Multi-Ferrocene-Containing Silanols as Redox-Active Anion

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Multi-Ferrocene-Containing Silanols as Redox-Active Anion Receptors Sonia Bruña,† Alberto F. Garrido-Castro,† Josefina Perles,‡ M. Merced Montero-Campillo,§ Otilia Mó,§ Angel E. Kaifer,⊥ and Isabel Cuadrado*,† †

Departamento de Química Inorgánica, Facultad de Ciencias, ‡Laboratorio de Difracción de Rayos X de Monocristal, Servicio Interdepartamental de Investigación (SIdI), and §Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain ⊥ Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431, United States S Supporting Information *

ABSTRACT: The ability of diferrocenylsilanediol, Fc2Si(OH)2 (5), and 1,1,3,3tetraferrocenyldisiloxane-1,3-diol, Fc2(HO)Si−O−Si(OH)Fc2 (6), to act as new electroactive anion receptors for either acetate or chloride anions has been investigated in solution, in the solid state, and in the gas phase. 1H NMR spectroscopic titrations with anions reveal that the binding interaction causes chemical-shift perturbations not only in the Si−OH hydrogen-bonding donor motif but also in the ferrocenyl protons of receptors 5 and 6. Square-wave voltammetric studies evidence that multiferrocenyl silanols 5 and 6 exhibit higher ability for electrochemical sensing of acetate than chloride, since the corresponding half-wave potentials (E1/2) for the successive ferrocene oxidations display a higher cathodic shift in the presence of such an anion. Furthermore, single-crystal X-ray diffraction analyses of the tetrabutylammonium salts of complexes [Fc2Si(OH)2·CH3COO]− (8), [Fc2Si(OH)2·Cl]− (9), [Fc2(HO)Si−O−Si(OH)Fc2· CH3COO]− (10), [{Fc2(HO)Si−O−Si(OH)Fc2}2·CH3COO]− (11), and [Fc2(HO)Si−O−Si(OH)Fc2·Cl]− (12) confirm that redox-active silanol receptors 5 and 6 can bind the acetate and chloride anions in the solid state. Electronic structure calculations were carried out for 5 to explore the intrinsic ability of the silanediol group to bind these anions in a vacuum.

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INTRODUCTION Silanols are receiving increasing attention in many areas of chemistry, as these silicon compounds of general formula RnSi(OH)4−n show impressive potential for a variety of interesting applications.1 First, organosilanols play important roles as intermediates in the industrial-scale production of silicone polymers and sol−gel processing.2 Furthermore, silanols are well known for their extensive self-organization in the solid state, which makes them valuable building blocks for supramolecular chemistry.3 Moreover, suitably substituted silanols are important as precursors of a new class of environmentally friendly surfactants.4 On the other hand, organosilanols can function as bioisosteres and transition-state analogues in drug design, where the enhanced acidity of the Si− OH group can improve binding to a receptor.5 For example, the Si−OH-containing compound (R)-sila-venlafaxine, the silicon analogue of venlafaxine, which is a serotonin/noradrenaline reuptake inhibitor, may provide therapeutic benefit in the treatment of various nervous system disorders.6 Antimicrobial activities of some triorganosilanols have also been evaluated, where silanols exhibit significantly higher biocidal properties relative to carbon analogues, which is attributed to their greater H-bond acidity and their enhanced lipophilicity.7 Very recently, Kondo, 8 Mattson,9 and Frank10 have demonstrated that organosilanols have great potential as new © XXXX American Chemical Society

scaffolds in molecular recognition and organocatalysis, due to their ability to act both as donor and as acceptor. Specifically, silanediols (R2Si(OH)2) are of interest since they contain a geminal silicon diol that is not commonly accessible for carbon analogues and has the potential to serve as a dual hydrogenbonding moiety. In addition, it has been highlighted that silanols have unique advantages as donors in palladiumcatalyzed cross-coupling reactions with a wide variety of substrates.1e Figure 1 shows some interesting examples of ferrocenefunctionalized silanols, including monosilanols 111 and 2, bissilanol 3,12 silanediols 4 and 5, and disiloxanediol 6. In particular, triferrocenylsilanol Fc3Si(OH) (2) and diferrocenylsilanediol Fc2Si(OH)2 (5) (Fc = Fe(η5-C5H4)(η5-C5H5)) were first mentioned in a very brief note nearly 50 years ago, and they were isolated in very low yields.13 Improved syntheses of these organometallic silanols were reported by the Manners group in the late 1990s.14,15 Independently, Pannell and coworkers prepared silanediol compounds of the type Fc(R)Si(OH)2 (4) and evaluated their capacity to form cyclic stannasiloxanes.16 Likewise, Manners and co-workers described the synthesis of tetraferrocenyldisiloxane-1,3-diol Fc2(HO)Si− Received: July 12, 2016

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DOI: 10.1021/acs.organomet.6b00559 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

This is because anions are ubiquitous in nature and play significant roles in a wide range of chemical, biological, medical, and environmental processes. Within this context, it is well established that anions can be sensed electrochemically. This goal can be achieved with suitable molecules bearing a redoxactive group in close proximity to the anion binding site, in such a way that their redox properties change when the binding event occurs. In particular, receptors containing the ferrocene moiety are especially valuable as electrochemical anion sensors,24 since this chemically robust metallocene undergoes fast and reversible one-electron oxidation to the positively charged ferrocenium form at accessible potentials. Surprisingly, and although a wide number of ferrocene-based redox-active receptors bearing N−H groups such as amide,25 urea,26 thiourea,27 triazole,28 amidine,29a guanidine,29b and aminosilane moieties30 have been reported, the use of ferrocene-containing receptors incorporating the Si−OH group as an anion recognition motif remains unexplored. Herein, we describe our recent studies in solution and in the solid state, combined with computational investigations in gaseous phase, regarding the anion binding ability of electroactive ferrocene-functionalized silanol receptors 5 and 6.

Figure 1. Structures of ferrocene-bearing monosilanols, silanediols, and a disiloxane-1,3-diol.

O−Si(OH)Fc2 (6) and studied its ability to undergo basecatalyzed condensations to afford hexaferrocenylcyclotrisiloxane, [Fc2SiO]3 (7),17 as well as boron- and zirconiumcontaining heterocyclosiloxanes.18,19 During the past few years, we have described synthetic methods for obtaining a variety of families of redox-active silicon-bridged multiferrocenyl molecules functionalized with vinylsilane and hydrosilane reactive groups.20−22 Over the course of their preparations we have noticed that, in addition to the desired Si−CHCH2- or Si−H-functionalized ferrocenyl compounds, other silanol-based ferrocenes were obtained as side products during the purification steps.21,22 Namely, monosilanol 2 and silanediols 5 and 6 were isolated in variable yields depending on the type of chlorosilane and nucleophilic ferrocenyl reagent used. Motivated by the excellent hydrogen-bonding abilities shown by organosilanols, we have decided to explore the use of multiferrocenyl silanols 5 and 6 as a novel type of redox-active receptors for anions. The development of new receptors capable of binding and sensing anionic guest species remains an ongoing challenge in the field of supramolecular chemistry.23

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RESULTS AND DISCUSSION Synthesis and Redox Chemistry of Multiferrocenyl Silanols 5 and 6. In general, silanols can be prepared from chlorosilanes, siloxanes, and hydrosilanes through hydrolysis, nucleophilic substitution, and oxidation reactions, respectively.1a−d While recent reports are focusing on the catalytic transformation of hydrosilanes into the corresponding silanols,31 chlorosilanes are the most commonly used precursors for the synthesis of this type of organosilicon compounds. Scheme 1 shows different synthetic approaches leading to the key diferrocenylsilanediol 5 used in this study, involving reactions of two different chlorosilanes: bis(N,Ndimethylamino)dichlorosilane and silicon tetrachloride. As mentioned above, Fc2Si(OH)2 (5) was initially obtained in 1967, presumably as a side product of the Friedel−Crafts

