Electrosynthesis and coordination chemistry of poly(ferrocenebipyridyl) films

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Journal of Electroanalytical Chemistry 484 (2000) 164–171

Electrosynthesis and coordination chemistry ofpoly(ferrocene�bipyridyl) films

Mihai Buda a,1, Jean-Claude Moutet a,*, Alain Pailleret a, Eric Saint-Aman a,Raymond Ziessel b

a Laboratoire d’Electrochimie Organique et de Photochimie Redox, UMR CNRS 5630, Uni6ersite Joseph Fourier Grenoble I, BP 53,38041 Grenoble Cedex 9, France

b Laboratoire de Chimie d’Electronique et de Photonique Moleculaires, Ecole de Chimie, Polymeres et Materiaux, ESA 7008,Uni6ersite Louis Pasteur, 1 rue Blaise Pascal, BP 296 F, 67008 Strasbourg Cedex, France

Received 22 September 1999; received in revised form 2 February 2000; accepted 3 February 2000

Abstract

The pyrrole-containing 1,1%-bis(bipyridyl)-substituted derivative of ferrocene L exhibits electrochemical recognition propertiestowards CuI and NiII in homogeneous solution. PolyL films synthesised by its oxidative electropolymerisation are able tocoordinate CuI ions, whereas they do not display coordinating ability for NiII. In contrast, electropolymerisation of L in thepresence of CuI or NiII ions readily obtain pre-structured metallated films. These films can be demetallated electrochemically, andthen partially remetallated. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Electrochemical recognition; Redox-active receptors; Functionalized polypyrrole; Ferrocene�bipyridyl ligand; Copper and nickelcomplexes

1. Introduction

Design and synthesis of host molecules containing aredox centre which can bind a particular metal cationat a specific coordination site and undergo a concurrentredox change have attracted considerable attention inhost–guest chemistry [1,2]. The derivatisation of elec-trode surfaces with these redox-active receptors mayopen the way to new electrochemical sensors. Conse-quently it is challenging to synthesise redox-active hostsystems functionalised by specific groups allowing theirimmobilisation onto an electrode surface. We havealready demonstrated [3,4] that the electropolymerisa-tion of a ferrocene�crown ether bearing a pyrrole groupallowed the coating of electrode surfaces with polymerfilms showing the same electrochemical recognitionproperties towards Group II metal cations as those

observed in homogeneous solution [5,6]. In this way,the electrochemical recognition properties are success-fully retained at the electrode � solution interface. Onthe other hand, we have recently shown that a 1,1%-bis-(bipyridyl) ester-bridged derivative of ferrocene affordsa convenient structure for the electrochemical recogni-tion of copper cations in homogeneous solution [7]. Inthis paper, we present results on the electropolymerisa-tion of its pyrrole-substituted derivative L, along withthe coordination and the electrochemical recognitionproperties of the resulting polymer film modified elec-trode towards copper and nickel cations.

2. Experimental

2.1. Synthesis

L was prepared according to Scheme 1.The 6,6%-bis-(hydroxymethyl)-2,2%-bipyridine (I) was

prepared following literature procedures [8–11]. Theintermediate compound (II) was synthesised by thestoichiometric reaction of (I) with 1,1%-bis-(chlorocar-

* Corresponding author. Tel.: +33-4-76514481; fax: +33-4-76514267.

E-mail address: [email protected] (J.-C. Moutet)1 Permanent address: Department of Applied Physical Chemistry

and Electrochemistry, Polytechnica University, Bucharest, Romania.

