Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Reaction of Silyl Hydrides with Tetrabutoxygermanium in the Presence of B(C6F5)3: Difference between Silicon and Germanium Chemistries and Easy Route to GeH4 Slawomir Rubinsztajn,* Marek Cypryk, Julian Chojnowski, Witold Fortuniak, Urszula Mizerska, and Piotr Pospiech† Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland S Supporting Information *
ABSTRACT: Dehydrocarbonative condensation reaction between alkoxysilanes and hydrosilanes catalyzed by tris(pentafluorophenyl)borane has been widely used for the formation of siloxane bonds. Our attempt to extend this chemistry to the preparation of Ge−O−Si bonds produced an unexpected outcome. The reaction of Ge(OBu)4 with PhMe2SiH in the presence of a catalytic amount of B(C6F5)3 at room temperature proceeded smoothly with the complete consumption of reactants and formation of GeH4 and PhMe2SiOBu in high yields. For the first time we have achieved selective exchange of functional groups between Ge−OR and Si−H. The discovered reaction features simple reaction conditions and can be used to prepare GeH4 in situ from easily available and safe substrates.
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INTRODUCTION One of the important reactions of silyl hydrides is their dehydrocarbonative condensation with alkoxysilanes promoted by tris(pentafluorophenyl)borane. This process, also known as the Piers−Rubinsztajn reaction,1,2 leads to the formation of a siloxane bond and release of a benign hydrocarbon as a byproduct. The dehydrocarbonative condensation opens a new route to the selective and efficient synthesis of various siloxane polymers and oligomers.3−6 The reaction of silyl hydrides with alkoxysilanes has been used to obtain functional oligosiloxanes of regular architecture5,7 and polysiloxane resins functionalized with reactive alkoxy groups.8,9 This reaction was successively extended to boron chemistry as a new route to borosiloxane resins by the reaction of diphenylsilane (Ph2SiH2) with trimethylborate catalyzed by B(C6F5)3.10 Since germanium is the element directly neighboring silicon within the same 14th group of the Periodic Table, a similarity might be expected between chemistries of these elements. Indeed, preliminary studies of Ignatovich et al.11 showed that both analogous dehydrocarbonative condensation reactions, i.e., reaction of germanium alkoxides with hydrosilanes and reaction of alkoxysilanes with germanium hydrides, occur and lead to the formation of Si−O−Ge bonds. These reactions proceed more slowly than the standard dehydrocarbonative condensation process, and their yields are low. Recently, Hreczycho et al. obtained germasiloxanes by dealkenative condensation of silanols with some allylgermanium derivatives mediated by scandium(III) triflate.12−14 This reaction demonstrated the stability of the Si−O−Ge bond, which may be generated by cleavage of the Ge−C bond. On the other hand, Gevorgyan et © XXXX American Chemical Society
al. and recently Oestreich et al. showed that hydrogermylation of alkynes and alkenes catalyzed by B(C6F5)3 occurs selectively in the presence of carboxyl and ether groups,15,16 which put in doubt the affinity of oxygen for germanium. The purpose of this study was to test dehydrocarbonative condensation of germanium tetraalkoxides with silyl hydride as a possible route to monomers and polymers containing siloxygermanium bonding.
