Synthetic, Structural, and Physical Investigations of the Large Linear

Article pubs.acs.org/Organometallics

Synthetic, Structural, and Physical Investigations of the Large Linear and Branched Oligogermanes Ph3GeGePh2GePh2GePh2H, Ge5Ph12, and (Ph3Ge)4Ge Christian R. Samanamu,† Monika L. Amadoruge,† Aaron C. Schrick,† Chao Chen,‡ James A. Golen,‡,§ Arnold L. Rheingold,‡ Nicholas F. Materer,† and Charles S. Weinert*,† †

Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, United States Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093-0358, United States § Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, Massachusetts 02747, United States ‡

S Supporting Information *

ABSTRACT: The syntheses of two linear oligogermanes, Ph3GeGePh2GePh2GePh2H and Ge5Ph12, were achieved using a hydrogermolysis reaction starting with HPh2GeGePh2GePh2H. The preparation of the hydride-terminated tetragermane indicates that selectivity is possible using the hydrogermolysis reaction, which had not been observed previously. The structures of both of these compounds were determined, and they were also characterized by UV/visible spectroscopy and electrochemical methods (CV and DPV). The pentagermane Ge5Ph12 exhibits four irreversible oxidation waves in both its CV and DPV, as was observed for other aryl-substituted oligogermanes. The successful synthesis of the neopentane analogue (Ph3Ge)4Ge was also achieved by starting from GeH4 and Ph3GeCH2CN. This material was structurally characterized; the structure of (Ph3Ge)4Ge is highly sterically congested and contains long Ge−Ge single-bond distances that average 2.497(6) Å and exhibits an nearly idealized tetrahedral geometry at the central germanium atom with an average Ge−Ge−Ge bond angle of 109.49(2)°. The UV/ visible spectrum of (Ph3Ge)4Ge exhibits a broad absorbance maximum centered at 250 nm, and DFT calculations indicate that this compound has a stabilized HOMO at −6.223 eV and a large HOMO−LUMO gap relative to those in other branched oligogermanes.

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which occurs via overlap of the relatively large sp3-hybridized orbitals. In addition, these compounds also absorb in the UV region and are electrochemically active as a result of their σ delocalization. The observed UV/visible spectra result from electronic transitions from the bonding σ orbital to the antibonding σ* orbital, both of which are localized on the element−element backbone. In the case of germanium, the longest discrete linear molecule that has been reported is Ge10H22,46 while the longest linear molecule that has been crystallographically characterized is Ge5Ph12.47 Branched oligogermanes are somewhat uncommon, and the first crystallographically characterized

INTRODUCTION Catenated compounds of the group 14 elements can be regarded as heavy analogues of simple organic molecules, and both “saturated” species having element−element single bonds and “unsaturated” species having element−element multiple bonds have been realized for silicon,1−11 germanium,12−22 and tin.23−42 In addition, various branched and cage-type structures are known for these elements. However, while long-chain hydrocarbons, highly branched structures, and polycyclic species are very common for carbon catenates, these types of materials are less known for the heavier group 14 elements. Heavy group 14 singly bonded catenates are of interest since they possess inherent σ delocalization,43−45 where the bonding electrons situated in the highest occupied molecular orbital are delocalized over part or all of the element−element framework © 2012 American Chemical Society

Received: May 7, 2012 Published: May 31, 2012 4374

dx.doi.org/10.1021/om300385n | Organometallics 2012, 31, 4374−4385

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Scheme 1

resolved. However, the 13C NMR spectrum of 1 contains the expected 16 signals for each of the magnetically nonequivalent carbon atoms present in 1. The UV/visible spectrum of 1 exhibits a broad λmax at 269 nm (ε = 4.5 × 104 M−1 cm−1) that is blue-shifted in comparison to the λmax value for Ge4Ph10 (3) reported at 282 nm,55 due to the presence of a terminal −GePh2H group in 1 versus a −GePh3 group in 3. Crystals of 1 suitable for X-ray analysis were obtained; an ORTEP diagram of 1 is shown in Figure 1, and selected bond

example of one of these compounds was (Ph3Ge)3GePh, which was reported in 2008.48 Since then, the structures of (Ph3Ge)3GeH,49,50 (Ph3Ge)3GeX (X = Cl, Br, I),49,50 (Me3Ge)3GeGe(GeMe3)3,51 (Me3Ge)3GeGeMe2Ge(GeMe3)3,51 and the salt complex K[(18-crown-6)2][(Me2Ge)2GeGe(GeMe2)2]51 have been described. We now wish to report the synthesis and characterization of the linear oligogermanes Ph3GeGePh2GePh2GePh2H (1) and Ge5Ph12 (2), which were obtained by the hydrogermolysis reaction. Compound 1 is a rare example of a structurally characterized hydride-terminated oligogermane. In addition, we also have obtained the highly sterically congested (Ph3Ge)4Ge (7), which is the first structurally characterized example of a germanium analogue of neopentane. This compound was thought to be an “impossible molecule” due to the steric environment at the central germanium atom, and our previous attempts to synthesize this molecule were not successful.50 In addition, density functional theory (DFT) calculations on this species did not converge,52 also leading to our postulating that the synthesis of 7 might not be possible. The synthesis and full characterization of 7 are also described herein.

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RESULTS AND DISCUSSION The hydride-terminated tetragermane Ph3GeGePh2GePh2GePh2H (1) was prepared from HGePh2GePh2GePh2H and Ph3GeNMe2 (Scheme 1). The starting oligogermane HGePh2GePh2GePh2H was obtained by the method of Satgé et al. by treatment of Ph2GeH2 with ButLi in Et3N solvent with a reaction time of 9.5 h.53 Curiously, the reaction of HGePh2GePh2GePh2H with 1 equiv of Ph3GeNMe2 in CH3CN solvent produced only the tetragermane 1 in 79% yield, with no evidence for the formation of the pentagermane Ge5Ph10 (2). This finding is interesting, since we have not previously observed a selective reaction of one terminal hydride site in an oligogermane with an incoming germanium α-germyl nitrile which is formed via the in situ reaction of a germanium amide with the CH3CN solvent. The digermane HPh2GeGePh2H reacts with Ph3GeNMe2 or Tol3GeNMe2 (Tol = p-tolyl) to provide the tetragermanes Ge4Ph10 (3) and Tol3GeGePh2GePh2GeTol3 (4), with no evidence for the formation of the hydrideterminated trigermane Ar3GeGePh2GePh2H.54 The 1H NMR spectrum of 1 contains a singlet at δ 5.67 ppm corresponding to the terminal hydrogen atom, and several of the signals assigned to the aromatic hydrogen atoms of the phenyl rings are not