0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S 0 0 2 2 -0728 (00 )00073 -5

M. Buda et al. / Journal of Electroanalytical Chemistry 848 (2000) 164–171 165

Scheme 1. Synthesis of L.

bonyl)ferrocene [12]. A solution of 1,1%-bis-chlorocar-bonyl ferrocene (10 mmol) in 150 ml of dry toluene wasadded dropwise to a solution of (I) (22 mmol) anddistilled triethylamine (20 mmol) in dry acetonitrile (200ml) for 12 h, under an inert atmosphere. The reactionmixture was stirred at room temperature (r.t.) for anadditional period of 48 h, then filtered and evaporatedto dryness. The crude solid was extracted with CH2Cl2and the organic phase was washed with H2O. Thesolvent was removed in vacuo, affording an orangesolid, which was purified by chromatography on aneutral alumina column eluted with a CHCl3+CH3OH(99:1) mixture. The product was further purified byrepeated recrystallisation in CHCl3 to yield pure (II) asan orange solid; yield 25%. 1H-NMR (CDCl3) d ppm(TMS): 4.03 (OH, t, 2H), 4.42 (Hb,b%, t, 4H), 4.79 (Ha%,d, 4H), 4.90 (Ha,a%, t, 4H), 5.43 (Ha, s, 4H), 7.19 (H5%, d,2H), 7.44 (H5, d, 2H), 7.79 (H4,4%, q, 4H), 8.30 (H3,3%, t,4H). FAB-MS, m/z : 671 (M+H+).

Under an inert atmosphere, 3-(pyrrol-1-yl)propionicacid (2.5 mmol) was dissolved in 5 ml of dry CH2Cl2and freshly distilled dicyclohexylcarbodiimine (2.5

mmol) was added. A white precipitate formed. A sus-pension of (II) (1 mmol) in 4 ml of dry CH2Cl2 wasthen added, followed by 1 mmol of dimethylaminopy-ridine. The reaction mixture was stirred for 48 h at r.t.under an inert atmosphere, then filtered and evaporatedto dryness. The crude solid was extracted with CH2Cl2,and the organic phase was washed with H2O. Thesolution was concentrated in vacuo and L was precipi-tated as an orange solid by the addition of diethylether;yield 94%. 1H-NMR (CD3Cl) d ppm (TMS): 2.91 (Hc%,t, 4H), 4.47 (Hb%, t, 4H), 4.41 (Hb,b%, t, 4H), 4.89 (Ha,a%,t, 4H), 5.29 (Ha%, s, 4H), 5.41 (Ha, s, 4H), 6.12 (Hv,v%, t,4H), 6.67 (Hg,g%, t, 4H), 7.17 (H5%, d, 2H), 7.45 (H5, d,2H), 7.79 (H4,4%, q, 4H), 8.33 (H3,3%, t, 4H). FAB-MS,m/z : 913 (M+H+).

Copper and nickel complexes were synthesised astheir perchlorate salts by the reaction at r.t. in CH2Cl2of stoichiometric amounts of L and [Cu(CH3CN)4]ClO4

or [Ni(CH3CN)4](ClO4)2 respectively, precipitated uponaddition of diethylether and collected as orange solidsby suction filtration. Warning: perchlorate salts are haz-ardous because of the possibility of explosion. FAB-MS

M. Buda et al. / Journal of Electroanalytical Chemistry 848 (2000) 164–171166

Fig. 1. (A) Cyclic voltammograms of L (1 mM) in CH3CN+0.1 MTBAP: (a) free L; (b) L+0.3 mM CuI; (c) L+1 mM CuI. (B) L (2mM)+0.5 mM CuI. Scan rate 0.1 V s−1.

