Reactivity of a Titanocene Pendant Si–H Group toward Alcohols

Jul 22, 2013 - J. Selye University, Faculty of Education, Bratislavská cesta 3322, 945 01 Komárno, Slovak Republic. •S Supporting Information. ABSTRAC...
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Reactivity of a Titanocene Pendant Si−H Group toward Alcohols. Unexpected Formation of Siloxanes from the Reaction of Hydrosilanes and Ph3COH Catalyzed by B(C6F5)3 Tomás ̌ Strašaḱ ,† Jan Sýkora,† Martin Lamač,‡ Jiří Kubišta,‡ Michal Horácě k,‡ Róbert Gyepes,‡,§ and Jiří Pinkas*,‡ †

Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, v.v.i., Rozvojová 135, 165 02 Prague 6, Czech Republic ‡ J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 2155/3, 182 23 Prague 8, Czech Republic § J. Selye University, Faculty of Education, Bratislavská cesta 3322, 945 01 Komárno, Slovak Republic S Supporting Information *

ABSTRACT: The reaction of [Cp(η5-C5H4CH2SiMe2H)TiCl2] (1; Cp = η5-C5H5) and methanol in the presence of catalytic amounts of B(C6F5)3 afforded a complex with a pendant silyl ether group, [Cp(η5-C5H4CH2SiMe2OMe)TiCl2] (2), in good yield. The analogous reaction of 1 and Ph3COH resulted in the unexpected formation of [CpTiCl2{μη5:η5-(C5H4)CH2SiMe2OSiMe2CH2(C5H4)}TiCl2Cp] (4). The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph3COH mediated by B(C6F5)3 has a general applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity toward Ph3COH in comparison with other alcohols can be attributed to the easy generation of the borate salt [Ph3C]+[(C6F5)3B(μ-OH)B(C6F5)3]− (5) under catalytic conditions. The intramolecular Si−H and Ti−Cl exchange in 1 is catalyzed by B(C6F5)3 in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride, which is either chlorinated to give [Cp(η5-C5H4CH2SiMe2Cl)TiCl2] (3) in CD2Cl2 or decomposes into several paramagnetic Ti(III) species in toluene-d8. Complex 3 was independently synthesized from 1 and Ph3CCl in a good yield.

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INTRODUCTION The preparation of tris(pentafluorophenyl)borane, B(C6F5)3, a unique and remarkably strong Lewis acid, was first reported in 1963 by Massey, Stone, and Park.1 However, the story of its successful application began more than 20 years later, after its ability to abstract the alkyl group from group 4 metallocene dialkyls was uncovered. These species, after forming cationic complexes, exhibited high activity in olefin polymerization.2−4 Since then, B(C6F5)3 as a catalyst in organic, organometallic, and polymer chemistry has drawn considerable attention, evidenced also by numerous review articles.5−9 In addition, catalytic processes utilizing hydrosilanes as reactants have also become very important in recent years. The main reaction types include (i) silylation of alcohols (Scheme 1)10,11 and thiols,12 (ii) formation of (poly)siloxanes from silyl hydrides and alkoxysilanes (known as the Piers−Rubinsztajn reaction),13,14 (iii) hydrosilylation of imines,15 olefins,16 and aldehydes, ketones, and esters,17 and (iv) activation of C−F bonds in fluoroalkanes.18 A general feature of all these reactions is the activation of the Si−H bond via the formation of the silane− borane adduct R3Siδ+H·Bδ−(C6F5)3 (see Scheme 1). Such an activated “silylium cation” then attacks the nucleophilic part of the substrate, which is followed either by the boron-bonded © XXXX American Chemical Society

Scheme 1. Mechanism of the Silylation of Alcohols Catalyzed by B(C6F5)3

hydride compensating the carbocation formed (in iii and iv) or releasing hydrogen or alkane, respectively (in i and ii). The present work is the result of our long-term interest in the preparation of group 4 metallocene and related complexes with pendant functional groups, which are modifiable directly Received: March 25, 2013

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within the organometallic framework.19−26 The pendant Si−H group seemed to be a convenient choice for further transformations, since it is easily introduced at early stages of the metallocene dichloride synthesis and is also reasonably stable toward air and moisture under laboratory conditions. The Si−H group on the metallocene periphery can subsequently be modified via either catalytic hydrosilylation27,28 or intramolecular dehydrocoupling reactions.29,30 Herein we introduce a new titanocene dichloride bearing a pendant Si−H group (1) as a convenient precursor for the preparation of titanocene dichlorides with a pendant silyl ether functionality using silylation of methanol catalyzed by B(C6F5)3. This reaction follows the mechanism described in Scheme 1.10 In contrast, we have found the reaction of titanocene 1 with Ph3COH and catalyzed by B(C6F5)3 to proceed in a different way, leading to the formation of a dinuclear complex having a bis(methylene)tetramethylsiloxane bridge between its two titanocene units. Extending the reaction scope to a variety of hydrosilanes showed that formation of siloxanes from the reaction of 2 equiv of hydrosilanes with Ph3COH in the presence of a catalytic amount of B(C6F5)3 is of general applicability. In addition, the reactivity of B(C6F5)3 toward Ph3COH with various stoichiometric ratios was evaluated.

Scheme 3. Methanol Silylation with 1 in CD2Cl2 Catalyzed by B(C6F5)3

RESULTS AND DISCUSSION The reaction of lithium (dimethylsilyl)methylenecyclopentadienide prepared in situ with [CpTiCl3] in THF (Scheme 2) yielded a titanocene dichloride with a

the signals corresponding to 1 and the formation of a new set of signals attributable to 2. In addition to the major product 2 (95 mol %), the presence of the byproduct [Cp(η 5 C5H4CH2SiMe2Cl)TiCl2] (3; 5 mol %) was also recognized (vide infra) in the 1H NMR spectrum. Complex 2 was isolated in a pure state in a moderate yield of 62% and was characterized by common spectroscopic methods. The presence of the methylsiloxy group was confirmed by both NMR in CD2Cl2 solution (SiOMe: δH 3.39 ppm; δC 50.77 ppm) and IR spectroscopy (ν(C−H) 2832 cm−1, νas(Si−O−C) 1187 and 1089 cm−1; Figure S1 in the Supporting Information). Signals corresponding to SiMe2 (δH 0.09 ppm, δC −2.58 ppm) and CH2Si (δH 2.28 ppm, δC 25.94 ppm) appeared as singlets in 1H NMR spectra due to the disappearance of the Si−H group. The identity of byproduct 3 was verified by its independent synthesis following Corey’s work on the chlorination of hydrosilanes with trityl chloride.31 The reaction of 1 with Ph3CCl in CDCl3 as a solvent (Scheme 4) ran to completion