Figure 1. ORTEP plot of Ph3GeGePh2GePh2GePh2H (1). Thermal ellipsoids are drawn at the 50% probability level.

distances and angles are collected in Table 1. Compound 1 is an unusual example of a catenated germanium compound having a refined Ge−H distance that measures 1.50(4) Å, which is similar to reported distances in monomeric germanium hydride compounds.56−59 The three Ge−Ge distances in 1 average 2.4531(6) Å, and the average Ge−Ge−Ge bond angle is 114.88(2)°. The bond angle at Ge(2) measuring 110.23(2)° is substantially more acute than that at Ge(3), which is 119.53(2)°, due to the presence of the terminal hydrogen atom bound to Ge(1) versus the third phenyl ring attached to Ge(4). The structure of 1 can be compared to those of the tetragermanes Ge4Ph10·2C6H6 (3·2C6H6)55 and Tol3GeGePh2GePh2GeTol3 4375


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Table 1. Selected Bond Distances (Å) and Angles (deg) for Ph3GeGePh2GePh2GePh2H (1) Ge(1)−Ge(2) Ge(2)−Ge(3) Ge(3)−Ge(4) Ge(1)−H(1) Ge(1)−C(1) Ge(1)−C(7) Ge(2)−C(13) Ge(1)−Ge(2)−Ge(3) Ge(2)−Ge(1)−H(1) Ge(2)−Ge(3)−Ge(4) C(1)−Ge(1)−C(7) C(1)−Ge(1)−H(1) C(7)−Ge(1)−H(1)

2.4466(5) 2.4577(6) 2.4551(5) 1.50(4) 1.956(3) 1.962(4) 1.969(3) 110.23(2) 107(1) 119.53(2) 106.0(1) 104(1) 108(1)

Ge(2)−C(49) Ge(3)−C(37) Ge(3)−C(43) Ge(4)−C(19) Ge(4)−C(25) Ge(4)−C(31) C(13)−Ge(2)−C(49) C(37)−Ge(3)−C(43) C(19)−Ge(4)−C(25) C(19)−Ge(4)−C(31) C(25)−Ge(4)−C(31)

1.962(3) 1.964(3) 1.969(3) 1.957(3) 1.961(3) 1.967(4) 108.2(1) 110.1(2) 107.3(1) 106.3(2) 107.5(1)

Figure 2. ORTEP plot of Ge5Ph12 (2). Thermal ellipsoids are drawn at the 50% probability level.

(4; Tol = p-CH3C6H4),60 which are both C2 symmetric. 3·2C6H6 has an average Ge−Ge bond distance of 2.462(3) Å and a Ge−Ge−Ge bond angle of 117.8(1)°, while in 4 the corresponding values are 2.454(4) Å and 117.2(2)°. The terminal Ge(1)−Ge(2) bond in 1 is shorter than the corresponding distance in 3·2C6H655 by 0.016 Å, and the Ge− Ge−Ge bond angle centered at Ge(2) in 1 is more acute than the corresponding angle in 3·2C6H6 by 7.8°. The average Ge−Ge bond length in 460 is only slightly shorter than that in 1, while the average Ge−Ge−Ge bond angle in 1 is more acute than the average angle in 4 by 3.0°. The shorter Ge−Ge bond distances and more acute Ge−Ge−Ge bond angles in 1 result from the the presence of a terminal hydrogen atom in 1, which lowers the overall steric congestion in the molecule and therefore results in the different metric parameters in 1 versus those in 3·2C6H6 and 4. The reaction of 1 with another 1 equiv of Ph3GeNMe2 yielded the pentagermane Ge5Ph12 (2) in 83% yield, and compound 2 could also be obtained directly from HPh2GeGePh2GePh2H using 2 equiv of Ph3GeNMe2 in 85% yield. The 1H and 13C NMR data for 2 agree with the data previously reported in the literature,47 where 12 resonances were observed in the 13C NMR spectrum of 2. The UV/visible spectrum of 2 exhibits λmax at 295 nm (ε = 4.5 × 104 M−1 cm−1), which is close to the previously reported value of 293 nm.47 As expected, the absorbance maximum of 2 is red-shifted relative to that for 1, due to the increase in catenation in the pentagermane. Crystals of 2 suitable for X-ray analysis were obtained via slow cooling of a refluxing benzene solution, and an ORTEP diagram of 2 is shown in Figure 2, while selected bond distances and angles are collected in Table 2. The structure of 2 was previously determined at room temperature,47 while the present data for 2 were collected at 100 K. Therefore, the bond distances and angles shown in Table 2 differ from the previously reported values. The average Ge−Ge bond distance in 2 is 2.4502(6) Å, and the average Ge−Ge−Ge bond angle is 115.52(2)°, which are nearly identical with those of the previously reported structure.47 An interesting aspect of the structure of 2 that was mentioned previously but described in a different fashion is the fact that the four atoms Ge(2)−Ge(3)− Ge(4)−Ge(5) are coplanar, while the fifth germanium atom Ge(1) is canted out of the plane, as shown in Figure 2. The P212121 space group is noncentrosymmetric, and the disposition of the Ge(1) atom in 2 clearly implies that the molecule as pictured in Figure 2 must be present, along with a second species that has the out-of-plane germanium atom canted in the opposite direction. Furthermore, this out-of-plane

Table 2. Selected Bond Distances (Å) and Angles (deg) for Ph3GeGePh2GePh2GePh2GePh3 (2) Ge(1)−Ge(2) Ge(2)−Ge(3) Ge(3)−Ge(4) Ge(4)−Ge(5) Ge(1)−C(55) Ge(1)−C(61) Ge(1)−C(67) Ge(2)−C(43) Ge(1)−Ge(2)−Ge(3) Ge(2)−Ge(3)−Ge(4) Ge(3)−Ge(4)−Ge(5) C(55)−Ge(1)−C(61) C(55)−Ge(1)−C(67) C(61)−Ge(1)−C(67)

