Electrochemical fabrication and characterization of

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PAPER

Electrochemical fabrication and characterization of thin films of redox-active molecular wires based on extended Rh–Rh bonded chains Frédéric Lafolet,a Sylvie Chardon-Noblat,*a Carole Duboc,a Alain Deronzier,a Florian P. Pruchnikb and Magdalena Rakb Received 23rd November 2007, Accepted 25th January 2008 First published as an Advance Article on the web 3rd March 2008 DOI: 10.1039/b718123j An original electrochemical synthesis of {[Rh4 (l-OOCCH3 )4 (phen)4 ]2+ }n (1) molecular wire films from a solution of binuclear bridged Rh complexes [Rh2 (l-OOCCH3 )2 (phen)2 (X)2 ](Y)2 (X = H2 O, Y = BF4 − (2a) and X = CH3 CN, Y = BF4 − (2b)) in MeCN electrolyte is reported. UV-vis spectroscopy and quartz crystal microbalance electrochemical coupled techniques have been used to demonstrate the electrosynthesis process. The resulting polymetallic compound 1 has been characterized on the basis of its physicochemical properties, which have been compared with those of a chemically synthesized sample. Furthermore, according to EPR, 1 H NMR and electrochemical behaviour, the mechanism of the oxidation of this polymetallic wire, containing mixed valent rhodium centers and alternatively acetate bridged Rh–Rh bonds, has been investigated in detail.

Introduction The strongly increasing demand for nanoscale molecular devices has given rise to an intense research activity in the area of lowdimensional molecular systems. Among new molecules developed, those based on infinite extended metal–metal bonded chains are of interest for their new and unusual properties. For instance, there is reason to believe that these polymetallic compounds will be useful for electronics applications, as conducting molecular wires.1 One of the main strategies for constructing this kind of chain compounds, based on transition-metal backbones, involves a bottom-up approach using a redox condensation of building blocks (monomer or dimer). Electrochemistry appears a convenient method to achieve this condensation.2 However, only a few examples demonstrating the validity of this concept have been reported. The best developed metal–metal bonded system is based on electroactive chains of non-bridged ruthenium or osmium atoms having the general formula [M0 (L)(CO)2 ]n (L = diimine ligand).3–5 The low metal oxidation state and the two vacant coordination sites, needed for metal chain formation, can be achieved by electrochemical reduction of a mononuclear or binuclear metallic precursor complex such as [MII (L)(CO)2 Cl2 ]. The overall electrochemical process is given in eqn (1). n [MII (L)(CO)2 Cl2 ] + 2n e− → [M0 (L)(CO)2 ]n + 2n Cl−

valence rhodium molecular wire (MW) {[Rh(CH3 CN)4 ](BF4 )1.5 }∞ obtained by slow galvanostatic reduction of the binuclear [Rh2 (CH3 CN)10 ](BF4 )4 complex at low currents at a Pt electrode.8 The resulting infinite metal–metal bonded polymer contains two ˚) different Rh–Rh interactions in the chain (2.8442 and 2.9277 A with Rh atoms in an average oxidation state of +1.5. More recently, other MWs with infinite Rh–Rh chains have been chemically synthesized and characterized.9–11 For instance, the Rh1.5+ polymetallic complex 1 as its PF6 − salt (Scheme 1; [Rh2 (lOOCCH3 )2 (phen)2 ]n n+ phen = 1,10-phenanthroline) has been prepared by refluxing the corresponding binuclear Rh2+ complex e.g. [Rh2 (l-OOCCH3 )2 (phen)2 (OH2 )2 ](OOCCH3 )2 in aqueous alcohol with 90% yield.9 In the solid state (X-ray structure) three different Rh–Rh bond distances are present in the polymetallic Rh chain. ˚ , the The distance for carboxylate bridged Rh–Rh bonds is 2.652 A dimeric units bound together with unbridged Rh–Rh distances ˚ to form tetranuclear fragments which are linked into of 2.739 A ˚ ). In an other infinite chains with the longer Rh–Rh bonds (2.832 A example ({[Rh4 (l-OOCH)4 (bpy)4 ](BF4 )}n (bpy = 2,2 -bipyridine)) Rh is in +1.25 oxidation state and three different metallic bond ˚ ).11 distances are also found (2.678, 2.793 and 2.921 A

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This type of polymer has proved to be highly selective and efficient as a catalyst for the water–gas shift reaction6 and for electroreduction of carbon dioxide in pure aqueous media.7 Another representative example is the one-dimensional unbridged mixeda Université Joseph Fourier Grenoble 1, Département de Chimie Moléculaire, UMR CNRS-5250, Institut de Chimie Moléculaire de Grenoble FR CNRS-2607, B. P. 53, 38041, Grenoble Cedex 9, France. E-mail: sylvie. [email protected]; Fax: +33 (4) 7651 4267; Tel: +33(4) 7651 4436 b Faculty of Chemistry, University of Wroclaw, Joliot Curie 14, 50–383, Wroclaw, Poland. E-mail: [email protected]; Fax: +48 71 3757232; Tel: +48 71 3757232

This journal is © The Royal Society of Chemistry 2008

Scheme 1 Chemical structure of 1.

