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

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.


Results and Discussions
When the stable digermyne 2 bearing bulky aryl substituents, Tbb groups (2,6-[CH(SiMe3)2]2-4t-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 1,3digerma-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

Results and Discussions
When the stable digermyne 2 bearing bulky aryl substituents, Tbb groups (2,6-[CH(SiMe3)2]2-4t-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 1,3digerma-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 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 C 2 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 GeCl 4 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 GeCl 4 in the dark [29].In addition, the reaction of 1 with CCl 4 also furnished the formation of 5 without any formation of the CCl 2 -insertion product 6. 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 1,2dichloro-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 The difference of the products in the reaction of 1 with ECl 4 (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 ECl 4 (E = Si, Ge, C) to give the insertion products, 3, 4, and 6, or the chlorination products, 5 and Cl 2 E: (calculated as 1/2 Cl 2 E=ECl 2 ) 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 Cl 2 Si=SiCl 2 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 Cl 2 E=ECl 2 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.

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]. 1 H, 13 C, and 29 Si NMR spectra were measured on a JEOL AL-300 spectrometer ( 1 H: 300 MHz, 13 C: 75 MHz, 29 Si: 59 MHz).Signals arising from residual C6D5H (7.15 ppm) in the C6D6 were used as an internal standard for the 1 H NMR spectra, and that of C6D6 (128.0 ppm) for the 13 C NMR spectra, and external SiMe4 0.0 ppm for the 29 Si NMR spectra.Highresolution 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].6), 2.002(6) Å) (Scheme 4) [30].Interestingly, reduction of the isolated 5 with lithium naphthalenide was found to reproduce 1,2digermacyclobutadiene 1 quantitatively, as evidenced by the 1 H NMR spectra.

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]. 1 H, 13 C, and 29 Si NMR spectra were measured on a JEOL AL-300 spectrometer ( 1 H: 300 MHz, 13 C: 75 MHz, 29 Si: 59 MHz).Signals arising from residual C6D5H (7.15 ppm) in the C6D6 were used as an internal standard for the 1 H NMR spectra, and that of C6D6 (128.0 ppm) for the 13 C NMR spectra, and external SiMe4 0.0 ppm for the 29 Si NMR spectra.Highresolution 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].GeCl 2 + Scheme 4. Reported reaction of GeCl 2 •(dioxane) with the highly strained alkyne to give the first example of chlorinated 1,2-digerma-3-cyclobutadiene derivative 7 [30].

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]. 1 H, 13 C, and 29 Si NMR spectra were measured on a JEOL AL-300 spectrometer ( 1 H: 300 MHz, 13 C: 75 MHz, 29 Si: 59 MHz).Signals arising from residual C 6 D 5 H (7.15 ppm) in the C 6 D 6 were used as an internal standard for the 1 H NMR spectra, and that of C 6 D 6 (128.0 ppm) for the 13 C NMR spectra, and external SiMe 4 0.0 ppm for the 29 Si 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].A C 6 D 6 solution of 1,2-Tbb 2 -1,2-digermacyclobutadiene 1 (32.9 mg, 0.0269 mmol) was treated with an excess amount of CCl 4 (0.2 mL, 2.1 mmol) at room temperature.After stirring of the reaction mixture for 10 min, the solvent and CCl 4 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).

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.

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 F 2 for all reflections (SHELXL-97 [33]).All hydrogen atoms were placed using AFIX instructions, while all

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 GeCl 4 or CCl 4 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 ECl 2 (E = Ge or C) moiety.Inorganics 2017, 5, 79 3 of 7