Co2(CO)6(μ,η2-HCCFc) as Precursor in the Synthesis of Multiredox

Cyclic and Linear VinylferrocenylsiloxanesCovadonga Blasco, Sonia Bruna, Isabel Cuadrado,* Esther Delgado,* and Elisa Hernandez

Departamento de Química Inorganica, Facultad de Ciencias, Universidad Autonoma de Madrid, Cantoblanco, 29049, Madrid, Spain

ABSTRACT: Novel vinylferrocenyl-functionalized cyclic andlinear polysiloxanes [MeSi{C(Fc)CH2}O]4 (1) and (Me3SiO)-(MeSi{C(Fc)CH2}O)n(SiMe3) (n ≈ 35) (3) (Fc = (η5-C5H4)Fe(η

5-C5H5) have been synthesized by hydrosilylationreactions of the heterometallic compound Co2(CO)6(μ,η

2-HCCFc) with the Si−H-functionalized polysiloxane frameworks[MeSiHO]4 and (Me3SiO)(MeSiHO)n(SiMe3). The solutionelectrochemical behavior of polysiloxanes 1 and 3 has beeninvestigated and showed that all the vinylferrocenyl units present inthe molecules are electrochemically independent. Both neutraltetrametallic cyclotetrasiloxane 1 and polysiloxane 3 becomeinsoluble upon complete oxidation to the correspondingpolycationic species, yielding electroactive films on platinum electrodes. In addition, stable electrode surfaces modified withthe polyvinylferrocenylsiloxane 3 have been prepared and electrochemically characterized. The well-defined and persistent redoxwaves of electrodes coated with the polyvinylferrocenylsiloxane films are characteristic of electrochemically stable, surface-confined reversible redox couples.

■ INTRODUCTIONHydrosilylation is one of the most important ways to form Si−C bonds in organosilicon chemistry;1 however its use for thesynthesis of transition metal-containing silanes and siloxaneshas been less studied.2,3 Cobalt carbonyls are among thetransition metal complexes that favor the hydrosilylation.1

Although it is known that the Co2(CO)6 fragment may act as aprotecting group of the CC bond, decomplexation of thealkynes in the presence of H−SiR3 may also generatevinylsilanes.4 The regio- and stereoselectivity of the alkynehydrosilylation depend on the reaction conditions. Thus, theformation of the β-isomer FcC(H)CH(SiEt3) has beendescribed,5 being the main compound by reaction ofCo2(CO)6(μ,η

2-HCCFc)6 with HSiEt3 in the presence ofSpeier's catalyst, whereas we have obtained only the α-isomerFcC(SiEt3)CH2 using Co2(CO)6 as catalyst.

7

On the other hand, there is an increasing interest inpreparing silicon-containing ferrocenyl polymers and macro-molecules due to their applications as precursors to functionalmagnetic ceramics,8 conducting materials,9 electrode modi-fiers,10 and also electrochemical biosensors.11 In the last years,we have been exploring different routes for the preparation ofnew families of multimetallic macromolecular structurescontaining ferrocenyl moieties together with linear,12 poly-hedral, and cyclic siloxanes.13,14 One of our approaches to thepreparation of such organometallic molecules was based onhydrosilylation reactions using Karstedt's catalyst. Also, recentlywe have described new carbosilane dendrimers functionalizedwith [Fc(CC)n-C(CHR)−] [R = H, n = 0; R = Fc, n = 1]groups,7 following this synthetic methodology but usingCo2(CO)6 as catalyst instead. Following our interest in

organometallic silicon chemistry, we have explored the role ofthe heterometallic compound Co2(CO)6(μ,η

2-HCCFc) asprecursor in the synthesis of new linear and cyclic ferrocenyl-functionalized siloxanes. Herein we report the synthesis,characterization, and redox properties of the novel polysilox-anes [MeSi{C(Fc)CH2}O]4 (1) and (Me3SiO)(MeSi{C-(Fc)CH2}O)n(SiMe3) (n ≈ 35) (3) containing, potentiallyreactive, pendant vinylferrocenyl units.

