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Oligogermanes Containing Only Electron-Withdrawing Substituents: Synthesis and Properties Kirill V. Zaitsev,*,† Kevin Lam,*,‡ Zhaisan Zhanabil,‡ Yerlan Suleimen,‡ Anastasia V. Kharcheva,† Viktor A. Tafeenko,† Yuri F. Oprunenko,† Oleg Kh. Poleshchuk,§,∥ Elmira Kh. Lermontova,# and Andrei V. Churakov# †

Department of Chemistry, Moscow State University, Leninskye Gory 1, Moscow 119991, Russia Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana, Kazakhstan, 010000 § National Research Tomsk Polytechnic University, Lenin Avenue, 30, Tomsk 634050, Russia ∥ Tomsk State Pedagogical University, Kievskaya Street, 60, Tomsk 634061, Russia # Russian Academy of Science, N.S. Kurnakov General and Inorganic Chemistry Institute, Leninskii pr., 31, Moscow 119991, Russia ‡

S Supporting Information *

ABSTRACT: A series of germanes Ar3GeX, containing electron-withdrawing substituents [Ar = p-FC6H4, 1a−d, 1a (X = Cl), 1b (X = Br), 1c (X = H), 1d (X = NMe2); p-F3CC6H4, 2a−d, 2a (X = Cl), 2b (X = Br), 2c (X = H), 2d (X = NMe2)], was synthesized and used to prepare symmetrical digermanes Ar3Ge−GeAr3, (p-FC6H4)3GeGe(C6H4F-p)3 (3), and (p-F 3 CC 6 H 4 ) 3 GeGe(C 6 H 4 CF 3 -p) 3 (4) and trigermane [(pF3CC6H4)3Ge]2Ge(C6F5)2 (5) by hydrogermolysis reaction. The properties of all compounds were investigated by multinuclear NMR and for oligogermanes by UV/vis and fluorescence spectroscopy, as well as by electrochemical methods. In addition, the molecular structures of 1a, 1b, 2b, 2c, and 3−5 were studied by X-ray diffraction analysis. Compound 5 showed a significantly shifted UV/visible absorption to the red field in comparison with previously described derivatives.

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INTRODUCTION Nowadays significant attention is drawn to the synthesis of novel types of compounds with the hope of finding new unique physical properties and to establish a possible “structure− property” relationship. Due to the σ-conjugation between the group 14 elements E (E = Si, Ge, Sn), molecules containing element−element bonds may be regarded as potentially important precursors for the construction of new and unique conductive or luminescent materials that display nonlinear optical properties, etc.1 In general, such materials are polymers instead of individual molecular compounds. Only recently were molecular catenated derivatives of group 14 elements used as building blocks for semiconductors.2 This reinforces the need for more studies on the physical properties of the individual molecular compounds with the hope to be able to identify the key structural features (nature of the substituents and E, number of the E atoms in the chain, etc.) that are influencing the physical properties of the material. Previous works by Satge,3 Castel,4 Draeger,5 Mochida,6 Weinert,7 Osakada,8 and Marschner9 have reported the synthesis and the studies of oligogermane derivatives bearing in general electron-donating groups. To the best of our knowledge, simple germanium compounds containing only electron-withdrawing substituents such as linear oligogermane Cl3Ge−GeCl3 and its higher analogues are almost unknown and highly unstable10 due to the instability of the Ge−Ge bond © 2016 American Chemical Society

in such species and due to the high thermodynamic stability of decomposition products such as GeCl2. In addition, branched catenated oligogermanes containing only electron-withdrawing groups (such as [Cl3Ge]4Ge)11 are also very rare. However, it is possible to assume that the presence of organic electronwithdrawing groups should stabilize linear oligogermanes. Surprisingly, to the best of our knowledge, only few related oligogermanes containing organic electron-withdrawing substituents (for example, (C6F5)3Ge−Ge(C6F5)312), have been reported in the literature, and unfortunately their physical properties have been only superficially investigated. Therefore, we would like to report the synthesis and investigation of the properties of new oligogermanes containing only electronwithdrawing substituents. It is well-known that physical properties of derivatives of the catenated group 14 elements heavily depend on the nature of their substituents (electronic properties and steric hindrance),13 on the number of the element atoms in the chain,1b and on the conformation of the molecule.14 In the present work, we would like to disclose the syntheses of a series of triarylgermanium derivatives and of new oligogermanes bearing aryl groups (p-FC6H4 and p-F3CC6H4) as well as their structural and physical studies. Received: October 1, 2016 Published: December 22, 2016 298

DOI: 10.1021/acs.organomet.6b00767 Organometallics 2017, 36, 298−309

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Organometallics

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RESULTS AND DISCUSSION The focused construction of Ge−Ge bonds is a challenging synthetic task. In the case of aryl-substituted germanes, the hydrogermolysis reaction (i.e., reaction between a germane and a germanium amide) developed by Weinert et al. represents one of the few ways to access such compounds under controlled conditions and with a good reproducibility.7a,f,g,15 This methodology leads to the desired compounds in high yields and produces only gaseous byproducts, which make it particularly convenient and attractive. In order to use this methodology, the first task is to prepare a series of novel triarylgermanium halogenides, which may then be easily transformed into the corresponding Ar3GeH and Ar3GeNMe2 compounds. Bromobenzenes para-substituted by electron-withdrawing groups such as p-FC6H4Br and p-F3CC6H4Br were used for the synthesis of the desired triarylgermanes. Several synthetic approaches could be used to prepare such compounds including a nucleophilic substitution at the Ge center (reaction of a Grignard or organolithium reagents with GeCl4) or an electrophilic substitution (desymmetrization of tetraarylgermane by GeCl4 in the presence of a Lewis acid). It should be noted that GeCl4 is a better starting material since it is significantly cheaper than GeBr4. The reaction between a Grignard reagent and GeCl4 (molar ratio 3:1) resulted in the formation of a mixture of the corresponding chlorides and bromides (i.e., 1a/1b and 2a/2b) (Scheme 1) as well as other byproducts (evidently, monoaryl-,

Scheme 2. Synthesis of Germanium Hydrides and Amides

are partially rotationally disordered, as frequently found in this type of compounds.16

Figure 1. Molecular structure of (p-F3CC6H4)3GeH (2c). Displacement ellipsoids are shown at the 50% probability level. Minor components of disordered CF3 groups are not shown. Hydrogen atoms except of H(1) are omitted for clarity.

Scheme 1. Synthesis of Halogenides, 1a/1b and 2a/2b, Using Grignard Reagents

Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for Compounds 1a, 1b, 2b, and 2c Ar X Ge(1)−X Ge(1)−Cav C−Ge(1)−Cav X−Ge(1)−Cav

diaryl-, and tetraarylgermanes) as the minor constituents. Such a complex mixture of compounds is typical when it comes to the preparation of organogermanes16 and unavoidable in many cases. To the best of our knowledge, there are only a few reported cases in which a halogenated triarylgermane was obtained as a sole product and in one step from the reaction between GeCl4 and a Grignard or an organolithium reagent (Mes 3 GeCl,17 (p-Tol) 3 GeCl,18 (C 6 F 5 ) 3 GeCl, 13 and (oPh2PC6H4)3GeCl19). The formation of the chloride and bromide derivatives could be explained by the exchange of halogen atoms that occurs during the reaction. This reaction happens due to the presence of MgHal2, which is formed in situ. Although the mixtures 1a/ 1b and 2a/2b could be separated from the other byproducts (by partial crystallization or distillation; see the Experimental Section), it is impossible to separate the chloride from the bromide due to the similarities in their physical properties. The ratios for 1a/1b and 2a/2b have been determined by 19F NMR spectroscopy and are respectively 1:4 and 6:5. Both of these mixtures were used as obtained without further purification and fully converted into the corresponding hydrides or amides by using an excess of LiAlH47d or LiNMe27d,20 in high yields (Scheme 2). The molecular structure of the hydride 2c was investigated in the solid state by X-ray analysis (Figure 1, Table 1, Table S1, Supporting Information). In this structure, the fluorine atoms

1a

1b

2b

2c

p-FC6H4 Cl 2.1873(6) 1.930(2) 112.86(8) 105.81(6)

Br 2.3126(8) 1.936(5) 112.69(19) 106.01(19)

p-F3CC6H4 Br 2.3095(8) 1.941(4) 111.71(19) 107.11(14)

