Bis(diethylphosphino)methane as a bridging ligand in complexes of Ir 2, Rh2, and IrRh: Geminal C-H activation of α-olefins

Article abstract of DOI:10.1021/om201214z

Bis(diethylphosphino)methane (depm, Et₂PCH₂PEt ₂) is used as a new bridging ligand for the syntheses of a series of diiridium, dirhodium, and iridium/rhodium complexes, starting from trans-[MM′Cl₂(CO)₂(depm)₂] (M = M′ = Ir (1); M = M′ = Rh (11); M = Ir, M′ = Rh (19)). The reduction of all three chloro compounds in aqueous KOH under an atmosphere of CO produces the neutral tricarbonyl complexes [MM′(CO)₃(depm)₂] (M = M′ = Ir (2); M = M′ = Rh (12); M = Ir, M′ = Rh (20)). Protonation of these tricarbonyl complexes by triflic acid (HOTf) or Brookhart's acid (HBAr^F₄) yields the hydride-bridged tricarbonyl complexes [MM′(CO)₃(μ-H)(depm)₂][X] (M = M′ = Ir (9); M = M′ = Rh (16); M = Ir, M′ = Rh (25); X = SO ₃CF₃ or B(3,5-(CF₃)₂C ₆H₃)₄). Reaction of 2 with methyl triflate yields the methylene/hydride product [Ir₂(H)(CO)₃(μ- CH₂)(depm)₂][OTf] (4), while the same reaction with the dirhodium analogue (12) produces the methyl complex [Rh₂(CH ₃)(CO)₃(depm)₂][OTf] (14) at below -20 °C, which upon warming to above -20 °C results in migratory insertion to yield an acetyl-bridged species, [Rh₂(CO)₂(μ-C(CH ₃)O)(depm)₂][OTf] (14a). The analogous Rh/Ir species(20) reacts with methyl triflate to give the methyl-tricarbonyl product [IrRh(CH ₃)(CO)₃(depm)₂][OTf] (22). Removal of a carbonyl from 4 and 22 yields [MIr(CH₃)(CO)₂(depm) ₂][OTf] (M = Ir (5); M = Rh (23)), which are highly susceptible to hydrolysis; however the dirhodium species 14 and 14a are not susceptible to CO loss under the conditions investigated. The reaction of [Ir₂(CO) ₃(μ-H)(depm)₂][BAr^F₄] (9) with a number of α-olefins results in double C-H bond activation of the geminal hydrogens to give the vinylidene-bridged trihydride products of the form [Ir₂(H)₂(CO)₂(μ-H)(μ-C=CRR′)(depm) ₂][BAr^F₄].