Scheme 2. Preparation of 1

Scheme 4. Chlorination of 1 with Ph3CCl

pendant (methylene)dimethylsilane group, 1, obtained as a redorange solid in good yield (72%) after workup. A characteristic feature of the complex is the presence of a SiH functionality, which is easily recognized in the solid state by IR spectroscopy (ν(Si−H) 2107 cm −1 ; Figure S1 in the Supporting Information) and in its CDCl3 solution by NMR spectroscopy (SiH: δH 3.93 ppm; δSi −11.0 ppm). The presence of an Si−H group led to a split of signals corresponding to SiMe2 (δH 0.09 ppm) and CH2Si groups (δH 2.32 ppm) into doublets with a low coupling constant (3JHH = 3.3 Hz), typical for vicinal coupling through the silicon atom.29,30 The composition of 1 was corroborated by EI-MS, where the [SiMe2H]+ (m/z 59) fragment was found as the base peak, whereas the abundance of the molecular ion (m/z 320) was low. Solid 1 was found to be sufficiently stable toward air and moisture, as no decomposition was detected by NMR even after it was stored without a protective atmosphere for a period of several months. The transformation of Si−H into Si−OR in 1 was performed by following the synthetic protocol for silylation of alcohols.10 The reaction of 1 with methanol in the presence of a catalytic amount of B(C6F5)3 (ca. 2 mol %) was conducted in CD2Cl2 (Scheme 3). Analysis of the reaction mixture after several hours by NMR spectroscopy revealed a complete disappearance of

within 4 days, affording 3 in 70% yield. The formation of triphenylmethane as the second reaction product was proved by NMR spectroscopy (Ph3CH: δH 5.55 ppm; δC 56.92 ppm). Complex 3 was identified by NMR, IR, and EI-MS. Replacement of the electron-donating hydride with the electron-withdrawing chloride led to a significant electron deshielding on the silicon atom, which resulted in a downfield shift of the SiMe2 group signals in 29Si NMR (δSi 29.3 ppm) as well as in 1H NMR for SiMe2 (δH 0.43 ppm) and CH2Si (δH 2.57 ppm). The presence of pendant chlorosilane was further supported by IR spectroscopy, where the strong stretching vibration ν(Si−Cl) could be found at 465 cm−1 (Figure S1 in the Supporting Information). The observed formation of byproduct 3 during the reaction of 1 with methanol (as depicted in Scheme 3) was rather surprising, since the formation of chlorosilanes has not been reported for the catalytic silylation of alcohols in chlorinated solvents.10 Therefore, we have conducted the reaction in the absence of methanol to evaluate the role of chlorinated solvent in the formation of 3. The treatment of 1 with ca. 4 mol % of B(C6F5)3 in CD2Cl2 in an NMR tube led to a complete consumption of 1 and formation of 3 as the main reaction product. Furthermore, a quintuplet (2JHD = 1.5 Hz) with a line ratio of ca. 1:2:3:2:1 positioned at 2.98 ppm in the 1H NMR

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similarly to methanol (with the formation of the corresponding siloxane) or rather to Ph3CCl. The reaction of 1 with an equimolar amount of Ph3COH in the presence of 4 mol % of B(C6F5)3 in CD2Cl2 solution (Scheme 6) afforded the

spectrum (Figure S2 in the Supporting Information) can be assigned to CD2HCl (for comparison, CH3Cl: δH(CDCl3) 3.01 ppm)32 as a product of CD2Cl2 reduction. The reaction proceeded in the same manner in CDCl3, giving rise to characteristic signals of CDHCl2 as a triplet (2JHD = 1.0 Hz, ca. 1:1:1 line ratio) at 5.28 ppm in the 1H NMR spectrum and a triplet (1JCD = 27.5 Hz, 1:1:1 line ratio, Figure S3 in Supporting Information) at 53.37 ppm in the 13C NMR spectrum in addition to the signals of 3. Notably, the reaction of 1 with a catalytic amount of B(C6F5)3 in toluene-d8 led to visible gas evolution and the 1H NMR spectrum did not reveal any sufficiently populated diamagnetic compound. In contrast, the EPR spectrum (Figure S4 in the Supporting Information) of the sample showed three signals (centered at g = 1.9886, 1.9767, and ca. 1.98) with the dominating one being the very broad (ΔH ≈ 20 mT) signal centered at g ≈ 1.98, which suggested the presence of several paramagnetic Ti(III) species. We suggest that in the absence of methanol the reaction proceeds following Scheme 5. The Si−H bond in 1 is polarized

