Electroactive polymer films of silver complexes with intertwined and interlocked ligands: the AgI–Ag0 redox couple as a topological probe† Martial Billon,a Bernadette Divisia-Blohorn,a Jean-Marc Kernb and Jean-Pierre Sauvageb aCEA, De�partement de Recherche Fondamentale sur la Matie`re Condense�e, S.I.3M., L aboratoire d’ElectrochimieMole�culaire, 17 rue desMartyrs, F-38054 Grenoble, France bL aboratoire de Chimie Organo-Mine�rale, UA 422 au CNRS, Institut L e Bel, Universite� L ouis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg, France The electrochemical properties of bis(2,9-diaryl-1,10-phenanthroline)silver complexes are reported.The redox potentials of the coordinated metal centre are strongly dependent on the topology of the organic backbone surrounding the metal centre and its set of ligands.This particular property can be used for the evaluation of the topological characteristics of dierent polypyrrole matrices built around entwined 2,9-diaryl-1,10-phenanthroline moieties. Although the functionalization of electronic conducting poly- that the metal is zerovalent. It simply means that the monomers (ECP) with a view to obtaining predetermined chemical valent complex has been reduced in a one-electron process to or physical properties has been extensively studied,1 there are aord a neutral species (overall charge=0) with no indication very few results on the topological properties of these materials, of whether this process is metal- or ligand-localized. More probably owing to the diversity of possible connections, and recently, electrochemical hysteresis properties have been consequently, the diculties in characterising the relation observed for catenates where one of the macrocyclic subunit between the particular properties of the material and their contains both di- and ter-dentate ligands12 Our previous topological features. To our knowledge, the only studies work13 has demonstrated the electropolymerisation of dierent reported concern the thermodynamic properties of specific metallic complexes with acyclic chelating fragments (end-functypes of polymers such as polyrotaxanes and polycatenanes.2 tionalized dpp) covalently linked to either the nitrogen13a atom Elaborations of conducting polymetallorotaxanes using supra- or the 3-carbon13b of the pyrrole unit, through a polymethylene molecular approaches, with the aid of transition metals as spacer.Removal of the gathering metal centre allows the templating agents, have been recently described.3 anchoring of preformed sites in a rigid polymeric conducting At present, there is a growing interest in topology in network [Scheme 1(B)]. These coordinating sites are able to molecular chemistry due to the challengeto build supramolecu- complex other transition-metal species.An increase in the lar edifices displaying new topological properties.4 Over the topological complexity of the precursor for electropolymeris- last few years, interesting topology related properties have ation, by the substitution of one acyclic ligand by the coordinat- been observed for certain molecular structures such as inter- ing macrocycle m-30 (a 30-membered 2,9-dpp-containing ring) locked macrocycles, especially the enhancement of basicity,5 [Scheme 1(C)], leads to the ECP films deposited on the kinetic eects,6 NMR properties7 and photochemical and electrodes exhibiting an enhancement of the electronic accessi- photophysical properties.4f,8 Recently, interesting electro- bility of the redox metal centres.3a These electropolymerisations chemical properties have been reported for purely organic produce random connections, which can be inter- or intra- catenanes or rotaxanes formed by a self-assembly process.9 molecular, between pyrrole units. Processes B and C allow the They display cathodic or anodic shifts of the half-wave poten- elaboration of ECP-modified electrodes with varying topologi- tials of the redox subunits included in the host complexing cal properties.Thus, electropolymerisation according to pro- macrocycle, reflecting either the intensity of the interaction cess B produces a multi-entangled criss-cross of molecular between aromatic units or the steric hindrance imposed by the wires with possible ring formation leading to ECP films rotaxane structure.Dierent behaviour has been observed10 containing local catenane- and rotaxane-like structures, whilst for catenands synthesized