Kinetics and mechanisms of some redox reactions of trans-dioxoruthenium(VI) complexes bearing macrocyclic tertiary amine ligands and a nitridoruthenium(VI) complex bearing a salen ligand

Abstract

The chemistry of oxo- and nitrido-metal complexes has received much attention in recent years. This project studies the kinetics and mechanisms of the reduction of dioxoruthenium(VI) and nitridoruthenium(VI) complexes by various inorganic and organic substrates. The oxidation of ascorbic acid (H2A) by trans-[RuVI(tmc)(O)2]2+ (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) has been studied in aqueous solutions under argon. The reaction occurs in two phases: trans-[RuVI(tmc)(O)2]2+ + H2A → trans-[RuIV(tmc)(O)(OH2)]2+ + A, trans-[RuIV(tmc)(O)(OH2)]2+ + H2A → trans-[RuII(tmc)(OH2)2]2+ + A. Further reaction involving anation by H2A occurs, and the species [RuIII(tmc)(A2−)(MeOH)]+ can be isolated upon aerial oxidation of the solution at the end of phase two. The rate laws for both phases are first-order in both RuVI and H2A, with the second-order rate constants k2 = (2.58 ± 0.04) × 103 M−1 s−1 at pH = 1.19 and k2’= (1.90 ± 0.03) M−1 s−1 at pH = 1.24, T = 298 K and I = 0.1 M for the first and second phase respectively. Studies on the effects of acidity on k2 and k2 ’ suggest that HA− is the kinetically active species. Kinetic studies have also been carried out in D2O, and the deuterium isotope effects for oxidation of HA− by RuVI and RuIV are 5.0 ± 0.3 and 19.3 ± 2.9 respectively; consistent with a hydrogen atom transfer (HAT) mechanism for both phases. A linear correlation between log(rate constants) for oxidation by RuVI and the O−H bond dissociation energies of HA− and hydroquinones is obtained, which also supports a HAT mechanism. The kinetics of the oxidation of thiocyanate (SCN−) by trans-[RuVI(N2O2)(O)2]2+ (N2O2 = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza -5,8-dioxacyclopentadecane) have been investigated in aqueous solutions as well as in CH3CN. In aqueous solutions, the reaction has the following stoichiometry: 3[RuVI(N2O2)(O)2]2+ + SCN− + 4H2O → 3[RuIV(N2O2)(O)(OH2)]2+ + SO4 2− + HCN + H+. The reaction follows the rate law −d[RuVI]/dt = kH2O[RuVI][SCN−]. kH2O increases with [H+] according to the equation: kH2O = ka + kb[H+]. At 298.0 K and I = 0.1 M, kb is (3.07 ± 0.06) × 105 M−2 s−1 and ka is close to zero. The activation parameters, ΔH‡ and ΔS‡, are found to be (4.4 ± 0.1) kcal mol−1 and (−27 ± 1) cal mol−1 K−1 at [H+] = 0.016 M and I = 0.1 M. The proposed mechanism involves an initial oxygen-atom transfer followed by protonation, and then the resulting [O=RuIV−OHSCN]2+ undergoes acid-catalyzed oxidation of SCN−. On a longer time scale, further reaction appears to occur which is due to the hydrolysis of the derivative of SCN−. In CH3CN, the reaction follows the rate law [Ru ] 1 [SCN ] [SCN ] d d[Ru ] VI CT CT VI − − + − = K kK t . In the proposed mechanism, the initial phase involves formation of a charge-transfer complex between RuVI and SCN−, followed by oxygen atom transfer to give [O=RuIV−OSCN]2+, which then undergoes relatively slow substitution to produce [O=RuIV−NCCH3]2+. ESI/MS shows the presence of [RuIV(N2O2)(O)(OSCN)]+ which provides evidence for oxygen-atom transfer mechanism. The kinetics and mechanisms of the reduction of [RuVI(N)(salchda)(MeOH)]+ (salchda = N,N’-bis(salicylidene)-o-cyclohexyldiamine dianion) to [RuIII(N(H)PPh3)(salchda)(MeOH)]+ have been investigated in CH3CN. In the reaction of [RuVI(N)(salchda)(MeOH)]+ with excess PPh3, the kinetics of the first phase are too fast to be followed while the rate law for the second phase is −d[RuVI]/dt = k2[RuVI][PPh3] where k2 is (1.21 ± 0.05) × 102 M−1 s−1 at 298 K. In the proposed mechanism, [RuVI(N)(salchda)(MeOH)]+ reacts rapidly with one equivalent of PPh3 to generate [RuIV(NPPh3)(salchda)(MeOH)]+, which is the first rapid phase. Then this species is converted to [RuIII(N(H)PPh3)(salchda)(MeOH)]+ by abstracting a hydrogen atom. This H-atom abstraction step is further confirmed by the reaction of [RuVI(N)(salchda)(MeOH)]+ with various phenols in excess in the presence of PPh3. 4-t-butylphenol is oxidized to 5,5’-bis(1,1-dimethylethyl)-1,1’-biphenyl-2,2’-diol. The reaction follows the rate law: −d[RuIV]/dt = Kk[RuIV][X-PhOH]. The kinetic isotope effect for the oxidation of C6H5OH/C6H5OD is 1.8 at 298 K. A linear correlation is obtained when log k2’ was plotted against the O−H BDEs of phenols with electron-donating substituents, while 2,6-di-t-butylphenol reacts much more slowly than expected. This arises from the steric crowding of the hydroxyl group and hence a hydrogen atom abstraction mechanism is proposed.