Oxidative Addition of CH₃I and CO Migratory Insertion in a Series of Ferrocene-Containing Carbonyl Phosphine β-Diketonato Rhodium(I) Complexes

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Abstract

A kinetic study of the oxidative addition reaction between CH3I and a series of ferrocene-containing β-diketonato carbonyl phosphine rhodium(I) complexes [Rh(FcCOCHCOR)(CO)(PPh3)], with R = CF3 (1), CH3 (2), Ph = phenyl (3), and Fc = ferrocenyl (4), showed that the mechanism consists of 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. The first reaction set involves oxidative addition itself, while the second and third reaction sets lead to different isomerization products involving carbonyl insertion or deinsertion in the Rh-CH3 or Rh-COCH3 bond. Second-order rate constants for the first oxidative addition step in CHCl3 at 25 °C were ki = 0.0061(1), 0.065(1), 0.077(2), and 0.16(1) dm3 mol−1 s−1 for 1, 2, 3, and 4, respectively. The latter is the fastest reaction of this type reported to date for any [Rh(β-diketonato)(CO)(PPh3)] complex. Activation parameters for all reactions from a variable temperature kinetic study resulted in 29 < ΔH# < 45 kJ mol−1 and −188 < ΔS# < −116 J mol−1 K−1. Utilizing kinetic results from this and several previously reported studies involving 13 different [Rh(β-diketonato)(CO)(PPh3)] complexes, it was possible to quantify the relationship between [Rh(R1COCHCOR2)(CO)(PPh3)] reactivity toward oxidative addition and the propensity of β-diketonato ligands to donate electrons to the Rh center as ln k1 = −3.14(χR1 + χR2) + 10.0, where χR is the group electronegativity on the Gordy scale of β-diketonato R groups. The pKa of the free β-diketone may be estimated from the relationship pKa = −3.484(χR1 + χR2) + 24.6.

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