Scheme 1. Different Synthetic Routes for Diferrocenylsilanediol 5

B


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Organometallics silylation reaction of ferrocene.13 In the original synthetic procedure of 5, ferrocene was treated with SiCl2(NMe2)2, in the presence of aluminum chloride catalyst (route A, Scheme 1), and the mixture was refluxed in n-octane. After hydrolysis, silanediol 5 was isolated in very low yield (4%) as a crystalline product, along with the related triferrocenylsilanol Fc3SiOH (2) (5% yield). Years later, silanediol 5 was successfully prepared in higher yield in a remarkable and elegant multistep synthesis as shown in route B of Scheme 1. Reaction of silicon tetrachloride with 1,1′-dilithioferrocene·TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) afforded the highly strained spirocyclic [1]silaferrocenophane [(η5-C5H4)2Fe]2Si.15b Relief of the ring strain drove the reaction of this ferrocenophane to dichlorodiferrocenylsilane, Fc2SiCl2, upon addition of an ethereal solution of hydrogen chloride. Subsequent hydrolysis of Fc2SiCl2 in the presence of triethylamine afforded the silanediol Fc2Si(OH)2 (5) in high yield.15a A third possible route to diferrocenyl silanediol 5 could be a direct salt metathesis reaction of SiCl4 with 2 equiv of monolithioferrocene (FcLi), in order to generate the intermediate dichlorosilane Fc2SiCl2, followed by hydrolysis to obtain Fc2Si(OH)2 (5), as shown in route C of Scheme 1. Such a direct procedure could be a good alternative for the preparation of silanediol 5 since it would avoid the formation of the highly reactive [1]silaferrocenophane precursor and especially its complex purification. Indeed, because of the low stability of this strained spirocycle on different types of silica gel and aluminum oxide, we have observed that its purification using standard column chromatographic methods is difficult to achieve. Monolithioferrocene was generated in situ from the reaction between ferrocene and t-BuLi, in a 10:3 mixture of THF and nhexane at −30 °C. Subsequently, without isolation of the pyrophoric solid FcLi, freshly distilled SiCl4 was added dropwise to the reaction mixture. After removing the LiCl byproduct, the red-orange solid was dissolved in diethyl ether and was added slowly to a mixture of triethylamine and water. Purification of the crude reaction product was successfully accomplished by column chromatography using silanized silica gel in order to avoid the facile reaction between the −Si−OH groups of both the ferrocenyl compounds formed and the silica surface. Following this approach, we were able to isolate the desired diferrocenilsilanediol 5 (eluted with n-hexane/CH2Cl2, 1:2) and tetraferrocenyldisiloxanediol 6 (with n-hexane/ CH2Cl2, 1:1) in high purity and in 32% and 10% yields, respectively. The disiloxanediol tetrametallic side product 6 was formed under the used reaction conditions, as a result of the condensation reaction of a single Si−OH bond in 5. Once isolated, silanols 5 and 6 were thoroughly characterized by elemental analysis, IR, 1H, 13C, and 29Si NMR spectroscopy, and MALDI-TOF mass spectrometry (see the Experimental Section). During purification of the target silanediol 5 we found that other silanols and siloxane oligomeric side products with different numbers of ferrocenyl units, including cyclotrisiloxane [Fc2SiO]3 (7), were formed in minor amounts, as we discerned through 1H NMR and MALDI-TOF analysis. This approach (route C, Scheme 1) is certainly useful, as it allows the formation of the target silanediol 5 in reasonable yield and because the requirement for a multistep synthesis as the alternative route B is avoided. Although 5 and 6 were structurally characterized in the late 1990s,15,17 the electrochemical behavior of these compounds

has yet to be reported, to the best of our knowledge. Consequently, before testing if these redox-active ferrocenecontaining silanols were capable of electrochemically recognizing anionic guest species, their anodic voltammetric response was examined in detail. Cyclic voltammetry (CV) was measured using dichloromethane as a non-nucleophilic solvent and tetra-n-butylammonium hexafluorophosphate and tetra-nbutylammonium tetrakis(pentafluorophenyl)borate as supporting electrolytes, containing anions of different coordinating ability. As seen in Figure 2A, the CV of diferrocenylsilanediol 5 in CH2Cl2 solution with the traditional supporting electrolyte

Figure 2. Cyclic voltammetric responses on a Pt-disk electrode of CH2Cl2 solutions containing (A) 5 + [n-Bu4N][PF6]; (B) 6 + [nBu4N][PF6]; (C) 5 + [n-Bu4N][B(C6F5)4]; and (D) 6 + [nBu4N][B(C6F5)4]. Scan rate: 0.1 V s−1.

tetra-n-butylammonium hexafluorophosphate ([n-Bu4N][PF6]) shows two closely spaced voltammetric waves. The shape of the cathodic peak for the second process (with an anodic peak at Epa = +0.64 vs SCE) suggests some adsorption of the fully oxidized form. Better resolution of the redox processes was observed in the CV response of 5 measured in CH2Cl2/ CH3CN with the same supporting electrolyte, which shows two well-separated reversible redox waves at 1E1/2 = +0.43 and 2E1/2 = +0.57 V (vs SCE) (see Figure S26 in the Supporting Information). The potential difference ΔE1/2 = 0.14 V (ΔE1/2 = 2 E1/2 − 1E1/2) is typical of ferrocenyl moieties experiencing moderate electronic interactions. In the case of the tetrametallic silanol 6, the CV in CH2Cl2 with [n-Bu4N][PF6] as supporting electrolyte (Figure 2B) exhibits a sequence of several overlapped and poorly resolved oxidation waves (from about +0.58 to +0.75 V vs SCE). The shape of the voltammetric waves departs from that expected for reversible oxidation processes. Specifically, the anodic and cathodic peak currents are unequal, and a very distorted cathodic peak is observed, different from the typical diffusional shape and closer to a surface wave. This clearly indicates that for disiloxane 6, with four ferrocenyl units, a drastic change in solubility accompanies the change in oxidation state, so that, upon scan reversal after the oxidation, the reduction peak appears as a sharp stripping peak.32 Thus, the full oxidation of 6 results in the precipitation of the oxidized species 64+ onto the electrode surface, and, on the reverse scan, it is redissolved upon reduction. To better clarify the redox behavior of multiferrocenyl silanols, we also used tetra-n-butylammonium tetrakisC


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Organometallics (pentafluorophenyl)borate ([n-Bu4N][B(C6F5)4]) as supporting electrolyte in CH2Cl2 solution. Our previous studies on silicon- and disiloxane-bridged polyferrocenyl compounds have shown that the use of [B(C6F5)4]− as the supporting electrolyte anion provided improved solubility and enhanced peak separation, in cyclic and square-wave voltammetric experiments.20a−c,21,22 Geiger and co-workers have performed remarkable and extensive studies on this fluoroarylborate anion and have ascribed the reported larger peak separation to its substantially lower coordinating or ion-pairing ability (as compared to the PF6− anion).33,34 In Figure 2C and D it becomes patently obvious that improved electrochemical reversibility and better resolution are achieved when [B(C6F5)4]− is employed instead of [PF6]−. With [B(C6F5)4]− no effects of adsorption on the working electrode were observed, which indicates an increase in solubility of the multiferrocenium electrogenerated products 52+ and 64+ in this solvent/ electrolyte medium. In this way, the CV of tetrametallic 6 clearly shows four well-separated and reversible one-electron oxidation waves, for each of the four ferrocenyl moieties. The mechanism for the electrochemical oxidation of this tetraferrocenyldisiloxane is shown in Scheme S1 (in the Supporting Information) and involves a first oxidation (at 1E1/2 = +0.39 V vs SCE) corresponding to the generation of monocationic species 6+. At a higher potential (2E1/2 = +0.52 V) a second electron is removed from a ferrocenyl moiety attached to the neighboring silicon atom, at the other end of the Si−O−Si bridge, yielding the dicationic species 62+. The third oxidation (at 3E1/2 = +0.73 V) occurs at one of the two remaining ferrocenyl moieties, that adjacent to the last oxidized ferrocenyl subunit.21 The final oxidation of the last neutral ferrocenyl center is the most difficult one and takes place at a more anodic potential (4E1/2 = +0.89 V), giving rise to tetracationic species 64+. The considerable spread of the four oxidations in disiloxane 6 suggests appreciable electronic interactions between the Siand Si−O−Si-bridged ferrocenyl moieties as they are gradually oxidized.21 Anion Recognition Properties in Solution of Ferrocene-Containing Silanols 5 and 6. Evidence of the anion recognition ability of silanols 5 and 6 in liquid phase was provided by 1H NMR spectroscopy and electrochemical studies. 1 H NMR Titration Studies. To measure the anion binding strength of ferrocene-containing silanols 5 and 6, we performed 1 H NMR spectroscopic titration experiments. Formerly, the possibility of intermolecular hydrogen bond formation for these two receptors in CDCl3 solution was discarded, as their silanol chemical shifts showed no dependence on concentration. NMR samples of the two compounds were prepared in CDCl3, to which aliquots of tetrabutylammonium salts of acetate (Yshaped) or chloride (spherical) anions were added. The chemical shifts of signals arising from the protons of the Si− OH donor motif and the cyclopentadienyl rings were monitored, and the results are shown in Figures 3 and 4. Addition of chloride anion results in a similar pathway for the silanol resonances of both compounds 5 and 6 (Figure 3A and C). Thus, as progressive equivalents of Cl− are added to the ferrocene-containing silanol, significant downfield shifts for the corresponding Si−OH signals are observed, suggesting complexation of the chloride anion and the silanol groups through hydrogen bonds. Namely, upon addition of 2 equiv of this anion, the shifts for silanol resonances (Δδ) range from 1.07 ppm (for 5) to 1.19 ppm (for 6). In addition,

Figure 3. Evolution of the 1H NMR spectra (CDCl3, 300 MHz) of silanols 5 (0.02 M) (A and B) and 6 (0.01 M) (C and D) upon addition of increasing amounts of [n-Bu4N][Cl].

