Synthesis of the Elusive Branched Fluoro-oligogermane (Ph3Ge

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis of the Elusive Branched Fluoro-oligogermane (Ph3Ge)3GeF: A Structural, Spectroscopic, Electrochemical, and Computational Study Ardalan Hayatifar,† F. Alexander Shumaker,† Sangeetha P. Komanduri,† Sydney A. Hallenbeck,† Arnold L. Rheingold,‡ 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

‡

S Supporting Information *

ABSTRACT: The fluorine-substituted branched oligogermane (Ph3Ge)3GeF was successfully synthesized from (Ph3Ge)3GeH and [Ph3C][BF4] after several unfruitful attempts using other synthetic methods and was formed as a mixture with Ph3GeF. Pure (Ph3Ge)3GeF could be obtained from the reaction mixture by successive recrystallizations and was characterized by elemental analysis and NMR (1H, 13C, and 19F) spectroscopy, including a variable-temperature 19F NMR study to investigate the presence or absence of hydrogen bonding in this species. The oligogermane (Ph3Ge)3GeF is indefinitely stable in the solid state under an inert atmosphere, but gradually decomposes to Ph3GeF and other unidentified products in solution. The X-ray crystal structure of (Ph3Ge)3GeF was obtained and represents the only crystallographically characterized germanium−fluorine compound having unsupported Ge−Ge bonds. The Ge4 framework of the oligogermane (Ph3Ge)3GeF is isostructural with the other previously prepared halogen-substituted analogues (Ph3Ge)3GeX (X = Cl, Br, I). The position of the fluorine atom in the structure of (Ph3Ge)3GeF is disordered by displacement of a chlorine atom 39% of the time. The UV/visible spectrum and the cyclic and differential pulse voltammograms of (Ph3Ge)3GeF were obtained, and the relative energies of the frontier orbitals were determined using DFT computations.

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INTRODUCTION Oligogermanes are catenated compounds of germanium that can be regarded as the germanium analogues of hydrocarbons. These species are of interest due to their inherent σdelocalization that results from the overlap of the diffuse 4sp3 orbitals of the germanium atoms.1−6 The degree of σdelocalization in these molecules is tunable and depends on the number of catenated germanium atoms and/or the electron-donating or -withdrawing nature of the organic substituents attached to the germanium−germanium backbone. The amount of σ-delocalization is also dependent on the conformation of the molecule, which is also influenced by the substituents and is maximized when a trans-coplanar arrangement of the atoms is present.3,7 The position of the absorbance maximum for these oligogermanes undergoes a red shift, and the molecules become easier to oxidize as the chain length is increased or when more inductively donating organic substituents are attached to the chain. These compounds have also been found to be luminescent and their emissive properties also are tunable by variation of their composition.8−13 In addition, they have the potential to serve as conductive materials and have been proposed to function as single-molecule conductors.14 © XXXX American Chemical Society

Like hydrocarbons, oligogermanes can adopt linear, cyclic, and branched geometries. Branched systems are somewhat uncommon,15−24 but several structurally characterized examples are known including (Ph3Ge)3GePh,15 (Ph3Ge)3GeH,23 and (Ph3Ge)3GeX (X = Cl, Br, I).23 In addition to their X-ray structures, all of these species have been characterized by UV/ visible spectroscopy and also by cyclic and differential pulse voltammetry.23 The energies of their frontier orbitals have also been calculated using DFT. Despite the relatively facile syntheses of the three (Ph3Ge)3GeX derivatives, the synthesis of the related fluoride analogue (Ph3Ge)3GeF (1) proved to be much more difficult. We now wish to report the successful synthesis and characterization of 1, and to compare its properties of those of the other members of the halide series (Ph3Ge)3GeCl (2), (Ph3Ge)3GeBr (3), and (Ph3Ge)3GeI (4).