2.4359(6) 2.4701(6) 2.4549(6) 2.4397(6) 1.941(4) 1.964(4) 1.957(4) 1.960(4) 116.70(2) 114.25(2) 115.61(2) 106.8(2) 110.5(2) 107.9(2)

Ge(2)−C(49) Ge(3)−C(1) Ge(3)−C(37) Ge(4)−C(25) Ge(4)−C(31) Ge(5)−C(7) Ge(5)−C(13) Ge(5)−C(19) C(43)−Ge(2)−C(49) C(1)−Ge(3)−C(37) C(25)−Ge(4)−C(31) C(7)−Ge(5)−C(13) C(7)−Ge(5)−C(19) C(13)−Ge(5)−C(19)

1.946(4) 1.964(4) 1.962(4) 1.982(4) 1.963(4) 1.965(4) 1.953(4) 1.957(4) 106.2(2) 107.1(2) 107.2(2) 105.5(2) 109.2(2) 110.0(2)

orientation must occur in a 50/50 left/right distribution but the other molecule differing from the one shown in Figure 2 was not found, indicating that the two individual species must spontaneously resolve. We have shown that aryl-substituted linear oligogermanes GenAr2n+2 (Ar = Ph, p-CH3C6H4) typically exhibit n − 1 irreversible oxidation waves in their cyclic voltammograms,60 and the same number of features is also typically observed in their differential pulse voltammograms (DPV). The CV of 1 exhibits two waves at 1690 and 1946 mV, but the expected third oxidation wave was not observed. Only two waves were observed in the DPV of 1 at 1710 and 1960 mV as well, despite the higher resolution of this method versus that of CV (Figure 3). Presumably the product present after the second oxidation event is unstable and decomposes rapidly, which could be due to the presence of the hydride at the terminal germanium atom. Compound 1 is slightly more difficult to oxidize than Ge4Ph10 (3), as the first oxidation wave for 3 was observed at 1644 mV.60 In contrast to 1, the CV and DPV for compound 2 follow the expected trend and are shown in Figure 4. The CV exhibits four waves at 1385, 1605, 1772, and 1998 mV, and the first oxidation wave is at a lower potential than the corresponding first oxidation for Ge4Ph10 (1644 mV),60 which is expected with an increase in catenation of one −GePh2− unit in 2. The DPV for 2 also exhibits four oxidation waves at 1290, 1510, 1680, and 1890 mV that are slightly shifted to potentials lower than 4376


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Figure 3. Differential pulse voltammogram of Ph3GeGePh2GePh2GePh2H (1) (pulse period 0.1 s, pulse width 0.05 s, sample time 0.02 s). Note that the weak features at ca. 1250 and 1350 mV are due to background noise.

had been reported earlier by Glockling et al.62,63 However, the four −GeMe3 groups attached to the central germanium atom in this species are significantly less sterically encumbering than four −GePh3 groups. In addition, the tin analogue of 7 (Ph3Sn)4Sn (9) had been prepared and structurally characterized,64 but in this case the larger covalent radius of tin (1.40 Å) versus that of germanium (1.22 Å) also allows the more facile accommodation of four triphenyl-substituted peripheral groups. Thus, we assumed that the synthesis of compound 7 might not be possible due to steric effects. The progress of the hydrogermolysis reaction has been shown to be hampered by sterically demanding substituents, as the digermane But3GeGePh3 could not prepared by this method, and only But3Ge[NHC(CH3)CHCN] was isolated.54 However, it now appears that this assumption was somewhat premature, since we have now prepared 7 via a hydrogermolysis reaction and have obtained its X-ray crystal structure. The first suggestion that the preparation of 7 was in fact possible was indicated by the outcome of the reaction of GeH4 with a large excess (ca. 20 equiv) of Ph3GeNMe2 in CH3CN. The major product isolated from this reaction was (Ph3Ge)3GeH (6) in 73% yield, and a significant amount of Ph3GeCH2CN (10) was also left over, which could be removed via distillation using a Kugelrohr oven. Compound 10 crystallized in the receiving flask, and an X-ray crystal structure of this material was obtained. An ORTEP diagram of 10 is shown in Figure 5, and selected bond distances and angles are collected in Table 3. The Ge−C distance in the bond to the α-carbon is 1.982(4) Å and is similar to those in two other crystallographically characterized α-germyl nitriles, [Mes*PC]GeBut(Tip)CH2CN (11)65 and [(Me3Si)2CH]2Ge(H)CH2CN (12),66 which measure 2.004(2) and 1.911(9) Å, respectively. The −CH2CN ligand is nearly linear, as shown by the C(19)− C(20)−N(1) bond angle, which is 178.9(5)°, and the Ge(1)− C(19)−C(20) bond angle is 116.6(3)°, which is similar to the

those found in the CV due to the suppression of the charging current using this technique. As we postulated for other arylsubstituted oligogermanes,60 the pentagermane 2 likely undergoes four consecutive germylene extrusions to yield cationic tetra-, tri-, and digermane species during the course of the sweep. In addition to the preparation of linear oligogermanes, we have employed the hydrogermolysis reaction for the successful preparation and structural characterization of several branched oligogermanes, including (Ph3Ge)3GePh (5),48 (Ph3Ge)3GeH (6),49,50 and (Ph3Ge)3GeX (X = Cl, Br, I).49,50 We were interested in the preparation of the germanium neopentyl analogue (Ph3Ge)4Ge (7) and initially attempted its synthesis via treatment of GeH4 with 4 equiv of Ph3GeNMe2 in CH3CN solvent. However, this method yielded only the compound 6. We then attempted to prepare 7 by the two methods illustrated in Scheme 2, but neither of these methods were successful.49,50 The route starting from 6 only furnished unreacted 6 and Ph3GeCH2CN even if a prolonged reaction time (ca. 5 days) or excess Ph3GeNMe2 was used in the reaction. The branched amide (Ph3Ge)3GeNMe2 was obtained in two steps from 6,49,50 but the reaction of (Ph3Ge)3GeNMe2 with Ph3GeH also did not provide 7. Instead, we isolated 3-aminocrotononitrile and 2,6-dimethylpyrimidine, which are oligomers of acetonitrile, and identified Ph3GeGePh3 as a product of the reaction.49,50 Several other unidentified germanium-containing products were also generated, suggesting that (Ph3Ge)3GeNMe2 had decomposed during the course of the reaction. We subsequently demonstrated that that Ph3GeCH2CN could be converted to Ph3Ge[NHC(CH3)CHCN] and 2,6-dimethyl-4(triphenylgermylamino)pyrimidine upon prolonged heating of Ph3GeCH2CN in CH3CN in the presence of HNMe2,61 which explains how the aforementioned products were formed. The methyl-substituted neopentane analogue (Me3Ge)4Ge (8) was prepared in 40% yield by Marschner et al.51 and also 4377


