Synthesis and Electrochemistry of ((Diferrocenylsilyl) propyl)-and

Article pubs.acs.org/Organometallics

Synthesis and Electrochemistry of ((Diferrocenylsilyl)propyl)- and ((Triferrocenylsilyl)propyl)triethoxysilanes ‡ ́ Marta Herrero,† Raquel Sevilla,† Carmen M. Casado,*,† José Losada,‡ Pilar Garcıa-Armada, § §,∥ ,† ́ Antonio Rodrıguez-Dié guez, David Briones, and Beatriz Alonso* †

Departamento de Quı ́mica Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain Departamento de Ingeniería Quı ́mica Industrial, Escuela Técnica Superior de Ingenieros Industriales, Universidad Politécnica de Madrid, 28006-Madrid, Spain § Departamento de Quı ́mica Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain ‡

S Supporting Information *

ABSTRACT: Triferrocenylsilane 2 was synthesized. Hydrosilylation reactions employing allyltriethoxysilane and diferrocenylmethylsilane (1) and triferrocenylsilane (2) yielded new ferrocenyltriethoxysilane compounds functionalized with two (3) and three (4) interacting ferrocenyl units, respectively. Characterization of 2 and the ethoxysilane derivatives 3 and 4 by elemental analysis, 1H, 13C{1H}, and 29Si{1H} NMR spectroscopy, and mass spectrometry supports their assigned structures. The crystal structure of 2 has been determined by a single-crystal X-ray diffraction study. The redox activity of the ferrocenyl centers in 2−4 has been characterized by cyclic voltammetry and square wave voltammetry in dichloromethane containing [n-Bu4N][PF6] or [n-Bu4N][B(C6F5)4] as electrolyte support. Voltammetric studies of 2−4 in solution exhibit the pattern of communicating ferrocenyl sites with two or three distinct, separated oxidation waves. Platinum oxide surfaces are covalently modified by redox-active 3 and 4.

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INTRODUCTION During the last several decades the synthesis of compounds in which two or more metal centers are simultaneously coordinated to a bridge has been a current area of active research,1 because they can be regarded as model compounds for the investigations of metal−metal interactions. The ferrocene unit has proved to be a versatile building block which combines chemical versatility and high thermal stability as well as unique and valuable redox properties. The continued interest in materials containing multiple ferrocene moieties stems in part from their electrochemical (with a very good stability in the neutral as well as oxidized state during one-electron-transfer processes), and optical properties2−5 combined with their potential applications in many fields such as organic synthesis, catalysis, and materials science.6 In this context, many compounds containing multiple ferrocenyl units bridged through carbon-based spacers,7 carbon atoms,8 and heteroatoms (e.g., Si,9 P,10 S,11 B12) have been synthesized. Extensive studies have also been performed on the respective polymeric derivatives with heteroatom bridges.13 As a part of a program focusing on the synthesis and applications of multiferrocenyl derivatives,14 our own group recently reported the synthesis of diferrocenylmethylsilane (1) and its inclusion in different dendritic15 and polymeric16 silicon-based structures. Research in the field of chemically modified electrodes17 has attracted attention over the past 30 years because of their broad and interesting practical applications. It was well-established that © 2013 American Chemical Society

the attachment of organosilane derivatives to a surface oxide via siloxane bonds represents one of the more versatile methods of surface modification.18 Successful silanizations have been carried out on different electrode materials,19 such as Au, Si, Pt, C, etc. Electrochemically reactive organometallic compounds are interesting candidates to attach covalently to an electrode surface and, among others, can be successfully immobilized through a siloxane spacer to a metallic electrode.20,21 Indeed, we have previously reported the chemical derivatization of anodized Pt electrodes and silica surfaces via siloxane linkages, using ((amidoferrocenyl)propyl)diethoxymethylsilane as the ferrocenyl derivative, which resulted in the persistent attachment of the amide-linked ferrocenyl moieties to the silica and platinum surfaces.22 More recently, several research groups have also studied the incorporation and electrochemistry of amidoferrocenyl derivatives in ZnO23 surfaces or silica nanoparticles.24 Examples of electrode surfaces which are covalently modified with ferrocenylsilane derivatives that contain a single redoxactive ferrocene unit have been described.25 However, to the best of our knowledge, no reports exist on such derivatives containing di- or triferrocenyl units. Special Issue: Ferrocene - Beauty and Function Received: May 3, 2013 Published: May 24, 2013 5826

