Silver–palladium cathode

Electrochimica Acta 56 (2010) 15–36

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Review article

Silver–palladium cathode Selective one-electron scission of alkyl halides: Homo-coupling and cross-coupling subsequent reactions Philippe Poizot a , Jacques Simonet b,∗ a b

Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 6007, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France Laboratoire MaCSE, UMR CNRS 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France

a r t i c l e

i n f o

Article history: Received 31 May 2010 Received in revised form 7 September 2010 Accepted 7 September 2010 Available online 16 September 2010 Keywords: Silver–palladium electrode Carbon–halogen bonds Cathodic bond cleavages Alkyl radicals

a b s t r a c t The formation of silver–palladium electrodes is described. It mainly corresponds to the palladization of silver by means of treatment with palladium salts (nitrate and sulphate) in acidic media. Other ways may exist such as the modification of solid conductors like carbons by deposition of a silver–palladium alloy. By using those electrodes in polar aprotic solvents, the one-electron cleavage of carbon–halogen bonds of most alkyl iodides and bromides may yield free alkyl radicals. Coupling and cross-coupling reactions can easily be carried out at such electrodes. The present review aims at discussing the electro-catalytic process as well as providing an update on the state of the art on this new mode of scission regarding carbon–heteroatom bonds. © 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

5.

Introduction: Why use new electro-catalytic reductions yielding one-electron processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silver–palladium alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Principle of the chemical doping of smooth silver by palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Textural and structural characterizations of the Ag–Pd interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental section: Building of Ag–Pd electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Generating Ag–Pd electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Chemical doping of silver particles by palladium and characterization of the as-produced surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Electrochemical procedure; salts and solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Electrochemical instrumentation and procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Working electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Coulometries and electrolyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of alkyl iodides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Cathodic reduction of primary alkyl iodides at palladized silver interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Reduction of secondary alkyl iodides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Evidence for free alkyl radicals as intermediates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Homo-coupling reactions from RI reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Cross-coupling reactions with RI mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Use of RIs for silverization of conducting substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of alkyl bromides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. One-electron processes with primary alkyl bromides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Secondary and tertiary alkyl bromides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. E-mail address: [email protected] (J. Simonet). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.020

16 17 17 18 18 18 18 18 20 20 21 21 21 21 23 23 24 24 25 27 27 29

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P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

5.3. Free radical trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Homo-coupling reactions from RBr reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Addition of the electrogenerated free radicals from RXs onto ␲-acceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion: Why a cathodic activation of the carbon-halide bond at palladization interfaces? Decisive contribution of silver . . . . . . . . . . . . . . . . . . 6.1. About the reactivity of organic halides with metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Evidences for a chemical reactivity of RXs towards silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Why an electro-catalytic synergy between palladium and silver? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction: Why use new electro-catalytic reductions yielding one-electron processes

Ar-N=N+, X -

The point in using organic electrochemistry is to determine a priori the potential of working electrodes and therefore control (in principle) reduction/oxidation reactions towards involved organic substrates. This was used for selective electrode reactions with multi-functional molecules and the cleavage of protecting groups [1,2]. Tailoring reactions were also found to selectively generate reactive transients of short-lived of great interest in organic synthesis [3]. Quite generally, such transient intermediates are carbanions and carbocations formed in the course of two-electron reactions occurring at cathodes and anodes, respectively. These reactions are also expected to produce radicals when their LUMO or HOMO levels have the required energy to stabilize those intermediates. More exceptional are the reactions producing free radical intermediates in the course of mono-electronic reactions. That is generally the case of the electrode involvement concerning charged substrates like organic cations in reduction or carbanions in oxidation. At first the oxidation of carboxylates at the platinum anodes (Kolbe reaction) is quite certainly the most famous and ancient reaction belonging to organic electro-chemistry [4]. Pt anode Dim. 1 R-COO− − e− −→ Rads • −→ R-R 2

(I)

Also within the anodic range, reactions of cleavage leading to stable free radicals were found. Another famous reaction is worth being quoted: 2,4,6-tertiobutyl-phenol oxidation yields the stable corresponding phenoxy radical [5]: −H+

Ar-OH − e− → Ar-OH• + −→Ar-O•

(II)

On the contrary, the domain of cathodic reductions appears less rich in one-electron processes. The use of strongly acidic media in organic electrochemistry showed many functions able to be protonated (such as aldehydes, ketones, Schiff bases). Their protonated forms could be reduced according to one-electron processes that could be further converted [6] to hydrodimers via ketyl radicals. However, in the past the development of such reactions has imposed, in many cases, the use of mercury cathodes owing to the large proton overvoltage specific to this metal. Staying in the field of positively charged substrates, onium salts (tetra substituted ammonium cations or sulfonium ions) were widely considered as sources of radicals under electron transfer, but the preparative aspect of those reactions occurring at rather negative potentials remains limited. Presently, the most popular reaction to generate aryl radicals would be to consider organic salts such as aryldiazonium salts, capable of yielding corresponding aryl radicals at much moderated reducing potentials with the aim of grafting surfaces [7,8].

e-

30 31 31 32 32 34 34 35 35 35

.

Ar + N2(g) + X-

(III) Modification of conducting surfaces by radicals Ar

.

Owing to the fact that free radicals are readily reduced, it is expected that the finding of specific electrodes should contribute to differently orientate electrochemical processes. Thus, the use of new chemically modified electrode materials could change the reduction processes and contribute to generate free alkyl or aryl radicals within a potential range where these species may show some stability (towards the electron, the solvent bulk and the solid material at which they are formed). The recent discovery and the practical use of electrodes giving priority to transition metals as main surface material push forward the development of new techniques and reactions, unexpected until now. Therefore, the growing interest in copper, palladium, silver, gold, iron, nickel, and tin in the cathodic domain certainly offers new perspectives. Basically, within the cathodic range, platinum and glassy carbons are still considered as standard materials in electrochemistry. However, it is now clear that electrochemistry takes advantage of modified electrodes by means of organic functions deposited at surfaces (grafted-organic [9]) and the use of complex metallic surface (all-inorganic) electrodes. Under different conditions (organic solvents), the development of palladium and palladized surfaces [10–12], as well as smooth silver [13–18], is of growing interest in analysis and electro-catalytic synthesis; this is mainly due to their very simple use (often just requiring regeneration by means of polishing) or preparation (commonly by simple galvanostatic deposition). Quite surprisingly, the electrochemical community seldom uses copper as cathode material. The use of activated copper (modified according to Dewarda) or nickel (employing Raney’s technique) for specific catalytic hydrogenations of organic functions brought a decisive improvement in the use of these metals as electrode materials [19,20]. In organic chemistry, this metal is known to react with aromatic halides involving (by reduction) halogen removal reactions (Ullmann reaction [21]). Under precise experimental conditions, a growing interest in using “copperized” electrodes is foreseen to achieve electro-catalytic reactions. Moreover, the recent development of copper-palladium electrodes (easy preparation of a layer of Cu–Pd alloy at solid surfaces [22,23]) is worth being quoted. In most cases, palladium takes part in the cathodic reaction and appears to activate copper. In this context it appears of high interest to suggest a general cathodic reaction to produce free alkyl radicals for mimicking Kolbe’s reaction. In the field of electrochemistry, proposals with regards to transition metals as cathode material to reduce nonactivated organic halides such as alkyl iodides (denoted RIs) were successfully improved at palladium and palladized interfaces.

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Especially large gains in energy were already noticed with RIs [10]. In N,N,-dimethylformamide, coupling reactions took place producing homo-dimers but they were followed by a growing inhibition phenomenon during reduction. Furthermore, with alkyl bromides (denoted RBrs), the use of smooth palladium as cathodic material does not display any fast electro-catalytic reduction. To overcome this drawback, sophisticated palladized materials, such as those first disclosed with copper [22,23] and very recently described with materials associating silver and palladium [24–29], proved to be efficient in the one-electron reductive cleavage of carbon-halogen bonds. Similar materials like nickel-palladium and gold-palladium are certainly useful and their development is in progress [30]. Moreover, the use of Ag–Pd alloys as electrode material lead to spectacular improvements towards reduction of alkyl iodides, which were easily converted into homo-dimers [31]. Under similar conditions, the one-electron cleavage of alkyl bromides could also be successfully achieved. It is highly unexpected that one-electron reductions of RIs and RBrs can be achieved within quite similar potential ranges. Thus, a facile generation of free alkyl radicals coming from alkyl bromides is of great interest, owing to a better stability of the starting substrate and a lower commercial cost for species considered as synthetic blocks. Thus, the aim of the present contribution is to first gather valid informations on the preparation of silver–palladium (Ag–Pd) cathodes used as microelectrodes or large surface electrodes. The Ag–Pd deposit could also be achieved onto substrates like carbons and many other interfaces (gold, platinum, and nickel), as well as fine powders to set up fluidized electrode beds. Evidence for the formation of free alkyl radicals as intermediate species is given by ESR studies. A large range of coupling [32] and cross-coupling reactions involving a wide palette of alkyl halides (denoted RXs) is presented for the first time. Lastly, addition reactions of transient free alkyl radicals onto unsaturated systems are also introduced [33].

2. Silver–palladium alloy 2.1. Principle of the chemical doping of smooth silver by palladium The easy modification of silver by palladium at the nanometric scale was previously demonstrated [24,25]. The process consists of a simple contact of silver (sheet or powder) with a fresh aqueous solution of palladium sulphate in sulphuric acid (e.g., 1 g of Pd(SO4 )·2H2 O dissolved in 0.1 dm3 of 0.1 N H2 SO4 aqueous solution). After a few seconds, a shiny layer progressively covers the silver surface (Fig. 1). The displacement of Ag0 by palladium cations then enables the coverage of silver with an Ag/Pd-based alloy. Palladium nitrate could be used as well. On the contrary, it must be stressed that palladium chloride cannot be considered. Actually, silver chloride (insoluble) is formed during the displacement, and its presence induces the deposit of silver onto the alloy surface at the start of electrolyses. Consequently, a change of silver ratio in the alloy, as well as a modification of the nature of the surface, leads to a drop in the catalytic properties of the interface. The layer thickness looked strongly dependent on the time of contact with the palladium sulphate solution. However, no study has been made yet to connect the alloy thickness to the time of contact. It can be roughly estimated to several micrometers for contact times of the order of 1 h. It also seemed interesting to make modified electrodes by silverization of surfaces followed by a palladization procedure using the treatment by palladium salts, as described above. These electrodes surfaces could be easily obtained by galvanostatic deposits of sil-

Fig. 1. SEM images of the smooth silver surface (A) and after immersion in a PdSO4 based solution during 30 s (B) and 5 min (C).

ver from a solution of AgNO3 (10 g dm−3 in a 0.1 N HNO3 solution). These deposits (between 0.5 and 4 × 10−3 C mm−2 ) were achieved onto glassy carbon, graphite, smooth silver, palladium, or platinum. Depositions of silver on industrial cokes were also carried out thus leading to consider very cheap materials to make the setting of fluidized bed electrodes possible.