by the template eect of a transition electropolymerisation of preformed rotaxanes produces, after metal coordinated to 2,9-diphenyl-1,10-phenanthroline (dpp) demetallation, a polyrotaxane [Scheme 1(C)].Consequently, units [Scheme 1(A)].They exhibit stabilization of the low it appears to be interesting to investigate and to compare the oxidation states of the metal ion concerned, this being due to topological properties of these dierent materials. the pseudo-tetrahedral geometry imposed by the phenanthro- Here, we report the electrochemical properties of the three line units around the metal centre.As a matter of fact, silver complexes 1, 2 and 3; with the same topographical transition-metal complexes of entwined phenanthroline-con- features but dierent topologies, by increasing the number taining ligands show very similar electrochemical properties. of macrocyclic ligands around the metal centre (Scheme 2). Particular electrochemical characteristics arising from the The entwined system 1, the threaded complex 2 and the silver topological properties of interlocked macrocyclic complexes catenate 3 exhibit dierent electrochemical behaviour for the have been reported11 and include the emergence of reversibility reduction process of the metal.Hence the formal AgI–Ag0 of the one-electron reduction of the formal CuI–Cu0 redox redox centre can be used as an electrochemical probe to couple and a cathodic shift of the reduction potential between evaluate the topological properties of the dierently func- a AgI–Ag0 catenate and its analogous entwined complex.It tionalized ECP films elaborated by utilizing dierent starting should be noted that the index 0 in Cu0 or Ag0 does not imply materials. The electrochemical response of the cobalt centre in cobalt-containing films (poly-5) prepared with cobalt(II) as a templating centre is not sensitive to the topological properties † Dedicated to Professor H.J. Scha�fer on the occasion of his 60th birthday. of the polymeric network. However, this resulting polyrotaxane J. Mater. Chem., 1997, 7(7), 1169–1173 1169Scheme 1 Synthetic strategy used for constructing: (A) a [2]catenate, by means of a strategy based on the three-dimensional template eect induced by a transition metal on the molecular fragment represented by fMf; f and g represent functional groups which are able to react to form a covalent bond; (B) multi-entangled criss-cross molecular wires produced by electropolymerisation of preformed entwined complexes; (C) polyrotaxanes, by means of electropolymerisation of suitable rotaxane monomers, where E represents the electropolymerisable unit like system is able to undergo a reversible complexing–decom- voltammetry.The modified electrodes were washed in pure solvent before further analytical experiments. plexing process.3a Cobalt–silver exchange has therefore been monitored by voltammetric measurements in order to observe As the electrochemical study of the silver complexes turned out to be especially dicult due to non-reproducible adsorp- the sensitive electrochemical response of silver and to use it as a probe of the topological properties of the network matrix.tion poisoning phenomena on the platinum electrode, electropolymerisation experiments were performed on carbon felt electrodes.The results reported herein concern the first Experimental cathodic CV made after electrodeposition of the film. Chemicals The synthetic procedures for the synthesis of the macrocycle Results and Discussion m-30,14 the dierent ligands13 and the catenate11 3 have been Electrochemical properties of free Ag+ and complexes previously reported.The complexes 1, 2, 4 and 5 were formed 1, 2 and 3 in solution immediately prior to the electrochements by addition of slightly less than the stoichiometric The voltammograms of the free Ag+ ion and complexes 1, 2 amount of Ag+ (AgBF4) or Co2+ [Co(BF4 )2 ] to the linear and 3 exhibit the same shape and are characteristic of thinand macrocyclic ligands. layer electrochemical behaviour due to the nature of the working electrode (carbon felt) and the low scan rate used.Reagents, electrochemical apparatus and procedure They exhibit an irreversible one electron reduction attributed to the AgI–Ag0 redox centre leading to formal Ag0 complexes. Electrochemical studies were performed in a dry-box under an In addition, the complexes 1 and 2 show irreversible oxidation argon