dx.doi.org/10.1021/om400387c | Organometallics 2013, 32, 5826−5833

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formula of 2 was corroborated by MALDI-TOF mass spectrometry, which showed the molecular ion at m/z 584.1. Crystal Structure of 2. The slow diffusion of n-hexane into a concentrated solution of 2 in dichloromethane at room temperature provided orange crystals suitable for a single-crystal X-ray diffraction study. A perspective view of the molecular structure is presented in Figure 1. A summary of crystallographic data and data collection parameters is included in Table S1 (Supporting Information).

We report herein full details on the synthesis of di- and triferrocenyl triethoxysilane derivatives (3 and 4, respectively) via hydrosilylation reactions26 of allyltriethoxysilane with the previously reported 1 and the triferrocenylsilane 2, both functionalized with a Si−H group. In addition, 3 and 4 have been used as derivatizing reagents for anodized Pt electrode surfaces. The X-ray crystal structure of 2 is also reported.

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RESULTS AND DISCUSSION Previously we have synthesized diferrocenylmethylsilane (1; Chart 1), which is accessible by reaction of ferrocenyllithium Chart 1. Diferrocenylmethylsilane 1

with methyldichlorosilane via a salt elimination reaction.15b,27 As a continuation of our search for redox-active molecules containing multiple electronically communicating ferrocenyl groups, we envisaged increasing to the maximum the number of ferrocenyl units in a simple Si−H functionalized ferrocenyl derivative.28 Thus, we have synthesized the silane 2, containing three ferrocenyl units linked by a silicon bridge and a reactive Si− H group. Synthesis of Triferrocenylsilane 2. This new organometallic fragment was synthesized via a salt elimination reaction of ferrocenyllithium (η5-C5H5)Fe(η5-C5H4Li) and trichlorosilane in THF at low temperature (Scheme 1). For this purpose

Figure 1. View of the silicon environment and disposition of the three ferrocenyl groups attached in 2.

In the structure of 2 the cyclopentadienyl rings pertaining to the three ferrocenyl groups attached to the silicon atom have a dihedral angle of 73.29° with respect to the plane formed by the three iron atoms. The cyclopentadienyl rings in the metallocenes are planar and nearly parallel (near 1.5°) and exhibit an almost eclipsed conformation, but in this case this information is not relevant due to the disorder problem present in the cyclopentadienyl rings (C15C16C17C18C19). In the ferrocenyl moieties, the Fe−C bond distances have values in the range 2.021(8)−2.077(7) Å, and the iron atoms are separated by 6.068 Å, similar to the case for other triferrocenylsilane complexes.28,9a The silicon coordination has essentially a tetrahedral SiHC3 geometry in which the C−Si−C bond angle is 109.92(13)° imposed by symmetry and the Si−C bond distance has a value of 1.878(3) Å. Selected bond distances and angles for this compound are given in Table S2 (Supporting Information). Synthesis of ((Diferrocenylmethylsilyl)propyl)triethoxysilane (3) and ((Triferrocenylsilyl)propyl)triethoxysilane (4). Compounds 1 and 2 with Si−H functions were used as hydrosilylation reagents. The hydrosilylation reaction of allyltriethoxysilane using stoichiometric amounts of 1 in toluene, in the presence of Karstedt catalyst at 65 °C overnight, led quantitatively to compound 3 (Scheme 2). As for 3, attempts were made to prepare 4 by hydrosilylation of allyltriethoxysilane with the respective triferrocenylsilane 2, in toluene and in the presence of Karstedt catalyst; however, the reaction did not occur. Efforts to achieve the functionalization by changing the temperature and reaction times were also carried out, without success. The permanence of the Si−H signal in the 1 H NMR spectrum indicated its lack of reactivity under these conditions. Finally, a solvent-free platinum-catalyzed hydrosilylation reaction of allyltriethoxysilane with 2, at 85 °C overnight, led to the formation of 4 in 40% yield. The 1H olefinic resonances were used to confirm the completion of the hydrosilylation reaction.

