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Article

Synthesis of a Dichlorodigermasilane: Double Si–Cl Activation by a Ge=Ge Unit

by
Tomohiro Sugahara
1,
Norihiro Tokitoh
1 and
Takahiro Sasamori
2,*
1
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
2
Graduate School of Natural Sciences, Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya, Aichi 467-8501, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2017, 5(4), 79; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics5040079
Submission received: 21 October 2017 / Revised: 8 November 2017 / Accepted: 10 November 2017 / Published: 14 November 2017
(This article belongs to the Special Issue Coordination Chemistry of Silicon)
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Abstract

:
Halogenated oligosilanes and oligogermanes are interesting compounds in oligosilane chemistry from the viewpoint of silicon-based-materials. Herein, it was demonstrated that a 1,2-digermacyclobutadiene derivative could work as a bis-germylene building block towards double Si–Cl activation to give a halogenated oligometallane, a bis(chlorogermyl)dichlorosilane derivative.

Graphical Abstract

1. Introduction

Halogenated oligosilanes and oligogermanes are attractive compounds as functionalized oligometallanes from the standpoint of oligosilane material chemistry [1,2,3,4,5,6,7]. In this regard, Si(II) or Ge(II) species should be an important building block for creating such halogenated oligosilanes/germanes because silylenes (divalent Si(II) species) or germylenes (divalent Ge(II) species) have been known to undergo ready Si–Cl insertion reactions, i.e., Si–Cl activation reactions [8,9,10,11]. For example, Iwamoto and Kira reported the facile Si–Cl insertion of the isolable dialkylmetallylenes towards SiCl4 under mild conditions [10]. However, especially in the germanium cases, it is difficult to isolate the insertion products, >Ge(Cl)SiR3, because the insertion reaction of a germylene toward a Si–Cl bond would be reversible in some cases [12]. Thus, the substituents on the Si atom (R of the R3Si–Cl species) should be bulky and/or electropositive to avoid the α-elimination of R3Si–Cl from the Ge moiety [8,12,13,14,15,16,17,18,19,20,21,22]. The requirement for the bulkiness on the Si–Cl moiety could make it difficult to create halogenated oligometallanes, such as >Ge(Cl)–SiCl2–Ge(Cl)< with utilizing the double Si–Cl activation of the Ge(II) species towards SiCl4, because of two unfavorable factors: (i) entropy, two Ge(II) moieties should react with one SiCl4 species; and (ii) the stability of the product, the Cl atoms on the Si atom should promote the α-Si–Cl elimination, i.e., the retro reaction (Scheme 1). In this paper, we chose a 1,2-digermacyclobutadiene derivative [23] as a suitable Ge(II) building block for the double Si–Cl activation of SiCl4 to yield >Ge(Cl)–SiCl2–Ge(Cl)< species, because the rigid cyclic skeleton should overcome the entropy-disadvantage, and the rigidness of the cyclic skeleton should suppress the α-Si–Cl elimination. Finally, it was found that the stable 1,2-digermacyclobutadiene 1 (1,2-Tbb2-3,4-Ph2-digermacyclobutadiene, Tbb = 2,6-[CH(SiMe3)2]2-4-t-Bu-C6H2, Scheme 2) [24] with SiCl4 afforded the corresponding 1,3-digerma-2-sila-cyclopent-4-ene derivative, the cyclic >Ge(Cl)–SiCl2–Ge(Cl)< compound. 1,2-Digermacyclobut-1-ene derivative was reacted with SiCl4 to give the double-Si–Cl-insertion product, and the following reduction reaction gave the corresponding > Ge = Si = Ge < species [11]. Although 1 could undergo facile double Si–Cl activation toward SiCl4, neither double Ge–Cl nor C–Cl activation could occur in the reaction of 1 with GeCl4/CCl4.

