Synthesis and reactivity of the coordinatively unsaturated methylene complex Ir=CH2[N(SiMe2CH2PPh2)2]

Article abstract of DOI:10.1021/ja00076a016

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 H₂ 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 H₂ 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)[(CH₂PPh₂CH₂SiMe₂NSiMe ₂CH₂PPh₂)], 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 (≤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(μ-AlMe₂)H[N(SiMe₂-CH₂PPh ₂)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 β-eliminates and releases ethylene to generate the hydride. The methylene complex also reacts with methyl iodide to generate the ethylene hydride iodide complex Ir(η²-C₂H₄)H(I)[N(SiMe₂CH ₂PPh₂)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 β-elimination to the iridium center, the amide unit abstracts the β-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 β-hydrogen is reversible since there is scrambling of the label equally into both the α and β positions of the coordinated ethylene.