73Ge NMR Spectral Investigations of Singly Bonded Oligogermanes

Apr 20, 2009 - and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093-0332, and Frontier. High School, Red Rock, Oklahoma 7...
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Organometallics 2009, 28, 3067–3073

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3067

Ge NMR Spectral Investigations of Singly Bonded Oligogermanes

Monika L. Amadoruge,§ Claude H. Yoder,† Julia Hope Conneywerdy,‡ Katie Heroux,⊥ Arnold L. Rheingold,⊥ and Charles S. Weinert*,§ Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, Department of Chemistry, Franklin and Marshall College, Lancaster, PennsylVania 17604-3003, Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093-0332, and Frontier High School, Red Rock, Oklahoma 74651 ReceiVed January 15, 2009

The synthesis of the two digermanes Bus3GeGePh3 and PhMe2GeGePh3, as well as the branched tetragermane PhGe(GeBun3)3, was achieved using the hydrogermolysis reaction. These species were fully characterized by NMR (1H, 13C) spectroscopy and elemental analysis, and the crystal structure of PhMe2GeGePh3 was determined. These three species, along with 11 other oligogermanes, were also characterized by 73Ge NMR spectroscopy. Chemical shifts of the 73Ge NMR resonances for these oligogermanes have been correlated with the substitution pattern at germanium and also with the number of germanium-germanium bonds at the individual Ge centers. Germanium centers having only one attached germanium atom result in resonances appearing in the range δ -30 to -65 ppm, while those having two or three bonded germanium atoms exhibit resonances in the respective ranges δ -100 to -120 and δ -195 to -210 ppm. Chemical shifts of resonances for germanium centers bearing phenyl substituents appear upfield from those having alkyl substituents. Introduction The acquisition of meaningful 73Ge NMR data has, until recently, only been accomplished with difficulty due to a variety of factors. The highly oblate charge distribution at the 73Ge nucleus is indicated by its large quadruple moment (q ) -0.2 × 10-28 m2), which typically results in the appearance of broad resonances upon interaction of the quadruple moment with the electric field gradient at the nucleus. The line broadening can be extreme, except in compounds where the germanium atom is symmetrically substituted, such as GeR4 (R ) Me, Et, Prn, Bun, OMe),1 GePh4,2 and GeX4 (X ) Cl, Br, I).3 The resonance frequency at a magnetic field strength of 2.3488 T (1H ) 100 MHz) is 3.488 MHz, and therefore a dedicated low-frequency probe is required for observation of the 73Ge nucleus. An additional complication involves acoustic ringing, which arises from transient responses in the probe resulting from the radio frequency pulse; however, several pulse sequences have been developed to suppress this problem.4-7 The advent of high-field instruments and advanced software technology has recently allowed the acquisition of 73Ge NMR * To whom correspondence should be addressed. E-mail: weinert@ chem.okstate.edu. § Oklahoma State University. † Franklin and Marshall College. ⊥ University of California, San Diego. ‡ Frontier High School. (1) Kaufmann, J.; Sahm, W.; Schwenk, A. Z. Naturforsch. A 1971, 26A, 1384–1389. (2) Takeuchi, Y.; Harazono, T.; Kakimoto, N. Inorg. Chem. 1984, 23, 3835–3836. (3) Kidd, R. G.; Spinney, H. G. J. Am. Chem. Soc. 1973, 95, 88–90. (4) Belton, P. S.; Cox, I. J.; Harris, R. K. J. Chem. Soc., Faraday Trans. 1985, 81, 63–75. (5) Mackay, K. M.; Watkinson, P. J.; Wilkins, A. L. J. Chem. Soc., Dalton Trans. 1984, 133–139. (6) Patt, S. L. J. Magn. Reson. 1982, 49, 161–163. (7) Wilkins, A. L.; Thomson, R. A.; Mackay, K. M. Main Group Met. Chem. 1990, 13, 219–236.

