Synthesis,structure and decomposition of the digermane Ph 3 GeGePh 2 H

1 Introduction

The replacement of an aryl substituent at each of the two germanium atoms to give compounds of the general formula Ge₂Ar₄X₂ is relatively facile, and Dräger et al. reported the conversion of Ge₂Ph₆ (1) to (Cl₃CCOO)Ph₂GeGePh₂(OOCCCl₃) (2) using trichloroacetic acid (TCA) in reasonable yield in 1984 [1]. Digermane 1 could also be converted to both Ge₂Ph₄Cl₂ and Ge₂Ph₂Cl₄ with liquid HCl under pressure [2]. However, replacement of only one aryl group to generate monosubstituted digermanes Ge₂Ar₅X represents a more difficult synthetic challenge. These substances are potentially useful synthons since they can be used as two-atom end-capping reagents for the construction of oligogermanes or mixed group 14 element oligomers or polymers. Recently, Zaitsev et al. have reported the successful synthesis of several mono-substituted digermanes, including Ph₂(OTf)GeGeMe₃, [3] ClPh₂GeGeMe₃, [3] Ph₂Ge(OTf)GePh₃, [4] Ph₂Ge(OTf)GeBu ^t Me₂, [4] and ClPh₂GeGePh₃ [4]. These species were fully characterized by spectroscopic techniques and the X-ray structure of Ph₂Ge(OTf)GePh₃ was determined [4].

We have recently found that the method of Dräger et al. [1] can be used to prepare the monosubstituted digermane Ph₃GeGePh₂(OOCCCl₃) (3) as a mixture with 2 by variation of the experimental conditions. We have converted the mixture of 2 and 3 to the chloro-substituted species ClPh₂GeGePh₂Cl (4) and Ph₃GeGePh₂Cl (5), which can subsequently be used to prepare the hydrides HPh₂GeGePh₂H (6) and Ph₃GeGePh₂H (7). The digermane 7 can be obtained in pure form by selective crystallization, and its X-ray crystal structure has been obtained. The potential use of 7 for the synthesis of higher oligogermanes was also investigated. However, this species decomposes under the thermal conditions of the hydrogermolysis reaction, [5–9] and the thermal decomposition products of 7 have been identified.

2 Experimental section

3 Results and discussion

The preparation of (Cl₃CCOO)Ph₂GeGePh₂(OOCCCl₃) (2) from Ge₂Ph₆ (1) reported by Dräger et al. was carried out using a 5 : 1 molar ratio of trichloroacetic acid (TCA) to 1 in boiling toluene [1]. If a molar ratio if 4.3 : 1 of TCA:1 is employed under the same conditions, the complete conversion of 1 to 2 does not occur and a mixture of 2 and Ph₃GeGePh₂(OOCCCl₃) (3) is obtained (Scheme 1). The digermanes 2 and 3 were not isolated but rather were converted to the corresponding chlorides ClPh₂GeGePh₂Cl (4) and Ph₃GeGePh₂Cl (5) by addition of ethereal HCl [1, 11]. Although it is not possible to determine the ratio of 4 to 5 that is formed by integration of their ¹H NMR resonances since the signals overlap the ¹³C NMR of the mixture indicates both species are present in the product mixture since there are twelve aromatic resonances that match the literature values for 4 [1] and 5 [4].

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Scheme 1

Synthesis of 6 and 7.

The mixture of 4 and 5 was subsequently treated with LiAlH₄ to yield a mixture of the digermanes HPh₂GeGePh₂H (6) and Ph₃GeGePh₂H (7). The ¹H NMR spectrum of the product mixture after treatment contains three resonances in the Ge – H region (Fig. 1), where the peak at δ 5.57 ppm corresponds to 6 [11] and the peak at δ 5.85 ppm corresponds to a small amount of Ph₃GeH that was generated during the course of the reaction (vide infra). The most intense peak present at δ 5.72 ppm corresponds to the digermane 7. Integration of these three resonances indicated that the mixture was ca. 79% 7, 10% 6, and 11% Ph₃GeH. Compound 7 was separated from 6 and Ph₃GeH by crystallization from a concentrated toluene solution at –35°C, and was isolated in 28% yield based on the starting digermane Ge₂Ph₆ (1). Pure 7 was prepared previously by the action of Ph₃GeLi on Ph₂GeHCl and the NMR data reported here is identical to that of the previously prepared material [12].

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Fig.1

¹H NMR spectrum of the Ge – H region after treating the mixture of 4 and 5 with LiAlH₄.

The crystals of 7 that were obtained were suitable for X-ray analysis and it was found that two different morphologies of 7 crystallized out of solution, and the bond distances and angles are collected in Tables 1 and 2. The physically larger crystals (7a) contained three independent molecules in the unit cell and the ORTEP diagrams of these are shown in Fig. 2. Both of the germanium atoms in all three of the independent molecules in 7a are disordered over two sites but each have occupancies of greater than 97.8% in one position versus the other such that the occupancy of the second position has virtually no effect on the bond distances and angles within each molecule. The ORTEP diagrams shown in Fig. 2 for 7a contain the germanium atoms drawn in their sites of major occupancy.

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Fig.2

ORTEP diagrams of the three crystallographically independent molecules of 7a and the single crystallographically independent molecule of 7b.