3. Results and discussion

3.1. Electrochemical beha6iour of L in homogeneoussolution and its complexation properties towards CuI

and NiII

The cyclic voltammetry (CV) curves for L wererecorded in CH3CN containing 0.1 M TBAP. They arecharacterised by a reversible redox wave correspondingto the regular ferrocene/ferricinium (Fc/Fc+) redoxcouple at E1/2=0.61 V (DEp=50 mV, n=0.1 V s−1;labelled II on Fig. 1(A), curve a). This value is slightlyhigher than that found for the parent pyrrole-freemolecule (0.56 V) [7], due to the additional substitutionat the 6% positions of the 2,2%-bipyridyl groups withcarboxy�ester groups. At higher potentials, an irre-versible anodic peak is seen at Epa=0.96 V correspond-ing to the irreversible oxidation of the pyrrole moieties(labelled IV on Fig. 1(A), curves a and b). As alreadyobserved with the parent pyrrole-free redox ligand [7],complexation of L with CuI ions leads to dramaticchanges in the CV curves (Fig. 1(A), curves b and c).Two new anodic peaks at Epa=0.45 and 0.83 V, re-spectively (labelled I and III on Fig. 1(A)), grow at theexpense of the original peak for free L. Peaks I and IIIreach full development after one molar equivalent ofCuI has been added to the electrolytic solution (Fig.1(A), curve c) and remain constant upon further addi-tion of copper salt. This behaviour strongly suggests theformation of a 1:1 L+Cu complex. This has beenconfirmed by FAB-MS measurements. The spectrum ofthe isolated [LCu]ClO4 complex displayed exclusively apeak at m/z=975 corresponding to [LCu]+. ProcessesI and III correspond to the oxidation of the complexedCuI and Fc centres, respectively. It is assumed that thepositive potential shift of the Fc/Fc+ redox coupleupon complexation by copper cations is due to theelectrostatic repulsion effect between the bound metalcation and the oxidised, positively charged ferriciniummoieties. The potential shift reflects the balance of theinteraction of the metal cation with the neutral and thecharged ferrocenoyl subunit [13]. In the parent pyrrole-free complex [7], the formal potential of the CuII/I redoxcouple is 100 mV lower than that measured with L. Thedisubstitution of the bipyridyl arms at the 6,6% positionsin L leads to a destabilisation of the correspondingcopper complex in its CuII form, and therefore to apositive shift of the formal potential of the CuII/I redoxcouple. It must be noticed that in the presence of onemolar equivalent of CuI (Fig. 1(A), curve c), the irre-versible oxidation of the pyrrole moieties (process IVon curves a and b) is no longer observed on the CVcurve. This suggests that the oxidised complexed recep-tor [LCu]3+ catalyses the oxidation of the pyrrolegroups which occurs at nearly the same potential, inaccordance with the abnormally high intensity of peak

spectra display exclusively the m/z peaks at 975([LCu]+) and 1069 ([LNi](ClO4)+), respectively.

2.2. Reagents, instrumentation and procedure

Acetonitrile (Rathburn, HPLC grade) was used asreceived. Tetra-n-butylammonium perchlorate (TBAP)was purchased from Fluka, recrystallised from an ethy-lacetate+cyclohexane mixture, and dried under vac-uum at 80°C for 3 days. Electrochemical experimentswere conducted in a conventional three-electrode cellusing a EG&G PAR model 273 potentiostat, at 20°Cunder an argon atmosphere. Ag � 10 mM AgNO3+0.1M TBAP in CH3CN was used as a reference electrode.The potential of the simple Fc/Fc+ redox couple usedas an internal standard was 0.07 V under our experi-mental conditions. The working electrode consisted of aplatinum disk (5 mm diameter) polished with 2 mmdiamond paste. Concentrated stock solutions of[Cu(CH3CN)4](ClO4) and [Ni(CH3CN)4](ClO4)2 inCH3CN were used for the addition of CuI or NiII

cations to electrolytic solutions of L. FAB (positivemode) mass spectra were recorded on an AEI KratosMS50 spectrometer fitted with an Ion Tech Ltd gun,using m-nitrobenzyl alcohol as the matrix. 1H-NMRspectra were recorded on a Brucker AM250 spectrome-ter. Electropolymerisation of L was accomplished byrepeated cyclic voltammetry scans between −0.4 and0.9 V, or by controlled potential electrolysis (0.8–0.85V) in unstirred millimolar solutions of L in CH3CN.The same procedures were used for electropolymerisa-tion of L in the presence of added metal cations.