In the present work, we have developed an original preparative electrochemical method to synthesise thin films of 1 (Scheme 1). The electrochemical fabrication of these films has Dalton Trans., 2008, 2149–2156 | 2149

been followed, and unambiguously demonstrated, by in-situ coupled electrochemical and physicochemical characterization techniques. 1 results from the one-electron reduction of [Rh2 (lOOCCH3 )2 (phen)2 (X)2 ](Y)2 binuclear precursor complex (X = H2 O, Y = CH3 COO− (2a)9 or X = CH3 CN, Y = BF4 − (2b)12 ) in CH3 CN containing Bu4 NPF6 or Bu4 NClO4 electrolyte (eqn (2)). During this work we have also studied, by spectroscopic and electrochemical techniques, the mechanism of the oxidation of 1. n [RhII,II 2 (l-OOCCH3 )2 (phen)2 (X)2 ]2+ + n e− → {[RhI,II,II,I 4 (l-OOCCH3 )4 (phen)4 ]2+ }n/2 + 2n X−

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The aim of the study reported here is to show the possibility of using an alternative and original electrochemical route, involving a Rh dimer one-electron reduction, for quantitative synthesis of Rh–Rh bonded molecular wire films.

Results and discussion Electrochemical fabrication of thin films of 1 The electrochemical behaviour of the aquo complex 2a (CH3 CN + 0.1 M TBAP), at a Pt or glassy carbon (GC) electrode, is strictly identical than the behaviour of 2b corresponding acetonitrile complex. This result indicates that the initial H2 O ligands in 2a are readily exchanged with coordinating CH3 CN molecules when 2a is dissolved in acetonitrile electrolyte. On a Pt electrode, the cyclic voltammogram (CV) of a solution of 2a reveals that the complex is irreversibly oxidized at a potential (E pa = 1.57 V; Fig. 1(a))

very close to the oxidation potential previously determined for bis chloride equivalent complexes.13 It has been previously reported that the complexes [Rh2 (lOOCCH3 )2 (L)2 (CH3 CN)2 ](BF4 )2 with various bpy or phen ligands (L), are reduced by a reversible one-electron process.12 In this previous study a rather large anodic–cathodic separation (DE p ∼190 mV) and a return wave less steep than the forward one were found. The authors have suggested that the peak asymmetry is likely due to a small structural change accompanying the reduction process, nevertheless the final reduced product has not been clearly identified. We will see here that the reduction of 2a and 2b, in acetonitrile electrolyte, is an irreversible one-electron process (1 mol per Rh2 2+ unit) and that the quite large peak separation, observed on the CV (Fig. 1(b)), is due to the formation of an electroactive polymer film on the working electrode (WE) surface, in accordance with the overall reaction (eqn (2)). As a demonstration, in the negative potential area, the CV of 2a (Fig. 1(b)) exhibits an one-electron cathodic peak (E pc = −0.86 V) associated on the reverse scan with an anodic peak (E pa = −0.66 V; DE p = 200 mV), typical of an electrodeposition–redissolution phenomenon. Iterative CVs, from −0.72 to −1.15 V, induce the formation of an adherent redox active species on the WE surface (Fig. 1(c)). This electrodeposition is characterized by regular growth of a reversible redox system at E 1/2 = −0.83 V (DE p ∼ 0 V). The adherence of the electrogenerated product, on conductive surfaces, depends strongly on experimental conditions, e.g. the nature of the electrolyte solution and the concentration of the precursor complex. Totally adherent films have been produced from a solution of ∼1 mM of 2a or 2b in CH3 CN + 0.1 M TBAP. The electrosynthesis of thin films of 1 can be also accomplished by applying a constant potential (−0.90 V) at conductive planar electrode surfaces (Pt or GC) in precursor complex solution. The electrochemical stability of the film is excellent and redox systems persist when CV potential scan limits are suitably chosen (Fig. 2(a)). In this range of potentials CV of 1 is essentially characterized by two well defined reversible systems at E 1/2 = −1.56 and −0.82 V, corresponding respectively to the reduction and oxidation of 1 deposited as a thin film on the WE surface. Successive scans beyond the oxidation-desorption potential (E pa = −0.52 V; Fig. 2(b)) result in a slow and progressive disappearance of the initial redox systems due to the desorption of the electroactive deposit. The complete oxidation of the polymer film, following eqn (3), can be achieved either via electrochemical and chemical ways (see further in the discussion). {[RhI,II,II,I 4 (l-OOCCH3 )4 (phen)4 ]2+ }n → 2n [RhII,II 2 (l-OOCCH3 )2 (phen)2 (CH3 CN)2 ]2+ + 2n e− (3)