H 1.48 1.942(5) 109.0(2) 109.9

Much to our surprise, only 11 X-ray structures of triarylgermanes have been previously reported, wherein the majority are ortho-substituted aryl derivatives ((o-(MeSCH2)C6H4)3GeH,21 (o-(t-BuOCH2)C6H4)3GeH,21 (o-(Me2NCH2)C 6 H 4 ) 3 G eH, 2 2 (o-(EtO)C 6 H 4 ) 3 GeH, 2 3 (o-(Ph 2 P)C 6 H 4 ) 3 GeH, 19,24 (o-(Me 2 N)C 6 H 4 ) 3 GeH, 25 (o-(t-Bu)C6H4)3GeH,7e (o-MeC6H4)3GeH26) as well as also other derivatives (Ph3GeH,27 Mes3GeH,28 (C6F5)3GeH13). No compound containing only para-substituted aryl rings has ever been described before. The average bond lengths for Ge− H (1.33−2.05 Å) and Ge−C (1.93−2.06 Å) are observed. When comparing these derivatives with 2c, it is evident that the structural parameters are close to the smallest value of the range (close to an unsubstituted phenyl derivative). Therefore, it seems that the presence of an electron-withdrawing substituent in the para-position of the aryl group does not significantly affect the structural parameters of the molecule. As expected, the germanium atom in 2c has a slightly distorted tetrahedral geometry. The aromatic rings are adopting an unusual conformation, which differs from the typical propeller-like structure. The two rings are almost coplanar 299


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Organometallics

for Ar3GeH. The 13C NMR spectra for the compounds 1a−d showed aromatic signals as four doublets (at δ 163.8−164.6, 136.0−137.1, 129.6−131.0, 115.7−116.2 ppm) due to high values of spin 13C−19F coupling constants (1J 248.7−251.0, 2J 20.2−20.6, 3 J 7.6−8.1, 4 J 3.3−3.8 Hz). For the (ptrifluoromethyl)phenyl derivatives 2a−d, the 13C NMR displayed aromatic signals as broad singlets (at δ 137.8− 139.4, 133.1−135.5 ppm) and quartets (in the δ range 131.1− 133.9, 125.2−125.7 ppm with 2J13C‑19F 32.2−35.9, 3J13C‑19F 3.7− 3.8 Hz), and the CF3 group was observed as a quartet (δ range 122.9−124.7 ppm, 1J13C‑19F 272.3−273.0 Hz). It is interesting to note that the 13C chemical shifts of the aromatic ring for the para-substituted (in relation to the Ge atom) CF3 derivatives 2a−d (ipso → ortho → para → meta) are close to the shifts of (p-F3CC6H4)4Ge23 or of the majority of unsubstituted phenyl derivatives.5e In contrast, 1a−d displayed chemical shifts (para → ortho → ipso → meta in relation to the Ge atom) that are characteristic for electron donor para-substituted compounds.23 In the 19F NMR 1a−d were characterized by a multiplet (δ range −108.7 to (−111.4) ppm) due to 1H−19F spin coupling, and 2a−d showed a singlet (δ range −62.7 to (−63.4) ppm). The molecular structures of the three halides, chloride 1a (Figure 2) and bromides 1b and 2b (Figures 3 and 4), were

with the Ge−H bond (torsion angles H−Ge−C−C are −13.50° and 11.62°), whereas the third ring is almost perpendicular (torsion angles H−Ge−C−C is 88.66°). Furthermore, in order to fully characterize 1a, 1b, 2a, and 2b it is possible to prepare those compounds individually by starting from the corresponding hydrides 1c and 2c. The hydrides were subjected to thermally induced radical halogenations and yielded the corresponding halides in high yields (Scheme 3). Such a chlorination17a,29 and bromination30 Scheme 3. Selective Synthesis of Germanium Chlorides 1a and 2a and Bromides 1b and 2b

methodology is easy to perform and may be considered as an alternative to the previously reported methods such as the ones using CuCl2/CuI,7e,31 AcCl/FeCl332 (for chlorination), bromine in CCl4,33 or allyl bromide/PdCl234 (for bromination). Several attempts were made in order to obtain (pF3CC6H4)3GeCl (2a) by reacting the corresponding organolithium reagent (formed in situ by the reaction of p-F3CC6H4Br with an equimolar amount of n-BuLi at −30 °C) with GeCl4 (in a molar ratio of 3:1).13 Nevertheless, all of the attempts resulted in a mixture of mono-, di-, tetra-, and target triarylgermanes, where (p-F3CC6H4)4Ge was the major constituent (according to the 19F NMR spectroscopy data). The formation of a significant amount of polymeric materials was also observed. In this case, in contrast to when a Grignard reagent was used (see above), we were not able to isolate the individual compounds. At the same time, the reaction between a hypercoordinate germanium alkoxide, such as the readily available compounds based only on an alkyl derivative, K2[Ge{OCH(Me)CH(Me)O}3], with p-F3CC6H4MgBr followed by a subsequent reduction7g,35 led, again, to an inseparable mixture of unidentified compounds containing the target hydride (pF3CC6H4)3GeH only in a very low amount. Furthermore, the attempts to perform the desymmetrization (Kocheshkov reaction) of (p-F3CC6H4)4Ge23 by reacting it with GeCl4 in the presence of AlCl3 failed since no reaction was observed. Similarly, no reaction occurred when HOTf36 or Br237 was reacted with (p-F3CC6H4)4Ge. The absence of electrophilic aromatic substitution in this case could be explained by the presence of the strong electron-withdrawing substituents, p-F3CC6H4, on the Ge atom. Thus, according to the data obtained, it is possible to state that the synthetic methodology using organomagnesium compounds, developed in this work, may be regarded as a most suitable way for the synthesis of triarylgermanes bearing electron-withdrawing groups. Compounds 1a−d and 2a−d were characterized by multinuclear (1H, 13C, and 19F) NMR spectroscopy and elemental analysis. For the p-fluorophenyl derivatives 1a−d, the 1H NMR spectra display the signals of aromatic protons as two multiplets (at δ 7.61−7.32 and 7.19−6.88 ppm) due to spin−spin coupling with H and F atoms. In the case of the p(trifluoromethyl)phenyl derivatives 2a−d the proton signals appeared as doublets (in a δ range 7.43−7.89 and 7.34−7.80 ppm, 3JH−H 7.8−8.6 Hz). The signals for Ge−H were observed at δ 5.70 (1c) and 5.88 (2c) ppm, which are typical shift values

Figure 2. Molecular structure of (p-FC6H4)3GeCl (1a). Displacement ellipsoids are shown at the 50% probability level.

investigated by X-ray analysis (Table 1, Table S1, Supporting Information) in order to determine the effect of the electronic nature of the substituents on the germanium atom and on the structural parameters of the germanes. Previously, only very few examples of triarylgermanes bearing electron-withdrawing groups were reported. There are only seven known substituted triarylgermanium chlorides that have been investigated by X-ray analysis (Ph3GeCl,38 (o-(MeOCH2)C6H4)3GeCl,29a (oEtOC6H4)3GeCl,23 (o-(t-Bu)C6H4)3GeCl,7e (C6Cl5)3GeCl,33 (C6F5)3GeCl13). The germanium atom in 1a displays a tetrahedral arrangement with its aryl rings adopting a propeller-like conformation (torsion angles Cl−Ge−C−C are 54.24°, 60.09°, and 66.97°). Such a conformation is typical for such derivatives. Other structural parameters, including d(Ge− 300


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Organometallics

unsubstituted Ph3GeBr, showing that the effect of the electronic nature of substituents on the aryl groups is mainly affecting the Ge bond length and the conformation of the aryl groups. At the same time, it is evident that the corresponding chlorides and bromides have very similar structures. The next goal of this research was to synthesize oligogermanes containing electron-withdrawing substituents using known precursors as well as those obtained in this work (1c, 1d; 2c, 2d). Our attempts to synthesize oligogermanes bearing electronwithdrawing groups by reacting (C6F5)3GeH with polyamide germanes such as Me2Ge(NMe2)2 or Ph2Ge(NMe2)2 in nhexane or MeCN were unsuccessful even under harsh conditions (up to 100 h at 90 °C). Furthermore, (Me2N)4Ge showed also to be unreactive when treated with Ph3GeH or (C6F5)3GeH in MeCN.7a Therefore, it seems that the hydrogermolysis using monoamidogermanes remains the easiest way to prepare oligogermanes. Moreover, when 1c was submitted to hydrogermolysis conditions in hexane13 with 1d or 2c with 2d, no reaction was observed, even after 26 h of heating. This greatly differs from the case of (C 6 F 5 ) 3 GeH, which we previously investigated.13 However, when hexane was replaced by acetonitrile, the corresponding symmetric digermanes 3 and 4 were obtained in high yields (Scheme 4).