III. This behaviour is similar to that already observedwith metal complexes containing pyrrole substituentson their ligands, such as RuII [14] and ReI [15]polypyridyl complexes, for example. In addition, itmust be emphasised that processes I and III appear tobe weakly reversible, the cathodic peaks associated withthe reduction of the complexed receptor being abnor-mally low. Moreover, even in the presence of one molarequivalent of CuI, the cathodic peak corresponding tothe reduction of the ferricinium moieties in the freereceptor is clearly seen at 0.59 V (Fig. 1(A), curve c).Due to the electrostatic repulsion effect between thebound metal cation and the oxidised, positively chargedferrocenoyl ligand, oxidation of the [LCu]+ complex ata potential higher than 0.80 V leads to its destabilisa-tion and, then, to the release in solution of the Cu2+

cations. It must be noticed that limiting the positivescan to the first oxidation of the [LCu]+ complex (e.g.0.7 V) leads to a strong improvement in the reversibilityof the CuII/I redox system, even in the presence of anexcess of L. The copper-localised redox system is thencharacterised by a well-behaved pair of peaks, which isseen at the foot of the free Fc/Fc+ redox wave (Fig.1(B); L+0.5 CuI).

Complexation of L with NiII ions is also revealed bythe appearance of new redox waves (Fig. 2(A)). Tworedox systems of increasing intensities are seen on theCV curves at −0.86 V (DEp=60 mV; n=0.1 V s−1)and −1.26 V (DEp=50 mV, n=0.1 V s−1), respec-tively. They reach full development after one molarequivalent of NiII has been added to the CH3CN solu-tion of L (Fig. 2(A), curve c). FAB-MS experimentsconfirm the 1:1 stoichiometry found by CV titration.The mass spectrum of the isolated [LNi](ClO4)2 com-plex showed an m/z peak at 1069 corresponding to the{[LNi](ClO4)}+ species. Coulometric measurements un-

ambiguously indicated the reductions to be one-electronprocesses leading to [LNi]+ and [LNi]0, respectively.Unfortunately the poor stability of the electroreducedspecies precluded the assignment from EPR studies ofthe redox orbitals involved in the course of these twoone-electron reductions. The electrochemical behaviourof [LNi]2+ is markedly different from that for unsubsti-tuted-2,2%-bipyridyl nickel(II) complexes which arecharacterised by a two-electron reduction [16], but rem-iniscent of that for a NiII catenate and its acyclicanalogue containing the 2,9-di-p-anisyl-1,10-phenan-throline framework [17]. However, the reduced [LNi]+

complex is less stable, since the NiII/I redox couple wasfound at a notably less negative potential (around−0.5 V) in the nickel catenate and in its acyclic ana-logue [17]. In the positive potential region of the CVcurve, the intensity of the original free Fc/Fc+ redoxwave decreases as the amount of nickel in the elec-trolytic solution increases (Fig. 2(A)). The oxidationwave for the ferrocene group in the complexed ligand(see below) is not seen in the potential area explored,from −1.5 to 0.8 V. It is noteworthy that the wavecorresponding to the uncomplexed Fc/Fc+ redox cou-ple is always seen, even when one molar equivalent ofNiII has been added to the solution (Fig. 2(A), curve c).Obviously, the [LNi]2+ complex is weaker than itscorresponding copper complex. This assumption is cor-roborated by the electrochemical study of the isolated,pure [LNi]2+ complex: when solubilised in CH3CN+TBAP electrolyte, its CV curve displays a weak redoxpeak system at 0.61 V corresponding to free Fc (Fig.2(B)). This behaviour indicates that a small amount ofnickel complex decomposes upon solubilisation in ace-tonitrile electrolyte. As in the case of the [LCu]+ com-plex, the wave corresponding to the oxidation of theferrocene group in the nickel complex appears weaklyreversible, and is fully overlapped by the irreversibleoxidation peak of the pyrrole moieties. However, thepresence of the complexed Fc/Fc+ wave under thepyrrole irreversible oxidation wave is evidenced by theappearance of the Fc+�Fc reduction peak at Ep=0.82 V.

Taking into account the complexation behaviour ofL towards NiII and CuI, the incorporation of thesemetal cations in polyL films deposited by electropoly-merisation onto an electrode surface has been tested.