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RESULTS AND DISCUSSION On the basis of earlier studies and preliminary theoretical calculations, we expected that the reaction of germanium tetraalkoxides with silyl hydride in the presence of B(C6F5)3 would proceed according to eq 1 with the possibility of further substitution of alkoxide by siloxide groups: R3SiH + Ge(OR′)4 → R3Si−O−Ge(OR′)3 + R′H
(1)
It is known that the dehydrocarbonative condensation is accompanied by functional group exchange.17 For that reason, potentially concurrent exchange reaction of reactive groups (eq 2) should be taken into consideration. R3SiH + Ge(OR′)4 → R3Si−OR′ + H−Ge(OR′)3
(2)
DFT calculations of thermodynamic parameters at 298 K were performed for the simple model systems where R and R′ were methyl (Table 1). Results of these calculations indicate that both dehydrocarbonative condensations, reactions 3 and 4, Received: March 13, 2018
A
DOI: 10.1021/acs.organomet.8b00156 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Table 1. DFT Calculations of Thermodynamic Parameters at 298 K for Reactions of Germanium and Silicon Tetramethoxides with Trimethylsilane reaction
Me3SiH + Ge(OMe)4 → Me3SiOGe(OMe)3 + H−Me Me3SiH + Si(OMe)4 → Me3SiOSi(OMe)3 + H−Me
Me3SiH + Ge(OMe)4 → Me3SiOMe + H−Ge(OMe)3
Me3SiH + Si(OMe)4 → Me3SiOMe + H−Si(OMe)3
(3) (4) (5) (6)
are strongly favorable. Moreover, thermodynamic quantities for both reactions are very similar. The reactions are enthalpydriven, which is not surprising if it is taken into account that the main driving force is transformations of weaker Si−H and C−O bonds into stronger Si−O and C−H bonds. Assuming the average values for bond dissociation enthalpies for these bonds, one can calculate that the net enthalpy effect upon this transformation is ca. −45 kcal/mol. It is little dependent on whether germanium or silicon alkoxide is involved because bonds around Ge or Si centers are not directly affected. The entropy change is not large, since the number of reagents does not change during reactions 3 and 4. Thermodynamic parameters for the competing group exchange processes (Table 1, reactions 5 and 6) were also calculated. The group exchange process between tetramethoxygermanium and trimethylsilane is also thermodynamically favorable. However, much more thermodynamically preferred is the dehydrocarbonative condensation between these two reactants. According to the above thermodynamic calculations, the reaction between a silyl hydride and germanium(IV) alkoxide should proceed according to eq 1. We decided to test this hypothesis by studying a model reaction between germanium(IV) butoxide and phenyldimethylsilane in the presence of a catalytic amount of B(C6F5)3. A premade solution of 2.4 × 10−2 mol of phenyldimethylsilane with 1.6 × 10−4 mol of the catalyst in 3.6 mL of toluene was added in four equal portions to a solution of 5.4 × 10−3 mol of germanium(IV) butoxide in 4 mL of toluene. A relatively high concentration of B(C6F5)3, 0.03 mol equiv, was used to ensure a rapid reaction. During this reaction, the generation of a gaseous product was observed. It was particularly intense after each batch of silane was added. The progress of the reaction was monitored by 1H NMR (Figures 1 and Figures S1−S4) and IR analysis (Figure 2 and Figure S5) of samples taken approximately 30 min after addition of each portion of silane. Additionally, selected samples were analyzed by 29Si NMR and GC/MS. The expected signal of the PhMe2SiH group in the 1H NMR at 4.43 ppm was not present in the recorded spectra (Figure 1), indicating the full conversion of dimethylphenylsilane. However, no signals could be attributed to the anticipated dehydrocarbonative condensation products (PhMe2SiO)nGe(OBu)4−n (reaction 7). Instead, a triplet, assigned to SiOCH2 of the butoxy group, appeared at 3.66 ppm. This assignment was confirmed by 1H NMR analysis of the independently prepared model compound PhMe2SiOC6H11 (Figure S14) and the 1H NMR spectrum of a mixture of Ge(OBu) 4 and PhMe2SiOC6H11 in toluene (Figure S19) and is consistent with the literature data.18 This signal is associated with PhMe2SiOBu, which must be formed by the exchange of Si− H and Ge-OBu groups (reaction 8). There were two singlets at 0.46 and 0.43 ppm, which were assigned respectively to Ph(CH3)2SiOBu and Ph(CH3)2SiOSi(CH3)2Ph. The presence of PhMe2SiOSiMe2Ph may be explained by the dehydrocarbo-
ΔH, kcal/mol
ΔG, kcal/mol
−52.9
−52.0
−53.9
−52.3
−14.4
−15.7
3.5
2.1
Figure 1. 1H NMR spectra of the reaction mixture after introduction of (A) 1.1, (B) 2.2, (C) 3.3, and (D) 4.4 mol equiv of PhMe2SiH to 1 mol equiv of Ge(OBu)4 in the presence of 0.03 mol equiv of B(C6F5)3 in toluene.