■ RESULTS AND DISCUSSIONA 1,2-dichloroethane solution of Co2(CO)6(μ,η

2-HCCFc) with1,3,5,7-tetramethylcyclotetrasiloxane, [MeSiHO]4, was heatedat 50 °C, and after 2 h of reaction, the new vinylferrocenyl-functionalized cyclotetrasiloxane [MeSi{C(Fc)CH2}O]4 (1)(Scheme 1) together with the known compoundsCo3(CO)9(μ3-CCH2Fc)

7 and Co4(CO)12 were separated by achromatography column.Compound 1 has been characterized by 1H and 29Si{1H}

NMR and IR spectroscopy, FAB+ mass spectrometry, andanalytical data. Tetrametallic 1 has been isolated as a mixture ofisomers, as was confirmed by the presence in its 1H NMRspectrum of two sets of resonances corresponding to allprotons present in the molecule. Thus, four signals at δ 6.15,5.75 (d, J = 2.6 Hz) and 6.09, 5.70 (br, s) ppm were assigned tothe olefinic protons, while those corresponding to theferrocenyl groups were observed at δ 4.50, 4.25, and 4.10

Special Issue: F. Gordon A. Stone Commemorative Issue

Received: October 21, 2011Published: January 6, 2012

Article

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ppm (C5H4 and C5H5, respectively). Additionally, four signalsthat appear at δ 0.43, 0.41, 0.37, and 0.34 ppm were assigned tothe CH3 groups. In the 29Si{1H} NMR spectrum only oneresonance, at δ −33.8 ppm, was observed, which is within therange expected for a D-type silicon atom in a cyclictetrasiloxane. The molecular peak at m/z 1079 [M+] as wellas several ones corresponding to the loss of C5H5 and C5H5Fegroups were exhibited in its FAB mass spectrum.Taking into account the successful result obtained in

preparing the cyclic siloxane 1 following this route, we studiedthe reaction between Co2(CO)6(μ,η

2-HCCFc) and 1,1,3,3,-tetramethyldisiloxane under the same conditions, but a mixtureof several, yet unidentified, organosilicon compounds wereformed together with the new heterometallic clusterCo4(CO)8(μ-CO)2(μ4,η

2-HCCFc) (2). This last compoundshows in its IR spectrum a pattern in the carbonyl region (νCO 2089w, 2055vs, 2040s, 2028s, 1991w, and 1877m) that is

similar to those observed in analogous compoundsM4(CO)8(μ-CO)2(μ4,η

2-RCCR′) [M = Co, HCCH,15

C6H5CCH;16 M = Rh, C6H5CCC6H5;17 M = Rh, R =

3-hexyne;18 M = Rh, 1-heptyne;18 M = Rh, phenylacetylene].16

Also, the resonances corresponding to all protons present in themolecule 2 (δ 8.15 (s, 1H, CH), 4.27 (s, 2H, C5H4), 4.17 (s,5H, C5H5), 3.99 (s, 4H, C5H4)) were shown in its 1H NMRspectrum. The blue compound Co4(CO)8(μ-CO)2(μ4,η

2-HCCFc) (2) is air sensitive in solution as well as in the solidstate and undergoes conversion to the known green compoundCo2(CO)6(μ,η

2-HCCFc). Taking into account that the hydro-silylation of Co2(CO)6(μ,η

2-HCCFc) with HMe2Si-O-SiMe2H,under the described reaction conditions, did not result in theformation of the desired divinylferrocenyl disiloxane com-pound, we repeated the experiment but using toluene assolvent; however it once again failed. Thinking that, probably,the presence of two methyl groups linked to the reactive Si−H

Scheme 1. Preparation of Vinylferrocenyl-Functionalized Cyclic and Linear Siloxanes 1 and 3

