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2016, Vol.89, No.11

1375-1384
BCSJ Award ArticleOpen Access

Reaction of a Stable Digermyne with Acetylenes: Synthesis of a 1,2-Digermabenzene and a 1,4-Digermabarrelene#

Tomohiro Sugahara ,1Jing-Dong Guo ,1,2Takahiro Sasamori ,*1Yusaku Karatsu ,3Yukio Furukawa ,3Arturo Espinosa Ferao ,4Shigeru Nagase ,2 and Norihiro Tokitoh 1
1Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011
2Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103
3Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
4Departmento de Química Orgánica, Universidad de Murcia, Campus de Espinardo, 30100 Murcia, Spain
https://doi.org/10.1246/bcsj.20160269

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Abstract

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Reactions between acetylenes and a stable digermyne bearing 4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2 (Tbb) groups afforded the corresponding stable 1,2-digermabenzenes together with the respective 1,4-digermabarrelenes. The properties of the obtained products and the reaction mechanism are discussed on the basis of experimental and theoretical results. Especially, the aromaticity of the newly obtained 1,2-digermabenzene has been discussed on the detailed calculations, revealing its aromatic character to some degree.

figure

Reactions between acetylenes and a stable digermyne afforded the corresponding stable 1,2-digermabenzenes together with the respective 1,4-digermabarrelenes. The properties of the obtained products and the reaction mechanism are discussed on the basis of experimental and theoretical results. Especially, the aromaticity of the newly obtained 1,2-digermabenzene has been discussed on the detailed calculations, revealing its aromatic character to some degree.

1. Introduction
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Multiple-bond compounds of heavier group 14 elements represent the heavier homologues of unsaturated organic compounds. For a long time, these compounds were considered hardly isolable as stable, monomeric compounds under ambient conditions, on account of their inherently high propensity towards auto-oligomerization and their high reactivity towards addition reactions with moisture and/or aerobic oxygen.1 However, this is only correct in the absence of appropriate stabilization methods, as the introduction of bulky substituents on the heavier group 14 elements offers kinetic protection, and their presence renders such multiple-bond compounds isolable and stable under ambient conditions. Especially, homonuclear triple-bond compounds of heavier group 14 elements, i.e., the heavier analogues of acetylene, have attracted much attention on account of their unique chemical and physical properties that arise from their trans-bent structures, which stand in strong contrast to the linear structures of carbon-based alkynes.1,2 Power and co-workers have achieved the synthesis and isolation of stable digermynes (ArGe≡GeAr), distannynes (ArSn≡SnAr), and diplumbynes (ArPb≡PbAr) using bulky m-terphenyl ligands.2,3 In 2004, Sekiguchi and Wiberg independently reported the synthesis of stable disilynes bearing bulky silyl groups.4 Later on, we reported the synthesis of the stable diaryldisilyne 15 and the diaryldigermynes 2,6 which bear the bulky aryl substituents Bbt or Tbb [Bbt: 2,6-[CH(SiMe3)2]2-4-[C(SiMe3)3]-C6H2, Tbb: 4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2] (Chart 1). So far, several stable examples of compounds with triple bonds between heavier group 14 elements have been synthesized and these have allowed us to study their intrinsic nature and properties.3–7 Accordingly, these species are no longer laboratory curiosities, but key components for advanced synthetic projects.

