Cubic Octasilsesquioxanes, Cyclotetrasiloxanes, and Disiloxanes

Article pubs.acs.org/Organometallics

Cubic Octasilsesquioxanes, Cyclotetrasiloxanes, and Disiloxanes Maximally Functionalized with Silicon-Bridged Interacting Triferrocenyl Units Sonia Bruña,† Daniel Nieto,† Ana Ma González-Vadillo,† Josefina Perles,‡ and Isabel Cuadrado*,† †

Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049, Madrid, Spain Servicio Interdepartamental de Investigación (SIDI), Universidad Autónoma de Madrid, Madrid, Spain

‡

S Supporting Information *

ABSTRACT: Redox-active, highly symmetrical cubic octasilsesquioxanes (OS) peripherally decorated with 24 ferrocenyl units, linked in threes around the periphery of a cubic cage, namely, [Fc 3 Si(CH 2 ) 2 Me 2 SiO]8 Si 8 O 12 (6) and [Fc 3 Si(CH2)2]8Si8O12 (7) (Fc = (η5-C5H4)Fe(η5-C5H5)), have been synthesized. Such integrally ferrocenyl-functionalized cubic macromolecules 6 and 7, as well as the related smallmolecule models hexaferrocenyldisiloxane [Fc 3 Si(CH2)2Me2Si]2O (4) and dodecaferrocenyl cyclotetrasiloxane [Fc3Si(CH2)2MeSiO]4 (5), have been prepared by covalently linking, via Karstedt’s-catalyzed hydrosilylation, triferrocenylvinylsilane (CH2CH)Si(Fc)3 (3) around the surface of octasilsesquioxane cages T8(OSiMe2H)8 and T8H8 and linear [Me2SiH]2O and cyclic [MeSiHO]4 siloxane scaffolds, respectively. All new polyferrocenyl oligosiloxanes have been thoroughly characterized using a combination of elemental analysis, multinuclear (1H, 13C, 29Si) NMR spectroscopy, FT-IR, and MALDI-TOF mass spectrometry. The molecular structure of disiloxane 4, in the solid state, has been determined by single-crystal X-ray analysis. Hexametallic 4 shows a bent arrangement of the ferrocenyl-substituted disiloxane linkage (Si−O−Si angle of 147.6(5)°). Polyferrocenyl-OS 6 and 7 show good thermal stability and form iron-containing ceramics when pyrolyzed under nitrogen. The electrochemical behavior of polyferrocenyl OS and model linear and cyclic siloxanes has been examined by cyclic and square wave voltammetries, in dichloromethane solution using PF6− and B(C6F5)4− as supporting electrolyte anions of different coordinating ability. The novel maximally ferrocenylfunctionalized oligosiloxanes exhibit a three-wave redox pattern, suggesting appreciable electronic interactions between the silicon-bridged triferrocenyl moieties as they are successively oxidized. OS 6 and 7 undergo remarkable oxidative precipitation in CH2Cl2/n-NBu4PF6 and are able to form stable electroactive films on platinum electrode surfaces. They are the first redox-active OS showing significant electronic interactions between metal sites on the cage surface.

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INTRODUCTION The design and synthesis of macromolecular systems containing multiple metallocene moieties are currently a subject of intense interest in various areas of research including molecular electronics, catalysis, biomolecules, and molecular recognition.1−5 Particularly, compounds containing multiple ferrocene units linked together by a variety of bridging spacer units6−8 represent challenging synthetic targets, because the well-behaved redox chemistry of the 18-electron ferrocene system9 provides a powerful electrochemical probe to investigate intramolecular electron-transfer processes and possible mixed-valence behaviors.5,6 Many synthetic strategies have been devised, enabling access to a large variety of polyferrocene macromolecular systems including oligomers and polymers10 as well as dendritic molecules,11−14 which have shown promise as multielectron redox catalysts and sensors. In sharp contrast, to date very little work has been directed toward developing polymetallocene © 2012 American Chemical Society

structures having high 3-D cubic symmetry. In this context, remarkable and novel multiferrocene cuboctaedra systems obtained via coordination-driven self-assembly have been recently reported by Stang and co-workers.15 Because of their specific, well-defined geometry, polyhedral oligomeric silsesquioxanes (POSS), T8 cubic octasilsesquioxanes of general structure (RSiO1.5)8 in particular, are unique three-dimensional cage molecules16−19 that promise to be versatile building blocks to create a wide variety of highly symmetrical polymetallocene macromolecules having nanometer-size architectures.20 Cubic octasilsesquioxanes (OS) have been extensively used in recent years as homogeneous models for silica, as biocompatible drug carriers, to generate novel catalysts and porous media, and also as anion receptors.16−18 In addition, POSS have proven to be valuable scaffolds for the Received: February 8, 2012 Published: March 19, 2012 3248

dx.doi.org/10.1021/om300101w | Organometallics 2012, 31, 3248−3258

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pounds,30 we report now the use of compound 3 in building large 2-D and 3-D silicon-based macromolecules maximally functionalized with as many interacting metallocene units as chemically possible. We present full details of the isolation, characterization, thermal properties, and redox behavior of three-dimensional, perfectly symmetrical octasilsesquioxanes peripherally functionalized with 24 ferrocenyl units, electronically interacting in threes. This is the highest possible number of ferrocenes that can be placed at the vertices of a cubic OS. In addition, we compare their properties with those of closely related siloxanebased small-model compounds, also completely functionalized with silicon-bridged triferrocenyl moieties, built on linear and cyclic oligosiloxane cores.

construction of dendrimers and hyperbranched dendritic structures.21−25 For instance, Morris and co-workers21 and Hong and co-workers22 have used cubic silsesquioxanes as octaarm core units, from which dendrons were grown divergently or to which they were appended via a convergent approach. In a dissimilar strategy, Dvornic et al. have prepared remarkable dendrimer-POSS nanostructures, where silsesquioxane cages are attached to the surface of a dendritic core unit.23 On the other hand, Marciniec and co-workers have recently prepared noteworthy functionalized octasilsesquioxanes via efficient and highly stereoselective cross-metathesis and silylative coupling with substituted styrenes.19d Likewise, Laine et al. have prepared octaalkynylsilsesquioxanes, which have been used in the synthesis of interesting polyphenylene macromolecules through Diels−Alder reactions.19c In this work, our efforts target the development of a new family of highly functionalized polyferrocenyl molecules, having a controlled number of electronically communicated redoxactive organometallic units, using different Si−H multifunctional scaffolds: cubic oligosilsesquioxanes as well as cyclic and linear siloxanes. This goal was motivated by our earlier work on incorporation of ferrocenyl moieties into polyfunctional siloxane frameworks, via hydrosilylation of vinylferrocene (η5C5H4CHCH2)Fe(η5-C5H5) with Si−H-containing cyclotetrasiloxane and octasilsesquioxanes.26,27 These reactions provided the well-defined multimetallic redox-active molecules 1 and 2 shown in Scheme 1, as well as related network

