Redox-Active Silica Nanoparticles. 9.1 Synthesis, Electrochemistry

Article pubs.acs.org/Organometallics

Redox-Active Silica Nanoparticles. 9.1 Synthesis, Electrochemistry, and Diffusion Properties of Caged Octakis(N‑ferrocenoyl-3aminopropyl)silsesquioxane David Ruiz Abad,† Jörg Henig,† Hermann A. Mayer,*,† Thomas Reißig,‡ and Bernd Speiser*,‡ †

Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

‡

ABSTRACT: The caged, cube-shaped silsesquioxane T8Fc8 with eight attached ferrocenoyl units has been synthesized. Its structure is supported by NMR and IR spectroscopy as well as mass spectrometry. Electrochemical results show separate signals for adsorbed and freely diffusing molecules. Estimation of the diffusion coefficient and comparison of electrochemical and PGSE NMR data lead to the conclusion that the ferrocene units undergo stepwise one-electron transfers with closely spaced formal potentials, where possibly not all electroactive moieties are oxidized.

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INTRODUCTION Silsesquioxane derivatives have been used in a variety of contexts in both molecular science,2−5 e.g. as catalysts, and materials science, as building blocks,6,7 or to support effects such as electrochromism,8 sensing with modified electrodes,9 and hole transport in organic light emitting diode materials.10 Here, we concentrate on derivatives of the cubic silsesquioxanes Si8O12R8 (T8R8). In particular, their ability to accommodate eight functional units R is of great interest. Successful catalytic11 and medical diagnosis12,13 applications have been reported. Our present work explores nonporous silica nanoparticulate materials which are surface modified by redox-active molecules (see ref 1 and earlier papers of this series). Spherical particles with sizes down to ∼50 nm have been achieved with the Stöber process,14,15 and covalent surface attachment by both condensation (resulting in Si−O−Si attachment)16 and alkene hydrosilylation (resulting in Si−C attachment)1,17−19 was successfully used to immobilize ferrocene,16,17,19 organometallic catalysts,16,19 and organic molecules.1,16,20 Electrochemical data support a structural model of particles immobilized on the electrode and redox processes that include electron-hopping transport on the particle surface. Silica particles with a redoxactive shell and comparable or even smaller sizes have been characterized by the Murray21 and Reinhoudt groups22 with similar results. Cubic silsesquioxanes can be regarded as molecular models of spherical silica nanoparticles with very small diameters (∼2 nm). In particular, their high symmetry resembles the spherical character of the larger Stöber particles. Thus, the investigation of the behavior of T8R8 with redox-active R might extend our © 2014 American Chemical Society

basic knowledge of highly structured modified silica materials and support their application. Ferrocene (Fc) units attached to the cubic core provide a simple example. Fc derivatives of cubic silsequioxanes with a dendrimeric structure of various generations have been described.23−27 In these papers, the electronic interactions between Fc units on the same “branch” of the dendrimer were highlighted. In contrast, our target molecule with R = −(CH2)3NHCOFc, designated as T8Fc8, will bear only a single Fc per side chain, and thus the interaction between the redox-active sites attached to the eight corners of the silsesquioxane cube is the goal of investigation. We are interested in the oxidation process of T8Fc8, in particular the question whether individual electron transfers can be observed or potential inversion phenomena occur.

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EXPERIMENTAL SECTION

General Instrumentation, Chemicals, and Methods. All manipulations were performed under an atmosphere of dry argon employing usual Schlenk techniques. If not otherwise stated, the solvents were dried according to common methods, distilled, and stored under argon. (3-Aminopropyl)triethoxysilane and Amberlite IRA-400 were purchased from Aldrich. Ferrocenecarboxylic acid and ammonia solution (ca. 25%) were bought from Fluka. IR data were obtained on Bruker Vertex 70 and Bruker Tensor 27 spectrometers. Elemental analyses were performed using a Vario EL analyzer. Mass spectra were recorded on a Bruker Esquire 3000+ mass analyzer equipped with an Special Issue: Organometallic Electrochemistry Received: January 15, 2014 Published: March 27, 2014 4777