Figure 2. Standardized FTIR spectra of the reaction system after introduction of 1.1, 2.2, 3.3, and 4.4 mol equiv of PhMe2SiH to 1 mol equiv of Ge(OBu)4 in the presence of 0.03 mol equiv of B(C6F5)3 in toluene.
native condensation of PhMe 2 SiH with the created PhMe2SiOBu (reaction 9). Comparison of the integrations of the signals of SiOCH2 with the total integration of SiCH3, which corresponds to the initial concentration of SiH substrate, allows evaluation of the PhMe2SiH fraction transformed into the group exchange product (Table 2). The GeOCH2 signal decreased and became broadened as the reaction progressed. This signal completely disappeared after addition of an excess of PhMe2SiH (see Figure 1D and Figure S4). nPhMe2SiH + Ge(OBu)4 → (PhMe2SiO)n Ge(OBu)4 − n + n BuH
(7) B
DOI: 10.1021/acs.organomet.8b00156 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Table 2. Analysis of the Progress of the Reaction of PhMe2SiH with Ge(OBu)4 Catalyzed by B(C6F5)3 in Toluene at 23 °C on the Basis of the Recorded 1H NMR Spectra (Figures S1−S4) sample
fraction of PhMe2SiH added, mol/mol of GeOBu
full time of reaction, min
fraction of PhMe2SiOBu relative to total PhMe2SiH added
fraction of Ge-OBu relative to total amount of butoxy group GeOBu/(GeOBu + SiOBu)
yield of GeH4 vs [Ge(OBu)4]0, %
disiloxane, mol %
A B C D
0.28 0.56 0.84 1.12
20 80 140 200
0.90 0.91 0.88 0.67
0.79 0.59 0.22 0.01
21 41 78 99
11 11 11 22
the model dimethylphenylalkoxysilane PhMe2SiOC6H11 (Figure S15) and concurred with the literature data.19−21 The presence of both compounds was also verified by GC/MS analysis (Figures S6−S8). The question arises as to what the germanium products are. In the first stage of the process, after 1.1 mol equiv of PhMe2SiH was introduced to Ge(OBu)4, the 1H NMR spectrum (Figure 1A and Figure S1) showed a large triplet at 4.03 ppm assigned to the Ge−OCH2 group of the substrate. A second, much smaller triplet shifted downfield by 0.06 ppm, presumably related to the germanium butoxides containing a Ge−H group, was also detected. These signals became more complex and diffuse after further addition of PhMe2SiH (Figure 1B,C and Figures S2 and S3), thus indicating replacement of other OBu groups on germanium. The Ge−OCH2 signals disappeared after an excess of SiH with respect to the butoxy group in the substrate was introduced. The expected Ge−H proton signals were not easily noticeable by 1H NMR. Small broad signals slightly shifted upfield from the Si−OCH2 triplet were observed in Figure 1B,C and Figures S2 and S3. These signals could be related to the various GeHx(OBu)4−x species, but their integration was very low in comparison to the added silane. Additionally, these signals disappeared completely as the reaction proceeded, which implies that a volatile GeHcontaining product must be formed. It could be GeH4, the boiling point of which is as low as −88 °C;22 thus, it could easily escape from the reaction mixture at room temperature. This conclusion was confirmed by FTIR spectra (Figure 2 and Figure S5). The spectra were standardized to the same absorbance at maximum of the SiCH3 band at 1250 cm−1. The SiCH3 group was retained in the reaction, but its concentration was changed due to the addition of the silane. The FTIR spectra showed fast disappearance of SiH in the reaction system. Indeed, the Si−H stretching band, which appears in PhMe2SiH spectra at 2150 cm−1, and any new band associated with the stretching vibration of Ge−H bond23 were not detected (Figure S5). At the same time the bands of the GeO bond at 1030 and 1081 cm−1 decreased and the band of the SiO bond at 1090 cm−1 increased after adding the next portion of silane (Figure 2).24 The almost quantitative
nPhMe2SiH + Ge(OBu)4 → nPhMe2SiOBu + H nGe(OBu)4 − n
(8) PhMe2SiH + PhMe2SiOBu → PhMe2SiOSiMe2Ph + BuH
(9)
1