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RESULTS AND DISCUSSION

We have employed several preparative routes for the attempted synthesis of 1 that were unsuccessful (Scheme 1). These included the reaction of (Ph3Ge)3GeH (5) with [Ph3C][BReceived: February 15, 2018

A

DOI: 10.1021/acs.organomet.8b00095 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Attempted Syntheses of (Ph3Ge)3GeF (1)

(C6F5)4] that abstracts the hydrogen atom from 5 to generate the cation [(Ph3Ge)3Ge]+, the formation of which was confirmed by the presence of Ph3CH in the 1H NMR spectrum of the product mixture. Attempts to then subsequently fluorinate the [(Ph3Ge)3Ge]+ cation with CH2F2, XeF2, or [(Me2N)3S][Me3SiF2] (TAS-F) were not successful. Instead an intractable mixture of products was obtained in each case, although the presence of Ph3GeF was confirmed in each reaction by the presence of a singlet at δ −202.3 ppm in the 19F NMR of the product mixtures. The synthesis of 1 was also attempted by the reaction of AgF with 2 but this was also unsuccessful (Scheme 1). The formation of Ph3GeF was again detected in the product mixture resulting from the reaction of 2 with AgF. All attempts to isolate the cation as the salt [(Ph3Ge)3Ge][B(C6F5)4], or as salts with other weakly coordinating nonnucleophilic anions including [CHB11H11]−, have been unsuccessful to date. This could be due to the fact that the [(Ph3Ge)3Ge]+ cation is labile and decomposes to Ph3Ge+ or another species, but we do not have direct evidence for this. When 5 undergoes hydrogen abstraction by [Ph3C][PF6] in CH2X2 (X = Cl, Br, or I) at 85 °C, the three oligogermanes 2− 4 were obtained in good yields suggesting that decomposition of [(Ph3Ge)3Ge]+ did not occur.23 However, in this case the halogenating reagent is present while [(Ph3Ge)3Ge]+ is being generated, such that the halogen atom is available for abstraction from the solvent immediately which could prevent decomposition of the tetragermyl cation. The successful synthesis of 1 was ultimately achieved using [Ph3C][BF4], where the source of the fluorine atom is the tetrafluoroborate anion itself (Scheme 2). In this case, the fluorinating agent is present simultaneously with the generation of the [(Ph3Ge)3Ge]+ cation, which again might prevent

Scheme 2. Synthesis of (Ph3Ge)3GeF (1)

decomposition of the cation. The formation of BF3 as a byproduct was confirmed by conducting the reaction in an NMR tube on a small scale in C6D6. A peak at δ −131.3 ppm in the 19F NMR spectrum confirmed the presence of BF3. A similar reaction was recently reported for the conversion of (TPFC)GeH to (TPFC)GeF (TPFC = tris(pentafluorophenyl)corrole) using [Ph3C][BF4].25 The reaction of 5 with 1 equiv of [Ph3C][BF4] was conducted in benzene solvent at room temperature for 48 h. Upon removal of the solvent a light brown solid resulted that not only was mostly amorphous but also contained a small amount of white needle-like crystals. The 19F NMR spectrum of the product mixture indicated the presence of two different fluorine-containing species as indicated by the presence of two singlets at δ −202.3 and −194.7 ppm in an integrated ratio of 6:1. The upfield signal is due to the presence of Ph3GeF26,27 while the downfield signal is assigned to the branched oligogermane 1. Resonances in the 13 C NMR spectrum of the product mixture at δ 134.6, 134.5, 130.8, and 128.9 ppm also indicate the presence of Ph3GeF.26 The formation of Ph3GeF indicates that the germanium− germanium bonds in either 5 or the cation [(Ph3Ge)3Ge]+ generated in situ are not stable in the presence of fluoride, since Ph3GeF is generated by decomposition of the Ge4 framework. Several other unidentified products were present in the product mixture as well as indicated by the 1H and 13C NMR spectra of the product mixture. Compound 1 was isolated in pure form by B

DOI: 10.1021/acs.organomet.8b00095 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Selected 19F NMR spectra of 1 in toluene-d7 in the temperature range −60 to +60 °C.