Scheme 6. Reaction of 1 with Ph3COH

dinuclear complex 4 with a bis(methylene)tetramethylsiloxane bridge between two titanocene dichloride units as the sole organometallic product. 1H and 13C NMR spectra of 4 showed a pattern similar to that for 2 with singlet signals for SiMe2 (δH 0.05 ppm, δC 0.42 ppm) and CH2Si (δH 2.21 ppm, δC 25.28 ppm). The SiMe2 signal observed in 29Si NMR spectra (δSi 5.9 ppm) is shifted considerably upfield in comparison with that for 2 (δSi 15.2 ppm); however, it is close to the value for hexamethyldisiloxane (δSi 7.4 ppm).38 Further evidence for the Me2SiOSiMe2 siloxane spacer comes from the presence of a strong infrared absorption band at 1059 cm−1 assigned to the ν as (Si−O−Si) vibration (Figure S1 in the Supporting Information).39 The unexpected formation of 4 led us to investigate the reaction in detail. We have used PhMe2SiH as a model hydrosilane for further study in order to simplify the reaction. A stoichiometric reaction of PhMe2SiH and Ph3COH catalyzed by B(C6F5)3 (0.5 mol % with respect to the silane) produced a mixture consisting of approximately equimolar amounts of (PhMe2Si)2O (tr = 16.1 min, m/z 286), Ph3CH (tr = 19.8 min, m/z 244) and Ph3COH (tr = 21.7 min, m/z 260), as was determined by GC-MS. A preparative-scale reaction of 2 equiv of PhMe2SiH with 1 equiv of Ph3COH catalyzed by B(C6F5)3 (0.5 mol % with respect to the silane) produced (PhMe2Si)2O and Ph3CH as the reaction products in a ca. 1:1 molar ratio (as determined by GC-MS and 1H NMR of the reaction mixture after 30 min). Finally, the siloxane (PhMe2Si)2O was isolated from the reaction mixture by column chromatography on silica gel (eluent hexane, Rf = 0.33) in a high yield (84%). Importantly, the GC-MS analysis of the reaction mixture showed also a peak with low population (ca. 2 mol % with respect to (PhMe2Si)2O) at tr = 8.4 min with m/z 152, which indicated the presence of the silanol PhMe2SiOH. This observation and the reaction stoichiometry indicate that the reaction proceeds in two consecutive steps and the silanol PhMe2SiOH is a crucial intermediate. Indeed, performing the reaction of silanol PhMe2SiOH and hydrosilane PhMe2SiH catalyzed by B(C6F5)3 in CD2Cl2 in an NMR tube led to hydrogen evolution and clean formation of the siloxane (PhMe2Si)2O (see Figure S5 in the Supporting Information). On the basis of the above observations we propose the reaction sequence as depicted in Scheme 7. To validate the proposed sequence, the reaction was monitored by 1H NMR in CD2Cl2 at low temperature (see Figure S6 in the Supporting Information). Immediately after all components were mixed, formation of Ph3CH was detected (a characteristic singlet signal of the methine group Ph3CH appeared at 5.60 ppm), which indicated hydroxyl transfer from the trityl group to silicon proceeding very quickly. Simulta-

Scheme 5. Proposed Mechanism for Catalytic Transformations of 1 in the Absence of Methanol

by the formation of the silane−borane adduct [Cp(η5C5H4CH2SiMe2H·B(C6F5)3)TiCl2], similarly as known for Me3SiH·B(C6F5)3 or Et3SiH·B(C6F5)3.9,13 The activated silicon atom becomes a sufficiently strong electrophile to attack the negatively charged chlorine ligand and induces an intramolecular hydrido chloride exchange between the titanium and silicon centers yielding a titanocene hydrido chloride with a pendant chlorosilane arm. A similar intermolecular hydrosilylation of decamethylscandocene chloride mediated by B(C6F5)3, leading to the corresponding decamethylscandocene hydride, was reported recently.33,34 The transiently formed titanocene hydrido chloride complex is a highly reactive species35 and undergoes chlorination with CD2Cl2 or CDCl3 to give 3, with concomitant reduction of the solvent to CD2HCl or CDHCl2, respectively. Indeed, the ability of titanium hydrides to activate the C−X bond (X = Cl, Br) in haloaromatic substrates has been well demonstrated.36,37 One can assume that in a less nucleophilic solvent such as toluene-d8 the titanocene hydrido chloride species reductively decomposes into paramagnetic Ti(III) species with the concomitant evolution of molecular hydrogen. An attempt to trap the transient titanocene hydrido chloride with diphenylacetylene failed, since the paramagnetic products detected in EPR yielded a spectrum with a spectral pattern very similar to that obtained in the previous experiment. In the next step we have examined the reactivity of 1 toward triphenylmethanol. We were curious whether it would react C

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B(C6F5)3·ROH8 does not hold for the particular case of Ph3COH. Therefore, we have investigated the reactivity of B(C6F5)3 toward Ph3COH at their different mutual ratios. As an initial effort, we reacted equimolar amounts of Ph3COH and B(C6F5)3 in CD2Cl2 (Scheme 8). Mixing both colorless solids led already to a yellowish mixture, which after addition of CD2Cl2 dissolved immediately to give a yellow solution. This solution was investigated by 1H and 19F NMR spectroscopy, performed at variable temperature. The 1H NMR spectrum at −100 °C (Figure S7 in the Supporting Information) showed two sets of signals: (i) a triplet signal at 6.57 ppm (JHF ≈ 17.5 Hz) and a very broad unresolved signal centered at ca. 7.8 ppm (ν1/2 ≈ 150 Hz) and (ii) a broad singlet at 3.29 ppm (ν1/2 ≈ 16 Hz) and a multiplet at 7.09−7.32 ppm. On the basis of previously published data,40 we have assigned the former set of signals to a hydroxo-bridged borate anion with the compensating tritylium cation [Ph3C]+[(C6F5)3B(μ-OH)B(C6F5)3]− (5). The assignment of the borate anion was further supported by 19F NMR spectra (Figure S8 in the Supporting Information). The second set of signals was slightly shifted with respect to those of the free triphenylmethanol (Ph3COH (CD2Cl2): δH 2.80 (s, 1H, OH); 7.20−7.31 (m, 15H, Ph)).41 Moreover, the signal of the hydroxyl group (δH 3.29 ppm at −100 °C) was found to be thermally dependent (Figure S9 in the Supporting Information). The aforementioned observations can be tentatively explained by the formation of the weak adduct [Ph3C]+[(C6F5)3B(μ-OH)B(C6F5)3]−·Ph3COH and its equilibrium with free triphenylmethanol as depicted on the left side in Scheme 8. The reaction between Ph 3 COH and B(C 6 F 5 ) 3 was subsequently performed also in a 1:2 ratio to maintain a proper stoichiometry for the hydroxo-bridged borate salt 5. The initial formation of a yellow color after mixing both solid reactants indicated the likely formation of the tritylium cation already in the solid state. The IR spectrum of the homogenized solid mixture (Figure S10 in the Supporting Information) showed a new vibration band for ν(O−H) at 3551 cm−1 (the starting Ph3COH showed ν(O−H) at 3473 cm−1). This observed value is closer to the value reported for [Ge(Dipp 2 nacnac)] + [(C 6 F 5 ) 3 B(μ-OH)B(C 6 F 5 ) 3 ] − (where Dipp2nacnac = {N(C6H3-2,6-i-Pr2)C(Me)}2CH}; ν(O−H) 3540 cm −1 ) 42 than to a range of values known for hydroxyborate anions in the salt [PPN]+[(C6F5)3B(OH)]− (where PPN = bis(triphenylphosphoranylidene)ammonium; ν(O−H) 3680−3700 cm−1);43 therefore, the formation of salt 5 is anticipated even in the solid state. 1H spectra of CD2Cl2 solution of 5 showed tritylium signals as slightly broadened peaks in the expected range at all studied temperatures (Figure S11 in the Supporting Information). The 19F NMR spectra at −100 °C (Figure S12 in the Supporting Information) showed a pattern typical for the [(C6F5)3B(μ-OH)B(C6F5)3]− anion, supportive of the proposed structure. Interestingly, temperature-dependent spectra of 5 did not give any evidence for the presence of free B(C6F5)3, which indicated that the equilibrium

