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 eta(1), eta(2)-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 Delta H* = 19 +/- 1 kcal.mol(-1) and Delta S* = 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 sigma-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 gamma-carbon of the pentenyl ligand is activated to form a substituted eta(3)-allyl complex Rh(eta(3)-anti-CH2=CHCHCMe3)(DBCOT), which subsequently isomerizes in solution to form the more stable Rh(eta(3)-syn-CH2=CHCHCMe3)(DBCOT).