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Figure 4. Cyclic voltammogram (bottom; scan rate 100 mV/s) and differential pulse voltammogram (top; pulse period 0.1 s, pulse width 0.05 s, sample time 0.02 s) for Ge5Ph12 (2).

disposition of the −CH2CN ligand in the germanium complexes 11 and 12. In these compounds, the C−C−N bond angles are 179.4(2)° (11) and 117(1)° (12), while the Ge−C−C bond angles are 113.5(1)° (11) and 115.2(7)° (12). The mixture of 6 and 10 also was contaminated with a trace amount of a second phenyl-containing product that we suspected was (Ph3Ge)4Ge (7). The presence of a large excess of Ph3GeNMe2 in the aforementioned reaction prompted us to attempt the synthesis of 7 as shown in Scheme 3. The key

feature of the successful preparation of 7 is the use of 10 itself as the reactant rather than Ph3GeNMe2. We have shown that Ph3GeNMe2 is completely converted to 10 within ca. 6 h in CH3CN solvent,54 and therefore a large excess of 10 would be present relative to GeH4 if ca. 20 equiv of Ph3GeNMe2 was employed as described above. However, the reaction shown in Scheme 3 is still very sluggish, and a prolonged reaction time of 5 days was necessary for 7 to be formed in an appreciable amount (32% yield). Curiously, the reaction of 6 4378


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Scheme 2

respectively, while the 13C NMR of this compound has four features at δ 139.8, 137.3, 128.5, and 127.8 ppm corresponding to the ipso, ortho, para, and meta carbon atoms of the phenyl rings, respectively. The 13C NMR data match very well with the chemical shift values predicted by Dräger et al. in 1987.67 Crystals of 7 suitable for X-ray analysis were obtained from a benzene/hexane solution; an ORTEP diagram of compound 7 is shown in Figure 6, and selected bond distances and angles are collected in Table 4. Compound 7 crystallizes with two independent molecules in the unit cell, which also incorporates a molecule of hexane that is disordered. The average Ge−Ge bond distance among the two molecules of 7 is 2.497(6) Å, which is longer than those in (Ph3Ge)3GeH (6) and (Ph3Ge)3GePh (5) that measure 2.4310(5) and 2.469(4) Å, respectively. This is expected, given the steric congestion at the central germanium atom in 7 resulting from the attachment of a fourth Ph3Ge− group to the central germanium atom. In fact, three of the Ge−Ge bond distances among the two molecules of 7 are longer than 2.50 Å. The Ge(1)−Ge(3) and Ge(1)− Ge(4) bonds in molecule 1 measure 2.5042(6) and 2.5136(6) Å, respectively, while the Ge(6)−Ge(7) and Ge(6)−Ge(7) bonds both measure 2.5136(6) Å. These bond distances are shorter than those found in the congested cyclotrigermanes Mes6Ge3 and But6Ge3, which average 2.537(2)68 and 2.563(1) Å,69 respectively. The Ge−Ge−Ge bond angles in 7 have an overall average value of 109.49(2)°, which matches the idealized tetrahedral bond angle exactly. It is likely that this is a necessary geometric requirement for the formation of the four Ge−Ge bonds that incorporate four −GePh3 moieties around the central germanium atom. This differs from the average bond angles about the central germanium atom in 6 and 5, which are 115.50(2)49,50 and 112.72(1)°,48 respectively. The most obtuse angle averaged between the two molecules of 8 is 113.28(2)°, while in 5 the most obtuse angle measures 115.70(1)° and in 6 it is 117.89(2)°. The degree of steric congestion in 7 can be seen in the space-filling diagram shown in Figure 7. The Ge5 framework in 7 is almost completely encapsulated by the peripheral phenyl rings, and the rings are oriented such that they adopt a twisted conformation. Compound 7 was further characterized using UV/visible spectroscopy and cyclic and differential pulse voltammetry, and

Figure 5. ORTEP diagram of Ph3GeCH2CN (9). Thermal ellipsoids are drawn at the 50% probability level.

Table 3. Selected Bond Distances (Å) and Angles (deg) for Ph3GeCH2CN (10) Ge(1)−C(1) Ge(1)−C(7) Ge(1)−C(13) C(1)−Ge(1)−C(7) C(1)−Ge(1)−C(13) C(7)−Ge(1)−C(13) C(1)−Ge(1)−C(19)

1.940(4) 1.929(4) 1.944(4) 110.5(2) 111.9(2) 111.5(2) 102.4(2)

Ge(1)−C(19) C(19)−C(20) C(20)−N(1) C(7)−Ge(1)−C(19) C(13)−Ge(1)−C(19) Ge(1)−C(19)−C(20) C(19)−C(20)−N(1)

1.982(4) 1.450(6) 1.149(5) 108.9(2) 111.3(2) 116.6(3) 178.9(5)

with either a stoichiometric amount or an excess of 10 lead only to the recovery of 6 without formation of any detectable amount of 7, even when a 5 day reaction time was employed. The 1H NMR of 7 exhibits resonances at δ 7.65, 7.11, and 7.04 ppm corresponding to the ortho, para, and meta protons, 4379


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Scheme 3

Figure 6. ORTEP diagram of the two crystallographically independent molecules of (Ph3Ge)4Ge·C6H14 (7·C6H14). Thermal ellipsoids are drawn at the 50% probability level.