Scheme 1. Synthesis of Triferrocenylsilane 2

(tri-n-butylstannyl)ferrocene was selected as the starting material in order to generate pure monolithioferrocene. After purification by column chromatography the desired compound 2, possessing three ferrocenyl units attached to the silicon atom, was isolated in high purity as an air-stable orange crystalline solid. The structural identity of the novel molecule 2 was straightforwardly established on the basis of elemental analysis and multinuclear (1H, 13C{1H}, 29Si{1H}) NMR spectroscopy and mass spectrometry and confirmed by X-ray diffraction analysis. The 1H NMR spectrum of 2 shows the pattern of resonances in the range 4.1−4.5 ppm characteristic of the unsubstituted and substituted cyclopentadienyl ligands in the ferrocenyl moieties and a resonance at 5.47 ppm corresponding to the Si−H functionality. The 29Si{1H} spectrum of this molecule shows only one signal at −21.7 ppm. The molecular 5827


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Scheme 2. Synthesis of 3 and 4

Scheme 3. Alternative Method for the Synthesis of 4

signals expected for the two different types of silicon atoms in the molecules. Thus, the silicon atoms bridging two or three ferrocenyl units resonate at −6.7 ppm (for 3) and −10.4 ppm (for 4), and a singlet is observed at −45.2 ppm (for 3) and at −45.3 ppm (for 4) corresponding to the T-type silicon atoms of the ethoxysilyl group.30 The structures of 3 and 4 were corroborated by FAB or MALDI-TOF mass spectrometry, which showed the molecular ions at m/z 618.4 and 788.2, respectively, as well as in the case of 3 some informative peaks assignable to reasonable fragmentation products. Electrochemical Behavior in Solution. The electrochemical behavior of the synthesized ferrocenylethoxysilanes 3 and 4 and that of the triferrocenyl derivative 2 has been investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV) in dichloromethane solution. The electrochemical parameters of the redox processes exhibited by 2−4 are summarized in Table 1 together with those of 1, measured in the same medium. The bimetallic 3 exhibits cyclic voltammograms in CH2Cl2 solution with [n-Bu4N][PF6] as supporting electrolyte (Figure 2) characterized by two well-separated and reversible oxidation waves of equal intensity, at 0.432 and 0.628 V (vs SCE). For both oxidation waves, the peak current ipa increased linearly with the square root of the scan rate (v1/2), indicating that the electron transfer is diffusion-controlled. The square wave voltammogram in this medium shows two separated oxidation waves of the same area, indicating that an equal number of electrons are transferred in both redox processes. This two-wave

After the appropriate workup, the crude products were purified by column chromatography with hexane as eluent by using special silanized silica in order to avoid the facile reaction of the ferrocenyl derivatives with the silica surface hydroxylic groups. Solvent removal afforded the desired di- and triferrocenyl derivatives 3 and 4, as red-orange oils. A second approach involving the prior hydrosilylation of the allyltriethoxysilane precursor with trichlorosilane and the subsequent reaction with ferrocenyllithium29 was performed with the aim of increasing the yield of 4 (Scheme 3). Surprisingly, the desired compound 4 obtained by this alternative procedure was always contaminated with compounds resulting from the hydrolysis of the ethoxy groups, as indicated by the MALDI mass spectra. All attempts to separate this derivative failed. The novel compounds 3 and 4 were characterized by 1H, 13 C{1H} and 29Si{1H}NMR spectroscopy and mass spectrometry, which confirmed the proposed structures. 1H NMR spectra of 3 and 4 show in all cases the pattern of resonances in the range 4.0−4.4 ppm characteristic of the unsubstituted and substituted cyclopentadienyl ligands in the ferrocenyl moieties. Moreover, the 1H NMR spectra display resonances corresponding to the protons of the propyl chain −(CH2)3−, in the region 1.6−0.6 ppm, which have been unambiguously assigned. The signals of the ethoxysilyl groups appear at around 3.8 and 1.2 ppm. The structures of 3 and 4 were also confirmed by the 13C NMR spectra, which display exclusively the resonances expected for the different carbon atoms (see the Experimental Section); the absence of characteristic allyl signals confirms complete hydrosilylation. The 29Si{1H} NMR spectra display only the 5828


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Table 1. Electrochemical Data for 1−4a,b compd

