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

摘要

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(CH_3)(I)], with Fc = ferrocenyl and R = Fc, CH_3 and CF_3, have been synthesized. The reactivity of complexes of the type [Rh(β-diketonato)(P(OPh)_3)_2] increase in the order: β-diketonato = (CF_3COCHCOCF _3)- < (CF_3COCHCOPh)- < (CF_3COCHCOCH_3)- < (PhCOCHCOPh)- < (CF_3COCHCOFc)- < (CH_3COCHCOPh) - < (CH_3COCHCOCH_3)- < (CH _3COCHCOFc)- < (FcCOCHCOFc)-, giving linear relationships between the kinetic parameter ln k_2 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 (χ_R + χ_R′) and the pK_a of the uncoordinated β-diketone RCOCH_2COR′. 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(FcCOCHCOCF_3)(P(OCH_3)_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.