Iridium reactions with halogens

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

Iridium-catalysed C-H-boronation can be carried out using either pinacol borane or pinacol diborane, both methods giving comparable results. Reaction occurs at a-positions of five-membered rings and is compatible with halogen substituents, as exemplified below. ... 

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

Halogen Electrodes.—The determination of the standard potentials of the halogens is simple in principle it involves measurement of the potential of a platinum electrode, coated with a thin layer of platinum or iridium black, dipping in a solution of the halogen acid or a halide, and surrounded by the free halogen. The uncertainty due to liquid junction can be avoided by employing the appropriate silver-silver halide or mercury-mercurous halide electrode as reference electrode. In practice, however, difficulties arise because of the possibility of the reactions... 

For M(V) and M(VI), no binary compounds with the heavier halogens and no oxides are known. Iridium(VI) fluoride is the precursor to [Ir(CO)g] +, the only example to date of a tripositive, binary metal carbonyl cation. Compare reaction... 

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