Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)]

The kinetics of oxidative addition of CH3I to [Rh(FcCOCHCOCF3)(CO)(PPh3)], where Fc = ferrocenyl and (FcCOCHCOCF3)− = 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 RhIII-alkyl and two different classes of RhIII-acyl species were observed. Rate constants for this reaction in CHCl3 at 25 °C, applicable to the reaction sequence below, were determined as k1 = 0.00611(1) dm3 mol−1 s−1, k−1 = 0.0005(1) s−1, k3 = 0.00017(2) s−1 and k4 = 0.0000044(1) s−1 while k−3 ≪ k3 and k−4 ≪ k4 but both ≠0. The indeterminable equilibrium K2 was fast enough to be maintained during RhI depletion in the first set of reactions and during the RhIIIalkyl2 formation in the second set of reactions. From a 1H and 31P NMR study in CDCl3, Kc1 was found to be 0.68, Kc2 = 2.57, Kc3 = 1.00, Kc4 = 4.56 and Kc5 = 1.65.Figure optionsDownload full-size imageDownload as PowerPoint slideThe 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)(PPh3)], although the equilibrium K2 may not necessarily always be fast. A temperature dependence study in chloroform led to the activation enthalpies, ΔH#, and activation entropies, ΔS#, 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 RhIII-alkyl and RhIII-acyl species are [Rh(FcCOCHCOCF3)(CH3)(CO)(PPh3)(I)] and [Rh(FcCOCHCOCF3)(COCH3)(PPh3)(I)], respectively (Fc = ferrocenyl), but the geometries of the two RhIII isomers and the initial RhI species all differ due to different coordination spheres. A crystal structure determination (Z = 4, monoclinic, P21/c) of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)], an isomer of the RhIII-alkyl2 species, is also reported.

Graphical abstractThe chemical kinetics, studied by UV/Vis, IR, 1H and 31P NMR techniques, of oxidative addition of CH3I to [Rh(FcCOCHCOCF3)(CO)(PPh3)] with Fc = ferrocenyl, consist of three definite sets of reactions and involves geometrical isomers of two different RhIII-alkyl and two different RhIII-acyl species.Figure optionsDownload full-size imageDownload as PowerPoint slide