Figure 1. Cyclic voltammograms of vinylferrocenyl-containing cyclotetrasiloxane 1 (A) and polysiloxane 3 (B) measured in CH2Cl2 solutions with0.1 M Bu4NPF6 at a scan rate of 0.1 V s−1; (C) CV of 1 in CH2Cl2/CH3CN (3:1 by volume) at 0.1 V s−1. (D) Cyclic voltammetric responses of a Ptelectrode modified with a film of polyvinylferrocenylsiloxane 3, measured in 0.1 M Bu4NPF6/CH2Cl2 at different potential scan rates (mV s−1). Scanrate dependence upon the anodic peak current is shown.

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bond in HMe2Si-O-SiMe2H may make difficult the hydro-silylation reaction, our following objective was to explore thereactivity of the heterometallic Co2(CO)6(μ,η

2-HCCFc)toward the poly(methylhydrosi loxane) (Me3SiO)-(MeSiHO)n(SiMe3) (n ≈ 35), in which the Si−H reactivebonds show the same chemical environment as that in thepreviously studied cyclotetrasiloxane [MeSiHO]4.In an analogous reaction, Co2(CO)6(μ,η

2-HCCFc) reactswith (Me3SiO)(MeSiHO)n(SiMe3) to give the new polysilox-ane (Me3SiO)(MeSi{C(Fc)CH2}O)n(SiMe3) (n ≈ 35) (3)together with the cobalt carbonyl compounds Co3(CO)9(μ3-CCH2Fc)

7 and Co4(CO)8(μ-CO)2(μ4,η2-HCCFc) (2)

(Scheme 1). The 1H NMR spectrum of polysiloxane 3 exhibitsthe expected signals for the ferrocenyl groups (at δ 4.41 and4.07 ppm), those corresponding to the CH2 at δ 6.12 and5.72 ppm, and the resonances for the CH3 groups in the rangeδ 0.30−0.10 ppm. The MALDI-TOF spectrum of polysiloxane3 shows 11 peaks corresponding to oligomers that possess 1−11 organometallic units attached to the main siloxane chain.Each of these peaks is equally spaced at 270 mass units apart,which corresponds to the mass of the Si(Me)(CFcCH2)Ounit.Electrochemical Studies. The anodic electrochemistry of

vinylferrocenyl-functionalized polysiloxanes 1 and 3 has beenexamined by cyclic voltammetry (CV) and square wavevoltammetry (SWV) in CH2Cl2 solution containing tetra-n-butylammonium hexafluorophosphate (Bu4NPF6).The CV of tetravinylferrocenyl-cyclosiloxane 1, shown in

Figure 1A, exhibits a single oxidation at Epa = +0.54 V vs SCE.In the same way, the major feature in the cyclic voltammetricresponse of polysiloxane 3 (Figure 1B) in CH2Cl2 solution is asingle redox process, associated with the FeII/FeIII oxidation ofthe ferrocenyl units. These results indicate that for both cyclicand linear vinylferrocenyl-containing polysiloxanes 1 and 3, themultielectron transfer occurs simultaneously and at the samepotential; therefore the pendant ferrocenyl groups along thepolysiloxane backbones do not interact with the neighboringones.19

As can be noticeably seen in the CVs of 1 and 3, Figure 1Aand B, while the anodic wave has the characteristic shape for adiffusion-controlled process, the return wave (cathodic) doesnot and shows product adsorption effects. This clearly indicatesthat, using the low-polarity dichloromethane solvent, thesolubility of the polysiloxane changes (reduces) upon oxidationand that the insoluble species 14+ and 3n+ precipitate on theelectrode surface, as PF6

− salts. Reversal of the potentialregenerates the soluble neutral polysiloxane species. Figure 1Balso shows that, as expected, the stripping peak is morepronounced for the neutral fully ferrocenyl-substitutedpolysiloxane 3, which becomes the most insoluble uponoxidation and has the largest precipitation effect. Similaradsorption phenomena have been observed previously in thevoltammetric behavior of several examples of related siloxane-based ferrocenyl polymers,11,12 silicon-containing polyferrocen-yl dendrimers,20 and polyvinylferrocene.21