E°1, V

E°2, V

1 2 3 4

0.463 0.482 0.432 0.432

0.624 0.638 0.628 0.617

E°3, V 0.760 0.764

ΔE (E°2 − E°1), mV 161 156 196 185

Nevertheless, in dichloromethane with [n-Bu4N][B(C6F5)4], the CV of 2 shows three well-separated, reversible oxidation− reduction waves (Figure 3C),35 at E°1 = 0.366, E°2 = 0.642, and E°3 = 0.935 V, resulting in the formation of the mono-, di-, and tricationic species [2]+, [2]2+ and [2]3+, respectively. The square wave voltammogram of 2 is also despicted in Figure 3D. With its better resolution it confirms three separated well-shaped oneelectron processes, indicating a good interaction between the ferrocenyl termini. While the reversibility of the oxidation and reduction processes were comparable in both electrolyte systems, the relative potentials and separations of the redox processes differed markedly. Observed shifts of ΔE°2−1 = E°2 − E°1 = 120 mV and ΔE°3−2 = E°3 − E°2 = 171 mV, when going from [PF6]− to [B(C6F5)4]− media, are consistent with increasingly reduced ion pairing of the FeIII centers with the borate anion (in comparison to hexafluorophosphate) as the ferrocenyl groups are progressively oxidized. Clearly, in agreement with the results reported by Geiger and co-workers,35 the combination of dichloromethane and [B(C6F5)4]− electrolyte anion provides more favorable conditions for electrochemical studies for polyferrocenyl compounds, minimizing ion-pairing interactions. The spreads of the three potentials in triferrocenylsilane 2 are 0.278 V (dichloromethane/[n-Bu4N][PF6]) and 0.569 V (dichloromethane/[n-Bu4N][B(C6F5)4]), implying appreciable interactions between the ferrocenyl moieties as they are successively oxidized. The triethoxysilane derivative 4 also exhibits three diffusioncontrolled ferrocenyl-related redox events independent of the electrolyte used (see Table 1 and Table S14 in the Supporting Information). Electrochemical Behavior of Derivatized Pt Electrodes. We have studied the immobilization of 3 and 4, which possess the −Si(OEt)3 functionality, on platinum oxide surfaces. The results are consistent with covalent bonding between the ferrocenyl moieties and the surface, via siloxane linkages, as illustrated in Scheme 4. The PtO layer of the electrode was prepared as described in the literature (see the Experimental Section).36 Functionalization of PtO surfaces with 3 and 4, 3/PtO and 4/ PtO (see the Experimental Section), results in detectable electroactive materials attached on the electrode surface which have been characterized by cyclic voltammetry in fresh dichloromethane solutions containing only supporting electrolyte. As shown in Figure 4, the voltammetric response of a 3/PtO electrode in CH2Cl2 with 0.1 M [n-Bu4N][PF6] is reminiscent of the corresponding compound 3 in solution (Table 1) displaying two successive well-defined reversible oxidation−reduction waves at formal potential values of E°1 = 0.443 and E°2 = 0.569 V due to the existence of appreciable interactions between the two iron centers. The linear dependence of peak current on v, found for all oxidation and reduction peaks (Figure 4), is typical for surface-confined electroactive species.37 The peak separation at a scan rate of 100 mV s−1 is approximately 58 mV for the first wave and 61 mV for the second wave. The amount of material bound to the electrode surface (Γ) is determined by measuring the amount of charge associated with the oxidation of the redox centers, by integrating the area under the oxidation waves, and was found to be 4.98 × 10−10 (mol of Fc)/cm2.37 With respect to stability, the shape of the features in the cyclic voltammograms is independent of the scan rate, and essentially no decay of electroactivity during continuous scanning for the 3/PtO electrodes over hours is observed.

ΔE (E°3 − E°2), mV 122 147

a

Obtained in dichloromethane solution containing 0.1 M [nBu4N][PF6]. bFormal potentials were calculated from the SWV peak potentials.