2. Results and Discussions

When the stable digermyne 2 bearing bulky aryl substituents, Tbb groups (2,6-[CH(SiMe3)2]2-4-t-Bu-C6H2), was treated with PhC≡CPh (tolan) at room temperature, 1,2-digermacyclobutadiene 1 was isolated as a stable crystalline compound [23,24,25,26] via formal [2+2] cycloaddition (Scheme 2). As one can see from the structure of 1, it is a cyclic 4π-electron conjugated, anti-aromatic compound incorporating Ge(II) moieties. On the basis of theoretical calculations, 1 has considerable –Ge=Ge–C=C– character rather than =Ge–Ge=C–C– [24]. Accodingly, as expected, 1 could work as a building block of the bis-Ge(II) moiety. Reaction of 1 with SiCl4 afforded digermadichlorosilane 3 quantitatively, which could be formed via double Si–Cl insertion reactions of the Ge(II) moieties of the 1,2-digermacyclobutadiene skeleton in 1. This reaction has been performed under the neat condition at 55 °C because the addition of small amount of SiCl4 or reaction at r.t. afforded very slow conversion of 3. The obtained dichlorosilane 3 has the >Ge(Cl)–SiCl2–Ge(Cl)< moiety in its 1,3-digerma-2-sila-cyclopent-4-ene skeleton, i.e., 2 should be one of a unique class of compounds of oligohalo-oligometallanes. Thus, 1 was found to work as a bis-germylene building block (>Ge: + :Ge<) towards a double Si–Cl activation.
The molecular structure of digermadichlorosilane 3 was definitively determined by X-ray crystallographic analysis (Figure 1). The two Tbb/Cl groups are oriented in (E)-geometry probably due to steric reasons. The five-membered ring skeleton in 3 exhibits the envelope geometry with a deviation of the Si atom from the Ge–C=C–Ge plane by ca. 1.27 Å. While the two Ge–Cl bond lengths are almost the same (Ge1–Cl1: 2.2094(14) Å, Ge2–Cl4: 2.2011(15) Å) within a range of standard deviations, the orientation of the two Cl atoms are slightly different to each other. That is, one of the Cl atom (Cl4) is oriented to outside of the five-membered ring skeleton, but another one (Cl1) is approaching to the central Si atom with the Cl1···Si distance of 3.25 Å, which is far from the other one (Cl4···Si = 3.66 Å) [27]. In addition, the two Cl–Ge–Si angles are considerably different from each other, (Cl1–Ge1–Si = 90.20(8), Cl4–Ge2–Si = 105.40(8)). These asymmetrical structural features indicate weak n(Cl1)···σ*(Si–Cl3) interaction. These structural features were reasonably reproduced by the theoretical structural optimization at B3PW91/6-311G(2d) [28]. The theoretically-optimized structure of the less hindered model 3′, which has Me groups instead of Tbb groups, exhibits a completely planar five-membered skeleton with C2 symmetry. Thus, these structural features observed in 3 could be due to the steric congestion.
In the expectation of obtaining the Ge analogue of 3, digermadichlorogermane 4, the reaction of 1 with GeCl4 was attempted. As a result, the expected product, 4, was not obtained, but the 1,2-dichloro-1,2-digermacyclobut-3-ene 5 was obtained as a predominant product even under the conditions of using only a small amount of GeCl4 in the dark [29]. In addition, the reaction of 1 with CCl4 also furnished the formation of 5 without any formation of the CCl2-insertion product 6. 1,2-Dichloro-1,2-digermacyclobutene 5 showed considerable stability in the air, and it can object to further purification by silica gel column chromatography. Although the reaction mechanism for the formation of 5 by the reaction of 1 with GeCl4 or CCl4 was not clear at present, the formation of 5 is most likely interpreted in terms of the double-chlorination of 1 with the elimination of ECl2 (E = Ge or C) moiety.
The difference of the products in the reaction of 1 with ECl4 (E = Si, Ge, and C) between E = Si and E = Ge, C cases should be of great interest. Although we could not draw a definitive conclusion, we performed the thermodynamic energy calculations (free energies) on the reaction of 1 with ECl4 (E = Si, Ge, C) to give the insertion products, 3, 4, and 6, or the chlorination products, 5 and Cl2E: (calculated as 1/2 Cl2E=ECl2) at the B3PW91/6-311G(2d) level of theory (Scheme 3) [28]. In the case of E = Si, the formation of 3 should be exothermic by 2.3 kcal/mol, and that of 5 with Cl2Si=SiCl2 was estimated as an endothermic reaction by 4.5 kcal/mol. However, in the case of E = Ge or C, the formation of 5 with Cl2E=ECl2 was thermodynamically favorable (E = Ge: ∆G = −27 kcal/mol, E = C: ∆G = −81 kcal/mol) relative to the formation of 4 or 6 (E = Ge (4): ∆G = −24 kcal/mol, E = C (6): ∆G = −63 kcal/mol). Thus, thermodynamic energy difference between cases of E = Si, Ge, and C could give us some hints on the difference of the reaction products, though the reasonable reaction mechanisms are not clear at present.
The structure of 1,2-dichloro-1,2-digermacyclobut-3-ene 5 was revealed by the X-ray crystallographic analysis. The two Tbb/Cl moieties are oriented in (E)-geometries, in the digermacyclobutene skeleton in 5. The Ge–Ge bond length is 2.4694(6) Å, which is within a range of singly-bonded Ge–Ge distances. The lengths of the two Ge–Cl bonds are almost identical as Ge1–Cl1 = 2.2098(11) Å and Ge2–Cl2 = 2.2049(11) Å, which are similar to those of 3. The Ge1–C2 and Ge2–C1 (1.984(4), 1.996(4) Å) bond lengths in the digermacyclobutene skeleton of 5 are slightly longer, and shorter relative to those of the only example of the previously reported chlorinated 1,2-digerma-3-cyclobutadiene derivative 7 (Ge–Cl: 2.145(2)–2.150(2), Ge–C: 1.998(6), 2.002(6) Å) (Scheme 4) [30]. Interestingly, reduction of the isolated 5 with lithium naphthalenide was found to reproduce 1,2-digermacyclobutadiene 1 quantitatively, as evidenced by the 1 H NMR spectra.