Results of the analysis of the H NMR spectra gathered in Table 2 show that the reaction of the silyl hydride and germanium(IV) butoxide went predominantly toward reactive group exchange (reaction 8), producing GeH4 in high yield. The competing dehydrocarbonative condensation between PhMe2SiH with GeOBu (reaction 7) was stalled. The subsequent dehydrocarbonative condensation of the formed alkoxysilane with the silyl hydride substrate (reaction 9) was much slower when GeOBu was present. However, the dehydrocarbonative condensation started to dominate when all GeOBu was consumed and a stepwise increase of the fraction of disiloxane was observed after addition of an excess of silane (Figure 1D). This conclusion is fully confirmed by the 29 Si NMR spectrum (Figure 3), which shows the main signal at
Figure 3. 29Si NMR spectrum of the reaction mixture after addition of 4.4 mol equiv of PhMe2SiH to 1 mol equiv of Ge(OBu)4 in the presence of 0.03 mol equiv of B(C6F5)3 in toluene.
6.98 ppm assigned to dimethylphenylbutoxysilane and a smaller signal at −0.7 ppm assigned to the corresponding disiloxane. The above assignment was validated by 29Si NMR analysis of
Table 3. DFT Calculations of Thermodynamic Parameters at 298 K for Consecutive Reactions of Functional Group Exchange between Tetramethoxygermanium and Trimethylsilane Leading to the Formation of GeH4 reaction
Me3SiH + Ge(OMe)4 → Me3SiOMe + HGe(OMe)3
(10)
ΔH, kcal/mol
ΔG, kcal/mol
−14.4
−15.7
Me3SiH + HGe(OMe)3 → Me3SiOMe + H 2Ge(OMe)2
(11)
−15.0
−15.2
Me3SiH + H 2Ge(OMe)2 → Me3SiOMe + H3Ge(OMe)
(12)
−14.2
−14.8
−20.1
−19.1
−63.7
−64.8
Me3SiH + H3Ge(OMe) → Me3SiOMe + GeH4
(13)
total C
DOI: 10.1021/acs.organomet.8b00156 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
One practical conclusion from this study may be drawn. Since the reaction studied leads to GeH4, which is formed rapidly under mild conditions and with almost theoretical yield, it may serve as a convenient method for generating this compound, alternative to previously described reactions.27,28 In addition, the substrates for these reactions are available and easy to handle. GeH4 is commonly used as a precursor in CVD generation of germanium, mixed silicon−germanium, or germanium-doped thin films that are widely used in various photonic and electronic devices.29,30
formation of GeH4 means that substituting each subsequent butoxy group in Ge(OBu)4 by hydrogen occurs more easily. To verify this concept, the thermodynamic parameters for reactions 10−13 were calculated (Table 3). Results of these calculations clearly indicate advantageous substitution of all alkoxy groups at germanium by hydrogen. It should be noted that formation of the transition state should be easier for germanium reactant with a higher number of hydrogens due to lower steric effects. Thus, from a kinetic point of view, an increase in the rate of reaction is also expected with the higher degree of alkoxy group substitution by hydrogen at germanium. Further confirmation of almost quantitative substitution of alkoxide groups by hydrogen came from a preparative experiment in which GeH4 was collected in an upturned buret filled with silicone oil (Figure S9) and identified by 1H NMR (Figure 4) and GC/MS (Figure S12). 1H NMR
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CONCLUSIONS The reaction of germanium tetraalkoxides with silyl hydride in the presence of a catalytic amount of B(C6F5)3 was investigated. DFT calculations of thermodynamic parameters for the model system (Ge(OMe)4 and Me3SiH) revealed that dehydrocarbonative condensation leading to the creation of Ge−O−Si bonds and release of alkane is strongly favorable. Indeed, the reaction of Ge(OBu)4 with PhMe2SiH in the presence of a catalytic amount of B(C6F5)3 proceeded to the complete consumption of reactants. However, the expected products containing Ge− O−Si bonds were not detected. Instead, GeH 4 and PhMe2SiOBu were produced almost quantitatively. For the first time, the selective exchange of functional groups between Ge−OR and Si−H was observed. This unexpected result can be explained by the higher barrier of free energy of Si−O−Ge bond formation in relation to the free energy barrier of the competing group exchange reaction. Further theoretical and experimental studies are underway to clarify this problem. The discovered reaction features simple reaction conditions and can be used to prepare GeH4 in situ from commercially available and nonhazardous substrates.
