Electrochemical study of carbonyl phosphine β-diketonato rhodium(I) complexes

Abstract

Cyclic voltammetry of [Rh(RCOCHCOR′)(CO)(PPh 3 )] complexes. The electrochemical oxidation of a series of [Rh(RCOCHCOR′)(CO)(PPh 3 )] complexes studied in acetonitrile, containing 0.100 mol dm −3 tetra- n -butylammonium hexafluorophosphate as supporting electrolyte, shows that Rh(I) is being oxidized in an electrochemically irreversible two-electron transfer process, at peak anodic potentials ranging from E pa (Rh) = 0.29–0.57 V vs. Fc/Fc + . R and R′ groups on the β-diketonato ligand (RCOCHCOR′) which are more electron withdrawing, reduce electron density on the rhodium atom to a larger extent than electron donating groups do. This leads to a higher, more positive, electrochemical oxidation potential, E pa (Rh). Relationships were established between the electrochemical quantity, E pa (Rh), and chemical oxidation, in terms of the second order kinetic oxidative addition rate constant, k 2 , as well as other parameters related to the electronic effect of the R and R′ groups through conjugation on rhodium in [Rh(RCOCHCOR′)(CO)(PPh 3 )]; such as the sum of the group electronegativities (Gordy Scale) of the R and R′ groups ( χ R + χ R′ ), the Hammett meta σ values ( σ R + σ R′ ) and the p K a of the free β-diketone R′COCH 2 COR. E pa can also be predicted by the density functional theory calculated HOMO energy and the two-electron adiabatic ionization potential, IP 2e− , of these [Rh(RCOCHCOR′)(CO)(PPh 3 )] complexes.