3. Materials and Methods

3.1. General Information

All manipulations were carried out under an argon atmosphere using either a Schlenk line techniques or glove boxes. Solvents were purified by the Ultimate Solvent System, Glass Contour Company (Laguna Beach, CA, USA) [31]. 1H, 13C, and 29Si NMR spectra were measured on a JEOL AL-300 spectrometer (1H: 300 MHz, 13C: 75 MHz, 29Si: 59 MHz). Signals arising from residual C6D5H (7.15 ppm) in the C6D6 were used as an internal standard for the 1H NMR spectra, and that of C6D6 (128.0 ppm) for the 13C NMR spectra, and external SiMe4 0.0 ppm for the 29Si NMR spectra. High-resolution mass spectra (HRMS) were measured on a Bruker micrOTOF focus-Kci mass spectrometer (on ESI-positive mode). All melting points were determined on a Büchi Melting Point Apparatus M-565 and are uncorrected. 1,2-digermacyclobutadiene 1 was prepared according to literature procedure [24].

3.2. Experimental Details

3.2.1. Reaction of 1,2-Tbb2-1,2-Digermacyclobutadiene 1 with an Excess of SiCl4

A solution of 1,2-Tbb2-1,2-digermacyclobutadiene 1 (56.0 mg, 0.046 mmol) in SiCl4 (1.0 mL, 8.8 mmol, excess) was treated at 55 °C for 48 h, and the color of the dark red solution disappeared. After removal of residual SiCl4 under the reduced pressure, the residue was recrystallized from THF at room temperature to give compound 3 as colorless crystals in quantitative yield (64.2 mg, 0.046 mmol).
Data for 3: colorless crystals, m.p. = 68.7–69.7 °C (dec.); 1H NMR (300 MHz, C6D6, r.t.): δ 0.11 (s, 36H, SiMe3), 0.32 (s, 36 H, SiMe3), 1.24 (s, 18H, t-Bu), 2.51 (bs, 4H, CH), 6.70–6.76 (m, 2H, ArH), 6.84–6.90 (m, 8H, ArH), 7.22 (d, 4H, J = 7.2 Hz, ArH); 13C NMR (75 MHz, C6D6, 298 K): δ 1.80 (SiMe3), 2.17 (SiMe3), 30.96 (CMe3), 30.99 (CH), 34.32 (CMe3), 124.20 (ArH), 126.87 (ArH), 127.81 (ArH), 130.97 (ArH), 132.28 (Ar), 140.70 (Ar), 150.13 (Ar), 151.14 (Ar), 162.66 (CAr); 29Si NMR (59 MHz, C6D6, 298 K): δ 3.61 (SiMe3), 3.84 (SiMe3), 28.83 (GeSiGe); MS (DART-TOF, positive mode): m/z calcd. for C62H10935Cl474Ge2Si9 1393.3630 ([M + H]+), found 1393.3681 ([M + H]+).