Modifications of unsaturated hydrocarbons such as alkenes and alkynes are of great importance in organic synthesis, as most methods for the introduction of functional groups start with modifications of a C–C multiple bond. Most modifications of unsaturated hydrocarbons thereby require transition-metal catalysts to activate the C–C π-bonds.8 Moreover, low-coordinated species of heavier group 14 elements (divalent species or multiple-bonded compounds) have recently attracted considerable attention as potential transition-metal-free reaction initiators for unsaturated hydrocarbons. Such low-coordinated species of heavier group 14 elements have already shown high electrophilic reactivity toward small unsaturated organic molecules such as ketones, alkenes, or alkynes.9 For example, Power et al. reported that the stable digermyne 3 as well as the distannyne 4 smoothly react in the absence of any transition-metal catalyst with two molecules of ethylene at room temperature to afford the corresponding 4-membered-ring cycloadducts 5 and 6, respectively (Scheme 1).10 The cycloaddition of Sn-analogue 6 was found to be thermally reversible to release 4 together with two molecules of ethylene. The stereoselective [2+2] cycloaddition of the stable disilyne RSiSi≡SiRSi (7; RSi: Si[CH(SiMe3)2](i-Pr)) with cis- and trans-2-butenes to generate disilenes 8 was reported by Sekiguchi and co-workers.11 Recently, we reported the reaction of stable disilyne 1 and digermyne 2a with ethylene to furnish cyclic products 9 and 10.12,13 Interestingly, 10 was converted into 11 at high ethylene pressures, while 11 underwent a retro-cycloaddition to release 10 and ethylene under ambient conditions.13 Furthermore, upon treatment with ethylene at low temperature in THF, 10 furnished four-membered ring species 12, which is an analogue of 5. It can thus be concluded that 11 and 12 should be the kinetic and thermodynamic products for the reaction between 2a and ethylene, respectively. In order to validate this hypothesis, we conducted systematic investigations on the reactivity of a disilyne and a digermyne with ethylene.

The reaction of disilynes (e.g., 1 and 7) and digermynes (e.g., 2) with acetylene are of great interest, as have been reported to afford the corresponding stable 1,2-disila- and 1,2-digermabenzenes (Scheme 2).11,14,15 In the case of the carbon analogues, the trimerization of acetylene to afford a six-membered cyclic π-conjugated system (i.e., an aromatic ring system) usually requires an appropriate transition-metal catalyst and severe conditions, as for example in the well-known Reppe reaction. In contrast, a cyclic six-membered π-conjugated aromatic ring system containing two heavier group 14 elements can be generated under ambient conditions in the absence of any transition-metal catalysts from the reaction of two molecules of acetylene and one heavier dimetallyne (vide supra).

Among the abundance of unsaturated organic compounds, benzene occupies a truly outstanding position. The replacement of carbon atoms in the benzene ring with heavier group 14 elements, generating so-called heavy aromatics,15 has generated much interest, even though this class of compounds is known to be highly susceptible towards auto-oligo- and polymerizations. So far, treatment of disilynes or digermynes with acetylene remains the only synthetic method to generate 1,2-disila- or 1,2-digermabenzenes.11,14,15 Such transformations are not only of great interest due to their unique reactivity, but the resulting heavy aromatic systems that include two heavy atoms are also highly interesting with respect to structural aspects, such as bond-alternation, planarity of the aromatic ring, and other features that may be affected by the double-bond character of the E=E bonds.

In a preliminary report, we described that the reaction of 2b with acetylene furnished the corresponding 1,2-digermabenzene (13) as the main product.15 However, this reaction also generated another unprecedented product, 1,4-digermabarrelene 14, which could be isolated and fully characterized. Herein, we report the reaction of the digermyne TbbGe≡GeTbb (2b) with acetylene in detail and we discuss the reaction mechanism on the basis of experimental and theoretical results.

2. Results and Discussion
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Reaction of Digermyne 2b with Acetylene.

The synthesis of TbbBr and its subsequent application to the stabilization of other reactive main-group element species is reported elsewhere.16 Digermyne 2b was prepared by a synthesis similar to that of BbtGe≡GeBbt.6 Dibromodigermene Tbb(Br)Ge=Ge(Br)Tbb (15) was obtained from the reaction of TbbLi with GeBr2·dioxane, followed by a subsequent reduction with KC8 in benzene at room temperature to furnish digermyne 2b in 99% yield as stable red crystals (Scheme 3).

The structural parameters of digermene 15 and digermyne 2b were unambiguously determined by single-crystal X-ray diffraction analysis (Figure 1).17 In both cases, two independent molecules were found in the unit cell, and each independent molecule contained a crystallographic center of symmetry. The Ge–Ge bonds in digermene 15 [2.4064(8) and 2.3969(8) Å] are slightly longer than those in carbon-substituted digermenes, indicating a weakened Ge=Ge bond.6 Conversely, digermyne 2b exhibits a trans-bent structure with Ge–Ge–C bond angles of ca. 130°, which is comparable to previously reported stable digermynes.3a,6 The observed Ge≡Ge bonds in 2b [2.2410(9) and 2.2221(9) Å] are comparable to those of previously reported stable digermynes [e.g., 2.2060(7) Å for BbtGe≡GeBbt (2a) or 2.2850(6) Å for ArDipGe≡GeArDip (3)].3a,6 The Raman spectrum of 2b revealed a strong Raman shift at 408 cm−1, which is comparable to that of 2a (398 cm−1).6b At the B3PW91/6-311G(3d,p) level of theory, a Ge–Ge vibrational frequency of 406 cm−1 was calculated for 2b. The spectral and structural features observed for 2b suggest a considerable triple-bond character for the Ge≡Ge bond in 2b, similar to those in 2a and 3.18