Scheme 7. Proposed Reaction Sequence for the Reaction of 2 equiv of PhMe2SiH and Ph3COH

neously, broad singlet signals centered at 3.05 and 2.81 ppm appeared. The former signal could be attributed to the residual adduct B(C6F5)3·Ph3COH. As the reaction evolved, the latter signal shifted downfield to 2.61 ppm, which is close to the value found for the hydroxyl proton in free PhMe2SiOH (δH 2.48 ppm) (see Figure S5 in the Supporting Information). Within approximately 10 min, the signal became very broad (in the range from ca. 2 to 3 ppm), indicating the presence of a rather complicated mixture of unidentified silanol−borane adducts and intermediates. The signal of the dihydrogen formed could be detected at later stages of the reaction (after ca. 4 min), and its intensity increased slowly as the reaction proceeded. Concurrently, the intensity of the signal corresponding to the hydride SiH steadily decreased, which indicated a rather slow rate of the second step of the reaction depicted in Scheme 7. The reaction reached completion in approximately 20 min, affording (PhMe2Si)2O and Ph3CH in the expected stoichiometric ratio. The reaction scope was further extended to three commercially available tertiary hydrosilanes; the results are given in Table 1. All hydrosilanes tested were converted cleanly into the corresponding siloxanes in high yields (78−91%); in all cases Ph3CH was found as a byproduct, as evidenced by 1H NMR. Table 1. Reactions of Tertiary Hydrosilanes with Ph3COH Catalyzed by B(C6F5)3a cat. B(C6F5)3

R3SiH (2 equiv) + Ph3COH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ R3SiOSiR3 −Ph3CH, −H 2

run

hydrosilane

product

yield, %b

1 2c 3 4 5

PhMe2SiH PhMe2SiH Et3SiH Ph3SiH i-Pr3SiH

(PhMe2Si)2O (PhMe2Si)2O (Et3Si)2O (Ph3Si)2O (i-Pr3Si)2O

91 84d 88 82 78

a

Reaction conditions: hydrosilane, 0.6 mmol; Ph3COH, 0.3 mmol; B(C6F5)3, 6 μmol; CH2Cl2, 5 mL; time, 30 min; room temperature. b Yield determined by 1H NMR. cPreparative scale: hydrosilane, 10 mmol; B(C6F5)3, 23 μmol; CH2Cl2, 20 mL, time, 30 min, room temperature. dIsolated yield.

Reactivity of B(C6F5)3 toward Ph3COH. The above results indicated a different reactivity of hydrosilanes toward Ph3COH in the presence of B(C6F5)3, in comparison to other alcohols. On the basis of these observations one can assume that the well-documented formation of the borane−alcohol adduct Scheme 8. Reactivity of B(C6F5)3 toward Ph3COH

D

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proposed by Saverio et al. (eq 1)40 either does not hold at all for the present case or is at least strongly shifted to the right side.

reaction could be extended to a variety of hydrosilanes. The easy generation of the borate salt [Ph3C]+[(C6F5)3B(μOH)B(C6F5)3]− (5) under catalytic conditions is proposed to be responsible for the catalysis of hydrosilane hydroxylation and in general for the twist of hydrosilane reactivity toward Ph3COH in comparison to other alcohols. An intramolecular mutual Si−H to Ti−Cl group exchange was found in 1 in the presence of a catalytic amount of B(C6F5)3. The transiently formed titanocene hydrido chloride was either chlorinated to give the titanocene dichloride with pendant chlorosilane group 3 in CD2Cl2 solution or decomposed into Ti(III) species in toluene-d8 solution. This indicates that titanocene hydrides can be generated in situ by the reaction of the corresponding metallocene dichlorides with a hydrosilane in the presence of a catalytic amount of B(C6F5)3, similarly as published recently for decamethylscandocene.

[(C6F5)3 B(OH)]− + B(C6F5)3 ⥂[(C6F5)3 B(μ‐OH)B(C6F5)3 ]−

(1)

The formation of 5 could be detected even in a mixture containing a huge excess of Ph3COH with respect to B(C6F5)3. The mixing of 50 equiv of Ph3COH with 1 equiv of B(C6F5)3 (i.e., in the ratio at which the catalytic reactions were performed) gave a yellow solid which dissolved in CD2Cl2, giving a green solution. The presence of the borate anion [(C6F5)3B(μ-OH)B(C6F5)3]− could be confirmed by the appearance of characteristic signals in the 19F NMR spectrum of the solution at −100 °C (Figure S13 in the Supporting Information). Moreover, signals of the tritylium cation were found in the 1H NMR spectrum at −100 °C (Figure S14 in the Supporting Information). From the aforementioned observations we propose that 5 is the key catalytic species in the B(C6F5)3-catalyzed formation of silanols from hydrosilanes. A tentatively proposed overall catalytic cycle is depicted in Scheme 9.

