Iodomethane oxidative addition and CO migratory insertion in monocarbonylphosphine complexes of the type [Rh((C 6H 5)COCHCO((CH 2) nCH 3))(CO)(PPh 3)]: Steric and electronic effects

Abstract

The chemical kinetics, studied by UV/Vis, IR and NMR, of the oxidative addition of iodomethane to [Rh((C 6H 5)COCHCOR)(CO)(PPh 3)], with R = (CH 2) n CH 3, n = 1–3, consists of three consecutive reaction steps that involves isomers of two distinctly different classes of Rh III-alkyl and two distinctly different classes of Rh III-acyl species. Kinetic studies on the first oxidative addition step of [Rh((C 6H 5)COCHCOR)(CO)(PPh 3)] + CH 3I to form [Rh((C 6H 5)COCHCOR)(CH 3)(CO)(PPh 3)(I)] revealed a second order oxidative addition rate constant approximately 500–600 times faster than that observed for the Monsanto catalyst [Rh(CO) 2I 2] −. The reaction rate of the first oxidative addition step in chloroform was not influenced by the increasing alkyl chain length of the R group on the β-diketonato ligand: k 1 = 0.0333 ([Rh((C 6H 5)COCHCO(CH 2CH 3))(CO)(PPh 3)]), 0.0437 ([Rh((C 6H 5)COCHCO(CH 2CH 2CH 3))(CO)(PPh 3)]) and 0.0354 dm 3 mol −1 s −1 ([Rh((C 6H 5)COCHCO(CH 2CH 2CH 2CH 3))(CO)(PPh 3)]). The p K a ′ and keto–enol equilibrium constant, K c, of the β-diketones (C 6H 5)COCH 2COR, along with apparent group electronegativities, χ R of the R group of the β-diketones (C 6H 5)COCH 2COR, give a measurement of the electron donating character of the coordinating β-diketonato ligand: (R, p K a ′, K c, χ R) = (CH 3, 8.70, 12.1, 2.34), (CH 2CH 3, 9.33, 8.2, 2.31), (CH 2CH 2CH 3, 9.23, 11.5, 2.41) and (CH 2CH 2CH 2CH 3, 9.33, 11.6, 2.22).