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(I)

(II)

(III)

Ag

GC

GC

Ag-Pd

_

Scheme 1. (I) Modified silver plate by a layer of Ag–Pd (reaction with a Pd2+ -based solution). (II) Modified glassy carbon (GC) surface by a layer of Ag (galvanostatically deposited) followed by a chemical reaction with Pd2+ . (III) Principle of a fluidised bed electrode by means of Ag–Pd grains in suspension in the electrolyte. The solid conductor (here glassy carbon) is electrified at a potential at which the organic substrate is not reduced in the absence of an electro-catalytic process.

2.2. Textural and structural characterizations of the Ag–Pd interface A combined SEM, HRTEM and EDS study was performed to clearly characterize the as-produced modified layer [24,25]. Since the displacement reaction occurs whatever the silver source used, TEM and HRTEM characterizations can directly be performed using a homemade silver sample-holder (silver grids for microscope being commercially unavailable). Diffraction patterns were performed using the selected area diffraction (SAED) mode or by Fourier transform of the HRTEM imaging. With quite short times of contact (e.g., less than one minute) the as-produced layer is quite stable. In the initial stages of the “palladization” very small grains of spheroid-like shape cover the surface of the substrate (Fig. 1). Although Pd and Ag exhibit similar energy, identification of the Pd incorporation was possible by EDS measurements since the Pd-L␣1 radiation (at 2.8387 eV in energy) is not overlapped (Fig. 2). Via a complete TEM study [25], it was possible to determine the texture, the structure and the composition of the as-produced deposit. For instance, one observes a dendritic-like growth of the metallic layer (size <20 nm) from a PdSO4 -based solution. As shown on the HRTEM image (Fig. 3) these nano-particles have a homogeneous morphology and dimension (around 5–10 nm) and are well-crystallized. The SAED pattern is composed of diffraction circles as all nano-particles are oriented in all directions. A line profile of the electron diffraction pattern enables to plot a graph similar to the X-ray diffraction and to determine that the layer is mainly composed of an Ag–Pd alloy [34–36] having, in the present case, the formal composition Ag0.8 Pd0.2 (lat˚ tice parameter: 4.0499(8) A). 3. Experimental section: Building of Ag–Pd electrodes 3.1. Generating Ag–Pd electrodes The different electrodes used in the course of preliminary works devoted to silver–palladium materials are summarized in Scheme 1. The global palladization process could be achieved directly onto a commercial sheet of silver, and then put in contact with a PdSO4 -based solution (as explained above) during 1 h, rinsed with water to eliminate residues of palladium salt and followed by a rather classical cleaning and drying procedure. The obtained electrode corresponds to the sketch (I) presented in Scheme 1. A useful method to increase the efficiency of electrodes (presumably by extension of the active surface area) is to achieve a silver deposit onto conducting substrates simply by constant current electrolyses (for example, j ∼ 50 ␮A mm−2 and Q ∼ 4 × 10−3 C mm−2 ). An easy treatment by a solution of palladium

sulphate during a few minutes generates quite stable electrodes. Deposition of Ag–Pd layers could be obtained on a large palette of conducting substrates, on condition that the considered substrate (in general metal) is not easily corroded by PdSO4 acidic solution. In the present paper, several examples of Ag deposits onto smooth palladium, platinum, copper, glassy carbon, gold and nickel before (Fig. 4) and after (Fig. 5) modification by palladium salt are considered. Organic conducting polymers like poly-pyrrole and poly-thiophene could also be used, but these experiments have not been carried out yet. Such surfaces are sketched in the picture (II) of Scheme 1. 3.2. Chemical doping of silver particles by palladium and characterization of the as-produced surface The purpose of this work was to achieve the reduction of alkyl halides thanks to a 3-D electrode constituted of a fine suspension of silver–palladium alloy grains constantly electrified (and then maintained) at a given potential by an inactive central cathode where organic halides cannot be reduced fast. In other words and owing to recent results, the standard system was first an electrified carbon sheet in contact with a relatively dense Ag–Pd alloy suspension. The modification of silver grains by palladium was simply obtained by contact of smooth silver metal with a fresh aqueous acidic solution of a palladium salt. This mode of palladization of silver has been already reported. In concrete terms, one gram of Pd(SO4 )·2H2 O (Alfa Aesar) was dissolved in 0.1 dm3 of 0.1 N H2 SO4 solution. When this solution comes in contact with silver metal, a shiny layer progressively covers the surface after a few tens of seconds; this is due to the displacement of Ag0 by palladium cations. The procedure of palladization gave a quasi-instantaneous dull deposit onto the smooth Ag surface apparently due to the displacement of silver by palladium (E◦ (Pd2+ /Pd) = 0.92 V/ENH and E◦ (Ag+ /Ag) = 0.799 V/ENH, in aqueous solution). The layer thickness looked strongly dependent on the time of contact with the palladium solution. With PdCl2 , the contact time is much shorter than with sulphate and nitrate. In order to be used as a 3-D electrode, a silver powder (size of grains: 250 ␮m, Alfa Aesar) was reacted with Pd salts (sulphate or nitrate) under violent shaking during 10 mn. After reaction and decantation, the formed Ag–Pd powder, considered now as a new substrate, was rinsed with water then alcohol and at last acetone. Finally, the powder was dried with a hot air flow for about 30 s. Under such conditions, the Ag–Pd powder was easily re-used or re-activated again with a Pd salt following the same recipe. The morphology of modified silver grains appears like a concretion of small-sized elements apparently amorphous. These particles, with an average size of the order of 1 ␮m, are assigned to Ag–Pd microstructures highly reactive with alkyl halides. It also appeared of interest to make, in the course of this study, electrodes modified by silver or Ag–Pd particles in surface. These electrodes were obtained at different metallic substrates by galvanostatic deposits of silver from a solution of AgNO3 (10 g dm−3 in a 0.1 N HNO3 solution). The imposed current was of the order of 0.5 mA mm−2 , and the average amount of electricity was in all cases 1.2 mC mm−2 . When necessary, the silvered layer was easily transformed into an Ag–Pd layer by contact with a palladium salt solution. 3.3. Electrochemical procedure; salts and solvents In all experiments, the supporting salt concentration was fixed at 0.1 M. In this study, the results mainly concern solutions of tetra-n-butylammonium tetrafluoroborate (TBABF4 ) and tetra-n-butylammonium hexafluorophosphate (TBAPF6 ) into different dipolar solvents. The purity of these salts (at least 98%) was

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Fig. 2. EDS spectrum together with the corresponding SEM picture of a smooth silver sample (a) and a modified silver surface by palladium after a contact time of 5 min (b).

Fig. 3. (a) Bright Field image of the Ag–Pd alloy together with a HRTEM picture (insert) showing the nano-particles of the alloy. (b) SAED pattern (and corresponding line profile) of the TEM picture showing the polycrystalline nature of the layer. Adapted from ref. [25].

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Fig. 5. Morphology of Ag–Pd grains obtained by treatment of silver powder (size 250 ␮m) with a PdSO4 -based solution during 30 min (see Section 2).

not require especially dry solutions. However, if reaching potentials more negative than −2 V, the solution needs to be dried more efficiently to limit the hydrogen evolution. For that, the use of activated neutral alumina is necessary according to an experimental procedure already reported [23]. All electrochemical experiments were performed under inert atmosphere (bubbling of dry argon).

3.4. Electrochemical instrumentation and procedures All potentials are referring to aqueous saturated calomel electrode (SCE). The electrochemical instrumentation has been described in previous reports [23,25,26]. Fig. 4. Morphologies of galvanostatic deposits of pure silver (A) onto nickel (Q = 0.3 C cm−2 ), (B) onto palladium (Q = 0.7 C cm−2 ) and (C) onto glassy carbon (Q = 0.6 C cm−2 ) after reaction with a PdSO4 -based solution.

considered suitable to achieve experiments, and they were used without any further purification. These salts were purchased from Aldrich. The solvents used were N,N-dimethylformamide (DMF) and acetonitrile (AN) (quality for analyses) purchased from SDS and propylene carbonate (quality for syntheses) purchased from Merck. It is worth mentioning that procedures given hereafter do

3.5. Working electrodes All electrodes used in voltametry had an apparent surface area of 0.8 mm2 . Glassy carbon, palladium, nickel, copper, gold, and platinum disks were always carefully polished with silicon carbide paper (Struers) or with Norton polishing paper (type 02 and 03) before a silver deposit was made. Before use, the electrodes were twice rinsed with water then alcohol and finally acetone. Lastly, they were dried with a hot air stream.