When a hexane solution of 2b was exposed to 1 atm of acetylene at room temperature, its deep red color changed immediately to pale yellow. After removing the solvent, signals for both 1,2-digermabenzene 13 and 1,4-digermabarrelene 14 were observed in the NMR spectra of the crude mixture,15 and judging from their 1H NMR spectra, yields of 61% (13) and 22% (14) were estimated (Scheme 3). Recrystallization of the crude mixture from benzene at room temperature afforded yellow crystals of 13 in 25% yield. Subsequently, the filtrate was exposed to air, and recrystallization of thus obtained residue from hexane at room temperature afforded 14 as colorless crystals in 20% yield. Both 13 and 14 were fully characterized by spectroscopic and single-crystal X-ray diffraction techniques.19

Reaction Mechanism.

It seems feasible to conclude that 1,2-digermabenzene 13 should be the major product from the reaction of digermyne 2b with acetylene, whereas 1,4-digemabarrelene 14 should be a by-product. The formation of 14 should probably be interpreted as a result of the presence of the intermediate 1,4-digermabenzene 16, which could readily undergo a [4+2] cycloaddition with one molecule of acetylene to give 14. In order to elucidate the underlying reaction mechanism, detailed theoretical calculations for the reaction of 2b with acetylene to generate 13 and 14 were carried out at the B3PW91/6-311G(3d,p)//B3PW91/3-21G(d) level of theory (Scheme 4 and Figure 2). The key intermediate in this reaction is digermacyclobutadiene INT-2, which is the common intermediate for both 13 and 14. The pathway from 2b to INT-2 proceeds via the formation of a reactant complex (a van der Waals complex), which is formed by the interaction of the out-of-plane π*-orbital of 2b with the π-orbital of acetylene.20 Subsequently, both the [2+1] cycloaddition affording intermediate INT-1, and the isomerization of INT-1 to INT-2 by migration of a carbon atom would occur as low-barrier or barrierless processes, respectively.

The formation of 1,2-digermacyclobutadiene (17) from the reaction of digermyne 3 with tolan (PhC≡CPh) via formal [2+2]cycloaddition has previously been reported (Scheme 5).11,21 Similarly, the reaction of 2b with tolan in C6D6 at room temperature resulted in the formation of 1,2-digerma-3,4-diphenylcyclobutadiene 18 as a stable, red, crystalline compound. The structural parameters of 18, determined by single-crystal X-ray diffraction, are similar to those of 17,22 exhibiting an almost planar [Ge2C2] ring (Figure 3). Moreover, 18 contains an intramolecular crystallographic C2 axis through the center of the Ge–Ge and C–C bonds. Considering the lengths of the Ge–Ge [2.4160(5) Å], Ge–C [2.022(2) Å], and C–C [1.362(5) Å] bonds, it seems feasible to assume that the predominant contribution to the observed structure arises from canonical structure 18A, which exhibits a Ge=Ge double bond character (Scheme 5). However, the results of detailed theoretical calculations on the parent 1,2-digermacyclobutadiene (19), including WBI, AIM, and NBO analyses, suggested a dominant contribution from canonical structure 19B.23 The long Ge–Ge bond (2.600 Å; WBI, 1.086), the relatively short C–C bond (1.426 Å; WBI, 1.349), in combination with the elongated Ge=C bond (1.854 Å; WBI, 1.419) clearly support this notion and the corresponding description as a 1,4-digerma-1,3-cyclobutadiene with a loose Ge–Ge bond. Especially the NBO calculations showed only one σ-bond between the Ge atoms, as well as σ- and π-bonds between the Ge and carbon atoms.24 Conversely, NBO and AIM calculations on 18 and INT-2 suggested the presence of Ge=Ge and C=C double bonds, as well as of Ge–C single bonds.24,25 In their entirety, these theoretical results indicate that, on account of the steric and/or electronic effect of the bulky Tbb groups, INT-2 should consist mainly of INT-2A with a –Ge=Ge–C=C– character. The parent 1,2-digermacyclobutadiene (19) exhibits a different electronic distribution, mostly based on 19B with a –Ge=C–C=Ge– character. The combined consideration of these theoretical results on 18 and 19 point to an increased C=C bond character in the case of the bis-C-phenyl substituted 18, in order to allow for a dominant stabilization effect from the conjugation between these aromatic rings, which in turn favors structure type A. Therefore, INT-2 should exhibit a Ge=C bond character (as in INT-2B), which is to some extent similar to 19B.

