Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF 3)(CO)(PPh 3)]: Structure of [Rh(FcCOCHCOCF 3)(CO)(PPh 3)(CH 3)(I)

Abstract

The kinetics of oxidative addition of CH 3I to [Rh(FcCOCHCOCF 3)(CO)(PPh 3)], where Fc = ferrocenyl and (FcCOCHCOCF 3) − = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, 1H and 31P NMR techniques. Three definite sets of reactions involving isomers of at least two distinctly different classes of Rh III-alkyl and two different classes of Rh III-acyl species were observed. Rate constants for this reaction in CHCl 3 at 25 °C, applicable to the reaction sequence below, were determined as k 1 = 0.00611(1) dm 3 mol −1 s −1, k −1 = 0.0005(1) s −1, k 3 = 0.00017(2) s −1 and k 4 = 0.0000044(1) s −1 while k −3 ≪ k 3 and k −4 ≪ k 4 but both ≠0. The indeterminable equilibrium K 2 was fast enough to be maintained during Rh I depletion in the first set of reactions and during the Rh IIIalkyl2 formation in the second set of reactions. From a 1H and 31P NMR study in CDCl 3, K c1 was found to be 0.68, K c2 = 2.57, K c3 = 1.00, K c4 = 4.56 and K c5 = 1.65. [Display omitted] The above reaction sequence represents a completely general reaction sequence for the oxidative addition of iodomethane to any β-diketonato complex of the type [Rh(β-diketonato)(CO)(PPh 3)], although the equilibrium K 2 may not necessarily always be fast. A temperature dependence study in chloroform led to the activation enthalpies, Δ H #, and activation entropies, Δ S #, for both the first and second sets of reactions. A solvent dependence study utilizing five different solvents showed the rate of the first set of reactions are directly proportional to the dielectric constant of the solvent. The molecular formula of all the Rh III-alkyl and Rh III-acyl species are [Rh(FcCOCHCOCF 3)(CH 3)(CO)(PPh 3)(I)] and [Rh(FcCOCHCOCF 3)(COCH 3)(PPh 3)(I)], respectively (Fc = ferrocenyl), but the geometries of the two Rh III isomers and the initial Rh I species all differ due to different coordination spheres. A crystal structure determination ( Z = 4, monoclinic, P2 1/c) of [Rh(FcCOCHCOCF 3)(CO)(PPh 3)(CH 3)(I)], an isomer of the Rh III-alkyl2 species, is also reported.