3.2. Electropolymerisation of L in CH3CN solution

The growth of polyL films onto the electrode surfacewas accomplished either by repeated scans over the−0.4 to 0.9 V potential range, or by controlled poten-tial electrolysis at 0.85 V in millimolar solutions of L.Repeated scans led to a continuous increase in theintensity of the redox wave corresponding to the Fc/Fc+ couple at 0.63 V, along with the rise of a new

Fig. 2. (A) Cyclic voltammograms of L (1 mM) in CH3CN+0.1 MTBAP: (a) free L; (b) L+0.5 mM NiII; (c) L+1 mM NiII. (B) Cyclicvoltammograms of a solution of [L�Ni]2+ prepared by dissolution inCH3CN+0.1 M TBAP of the isolated complex. Scan rate 0.1 V s−1.


Fig. 3. (A) Oxidative electropolymerisation of L (2 mM) inCH3CN+0.1 M TBAP by repeated scans between −0.4 and 0.9 V.(B) Cyclic voltammogram of the resulting Pt � polyL modified elec-trode in CH3CN+0.1 M TBAP. Scan rate 0.1 V s−1.

unit, 0.66 for the oxidation of the two pyrrole groups inthe resulting polypyrrole matrix [18], and one for theoxidation of the ferrocene group). The polymerisationyield was dependent on the charge passed for theelectropolymerisation. It decreased from 52 to 18%when the polymerisation charge was increased from 0.1to 10 mC in 2 mM solutions of L. The rather lowpolymerisation ability of L is not surprising, since it isknown that the electropolymerisation of pyrrole deriva-tives containing polypyridyl groups is poorly efficient[19], due to the deprotonation by the pyridyl moietiesof the first-formed pyrrole radical cation which pre-vents regular pyrrole polymerisation [20].

3.3. Complexation properties of polyL films towardsCuI

The complexation properties of polyL films havebeen checked by immersing Pt/polyL electrodes for 20min in dimethylsulfoxide (DMSO) containing 0.5 M[Cu(CH3CN)4]+. The resulting modified electrodes werestudied by cyclic voltammetry in copper-free electrolyte.They present the electrochemical response of a partiallymetallated film (Fig. 4, curve b). The electrochemicalsignal due to the complexed CuII/I reversible redoxcouple system located at E1/2=0.45 V appears as shoul-ders on the main redox peak system corresponding tothe immobilised free Fc/Fc+ couple at 0.63 V, followedby the reversible oxidation wave for the complexed Fccentre at E1/2=0.83 V. These electrochemical featuresare clearly observed with thin polyL films, in the 2×10−10 to 4×10−9 mol cm−2 range. Evidence for thecomplexation of CuI cations in thicker films (GL around6×10−9 mol cm−2) is given only by a decrease in theanodic and cathodic peak currents corresponding to thefree Fc/Fc+ wave, which is accompanied by a strongbroadening of this redox wave. It is noteworthy thatfull complexation of the polymer with CuI could not beobtained, even with very thin films (GL=2×10−10 molcm−2; Fig. 4, curve b). The rise of a new CuII/I redoxcouple leads to an increase of the charge recordedunder the CV wave in the −0.2 to 0.9 V potentialrange, as compared to that measured with free polyLfilms. Thus, coulometric measurements allowed us toestimate the percentage of complexed bpy sites in thepolymer, which remained limited to 40%. This be-haviour is probably due to steric constraints and lowerconformational mobility of the bpy ligands in polyL.

The poor complexation ability of polyL filmsprompts us to study the synthesis of pre-structuredfilms by electropolymerisation of L in the presence ofCuI. The growth of a film on a Pt electrode by cyclingthe potential from −0.3 to 0.75 V in a solution of Lcontaining 5 molar equivalents of [Cu(CH3CN)4]+ isshown in Fig. 5(A). The anodic peak, which graduallyincreases, corresponds to the oxidation of both the

Fig. 4. Cyclic voltammograms of a Pt � polyL modified electrode(GL=2×10−10 mol cm−2) in CH3CN+0.1 M TBAP: (a) freepolyL; (b) after soaking the modified electrode in DMSO containing0.5 M [Cu(CH3CN)4](ClO4) for 10 min. Scan rate 0.1 V s−1.