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EXPERIMENTAL SECTION

General Considerations. All reactions with moisture- and airsensitive compounds were carried out under argon (99.998%) using standard Schlenk techniques. Solvents (hexane, heptane, toluene, diethyl ether (Et2O), and tetrahydrofuran (THF)) were dried with sodium/benzophenone and stored over a sodium mirror under argon. Chloroform and dichloromethane were dried with CaH2 and stored over molecular sieves 4 Å. Methanol was dried with sodium and freshly distilled prior to use. n-BuLi (1.6 M in hexane) was obtained from Aldrich. [CpTiCl3] (99%) was obtained from Acros Organics and used as received. Diphenylacetylene (Fluka) was dried under high vacuum overnight. B(C6F5)3 was obtained from Strem and used as received. PhMe2SiH, Et3SiH, and i-Pr3SiH (Aldrich) were dried with LiAlH4 and distilled under vacuum. PhMe2SiOH (Aldrich) was distilled under vacuum (98−100 °C/10 Torr) prior to use. Ph3COH and Ph3CCl were obtained from Alfa Aesar and dried under vacuum for 8 h prior to use. (Dimethylsilyl)methylenecyclopentadiene (a mixture of isomers) was prepared according to the literature procedure.44 1H, 13C, 19F, and 29 Si NMR spectra were recorded on a Varian Mercury 300 spectrometer at 300.0, 75.4, 282.2, and 59.9 MHz, respectively, at 298 K if not otherwise stated. Chemical shifts (δ/ppm) are given relative to solvent signals (toluene-d8, δH 2.08 ppm, δC 20.43 ppm for CHD2; CDCl3, δH 7.26 ppm, δC 77.16 ppm; CD2Cl2, δH 5.32 ppm, δC 53.84 ppm). EPR spectra were measured on an ERS-220 spectrometer (Center for Production of Scientific Instruments, Academy of Sciences of GDR, Berlin, Germany) operated by a CU-3 unit (Magnettech, Berlin, Germany) in the X-band. g values were determined using an Mn2+ standard signal at g = 1.9860 (MI = −1/2 line). EI-MS spectra were obtained on a VG-7070E mass spectrometer at 70 eV. Crystalline samples in sealed capillaries were opened and inserted into the direct inlet under argon. IR spectra of samples either in a Nujol mull or in KBr were taken on a Nicolet Avatar FTIR spectrometer in the range 400−4000 cm−1. Melting points of samples (under air or in sealed capillaries under nitrogen) were measured on a Koffler block and were not corrected. Elemental analyses were carried out on a FLASH 2000 CHN-O Automatic Elemental Analyzer (Thermo Scientific). [Cp(η 5 -C 5 H 4 CH 2 SiMe 2 H)TiCl 2 ] (1). To a mixture of (dimethylsilyl)methylenecyclopentadienes (2.00 g, 14.5 mmol) in THF (20 mL) was slowly added dropwise a solution of n-BuLi in hexane (9.1 mmol, 1.6 M, 14.5 mmol). The resulting solution was stirred for 2 h and then slowly dropped into a cold (−78 °C) solution of [CpTiCl3] (3.17 g, 14.5 mmol) in THF (30 mL). The reaction mixture was warmed to room temperature and stirred for a further 20 h. The final red mixture was filtered, and the filtrate was evaporated to dryness under vacuum. The crude product was extracted with a toluene/CH2Cl2 mixture (1/1 v/v, 10 mL). The volume of the obtained dark red extract was reduced to a minimum and left in the freezer (−78 °C) for several days. The formed red-orange solid that formed was isolated, washed with toluene (5 mL) and hexane (3 × 5 mL), and dried under vacuum. Yield: 3.36 g (72%).

Scheme 9. Proposed Catalytic Cycle for Hydroxylation of Silanes (Left Cycle) and Subsequent Siloxane Formation from Silane and in Situ Formed Silanol (Right Cycle)

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CONCLUSION This work has demonstrated the ability of a novel titanocene dichloride bearing a pendant methylenedimethylsilane group, 1, to achieve methanol silylation in the presence of a catalytic amount of B(C6F5)3 in a way similar to that known for commercially available hydrosilanes. A possible extension of the reaction scope to a variety of alcohols could lead to a direct modification of the titanocene periphery with various alkoxo groups in one reaction step. However, Ph3COH reacted with 1 in a different way, and the dimeric titanocene complex 4 with a bis(methylene)tetramethylsiloxane bridge between cyclopentadienyl rings was formed. A detailed study of the B(C6F5)3mediated reaction of Ph3COH with the model hydrosilane PhMe2SiH showed that the siloxane formation proceeds in two consecutive steps. The fast initial step consists of hydroxyl group transfer from the trityl moiety to the silicon atom with concomitant formation of the silanol PhMe2SiOH and Ph3CH. In the next step, the in situ formed silanol undergoes a B(C6F5)3-catalyzed silylation with a second equivalent of hydrosilane to form a siloxane and H2. The scope of the E