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Figure 4. 1H NMR spectrum of the captured volatile products of the reaction of 1 mol equiv of Ge(OBu)4 with 4.6 mol equiv of PhMe2SiH in the presence of 0.039 mol equiv of B(C6F5)3 in toluene.
EXPERIMENTAL SECTION
Materials. Tetrabutoxygermanium (95% purity, ABCR), phenyldimethylsilane (98% purity, Fluorochem), and tris(pentafluorophenyl)borane (97% purity, TCI) were used as received. Toluene (99.9% purity, HPLC Plus, Sigma-Aldrich) was additionally dried on the column system NBSPS-800 of MBraun drybox, Model UNILAB. Deuterated solvents CDCl3 and toluene-d8 (Sigma-Aldrich) were dried over 4A molecular sieves. Instrumentation. NMR Spectroscopy. 1H NMR spectra in CDCl3 or toluene-d8 were obtained with a Bruker AV 200 MHz spectrometer working at 200.16 MHz. 29Si NMR spectra were acquired on a Bruker AV III 500 MHz spectrometer working at 99.36 MHz using an ingated decoupling technique with 20 s delay and 30° pulse. FTIR Spectroscopy. FTIR spectra of film placed between KBr plates were recorded using a Thermo Electron Corporation FTIR Nicolet Model 380 instrument. Gas Chromatography−Mass Spectrometry (GC/MS). GC/MS analysis was performed using a Shimadzu QP2010 ultra apparatus equipped with Zebron ZB-5MSi capillary GC column (30 m × 0.25 mm × 0.25 μm). The carrier gas was helium. The following temperature program was used: hold at 50 °C for 3 min, heat to 250 °C at a rate of 10 °C/min, hold at 250 °C for 20 min, heat to 280 °C at a rate of 20 °C/min. A quadrupole mass spectrometer, Shimadzu QP2010 Ultra, with electron ionization was connected to a GC system. Procedure 1: Reaction of 1 mol equiv of Ge(OBu)4 with 4.4 mol equiv of PhMe2SiH Promoted by 0.030 mol equiv of B(C6F5)3. All operations in the preparation of the reaction mixture were performed under an atmosphere of nitrogen. A solution of germanium(IV) butoxide (1.97 g, 5.41 × 10−3 mol) dissolved in 4.0 mL of dried toluene was placed in a flask purged with nitrogen, equipped with a magnetic stirrer, nitrogen gas inlet, and gaseous products outlet through a bubbler. Separately phenyldimethylsilane (3.30 g, 2.42 × 10−2 mol) and tris(pentafluorophenyl)borane (0.0831
spectrum of the volatiles dissolved in cold deuterated chloroform gave a singlet at −3.20 ppm, in accord with the literature data.25 A small amount of butane, which is formed in reaction 9, was also detected. The purity of the collected GeH4 was estimated as 95% on the basis of 1H NMR analysis. GC/ MS of the gaseous product displayed a mass spectrogram showing the peak pattern characteristic for GeH4 with maximum at m/z 76, which is consistent with the literature data26 (Figure S13). Although the siloxane bond is easily formed by dehydrocarbonative condensation, the Si−O−Ge bond is not as readily available by the analogous reaction of silyl hydride with the alkoxy germanium reagent. The reason lies not in the thermodynamics of this process but rather in the higher barrier of free energy of the Si−O−Ge bond formation in relation to the free energy barrier of the competing group exchange reaction. Germanium is a softer electrophilic center in comparison to silicon and is more susceptible to accepting a hydride ion, which is a relatively soft nucleophile, thus making the group exchange process easier. Another reason may be that alkoxy germanium reactants may interact more strongly with the borane catalyst, reducing its ability to mediate the transfer of hydride from silicon to alkoxide carbon, which is vital for the dehydrocarbonative condensation. Further theoretical and experimental studies are underway to clarify this problem. D