broad reversible electrochemical signal around 0.3 Vcorresponding to the electrochemical response of thepolypyrrole matrix (Fig. 3(A)). This behaviour is inaccordance with the growth of a polyL film onto theelectrode surface. The electroactivity of polyL films hasbeen characterised by cyclic voltammetry after transferof the modified electrode in monomer-free electrolyte.The CV curve displays the electrochemical response ofthe immobilised Fc/Fc+ redox system at E1/2=0.64 V,which adds to the weak reversible oxidation wave cor-responding to the electrochemical activity of thepolypyrrole matrix (Fig. 3(B)). The apparent surfaceconcentration of the immobilised redox ligand, GL, wasdetermined from the integration of the current underthe Fc/Fc+ oxidation peak. Coatings containing be-tween 5×10−10 and 1.7×10−8 mol cm−2 were ob-tained by using polymerisation charges between 0.1 and10 mC at 0.85 V. The film deposition efficiency, whichrepresents the polymerisation yield, was evaluated fromthe amount of L immobilised divided by the amount ofL oxidised during the electropolymerisation process,determined from the total charge passed during elec-trolysis and using a value of 5.66 for the number ofelectrons transferred for one immobilised Fc unit (fourfor the polymerisation of the two pyrrole groups per Fc


copper complex and the ferrocene moieties. The ca-thodic response is characterised by the continuous in-crease of a reduction peak at 0.63 V corresponding tothe immobilised free Fc/Fc+ redox system, along with abroad cathodic wave around 0.4 V, which can beattributed to the reduction of the Cu2+ cations, re-leased during the positive scans. When transferred to apure electrolyte, the resulting modified electrode dis-plays the electrochemical response of a partially com-plexed film (Fig. 5(B)). Similar films could besynthesised by controlled potential polymerisation of Lin the presence of CuI (Fig. 6(A)). We found thatincreasing the amount of CuI in the polymerisationsolution from 1 molar equivalent up to 5 molar equiva-lents, led to a significant increase of the amount ofcomplexed bpy sites in the films, as judged by thehigher intensity of the peak systems attributed to theCuII/I and the complexed Fc/Fc+ redox couples on theCV waves for films synthesised in the presence of anexcess of copper cations.

Films prepared under these experimental conditionsact as redox switchable devices. Oxidation of the par-tially metallated film leads to a destabilisation of the[L�Cu] complexes in the polymer and therefore to therelease of copper cations from the film to the solution.This is illustrated in Fig. 6(A), where we see a continu-ous decrease in the intensity of the CuII/I and thecomplexed Fc/Fc+ redox waves upon repeated scansover the −0.3 to 0.9 V potential range. Full decom-plexation was reached in ten scans, and the regular CVcurve for a free polyL film was restored (Fig. 6(B)).This behaviour can be attributed to repulsive electro-static forces between the bound CuII metal cation andthe oxidised ferrocenoyl ligand, as was already seen forthe oxidation in solution of the [LCu]+ complex (seeabove). Furthermore, demetallation of complexedpolyL films might be due in part to a modification inthe coordination geometry of the bis(6,6%-substituted-2,2%-bipyridyl) copper complex after oxidation of CuI toCuII from a distorted-tetrahedral geometry towards asquare-pyramidal one [21], the change in the coordina-tion geometry leading to a destabilisation of the coppercomplex in the polymer film.

Demetallated films are capable of re-coordinatingCuI ions. This is evidenced by the presence of the redoxpeak systems due to the complexed CuI/II and Fc/Fc+

redox couples on the CV wave of a film which has beenfirst demetallated, then soaked for 10 min in CH3CNcontaining 20 mM [Cu(CH3CN)4]+ and finally trans-ferred into copper-free electrolyte (Fig. 6(C)). Longersoaking times did not improve the remetallation processsignificantly.

From coulometric measurements it could be esti-mated that 65% of the bpy sites were complexed in thefilm synthesised by oxidation of L in the presence of 5molar equivalents of CuI, and that 45% could be re-complexed in the demetallated film. Thus, despite adestabilisation of the copper complex upon oxidativeelectropolymerisation, the forced pre-structuring of thepolymer synthesised by electropolymerisation of the[L�CuI] complex leads to an increased complexationability of the resulting films towards copper cations, ascompared to polyL films prepared in copper-freeelectrolyte.

