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Synthesis, characterisation and reactivity of phosphide and methylidene complexes of iridium Joshi, Kiran


The iridium(III) methyl diarylphosphide complexes, Ir(CH₃PR₂-[N(SiMe₂CH₂PPh₂)₂] (2a: R = phenyl, 2b: R = meta-tolyl) had been prepared previous to this work. The iridium(III) dimethylphosphide complex, Ir(CH₃)PMe₂-[N(SiMe₂CH₂PPh₂)₂], 2c, is readily prepared in situ by transmetallation of the Ir(CH₃)I[N(SiMe₂CH₂PPh₂)₂] with KPMe₂ at -30°C. The synthesis of the phenylphosphide complex Ir(CH₃)PHPh[N(SiMe₂CH₂PPh₂)₂], 2d, involves deprotonation of the six-coordinate iridium(III) phenylphosphine complex, Ir(CH₃)I-(PH₂Ph)[N(SiMe₂CH₂PPh₂)₂], with KO¹Bu. Thermolysis of 2a and 2b yields the six-coordinate iridium(III) cyclometallated hydride complexes fac-Ir(ɳ²-CH₂PR₂)H[N(SiMe₂CH₂PPh₂)₂], 3a and 3b. The dimethylphosphide complex 2c undergoes the same rearrangement to afford 3c but more rapidly. Thermolysis of 3a-3c yields the square planar iridium(I) phosphine complexes of the formula, Ir(PCH₃R₂)[N(SiMe₂CH₂PPh₂)₂], 4a-4c. Some of the intermediates proposed in the thermolysis of 2a are synthesised independently by the reaction of iridium methylidene complex, Ir=CH₂[N(SiMe₂CH₂PPh₂)₂]. 10, with PHPh₂. The complex fac-Ir(ɳ²-CHPhPMe₂)H[N(SiMe₂CH₂PPh₂)₂] is generated from the reaction of Ir(CH₂Ph)Br[N(SiMe₂CH₂PPh₂)₂] with KPMe₂ without intermediacy of the corresponding phosphide complex. The photolysis of 2a-2c also yields species 4a-4c; however, no intermediacy of the cyclometallated hydride complexes 3a-3c is observed during this transformation. Upon thermolysis of the phenylphosphide complex 2d, only the corresponding iridium(I) phosphine complex, Ir(PHCH3Ph)[N(SiMe2CH2PPh2)2], 4d, is obtained, which is also the photolysis product of 2d. Ir(CH₃)PPh₂[N(SiMe₂CH₂PPh₂)₂], 2a, reacts at -78°C with dimethyl-acetylenedicarboxylate to yield an octahedral iridium(III) complex in which the alkyne has bridged between the phosphide ligand and the phosphine group of the chelating ligand. In addition, one of the phenyl groups from the chelating phosphine has migrated to the metal. On the other hand, Ir(CH₃)PMe₂[N(SiMe₂CH₂PPh₂)₂], 2c, reacts with the same alkyne to yield a product in which the alkyne has bridged between the phosphide group and the iridium centre. The reaction of 2a with diphenylacetylene affords Ir(PhC≡CPh)[N(SiMe₂CH₂PPh₂)₂] and free methyl-diphenylphosphine. Complex 2a reacts with terminal alkynes (RC≡CH; R = H, Ph, ¹Bu) to yield acetylide complexes of formula Ir(CH₃)PHPh₂(C≡CR)[N(SiMe₂CH₂PPh₂)₂]- The methylidene complex, lr=CH₂[N(SiMe₂CH₂PPh₂)₂], 10, prepared by the reaction of Ir(CH₃)I[N(SiMe₂CH₂PPh₂)₂] with KO¹Bu, reacts with phosphines PHR₂ (R = Ph, ¹Bu) to afford the cyclometallated hydride complexes, fac-Ir(ɳ²-CH₂PR₂)H[N(SiMe₂CH₂PPh₂)₂], via a five-coordinate methylidene phosphine intermediate. The reaction of 10 with PH₂Ph yields similar cyclometallated hydride product, but in this case the five-coordinate intermediate is not observed. The methylidene complex 10 reacts with the electrophiles MeI and AlMe₃ to yield Ir(ɳ²-C₂H₄)H(I)[N(SiMe₂CH₂PPh₂)₂] and Ir((µ-AlMe₂)H[N(SiMe₂CH₂PPh₂)₂], respectively. Reaction of 10 with HC≡CH affords an ɳ³˗allyl acetylide complex Ir(ɳ³-C₃H₅)(C≡CH)[N(SiMe₂CH₂PPh₂)₂]. A trimethylenemethane complex, fac-Ir{ɳ⁴-C(CH⁴₂)₃}[N(SiMe₂CH₂PPh₂)₂], is obtained readily upon exposing 10 to 1,2-propadiene. The reaction of 10 with 1,3-butadiene affords a pentenyl product, Ir(σ-ɳ³-C₅H₈)[N(SiMe₂CH₂PPh₂)₂]. In previous studies, the iridium(I) ɳ²-cyclooctene species, Ir(ɳ²-C₈H₁₄)-[N(SiMe₂CH₂PPh₂)₂], 25, has served as a useful starting material in the preparation of a number of iridium(I) and iridium(III) amide complexes. This complex is thermally stable, but upon photolysis, it yields Ir(H)₂[N(SiMe₂CH₂PPh₂)₂] and a mixture (2:1) of free 1,3-and 1,5-cyclooctadiene. This dehydrogenation process proceeds through ɳ³-allyl hydride intermediate, Ir(ɳ³-C₈H₁₃)H[N(SiMe₂CH₂PPh₂)₂]- The cyclo-octene ligand in 25 can be replaced by 1,3-butadiene and. 1,2-propadiene. The products obtained from these reactions are Ir(ɳ⁴-C₄H₆)[N(SiMe₂CH₂PPh₂)₂] and Ir(ɳ²-C₃H₄)[N(SiMe₂CH₂PPh₂)₂]. respectively. The reaction of 25 with AlMe₃ affords Ir(µ-AlMe₂)Me[N(SiMe₂CH₂PPh₂)₂].

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