalized NHC alkylammonium-imidazolium chloride salts NHC pincer complexes, pyridine-functionalized N-heterocyclic carbenes (also with iridium), and bis(dichloroimidazolylidene) (also with iridium). 

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