Breakdown kinetics of the tri-chromium(III) oxo acetate cluster ([Cr3O(OAc)6]+) with some ligands of biological interest

Kinetics for the breakdown of the trinuclear chromium acetate cluster, [Cr3O(OAc)6]+, with a series of monoprotic and diprotic ligands in weakly acidic aqueous media (pH ∼ 4 or ∼5) have been investigated spectrophotometrically at 40–60 °C. The results point to an ion-pair equilibrium as the first step followed by associative interchange mechanism forming the mononuclear product of the reaction. Pseudo-first-order rates were determined from absorbance data and associated activation parameters were calculated using the Eyring equation. Enthalpy and entropy terms of the reactions (e.g., histidine, ΔH‡ = 75 ± 15 kJ mol−1, ΔS‡ = −130 ± 25 J K−1 mol−1; lactic acid, ΔH‡ = 66 ± 13 kJ mol−1, ΔS‡ = −155 ± 30 J K−1 mol−1; glycine, ΔH‡ = 31 ± 6 kJ mol−1, ΔS‡ = −225 ± 45 J K−1 mol−1) are consistent with an associative interchange (Ia) mechanism, and produce a linear isokinetic plot (slope = 50 °C). Rates and activation parameters are comparable to those of substitution reactions of the chromium(III) hexaaqua cation. Other ligands studied included malonic acid and the amino acid, aspartic acid. Observed rates are faster than water exchange rates, but typically slower than anion substitution rates, and indicate that trinuclear chromium(III) clusters are expected to be kinetically stable in neutral to slightly acidic conditions.