data for numerous nonsymmetric compounds. These include the vinyl and alkynyl species MexGe(CHdCH2)4-x8 and MexGe(CtCH)4-x,9 the heteroaromatic germanes RxGeMe4-x (R ) 2-furyl, 2-thienyl, 4,5-dihydro-2-furyl),10 various Ge-substituted germacyclohexanes,11 several hypervalent germanium compounds,12-14 and a number of arylgermanes ArxGeH4-x.15-17 The 1J(Ge-H) coupling constants for several of these ArxGeH4-x compounds have recently been determined, including those for (p-MeOC6H4)GeH3 (97 Hz), (p-H3CC6H4)GeH3 (96 Hz), MesGeH3 (95 Hz), PhGeH3 (98 Hz), Ph2GeH2 (94 Hz), and Ph3GeH (98 Hz).17 These Ge-H coupling constants are similar to that observed for GeH4 (97.6 Hz).5 In 1999, the first 73Ge NMR data for compounds containing Ge-Ge bonds was reported.18 Resonances for Me3GeGeMe3, Ph3GeGePh3, and (Ph3Ge)3GeH were observed at δ -59, -67, and -314 ppm, where the latter peak corresponds to the central germanium atom of (Ph3Ge)3GeH. A resonance corresponding to the Ph3Ge- atoms was not observed. (8) Takeuchi, Y.; Inagaki, H.; Tanaka, K.; Yoshimura, S. Magn. Reson. Chem. 1989, 27, 72–74. (9) Liepins, E.; Petrova, M. V.; Bogoradovsky, E. T.; Zavgorodny, V. S. J. Organomet. Chem. 1991, 410, 287–291. (10) Liepins, E.; Zicmane, I.; Ignatovich, L. M.; Lukevics, E. J. Organomet. Chem. 1991, 389, 23–28. (11) Takeuchi, Y.; Shimoda, M.; Tanaka, K.; Tomoda, S.; Ogawa, K.; Suzuki, H. J. Chem. Soc., Perkin Trans. 2 1988, 7–13. (12) Kupce, E.; Ignatovich, L. M.; Lukevics, E. J. Organomet. Chem. 1989, 372, 189–191. (13) Kupce, E.; Lukevics, E. J. Magn. Reson. 1988, 79, 325–327. (14) Yoder, C. H.; Agee, T. M.; Schaeffer, C. D.; Carroll, M. J.; Fleisher, A. J.; DeToma, A. S. Inorg. Chem. 2008, 47, 10765–10770. (15) Takeuchi, Y.; Yamamoto, H.; Tanaka, K.; Ogawa, K.; Harada, J.; Iwamoto, T.; Yuge, H. Tetrahedron 1998, 54, 9811–9822. (16) Takeuchi, Y.; Tanaka, K.; Aoyagi, S.; Yamamoto, H. Magn. Reson. Chem. 2002, 40, 241–243. (17) Riedmiller, F.; Wegner, G. L.; Jockisch, A.; Schmidbaur, H. Organometallics 1999, 18, 4317–4324. (18) Thomson, R. A.; Wilkins, A. L.; Mackay, K. M. Phosphorus, Sulfur, Silicon 1999, 150-151, 319–324.

10.1021/om900035r CCC: $40.75  2009 American Chemical Society Publication on Web 04/20/2009

3068 Organometallics, Vol. 28, No. 10, 2009

Amadoruge et al. Scheme 1

We have recently synthesized and characterized a variety of linear and branched oligogermanes containing Ge-Ge single bonds via the hydrogermolysis reaction, either alone or coupled with a germanium hydride protection/deprotection strategy.19-24 We have now characterized a number of these species by 73Ge NMR spectroscopy, including the digermanes R3GeGePh3 (R3 ) Bun3 (1), Bus3 (2), Et3 (3), Pri3 (4), PhMe2 (5), Bun2CH2CH2OEt (6)), the trigermanes Ph3GeGeBun2GeR2CH2CH2OEt (R ) Bun (7) or Ph (8)) and EtOCH2CH2R2GeGePh2GeR2CH2CH2OEt (R ) Bun (9) or Ph (10)), and the branched species PhGe(GePh3)3 (11), PhGe(GeBun3)3 (12), PhGe(GeBun2CH2CH2OEt)3 (13), and PhGe(GeBun2GeBun2CH2CH2OEt)3 (14). The compounds Bus3GeGePh3 (2), PhMe2GeGePh3 (5), and PhGe(GeBun3)3 (12) were newly synthesized for these investigations, and the X-ray crystal structure of compound 5 was obtained.

