Kinetic Study of the Oxidative Addition Reaction between Methyl Iodide and [Rh(imino-β-diketonato)(CO)(PPh)3] Complexes, Utilizing UV–Vis, IR Spectrophotometry, NMR Spectroscopy and DFT Calculations

Abstract

The oxidative addition of methyl iodide to [Rh(β-diketonato)(CO)(PPh)₃ ] complexes, as modal catalysts of the first step during the Monsanto process, are well-studied. The β-diketonato ligand is a bidentate (BID) ligand that bonds, through two O donor atoms (O,O-BID ligand), to rhodium. Imino-β-diketones are similar to β-diketones, though the donor atoms are N and O, referred to as an N,O-BID ligand. In this study, the oxidative addition of methyl iodide to [Rh(iminoβ-diketonato)(CO)(PPh)₃ ] complexes, as observed on UV–Vis spectrophotometry, IR spectrophotometry and NMR spectrometry, are presented. Experimentally, one isomer of [Rh(CH₃COCHCNPhCH₃ ) (CO)(PPh₃ )] and two isomers of [Rh(CH₃COCHCNHCH₃ )(CO)(PPh₃ )] are observed—in agreement with density functional theory (DFT) calculations. Experimentally the [Rh(CH₃COCHCNPhCH₃ )(CO) (PPh₃ )] + CH₃ I reaction proceeds through one reaction step, with a rhodium(III)-alkyl as the final reaction product. However, the [Rh(CH₃COCHCNHCH₃ )(CO)(PPh₃ )] + CH₃ I reaction proceeds through two reaction steps, with a rhodium(III)-acyl as the final reaction product. DFT calculations of all the possible reaction products and transition states agree with experimental findings. Due to the smaller electronegativity of N, compared to O, the oxidative addition reaction rate of CH₃ I to the two [Rh(imino-β-diketonato)(CO)(PPh)₃ ] complexes of this study was 7–11 times faster than the oxidative addition reaction rate of CH₃ I to [Rh(CH₃COCHCOCH3 )(CO)(PPh₃ )].