Characterization and oxidative addition of different rhodium(I) carbonyl diphenyl-2-pyridylophosphine complexes

English: The aim of the study was to synthesize different [Rh(LL’)(CO)(DPP)] complexes [LL’ = cupf (N-phenyl-N-nitrosohydroxylamine ammonium salt, acac (2,4- pentanedione) and DPP (diphenyl-2-pyridylphosphine)] and to characterize the complexes by means of IR, NMR and crystallographic data. A comparison between the IR spectra of the rhodium dicarbonyl complex and that of the substituted carbonyl complex clearly showed the disappearance of one of the carbonyl stretching frequencies, confirming thus the displacement of one of the carbonyl ligands by the phosphine ligand. In both cases the stretching frequency of the mono substituted carbonyl shifted to a lower wavelength. The 1H-NMR results for the mono carbonyl complexes obtained in this study clearly show an down-field shift of new peaks compared to the dicarbonyl complexes and confirm a change in the chemical environment in the metal complex which correlates with the substitution of one of the carbonyl ligands by a phosphine ligand. [Rh(acac)(CO)(DPP)] crystallized into a monoclinic (P21/n ) and triclinic (Pī) space group with final R values of 2.81 and 3.08% respectively. The triclinic space group showed two isomorphic species. Secondly, the oxidative addition of methyl iodide to [Rh(cupf)(CO)(DPP)] in different solvents and at different temperatures was studied to determine a possible mechanism for this reaction. [Rh(cupf)(CO)(DPP)] undergoes oxidative addition by methyl iodide, forming a Rh(I)*- intermediate species via a very fast equilibrium, followed by the formation of a Rh(III)-alkyl species and finally the formation of a Rh(III)-acyl species as was observed for acetonitrile, acetone and chloroform. Only one reaction was observed for ethyl acetate as solvent with only the formation of the alkyl complex as final product. The results obtained show that the increase in nucleophilicity of rhodium caused by the DPP ligand led to an increase in the rate of formation of the alkyl and acyl in the [Rh(cupf)(CO)(DPP)] complex.