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RESULTS AND DISCUSSION Synthesis and Characterization of Disiloxane-Bridged Model Compound 4 and Dodecaferrocenyl Cyclosiloxane 5. The key starting compound selected for the synthesis of the new polyferrocenyl OS was triferrocenylvinylsilane (3), which was successfully synthesized via the lowtemperature salt metathesis reaction of monolithioferrocene and the chlorosilane (CH2CH)SiCl3 (Scheme 2). This trimetallic molecule contains a reactive vinylsilyl group, which enables its incorporation into Si−H-polyfunctionalized frameworks. From an electronic point of view, the Si−CHCH2 group of 3, bonded to three electron-donating ferrocenyl moieties, is particularly electron-rich and, accordingly, should be highly reactive toward hydrosilylation reactions.31 Nevertheless, since we were working under the assumption that the reactive Si−CHCH2 group of 3 is in a sterically hindered environment, surrounded by three bulky ferrocenyl moieties that might well affect the effectiveness of hydrosilylation, we, at first, targeted well-defined polyferrocenyl siloxane small models in order to determine whether a similar strategy might provide access to polymetallo-OS of higher nuclearity. Hydrosilylation of 3 with 1,1,3,3-tetramethyldisiloxane was effected in toluene solution and in the presence of Karstedt′s catalyst. 1H NMR spectroscopy was utilized to follow the progress of the reaction, and it was established that complete transformation of the Si−H functionalities of [Me2SiH]2O occurred effectively after 12 h at 45 °C. Purification of the resulting orange, oily mixture was effected by column chromatography on silica gel (with hexane/CH2Cl2, 100:50), affording the desired hydrosilylated model compound [Fc3Si(CH2)2Me2Si]2O (4), containing silicon-bridged triferrocenyl groups linked by −(CH2)2− bridges (Scheme 2), which was isolated as an air-stable, highly pure, orange product, in relatively high yield (87%). Similarly, Karstedt's-catalyzed hydrosilylation of an excess of 3 with 1,3,5,7-tetramethylcyclotetrasiloxane occurred effectively in toluene solution at 60 °C, with full conversion of the Si−H groups on the cyclic siloxane framework obtained in an overnight reaction. After careful column chromatographic purification on silica gel (using hexane/CH2Cl2, 100:60) the targeted dodecaferrocenyl-cyclotetrasiloxane [Fc3Si(CH2)2MeSiO]4 (5) was isolated as an airstable, orange, crystalline product in 81% yield (Scheme 2). Multinuclear NMR (1H, 13C, and 29Si) analyses of 4 and 5 were consistent with the desired hydrosilylated polyferrocenyl linear and cyclic model compounds. On the basis of the 1H and 13 C NMR spectra of 4 and 5 it was determined that addition of Si−H to the vinyl group of 3 proceeded selectively according to the anti-Markownikov rule, so that disiloxane 4 and cyclo-

Scheme 1. Cyclic and Cubic Oligosiloxanes with Electronically Isolated Ferrocenylethyl Units, Prepared from Vinylferrocene

structures composed of ferrocene and cyclosiloxane or spherosilicate moieties, which were used as mediators in amperometric enzyme electrodes for the detection of glucose.28 Both compounds 1 and 2 undergo a simultaneous multielectron transfer at the same potential, of four (for 1) and of eight (for 2) electrons, so that the multiple ferrocenylethyl moieties in each molecule undergo electron transfer independently, and no electronic coupling among them can be discerned.26,27 These were the first examples of ferrocenyl fully substituted cyclic and cubic oligosiloxanes. In addition, very recently we have successfully synthesized and characterized the vinylsilyl-functionalized trimetallic molecule (CH2CH)Si(Fc)3 (3) (Fc = (η5-C5H4)Fe(η5C5H5)).29 In particular, we found that its electrochemical behavior indicated a considerable degree of electronic communication between the silicon-bridged ferrocenyl centers, as they are oxidized. With the aim to extend our studies on the chemistry of vinylsilyl-functionalized organometallic com3249


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Scheme 2. Synthesis of Disiloxane (4) and Cyclotetrasiloxane (5) Model Compounds, Fully Functionalized with Si-Bridged Triferrocenyl Units

tetrasiloxane 5 are truly β-addition products. The 1H NMR spectrum of the purified model 4 (Figure S1) shows that the characteristic peak for the −Si−H protons at δ 4.69 ppm has disappeared. In the same way, transformation of the terminal vinyl groups of 3 to internal single bonds (−CH2−CH2− bridges between δ 0.9 and 1.4 ppm) is clearly observed. Likewise, the 13C NMR spectrum of 4 shows signals at δ 7.7 and 11.4 ppm due to the new CH2 carbon signals and a complete absence of the vinyl signals at δ 133.2 and 136.4 ppm. 29 Si NMR of starting vinylsilyl 3 showed a single peak centered at δ −17.0 ppm arising from a (Fc)3Si−vinyl unit. After hydrosilylation, the 29Si NMR spectrum of the resulting disiloxane 4 shows two peaks centered at δ −8.4 (for the (Fc)3Si−CH2CH2 group) and at 8.5 ppm (for the (CH3)2−Si− O unit), reflecting the change in the electronic environment of Si units due to the formation of the new Si−O−Si−CH2−CH2 bridge. The 1H NMR spectrum of cyclic compound 5 shows the characteristic peaks for the Si−CH2−CH2 protons at δ 1.04− 1.34 ppm and the characteristic resonance peaks for the methyl and ferrocenyl protons. The CH2 carbons adjacent to silicon atoms are shifted at δ 7.6 and 10.0 ppm in the 13C NMR spectrum. The 29Si NMR spectrum shows two signals; the first one at δ −8.3 ppm due to the Fc−Si unit presents the same value as the one found in model 4, δ −8.4 ppm. The second signal, at δ −18.4 ppm, corresponds to a D-type silicon atom and is considerably shifted high-field in comparison to model compound 4 (δ 8.5 ppm), due to the different environment of the Si atoms. Furthermore, the IR spectra of models 4 and 5 exhibit strong Si−O stretching vibrations, characteristic of Si− O−Si disiloxane linkages, namely, at 1033−1050 cm−1for 4 and at 1034−1106 cm−1 for 5, and also confirm the disappearance of the characteristic bands at approximately 2110 cm−1 due to Si−H bonds. The MALDI-TOF mass spectra show major peaks at m/z 1354.2 for 4 (S5) and at m/z 2680.9 for 5 (Figure 1A), corresponding to molecular ions M+, with excellent agreement between the experimental and calculated isotopic patterns. These analyses also confirm their precisely defined linear and cyclic structures and the presence of 6 and 12 ferrocenyl units, respectively. It is worth noting that the high yields of hydrosilylations, in combination with the high degree of regioselectivity (exclusive

Figure 1. MALDI-TOF mass spectra of (A) cyclotetrasiloxane model 5 and (B) cubic OS 6. The insets in the figure show the experimental (top) and calculated (bottom) isotopic patterns.

formation of β-isomers 4 and 5), and the lack of any side reaction make trimetallic vinylsilane 3 an ideal candidate for the construction of highly symmetric, ferrocenyl fully functionalized OS. The solid-state structure of hexaferrocenyl disiloxane model 4 was determined by single-crystal X-ray diffraction. Suitable crystals of 4 for X-ray analysis were obtained at 4 °C from a solution of the corresponding compound in hexane/CH2Cl2 (10:5). The molecular structure of 4 is illustrated in Figure 2. A

Figure 2. Simplified view of the structure of hexaferrocenyl disiloxane 4 (hydrogen atoms omitted for clarity). 3250


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summary of crystallographic data and data collection parameters is included in Table 1. Table 2 contains a

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Hexaferrocenyl Disiloxane 4 length

Table 1. Selected Crystallographic Data for Disiloxane Model Compound 4 empirical formula fw temp, K wavelength, Å cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z density (calcd), mg cm−3 F(000) no. of reflns collected no. of indep reflns completeness no. of data/restraints/params goodness-of-fit on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data) largest diff peak and hole, e Å−3

