Cationic complexes of iridium: Diiodobenzene chelation, electrophilic behavior with olefins, and fluxionality of an Ir(I) ethylene complex

Article abstract of DOI:10.1021/ic025506s

The synthesis of a series of dicationic Ir(III) complexes is described. Reaction of Ir(CO)(dppe)I (dppe = 1,2-bis-(diphenylphosphino)ethane)) with RI (R = CH₃ and CF₃) results in formation of the Ir(III) precursors IrR(CO)-(dppe)(I)₂ (R = CH₃ (1a) and CF₃ (1b)). Subsequent treatment with AgOTf (OTf = triflate) generates the bis(triflate) analogues IrR(CO)(dppe)(OTf)₂ (R = CH₃ (2a) and CF₃ (2b)), which undergo clean metathesis with NaBARF (BARF = B(3,5-(CF₃)₂C₆ H₃)₄^-) in the presence of 1,2-diiodobenzene (DIB) forming the dicationic halocarbon adducts [IrR(CO)(dppe)(DIB)][BARF]₂(R = CH₃ (3a) and CF₃ (3b)). Complexes 3a and 3b demonstrate facile exchange chemistry with acetonitrile and carbon monoxide forming complexes 4 and 5, respectively. NMR investigation of the mechanism reveals that the process proceeds through an η¹-diiodobenzene adduct, where labilization at the coordination site trans to the alkyl group occurs first. Complex 3a reacts with ethylene forming the cationic iridium(I) product [Ir(C₂H₄)₂(CO)(dppe)][BARF] (6), which demonstrates fluxional behavior. Variable-temperature NMR studies indicate that the five-coordinate complex 6 undergoes three dynamic processes corresponding to ethylene rotation, Berry pseudorotation, and intermolecular ethylene exchange in order of increasing temperature based on NMR line shape analyses used to determine the thermodynamic parameters for the processes. The DIB adducts 3a and 3b were also found to promote olefin isomerization of 1-pentene, and polymerization/oligomerization of styrene, α-methylstyrene, norbornene, β-pinene, and isobutylene via cationic initiation.