Electrochimica Acta 54 (2009) 2164–2170 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Spontaneous grafting of diazoates on metals Fetah I. Podvorica a , Frédéric Kanoufi b , Jean Pinson b , Catherine Combellas b,∗ a Chemistry Department of Natural Sciences Faculty, University of Prishtina, rr. “Nëna Tereze” nr. 5, Prishtina, Kosovo b Laboratoire Environnement et Chimie Analytique, CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France article info Article history: Received 30 May 2008 Received in revised form 9 October 2008 Accepted 10 October 2008 Available online 19 October 2008 Keywords: Surface chemistry Electrografting Thin layer Diazoate Diazonium abstract Hydroxyl ions react with diazonium salts to give transient diazohydroxides, which are deprotonated to diazoates. The latter react spontaneously with Fe and Au to give surfaces modified with aryl groups. These surfaces are characterized by XPS and IR spectroscopies. The mechanism involves the homolytic reduction of the diazoate to give a radical that binds to the substrate. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction Diazonium salts react with various strong nucleophiles at the terminal nitrogen to give azo adducts [1,2]. For example, they react with oxygen nucleophiles such as hydroxide and methoxide ions, sulfur nucleophiles such as thiolates and carbon nucleophiles such as cyanide. These reactions lead respectively to diazohydroxides, diazomethoxides, diazosulfides and diazocyanides. Diazohydroxides and diazoates are obtained by addition of OH - to the diazonium salt along the simplified mechanism shown in Scheme 1. The attack of the hydroxide ion on the nitrogen atom leads to the Z-isomer of the diazo hydroxide Ar-N N-OH. The rate of the reaction, between two oppositely charged, ions is quite fast (from 1.2 × 10 3 to 5.4 × 10 5 M -1 s -1 for, respectively, Ar NO 2 - C 6 H 4 and CH 3 -C 6 H 4 ). The kinetic behaviour indicates that the rate-determining step is the formation of the diazohydroxide, the diazoate being formed in a rapid subsequent equilibrium. In basic water the diazohydroxide never exceeds a few percents [1]. The reaction leads to the exclusive formation of Z-diazoates, some of them have been isolated and crystallized [3]. The overall equi- librium constants, K, for the reaction of Scheme 1 have been measured: K = 2.8 × 10 4 , 1.4 × 10 6 , 2.0 × 10 10 M -2 for respectively Ar C 6 H 5 , BrC 6 H 4 , 4-NO 2 C 6 H 4 [1]. The pK a obtained by titration of diazoate with an acid or the kinetic pK a measured from the ∗ Corresponding author. E-mail address: Catherine.Combellas@espci.fr (C. Combellas). observed rate of transformation of diazoate to diazonium are quite similar: for Ar C 6 H 5 ,pK a =7.25 and 7.29 and for Ar C 6 H 4 NO 2 , pK a =6.25 and 6.13, respectively [1]. Therefore, in a pH 10 buffer, as in the following experiments, the species in solution after disso- lution of a diazonium salt is mainly the diazoate. As stated above, the Z-isomer is formed directly when a diazonium salt is dis- solved in a basic medium, its isomerization to the E-isomer is very slow (k = 30.5 × 10 -3 s -1 for 4-NO 2 -C 6 H 4 O - and K iso = [Z]/[E] = 600). Therefore, during our experiments, the only Z-isomer is present in solution [4–6]. Diazonium salts (ArN 2 + ) have been used to attach aryl groups to a variety of surfaces (carbon, metals, semiconductors or poly- mers) [7,8]. This reaction takes place when the diazonium salts are reduced either electrochemically or chemically (by the substrate to be modified or by a reducing agent in solution [9]). The reactive species is an aryl radical Ar • produced upon transfer of one electron to ArN 2 + ; it binds to the surface through a covalent bond [10–12]. This reaction has lead to industrial applications for the modifica- tion of carbon blacks [13] or for the surface modification of drug eluting stents [14]. The reduction of some N-substituted derivatives of diazonium salts is described in the literature: (i) diazosulfides, which upon electrochemical reduction, give rise to S RN 1 and izomerization reactions and no sign of electrografting [15–17] and (ii) triazenes, which are obtained upon reaction of amines on diazonium salts and react spontaneously in HF on silicon to give a modified sil- icon surface [18]. Recently, Tour has examined the reactivity of benzenediazonium at pH 10 on Single Wall Carbon NanoTubes [19]. To our knowledge, the reactivity of diazoates towards glassy 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.10.017