atmosphere.Acetonitrile (MeCN) (BDH, HiPer Solv) of the pyrrole units at 0.80 V. The relative intensities of the was distilled over P4O10. Tetraethylammonium tetrafluorobo- peaks associatedwith the silver redox process and the oxidation rate (Et4NBF4 ) (Fluka purum) was dried at 100 °C under of the pyrrole units are consistent with the stoichiometry of vacuum for 1 day prior to use.The concentration of the each complex (number of pyrrole nuclei per metal) and the pyrrole units was 4×10-1 mol l-1 for complexes 1, 2, 4 and 5. number of electrons transferred for each electrochemical pro- The electrochemical apparatus consisted of PAR 273A from cess. The half-potential values E1/2, defined as (Epc+Epa)/2 EG&G Princeton Applied Research, monitored by a Hewlett (Epc and Epa are the cathodic and anodic peak potentials Packard 9836C computer ensuring data acquisition and con- respectively), and the dierence between the cathodic and the nected to a Kipp & Zonen recorder. All potentials are relative anodic peak potential values DEp for the three complexes 1, 2 to a 10-2 mol l-1 Ag+–Ag reference electrode.The working and 3 and for the free silver ion are reported in Table 1.electrodes were made of carbon felt (s=3 cm2). The redox potential of free Ag+ at carbon felt determined in CH3CN is in good agreement with previous published data Elaboration of the films at a dropping mercury electrode.15 Only a slight irreversibility of the electron transfer process is observed due to silver Anodic electropolymerisation of the functionalized ECP films was performed in MeCN either by repeated potential linear desorption during the anodic scan.As the three complexes 1, 2 and 3 exhibit the same voltammo- scanning or by electrolysis at a controlled potential of 0.850 V vs. the Ag+–Ag reference electrode, [Ag+]=10-2 mol l-1 in gram shape as the free Ag+ ion, it is not possible to determine if their reoxidation occurs with or without partial silver MeCN.The film growth was monitored by anodic cyclic 1170 J. Mater. Chem., 1997, 7(7), 1169–1173Fig. 1 Cyclic voltammetric response of the silver redox centre of 2. 2×10-3 mol l-1 in MeCN–Et4NBF4 (0.1 mol l-1), sweep rate v= 5mV s-1 on a carbon felt electrode, Ag+ (10-2 mol l-1)/Ag as reference. desorption, free silverpossibly being formedfrom the complexes by demetallation during the reductive scan.The E1/2 and DEp values for the silver redox centre of complex 1 are in agreement with those reported by El Hajbi et al.16 for several phenanthroline complexes of silver which undergo an irreversible one-electron reduction at -0.17 V vs. SCE with low DEp. Surprisingly, concerning the metal redox response, a relatively strong cathodic shift (0.21 V) of the Epc values is observed between 1 and 2, together with a remarkable cathodic shift (0.70 V) between 1 and 3 (Table 1).In addition, DEp increases slightly (0.13 V) between 1 and 2 but increases drastically (0.30 V) between 1 and 3. This observation shows that each successive substitution of an acyclic phenanthroline ligand by a macrocyclic one induces a strong stabilization of the corresponding silver complex.This remarkable topological eect on the redox properties of the complexed AgI–Ag0 centre led us to use it as an electrochemical probe for evaluating the topology of complexes linked to dierent polypyrrole matrices, i.e. their degree of interlocking and entanglement. Electrochemical properties of silver (I ) complexes covalently linked to polypyrrole matrices The polypyrrole films modified by 1 and 2 display very similar cyclic voltammograms composed of both the characteristic anodic and cathodic response of the N-substituted polypyrrole matrix centred at 0.30 V and, in the cathodic potential range, an irreversible one-electron transfer attributed to the reduction of the metal centre (Fig. 2). Further scans produce main changes in the metal centre response, indicating silver electrodeposition. As the formal zerovalent silver complexes are very unstable, all reported results concern the first scan. The Scheme 2 Structural formulae of the catenate 3 and of the precursors of the polymers Table 1 Cyclic voltammetry data for AgBF4 and complexes 1, 2 and 3.All potentials refer to Ag+ (10-2 mol l-1)/Ag reference in MeCN–Et4NBF4 (0.1 mol l-1), v=5 mV s-1; potentials were determined by CV on a carbon felt electrode complex c/mol l-1 Epc/V Epa/V E1/2/V DEp/V Fig. 2 