Figure 4. 1H NMR spectra (cyclopentadienyl region) of receptors 5 (0.02 M) (A) and 6 (0.01 M) (B) recorded in the presence of increasing concentrations of [n-Bu4N][AcO] (CDCl3, 300 MHz).

perturbations of the ferrocenyl protons are also detected with increasing anion concentration. While the resonances corresponding to the hydrogen atoms of the furthest η5-C5H5 rings remain almost unaltered for 5 and 6, the η5-C5H4 protons are considerably affected. In the case of diferrocenylsilanediol 5, the two signals at δ 4.31 and 4.41 ppm gradually approach each other with the progressive addition of chloride anion (Figure 3B), until they finally merge into a single resonance (at δ 4.34 ppm) upon addition of 2 equiv. The sequence of complex formation could be summarized as 5 → 5·Cl− (Scheme 2). Meanwhile, the addition of up to 2 equiv of Cl− to tetraferrocenyldisiloxanediol 6 (Figure 3D) involves the transformation of the two initial signals (at δ 4.32 and 4.40 ppm) of intensity 1:3 into three of intensity 1:2:1 (at δ 4.31, D


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Organometallics Scheme 2. Schematic Representation of the Possible Anion Binding Modes for Silanols 5 and 6

4.36, and 4.42 ppm), suggesting the transformation 6 → 6·Cl−. Addition of up to 5 equiv of Cl− revealed no significant proton modifications for both silanols 5 and 6. Upon addition of acetate anion (AcO−) as a guest, a major downfield shift of the Si−OH resonance of diferrocenylsilanediol 5 is noticed (see Figure S29 in the Supporting Information), from δ 2.62 ppm (in the free receptor) to δ 4.56 ppm (when 1 equiv of acetate is added), inferring a difference of Δδ = 1.94 ppm. At the same time, the OH resonance progressively broadens. This trend is considerably greater in the case of compound 6 (Figure S31), leading to the disappearance of the signal after the addition of 1 equiv of AcO− and preventing shift measurements. Fortunately, the perturbations in the cyclopentadienyl area are clear and point to several assumptions. The two initial C5H4 signals at δ 4.31 and 4.41 ppm of compound 5, in the presence of acetate, experience an interesting alteration. As shown in Figure 4A, after 0.5 equiv of acetate is added a new resonance at δ 4.33 ppm is observed, while upon the addition of 1 equiv of acetate, this resonance is split into two new signals of relative intensity 1:1 (at δ 4.27 and 4.30 ppm). These results suggest the formation of different complexes that may well correspond to stoichiometries of the type 5·AcO·5 and 5·AcO (Scheme 2) as argued below. The addition of acetate to the solution of siloxanediol 6 gives rise to similar trends in the η5-C5H4 region (Figure 4B). Again, after 0.5 equiv of acetate three signals (at δ 4.32, 4.35, and 4.39 ppm) of intensity 1:2:1 can be noticed. These resonances still suffered modifications until 1 equiv is added, again resulting in only two signals, but this time of intensity 3:1 (at δ 4.32 and 4.39 ppm). This fact seems to agree with the process 6 → 6·AcO−·6 → 6· AcO−, where two new supramolecular species of different proportions are progressively formed. Host:guest complexes with stoichiometry 2:1, similar to 6·AcO−·6, have been proposed by Kondo and Unno in the anion recognition studies of the closely structurally related receptor Ph2(HO)Si−O− Si(OH)Ph2 having the same disiloxane-1,3-diol skeleton.8b Finally, the addition of more than 1 equiv of acetate anion caused no significant modifications in the 1H NMR spectra of silanols 5 and 6. The binding modes in which Cl− and AcO−

bind silanol 5, as well as findings related to the 5·AcO−·5 complex, are described in the theoretical studies section. Fitting of the data obtained from the 1H NMR spectroscopic titrations to binding isotherms allowed us to determine the equilibrium association constants (K) for the formation of the 5·Cl− and 6·Cl− 1:1 complexes (Figures S32 and S33).35 The K values obtained were 47 and 6 M−1. Clearly, the main structural difference between receptors 5 and 6 is that in 6 the two OH groups are bonded to different silicon atoms in the Si−O−Si bridge, whereas, in compound 5, the two OH are closer to each other since they are linked to the same silicon atom. Probably, the cooperative action of the two OH motifs in 5 facilitates binding of a relatively small anion, such as the spherical monatomic Cl−. The fits of the experimental data to the 1:1 binding isotherms were excellent, suggesting that species of other stoichiometries, such as Cl−·5·Cl− and Cl−·6·Cl−, do not play a significant role in the complexation process. In contrast to Cl− binding, the NMR titration data between compound 5 and AcO− could not be fitted accurately to a 1:1 binding isotherm, probably because of a more significant involvement by complexes of other stoichiometries, such as the 2:1 complex 5·AcO−·5. Electrochemical Anion Recognition Studies. A remarkable structural feature of molecules 5 and 6 is the presence of multiple redox-active ferrocene moieties in close proximity to the hydrogen bond donor silanols. This gives these organometallic receptors the potential ability to electrochemically sense anionic guests through cathodic perturbations of the corresponding ferrocene/ferrocenium (Fc/Fc+) redox couples. To investigate the electrochemical anion recognition properties of multiferrocenyl silanols 5 and 6, aliquots of the same anions used in the 1H NMR spectroscopic investigations were added to samples of the ferrocene-containing silanols. Squarewave voltammetric (SWV) experiments were carried out in mixed CH2Cl2/CH3CN electrolyte solutions of 0.1 M [nBu4N][PF6]. SWV was used, instead of CV, because of its higher sensitivity and in order to obtain well-resolved individual redox processes. Unfortunately, the lack of solubility precluded analogous measurements being carried out in more polar E


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separated oxidation events, of different size at half-wave potential (E1/2) values of +0.44, +0.50, and +0.66 V vs SCE (see Figure 5C). The third peak is broader than the first two, suggesting that this wave involves two unresolved one-electron processes. The addition of increasing amounts of AcO− leads to a progressive negative shift in the potential of the redox waves. These cathodic shifts can be attributed to the binding of the anion by the Si−OH protons, facilitating the ferrocene to ferrocenium conversion. It is interesting to note that the coordination of an acetate anion induces a different effect on the oxidation potential of the ferrocenyl moieties in this silanediol receptor. Thus, the first and second redox waves merge in a new one at +0.44 V involving two redox processes. In contrast, the more anodic wave in the free receptor is transformed into two new waves at around +0.60 V after 1 equiv of AcO− is added. As compared to compound 5, the more moderate anion-induced potential shifts in this case suggest that AcO− binding to 6 is weaker. The addition of substoichiometric equivalents of Cl− anion to solutions of both electroactive silanols 5 and 6 also causes perturbations in the SWV responses, although the magnitude of cathodic shifts is smaller (see Figure S34), probably as a result of the weak association of the silanols with chloride. Thus, the more anodic peak is slightly shifted cathodically by −40 mV (for 5) and −20 mV (for 6), whereas the first oxidation peaks remain almost unaltered for both compounds. In all cases the addition of more than 1 equiv of anion results in the precipitation of the corresponding species. To the best of our knowledge, compounds 5 and 6 are the first examples of redox-active silanol-based systems capable of electrochemically sensing acetate and chloride anions. Solid-State Structures of Complexes 8−12. Reaction of the corresponding silanol 5 or 6 and tetrabutylammonium acetate or chloride in CH2Cl2 afforded the five complexes shown in Figure 6 with the formal stoichiometries of [nBu4N][Fc2Si(OH)2·CH3COO] (8), [n-Bu4N][Fc2Si(OH)2·Cl] (9), [n-Bu4N][Fc2(HO)Si−O−Si(OH)Fc2·CH3COO] (10), [n-Bu 4N][{Fc2(HO)Si−O−Si(OH)Fc 2}2·CH3COO] (11), and [n-Bu4N][Fc2(HO)Si−O−Si(OH)Fc2·Cl] (12). Interestingly, treatment of 6 with acetate afforded two compounds of different stoichiometry, 10 and 11. Further support for the complexation of acetate and chloride anions by silanol-based receptors 5 and 6 was obtained from single-crystal X-ray diffraction analyses. Crystallization in CH2Cl2/n-hexane (1:2) at −20 °C yielded diffraction-grade single crystals of complexes 8−12. The resulting structures are shown in Figures S35−S39 and provide conclusive evidence of the solid-state arrangement of the molecules in the crystal. Complete structural information is collected in the Supporting Information, and Table S1 contains selected structural parameters for compounds 8−12. The crystal structures of 8 and 9 both contain the same diferrocenylsilanediol 5 molecule, plus an anionic moiety (acetate in 8 and chloride in 9), and tetrabutylammonium as a cationic unit in a 1:1:1 stoichiometric ratio. Some selected structural parameters are collected in Tables S8 and S9 in the Supporting Information. Bond distances and angles around the silicon atoms are within the expected values, as well as distances between hydroxyl oxygen atoms (Tables S8−S10). Compound 8 crystallizes in the monoclinic P21/c space group with one diferrocenylsilanediol 5 molecule, one acetate anion, one tetrabutylammonium cation, and two halves of water molecules per asymmetric unit (Figure S35), and the distance