HOMO among the phenyl-substituted species 5−7, resulting from the effective increase in catenation relative to that in the tetragermanes 5 and 6. This results in a larger HOMO−LUMO gap for 7 compared to those in 5 and 6, and thus the calculated energy of 220 nm for 7 is blue-shifted relative to the corresponding energies for 5 and 6. Time-dependent DFT calculations reveal the presence of three primary transitions in the UV region from the HOMO, HOMO- 1, and HOMO-2 to the LUMO, where the three HOMO levels are approximately degenerate in energy and each contains a different p orbital on the central germanium atom. The calculated λmax with the highest oscillator strength in 7 is 245 nm, and the observed UV/visible absorbance maximum for 7 is a broad band centered at 250 nm (ε = 1.59 × 10 4 M−1 cm−1) that tails to ca. 325 nm. The CV of 7 exhibits a single irreversible oxidation wave at 1954 ± 11 mV, while the DPV of 7 contains a single wave at 1923 ± 7 mV. Therefore, 7 is more difficult to oxidize than the other phenyl-substituted tetragermanes 5 and 6 but is easier to oxidize than the methyl-substituted species 8. These data are consistent with the calculated HOMO energies of these species, as the HOMO of 7 is stabilized relative to those of 5 and 6 but destabilized relative to the HOMO of 8. Therefore, the experimental and theoretical data for 7 are in excellent agreement.

its structure was computed using density functional theory (DFT) calculations. The calculated structure of 7 resembles the crystallograhically determined structure, having an encapsulated central germanium atom and a twisted disposition of the phenyl substituents. The structure is more symmetrical than the closely packed crystal structure and has significant interaction between the phenyl rings, which are rotationally staggered around each of the four peripheral germanium atoms. The peripheral germanium atoms are essentially neutral, and there is a positive charge on the central germanium atom. The Ge−C bond distances are identical with those in the X-ray structure. However, the calculated Ge−Ge bond distances are shorter than those in the X-ray structure of 7 by an average of 0.06 Å and measure 2.44 Å. Figures depicting the HOMO and LUMO of 7 are shown in Figure 8. The HOMO in 7 is composed primarily of the p orbital on the central germanium atom, while the LUMO contains p orbital density on the four peripheral germanium atoms as well as a contribution from the 4s orbital on the central germanium atom. The HOMO of 7 lies at −6.220 eV, and the LUMO is at −0.571 eV; thus, the HOMO− LUMO gap is 5.653 eV (220 nm). The energy values for the frontier orbitals of 7, as well as those calculated for (Ph3Ge)3GePh (5),50 (Ph3Ge)3GeH (6),50 and (Me3Ge)4Ge (8),70 are collected in Table 5. The methyl-substituted compound 8 has the lowest-lying HOMO and the most destabilized LUMO among these four oligogermanes. Compound 7 has the most highly stabilized 4380


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Table 4. Selected Bond Distances (Å) and Angles (deg) for (Ph3Ge)4Ge·C6H14 (7·C6H14) molecule 1

molecule 2

molecule 1

molecule 2

average Ge(1)−Ge(2) Ge(1)−Ge(3) Ge(1)−Ge(4) Ge(1)−Ge(5) Ge(2)−C(1) Ge(2)−C(7) Ge(2)−C(13) Ge(3)−C(19) Ge(3)−C(25) Ge(3)−C(31) Ge(4)−C(37) Ge(4)−C(43) Ge(4)−C(49) Ge(5)−C(55) Ge(5)−C(61) Ge(5)−C(67) Ge(2)−Ge(1)−Ge(3) Ge(2)−Ge(1)−Ge(4) Ge(2)−Ge(1)−Ge(5) Ge(3)−Ge(1)−Ge(4) Ge(3)−Ge(1)−Ge(5) Ge(4)−Ge(1)−Ge(5) C(1)−Ge(2)−C(7) C(1)−Ge(2)−C(13)

2.4948(6) 2.4945(6) 2.5042(6) 2.5136(6) 1.972(4) 1.961(4) 1.970(4) 1.956(4) 1.950(4) 1.956(4) 1.954(4) 1.967(4) 1.962(4) 1.963(4) 1.974(4) 1.962(4) 108.34(2) 113.90(2) 105.08(2) 105.16(2) 113.50(2) 111.04(2) 107.6(2) 102.7(2)

Ge(6)−Ge(7) Ge(6)−Ge(8) Ge(6)−Ge(9) Ge(6)−Ge(10) Ge(7)−C(73) Ge(7)−C(79) Ge(7)−C(85) Ge(8)−C(91) Ge(8)−C(97) Ge(8)−C(103) Ge(9)−C(109) Ge(9)−C(115) Ge(9)−C(121) Ge(10)−C(127) Ge(10)−C(133) Ge(10)−C(139) Ge(7)−Ge(6)−Ge(8) Ge(7)−Ge(6)−Ge(9) Ge(7)−Ge(6)−Ge(10) Ge(8)−Ge(6)−Ge(9) Ge(8)−Ge(6)−Ge(10) Ge(9)−Ge(6)−Ge(10) C(73)−Ge(7)−C(79) C(73)−Ge(7)−C(85)

2.5136(6) 2.4842(6) 2.4957(6) 2.4977(6) 1.966(4) 1.959(4) 1.968(4) 1.978(4) 1.956(4) 1.954(4) 1.965(4) 1.953(4) 1.971(4) 1.961(4) 1.961(4) 1.963(4) 106.04(2) 112.65(2) 111.69(2) 111.88(2) 108.74(2) 105.87(2) 109.6(2) 103.4(2)

average

2.5042(6) 2.4894(6) 2.4910(6) 2.5057(6) 1.968(4) 1.960(4) 1.969(4) 1.967(4) 1.953(4) 1.955(4) 1.960(4) 1.960(4) 1.967(4) 1.962(4) 1.968(4) 1.963(4) 107.19(2) 113.28(2) 108.39(2) 108.52(2) 111.12(2) 108.46(2) 108.6(2) 103.1(2)

C(7)−Ge(2)−C(13) C(19)−Ge(3)−C(25) C(19)−Ge(3)−C(31) C(25)−Ge(3)−C(31)

107.4(2) 106.9(2) 108.6(2) 106.3(2)

C(79)−Ge(7)−C(85) C(91)−Ge(8)−C(97) C(91)−Ge(8)−C(103) C(97)−Ge(8)−C(103)

106.0(2) 106.5(2) 104.9(2) 107.9(2)

106.7(2) 106.7(2) 106.8(2) 107.1(2)