Cyclic voltammetry of poly-2 on a carbon felt electrode (s= AgBF4 10-2 -0.07 0.00 -0.04 0.07 3 cm2) in MeCN–Et4NBF4 (0.1 mol l-1 ), v=5 mVs-1, Ag+(10-2 mol 1 10-3 -0.24 -0.05 -0.15 0.20 l-1)/Ag as reference.Synthesis of the film performed at 0.85 V in 2 2×10-3 -0.40 -0.12 -0.26 0.33 MeCN–Et4NBF4 (0.1 mol l-1), at a monomer concentration of 3 10-3 -0.94 -0.44 -0.69 0.50 2×10-3 mol l-1, by passing 14 mC cm-2. J. Mater. Chem., 1997, 7(7), 1169–1173 1171Table 2 Cyclic voltammetry data of the substituted polypyrroles deter- 3-substituted polypyrrole which occurs near -0.20 V, loss of mined by CV on a carbon felt electrode vs.Ag+ (10-2 mol l-1)/Ag electrochemical activity as a result of the removal of cobalt reference in MeCN–Et4NBF4 (0.1 mol l-1), v=5 mVs-1 and the appearance of a reduction process located, here again, at -1.10 V and assigned to the complexed AgI–Ag0 electro- metal centre phore. This identical reduction potential value for the com- ppy matrix polymer Epc/V Epa/V E1/2 plexed silver ion in three films issued from three dierent precursors, the entwined complex 1, the thread and ring poly-1a -1.10 -0.35 +0.30 complex 2 and the thread and ring 5, after silver–cobalt poly-2b -1.10 -0.20 +0.30 exchange, suggests that these films have very similar polyrotax- poly-5 after -1.10 -0.10 -0.20 ane network structures.Consequently, in the specific case of Co2+–Ag+ exchangec the poly-1, inter- or intra-molecular ring formation must occur free-ligand film <-2.0 -0.20 during electropolymerisation. The linkage position of the silver complex through the alkyl chain on the pyrrole ring, i.e. 3- a,bSynthesis of the film performed at 0.85 V in MeCN+Et4NBF4 0.1 mol l-1+4×10-3 mol l-1 pyrrole units by passing 14 mC cm-2, substituted or N-substituted pyrrole, has no influence on the as=4 cm2, bs=3 cm2.cSynthesis of the film by ion exchange within an topological properties of the films. analogous film polymer with cobalt as the complexing transition metal (see text). Conclusion cathodic peak is situated at -1.1 V for the two films while, We had previously noticed that the redox potential of the on the anodic scan, the associated reoxidation peak is observed AgI–Ag0 couple was strongly dependent on the topology of in the potential range where the polymer matrix is conductive its 2,9-diphenyl-1,10-phenanthroline based ligands, being more (Table 2).These reduction peaks are not due to the reduction negative in the case of the catenate structure than in the case of protons which are formed during electropolymerisation and of the entwined one by 700 mV.In this study we took which could be trapped by the polymer matrix and/or initiate advantage of this particular property to characterise the partial decomplexation of silver, despite the relative stabilities topology of bis(2,9-diaryl-1,10-phenanthroline)silver(I) based of silver catenates and proton [2]catenates.Indeed, dipping complexes covalently linked to polypyrrole matrices. In the of the modified electrodes into a basic 10-3 mol l-1 solution case of coordinating polymers built around other metallic of 2,4,6-trimethylpyridine in MeCN before the first voltam- templates, the topology could be evidenced by exchanging metric measurement led to no significant modification of the the metallic centre for AgI and determining the redox voltammogram. For each film, the ratio of the metal centre potential of the corresponding electroactive polymer.This reduction peak area to the area of the polypyrrole matrix observation is of course valid for the present family of redox peak is in agreement with its own stoichiometry, the coordinating polymers only. number of electrons transferred for the electrochemical process concerned and the doping level values (ca. 0.2).13a As the reoxidation of the zerovalent complex occurs in the References conducting matrix potential range, DEp has no significance 1 G. Bidan in Polymer films in sensor applications, ed G. Harsanyi, and it is not possible to determine E1/2.So the discussion will Technomic Publishing Co., Lancaster, Basel, 1995, p. 206 and ref- concern only the Epc values of the one-electron transfer of the erences therein. AgI–Ag0 centre. Both polymers exhibit the same one-electron 2 (a) H. L. Frisch,New. J. Chem., 1993, 17, 697; (b) S. J. Clarson, New. J. Chem., 1993, 17, 319; (c) Y. Lipatov and Y. Nizel’sky, New. reduction potential value of the silver