Fig. 1 CVs of 2a, 1 mM in CH3 CN + 0.1 M TBAP on a Pt electrode (diameter 5 mm), (a) between 0 and 1.70 V (b) 1st and 2nd cycle between 0, −1.05 and −1.60 V, respectively; (c) 3rd to 21st successive scans from −0.72 to −1.15 V; m = 100 mV s−1 .

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An exhaustive electrolysis, of 2a or 2b, carried out at −0.90 V on a large WE (one mol of electron per mol of 2a or 2b dimer has been consumed), leads quantitatively to the formation of an adherent and highly coloured film of 1. Measurement of the coulometry during this electrosynthesis confirms the number of exchanged electron during the reduction process (one mol per mol of dimer precursor complex). Elemental analysis (EA), infrared (IR), UVvis, 1 H NMR, X-band EPR and electrochemical behaviour (cf . Experimental section and below) confirm, that the structure of the resulting solid compound electrochemically synthesized, is exactly the same than that of 1 chemically prepared.9 This journal is © The Royal Society of Chemistry 2008

Fig. 3 CVs of chemically (—) and electrochemically ( · · · ) synthesized 1 powder recorded using a Pt CME (diameter 60 lm; depth 20 lm) in CH3 CN + 0.1 M TBAP, m = 50 mV s−1 .

Fig. 2 CVs of 1 film (sRh = 1.9 × 10−7 mol cm−2 ) recorded with the ME (Fig. 1(c)) and transferred into CH3 CN + 0.1 M TBAP electrolyte: (a) 1st cycle from −0.75 to −1.60 V; (b) following cycles from −1.60 to 0.60 V, m = 100 mV s−1 .

Powder EPR spectra have been recorded at 100 K on 1 chemically obtained or electrochemically deposited on a Pt wire. In strictly deoxygenated experimental conditions, 1 is EPR silent demonstrating that the metal–metal bonded species {[Rh4 (lOOCCH3 )4 (phen)4 ]2+ }n is diamagnetic. On the basis of the X-ray characterization, showing in 1 the presence of three different Rh–Rh distances,9 we suggest an arrangement of the rhodium metallic centers in tetranuclear cores with oxidation levels {RhI∧ RhII –RhII∧ RhI } or {RhII ∧ RhI – RhI ∧ RhII } (Rh∧ Rh = acetate bridged Rh–Rh bonds). Indeed an alternative disposition like {RhI∧ RhII –RhI∧ RhII } along the chain should generate a paramagnetic system. After a short exposure to air 1 displays a rhombic EPR signal characterized by gx = 2.075, gy = 2.037 and gz = 2.002. This signal originates from an oxidation of some Rh metallic centers in the polymer and can be assigned to the low-spin Rh(II) d7 ions. The rhombic pattern is due to the distorted octahedral geometry around the metallic ions. Even if the majority of Rh(II) compounds described in the literature displays axial EPR signatures, rhombic spectra have already been reported.14,15 This result underlines the molecular oxygen sensitivity of Rh redox states in 1. To compare the electroactivity of samples of 1, obtained by electrochemical reduction of 2a and 2b with the electroactivity of 1 chemically synthesized,9 CVs of powder materials have been investigated using cavity microelectrodes (CMEs) as WEs. Typical voltammogram profiles are presented in Fig. 3. CVs display two close broad quasi-reversible systems at potentials (E 1/2 = −1.10 and −0.68 V) different to those of 1 deposited as thin film on planar classical electrodes (Fig. 2(a)). In a general manner, the origin of the shift potentials and shapes of the powder CVs obtained with CMEs are related to the structural parameters of the compound studied (size and shape of the grains), the configuration of the powder within the cavity (space distribution of the electroactive This journal is © The Royal Society of Chemistry 2008

sites) and also, to the geometry of the CME used (Pt diameter and deepness of the microcavity).16 In order to characterize the film formation onto conductive surfaces, the redox properties of 2a and 2b were investigated by electrochemical quartz crystal microbalance (EQCM) measurements. Typical current–potential and frequency–potential profiles recorded on a gold quartz WE are illustrated in Fig. 4, curves (a) and (b).