C(37)−Ge(4)−C(43) 106.9(2) C(109)−Ge(9)−C(115) 106.0(2) 106.5(2) C(37)−Ge(4)−C(49) 107.3(2) C(109)−Ge(9)−C(121) 106.0(2) 106.7(2) C(43)−Ge(4)−C(49) 105.4(2) C(115)−Ge(9)−C(121) 104.7(2) 105.1(2) C(55)−Ge(5)−C(61) 105.6(2) C(127)−Ge(10)−C(133) 105.0(2) 105.3(2) C(55)−Ge(5)−C(67) 101.2(2) C(127)−Ge(10)−C(139) 105.5(2) 103.4(2) C(61)−Ge(5)−C(67) 108.2(2) C(133)−Ge(10)−C(139) 106.7(2) 107.5(2) C(1)−Ge(2)−Ge(1) 111.1(1) C(73)−Ge(7)−Ge(6)

114.0(1) 112.6(1)

C(7)−Ge(2)−Ge(1) 110.2(1) C(79)−Ge(7)−Ge(6)

110.8(1) 110.5(1)

C(13)−Ge(2)−Ge(1) 117.3(1) C(85)−Ge(7)−Ge(6)

112.5(1) 114.9(1)

C(19)−Ge(3)−Ge(1) 113.6(1) C(91)−Ge(8)−Ge(6)

116.7(1) 115.2(1)

C(25)−Ge(3)−Ge(1) 113.7(1) C(97)−Ge(8)−Ge(6)

108.0(1) 110.9(1)

C(31)−Ge(3)−Ge(1) 107.4(1) C(103)−Ge(8)−Ge(6)

112.5(1) 110.0(1)

C(37)−Ge(4)−Ge(1) 108.9(1) C(109)−Ge(9)−Ge(6)

112.2(1) 110.6(1)

C(43)−Ge(4)−Ge(1) 116.1(1) C(115)−Ge(9)−Ge(6)

115.4(1) 115.8(1)

C(49)−Ge(4)−Ge(1) 111.8 (1) C(121)−Ge(9)−Ge(6)

111.8(1) 111.8(1)

C(55)−Ge(5)−Ge(1) 116.3(1) C(127)−Ge(10)−Ge(6) 114.6(1) 115.5(1) C(61)−Ge(5)−Ge(1) 111.9(1) C(133)−Ge(10)−Ge(6) 109.6(1) 110.8(1) C(67)−Ge(5)−Ge(1) 112.8(1) C(139)−Ge(10)−Ge(6) 114.7(1) 113.8(1)

Figure 8. Calculated HOMO and LUMO molecular orbitals for 7.

Four of the five germanium atoms in 2 are coplanar, while the fifth atom is canted out of the plane. Since 2 crystallizes in a noncentrosymmetric space group, the experimentally determined structure for 2 contains a single conformation of 2 that is present via spontaneous resolution with the second conformation that must also be present. The CV and DPV of 2 exhibit the expected pattern for aryl-substituted oligogermanes, each having four irreversible oxidation waves that correspond to the expected n − 1 pattern for oligogermanes of the formula GenAr2n+2.60 However, the CV and DPV for 1 exhibit only two waves rather than the expected three, suggesting that the terminal hydride in 1 imparts instability to the species generated after the second oxidation event takes place. The synthesis of the germanium neopentane analogue (Ph3Ge)4Ge (7) was also achieved via a hydrogermolysis reaction starting with GeH4 and the α-germyl nitrile Ph3GeCH2CN (10). The successful preparation of 7 seemed unlikely, since we had previously shown that 7 could be synthesized neither by the reaction of (Ph3Ge)3GeH (6) with Ph3GeNMe2 nor via reaction of (Ph3Ge)3GeNMe2 with Ph3GeH.49,50 Furthermore, treatment of GeH4 with 4 equiv of Ph3GeNMe2 yielded

Figure 7. Space-filling diagram of (Ph3Ge)4Ge·C6H14 (7·C6H14).

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CONCLUSIONS The hydride-terminated tetragermane Ph3GeGePh2GePh2GePh2H (1) was prepared via the selective reaction of 1 equiv of Ph3GeNMe2 with HPh2GeGePh2GePh2H and was converted to the pentagermane Ge5Ph12 (2) by reaction with a another 1 equiv of Ph3GeNMe2. Compound 2 could also be obtained by treatment of HPh2GeGePh2GePh2H with 2 equiv of Ph3GeNMe2. The crystal structures of 1 and 2 have been determined, and the Ge−H bond distance in 1 measures 1.50(4) Å. The Ge−Ge bond distance between the −GePh2H group and its neighboring germanium atom was found to be shorter than that in the perphenyl-substituted tetragermane Ge4Ph10 (3), and the Ge−Ge−Ge bond angle at the germanium atom adjacent to the −GePh2H moiety in 1 is also more acute than that in 3. The structure of 2 was also determined and found to be similar to the previously reported structure by Dräger et al.47 4381


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Table 5. Theoretical and Experimental Data for Compounds 5−8 compd

HOMO (eV)

LUMO (eV)

HOMO−LUMO gap (eV)

HOMO−LUMO gap (nm)

λmax(calcd) (nm)

λmax (nm)

(Ph3Ge)3GePh (5) (Ph3Ge)3GeH (6) (Ph3Ge)4Ge (7) (Me3Ge)4Ge (8)

−5.907 −6.003 −6.223 −6.529

−0.593 −0.576 −0.570 −0.527

5.314 5.427 5.653 6.002

233 228 220 207

262 253 245 206

256 251 250 212

Eox (mV) 1435 1921 1954 2090

± ± ± ±

14 8 11 2

Table 6. Crystallographic Data for Compounds 1, 2, 7·C6H14, and 10 1 empirical formula formula wt temp (K) wavelength (Å) cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρ (g cm−3) abs coeff (mm−1) F(000) cryst size (mm3) θ range for data collection (deg) index ranges

no. of rflns collected no. of indep rflns (Rint) completeness to θ = 25.00° (%) abs cor max and min transmission refinement method no. of data/restraints/params goodness of fit on F2 final R indices (I < 2σ(I)) R1 wR2 final R indices (all data) R1 wR2 largest diff peak and hole (e Å−3)