DOI: 10.1021/acs.organomet.8b00156 Organometallics XXXX, XXX, XXX−XXX
Organometallics
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g, 1.62 × 10−4 mol) were dissolved in 3.6 mL of dried toluene. This solution was divided into four equal fractions. These fractions, in time intervals shown in Table 1, were introduced to the stirred solution of germanium(IV) butoxide by means of a syringe through a septum. Gas evolution was observed during the reaction. Samples were withdrawn 30 min after addition of each portion of silane by means of a hypodermal syringe through the septum and subjected to analysis by 1 H NMR and FTIR. In addition, the last sample was subjected to GC/ MS (EI) (Figures S6−S8) and 29Si NMR (Figure 3). Procedure 2: Capture of the Formed GeH4 in Upturned Buret during Reaction of Ge(OBu)4 with PhMe2SiH Promoted by B(C6F5)3. All operations in the preparation of the reaction mixture were performed under an atmosphere of dry nitrogen in a glovebox. The reaction system (Figure S9) was used to measure the volume of GeH4 formed in the reaction of Ge(OBu)4 with PhMe2SiH promoted by B(C6F5)3. A solution of germanium(IV) butoxide (1.804 g, 4.93 × 10−3 mol) dissolved in 3.0 mL of dried toluene was placed in a 50 mL three-necked flask purged with nitrogen and equipped with a magnetic stirrer, cold finger (−32 °C), and gaseous products outlet connected via a double-tipped needle to an upturned buret filled with silicone oil. Separately, 12 mL of a toluene solution containing 3.09 g (0.0227 mol) of phenyldimethylsilane and 0.0974 g (1.902 × 10−4 mol) of B(C6F5)3 was transferred to a 20 mL Hamilton syringe. This solution was slowly added over a period of 2 h to the stirred solution of germanium(IV) butoxide by means of a syringe pump through a septum. Gas evolution was observed during addition of the silane. A 113.7 mL portion of the gas was collected when the addition was completed. Reaction was continued for an additional 1 h. The total volume of the collected gas was 118 mL, which corresponds to about 95% of the expected volume of GeH4, assuming 95% conversion of Ge(OBu)4 and high solubility of the formed butane in the reaction mixture. A sample of the final reaction mixture was analyzed by 1H NMR (Figure S10) and 29Si NMR (Figure S11). A sample of the captured gas was withdrawn by means of a hypodermal syringe from the buret, mixed with cold CDCl3, and subjected to analysis by 1H NMR (Figure 4) and GC/MS (EI) (Figures S12 and S13). 1H NMR (200 MHz, CDCl3): δ 3.20 ppm (s), The purity of the collected GeH4 was estimated as 95% on the basis of 1H NMR analysis. Theoretical Methods. All quantum mechanical calculations were performed using the Gaussian 09 suite of programs.31 Geometries of all model compounds were optimized using the hybrid B3LYP density functional32 corrected for dispersion interactions using the Grimme GD3 empirical term,33 with the Def2-SVP basis set34 in the gas phase. All stationary points were identified as stable minima by frequency calculations. The vibrational analysis provided thermal enthalpy and entropy corrections at 298 K within the rigid rotor/harmonic oscillator/ideal gas approximation.31 Thermochemical corrections were scaled by a factor of 0.985.35 More accurate single-point electronic energies were obtained using the B3LYP functional, including the Grimme GD3 dispersion correction,33 with the larger Def2-TZVP basis set for the Def2-SVP optimized geometries.36 This level of theory is denoted as B3LYP-GD3/Def2TZVP//B3LYP-GD3/ Def2SVP. The integration grid was set to ultrafine. The basis set superposition error (BSSE) has been neglected, since it is small (