Disregarding the minor contribution of the disordered atoms the Ge – Ge bond distance in molecule 1 is 2.4219(7) Å, in molecules 2 and 3 these distances are 2.4205(7) and 2.4279(7) Å, respectively. The average Ge – Ge bond distance among the three molecules of 7a is 2.4234(7) Å. The Ge – H bond distances among the three independent molecules are 1.35(4), 1.41(4), 1.38(4) Å for molecules 1, 2, and 3, respectively, which average 1.38(4) Å. The Ge – C _ipso bond distances in the three molecules average 1.956(4), 1.956(4), and 1.959(4) Å for molecules 1, 2, and 3, respectively, and average 1.957(4) Å, which are typical bond distances for germanium-bound phenyl rings. The environment around Ge(1) in all three molecules of 7a is nearly idealized tetrahedral, and the C – Ge – C bond angles at Ge(1) average 109.0(2)° among the three molecules. The environment at Ge(2) is slightly distorted from tetrahedral with an average bond angle of 106(1)° due to the presence of the hydrogen atom.

The second crop of smaller crystals (7b) crystallized with only one independent molecule in the unit cell and the atoms were again disordered with occupancies of one site greater than 97.3%. Again ignoring the contribution of the less occupied atoms in 7b, the Ge – Ge bond distance is 2.4213(5) Å and the Ge – H bond distance is 1.49(3) Å, while the Ge – C _ipso bond distances average 1.954(3) Å. The average C – Ge – C bond angle at Ge(1) in 7b is 108.5(1)° and the average bond angle of 107(1)° at Ge(2). Thus, both 7a and 7b are structurally similar, although the Ge – H distance in 7b is longer than each of the three Ge – H bonds in 7a.

The Ge – Ge bond distance in Ge₂Ph₆ (1) is 2.437(2) Å and measures 2.446(1) Å [14] and in its dibenzene solvate 1•2C₆H₆, while the C – Ge – C bond angles average 108.6 and 108.1° for 1 and 1•2C₆H₆, respectively. The replacement of one phenyl substituent with a less sterically encumbering hydrogen atom results in the shortening of the Ge – Ge bond by only ca. 0.02 Å, and slightly distorts the environment at the germanium atom that is bound to the hydrogen atom. The Ge – H bond distances found in the three molecules of 7a and the single molecule of 7b are typical for refined Ge – H bond distances. For example, the Ge – H bond distances in Ph₃GeH, (o-Bu ^t C₆H₄)₃GeH, (Ph₃Ge)₃GeH, Ph₃Ge(GePh₂)₃H are 1.48(3), [15] 1.37(2), [16] 1.45(3), [17] and 1.50(4) Å, [18] respectively.

We had envisioned using 7 as a two-germanium atom building block for the construction of oligogermane chains, such that we might be able to prepare a heptagermane Ge₇Ph₁₆ or an octagermane Ge₈Ph₁₈ from 7 and the amides Me₂N(GePh₂)₃NMe₂ and Me₂N(GePh₂)₄NMe₂, which could be the longest structurally characterized germanes that have been reported. As a proof of concept experiment, we treated 7 with one equiv. of Ph₃GeNMe₂ in an attempt to prepare the known trigermane Ge₃Ph₈ [19, 20]. However, after stirring the reaction mixture at 85°C for 48 h in CH₃CN solvent and removing the volatiles in vacuo an intractable mixture of products was obtained, although Ph₃GeH could be identified by ¹H NMR spectroscopy due to the presence of a resonance at δ 5.85 ppm. These findings suggested that 7 might be thermally unstable and decomposed under the reaction conditions employed.

This was investigated by heating a sample of 7 alone in acetonitrile for 48 h in CH₃CN solvent. A yellow oil resulted after evaporation of the volatiles, and the ¹H NMR spectrum of the product contained multiple overlapping aromatic resonances as well as a peak at δ 5.85 ppm indicating the presence of Ph₃GeH. A solid sample of 7 was also heated in a Kugelrohr oven at 160°C at a pressure of 0.010 torr, and a white solid had collected in the receiving flask after heating for 2 h. This was identified as Ph₃GeH and it was obtained in 86% yield based on 7. A thick oil remained in the distillation pot, and the ¹H NMR of this material contained numerous overlapping resonances in the aromatic region suggesting that it was a polygermane.

These findings indicate that 7 is thermally unstable and decomposes both in the solid state and in solution. In order to determine decomposition pathway, a sample of 7 was heated in toluene solution in a sealed tube at 95°C in the presence of 10 equiv. of 2,3-dimethyl-1,4-butadiene (DMB). After heating for 72 h and removal of the volatiles, the ¹H NMR spectrum of the resulting product mixture indicated that Ph₃GeH and a germacyclopentene 8 [21] were present, along with some polymeric material (Scheme 2). The presence of 8 and the polymer indicate that 7 decomposes by extrusion of the germylene Ph₂Ge: to generate Ph₃GeH, and the Ph₂Ge: is subsequently trapped by DMB to provide 8 and also undergoes rapid polymerization to yield the polygermane [Ph₂Ge]_n.

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Scheme 2

Thermal decomposition of 7 in solution.

The digermane Ph₃GeGePh₂H (7) was separated from a mixture with HPh₂GeGePh₂H (6) by selective crystallization. The mixture of 6 and 7 was synthesized via treatment of Ge₂Ph₆ (1) with 4.3 equiv. of trichloroacetic acid to provide (Cl₃CCOO)Ph₂GeGePh₂(OOCCCl₃) (2) and Ph₃GeGePh₂(OOCCCl₃) (3) that were converted to ClPh₂GeGePh₂Cl (4) and Ph₃GeGePh₂Cl (5) with ethereal HCl, and finally to 6 and 7 using LiAlH₄. The digermane 7 was found to crystallize in two different morphologies, where one contains three unique molecules in the unit cell and the other contains only one. The average Ge – Ge bond length among the two structures of 7 is 2.4223(5) Å. Although 7 might be expected to serve as a two-germanium atom building block for the synthesis of higher oligomers, it undergoes thermal decomposition under the conditions of the hydrogermolysis reaction to produce Ph₃GeH and Ph₂Ge:, the latter of which polymerizes or can be trapped with DMB.