dx.doi.org/10.1021/om5000419 | Organometallics 2014, 33, 4777−4783

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electron spray ionization (ESI) source. High-resolution mass spectroscopy analyses (HR ESI-MS) were carried out on a Bruker Daltonics APEX II Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. FAB analyses were carried out with a Thermo Finnigan TSQ70 mass spectrometer. For the FAB measurements, 3-nitrobenzyl alcohol (NBA) was used as the matrix. Solution nuclear magnetic resonance spectra (NMR) were recorded on Bruker DRX 250 and Bruker Avance II 400 spectrometers at 298 K. Frequencies and standards were as follows: 1H NMR, 250.13 and 400.13 MHz; 13C{1H} NMR, 62.90 and 100.62 MHz; 29Si{1H} NMR, 49.66 and 99.36 MHz. Chemical shifts are reported in δ values in ppm relative to external tetramethylsilane (TMS) using the chemical shift of the solvent 2H resonance frequency. For the 29Si{1H} experiments the DEPT45 pulse sequence was applied. Solid-state NMR spectra were recorded on a Bruker ASX 300 multinuclear spectrometer equipped with a wide-bore magnet. Proton high-power decoupling (HPDEC) was applied. Magic angle spinning was performed at 4.5 kHz (29Si) and 10 kHz (13C), respectively. Chemical shifts are reported in δ values in ppm relative to external TMS, using Q8M8 (low-field peak at 12.05 ppm) and glycin (176.03 ppm) as secondary reference. For the 1H PGSE NMR measurements a saturated solution of T8Fc8 in dry deuterated DMSO was used. The measurements were performed by using the standard stimulated echo pulse sequence on a 500 MHz Bruker Avance II+ spectrometer equipped with a gradient unit and a multinuclear inverse probe with a Z-gradient coil. The temperature was 298 K, and the sample was not spun. The shape of the gradient pulses was rectangular, their duration (δ) was 2.5 ms, and their strength (G) was varied during the experiments. The delay between the midpoints of the gradients (Δ) was set to 250 ms. All spectra were recorded using 128 scans, 32 K points, a relaxation delay of 3 s, and a spectral width of 7500 Hz and were processed with a line broadening of 2. The intensity of the 1H resonance of the methylene group closest to the silsesquioxane core was analyzed in all spectra. The plots of ln(I/I0) versus G2 were fitted using a linear regression algorithm to obtain the diffusion coefficient D according to eq 1, where I = intensity of the observed spin−echo, I0 = intensity of the spin−echo without gradients, γ = magnetogyric ratio, and δ = length of the gradient pulse.