Table 1. Variable Temperature 19F NMR Spectral Data for 1

addition of hexane to a toluene solution of the crude product mixture that was allowed to evaporate to approximately onethird its original volume, after which time a colorless solid material was obtained that was identified as pure 1. Oligogermane 1 is stable in the solid state under an inert atmosphere, but slowly decomposes in solution to generate Ph3GeF. The 19F NMR spectrum of a pure sample of 1 was recorded in a solution of benzene-d6 after a period of 1 week, and two resonances corresponding to 1 and Ph3GeF were observed in an integrated ratio of 1:1. Further decomposition of 1 was observed over longer periods of time, and the decomposition of 1 occurs when the sample is stored in the presence of light or in the dark, although the process is slower in the absence of light. The decomposition of 1 is likely promoted by the presence of the electronegative fluorine atom that polarizes and therefore weakens the germanium− germanium bonds. This was not observed for the other halogen-substituted oligogermanes 2−4, nor has it been reported for other germanium−fluorine compounds to our knowledge. However, there are very few fluorine-substituted germanium compounds that also contain germanium− germanium bonds that have been reported (vide inf ra). A variable temperature 19F NMR experiment was performed in the temperature range −60 to +60 °C in toluene-d8, and selected spectra are shown in Figure 1 while spectral data are collected in Table 1. There is a clear upfield shift of the singlet in the 19F NMR spectrum of 1 as the temperature is increased as well as a sharpening of the resonance with increasing temperature. The chemical shift for the fluorine atom of 1 is at δ −193.16 ppm at −60 °C, which shifts upfield to δ −194.94 ppm at +60 °C, indicating that the fluorine atom of 1 becomes more shielded as the temperature increases. Since rotation of the phenyl rings increases with increasing temperature, this change in chemical shift is likely caused by increased shielding of the fluorine atom in 1 by the phenyl rings as their degree of rotation increases.

temp (°C)

δ (ppm)

Δ1/2 (Hz)

−60 −40 −20 0 20 40 60

−193.16 −193.92 −194.41 −194.73 −194.89 −194.94 −194.94

28 16 8 8 6 6 6

X-ray quality crystals of 1 were obtained by the slow evaporation of a toluene solution of the solid material isolated as described above, and an ORTEP diagram of 1·C6H6 is shown in Figure 2 and selected bond distances and angles are collected in Table 2. The structure of 1·C6H6 is disordered and contains a chlorine atom in place of the fluorine atom 39% of the time. We have identified the source of the chlorine atom as trace lithium chloride that remained from the synthesis of Ph3GeNMe2 from Ph3GeCl and LiNMe2 that persists upon the synthesis of 5 from Ph3GeNMe2 and GeH4. Once the cation [(Ph3Ge)3Ge]+ is generated from 5 it reacts with the residual chloride present to generate (Ph3Ge)3GeCl (2). Although this is unsurprising given the chlorophilicity of germanium, it occurs only to a very small extent since the bulk material of 1 was shown to be pure by NMR (1H and 13C) NMR spectroscopy as well as by elemental analysis. Since only a small fraction of the bulk material crystallized to give X-ray quality crystals, and in this compound 2 is present only 39% of the time, it is expected that the presence of 2 would not be seen in the 13C or 1H NMR spectra of the bulk material. We were unable to obtain nondisordered crystals of 1 despite numerous attempts. The structure of 1 was solved with two different positions for the fluorine and chlorine atoms. The Ge4 skeleton in 1·C6H6 is isostructural with those of the other halogen-substituted C