Si(1)−C(1) Si(1)−C(9) Si(1)−C(19) Si(1)−C(29) Si(2)−C(2) Si(2)−C(5) Si(2)−C(6) Si(2)−O(1) Si(3)−C(3) Si(3)−C(7) Si(3)−C(8) Si(3)−O(1) Si(4)−C(4) Si(4)−C(39) Si(4)−C(49) Si(4)−C(59) C(1)−C(2) C(3)−C(4) Fe(1)−Fe(2) Fe(1)−Fe(3) Fe(2)−Fe(3) Fe(4)−Fe(5) Fe(4)−Fe(6) Fe(5)−Fe(6) Fe(1)−Fe(4) Fe(1)−Fe(5) Fe(1)−Fe(6) Fe(2)−Fe(4) Fe(2)−Fe(5) Fe(2)−Fe(6) Fe(3)−Fe(4) Fe(3)−Fe(5) Fe(3)−Fe(6)

C69H77Fe6OSi4 1376.28 296 0.71073 triclinic P1̅ 15.3608(5) 16.6451(6) 18.190(1) 103.746(2) 104.557(2) 111.570(2) 3894.8(3) 2 1.174 1429 50 173 11 255 [R(int) = 0.0508] 93.4% (to θ = 23.87°) 11 255/154/746 1.574 0.1152, 0.3535 0.1685, 0.4033 4.035 and −2.730

comparison of selected bond lengths and angles of 4. Hexametallic compound 4 crystallizes in the triclinic space group P1̅ with Z = 2. The molecular structure of hexaferrocenyl disiloxane model 4 shows that in each siloxane moiety the bulky Si-bridged triferrocenyl groups are located in an approximately orthogonal disposition to avoid steric congestion and that the disiloxane linkage adopts a bent arrangement (Si−O−Si angle of 147.6(5)°). The Fe···Fe separations vary from 5.49−6.26 Å, for neighboring ferrocenyl groups on the same Si atom, to 6.40−15.33 Å for ferrocenyl groups attached to different Si centers. The cyclopentadienyl rings are parallel (with a maximum deviation of 6.09° in the ferrocenyl group containing Fe5) and are eclipsed in every ferrocenyl moiety. The molecular arrangement of 4 in the crystal structure, shown in Figure 3, is of interest. An examination of the crystalpacking diagram along the a axis shows that the molecules are folded into a “V” shape and located in sheets parallel to the bc plane. Likewise, in the molecular packing along the b axis (Figure S31) the voids among the molecules can be seen. Disordered hexane molecules are placed in the voids (in a ratio hexane:4 of 1:4) as well as other residual density peaks that could not be identified. Synthesis and Characterization of OS Containing 24 Ferrocenyl Units. After establishing that 3 is capable of forming ferrocenyl fully functionalized linear and cyclic siloxanes via hydrosilylation chemistry, we undertook the synthesis of the targeted polyferrocenyl cubic silsesquioxanes. To this end, two different Si−H-octafunctionalized OS were selected as frameworks: octakis(hydridodimethylsiloxy)silsesquioxane (sp herosilicate) [HMe 2 SiO] 8 Si 8 O 1 2 (T8(OSiMe2H)8) and octa(hydrido)silsesquioxane H8Si8O12 (T8H8). Both OS have silyl hydride reactive groups at the

angle 1.872(9) 1.868(9) 1.836(9) 1.869(9) 1.87 (1) 1.86(1) 1.88(1) 1.639(7) 1.81(2) 1.87(2) 1.79(2) 1.601(7) 1.99(2) 1.85 (1) 1.75(1) 1.850(8) 1.52(1) 1.54(2) 5.884 6.261 5.492 5.525 5.989 5.989 6.404 10.804 9.050 11.830 15.328 10.813 9.376 11.945 6.397

C(1)−Si(1)−C(9) C(1)−Si(1)−C(29) C(19)−Si(1)−C(29) C(9)−Si(1)−C(19) C(1)−Si(1)−C(19) C(9)−Si(1)−C(29) Si(1)−C(1)−C(2) C(1)−C(2)−Si(2) C(2)−Si(2)−C(5) C(2)−Si(2)−C(6) C(5)−Si(2)−O(1) C(6)−Si(2)−O(1) C(2)−Si(2)−O(1) C(5)−Si(2)−C(6) Si(2)−O(1)−Si(3) C(3)−Si(3)−C(7) C(3)−Si(3)−C(8) C(7)−Si(3)−O(1) C(8)−Si(3)−O(1) C(3)−Si(3)−O(1) C(7)−Si(3)−C(8) Si(3)−C(3)−C(4) C(3)−C(4)−Si(4) C(4)−Si(4)−C(39) C(4)−Si(4)−C(59) C(49)−Si(4)−C(59) C(39)−Si(4)−C(49) C(4)−Si(4)−C(49) C(39)−Si(4)−C(59)

114.0(4) 108.1(4) 112.4(4) 106.9(4) 109.5(4) 106.0(4) 116.0(7) 115.4(7) 109.9(6) 110.8(5) 109.3(5) 110.8(6) 106.8(5) 109.2(7) 147.6(5) 99.9(9) 110.2(9) 111.4(6) 111.1(7) 117.1(9) 106.1(1) 111.3(1) 114.1(1) 100.0(6) 113.8(8) 113.1(5) 113.0(6) 108.0(9) 108.3(4)

Figure 3. Crystal-packing diagram of 4: view along the crystallographic a axis of the molecular packing.

eight corners of the cube, but their Si−H bonds show a different chemical environment. T8(OSiMe2H)8 was prepared in two steps by adapting the literature procedures described by Marciniec et al.,19a Caetano et al.,32 and our own group27a by the treatment of tetramethylammonium hydroxide with tetraethoxysilane and dimethylchlorosilane in a mixture of methanol, water, and hexane as solvent. Concerning T8H8 this cubic silsesquioxane was synthesized by hydrolysis of HSiCl3 in a biphasic system (methanol/toluene) in the presence of FeCl3 3251


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Scheme 3. Synthesis of Cubic Octasilsesquioxanes 6 and 7, Fully Functionalized with Si-Bridged Triferrocenyl Units

by adapting reported procedures described by Matisons et al.33 and Schriver et al.34 The identities of both T8(OSiMe2H)8 and T8H8 were confirmed on the basis of elemental analysis, IR spectra, mass spectrometry, and 1H and 29Si NMR spectra (see Supporting Information), which afforded data consistent with the assigned structures reported in the literature. We first tried the reaction between 3 and octafunctional spherosilicate T8(OSiMe2H)8, in which the Si−H reactive groups are separated from the rigid cubic Si8O12 core by a dimethylsiloxy spacer, in order to prevent the possibility of steric congestion. We found that Karstedt's-catalyzed reaction of 3 with T8(OSiMe2H)8 was a successful process with full consumption of Si−H groups located at the vertexes of the cube, affording hydrosilylated cubic-shaped spherosilicate [Fc3Si(CH2)2Me2SiO]8Si8O12 (6) (Scheme 3). The reaction mixture was heated at 60 °C and periodically monitored by 1H NMR. During the course of reaction a darkening in the color of the mixture was observed. After 21 h, analysis of the reaction mixture by 1H NMR confirmed the complete consumption of the Si−H protons of T8(OSiMe2H)8 and the appearance of a set of new resonances at δ 0.8−1.2 ppm corresponding to a new hydrosilylated product with CH2−CH2 linkages, indicating quantitative transformation of the Si−H reactive groups and the formation of OS 6. Once separated, OS 6, containing 24 ferrocenyl moieties on the octahedral surface, linked in threes through a silicon atom, was isolated as a crystalline, yellow-orange solid in relatively high yield (68%). Scanning electron microscopy (SEM) images of the crystals obtained for cubic 6 (see Figure S39) clearly show that this compound tends to crystallize as nearly cubic particles with side dimensions of ca. 60 μm. Unfortunately, these orange crystals diffracted weakly and were not suitable for X-ray studies. In the ensuing, complementary reaction, we were also interested in testing the reactivity of 3 toward T8H8. The choice of this cubic cage was intentional since the eight reactive Si−H groups that enable chemical modifications are directly linked to three oxygen atoms, and it is known that the Si8O12 core acts as an electron-withdrawing group equivalent to a CF3 substituent.35 Consequently, one would normally expect an increased reactivity to the addition of T8H8 to the vinyl group in 3.36 Accordingly, hydrosilylation of T8H8 with an excess of 3 occurred with good conversion of the Si-vinyl group after only 12 h. Nevertheless, targeted fully substituted [Fc 3 Si(CH2)2]8Si8O12 (7) (Scheme 3) was obtained together with a

