SYNTHESIS AND REACTIVITY OF THE COORDINATIVELY UNSATURATED METHYLENE COMPLEX IR=CH2[N(SIME2CH2PPH2)2]

The reactions of the coordinatively unsaturated iridium methylene complex Ir=CH2[N(SiMe2CH2PPh2)2] are described. This methylene complex is prepared by the reaction of 2 equiv of KOBu(t) with the methyl iodide derivative Ir(CH3)I[N(SiMe2CH2PPh2)2] in toluene; the extra equivalent of KOBu(t) serves to coordinate the HOBu(t) that is produced as the precipitate KOBu(t).HOBu(t). The reaction of H-2 with the methylene complex generates the trihydride amine IrH3[HN(SiMe2CH2PPh2)2] via a series of oxidative addition and migratory insertion steps; the trihydride derivative loses H-2 upon workup to generate the iridium(III) dihydride IrH2[N(SiMe2CH2PPh2)2]. The reaction of the methylene complex with PMe3 produces the square planar PMe3 derivative Ir(PMe3)[N(SiMe2CH2PPh2)2] and ethylene (0.5 equiv); monitoring this reaction at low temperatures shows the presence of a number of intermediates that suggest the mechanism involves the generation of free H2C=PMe3 which reacts with the methylene complex to ultimately give the observed products. The reaction of the methylene complex with CO results in the formation of Ir(CO)[(CH2PPh2CH2SiMe2NSiMe2CH2PPh2)], in which the methylene unit has inserted into one of the iridium-phosphine bonds of the ancillary tridentate ligand. Also produced in this reaction is a small and variable amount (less-than-or-equal-to 15%) of the iridium(I) carbonyl complex Ir(CO)[N(SiMe2CH2PPh2)2]; the fate of the coordinated methylene unit for this side reaction could not be determined. Oxidative addition reactions were also examined; addition of Al2Me6 to the methylene derivative resulted in the formation of the hydride-aluminum complex Ir(mu-AlMe2)H[N(SiMe2CH2PPh2) 2]; this heterobimetallic species has the AlMe2 unit directly bound to iridium and bridged by the amide donor of the backbone. A mechanism is proposed that involves oxidative addition of AlMe3 to the iridium center followed by migratory insertion of the methylene unit and the methyl to generate an ethyl ligand which beta-eliminates and releases ethylene to generate the hydride. The methylene complex also reacts with methyl iodide to generate the ethylene hydride iodide complex Ir(eta2-C2H4)H(I) [N(SiMe2CH2PPh2)2]; the proposed mechanism also involves oxidative addition as the first step followed by migratory insertion to generate an ethyl moiety; however, rather than simple beta-elimination to the iridium center, the amide unit abstracts the beta-hydrogen to form an amine-olefin complex that subsequently inverts at nitrogen and then oxidatively adds the N-H unit to generate the hydride complex. Deuterium-labeling experiments show that the abstraction of the beta-hydrogen is reversible since there is scrambling of the label equally into both the alpha and beta positions of the coordinated ethylene.