⎛ δ⎞ ln(I /I0) = − (γδ)2 D⎜Δ − ⎟G2 ⎝ 3⎠

The column was washed several times with cold methanol, and the collected methanol fractions were combined. This solution contained the deprotected octakis(propylamine)silsesquioxane T8Amine8. A part of this solution was taken to remove the solvent, giving 100 mg (0.113 mmol) of octakis(propylamine)silsesquioxane, which was dissolved in 50 mL of dry THF. A 0.320 mL portion of diisopropylethylamine and 280 mg (1.13 mmol) of ferrocene acyl chloride were added to this solution. The reaction mixture was stirred for 2 days, and the solvent was removed. The solid part was washed several times with chloroform to eliminate the excess of ferrocene acyl chloride and with methanol to eliminate the nonreacted octakis(propylamine)silsesquioxanes as well as partially substituted silsesquioxanes. Yield: 126 mg (43%). Mp: 247 °C dec. 1 H NMR (400 MHz, DMF-d7): δ 0.89 (m, 16 H, SiCH2), 1.87 (m, 16 H, SiCH2CH2CH2), 3.46 (m, 16 H, SiCH2CH2CH2), 4.37 (s, 40 H, C5H5), 4.53 and 5.10 (m, 32 H, C5H4). 13C CP/MAS NMR (50.32 MHz): δ 11.8 (s, SiCH2), 25.8 (s, SiCH2CH2CH2), 45.6 (s, SiCH2CH2CH2), 65.8−82.4 (br, C5H4, C5H5), 173.9 (s, CO). 29Si CP/MAS NMR (59.62 MHz): −64 to −68. IR (KBr, cm−1): ν(C−H) 2926, ν(CO) 1635, δ(NH) 1543, νAS(Si−O−Si) 1116, δ(Si−O−Si) 486. Anal. Calcd for C112H128Fe8N8Si8O20: C, 52.19; H, 5.01, N, 4.35. Found: C, 51.31; H, 4.75, N, 3.96. HR ESI-MS (positive mode), m/z 1300.6080 [M + H + Na]2+, calcd for [C112H129Fe8N8Si8O20N]2+ 1300.6097. Electrochemical Experiments. Dimethylformamide (DMF) was first dried three times for 2−3 days over molecular sieves (3 Å; Merck; activated for 12−15 h at 400 °C under vacuum). For 2 L of DMF, 100 g of molecular sieves was used, and the drying agent was replaced for each step. The predried DMF was distilled at ∼100 mbar through a 1 m (diameter 5 cm) column filled with glass rings. The main fraction comprises about 60%. Dimethyl sulfoxide (DMSO) was purified by a combined freezing/ distillation procedure.29,30 A 500 mL portion of DMSO was cooled to 6 °C in a 1 L flask for 6−8 h. About 80% of the liquid had then formed clear, needle-shaped crystals. The remaining liquid phase was decanted, and the crystals were melted at room temperature. The resulting liquid was refluxed at ∼90 °C and a pressure of ∼400 mbar for 2 h and then distilled until ∼90% of the DMSO was collected. Finally, another freezing step similar to the first one was performed. Cyclic voltammetry (CV), chronoamperometry (CA), and differential pulse voltammetry (DPV) were performed at room temperature and under an argon atmosphere. A gastight full glass cell with three electrodes was used. The working electrode was a Pt-disk electrode tip by Metrohm (nominal diameter 3 mm; electroactive area from ferrocene voltammograms recorded with various concentrations and scan rates in DMF/0.1 M NBu4PF6: A = (7.9 ± 0.9) × 10−2 cm2) mounted on a Teflon-coated steel shaft. The Pt disk was polished prior to measurement with 0.3 μm α-Al2O3 (Buehler). The counter electrode was a Pt wire (diameter 1 mm). A Haber−Luggin dual reference electrode system31 with a Ag/Ag+ (0.01 M AgClO4 in 0.1 M NBu4PF6/CH3CN) electrode was used. All potentials were rescaled to the Fc/Fc+ potential, E0(Fc/Fc+) vs Ag/Ag+, measured in separate, reproducible cyclic voltammetric experiments (external reference

(1)

Syntheses. Octakis(3-chloroammoniumpropyl)silsesquioxane (T8(NH3Cl)8). This compound has been prepared according to reported procedures.28 Synthesis of T8Fc8. The ion exchanger Amberlite IRA-400 was activated by means of washing successively with water, a solution of NaOH (1 M), and methanol. A part of this ion exchanger was deposited inside a column, whereas the rest of it was used suspended in ice-cold methanol, where 0.6 g of T8(NH3Cl)8 was dissolved. This suspension was stirred for 15 min and then poured into the column.

Scheme 1. Synthesis of T8Fc8

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system). In DMF, the result was E0(Fc/Fc+) = 0.075 ± 0.001 V; in DMSO E0(Fc/Fc+) = 0.051 ± 0.002 V was found. In all electrochemical experiments, NBu4PF6 (synthesized according to ref 32 with minor modifications) was used as the supporting electrolyte in a concentration of 0.1 M. CV, CA, and DPV were performed with a Bioanalytical Systems BAS 100 B/W electrochemical workstation and evaluated with the respective instrument software.