Once INT-2 is formed via an exothermic step (27.3 kcal mol−1) from 2b, the second molecule of acetylene should approach giving rise to the intermediate complex INT-2′ due to charge transfer, which is energetically more unfavorable (5.6 kcal mol−1) than INT-2 (Scheme 6). The calculated Ge1⋯HC4 (2.956 Å) and Ge1⋯C4 (4.020 Å) distances in INT-2′ are relatively long, probably due to the steric repulsion that also leads to the energy increase. Nevertheless, this intermediate should be formed due to the existence of stabilizing Ge⋯HC interactions. This interaction seems to be a requisite for the reaction to occur and accordingly the reason why no further reaction proceeded in the reaction of 2b with tolan, which does not contain HC≡ bonds. Subsequently, Dewar-1,2-digermabenzene INT-3 would be generated from INT-2′ through a [2+2] cycloaddition of INT-2′ with acetylene via TS1, which includes an activation barrier of 3.8 kcal mol−1.26 The last step, i.e., the dissociation of a Ge–C bond in INT-3 to form 1,2-digermabenzene 13, involves an activation barrier of 7.7 kcal mol−1.

Starting from INT-2′, the reaction pathway to 1,4-digermabarrelene 14 contains INT-2′′ as another important intermediate (Scheme 6). While the Ge–Ge bond length increases significantly from INT-2′ (2.558 Å) to INT-2′′ (2.856 Å), the C–C bond length in the coordinated acetylene (1.225 Å) is just slightly longer than that calculated for free C2H2 (1.205 Å). These conformational changes are probably due to a strong charge transfer (CT) between the Ge atom and the second C2H2 moiety.27 The main contribution to this CT should arise from the overlapping of the π-orbitals of the C3–C4 bond with the unoccupied π*-orbital located at the Ge2 atom. On the basis of NBO calculations (second order perturbation theory), a stabilization energy of 2.4 kcal mol−1 was calculated for the CT.24 However, INT-2′′ is still 4.1 kcal mol−1 less stable than INT-2′ and 9.7 kcal mol−1 less stable than INT-2. This may be attributed to the energetically demanding conformational changes, even after considering the CT effect.

Accordingly, the Ge–Ge bond weakens substantially upon coordination of the second molecule of acetylene in INT-2′′, and INT-4 containing a three-membered [C2Ge] ring is readily formed given its very small activation barrier. Subsequently, INT-4 can readily undergo isomerization to afford 1,4-digermabenzene 16, which exhibits a planar geometry at a singlet closed-shell state confirmed by stable wave functions.28 The other product, 1,4-digermabarrelene 14, can be formed via a [4+2] cycloaddition between 16 and a third molecule of acetylene, which includes an activation barrier of 12.3 kcal mol−1. Overall, both pathways in Scheme 4 show highly exothermic processes. Although the pathway toward 14 is by 33.2 kcal mol−1 more exothermic than that furnishing 13, the main product was 13 and not 14. The theoretically calculated pathways showed that the activation energy from key intermediate INT-2′ to INT-3 (3.8 kcal mol−1), which is involved in the formation of 13, should be slightly smaller (0.3 kcal mol−1) relative to that from INT-2′ to INT-2′′. As a result of these theoretical insights, we examined the product ratios obtained at higher and lower reaction temperature. A 13:14 product ratio of 2:1 and 7:1 was observed at 50 °C and −78 °C, respectively, thus supporting the calculated reaction mechanism.

Molecular Structures of 1,2-Digermabenzene 13 and 1,4-Digermabarrelene 14.

