The electroreduction of bromothiophenes on Au and Ag provides a striking model of synergy in electrocatalysis, when a second group specifically interacting with the catalytic surface is present besides the reacting one, providing an auxiliary anchoring effect. The high catalytic activity of Ag for bromobenzene reduction is enhanced in the bromothiophene case. Moreover, Au, having for bromobenzene a much lower and less reproducible catalytic effect than Ag on account of the repulsive effect of its very negative surface charge in the working potential range, approaches Ag activity in the case of 2-bromothiophene, where the anchoring S group is adjacent to the Br group to be cleaved. The beneficial anchoring effect is lower when it has to be shared between two Br leaving groups adjacent to the S group, and becomes negligible in the case of a bromide leaving group in 3-position. Keywords: Molecular electrocatalysis, Dissociative electron transfer, Anchoring groups, Bromothiophenes, Gold electrodes, Silver electrode

Abdirisak Ahmed Isse

Alberto Bonetti

Armando Gennaro

Ester Giussani

Patrizia Romana Mussini

Serena Arnaboldi

Simona Rizzo

Tiziana Benincori

English

The electroreduction of bromothiophenes on Au and Ag provides a striking model of synergy in electrocatalysis, when a second group specifically interacting with the catalytic surface is present besides the reacting one, providing an auxiliary anchoring effect. The high catalytic activity of Ag for bromobenzene reduction is enhanced in the bromothiophene case. Moreover, Au, having for bromobenzene a much lower and less reproducible catalytic effect than Ag on account of the repulsive effect of its very negative surface charge in the working potential range, approaches Ag activity in the case of 2-bromothiophene, where the anchoring S group is adjacent to the Br group to be cleaved. The beneficial anchoring effect is lower when it has to be shared between two Br leaving groups adjacent to the S group, and becomes negligible in the case of a bromide leaving group in 3-position

Arnaboldi, S.

Bonetti, A.

Giussani, E.

Mussini, P. R.

Rizzo, S.

Isse, A. A.

Gennaro, A.

BENINCORI, TIZIANA

Archivio istituzionale della ricerca - Università dell'Insubria

Electrocatalytic reduction of bromothiophenes on gold and silver electrodes: an example of synergy in electrocatalysis

The electroreduction of bromothiophenes on Au and Ag provides a striking model of synergy in electrocatalysis,
when a second group specifically interacting with the catalytic surface is present besides the reacting one, providing an auxiliary anchoring effect. The high catalytic activity of Ag for bromobenzene reduction is enhanced in the bromothiophene case.Moreover, Au, having for bromobenzene a much lower and less reproducible catalytic effect than Ag on account of the repulsive effect of its very negative surface charge in the working potential range,
approaches Ag activity in the case of 2-bromothiophene, where the anchoring S group is adjacent to the Br group
to be cleaved. The beneficial anchoring effect is lower when it has to be shared between two Br leaving groups
adjacent to the S group, and becomes negligible in the case of a bromide leaving group in 3-position

AHMED ISSE, ABDIRISAK

GENNARO, ARMANDO

Archivio istituzionale della ricerca - Università di Padova

Electrocatalytic reduction of bromothiophenes on gold and silver electrodes: An example of synergy in electrocatalysis