In addition, Figure 1C shows that, when a small amount ofacetonitrile is added to the CH2Cl2/electrolyte medium, thecathodic stripping peak disappears, and the CV of tetrametallic1 shows a redox wave (at E1/2 = 0.45) having diffusionalfeatures. The voltammetric features (ipc/ipa essentially equal tounit, ΔEpeak values about 70−100 mV, and Epeak independent ofthe scan rate) clearly show that oxidation of 1 in this CH2Cl2/CH3CN medium (3:1 by volume) is chemically and electro-

chemically reversible, resulting in the production of the soluble,stable tetracationic cyclic siloxane species [14+][PF6

−]4. Square-wave voltammetric measurements in this medium gave onlyone wave, also indicating that the oxidation of the fourvinylferrocenyl units attached to the cyclotetrasiloxane frame-work occurs at the same potential.We have found that polyvinylferrocenylsiloxane 3 deposited

electroactive films onto platinum electrode surfaces. Theelectrodeposition of the polysiloxane films was carried out byrepeatedly cyclically scanning the working electrode 30 times(at v = 50 mV s−1) between 0.0 and +0.9 V vs SCE, in degassedCH2Cl2 solutions of 3. The electrode was then rinsed withCH2Cl2 to remove any adhering solution and placed in apolyvinylferrocenylsiloxane-free Bu4NPF6/CH2Cl2 solution.The voltammetric response of a film of 3 is shown in Figure1D. A well-defined symmetrical oxidation−reduction wave,corresponding to the ferrocene/ferricinium couple, is observed,with a formal potential value E1/2 = +0.45 V vs SCE, which issimilar to the formal potential of the polysiloxane in solution.The wave shape is independent of the scan rate and typical of asurface-confined reversible couple,22,23 with the expected linearrelationship of peak current to potential sweep rate, for valuesup to 500 mV s−1. The peak to peak separation values (ΔEpeak)vary from 12 mV at a scan rate of 25 mV s−1 to 30 mV at 500mV s−1. In addition, the full width at half-maximum (Efwhm) ofthe surface voltammetric wave is about 160 mV at a scan rate of100 mV s−1 and is considerably larger than Efwhm = 90.6, whichis the ideally expected value for a one-electron transfer ofsurface-confined redox species.22,23 This larger value can beattributed to repulsive interactions between the electroactiveferrocenyl sites attached to the electrode surface.24 Platinumelectrodes modified with films of 3 are considerably durableand reproducible. In fact, extended cyclic voltammetric scanscan be carried out in CH2Cl2 and CH3CN solutions with nodetectable loss of electrodeposited ferrocenyl-containing ma-terial.In summary, we have found a new strategy to prepare new

cyclic and linear polyvinyl-ferrocenylsiloxanes 1 and 3 byhydrosilylation reactions of heterometallic Co2(CO)6(μ,η

2-HCCFc) in the presence of polymethylsiloxanes [MeSiHO]4and (Me3SiO)(MeSiHO)n(SiMe3) (n ≈ 35). Solution electro-chemical studies showed that all the vinylferrocenyl redoxcenters attached to the siloxane frameworks are electrochemi-cally independent and that, in CH2Cl2 as solvent, the neutralpolysiloxanes undergo oxidative precipitation onto theelectrode. Additionally, we have prepared and characterizedplatinum electrode surfaces modified with electroactive andstable films of polyvinylferrocenylsiloxane 3. Further syntheticwork is in progress with the aim to explore the reactivity of thevinylferrocenyl group in the described 1 and 3, with siloxane-based frameworks, as well as in related vinylferrocenyl-functionalized silicon-containing molecules.