The structural parameters of 1,2-digermabenzene 13 and 1,4-digermabarrelene 14 were determined unambiguously by single-crystal X-ray diffraction analysis.19 Single crystals of 13 were obtained by recrystallization from benzene. The structure of 13 (Figure 4) exhibits a crystallographic C2 axis that bisects the Ge–Ge* and C–C* bonds.29 The Ge=Ge distance in 13 is 2.3117(6) Å, which is slightly longer than those in previously reported digermenes (e.g., 2.2856(8) Å for Mes2Ge=GeMes2, Mes: mesityl30), showing a distinct, albeit weakened, Ge=Ge π-bond character. Within the central [Ge2C4] ring, a Ge–C bond length of 1.897(3) Å was observed, which falls between previously reported average values for Ge–C (ca. 1.95 Å) and Ge=C bonds (ca. 1.83 Å).31 In the central [Ge2C4] ring, C–C bond lengths of 1.359(5) Å (C1–C2) and 1.417(7) Å (C2–C2*) were observed, which are similar to those in benzene (1.39–1.40 Å), suggesting a delocalization of π-electron density over the [Ge2C4] moiety. It should also be noted that the central [Ge2C4] ring in 1,2-digermabenzene 13 exhibits a non-planar geometry, wherein the Ge–Ge axis comprises an angle of 8.6° relative to the [C4] plane (Figure 4). This result stands in sharp contrast to stable 1,2-disilabenzenes,14 which contain a virtually planar [Si2C4] ring.

The 1H NMR spectrum of 13 in C6D6 suggested a C2-symmetric structure in solution on the basis of the diastereomeric non-equivalency of the SiMe3 groups, which is consistent with the C2-structure with a non-planar geometry at Ge atoms as observed in the solid state.29,32 For the parent 1,2-digermabenzene C4Ge2H6 (20), theoretical calculations at the B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p) level of theory suggested a planar geometry (C2v) for the transition state, which was ca. 0.36 kcal mol−1 more unstable relative to the non-planar structure (C2) at the minimum. Conversely, disilabenzene C4Si2H6, exhibited a C2v symmetric structure at the energetic minimum. Thus, it can be concluded that the non-planar structure of 13 should not be due to steric reasons and/or crystal packing, but arises from the intrinsic nature of the 1,2-digermabenzene ring system. The considerable aromaticity of the [Ge2C4] ring in 13 is reflected experimentally in its characteristic NMR spectral features, i.e., low-field-shifted 1H NMR signals (δH = 7.89 and 8.41) of the protons in the [Ge2C4] ring, which arise from the ring-current effect of the 6π-electrons. It is moreover reflected theoretically in the calculated NICS values and the ideal hydrogenation heat.29 In order to compare their aromaticity, calculated NICS(r) and NICSzz(r) values33 were plotted for parent models of a 1,2-digermabenzene (20; non-planar C2 symmetry), a 1,2-disilabenzene (21; planar C2v symmetry), and for benzene [GIAO-B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p)] (Figure 5).34 The NICS(r) and NICSzz(r) profiles for benzene were almost identical to those previously reported.34 The NICSzz(r) profile for benzene exhibits the highest absolute value (−30.0) at r = 1 Å above the center of the C6 plane, while the highest absolute value (−10.8) for the corresponding NICS(r) profile is observed at r = 0.8 Å. The profiles of 1,2-digermabenzene 20 are similar, albeit that the absolute values are slightly smaller relative to those of 1,2-disilabenzene 21. The NICSzz(r) profiles for 20 and 21 exhibit highest values at r = 1.3 and 1.1 Å, respectively, which is slightly further away from the central ring than in the case of benzene, reflecting the larger π-orbitals of Si and Ge relative to C. Even though the highest absolute NICS(r) and NICSzz(r) values for 20 and 21 are smaller than those of benzene, they still suggest considerable aromaticity for 1,2-disilabenzene and 1,2-digermabenzene. Moreover, the NICS(r) values of 20 and 21 are comparable and slightly higher than that of benzene. The most negative NICS values for 20 and 21 were found at r = 0.8 and 0.7 Å, which is similar to that of benzene (r = 0.8 Å). On the basis of the highest absolute NICS(r) and NICSzz(r) values, it can be concluded that the aromaticity in the 1,2-digermabenzene and the 1,2-disilabenzene is considerable, albeit lower relative to benzene.