Crossref

Electrochemistry Communications

Open Access Repository

Electrochemistry Communications 38 (2014) 100–103Contents lists available at ScienceDirectElectrochemistry Communicationsj ourna l homepage: www.e lsev ie r .com/ locate /e lecomShort communicationElectrocatalytic reduction of bromothiophenes on gold and silverelectrodes: An example of synergy in electrocatalysisSerena Arnaboldi a, Alberto Bonetti a, Ester Giussani a, Patrizia Romana Mussini a,⁎, Tiziana Benincori b,Simona Rizzo c, Abdirisak Ahmed Isse d,⁎⁎, Armando Gennaro da Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italyb Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italyc ISTM-CNR, via Golgi 19, 20133 Milano, Italyd Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy⁎ Corresponding author. Tel.: +39 02 50314211.⁎⁎ Corresponding author. Tel.: +39 049 8275677.E-mail addresses: patrizia.mussini@unimi.it (P.R. Mussabdirisak.ahmedisse@unipd.it (A.A. Isse).1388-2481/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.elecom.2013.10.025a b s t r a c ta r t i c l e i n f oArticle history:Received 12 August 2013Received in revised form 9 October 2013Accepted 18 October 2013Available online 26 October 2013Keywords:Molecular electrocatalysisDissociative electron transferAnchoring groupsBromothiophenesGold electrodesSilver electrodesThe electroreduction of bromothiophenes on Au and Ag provides a striking model of synergy in electrocatalysis,when a second group specifically interactingwith the catalytic surface is present besides the reacting one, provid-ing an auxiliary anchoring effect. The high catalytic activity of Ag for bromobenzene reduction is enhanced in thebromothiophene case. Moreover, Au, having for bromobenzene a much lower and less reproducible catalytic ef-fect than Ag on account of the repulsive effect of its very negative surface charge in the working potential range,approaches Ag activity in the case of 2-bromothiophene, where the anchoring S group is adjacent to the Br groupto be cleaved. The beneficial anchoring effect is lower when it has to be shared between two Br leaving groupsadjacent to the S group, and becomes negligible in the case of a bromide leaving group in 3-position.© 2013 Elsevier B.V. All rights reserved.1. IntroductionReductive cleavage of carbon–halogen bonds (C\X) on Ag elec-trodes provides a good model for dissociative electron transfer (DET)in electrocatalytic conditions. In recent years the process has beenshown to bemodulated by many factors, amongwhich is themolecularstructure, in terms of both the halide leaving group [1–3] and the aro-matic [4], aliphatic [5], or benzylic residue [3]. Recently, a detailedmechanistic study led to propose a rationalization of the process forthe case of aryl bromides in acetonitrile [4]. Catalytic effects linearlyincrease with increasing significance of the heterogeneous ET barrierin the overall DET kinetics and decrease with increasing electron-withdrawing ability of substituents on the aryl ring, implying increasinglocalization of the negative charge away from the halide leaving group.We are extending our investigations to the heteroaromatic halides, inwhich the heteroatom (a) makes the aromatic ring asymmetric fromthe perspective of the electron density and (b) can itself have specificini),ghts reserved.interactions with the electrode surface, in addition to those of the C\Xgroup, thus acting as an “adsorption auxiliary group” [6] on the catalyticsurface.A good example is provided by halothiophenes, since both Au andAg,catalytic for C\X bond cleavage, also have strong specific interactionswith sulfur atoms, promoting formation of self-assembled monolayers(SAMs) [7,8]. Studies on thiophene SAMs on Au(111) confirm a directchemical interaction between the sulfur head groups in the thiophenerings and the metal surface [9]. Moreover, it is confirmed that, inthiophene-SAM formation, the involvement of the S atom results in sim-ple molecular adsorption