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21

3.6. Coulometries and electrolyses Coulometric determinations and electrolyses of alkyl halides were carried out using three-electrode system cells with a cathodic compartment volume of about 5–8 cm3 . A frit with medium porosity isolated the anodic compartment. Experiments were completed on small substrate amounts (typically about 0.1 millimole). Efficient argon bubbling was completed in all cases in the course of voltametries and coulometries. 3.7. Chemicals All the alkyl halides studied in the present work were obtained from Aldrich, with a purity minimum of 95% and used as such. 4. Reduction of alkyl iodides 4.1. Cathodic reduction of primary alkyl iodides at palladized silver interfaces Electrochemical data obtained with primary alkyl iodides RIs at Ag–Pd surfaces are given using DMF as solvent (Table 1). One may observe a very large potential shift between glassy carbon and Ag–Pd used as cathode materials. The average shift toward less negative potential is of the order of 1 V. Peak currents observed with Ag–Pd were found to be diffusion-controlled even when organic substrate concentrations are higher than 10 mmol dm−3 . It was already reported that reduction potentials at Ag–Pd electrodes were slightly less negative (E > 0.1 V) than the ones observed under the same conditions at smooth silver [25]. As shown hereafter, the use of propylene carbonate (PC) as solvent is of great interest owing to its large dielectric constant (ε = 66), and its polarizing power excludes strong inhibition effect observed with all RIs in DMF assigned to the presence of [R-Ag+ , I− ], which is already expected to be the crucial organo-metallic intermediate in the global reduction process. As shown in the course of this presentation, the use of PC as solvent allows getting almost identical data with smooth silver (as later stressed in Section 5, reduction potentials with RIs and RBrs were very close, if not identical). Thus, at the interface, and depending on the nature of the solvent (DMF vs. PC), it is suggested that, after dissolution of the layer, there is an anion exchange between the iodide and the anion of the electrolyte A+ X− (to generate another electroactive species [R-Ag+ , X− ]). Supports about this point are presented along this paper with RIs and RBrs. Voltametries of RIs in DMF at smooth silver show very sharp peaks probably due to the sudden reduction of the insoluble layer on the cathode allowing then the pursuit of the regular reduction process (Fig. 6, curve C). The formation, at the interface, of an insoluble (and probably thick) layer of [R-Ag+ , I− ] could be evidenced by means of experiments achieved hereafter: a commercial silver sheet is placed in contact with RIs (pure or dissolved in a polar solvent like DMF). Prolonged contacts in the dark (e.g., >30 min) yield/lead to very large changes at the metal surface as displayed by SEM (Fig. 7, images B and C). Large black zones (assigned to the formation of an organic non-conducting material) progressively cover the metal surface. The importance of silver surface modification depends on the contact time between Ag and RI. Quite long contact times between RIs (even diluted in DMF) and silver lead to a huge corrosion of the metallic surface. It is then expected that RI be chemically reduced by silver (this point will be discussed in Section 7). Afterwards, the modified silver sheet is thoroughly rinsed with petrol ether and then alcohol to eliminate any traces of RI at the Ag interface. The cathodic reduction of the layer was achieved in DMF containing a tetraalkylammonium salt, and crystals of correspond-

Fig. 6. Voltametries of 1-iodobutane (concentration: 7 mmol dm−3 ) in 0.1 M TBAPF6 /DMF (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon, (B) Ag–Pd electrode (two first scans), (C) smooth silver electrode (three first scans with crossing).

ing dimers R-R (Fig. 7, image D) are formed. They were identified by X-ray diffraction measurement. Such experiments undoubtedly show that primary RIs react with silver. This process (previous reaction of RI with Ag) appears to be the basic activation process for the cathodic reduction of alkyl iodides at this metal. Therefore, the reaction of RIs with Ag seems close to an oxidative insertion involving the carbon–halogen bond as already evidenced with many metals (such as Pd, Ni, Mg) especially with aromatic halides. This enables to suggest a reduction mechanism in the case of alkyl iodides (Scheme 2) where the first and second electron transfers are well separated. This point is well supported by voltametries achieved in propylene carbonate. Thus, these data display two reduction steps, especially with long chain alkyl iodides with E1 > E2 (Fig. 8, curve B, case for long chain RIs). The peak 1 is assigned to the formation of the free alkyl radical whereas the second one (at potential E2 ) could account for the reduction of the free radical. Fixed potential reductions at the level of the first peaks have enabled to get, especially with long chain RIs by using PC, thick and organized dimer layers at the silver interface [24]. The potential range found for peak 2, Fig. 8 (around −1.5 V for the threshold of this peak) agrees with reduction potentials given by Lund [37,38] for a large panel of free alkyl radicals under quite comparable conditions. Peak 2 vanishes in the course of recurrent scans (as shown in curve B). Additionally, fixed potential coulometry values at smooth silver (ERed < E2 ) have shown that the global reduction yields coulometric values close to 2 F mol−1 whatever the solvent used. On the contrary, the reduction of RIs at Ag–Pd electrodes displays a well-marked step (Fig. 6, curve B and Fig. 8, curve D). This step was shown to be diffusion-controlled, and the peak currents were proportional to the organic iodide concentration. The first electron transfer is clearly shifted towards less cathodic potentials. Thus, these reduction peaks are now located within a potential range close to −1.2 to −1.3 V vs. SCE. As checked by means of SEM analysis, the presence of RIs at Ag–Pd surfaces does not yield any visible traces of organometallic deposits. This could support that

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Table 1 Voltammetric and coulometric data relative to some primary alkyl iodides in 0.1 M TBABF4 /DMF. Scan rate: 50 mV s−1 . Potentials are referred to SCE. Apparent area of all electrodes: 0.8 mm2 . The treatment of the Ag electrode was achieved by a dipping time of five minutes in a Pd sulphate acidic solution (see Section 2). Entry

1 2 3 4 5 6 a b c d

RI (mmol dm−3 )

1-iodopropane (10) 1-iodobutane (8.8) 1-iodohexane (7.8) 1-iodooctane (7.7) 1-iododecane (7.5) 1-iodohexadecane (7.6)

Glassy carbon electrodea

Ag–Pd electrodea

Coulometric measurementsb , c Ag–Pd cathoded

Ep/2 (V)

Ip (␮A)

Ep/2 (V)

Ip (␮A)

ERed (V)

F mol−1

−1.96 −1.96 −2.00 −1.93 −1.99 −1.88

14.8 13.6 12.0 12.3 12.2 9.1

−1.00 −1.04 −0.98 −1.11 −1.11 −1.17

9.2 8.6 7.5 7.2 9.6 8.8

−1.6 −1.6 −1.6 −1.5 −1.6 −1.5

1.1 1.1 1.2 1.2 1.3 1.1

Potential and current of the first peak. All coulometries were achieved in DMF containing 0.1 M TBABF4 . In all cases, the area of working electrodes was 4 cm2 . At −1.5 V, electrolyses at smooth silver under the same conditions, yielded in all cases, two-electron processes: 2 ± 0.2 F mol−1 [25].

Fig. 7. SEM images of silver plates immerged in alkyl iodides: (A) pure commercial silver, (B) silver after contact with pure 1-iodooctane (30 min), (C) silver after contact with 1-iodohexane (20%) in solution in DMF during two days in the dark and (D) silver interface treated by 1-iodooctane (30 min) and then electrochemically reduced at −1.4 V vs. SCE.

R-Ag+, I-

R-I + Ag0

E1

R-Ag+, I - or/and R-Ag+, A- + e-

M+, A- (salt)

R-Ag+, A-

I-/A- + {R-Ag}

(1)

(2) With activation

R + Ag0

{R-Ag}

R + e-

E2

(2')

R-

R-H

(3)

R-I + 2 e- + proton source

R-H

(4)

Scheme 2.

Without activation

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Fig. 8. Voltametries of 1-iodododecane in 0.1 M TBABF4 /PC (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon, (B) smooth silver (two first scans) with separation of two steps 1 and 2 in the course of the first scan. (C) and (D) Comparison between smooth silver (C) and Ag–Pd (D).

in such cases, reaction product(s) between the interface and RIs is (are) soluble. This point will be demonstrated later by using a specific process (see Section 6). Coulometric measurements on Ag–Pd achieved at the level of the first reduction peak enable us (Table 1) in all cases to check that RI reduction well corresponds to one-electron processes. This very crucial and unexpected point is quite important: the use of palladized silver specifically makes the formation of free alkyl radicals possible. This proposal appears fully checked in ESR trapping of reduction intermediates by nitrones (see Section 4.3). Homo-coupling and cross-coupling reactions were also carried out in high yield (see Sections 4.4 and 4.5). These reduction steps are sometimes unique and well separated from the cathodic limit at about −2 V. This limit is usually noticed with palladized surfaces under quite similar conditions and assigned to a relatively low hydrogen over-voltage. The absence of a neat second cathodic step assigned to the free radical remains intriguing. One may explain this point by a fast coupling reaction (shifting of the reduction potential) or/and a strong adsorption of those radicals, which leads to a compact coverage of the interface. The existence of such layers would strongly slow down the reduction of those transient free radicals. Similar observations have been made with other palladized surfaces [23]. Short chain iodo-alkanes like methyl and ethyl iodides certainly deserve additional information regarding their electrochemical reduction at Ag–Pd electrodes. Thus, Me-I exhibits a one-electron step at −0.88 V vs. SCE. Using the conditions developed in this review, its reduction could appear as an easy source of methyl radical. Additionally, there are some evidences that all palladized electrodes exhibit an outstanding easiness in the cleavage of the C–I bond (starting from −0.7 V), thus reminding us of the reduction of organic halides possessing a mobile halogen at mercury drop electrode. Anyway, this point certainly deserves to be clarified in the future, but it underlines that very short RI chains are easily cleaved at all palladized electrodes whatever their structure. Let us quote also the case of ethyl iodide: the potential shift when using

23

Fig. 9. Voltametries of iodocyclohexane (concentration: 12 mmol dm−3 ) in 0.1 M TBABF4 /DMF (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon, (B) palladized platinum, (C) smooth silver, (D) AgPd surface.

