Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF(3))(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF(3))(CO)(PPh3)(CH3)(I)]

摘要

The kinetics of oxidative addition of CH3I to [Rh(FcCOCHCOCF(3))(CO)(PPh3)] where Fc = ferrocenyl and (FcCOCHCOCF(3))(-) = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, H-1 and P-31 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 CHCl3 at 25 degrees 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 not equal 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-III alkyl2 formation in the second set of reactions. From a H-1 and P-31 NMR study in CDCl3, 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. [GRAPHICS] The above reaction sequence represents a completely general reaction sequence for the oxidative addition of iodomethane to any Pdiketonato complex of the type [Rh(beta-diketonato)(CO)(PPh3)], although the equilibrium K-2 may not necessarily always be fast. A temperature dependence study in chloroform led to the activation enthalpies, Delta H#, and activation entropies, Delta 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))(CH3)(CO)(PPh3)(I)] and [Rh(FeCOCHCOCF3)(COCH3)(PPh3)(1)], 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)(PPh3)(CH3)(I)], an isomer of the RhIII-alkyl2 species, is also reported. A (C) 2007 Elsevier Ltd. All rights reserved.