Figure 2. Cyclic (scan rate 100 mV/s) and square wave voltammograms at a Pt-disk electrode of 3 in CH2Cl2/[n-Bu4N][PF6].

redox response is similar to that found in other diferrocenylsilanes.9,15,27,31,32 The redox splitting between the first and second redox processes33 (ΔE°2−1 = E°2 − E°1) is 196 mV. The value of the comproportionation constant Kc (Kc = exp(ΔE)F/RT) of 2058 allows us to classify [3]+ as a partially delocalized (on the electrochemical time scale) Robin-Day class II system.34 This suggests the presence of significant electronic interactions between the two ferrocenyl units linked together by the bridging silicon atom, also observed in the starting species 1 (Table 1). The voltammetry of triferrocenylsilane 2 displays three reversible oxidation waves in CH2Cl2 solutions containing either [PF6]− or [B(C6F5)4]− electrolyte anions (Figure 3). When [nBu4N][PF6] is used as the supporting electrolyte in dichloromethane, the CV of 2 shows three closely spaced waves having diffusional features (Figure 3A). The diffusion-controlled shapes of all three waves for the experiment with [PF6]− media attest to the fact that even the tricationic [2]3+ is soluble, in contrast to the clear evidence of precipitation of the trication and subsequent cathodic stripping on the return sweep for other related triferrocenyl derivatives in the same media.28,9a Likewise, square wave voltammetric measurements exhibit three slightly separated oxidation events (Figure 3B). 5829


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Figure 3. (left) Cyclic (scan rate 100 mV/s) (A) and square wave voltammograms (B) at a Pt-disk electrode of 2 in CH2Cl2/[n-Bu4N][PF6]. (right) cyclic (scan rate 100 mV/s) (C) and square wave voltammograms (D) at a Pt-disk electrode of 2 in in CH2Cl2/[n-Bu4N][B(C6F5)4].

Scheme 4. Model for Attachment of 3 and 4 to an Anodically Generated Pt Oxide Electrode

currents decay during continuous scanning, indicating that platinum surfaces derivatized with 4 are qualitatively less durable than those using 3. Three successive symmetrical, reversible oxidation−reduction waves of equal intensity are observed, with formal potential values of E°1 = 0.463, E°2 = 0.603 V, and E°3 = 0.729 V (see Figure S15 in the Supporting Information).

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CONCLUSIONS

In summary, we have prepared and adequately characterized the triferrocenylsilane 2, containing a Si−H function, and di- and triferrocenyl derivatives 3 and 4, possessing triethoxysilane groups. Solution voltammetric studies of 3 show two wellseparated reversible redox processes, indicating electronic communication between the metal sites. The solution electrochemistry of the triferrocene derivatives 2 and 4 in different supporting electrolytes shows three diffusional redox processes, with the more favorable conditions being provided by using a very poorly ion pairing anion of low nucleophilicity. In addition, we have demonstrated the feasibility of modifying platinum oxide electrode surfaces with stable electroactive films of the

Figure 4. Cyclic voltammograms at different scan rates of a 3/PtO electrode in CH2Cl2/0.1 M [n-Bu4N][PF6].

In contrast, even though the most forcing conditions are used in the derivatization procedure, for 4/PtO electrodes the peak 5830


