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

The reactions of the coordinatively unsaturated iridium methylene comp lex 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 p recipitate KOBu(t).HOBu(t). The reaction of H-2 with the methylene com plex generates the trihydride amine IrH3[HN(SiMe2CH2PPh2)2] via a seri es of oxidative addition and migratory insertion steps; the trihydride derivative loses H-2 upon workup to generate the iridium(III) dihydri de IrH2[N(SiMe2CH2PPh2)2]. The reaction of the methylene complex with PMe3 produces the square planar PMe3 derivative Ir(PMe3)[N(SiMe2CH2PPh 2)2] and ethylene (0.5 equiv); monitoring this reaction at low tempera tures 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 re action 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 trid entate 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 f or this side reaction could not be determined. Oxidative addition reac tions were also examined; addition of Al2Me6 to the methylene derivati ve resulted in the formation of the hydride-aluminum complex Ir(mu-AlM e2)H[N(SiMe2CH2PPh2) 2]; this heterobimetallic species has the AlMe2 u nit directly bound to iridium and bridged by the amide donor of the ba ckbone. A mechanism is proposed that involves oxidative addition of Al Me3 to the iridium center followed by migratory insertion of the methy lene unit and the methyl to generate an ethyl ligand which beta-elimin ates and releases ethylene to generate the hydride. The methylene comp lex also reacts with methyl iodide to generate the ethylene hydride io dide complex Ir(eta2-C2H4)H(I) [N(SiMe2CH2PPh2)2]; the proposed mechan ism also involves oxidative addition as the first step followed by mig ratory insertion to generate an ethyl moiety; however, rather than sim ple beta-elimination to the iridium center, the amide unit abstracts t he beta-hydrogen to form an amine-olefin complex that subsequently inv erts at nitrogen and then oxidatively adds the N-H unit to generate th e hydride complex. Deuterium-labeling experiments show that the abstra ction of the beta-hydrogen is reversible since there is scrambling of the label equally into both the alpha and beta positions of the coordi nated ethylene.