Oxidative addition and co insertion of rhodium cupferrate complexes containing arsine ligands

[摘要] Results obtained for the reaction between [Rh(cupf)(CO)(AsPh3)] and CH3I in a rangeof solvents indicated that the second-order rate constants are comparable with itsphosphine analogue. A likely explanation given was that steric factors are probablyovershadowed by electronic ones. This suggestion is supported by literature findingsspecifying the σ-donor order of group 15 ligands as PR3 > AsR3 > SbR3.Rhodium complexes with phosphine ligands should thus have more electron densitythan those with corresponding arsine ligands. The less steric crowding at the Rhcentre of the starting complex was proved by rate enhancement (relative to othersimilar rhodium complexes) of oxidative addition of the bulky substrate, iodoethane,to the complex. The oxidative addition kinetics showed insignificant intercepts forthe plots of kobs vs [CH3I] which implied a reaction thermodynamically favouredtowards the Rh(III)-alkyl complex product.Electron density and steric constraints of the rhodium(I) species were manipulated inthe study by using different arsine ligands. [Rh(cupf.CH3)(CO)(AsPh3)] and[Rh(cupf)(CO)(AsMePh2)] were thus prepared and analysed. Both complexesenhanced the rate of oxidative addition, which is in agreement with the higherelectron density at the metal due to the more efficient electron donating property ofthe methyl group compared to the phenyl group. The increased reactivity of[Rh(cupf)(CO)(AsMePh2)] could also be ascribed to the smaller cone angle ofAsMePh2 compared to AsPh3. The rate of oxidative addition of iodoethane waslower, which indicated that the first step of the reaction was a nucleophilic attack bythe metal centre (rhodium) on the α-carbon of the haloalkanes.The first-order rate constant for CO-insertion was determined from IR-multi-scans inacetonitrile and chloroform. Although UV/VIS spectra of the reaction showedbiphasic kinetics in acetonitrile and methanol, the second step after the initialoxidative addition reaction was not only CO-insertion but, as revealed from IR-multiscannedspectra, also isomerisation from a Rh(III)-acyl to alkyl species in the samestep. In other words, the UV/VIS spectra did not show clearly for which species thespecific rate was measured in the second part of the biphasic plots. However, IRmulti-scanned spectra of the reaction at fixed [CH3I] for the oxidative addition step(Rh(I)–CO peak disappearance) was possible since it was proved that the graph of kobsvs [CH3I] was linear with insignificant intercepts. The IR-multi-scanned spectratogether with the UV/VIS rate data indicated that the fast equilibrium for theformation of the alkyl complex was followed by a second slower CO-insertionreaction. This was consistent with the proposed mechanism of the title reaction asshown below.See Scheme in full text.The observed rate enhancement due to the increase in electron density of the startingcomplex together with the activation parameters found (relatively large negative ΔS#)and the type of IR-multi-scans spectra obtained are all indicative of an associativemechanism. Furthermore, the activation of the second-order rate constants fromnon-polar to highly polar solvents could be taken as an indication of a polar transitionstate. Although activation parameters and IR multi-scanned spectra were performedin different solvents, i.e. in acetone and chloroform, it would not be expected to affectthe proposed mechanism. On the basis of a combined solvent, temperature andpressure dependence study conducted, activation parameters for the reaction betweenCH3I and [Rh(cupf)(CO)(PPh3)] in acetone and chloroform were very close to thoseobtained for the title reaction in this investigation.In this study, the effect of organic halide substrates on the rate of the reaction bychanging the substrate from CH3I to CH3Br and C2H5I was also investigated. Incomparison to literature data, the rate of the reaction with C2H5I was faster, i.e. only a40-fold deactivation was found. Both CH3Br and C2H5I retarded the reaction.Possible explanations could be due to electronic factors (bond energy, BE, of CH3–Br= 70 vs CH3–I = 56 kcal/mol). Since BE of CH3CH2–I is53 kcal/mol, decreased reactivity observed could be interpreted in terms of the largesize of the ethyl group. With regard to the mode of addition, oxidative addition ofCH3I to [Rh(cupf)(CO)(AsPh3)] is expected to proceed by addition of iodide (I-) andCH3 adjacently (cis to one another) like in its phosphine analogue.Another aspect of this study was kinetic runs of the oxidative addition product,[IRh(cupf)(CO)(CH3)(AsPh3)], performed in a range of solvents. Most kinetic studiesonly employ Rh(I) or Ir(I) complexes as a starting material, but in this study theRh(III)-alkyl product was also utilized as a starting complex. In all solventsemployed, the product was depleted and gave rise to a small peak at ca. 1712 cm-1corresponding to a Rh(III)-acyl species. The rate of acyl formation was thusdetermined by the slow appearance of this peak.

[发布机构] University of the Free State