■ EXPERIMENTAL SECTIONMaterials and Instrumentation. All reactions and compound

manipulations were performed in an oxygen- and moisture-freeatmosphere (N2 or Ar) using standard Schlenk techniques. Solventswere dried by standard procedures over the appropriate drying agentsand distilled prior to use. 1,1,3,3-Tetramethyldisiloxane, 1,3,5,7-t e t r am e t h y l c y c l o t e t r a s i l o x a n e ( ABCR ) , a n d p o l y -(methylhydrosiloxane) (Me3SiO)(MeSiHO)n(SiMe3) (Mn 1700−3200, n ≈ 35) (Aldrich) were used as received. The heterometallicCo2(CO)6(μ,η

2-HCCFc) was synthesized as previously described.6

NMR spectra were recorded on a Bruker-AMX-300 spectrometer.

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Chemical shifts were reported in parts per million (δ) with referenceto residual solvent resonances for 1H NMR (CDCl3,

1H, δ 7.27 ppm).29Si NMR spectra were recorded with inverse-gated proton decouplingin order to minimize nuclear Overhauser effects. Silicon NMRassignment was confirmed by a 1H−29Si HMBC experiment. FABmass spectra analyses were conducted on a VG Auto Spec massspectrometer equipped with a cesium ion gun. MALDI-TOF massspectra were obtained using a Reflex III (Bruker) mass spectrometerequipped with a nitrogen laser emitting at 337 nm. IR spectra wererecorded on a Perkin-Elmer Spectrum BX FT-IR spectrometer usingNaCl cells and KBr pellets. Elemental analyses were performed by theMicroanalytical Laboratory, SIDI, Universidad Autonoma de Madrid,Spain.Electrochemical Measurements. Cyclic voltammetric experi-

ments were recorded on a BASCV-50W potentiostat. CH2Cl2 andacetonitrile (spectrograde) for electrochemical measurements werefreshly distilled from calcium hydride under argon. The supportingelectrolyte used was tetra-n-butylammonium hexafluorophosphate(Fluka), which was purified by recrystallization from ethanol anddried under vacuum at 60 °C. The supporting electrolyteconcentration was typically 0.1 M. A conventional three-electrodecell connected to an atmosphere of prepurified nitrogen was used. AllCV experiments were performed using a Pt-disk working electrode (A= 0.020 cm2) (BAS), polished with a Buehler polishing cloth withMetadi II diamond paste, rinsed with purified water and acetone, anddried. A coiled platinum wire was used as a counter electrode.Solutions were, typically, 10−3 M in the redox-active species. Allpotentials were referenced to the saturated calomel electrode (SCE),which was separated from organic solutions by a fine frit. Under ourconditions, the ferrocene redox couple [FeCp2]

0/+ is +0.462, and thedecamethylferrocene redox couple [FeCp*2]

0/+ is −0.056 V vs SCE inCH2Cl2/0.1 M n-Bu4NPF6.Reaction of Co2(CO)6(μ,η

2-HCCFc) with [MeSiHO]4. 1,3,5,7-Tetramethylcyclotetrasiloxane (46 mg, 0.19 mmol) dissolved in 2 mLof 1,2-dichloroethane was added dropwise to a solution ofCo2(CO)6(μ,η

2-HCCFc) (300 mg, 0.6 mmol) in 35 mL of the samesolvent and was kept at 50 °C for 2.5 h, under an argon atmosphereand in the absence of light. Then, the solvent was removed and theresidue purified by TLC. Using a mixture of hexane/dichloromethane(3:1) as eluent, a red band corresponding to the compound Co3(CO)9(μ3-CCH2Fc) (15 mg, 0.024 mmol, 4%) was separated, followed by asecond, brown one of the cluster Co4(CO)12 (38 mg, 0.07 mmol,11.1%); finally, the third, orange one yielded the desired cyclo-tetrasiloxane [MeSi{C(Fc)CH2}O]4 (1) (75 mg, 0.07 mmol, 36.9%yield). Anal. Calcd (found) for C52H56Fe4O4Si4·2CH2Cl2: C, 51.75(51.86); H, 5.17 (4.84). IR (KBr, cm−1): ν Si−O−Si (1093−1028)and ν Si−C (800). 1H NMR (CDCl3, 300 MHz, 22 °C): δ 6.15, 5.75(d, J = 2.6 Hz, 8H,CH2), 6.09, 5.70 (br s, 8H,CH2), 4.50 (br s, 8× 2H, C5H4), 4.25 (br s, 8 × 2H, C5H4), 4.10 (s, 8 × 5H, C5H5), 0.43,0.41, 0.37, 0.34 (s, 8 × 3H, CH3).