Ag–Pd electrodes towards glassy carbon is actually huge: it exceeds 1.3 V. 4.2. Reduction of secondary alkyl iodides The reduction of secondary alkyl iodides at silver and Ag–Pd electrodes has not been systematically investigated so far. Whatever the cathodic material, it appears that these RIs always generate a one-electron reduction step. Interestingly, we can compare the peak currents of cyclohexyl iodide at glassy carbon and palladized platinum (Fig. 9, curves A and B). At the silver electrodes, curve C, a very sharp peak is observed and is still explained by the formation of an organo-metallic salt at the surface, followed by a sudden reduction (see Section 4.1). As expected, the use of Ag–Pd electrodes (curve D) gives the largest potential shifts towards carbon. The situation is quite clear with 2-iodopropane since both reduction steps at glassy carbon and Ag–Pd electrodes (diffusion controlled processes) give equal currents and exhibit one-electron processes (Fig. 10, curves A and C). This point was verified by coulometry. Additionally, a potentiostatic reduction of iodo-cyclopropane at an Ag–Pd electrode in DMF-TBABF4 produces near exclusively homodimers. 4.3. Evidence for free alkyl radicals as intermediates The presence of free alkyl radical in the course of the oneelectron reduction of RIs was efficiently checked thanks to the spin trapping method (formation and ESR determination of paramagnetic nitroxide radicals when the reduction was completed in the presence of nitrone (N-tert-butyl-␣-phenylnitrone, TBPN) or nitroso compounds (tert-nitrosobutane). These reactions were achieved until total current depletion (obtained at 1 F mol−1 ). This method has already been successfully developed with other palladized surfaces [10] and checks the existence of a one-electron overall process (effective trapping, in this case, of the free radical R• by the nitrone). More specifically, the reduction of 1-iododecane

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(DMF/tetraalkylammonium salts as media). Ag−Pd

Nitrone + R-I + e− −→ Nitroxide In situ reduction

TBPN

of R-Iat Ag-Pd

N O

(IV) Ph H R

(V)

4.4. Homo-coupling reactions from RI reductions Coupling reactions were achieved with primary alkyl iodides Cn H2n+1 I compounds thanks to Ag–Pd electrodes. Such reductions were carried out in DMF by using either silver plates or glassy carbon (GC) substrates covered with an Ag–Pd alloy (Table 2, [41] and Table 3, [31]). Coupling yields are excellent and some are quantitative. Preliminary data have stressed that the use of GC/Ag–Pd electrodes appears to favour coupling and virtually suppresses radical disproportionation, at least with short alkyl chains (n ≤ 12). It is worth underlining that smooth silver could also be used as cathode material when electrolyses are achieved in PC; the applied potential was strictly controlled (ERed = −1.3 V vs. SCE) in order to avoid the free alkyl radical reduction (no hydrogenolysis reaction) [24]. Ag-Pd

R + I-

R-I + e-

Fig. 10. Voltametries of 2-iodopropane (concentration: 12 mmol dm−3 ) in 0.1 M TBABF4 /DMF (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon, (B) smooth silver, (C) Ag–Pd surface.

Dim.

1/2 R-R at Ag–Pd in the presence of an excess of TBPN (3 fold) yields a paramagnetic signal (Fig. 11) (coupling constants: aH = 3.17 and aN = 14.75 G). Secondary iodides behave similarly. In the presence of TBPN, iodocyclohexane gives, for instance, six-ray ESR responses (coupling constants: aH = 2.386 and aN = 14.775 G). These values are in good agreement with the data obtained with the trapping of alkyl radicals [39,40] under similar conditions

Disp.

(VI)

R(H) + R(-H)

The coupling selectivity seems almost exclusively controlled by the Ag–Pd alloy layers deposited onto glassy carbon (certainly of much larger active surface). On the contrary, amounts of alkanes and alkenes were obtained in quite comparable proportions at smooth silver in PC (Table 2). This strongly supports the presence of free alkyl radicals as intermediates. The difference between substrates used as cathode materials specificity of the solid substrate (Ag–Pd vs. smooth Ag) could be due, in the case of smooth silver, to a slower reaction 2 , Scheme 2. 4.5. Cross-coupling reactions with RI mixtures

Fig. 11. ESR response (A) after electrolysis of 1-iododecane (concentration: 15 mmol dm−3 ) using a large surface area Ag–Pd cathode in 0.1 M TBABF4 /DMF with TBPN in a threefold excess (ERed = −1.3 V vs. SCE, j = −3 mA cm2 ). (B) Corresponding simulated spectrum.

Preliminary attempts to achieve cross-coupling electrolyses implying two electrogenerated free alkyl radicals from mixtures of RIs were tested. From R1 I/R2 I mixtures in equivalent molar ratio and defined experimental conditions, one could get appreciable amounts of mixed dimers R1 -R2 (Table 4, entries 1, 2, 8), especially when the dimer of one component of the initial mixture is volatile. Experiments on short chain RIs in excess toward long chain iodides in order to favour mixed dimer formation apparently failed, possibly because of higher diffusion rates than those of RIs at the interface. Attempts at methylation by using methyl iodide also failed; the one-electron reduction of this RI takes place at a potential much less negative than most other RIs (E ≈ 0.3 V) leading probably to the exclusive formation of ethane. In such cases, there is no evidence for the reduction of long chain of RIs at the applied potential. To prevent this, perhaps it could be necessary to achieve electrolyses with defect of MeI and constantly re-equilibrate ratios in the course of electrolyses. Let us also note that mixed couplings could not be achieved with long chain iodides (see entry 6 relative to 1-iodo-octadecane in the presence of iodo-propane) owing to the evidence that long chain alkyl radicals may disproportionate fast under the given experimental conditions (as underlined in Section 4.5). Lastly, data displayed in the right column of Table 4 imply that any hetero-disproportionation (between R1 • and R2 • )

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25

Table 2 Potentiostatic reductions of primary alkyl iodides. The experimental conditions are those given in Table 1 [25,31]. Substrates

Experimental conditions

R(H) %

R(-H)%

R-R %

C6 H13 -I C8 H17 -I C10 H21 -I C16 H33 -I Ph-(CH2 )3 -I

Smooth Ag cathode PC + TBAPF6 ERed = −1.30 V/SCE

3.6 2.7 3.2 7.7 –

3.2 5.1 3.3 6.7 –

93.1 92.2 93.4 85.6 >99

C8 H17 -I C10 H21 -I Ph-(CH2 )3 -I

Ag–Pd cathode DMF + TBABF4 ERed = −1.25 V/SCE

– 1.1 –

0.1 1.6 –

99.9 97.3 >99

Table 3 Reduction of primary alkyl iodides at GC/Ag–Pd electrodes. Solvent: DMF + 0.1 M TBABF4 . Applied potential ERed = −1.3 V vs. SCE. The experimental conditions are given in text [31]. RI [Cn H2n+1 -I]

R(H) (%)

R(-H) (%)

R-R (%)

Passed charge (F mol−1 )

n=4 n=6 n=8 n = 10 n = 12 n = 16 n = 18 Ph−(CH2 )3 −I

– – – 1.1 1.0 26.7 27.8 –

– – – – – 41.7 39.4 1.0

100 100 100 98.9 99.0 26.6 32.8 99.0

1.1 1.05 1.1 1.1 1.05 1.15 1.15 1.0

was noticed:

R1-I + R2-I +e-

Ag-Pd

R1 + R2 Disp.

Disp.

(VI)

Mixed Dim.

R1(H) + R1(-H)

R1-R2

R2(H) + R2(-H)

However, the present method cannot be regarded as great to obtain selectively R1 -R2 owing to the unavoidable formation of homo-dimers.

R-I + Pd (powder)

[R-PdII-I]0

the carbon–halogen bond. We expect that palladium complex is chemisorbed at the particles metal surfaces, which leads to the discharge of the complex and then deposition of Pd0 at the conductor interface. Nanoparticles of palladium, often <10 nm, could cover numerous substrates such as platinum, gold, nickel, palladium, iron, and graphite [41]. Blank experiments have checked that both RIs and an applied potential permitting the RI reduction at a palladium interface are necessary to observe Pd nano-particle deposits.

eSolid conductor

R +Pd electrodeposited

(VIII)

1/2 R-R

4.6. Use of RIs for silverization of conducting substrates First, it is necessary to recall that the palladization reaction of solid conductors has been recently achieved with RIs in the presence of fine Pd powder and palladium black maintained in suspension in the electrolysis cell. The principle is based on the Heck reaction with an oxidative insertion of palladium inside

Scheme 3 and voltametries shown in Fig. 12 aim to summarize the procedure used for easily silverizing different metallic interfaces with electrogenerated Ag nanoparticles, in particular by means of strongly stirred Ag–Pd powders in the cathodic compartment, and then considered as a fluidized bed electrode. This way, deposits of silver onto iron, copper, and gold surfaces were achieved in one cell and further tested in another cell devoted to electrochemical analysis. It is worth noting that palladium (possi-

Table 4 Mixed electrolyses of alkyl iodides R1 I/R2 I. Cross coupling reactions by means of potentiostatic electrolyses implying some RIs compounds. Solvent/electrolyte: DMF/0.1 M TBABF4 . Ag–Pd electrodes (surface area: 4 cm2 ). Applied potential ERed = −1.3 V vs. SCE. Electrolyses were achieved on about 0.5 millimole of each component until nil current [41]. Entry

1 2 3 4 5 6 7 8 9

Ratio R1 I/R2 I

1-iododecane/1-iodopropane 1-iododecane/cyclohexyl-iodide 1-iodo-dodecane/methyl iodide 1-iodo-dodecane/1-iodooctane 1-iodo-dodecane/1-iodobutane 1-iodooctadecane/1-iodopropane cyclohexyl-iodide/1-iodopropane 1-iodophenylpropane/cyclohexyl iodide 1-iodophenylpropane/methyl iodide

Coupling products (%)

Other products (%)

R1 -R1

R1 -R2

R2 -R2

R1 (H)

R1 (-H)

15 22 98 7 3 – 89 16 100

47 37 1 11 22 9 11 54 –

– 6 – 5 – traces – 29 –

– 35 – 40 29 37 – – –

5 5 – 37 45 44 – – –

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Pd2+ Ag

n RI Ag-Pd + -

≡ R-Ag ,I 0

n Pd

R−R

neSolid conductor M at E Red

≡ Ag nanoparticle Scheme 3. Proposal for the formation of Ag nanoparticles and aggregates at an electrified conductor inert towards RI compounds with an applied potential (ERed ). Note that the conversion Ag → Ag–Pd is not completed inside the electrochemical cell.

bly coming from the splitting of Ag–Pd bonds) was not deposited this way, and the cathodic coverage is strictly selective (checked by EDS analysis). Moreover, these experiments demonstrate that the nature of the catalysis is principally due to the insertion of silver thanks to an initial Pd activation. The procedure (Fig. 12) is very simple: a solution of DMF containing a salt like a tetraalkylammonium salt to insure conductivity, a RI such as 1-iodobutane,

Fig. 12. Voltametries of 1-iodohexane (concentration: 8 mmol dm−3 ) at several silverized electrodes (v = 50 mV s−1 , S = 0.8 mm2 ). Note that voltametry at glassy carbon (denoted GCWS) corresponds to a basic response without any silver deposit. With electrodes covered with silver such as glassy carbon (GC), iron (Fe), copper (Cu), and gold (Au), the silverization was completed in another cell according to the following recipe: 1-iodopropane (24 mmol dm−3 ) is dissolved in a solution of TBABF4 in DMF. After addition of about 20 mg Ag–Pd powder, the solution is efficiently stirred and applied to the electrode ERed = −1.3 V. The current obtained is of the order of 2 to 5 ␮A. Silverization was stopped after an average electricity amount of 0.5 × 10−2 C. Electrodes were sonicated in water during 10 min, and then rinsed with alcohol, acetone and dried under hot air stream. It is remarkable that limit currents correspond more or less to half of the wave obtained with bare GC. Also, peak potentials are different. In the reported examples, the “catalytic” effect is the largest with Fe. The implication of the solid substrate in the electro-catalytic activity has not been demonstrated so far. The thickness of the Ag deposit was not estimated.