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common, with only ca. 35 hits.25−49 The Ge−F bond in Ph3GeF is 1.749(2) Å,27 and the other Ge−F distances that have been reported for organometallic germanium fluoride compounds range from 1.629(3) Å in (o-Mes2C6H3)2Ge(H)F40 to 1.839(2) Å in (3-But-6-(OMe)C6H3)3CGeF3,32 although the zwitterionic complex pentafluoro(4-methyl-1,4-diazoniacyclohex-1-yl)methylgermanate has a Ge−F bond measuring 1.867(2) Å.48 Of the compounds reported, only [Ag2{C6H32,6-(C6H3-2,6-Pri2)2}GeGe(F){C6H3-(C6H3-2,6-Pri2)2}][SbF6] contains a direct germanium−germanium single bond, but this is bridged by two silver atoms.49 Therefore, 1 is the only structurally characterized example of a germanium−fluorine compound containing unsupported Ge−Ge bonds. The CV and DPV of 1 were measured in CH2Cl2 solvent using 0.1 M [Bun4N][PF6] as the supporting electrolyte, and the CV and DPV are shown in Figure 3. The CV of 1 exhibited a single irreversible oxidation wave at 1725 mV and an irreversible oxidation wave at 1680 mV in its DPV. As was expected due to the presence of the highly electronegative fluorine atom, 1 is more difficult to oxidize than the other halogen-substituted branched oligogermanes that have oxidation waves at 1668 (2), 1656 (3), and 1643 (4) in their CVs.23 The absorbance maximum in the UV/visible spectrum of 1 was observed as a shoulder at 240 nm, while the UV/visible spectra of 2−4 each contained a well-defined maximum at 245 (2), 264 (3), and 271 (4) nm.23 Therefore, the absorbance maximum of 1 is only slightly blue-shifted relative to that of its chlorinesubstituted analogue 2. To augment the experimental data for 1, DFT calculations were performed in order to determine the energies and nature of the frontier orbitals. Diagrams of the HOMO and LUMO of 1 and 2 are shown in Figure 4, and DFT data for 1−4 are collected in Table 3. The HOMO of 1 is primarily distributed on the fluorine atom and also on the phenyl rings and is only slightly distributed on the Ge4 framework. However, the LUMO of 1 is mainly localized on the Ge4 framework with some distribution on the phenyl rings and the fluorine atom. The composition of the HOMO and LUMO of 1 are very similar to those of the chlorine-containing tetragermane 2. A survey of the relative energies of the frontier orbitals of 1− 4 indicates that the HOMO−LUMO gaps of 1, 2, and 3 are similar, and therefore the UV/visible absorption maxima are expected to be nearly identical. The absolute values of the HOMO and LUMO energies depend on the basis set and computational method employed, and the difference between the number of orbitals and their diffuseness is responsible for anonymous ordering. Fundamentally the Kohn−Sham energy differences are what is physically relevant; see a recent review for a discussion for the use of orbital energy gaps from density functional theory.50 The computational trend in λmax was also observed experimentally, as the λmax for 1 was observed at 240 nm while that for 2 was observed at 245 nm.23 However, the calculated band gap for 3 is 243.1 nm, which is similar than that of 1 and 2, but the observed absorbance maximum for 3 was at 264 nm. As the electronegativity of the halogen atom increases, it is expected that the energy of the HOMO will become more stabilized (i.e., more negative in energy) and also that the HOMO−LUMO gap will increase. It was anticipated that 1 would have the most stabilized HOMO and the largest HOMO−LUMO gap in the series 1−4. However, this was not found to be the case upon analysis of 1 by DFT. The energy of the HOMO of 1 was calculated to be −5.958 eV, which is actually more destabilized than the

Figure 2. ORTEP diagram of 1·C6H6. Thermal ellipsoids are drawn at 50% probability. The disordered chlorine atom and the solvent molecule are not shown.

Table 2. Selected Bond Distances (Å) and Angles (deg) for (Ph3Ge)3GeF (1)·C6H6 Ge(1)−Ge(2) Ge(1)−Ge(3) Ge(1)−Ge(4) Ge(1)−F(1) Ge(1)−Cl(1) 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)

2.4751(8) 2.4708(8) 2.4637(8) 1.801(1) 2.192(1) 1.946(5) 1.961(5) 1.925(6) 1.964(5) 1.946(5) 1.950(5) 1.942(5) 1.953(5) 1.956(5)