organic media such as pure CH3CN or DMSO solutions. Representative SWV responses of silanol receptors 5 and 6 in the presence of the oxoanion AcO− and the halide anion Cl− are shown in Figures 5 and S34, respectively.

Figure 5. Square-wave voltammograms of 5 (A and B) and 6 (C and D) (10−3 M) on a Pt-disk working electrode recorded in CH2Cl2/ CH3CN (1:1 for 5 and 1:2 for 6) containing 0.1 M [n-Bu4N][PF6], upon addition of increasing amounts of AcO− added as the [n-Bu4N]+ salt.

In the absence of anionic guest species (black lines in Figure 5), the SWV of diferrocenylsilanediol 5 in the solvent mixture of 1:1 CH2Cl2/CH3CN shows two well-resolved one-electron oxidations for the ferrocenyl moieties, at half-wave potential values of 1E1/2 = +0.43 and 2E1/2 = +0.58 V vs SCE. As it is apparent from Figure 5A and B, both redox waves are strongly affected by the addition of increasing substoichiometric amounts of [n-Bu4N][AcO], resulting in the observation of a “two-wave behavior”36 electrochemical response for the two oxidation processes. The current intensity of the new anodic peaks increases gradually, reaching its maximum at 1 equiv of AcO− anion. The new waves clearly grow to the detriment of the original redox waves, which disappear upon the addition of 1 equiv of AcO−. The appearance of two new peaks at more negative potentials 1E1/2 = +0.34 V vs SCE (Δ1E1/2 = 1E1/2(5 free) − 1E1/2(5+AcO-) = +90 mV) and 2E1/2 = +0.48 V (Δ2E1/2 = 2 E1/2(5 free) − 2E1/2(5+AcO-) = +100 mV), along with the original peaks corresponding to free silanediol 5, indicates that coordination of the AcO− anionic guest to the Si−OH protons, in close proximity to the ferrocenyl moieties, facilitates their oxidation to ferrocenium cations. The binding affinity of the AcO− anion and the neutral diferrocenylsilanediol 5 prior to oxidation is considerable, as evidenced from the quantitative two-wave voltammetric behavior observed (Figure 5B). Therefore, these findings plainly indicate the formation of a stable complex composed of one AcO− anion and one molecule of silanediol 5, in good agreement with the NMR spectroscopic data. Using the acetate-induced shift in the half-wave potential for the oxidation of the first ferrocene unit, we can estimate that the binding constant is enhanced by a factor of 33 upon oneelectron oxidation. The electrochemical response of tetraferrocenyldisiloxanediol 6 in the presence of anions is more complex. The SWV of free receptor 6 (in 1:2 CH2Cl2/CH3CN) exhibits three slightly F


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Figure 7. Hydrogen bond interactions (in blue) found in the crystal structures of 8 (top) and 9 (bottom) between diferrocenylsilanediol molecules and the present anionic units.

of arrangement, instead of the expected location of the anion between the two hydroxyl groups, had been previously reported9d for isoquinolinium bis(1-naphthyl)silanediol chloride. The main difference between positions A and B for Cl1 in the structure of 9 is that Cl1B acts as a bridge between two diferrocenylsilanediol molecules, while Cl1A interacts with only one molecule (see Figure 7). On the other hand, the structures of 10, 11, and 12 display tetraferrocenyldisiloxanediol 6, with anionic moieties (acetate in 10 and 11 and chloride in 12), and tetrabutylammonium as a cationic unit. Compounds 10 and 12 present a 1:1:1 stoichiometry, while in compound 11 the ratio 6:acetate:tetrabutylammonium is 2:1:1. Some selected structural parameters for these structures are collected in the Supporting Information. Bond distances and angles around the Si atoms are within the expected values, as well as the distances between hydroxyl oxygen atoms. Compound 10 crystallizes in the monoclinic P21/n space group with one tetraferrocenyldisiloxanediol 6 molecule, one acetate anion, one tetrabutylammonium cation, and two water molecules per asymmetric unit (Figure S37), and the distances between iron atoms in ferrocenyl groups linked by the same silicon atom are 6.404 Å (Fe1−Fe2) and 6.131 Å (Fe3−F4). Interestingly, one of the distances between Fe atoms from different silicon atoms is considerably shorter (Fe2−Fe4 = 6.388 Å) than the others (Fe1−Fe4 = 6.952 Å, Fe2−Fe3 = 7.556 Å, Fe1−Fe3 = 8.733 Å) and even shorter than the one from the two ferrocenyl moieties in Si1. There are several O− H···O hydrogen bonds in the crystal structure of 10, between hydroxyl oxygen atom O2 and carboxylic O4, between one of the water molecules and the other hydroxyl group (O1−H1··· O6), and also between solvent water molecules themselves and with the acetate anion. This leads to an arrangement of the anionic unit and the tetraferrocenyldisiloxanediol molecule

Figure 6. Schematic representation of complexes formed between chloride or acetate anions and redox-active silanol receptors 5 and 6 (expected structures on the basis of NMR and MS data).

between Fe atoms in the same molecule is 6.016 Å. There are O−H···O hydrogen bonds between hydroxyl oxygen atoms O1 and O2, and carboxylic ones O3 and O4, as expected, leading to bimolecular synthons (see Figure 7, top). This type of supramolecular association had been reported for molecules containing methanediol moieties, for instance, but a survey in the Cambridge Structural Database of silanediol derivatives displaying this arrangement with acetate ions yielded no result. The crystal structure of 9 belongs to the C2/c space group and contains one diferrocenylsilanediol 5 molecule, one chloride anion, and one tetrabutylammonium cation (Figure S36) with a distance between iron atoms of 6.192 Å. It is important to note that the chlorine atom is statistically disordered in two alternative positions (Cl1A and Cl1B). As the electron density found in the Fourier map from the two halves of the chlorine atom could theoretically also correspond to two oxygen atoms from interstitial water molecules, TXRF measurements were performed on the same crystal used for the single-crystal X-ray diffraction experiments. The characteristic emission of the chlorine atom was found (Figure S40), thus confirming the presence of this anion in the structure, as well as the silanediol:Cl 1:1 stoichiometric ratio, which is consistent with the amount of Fe, Si, and Cl detected (Table S11). In this structure there are O−H···Cl hydrogen bonds between hydroxyl oxygen atoms O1 and O2 and the two alternative positions of Cl. The two possible locations of the chlorine atoms allow interactions with very similar donor−acceptor distances, thus indicating that both positions are equally advantageous in terms of supramolecular bonding. This type G


