Formyl iridium complex

The sole example of a cationic formyl complex was reported recently by Thorn (67). It was obtained by the oxidative addition of formaldehyde to a coordinatively unsaturated iridium cation, as shown in Eq. (14). Characterization included IR and H, l3C, and 3IP NMR. Formyl 49 is stable as a solid to 146°C. 

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). 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

Reactions of NH3 with iridium(I) complexes have also been reported. Formaldehyde has been oxidatively added to iridium(I), resulting in hydrido, formyl complexes ... 

Exchange of formyl hydrogens for tritium is observed to occur in both aryl aldehydes (with concurrent ortho labeling in appropriate cases) and aliphatic aldehydes using [(cod)Ir(PCy3)(py)]PF6. Partial reduction of aldehydes to alcohols may occur in some cases. Labeling by other iridium phosphine catalysts has not been reported but is likely to occur. This type of catalytic activity, which likely involves reversible oxidative addition of the iridium center into the formyl C—H bond, is different in outcome from that of organorhodium complexes, whose insertion into formyl C—H bonds proceeds instead to decarbonylation. 

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