3.2.2. Reaction of 1,2-Tbb2-1,2-Digermacyclobutadiene 1 with an Excess of GeCl4

A C6D6 solution of 1,2-Tbb2-1,2-digermacyclobutadiene 1 (38.3 mg, 0.0313 mmol) was treated with an excess amount of GeCl4 (0.3 mL, 2.6 mmol) at room temperature. After stirring of the reaction mixture for 10 min, the solvent and GeCl4 were removed under reduced pressure. The residue was recrystallized from benzene at room temperature to give compound 5 as main product in 61% yield (24.6 mg, 0.0190 mmol).
Data for 5: colorless crystals, m.p. 90.4–91.4 °C; 1H NMR (300 MHz, C6D6, r.t.): δ 0.13 (s, 36H, SiMe3), 0.27 (s, 36 H, SiMe3), 1.26 (s, 18H, t-Bu), 2.58 (s, 4H, CH), 6.88–6.93 (m, 6H, ArH), 7.00 (t, 4H, J = 7.2 Hz, ArH), 7.39 (d, 4H, J = 7.2 Hz, ArH); 13C NMR (75 MHz, C6D6, 298 K): δ 1.83 (SiMe3), 1.86 (SiMe3), 30.26 (CMe3), 31.03 (CH), 34.39 (CMe3), 124.16 (ArH), 128.00 (ArH), 128.67 (ArH), 129.79 (ArH), 133.71 (Ar), 139.21 (Ar), 150.45 (Ar), 151.29 (Ar), 167.29 (CAr); MS (DART-TOF, positive mode): m/z calcd. for C62H10935Cl274Ge2Si8 1295.4484 ([M + H]+), found 1295.4492 ([M + H]+).

3.2.3. Reaction of 1,2-Tbb2-1,2-Digermacyclobutadiene 1 with an Excess of CCl4

A C6D6 solution of 1,2-Tbb2-1,2-digermacyclobutadiene 1 (32.9 mg, 0.0269 mmol) was treated with an excess amount of CCl4 (0.2 mL, 2.1 mmol) at room temperature. After stirring of the reaction mixture for 10 min, the solvent and CCl4 were removed under reduced pressure. The residue was recrystallized from benzene at room temperature to give compound 3 as main product in 55% yield (22.8 mg, 0.0175 mmol).

3.3. Computational Methods

The level of theory and the basis sets used for the structural optimization are contained within the main text. Frequency calculations confirmed minimum energies for all optimized structures. All calculations were carried out using the Gaussian 09 program package [28]. Computational time was generously provided by the Supercomputer Laboratory in the Institute for Chemical Research of Kyoto University.