centre, -1.1 V, corre- J.Chem., 1993, 17, 715. sponding respectively to a cathodic potential shift of 0.86 and 3 (a) J. M. Kern, J. P. Sauvage, G. Bidan, M. Billon and B. Divisia- 0.70 V between the precursors 1 and 2 and their polymers. Blohorn, Adv. Mater., 1996, 8, 580; (b) S. S. Zhu, P. J. Caroll and This potential is measured for a solid-phase species, in a T.M. Swager, J. Am. Chem. Soc., 1996, 118, 8713. potential range in which the polymer matrix is electrically 4 (a) D. B. Amabilino and J. F. Stoddart, Chem. Rev., 1995, 95, 2725; insulating, which is expected to create an overpotential as (b) J. P. Sauvage, Acc. Chem. Res., 1990, 23, 319; (c) H. W. Gibson and H. Marand, Adv. Mater., 1993, 5, 11; (d) C. Dietrich-Buchecker compared to the same redox species in solution.Nevertheless, and J. P. Sauvage, New J. Chem., 1992, 16, 277; (e) P. L. Anelli, this strongly negative value of the silver(I) complex reduction P. R. Ashton, R. Ballardini, V. Balzani, M. Delgado, M. T. potential Epc for the polymers is remarkable. It reflects the Gandolfi, T. T. Goodnow, A. E. Kaifer, D. Philp, pronounced stabilization of the complex as compared to the M.Pietraszkiewicz, L. Prodi, M. V. Reddington, A. M. Z. Slawin, monomer in solution and is likely to originate from the highly N. Spencer, J. F. Stoddart, C. Vicent and D. J. Williams, J. Am. entangled nature of the matrix. The fact that this reduction Chem. Soc., 1992, 114, 193; ( f ) A. C. Benniston, A. Harriman and V. M. Lynch, J.Am. Chem. Soc., 1995, 117, 5275. occurs in all the polymers at the same potential values suggests 5 M. Cesario, C. O. Dietrich, A. Edel, J. Guilhem, J. P. Kintzinger, that the two films poly-1 and poly-2 have very similar multi- C. Pascard and J. P. Sauvage, J. Am. Chem. Soc., 1986, 106, 6250. catenate-like network structures. 6 A. M. Albrecht-Gary, C. Dietrich-Buchecker, Z. Saad and To take advantage of these observations and of the special J.P. Sauvage, J. Am. Chem. Soc., 1988, 110, 1467. relationship between the topological properties of the matrix 7 C. O. Dietrich-Buchecker, P. A. Marnot, J. P. Sauvage, backbone and the reduction potential of the silver(I) complex J. P. Kintzinger and P. Malte`se, New. J. Chem., 1984, 8, 573. 8 N. Armaroli, L. De Cola, V.Balzani, J. P. Sauvage, C. O. Dietrich- used as a probe, it can be useful to substitute any metal of a Buchecker, J. M. Kern and A. Bailal, J. Chem. Soc., Dalton T rans., polymer to be tested by silver(I). To demonstrate the usefulness 1993, 3241. of the technique, ion-exchange experiments have been per- 9 (a) D. B. Amabilino, P. R. Ashton, C. L. Brown, E. Cordova, L. A.formed on analogous 3-substituted polypyrroles covalently Godinez, T. T. Goodnow, A. E. Kaifer, S. P. Newton, M. linked to Co(dpp,m-30)2+. The successive cobalt exclusion– Pietraszkiewicz, D. Philp, F. M. Raymo, A. S. Reder, J. F. Stoddart silver inclusion reactions have been performed on a poly-5 and D. J. Williams, J. Am. Chem. Soc., 1995, 117, 1271; (b) E. Cordova, R. A. Bissell and A.E. Kaifer, J. Org. Chem., 1995, modified electrode. The poly-5 is demetallated by action of 60, 1033. thiocyanate ions by dipping the polymer film into an 10 C. O. Dietrich-Buchecker, J. P. Sauvage and J. M. Kern, J. Am. MeCN–H2O/KSCN (0.1 mol l-1) solution and subsequently Chem. Soc., 1984, 106, 3043. metallated by silver ion by dipping in an MeCN–AgBF4 11 C. Dietrich-Buchecker, J. P. Sauvage and J. M. Kern, J. Am. Chem. (0.1 mol l-1) solution. Cobalt demetallation and silver metall- Soc., 1989, 111, 7791. ation were monitored by cyclic voltammetry. After treatment, 12 A. Livoreil, C. O. Dietrich-Buchecker and J. P. Sauvage, J. Am. Chem. Soc., 1994, 116, 9399. the film always displays the characteristic electroactivity of 1172 J. Mater. Chem., 1997, 7(7), 1169–117313 (a) G. Bidan, B. Divisia-Blohorn, M. Lapkowski, J. M. Kern and 15 I. M. Koltho and J. F. Coetzee, J. Am. Chem. Soc., 1957, 79, 1852. 16 A. El Hajbi, N. Alonso Vante, P. Chartier, G. Goetz-Grandmont, J. P. Sauvage, J. Am. Chem. Soc., 1992, 114, 5986; (b) G. Bidan, R. Heimburger and M. J. F. Leroy, J. Electroanal. Chem., 1986, B. Divisia-Blohorn, M. Billon, J. M. Kern and J. P. Sauvage, 206, 127. J. Electroanal. Chem., 1993, 360, 189. 14 C. Dietrich-Buchecker and J. P. Sauvage, T etrahedron, 1990, 46, 503. Paper 7/00014F; Received 2nd January, 1997 J. Mater. Chem., 1997, 7(7), 1169–1173 1173