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

[摘要] English: A number of dicarbonylrhodium complexes of the type[Rh(CH3cupf)(CO)2] as well as their substituted monocarbonylproducts [Rh(CH3cupf)(CO)(PX3)] (where X = Ph, p-MeOph, p-Tol, o-Tol and Cy) have been prepared and identified by IR and NMRtechniques. The square planar substitution of the carbonyls inthese dicarbonylrhodium complexes by different tertiary phosphineligands have also been identified using UV/Visible and IRspectroscopic techniques. One of the aims of this study was todetermine 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 andsolvent, as well as the steric and electronic effect of the phosphineligands on this reaction.The [Rh(CH3cupf)(CO)(PPh3)] complex is expected to have a similargeometric 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 transto the nitroso group. It can be concluded that the electronicproperties of the cupferrate ligand overshadows the steric effect ofthe different phosphine ligands.The different Rh(I)-CH3cupf complexes underwent oxidativeaddition with iodomethane to form the corresponding Rh(III)-alkylspecies followed by the slower formation of Rh(III)-acyl species according to the scheme below. All the Rh(I) and Rh(III) specieswere characterised by infrared spectroscopy.The rate constant for the oxidative addition of[Rh(CH3cupf)(CO)(PX3)] with iodomethane increased with increasingpolarity of solvents. At 25.0 ºC, this reaction proceeds at a rate ofk1 = 3.94(5) x 10-3 M-1s-1 in the highly polar methanol and at a rateof k1 = 1.33(4) x 10-3 M-1s-1 in less polar acetone, compared to theleast polar benzene with a rate of k1 = 0.101(2) x 10-3 M-1s-1.Activation parameters (ΔH# and ΔS#) were determined for thetemperature 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 formationof a linear, polar transition state with subsequent formation of anion-pair intermediate is postulated for the intrinsic mechanism.The rate of formation of the Rh(III) acyl species was found to beindependent of iodomethane concentrations.The rate constant of the oxidative addition was also affected byelectronic and steric manipulations. The electronic effect wasachieved by interchanging, for example, PPh3 with P(p-MeOC6H4)3which resulted in a more than fivefold increase in magnitude forthe rate of oxidative addition. An elevenfold decrease in the k1value for P(o-Tol)3, when compared to P(p-MeOC6H4)3, showed theimpact of the steric effect.

[发布机构] University of the Free State