with no chemical reactions, unlike disulfidesthat might be reductively cleaved, and alkanethiols which might bereduced to thiolates or oxidized to disulfides [10].In this context, the whole bromothiophene family (Fig. 1) is investi-gated at (i) glassy carbon (GC), assumed as a non-catalytic reference ac-counting for intrinsic reactivity only [11], (ii) Ag, highly catalytic for DETto C\X bonds [1–3,12] and (iii) Au, having the highest intrinsic affinityfor both S and X ions, but with much lower catalytic effects than Ag forthe reduction of organic halides on account of its high negative surfacecharge in the working potential range [12,13]. It is particularly interest-ing to see whether this scenario changes if the molecule contains “ad-sorption auxiliary groups” such as sulfur capable of interacting stronglyFig. 1. A synopsis of the investigated bromothiophenes.Fig. 2. Normalized CV features of the investigated bromothiophenes (0.00075 M) recordedon GC (– •• –), Au (−) and Ag (– –) electrodes in CH3CN + 0.1 M (Et4N)BF4 at 0.2 V s−1.101S. Arnaboldi et al. / Electrochemistry Communications 38 (2014) 100–103with themetal. To our knowledge this preliminary study is the first workspecifically focusing on the effect of anchoring groups on electrocatalysis.2. ExperimentalVoltammograms were recorded for each substrate (Sigma-Aldrich orsynthesized according to literature procedures [14–19]) in deaerated0.00075 M solutions in CH3CN (Merck and Sigma-Aldrich, HPLC grade)with 0.1 M (Et4N)BF4 (Fluka 99%) at 0.05 − 2 V s−1. The investigationswere carried out by an Autolab PGSTAT potentiostat/galvanostat withpositive feedback (EcoChemie, The Netherlands) run by a PC with GPESsoftware. The working electrodes were Teflon®-embedded disks of GC(Sigradur, grade G), Au and Ag (AMEL, d = 0.2 cm for Au and 0.3 cmfor GC and Ag) polished with 1-μm diamond powder (Sigma-Aldrich)on a wet cloth (DP-Nap, Struers). The reference electrode was an aque-ous saturated calomel electrode with a potential of −0.39 V vs Fc+|Fc,jacketed in a compartment filled with the working medium to preventleakage of water and Cl− into the working solution. The counter elec-trode was a platinum wire or disk.3. Results and discussionA synopsis of the CV features of the investigated bromothiophenes onthe three electrodes is provided in Fig. 2 and Table 1, which reports dataon the first and second reduction peaks. For comparison, bromobenzenedata are also reported.3.1. Dissociative electron transfer mechanismAs for the electrochemical reduction of aryl halides [4], all the CVpatterns account for an overall irreversible process, following a stepwiseDET mechanism [19]:ArBrþ e−⇄ArBr•− ð1ÞArBr•−→Ar• þ Br−: ð2ÞThe relative heights of the activation barriers of reactions (1) and(2) in the global DET process have recently been shown to be nicelyaccounted for by a kinetic parameter κ [4], calculated along two possiblecriteria:κ 0 ¼ − 1:51RT=F∂Ep=∂ logv¼ − 0:0296∂Ep=∂ logvat 25 C ð3Þκ 00 ¼ − 1:857RTF Ep−Ep=2  ¼ − 0:0477Ep−Ep=2at 25 C: ð4ÞThe higher the κ value, the more determining the chemical barrier,which can be assumed to prevail in the 0.5 b κ b 1 range, whereasthe electrochemical barrier prevails for 0.35 b κ b 0.5; lower κ valuespoint to a concertedmechanism inwhich the electron uptake is concur-rent with bond breaking:ArBrþ e−→Ar• þ Br−: ð5ÞTable 1Key CV features of the investigated bromothiophenes recorded on GC, Au, and Ag electrodes in CH3CN + 0.1 M Et4NBF4 at 0.2 V s−1.Entry R-Br Electrode EpI vs Fc+|Fc/V κa EpI(M)–EpI(GC)/V EpII vs Fc+|Fc/V1 2-BT GC −2.74 0.352 Au −1.89 0.54 0.853 Ag −1.78 0.45 0.964 3-BT GC −2.93 0.415 Au −2.42 0.35 0.516 Ag −1.98 0.46 0.957 2,5-DBT GC −2.44 0.33 −2.738 Au −1.76 0.41 0.68 −1.919 Ag −1.54 0.47 0.90 −1.7410 3,4-DBT GC −2.64 0.41 −2.9311 Au −2.02 0.48 0.62 −2.3212 Ag −1.80 0.43 0.84 −2.0513 2,3-DBT GC −2.40 0.35 −2.9614 Au −1.87 0.40 0.53 −2.3915 Ag −1.56 0.46 0.84 −2.0316 2,4-DBT GC −2.43 0.32 −2.9317 Au −1.77 0.36 0.66 −2.4118 Ag −1.64 0.49 0.79 −2.0819 2,3,4-TBT GC −1.92 0.44 −2.6220 Au −1.26 0.66 −2.1021 Ag −1.38 0.54 −1.8322 2,3,5-TBT GC −1.99 0.43 −2.4923 Au −1.83 0.16 −1.7524 Ag −1.40 0.59 −1.5525 TeBT GC −1.97 0.23 −2.6726 Au −1.63 0.43 0.38 −2.0927 Ag −1.37 0.49 0.64 −1.8028 BBb GCc −3.00 0.4529 Au −2.47 0.25 0.5330 Agc −2.15 0.29 0.85Bold characters have been used to highlight the compound acronyms to facilitate table reading, considering that there are three entries for each compound.a Average of the values calculated from Eqs. (3) and (4) for GC; values from Eq. (3) for Au and Ag.b BB = bromobenzene.c From ref. [4].102 S. Arnaboldi et al. / Electrochemistry Communications 38 (2014) 100–103The concerted mechanism is typical of alkyl halides, but can beobserved in some cases also with aryl halides on a catalytic surface. Ina previous study of halobenzenes [4] κ values were found to be in the0.5–1 range on GC (except for a few cases like bromobenzene,κ = 0.45), but in many cases b 0.5 on Ag, pointing to the stepwise pro-cess on the catalytic electrode being controlled by the ET step, and, insome cases, to a mechanism change from stepwise to concerted.In the investigated range of v, plots of Ep versus logv give straightlines for all compounds regardless of the electrode type. In addition, Ipvaries linearly with v1/2, indicating that the overall process is under dif-fusion control. In this circumstance, Eqs. (3) and (4) can be applied tocalculate κ [4] and indeed the two methods gave comparable valueson GC (the average values are reported in the table). Since in severalcases sharp peaks have been observed on the catalytic electrodes, weconsidered the method based on half-peak width not fully reliable andhence data calculated from Eq. (3) are reported for Ag and Au. The κvalues reported in the table are in the 0.35–0.50 range for nearly all sub-strates, on both GC and the two catalytic electrodes. This points to astepwise DETmechanismwith the heterogeneous electron transfer bar-rier prevailing on that of the C\X bond cleavage even in the absence ofcatalytic effects.3.2. Reactivity in non-catalytic conditionsThe CV features recorded at the non-catalytic GC electrode accountfor the intrinsic reactivity of bromothiophene molecules. ComparingCV patterns for a decreasing number of Br groups confirms that on GCthe DET processes involving the C\Br bonds result in progressivehydrodebromination, consistent with former literature [19,20]. All re-duction potentials of the C\Br bonds are consistent with the inductiveeffects of the S atom and of the Br atoms present in the molecule: the2-position is much more reactive than the 3-position (Table 1, entry1 vs 4), and increasing the number of Br atoms shifts EpI in the posi-tive direction (entries 1, 13, 22, 25).3.3. Reactivity in catalytic conditionsThe catalytic effects of Au and Ag for the first C\Br cleavage, calcu-lated as EpI(M) − EpI(GC), are reported in Table 1. It is important to stressthat this potential difference does not contain any contribution from in-ductive substituent effects but accounts only for the specific surface–substrate interactions leading to the observed anodic shift of Ep.A conspicuous catalytic effect, which is much higher than in thebromobenzene case, is observed on Au for 2-BT (entries 2, 29). This cat-alytic enhancementmust arise from synergic interactions of S and C\Brwith themetal, which of course is not possible for bromobenzene. In the3-BT case the catalytic effect ismuch lower than that found for 2-BT. Theremarkable difference in catalytic activity for positions 2 and 3 must bejustified in terms of different substrate–surface interactions. In particu-lar, assuming the reductive cleavage process to evolve through an inter-mediate state inwhich Au coordinates the S atom of the thiophene ring,as in SAMs, the highest catalytic effect is expected when the molecularstructure allows optimal synergic interactions of S and C\Br with themetal. Now, when