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RESULTS AND DISCUSSION Synthesis of T 8 Fc 8 . Freshly prepared octakis(3aminopropyl)silsesquioxane (T8Amine8) was treated with ferrocene acyl chloride in THF in the presence of diisopropylethylamine. Pure octakis(N-ferrocenoyl-3aminopropyl)silsesquioxane (T8Fc8) could be isolated in good yield (Scheme 1). The NMR spectra of the polyhedral silsesquioxane T8Fc8 confirm the structure of the compound displayed in Scheme 1. The number of multiplets in the 1H NMR spectrum and the resonances in the 13C{1H} NMR spectrum of T8Fc8 agree with a high symmetry and thus with a complete functionalization of all eight corners of the T8 cage. In the 1H NMR spectrum of T8Fc8 three broad peaks were detected for the propylene protons at room temperature. Increasing the temperature of the sample to 100 °C gradually reduces the line broadening without a complete resolution of the multiplet structure. In addition to the hindered rotation of the functional groups, a reduction of the transverse relaxation time (T2) may be responsible for the line broadening, because T8Fc8 becomes so large that slow tumbling of the molecule leads to a small correlation time.13 This is supported by the fact that it was not possible to detect a 13 C NMR spectrum of T8Fc8 due to the large T1 and small T2 relaxation times, a behavior typical for molecules with large molecular mass. Unfortunately, the solubility of the ferrocenesubstituted silsesquioxane T8Fc8 is too poor to achieve a solution 29Si NMR spectrum in a reasonable time. In the IR spectrum of T8Fc8 the T8 cage gives rise to asymmetric stretching frequencies of the Si−O−Si units between 1124 and 1109 cm−1, while the Si−O−Si bending modes are observed between 486 and 466 cm−1.33 Moreover, the carbonyl stretching frequency at 1635 cm−1 provides evidence that the ferrocenoyl groups have been efficiently bound to the amine functions of the silsesquioxane T8Amine8. Combustion analyses of the silsesquioxane T8Fc8 as well as high-resolution ESI mass spectrometry verify the elemental composition of the compound and prove that it can be synthesized with high purity. In general, the molecular symmetry of octahydrosilsesquioxanes in the crystalline phase is reduced34 by small distortions of the cube, explained as a result of packing processes. Thus, it is not surprising that such distortions increase upon substitution of hydride by a sterically more demanding group as in T8Fc8. Moreover, similar to the hindrance of rotation around a C−C bond by larger substituents, a number of symmetrically independent molecules are generated. This leads to an increase of the number of resonances in the 29Si solid-state NMR spectrum in the typical region for T3 groups (Figure 1). For the same reason, the number of resonances for a single carbon site increases, resulting in broad lines in the 13C CP/MAS NMR spectrum of T8Fc8 (Figure 1), similar to that in ref 34. Electrochemistry of T8Fc8. The solubility of T8Fc8 in most nonaqueous solvents commonly used in electrochemistry is low, and the intensity of signals in cyclic voltammograms is

Figure 1. T8Fc8.

13

C CP/MAS and

29

Si CP/MAS NMR (inset) spectra of

insufficient for further analysis (see Figure 2 for the example of a saturated solution of T8Fc8 in DMF/0.1 M NBu4PF6 at a Pt

Figure 2. Cyclic voltammetry of T8Fc8 in DMF/0.1 M NBu4PF6 (saturated solution; v = 0.1 V s−1).

electrode). Only a very minor deviation from the background current at potentials around +0.25 V is visible (all potentials are given vs an external Fc/Fc+ standard). The only solvent in which a substantially higher solubility was achieved is DMSO. Still, the high molar mass of T8Fc8 and the low availability of this compound limits the concentrations in practice. Thus, only thoroughly purified electrolyte components (solvent, supporting electrolyte) were used. Several purification methods reported in the literature29,30,35,36 for DMSO were tested. Batches resulting from the mixed freezing/crystallization/distillation protocol were found to provide minimum artifacts in the potential window necessary for voltammetry of T8Fc8. Concentrations up to 0.47 mM were accessible, and even solutions with c values as low as 0.023 mM provided clearly defined current peaks (Figure 3). The cyclic voltammograms exhibit two redox processes at E ≈ −0.25 V (peaks I, IV) and E ≈ +0.15 V (peaks II, III), respectively. Both are clearly more complex than simple reversible electron transfers. 4779


