Characterization and oxidative addition reactions of rhodium(I) carbonyl cupferrate diphenyl-2-pyridylphoshine complexes

Abstract

The oxidative addition of CH3I to the [Rh(cupf)(CO)(DPP)] complex (DPP = diphenyl-2-pyridylphoshine and cupf = N-nitroso-N-phenylhydroxylaminen) was kinetically investigated using UV/vis and infrared spectroscopy. The kinetics followed in chloroform, acetonitrile and acetone as solvents, indicated three different consecutive reactions. Firstly, a very fast reaction (intermediate formation, IM), secondly, a slower reaction with the formation of the Rh(III) alkyl complex (5.0(1) × 10−3 M−1 s−1 for acetonitrile at 20 °C) and thirdly a very slow formation of the Rh(III) acyl complex as final product with a rate constant of 2.9(6) × 10−4 s−1. The same oxidative addition reaction in ethyl acetate as solvent exhibited only two reactions. Firstly the Rh(III) alkyl formation (1.03(3) × 10−3 M−1 s−1), which was five times slower than in the other solvents. Secondly, the Rh(III) acyl formation, which was masked by solvent IR stretching frequencies in the detection area. Rh(III) acyl, however, was isolated from ethyl acetate. There was no indication of intermediate formation with ethyl acetate. This apparent discrepancy between the rate and the mechanism for the same reaction prompted a DFT study to gain more insight into the reactants and products of the reaction, as well as to try and determine the geometry of the transition state. The DFT study predicted the formation of a linear transition state (TS), followed by the formation of the cationic five-coordinate [Rh(cupf)(CO)(DPP)(CH3)]+ intermediate with the CH3 group in the apical position and with the iodide ion drifting away into the solvent sphere. This was in agreement with the experimental very fast first reaction. The experimentally observed difference in the rates and mechanisms of the [Rh(cupf)(CO)(DPP)] + CH3I reaction in ethyl acetate relative to the other solvents can be attributed to the rate of the formation and/or the build-up and conversion of the I.M.