Inversion at Selenium in the Bis(β-diketonato)cobalt(III) Ion

Article abstract of DOI:10.1246/bcsj.59.3505

The selenide (selenoether) complexes, ⁺ (β-dik: acac=2,4-pentanedionate, Clacac=3-chloro-2,4-pentanedionate, Meacac=3-methyl-2,4-pentanedionate, and dbme=1,3-diphenyl-1,3-propanedionate) were prepared and separated into two racemic pairs of the diastereomers (Δ(R)Λ(S) and Δ(S)Λ(R) isomers).The complexes in solution epimerize by inversion at the selenium atom, and the rates were followed by high-performance liquid chromatography in the temperature range of 30.3-55.0 deg C in 0.05 mol dm^-3 Na2SO4 (acac, Clacac, and Meacac complexes) or in methanol (dbme complex).The ΔG^%_{25 deg C} values for inversion of ⁺ decrease in the order β-dik=Clacac(108)>acac(106)>Dbme(104)>Meacac(100 kJ mol^-1), and the decrease in ΔG^%_{25 deg C} comes mainly from a decrease in ΔH^%.The effect of substituents is electronic and the Hammett relationship was found to hold for the inversion rates of the acac, Clacac, and Meacac complexes.