Figure 3. Molecular structure of (p-FC6H4)3GeBr (1b). Displacement ellipsoids are shown at the 50% probability level.

Scheme 4. Synthesis of Digermanes 3 and 4

Figure 4. Molecular structure of (p-F3CC6H4)3GeBr (2b). Displacement ellipsoids are shown at the 50% probability level. Minor components of disordered CF3 groups are not shown. Hydrogen atoms are omitted for clarity.

Thus, the result of hydrogermolysis is strongly dependent not only on the nature of the germanes but also on the type of solvent. Unexpected results were obtained during our attempts to prepare unsymmetrical oligogermanes. The reaction between 1d and 2c or between 1c and 2d gave only mixtures of the corresponding symmetrical derivatives, 3 and 4. The reaction of 1d with (C6F5)3GeH led to a mixture, where the symmetric derivative 3 was identified. Evidently, in our case, (C6F5)3GeH may act as a proton transfer reagent. On the contrary, when (C6F5)2GeH2 was used as the germanium hydride source in the hydrogermolysis, the reaction resulted in the formation of the desired product (Scheme 5). Compounds 3−5 were characterized by 1H, 13C, and 19F NMR, UV/visible, and fluorescence spectroscopy, elemental analysis, and electrochemical methods. The NMR spectra of oligogermanes 3, 4, and 5 are similar to the corresponding

Cl) and d(Ge−C), are very similar to unsubstituted Ph3GeCl (2.1873(6) vs 2.187(2)38a and 1.930(2) vs 1.933(5)38b Å). To the best of our knowledge, only four triarylgermanium bromides have been investigated by X-ray analysis, and none of them are para-substituted. The molecular structure of 2b is partially disordered due to the presence of the CF3 groups. In the bromide derivatives 1b and 2b, the germanium atom has a slightly distorted tetrahedral geometry. Introduction of electron-withdrawing groups such as F or CF3 in para-position of the phenyl group in aryl germanes results in a decrease of the Ge−Br bond length in comparison with triaryl bromides that were previously described (2.3095(8) Å for 2b and 2.3126(8) Å for 1b vs 2.319(3) Å for Ph3GeBr39, 2.3618(5) Å for [o(MeOCH 2 (C 6 H 4 ) 3 GeBr], 29a 2.3791(6) Å for [(2,6-(tBuO) 2 C 6 H 3 ) 3 GeBr], 4 0 2.362(1) Å for [(o-(t-Bu)C6H4)3GeBr]7e). In 2b, the conformation of the aryl groups is similar to that of 2c, where one of the aryl cycles is almost perpendicular to the Ge−Br bond (the torsion angles Br−Ge− C−C are 29.87°, 40.75°, and 77.20°). In sharp contrast, in 1b the aryl rings adopt a typical propeller-like conformation (the torsion angles Br−Ge−C−C are 51.72°, 58.91°, and 65.12°). Other structural parameters of 1b and 2b are similar to

Scheme 5. Synthesis of Trigermane 6

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Organometallics germanes. The molecular structures of digermanes 3 and 4 and trigermane 5 (Figures 5−7, Table S2, Supporting Information)

Figure 5. Molecular structure of (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3). Minor components of disordered phenyl groups are not shown. Displacement ellipsoids are shown at the 50% probability level.

Figure 7. Molecular structure of [(p-F3CC6H4)3Ge]2Ge(C6F5)2 (5). Minor components of disordered −CF3 groups are not shown. Displacement ellipsoids are shown at the 30% probability level. Selected bond lengths (Å) and bond angles (deg): Ge(1)−Ge(2) 2.4641(9), Ge(1)−C(19) 1.996(4), Ge(2)−Cav 1.958(4); Ge(2)− Ge(1)−Ge(2A) 122.05(6), C(19)−Ge(1)−C(19A) 110.8(3), C(19)−Ge(1)−Ge(2) 101.06(14), C(19A)−Ge(1)−Ge(2) 111.00(14), C−Ge(2)−Cav 109.9(2), C−Ge(2)−Geav 109.07(14).

Table 2. Structural Parameters of Hexaaryldigermanes Investigated by XRD compound

d(Ge−Ge), Å

ref

(p-Tol)3GeGePh3 (p-Tol)3GeGe(Tol-p)3 Ph3GeGePh3 Ph3GeGePh3*2C6H6 (C6F5)3GeGePh3 (C6F5)3GeGe(Tol-p)3

2.408(1) 2.419(1) 2.437(2) 2.446(1) 2.4623(4) 2.4652(11)

7d 42 5a 5b 13 13

Both germanium atoms in 3 and 4 exhibit a tetrahedral geometry. The d(Ge−Ge) bonds in these compounds are very close to the shortest reported ones. The introduction of electron-withdrawing groups has a small effect and results in a shortening of the Ge−Ge bond, especially in comparison with the related donor−acceptor digermanes (compare Tables 2 and 3). Apparently, due to the introduction of electron-withdrawing substituents, the electron density is moved away from the Ge atoms to the substituents. This results in an increase of the positive charge at the Ge center and an increase in the covalency of the Ge−Ge bond and, therefore, as a consequence, also in a shortening of germanium−germanium bond.

Figure 6. Molecular structure of (p-F3CC6H4)3Ge−Ge(C6H4CF3-p)3 (4). Minor components of disordered CF3 groups are not shown. Displacement ellipsoids are shown at the 30% probability level.

in the solid state were investigated by single-crystal X-ray diffraction analysis. Compounds 3−5, containing only electronwithdrawing substituents at the Ge atoms, are the first examples of oligogermanes of such type investigated by X-ray diffraction (XRD). According to the Cambridge Structural Database, the Ge−Ge bond length in digermanes varies between 2.393 Å (for Ph2(O2CCCl3)GeGe(O2CCCl3)Ph25c) and 2.713 Å (for (tBu)6Ge241). Sterically hindered substituents increase this bond length. Usually, the introduction of several electron-withdrawing substituents decreases the bond length. At the same time, the presence of a donor and an acceptor substituent at the Ge center elongates the Ge−Ge bond. To the best of our knowledge, only five hexaaryldigermanes have been previously investigated by XRD (Table 2).

Table 3. Selected Bond Distances (Å) and Bond Angles (deg) for (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3) and (pF3CC6H4)3Ge−Ge(C6H4CF3-p)3 (4)

Ge−Ge Ge−Cav C−Ge−Geav C−Ge−Cav 302

3

4

2.4209(8) 1.966(8) 110.3(4) 108.6(6)

2.433(8) 1.96(4) 109.5(13) 109.4(18) DOI: 10.1021/acs.organomet.6b00767 Organometallics 2017, 36, 298−309

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Organometallics Table 4. Selected Structural Parameters for Known Trigermanes

a

trigermane

d(Ge−Ge), Å

angle Ge−Ge−Ge, deg

angle C−Ge(center)−Cav, deg

ref

[Ph3Ge]2GePh2 [Ph3Ge]2GeMe2 [ClPh2Ge]2GePh2 [Me(t-Bu)2Ge]2Ge(t-Bu)2 [Br(t-Bu)2Ge]2Ge(t-Bu)2 [I(t-Bu)2Ge]2Ge(t-Bu)2 [(p-Tol)3Ge]2GePh2 [(p-Tol)3Ge]2Ge(p-Tol)2 [(p-Tol)3Ge]2Ge(C6F5)2 [(Me3Si)3Ge]2GeMe2 [Ph3Ge]2Ge(SiMe3)2

2.440(2) 2.429(1) 2.413−2.437 2.620(3) 2.6223(1), 2.595(1) 2.660(1), 2.622(1) 2.4328(5) 2.4404(5) 2.459(5) 2.4616(8) 2.4494(3)

121.3(1) 120.3(1) 110.4−116.7 118.6(1) 113.5(1) 115.38(10) 114.80(2) 117.54(1) 124.10(3) 125.00(4) 111.949(10)

108.7(4) 109.2(2) 111.82(13) 110.31(15) 109.25(12) 109.36(12) 106.2(1) 106.45(9) 108.0(2) 105.35(4) 104.78(2)a

5f 5d 5e 43 43 44 7d 7d 13 9a 45

Si−Ge−Si angle.