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

103.3(6) 104.2(6) 102.1(5) 111.89(3) 118.82(3) 114.16(3) 107.0(2) 109.4(2) 111.2(2) 107.8(2) 109.1(2) 107.7(2) 109.6(2) 108.4(2) 104.6(2)

branched oligogermanes 2−4.23 The germanium−germanium bond distances in 1·C6H6 average 2.4699(8) Å and this is nearly identical to those in 2, 3, and 4 that are 2.4636(7), 2.4698(4), and 2.4689(6) Å, respectively. The Ge−Ge−Ge bond angles in 1·C6H6 average 114.96(3)° that is only slightly more acute than the corresponding values for 2 (116.22(2)°), 3 (116.21(1)°), and 4 (116.54(2)°). The Ge−Ge−F bond angles in 1·C6H6 average 103.2(6)°, which is more obtuse than the average of the Ge−Ge−X bond angles in 2 (101.34(4)°), 3 (101.38(2)°), and 4 (100.80(2)°). The Ge−Cl bond distance for the disordered chlorine atom in 1 is 2.192(1) Å and is slightly shorter than the values of 2.230(1) and 2.215(2) Å previously reported for 2.23 The crystal lattice for 1 is also different from the two forms of 2, where the space group of 1 is P21/n while the space group for the non-C3 symmetric form of 2 is P21 and the C3-symmetric form is in the P63 space group. The Ge−F bond in 1·C6H6 is 1.801(1) Å, and this is in the range of other Ge−F bonds that have been reported in the literature. A survey of the Cambridge Structural Database indicates that crystallographically characterized compounds containing a germanium−fluorine bond are somewhat unD

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Figure 3. CV and DPV of 1 in CH2Cl2 solvent using 0.1 M [Bun4N][PF6] as the supporting electrolyte.

shifted relative to the other members of the series (Ph3Ge)3GeX (X = Cl (2), Br (3), I (4)), and 1 is also more difficult to oxidize than 2−4. DFT studies indicate that although the HOMO−LUMO gap of 1 is larger than 2−4 as expected, the energy of the HOMO is destabilized relative to those of 2−4.

HOMOs of all of the other halogen-substituted oligogermanes 2−4. The fluorine 2p orbital is significantly contracted relative to the larger np orbitals on the halogen atoms of 2−4,23 and therefore the overall contribution of the fluorine 2p orbital to the calculated HOMO of 1 is minimal. The chlorine, bromine, and iodine p-orbitals in 2, 3, and 4, respectively, are significantly larger and thus make a more significant contribution to the calculated HOMOs of these three oligogermanes. Consequently, since there is little to no contribution from the halogen orbital in 1 to the calculated HOMO of 1, it is destabilized relative to those of 2−4 and is primarily centered on the Ge4 skeleton. Despite the fact that the HOMO of 1 is the highest in energy among 1−4, 1 is more difficult to oxidize than its heaver analogues 2−4 (Table 3). This can be attributed to the fact that the calculated HOMO and LUMO energies are relative, rather than absolute, in nature. Therefore, with regard to the observed oxidation potential of 1, the theoretical and experimental data do not correlate with one another.

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EXPERIMENTAL SECTION

General Considerations. The reagents [Ph3C][BF4], [Ph3C][B(C6F5)4], [Ph3C][PF6], XeF2, CH2F2, [(Me2N)3S][Me3SiF2], and AgF were purchased from Aldrich and used without further purification. The oligogermanes (Ph3Ge)3GeH and (Ph3Ge)3GeCl were prepared according to the literature procedure.23 All manipulations were carried out using standard Schlenk, syringe, and glovebox techniques, and solvents were dried using a Glass Contour solvent purification system. The purity of the isolated bulk material for (Ph3Ge)3GeF (1) was determined by NMR (1H, 13C, and 19F) spectroscopy and elemental analysis. NMR spectra were recorded on a Bruker Avance III spectrometer operating at 400.00 MHz (1H), 376.31 (19F), or 100.57 (13C) MHz. Variable temperature 19F NMR spectra were recorded on an Agilent INOVA 400 spectrometer operating at 376.31 MHz. UV/visible spectra were recorded using an Ocean Optics Red Tide USB650UV spectrometer. Electrochemical data (CV and DPV) were obtained using a DigiIvy DY2312 potentiostat using a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode in CH2Cl2 solution using 0.1 M [Bu4N][PF6] as the supporting electrolyte. Elemental analysis was conducted by Galbraith Laboratories.