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Mp: 138 °C. 1H NMR (CDCl3): 0.09 (d, 3JHH = 3.3 Hz, 6H, SiMe2); 2.32 (d, 3JHH = 3.3 Hz, 2H, CH2); 3.93 (m, 1H, SiMe2H); 6.16, 6.41 (2 × pseudo t, 2 × 2H, C5H4); 6.54 (s, 5H, C5H5). 13C{1H} NMR (CDCl3): −4.31 (SiMe2); 20.74 (CH2); 115.76 (CH, C5H4); 119.57 (C5H5); 121.98 (CH, C5H4); 139.05 (Cipso, C5H4). 29Si{1H} NMR (CDCl3): −11.0 (Me2SiH). EI-MS, m/z (relative abundance): 322 (17); 321 (11); 320 (M•+, 21); 284 (24); 284 (20); 283 (34); 259 (14); 257 (63); 256 (33); 255 ([M − Cp]+, 75); 253 (36); 244 (23); 242 (32); 224 (28); 223 (18); 222 (64); 191 (24); 190 (46); 185 (24); 183 ([CpTiCl2]+, 45); 163 (21); 161 (23); 150 (75); 149 (69); 148 ([CpTiCl]•+, 85); 137 ([C5H4CH2SiMe2H]+, 37); 85 (18); 83 ([TiCl]+, 32); 78 (40); 65 ([Cp]+, 50); 59 ([SiMe2H]+, 100). IR (Nujol, cm−1): 3107 (m), 2107 (ν(Si−H), vs), 1490 (m), 1248 (m), 1151 (m), 1060 (m), 1026 (m), 1014 (m), 940 (m), 895 (vs), 886 (vs), 860 (m), 838 (w), 824 (s), 777 (w), 459 (w), 418 (w), 409 (vw). Anal. Calcd for C13H18Cl2SiTi (321.16): C, 48.61; H, 5.65. Found: C, 48.94; H, 5.78. [Cp(η5-C5H4CH2SiMe2OMe)TiCl2] (2). To a solid mixture of 1 (25 mg, 78 μmol) and a catalytic amount of B(C6F5)3 (ca. 1 mg, 2 μmol) were added CD2Cl2 (0.7 mL) and methanol (3.2 μL, 78 μmol). The resulting red solution was stirred for 5 min, transferred into an NMR tube, and degassed by freeze−pump−thaw process (three times), and the tube was sealed off by flame. After ca. 1 h, the complete consumption of 1 and formation of 2 was proved by NMR spectroscopy. After 1H, 13C, and 29Si spectra were measured, the tube was opened under argon and its contents were transferred into a Schlenk flask and layered with heptane (3 mL). Standing of the mixture for several days in the refrigerator led to the formation of a red solid. The solid was isolated, washed with heptane (4 × 2 mL), and dried under vacuum. Yield: 17 mg (62%). Similarly, complex 2 was obtained from 1 (34 mg, 106 μmol), methanol (4.3 μL, 106 μmol), and B(C6F5)3 (ca. 1 mg, 2 μmol) in a spectroscopic purity of >90% in toluene-d8 as a solvent. Mp: 176 °C. 1H NMR (CD2Cl2): 0.09 (s, 6H, SiMe2); 2.28 (s, 2H, CH2); 3.39 (s, 3H, OMe); 6.16 (pseudo t, 2H, C5H4); 6.42 (pseudo t, 2H, C5H4); 6.52 (s, 5H, C5H5). 1H NMR (toluene-d8): −0.04 (s, 6H, SiMe2); 2.33 (s, 2H, CH2); 3.19 (s, 3H, OMe); 5.69 (pseudo t, 2H, C5H4); 5.87 (pseudo t, 2H, C5H4); 6.01 (s, 5H, C5H5). 13C{1H} NMR (CD2Cl2): −2.58 (SiMe2); 25.94 (CH2); 50.77 (OMe); 116.32 (CH, C5H4); 119.95 (C5H5); 122.47 (CH, C5H4); 138.33 (Cipso, C5H4). 29 Si{1H} NMR (CD2Cl2): 16.0 (SiMe2). 29Si{1H} NMR (toluene-d8): 15.2 (Me2SiOMe). EI-MS, m/z (relative abundance): 350 (M•+, 6); 335 ([M − Me]+, 5); 315 ([M − Cl]+, 8); 300 ([M − MeCl]•+, 5); 291 (18); 289 (38); 287 (88); 286 (49); 285 ([M − Cp]+, 100); 274 (11); 272 (18); 257 (13); 235 (13); 224 (16); 222 (25); 191 (23); 190 (32); 185 (32); 183 ([CpTiCl2]+, 36); 167 ([C5H4CH2SiMe2OMe]+, 18); 150 (31); 148 ([CpTiCl]•+, 73); 95 (19); 93 (32); 91 (44); 90 (75); 89 ([SiMe2OMe]+, 86); 65 (28); 59 (98). IR (KBr, cm−1): 3107 (m, sh), 2956 (m), 2899 (m), 2832 (m), 1490 (s), 1443 (w), 1425 (vw), 1392 (vw), 1254 (m), 1187 (vw), 1164 (m), 1089 (s), 1033 (w), 1015 (w), 938 (w), 841 (vs), 826 (vs), 799 (m), 750 (w), 734 (w), 637 (vw), 415 (vw). Anal. Calcd for C14H20Cl2OSiTi (351.18): C, 47.88; H, 5.74. Found: C, 47.79; H, 5.87. [Cp(η5-C5H4CH2SiMe2Cl)TiCl2] (3). To a mixture of 1 (32 mg, 100 μmol) and Ph3CCl (28 mg, 100 μmol) was added CDCl3 (0.7 mL). The resulting red solution was transferred into an NMR tube and degassed by a freeze−pump−thaw process (three times), and the tube was sealed off by flame. After the reaction was completed (ca. 4 days as determined by 1H NMR spectroscopy), the NMR tube was opened and its contents were transferred into a Schlenk flask and then layered with heptane (4 mL). Standing of the mixture for several days led to a formation of red solid and a white fluffy powder. The mother liquor and the white powder were removed, and the residual red solid was washed with heptane (2 × 2 mL) and dried under vacuum. Yield: 25 mg (70%). Mp: 195 °C. 1H NMR (CDCl3): 0.43 (s, 6H, SiMe2); 2.57 (s, 2H, CH2); 6.22 (pseudo t, 2H, C5H4); 6.42 (pseudo t, 2H, C5H4); 6.55 (s, 5H, C5H5). 13C {1H}(CDCl3): 1.66 (SiMe2); 24.97 (CH2); 114.92 (CH, C5H4); 119.64 (C5H5); 123.11 (CH, C5H4); 135.37 (Cipso, C5H4).