Fig. 4 (a) CV of 2a 1 mM in CH3 CN + 0.1 M TBAP, gold quartz electrode (diameter 5.1 mm) between 0 and −0.90 V, m = 100 mV s−1 ; (b) quartz frequency response; (inset) electrodeposition mass–charge plot for cathodic current area.

In the negative area, the CV (curve a) is very much like that on Pt presented in Fig. 1(a). During the first negative scan the reduction-deposition process is activated from −0.82 V and the QCM frequency decreases (Fig. 4(b)). To interpret the EQCM results we have used the Sauerbrey equation17 (cf . Experimental section). The adhered polymer forms a rigid film on the gold surface. The efficiency of the electrodeposition process has been determined from the mass–charge plot (7 mg C−1 ; Fig. 4 inset). The mass change during the electrodeposition process (cathodic current area) has confirmed that the faradic electropolymerization yield was quantitative and identical to that obtained on a Pt electrode (see above). On the return CV scan, from −0.78 V, a main oxidation-desorption process is observed and the frequency Dalton Trans., 2008, 2149–2156 | 2151

increases. The maximum change in WE mass during this CV scan has been estimated at 420 ng (300 Hz) i.e. 2 lg cm−2 . However, as shown in curve (b), the frequency does not return to its original value. This behaviour is due to the slow oxidation process. By using a lower potential scanning rate (10 mV s−1 ) an increase of the mass deposited on the WE (1600 Hz; 2.24 lg) and a complete oxidation desorption process, with a frequency return to the original value at 0 V (0 Hz), are observed. Furthermore, we have observed a linear change of frequency (mass) during a continuous cycling, in the region where the film is formed (−0.75 to −0.90 V; m = 100 mV s−1 ). These results give strong supplementary evidence that the polymer film is rigidly attached to the electrode surface and demonstrates that the deposit mass and the number of CV cycles are linearly related. However, the electrodeposition process is limited, and the optimum mass deposit was 2.94 lg (quartz frequency saturation; Df max = 2100 Hz). Unfortunately, due to the lower adherence of the deposit of 1 on the gold surface, it has not been possible to study MEs in clean electrolytes and consequently, any specific information concerning the charges and ion exchanges in the film during the redox processes have not been obtained. Polymer film formation by reduction of 2a and 2b has been also demonstrated by UV-vis spectroelectrochemical investigations. For instance, during the electrolysis at applied potential (−1.15 V) on an indium tin oxide coated aluminosilicate glass optically transparent electrode (OTE) surface, the absorbance increases over the whole visible range (Fig. 5A; inset).

Fig. 5 (A) UV-vis spectra recorded during the reduction of 2a (1 mM in CH3 CN + 0.1 M TBAP) at −1.15 V on an OTE surface; (inset) absorbance at 475 nm–charge plot; (B) Spectra obtained on the ME (sRh = 5.4 × 10−8 mol cm−2 ) after transfer in a clean CH3 CN + 0.1 M TBAP electrolyte at different applied potentials: (a) −0.85, (b) −1.15 and (c) −1.60 V.

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The intensity of successive curves regularly and quasi-linearly grows (quantitative Faradaic yield) and shows the emergence of broad absorption bands. After the electrodeposition, the solution of binuclear species (2a) was replaced in the spectroelectrochemical cell18 by a clean electrolyte solution and the UV-Vis spectra were recorded at different applied potentials (Fig. 5(a)–(c)). By analogy with the behaviour of Ru–Ru bonded MWs19 we suggest that the oxidation is localized on metallic Rh1.5+ centres along the chains, whereas the reduction leads to radical anions localized on diimine phen ligands. This hypothesis is supported by the absorption changes observed during the reduction and oxidation of the film. While the reduction induces the appearance of characteristic absorptions of the phen• − radical anion above 800 nm (curve (c)),20 the oxidation of the film induces only slight absorbance modifications (curve (a)), in agreement with a metal-localized oxidation process.3