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CONCLUSIONS The branched germanium fluoride compound (Ph3Ge)3GeF (1) was synthesized from (Ph3Ge)3GeH (5) and [Ph3Ge][BF4] after many unsuccessful attempts by alternate synthetic routes. The X-ray crystal structure of 1 was determined, and 1 was further characterized by NMR (1H, 13C, and 19F) and UV/ visible spectroscopy, cyclic and differential pulse voltammetry, and elemental analysis. The absorbance maximum of 1 is blueE

DOI: 10.1021/acs.organomet.8b00095 Organometallics XXXX, XXX, XXX−XXX

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was stirred for 24 h at 85 °C in a Schlenk tube, after which time the Schlenk tube was opened in the glovebox and XeF2 (0.060 g, 0.35 mmol) was added. The reaction mixture was then heated with stirring for an additional 24 h at 85 °C, the volatiles were removed in vacuo, and the resulting solids were washed with hexane (4 × 10 mL). The residue was dissolved in benzene (5 mL) and filtered to remove [XeF][B(C6F5)4], and the volatiles were removed from the filtrate in vacuo to yield a colorless oil that contained an intractable product mixture as shown by 1H and 13C NMR spectroscopy. Attempted Synthesis of 1 Using [Ph3C][B(C6F5)4] and [(Me2N)3S][Me3SiF2]. To a solution of 5 (0.453 g, 0.460 mmol) in toluene (15 mL) was added [Ph3C][B(C6F5)4] (0.430 g, 0.465 mmol). The reaction mixture was stirred for 24 h at 85 °C in a Schlenk tube, after which time the Schlenk tube was opened in the glovebox and [(Me2N)3S][Me3SiF2] (0.130 g, 0.472 mmol) was added. The reaction mixture was then stirred at 85 °C for an additional 24 h, the volatiles were removed in vacuo, and the resulting solids were washed with hexane (5 × 10 mL). The residue was dissolved in benzene (10 mL) and filtered, and the volatiles were removed from the filtrate in vacuo to yield a colorless solid. The solid was recrystallized from toluene at −35 °C to yield colorless crystals that were shown to be [(Me2N)3S][B(C6F5)4] by X-ray analysis. The supernatant contained an intractable mixture of products as shown by 1H and 13C NMR spectroscopy. Attempted Synthesis of 1 Using [Ph3C][B(C6F5)4] and CH2F2. To a solution of 5 (0.657 g, 0.667 mmol) in toluene (15 mL) was added [Ph3C][B(C6F5)4] (0.625 g, 0.678 mmol). The reaction mixture was stirred for 24 h at 85 °C in a Schlenk tube. The reaction mixture was allowed to cool to room temperature, and the volatiles were removed in vacuo. The Schlenk tube was cooled to −78 °C and evacuated, and CH2CF2 (13.67 g, 0.262 mol) was condensed into the tube under static vacuum. The Schlenk tube was sealed, and the reaction mixture was stirred at −78 °C for 5 h, after which time the reaction mixture was allowed to warm to room temperature and the volatiles were removed in vacuo to yield a light brown solid. The solid was an intractable mixture of products as shown by 1H and 13C NMR spectroscopy. X-ray Crystal Structure Determination of (Ph3Ge)3GeF (1). Diffraction intensity data were collected with a Siemens P4/CCD diffractometer. Crystallographic data for the X-ray analysis of 1 are collected in the Supporting Information (Table S1). The crystal-todetector distance was 60 mm, and the exposure time was 20 s per frame using a scan width of 0.5°. The data were integrated using the Bruker SAINT software program. Solution by direct methods (SIR2004) produced a complete heavy-atom phasing model consistent with the proposed structures. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares (SHELXL-97). 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. A global RIGU command was used to stabilize the refinement of the thermal ellipsoids and accounts for the substantial number of restraints employed. Computational Details. Gaussian 03 was utilized for all computations.51 Energy calculations, geometry optimizations, and frequency calculations are performed using the hybrid density functional method including Becke’s three-parameter nonlocalexchange functional52 with the correlation functional of Lee−Yang− Parr, B3LYP.53 The 6-31G* basis set54 was employed for all atoms. All atomic positions are optimized without geometry constraints.