Fig. 13. Silverization of carbon by reduction of 1-iodobutane in the presence of a few tens mg of Ag–Pd powder (ERed = −1.3 V vs. SCE). Electrolyte: DMF + TBABF4 . (A) and (B) show deposits of silver onto pieces of glassy carbon. The shiny aspect of the produced silverization depends on the amount of electricity used. The sample (A) needed an electricity amount of 2.5 C cm−2 while (B) corresponded to a much smaller value: 1 C cm−2 . With these two samples, the mass increase was found to be extremely small ( 0.5 mg).

a small amount (about 20 mg) of Ag–Pd powder, and finally an applied potential >−1.3 V vs. SCE. Under stirring, current at the microelectrode increases and may easily reach 50 ␮A. In principle, one explained this increase by the progressive activated silver Ag* deposit that makes the electrode more and more reactive towards the alkyl iodide (Scheme 3). By convention, the electrode silverization was stopped for a fixed charge of 2 × 10−2 C/mm2 . These very small amounts of electricity involved authorize the use of undivided cells. Galvanostatic procedures, completed until the same total electricity consumption is reached, can be used as well. The silver deposit was found to be chemically and mechanically quite stable. Silvered electrodes could be used and re-used several times without noticeable discrepancies. Interfaces modified by silver coverage onto different substrates are excellent in terms of electro-catalysis (Fig. 12). We may note, however, that the best catalytic results in terms of potential shift are obtained with both gold-silver and iron-silver interfaces. The inorganic structure of the as-produced interfaces using this recipe has not been specified so far. Thicker and uniform deposits of silver were also easily achieved, for example, onto glassy carbon (Fig. 13). SEM images of pure silver deposits may show that silver nanoparticles are extremely small (Fig. 14). The deposit looks then as a kind of paint layer. Such interfaces modification by silver could be considered as a convenient way to build new silver electrodes (silver nano-particles modified electrodes, denoted SNPME). Attempts to check their catalytic efficiency were carried out: an example is presented in Fig. 15. Thus, the reduction of an alkyl bromide (RBr) is weakly catalyzed at a smooth silver surface (curve B) but affords contrariwise a catalysis almost as efficient as the one observed with an Ag–Pd system (compare curves C and D). The one-electron reduction of alkyl bromides is developed in Section 5.1).

P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

27

Fig. 15. Voltametries of 1-bromobutane (concentration: 12 mmol dm−3 ) in 0.1 M TBABF4 /DMF (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon. (B) smooth silver, (C) silvered surface by deposit of nano-particles, (D) response at a silver cathode modified by a layer of Ag–Pd.

Fig. 14. Two SEM images after a reduction of 1-iodooctane (concentration: 10 mmol dm−3 ) in 0.1 M TBABF4 /DMF at a glassy carbon electrode (S = 3 cm2 , ERed = −1.35 V vs. SCE, Q = 5 C cm−2 ) in the presence of Ag–Pd powder (50 mg). Volume of solution: 5 cm3 .

5. Reduction of alkyl bromides 5.1. One-electron processes with primary alkyl bromides It has been reported [42–44] that alkyl bromides generally display a two-electron step at glassy carbon electrodes at quite reducing potentials in DMF containing tetraalkylammonium salts (typically at E < −2.4 V vs. SCE). This step corresponds to the twoelectron irreversible scission of the C-Br bond (Fig. 16, curve A). The use of smooth silver as a cathode, under quite comparable conditions, was reported to give more or less significant potential shifts towards less cathodic potentials [13,14]. More specifically in DMF, two reduction steps (I) and (II) are generally obtained in DMF with primary RBrs (Fig. 16, curves B and D with 1-bromoheptane and 1-bromooctane, respectively). Step (I) located at a less negative potential (Ep/2 ∼ −1.3 V) is usually small. Generally, the sum of currents of peaks (I) and (II) is roughly equal to the current of the two-electron step usually observed using glassy carbon. The presence of peak (I) (Scheme 4, equation 6) is consistent with a slow activation process by silver. The activation presumably corresponds to a preliminary reaction between Ag and the alkyl bromide (i.e., quite similar RI compounds [38], but much slower). The reaction of primary alkyl bromides with silver could be evidenced by SEM. Thus, a sample of silver was reacted with 1-bromooctane for

Fig. 16. Typical voltametries of RBrs in DMF using several solid electrodes (v = 50 mV s−1 , S = 0.8 mm2 ). (␣): 1-bromoheptane (concentration: 10 mmol dm−3 , electrolyte: TBABr): (A) at glassy carbon electrode, (B) at smooth silver electrode, (C) At Ag–Pd electrode. (␤): 1-bromooctane (concentration: 7 mmol dm−3 , electrolyte: TBABF4 ): (A) at smooth silver electrode, (B) at Ag–Pd electrode.

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R-Br + Ag0

Ag metal (k)

R-Ag+ , Br -

Slow activation

M+A-

R-Ag+ , A-

(5)

R-(Ag)0

(6)

(electrolyte)

R-Ag+, Br- and/or R-Ag+, A- + e-

.

R-(Ag0)

R + Ag0

.

R + e- + proton source

(7)

R-H

R-Br + 2e- + proton source

R-H

(8)

(9)

Scheme 4.

one day and then, after thoroughly rinsing with acetone, electrified in an electrochemical cell at the level of step (I). The as-produced layer was examined by SEM whereas the crystal deposit has been clearly identified to correspond to the C16 homo-dimer. Step (II) at more negative potentials (quite close to those observed with glassy carbon) presumably deals with the RBr reduction in the absence of a noticeable activation process. As shown in Table 5, macro-reductions of RBrs at smooth silver obey a global two-electron process. Rather negative potentials are necessary to obtain in DMF a reasonable current density (E < −1.6 V). Therefore, as two-electron processes with major formation of the alkane RH were pointed out with several alkyl bromides, it appears difficult to discriminate between the two processes (involvement or not of the cathode material) as suggested in Scheme 4. As already stressed above, free alkyl radicals are readily reduced. The potentials necessary for the conversion R• /R− given by Lund [37,38] are higher than −1.5 V vs. SCE). If now the silver surface is palladized, dramatic changes are observed: appearance in voltametry of a large step that replaces step (I) previously observed at smooth silver (see Fig. 16, curves C and E relative to two alkyl bromides) concomitantly with a potential shift to less cathodic potentials (∼0.1 to 0.2 V). It is noteworthy that primary alkyl bromides are reduced according to a one-electron step (e.g., coulometric data obtained in DMF, Table 1). Two steps of equal current are generally obtained, presumably corresponding to the formation of the alkyl radical and then its subsequent reduction. The potential of the second step is a little bit more negative than the one expected for radical reduction, which could be explained by a fast radical coupling process or/and the concomitant adsorption (or grafting) of the transient onto the Ag surface. It may happen that large electrode coverage slows down the cathodic process although silver doped by palladium specifically yields a significantly faster catalytic process. In many cases, however, electrode processes stay at the limit of the diffusioncontrolled zone. During the preparation of Ag–Pd electrodes, longer immersion times of Ag in a PdII -based solution (efficient coverage of silver surface) contribute to totally delete the kinetic character of the electro-catalytic step. Larger thicknesses of palladized layers (∼0.2 to 2 ␮m) as well as a better coverage of the silver surface seem to increase the global rate of the process. Thus, the contribution of Pd is essential and certainly governs the overall kinetics of the process. Preliminary experiments with galvanostatic deposits of silver onto gold, platinum, palladium, nickel and glassy carbon were

also completed. Such silvered interfaces give systematically very reproducible results somewhat better in terms of catalysis than smooth silver. Similarly, the activation of these silver deposits with a PdII -based solution has been achieved. For instance, coverage of glassy carbon, platinum, or gold with thin Ag–Pd layers by means of this procedure, led to results comparable to the ones described above with massive Ag–Pd electrodes. In other words, very thin galvanostatic deposits of silver at many solid-conducting surfaces can be obtained (average thickness of Ag often less than 1 ␮m). By using such procedures, many conducting interfaces can be converted into efficient and relatively cheap Ag–Pd supported electrodes. It has been put forward that RIs and RBrs (Schemes 2 and 4 including metal insertion into the C–X bond) in contact with silver could possibly induce the same intermediate when an exchange with the anion of the electrolyte occurs (see equations (1) and (5)). The use of a solvent of high dielectric constant could in principle lead both to a larger polarization of the carbon-halogen bond (faster catalysis) and also favour the dissociation of organo-silver salts (faster anion exchange with the electrolyte). In order to bring some support to this crucial proposal, the use of propylene carbonate containing tetraalkylammonium tetrafluoroborate was chosen as liquid electrolyte. As a matter of fact, Fig. 17 clearly indicates that both alkyl iodides and alkyl bromides are reduced at smooth silver at the same potential. For example, 1-iododoctane and 1bromooctane give respectively Ep/2 = −1.26 V and −1.28 V vs. SCE (if one does not take into account the small adsorption peak observed at the summit of the iodide step). Additionally, the threshold potentials with these two RXs were found to be the same at −1.20 V. Thus, combination between the use of a strongly dissociating solvent and the use of the silver electrode enables to observe the reduction of RIs and RBrs at the same potential. Under the conditions of Fig. 17, E is found larger than 1 V with the RBr and only 0.5 V with the RI. The use of PC as solvent is therefore an excellent way to boost up the activation of alkyl bromides. It is also noticed that smooth silver and Ag–Pd electrode show strictly identical results with RXs (Fig. 17, part A): the electrode process turns out to be independent of the halogen, and this finding supports the mechanisms proposed in Schemes 2 and 4. The implication of silver in the cathodic reaction using an Ag–Pd modified electrode (Scheme 4, eq. (6)) has been checked: one observes, when completing RBr reduction, both the appearance and progressive growing of silver nanoparticles (staying probably outside the metallic layer) of very similar sizes (Fig. 18). The as-obtained silver layers exhibit a great compactness.

