Kinetics and mechanism of the aquation of a series of mixed-metal oxo-centered trinuclear cations, μ_3-oxo-triaqua-hexakis(carboxylato) bis-(chromium(III)(iron(III))~+, [(Cr_2~(III)Fe~(III)O(RCOO)_6(H_2O)_3]~+, (R=H, CH_3, CH_3CH_2 and (CH_3)_2CH) in perchloric acid media

摘要

The aquation of a series of [Cr_2Fe(μ_3-O)μ_2-(RCO_2)_6(H_2O)_3]~+ cations, where R = H, CH_3, CH_3CH_2, and (CH_3)_2CH was investigated in aqueous perchloric acid, 0.01 ≤ [H~+] ≤ 0.17 mol dm~(-3) and 25 ≤θ≤35 ℃. 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℃ and 0.5 mol dm~(-3) ionic strength (NaClO_4) and corresponding activation parameters are: R = (CH_3)_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 = CH_3CH_2; 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 = CH_3; 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 ℃ and 0.5 mol dm~(-3) ionic strength (NaClO_4): R = (CH_3)_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 = CH_3CH_2; 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 = CH_3; 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)).