Solid-state Raman spectra of 13 were recorded under excitation from a He/Ne laser at 633 nm. The broadened Raman shift observed at ca. 320–340 cm−1 should be assigned to Ge–Ge vibrational frequencies, given that it is comparable to those of previously reported digermenes (e.g., 404 cm−1 for Me2Ge=GeMe2).35 Moreover, the results of calculations for 13 at the B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p) level of theory predict Raman shifts at 304 and 324 cm−1 (Figure 6). The Raman shifts observed at 598 and 1469 cm−1 were assigned to the skeletal vibrations that are predominantly due to the [C4] moiety in the 1,2-digermabenzene, and for which values of 602 and 1503 cm−1 were calculated.

So far, examples and structural analyses of isolable 1,4-digermabarrelenes still remain elusive. Conversely, the benzo-fused analogs, 1,4-dimetallatriptycenes, have already been structurally characterized.36 Single crystals of 1,4-digermabarrelene 14, suitable for single-crystal X-ray diffraction, were obtained by recrystallization from benzene, and its molecular structure, as well as selected structural parameters are shown in Figure 7. The [Ge2C6] core in 14 exhibits the characteristic structure of a 9,10-dimetallatriptycene containing heavier group 14 elements.36 For example, the average internal angles at the bridging parts β(Ge–C–C; see Figure 7) are slightly smaller than 120°, while the average angles α(C–Ge–C) are substantially smaller than 109.5°. The averages lengths of the Ge–C (ca. 1.97 Å) and C–C bonds (ca. 1.33 Å) are consistent with typical single Ge–C and double C=C bond values. Although the two germanium atoms were observed to be relatively close [Ge1⋯Ge2 = 3.1589(5) Å] compared to the sum of their van der Waals radii (ca. 4.5 Å),37 the theoretical calculations on the H-substituted parent model for the 1,4-digermabarrelene (22) did not indicate any favorable electronic interactions.

One of the most intriguing intrinsic properties of the 1,4-digermabarrelene is its low-lying degenerated LUMOs. Their energy levels are much lower than those of the parent carbon analog, i.e., barrelene 23,38 and comparable to those of silicon analogue 2439 (Figure 8). One of the LUMOs is composed of π*-orbitals of the C=C moieties, while the other includes the two C=C π*-orbitals and the Ge–C σ*-orbitals. Moreover, the HOMO of 22, which comprises the three π-orbitals of the C=C bonds, lies also below that of 23. Currently, investigations into the redox properties of 14 are in progress in our laboratories.

3. Conclusion
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The reaction of digermyne 2b with acetylene afforded stable 1,2-digermabenzene 13 together with 1,4-digermabarrelene 14. The reaction mechanism for the formation of 13 and 14 was investigated by detailed theoretical calculations. The results allow drawing the conclusion that 1,2-digermacyclobutadiene INT-2 is the key intermediate for both 13 and 14, whereby 13 represents the kinetically slightly favored product. Both 13 and 14 were structurally characterized and the aromaticity of the 1,2-digermabenzene was evaluated by theoretical calculations.

4. Experimental
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General Remarks.

All manipulations were carried out under an argon atmosphere using either Schlenk line techniques or glove boxes. All solvents were purified by standard methods and/or the Ultimate Solvent System, Glass Contour Company. Remaining trace amounts of water and oxygen in the solvents were thoroughly removed by bulb-to-bulb distillation from a potassium mirror prior to use. 1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded on a JEOL JNM AL-300 spectrometer. High-resolution mass spectra (HRMS) were obtained on a Bruker micrOTOF focus-Kci mass spectrometer (DART) or a JEOL JMS-700 MStation (FAB). Raman spectra were measured on a Raman spectrometer consisting of a Spex 1877 Triplemate and an EG&G PARC 1421 intensified photodiode array detector. An NEC GLG 108 He/Ne laser (633 nm) was used for Raman excitation. All melting points were determined on a Büchi Melting Point Apparatus M-565 and are uncorrected. Elemental analyses were carried out at the Microanalytical Laboratory (Institute for Chemical Research) of Kyoto University. Digermyne 2b was prepared according to literature procedures.15

Reaction of 1,2-Tbb2-digermyne 2b with an Excess of Acetylene in Hexane.

