Kinetics and mechanism of the aquation of a series of mixed-metal oxo-centered trinuclear cations, μ3-oxo-triaqua-hexakis(carboxylato)bis-(chromium(III)(iron(III))+, [(RCOO)6(H2O)3]+, (R=H, CH3, CH3CH2 and (CH3)2CH) in perchloric acid media

Abstract

The aquation of a series of [Cr2Fe(µ3-O)µ2-(RCO2)6(H2O)3]+ cations, where R = H, CH3, CH3CH2, and (CH3)2CH was investigated in aqueous perchloric acid, 0.01 ≤ [H+] ≤ 0.17 mol dm−3 and 25 ≤ θ ≤ 35°C. The mechanism is postulated as an equilibrium between the protons and the complex cation, where the oxygens of the carboxylate coordinated to the metal center are protonated, followed by Fe–O bond cleavage. This is followed by rapid decomposition to produce aqueous iron(III), a dinuclear chromium(III) species (which is further hydrolyzed), and carboxylate ions. First-order rate constants for the reactions at 25°C and 0.5 mol dm−3 ionic strength (NaClO4) and corresponding activation parameters are: R = (CH3)2CH; k 1 = (7.18 ± 0.07) × 10−5 s−1 (ΔH ‡= 52 ± 2 kJ mol−1, ΔS ‡= −151 ± 8 J K−1 mol−1), R = CH3CH2; k 1 = (13.67 ± 0.02) × 10−5 s−1 (ΔH ‡= 57.8 ± 0.6 kJ mol−1, ΔS ‡= −125 ± 2 J K−1 mol−1), R = CH3; k 1 = (17.6 ± 0.3) × 10−5 s−1 (ΔH ‡= 30 ± 5 kJ mol−1, ΔS ‡= −216 ± 16 J K−1 mol−1), R = H; k 1 = (3.86 ± 3.01) × 10−2 s−1 (ΔH ‡= 75 ± 1 kJ mol−1, ΔS ‡= −22 ± 5 J K−1 mol−1). Spontaneous hydrolysis rate constants and activation parameters were also determined at 25°C and 0.5 mol dm−3 ionic strength (NaClO4): R = (CH3)2CH; k 0 = (3.18 ± 0.05) × 10−5 s−1 (ΔH ‡= 12.0 ± 0.1 kJ mol−1, ΔS‡= −291 ± 1 J K−1 mol−1), R = CH3CH2; k 0 = (4.04 ± 0.01) × 10−5 s−1 (ΔH ‡= 22.4 ± 0.9 kJ mol−1, ΔS ‡= −254 ± 3 J K−1 mol−1), R = CH3; k 0 = (4.05 ± 0.17) × 10−5 s−1 (ΔH ‡= 34.1 ± 0.1 kJ mol−1, ΔS ‡= −214 ± 1 J K−1 mol−1), R = H; k 0 = (3.4 ± 0.2) × 10−3 s−1 (ΔH ‡= 25.3 ± 0.4 kJ mol−1, ΔS ‡= −207 ± 1 J K−1 mol−1).