3.4. Complexation properties of polyL films towardsNiII

We found that polyL films do not display coordinat-ing ability for NiII. Pt � polyL modified electrodes whichhave been immersed in concentrated solutions of[Ni(CH3CN)4](ClO4)2 in acetonitrile for several hoursexhibit the regular electrochemical features of freepolyL films. It should be noted that nickel ions candiffuse through thin polyL films, as shown by thepresence of a reduction wave (Epc= −1.5 V) to which

Fig. 5. Growth of a partially metallated poly[L�Cu]+ film inCH3CN+0.1 M TBAP containing 2.6 mM L+13 mM CuI, byrepeated scans between −0.3 and 0.75 V. (B) Cyclic voltammogramof the resulting modified electrode transferred in CH3CN+0.1 MTBAP. Scan rate 0.05 V s−1.

Fig. 6. Cyclic voltammograms in CH3CN+0.1 M TBAP of Ptmodified electrodes synthesised by oxidative electropolymerisation ofL (2.6 mM in CH3CN+0.1 M TBAP) at 0.8 V (charge consumed 1mC) in the presence of 5 molar equivalents of CuI. (A) First to sixthscan. (B) Tenth scan. (C) Demetallated film soaked for 10 min inCH3CN containing 20 mM [Cu(CH3CN)4]+. Scan rate 0.05 V s−1.


Fig. 7. (A) Cyclic voltammograms in CH3CN+0.1 M TBAP of a Ptmodified electrode synthesised by oxidative electropolymerisation ofL (1 mM in CH3CN+0.1 M TBAP)+1 molar equivalent of NiII at0.8 V (charge consumed 0.5 mC): (a) second cycle between −0.2 and−1.5 V, followed by a scan in the positive potential region; (b) cycle−0.2�0.9�−1.5�−0.2 V following scan (a); scan rate 0.01 Vs−1. (B) Demetallation by repeated scans in CH3CN+0.1 M TBAPof a poly[L�Ni]2+ film synthesised by oxidative electropolymerisationat 0.8 V (charge consumed 1 mC) of L (3 mM)+1 molar equivalentof NiII in CH3CN+0.1 M TBAP. Scan rate 0.1 V s−1.

potential region are seen the peak systems due to theimmobilised free and complexed Fc/Fc+ couples atE1/2=0.64 and E1/2=0.85 V, respectively. Prepeaks(denoted pa

1 and pc1) appear at the foot of the [L�Ni]2+/

+ and free Fc/Fc+ waves. Such a pair of prepeaks isoften seen in functionalised polypyrrole films, especiallywhen the electroactivity of the polypyrrole matrix islow [18,22]. This behaviour is also reminiscent of pre-peak phenomena observed in redox bilayer systems[23]. If the potential scan range is restricted to thenegative (or positive) region, the cathodic (or anodic)prepeak vanishes from the second scan (Fig. 7(A),curve a), but is restored after a sweep in the positive (ornegative) potential area (Fig. 7(A), curve b). Prepeaksassociated with redox events occuring in films haveoften been explained in terms of resistive effects [24].Reversible oxidation (or reduction) events in films hav-ing an initial large resistivity can cause a decrease inresistivity. The decrease in resistance could make feasi-ble a flow of charge previously delayed because of IRdrop. As a consequence, prepeaks emerge on the CVcurve. Thus prepeaks could be a consequence of medi-ated reduction and oxidation of oxidised and reducedspecies, trapped and non-uniformly distributed into thepolymer film, through the redox levels of the nickelcomplex.

