Kinetics and mechanisms of some redox reactions of nitridoruthenium(VI) and nitridoosmium(VI) complexes bearing a salen ligand in aqueous solutions

含希夫堿的氮化釕以及氮化鋨絡合物在水相中的氧化還原反應動力學及機理

Student thesis: Doctoral Thesis

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Author(s)

  • Qian WANG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date2 Oct 2015

Abstract

The chemistry of metal nitrido complexes has received much attention in recent years. This thesis describes the kinetics and mechanisms of the reactions of [Ru(N)(salchda)(MeOH)]+ and [Os(N)(salchda)(H2O)]+ (salchda = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) with L-ascorbic acid, sulfite, hydroxide and L-cysteine in aqueous media. The oxidation of L-ascorbic acid (H2A) by [RuVI(N)(salchda)(MeOH)]+ (RuVIN) has been studied in aqueous acidic solutions from pH range of 1 to 6 under argon atmosphere. The reaction has the following stoichiometry: 2RuVIN + 3H2A → 2[RuIII(NH2-HA)]+ + A. The pseudo-first-order rate constant, kobs, depends linearly on the concentration of H2A. The second-order rate constants, k2 are dependent on [H+] according to the relationship k2 = (ka[H+] + kbKa) / ([H+] + Ka), where ka and kb are the rate constants for the oxidation of H2A and HA-, respectively. At 298.0 K and I = 0.1 M, ka = (6.47 ± 6.25) × 101 M-1 s-1, kb = (4.84 ± 0.10) × 103 M-1 s-1 and Ka = (4.23 ± 0.40) × 10-5. Kinetic studies have also been carried out in D2O. The solvent kinetic isotope effects are 3.32 and 0.98 for ka and kb, respectively. Mechanisms involving HAT/N-rebound at low pH (≤ 2) and nucleophilic attack at the nitrido at high pH (≥ 5) are proposed. The kinetics of oxidation of sulfite (SIV, SO32- + HSO3-) by [RuVI(N)(salchda)(MeOH)]+ (RuVIN) and [OsVI(N)(salchda)(H2O)]+ (OsVIN) have been studied in aqueous solutions. For the oxidation of SIV by RuVIN, the rate law is -d[RuVIN]/dt = k2[RuVIN][SIV]. The second-order rate constant, k2, is found to be pH dependent with k2 = (ka[H+] + kbKa)/([H+] + Ka), where ka and kb are the rate constants for the oxidation of HSO3- and SO32-, respectively. At 298.0 K and I = 1.0 M, ka = (1.63 ± 0.92) × 102 M-1 s-1 and kb = (6.17 ± 0.02) × 106 M-1 s-1, respectively. For OsVIN, the reaction has the following stoichiometry: [OsVI(N)]+ + SO32- + 2H+ → [OsIV(NH2SO3)]+. The rate law is -d[OsVIN]/dt = k2[OsVIN][SIV]. The dependence of k2 on [H+] shows a similar relationship with ruthenium(VI) nitrido where ka' and kb' were found to be (2.79 ± 0.97) × 10-1 M-1 s-1 and (1.14 ± 0.03) × 101 M-1 s-1 respectively at 298.0 K and I = 1.0 M. For both RuVIN and OsVIN, sulfite undergoes nucleophilic attack at the nitrido ligand to afford the sulfamato species, [RuIII(NH2SO3)(salchda)(OH2)] and [OsIV(NH2SO3)(salchda)(OH2)]+, respectively. Details of the reaction mechanism will be discussed. The reactions of RuVIN and OsVIN with hydroxide (OH-) have been studied in aqueous alkaline solutions. For RuVIN, the reaction has the following stoichiometry: RuVIN + 2OH- → RuIINO + H2O. The rate law is -d[RuVIN]/dt = k2[RuVIN][OH-]. It is proposed that OH- undergoes nucleophilic attack at the nitrido ligand to afford the nitrosyl species, RuII(NO)(salchda)(OH). For OsVIN, the reaction follows the rate law -d[OsVIN]/dt = k2[OsVIN][OH-]. The mechanism involves initial N∙∙∙N coupling of [OsVI(N)(L)(OH)], followed by N2 releasing and attacked by another OH- to form the peroxo species, Os(O2)(salchda). The oxidation of L-cysteine (Cys) by [RuVI(N)(salchda)(MeOH)]+ has been studied in anaerobic aqueous solutions over the pH range of 1.00-5.32. The electrophilic ruthenium nitrido species undergoes stepwise reactions with L-cysteine to produce the sulfilamido ([RuIV(NH)(Cys)(salchda)]+), sulfilamine ([RuIII(NH2)(Cys)(salchda)]+), and amine ([RuIII(NH3)(salchda)]+) complexes of ruthenium. At low pH (pH ≤ 4), only two phases were observed. However, three phases were observed at high pH (pH ≥ 4). The rate laws for all three phases are phases were observed at high pH (pH ≥ 4). The rate laws for all three phases are 0.09) × 105 M-1 s-1, k2' = (1.78 ± 0.21) × 102 M-1 s-1, and k2'' = (6.52 ± 0.30) × 101 M-1 s-1 at pH = 5.32, T = 298.0 K and I = 0.1 M for the first, second, and third phase respectively. The dependence of k2 on [H]+ indicated that both protonated (Cys-H+) nd neutral species (Cys) are involved in the reaction with the relationship k2 = (ka[H+] + kbKa) / ([H+] + Ka), where ka and kb are the rate constants for the oxidation of Cys-H+ and Cys, respectively. The reactivity of Cys is almost 103 times greater than that of Cys-H+. Kinetic studies have also been carried out in D2O. For the first phase, the solvent kinetic isotope effects (ka(H2O)/ka(D2O) are 2.03 and 1.74 at pH = 1.0 and 5.4, respectively. For the second phase, the solvent kinetic isotope effect (ka'(H2O)/ka'(D2O) is 2.52 at pH = 1.0. For the third phase, the solvent kinetic isotope effect (ka"(H2O)/ka"(D2O) is 2.27 at pH = 5.4. Hydrogen atom transfer (HAT) mechanisms are proposed at low pH.

    Research areas

  • Oxidation, Solution (Chemistry), Ligands (Biochemistry), Ruthenium compounds, Osmium compounds, Oxidation-reduction reaction