mixture of partially functionalized OS ([Fc3Si(CH2)2]7HSi8O12, [Fc3Si(CH2)2]6H2Si8O12, and [Fc3Si(CH2)2]5H3Si8O12) containing 21, 18, and 15 ferrocenyl units, respectively, located at the vertexes of the cube, as established by the MALDI-TOF mass spectrum of the mixture resulting from the reaction of 3 and T8H8 (Figure 4A). Increased reaction time, temperature, and catalyst concentration did not enable the reaction to go to completion. We believe that steric reasons were solely responsible for this, not unexpected, result. After careful column chromatographic purification, OS 7, which contains 24 ferrocenyl moieties on the octahedral surface, was isolated as an air-stable, crystalline, orange solid in 22% yield.

Figure 4. (A) MALDI-TOF mass spectrum of the reaction mixture of 3 and T8H8. Molecular ion peaks corresponding to octasilsesquioxanes functionalized with 24 Fc ([Fc3Si(CH2)2]8Si8O12) (7); 21 Fc ([Fc3Si(CH2)2]7HSi8O12); 18 Fc ([Fc3Si(CH2)2]6H2Si8O12); and 15 Fc ([Fc3Si(CH2)2]5H3Si8O12). (B) MALDI-TOF mass spectrum for octasilsesquioxane 7. Isotopic distributions, experimental (top) and calculated (bottom). 3252


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Multinuclear (1H, 13C, and 29Si) NMR analyses revealed the formation of pure and highly symmetric polyferrocenyl cubic OS species 6 and 7. The 1H NMR and 13C NMR spectra of the two purified cubic cages display the characteristic peaks for the ferrocenyl and CH2 fragments (see Supporting Information). In addition, the 1H NMR spectra of both compounds show the disappearance of the terminal protons, at δ 4.73 and 4.25 ppm, respectively, confirming the entire functionalization of the Si− H reactive groups. The 29Si NMR spectra of 6 (S18) and 7 (S27) show a singlet at −108.4 (for 6) and at −65.8 ppm (for 7), corresponding to the Q- or T-type silicons of the silsesquioxane cages, which confirms that (i) all of the silicon atoms in each silsesquioxane core are magnetically equivalent and (ii) the cubic structure remained intact during the reaction. In addition, the peak observed at −8.2 ppm (for 6) and at −8.0 ppm (for 7) supports the incorporation of triferrocenylsilyl moieties around the cubic silsesquioxane frameworks. In the case of 6 a third signal can be observed at δ 13.4 ppm corresponding to the M-type silicon atom. In the IR spectra of both compounds, the ν (Si−O−Si) bands of the cage cores can be seen at 1033−1130 cm−1 for 6 and at 1034−1106 cm−1 for 7. MALDI-TOF mass spectrometry provides further evidence of the formation of the new polyferrocenyl OS. The mass spectra of 6 and 7 show the most intense peak at m/z 5898.0 (Figure 1B) and at m/z 5305.0 (Figure 4B), respectively, which corresponds to the M+ ion. Their isotopic masses agree well with those of the calculated ones, confirming their precisely defined cubic structures and the presence of 24 pendant ferrocenyl moieties, grouped in threes at the vertexes of the OS cubic cages. Other less intense peaks, assignable to fragmentation products, can also be seen, highlighting the peak at m/z 2948.8 (for 6), which is just half the value of M+ and corresponds to the M2+ ion. The described structural results suggest that cubic silsesquioxanes T8 (OSiMe 2 H) 8 and T 8 H8 offer unique opportunities to build new types of perfectly symmetrical 3-D polyferrocenyl structures around a nanometer-size octafunctional core with cubic symmetry. It is known that thermogravimetric analysis (TGA) measured under nitrogen of both T8(OSiMe2H)8 and T8H8 precursors shows rapid and complete mass loss near 200 °C, which is due to sublimation.16b,17,37 Concerning novel polyferrocenyl 6 and 7, we have found that these OS show a high thermal stability. TGA analysis of 6 under nitrogen displayed two weight losses (Figure 5). The first loss, beginning near 400 °C, can be attributed to cleavage of the organic component of the peripheral arms, whereas the second weight loss beginning at 530 °C can be attributed to breaking down of the inorganic OS core structure. The octasilsesquioxane 7 suffers a first slight weight loss around 100 °C, which can be ascribed to evaporation of toluene trapped in the polyferrocenyl cubic cage. The major weight loss of this octasilsesquioxane occurs in the temperature region between 300 and 650 °C, after which the TGA curve is almost leveled off. The ceramization yield of OS 6 at 1000 °C is 50 wt %, whereas OS 7 yielded 66% of ceramic residues. We further studied the residues obtained via pyrolysis of OS 6 under a nitrogen stream at 1000 °C, by SEM and energy-dispersive X-ray analysis. The pyrolyzed material obtained from 6 presents magnetic properties, being readily attracted to a magnet bar, and adopts a structure that resembles a piece of a broken ceramic vase (S40) with different

Figure 5. TGA thermograms of OS 6 and 7 recorded under N2 at a heating rate of 10 °C min−1.

proportions of Si and Fe depending on the area studied (see SI). Electrochemical Studies of Linear and Cyclic Model Compounds 4 and 5 and Polyferrocenyl OS 6 and 7. The electrochemical behavior of polyferrocenyl octasilsesquioxanes 6 and 7 is also of considerable interest and was examined by cyclic voltammetry (CV) and square wave voltammetry (SWV) and compared with the redox behavior of the linear and cyclic model compounds 4 and 5. The cyclic voltammetric responses of both octasilsesquioxanes 6 and 7, in dichloromethane solution with 0.1 M nBu4NPF6 as the supporting electrolyte, are complex and proceed by three poorly resolved redox processes. As an example, the CV of 6 is shown in Figure 6A. The shape of the voltammetric waves observed in the CV departs from that expected for a reversible electrochemical oxidation process. Specifically, the anodic and cathodic peak currents are dissimilar, and the two more anodic waves have a shape clearly indicative of precipitation of the oxidized, highly charged species 624+ onto the electrode, followed by cathodic stripping on the return scan. A slightly better resolution of the anodic processes was observed in the CV responses, measured in the same medium, of oligosiloxane model compounds 4 and 5, having fewer ferrocenyl units, although in both cases noticeable stripping peaks due to adsorption are also observed (Figure 7A). By enriching the solvent mixture with acetonitrile, the cathodic stripping peaks progressively tend to disappear, and the CVs of model compounds 4 and 5, as well as of OS 6 and 7, show three well-resolved anodic waves, which are indicative of appreciable electronic interactions between the silicon-bridged triferrocenyl moieties as they are successively oxidized.8a−d Specifically, in the CVs of hexaferrocenyl disiloxane 4 and cyclotetrasiloxane 5 recorded in a 3:0.5 CH2Cl2/CH3CN mixture, each of the three oxidation waves exhibits the characteristics of freely diffusing soluble species undergoing reversible charge transfer (see Figure 7B and S33). Likewise, the corresponding square wave voltammograms of 4 and 5, measured in the same medium (Figure 7C and S33), show three well-separated redox processes at E1/2 = 0.372, 0.544, and 0.668 V vs SCE (for 4) and at E1/2 = 0.400, 0.568, and 0.692 V vs SCE (for 5), due to the redox couples 4/42+, 4/44+, 4/46+ and 5/54+, 5/58+, 5/512+, respectively. Figure 6B shows the striking improvement observed for the anodic electrochemistry of 6 when n-Bu4NB(C6F5)4 is 3253