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Figure 3. Cyclic voltammetry of T8Fc8 in DMSO/0.1 M NBu4PF6; (a) c = 0.12 mM, v = 0.01 V s−1; (b) c = 0.12 mM, v = 0.2 V s−1; (c) c = 0.47 mM, v = 0.5 V s−1; (d) c = 0.023 mM, v = 1 V s−1.

broad and shows strong overlap with peak II at low c (Figure 3d). The scan rate dependency of ipI is linear in the v range where a defined peak is observed. This leads us to the conclusion that peak couple I/IV corresponds to the redox process of T8Fc8 adsorbed on the surface of the Pt electrode. Despite these complications, the midpoint potential for the respective redox process is E̅ = −0.22 ± 0.02 V. The formal potentials E° (assumed to be identical with E̅) are confirmed by DPV data, resulting in E° values of −0.21 and +0.155 V, respectively. The diffusion-controlled peak couple II/III is found at a potential that seems to be characteristic for dissolved Nalkylferrocenecarboxamide derivatives (1, +0.128 V in DMSO (experiments in this work) +0.136 V in DMF;38 2, +0.149 V in DMSO, +0.146 V in DMF;37 N-ferrocenoyl-N′-ω-decenoylethylendiamine: +0.174 V in CH3CN, +0.163 V in propylene carbonate;39 all potentials vs Fc/Fc+ in the respective solvent). Comparison of these values is made possible by the use of the Fc/Fc+ reference redox system40,41 and the extra-thermodynamic “ferrocene assumption”, becauseas used herethe influence of unknown liquid junction potentials should be minimized. On the other hand, ion pairing effects42 cannot be fully excluded. It appears that, of the carboxamides mentioned, only 1 shows an appreciable deviation in the midpoint potential to less positive potentials, possibly owing to the fact that it is the only N-methyl derivative, with the other examples and T8Fc8 having a longer carbon chain attached to the amide nitrogen atom. Other substitution patterns on Fc show strongly deviating potentials. For example, ring-Si-substituted ferrocenes linked to a silsesquioxane core24 are oxidized at +0.09 V (recalculated from a decamethylferrocene internal standard43): i.e., considerably less positive as compared to T8Fc8.

If we compare current/potential curves recorded at different scan rates v, it becomes evident that the peak couple II/III observed at higher v (Figure 3b) changes into a single oxidation peak II at low v (Figure 3a). Obviously, an irreversible followup reaction destroys the oxidation product generated in peak II. Similar reactivity is also observed for simple model compounds, such as N-methylferrocenecarboxamide (1) or N,N′-bis(ferrocenoyl)-1,2-diaminoethane (2),37 characterized by the Fc−CO−NH moiety, in both DMF and DMSO electrolytes. We thus conclude that, in the present case, the follow-up reaction of an oxidation product of T8Fc8 is related to the presence of the Fc-substituted amide functions.

For scan rates large enough for both peaks II and III to be observed, the midpoint potential E̅ is +0.155 ± 0.009 V. The peak potential difference is significantly larger than the 58 mV expected for a reversible one-electron transfer. Its concentration and scan rate dependence, however, is weak. The peak current ipII is proportional to the square root of the scan rate (Figure 4a), indicating a diffusion-controlled process. In contrast, the behavior of peak I is more complicated. With increasing scan rate the peak intensity in comparison to that of peak II strongly increases (Figure 3a,b). Its corresponding reduction signal IV splits and exhibits two maxima (Figure 3b). At high concentrations it almost vanishes (Figure 3c), while it becomes 4780


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Figure 4. Scan rate dependence of cyclic voltammetric peak currents, in DMSO/0.1 M NBu4PF6 at a Pt electrode for c = 0.023, 0.071, 0.116 mM: (a) ipII; (b) ipI.