Oxidation of 1 The full oxidation of 1 (eqn (3)), can be achieved by three different ways: (i) an electrochemical oxidation by exhaustive electrolysis, (ii) a chemical oxidation by O2 or a diazonium salt, (iii) an associated chemical–electrochemical oxidation. Oxidation of 1, deposited on a large WE, into 2b has been performed either by a potentiostatic electrolysis (E app = +0.80 V in CH3 CN + 0.1 M TBAP) or a chemical oxidation by bubbling O2 in the electrolyte solution during 24 h. This oxidation can be also performed, in a few seconds, in the presence of an excess of a mild oxidizing agent such as diazonium salt (PhN2 + ).21 Both electrochemical and chemical oxidations induce the cleavage of unbridged metallic bonds in MWs and lead quasi-quantitatively to the regeneration of the soluble binuclear bis-CH3 CN complex (2b). The resulting sample 2b has been identified by its UV-vis spectrum and electrochemical behaviour, which are strictly identical as those of an authentic sample.12 A step-by-step oxidation of a measured amount of 1, anchored on an electrode surface, can be also obtained. A first exhaustive electrolysis, carried out at 0 V in CH3 CN + 0.1 M TBAP, liberates one mol of electron per mol of Rh4 6+ core (coulometry measurement) and leads to a brown–orange solution (broad absorption bands at kmax ∼363 and 417 nm). In view of the oxidation coulometry measurement, the soluble oxidized multimetallic complex may contain tetranuclear Rh4 7+ core with formal Rh1.75+ metallic centers (eqn (4)). The CV of the resulting solution (Fig. 6(b)) exhibits a poorly reversible one-electron per Rh4 7+ core reduction system (E 1/2 = −0.86 V; DE p = 0.18 V), very close to the potential of the 2b irreversible one-electron reduction (Fig. 6(a)). In addition a broad irreversible one-electron oxidation peak (E pa ∼ +0.46 V) has been observed. The subsequent one-electron oxidation at +0.80 V induces the formation of 2b (eqn (5)). The UV-vis spectroscopic data and CV of the fully electrooxidized solution (Fig. 6(c)) are characteristic of 2b. These results support the mechanism of the oxidation of 1 (eqn (4) and (5)) via intermediates containing Rh4 7+ cores (formally Rh1.75+ ). This hypothesis is also supported by experiments conducted in DMF (see below). At this stage of the study, the value of n remains unknown, but the hypothesis of the formation of a mixture of This journal is © The Royal Society of Chemistry 2008

Fig. 6 CVs of (a) 2b, 1.2 mM in CH3 CN + 0.1 M TBAP on a Pt electrode (diameter 5 mm), solution produced after successive exhaustive oxidations of 1 (b) at 0 V and (c) at +0.8 V (b); m = 100 mV s−1 .

oligomer fragments, with n = 1 or n >1, containing tetranuclear core(s) must be considered. {[RhI,II,II,I 4 (l-OOCCH3 )4 (phen)4 ]2+ }n → {[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ }n + n e−

chemical reduction of 2a with chromium(II) acetate in ethanol. By analogy with the Rh–bpy equivalent complex,22 the major electronic band (Fig. 7(a)) has been assigned to the [{RhI,II,II,I 4 (lOOCCH3 )4 (phen)4 }2+ ] core. The CV and voltammogram on RDE of the 1–DMF blue solution containing 0.1 M TBAP exhibit a quasi-reversible oxidation system at E 1/2 = −0.89 V (Fig. 8(a)); DE p = 0.11 V), a potential close to that of the irreversible one-electron reduction of the initial 2b complex. However this solution is unstable, even in the absence of oxygen, and after 60 min an orange–brown solution (broad absorption band around 370 nm; Fig. 7(b)) is produced. This species presents a UV-vis spectrum close to that of {[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ }n which was generated by electro-oxidation of 1 at 0 V in CH3 CN (eqn (4); see above). The RDE voltametry experiments conducted in the latter orange–brown solution (Fig. 8(b)) confirm that 1 was oxidized to {[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ }n , containing the Rh4 7+ core. For this oxidation process the chemical oxidant was probably the residual water of the electrolyte since addition of water, or of a weak acid such as acetic acid, accelerated the process. This type of reactivity is reminiscent of the formation of a blue species during the photocycle for H2 production from mixed-valence Rh complexes.23 However, it should be noted, in our case, that the same evolution occurs in absence of light.

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{[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ }n → 2n [RhII,II 2 (l-OOCCH3 )2 (phen)2 (CH3 CN)2 ]2+ + n e− (5) It should be noted that the growth of electroactive polymer films of 1 can be ensured by reduction of the oligomer fragment mixture. If 1 (chemically or electrochemically synthesized) is insoluble in dry CH3 CN, it can be dissolved in a few seconds in strictly deoxygenated DMF giving a deep blue solution. The resulting UV-vis electronic spectrum presents an intense absorption band at 590 nm and broad bands of lower intensity at ∼450 and ∼770 nm (Fig. 7(a)). A similar UV-vis spectrum has been obtained after the

Fig. 8 CVs (m = 100 mV s−1 ) and voltammogram (x = 360 rad min−1 , m = 10 mV s−1 ) of 1 (1.6 mM) in DMF + 0.1 M TBAP on Pt (diameter 3 mm): (a) blue solution just after dissolution, (b) after 60 min, (c) after exhaustive electrolysis at 0.80 V.

Fig. 7 UV-vis spectra of electrochemically prepared 1 polymer film after dissolution in DMF + 0.1 M TBAP: (a) t = 0, 10, 18, 36 min, (b) 60 min, (c) after exhaustive oxidation at 0.80 V.