P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

29

Table 5 Voltammetric data of bromoalkanes in 0.1 M TBABF4 in DMF. Scan rate: 0.1 V s−1 . Potentials are referred to aqueous SCE. Apparent surface area of electrodes used in voltammetry: 0.8 mm2 . Entry

1 2 3 4 5 6 7 8

RBr (mmol dm−3 )

2-bromopropane (8.5) 1-bromopentane (5) 2-bromopentane (6) 1-bromohexane (11) 1-bromoheptane (10) 1-bromooctane (10) 1-bromodecane (9) 1-bromododecane (6)

Glassy carbon electrode

Ag–Pd electrode

Coulometric measurementsb

Ep/2 (V)

Ep/2 (V)a

Ag–Pd electrodeb

−2.25 −2.36 −2.25 −2.42 −2.56 −2.54 −2.42 −2.47

Ilim (␮A)

25 15 17 27 25 25 22 20

−1.30 −1.24 −1.32 −1.48 −1.33 −1.40 −1.30 −1.26

Ilim (␮A)

11 8 8 13 10 14 13 7

Smooth Ag electrodec −1

ERed (V)

Q (F mol

– −1.5 −1.5 −1.5 – −1.5 −1.5 –

– 1.2 1.3 1.3 – 1.4 1.2 –

)

ERed (V)

Q (F mol−1 )

– −1.5 −1.6 – −1.5 −1.6 −1.9 −1.8

– 1.8 2.2 – 2.0 2.0 2.0 1.9

a

Existence of two peaks neatly separated, the first one being a one-electron step. Coulometries were completed until neal current by using silver plates activated or not in fresh solution of palladium sulphate during two minutes (see experimental section) [26]. Fixed potential was that of the main step at the corresponding electrode material. b ,c

5.2. Secondary and tertiary alkyl bromides Secondary Rs Brs and tertiary Rt Brs alkyl bromides display slow catalysis processes at silver and Ag–Pd surface both in DMF and acetonitrile (AN). For instance, voltammetric data of 2-bromopropane points out a significant change towards primary alkyl bromides (Fig. 19). At smooth Ag, three reduction steps are observed in DMF. Within a potential range going from −1 to −1.6 V vs. SCE, two small current reduction steps (I) and (II) are observed, which could be explained by a slow surface reaction of 2-bromopropane at the silver interface. The occurrence of an anion exchange between the organometallic ion and the electrolyte has been previously discussed (Fig. 19), and the existence of two different ion pairs could explain the presence of two additional steps of small currents. Step (III) in Fig. 19 would correspond to the alkyl bromide scission without any surface catalysis: one may note that its potential lies within a potential range close to that observed with a carbon electrode. The

Fig. 17. Comparison, under similar experimental conditions, between voltammetric responses of 1-iodooctane (concentration: 6.6 mmol dm−3 ) and 1-bromooctane (concentration: 5.7 mmol dm−3 ) in 0.1 M TBABF4 /PC (v = 50 mV s−1 , S = 0.8 mm2 ). (A) 1-bromooctane at GC, Ag–Pd, and smooth Ag electrodes. (B) 1-iodooctane at GC and smooth Ag. The shift (E) between half-peak potentials obtained at smooth Ag and GC is quantified with the two RXs.

proposal of a slow exchange with the electrolyte is well supported when secondary alkyl bromides are reduced in a tetraalkylammonium bromide (Fig. 19, curve C): only one kinetic step appears within the potential values ranging from −1 to −2 V. The use of an Ag–Pd surface successfully checks that the catalysis process starts to be much faster. Two equal waves, presumably of one-electron each, are obtained and the main step is observed with a large

Fig. 18. SEM images (two magnifications) after reduction of 1-bromoheptane in 0.1 M TBABF4 /DMF at a glassy carbon modified by a thin Ag–Pd layer (Q = 7 C cm−2 ). Spheroid particles were checked by means of EDS to be only pure silver.

30

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Fig. 19. Voltametries of 2-bromopropane (concentration: 12.5 mmol dm−3 ) in 0.1 M TBA+ X− /DMF (v = 50 mV s−1 , S = 0.8 mm2 ). (A) and B) Responses at glassy carbon and smooth silver, respectively (electrolyte: TBABF4 ). (C) Response at smooth silver electrode (electrolyte: TBABr). (D) Response using an Ag–Pd electrode (electrolyte: TBABF4 ). Insert: responses (first step) at an Ag–Pd electrode in TBABr-DMF. Increase in the scan rate from 20 to 500 mV s−1 .

potential shift toward GC and smooth Ag. However, a clear kinetic aspect of the first electrode process has to be underlined since small current changes upon scan rate (20 mV s−1 ≤ v ≤ 500 mV s−1 ) are observed (Fig. 19, inset). Thus, secondary and tertiary alkyl bromides display slow catalysis processes in DMF and AN, especially at smooth silver. We noted that the concomitant use of Ag–Pd electrodes and PC containing tetraalkylammonium salts (TAAX with X− = ClO4 − , PF6 − , and BF4 − ) yields pure one-electron processes. Two examples are quoted hereafter: tert-butyl bromide (Fig. 20) that displays a one-electron system clearly checked by coulometry, and ESR, quite certainly generating tert-butyl radical. The formation of the radical may be achieved at very moderate potentials: Ep/2 = −1.25 V vs. SCE. Let us note that Saveant [45] has already reported that tert-butyl bromide could be reduced at quite negative potentials, in two steps at glassy carbon using dipolar solvents. Fig. 20, curve B: the potential difference in the reduction potential steps successively obtained at GC and Ag–Pd is huge (E ≈ 1.2 V) and gives for the second step potential (assigned to the reduction of the free radical) a value close to −2.5 V, rather in accordance with previous data [45]. t-Bu-Br + e−

Ag-Pd cathode

−→

PC,TAAX

t-Bu• + Br−

(IX)

1-Bromoadamantane displays a similar behaviour. As already shown, smooth silver and Ag–Pd give similar results displaying two separate steps with a noticeable potential gain towards glassy carbon. However, the use of Ag–Pd certainly favours the radical formation. The coupling of 1-adamantyl radicals was already achieved via the corresponding RT I species at silver electrode [46] in tetrahydrofuran (THF) under ultrasonic conditions. The experimental conditions presented here might possibly be an alternative to the homo-coupling of tertiary radicals. For the moment, such experiments remain to be undertaken.

Fig. 20. Voltametries of tertio-butyl bromide (concentration: 12.5 mmol dm−3 ) in 0.1 M TBABF4 /DMF using several solid electrodes (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Glassy carbon electrode. (B) Platinum electrode modified by a layer of Ag–Pd (average thickness: 0.5 ␮m). (C) Ag–Pd electrode produced according to the process given in Section 2. (D) Response after a coulometric measurement achieved at the potential of the arrow. (Curve C) after a passed charge of 1.1 F mol−1 .

1-bromo-adamantane + e-

Ag-Pd PC

(X)

5.3. Free radical trapping The trapping technique was used once more for the ESR study of the transients produced when achieving the one-electron reduction of RBrs; it revealed that paramagnetic species are produced. The method is the same as the one already described in Section 4.3 with alkyl iodides in the presence of TBPN and the as-obtained results are similar. Coulometric balances of reactions found with RBrs in the presence of an excess nitrone (cathodically inactive at the fixed potential of −1.4 V vs. SCE) are very close to 1 F mol−1 . At this potential, the reduction of free alkyl radicals is unlikely like the formation and addition of the R− anion onto nitrone (followed by the oxidation by air of adduct after electrolysis). Only the coupling reaction could compete with the trapping addition. In DMF containing TBABF4 , paramagnetic responses are rather intense when an Ag–Pd cathode is specifically used. Six-ray spectra were, by far, the main response, but the trapping reaction is not specific (two or three paramagnetic side species were also detected). Interestingly, the reduction of 1-bromopentane yields a main signal (86%) which coupling constants are aH = 2.89 G and aN = 14.61 G. Similarly, 2-bromopentane gives a similar signal (relative intensity: 75%) with the corresponding constants: aH = 2.40 G and aN = 14.27 G. These findings are in agreement with the previous data obtained with alkyl iodides under similar experimental conditions (see Section 4.3). The partial lack of specificity of the trapping (obtaining of a three-ray response showing the absence of a coupling with a proton) has not been satisfactorily explained so far.