Compounds 3 and 4 display an almost ideal gauche conformation46 along the Ge−Ge bond with the torsion angle of C−Ge−Ge−Cav equal to 60.7(8)° and 60.0(19)°. All phenyl rings are in a propeller-like conformation relative to the Ge−Ge bond (average torsion angles are 44.03(8)° and 53.96(18)°, respectively). The molecular structure of 5 in the solid state was also studied by XRD analysis. Trigermanes are rare, and there are only 11 reported compounds that have been studied by XRD (Table 4). According to these data, in general, steric bulk of the substituents mainly affects the structural properties of trigermanes, while the electronic effects have a lesser impact. The d(Ge−Ge) varied from 2.413 to 2.660 Å, and angles Ge− Ge−Ge and C−Ge(central)−C are 110.40−125.00° and 104.78−111.82°, respectively. In a crystal, the molecules of 5 lie on a 2-fold axis. The Ge− Ge bond length (2.4641(9) Å) for this compound is the biggest for the known octaaryltrigermanes (see Table 2). Thus, it seems that the introduction of electron-withdrawing substituents at the Ge center in oligogermanes results in an elongation of the Ge−Ge bond length. The value of the Ge(2)−Ge(1)−Ge(2A) angle (122.05(6)°) in 5 is typical of an effective σ-conjugation in the oligogermanium chain. Unlike for 3 and 4 the substituents in 5 on the neighboring Ge atoms are in cis-conformation (the average C−Ge(1)−Ge(2)−C torsion angle is 20.74(14)°), and therefore, this may result in some Ge−Ge elongation of the Ge−Ge bond. Interestingly, this conformation is atypical for oligogermanes and apparently is caused by crystal packing. At the same time, this conformation may be explained by the second electronic interaction in a molecule with highly electron-withdrawing groups. Really, according to density functional theory (DFT) data obtained earlier,13 the HOMO and LUMO are not only on Ge chain atoms but also on aromatic rings. The angles around the central Ge atom, Ce− Ge(1)−Ge and C−Ge(1)−C, are significantly different (122.05(6)° and 110.8(3)°), and in the absence of sterically demanding substituents such as SiMe3 groups, it is possible to state that the electron-withdrawing substituent strongly affects this parameter. Therefore, in addition to the transoid conformation adopted by the catenated compounds of group 14 bearing large end groups and reported by Marschner et al.,14,47 it is evident that the molecular geometry may be influenced by electronic factors, i.e., by the introduction of an electron-withdrawing substituent in the core of the molecule. As has been described before, the introduction of an electron-withdrawing aryl substituent into a catenated

oligogermane results not only in the stabilization of the HOMO (and hence in an increase of the oxidation potential of oligogermanes) but also in a stabilization of the LUMO.13 Therefore, the HOMO/LUMO gap, which corresponds to the UV/visible absorption, shows some significant changes. UV/visible absorption spectra for catenated organogermanium compounds 3−5 are presented in Figure 8.

Figure 8. UV/visible absorption spectra for compounds 3−5 (CH2Cl2, rt).

As expected, the elongation of the germanium chain resulted in a red shift in UV absorption (compare with 237 nm for 3, 239 nm for 4 vs 264 nm for 5). It should be noted that the absorption of digermanes 3 and 4 bearing electron-withdrawing groups is slightly blue-shifted in comparison with the known digermanes (usually in the 2385f−240 nm7d range, which may be regarded as a typical feature) due to some HOMO stabilization. However, for trigermane 5 the absorption (264 nm) is significantly red-shifted (compare to Ph8Ge3, 238 nm; (p-Tol)8Ge3, 253 nm; (p-Tol)3GeGePh2Ge(Tol-p)3, 251 nm).7d This could be explained by a significant LUMO stabilization. Furthermore, the comparison between the UV absorption of 5 with [(Me3Si)3SiGeMe2]2GeMe2 (253 nm)9a or even with Me(Me2Ge)6Me (255 nm)6a suggests that electron-withdrawing substituents exhibit a significant effect on the absorption properties of oligogermanes. It is interesting to note that a similar effect including bathochromic shift has been observed earlier for molecular oligosilanes when electron-withdrawing substituents (chlorine, nitrophenyl groups) are introduced into their structures.48 Electrochemical investigations for compound 3 under standard conditions in dichloromethane containing [NBu4][PF6] (0.10 M) as supporting electrolyte did not show any reduction, but a chemically irreversible anodic oxidation was observed at Epa = 1.20 V Fc/Fc+50 (approximately 1.69 V vs Ag/AgCl) (see Supporting Information, Figure S44). No rereduction wave was observed during the back scan. This 303


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Organometallics value is close to the range of 1.48−1.96 V vs Ag/AgCl of potentials previously measured for known digermanes (compare with 1.48 V for Ph3GeGe(Tol-p)3, 1.76 V for (pTol)6Ge2, 1.96 V for Ph6Ge27d). The electrochemistry of compound 3 was then carried out in dichloromethane containing [NBu4][B(C6F5)4] (0.05 M) as supporting electrolyte. The advantages of using a weakly coordinating anion as supporting electrolyte such as the “TFAB” anion, [B(C6F5)4]−, have been previously described.51 In the present case, we were hoping to stabilize the oxidation product in order to be able to detect it during the back scan. Unfortunately, even at a higher sweeping rate (1000 V/s) and at lower temperature (−50 °C) we were not able to observe any product during the rereduction. Again, a chemically irreversible anodic wave was observed at Epa = 1.41 V vs Fc/ Fc+ (approximately 1.90 V vs Ag/AgCl). The formation of a transient radical-cation 3+ would explain why the oxidation is more difficult in [NBu4][B(C6F5)4] than in [NBu4][PF6] since [PF6]− would better stabilize 3+ due to its higher coordination ability. In addition to the chemical irreversibility, 3 displays an important degree of electrochemical irreversibility. The measured Epa − Epa/2 value for the oxidation of 3 is a diagnostic of a slow charge transfer process with a coefficient (β) of 0.34 in the presence of [NBu4][PF6] and of 0.30 in the presence of [NBu4][B(C6F5)4].52 The electrochemistry of compound 4 was investigated in several media such as dichloromethane ([NBu4][B(C6F5)4] (0.05 M)), dichloromethane ([NBu4][PF6] (0.10 M)), and acetonitrile ([NBu4][PF6] (0.10 M)). Unfortunately, none of these conditions allowed us to observe any anodic process. The presence of a strong electron-withdrawing group on the aromatic ring in this case increased the oxidation potential beyond the oxidation of the solvent itself. Apparently, this fact is caused by a significant HOMO stabilization (see Table 7).

Table 6. Luminescence Emission Data for Known Molecular Oligogermanes compound cyclo-Ge6Me12 (iPr)3Ge[GePh2]4Ge(Pri)3 (p-Tol)3GeGeMe3

a

compound (p-FC6H4)3Ge− Ge(C6H4F-p)3 (3) (p-F3CC6H4)3Ge− Ge(C6H4CF3-p)3 (4) (p-F3CC6H4)3Ge− Ge(C6F5)2Ge(C6H4CF3-p)3 (5) a b

λem (nm)b

Stokes shift (nm)

solutiona

378

141

437

198

476

212

λem (nm) 350/364 sh 335/345 sh 376

Stokes shift (nm)

Φf (%)c

113/127

3.57

96/106

11.77

112

12.36

310, CH2Cl2 232, CH2Cl2

λem (nm),a details 293 (250); cyclohexane, 293 K 370 (312), CH2Cl2 286 (270), CH2Cl2; Φf 3.27% 357, 373, 393 (300), solid

ref 49 7h 45 45

Excitation wavelength λex in parentheses.

fluorescent in solution (Figure 9) as well as in the solid state (Figure 10). Nowadays the studies of fluorescence of molecular oligogermanes (linear7h,45 or cyclic49) (Table 6) are very limited in comparison with the studies conducted on polymeric materials.53 But in general it is possible to affirm that substituted aryl digermanes display luminescence. Therefore, the investigation of the effect of the nature of substituents at the Ge atom on the emission is relevant, and this would help to identify the main structural parameters that influence the emissive properties of oligogermanes. This would also allow improving the optical properties of those compounds (increase the quantum yield, shift into the red field, and narrow the emissive line). It is possible to state that the increase of the length of the catenated chain in molecular oligogermanes resulted in a red shift of the emission, which is comparable to that found in the related polymers. Furthermore, the introduction of electron-withdrawing groups also led to the same trend; in addition, the quantum yield in solution is also increased (Table 5). The fluorescence of the molecular digermanes 3−5 is comparable to that of polygermanes (369 nm in solution and 367 nm in film for [(Me3SiOC6H4)2Ge]n54). The luminescence properties of the oligogermanes were also studied by DFT. The influence of the nature of the substituent on the fluorescence properties was studied (Table 7). According to these data, it is evident that the introduction of an alkyl electron-withdrawing group results in a red shift of the emission wavelength. At the same time, aromatic acceptor substituents lead to a red-shifted emission in comparison to donors or unsubstituted ones. Finally, the predicted values for 3 and 4 are in a good agreement with the experimental values (Tables 5 and 7). Furthermore, it should be noted that the structural parameters in oligogermanes in fundamental (S0) and excited (S 1) states differ significantly (see Supporting Information). For instance, the Ge−Ge bond lengths (2.44− 2.48 vs 2.66−2.68 Å) are elongated, wherein the energy in S1 increased significantly.

