Malcolm P. Heyward

Kinetics of the oxidation of isopropyl alcohol by bis(2,2′-bipyridine)silver(II) ions

The oxidation of isopropyl alcohol by [Ag(bipy)2]2+ is first order in [Ag(bipy)2]2+ and the order in PriOH is between zero and unity. The kinetics show that intermediate complexes are involved and from the variation of rate with acidity one formed between [Ag(bipy)]2+ and PriOH can be specified as the only one involved. At high added [AgI] a back reaction is found and this is discussed in terms of the influence of AgI ions in the solvent wall of a cage before the AgI and radical formed in the oxidation can diffuse out. Values for ΔH* and ΔS* for the oxidation at low [AgI] are compared with ΔH* and ΔS* for the oxidation of PriOH and aqua-cations.

Kinetics of the oxidation of quinol by bis(2,2′-bipyridine)manganese(III)ions in aqueous perchlorate media

The kinetics of the oxidation of quinol by Mn(bipy)23+aq ions have been investigated at 25 °C in aqueous perchlorate media using a high excess of H2Q. The observed kinetic order in [Mn(bipy)23+] is unity and the linear plots of the reciprocal of the pseudo first order rate constant against [H2Q]−1 show that intermediate complexes are involved. From the manner in which the slopes of these latter plots vary with acidity, it is suggested that these reactive intermediate complexes are Mn(bipy)2HQ2+aq and Mn(bipy)2(OH)HQ+aq. The mechanism of this reaction is compared with those suggested for the oxidation of quinol by aqua cations and by cations without a water molecule present in the inner sphere.

Kinetics of the oxidation of water by silver(II) ions complexed with 2,2′-bipyridine in aqueous nitrate media

The kinetics of the oxidative decay of silver(II) ions complexed by 2,2′-bipyridine (bipy) when present in aqueous acidic media have been investigated. The reaction is found to be second order in [AgII] and zero order in [H+] with an inverse dependence on [AgI]. From a detailed consideration of the experimental data, including the overall enthalpy and entropy of activation, it is concluded that AgIII is not involved as an intermediate and that the essential redox step consists of the interaction of two molecules of [Ag(bipy)(OH)]+ formed in the hydrolysis of [Ag(bipy)]2+(aq), which has been found to be the active bipy complex of AgII involved in the oxidation of substrate ligands added to aqueous solutions.

Kinetics of the oxidation of hydrogen peroxide by bis(2,2′-bipyridine)-silver(II) ions

The kinetics of the oxidation of hydrogen peroxide by [Ag(bipy)2]2+ have been investigated in aqueous nitric acid. It is found that the principal reactive species of AgII is [Ag(bipy)]2+ which has been shown to exist in equilibrium with [Ag(bipy)2]2+. There is no direct evidence that the reaction involves intermediate ternary complexes AgII(bipy) H2O2. The mechanism and the transition-state parameters are contrasted with those for the oxidation of H2O2 by aqua-cations and compared with those for the oxidation of H2O2 by other cations complexed by nitrogen-containing ligands, including some of biological significance.

Kinetics of oxidation of hydrogen azide (hydrazoic acid) by tris(2,2′-bipyridine)nickel(III) ions in aqueous perchlorate media: comparison with oxidation by aqua-cations

E.s.r. measurements on solutions of NiII containing 2,2′-bipyridine (bipy) confirm that anodic oxidation produces NiIII. The stoicheiometry and kinetics of oxidation of HN3 by [Ni(bipy)3]3+ have been investigated. Unlike the oxidations of H2O2 and Br– by this cation, with HN3 the rate varies with the concentration of HClO4 at constant ionic strength. Under conditions where specific cation effects are eliminated, the rate is directly proportional to [H+]–1, suggesting that the kinetically active entity of the hydrazoic acid is N3–. No removal of bipy from the NiIII occurs before the slow oxidative step, even though an outer-sphere intermediate complex may be formed between [Ni(bipy)3]3+ and N3–. This system is compared with the oxidations of HN3 by aqua-cations and with the oxidations of other ligands by [Ni(bipy)3]3+.