7

2

10

C72H60Ge5 C150H134Ge10 C54H46Ge4 985.27 1288.15 2662.47 123(2) 100(2) 100(2) 0.71073 0.71073 0.71073 monoclinic orthorhombic monoclinic P21/c P212121 P21/n 13.193(1) 14.303(1) 13.3210(6) 17.557(2) 19.159(2) 49.968(2) 19.868(2) 21.398(2) 18.4106(8) 90 90 90 105.178(1) 90 90.028(2) 90 90 90 4441.6(8) 5863.5(9) 12254(1) 4 4 4 1.473 1.459 1.443 2.717 2.576 2.468 1992 2608 5416 0.24 × 0.23 × 0.20 0.32 × 0.30 × 0.28 0.15 × 0.10 × 0.10 1.57−25.38 3.19−25.42 2.25−26.83 −15 ≤ h ≤ 15 −17 ≤ h ≤ 16 −16 ≤ h ≤ 14 −20 ≤ k ≤ 21 −23 ≤ k ≤ 16 −63 ≤ k ≤ 63 −23 ≤ l ≤ 23 −25 ≤ l ≤ 25 −23 ≤ l ≤ 23 36 823 35 345 153 538 8125 (0.0585) 10 770 (0.0441) 25 464 (0.0918) 100.0 99.5 98.7 multiscan multiscan multilayer/SADABS 0.6795 and 0.5654 0.762 and 0.683 0.7904 and 0.7085 full-matrix least squares on F2 full-matrix least squares on F2 full-matrix least squares on F2 8125/0/527 10 770/0/694 25 464/10/1425 1.002 1.060 1.033

C20H17GeN 343.96 100(2) 0.71073 triclinic P1̅ 9.412(2) 9.440(2) 9.779(2) 92.021(3) 108.696(3) 98.341(3) 811.2(3) 2 1.408 1.844 352 0.18 × 0.15 × 0.10 2.19−28.36 −12 ≤ h ≤ 12 −12 ≤ k ≤ 12 −12 ≤ l ≤ 12 9828 3695 (0.0862) 99.8 semiempirical from equivalents 0.8340 and 0.7280 full-matrix least squares on F2 3695/0/199 1.000

0.0340 0.0748

0.0320 0.0847

0.0482 0.1068

0.0545 0.1102

0.0540 0.0841 0.486 and −0.630

0.0350 0.0861 1.522 and −0.557

0.0801 0.1256 1.225 and −0.803

0.0831 0.1249 0.971 and −0.879

The UV/visible spectrum of 7 exhibits λmax at 250 nm, and a single oxidation wave was observed for 7 at 1923 mV in its CV; these are consistent with the energies of the HOMO and LUMO in 7 computed by DFT calculations. These data indicate that the HOMO in 7 is slightly lower in energy than those of the other phenyl-substituted oligogermanes (Ph3Ge)3GePh (5) and (Ph3Ge)3GeH (6) by an average of −0.283 eV but is higher in energy than the methyl-substituted neopentane analogue (Me3Ge)4Ge (8) by 0.340 eV. As a result, 7 is also more difficult to oxidize than 5 and 6 but is easier to oxidize than 5. The findings reported here represent some significant advances in the synthesis of oligogermanes via the hydrogermolysis reaction. The selectivity observed in the reaction of Ph3GeNMe2 with HPh2GeGePh2GePh2H to yield 1 might

only the hydride 6.49,50 However, the key to the successful preparation of 7 is the use of the α-germyl nitrile 10 directly with a longer (5 days) reaction time. The X-ray crystal structure of 7 was obtained and represents the first structurally characterized example of a germanium neopentane analogue. The molecular structure of 7 indicates that this species is highly congested at the central germanium atom, and this was also confirmed by DFT computations. The Ge−Ge distances in 7 are on average longer than those typically found in other linear and branched oligogermanes, and the central germanium atom in 7 is in a nearly idealized tetrahedral environment. We also obtained the X-ray crystal structure of 10, which contains a tetrahedral germanium atom, a linear −CH2CN ligand, and a Ge−Cα bond distance of 1.982(4) Å. 4382


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vacuo to yield 3 (1.37 g, 32%). 1H NMR (C6D6, 25 °C): δ 7.65 (d, J = 7.6 Hz, 24H, o-H), 7.11 (t, J = 7.5 Hz, 12H, p-H), and 7.04 (t, J = 7.8 Hz, 24H, m-H). ppm. 13C NMR (C6D6, 25 °C): δ 139.8 (ipso-C), 137.3 (o-C), 128.5 (p-C), and 127.8 (m-C) ppm. UV/visible (CH2Cl2): 250 nm (ε = 1.59 × 104 M−1 cm−1). Anal. Calcd for C72H60Ge5: C, 67.10; H, 4.70. Found: C, 67.05; H, 4.74. Computational Details. Gaussian 03 was utilized for all computations.72 The total electronic energy and frequency calculations were performed using the hybrid density functional method, including Becke’s three-parameter nonlocal exchange functional73 with the correction functional of Lee−Yang−Parr B3LYP.74 The 6-31G* basis set was employed for all atoms.75 All atomic positions were optimized without geometry constraints to a minimum in the total force. The frequency calculation was performed at a lower level due to the size of the system. Once the optimized structure had been found, timedependent density functional calculations as implemented by Gaussian 03 were utilized to explore the excited state manifold and compute the possible electronic transitions and oscillator strengths. GaussSum was used to compute the UV/visible spectra.76 X-ray Crystal Structure Determinations. Diffraction intensity data were collected with a Siemens P4/CCD diffractometer. Crystallographic data for the X-ray analysis of 1, 2, 7·C6H14, and 10 are collected in Table 6. The crystal-to-detector distance was 60 mm, and the exposure time was 20 s per frmae using a scan width of 0.5°. The data were integrated using the Bruker SAINT software program. Solution by direct methods (SIR-2004) produced a complete heavyatom phasing model consistent with the proposed structures. All nonhydrogen atoms were refined anisotropically by full-matrix least squares (SHELXL-97). Aside from the germanium-bound hydrogen in 1, all hydrogen atoms were placed using a riding model. Their positions were constrained relative to their parent atom using the appropriate HFIX command in SHELXL-97.