Table 5. Luminescence Emission Data for Compounds 3−5 solid state

λabs (nm), details

Spectra were recorded in CH2Cl2, excitation wavelength λex 290 nm. Excitation wavelength λex 320 nm. cQuantum yield.

■

Electrochemical investigations for 5 were performed in conditions found for 3. As expected, in this case there are two oxidation waves, E pa = 1.27 and 1.63 V vs Fc/Fc + (approximately 1.76 and 2.09 V vs Ag/AgCl) (see Supporting Information, Figure S45). These data are higher than the ones obtained earlier for trigermanes7d (1.696 and 2.052 V for Ph8Ge3, 1.498 and 1.860 V for (p-Tol)3GeGePh2Ge(Tol-p)3, 1.542 and 1.865 V for (p-Tol)8Ge3 vs Ag/AgCl), which shows the stabilization of the HOMO in 5 due to the presence of electron-withdrawing groups. Finally, the fluorescence of the compounds 3−5 was studied by fluorescence spectroscopy (Table 5). All compounds are

CONCLUSIONS A series of electron-deficient triarylgermanes (1a−d, 2a−d), containing p-fluorophenyl and (p-trifluoromethyl)phenyl groups, have been synthesized, wherein organomagnesium reagents gave suitable results. These derivatives were used in hydrogermolysis reactions in order to prepare di- (3, 4) and trigermanes (5). Introduction of electron-withdrawing substituents into the oligogermane core resulted in a red shift in UV absorption in comparison to the known related derivatives. Furthermore, oligogermanes bearing electron-withdrawing 304


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Organometallics Table 7. Luminescence Emission Data for Substituted Oligogermanes According to DFT Calculations compound Me3GeGeMe3 (F3C)3GeGe(CF3)3 Ph3GeGePh3

(p-Tol)3GeGe(Tol-p)3

(p-FC6H4)3GeGe(C6H4F-p)3 (3)

(p-F3CC6H4)3GeGe(C6H4CF3-p)3 (4)

fluorescence, λ, nm

oscillator strength

transition

HOMO, eV

350 373 391 342 338 335 305 306 389 350 343 402 357 349

0.47 0.59 0.36 0.17 0.16 0.27 0.23 0.22 0.23 0.01 0.14 0.39 0.30 0.30

LUMO−HOMO LUMO−HOMO LUMO−HOMO LUMO+1−HOMO LUMO+2−HOMO LUMO−HOMO LUMO+1−HOMO LUMO+2−HOMO LUMO−HOMO LUMO+1−HOMO LUMO+2−HOMO LUMO−HOMO LUMO+1−HOMO LUMO−HOMO

−6.37 −9.23 −5.62

−6.68

last cases, the absence of detectable amounts of organic impurities was established by high-field multinuclear NMR spectroscopy. 1 H NMR (400.130 MHz), 13C NMR (100.613 MHz), and 19F (376.498 MHz) spectra were recorded with a Bruker 400 or Agilent 400MR spectrometer at 298 K. Chemical shifts are given in ppm relative to internal Me4Si (1H and 13C NMR spectra) or external CFCl3 (19F spectra). Mass spectra (EI-MS, 70 eV) were recorded on a Finnigan MAT INCOS 50 quadropole mass spectrometer with direct insertion; all assignments were made with reference to the most abundant isotopes. Elemental analyses were carried out by the Microanalytical Laboratory of the Chemistry Department of Moscow State University using a HeraeusVarioElementar instrument. MALDI mass spectra were recorded on an Autoflex II Bruker spectrometer (fwhm resolution 18 000) equipped with a nitrogen laser operating at a wavelength of 337 nm and with a time-of-flight mass analyzer operating in reflection mode. The accelerating voltage was 20 kV, and anthracene (Acros, 99%) was used as the matrix. UV/visible spectra were obtained using an Evolution 300 Thermo Scientific two-ray spectrophotometer with a 0.10 cm long cuvette. Fluorescence spectra at room temperature were recorded with a Hitachi F-7000 spectrofluorimeter. The fluorescence quantum yields were measured with respect to rhodamine. Known compounds (C6F5)2GeH2, (C6F5)3GeH,13 Me4Ge,45 and Ph4Ge36 were obtained by employing published procedures. Dimethyldichlorogermane, Me2GeCl2. The improved procedure with detailed conditions was used.55 At 0 °C under strong stirring acetyl chloride (51.00 mL, 56.50 g, 0.72 mmol) was added dropwise to the mixture of Me4Ge (26.00 g, 200.00 mmol) and anhydrous AlCl3 (112.00 g, 840.00 mmol). The reaction mixture was slowly warmed to room temperature and stirred for 5 h, then heated at 100 °C for 1 h. Volatile materials were distilled twice using an effective condenser, giving dimethyldichlorogermane as a colorless liquid, bp 119−123 °C (12 mmHg). Yield: 18.40 g (53%). 1H NMR (δ, ppm, CDCl3): 1.20 (s, 6H, CH3). 13C NMR (δ, ppm, CDCl3): 10.77 (CH3). Diphenyldichlorogermane, Ph2GeCl2. GeCl4 (2.20 g, 10.20 mmol) was added to the mixture of Ph4Ge (3.93 g, 10.30 mmol) and AlCl3 (0.24 g, 1.80 mmol). The procedure of freezing in liquid nitrogen, evacuation, and slow warming to room temperature was repeated three times, and then the solution obtained was heated at 125 °C (oil bath temperature) for 4 h. After cooling to room temperature the reaction mixture was treated with concentrated HCl and extracted with benzene (3 × 30 mL). Combined organic phases were dried over MgSO4; then all volatile materials were removed under reduced pressure, and the residue was fractionized using an oil bath for heating (150−180 °C). Diphenyldichlorogermane (2.23 g, 73%) was isolated as a colorless oil, bp 115−120 °C (0.17 mmHg), bp 120−124 °C (0.01 mmHg).56 1H NMR (δ, ppm, CDCl3): 7.77−7.79 (m, 4H, aromatic protons), 7.51−7.58 (m, 6H, aromatic protons). 13C NMR (δ, ppm, CDCl3): 129.01, 131.80, 132.67, 134.29 (aromatic carbons). NMR spectra correspond to known data.5c

Figure 9. Florescence emission spectra for 3−5 (λex 290 nm) in solution in CH2Cl2.

Figure 10. Fluorescence emission spectra for 3−5 (λex 320 nm) in the solid state.

groups displayed luminescence properties. The presence of acceptor groups results in a red-shifted emission. The found results would stimulate investigations in design and further investigation of materials based on group 14 elements.

