Imidazolium salts iridium complexes

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

For imidazolium salts 1, an alternative pathway with deprotonation and carbene generation at the C4/C5-position was observed previously the carbenes thus generated are called abnormal carbenes [79-81]. Likewise, suitably substituted imidazo[l,5-a]pyridinium salts can be deprotonated to mesoionic carbenes 10b and the corresponding silver, iridium and rhodium complexes were formed. 

Crabtree and Chianese have extended the scope of Hoveyda s ligand by making the imidazolium salt 39 in two steps from l,l/-diamino-2,2/-binaphthyl (Fig. 10) [80]. They prepared neutral rhodium and iridium complexes with that ligand precursor and applied these complexes in the asymmetric hydrosilylation of acetophenone. Moderate enantioselectivities were obtained with the iridium derivative (up to 60% ee) whilst the rhodium catalysts only gave low enantioselectivities. 

The ligand was then used to form a variety of transition metal carbene complexes [207] (see Figure 3.72). Interestingly, more than one method for the formation of transition metal carbene complexes was successfully employed presence of an inorganic base (IC COj) to deprotonate the imidazolium salt and the silver(I) oxide method with subsequent carbene transfer to rhodium(I), iridium(I) and copperfi), respectively. The silver(I) and copper(I) carbene complexes were used for the cyclopropanation of styrene and indene with 1,1-ethanediol diacetate (EDA) giving very poor conversion with silver (< 5%) and qnantitative yields with copper. The diastereomeric ratio (endolexo) was more favonrable with silver than with copper giving almost a pnre diastereomer for the silver catalysed reaction of indene. 

We are in for a few surprises with the chemistry of rhodium and iridium. Let us stay with the phosphino functionalised imidazolium salt PhjPCHjCHJmMes and react it with the precursor complexes [M(cod)OEt]j (M = Rh, Ir) [296]. As expected, the corresponding cationic phosphino functionalised carbene complexes are formed (see Figure 3.96). 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

In both cases, no catalytic reactions were performed with the iridium(I) and palladium(II) complexes synthesised with these ligands. However, Peris and coworkers used the synthesis of their iridium carbene complex to show that the formation of the M-NHC bond is an oxidative addition that can be assisted by a weak base. This base does not deprotonate the imidazolium salt, but traps the protons formed in the process [152],... 

Peracetylated glucose, brominated in the 1-position, was reacted with V-methylimi-dazole to yield the corresponding carbohydrate functionalised imidazolium salt [114], Reaction with silver(l) oxide and subsequently with [Cp lrCyj yields the silver(I) and iridium(lll) NHC complexes, respectively (see Figure 6.47). The structural parameters of the Cp lr(NHC) complex are similar to those of analogous Cp lr(NHC) complexes featuring nonfunctionalised carbenes [115,116]. 

Recently, Peris and coworkers reported the synthesis of highly stable orthometallated Cp -Ir NHC complexes and a catalytic application in the deuteration of organic molecules. Complexes (227) were synthesized from imidazolium salt (225) and [IrCp Cl2]2 as metal precursor either by a one-step procedure with sodium iodide to minimize the mixture I versus Cl or by transmetallation from the corresponding silver carbene affording (226) followed by the C-H activation of the phenyl ring by the iridium... 

The direct oxidative addition of the C2-H of azolium salts to electron-rich late transition metals is another method for the preparation of NHC-metal complexes. For example. Peris showed that a bis(imidazolium) salt could oxidatively add to an iridium(i) centre to form an iridium(iii) hydride complex... 

Direct C-H activation at abnormal carbene positions was achieved by heteroatom-directed cyclometalation reactions using pyridyl-functionalized azolium salts and [IrCp Cl2]2 (Cp = pentamethylcyclopentadienyl Scheme 3.4). Cyclometalation proceeded well with both triazolium salt 20 and imidazolium salt 22 and yielded iridium complexes 21 and 23 in good yields. Transmetalation from silver was not efficient in either case. The imidazolium precursor underwent activation and oxidation of the exocyclic C2-bound CH3 group, whereas triazolium salt 20 formed a mixture of compounds upon reaction with Ag20, probably due to poor selectivity and competitive coordination of the pyridyl nitrogen. 

Scheme 3.4 Cyclometalation of triazolium and imidazolium salts to form abnormal carbene iridium(III) complexes 21 and 23.

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

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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