The exhaustive electrolysis of the intermediate species {[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ }n at 0.80 V liberates one mol of electron per mol of Rh4 7+ tetranuclear core and leads to the formation of the binuclear Rh(II) complex which contains probably DMF as axial ligands (2c) with a quasi-quantitative Faradaic yield (eqn (5); Fig. 7 and 8, curves (c)). This latter oxidation can be also chemically achieved in the presence of air in a few hours. The direct oxidation of 1 dissolved in DMF into 2c by air has been followed by 1 H NMR and EPR spectroscopies. In strictly deoxygenated and at room temperature the blue solution of 1 in deuterated DMF shows broad NMR signals of phen protons as was previously described in CD3 CN.9,12 After air exposure (few Dalton Trans., 2008, 2149–2156 | 2153

hours), a highly resolved 1 H NMR spectrum, characteristic of a diamagnetic Rh(II) binuclear complex is obtained. X-Band EPR experiments have been performed in frozen DMF solution. 1, in strictly deaerated conditions, is EPR silent as was observed for 1 polymer deposit (see above; fig. 9(B)). After a short exposure to air (1 min), two independent signals are observed showing the formation of a mixture of paramagnetic species. The strongest signal presents g-values (gx = 2.083, gy = 2.033 and gz = 2.002) close to those found on the oxidized powder sample (Fig. 9(A)). A second signal can be observed with visible gx and gy components at 2.102 and 2.022, respectively, and gz component being hidden by the major signal at g = 2. As the time of the exposure to air increases, the intensity of the strongest signal decreases and other features appear. The origin of these later EPR signals comes from oligomers, having different lengths and resulting from the breaking of some Rh–Rh non-supported bonds in 1. After 1 h, a silent EPR spectrum is obtained, in agreement with the formation of the diamagnetic binuclear Rh(II) complex (2c).

tion of the one-dimensional multimetallic Rh complex [Rh2 (lOOCCH3 )2 (phen)2 ]n (PF6 )n . It shows clearly that the electrochemical reduction of 2 causes the formation of an insoluble film of 1 on the WE. This adherent film is removed from the surface by oxidation. According to EPR, 1 H NMR and spectroelectrochemistry data the oxidation of Rh metallic centers in the polymetallic 1 compound, induces the breaking of non-supported Rh–Rh bonds and produces the l-bridged acetate binuclear complex 2. Spectroscopic features as well as electrochemical behaviour show that this oxidation occurs through the formation of intermediates containing a tetranuclear {[RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 ]3+ } core. This oxidation process may be in favour of the following mechanism for the electrochemical synthesis of 1. The initial step involves a metal–metal bond formation leading to a Rh4 7+ tetranuclear species (eqn (6)) containing formal Rh1.75+ metallic centers. This latter species results from the coupling of an electrogenerated one-electron-reduced Rh2 3+ species and Rh2 4+ . Then, the Rh4 7+ intermediate is reduced, by one electron, to Rh4 6+ (eqn (7)) and polymerizes into 1 (eqn (8)). The electropolymerization process is a consequence of the high insolubility of 1 in acetonitrile electrolyte and also of the close reduction potential values of Rh2 4+ and Rh4 7+ species. 2n [RhII,II 2 (l-OOCCH3 )2 (phen)2 (CH3 CN)2 ]2+ + n e− → n [RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 (CH3 CN)2 ]3+ + 2 nCH3 CN

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n [RhI,II,II,II 4 (l-OOCCH3 )4 (phen)4 (CH3 CN)2 ]3+ + n e− → n [RhI,II,II,I 4 (l-OOCCH3 )2 (phen)2 (CH3 CN)2 ]2+

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n [RhI,II,II,I 4 (l-OOCCH3 )2 (phen)2 (CH3 CN)2 ]2+ → {[RhI,II,II,I 4 (l-OOCCH3 )4 (phen)4 ]2+ }n + 2n CH3 CN

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Work is in progress to prepare new Rh–Rh MWs, by using electrochemical synthesis, in view to improve knowledge of metal– metal bonded one-dimensional materials.

Experimental General 1,10-Phenanthroline and [Rh2 (l-OOCCH3 )4 (OH2 )2 ] were obtained from Aldrich and Pressure Chemical Co. respectively and used without further purification. All the supporting electrolytes, tetra-n-butylammonium perchlorate, Bu4 NClO4 (TBAP), and tetra-n-butylammonium hexafluorophosphate, Bu4 NPF6 (TBAPF6 ), were purchased from Fluka and used without purification. HPLC grade acetonitrile (Rathburn), spectrograde or extra-dry dimethylformamide (Acros) were used as received.