open new pathways for the preparation of higher oligomers. Furthermore, the successful preparation of 7 using Ph3GeCH2CN directly also suggests that the synthesis of additional highly sterically congested branched oligogermanes, possibly including dendridic structures, might also be possible.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using standard Schlenk, syringe, and glovebox techniques. Germane was purchased from Gelest, and Ph3GeNMe2,71 Ph3GeCH2CN,54 and HPh2GeGePh2GePh2H53 were prepared by literature methods. 1H and 13 C NMR spectra were recorded at 300 and 75.46 mHz using an INOVA Gemini 2000 spectrometer. IR spectra were obtained using a Perkin-Elmer 1720 infrared spectrometer, and UV/visible spectra were recorded using a Hewlett-Packard 8453 diode array spectrometer. CV and DPV data were obtained using a DigiIvy DY2300 potentiostat with a glassy-carbon working electrode, a platinum-wire counter electrode, and an Ag/AgCl reference electrode using 0.1 M [Bu4N][PF6] as the supporting electrolyte. Elemental analyses were conducted by Gailbraith Laboratories. Synthesis of Ph3GeGePh2GePh2GePh2H (1). To a solution of HPh2GeGePh2GePh2H (0.100 g, 0.147 mmol) in acetonitrile (10 mL) was added a solution of Ph3GeNMe2 (0.051 g, 0.147 mmol) in acetonitrile (10 mL). The reaction mixture was sealed in a Schlenk tube and was heated at 85 °C for 48 h, after which time the volatiles were removed in vacuo to yield 1 (0.114 g, 79%) as a white solid. 1H NMR (C6D6, 25 °C): δ 7.62 (d, J = 7.8 Hz, 6H, o-C6H5), 7.52 (t, J = 8.1 Hz, 6H, m-C6H5), 7.40 (d, J = 6.6 Hz, 4H, o-C6H5), 7.31 (d, J = 7.2 Hz, 4H, o-C6H5), 7.16−7.06 (m, 25H, m-C6H5 and p-C6H5), 5.67 (s, 1H, −GePh2H) ppm. 13C NMR (C6D6, 25 °C): δ 138.3 (ipso-C Ph3Ge), 138.1 (ipso-C Ph2Ge), 138.0 (ipso-C Ph2Ge), 137.0 (ipso-C HPh2Ge), 136.9 (o-C Ph3Ge), 136.7 (o-C Ph2Ge), 136.1 (o-C Ph2Ge), 136.0 (o-C HPh2Ge), 129.0 (m-C Ph3Ge), 128.8 (m-C Ph2Ge), 128.7 (m-C Ph2Ge), 128.6 (m-C HPh2Ge), 128.5 (p-C Ph3Ge), 128.4 (p-C Ph2Ge), 128.3 (p-C Ph2Ge), 128.1 (p-C HPh2Ge) ppm. IR (Nujol mull): 2009 cm−1 (νGe−H). UV/visible (CH2Cl2): λmax 269 nm (ε = 4.5 × 104 M−1 cm−1). Anal. Calcd for C54H46Ge4: C, 65.79; H, 4.71. Found: C, 65.91; H, 4.62. Synthesis of Ge5Ph12 (2). To a solution of 1 (0.114 g, 0.116 mmol) in acetonitrile (10 mL) was added a solution of Ph3GeNMe2 (0.040 g, 0.116 mmol) in acetonitrile (5 mL). The reaction mixture was sealed in a Schlenk tube and was heated at 85 °C for 48 h, after which time the volatiles were removed in vacuo to yield 2 (0.124 g, 83%) as a white solid. 1H NMR (C6D6, 25 °C): δ 7.71 (d, J = 6.0 Hz, 6H, o-C6H5), 7.60 (d, J = 6.6 Hz, 4H, o-C6H5), 7.51(t, J = 7.5 Hz, 6H, m-C6H5), 7.48 (d, J = 6.6 Hz, 4H, o-C6H5), 7.32−7.03 (m, 40H, mC6H5 and p-C6H5) ppm. 13C NMR (C6D6, 25 °C): δ 138.5 (ipso-C (Ph3Ge)2), 138.3 (ipso-C (Ph2Ge)2), 137.2 (ipso-C (Ph2Ge)), 137.0 (o-C (Ph2Ge)2), 136.2 (o-C (Ph3Ge)2), 128.7 (m-C (Ph3Ge)2), 128.5 (p-C (Ph3Ge)2), 128.4 (p-C (Ph2Ge)2), 128.3 (p-C (Ph2Ge)), 127.8 (m-C (Ph2Ge)), 127.7 (m-C (Ph2Ge)2), 126.7 (o-C (Ph2Ge)) ppm. UV/visible (CH2Cl2): λmax 295 nm (ε = 4.5 × 104 M−1 cm−1). Anal. Calcd for C72H60Ge5: C, 67.10; H, 4.70. Found: C, 66.91; H, 4.84. Alternate Synthesis of Ge5Ph12 (2). To a solution of HPh2GeGePh2GePh2H (0.288 g, 0.293 mmol) in acetonitrile (10 mL) was added a solution of Ph3GeNMe2 (0.203 g, 0.584 mmol) in acetonitrile (10 mL). The reaction mixture was sealed in a Schlenk tube and was heated at 85 °C for 48 h, after which time the volatiles were removed in vacuo to yield 2 (0.316 g, 84%) as a white solid. Synthesis of (Ph3Ge)4Ge (7). Germane gas (GeH4, 0.256 g, 3.34 mmol) was condensed into an evacuated Schlenk tube at 77 K. A nitrogen atmosphere was established, and subsequently a solution of Ph3GeCH2CN (9.20 g, 26.7 mmol) in acetonitrile was transferred via cannula to the Schlenk tube and a nitrogen atmosphere was established. The reaction mixture was allowed to come to room temperature and then was heated in an oil bath at 85 °C for 5 days. The volatiles were removed in vacuo, and the resulting solid was distilled using a Kugelrohr oven (175 °C, 0.01 Torr) to remove unreacted Ph3GeCH2CN. The resulting solid was washed with hexane and dried in

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving crystallographic data for 1, 2, 7·C6H14, and 10. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by a CAREER grant from the National Science Foundation (No. CHE-0844758) and is gratefully acknowledged.

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REFERENCES

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