Synthesis and Characterization of 2,2-Dimethylpent-4-en-1-yl Complexes of Rhodium and Iridium: Reversible Olefin Decomplexation and C-H Bond Activation

We describe the synthesis and characterization of four η1,η2-2,2-dimethylpent-4-en-1-yl complexes of stoichiometry M(CH2CMe2CH2CH═CH2)(diene), where M is Rh or Ir, and the diene is dibenzo[a,e]cyclooctatetraene (DBCOT), 1,5-cyclooctadiene (COD), or norbornadiene (NBD). All these compounds have four-coordinate square-planar structures. We also prepared a binuclear complex [Ir(CH2CMe2CH2CH═CH2)(DBCOT)]2(C14H26), in which the iridium centers are five-coordinate owing to the presence of a bridging diolefin ligand. In all five complexes, the C═C bond of the pentenyl ligand reversibly decomplexes in solution so that the two faces of the olefin are exchanged; for Rh(CH2CMe2CH2CH═CH2)(DBCOT) (2), the activation parameters for olefin decomplexation are ΔH‡ = 19 ± 1 kcal·mol-1 and ΔS‡ = 8 ± 4 cal·mol-1 K-1. Spin saturation transfer studies show that the decomplexation preserves the broken symmetry of the DBCOT ligand, which suggests that the olefin decomplexation in 2 most likely occurs by means of a C-H σ-complex, or, less likely, a T-shaped intermediate which is too short-lived for the alkyl group to migrate to the vacant coordination site. When this Rh compound is heated in solution, a C-H bond on the γ-carbon of the pentenyl ligand is activated to form a substituted η3-allyl complex Rh(η3-anti-CH2═CHCHCMe3)(DBCOT), which subsequently isomerizes in solution to form the more stable Rh(η3-syn-CH2═CHCHCMe3)(DBCOT).