determined, because the diffusion coefficient D of the silsesquioxane and the spacings of the E° values are unknown. Diffusion Properties of T8Fc8. If we assume that T8Fc8 were oxidized during CV in a single one-electron step, we could use the Randles−Ševčik relationship47,48 or Nicholson and Shain’s equation49 to determine D from peak current data. This analysis is, however, severely hampered by the overlap of inseparable peaks I and II at low concentrations. Consequently, the normalized peak current ipII/√vc shows a high standard deviation of 48% and the resulting diffusion coefficient of 9 × 10−6 cm2 s−1 can only be regarded as a rough estimate. Moreover, it attains an order of magnitude which is almost twice that of the much smaller molecule Fc in DMSO: DFc(DMSO) = (4.9 ± 0.1) × 10−6 cm2 s−1.50 It is thus rather unlikely that T8Fc8 undergoes a simple oneelectron oxidation to a monocationic species, and the hypothesis of several stepwise redox steps with a resulting larger current is supported. In the ideal case of vanishing interaction between the Fc units in T8Fc8, i.e. statistical distribution of redox states, the E° values are expected to be closely spaced.46,51 The voltammetric peaks will have a shape identical with that of the one-electron case,46 while the current is proportional to n.46,52 Thus, the peak current is n times that of a one-electron voltammogram. Application of these considerations, assuming n = 8, yields DT8Fc8(DMSO, CV) = (0.24 ± 0.36) × 10−6 cm2 s−1, now significantly smaller than DFc(DMSO). Again, the uncertainty in the peak current data is reflected in the high standard deviation. It is thus indispensable to check this value by comparison. Diffusion coefficients within series of similar compounds have been approximately correlated with molar masses raised to various powers: e.g., linear53,54 or inversely proportional.55 Starting from DFc(DMSO)50 and using the latter assumption as well as the molar masses of Fc and T8Fc8, we find an estimated DT8Fc8(DMSO, molar mass) = 0.35 × 10−6 cm2 s−1, close to the electrochemically determined value calculated above. It should, however, be noted that the inverse proportionality between molar mass and D was also critically evaluated55 and may not generally be applicable. Another estimate could be derived from the Stokes−Einstein relation

We conclude that peak couple II/III corresponds to the oxidation of freely diffusing T8Fc8 and that the silsesquioxane central unit excerts only a minor influence on the redox potential. The latter is shifted from the value for unsubstituted ferrocene (0.0 V) into a positive direction by the electronwithdrawing carboxamide substituent on one of the cyclopentadienyl rings. The molecule T8Fc8 features eight redox-active ferrocenoylamino groups. However, only a single diffusion-controlled peak couple is found in the cyclic voltammograms. It is thus not obvious to define the number of electrons transferred per molecule in peak II. In such multiredox site systems, depending on the relative values of the formal potentials (which again are related to the electronic interaction between the redox centers37), several situations may be encountered. (i) With strong interactions, clearly separated peaks for successive transfers of single electrons are observed. This is obviously not the case in the T8Fc8 system. (ii) In the other extreme, with substantial structural changes upon electron transfer, “potential inversion”44,45 could occur. This situation can also be excluded for T8Fc8, because it would result in a ΔEp(II/III) value which is significantly lower than the 58 mV for a one-electron transfer. In contrast, the experimental ΔEp(II/III) value is increased to almost 100 mV without being scan rate dependent. From this v independence another complication, quasi-reversibility, i.e. partial electron transfer kinetic control, can be excluded. The weak increase of ΔEp(II/III) with concentration (from 92 mV at c = 0.116 mM to 102 mV at c = 0.469 mM, both for v = 1 V s−1) may be related to some uncompensated iR drop which remains after instrumental compensation. Even without compensation, however, the solution resistance between the working electrode and the tip of the Haber−Luggin capillary (∼1000 Ω) on one hand and the peak current (2.6 μA at v = 0.05 V s−1 and c = 0.116 mM) on the other hand would result in an error of only 2.6 mV in the oxidation peak potential. This could not account for the increased value of ΔEp(II/III), which is ∼90 mV even at such low scan rates and concentrations. A reasonable explanation is the following, situation (iii): peaks II and III correspond to the envelope curve generated by overlap of several (up to eight) reversible one-electron signals shifted only slightly by closely spaced formal potentials.46 The difference in the E° is unknown but is small enough to avoid wave splitting. Thus, the peak current ipII is composed of contributions from all (up to eight) redox processes passed through during oxidation of T8Fc8. In this situation, how many electrons are transferred per molecule, n, however, is not easily