From the second scan over the −1.5 to 0.9 Vpotential region (Fig. 7(A), curve b), the decrease inintensity of the [L�Ni]2+/+ and complexed Fc/Fc+

waves shows the redox instability of the immobilisedcomplex, leading to the release of nickel ions from thefilm into the solution. As for poly[L�Cu] films, electro-static repulsion effects and changes in complex geome-try can be proposed to explain the electrochemicaldecomplexation of poly[L�Ni] films. Full decomplexa-tion can be reached upon repeated scans, as shown bythe complete vanishing of the [L�Ni]2+/+ redox peaksystem on the CV curves, along with the increase of thefree Fc/Fc+ wave (Fig. 7(B)). Prepeak phenomena areresponsible for the complicated features of the CVcurves in the positive potential area and of the abnor-mally high intensity of the [L�Ni]2+/+ reduction wave.Demetallation of poly[L�Ni]2+ films can also be ef-fected by cycling through the [L�Ni]2+/+ redox couple(Fig. 8(A)). However, the demetallation process ap-pears less efficient than when the potential is scannedthrough the Fc/Fc+ redox system, as judged from theslower decrease in the intensity of the [L�Ni]2+/+

redox peaks. These observations confirm the impor-tance of electrostatic repulsion forces between electro-generated Fc+ moieties and the bound metal cations inthe decomplexation process.

In contrast to polyL synthesised in nickel-free elec-trolyte, demetallated polyL films are capable of re-coor-dinating some NiII ions. This is shown in Fig. 8(B),where is seen the growth of the [L�Ni]2+/+ wave upon

corresponds a broad oxidation wave (Epa=0.2 V) char-acteristic of free nickel ions. These electrochemical fea-tures are similar to that observed on naked Pt. Thisbehaviour is in accordance with the fact that the[L�NiII] complex is weaker than the CuI complex, asshown above from electrochemical experiments carriedout in homogeneous solutions of L with copper andnickel metal cations. Moreover, steric constraints andlower conformational mobility of the bpy in polyLmight disfavour the appropriate geometry of the lig-ands around NiII.

In contrast, the oxidative electropolymerisation of Lin the presence of 0.5 to 2 molar equivalents of NiII

allows the synthesis of partially metallated films. TheCV curves for the resulting modified electrodes clearlyexhibit the electrochemical signals of both free andcomplexed L (Fig. 7(A)). The wave corresponding tothe [L�Ni]2+/+ reversible one-electron process appearsat E1/2= −1.20 V (Fig. 7(A), curve a). In the positive


Fig. 8. (A) Demetallation by repeated scans in CH3CN+0.1 MTBAP of a poly[L�Ni]2+ film, synthesised by oxidative electropoly-merisation at 0.85 V (charge consumed 1 mC) of L (1.4 mM)+1molar equivalent of NiII in CH3CN+0.1 M TBAP (the first threecycles are not shown). (B) Cyclic voltammograms of the demetalledPt � polyL electrode in CH3CN+0.1 M TBAP containing 5 mM[Ni(CH3CN)4]2+. Scan rate 0.1 V s−1.

complexation ability, as compared to those grown inmetal-free electrolyte. This molecular material behavesas if it retains a structural memory, owing to an ade-quate disposition of the bpy subunits within the nano-structured matrix.

Acknowledgements

The authors thank Region Rhone-Alpes for partialfinancial support through the TEMPRA Program.

References

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cycling a demetallated polyL film in acetonitrile con-taining 5 mM [Ni(CH3CN)4](ClO4)2. The complexationof NiII is also responsible for the decrease of the freeFc/Fc+ wave. However, remetallation is incomplete(less than 20% under these experimental conditions), asjuged by the large difference in intensity of the[L�Ni]2+/+ before demetallation (Fig. 8(A)) and afterremetallation (Fig. 8(B)).

4. Conclusions

The complexation in homogeneous solution of aferrocene functionalised with bipyridyl arms with CuI

and NiII can be electrochemically monitored throughmodifications in the cyclic voltammetric behaviour ofthe redox-active receptor. Complexation gives rise tonew electrochemical signals along with the decrease ofthe original signal for the free receptor. Thin polymerfilms immobilised on the electrode surface by the elec-tropolymerisation of its pyrrole derivative are able tocoordinate and to recognise copper cations electro-chemically. In contrast, they do not display complexa-tion ability for nickel ions. The best metallatedpolymers were obtained by growing films in solutionscontaining a metal cation. PolyL films act as redoxswitchable devices. Due to the redox instability of thecomplexed films, demetallation could be achieved elec-trochemically. The resulting films present improved

Documents

Electrosynthesis and coordination chemistry of poly(ferrocenebipyridyl) films