29Si{1H} NMR (CDCl3, 59.62 MHz,22 °C): δ −33.8 (Si−O). MS (FAB+) m/z: 1079 [M+], 1014 [M+ −C5H5], 958 [M+ − C5H5-Fe], 893 [M+ − 2C5H5-Fe], 828 [M+ −3C5H5-Fe], 763 [M+ − 4C5H5-Fe], 707 [M+ − 4C5H5-2Fe].Reaction of Co2(CO)6(μ ,η

2-HCCFc) with (Me3SiO)-(MeSiHO)n(SiMe3). Following the procedure indicated before, adichloroethane solution of Co2(CO)6(μ,η

2-HCCFc) (1.19 g, 2.37mmol) and poly(methylhydrosiloxane) (Me3SiO)(MeSiHO)n(SiMe3)(95 mg) reacted during one hour at 50 °C. The compounds obtainedin this reaction were separated by silica gel chromatography. A firstband corresponding to the compound Co4(CO)12 (170 mg, 0.29mmol, 12.5%) was eluted using a 20:1 hexane/dichloromethanemixture. By modification of the solvent ratio (10:1 instead of 20:1) thecompounds Co3(CO)9(μ3-CCH2Fc) (18 mg, 0.03, 1.3%) andCo4(CO)8(μ-CO)2(μ4,η

2-HCCFc) (2) (38 mg, 0.05 mmol, 2.2%)went down from the column as a second and third band, respectively.Finally, using a hexane/dichloromethane solvent mixture (1:1) thenew ferrocenyl-polysiloxane (Me3SiO)(MeSi{C(Fc)CH2}-O)n(SiMe3) (n ≈ 35) (3) was separated as a yellow-orange solid(80 mg). IR (KBr, cm−1): ν Si−O−Si (1090−1044) and ν Si−C(803). 1H NMR (CDCl3, 300 MHz, 22 °C): δ 6.12 (br s, CH2),

5.72 (br s, CH2), 4.41 (br s, C5H4), 4.07 (br s, C5H4 + C5H5),0.30−0.10 [s, Si(CH3)3, Si(CH3)]. No signal was observed in the29Si{1H} NMR spectrum (CDCl3, 59.62 MHz, 22 °C). MS (MALDI-TOF) m/z: 432 [M+, n = 1], 702 [M+, n = 2], 972 [M+, n = 3], 1242[M+, n = 4], 1512 [M+, n = 5], 1782 [M+, n = 6], 2052 [M+, n = 7],2322 [M+, n = 8], 2592 [M+, n = 9], 2862 [M+, n = 10], 3132 [M+, n =11].

■ AUTHOR INFORMATION

Corresponding Author*E-mail: esther.delgado@uam.es; isabel.cuadrado@uam.es.

■ ACKNOWLEDGMENTSWe thank the Spanish Ministerio de Ciencia e Innovacion,projects MAT2010-20843-C02-01 and CTQ2009-09125/BQU,for financial support. S.B. acknowledges the Ministerio deEducacion y Ciencia for a FPU grant.

■ DEDICATIONDedicated to the memory of Prof. F. Gordon A. Stone inrecognition of his outstanding contribution to organometallicchemistry.

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Organometallics Article

dx.doi.org/10.1021/om201018h | Organometallics 2012, 31, 2715−27192719