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hydrogen bonds arising from the two pairs of hydroxyl oxygen atoms (O1 and O2; O4 and O5) to the acetate oxygen atoms O7 (in the case of O1 and O2) and O8 (for O4 and O5) as depicted in Figure 8 (middle). The distances between Fe atoms in tetraferrocenyldisiloxanediol molecules are Fe1−Fe2 = 5.922 Å, Fe3−Fe4 = 6.111 Å, Fe5−Fe6 = 6.172 Å, and Fe7−Fe8 = 6.358 Å; for nonadjacent ferrocenyl fragments, Fe1−Fe3 = 7.795 Å, Fe1−Fe4 = 6.021 Å, Fe2−Fe3 = 6.276 Å, Fe2−Fe4 = 8.047 Å, Fe5−Fe7 = 6.642 Å, Fe5−Fe8 = 9.445 Å, Fe6−Fe7 = 6.865 Å, and Fe6−Fe8 = 7.089 Å. In one of the molecules, as it occurred in 10, a diagonal distance (Fe1−Fe4) is exceptionally short and even shorter than one of the distances (Fe3−Fe4) in adjacent ferrocenyl units. Compound 12 crystallizes in the monoclinic P21/n space group with one tetraferrocenyldisiloxanediol 6 molecule, one chloride anion, and one tetrabutylammonium cation, but without any solvent molecules. In this structural type, the chloride anion is closely located to both hydroxyl groups to yield highly symmetrical O−H···Cl interactions (see Figure 8, bottom, and Table S7). Similar interactions had previously been described in the crystal structure of 1,1,3,3-tetraphenyl1,3-disiloxanediol pyridinium chloride.38 The distances between Fe atoms in the tetraferrocenyldisiloxane molecule are Fe1− Fe2 = 6.044 Å, Fe3−Fe4 = 6.082 Å, and Fe1−Fe3 = 7.641 Å, Fe1−Fe4 = 8.916 Å, Fe2−Fe3 = 6.803 Å, Fe2−Fe4 = 6.814 Å, for ferrocenyl fragments in different silicon atoms. Through careful analysis of the interactions found in the crystal structures of compounds 8 and 9 we can conclude that selective interactions are established between diferrocenylsilanediol 5 molecules and the anionic moieties by hydrogen bond interactions involving the hydroxyl groups. The same situation occurs also in the structures of compounds 10−12 with the OH groups in tetraferrocenyldisiloxanediol 6. Mass spectrometric analyses (ESI-MS, negative mode) provided further evidence for the formation of compounds 8−12 (see Experimental Section). Theoretical Studies. We have previously shown that silanols 5 and 6 effectively bind acetate and chloride anions in solution and in the solid state. According to these observations, diferrocenylsilanediol 5 has been studied through electronic structure calculations with the purpose of exploring its binding patterns with the aforementioned anions in a vacuum. This study will shed some light on the intrinsic ability of diferrocenylsilanediol 5 as an acceptor, which can serve as a suitable example of the behavior of the silanediol group in this sense. In the gas phase we find two stable complexes with a Cl− anion. Figure 9 shows diferrocenylsilanediol 5 and its complexes with one and two Cl− anions, along with the most relevant distances. In the presence of one Cl−, both oxygens from the hydroxyl groups interact simultaneously with the anion. This type of binding is similar to the one found for compound 12 (see Figure 8). In the presence of two Cl−, both anions remain separated as much as possible, thus changing the orientation of the OH groups with respect to the previous situation. Regarding acetate, this oxoanion can bind silanediol Fc2Si(OH)2 through one (monodentate, κ1-O2CCH3−) or two (bidentate, κ2-O2CCH3−) oxygen atoms, as shown in Figure 10. In terms of free energy, [Fc2Si(OH)2·κ2-OAc]− is more stable than [Fc2Si(OH)2·κ1-OAc]− by 27.05 kJ/mol. The O−H distances in the diol moiety of Fc2Si(OH)2 are a measure of the

different from the expected one (see Figure 6). In this case, instead of forming pairs, as it occurred in the structure of 8, the supramolecular units involve two tetraferrocenyldisiloxanediol molecules at the ends of the motif with two acetate and four water molecules located between them (Figure 8, top). It is

Figure 8. Hydrogen bond interactions (in blue) found in the crystal structures of 10, 11, and 12 between tetraferrocenyldisiloxane-1,3-diol molecules and the anionic units.

remarkable that none of the few examples found in the CSD of structures containing disiloxanediol derivatives and acetate anions presented the expected synthon formation analogous to the one found in compound 8. This fact can be attributed to the logically longer distances between hydroxyl groups in this type of disiloxanediols (ranging from 3.398 Å in the structure with CSD named WISPET37a to 4.844 Å in KOJDAO)37b compared to the ones found in silanediols (between 2.566 Å in PEJQEC10a and 2.749 in DEVZIO).37c The structure of 11 contains the same molecular units as 10 (6, acetate, and tetrabutylammonium) but in a 2:1:1 ratio, plus one solvent molecule of dichloromethane (see Figure S38). It crystallizes in the triclinic P1̅ space group, with the acetate ion located between the two tetraferrocenyldisiloxanediol molecules and acting, as expected, as a bridge through O−H···O H


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OAc− (0.988 Å) < κ2-OAc− (0.995 Å). Accordingly, the hydrogen bond between the diol moiety and the anion is reinforced in the same way: Cl− (2.261 Å) < κ1-OAc− (1.816 Å) < κ2-OAc− (1.698 Å). A very efficient tool to analyze the interplay between the different noncovalent interactions involved in the aggregates is the topology of the electron density. By looking at the electron density changes at the bond critical points in the O−H bonds and the intermolecular hydrogen bonds, we can compare the strength of the interaction in the mono- and bidentate complexes. The O−H bond in hydroxyl groups is significantly weakened by the interaction with acetate in the [Fc2Si(OH)2·OAc]− complex, whose density ρ passes from 0.370 in Fc2Si(OH)2 to 0.311 (κ1OAc−) and 0.319 (κ2-OAc−) e·bohr−3 in the complexes. Hydrogen bonds between oxygen from the hydroxyl groups and acetate are stronger in the bidentate binding mode (0.046 and 0.046 e·bohr−3) than in the monodentate fashion (0.035 and 0.039 e·bohr−3). These variations have an influence on the relative energy changes between binding patterns previously described. Figure 10D includes the structure obtained for the complex formed by two diferrocenylsilanediol units and one acetate anion that brings them together (5·AcO−·5 complex in Scheme 2). The calculations allow elucidating the pattern adopted by the anion to bind both Fc2Si(OH)2 units. Interestingly, oxygen atoms from acetate interact with one of the diols through hydrogen bonds as in the bidentate binding mode, whereas the second diol also forms hydrogen bonds with the first diol and the anion at slightly larger distances. The arrangement of the six oxygen atoms along with the corresponding four hydrogen atoms involved in the net of interactions resembles very much the one obtained in crystals when water molecules are present, as shown and explained in Figure 8 for compound 10 in the Xray section. Figure 11 summarizes the similarities on binding found for acetate with compounds 5 and 6 in our theoretical and X-ray diffraction studies.

Figure 9. (A) Isolated 5; (B) 5·Cl−; (C) Cl−·5·Cl−. O−H and hydrogen bond distances (Å) are shown. Electronic structure calculations were carried out at the B3LYP/6-311+G(d) level of theory.

Figure 10. (A) Isolated diferrocenylsilanediol 5; (B) [Fc2Si(OH)2·(κ1OAc)]−; (C) [Fc2Si(OH)2·(κ2-OAc)]−, all of them carried out at the B3LYP/6-311+G(d) level of theory. (D) [{Fc2Si(OH)2}2·OAc]− and a simplified representation of the binding pattern, obtained at the ONIOM(B3LYP/6-31+G(d):B3LYP/6-31G(d)) level of theory (see computational details). O−H and hydrogen bond distances (Å) are also shown.

strength of the interaction with the anions. The longer this bond is compared to the isolated molecule, the stronger the hydrogen bond with the corresponding anion will be. The O− H bond distance increases in the order Cl− (0.981 Å) < κ1-

Figure 11. Binding patterns found for AcO− with silanols 5 and 6 in different complexes (5·AcO−·5, 10, 11) on the basis of the theoretical and X-ray diffraction studies. I


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useful in the design of novel electroactive anion receptors based on the silanol binding motif.

A disiloxanediol group O(SiOH)2 like that of compound 10 can follow the same pattern observed in the 5·AcO−·5 complex or present a different pattern, like that of compound 11. Compound 10 exhibits formally a κ1 binding mode with respect to the AcO− anion, but in practice it is a κ2-like one, due to the way water molecules are included in the net of intermolecular interactions between disiloxanediol units. A stoichiometry of 2:1 like that of compound 11, unlike the 1:1 observed in 10, favors a κ1 binding mode. The reason behind the preference for one of these two possibilities seems to depend on the availability of enough molecules (acetate anions or water molecules) to display that six-oxygen-atom network. In addition, we have explored the structures obtained by including the n-Bu4N+ cation in the complexes (see Figure S41). It is interesting to note that the Cl− anion in [nBu4N][Fc2Si(OH)2·Cl] (compound 9) does not equally bind both hydroxyl groups from the diol group, as the presence of the n-Bu4N+ cation introduces a certain degree of asymmetry in the system. In the solid state, two possible locations for the Cl− anion were found, as described in the previous section. Additional information regarding complexes including nBu4N+ can be found in the Supporting Information. The theoretical studies allow us to conclude that a system formed only by Fc2Si(OH)2 (5) and acetate would favor the bidentate binding mode, whereas the Cl− anion prefers to bind both hydroxyl groups of the diol moiety. This means that compound 5 would effectively bind and recognize these anions also in the gas phase. This conclusion could be extended to the 5·AcO−·5 supramolecular system, in good agreement with the X-ray diffraction studies.

