Efectrachimicn Acta, Voi. 42, Nos. 13-14, pp. 20652071, 1997 0 1997 Elsevier Science Ltd. All rights reserved Pergamon PII: soo13-4686(%)00482-3 Printed in Great Britain 0013-4686/97 $17.00 + 0.00 Electrochemical reduction of Schiff base ligands Hzsalen and Hzsalophen Abdirisak Ahmed Isse, Armando Cennaro and Elio Vianello* Dipartimento di Chimica Fisica, Universiti di Padova, via Loredan 2, 35131 Padova, Italy (Received 11 November 1996) Abstract-The electrochemical reduction of the Schiff base ligands N,N’-1,2-ethylenebis(salicylideneimine) and N,N’-1,2-phenylenebis(salicylideneimine) has been investigated in DMF by cyclic voltammetry, coulometry and controlled potentia1 electrolysis. The process involves a self-protonation mechanism whereby the two-electron reduction product, a cyclic derivative, is formed together with the conjugate base of the substrate, as a consequence of proton transfer from the substrate itself to the basic inte~ediates 0 1997 Elsevier Science Ltd. Key words: Schiff base ligands, electrochemical reduction, self-protonation, reductive cyclisation. INTRODUCTION Tetradentate Schiff bases such as N,N’-1,2- ethylenebis(salicylideneimine) (Hzsalen) and N,N’- 1,2-phenylenebis(salicylideneimine) (Hzsalophen) are widely used as ligands in many transition metal complexes. Schiff base complexes of cobalt and nickel play an important role in many electrocatalytic processes. Nickel complexes have been recognized as powerful catalysts in the chemical and electrochemi- cal reduction of alkyl halides [l-4]. Cobalt complexes have been extensively investigated as model com- pounds of vitamin BQ [5,6]. Recently, complexes such as Co(salen) and Co(salophen) have attracted considerable interest as catalysts for the electrochem- ical activation of carbon dioxide [7-121. A key factor in all catalytic processes is the reduction of the transition metal to a low oxidation state. Catalytic efficiency hinges on the reducibiiity of the metal center as well as on the reactivity of the reduced metal complex. In certain cases, however, preferential reduction of the ligand takes place, resulting in reduced catalysis or no catalysis at all [13]. For this reason knowledge of the electrochemical behaviour of the ligands may be very helpful in the set-up of eficient catalytic systems. The electrochemical behaviour of Schiff bases has been widely investigated, particularly in dipolar aprotic solvents, where hydrolysis of the iminic *Author to whom correspondence should be addressed. moiety is hampered [14-191. The process involves hydrogenation of the C==N bond, leading to a saturated amine, although formation of dimeric diamines were observed in a number of cases. The overall two-electron process gives rise to a single reduction wave or it takes place in two separate one-electron steps, depending on the structure of the substrate and on the proton-donor ability of the medium. However, with N-benzylideneanilines bear- ing a hydroxy group on either of the two phenyl rings, a wave splitting is invariably observed [ 19-221. As we have recently shown, the latter is not att~butable to a stepwise reduction of the C=N bond but is due to the separate reduction of the OH-substituted anil and of its conjugate base stemming from a self-protona- tion mechanism involving proton transfer from the acidic substrate to the basic reduction intermediates zyxwvutsr [231. El~trochemical reduction of diimines was studied by Koch and Dessy [24] in DMF and 1,2- dimethoxyethane. They found that, depending on the solvent used and on the structure of the Schiff base, reduction leads either to open-chain diamines or to cyclic diamines resulting from intramolecular coup- ling of radical species. More recently, Shono and co-workers [25,26] have studied the electrochemical reduction products of a series of substituted N,N’-bis(benzylidene)ethylenediamines, including the ligand Hzsalen, in DMF. All compounds were found to give, with good yields, cyclic diamines resulting from intramolecular reductive couplings.