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Figure 6. Cyclic voltammograms, at a scan rate of 0.05 V s−1, of CH2Cl2 solutions of OS 6 also containing (A) 0.1 M n-Bu4NPF6 and (B) 0.1 M nBu4NB(C6F5)4. (C) CV responses, at different scan rates, of a platinum-disk electrode modified with a film of 6, measured in 0.1 M n-Bu4NPF6; Γ = 6.05 × 10−12 mol Fe/cm2. (D) Plot of ipa against scan rate (v) of the first anodic process.

provides more favorable conditions for electrochemical studies of polyferrocenyl OS 6 and 7, minimizing ion-pairing interactions between the fluoroarylborate anion electrolyte and cationic species 624+ and 724+generated in the oxidation processes. From the wave splitting (ΔE1/2 = 1E1/2 − 2E1/2) between the first and second oxidations (ΔE1/2 = 272 mV for 6 and 268 mV for 7 in CH2Cl2/n-Bu4NB(C6F5)4), the value of the comproportionation constant (K c ) relative to the equilibrium [FeII−FeII−FeII] + [FeIII−FeIII−FeII] ⇆ 2[FeII− FeIII−FeII] can be calculated.39,40 From an electrochemical point of view, based on the resulting values of Kc = 39.60 × 103 and 33.89 × 103, partially oxidized OS 68+ and 78+ belong to the slightly delocalized class II mixed-valence species, according to the Robin−Day classification.5,6,41 The ability of OS 6 and 7 to undergo oxidative adsorption is remarkable and has allowed the preparation of electrodes modified with films of polyferrocenyl-OS (see Experimental Section). The electrochemical deposition of octasilsesquioxanes 6 and 7 was clearly shown by continuous increases in both the anodic and cathodic peak currents during consecutive cyclic voltammetric scans. A significant advantage of such redox behavior is that the polyferrocenyl-OS film thickness can be controlled by varying the number of scans in the CV. We have evaluated the stability of films of polyferrocenyl-OS 6 and 7 by transferring functionalized platinum electrodes into fresh CH2Cl2/electrolyte solutions. A representative example of the CV responses of an electrodeposited film of OS 6 is shown in Figure 6C. Three successive well-resolved, reversible oxidation−reduction waves are observed, with formal potential values of 1E1/2 = 0.409, 2E1/2 = 0.555, and 3E1/2 = 0.663 V, vs SCE. The surface-confined nature of electroactive OS 6 and 7 films is proved by the linear dependences of the anodic and cathodic peak currents, of the three waves, on the potential sweep rate v.42 Electrodeposited films of OS 6 and 7 are

Figure 7. (A) CV response of disiloxane 4 (10−3 M) recorded in CH2Cl2 containing 0.1 M n-Bu4NPF6; (B) CV response of 4 (10−3 M) recorded in CH2Cl2/CH3CN (3:0.5) containing 0.1 M n-Bu4NPF6; (C) SWV response of 4 in the same medium.

employed as supporting electrolyte. In the figure it becomes patently obvious that better resolution, improved electrochemical reversibility, and very little electrode adsorption occurred, which is indicative of a better solubility of the polyferrocenium electrogenerated OS 624+ in this solvent/ electrolyte medium. Similar redox responses are observed for polyferrocenyl octasilsesquioxane 7 when B(C6F5)4− is used as electrolyte anion. Clearly, in agreement with the results reported by Geiger and co-workers,38 the combination of CH2Cl2 and n-Bu4NB(C6F5)4 as solvent/electrolyte medium 3254


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droplet method. The positive ion and the reflectron mode were used for these analyses. EI mass spectra were recorded on a GCT (Waters) spectrometer. The thermogravimetric analyses were performed using a TGA Q500 instrument coupled with an EGA oven. Samples (5−10 mg) were loaded in platinum pans. The measurements were carried out under N2 (90 mL min−1) with a heating rate of 10 °C min−1. The morphology of the ceramic residues was investigated with SEM using a Philips XL30 instrument coupled with an EDAX DX4i analyzer. Electrochemical Measurements. Cyclic voltammetric and square wave voltammetric experiments were recorded on a Bioanalytical Systems BASCV-50W potentiostat. CH 2Cl2 and CH3CN (SDS, spectrograde) for electrochemical measurements were freshly distilled from calcium hydride under argon. The supporting electrolytes used were tetra-n-butylammonium hexafluorophosphate (Fluka), which was purified by recrystallization from ethanol and dried under vacuum at 60 °C, and tetra-n-butylammonium tetrakis(pentafluorophenyl)borate, which was synthesized as described in the literature, 43 by metathesis of [NBu 4 ]Br with Li[B(C6F5)4]·(nOEt2) (Aldrich) in methanol and recrystallized twice from CH2Cl2/hexane. The supporting electrolyte concentration was 0.1 M. A conventional three-electrode cell connected to an atmosphere of prepurified nitrogen was used. 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) (both Bioanalytical Systems), each of which were polished on a Buehler polishing cloth with Metadi II diamond paste, rinsed thoroughly with purified water and acetone, and dried. All potentials were referenced to the saturated calomel electrode (SCE). Under our conditions, the ferrocene redox couple [FeCp2]0/+ is +0.462, and the decamethylferrocene redox couple [FeCp*2]0/+ is −0.056 V vs SCE in CH2Cl2/0.1 M n-Bu4NPF6. A coiled platinum wire was used as a counter electrode. Solutions were, typically, 10−3 or 10−4 M in the redox-active species. The solutions for the electrochemical experiments were purged with nitrogen and kept under an inert atmosphere throughout the measurements. Square wave voltammetry was performed using frequencies of 10 Hz. The preparation of electrode surfaces modified with films of OS 6 and 7 was accomplished by electrodeposition of the cubic species on Pt electrodes. The electroactive films have been prepared by cyclically scanning the potential (between +0.0 and +1.3 V vs SCE) in degassed CH2Cl2 solutions of the corresponding OS at different numbers of times (10, 20, 30, or 50 scans). The electrochemical behavior of the films was then studied by CV in OS-free CH2Cl2 solution containing only supporting electrolyte ([n-Bu4N][PF6]). From the CVs of the modified electrodes, the surface coverages, Γ (mol/cm2), of the ferrocenyl sites were calculated from the charge, Q, under the voltammetric current peaks, using Γ = Q/nFA. X-ray Crystal Structure Determination. Disiloxane 4 was structurally characterized by single-crystal X-ray diffraction. A suitable orange crystal of dimensions 0.13 mm × 0.19 mm × 0.21 mm was coated with Paratone oil and mounted on a Mitegen MicroLoop. The sample was transferred to a Bruker D8 KAPPA series II with APEX II area-detector system equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). A total of 11 255 independent reflections (Rint = 0.0508) were collected in the range 3.00° < θ < 23.16°. X-ray data were collected at room temperature, because the crystal was found to be unstable at low temperature and cracked when it was cooled to collect data at 100 K. This was probably due to the formation of destructive solvent crystals from the disordered unidentified solvent molecules present in the voids. The frame width was set to 0.5° for data collection with a crystal-to-detector distance of 3.5 cm. Absorption corrections (SADABS)44 were applied to the collected data. The raw intensity data frames were integrated with the SAINT program,45 which also applied corrections for Lorentz and polarization effects. The software package Bruker SHELXTL was used for space group determination, structure solution, and refinement.46 The space group determination was based on a check of the Laue symmetry, and systematic absences were confirmed using the structure solution. The structure was solved by direct methods (SHELXS-97),

considerably robust, persisting after rinsing or soaking the electrode in CH2Cl2 (with or without dissolved electrolyte). The stability of electroactive films of 6 and 7 was further demonstrated by their nearly quantitative persistence after continuous CV scans (for more than 6 h) in OS-free CH2Cl2 with PF6− electrolyte anion. To the best of our knowledge, these are the first examples of electrode surfaces modified with films of redox-active OS, possessing a controlled number of electronically interacting metal centers.