Spectrophotometric investigation of equilibria between silver(II) and 2,2′-bipyridine

A spectrophotometric study has been made of an aqueous solution of silver(II) containing nitric acid and an excess of 2,2′-bipyridine over the wavelength range 350–550 nm. The data are consistent with the existence of only two complexes related by the equilibrium [Ag(bipy)2]2++ H+⇌[Ag(bipy)]2++ Hbipy+. At I= 1.00 mol dm–3 and at 25 °C, Kh for this equilibrium is found to be (3.3 ± 0.5)× 10–3. From the variation of Kh with temperature the values for the enthalpy and entropy of the above reaction are found to be 11.5 ± 2.6 kJ mol–1 and –9 ± 10 J K–1 mol–1 respectively.

The kinetics of the oxidation of hydrogen peroxide by bis(2,2′-bipyridine)manganese(III) ions in aqueous perchlorate media

The kinetics of the oxidation of hydrogen peroxide by bis(2,2′-bipyridine)manganese(III) ions have been investigated in aqueous perchlorate media. The kinetics are found to vary with the initial ratio [H2O2]/[Mn(bipy)3+2aq·]: if k0 is the observed pseudo-first-order rate constant, linear plots are found for k–10 against [H2O2]–1 at low [H2O2] and linear plots of k0 against [H2O2] with an intercept on the k0 axis are found at high [H2O2]. From a detailed consideration of the variation of the kinetics with [H2O2] and [H+], the first-order decomposition of three complexes, [Mn(bipy)2HO2]2+aq·, [Mn(bipy)2(H2O2)2]3+aq· and [Mn(bipy)2H2O2HO2]2+aq· are found to contribute to the oxidation.

Kinetics of the oxidation of hydrogen azide (hydrazoic acid) by bis(2,2′-bipyridine)manganese(III) ions in aqueous perchlorate media

On mixing HN3 with solutions containing [Mn(bipy)2]3+(aq)(bipy = 2,2′-bipyridine) the absorbance at 400 nm first increases before decreasing to zero. The analysis of the absorption coefficient extrapolated to zero time for the rise in absorbance shows that two 1 : 1 complexes are formed between N3– and [Mn(bipy)2]3+. The kinetics of the subsequent decay in absorbance shows that both these complexes, [Mn(bipy)2(N3)]2+ and [Mn(bipy)2(OH)(N3)]+, are involved in the oxidation process producing free radicals N3˙. This oxidation is compared with the kinetics of the oxidation of HN3 by aqua cations and by [Ni(bipy)3]3+.

Kinetics of the oxidation of 2-hydroxy-2-methylpropanoic acid by silver(II) ions complexed with 2,2′-bipyridine in aqueous nitrate media

Stopped-flow traces show that the oxidation of 2-hydroxy-2-methylpropanoic acid (hmpa) by [Ag(bipy)2]2+ proceeds in two consecutive reactions. Both are found to be first order in [AgII] and first order in [hmpa]. The first rapid reaction is ascribed to complex formation between AgII and hmpa and the second to a slower redox step. A mechanism is proposed to account for the observed orders in [AgII], [hmpa] and [H+] for each reaction and values for the enthalpies and entropies of activation are determined. To investigate the effect on the redox kinetics of oxidatively inert species close to the cation, these are compared with the transition-state parameters for the oxidation of hmpa by aqua-metal cations and for the oxidation of other substrates by metal cations complexed with 2,2′-bipyridine.

Kinetics of the oxidation of 2,3-dimethylbutane-2,3-diol by bis(2,2′-bipyridine)silver(II) ions in aqueous nitrate media

The stoichiometry and the kinetics of the oxidation of 2,3-dimethylbutane-2,3-diol (pinacol) by [Ag(bipy)2]2+ ions have been investigated. It is shown that the oxidation proceeds through intermediate complexes involving AgII and pinacol. Although [Ag(bipy)2]2+ itself is eliminated as a participant in these oxidatively active complexes, [Ag(bipy)(pinacol)]2+ remains as a possible intermediate. The kinetics show that one of the principal pathways involves Ag2+aq complexed with un-ionised pinacol, but they do not distinguish between the involvement of [Ag(bipy)(pinacol)]2+ and [Ag2+(CH3)2COH·(CH3)2CO–] in the other principal path: the linear Arrhenius plots suggest that the latter two are alternatives and not parallel courses. Values for the enthalpies and entropies of activation are determined. This oxidation is compared with the oxidation of other substrates by [Ag(bipy)2]2+; the pathway involving oxidation by Ag2+ with all the bipyridine removed is compared with the oxidation of other substrates by Ag2+aq and the oxidation of pinacol and other substrates by other aquacations.