D= 4781

kBT 6πηr

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Organometallics with η(DMSO) = 1.99 mPa s (at 25 °C; without supporting electrolyte),56 if we use a reasonable assumption for the radius r of the close to spherical T8Fc8. An X-ray crystal structure of T8Ts8 (Ts = −CH2CH2OSO3-p-C6H4CH3) with a size similar to that of T8Fc8 shows57 a distance between the extreme substituents at the cube core of d = 2.3 nm. With r = 1.15 nm as a lower estimate, the radius of T8Fc8 (neglecting any differences between the size in the crystal and in solution, as well as solvation effects leading to a possible increase of the hydrodynamic radius) we calculate an upper estimate of DT8Fc8(DMSO, size) = 0.95 × 10−6 cm2 s−1. This is higher than previous values but still within the same order of magnitude. Consequently, an assumption that several Fc groups in T8Fc8 are oxidized is not unreasonable. Finally, PGSE NMR data yield DT8Fc8(DMSO, NMR) = (1.11 ± 0.02) × 10−6 cm2 s−1, in close agreement with the sizederived estimate. The PGSE experiment avoids any artifacts resulting from adsorption at the electrode surface or influence of peak overlap and E° spacing in the cyclic voltammograms. On the other hand, no supporting electrolyte was used in the NMR experiments. The values of the two diffusion coefficient estimates that do not rely on current measurement are larger than the electrochemically derived estimate, which might indicate a more complex redox behavior, where on average not all eight Fc units in T8Fc8 are oxidized during the CV experiment. In addition, although interaction between the attached Fc moieties is weak, small increases of the formal potentials of subsequent electron transfer steps with increasing positive charge of the oxidation product will decrease the current maximum of the envelope curve, decreasing the electrochemically determined D. Changes of the viscosity of DMSO without (D from molecule size and from PGSE NMR) and with 0.1 M NBu4PF6 (electrochemical results) might contribute to the difference, as has recently been shown for experiments in acetonitrile.58 However, even with a supporting electrolyte concentration of 0.2 M, the effect on η and D was only about 4%, which does not account for the factor of 3−4 found for the D values determined in the present work.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg “Chemie in Interphasen” (GK 441). D.R.A. acknowledges a doctoral fellowship of the Graduiertenkolleg. T.R. acknowledges a fellowship for Lehramtskandidaten and J.H. a doctoral fellowship of the Stiftung Stipendienfonds of the Verband der Chemischen Industrie. We thank Judith Schäfer for the preparation of 1 as described in ref 37.

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CONCLUSIONS The caged silsesquioxane T8Fc8 is regarded as a small-diameter molecular model for Fc-modified Stöber silica nanoparticles. The Fc units remain electroactive, although comparison of PGSE NMR and electrochemical results indicate that probably less than the maximum number of electrons is transferred. On longer time scales, a chemical follow-up reaction interferes, which is attributed to the presence of the ferrocenoyl amide substituents. However, the data show that the interaction between the redox-active moieties attached to the corners of the cube core is weak. Checking electrochemically determined diffusion coefficients by values generated by independent methods (here, PGSE NMR) does provide valuable additional information.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.A.M.: [email protected]. *E-mail for B.S.: [email protected]. Notes

The authors declare no competing financial interest. 4782


Organometallics

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Redox-Active Silica Nanoparticles. 9.1 Synthesis, Electrochemistry

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