Electrochemical, kinetic and molecular mechanic aspects of Rhodium(I) and Rhodium(II) complexes

[摘要] English: Four Rh(I)(β-diketonato)(CO)(PPh3) complexes, in which the β-diketone has different electronegativities and steric hindrances, were successfully synthesised and characterised by means of Infrared and NMR spectroscopy. The crystal structure of (1 ,3-Diphenyl-1 ,3-propanedionato-KO, KO) Triphenyl Phosphine Rhodium(I) showed that the asymmetric unit consists of two centrosymmetric pair, but not a crystallographic inversion centre. The partial overlapping of the chelate ring of the one molecule with the phenyl ring of the other molecule can be attributed to π-π interactions between atoms of the two molecules. The oxidative addition kinetics of CH31to Rh(I)(β-diketonato)(CO)(PPh3) complexes was studied in the Visible, Infrared and NMR regions. The proposed oxidative addition mechanism of CH31 to Rh(I)(β-diketonato)( CO)(PPh3) complexes is a nucleophilic attack by the Rhodium atom on the Carbon atom of the methyl iodide, where a linear or a three-centred polar transition state is formed which leads to trans and cis addition respectively. The main purpose of the development of a Rh(I) Carbonyl Phosphine force field was not only to predict the molecular structure of Rh(I) complexes, but also to compute a possible transition states during the oxidative addition of CH31 to Rh(I)(β-diketonato)( CO)(PPh3) complexes. Steric energy calculations indicated that the oxidative addition of CH31to the Rh(I) Carbonyl Phosphine complexes would rather occur via a SN2 mechanism than a concerted three-centred mechanism. The negative entropy of activation and the negative experimental volume of activation obtained during oxidative addition of CH31 to Rh(I)(β-diketonato)( CO)(PPh3) complexes pointed towards an associative mechanism. An increase in solvent polarity leads to an increase in the second order rate constant. These results are indicative of a mechanism where a polar transition state is formed. During the electrochemical oxidation of Rh(I)(β-diketonato)(CO)(PPh3) the only possible co-ordination ligand was the solvent, CH3CN. The addition of solvents with different donosities revealed the co-ordination of the solvents during electrochemical oxidation. The addition of solvents with higher donosities and therefore better coordinating properties made it more difficult to reduce the Rh(III) species formed, back to Rh(I). The two electron irreversible electrochemical oxidation of Rh(I) to Rh(III) was confirmed during bulk electrolysis and Cyclic Voltammetric studies, where the addition of [BzN(Ethtcr to the solution stabilised the formation of Rh(III). The electronic effect of the substituents on the β-diketones showed that the reaction rate increases with the increase of the pKa-values of the freeβ-diketones. The effect of more electronegative substituents on the reactivity of the Rh(I) centre is explained by the fact that electron density is removed from the Rhodium metal, making the complex a stronger Lewis acid and less reactive towards oxidative addition. Steric effects were also perceptible in the present study where the β-diketones, btfa and tfaa, had the same pKa-values. The reaction rate of the bulkier btfa was slower than that of tfaa. Electrochemical oxidation justified the electronic effect, with the exception that steric parameters had no effect during electrochemical oxidation. The oxidation potentials of Rh(btfa)(CO)(PPh3) and Rh(tfaa)(CO)(PPh3), which have the same pKa-values, are the same, within experimental error. The technique of electrochemical oxidation can therefore be used to quantify steric effects during chemical oxidation. The final isomer formed during oxidative addition of CH31 to Rh(I)(β-diketonato)( CO)(PPh3) complexes depends on the nature and nucleophilicity of the ligands. A linear transition state leads to trans addition of CH31, and isomerisation forms a cis Rh(III) Carbonyl Phosphine isomer in the case of Rh(dbm)(CO)(PPh3) and Rh(ba)(CO)(PPh3) as final product. From IR spectroscopy it is clear that the final oxidative addition product of Rh(btfa)(CO)(PPh3) and Rh(tfaa)(CO)(PPh3) is the Rh(III) acyl complex.

[发布机构] University of the Free State