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CONCLUSIONS In summary, we have designed and successfully synthesized the first redox-active OS, peripherally decorated with 24 ferrocenyl units, which are electronically communicated in threes through a silicon atom. Such integrally ferrocenyl-functionalized cubic macromolecules 6 and 7, as well as related model compounds 4 and 5, have been prepared by covalently linking, via hydrosilylation, triferrocenylvinylsilyl 3 around the surface of T8 cages and linear and cyclic siloxane scaffolds, respectively. Polyferrocenyl-OS show good thermal stability and form ironcontaining ceramics when pyrolyzed under nitrogen. In addition, we have demonstrated that 6 and 7 are able to form stable electrodes modified with electroactive films of polyferrocenyl-OS. The stability of films of OS 6 and 7 will enable their use in a wide variety of applications, including molecular recognition and biosensing. We are currently preparing 3-D dendritic structures, maximally functionalized with interacting ferrocenyl units, by using cubic OS as octa-arm cores.

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EXPERIMENTAL SECTION

Materials. Toluene and THF were distilled over sodium/ benzophenone under argon before use. Hexane and dichloromethane were dried by standard procedures over the appropriate drying agents and distilled under argon, immediately prior to use. Ferrocene (Aldrich) was purified by sublimation prior to use. Trichlorovinylsilane and tert-butyllithium (1.7 M solution in pentane) (Aldrich) were used as received. Platinum-divinyltetramethyldisiloxane complex in xylene (3−3.5% Pt concentration) (Karstedt′s catalyst, available from Aldrich), 1,1,3,3-tetramethyldisiloxane (ABCR), and 1,3,5,7-tetramethylcyclotetrasiloxane (available from ABCR) were used as received. Tetramethylammonium hydroxide, tetraethoxysilane, dimethylchlorosilane, anhydrous ferric chloride, sodium dodecylsulfate, and trichlorosilane (Aldrich) were used as starting materials for the synthesis of cubic precursors. The vinyl-functionalized precursor compound (CH2CH)Si(Fc)3 (3) was synthesized from monolithioferrocene (generated in situ from the reaction between ferrocene and t-BuLi, at low temperature) according to the procedure already described.29 Silica gel (70−230 mesh) (Merck) was used for column chromatographic purifications. Characterization. Infrared spectra were recorded on Bomem MB100 FT-IR and on Perkin-Elmer 100 FT-IR spectrometers. Elemental analyses were performed in a LECO CHNS-932 elemental analyzer, equipped with a MX5METTLER TOLEDO microbalance. All NMR spectra were recorded on Bruker-AMX-300 and Bruker DRX-500 spectrometers. Chemical shifts were reported in parts per million (δ) with reference to CDCl3 residual solvent resonances for 1H, δ 7.27 ppm and 13C, δ 77.0 ppm. 29Si NMR spectra were recorded with inverse-gated proton decoupling in order to minimize nuclear Overhauser effects. The matrix-assisted laser desorption/time-of-flight (MALDI-TOF) mass spectra were recorded using a Reflex III (Bruker) mass spectrometer equipped with a nitrogen laser emitting at 337 nm. Dichloromethane solutions of the matrix (dithranol, 10 mg/mL) and dichloromethane solutions of the corresponding compound (1 mg/mL) were mixed in the ratio 20:5. Then, 0.5−1 μL of the mixture was deposited on the target plate using the dried 3255


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monitored by 1H NMR from the progressive disappearance of the Si− H signal of the corresponding oligosiloxane precursor. This was done by the periodic removal of small aliquots from the reaction; toluene and other volatiles were removed under vacuum prior to obtaining the 1 H NMR spectra. After appropriate time, the 1H NMR spectroscopy showed the complete disappearance of the Si−H signal; at this moment the reaction was stopped. The mixture was allowed to cool to room temperature, and the solvent was removed under vacuum. Synthesis of Disiloxane [Fc3Si(CH2)2Me2Si]2O (4). This compound was prepared as above from 3 (0.3 g, 0.49 mmol) in 6 mL of toluene, 30 μL of Karstedt’s catalyst, and 40.86 μL of 1,1,3,3tetramethyldisiloxane (0.22 mmol). After 12 h at 45−50 °C, the orange-brown, oily residue was purified by column chromatography on silica gel (2 cm × 16 cm) using a mixture of hexane and CH2Cl2 as eluent. A first band containing the excess of compound 3 was eluted with hexane/CH2Cl2 (100:20). Then, on eluting with hexane/CH2Cl2 (100:50), a second major band was collected. Solvent removal afforded the desired compound 4 as an analytically pure, air-stable, orange, crystalline solid. (Yield: 0.26 g, 87%.) Anal. Calcd for C68H74Fe6OSi4: C, 60.26; H, 5.51. Found: C, 60.38; H, 5.31. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.25 (s, 12H, CH3), 0.93−1.27 (m, 8H, CH2), 4.03 (s, 30H, C5H5), 4.26, 4.39 (m, 24H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ −0.1 (CH3), 7.7, 11.4 (CH2), 68.5 (C5H5), 70.1 (ipso-Fc), 70.5, 74.0 (C5H4). 29Si{1H} NMR (CDCl3, 59 MHz, ppm): δ −8.4 (Fc−Si), 8.5 (CH3−Si−CH3). IR (KBr, cm−1): δ(SiM− C) 1252 νas(Si−O−Si) 1033−1050, ν(SiM−C) 832, 757. MS (MALDI-TOF): m/z 1354.2 [M+]. Synthesis of Dodecaferrocenyl Cyclotetrasiloxane [Fc3Si(CH2)2MeSiO]4 (5). Compound 3 (0.3 g, 0.49 mmol) in 6 mL of toluene, Karstedt’s catalyst (30 μL), and 1,3,5,7-tetramethylcyclotetrasiloxane (27.89 μL, 0.11 mmol) were used. After the reaction mixture was stirred for 12 h at 60 °C, the orange-brown, oily residue was purified by column chromatography on silica gel (2 cm × 20 cm) using a mixture of hexane and CH2Cl2 as eluent. A first band containing the excess of 3 was eluted with hexane/CH2Cl2 (100:20). Subsequently a second orange band was separated (hexane/CH2Cl2, 100:60), affording, after solvent removal, the desired compound 5 as an analytically pure, air-stable, orange, crystalline solid. (Yield: 0.24 g, 81%.) Anal. Calcd for C132H136Fe12O4Si8: C, 59.10; H, 5.11. Found: C, 59.19; H, 5.39. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.37 (s, 12H, CH3), 1.04−1.34 (m, 16H, CH2), 4.04 (s, 60H, C5H5), 4.24, 4.35 (m, 48H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ −0.6 (CH3), 7.6, 10.0 (CH2), 68.6 (C5H5), 69.8 (ipso-Fc), 70.5, 74.0 (C5H4). 29 Si{1H} NMR (CDCl3, 59 MHz, ppm): δ −18.4 (CH3−Si−O), −8.3 (Fc−Si). IR (KBr, cm−1): δ(SiD−C) 1259, νas(Si−O−Si) 1034−1106, ν(SiD−C) 819, 734. MS (MALDI-TOF): m/z 2680.9 [M+], 2070.9 [M − (Fc3SiC2H3)+], 583.1 [Fc3Si+]. Hydrosilylation of 3 and T8(OSiMe2H)8: Synthesis of Cubic OS [Fc3Si(CH2)2Me2SiO]8Si8O12 (6). The same hydrosilylation procedure was employed in the reaction of 3 (0.4 g, 0.65 mmol) in 8 mL of toluene, 40 μL of Karstedt’s catalyst, and 75.81 mg of octakis(hydridodimethylsiloxy)silsesquioxane in 2 mL of toluene (0.07 mmol). After 21 h at 60 °C, the orange-brown, oily residue was purified by column chromatography on silica gel (2 cm × 15 cm) using a mixture of hexane and CH2Cl2 as eluent. A first band containing the excess of compound 3 was eluted with hexane/CH2Cl2 (100:20). Then, on eluting with hexane/CH2Cl2 (100:70), a second major band was collected. Solvent removal afforded the desired cubic OS 6 as an analytically pure, air-stable, orange, crystalline solid. (Yield: 0.28 g, 68%.) Anal. Calcd for C272H296Fe24O20Si24: C, 55.36; H, 5.06. Found: C, 55.28; H, 5.20. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.28 (s, 48H, CH3), 0.88−1.20 (m, 32H, CH2), 4.02 (s, 120H, C5H5), 4.27, 4.38 (m, 96H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ −0.5 (CH3), 7.8, 10.4 (CH2), 68.7 (C5H5), 70.0 (ipso-Fc), 70.7, 74.1 (C5H4). 29 Si{1H} NMR (CDCl3, 59 MHz, ppm): δ −108.4 (Si−O4), −8.2 (Fc−Si), 13.4 (CH3−Si−CH3). IR (KBr, cm−1): δ(SiMQ−C) 1252, νas(Si−O−Si) 1033−1130, ν(SiMQ−C) 820, 758. MS (MALDI-TOF): m/z 5898.0 [M+], 2948.8 [M2+], 582.7 [Fc3Si+]. Hydrosilylation of 3 and T8H8: Synthesis of Cubic OS [Fc3Si(CH2)2]8Si8O12 (7). Compound 7 was synthesized following