Results and Discussion The digermanes Bus3GeGePh3 (2) and PhMe2GeGePh3 (5) and the branched tetragermane PhGe(GeBun3)3 (12) were prepared via the hydrogermolysis reaction as shown in Scheme 1. The triakylchlorogermane precursor Bus3GeCl for the digermane 2 was synthesized from GeCl4 and BusMgCl, which resulted in the formation of a mixture of products BusnGeCl4-n as well as a polymeric material. Analytically pure Bus3GeCl was separated from the crude reaction mixture in 27% yield via fractional distillation at 47 °C (0.25 Torr), which is consistent with the previously reported boiling point of 93 °C (3 Torr).25 The 1H NMR of Bus3GeCl in benzene-d6 is complex since each of the three R-carbon atoms of the sec-butyl groups is chiral, resulting in eight possible diastereomers of this species. Resonances for the protons of the R-carbon, the methylene group, and the β-methyl group appear as multiplets centered at δ 1.71, 1.35, and 1.11 ppm (respectively), while that for the γ-methyl group appears as a broad triplet at δ 0.89 ppm (J ) (19) Subashi, E.; Rheingold, A. L.; Weinert, C. S. Organometallics 2006, 25, 3211–3219. (20) Amadoruge, M. L.; DiPasquale, A. G.; Rheingold, A. L.; Weinert, C. S. J. Organomet. Chem. 2008, 693, 1771–1778. (21) Amadoruge, M. L.; Gardinier, J. R.; Weinert, C. S. Organometallics 2008, 27, 3753–3760. (22) Amadoruge, M. L.; Golen, J. A.; Rheingold, A. L.; Weinert, C. S. Organometallics 2008, 27, 1979–1984. (23) Amadoruge, M. L.; Weinert, C. S. Chem. ReV. 2008, 108, 4253– 4294. (24) Weinert, C. S. Dalton Trans. 2008, Advance Articles, DOI: 10.1039/ b816067h. (25) Yarosh, O. G.; Voronkov, M. G.; Zhilitskaya, L. V.; Yarosh, N. O.; Albanov, A. I.; Korotaeva, I. M. Russ. J. Gen. Chem. 2005, 75, 714–718.

