Iodomethane oxidative addition to β-diketonatobis(triphenylphosphite)rhodium(I) complexes: A synthetic, kinetic and computational study

New rhodium(I)- and rhodium(III)-β-diketonato complexes of the type [Rh(FcCOCHCOR)(P(OPh)3)2] and [Rh(FcCOCHCOR)(P(OPh)3)2(CH3)(I)], with Fc = ferrocenyl and R = Fc, CH3 and CF3, have been synthesized. The reactivity of complexes of the type [Rh(β-diketonato)(P(OPh)3)2] increase in the order: β-diketonato = (CF3COCHCOCF3)− < (CF3COCHCOPh)− < (CF3COCHCOCH3)− < (PhCOCHCOPh)− < (CF3COCHCOFc)− < (CH3COCHCOPh)− < (CH3COCHCOCH3)− < (CH3COCHCOFc)− < (FcCOCHCOFc)−, giving linear relationships between the kinetic parameter ln k2 and the parameters that are related to the electron density on the rhodium centre; the sum of the group electronegativities of the β-diketonato side groups (χR + χR′) and the pKa of the uncoordinated β-diketone RCOCH2COR′. The large negative values of the volume and entropy of activation indicated a mechanism which occurs via a polar transition state. A density functional theory study, at the PW91/TZP level of theory, indicates that oxidative addition of iodo methane to [Rh(FcCOCHCOCF3)(P(OCH3)3)2] occurs via a two-step mechanism. This mechanism involves a nucleophilic attack by the metal on the methyl carbon to displace iodide to form a metal-carbon bond and the coordination of iodide to the five-coordinated intermediate to give a six-coordinated trans alkyl product.

The rate of oxidative addition of iodomethane to [Rh(β-diketonato)(P(OPh)3)2] complexes is related to the electron density on the rhodium(I) centre as expressed by the sum of the group electronegativities of the β-diketonato side groups or the pKa of the uncoordinated β-diketone.Figure optionsDownload as PowerPoint slideHighlights
► Novel ferrocene-containing [Rh(FcCOCHCOR)(P(OPh)3)2] complexes.
► High reactivity of [Rh(FcCOCHCOFc)(P(OPh)3)2] towards oxidative addition of iodomethane.
► Density functional theory results indicate trans addition of CH3I to [Rh(FcCOCHCOCF3)(P(OCH3)3)2].