Iridium dihydrides

In these reactions, the major diastereomer is formed by the addition of hydrogen syn to the hydroxyl group in the substrate. The cationic iridium catalyst [Ir(PCy3)(py)(nbd)]+ is very effective in hydroxy-directive hydrogenation of cyclic alcohols to afford high diastereoselectivity, even in the case of bishomoallyl alcohols (Table 21.4, entries 10-13) [5, 34, 35]. An intermediary dihydride species is not observed in the case of rhodium complexes, but iridium dihydride species are observed and the interaction of the hydroxyl unit of an unsaturated alcohol with iridium is detected spectrometrically through the presence of diastereotopic hydrides using NMR spectroscopy [21]. 

Figure 9.16 A series of iridium dihydrides that were fully characterized using standard NMR techniques [33].

MLX plots and electron count, 1, 36 MLX plots and ligand bond number, 1, 42 MLX plots and reactivity patterns, 1, 42 MLX plots and valence, 1, 40 multicenter bonding, 1, 32 pi- and delta-backbonding, 1, 29 pi-donation considerations, 1, 29 Covalent character, dn configuration compounds, 1, 20 Covalent interaction studies, and bis(arene)chromium, 5, 345 CPPL, see Circularly polarized photoluminescence Crabtree s catalyst, for iridium dihydride complexes,... 

Another example of nucleophilic abstraction is the use of trimethylamine A-oxide (Me3NO), a widely-used reagent for decarboxylation of metal carbonyl complexes (equation 8.53).73 Equation 8.54 describes a reaction where the iridium dihydride complex undergoes CO abstraction while a similar monohydride complex undergoes hydride extraction. The reason for the difference in reactivity of Me3NO is not clear.74... 

Scheme 3 Selective formation of specific iridium dihydride species due to electronic and steric differentiation...

The oxidative addition of alkanes with formation of alkyl hydride complexes was first demonstrated directly in studies using indium complexes [14], Thus the iridium dihydride derivative Cp lr(H)jPMe3 (Cp = pentamethylcyclopenta-dienyl) after irradiation in solution in cyclohexane or neopentane, produces the complexes Cp (PMe3)Ir(H)(C6Hn) and Cp (PMe3)Ir(H)CH2CMe3 in a satisfactory yield. Other saturated hydrocarbons and benzene also readily add to an iridium complex. 

In the Graham system, irradiation of the carbonyl iridium complex 1 extrudes a CO molecule and leads presumably to the coordinatively unsaturated intermediate 2 which then inserts into the CH bonds of cyclohexane or neopentane giving the compounds 3. Similarly, Bergman found that the iridium dihydride 4 loses H2 upon irradiation when the reaction takes place in cyclohexane or neopentane, the hydrido alkyl compounds 6 are formed, resulting from the oxidative addition of a CH bond to the intermediate 5. 

The reactivity of these iridium dihydride compounds has been largely studied in both stoichiometric and catalytic processes. A few selected examples are illustrated below. 

P. Kang, C. Cheng, Z.E Chen, C.K. Schauer, T.J. Meyer and M. Brookhart, Selective electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride Pincer complexes,. Am. Chem. Soc. 134,2012, 5500-5503. 

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