9.6 Hz). The 13C NMR spectrum of Bus3GeCl in benzene-d6 recorded at 100.6 MHz exhibits only eight resolved resonances (Vide infra) instead of the predicted 32 peaks since several of these features overlap (as expected) due to their nearly identical magnetic environments. Treatment of Bus3GeCl with LiNMe2 yielded the amide Bus3GeNMe2 in 78% yield (Scheme 1). As found for Bus3GeCl, three of the four resonances for the sec-butyl groups in the 1H NMR spectrum of Bus3GeNMe2 appear as multiplets centered at δ 1.73, 1.34, and 1.12 ppm, while the peak for the γ-methyl group is a broad triplet at δ 0.89 ppm (J ) 7.2 Hz). Three separate resonances for the -N(CH3)2 protons of the amide groups were visible at δ 2.67, 2.66, and 2.65 ppm in an intensity ratio of 1:1.3:3, and the expected eight individual peaks could not be resolved at a spectrometer frequency of 600 MHz. The alkyl region of the 13C NMR spectrum of Bus3GeNMe2 resembles that of Bus3GeCl, having eight resolved peaks, while two broad features at δ 42.4 and 42.1 ppm correspond to the carbon atoms of the -N(CH3)2 groups. The hydrogermolysis reaction of Bus3GeNMe2 with Ph3GeH in CH3CN yielded the digermane Bus3GeGePh3 in 81% yield via the in situ conversion of Bus3GeNMe2 to the R-germyl nitrile Bus3GeCH2CN (Scheme 1).19-24 In contrast with most digermanes of the general formula R3GeGePh3, compound 2 is a highly viscous oil at room temperature instead of a solid, which can be attributed to the presence of multiple diastereomers of 2 due to the chiral nature of the three sec-butyl groups. The aromatic region of the 1H NMR spectrum of 2 is similar to that for other digermanes R3GeGePh3, with a triplet and a doublet at δ 7.68 (J ) 7.2 Hz, p-H) and 7.63 (J ) 7.2 Hz, o-H) ppm (respectively), and a multiplet centered at δ 7.11 ppm (m-H). The alkyl region is again complex, with three multiplets centered at δ 1.75, 1.41, and 1.08 ppm and a broad triplet at δ 0.77 (J ) 7.6 Hz) ppm, and the corresponding region in the 13C NMR region of 2 exhibits six clearly resolved resonances at δ 28.8, 28.6, 26.8, 14.5, 14.4, and 13.8 ppm. The amides PhMe2GeNMe2 and Bun3GeNMe219 were prepared from the corresponding commercially available germanium chlorides and were used for the synthesis of the digermane 5 and the branched tetragermane 12 in yields of 76% and 98%, respectively (Scheme 1). The branched species 12 is a viscous colorless oil at room temperature, while 5 is a colorless solid. The UV/visible spectrum of 12 exhibits an absorption maximum at 233 nm, which is higher in energy than that of the phenylsubstituted species (Ph3Ge)3GePh (11), which was observed at 256 nm.22 As shown for linear oligogermanes, the presence of phenyl versus alkyl substituents stabilizes both the HOMO and

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Ge NMR Spectral InVestigations of Oligogermanes

Organometallics, Vol. 28, No. 10, 2009 3069 Table 2. Previously Determined 73Ge NMR Chemical Shift Data compound

δ 73Ge (ppm)b

ref

Ge2H6 Ge2D6 Ge3H8 Ge4H10 MeH2GeGeH3 Me2HGeGeMe2H Me2HGeGeH3 Me2ClGeGeH3 Me3GeGeH3 (MeH2Ge)2GeMeH Me3GeGeMe3 Et3GeGeEt3 Ph3GeGePh3 (Ph3Ge)3GeH H3GeGeH2Mn(CO)5 H3GeGeMeHMn(CO)5

-311.8 (95.5) -318.0 -298 (94), -310 (90) -284, -300 -306.2 (90.0) -209 -127, -296 (85) -280.5 -47.7, -295.6 (90.7) -125, -206.2 -59 -34.7 -67 -314 -291.8 -277.9

28 18 28 28 28 28 28 18 28 28 18 30 18 18 18 18

a Chemical shifts are relative to GeMe4. coupling constants are given in parentheses.

Figure 1. ORTEP plot of PhMe2GeGePh3 (5). Thermal ellipsoids are drawn at 50% probability. Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for PhMe2GeGePh3 (5) Ge(1)-Ge(2) Ge(1)-C(1) Ge(1)-C(7) Ge(1)-C(13) Ge(2)-C(19) Ge(2)-C(25) Ge(2)-C(26) C(1)-Ge(1)-C(7) C(1)-Ge(1)-C(13) C(7)-Ge(1)-C(13)

2.4216(4) 1.965(4) 1.953(3) 1.955(4) 1.959(3) 1.955(4) 1.951(4) 109.5(2) 109.7(1) 106.9(2)

