Iridium carbonyl-dihydride

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

The analogous iridium formate complex was synthesized by Kaska and coworkers by reacting CO2 with a Ir(lll) dihydride complex. However, in this case, the formate complex proved to be unstable, undergoing disproportionation to form the hydrogen carbonate complex and the carbonyl dihydride, overall corresponding to the reverse water gas shift reaction CO2 -H H2 - CO-P H20[45j. Reduction of CO2 to the methanol level has since been effected using an aromatic nickel pincer complex and a cascade reaction involving a ruthenium pincer complex in one step [46]. 

Thus, to understand how this compound is produced, the reactivity of (PNP)Ir(H)2 complex (101) was explored with carbon monoxide. The addition of CO to a CH2CI2 solution of complex (101) resulted in the immediate and quantitative formation of the cis-dihydride carbonyl complex (102). This compound is not stable in solution at room temperature, even under an atmosphere of hydrogen, and undergoes reductive elimination of hydrogen to yield the iridium carbonyl complex (103) (Scheme 2.47). 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

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... 

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 mechanism of the iridium-catalyzed hydroformylation has been studied by NMR spectroscopy. Species including iridium acyl and alkyl dihydride intermediates were detected. The results confirmed that the CO-deficient atmosphere favors hydrogenation over carbonylation [76]. 

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