■

−5.74

EXPERIMENTAL SECTION

General Remarks. All manipulations were performed under a dry, oxygen-free argon atmosphere using standard Schlenk techniques. GeCl4 (Aldrich), p-FC6F4Br (Aldrich), and p-F3CC6F4Br (Aldrich) were distilled prior to use. n-BuLi (Aldrich) and LiAlH4 (Aldrich) were used as supplied from commercial sources. Solvents were dried using the usual procedures. Tetrahydrofuran, diethyl ether, and triethylamine were stored under solid KOH and then distilled over sodium/ benzophenone. Toluene and n-hexane were refluxed and distilled over sodium. Dichloromethane, CCl4, and acetonitrile were distilled under CaH2. C6D6 was distilled over sodium under argon. CDCl3 was distilled over CaH2 under argon. The purity of the new compounds has been established based on elemental analysis or MALDI mass spectrometry. Me2GeCl2 and Ph2GeCl2 are known compounds; for Me2Ge(NMe2)2 and Ph2Ge(NMe2)2 elemental analysis was not performed due to their extreme sensitivity; for compounds 1d and 2d elemental analysis data are also sensitive to moisture traces. In the 305


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Organometallics

Tris(p-fluorophenyl)germanium Dimethylamide, (pFC 6 H 4 ) 3 GeNMe 2 (1d). A solution of the mixture of (pFC6H4)3GeCl/(p-FC6H4)3GeBr (1a/1b) (5.36 g, 12.53 mmol) in toluene (40 mL) was added to a suspension of LiNMe2 (0.80 g, 15.68 mmol) in toluene (40 mL). The reaction mixture was stirred at room temperature for 3 d and then filtered. All volatile materials were removed under reduced pressure. Compound 2d (4.33 g, 86%) was isolated as a colorless, low-melting substance. 1H NMR (δ, ppm, C6D6): 7.37−7.32, 6.91−6.86 (2m, each 6H, aromatic protons), 2.59 (s, 6H, NMe2). 13C NMR (δ, ppm, C6D6): 164.44 (d, 1J13C‑19F = 248.7 Hz), 137.07 (d, 3J13C‑19F = 7.6 Hz), 130.96 (d, 4J13C‑19F = 3.8 Hz), 115.90 (d, 2J13C‑19F = 20.2 Hz) (aromatic carbons), 41.32 (NMe2). 19F NMR (δ, ppm, C6D6): −110.58 to (−110.66) (m, 3F, 3 p-C6H4F). Anal. Calcd for C20H18F3GeN (401.9988): C, 59.75; H, 4.51; N, 3.48. Found: C, 57.78; H, 4.73; N, 3.06. The Mixture of (p-F3CC6H4)3GeCl/(p-F3CC6H4)3GeBr, 2a/2b. Synthesis of p-F3CC6H4MgBr. A solution of p-F3CC6H4Br (42 mL, 67.52 g, 300.00 mmol) in THF (40 mL) was added dropwise to a suspension of magnesium (7.30 g, 300.00 mmol) in THF (100 mL). After the addition reaction the mixture was refluxed for 2 h. The black solution of the Grignard reagent was filtered and used further without purification. Synthesis of the Mixture of (p-F3CC6H4)3GeCl/(p-F3CC6H4)3GeBr (2a/2b). At 0 °C the solution of the Grignard reagent obtained as stated above was added dropwise to a solution of GeCl4 (11.18 mL, 98 mmol) in THF (120 mL) during 2 h. The reaction mixture was warmed to room temperature and refluxed for 12 h. Then THF (120 mL) was distilled off, and hexane (150 mL) was added. The mixture obtained was filtered. The volatile materials were removed under reduced pressure, and the residue was fractionalized, giving two fractions. The first fraction, 5.81 g (20% based on GeCl4), bp 110−150 °C (0.08 mmHg), is a mixture of 2a/2b (6:5). The second fraction, 2.09 g (8% based on GeCl4), bp 155−170 °C (0.08 mmHg), is 2b, which solidifies on standing at room temperature. Compound 2b may be recrystallized from hexane, giving an analytically pure substance. Compound 2a, 1H NMR (δ, ppm, CDCl3): 7.89, 7.80 (2d, each 6H, 3 JH−H = 7.8 Hz, aromatic protons). 13C NMR (δ, ppm, CDCl3): 138.74 (ipso-Ge), 133.86 (q, 2J13C‑19F = 32.8 Hz), 133.13 (ortho-Ge), 125.76 (q, 3J13C‑19F = 3.8 Hz) (aromatic carbons), 123.47 (q, 1J13C‑19F = 272.8 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.40 (s, 9F, 3CF3). Compound 2b, 1H NMR (δ, ppm, CDCl3): 7.76, 7.73 (2d, each 6H, 3 JH−H = 8.6 Hz, aromatic protons). 13C NMR (δ, ppm, CDCl3): 137.80 (ipso-Ge), 134.49 (ortho-Ge), 133.11 (q, 2J13C‑19F = 32.9 Hz), 125.60 (q, 3J13C‑19F = 3.7 Hz) (aromatic carbons), 123.70 (q, 1J13C‑19F = 272.6 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.31 (s, 9F, 3CF3). Tris(p-(trifluoromethyl)phenyl)chlorogermane, (pF3CC6H4)3GeCl (2a). A catalytic quantity of AIBN (0.0226 g, 0.14 mmol, 5 mol %) was added to the solution of (p-F3CC6H4)3GeH (2c) (1.40 g, 2.75 mmol) in CCl4 (40 mL). The reaction mixture was frozen in liquid nitrogen, evacuated under high vacuum, slowly warmed to room temperature, and heated at 95 °C for 2 h. Then volatile materials were removed under reduced pressure, and the residue was recrystallized from n-hexane. Compound 2a (1.34 g, 90%) was isolated as a colorless oil, which solidifies on standing. Anal. Calcd for C21H12ClF9Ge (543.3986): C, 46.42; H, 2.23. Found: C, 46.24; H, 2.25. Tris(p-(trifluoromethyl)phenyl)bromogermane, (pF3CC6H4)3GeBr (2b). N-Bromosuccimide (0.54 g, 3.03 mmol) was added to a solution of (p-F3CC6H4)3GeH (2c) (1.40 g, 2.75 mmol) and a catalytic quantity of benzoyl peroxide (0.0339 g, 0.14 mmol, 5 mol %) in toluene (20 mL). The reaction mixture was refluxed for 2 h. Then volatile materials were removed under reduced pressure, and the residue was extracted with n-hexane and recrystallized from n-hexane. Compound 2b (1.47 g, 92%) was isolated as a white powder. Anal. Calcd for C21H12BrF9Ge (587.8496): C, 42.91; H, 2.06. Found: C, 42.87; H, 1.98. Crystals suitable for X-ray analysis were obtained after recrystallization from n-hexane. Tris(p-(trifluoromethyl)phenyl)germane, (p-F3CC6H4)3GeH (2c). At 0 °C LiAlH4 (0.05 g, 1.20 mmol) was added to a solution of the mixture of 2a/2b (0.50 g, 0.89 mmol) in ether (40 mL). The

