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Issue 8, 1998
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A kinetic study of the reaction between noradrenaline and iron(III): an example of parallel inner- and outer-sphere electron transfer

Abstract

In anaerobic acid solution noradrenaline [norepinephrine, 4-(2-amino-1-hydroxyethyl)benzene-1,2-diol, H2LH+ (in which the phenolic protons are written on the left of L)] reacts with iron(III) [in the form of Fe(OH)2+] to yield iron(II) and the semiquinone form of noradrenaline which is in turn oxidised rapidly by more iron(III) to ‘noradrenoquinone’. This reaction proceeds both directly (i.e.via an ‘outer-sphere’ reaction) and after prior formation of the complex Fe(LH)2+ which then decomposes via intramolecular electron transfer. [The observed rate of formation of the complex (monitored at 714 nm) is faster than the rate of its decomposition by a factor of about 200.] The quinone then cyclises by an intramolecular Michael addition giving the (UV transparent) leuconoradrenochrome (indoline-3,5,6-triol), which is able to react with iron(III) at high pH to give noradrenochrome (3,5-dihydro-3,6-dihydroxy-2H-indol-5-one). At lower pH values the presence of chloride ions shows a marked effect on the rate of complex formation because the species FeCl2+ is also able to react with noradrenaline to form the complex although chloride is not involved in the reverse reaction. The stability constant for the formation of FeCl2+ (K 1Cl) was found to be 35, i.e. log K 1Cl = 1.54 (identical to the value obtained from previous work with dopamine). The ring-closure reaction was studied by following the rate of quinone decomposition monitored at 380 nm, and a mechanism for this cyclisation is proposed. The following rate constants have been evaluated: (i) for the reversible formation of the iron–noradrenaline complex [via Fe(OH)2+ + H2LH+] k1 = 2170 ± 20 dm3 mol–1 s–1 and k–1 = 21 ± 2 dm3 mol–1 s–1, from which the stability constant of the Fe(LH)2+ complex has been calculated (log K 1M = 21.2), (ii) rate of formation of the complex via FeCl2+ + H2LH+, kCl = 48 ± 3 dm3 mol–1 s–1, (iii) rate of decomposition of the complex Fe(HLH)3+, k2 = 2.6 ± 0.1 s–1 [protonation constant for Fe(LH)2+, K MH = 34 ± 1 dm3 mol–1], (iv) rate of outer-sphere redox reaction, k2′ = 100 ± 2 dm3 mol–1 s–1, (v) rate of indole formation (ring-closure reaction), kcyc = 1400 ± 20 s–1 (for quinone) and kcycH = (2.0 ± 0.1) × 105 s–1 (for protonated quinone). All measurements were carried out at 25.0 °C in solutions of ionic strength 0.10 mol dm–3 (KNO3 serving as inert electrolyte).

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Article type: Paper
DOI: 10.1039/A708639C
Citation: J. Chem. Soc., Dalton Trans., 1998,0, 1315-1320
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    A kinetic study of the reaction between noradrenaline and iron(III): an example of parallel inner- and outer-sphere electron transfer

    U. El-Ayaan, R. F. Jameson and W. Linert, J. Chem. Soc., Dalton Trans., 1998, 0, 1315
    DOI: 10.1039/A708639C

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