In a Schlenk tube with a J-Young tap, degassed water was added to CaC2 at r.t. under an argon atmosphere to generate acetylene, which was dried by passing through a column of P2O5. In an NMR tube with a J-Young tap, an n-hexane solution (0.3 mL) of 2b (19.6 mg, 18.8 µmol) was degassed via freeze–pump–thaw cycles. Then, the solution was exposed to acetylene gas (1 atm), generated as described above, and the tube was shaken at room temperature. The color of the solution changed from dark red to pale yellow after 10 min of shaking. All volatiles were removed under reduced pressure, and the pale yellow residue was recrystallized from benzene to afford 1,2-digermabenzene 13 (5.2 mg, 4.7 µmol, 25%, NMR yield 61%). Subsequently, the filtrate was exposed to air, and recrystallization of thus obtained residue from hexane at room temperature afforded 1,4-digermabarrelene 14 (4.2 mg, 3.8 µmol, 20%, NMR yield 22%). 13: yellow crystals, mp 198.1–199.0 °C (dec); 1H NMR (300 MHz, C6D6, 298 K) δ 0.20 (s, 36H), 0.22 (s, 36H), 1.34 (s, 18H), 2.58 (s, 4H), 7.04 (s, 4H), 7.89 (AA′BB′, J = 3.9, 13.5 Hz, 2H), 8.41 (AA′BB′, J = 3.9, 13.5 Hz, 2H); 13C NMR (75 MHz, C6D6, 298 K) δ 1.37 (q), 1.89 (q), 31.33 (q), 34.57 (d), 34.81 (s), 122.10 (d), 137.04 (d), 144.24 (s), 149.54 (s), 151.29 (s), 157.40 (d). UV–vis (hexane) λmax (nm, ε) = 383 (9600). Anal. Calcd for C52H102Ge2Si8: C, 56.92; H, 9.37. Found: C, 56.97; H, 9.36. Anal. MS (DART-TOF, positive mode): m/z calcd for C52H102 74Ge2Si8 1098.4559 ([M]+), found 1098.4599 ([M]+). 14: colorless crystals; mp 79.7–81.3 °C; 1H NMR (300 MHz, C6D6, 298 K) δ 0.19 (s, 72H), 1.34 (s, 18H), 2.54 (s, 4H), 7.01 (s, 4H), 7.96 (s, 6H); 13C NMR (75 MHz, C6D6, 298 K) δ 0.84 (q), 29.89 (q), 31.26 (d), 34.40 (s), 121.80 (d), 129.01 (d), 149.56 (d), 150.80 (s), 151.43 (s). UV–vis (hexane) λmax (nm, ε) = 287 (4100), 296 (4600). MS (DART-TOF, positive mode): m/z calcd for C54H105 74Ge2Si8 1125.4794 ([M+H]+), found 1125.4810 ([M+H]+).

Reaction of Digermyne 2b with Diphenylacetylene.

In a J-Young NMR tube, a solution of 2b (30.3 mg, 29.2 µmol) in C6D6 (0.5 mL) was treated with diphenylacetylene (6.0 mg, 0.034 mmol, 1.1 equiv). After 10 min, the quantitative formation of 1,2-digermacyclobutadiene 18 was confirmed by 1H NMR spectroscopy. The mixture was recrystallized from hexane to afford 18 as stable dark red crystals (13.0 mg, 10.7 µmol, 37%). 18: dark red crystals, mp 68.6–68.7 °C; 1H NMR (300 MHz, C6D6, r.t.): δ 0.23 (s, 72H), 1.37 (s, 9H), 2.86 (s, 4H), 6.85–7.02 (m, 10H), 7.18–7.21 (m, 4H); 13C NMR (75 MHz, C6D6): δ 1.65 (q), 31.32 (q), 32.76 (d), 34.47 (s), 122.67 (d), 128.68 (d), 129.12 (d), 129.90 (d), 139.93 (s), 146.92 (s), 149.12 (s), 151.52 (s), 184.18 (s). UV–vis (hexane) λmax (nm, ε) = 369 (sh, 12000). Anal. MS (DART-TOF, positive mode): m/z calcd. for C62H10874Ge2Si8: 1224.5029 ([M+H]+); found: 1224.5045 ([M+H]+).