29

Si{1H} NMR (CD2Cl2): 29.3 (Me2SiCl). EI-MS, m/z (relative abundance): 354 (M•+, 4); 321 (8); 319 ([M − Cl]+, 10); 295 (9); 294 (8); 293 (28); 292 (19); 291 (78); 290 (25); 289 ([M − Cp]+, 75); 254 ([M − Cp-Cl]•+, 6); 222 (13); 185 (13); 183 ([CpTiCl2]+, 19); 173 (12); 171 ([C5H4CH2SiMe2Cl]+, 27); 150 (21); 148 ([CpTiCl]•+, 60); 95 (58); 94 (18); 93 ([SiMe2Cl]+, 100); 83 (22); 65 ([Cp]+, 28). IR (Nujol, cm−1): 3113 (s), 1491 (s); 1444 (s), 1426 (m), 1255 (s), 1170 (m), 1066 (w), 1048 (m), 1023 (w), 1016 (w), 935 (w), 904 (w), 853 (s, sh), 825 (vs), 804 (s), 759 (w), 738 (vw), 660 (w), 591 (vw), 465 (m), 418 (w), 405 (w). Anal. Calcd for C13H17Cl3SiTi (355.60): C, 43.91; H, 4.82. Found: C, 44.12; H, 5.89. [CpTiCl2{μ-η5:η5-(C5H4)CH2SiMe2OSiMe2CH2(C5H4)}TiCl2Cp] (4). To a solid mixture of 1 (17 mg, 53 μmol), Ph3COH (14 mg, 53 μmol), and a catalytic amount of B(C6F5)3 (ca. 1 mg, 2 μmol) was added CD2Cl2 (0.7 mL). The resulting red solution was stirred for 5 min and transferred into an NMR tube and degassed by a freeze− pump−thaw process (three times), and the tube was sealed off by flame. After 2 h at room temperature the 1H, 13C, and 29Si spectra showed a complete reaction (formation of 4 and Ph3CH). Then, the tube was opened under argon and its contents were transferred into a Schlenk flask and layered with heptane (4 mL). Standing of the mixture for several days in a refrigerator (at 4 °C) led to the formation of a red microcrystalline solid and a voluminous brownish mud. The mud was carefully removed by syringe, and the remaining red microcrystals were washed several times with heptane and dried under vacuum. Yield: 15 mg (87%). Mp: 238 °C. 1H NMR (CD2Cl2): 0.05 (s, 6H, SiMe2); 2.21 (s, 2H, CH2); 6.12 (pseudo t, 2H, C5H4); 6.42 (pseudo t, 2H, C5H4); 6.53 (s, 5H, C5H5). 13C{1H} NMR (CD2Cl2): 0.42 (SiMe2); 25.28 (CH2); 116.32 (CH, C5H4); 119.88 (C5H5); 122.41 (CH, C5H4); 138.67 (Cipso, C5H4). 29Si{1H} NMR (CD2Cl2): 5.9 (SiMe2). IR (KBr, cm−1): 3110 (m), 2955 (m), 2922 (m), 1491 (s), 1444 (m), 1426 (w), 1392 (w), 1256 (s), 1168 (m), 1155 (m), 1059 (s), 1017 (m), 937 (w), 840 (vs), 822 (vs), 757 (w), 734 (w), 693 (vw), 669 (vw), 651 (vw), 606 (vw), 522 (vw), 416 (w), 408 (w). Anal. Calcd for C26H34Cl4OSi2Ti2 (656.30): C, 47.58; H, 5.22. Found: C, 47.71; H, 5.27. Note: temperatures above 280 °C were necessary to evaporate the sample in a mass spectrometer, which obviously caused complex redistribution/decomposition. The only acceptable ions found in the spectrum were [CpTiCl2]+ (m/z 183) and [CpTiCl]+ (m/z 148). NMR Tube Reaction of 1 with a Catalytic Amount of B(C6F5)3 in CD2Cl2 or CDCl3 (Generation of 3). In a J. Young NMR tube, a solid mixture of 1 (18 mg, 56 μmol) and B(C6F5)3 (1 mg, 2 μmol) was cooled by liquid nitrogen and CD2Cl2 was distilled into the tube under vacuum. The mixture was warmed to room temperature, giving a red solution. 1H and 29Si NMR spectroscopy of the mixture confirmed the formation of 3 and CD2HCl. CD2HCl: 1H NMR (CD2Cl2) 2.98 (quintuplet, 2JHD = 1.5 Hz, 1H, CD2HCl). The reaction was conducted in an identical manner in CDCl3 starting from 1 (70 mg, 218 μmol) and B(C6F5)3 (4 mg, 8 μmol) and gave rise to 3 and CDHCl2. CDHCl2: 1H NMR (CDCl3) 5.28 (t, 2JHD = 1.0 Hz, 1H, CDHCl2); 13 C{1H} NMR (CDCl3) 53.37 (t, 1JCD = 27.5 Hz, CDHCl2). NMR Tube Reaction of 1 with a Catalytic Amount of B(C6F5)3 in Toluene-d8. To a mixture of 1 (17 mg, 53 μmol) and B(C6F5)3 (ca. 1 mg, 2 μmol) was added toluene-d8 (0.7 mL), which caused an immediate intense gas evolution. The mixture changed color from intense red to brown within several minutes. After 10 min of stirring, the mixture was transferred into an NMR tube and degassed by a freeze−pump−thaw process (three times) and the tube was sealed off by flame. The 1H NMR of the sample was almost silent. EPR (toluene-d8, 22 °C; three species): g = 1.9767 (ΔH = 0.53 mT); ca. 1.98 (ΔH ≈ 20 mT); g = 1.9886 (ΔH = 0.35 mT). Attempted Trapping of the Transient Titanocene Hydrido Chloride with Diphenylacetylene. To a solution of 1 (38 mg, 118 μmol) with a 5-fold molar excess of diphenylacetylene (105 mg, 590 μmol) in toluene-d8 (0.7 mL) was added a solution of B(C6F5)3 (ca. 2 mg, 4 μmol) in toluene-d8 (0.3 mL). The reaction mixture turned brown immediately, and gas evolution was observed. The 1H NMR F