Synthesis of Functionalized Triarylgermanes. The Mixture of (p-FC6H4)3GeCl/(p-FC6H4)3GeBr, 1a/1b. Synthesis of pFC6H4MgBr. A solution of p-FC6H4Br (33.00 mL, 300.00 mmol) in THF (100 mL) was added dropwise to a suspension of Mg (7.05 g, 290.00 mmol) in THF (40 mL). Then the reaction mixture was refluxed for 2 h, cooled to room temperature, and filtered. The solution of p-FC6H4MgBr in THF obtained was used without further purification. Synthesis of the Mixture of (p-FC6H4)3GeCl/(p-FC6H4)3GeBr, 1a/ 1b. At 0 °C the solution of p-FC6H4MgBr in THF obtained as stated above was added dropwise to a solution of GeCl4 (11.00 mL, 97.00 mmol) in THF (120 mL). The reaction mixture was refluxed for 19 h. Then THF (180 mL) was distilled off, hexane (200 mL) was added, the mixture was filtered, and the solid was washed with hexane (2 × 50 mL). All volatile materials were removed from the mother liquor, and the residue was recrystallized twice from n-hexane. The mixture of the compounds 1a/1b (1:4, according to 19F NMR) was obtained (9.16 g, 22% based on GeCl4) as a white glass. This mixture was used for synthesis of 1c without further purification. Tris(p-fluorophenyl)germanium Chloride, (p-FC6H4)3GeCl (1a). Catalytic quantities of 2,2′-azobisisobutyronitrile (AIBN) (0.0457 g, 0.28 mmol, 5 mol %) were added to the solution of tris(p-fluorophenyl)germane, 1c (2.00 g, 5.57 mmol), in CCl4 (40 mL). The procedure of freezing in liquid nitrogen, evacuation, and slow warming was repeated three times, and then the solution obtained was heated at 90 °C for 3 h. After cooling to room temperature all volatile materials were removed under reduced pressure. The residue was recrystallized from n-hexane. Compound 1a (2.06 g, 94%) was isolated as a white powder. 1H NMR (δ, ppm, CDCl3): 7.61−7.56, 7.19−7.14 (2m, each 6H, aromatic protons). 13C NMR (δ, ppm, CDCl3): 164.59 (d, 1J13C‑19F = 251.0 Hz), 136.00 (d, 3 J13C‑19F = 8.1 Hz), 129.73 (d, 4J13C‑19F = 3.7 Hz), 116.15 (d, 2J13C‑19F = 20.5 Hz) (aromatic carbons). 19F NMR (δ, ppm, CDCl3): −108.79 to (−108.87) (m, 3F, 3 p-C6H4F). Anal. Calcd for C18H12ClF3Ge (383.3761): C, 54.96; H, 3.07. Found: C, 55.18; H, 2.88. Crystals suitable for X-ray analysis were obtained after recrystallization from nhexane. Tris(p-fluorophenyl)germanium Bromide, (p-FC6H4)3GeBr (1b). Catalytic quantities of benzoyl peroxide (0.0678 g, 0.28 mmol, 5 mol %) were added to the solution of tris(p-fluorophenyl)germane, 1c (2.00 g, 5.57 mmol), and N-bromosuccinimide (1.09 g, 5.13 mmol) in toluene (40 mL). The solution obtained was refluxed for 3 h. After cooling to room temperature all volatile materials were removed under reduced pressure. The residue was recrystallized from n-hexane. Compound 1b (2.15 g, 88%) was isolated as a white powder. 1H NMR (δ, ppm, CDCl3): 7.61−7.56, 7.18−7.13 (2m, each 6H, aromatic protons). 13C NMR (δ, ppm, CDCl3): 164.52 (d, 1J13C‑19F = 251.0 Hz), 136.11 (d, 3J13C‑19F = 8.1 Hz), 129.64 (d, 4J13C‑19F = 3.3 Hz), 116.09 (d, 2J13C‑19F = 20.5 Hz) (aromatic carbons). 19F NMR (δ, ppm, CDCl3): −108.90 to (−108.97) (m, 3F, 3 p-C6H4F). Anal. Calcd for C18H12BrF3Ge (437.8271): C, 49.38; H, 2.76. Found: C, 49.02; H, 2.82. Crystals suitable for X-ray analysis were obtained after recrystallization from n-hexane. Tris(p-fluorophenyl)germane, (p-FC6H4)3GeH (1c). At 0 °C LiAlH4 (0.06 g, 1.58 mmol) was added to a solution of the mixture of 1a/1b (0.65 g, 1.52 mmol) in ether (40 mL). The reaction mixture was stirred for 6 h; then 2 M aqueous H2SO4 (40 mL) was added dropwise, the organic phase was separated, and the aqueous layer was extracted with ether (3 × 20 mL). Combined organic phases were dried over anhydrous Na2SO4; then the solvent was removed under reduced pressure, and the residue was recrystallized from n-hexane. Compound 1c (0.46 g, 84%) was isolated as a white powder. 1H NMR (δ, ppm, CDCl3): 7.48−7.43, 7.12−7.07 (2m, each 6H, aromatic protons), 5.70 (s, 1H, GeH). 13C NMR (δ, ppm, CDCl3): 163.87 (d, 1 J13C‑19F = 248.7 Hz), 136.75 (d, 3J13C‑19F = 7.6 Hz), 130.22 (d, 4J13C‑19F = 3.8 Hz), 115.75 (d, 2J13C‑19F = 20.6 Hz) (aromatic carbons). 19F NMR (δ, ppm, CDCl3): −111.25 to (−111.33) (m, 3F, 3 p-C6H4F). Anal. Calcd for C21H13F9Ge (508.9535): C, 49.56; H, 2.57. Found: C, 49.58; H, 2.48. 306


Article

Organometallics

3), 169 ([(p-FC6H4)Ge + H]+, 10). UV/vis (CH2Cl2), λmax nm (ε, M−1 cm−1): 237 (4.1 × 104). Anal. Calcd for C36H24F6Ge2 (715.8462): C, 60.40; H, 3.38. Found: C, 58.42; H, 3.08. MALDI, m/z (rel %): 716 ([M]+, 100). Crystals suitable for X-ray analysis were obtained after recrystallization from chloroform. Hexa(p-(trifluoromethyl)phenyl)digermane, (p-F3CC6H4)3Ge−Ge(C6H4CF3-p)3 (4): white powder, yield 0.37 g (46%). 1H NMR (δ, ppm, CDCl3): 7.59, 7.34 (2d, 3JH−H = 7.7 Hz, each 12H, aromatic protons). 13 C NMR (δ, ppm, CDCl3): 139.56 (ipso-Ge), 135.44 (ortho-Ge), 132.21 (q, 2J13C‑19F = 32.6 Hz), 125.63 (q, 3J13C‑19F = 3.7 Hz) (aromatic carbons), 123.80 (q, 1J13C‑19F = 272.4 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.21 (s, 18F, 6 p-C6H4CF3). MS EI, m/z (rel %): 1016 ([M] +, 3), 509 ([(p-F3CC 6H4)3 Ge + H] +, 100), 490 ([(pF3CC6H4)3Ge + H − F]+, 38), 219 ([(p-F3CC6H4)Ge + H]+, 15). UV/vis (CH2Cl2), λmax nm (ε, M−1 cm−1): 239 (2.3 × 104). Anal. Calcd for C42H24F18Ge2 (1015.8912): C, 49.66; H, 2.38. Found: C, 49.12; H, 2.13. MALDI, m/z (rel %): 1016 ([M]+, 100). Crystals suitable for X-ray analysis were obtained after recrystallization from CH2Cl2/n-hexane. 2,2-Bis(pentafluorophenyl)-1,1,1,3,3,3-hexakis(p-(trifluoromethyl)phenyl)trigermane, (p-F3CC6H4)3Ge−Ge(C6F5)2-Ge(C6H4CF3p)3 (5). (C6F5)2GeH2 (0.16 g, 0.40 mmol) was added to a solution of (p-F3CC6H4)3GeNMe2 (0.44 g, 0.80 mmol) in MeCN (30 mL). The procedure of freezing in liquid nitrogen, evacuation, and warming to room temperature was repeated three times. The mixture obtained was heated at 100 °C for 46 h. Then all volatile materials were removed under reduced pressure, and the residue was extracted with CH2Cl2, filtered, and recrystallized from CH2Cl2/n-hexane. Compound 6 was isolated as orange crystals, yield 0.32 g (57%). 1H NMR (δ, ppm, CDCl3): 7.54, 7.30 (2d, 3JH−H = 7.6 Hz, each 12H, aromatic protons). 13C NMR (δ, ppm, CDCl3): 138.27 (ipso-Ge), 135.10 (ortho-Ge), 132.56 (q, 2J13C‑19F = 32.9 Hz), 125.40 (br s) (aromatic carbons), 123.59 (q, 1J13C‑19F = 273.0 Hz, CF3). Carbons of C6F5 were not found due to low intensity and the high value of nuclear coupling. 19 F NMR (δ, ppm, CDCl3): −63.31 (s, 18F, 6 p-C6H4CF3), −123.21 to (−123.26) (m, 4F, C6F5), −148.26 to (−148.34) (m, 2F, p-C6F5), −158.76 to (−158.92) (m, 4F, C6F5). MS EI, m/z (rel %): 509 ([(pF3CC6H4)3Ge + H]+, 100), 218 ([(p-F3CC6H4)Ge]+, 12), 167 ([C6F5]+, 9). UV/vis (CH2Cl2), λmax nm (ε, M−1 cm−1): 264 (2.4 × 104). Anal. Calcd for C54H24F28Ge3 (1422.6436): C, 45.59; H, 1.70. Found: C, 45.08; H, 1.82. MALDI, m/z (rel %): 1423 ([M]+, 100). Crystals suitable for X-ray analysis were obtained after recrystallization from CH2Cl2/n-octane. X-ray Crystallography. Experimental intensities were measured on a Bruker SMART APEX II (for 1a, 2b, 2c, and 3) and on a STADIVARI Pilatus (for 1b, 4, and 5) diffractometer using ω-scan mode. Absorption correction based on measurements of equivalent reflections was applied. The structures were solved by direct methods and refined by full matrix least-squares based on F2 with anisotropic thermal parameters for all non-hydrogen atoms. All aromatic hydrogen atoms were placed in calculated positions. In 2c, the germanium H atom was found from difference Fourier synthesis. All H atoms were refined using a riding model. In the structures 2b, 2c, 4, and 5 all −CF3 groups are rotationally disordered over two or three positions. They were refined with restrained C−F and F···F distances (SADI). In 3, all phenyl groups are rotationally disordered over two positions. Minor components of these disordered substituents were refined with constrained C···C bond lengths (AFIX 66). Details of X-ray studies are given in Tables S1 and S2 (Supporting Information). The crystals of 2c were pseudomerohedrally twinned (β = 90.063(1)°) with twin law 1 0 0 0 −1 0 0 0 −1 and domain ratio 0.635(4)/0.365(4). Crystal data are deposited in the Crystallographic Data Centre as supplementary publications under the CCDC numbers 1505504− 1505509. This information may be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Electrochemistry. Electrochemical measurements were carried out using an Autolab 302N potentiostat interfaced through Nova 2.0 software to a personal computer. Electrochemical measurements were performed in a glovebox under oxygen levels of less than 5 ppm using

