Experimental evidence of formation of imines in the course of reduction of hydrazones M.S. Baymak a , H. Celik a , H. Lund b , P. Zuman a, * a Department of Chemistry, Clarkson University, Potsdam, 8 Clarkson Avenue, NY 13699-5810, USA b Department of Organic Chemistry, Aarhus University, Aarhus, DK, Denmark Received 2 February 2005; received in revised form 21 April 2005; accepted 21 April 2005 Available online 15 June 2005 Abstract The reduction of hydrazones is generally suggested to proceed through a reductive cleavage of the nitrogen–nitrogen bond followed by a reduction of the carbon–nitrogen bond. This sequence of reduction processes is here supported for fluorenone (V) and benzophenone (VI) hydrazones as well as by a comparison of the reduction of fluorenone and benzophenone hydrazo- nium ions (I,III) with corresponding imines (II,IV). Another proof of the presence of imines as intermediates is the splitting of four-electron waves of hydrazones V and VI and hydrazonium ions I and VIII into two waves at pH < 2. This has been inter- preted as due to differences in slopes dE 1/2 /dpH and pK a -values of protonated hydrazine derivatives on one side and correspond- ing imines on the other. In this pH-range imines formed in reductions of VI and VIII are reduced in a single two-electron wave, those of I and V in two one-electron steps. Fluorenone imine (II) is sufficiently stable to allow recording of time-independent current–voltage curves between pH 6 and 11. In this pH-range the imine (II) is reduced in two one-electron steps. Benzophenone imine (IV) has been found stable between pH 4.6 and 12. At pH 4.6–8 the reduction of the imine IV takes place in a single two- electron step, at pH 8–12 in two one-electron steps. Final proof of the initial cleavage of the N–N bond is presented by compar- ison with the reduction of nitrones. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Hydrazones; Imines; Electroreduction; Reactive intermediate; Polarography 1. Introduction The reduction of most studied hydrazones of aro- matic aldehydes and ketones occurs in a single four- electron step [1,2], in which the diprotonated form [3] is reduced to an amine. Such conversion can be initiated: (a) either by the hydrogenation of the azomethine group conjugated with the aromatic ring or (b) by the cleavage of the N–N bond. (a) The hydrogenation of the azomethine group (1) would yield a hydrazine derivative, which would have to be reduced in the second step (2). The overall reaction steps would be: ArCðRÞ@NNH 2 þ 2H þ þ 2e ! ArCHðRÞANHNH 2 ð1Þ ArCHðRÞANHNH 2 þ 2H þ þ 2e ! ArCHðRÞANH 2 þ NH 3 ð2Þ Initially, this interpretation seemed attractive, be- cause it is known that reductions of the conjugated azomethine bonds are relatively easy, whereas the 0022-0728/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelechem.2005.04.030 * Corresponding author. Tel.: +1 315 268 2340; fax: +1 315 268 6610. E-mail address: zumanp@clarkson.edu (P. Zuman). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 581 (2005) 284–293 Journal of Electroanalytical Chemistry