P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

31

5.4. Homo-coupling reactions from RBr reductions Potentiostatic electrolyses were achieved on a rather large series of RBrs at Ag–Pd electrodes. Fixed potentials necessary to carry out these reductions are less negative than reduction potentials reported for free alkyl radicals expected as intermediates. With primary alkyl bromides possessing a rather short chain (n ≤ 12), reduction processes produce mainly homo-dimers (Table 6). Beyond C12 , the coupling route clearly collapses and disproportionation products R(H) and R(-H) are alternatively obtained, often in almost equal proportions (entries 6, 10, and 11). It has to be pointed out that the used solvent and the nature of the electrode (Ag–Pd layer deposited onto carbon) might drastically change the product distribution. Thus, with a given chain length, PC seems to favour coupling (entry 8) while GC/Ag–Pd would dramatically turn out the electrode process (entry 6) when compared to an Ag–Pd layer simply deposited onto silver. Presumably, superficial concentration values for a given current density (larger when palladization is made over a smooth surface or when methylene chains are short and therefore less bulky) might favour the increasing ratio of homo-dimer. These preliminary data are presently sketchy and certainly deserve further studies. Additionally, very preliminary experiments on cross-coupling reactions were achieved with RBrs. Some elements relative to those couplings are shown in Table 7. The material chosen for those experiments was exclusively glassy carbon covered with an Ag–Pd layer. As expected, the percentage of cross-coupling products yield was obtained in a ratio close to what can be expected for this kind of radical coupling reaction (entries 1 and 2). There is no evidence for the occurrence of disproportionation reaction(s) relative to the various formed radicals. The most interesting finding is the merging of derivatives with no obvious connection with the products initially introduced in the cell. Therefore, it seems that such a noteworthy product distribution is partly governed by the participation of the carbon electrode (i.e., addition of free radicals onto and in the carbon fractals followed by a progressive collapse). However, reactions of radicals with the salt and/or the solvent cannot be presently excluded. 5.5. Addition of the electrogenerated free radicals from RXs onto -acceptors Very recently, a comparison between the redox catalysis (homogeneous reduction of an RI or RBr by the anion radical of a ␲-acceptor A) and the heterogeneous one-electron catalysis (H1EC, by use of the Ag–Pd electrode) has been achieved [32]. For the first time, voltametries of {A + RX} mixtures were considered both at inert electrodes (like glassy carbon) and Ag–Pd electrodes (Fig. 21). Thus, under aprotic conditions and when used on GC substrate, anthracene typically exhibits two reduction peaks (one-electron each), the first being fully reversible (formation of the anionradical). In the presence of an alkyl bromide RBr, the redox catalysis process may take place and then provokes the current increase in this first peak. Additionally, one notes the progressive irreversibility of this reduction step while RBr concentration is made larger and larger. This quite agrees with a first paper connected to the possibilities and the principle of redox catalysis [47]. The reaction (homogeneous reduction of RBr) corresponds to equations (1) and (2) of Scheme 5. Subsequent reactions such as the reduction of the alkyl radical R• were shown to occur owing to the potential range generally observed with most of arenes: E◦ arenes < −1.5 V vs. SCE. In the case where the free alkyl radical is not reduced fast, the coupling reaction according to equation (3) may occur. Lastly, the transient organic anion issued from the coupling may also afford a SN 2 reaction with RXs possessing a sufficient electrophilic affinity [48] (eq. 4). Such a reaction gives rise to the same behaviour as the coupling

Fig. 21. Voltametries of anthracene (concentration: 6.2 mmol dm−3 ) in 0.1 M TBAPF6 /DMF using several solid electrodes (v = 50 mV s−1 , S = 0.8 mm2 ). (A) Response of anthracene at glassy carbon. (B) After addition of 1-bromopentane (concentration: 6.5 mmol dm−3 ). The reduction potential peak (not shown) is located at −2.48 V vs. SCE. (C) Preceding solution response at an Ag–Pd electrode. Curves B and C are gathered to underline the fundamental difference between the two solid electrodes (redox catalysis versus one-electron heterogeneous catalysis).

Redox catalysis E°A

A + e-

-.

(1)

A kf

-.

A + R-X

A+R +X

-

(2)

kb -.

A +R

A-R

-

A-R + R-X

-

(3)

AR2 + X

-

-

A-R + Proton donating species

(4) A-R-H

(5)

Heterogeneous one-electron catalysis (E R >> E° A and E°R /R-)

R-X + e-

R +X

R +A

A-R

.

AR + R -

A-R + e -

-

(6)

-

(7)

AR2 -

A-R + R-X

A-R

(8) -

AR 2 + X Scheme 5.

(9) -

(10)

32

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Table 6 Potentiostatic reductions of alkyl bromides at Ag–Pd cathodes by use of two compartment cells. Bubbling of argon. The layer of Ag–Pd is deposited onto pure silver (see Section 2) or onto glassy carbon (preliminary deposition of silver followed by a treatment by palladium sulphate). Surface areas: about 4 to 6 cm2 . Applied potentials: between −1.2 V and −1.3 V vs. SCE. Reductions were achieved on about 0.5 millimoles of RBr. Concentration of the organic substrate was about 10−2 mol dm−3 . Here, the salt used is always 0.1 M tetra-n-butylammonium tetrafluoroborate. The initial current density was estimated to 10 mA cm−2 . Electrolyses were considered as fully completed when the current density reached 5% of the initial value [41]. Entry

RBr [Cn H2n+1 -Br]

RH (%)

R(-H) (%)

R-R (%)

Cathode material

Experimental conditions (passed charge)

1 2 3 4 5 6 7 8 9 10 11 12

n=5 n=6 n=7 n=8 n = 10

– – 15 – 2 42 66 16 68 58 59 –

– – – – – 35 34 5 28 39 41 –

88 100 55 100 98 12 – 79 3 – <1 70

GC/Ag–Pd AgPd GC/Ag–Pd Ag–Pd Ag–Pd GC/Ag–Pd Ag–Pd Ag–Pd Ag–Pd Ag–Pd Ag–Pd Ag–Pd

DMF (1.05 F mol−1 ) DMF (1.1 F mol−1 ) DMF (1.2 F mol−1 ) DMF (1.05 F mol−1 ) DMF (1.1 F mol−1 ) DMF (1.1 F mol−1 ) DMF (1.1 F mol−1 ) PC (1.0 F mol−1 ) DMF (1.1 F mol−1 ) DMF (1.2 F mol−1 ) DMF (1.1 F mol−1 ) DMF (1.05 F mol−1 )

n = 12 n = 14 n = 16 n = 18 1-hexenyl bromide

reaction. In other cases, the protonation reaction may take place giving a mono-alkylation (eq. 5). Thus, reactions 3 and 4 might be considered as a side-reaction and strongly perturbs a pure redoxcatalysis process (progressive consumption of the redox mediator). With alkyl iodides, the redox catalysis process can be considered as fast with most arenes. Fig. 21, more precisely depicts the mixed reduction of anthracene in the presence of 1-bromopentane according to the two modes of formation of R• . While the large current increase at −2 V (curve B) is provoked by redox catalysis (indirect reduction), the H1EC reaction relative to RBr already happens at −1.3 V vs. SCE (curve C). Electrolysis achieved at this potential enables to check the progressive consumption of the arene provoked by R• radical addition(s). Still in voltametry, the presence of free radicals at the interface of the Ag–Pd electrode suppresses both the reversibility of the pyrene first step (coupling of R• with the arene anion radical) and ends the redox catalysis process. Similar pathways could be shown to occur with alkyl iodides with convenient mediators. For example, mixed electrolyses achieved on mixtures [A + RX] generate (case where X = I) monoalkylation and dialkylation products (reaction 7, 8, and 10, Scheme 5) in weak but significant yield of about 10% (after 2 F mol−1 of arene).

6. Discussion: Why a cathodic activation of the carbon-halide bond at palladization interfaces? Decisive contribution of silver

(a) Magnesium used at the metallic state “M” is well known to lead to Grignard reagents [52]. In general, alkyl halides as well as aryl halides give “metallo-dehalogenation” reactions R-X + M → R-M + X−

First of all, it should be stressed that most alkyl and aryl halides may be reduced by metals. Alkali metals and earth-alkali metals are known [49,50] to react with most organic halides (Wurtz-Wittig reaction) to yield coupling products. However, sodium cannot be used to achieve aryl-aryl couplings. (XI)

(XII)

in the special case of magnesium: R-X + Mg → R-Mg-X

(XIII)

It has been expected that the reaction of the metal with the RX corresponded to an electron transfer reaction, possibly through a concerted electron transfer [53], with formation of a free radical R• as a reaction transient. As an important application of the Grignard reagent, we may quote the action of many metals (M ) like Zn, Hg, As, Sb, or Sn leading to a kind of “trans-metalation” reaction [54–56]. R-M + M → R-M + M

6.1. About the reactivity of organic halides with metals

2R-X + Na(excess) → R-R + 2X−

The reduction of organic halides was reported with many transition metals [50]. Thus, silver, zinc, and iron could react with RIs and RBrs. Pyrophoric lead was used as well [51]. Salts of low metal valency were alternatively used to reduce organic halides and produce dimers. Let us also underline that reductions of that kind were carried out in ethers of generally low dielectric constant (ε < 10). Under these conditions, in the absence of a sufficient polarization of the carbon-halogen bond, the use of high boiling point ethers at reflux may compensate for the low activation energies. For obvious reasons, special comments have to be presented regarding magnesium, copper, and palladium.

(XIV)

Thus, the Grignard reagent could be considered as an intermediate (or mediator) in the synthesis of alkyl metals RM . (b) Copper reacts with aryl halides (Ullmann reaction [21]) to yield biaryl compounds. The intermediate was expected to be an aryl copper intermediate: Ar-l + Cu → Ar-Cu

(XV)

Table 7 Some preliminary potentiostatic reductions of alkyl bromides in mixture at carbon cathodes covered with an Ag–Pd layer. Use of two compartment cells. Bubbling of argon. Applied potentials: between −1.2 V and −1.3 V vs. SCE. Reductions were achieved on about 0.5 millimole of RBr. Concentrations of both organic substrates were about 10−2 mol dm−3 . Electrolyte: tetra-n-butylammonium tetrafluoroborate at 0.1 M concentration. The initial current density was found to be about 10 mA cm−2 . Electrolyses were considered as fully completed when the current density reached 5% of the initial value [33]. Electricity amounts found: 1.05 ± 0.1 F mol−1 . Entry 1 2

R1 Br + R2 Br



C5 H11 Br  C8 H15 Br C5 C11 Br C12 H25 Br

R1 -R2 %

Presence of other products

Cathode material

36

Dimer in C16 (50%). Other oligomers in C14 (2%), C24 (3%), C25 (4%), and C26 (4%).