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PLATON-94.44 Refinement reduced R1 to 0.057. Final R(F), Rw(F2), and goodness of fit agreement factors and details of the data collection and analysis can be found in Table S1 (Supporting Information). Crystallographic data (excluding structure factors) for the reported structure have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 930257. Copies of the data can be obtained free of charge upon application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223 336-033; e-mail, [email protected]). Synthesis of Triferrocenylsilane 2. A solution of (tri-nbutylstannyl)ferrocene (5.3 g, 11.15 mmol) in 15 mL of THF was cooled to −78 °C and treated with n-butyllithium (6.9 mL, 1.6 M solution in hexane). The reaction mixture was stirred at that temperature for 90 min and then warmed to −20 °C, and a THF solution of trichlorosilane (0.2 mL, 1.98 mmol) was added dropwise. The mixture was stirred for 1 h, warmed to room temperature, and stirred for a further 24 h. The solvent was removed under vacuum, the orange residue was treated with hexane, and the solution was then filtered to remove the lithium chloride byproduct. Solvent was removed under vacuum, and the orange residue was chromatographed on a silica gel column using hexane and a hexane/dichloromethane mixture as eluents. Elution with hexane produced a yellow band, which gave ferrocene. The orange band eluted with hexane/dichloromethane 5/1 afforded compound 2 as an orange air-stable crystalline solid, mp 188 °C. Yield: 0.76 g (1.30 mmol, 66%). Anal. Calcd (found) for C30H28Fe3Si: C, 61.68 (61.43); H, 4.83 (4.81). 1H NMR (CDCl3, 300 MHz): δ 5.47 (s, 1H, SiH), 4.41 (t, 6H, C5H4), 4.32 (t, 6H, C5H4), 4.12 (s, 15H, C5H5). 13 C{1H} NMR (CDCl3, 75 MHz): δ 74.20, 70.99, 66.36 (C5H4), 68.69 (C5H5). 29Si{1H} NMR (CDCl3, 59.3 MHz): δ −21.7 (SiH). MS (MALDI-TOF): m/z 584.1 [M+]. Synthesis of ((Diferrocenylmethylsilyl)propyl)triethoxysilane (3). A 50 μL portion of Karstedt’s catalyst was added to 0.11 g (0.53 mmol) of allyltriethoxysilane, and the mixture was stirred for 5 min at room temperature. To this solution was added dropwise diferrocenylmethylsilane (1; 0.20 g, 0.48 mmol) in dry toluene (5 mL), and the mixture was stirred for 12 h at 65 °C and then cooled to room temperature. The solvent was removed under vacuum. The reddish brown oil residue was treated with hexane and subjected to column chromatography on silanized silica. The orange band eluted with hexane afforded compound 3 as an orange oil. Yield: 0.27 g (0.43 mmol, 90%). 1 H NMR (CDCl3, 500 MHz): δ 4.33 (m, 4H, C5H4), 4.14 (m, 4H, C5H4), 4.09 (s, 10H, C5H5), 3.82 (q, 6H, OCH2CH3), 1.65 (m, 2H, CH 2 CH 2 CH 2 SiO), 1.23 (t, 9H, OCH 2 CH 3 ), 1.06 (m, 2H, CH2CH2CH2SiO), 0.77 (t, 2H, CH2CH2CH2SiO), 0.48 (s, 3H, SiCH3). 13C{1H} NMR (CDCl3, 125 MHz): δ 73.91, 71.16 (C5H4), 68.88 (C5H5), 58.35 (SiOCH2CH3), 20.23 (CH2CH2CH2SiO), 18.41 (SiOCH2CH3), 17.91 (CH2CH2CH2SiO), 15.11 (CH2CH2CH2SiO), −2.66 (SiCH3). 29Si{1H} NMR (CDCl3, 99 MHz): δ − 6.7 (SiCH3), −45.2 (SiO). MS (FAB+): m/z 618.4 [M+]. Synthesis of ((Triferrocenylsilyl)propyl)triethoxysilane 4: Method A. In a Schlenk flask, allyltriethoxysilane (0.56 g, 1.36 mmol), 2 (0.19 g, 0.33 mmol), and 50 μL of Karstedt’s catalyst were mixed and stirred overnight at 85 °C. The volatiles were removed in vacuo to give a red oil, which was treated with hexane. After purification by column chromatography using hexane as eluent, 4 was isolated as an air-stable orange solid. Yield: 0.10 g (0.13 mmol, 40%). 1H NMR (CDCl3, 500 MHz): δ 4.36 (m, 6H, C5H4), 4.24 (m, 6H, C5H4), 4.01 (s, 15H, C5H5), 3.87 (q, 6H, OCH2CH3), 1.96 (m, 2H, CH2CH2CH2SiO), 1.38 (m, 2H, CH2CH2CH2SiO), 1.27 (t, 9H, OCH2CH3), 0.90 (t, 2H, CH2CH2CH2SiO). 13C{1H} NMR (CDCl3, 125 MHz): δ 73.85, 70.45, 40.26 (C 5 H 4 ), 68.47 (C 5 H 5 ), 58.39 (SiOCH 2 CH 3 ), 20.16 (CH2CH2CH2SiO), 18.47 (SiOCH2CH3), 18.29 (CH2CH2CH2SiO), 15.65 (CH2CH2CH2SiO). 29Si{1H} NMR (CDCl3, 99 MHz): δ − 10.4 (SiFc), − 45.2 (SiO). MS (MALDI-TOF): m/z 788.2 [M+].

triethoxysilane derivatives 3 and 4 covalently bound through siloxane coupling.

