Interaction of phenylhydrazine with Ru-III-EDTA complexes: Reduction of phenylhydrazine to ammonia and aniline in aqueous acidic conditions

Reaction of the phenylhydrazinium (PhN(2)H(4)(+)) ion with the labile aquo complexes of [Ru-III(HEDTA)(H2O)] (EDTA = ethylenediaminetetraacetate) was studied electrochemically employing sampled DC, cyclic voltammetry and differential pulse techniques in 0.2 M CH3COONa and H2SO4 mixture between pH 1.0 and 4.5 at 25 degrees C. It produced two phenylhydrazinium complexes, namely [Ru-III(HEDTA)(N(2)H(4)Ph)](+) and [Ru-III(EDTA)(N(2)H(4)Ph)] in the rapid aquo-substitution reactions. The second-order rate constants, k(1), for the formation of the former, and k(2) for the latter are 0.98 and 36.85 M(-1) s(-1), respectively, as determined spectrophotometrically by following the intensity of the LMCT band at 420 nm. The complex, [Ru-III(HEDTA)(N(2)H(4)Ph)]Cl . 2H(2)O was prepared and characterised by physicochemical methods. These phenylhydrazinium complexes were reduced by two electrons at E(1/2) = -0.175 V vs SCE, producing one mole of NH3 and the corresponding Ru-III-NH(2)Ph complexes in subsequent rapid decomposition steps. The unstable Ru-III-NH(2)Ph complexes were rapidly hydrolysed to one mole of NH(2)Ph and the respective aquo complex. One-electron reduction steps for the regenerated aquo complex at (E(1/2)) -0.254 V and the unhydrolysed Ru-III-NH(2)Ph complex at -0.417 V were also observed. Phenylhydrazine was reduced to ammonia and aniline by constant potential coulometry at -0.200 V(Hg) vs SCE, in the presence of [Ru-III(HEDTA)(H2O)] at 100:1 molar ratio. The turnover number (per mole of catalyst per hour) with respect to ammonia is 5.98 and 2.82 at pH 2.8 and 1.9, respectively. The probable mechanisms for the electroreduction of the above phenylhydrazinium complexes and the catalytic reduction of phenylhydrazine to NH3 and NH(2)Ph, are proposed. Copyright (C) 1997 Elsevier Science Ltd.