completed with different Fourier syntheses, and refined with fullmatrix least-squares using SHELXL-97, minimizing w(Fo2 − Fc2)2.47,48 Weighted R factors (Rw) and all goodness of fit S are based on F2; conventional R factors (R) are based on F. Despite our best efforts to select a good crystal and optimize the collection parameters, final data collected from the selected single crystal fell short of excellence. As a result, the values of R indices are not as good as expected; nevertheless they are within the acceptable limits. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atom positions were calculated geometrically and were allowed to ride on their parent carbon atoms with fixed isotropic U. All scattering factors and anomalous dispersion factors are contained in the SHELXTL 6.10 program library. The crystal structure of 4 has been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition number CCDC 858006. Synthesis of Octakis(hydridodimethylsiloxy)silsesquioxane (T8(OSiMe2H)8). This OS was synthesized by adapting the literature procedures described by Caetano et al.,32 Marciniec et al.,19a and by our own group.27a In a 2 L round-bottomed flask were placed 41.0 mL of tetramethylammonium hydroxide (0.39 mol, 25 wt % solution in methanol), 20 mL of methanol, and 15 mL of water. The mixture was stirred and cooled with an external ice bath. Then, 22.0 mL of tetraethoxysilane (0.097 mol) was added. The solution was stirred during 24 h at room temperature. After this time, this mixture was added dropwise to another flask containing a solution of 42.9 mL of dimethylchlorosilane (0.40 mol) in 250 mL of hexane, also placed in an ice bath. The new mixture was stirred at room temperature another 3 h. Then, the organic layer was extracted and the solvent removed under vacuum, affording a white powder that was washed three times with methanol and dried. (Yield: 11.2 g, 89%.) 1H NMR (CDCl3, 300 MHz, ppm): δ 0.25, 0.26 (d, 48H, CH3), 4.73 (m, 8H, Si−H). 13 C{1H} NMR (CDCl3, 75 MHz, ppm): δ 0.02 (CH3). 29Si{1H} NMR (CDCl3, 59 MHz, ppm): δ −108.7 (Si−O4), −1.6 (Si−H). IR (KBr, cm−1): ν(Si−H) 2144, δ(Si−C) 1257, νas(Si−O−Si) 1099, δ(Si−H) 902, ν(Si−C) 838, 772. MS (MALDI-TOF): m/z 1041.0 [M + Na+]. Synthesis of Octa(hydrido)silsesquioxane (T8H8). This cube was synthesized by adapting the literature procedures described by Matisons et al.33 and by Shriver et al.34 In a three-necked, 2 L, roundbottomed flask equipped with a fritted gas dispersion tube, a pressure equalizing addition funnel, an Allihn condenser topped with gas inlet and bubbler, and a Teflon-covered magnetic stir bar, 50 mL of methanol, 140 mL of toluene, and 72 mL of hydrochloric acid (35%) were placed. To this rapidly stirred mixture, 30 g of anhydrous ferric chloride (0.18 mol) was slowly added. After the mixture was allowed to cool, a solution of sodium dodecylsulfate (0.5 g) in 480 mL of hexane was added, and the biphasic mixture was heavily stirred. A solution of 24.6 mL of trichlorosilane (0.24 mol) in 180 mL of cold hexane was added dropwise to the stirred mixture (over 3 h). After an additional hour of stirring, the reaction mixture was transferred into a separating funnel. The upper hexane layer was removed, filtered, and dried with anhydrous K2CO3 and then with CaCl2. The mixture was filtered for a second time, and the solvent was removed until it was ca. 60 mL. The white crystals formed, after cooling, were washed twice with cold hexane and dried. (Yield: 1.8 g, 14%.) 1H NMR (CDCl3, 300 MHz, ppm): δ 4.25 (s, 8H, Si−H). 29Si{1H} NMR (CDCl3, 59 MHz, ppm): δ −85.0 (H−Si−O3). IR (KBr, cm−1): ν(Si−H) 2294, νas(Si− O−Si) 1119−1182, δ(Si−H) 859. MS (EI): m/z 422.81 [M+]. General Hydrosilylation Reaction Procedure. All the hydrosilylation reactions were performed in all-glass apparatus, under an oxygen- and moisture-free atmosphere (Ar) using standard Schlenk techniques. A typical experimental procedure was as follows. In a 25 mL, three-necked, round-bottomed flask equipped with a gas inlet, a 25 mL pressure equalizing addition funnel, an Allihn condenser topped with gas inlet and bubbler, and a Teflon-covered magnetic stir bar, trimetallic compound 3 was dissolved in freshly distilled toluene. To the resulting orange solution, Karstedt’s catalyst was injected using a Hamilton precision syringe, under a flow of argon, and the mixture was aged at room temperature during 30 min. Subsequently the corresponding siloxane was slowly added dropwise. The hydrosilylation mixture was heated, and the progress of the reaction was 3256