the C\Br bond to be cleaved is adjacent to the Satom, it is in a more favorable position than 3-Br to benefit from theadditional interaction with the catalytic surface offered by sulfur.On Ag, the catalytic effects are even greater than on Au, and higherthan those already observed for bromobenzene, but nearly identicalfor 2-Br and 3-Br.We assume that, also for Ag, the affinity of the surfacefor S results in a stronger coordination of the molecule and therefore inan increased catalytic effect for C\Br bond cleavage, but in this case thesurface interactionwith the Br atom ismore determining than the Ag∙∙∙Sinteraction.The dibromothiophene reductions on Au appear consistent with theassumptions made in the case of the monobromothiophenes. On both103S. Arnaboldi et al. / Electrochemistry Communications 38 (2014) 100–103metals the highest catalytic effects are observed for 2,5-DBT, probablybecause both C–Br groups are in the most favorable position forsulfur-assisted surface interactions. It is important to stress, however,that the catalytic effect is lower than that observed for 2-BT (0.68 V vs0.85 V for Au and 0.90 vs 0.96 for Ag), possibly on account of the anchor-ing group effect being sharedby the two redox sites. Another interestingpoint is that high catalytic effects are observed for 3,4-DBT at both elec-trodes. This might suggest a coplanar disposition of the molecule withrespect to the surface, affording surface interaction with the three het-eroatoms at the same time. Finally, in the 2,3-DBT and 2,4-DBT cases,a higher catalytic effect is observed on Auwhen the Br atoms are in dis-tal positions, whereas the opposite is true for Ag.On both catalytic electrodes the tri- and tetra-bromo derivativecases are complicated by the presence of multiple specific adsorptions,as a consequence of the availability of at least 4 groups capable of specif-ic interactionswith the electrode surface in combinationwith the signif-icantly less negative reduction potential range, allowing adsorptionphenomena [21–23].4. ConclusionsThe presence of an anchoring group promotes the electrocatalyticcleavage of C\X bonds on both Au and Ag, to an extent depending onthe relative positions between the S atom and the halide leaving group.High catalytic effects for bromothiophene reduction, even ap-proaching those of Ag in a few cases, are observed on Au, which insteadhas a significantly lower catalytic activity than Ag for bromobenzene re-duction. The anchoring sulfur atom allows the Au surface to recovermost of its intrinsically high catalytic activity, overcoming the repulsiveeffect of the surface charge in the working potential range which, beingmuchmore negative than the PZC, significantly hampers Au catalysis foraryl halides. Such an effect appears neatly modulated by the relativeposition of the Br leaving groupwith respect to the S atom: the catalyticeffects for the reduction of 2-Br positions are significantly higher thanthose for the 3-positions (nearly approaching the very high catalyticeffects of Ag); moreover, they decrease when the beneficial anchoringeffect is to be shared between two redox sites.Also Ag catalytic effects are significantly enhanced in thebromothiophene case with respect to the bromobenzene one, albeitthe position of the leaving group is less determining, on account of pre-vailing silver−halide interactions.The study provides a nice example of the important synergic role ofanchoring groups in enhancing molecular electrocatalysis.References[1] A.A. Isse, L. Falciola, P.R. Mussini, A. Gennaro, Chem. Commun. 3 (2006) 344–346.[2] A.A. Isse, G. Berzi, L. Falciola, M. Rossi, P.R. Mussini, A. Gennaro, J. Appl. Electrochem.39 (2009) 2217–2225.[3] A.A. Isse, A. De Giusti, A. Gennaro, L. 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Electrocatalytic reduction of bromothiophenes on gold and silver electrodes: An example of synergy in electrocatalysis

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