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Organometallics

Article

−137.9, −140.3 (6 × 2F, CFortho); −157.3, −158.0, −159.3 (3 × 2F, CFpara); −162.6 (2F, CFmeta); −164.2 (2 × 2F, CFmeta); −164.9, −165.2, −165.6 (3 × 2F, CFmeta). IR (KBr, cm−1): 3551 (w), 3066 (vw), 3034 (vw), 1647 (s), 1584 (s), 1563 (w), 1520 (vs), 1472 (vs), 1451 (vs), 1380 (s), 1359 (vs), 1321 (m), 1295 (m), 1230 (vw), 1188 (s), 1168 (m), 1111 (m), 1089 (s), 997 (m), 981 (vs), 915 (vw), 844 (vw), 807 (vw), 786 (m), 766 (m), 729 (vw), 705 (m), 670 (m), 623 (w), 609 (vw), 576 (vw), 484 (vw), 472 (vw), 403 (vw). 1 equiv of B(C6F5)3 and 50 equiv of Ph3COH. In a glovebox, a solid mixture of Ph3COH (717 mg, 2.76 mmol) and B(C6F5)3 (28 mg, 55 μmol) was homogenized with a mortar and pestle until the yellow color did not appear (ca. 15 min); 80 mg of the mixture was dissolved in CD2Cl2 (0.7 mL) in a J. Young NMR tube to give a green solution. The formation of 5 was detected by 19F and 1H NMR spectroscopy at −100 °C (see the Supporting Information, Figures S13 and S14).

spectrum showed diphenylacetylene as the only diamagnetic species. The EPR spectroscopy revealed the same paramagnetic species as in the above experiment, although in a slightly different ratio. Preparative-Scale Reaction of 2 equiv of PhMe2SiH with Ph3COH Catalyzed by B(C6F5)3. Solid Ph3COH (1.292 g, 4.97 mmol) and B(C6F5)3 (26 mg, 50 μmol) were mixed in a Schlenk tube, which caused the formation of a yellow mixture. The mixture was dissolved in CH2Cl2 (20 mL), and the resulting green solution was cooled to 0 °C. PhMe2SiH (1.362 g, 10 mmol) was added, and the cooling bath was removed. In a few minutes gas evolution started and continued for ca. 10 min. The reaction was finished after an additional 15 min, as was determined by GC-MS (the total reaction time was 30 min). The volatiles were evaporated under vacuum, and the waxy residue was purified by column chromatography on silica gel with hexane as eluent (Rf = 0.33). The yield of yellowish liquid was 1.185 g (84%). The identity of the product was confirmed by NMR and IR spectroscopy. 1H NMR and IR spectra are given in the Supporting Information. NMR Tube Reaction of PhMe2SiH with PhMe2SiOH Catalyzed by B(C6F5)3. A solution of PhMe2SiH (82 μL, 540 μmol) and PhMe2SiOH (84 μL, 540 μmol) in CD2Cl2 (0.7 mL) was added to solid B(C6F5)3 (3 mg, 6 μmol) in a J. Young NMR tube. Immediate gas evolution and a slightly exothermic reaction was observed. The NMR tube was closed with a Teflon valve, and NMR measurements were performed. The 1H NMR spectrum showed the presence of (PhMe2Si)2O and H2 (signal at 4.71 ppm). 1 H NMR Monitoring of the Reaction of 2 equiv of PhMe2SiH with Ph3COH Catalyzed by B(C6F5)3. To a cold (−78 °C) solution of Ph3COH (78 mg, 300 μmol) and B(C6F5)3 (3 mg, 6 μmol) in CD2Cl2 (0.5 mL) placed in a J. Young NMR tube was quickly added a cold (−78 °C) solution of PhMe2SiH (92 μL, 600 μmol). The tube was shaken quickly, which caused a color change from green to yellow. The tube was immediately inserted into the NMR probe precooled to −10 °C, and 1H NMR spectra were periodically acquired (see the Supporting Information). General Procedure for the B(C6F5)-Catalyzed Reaction of Ph3COH with Hydrosilanes. To a solution of Ph3COH (78 mg, 300 μmol) and B(C6F5)3 (3 mg, 6 μmol) in CH2Cl2 (5 mL) was added hydrosilane (600 μmol). The mixture was stirred for 30 min, and then a sample was taken for GC-MS (0.1 mL). The rest of the mixture was evaporated to dryness, and the residue was dissolved in CDCl3 and transferred into an NMR tube. THF (30 μL) was added as an internal standard, and the yield of the formed siloxane was determined by 1H NMR spectroscopy. Reaction of B(C6F5)3 with Ph3COH at Various Ratios. 1 equiv of B(C6F5)3 and 1 equiv of Ph3COH. To a mixture of Ph3COH (12 mg, 46 μmol) and B(C6F5)3 (24 mg, 46 μmol) was added CD2Cl2 (0.7 mL). The yellow solution that formed was stirred for 5 min and then transferred into an NMR tube and partially degassed and the tube was sealed off with flame. The presence of 5 in the reaction mixture was supported by 1H and 19F NMR spectroscopy in the temperature range from +25 to −100 °C (see the Supporting Information, Figures S7− S9). 2 equiv ofB(C6F5)3 and 1 equiv of Ph3COH: Generation of [Ph3C]+[(C6F5)3B(μ-OH)B(C6F5)3]− (5). In a glovebox, a solid mixture of Ph3COH (20 mg, 77 μmol) and B(C6F5)3 (79 mg, 154 μmol) was homogenized in a vibration mill for 5 min. The resulting intense yellow solid was checked by IR spectroscopy (Figure S10 in Supporting Information) and then transferred into a Schlenk vial, and CD2Cl2 (0.7 mL) was added. The resulting intense yellow solution was transferred into an NMR tube and partially degassed, and the tube was sealed off with flame. The formation of 5 was observed by 1H and 19 F NMR spectroscopy in the temperature range from +25 to −100 °C (see the Supporting Information, Figures S11 and S12). The signals of 5 persisted in 1H NMR for several days, although a slow increase of signals belonging to Ph3CH indicated some decomposition. However, an attempt to isolate 5 from the solution failed. 1 H NMR (298 K, CD2Cl2): 6.64 (br s, 1H, OH); 7.68 (br d, 6H, CHortho); 7.89 (t, 3JHH = 7.5 Hz, CHmeta); 8.28 (br t, 3H, CHpara). 19F NMR (282 MHz, 173 K, CD2Cl2) −129.7, −131.1, −131.5, −133.1,

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving the 1H, 13C, and 19F NMR, EPR, and IR spectra mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*J.P.: tel, (+420) 266053735; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Academy of Sciences of the Czech Republic (projects GA203/09/1574 and GAP207/12/2368) is gratefully acknowledged. We thank Dr. Lidmila Petrusová for melting point determinations.

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REFERENCES

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