C(19)-Ge(2)-C(25) C(19)-Ge(2)-C(26) C(25)-Ge(2)-C(26) C(1)-Ge(1)-Ge(2) C(7)-Ge(1)-Ge(2) C(13)-Ge(1)-Ge(2) C(19)-Ge(2)-Ge(1) C(25)-Ge(2)-Ge(1) C(26)-Ge(2)-Ge(1)

107.6(2) 111.3(2) 108.6(2) 106.7(1) 111.5(9) 112.3(1) 107.1(2) 109.0(1) 113.2(1)

the LUMO, resulting in a lower energy for the σ f σ* electronic transition.21 Therefore, the λmax for 12 is expected to be blueshifted relative to that for 11. Crystals of 5 suitable for X-ray analysis were obtained from a hot benzene solution, and an ORTEP diagram of 5 is shown in Figure 1, while selected bond distances and angles are collected in Table 1. The Ge-Ge bond distance in 5 is 2.4216(4) Å, which is slightly longer than those in the related compounds Me3GeGePh3 (2.418(1) Å)26 and Bun3GeGePh3 (1) (2.4208(8) Å, average of two independent molecules),19 but shorter than the Ge-Ge bond distance in Et3GeGePh3 (3) (2.4253(7) Å).19 The ethyl and phenyl substituents in 3 are eclipsed, resulting in the longer Ge-Ge distance, while the substituents at each of the germanium atoms in 5, Me3GeGePh3, and 1 are arranged in a gauche conformation. The presence of a phenyl substituent at Ge(2) in 5, versus three methyl groups in Me3GeGePh3 or three n-butyl groups that are attached to germanium via a methylene group in 1, results in the slightly elongated Ge-Ge distance in 5. Two of the C-Ge-C bond angles at Ge(1) very closely match the ideal angle of 109.5°, while the third is slightly more acute. The C-Ge-C bond angles at Ge(2) are more highly distorted from the ideal tetrahedral geometry, although the average bond angle is 109.2°. These distortions result from the steric interaction of the single phenyl substituent at Ge(2) with the three phenyl groups attached to Ge(1). Although observation of the 73Ge nucleus by NMR spectroscopy is challenging for the reasons described above, a significant (26) Pa´rka´nyi, L.; Ka´lma´n, A.; Sharma, S.; Nolen, D. M.; Pannell, K. H. Inorg. Chem. 1994, 33, 180–182.

b

Experimental 1J(Ge-H)

amount of information has been accumulated in the last several years, including chemical shifts, coupling constants, and relaxation times for various compounds that do not have a symmetric environment about the germanium atom. In contrast to simple germanes, 73Ge NMR data for compounds containing Ge-Ge single bonds are relatively scarce. Chemical shift data for 16 different compounds is collected in Table 2, and some trends in these values can be identified. Resonances for germanium atoms bearing hydride substituents appear upfield relative to those having methyl or ethyl substituents, while the signal for the two equivalent germanium atoms of Ph3GeGePh3 (δ -67 ppm) appears upfield from that of Me3GeGeMe3 (δ -59 ppm). Peaks for some of the germanium atoms in the tabulated compounds were not observed, as found for the branched germanium hydride (Ph3Ge)3GeH, which exhibits a peak for the central germanium atom at δ -314 ppm.18 No resonance was observed for the germanium atoms of the peripheral Ph3Ge- groups. We have obtained 73Ge NMR data for several linear and branched oligogermanes that we have prepared via the hydrogermolysis reaction. The structures of these species are shown in Scheme 2, and chemical shifts and half-height linewidths for these compounds are collected in Table 3. An acquisition time of 0.01 s, zero-filling, and a line broadening of 20 were used for all spectra, except for the digermane Bus3GeGePh3 (2), where an acquisition time of 0.1 s was employed. The lines are broad, as expected, and this is in part a result of the short acquisition time. The chemical shifts reported in Table 3 have an associated error of (3 ppm. Spectra for several samples were run using longer acquisition times, but this did not allow for the acquisition of a sufficient number of scans to obtain a reasonable signalto-noise ratio. No correction was applied in cases where the resonances overlap, but the chemical shift data obtained in spectra where overlapping resonances occur were run several different times and the data were consistently reproducible. From the chemical shifts observed for the digermanes 1-6, it is apparent that triakyl-substituted germanium atoms resonate at lower field than triphenyl-substituted germanium atoms, and these findings are consistent with the results of previous 73Ge NMR spectral investigations of simple alkyl- and aryl-germanes.1,2,5,27-31 The 73Ge NMR spectra obtained for the digermanes 1-3 are shown in Figure 2. The single 73Ge NMR resonance observed for compound 2 at δ -52 ppm corresponds to the sec-butylsubstituted germanium atom, while the single features present in the 73Ge NMR spectra of 3 at δ -64 ppm correspond to the triphenyl-substituted germanium atom. Compound 1 exhibits a