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EXPERIMENTAL SECTION

General Procedures and Equipment. All reactions and compound manipulations were performed in an oxygen- and moisture-free Ar atmosphere using standard Schlenk techniques. THF was distilled over sodium/benzophenone under Ar before use. nHexane and dicloromethane were dried by standard procedures over the appropriate drying agents and distilled under argon, immediately prior to use. Ferrocene (Sigma-Aldrich) was purified by sublimation prior to use. tert-Butyllithium (1.7 M solution in n-pentane) and tetrabutylammonium salts of acetate and chloride (Sigma-Aldrich) were used as received. Silicon tetrachloride and triethylamine were purchased from Sigma-Aldrich and distilled prior to use. Silanized silica gel 60 (0.063−0.200 mm), purchased from Merck, was used for column chromatography purifications. Infrared spectra were recorded on a PerkinElmer 100 FT-IR spectrometer. Elemental analyses were performed in a LECO CHNS-932 elemental analyzer, equipped with an MX5Mettler Toledo microbalance. All NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer. Chemical shifts were reported in parts per million (δ) with reference to CDCl3 residual solvent resonances for 1H (δ 7.26 ppm) and 13C (δ 77.2 ppm). 29Si NMR resonances were recorded with inverse-gated proton decoupling in order to minimize nuclear Overhauser effects and were referenced externally to tetramethylsilane. For 1H NMR titrations, a solution of the corresponding compound (0.02 M for 5 and 0.01 M for 6) in CDCl3 was prepared. Progressive additions of molar equivalents of the corresponding tetrabutylammonium salt of acetate or chloride anion (in CDCl3 solution) were performed, and the spectra were recorded. The equilibrium association constants were calculated from the chemical shifts of the silanol protons (except for AcO−) by fitting the experimental data to a 1:1 binding isotherm using a standard nonlinear least-squares-fitting program. MALDI-TOF mass spectra were recorded using a Bruker-Ultraflex III TOF/TOF mass spectrometer equipped with a nitrogen laser emitting at 337 nm. Dicloromethane solutions of the matrix (trans-2[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, 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 droplet method. The positive ion and the reflectron mode were used for these analyses. Electrospray ionization mass spectra (ESI-MS) were recorded with a QSTAR-pulsar i (Applied Biosystems) spectrometer, using either methanol (for 8 and 10−12) or acetonitrile (for 9) as ionizing phase. Samples were prepared in dicloromethane. TXRF analysis was performed with a benchtop S2 PicoFox TXRF spectrometer from Bruker Nano GmbH (Germany), equipped with a molybdenum X-ray source working at 50 kV and 600 μA, a multilayer monochromator with 80% reflectivity at 17.5 keV (Mo Kα), an XFlash SDD detector with an effective area of 30 mm2, and an energy resolution better than 150 eV for Mn Kα. The Spectra 7 software package, also from Bruker, was used for control, acquisition, deconvolution, and integrations. Electrochemical Measurements. Cyclic voltammetry and square-wave voltammetry experiments were recorded on a Bioanalytical Systems BAS CV-50W potentiostat. CH2Cl2 and CH3CN (SDS, spectrograde) were freshly distilled from calcium hydride under Ar for electrochemical measurements. The supporting electrolytes used were tetra-n-butylammonium hexafluorophosphate (Alfa-Aesar), which was purified by recrystallization from ethanol and dried in a vacuum at 60 °C, and tetra-n-butylammonium tetrakis(pentafluorophenyl)borate, which was synthesized as described in the literature,39 by metathesis of [n-Bu4N]Br with Li[B(C6F5)4]· (nOEt2) (Boulder Scientific Company) 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. The counter electrode was a coiled Pt wire, and the reference electrode was a BAS saturated calomel electrode (SCE). All cyclic voltammetric experiments were

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CONCLUSIONS In summary, the first examples of electroactive anion receptors bearing silanediol (Si(OH)2) or disiloxanediol (O(SiOH)2) groups as recognition sites and ferrocene moieties as redoxactive functions have been investigated in organic media, in the gas phase, and in the solid state. 1H NMR solution studies reveal that silanols 5 and 6 can bind a spherical monatomic chloride ion through hydrogen bonding with a 1:1 receptor:anion stoichiometry, while they bind Y-shaped acetate with 2:1 and 1:1 ratios. Receptor 5 exhibits association constants higher than those of 6 for the chloride anion guest. It has also been demonstrated that the incorporation of the ferrocene redox center into the silanol-based structural framework enables the silanol receptors 5 and 6 to electrochemically detect biologically relevant anions such as AcO− and Cl− at submillimolar concentrations via cathodic perturbation of the ferrocene/ ferrocenium redox couples. Silanol receptors 5 and 6 exhibit higher ability for electrochemical sensing of acetate anions than chloride, as manifested by significantly larger cathodic ΔE1/2 perturbations of the respective ferrocene/ferrocenium redox couples. In addition, diferrocenylsilanediol receptor 5 displays a higher affinity toward Cl− and AcO−, compared with disiloxanediol 6. Single-crystal X-ray diffraction analyses of the supramolecular complexes 8−12 support the notion that silanol-based neutral receptors 5 and 6 do bind the acetate and chloride anions in the solid state. Furthermore, computational studies concerning diferrocenylsilanediol 5 provided a detailed description of the binding modes with Cl− and AcO−, supporting the conclusions derived from the electrochemical studies and X-ray diffraction analysis. The present results, both experimental and theoretical, provide insights that might prove J


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Organometallics performed using a platinum-disk working electrode (A = 0.020 cm2) (BAS). The working electrode was polished on a Buehler polishing cloth with Metadi II diamond paste for about 3 min followed by sonication in absolute ethanol, rinsed thoroughly with purified water and acetone, and allowed to dry. 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 [nBu4N][PF6]. Solutions were, typically, 10−3 M in the redox-active species and were purged with nitrogen and kept under an inert atmosphere throughout the measurements. No IR compensation was used. SWV was performed using frequencies of 10 Hz. For electrochemical titrations, a CH2Cl2/CH3CN solution (3 mL) of the corresponding ferrocenylsilanol (10−3 M) and 0.1 M [n-Bu4N][PF6] was placed in an electrochemical cell, purged with nitrogen for 10 min, and stirred. Stirring was stopped and a nitrogen atmosphere was maintained above the solution while the experiment was in progress. The electrodes were cleaned after each run. Anion recognition studies were performed by consecutive additions of variable molar equivalents of the tetrabutylammonium salt of acetate or chloride anion. X-ray Crystal Structure Determination. Suitable orange crystals of 8−12 were coated with mineral oil and mounted on Mitegen MicroMounts. The samples were transferred to a Bruker D8 KAPPA series II with the APEX II area-detector system equipped with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Full details of the data collection and refinement can be found in the Supporting Information (section 4). The substantial redundancy in data allows empirical absorption corrections (SADABS)40 to be applied using multiple measurements of symmetry-equivalent reflections. Raw intensity data frames were integrated with the SAINT program,41 which also applied corrections for Lorentz and polarization effects. The Bruker SHELXTL software package was used for space group determination, structure solution, and refinement.42 The space group determination was based on a check of the Laue symmetry, and systematic absences were confirmed using the structure solution. The structures were solved by direct methods (SHELXS-97), completed with different Fourier syntheses, and refined with fullmatrix least-squares using SHELXS-97, minimizing w(Fo2 − Fc2)2.43,44 Weighted R factors (Rw) and all goodness of fit S are based on F2; conventional R factors (R) are based on F. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen positions for water molecules involved in hydrogen bonds, as well as the hydrogen attached to O5 in compound 11, were located in the electron density maps, and the rest were calculated geometrically and 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 structures of compounds 8−12 have been deposited at the Cambridge Crystallographic Data Centre with deposition numbers CCDC 1490199− 1490203. Computational Details. Fully optimized geometries were obtained at the B3LYP/6-311+G(d) level of theory using the Gaussian09 program45−47 with the only exception of the AcO−·5· AcO− structure, obtained at the ONIOM(B3LYP/6-31+G(d):B3LYP/ 6-31G(d)) level of theory with the same code.48 The B3LYP functional has been shown to be able to accurately describe ferrocene-containing compounds.49 Single-point SMD calculations50 have been carried out on top of the gas-phase-optimized geometries. Harmonic vibrational frequencies in the gas phase were obtained at the same level of theory. Reaction between 1-Lithioferrocene and SiCl4: Synthesis of Fc2Si(OH)2 (5) and Fc2(HO)Si−O−Si(OH)Fc2 (6). A 10 g amount of sublimed ferrocene (53.75 mmol) was dissolved in a mixture of 100 mL of dry THF and 30 mL of dry n-hexane under argon at room temperature and then cooled to −30 °C. To this stirred system was added dropwise a 1.7 M solution of t-BuLi in n-pentane (31.6 mL, 53.75 mmol). The mixture was stirred for another 30 min before adding SiCl4 (2.8 mL, 24.19 mmol). The solution was allowed to warm to room temperature and stirred overnight. The dark red mixture was filtered, treated with n-hexane, and then filtered once again to remove the LiCl byproduct. Solvent removal under reduced