Monitoring of the Reaction of 1,2-Tbb2-digermyne 2b with an Excess of Acetylene at −78 °C.

A solution of 2b (31.6 mg, 28.8 µmol) in n-hexane (0.5 mL) was degassed in a J-Young NMR tube, before being exposed to an excess of acetylene (ca. 1 atm) at −78 °C for 3 min. Subsequently, the solution was warmed to room temperature and all volatiles were removed under reduced pressure. The yield of 13 and 14 was estimated by 1H NMR spectroscopy (13:14 = 7:1).

Monitoring of the Reaction of 1,2-Tbb2-digermyne 2b with an Excess of Acetylene at 50 °C.

A solution of 2b (31.6 mg, 28.8 µmol) in toluene (0.5 mL) was degassed in a J-Young NMR tube, before being exposed to acetylene. The solution was treated with an excess of acetylene (ca. 1 atm) at 50 °C for 10 min. Subsequently, the solution was warmed to room temperature, before all volatiles were removed under reduced pressure. The yield of 13 and 14 was estimated by 1H NMR spectroscopy (13:14 = 2:1).

Computational Calculations.

The level of theory and the basis sets used for the structural optimization are contained within the references of the main text. Frequency calculations confirmed minimum energies for all optimized structures. All calculations were carried out using the Gaussian 09 program package40 except the structural and electronic analysis of the parent 1,2-digermacyclobutadiene (19) that was conducted with ORCA.41

X-ray Crystallographic Analysis.

Single crystals of [2b·C6H6], [13·C6H6], [14·C6H6], [15·C6H6], and 18 were obtained from recrystallization from benzene. Intensity data were collected on a RIGAKU Saturn70 CCD system with VariMax Mo Optics using MoKα radiation (λ = 0.71075 Å). Crystal data are shown in the references. The structures were solved by a direct method (SIR200442) and refined by a full-matrix least square method on F2 for all reflections (SHELXL-9743). 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: CCDC1035078, 1035079, 1485559, 1035077, 1485558 for [2b·C6H6], [13·C6H6], [14·C6H6], [15·C6H6], and 18, respectively) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request.cif.

This work was partially supported by the following grants: Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Numbers JP22350017 and JP15H03777), Grant-in-Aid for Challenging Exploratory Research (JSPS KAKENHI Grant Number JP15K13640), Grant-in-Aid for JSPS Fellows (JSPS KAKENHI Grant Number JP16J05501), Scientific Research on Innovative Areas, “New Polymeric Materials Based on Element-Blocks” [#2401] (JSPS KAKENHI Grant Numbers JP25102519, JP15H00738), “Stimuli-Responsive Chemical Species for the Creation of Functional Molecules” [#2408] (JSPS KAKENHI Grant Number JP24109013), the Integrated Research on Chemical Synthesis project of 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 (ICR), Kyoto University. The Preliminary X-ray diffraction data of 18 were collected at the BL40XU in Spring-8 (JASRI, 2015B1074). Computation time was partially provided by the Supercomputer Laboratory, Institute for Chemical Research, Kyoto University. We would like to thank Mr. Toshiaki Noda and Ms. Hideko Natsume at Nagoya University for the expert manufacturing of custom-tailored glassware. The manuscript was partially written at the Rheinische Friedrich-Wilhelms-Universität Bonn during the tenure of a Friedrich Wilhelm Bessel Research Award (TS). TS would like to express his gratitude to Prof. Rainer Streubel and his co-workers for their kind hospitality, as well as to the Alexander von Humboldt Stiftung for their generosity. One of the authors (AE) is grateful to the Kyoto University Research Coordination Alliance for their support of the collaboration and to “Servicio de Cálculo Científico” at the University of Murcia for computational resources and technical support.

Supporting Information

NMR charts and details of theoretical calculations are shown in Supporting Information. This material is available on http://dx.doi.org/10.1246/bcsj.20160269.

Takahiro Sasamori

Takahiro Sasamori received Ph.D. degree from Kyushu University in 2002. He got a position as Assistant Professor in 2003 and as Associate Professor in 2009 at Institute for Chemical Research, Kyoto University. His main research fields are organometallic and main-group element chemistry.

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