Synthesis and kinetic study of rhodium(I) complexes containing substituted cupferrate ligands

English: A number of dicarbonylrhodium complexes of the type [Rh(CH3cupf)(CO)2] as well as their substituted monocarbonyl products [Rh(CH3cupf)(CO)(PX3)] (where X = Ph, p-MeOph, p-Tol, o- Tol and Cy) have been prepared and identified by IR and NMR techniques. The square planar substitution of the carbonyls in these dicarbonylrhodium complexes by different tertiary phosphine ligands have also been identified using UV/Visible and IR spectroscopic techniques. One of the aims of this study was to determine the mechanism for the oxidative addition of [Rh(CH3cupf)(CO)(PX3)] ( X = Ph, p-MeOPh, p-Tol, o-Tol and Cy) with iodomethane and to investigate the effect of temperature and solvent, as well as the steric and electronic effect of the phosphine ligands on this reaction. The [Rh(CH3cupf)(CO)(PPh3)] complex is expected to have a similar geometric configuration as that of [Rh(cupf)(CO)(PX3)]. Extrapolation of this structural data predicted that the [Rh(CH3cupf)(CO)(PPh3)] complex also contain the CO group trans to the nitroso group. It can be concluded that the electronic properties of the cupferrate ligand overshadows the steric effect of the different phosphine ligands. The different Rh(I)-CH3cupf complexes underwent oxidative addition with iodomethane to form the corresponding Rh(III)-alkyl species followed by the slower formation of Rh(III)-acyl species according to the scheme below. All the Rh(I) and Rh(III) species were characterised by infrared spectroscopy. The rate constant for the oxidative addition of [Rh(CH3cupf)(CO)(PX3)] with iodomethane increased with increasing polarity of solvents. At 25.0 ºC, this reaction proceeds at a rate of k1 = 3.94(5) x 10-3 M-1s-1 in the highly polar methanol and at a rate of k1 = 1.33(4) x 10-3 M-1s-1 in less polar acetone, compared to the least polar benzene with a rate of k1 = 0.101(2) x 10-3 M-1s-1. Activation parameters (ΔH# and ΔS#) were determined for the temperature dependence of k1 in acetone. A large negative ΔS# value (ΔS# = -137(1) JK-1mol-1) and a positive ΔH# (ΔH# = 48(1) kJmol-1) were obtained which clearly point to an associative mechanism. Considering the experimental results, the formation of a linear, polar transition state with subsequent formation of an ion-pair intermediate is postulated for the intrinsic mechanism. The rate of formation of the Rh(III) acyl species was found to be independent of iodomethane concentrations. The rate constant of the oxidative addition was also affected by electronic and steric manipulations. The electronic effect was achieved by interchanging, for example, PPh3 with P(p-MeOC6H4)3 which resulted in a more than fivefold increase in magnitude for the rate of oxidative addition. An elevenfold decrease in the k1 value for P(o-Tol)3, when compared to P(p-MeOC6H4)3, showed the impact of the steric effect.