pressure yielded a red-orange solid, which was dissolved in diethyl ether. The remaining solution was added slowly to a mixture of triethylamine (5.2 mL, 37.31 mmol) and water (1.2 mL, 66.59 mmol). Vapor (hydrochloric acid) and a white precipitate (triethylamine hydrochloride) appeared upon combination of both solutions. The resulting orange mixture was filtered. Solvent removal yielded a redorange solid, which was purified by column chromatography on silanized silica gel (3 cm × 11 cm). An initial orange band, of unreacted ferrocene, was obtained with n-hexane, followed by another major band (eluted with n-hexane/CH2Cl2, 1:1), which contained a mixture of different products. The latter was subjected to a second column chromatography on silanized silica gel, allowing the separation and isolation of 6 (eluted with n-hexane/CH2Cl2, 1:1) and diferrocenylsilanediol 5 (with n-hexane/CH2Cl2, 1:2) as analytically pure, air-stable, orange powdery solids. Diferrocenylsilanediol, 5. Yield: 3.34 g (32%). Anal. Calcd for C20H20O2SiFe2: C 55.58; H 4.67. Found: C 55.55; H 4.72. 1H NMR (CDCl3, 300 MHz, ppm): δ 2.62 (s, 2H, OH), 4.19 (s, 10H, C5H5), 4.31, 4.41 (m, 8H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 66.5 (ipso-Fc), 68.7 (C5H5), 71.6, 73.5 (C5H4). {1H−29Si} HMBC (CDCl3, 59 MHz, ppm): δ −16.8. IR (KBr, cm−1): ν(O−H) 3619, 3405, ν(Si−O) 925, ν(Si−OH) 874, ν(Si−C) 820. MS (MALDITOF): m/z 432.0 [M+]. 1,1,3,3-Tetraferrocenyldisiloxane-1,3-diol, 6. Yield: 2.05 g (10%). Anal. Calcd for C40H38O3Si2Fe4: C 56.77; H 4.53. Found: C 56.92; H 4.57. 1H NMR (CDCl3, 300 MHz, ppm): δ 2.50 (s, 2H, OH), 4.25 (s, 20H, C5H5), 4.37, 4.47 (m, 16H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 68.3 (ipso-Fc), 69.3 (C5H5), 71.9, 74.1, 74.3 (C5H4). {1H−29Si} HMBC (CDCl3, 59 MHz, ppm): δ −25.0. IR (KBr, cm−1): ν(O−H) 3600, 3428, ν(Si−O) 1105, ν(Si−O) 1033, ν(Si−OH) 862, ν(Si−C) 818. MS (MALDI-TOF): m/z 846.0 [M+]. Synthesis of Complexes 8−12. Diferrocenylsilanediol 5 (10 mg, 0.023 mmol) or tetraferrocenyldisiloxane-1,3-diol 6 (10 mg, 0.012 mmol) and 1 equiv of the corresponding tetrabutylammonium salt ([n-Bu4N][CH3COO] or [n-Bu4N][Cl]) were dissolved in dry CH2Cl2 (1 mL). After stirring, cautious solvent removal afforded compounds 8−12 as pure orange solids in quantitative fashion. Suitable crystals for X-ray analysis of complexes 8−12 were obtained by solving the corresponding compound in CH2Cl2 and then adding dry n-hexane dropwise, until light turbidity was observed. The solutions were stored at −20 °C overnight. [n-Bu4N][Fc2Si(OH)2·CH3COO] (8). Anal. Calcd for C38H59O4SiFe2N: C 62.21; H 8.11; N 1.91. Found: C 62.43; H 8.05; N 1.79. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.99 (s, 12H, CH3CH2), 1.41 (s, 8H, CH3CH2CH2), 1.57 (s, 8H, CH2CH2CH2), 1.98 (s, 3H, CH3COO), 3.19 (s, 8H, CH2CH2N), 4.19 (s, 10H, C5H5), 4.27, 4.30 (m, 8H, C5H4), 4.57 (s, 2H, OH). 13C{1H} NMR (CDCl3, 75 MHz): δ 1.2 (CH3COO), 13.9 (CH3CH2), 19.8, 24.1 (CH3CH2CH2), 58.7 (CH2N), 68.8 (C5H5), 70.4 (C5H4), 70.7 (ipsoFc), 73.7 (C5H4), 131.1 (CH3COO). Negative-ion ESI-MS for C22H23O4SiFe2 (calcd 491.0): m/z 491.0 [Fc2Si(OH)2·CH3COO]−. [n-Bu4N][Fc2Si(OH)2·Cl] (9). Anal. Calcd for C36H56O2SiFe2NCl: C 60.89; H 7.95; N 1.97. Found: C 60.53; H 7.93; N 1.87. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.01 (s, 12H, CH3CH2), 1.46 (s, 8H, CH3CH2CH2), 1.67 (s, 8H, CH2CH2CH2), 3.20 (s, 2H, OH), 3.36 (s, 8H, CH2CH2N), 4.20 (s, 10H, C5H5), 4.34, 4.39 (m, 8H, C5H4). 13 C{1H} NMR (CDCl3, 75 MHz): δ 13.9 (CH3CH2), 20.1, 24.5 (CH3CH2CH2), 59.6 (CH2N), 66.8 (ipso-Fc), 68.7 (C5H5), 71.5, 73.5 (C5H4). Negative-ion ESI-MS for C20H20O2SiFe2Cl (calcd 466.9): m/z 467.0 [Fc2Si(OH)2·Cl]−. [n-Bu4N][Fc2(HO)Si−O−Si(OH)Fc2·CH3COO] (10). Anal. Calcd for C58H77O5Si2Fe4N: C 60.96; H 6.76; N 1.22. Found: C 60.61; H 6.83; N 1.11. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.98 (t, 12H, CH3CH2), 1.41 (m, 8H, CH3CH2CH2), 1.56 (m, 8H, CH2CH2CH2), 2.02 (s, 3H, CH3COO), 3.26 (m, 8H, CH2CH2N), 3.79 (br, 2H, OH), 4.19 (s, 20H, C5H5), 4.32 (m, 12H, C5H4), 4.39 (m, 4H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz): δ 1.2 (CH3COO), 13.9 (CH3CH2), 19.9, 24.2 (CH3CH2CH2), 59.1 (CH2N), 68.8 (C5H5), 69.3 (ipso-Fc), 70.8, 73.8 (C 5 H 4 ), 131.0 (CH 3 COO). Negative-ion ESI-MS for K