GC/AgPd/DMF

65

Dimers in C10 (10%) and C24 (19%). Other oligomers in C16 (3%) and C18 (3%). No traces of reduced forms.

GC/AgPd/DMF

P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

33

reduction of RBrs thanks to highly activated copper Cu* ): R-Br + Cu∗ → R-CuI , Br−

(XVII)

In electrochemistry, transients of formula R-CuI were already suggested in the cathodic reduction of alkyl bromides and ␣,␤-dibromoalkanes at Cu cathodes [28,29]. Their disproportionation at smooth copper cathodes with the continuous formation of highly reactive Cu* has been expected to be the key of the catalysis. The employment of Cu-Pd surfaces was found to strongly enhance electrode kinetics. It should be underlined that RXs [42] when reduced at mercury as electrode material give mercury alkyl compounds presumably through transients such as RHgI and RHgII . (c) Lastly, the interest of palladium in the chemistry and electrochemistry of aryl halides is now definitely established since applications are various and numerous. Most of the benefit lies in the reactivity of the organo-palladium intermediate (Heck’s reaction) obtained in solution via an oxidative insertion of palladium in the C-X bond, typically in the presence of appropriate ligands [58–61]: Ar-X + Pd0 + ligands → Ar-PdII -X Scheme 6.

followed by: 

Ar-Cu + Ar-I−→Ar-Ar

(XVI)

More in connection with the present review are Rieke’s works [56,57] relative to the formation of alkyl cuprous salts (via

(XVIII)

The mechanisms of coupling with ArX and RX with formation of C–C bonds were recently discussed including the interest in using electrochemistry [62,63] to favour catalytic cycles. On the contrary, the chemistry of alkyl halides in the presence of Pd metal has been scarcely explored. A new approach of the electrochemistry of RXs (especially RIs) at smooth palladium (or palladized surfaces) underlined, for the first time, the point in having such cathodic interfaces in the one-electron reduction of alkyl

Scheme 7.

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iodides. This process, which does not apparently need specific ligands, was interpreted as a surface phenomenon with an efficient lengthening of the C-I bond at the palladium surface. Similarly to ArX compounds, the possibility of an oxidative insertion of palladium was suggested although alkyl bromides were found to be too poorly reactive under such conditions. 6.2. Evidences for a chemical reactivity of RXs towards silver It was previously shown that RBrs and RIs undoubtedly react with numerous metals. In particular, the reduction of alkyl halides at a large palette of conducting materials (GC, but also Pt, Fe, Cu, Au, Zn), in the presence of a suspension of black palladium or palladium powder, induces the deposit of Pd0 nanoparticles on surfaces where any electro-catalysis occurs. Those experiments support the transient formation of organopalladium salts readily discharged at any solid conductors.

R-I + Pd0(suspension)

"R-PdII,I-"

R +[Pd 0]nanoparticles

(XIX) 1/2 R-R Thus, the motion of metals from a mobile or fixed location of a piece of metal immerged in the electrolysis cell was achieved with a series of metals (Cu, Pt, Au, Pd) when organic vectors were alkyl iodides, ␣,␻-dibromo and ␣,␻-diiodoalkanes. Current could be sometimes quite low, but the metal displacement through the cell was shown to occur. With those RX substrates, such works certainly demonstrated that R-MI , X− and R-MII , X− could present certain solubility. What about silver? There are several evidences for the chemical transient chemical involvement of silver in the catalytic reduction of RBrs and RIs (e.g., reaction onto smooth silver, see Fig. 7, same voltametric behaviour in PC from RBr and the parent RI, both possessing the same alkyl chain, Fig. 17). Moreover, the participation of the metal together with the exchange of the electrolyte anion in large excess was pointed out in Schemes 2 and 4. When the reduction of numerous RIs was achieved in the presence of silver suspensions by using powders of different size grains, electrodeposition of silver nano-particles was observed at electrified glassy carbon. However, the discharge of silver grains (covered with silver-organo salt) by impact onto electrified surfaces could not be considered as an absolute proof of the metal implication in the overall catalytic process. In order to bring out an ultimate support to this organosilver intermediate, we have successfully carried out experiments designed in Scheme 6. For that, RIs or RBrs dissolved in PC + TBABF4 were put in contact with Ag–Pd and Ag sheets immobilized at the bottom of the cell and therefore in no contact whatsoever with a working cathode. After electrolysis of the resulting solution at an electrified carbon (E = −1.5 V, Fig. 22) the collecting of pure silver could be observed at the carbon surface. On the contrary, in DMF, the use of pure silver sheets placed in the cell did not permit to observe any deposit of silver onto the carbon electrode. This strongly supports the fact that the salt [R-Ag+ , X− ] is practically insoluble and forms a non conductive layer at the metal surface. In this solvent, such surface coverage could strongly inhibit the cathodic process, and this point has been discussed in Section 4.3. 6.3. Why an electro-catalytic synergy between palladium and silver? In our opinion, the activation of RX at an Ag–Pd electrode occurs in two well different steps: (a) Insertion of Pd atom inside the C-X bond with a possible participation of silver atom(s) as donors:

Fig. 22. SEM images after reductions of RIs (concentration 30 mmol dm−3 ) at glassy carbon electrodes (ERed = −1.55 V vs. SCE). Deposits of pure silver particles checked by EDS. (A) After reduction of 1-iodohexane in PC + TBABF4 in the presence of a sheet covered in Ag–Pd (S = 4 cm2 , Q = 7 C cm−2 ). (B) After reduction of 1-iodooctane in PC + TBABF4 in the presence of a sheet (3 cm2 ) of pure silver (S = 3 cm2 , Q = 6 C cm−2 ). Note that there is no electric contact between the GC plate and the piece of metal in both cases.

R-X + Ag–Pd → R-Pd-X + Ag0∗

(XX)

(b) Followed – especially in the case of RIs – by a “transmetalation” that actually corresponds to a redox reaction

R-Pd-X + Ag0*

[R-Ag+, X-] + Pd0

followed by: (XXI) +

-

-

0

[R-Ag , X ] + e

R + Ag * + X

-

(and depending on the nature of X, possible direct reduction by silver) in competition with: (XXII) R-X + Ag0* R-I + Ag

slow

[R-Ag+, X-]

[R-Ag+, I -]0

[R-Ag+, I -]sol

(XXIII) ET at GC

R + I- + Ag0 NP (XXIV)

P. Poizot, J. Simonet / Electrochimica Acta 56 (2010) 15–36

Reaction (XXI) is equivalent to reaction (XIV), quite classical with Grignard organometallic compounds in the presence of a large series of metals via reduction. Let us recall that mercury alkyls R2 Hg can yield metal displacements with alkali metals, Al, Zn, Sn, etc. Moreover, it is totally thermodynamically compatible if we consider the redox potentials reported in aqueous solution vs. NHE of Ag+ /Ag0 (E◦ = 0.7996 V/SHE) and Pd2+ /Pd0 (E◦ = 0.991 V/SHE) [64]. Lastly, metal displacements are apparently the cause of some curious corrosion at Ag–Pd surface. It appears that RBrs necessarily need activation by Pd. Their reduction surprisingly leads to pinholes and tunnels (Scheme 7) at Ag–Pd surfaces. The process would start at Pd crystals present at the surface (I), followed by the digging of the Ag–Pd layer corresponding to reaction (XXI). In the absence of a fast catalysis by silver nano-particles, the reaction consumes more and more Ag–Pd and therefore produces channels (III). The further deposit of particles of Pd and Ag at tunnel edges is expected. If the synergy between Pd (first) and silver (second) is obvious, the mechanism is certainly complex since, in the course of electrolyses, Ag–Pd and Ag nano-particles may have a rather equivalent role in pursuing the electro-catalysis process. 7. Conclusions The easy formation of a silver–palladium alloy layer and its use as an electro-catalytic surface undeniably make important developments in the cathodic scissions of carbon-heteroatom bonds possible. The contribution of palladium in the Ag–Pd alloy certainly leads to a decisive improvement of silver cathodes in the reduction of iodides especially when one-electron cleavages are sought. Thus, the main interest of Ag–Pd electrodes is to reveal a general procedure for producing free alkyl radicals in reducing media. This way, a new radical chemistry appears to be possible under the specific conditions of electrochemistry (i.e., in organic polar solvents). The present review underlined possibilities of homo-coupling and cross-coupling reactions rather similar to those previously reported within the anodic range (oxidation of carboxylates at platinum anodes, Kolbe reaction). On the other hand, the possibility to achieve, under the same conditions, one-electron reactions with alkyl bromides is novel and holds potential interests. The cathodic conversion of RBrs into free alkyl radicals is obviously valuable owing to their better chemical stability compared to that reported with most organic iodides. Another advantage of Ag–Pd particles lays in their possible deposition onto different carbons as well as on other solid substrates not reactive with Pd2+ and therefore enables the production of efficient and cheap electrodes. In parallel, the use of fluidized bed electrodes (Ag–Pd powders or use of fine carbon pieces covered with this alloy) considerably enlarges possibilities to make deposits of both radicals and nano-particles onto conducting surfaces. Additions of mono-radicals as well as bi-radicals on aromatic systems via this procedure [29] were successful (although not developed here). As discussed in this paper, the synergist combination [silver–palladium] is expected to boost the idea of using those metals. Other combined palladium alloys (such as Cu-Pd, Ni-Pd or Au-Pd) may lead, at least with RIs, to similar results in comparison with Ag–Pd. Furthermore, experimental conditions developed in this general paper enable to extend one-electron scissions to benzyl halides and master the coupling of arylhalides. Thus, the cathodic generation in situ of free benzyl radicals has induced for the first time their efficient grafting onto carbons and graphites [65]. The ablation of carbons by free radicals such as R• could be successfully undertaken [66]. Moreover, the use of some metallic electrodes (Cu, Ag, Ag–Pd) in PC enabled us to suggest a kind of electrochemical Ullmann reaction with the conversion of aryl iodides ArI into Ar–Ar in good yield [67]. Further works relative to the use of alloys of palladium with a large palette of transition metals as cathode materials is certainly a valuable field for applications in the future.

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