Kinetics and mechanisms of the oxidation of hypophosphite and phosphite with trans-[RuVI(L)(O)2]2+ (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane)
“…In the presence of excess hydrocarbon, trans -[Ru VI (L)(O) 2 ] 2+ is reduced to trans -[Ru IV (L)(O)(CH 3 CN)] 2+ in CH 3 CN. No intermediate Ru V species was observed, as in the reaction of trans -[Ru VI (L)(O) 2 ] 2+ with a number of other reducing agents. −,− The failure to detect a Ru V intermediate can be due to a number of possibilities. If the reaction involves H - transfer, as in the oxidation of alcohols 14 and phosphite, the intermediate would be trans -[Ru IV (L)(O)(OH)] + .…”
Section: Discussionmentioning
confidence: 99%
“…No intermediate Ru V species was observed, as in the reaction of trans -[Ru VI (L)(O) 2 ] 2+ with a number of other reducing agents. −,− The failure to detect a Ru V intermediate can be due to a number of possibilities. If the reaction involves H - transfer, as in the oxidation of alcohols 14 and phosphite, the intermediate would be trans -[Ru IV (L)(O)(OH)] + . In the oxidation of excess [Ru II (NH 3 ) 4 (isn) 2 ] 2+ (isn = isonicotinamide) in aqueous acidic solution, trans -[Ru V (L)(O) 2 ] + is first produced, which is then protonated to give trans -[Ru V (L)(O)(OH)] 2+ …”
The oxidations of a series of 21 alkylaromatic compounds by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in CH(3)CN. Toluene is oxidized to benzaldehyde and a small amount of benzyl alcohol. 9,10-Dihydroanthracene is oxidized to anthracene and anthraquinone. Other substrates give oxygenated products. The kinetics of the reactions were monitored by UV-vis spectrophotometry, and the rate law is: -d[Ru(VI)]/dt = k(2)[Ru(VI)][ArCH(3)]. The kinetic isotope effects for the oxidation of toluene/d(8)-toluene and fluorene/d(10)-fluorene are 15 and 10.5, respectively. A plot of Delta H(++) versus Delta S(++) is linear, suggesting a common mechanism for all the substrates. In the oxidation of para-substituted toluenes, a linear correlation between log k(2) and sigma(0) values is observed, consistent with a benzyl radical intermediate. A linear correlation between Delta G(++) and Delta H(0) (the difference between the strength of the bond being broken and that being formed in a H-atom transfer step) is also found, which strongly supports a hydrogen atom transfer mechanism for the oxidation of these substrates by trans-[Ru(VI)(L)(O)(2)](2+). The slope of (0.61 +/- 0.06) is in reasonable agreement with the theoretical slope of 0.5 predicted by Marcus theory.
“…In the presence of excess hydrocarbon, trans -[Ru VI (L)(O) 2 ] 2+ is reduced to trans -[Ru IV (L)(O)(CH 3 CN)] 2+ in CH 3 CN. No intermediate Ru V species was observed, as in the reaction of trans -[Ru VI (L)(O) 2 ] 2+ with a number of other reducing agents. −,− The failure to detect a Ru V intermediate can be due to a number of possibilities. If the reaction involves H - transfer, as in the oxidation of alcohols 14 and phosphite, the intermediate would be trans -[Ru IV (L)(O)(OH)] + .…”
Section: Discussionmentioning
confidence: 99%
“…No intermediate Ru V species was observed, as in the reaction of trans -[Ru VI (L)(O) 2 ] 2+ with a number of other reducing agents. −,− The failure to detect a Ru V intermediate can be due to a number of possibilities. If the reaction involves H - transfer, as in the oxidation of alcohols 14 and phosphite, the intermediate would be trans -[Ru IV (L)(O)(OH)] + . In the oxidation of excess [Ru II (NH 3 ) 4 (isn) 2 ] 2+ (isn = isonicotinamide) in aqueous acidic solution, trans -[Ru V (L)(O) 2 ] + is first produced, which is then protonated to give trans -[Ru V (L)(O)(OH)] 2+ …”
The oxidations of a series of 21 alkylaromatic compounds by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in CH(3)CN. Toluene is oxidized to benzaldehyde and a small amount of benzyl alcohol. 9,10-Dihydroanthracene is oxidized to anthracene and anthraquinone. Other substrates give oxygenated products. The kinetics of the reactions were monitored by UV-vis spectrophotometry, and the rate law is: -d[Ru(VI)]/dt = k(2)[Ru(VI)][ArCH(3)]. The kinetic isotope effects for the oxidation of toluene/d(8)-toluene and fluorene/d(10)-fluorene are 15 and 10.5, respectively. A plot of Delta H(++) versus Delta S(++) is linear, suggesting a common mechanism for all the substrates. In the oxidation of para-substituted toluenes, a linear correlation between log k(2) and sigma(0) values is observed, consistent with a benzyl radical intermediate. A linear correlation between Delta G(++) and Delta H(0) (the difference between the strength of the bond being broken and that being formed in a H-atom transfer step) is also found, which strongly supports a hydrogen atom transfer mechanism for the oxidation of these substrates by trans-[Ru(VI)(L)(O)(2)](2+). The slope of (0.61 +/- 0.06) is in reasonable agreement with the theoretical slope of 0.5 predicted by Marcus theory.