Fig. 9 X-Band EPR spectra of (A) powdered 1 recorded at 100 K after a short exposure to air and (B) 1 in frozen DMF solution recorded under deaerated conditions (top) and after exposure to air for 1, 10 and 60 min.

Synthesis of {[Rh4 (l-OOCCH3 )4 (phen)4 ]2+ }n (1) Chemical synthesis of powders 1 was synthesized and characterized as previously described.9

Conclusion

Electrochemical synthesis of films

The present study complements and clarifies the previous reported work on chemical synthesis and structural characteriza-

An exhaustive electrolysis of a solution of 2a or 2b (1–1.5 mM in CH3 CN + 0.1 M TBAPF6 ) at −0.90 V, on a Pt sheet (electroactive

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surface area 5 cm2 ), leads to the quantitative formation of 1 (Faradaic yield ∼100%). The film of 1 deposited on the WE was washed with CH3 CN and dried under vacuum. The electrode was scratched to give a black powder. IR (CsI pellet, cm−1 ): 1636 (vs, br), 1395 (vs, br), 1352 (s), 993 (s), 833 (s), 695 (s), 557 (m). UV-vis (solid state, nm) 490, 805. Anal. Calc. for C56 H44 N8 O8 Rh4 P2 F12 ·2H2 O: C, 39.6; H, 2.86; N, 6.61. Found: C, 39.89; H, 3.41; N, 6.72%.

sources were halogen CLH 500 (20 W) and deuterium CLD 500 lamps with an optical fiber 041.002-UV SN 012105 or using an additional 1 mm quartz immersion probe (Hellma) and an automatic shutter. A thin layer of polymer was electrodeposited on transparent working electrodes which were placed in a conventional sandwich-type spectro-electrochemical cell equipped with a Pt wire counter electrode and a Ag/Ag+ reference electrode.18 IR spectra were recorded with a Perkin-Elmer Spectrum GX FTIR spectrometer.

Synthesis of 2a EQCM 2a was prepared using a adapted method from the literature procedure.24 A mixture of [Rh2 (l-OOCCH3 )4 (OH2 )2 ] (2.0 mmol) and phen (4.0 mmol) was refluxed in ethanol until a deep blue– green solution was formed, then the solution was air-oxidized. NaBF4 (4 mmol) in EtOH–H2 O was added, and the mixture was concentrated. The resulting solid was recrystallized from water giving red needles. Anal. Calc. for C28 H22 B2 F8 N4 O4 Rh2 ·5H2 O: C, 35.48; H, 3.40; N, 5.91. Found: C, 35.15; H, 3.52; N, 5.83%. Electrochemistry All measurements were performed under argon, in a dry-box (Jaram). Cyclic voltammetry (CV) measurements and bulk electrolysis were performed using a PAR Model 173 potentiostat equipped with a digital coulometer. All cyclic voltammograms were recorded with a conventional single-compartment threeelectrode cell. All electrode potentials reported in this work are given relative to an Ag/Ag+ reference (0.01 M in CH3 CN containing 0.1 M supporting electrolyte). Conversion into the ferrocene–ferrocenium reference system can be done by adding −0.087 V. The working electrodes for CV measurements were platinum or GC discs (active surface areas of 0.19 and 0.07 cm2 respectively), glassy carbon disc (active surface area 0.07 cm2 ) for RDE experiments, polished with a 2 lm diamond paste (Mecaprex Presi) and for UV-vis spectroelectrochemistry indium tin oxide coated optically transparent aluminosilicate glass (Aldrich). Exhaustive electrolyses were carried out on a platinum sheet (2 cm2 ). The auxiliary electrode was a Pt wire in acetonitrile + 0.1 M of supporting electrolyte. sRh is the number of mol of metal atom per unit area of electrode and was determined using Faraday’s law (Q/nF) where Q is the electrolysis charge considering an electropolymerization–deposition quantitative yield, as was determined for electrodeposition on Pt electrode. Electrochemistry of solid powder polymer was performed using Pt cavity microelectrodes CMEs16 (diameter 60 lm). The cavity (∼6 × 10−8 cm−3 ) was filled up with powdered material using the electrode as a pestle. The Pt CMEs were obtained from the Cavity Microelectrode Users Network, CNRS-France (http://www.lecso.cnrs.fr/eso/9_um/91_umec_fr/910_umec_ acc_fr.html) Spectroscopy 1

H NMR spectra were recorded on a Bruker Avance 300 MHz, using solvents as internal references. X-Band EPR spectra were recorded on a Bruker ESP 300E spectrometer equipped with a Bruker nitrogen flow cryostat. UV/Vis spectroelectrochemical experiments were carried out with a photodiode array UV-visibleNIR spectrometer MCS 501 UV-NIR (Carl Zeiss). The light


EQCM experiments were carried out using a CHI440 EQCM (Chi Instruments). Cyclic voltammograms were recorded with a potentiostat/galvanostat CHI440 equiped with an external box with oscillator circuitry using time-resolved mode. The working electrode was a 8 MHz quartz crystal disc (diameter 13.7 mm) coated with gold on both sides. The oscillating area and the density of the crystal was 0.205 cm2 and 2.648 g cm−3 , respectively. The mass change is 0.14 ng for 0.1 Hz frequency change. A platinum wire served as a counter electrode and all potentials were measured against an Ag/Ag+ reference electrode.