3.4. X-ray Crystallographic Analysis

Single crystals of [3·(thf)] and [5·2(benzene)] were obtained from recrystallization from THF and benzene, respectively. Intensity data were collected on a RIGAKU Saturn70 CCD system with VariMax Mo Optics using Mo Kα radiation (λ = 0.71075 Å). The structures were solved by a direct method (SIR2004 [32]) and refined by a full-matrix least square method on F2 for all reflections (SHELXL-97 [33]). All hydrogen atoms were placed using AFIX instructions, while all other atoms were refined anisotropically. Supplementary crystallographic data were deposited at the Cambridge Crystallographic Data Centre (CCDC; under reference numbers: CCDC-1578241 and 1578242 for [3·(thf)] and [5·2(benzene)], respectively) and can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. X-ray crystallographic data for [3·(thf)] and [5·2(benzene)]. Data for [3·(thf)] (C66H116Cl4Ge2OSi9): M = 1465.37, triclinic, P–1 (no.2), a = 12.6367(7) Å, b = 16.9170(6) Å, c = 20.4887(10) Å, α= 91.3815(14)°, β = 105.252(2)°, γ = 109.642(3)°, V = 3949.2(3) Å3, Z = 2, Dcalc. = 1.232 g·cm−3, μ = 1.070 mm−1, 2θmax = 51.0°, measd./unique refls. = 83580/14641 (Rint. = 0.1095), param = 767, GOF = 1.117, R1 = 0.0683/0.1122 [I>2σ(I)/all data], wR2 = 0.1188/0.1359 [I>2σ(I)/all data], largest diff. peak and hole 1.681 and −0.592 e.Å–3 (CCDC-1578241). Data for [5·2(benzene)] (C74H120Cl2Ge2Si8): M = 1450.49, triclinic, P–1 (no.2), a = 11.6792(2) Å, b = 15.7581(3) Å, c = 24.7906(5) Å, α = 76.2640(10)°, β = 88.0800(10)°, γ = 70.2510(10)°, V = 4165.89(14) Å3, Z = 2, Dcalc. = 1.156 g·cm−3, μ = 0.937 mm−1, 2θmax = 50.0°, measd./unique refls. = 64887/14555 (Rint. = 0.0810), param = 805, GOF = 1.289, R1 = 0.0637/0.0804 [I>2σ(I)/all data], wR2 = 0.1247/0.1311 [I>2σ(I)/all data], largest diff. peak and hole 0.983 and −0.689 e.Å22123 (CCDC-1578242).

4. Conclusions

It was demonstrated that 1,2-digermacyclobutadiene 1 could work as a bis-germylene building block (>Ge: + :Ge<) towards double Si–Cl activation in the reaction of 1 with SiCl4 to give the halogenated oligometallane, bis(chlorogermyl)dichlorosilane 3. Conversely, GeCl4 and CCl4 were found to work as double-chlorinating reagents towards 1 giving dichlorodigermacyclobutene 5. Thus, 1 would be an interesting building block for oligohalo-oligometallanes.

Supplementary Materials

The following are available online at www.mdpi.com/2304-6740/5/4/79/s1. Cif and cif-checked files. Figures S1–S5.

Acknowledgments

This work was partially supported by the following grants: Grant-in-Aid for Scientific Research (B) (no. 15H03777), Grant-in-Aid for Challenging Exploratory Research (no. 15K13640), Scientific Research on Innovative Areas, “New Polymeric Materials Based on Element-Blocks” (#2401) (no. 25102519), “Stimuli-Responsive Chemical Species for the Creation of Functional Molecules” (#2408) (no. 24109013), and the project of Integrated Research on Chemical Synthesis from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT), as well as by the “Molecular Systems Research” project of the RIKEN Advanced Science Institute and the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. We would like to thank Toshiaki Noda and Hideko Natsume at Nagoya University for the expert manufacturing of custom-tailored glasswares. T. Sugahara would like to thank the Japan Society for the Promotion of Science (JSPS) for a fellowship (no. JP16J05501).