reaction mixture was stirred for 6 h; then 2 M H2SO4 (40 mL) was added dropwise, the organic phase was separated, and the aqueous layer was extracted with ether (3 × 20 mL). Combined organic phases were dried over anhydrous Na2SO4. Then the solvent was removed under reduced pressure, and the residue was recrystallized from nhexane. Compound 2c (0.34 g, 76%) was isolated as a white powder. 1 H NMR (δ, ppm, CDCl3): 7.67, 7.64 (2d, each 6H, 3JH−H = 8.3 Hz, aromatic protons), 5.88 (s, 1H, GeH). 13C NMR (δ, ppm, CDCl3): 138.65 (ipso-Ge), 135.36 (ortho-Ge), 131.98 (q, 2J13C‑19F = 32.5 Hz), 125.25 (q, 3J13C‑19F = 3.7 Hz) (aromatic carbons), 122.96 (q, 1J13C‑19F = 273.0 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.17 (s, 9F, 3CF3). Anal. Calcd for C21H13F9Ge (508.9535): C, 49.56; H, 2.57. Found: C, 49.58; H, 2.48. Crystals suitable for X-ray analysis were obtained after recrystallization from n-hexane. Tris(p-(trifluoromethyl)phenyl)germanium Dimethylamide, (p-F3CC6H4)3GeNMe2 (2d). A solution of (p-F3CC6H4)3GeCl (2a) (2.86 g, 5.26 mmol) in toluene (15 mL) was added to a suspension of LiNMe2 (0.32 g, 6.30 mmol) in toluene (40 mL). The reaction mixture was stirred at room temperature for 4 d and then filtered. All volatile materials were removed under reduced pressure. Compound 2d (2.56 g, 88%) was isolated as a colorless oil that solidifies on standing at room temperature. 1H NMR (δ, ppm, C6D6): 7.43, 7.34 (2d, 3JH−H = 8.1 Hz, each 6H, aromatic protons), 2.52 (s, 6H, NMe2). 13 C NMR (δ, ppm, C6D6): 139.38 (ipso-Ge), 135.43 (ortho-Ge), 132.20 (q, 2J13H‑19F = 32.2 Hz), 125.34 (q, 3J13H‑19F = 3.7 Hz) (aromatic carbons), 124.72 (q, 1J13H‑19F = 272.3 Hz, CF3), 41.19 (NMe2). 19F NMR (δ, ppm, C6D6): −62.76 (s, 9F, 3CF3). Anal. Calcd for C23H18F9GeN (551.0213): C, 50.04; H, 3.29; N, 2.54. Found: C, 49.24; H, 3.03; N, 2.38. Bis(dimethylamido)dimethylgermane, Me 2 Ge(NMe 2 ) 2 . Me2GeCl2 (6.03 g, 35.00 mmol) was added dropwise to a suspension of LiNMe2 (3.90 g, 76.40 mmol) in ether (40 mL). The reaction mixture was stirred at room temperature for 20 h and then was refluxed for 5 h. The mixture was filtered, and the residue was carefully fractionized. Me2Ge(NMe2)2 (5.17 g, 78%) was isolated as a colorless liquid, bp 142−143 °C. 1H NMR (δ, ppm, C6D6): 2.54 (s, 12H, NMe2), 0.18 (s, 6H, Me2Ge). 13C NMR (δ, ppm, C6D6): 40.41 (NMe2), −5.24 (Me2Ge). Elemental analysis data are unsatisfactory due to high sensitivity of the compounds to the traces of moisture. Bis(dimethylamido)diphenylgermane, Ph2Ge(NMe2)2. At 0 °C a solution of Ph2GeCl2 (7.19 g, 24.15 mmol) in ether (20 mL) was added dropwise to a suspension of LiNMe2 (2.71 g, 53.00 mmol) in ether (40 mL). The reaction mixture was stirred at room temperature overnight and then was refluxed for 5 h. All volatile materials were removed under reduced pressure, toluene (40 mL) was added, and the suspension obtained was filtered. Then the solvent was removed under reduced pressure. Ph2Ge(NMe2)2 (6.67 g, 88%) was isolated as a colorless liquid. 1H NMR (δ, ppm, C6D6): 7.71−7.65 (m, 4H, aromatic protons), 7.24−7.16 (m, 6H, aromatic protons), 2.71 (s, 12H, NMe2). 13C NMR (δ, ppm, C6D6): 135.21, 135.01, 129.72, 128.56 (aromatic carbons), 40.87 (NMe2). Elemental analysis data are unsatisfactory due to high sensitivity of the compounds to the traces of moisture. Synthesis of Oligogermanes with Electron-Withdrawing Groups. General Procedure for Oligogermane Synthesis. Corresponding triarylgermane (0.80 mmol) was added to the solution of triarylgermanium dimethylamide (0.80 mmol) in MeCN (20 mL). The procedure of freezing in liquid nitrogen, evacuation, and warming to room temperature was repeated three times. The mixture obtained was heated at 100 °C for 86 h; then all volatile materials were removed under reduced pressure, and the residue was extracted with CH2Cl2, filtered, and recrystallized from CH2Cl2/n-hexane. Hexa(p-fluorophenyl)digermane, (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3): white powder, yield 0.32 g (56%). 1H NMR (δ, ppm, CDCl3): 7.18− 7.13, 7.02−6.96 (2m, each 12H, aromatic protons). 13C NMR (δ, ppm, CDCl3): 163.81 (d, 1J13C‑19F = 249.5 Hz), 136.93 (d, 3J13C‑19F = 7.6 Hz), 131.49 (d, 4J13C‑19F = 3.8 Hz), 115.95 (d, 2J13C‑19F = 19.8 Hz) (aromatic carbons). 19F NMR (δ, ppm, CDCl3): −111.16 to (−111.25) (m, 6F, 6 p-C6H4F). MS EI, m/z (rel %): 716 ([M]+, 12), 359 ([(p-FC6H4)3Ge + H]+, 100), 264 ([(p-FC6H4)2Ge + H]+, 307


Article

Organometallics solvent that had been purified by passing through an alumina-based purification system. Diamond-polished glassy carbon electrodes of 3 mm diameter were employed for cyclic voltammetry (CV) scans. CV data were evaluated using standard diagnostic criteria for diffusion control and for chemical and electrochemical reversibility. The experimental reference electrode was a silver wire coated with anodically deposited silver chloride and separated from the working solution by a fine glass frit. The electrochemical potentials in this paper are referenced to the ferrocene/ferrocenium couple, as recommended elsewhere.57 The ferrocene potential was obtained by its addition to the analyte solution58 at an appropriate time in the experiment. [NBu4][B(C6F5)4] was prepared as previously described.59

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00767. NMR spectra for the compounds synthesized in this work (Figures S1−S41); tables of crystallographic data (Tables S1, S2) for compounds 1a,b, 2b,c, 3, 4, and 5; electrochemistry CV curves (Figures S42−S44); computational details; calculated structures (Figures S45−S56) (PDF) Crystallographic data for compounds 1a,b, 2b,c, 3, 4, and 5 (CIF) Calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kirill V. Zaitsev: 0000-0003-3106-8692 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Russian President Grant for Young Russian Scientists (MK-1790.2014.3, K.V.Z.), in part by M.V. Lomonosov Moscow State University Program of Development (K.V.Z.), Nazarbayev University (ORAU grant for Medicinal Electrochemistry, K.L.), and the Ministry of Education and Science of Kazakhstan (K.L.). We are grateful to Dr. S.V. Gruener (MSU) for his valuable advice and assistance in obtaining the initial germanes.

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REFERENCES

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