Acknowledgements This work was supported by a PAI POLONIUM program (Minister of foreign affairs of France) and Ministry of Science and Higher Education of Poland (grant PBZ-KBN-118/T09/09). F. L. acknowledges support from a CNRS postdoctoral fellowship. The authors are also grateful for the help of P. Labbé for EQCM experiments.

Notes and references 1 D. R. Tyler, Frontiers in Transition Metal-Containing Polymers, ed. A. S. Abd-El-Aziz and I. Manners, Wiley-Interscience, Weinheim, 2007, p. 287. 2 J. K. Bera and K. R. Dunbar, Angew. Chem., Int. Ed., 2002, 41, 4453. 3 S. Chardon-Noblat, A. Deronzier and R. Ziessel, Collect. Czech. Chem. Commun., 2001, 66, 207, and references therein. 4 F. Hartl, T. Mahabiersing, S. Chardon-Noblat, P. Da Costa and A. Deronzier, Inorg. Chem., 2004, 43, 7250. 5 S. Myllynen, M. Wasberg and M. Haukka, J. Electroanal. Chem., 2006, 586, 217. 6 S. Luukkanen, P. Homanen, M. Haukka, T. A. Pakkanen, A. Deronzier, S. Chardon-Noblat, D. Zsoldos and R. Ziessel, Appl. Catal., A, 1999, 185, 157. 7 S. Chardon-Noblat, A. Deronzier, R. Ziessel and D. Zsoldos, J. Electroanal. Chem., 1998, 444, 253. 8 G. M. Finniss, E. Canadell, C. Campana and K. R. Dunbar, Angew. Chem., Int. Ed. Engl., 1996, 35, 2772. 9 F. P. Pruchnik, P. Jakimowicz and Z. Ciunik, Inorg. Chem. Commun., 2001, 4, 726. 10 F. P. Pruchnik, P. Jakimowicz, Z. Ciunik, K. Stanislawek, L. A. Oro, C. Tejel and M. A. Ciriano, Inorg. Chem. Commun., 2001, 4, 19. 11 F. P. Pruchnik, A. Jutarska, Z. Ciunik and M. Pruchnik, Inorg. Chim. Acta, 2004, 357, 3019. 12 C. A. Crawford, J. H. Matonic, J. C. Huffman, K. Folting, K. R. Dunbar and G. Christou, Inorg, Chem., 1997, 36, 2361. 13 A. Szymaszek and F. P. Pruchnik, Pol. J. Chem. Phys. Chem., 1992, 66, 1859. 14 M. Gerish, J. R. Krumper, R. G. Bergman and T. D. Tilley, Organometallics, 2003, 22, 47. 15 D. G. H. Hetterscheid, J. M. M. Smits and B. de Bruin, Organometallics, 2004, 23, 4236. 16 V. Vivier, C. Cachet-Vivier, D. Michel, J.-Y. Nedelec and L. T. Yu, Synth. Met., 2002, 126, 253.

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17 G. Sauerbrey, Z. Phys., 1959, 155, 206. 18 G. A. Gruner and T. Kuwana, J. Electroanal. Chem. Interfacial Electrochem., 1972, 36, 85. 19 C. Caix-Cecillon, S. Chardon-Noblat, A. Deronzier, M. Haukka, T. A. Pakkanen, R. Ziessel and D. Zsoldos, J. Electroanal. Chem., 1999, 466, 187. 20 P. S. Braterman, J.-I. Song and R. D. Peacock, Inorg. Chem., 1992, 31, 555.

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21 H. Cano-Yelo and A. Deronzier, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 3011. 22 E. Gałdecka, Z. Gałdecki, F. P. Pruchnik and P. Jakimowicz, Transition Met. Chem., 2000, 25, 315. 23 A. J. Esswein, A. S. Veige and D. G. Nocera, J. Am. Chem. Soc., 2005, 127, 16641. 24 M. Bien, F. P. Pruchnik, A. Seniuk, T. M. Lachowicz and P. Jakimowicz, J. Inorg. Biochem., 1999, 73, 49.