Author Contributions

Takahiro Sasamori and Norihiro Tokitoh conceived and designed the experiments; Tomohiro Sugahara performed the experiments; Takahiro Sasamori and Tomohiro Sugahara performed the XRD analysis and wrote the paper; and Takahiro Sasamori performed theoretical calculations.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Depictions for Si–Cl activations of Ge(II) species and Si–Cl α-elimination from a chlorosilylgermane.
Scheme 1. Depictions for Si–Cl activations of Ge(II) species and Si–Cl α-elimination from a chlorosilylgermane.
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Scheme 2. Preparation of 1,2-digermacyclobutadiene 1, and its reaction with ECl4 giving digermadichlorosilane 3 (E = Si) and dichlorodigermacyclobutene 5 (E = C, Ge), respectively.
Scheme 2. Preparation of 1,2-digermacyclobutadiene 1, and its reaction with ECl4 giving digermadichlorosilane 3 (E = Si) and dichlorodigermacyclobutene 5 (E = C, Ge), respectively.
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Figure 1. Molecular structures of (a) digermadichlorosilane 3 and (b) dichlorodigermacyclobutene 5 with atomic displacement parameters set at 50% probability. All hydrogen atoms and solvent molecules (THF and benzene) were omitted for clarity and only selected atoms are labeled. Selected bond lengths (Å) and angles (deg.): (a) 3: Ge1–Si, 2.3734(16); Ge2–Si, 2.3938(15); Ge1–Cl1, 2.2094(14); Ge2–Cl4: 2.2011(15); Si–Cl2, 2.052(2); Si–Cl3, 2.053(2); Ge1–Si–Ge2, 91.15(5); Cl1–Ge1–Si, 90.29(5); Cl4–Ge2–Si, 105.40(6); Cl2–Si–Cl3, 106.04(9); (b) 5: Ge1–Ge2, 2.4694(6); Ge1–Cl1, 2.2098(11); Ge2–Cl2, 2.2049(11); Ge1–C2, 1.984(4); Ge2–C1, 1.996(4); C2–Ge1–Ge2, 74.24(13); Ge1–Ge2–C1, 73.02(12); Ge2–C1–C2, 106.8(3); C1–C2–Ge1, 105.6(3).
Figure 1. Molecular structures of (a) digermadichlorosilane 3 and (b) dichlorodigermacyclobutene 5 with atomic displacement parameters set at 50% probability. All hydrogen atoms and solvent molecules (THF and benzene) were omitted for clarity and only selected atoms are labeled. Selected bond lengths (Å) and angles (deg.): (a) 3: Ge1–Si, 2.3734(16); Ge2–Si, 2.3938(15); Ge1–Cl1, 2.2094(14); Ge2–Cl4: 2.2011(15); Si–Cl2, 2.052(2); Si–Cl3, 2.053(2); Ge1–Si–Ge2, 91.15(5); Cl1–Ge1–Si, 90.29(5); Cl4–Ge2–Si, 105.40(6); Cl2–Si–Cl3, 106.04(9); (b) 5: Ge1–Ge2, 2.4694(6); Ge1–Cl1, 2.2098(11); Ge2–Cl2, 2.2049(11); Ge1–C2, 1.984(4); Ge2–C1, 1.996(4); C2–Ge1–Ge2, 74.24(13); Ge1–Ge2–C1, 73.02(12); Ge2–C1–C2, 106.8(3); C1–C2–Ge1, 105.6(3).
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Scheme 3. Theoretical calculations on ∆G values (in kcal/mol) in the reactions of 1 with ECl4 (E = Si, Ge, C) to give insertion products (3, 4, 6) or chlorinated product 5.
Scheme 3. Theoretical calculations on ∆G values (in kcal/mol) in the reactions of 1 with ECl4 (E = Si, Ge, C) to give insertion products (3, 4, 6) or chlorinated product 5.
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Scheme 4. Reported reaction of GeCl2·(dioxane) with the highly strained alkyne to give the first example of chlorinated 1,2-digerma-3-cyclobutadiene derivative 7 [30].
Scheme 4. Reported reaction of GeCl2·(dioxane) with the highly strained alkyne to give the first example of chlorinated 1,2-digerma-3-cyclobutadiene derivative 7 [30].
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Sugahara, T.; Tokitoh, N.; Sasamori, T. Synthesis of a Dichlorodigermasilane: Double Si–Cl Activation by a Ge=Ge Unit. Inorganics 2017, 5, 79. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics5040079

AMA Style

Sugahara T, Tokitoh N, Sasamori T. Synthesis of a Dichlorodigermasilane: Double Si–Cl Activation by a Ge=Ge Unit. Inorganics. 2017; 5(4):79. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics5040079

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Sugahara, Tomohiro, Norihiro Tokitoh, and Takahiro Sasamori. 2017. "Synthesis of a Dichlorodigermasilane: Double Si–Cl Activation by a Ge=Ge Unit" Inorganics 5, no. 4: 79. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics5040079

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