Kinetics of substitution of ferrocenyl-containing β-diketonato ligands by phenanthroline from β-diketonato-1,5-cyclooctadienerhodium(I) complexes

Second-order rate constants, k2, for the substitution of the ferrocene-containing β-diketonato ligands FcCOCHCOR− with R=CF3 (ferrocenoyltrifluroacetonato, fctfa, pKa 6.56), CCl3 (ferrocenoyltrichloroacetonato, fctca, 7.13), CH3 (ferrocenoylacatonato, fca, 10.01), Ph (anion of benzoylferrocenoylmethane, bfcm, 10.41) and Fc (anion of diferrocenoylmethane, dfcm, 13.1) (Ph=phenyl, Fc=ferrocenyl, values in brackets are the pKa values of the free β-diketones) from the complexes [Rh(cod)(FcCOCHCOR)] with 1,10-phenanthroline (phen, cod=1,5-cyclooctadiene) at 25 °C were found to be 560 (R=CF3), 1370 (CCl3), 30 (Ph), 18 (CH3) and 7.0 dm3 mol−1 s−1 (Fc), respectively. The temperature dependence of each reaction was determined and the large negative values obtained for activation, ΔS#<−100 J K−1 mol−1 for all but R=CCl3 (ΔS#CCl3=−81 J K−1 mol−1), suggests an associative substitution mechanism. The rate law of the reaction was found to be R={ks+k2[phen]}[Rh(cod)(FcCOCHCOR)]. Since the solvent-associated rate constant ks≈0 for all R except Ph (ks,RPh=0.06 s−1) the solvent, methanol, plays a limited role in the reaction. Results are interpreted to imply that the rate-determining step during substitution is breaking of an RhO bond and not the formation of an RhN bond. The role of β-diketone pKa and group electronegativity, χ, of each R group on the rate of substitution are also discussed.