Figure 4. Frontier molecular orbitals of (Ph3Ge)3GeF (1) and (Ph3Ge)3GeCl (2). Shown are the HOMO of 1 (a), the LUMO of 1 (b), the HOMO of 2 (c), and the LUMO of 2 (d). Synthesis of (Ph3Ge)3GeF (1). To a solution of 5 (0.37 g, 0.37 mmol) in benzene (10 mL) was added a solution of [Ph3C][BF4] (0.39 g, 1.2 mmol) in benzene (10 mL) resulting in a yellow solution. The reaction mixture was stirred at room temperature for 48 h, and the volatiles were removed in vacuo. The resulting light brown residue was washed with hexane (3 × 15 mL) to remove Ph3CH. Analytically pure material was obtained by dissolving the reaction mixture in toluene (5 mL) in a vial and then layering this solution with hexane (15 mL). The resulting solution was allowed to evaporate to a volume of ca. 8 mL at which time crystals had formed on the walls of the vial. The solution was decanted from the crystals, and the crystals were washed with hexane (3 × 5 mL) and allowed to dry, which provided pure 1 as colorless crystals (0.24 g, 65%). 1H NMR (C6D6, 25 °C) δ 7.33 (d, J = 7.6 Hz, 18H, o-C6H5), 7.07−7.03 (m, 18H, m-C6H5), 6.95 (t, J = 6.8 Hz, 9H, p-C6H5) ppm. 13C NMR (C6D6, 25 °C) δ 136.3 (ipso-C6H5), 136.2 (o-C6H5), 128.5 (m-C6H5), 127.9 (p-C6H5) ppm. 19 F NMR (C6D6, 25 °C) δ −194.72 (Ge-F) ppm. UV/vis (CH2Cl2, 25 °C): 240 nm (sh). Anal. calcd for C54H45FGe4: C, 64.61; H, 4.52. Found: C, 64.68; H, 4.57. Attempted Synthesis of 1 Using [Ph3C][B(C6F5)4] and XeF2. To a solution of 5 (0.290 g, 0.294 mmol) in toluene (10 mL) was added [Ph3C][B(C6F5)4] (0.326 g, 0.353 mmol). The reaction mixture

Table 3. Frontier Molecular Orbital Energy Data for 1−4a

a

compound

HOMO (eV)

LUMO (eV)

HOMO−LUMO gap (eV)

HOMO−LUMO gap (nm)

λmax (nm)

Eox (CV, mV)

(Ph3Ge)3GeF (1) (Ph3Ge)3GeCl (2) (Ph3Ge)3GeBr (3) (Ph3Ge)3GeI (4)

−5.958 −6.069 −6.050 −5.997

−0.850 −0.990 −0.950 −1.315

5.108 5.079 5.100 4.682

242.7 244.1 243.1 264.8

240 (sh) 245 264 271

1725 1668 1656 1643

Data for 2−4 are taken from ref 22. F

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Organometallics Frequency calculations were performed at a lower level to confirm that the stable geometries have real vibrational frequencies. The time dependent density functional computations, as implemented by Gaussian 03, were utilized to explore the excited manifold and compute the possible electronic transitions and oscillator strengths.

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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.8b00095. Table of crystallographic data for 1 (Table S1) and NMR spectra of oligogermane 1 (PDF) Accession Codes

CCDC 1824305 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (405) 744-6543. ORCID

Charles S. Weinert: 0000-0003-0754-8688 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Science Foundation (CHE-1464462) and is gratefully acknowledged. The computing for this project was performed at the OSU High Performance Computing Center at Oklahoma State University supported in part through the National Science Foundation grant OCI−1126330. C.S.W. thanks Prof. Nicholas F. Materer (Department of Chemistry, Oklahoma State University) for helpful discussions.

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DOI: 10.1021/acs.organomet.8b00095 Organometallics XXXX, XXX, XXX−XXX