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the same procedure described for 6. In this case, 0.4 g of 3 (0.65 mmol) in 8 mL of toluene, 40 μL of Karstedt’s catalyst, and 35.20 mg of octa(hydrido)silsesquioxane in 2 mL of toluene (0.07 mmol) were employed. After 12 h at 60 °C, the orange-brown, oily residue was purified by column chromatography on silica gel (2 cm × 14 cm) using a mixture of hexane and CH2Cl2 as eluent. After collecting the excess of compound 3 with hexane/CH2Cl2 (100:20), a second major band was eluted with hexane/CH2Cl2 (100:90) corresponding to a mixture of cubic species with different degrees of functionalization (from 5 to 8 substituted Si−H groups). A second column chromatography on silica gel (2 cm × 15 cm) of the cubic mixture was carried out. A major orange band was eluted (hexane/CH2Cl2 100:80), and solvent removal afforded the desired cubic 7. Further bands, eluted with different proportions of hexane/CH2Cl2, included mixtures of incompletely substituted cubic species [Fc3Si(CH2)2]7HSi8O12, [Fc3Si(CH2)2]6H2Si8O12, and [Fc3Si(CH2)2]5H3Si8O12, with 21, 18, and 15 ferrocenyl units, respectively. Octasilsesquioxane 7 was obtained as an orange solid. (Yield: 0.08 g, 22%.) Anal. Calcd for C256H248Fe24O12Si16: C, 57.92, H; 4.71. Found: C, 57.64; H, 4.55. 1 H NMR (CDCl3, 300 MHz, ppm): δ 1.00−1.50 (m, 32H, CH2), 4.00 (s, 120H, C5H5), 4.19, 4.30 (m, 96H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 7.5, 8.8 (CH2), 68.5 (C5H5), 68.9 (ipso-Fc), 70.5, 74.0 (C5H4). 29Si{1H} NMR (CDCl3, 99 MHz, ppm): δ −65.8 (Si− O4), −8.0 (Fc−Si). IR (KBr, cm−1): νas(Si−O−Si) 1034−1106. MS (MALDI-TOF): m/z 5305.0 [M+].

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10374−10382. (b) Hildebrandt, A.; Schaarschmidt, D.; Lang, H. Organometallics 2010, 29, 4900−4905. (c) Santi, S.; Orian, L.; Donoli, A.; Bisello, A.; Scapinello, M.; Benetollo, F.; Ganis, P.; Ceccon, A. Angew. Chem., Int. Ed. 2008, 47, 5331−5334. (8) Representative examples of compounds with multiple ferrocenyl groups bridged by p-block elements (E). E = Si: (a) Herbert, D. E.; Gilroy, J. B.; Chan, W. Y.; Chabanne, L.; Staubitz, A.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2009, 131, 14958−14968. (b) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683−12695. (c) Pannell, K. H.; Dementiev, V. V.; Li, H.; Cervantes-Lee, F.; Nguyen, M. T.; Diaz, A. F. Organometallics 1994, 13, 3644−3650. (d) Alonso, B.; Ramírez, E.; Zamora, M.; Casado, C. M.; Cuadrado, I. J. Organomet. Chem. 2001, 637−639, 642−652. E = P: (e) Barrière, G. F.; Kirss, R. U.; Geiger, W. E. Organometallics 2005, 24, 48−52. (f) Seibert, A. R.; Cain, M. F.; Glueck, D. S.; Nataro, C. J. Organomet. Chem. 2011, 696, 2259−2262. E = N: (g) Alvarez, J.; Ren, T.; Kaifer, A. E. Organometallics 2001, 20, 3543−3549. (9) Geiger, W. E. Organometallics 2007, 26, 5738−5765. (10) For leading reviews on ferrocene-containing macromolecules and polymers, see for example: (a) Nguyen, P.; Gómez-Elipe, P.; Manners, I. Chem. Rev. 1999, 99, 1515−1548. (b) Pittman, C. U. J. Inorg. Organomet. Polym. 2005, 15, 33−55. (c) Bellas, V.; Rehahn, M. Angew. Chem., Int. Ed. 2007, 46, 5082−5104. (d) Abd-El-Aziz, A. S.; Manners, I. J. Inorg. Organomet. Polym. 2005, 15, 157−195. (e) Bellas, V.; Rehahn, M. Angew. Chem., Int. Ed. 2007, 46, 5082−5104. (f) Whittell, G. R.; Manners, I. Adv. Mater. 2007, 19, 3439−3468. (g) Casado, C. M.; Cuadrado, I.; Morán, M.; Alonso, B.; García, B.; González, B.; Losada, J. Coord. Chem. Rev. 1999, 185−186, 53−79. (11) (a) Cuadrado, I. In Silicon-Containing Dendritic Polymers; Dvornic, P.; Owen, M. J., Eds.; Springer: Berlin, 2009; pp 141−196. (b) Cuadrado, I.; Casado, C. M.; Alonso, B.; Morán, M.; Losada, J.; Belsky, V. J. Am. Chem. Soc. 1997, 119, 7613−7614. (12) Astruc, D.; Ornelas, C.; Ruiz, J. Acc. Chem. Res. 2008, 41, 841− 856. (13) (a) Wang, W.; Sun, H.; Kaifer, A. E. Org. Lett. 2007, 1, 2657− 2660. (b) Wang, Y.; Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 9756−9757. (14) Ornelas, C.; Ruiz, J.; Belin, C.; Astruc, D. J. Am. Chem. Soc. 2009, 131, 590−601. (15) Ghosh, K.; Hu, J.; White, H. S.; Stang, P. J. J. Am. Chem. Soc. 2009, 131, 6695−6697. (16) For excellent recent reviews on silsesquioxanes see: (a) Laine, R. M.; Roll, M. F. Macromolecules 2011, 44, 1073−1109. (b) Cordes, D. B.; Lickiss, P. D.; Rataboul, F. Chem. Rev. 2010, 110, 2081−2173. (c) Lickiss, P. D.; Rataboul, F. Adv. Organomet. Chem. 2008, 57, 1− 116. (17) Applications of Polyhedral Oligomeric Silsesquioxanes; HartmannThompson, C., Ed.; Springer: New York, 2011. (18) (a) Li, G.; Wang, L.; Ni, H.; Pittman, C. U. J. Inorg. Organomet. Polym. 2001, 11, 123−154. (b) Pielichowski, K.; Njuguna, J.; Janowski, B.; Pielichowski, J. Adv. Polym. Sci. 2006, 201, 225−296. (c) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 95, 1409− 1430. (d) Voronkov, M. G.; Lavrent’yev, V. I. Top. Curr. Chem. 1982, 102, 199−236. (e) Feher, F. J. In Gelest Catalog. Polyhedral Oligosilsesquioxanes and Heterosilsesquioxanes; Gelest Inc.: Tullytown, PA, 2000; pp 43−59. (19) For recent, remarkable examples of functionalized octasilsesquioxanes see, for example: (a) Dutkiewicz, M.; Maciejewski, H.; Marciniec, B.; Karasiewicz, J. Organometallics 2011, 30, 2149−2153. (b) Zhang, W.; Li, Y.; Li, X.; Dong, X.; Yu, X.; Wang, C. L.; Wesdemiotis, C.; Quirk, R. P.; Cheng, S. C. Macromolecules 2011, 44, 2589−2596. (c) Roll, M. F.; Kampf, J.; Laine, R. M. Macromolecules 2011, 44, 3425−3435. (d) Zak, P.; Marciniec, B.; Majchrzak, M.; Pietraszuk, C. J. Organomet. Chem. 2011, 696, 887−891. (e) Liu, H.; Kondo, S.; Takeda, N.; Unno, M. J. Am. Chem. Soc. 2008, 130, 10074− 10075. (f) Ervithayasuporn, V.; Tomeechai, T.; Takeda, N.; Unno, M.; Chaiyanurakkul, A.; Hamkool, R.; Osotchan, T. Organometallics 2011, 30, 4475−4478.

ASSOCIATED CONTENT

S Supporting Information *

Supplementary figures referenced in the text; experimental procedures; spectroscopic and X-ray crystallographic data (for 4), and CV and SWV of 4−7. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Spanish Ministerio de Ciencia e Innovación, project CTQ2009-09125/BQU. S.B. acknowledges the Ministerio de Educación for a FPU grant.

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REFERENCES

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