Ir-H bond

The iridium(II) complexes are rarer that those of rhodium(II). Iridium does not seem to form carboxylates Ir2(02CR)4 with the lantern structure complexes analogous to trans-RhX2 (PR3 )2 are not formed with bulky tertiary phosphines, probably because the greater strength of Ir-H bonds leads to IrHX2(PR3)2. 

Reactions of the Ir1 complexes (289) and (290) with H2 and subsequent reaction of the dihydride products with dimethyl acetylenedicarboxylate, Me02CC CC02Me (dma), give the Ir11 complexes (291), (292), (293), and (294), as shown in Reaction Scheme 26.482 The X-ray crystal-structure determination of (294) confirms the double insertion of the alkyne into each of the Ir—H bonds of (293). 

At shorter distances the H—H bond is completely ruptured and replaced by the two Ir—H bonds of the equilibrium product species (Fig. 4.63). The geometry at the metal atom exhibits the expected sd2-like geometry (cf. Fig. 4.2(b)), and the final airn NHOs and NBO conform closely to this description, as shown in Fig. 4.66. 

The predominance of isomer 2c over 2a or 2b is consistent with Crabtree s findings, which convincingly demonstrated that in the reaction of H2 with [Ir(pyridine) (PR3)(COD)]PFg, the formation of an Ir-H bond trans to the N ligand is electronically favored [23]. Highly selective formation of isomer 2c results from H2 addition to the... 

The reaction may proceed as follows (see Scheme 10.4). An acrylate 51 coordinates to the iridium-hydride complex generated in situ, and then inserts into the Ir-H bond to form a o-lr complex 55 the coordination and insertion of another acrylate to 55 leads to an iridium complex, 56. A (5-hydride elimina-hon of the iridium-hydride from the intermediate 56, followed by isomerization of the double bond to a more stable internal aUcene, results in a head-to-tail dimer. 

The position of conformational equilibrium in OH-substituted heterocycles may be estimated by IR H-bonding studies.1428 For example, for the equilibrium 95 96 97, two bands at 3609 cm 1 (free OH) and 3475 cm 1 (intramolecular H-bonded OH) are observed. The % trans-fused conformer (60%) was calculated by comparison of the integrated area of the 3609 cm "1 band with that of the model compound 98.143... 

An iridium(III) hydride, I, coordinates the imine in a bidentate fashion to give complex II. The coordinated C=N imine bond inserts into the Ir-H bond to give the iridium complex III which after hydrogenolysis, probably via hetero-lytic splitting of the H-H bond, eliminates the amine adduct and regenerates the Ir(III) hydride. 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

More intriguing is that the prodnct of fiiran addition to [Ir(COD)(PMe3)3]Cl (106), undergoes an insertion reaction with t-bntylacetylene, not into the Ir-H bond bnt into the Ir ... 

IR H bonding of acetic acid, benzoic acid (and deutero-), GCI3GOOH, 2.5-5 fi. 

V. M. Chulanovslut. Doklady Akad. Nauk S.S.S.R, 101, 649-52 (1955). IR H bond effect on methanol C—O stretch, liquid, solution in acetone, CCI4, hexane. 

E. Halpern, J. B. Bouck, H. Finegold, and J. Goldenson. J. Am. Chem. Soc. 77, 4472-4 (1955). IR H bonding of phos-phoryl compounds with CHCls, C2CI6H, C2CI4H2. 

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