“…The reduced oxidation states Ru V , Ru IV , and Ru III have been isolated and/or characterized spectroscopically. The oxidation of aromatic hydrocarbons, alcohols, alkenes, phosphites, and hypophosphites by trans -[Ru VI (L)(O) 2 ] 2+ have been reported. 1 Structure of L.…”
The kinetics of the oxidation of phenols by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions and in acetonitrile. In H(2)O the oxidation of phenol produces the unstable 4,4'- biphenoquinone, as evidenced by a rapid increase and then a slow decrease in absorbance at 398 nm. The first step is first-order in both Ru(VI) and phenol, and rate constants are dependent on [H(+)] according to the relationship k(f) = k(x) + (k(y)K(a)/[H(+)]), where k(x) and k(y) are the rate constants for the oxidation of PhOH and PhO(-), respectively. At 298 K and I = 0.1 M, k(x) = 12.5 M(-1) s(-1) and k(y) = 8.0 x 10(8) M(-1) s(-1). At I = 0.1 M and pH = 2.98, the kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.8 and 0.74 for k(x) and k(y), respectively, and k(f)(C(6)H(5)OH)/k(f)(C(6)D(5)OH) = 1.1. It is proposed that the k(x) step occurs by a hydrogen atom abstraction mechanism, while the k(y) step occurs by an electron-transfer mechanism. In both steps the phenoxy radical is produced, which then undergoes two rapid concurrent reactions. The first is a further three-electron oxidation by Ru(VI) and Ru(V) to give p-benzoquinone and other organic products. The second is a coupling and oxidation process to give 4,4'-biphenoquinone, followed by the decay step, k(s). A similar mechanism is proposed for reactions in CH(3)CN. A plot of log k(x) vs O-H bond dissociation enthalpies (BDE) of the phenols separates those phenols with bulky tert-butyl substituents in the ortho positions from those with no 2,6-di-tert-butyl groups into two separate lines. This arises because there is steric crowding of the hydroxylic groups in 2,6-di-tert-butyl phenols, which react more slowly than phenols of similar O-H BDE but no 2,6-tert-butyl groups. This is as expected if hydrogen atom abstraction but not electron transfer is occurring.
“…The oxidation of phosphorus acids by various oxidants involves either a single‐electron transfer or hydride abstraction . The hydride abstraction generally occurs when the oxidant undergo more than two electron change since in the present study [H 5 Co III Mo 5 O 20 ] 2− is a single‐electron oxidant; the direct electron transfer mechanism is preferred.…”
The oxidation of hypophosphite and phosphite by the Anderson‐type hexamolybdocobaltate(III), [H6CoIIIMo6O24]3−, anion was investigated at pH 2 and 1, respectively, in aqueous medium. The reaction is found to occur through an outer‐sphere mechanism with a prior weak complex formation between the reactants. Under the reaction conditions, the oxidant exists in the [H5CoIIIMo5O20]2−, [H6CoIIIMo6O24]3−, and [H4CoIII2Mo10O38]6−(dimer) forms, and [H5CoIIIMo5O20]2− is the active species. Inhibition of the reaction by the oxidant anion and added molybdate ion kinetically indicates existence of prior equilibria between various forms of the oxidant. Both hypophosphite and phosphite exists in their protonated forms. The reaction involves direct electron transfer from the phosphorus center to the anion‐generating free radicals in a rate‐determining step. The effect of ionic strength and change in the solvent polarity did not affect the rate of the reaction. A probable mechanism was proposed leading to a complicated rate law as a result of involvement of prior equilibria between various forms of the oxidant. The activation parameters were also determined and are in support of the proposed mechanism.
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