Iridium complexes halides

The pattern of iridium halides resembles rhodium, with the higher oxidation states only represented by fluorides. The instability of iridium(IV) halides, compared with stable complexes IrCl4L2 and the ions IrX (X = Cl, Br, I), though unexpected, finds parallels with other metals, such as plutonium. Preparations of the halides include [19]... 

Other halides can be introduced by metathesis. Figure 2.55 summarizes some of the complexes isolable with dimethylphenylphosphine, similar in general to the corresponding iridium complexes (section 2.13.3), including the photochemical isomerization of the mer-isomer. 

Remarkable carbon-boron bond-forming reactions are catalyzed by iridium complexes and proceed at room temperature with excellent regioselectivity, governed by steric factors. Heteroarenes are borylated in the 2-position and this reaction is generally tolerant of halide substituents on the arene (Equations (87) and (88)). 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Since it is known that halo(phenyl)acetylenes add oxidatively to Vaska s complex to give (7-phenylethynyl iridium(III) halides, 32112, the intervention of phenyliodonium iridium(III) and rhodium(III) intermediates, 33, in the alkynyliodonium reactions seems plausible. In any case, the production of cr-alkynyl complexes with alkynyl(phenyl)-iodonium triflates appears to be both more general and efficient21. 

The cationic complexes [Ir(CO)2(SbPh3)3]+ and [Ir(CO)2(PR3)2]+ (R = Ph, C6Hn) have been synthesized from iridium carbonyl halide derivatives with halogen acceptors in the presence of CO.109... 

The iridium(I) halide complexes [IrCl], [IrBr] and [Irl] are believed to exist. Heat treatment (to 1043 K) of [IrCl3] causes sublimation of [IrCl], which decomposes above 1071 K and is insoluble in both alkaline and acidic media.172 [IrBr] may be obtained by heating the iridium(III) bromide to 758 K in an HBr atmosphere. The monobromide complex is stable at lower temperatures, and is slightly soluble in water, in acidic and in alkaline media.173 [Irl] forms upon heating either [Irl2] or [Irl3] to 628 K in an HI atmosphere.173... 

In an important report, Janowicz and Bergman have described the photochemical incorporation of an alkyl group from an alkane into an iridium complex. The process does not appear to involve alkyl radicals, but is thought to occur via two successive concerted steps. This report may point the way to new catalytic processes for the functionalization of alkanes. The photoaddition of alkyl halides to alkenes catalysed by Cu complexes represents a novel process for formation of C-C bonds, and may have useful synthetic applications (Mitani et ai). The possibility of analogous intramolecular reactions comes readily to mind. 

Evidence for a radical pathway includes the observation that the reaction is accelerated by radical initiators (such as oxygen or peroxides) and the presence of UV light. Moreover, the order of reactivity for the R group is IIP > II0 > 1°, which is inconsistent with a direct displacement mechanism, but is in accord with the stability of alkyl radicals. Radical inhibitors (such as steri-cally hindered phenols) retard the rate of reaction with sterically-hindered alkyl halides, but not when R = methyl, allyl, and benzyl. When stereoisomerically pure alkyl halides are used, OA results in the formation of a 1 1 mixture of stereoisomeric alkyl iridium complexes, consistent with the formation of an intermediate radical R-. 

The existence of iridium(II) halide complexes remains somewhat doubtful, although the preparation of both [IrBrj] and [Irl2] has been described, Heating [IrBrj] at 713 K in a stream of HBr reportedly yields the reddish-brown species [IrBr,], while [Irl2] was similarly obtained from the iridium(II) iodide in HI at 603 K. EPR studies on the product of electron irradiation of [IrCl ] in an NaCl matrix reveal a paramagnetic species formulated [1106] . ... 

A more prevalent approach has been the nucleophilic alkylation of a metal halide, as in the metathesis of 344 with methyllithium to afford Tp Rh(Me) (rj -C Rs) (348). This approach was also employed to prepare the iridium complex Tp Ir(Me)Br(PMe3) (372) from the respective dibromide. Similarly, a range of alkyl halides of general formula Tp Rh(R)X(CNCH2 Bu) (374-385,... 

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