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EXPERIMENTAL SECTION

General Methods. All reactions were performed under an inert atmosphere (prepurified N2 or Ar) using standard Schlenk techniques. Solvents were dried by standard procedures over the appropriate drying agents and distilled immediately prior to use. Diferrocenylmethylsilane15b and (tri-n-butylstannyl)ferrocene38 were prepared as described in the literature with slight modifications. Electrochemical Measurements. Electrochemical measurements were performed using an Autolab PGSTAT 128N instrument. CH2Cl2 (spectrograde) for electrochemical measurements was freshly distilled from calcium hydride under nitrogen. The supporting electrolytes used were tetra-n-butylammonium hexafluorophosphate ([n-Bu4N][PF6]), which was purchased from Fluka, and purified by recrystallization from ethanol, and dried under vacuum at 60 °C, and tetra-n-butylammonium tetrakis(pentafluorophenyl)borate ([n-Bu4N][B(C6F5)4]), which was synthesized as described in the literature35,39 by metathesis of an aqueous solution of Li[B(C6F5)4]·nEt2O (Boulder Scientific) with [nBu4N]Br in methanol and recrystallized twice from CH2Cl2/Et2O. The supporting electrolyte concentration was typically 0.1 M. A conventional sample cell operating under an atmosphere of prepurified nitrogen was used for cyclic voltammetry. All cyclic voltammetric experiments were performed using either a platinum-disk working electrode (A = 0.020 cm2) or a glassy-carbon-disk working electrode (A = 0.070 cm2), each of which was polished prior to use with either 0.05 μm alumina/water slurry or 1 μm diamond paste (Buehler) and rinsed thoroughly with purified water and acetone. All potentials are referenced to the saturated calomel electrode (SCE). A coiled platinum wire was used as a counter electrode. Solutions for cyclic voltammetry were typically 0.5 mM in the redox-active species and were deoxygenated by purging with prepurified nitrogen. No iR compensation was used. SWV was done with a step potential of 1 mV, square wave frequency of 25 Hz, and square wave amplitude of 10 mV. Preparation of Derivatized Pt Electrodes. For the chemical modification of the platinum surface via silanization, the Pt electrodes were anodized at +1.9 V vs SCE in 0.5 M sulfuric acid to form an oxide layer at the surface. This was them followed by cycling between +1.23 and −0.25 V at 100 mV s−1 until constant cyclic voltammograms were obtained. The potential was finally held at +1.1 V until the current decayed to small values. The platinum electrode containing the oxide layer was then removed from the electrochemical cell, washed thoroughly with distilled water and methanol, and dried at 50 °C in a vacuum oven for 1 h. The PtO electrodes were placed in Schlenk tubes maintained under argon and containing the triethoxysilyl compound 3 or 4 in toluene solution. The reaction was carried out at 75 °C for times varying from a few hours to a few days. After derivatization, the electrodes were rinsed several times with dry dichloromethane and dried in an argon stream. The samples were then stored under argon until used. X-ray Structure Determination of 2. Suitable crystals of 2 were mounted on glass fibers and used for data collection. Data were collected with a Bruker AXS APEX CCD area detector equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) by applying the ωscan method. The data were processed with APEX240 and corrected for absorption using SADABS.41 The structure was solved by direct methods using SIR97,42 revealing positions of all non-hydrogen atoms. These atoms were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters.43 All hydrogen atoms were located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 times those of the respective atom. The structure has disorder problems with one of the cyclopentadienyl rings (C15C16C17C18C19) in the metallocene. We have treated the twin problem (Figure S13, Supporting Information). Attempts to identify the solvent molecules (dichloromethane) failed in compound 2. Instead, a new set of F2 (hkl) values with the contribution from solvent molecules withdrawn was obtained by the SQUEEZE procedure implemented in

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and a CIF file giving additional experimental details, NMR and mass spectra for 2−4, X-ray crystallographic 5831


Organometallics

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data for 2, and additional CVs and SWVs for 3 and 4. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.A.); carmenm.casado@uam. es (C.M.C.). Present Address

∥ Departamento de Tecnologı ́a Quı ́mica y Energética, Universidad Rey Juan Carlos, 28933 Móstoles, Madrid, Spain.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Ministerio de Ciencia e Innovación (CTQ2009-12332-C02). M.H. acknowledges the Ministerio de Ciencia e Innovación for a predoctoral fellowship.

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REFERENCES

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Organometallics

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Synthesis and Electrochemistry of ((Diferrocenylsilyl) propyl)-and

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