Iridium complexes orthometallated

Other examples of C-H H-M short contacts [44] were found in iridium complexes, and the competition between agostic interactions and hydrogen bonds was tested by QMMM calculations for several R groups hanging from an orthometallated benzoquinoline ligand. The agostic structure is favored for t-Bu, but the hydrogen bond is preferred by isopropyl [44b]. 

Gold(III) usually displays a square planar geometry, typically observed in d8 metallic complexes such as palladium(II), platinum(II), rhodium(I) and iridium (I), for which an enormous number of liquid crystals have been described [3-5], mainly as orthometallated compounds. However, only a gold(III) metallomesogen has been published. Since the first gold mesogen was reported in 1986, many other compounds have been described. 

Different from the ruthenium system previously discussed, [Ir(COD)Cl]2 reacted with the electron-rich olefin (217) to afford the homoleptic organoiridium tricycle complex (223) with an increase of the iridium oxidation state from (I) to (III) (equation 41). This spontaneous orthometallation process... 

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 reactivity of iridium is often associated with that of rhodium however, as shown in Scheme 36, iridium is more favorable toward activating the C-H bond and so this metal has been widely used for this reaction. Reaction of [Ir(COD)Cl]2 with one equivalent of the pyridine-fimctionalizedNHC (232) gave in quantitative yield the complex (233) by cleavage of the metallic precursor dimer and orthometallation of the pyridine moiety (Scheme 36). In contrast, the same reaction carried ont with [Rh(COD)Cl]2 as the metal sonrce afforded (234), for which a strong interaction between the rhodinm center and the ortho hydrogen of the pyridine was observed. Treatment of (234) with a sodinm salt did not lead to a C-H activation, bnt led to the coordination of the pyridine onto the metal center yielding the air stable (235). 

Following their work on orthometallation with iridium-based complexes. Peris etal. investigated the difference between aromatic and aliphatic C-H activation processes. They reported the preparation and reactivity of alkyl-functionalized imidazolylidene complexes of iridium (III) undergoing intramolecular cyclometallation. As shown in... 

When the iridium hydride is reacted with a hydrogen acceptor, simple oxidative addition adducts can be seen for aromatic and vinylic C-H containing substrates. With nitrobenzene, although a thermodynamic preference is seen for an orthometallated chelate product, the kinetic preference is for meta- and para-Gr-W activation, which is then followed by rearrangement to the o/n4n-activated product, which in turn coordinates the nitro group. Hence, chelate assistance is found to have no kinetic benefit for C-H activation in this complex (Equation (22)). 

A recent example of this type of orthometallation is provided by the second stage of the reaction of IrCl(cyclo-octene) with triphenylphos-phite. Some iridium(i)-phosphite complexes react to form not just one but two or three iridium-carbon bonds per molecule. The reaction sequence then involves hydrogen elimination between intramolecular oxidative additions ... 

The first example of a mononuclear iridium carbonyl hydride complex with two orthometallated phosphite ligands r-[IrHCO P(0-o-Bu C6H3)(0-o-Bu C6H4)2 2] 94 was described by Claver et al. by treating [Ir(/r-OMe)(cod)]2 with tris-ortV o-ferf-butylphenyl phosphite under carbon monoxide (Scheme 12). The complex shows great stability even under high pressures of hydrogen and carbon monoxide. ... 

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