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Organometallics C42H41O5Si2Fe4 (calcd 905.0): m/z 905.0 [Fc2(HO)Si−O−Si(OH)Fc2·CH3COO]−. [n-Bu4N][{Fc2(HO)Si−O−Si(OH)Fc2}2·CH3COO] (11). Anal. Calcd for C98H115O8Si4Fe8N: C 59.03; H 5.81; N 0.70. Found: C 58.85; H 5.64; N 0.65. 1H NMR (CDCl3, 300 MHz, ppm): δ 0.97 (t, 12H, CH3CH2), 1.40 (m, 8H, CH3CH2CH2), 1.56 (m, 8H, CH2CH2CH2), 2.04 (s, 3H, CH3COO), 3.25 (m, 8H, CH2CH2N), 4.18 (s, 40H, C5H5), 4.32 (m, 8H, C5H4), 4.35 (m, 16H, C5H4), 4.39 (m, 8H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz): δ 1.2 (CH3COO), 13.9 (CH3CH2), 19.9, 24.2 (CH3CH2CH2), 58.9 (CH2N), 68.8 (C5H5), 69.3 (ipso-Fc), 70.9, 73.8 (C5H4), 131. Negative-ion ESI-MS for C82H79O8Si4Fe8 (calcd 1750.9): m/z 1751.0 [Fc2(HO)Si−O−Si(OH)Fc2}2·CH3COO]−. [n-Bu4N][Fc 2(HO)Si−O−Si(OH)Fc 2 ·Cl] (12). Anal. Calcd for C56H74O3Si2Fe4NCl: C 59.83; H 6.63; N 1.25. Found: C 60.07; H 6.30; N 1.49. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.00 (t, 12H, CH3CH2), 1.46 (m, 8H, CH3CH2CH2), 1.67 (m, 8H, CH2CH2CH2), 3.37 (m, 8H, CH2CH2N), 3.44 (s, 2H, OH), 4.17 (s, 20H, C5H5), 4.31, 4.38, 4.41 (m, 16H, C5H4). 13C{1H} NMR (CDCl3, 75 MHz): δ 13.9 (CH3CH2), 20.0, 24.5 (CH3CH2CH2), 59.7 (CH2N), 68.0 (ipsoFc), 68.8 (C5H5), 71.3, 73.8 (C5H4). Negative-ion ESI-MS for C40H38O3Si2Fe4Cl (calcd 880.9): m/z 880.9 [Fc2(HO)Si−O−Si(OH)Fc2·Cl]−.

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Organometallic, and Polymer Chemisty; John Wiley & Sons: New York, 2000. (3) For hydrogen-bonding properties of silanols and their use as building blocks in supramolecular chemistry, see: (a) Wilson, S. O.; Tran, N. T.; Franz, A. K. Organometallics 2012, 31, 6715−6718. (b) Tran, N. T.; Wilson, S. O.; Franz, A. K. Chem. Commun. 2014, 50, 3738−3740. (c) Fukawa, M.; Sato, T.; Kabe, Y. Chem. Commun. 2015, 51, 14746−14749. (d) Sato, N.; Kuroda, Y.; Abe, T.; Wada, H.; Shimojima, A.; Kuroda, K. Chem. Commun. 2015, 51, 11034−11037. (e) Beckmann, J.; Duthie, A.; Reeske, G.; Schürmann, M. Organometallics 2004, 23, 4630−4635. (4) Hurkes, N.; Ehmann, H. M. A.; List, M.; Spirk, S.; Bussiek, M.; Belaj, F.; Pietschnig, R. Chem. - Eur. J. 2014, 20, 9330−9335. (5) For an excellent review on the medicinal applications of organosilicon compounds, including organosilanols, see: Franz, A. F.; Wilson, S. O. J. Med. Chem. 2013, 56, 388−405. (6) Daiss, J. O.; Burschka, C.; Mills, J. S.; Montana, J. G.; Showell, G. A.; Warneck, J. B. H.; Tacke, R. Organometallics 2006, 25, 1188−1198. (7) (a) Kim, Y. M.; Farrah, S.; Baney, R. H. Int. J. Antimicrob. Agents 2007, 29, 217−222. (b) Kim, Y.-M.; Farrah, S.; Baney, R. H. Electron. J. Biotechnol. 2006, 9, 176−180. (8) (a) Kondo, S.; Harada, T.; Tanaka, R.; Unno, M. Org. Lett. 2006, 8, 4621−4624. (b) Kondo, S.; Fukuda, A.; Yamamura, T.; Tanaka, R.; Unno, M. Tetrahedron Lett. 2007, 48, 7946−7949. (c) Kondo, S.; Okada, N.; Tanaka, R.; Yamamura, M.; Unno, M. Tetrahedron Lett. 2009, 50, 2754−2757. (d) Kondo, S.; Bie, Y.; Yamamura, M. Org. Lett. 2013, 15, 520−523. (e) Yamamura, M.; Kondo, S.; Unno, M. Tetrahedron Lett. 2014, 55, 646−649. (9) (a) Hardman-Baldwin, A. M.; Mattson, A. M. ChemSusChem 2014, 7, 3275−3278. (b) Wieting, J. M.; Fisher, T. J.; Schafer, A. G.; Visco, M. D.; Gallucci, J. C.; Mattson, A. E. Eur. J. Org. Chem. 2015, 2015, 525−533. (c) Schafer, A. G.; Wieting, J. M.; Mattson, A. E. Org. Lett. 2011, 13, 5228−5231. (d) Schafer, A. G.; Wieting, J. M.; Fisher, T. J.; Mattson, M. A. Angew. Chem., Int. Ed. 2013, 52, 11321−11324. (10) (a) Tran, N. T.; Min, T.; Franz, A. K. Chem. - Eur. J. 2011, 17, 9897−9900. (b) Tran, N. T.; Wilson, S. O.; Franz, A. K. Org. Lett. 2012, 14, 186−189. (11) Beemelmanns, C.; Husmann, R.; Whelligan, D. K.; Oezcubukcu, S.; Bolm, C. Eur. J. Org. Chem. 2012, 18, 3373−3376. (12) (a) Rausch, M. D.; Schloemer, C. G. Org. Prep. Proc. 1969, 1, 131−136. (b) Sharma, H. K.; Cervantes-Lee, F.; Haiduc, I.; Pannell, K. H. Appl. Organomet. Chem. 2005, 19, 437−439. (13) Sollott, G. P.; Peterson, W. R. J. Am. Chem. Soc. 1967, 89, 6783−6784. (14) MacLachlan, M. J.; Zheng, J.; Thieme, K.; Lough, A. J.; Manners, I.; Mordas, C.; LeSuer, R.; Geiger, W. E.; Liable-Sands, L. M.; Rheingold, A. L. Polyhedron 2000, 19, 275−289. (15) (a) MacLachlan, M. J.; Ginzburg, M.; Zheng, J.; Knöll, O.; Lough, A. J.; Manners, I. New J. Chem. 1998, 22, 1409−1415. (b) MacLachlan, M. J.; Lough, J.; Geiger, W. E.; Manners, I. Organometallics 1998, 17, 1873−1883. (16) Reyes-García, E. A.; Cervantes-Lee, F.; Pannell, K. H. Organometallics 2001, 20, 4734−4740. (17) MacLachlan, M. J.; Zheng, J.; Lough, A. J.; Manners, I.; Mordas, C.; LeSuer, R.; Geiger, W. E.; Liable-Sands, L. M.; Rheingold, A. L. Organometallics 1999, 18, 1337−1345. (18) Thieme, K.; Bourke, S. C.; Zheng, J.; MacLachlan, M. J.; Zamanian, F.; Lough, A. J.; Manners, I. Can. J. Chem. 2002, 80, 1469− 1480. (19) For structurally related examples of ferrocenyl-substituted digermoxanediols FcR(HO)Ge−O−Ge(OH)RFc see: Zhang, Y.; Cervantes-Lee, F.; Pannell, K. H. Organometallics 2003, 22, 510−515. (20) (a) Bruña, S.; Nieto, D.; González-Vadillo, A. M.; Perles, J.; Cuadrado, I. Organometallics 2012, 31, 3248−3258. (b) Bruña, S.; González-Vadillo, A. M.; Nieto, D.; Pastor, C. J.; Cuadrado, I. Macromolecules 2012, 45, 781−793. (c) Bruña, S.; Martínez-Montero, I.; González-Vadillo, A.M.; Martín-Fernández, C.; Montero-Campillo, M. M.; Mó, O.; Cuadrado, I. Macromolecules 2015, 48, 6955−6969.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00559. Supplementary figures referenced in the text; spectroscopic, theoretical, X-ray crystallographic, and CV and SWV data of the compounds (PDF) Crystallographic data (CIF)

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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 The authors are grateful to the Spanish Ministerio de Economiá y Competitividad (MINECO) (projects CTQ2012-30728, CTQ2015-63997-C2-1-P, and CTQ2013-43698-P) for the generous support of this work. M.M.M.-C. and O.M. thank the STSM COST Action CM1204 and the Project FOTOCARBON-CM S2013/MIT-2841 of the Comunidad Autónoma de Madrid. Computational time at Centro de ́ Computación Cientifica (CCC) of Universidad Autónoma de Madrid is acknowledged. A.E.K. acknowledges the support from the National Science Foundation (CHE-1412455).

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REFERENCES

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Multi-Ferrocene-Containing Silanols as Redox-Active Anion

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