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Amadoruge et al. Scheme 2

Table 3.

Ge NMR Dataa for Various Oligogermanes (L ) CH2CH2OEt)

73

compound

δ (ppm)

∆ν1/2 (Hz)

assignment

1

-58 -65 -52 -64 -56 -65 -65 -57 -64 -57 -63 -54 -65 -111 -121 -202 -33 -195 -43 -203 -38 -209

100 340 30b 270 80 240 90 90 330 310 310 120 390 180 170 290 100 240 90 380 110 320

Bun3GeGePh3 Bun3GeGePh3 Bus3GeGePh3 Et3GeGePh3 Pri3GeGePh3 Pri3GeGePh3 PhMe2GeGePh3 Ph3GeGeBun2L Ph3GeGeBun2L Ph3GeGeBun2GeBun2L Ph3GeGeBun2GeBun2L Ph3GeGeBun2GePh2L Ph3GeGeBun2GePh2L LGeBun2GePh2GeBun2L LGePh2GePh2GePh2L PhGe(GePh3)3 PhGe(GeBun3)3 PhGe(GeBun3)3 PhGe(GeBun2L)3 PhGe(GeBun2L)3 PhGe(GeBun2GeBun2L)3 PhGe(GeBun2GeBun2L)3

2 3 4 5 6 7 8 9 10 11 12 13 14

a Chemical shifts are relative to external GeMe4 by substitution. Chemical shift values are (3 ppm, and ∆ν1/2 are (10%. No correction for overlapping peaks was applied. Acquisition time ) 0.01 s. b Acquisition time ) 0.1 s.

peak for each type of germanium atom, and the resonance for the tri-n-butyl-substituted germanium atom appears at δ -58 ppm, while that for the triphenyl-substituted germanium atom appears upfield at δ -65 ppm. The spectrum of the digermane 5 exhibits a single peak for the Ph3Ge- atom at δ -65 ppm, while that for 4 contains two resonances corresponding to each type of germanium atom. Although chemical shifts observed in 73Ge NMR spectroscopy are highly sensitive to the substitution pattern at germanium, it

has been demonstrated that there is not a linear correlation between inductive effects and 73Ge NMR chemical shift values.28-30 For example, the tetraalkyl germanes GeR4 exhibit chemical shifts of δ 0.0 (R ) Me), 17.8 (R ) Et), 2.4 (R ) Prn), and 6.0 (R ) Bun) ppm.1 However, among the digermanes 1-4, the observed chemical shift values exhibit a consistent upfield shift as the calculated inductive (σI) and polar (σ*) substituent constants32 of the alkyl groups become more negative, and the relationship between δ(73Ge) for the R3Gegroups and each of these constants is nearly linear (Figure 3). Germanium-73 NMR spectra for the two linear compounds Ph3GeGeBun2CH2CH2OEt (6) and Ph3GeGeBun2GePh2CH2CH2OEt (8) and the branched oligomers PhGe(GeBun2CH2CH2OEt)3 (13) and PhGe(GeBun2GeBun2CH2CH2OEt)3 (14) are shown in Figures 4 and 5, respectively. The digermane 6 exhibits two resonances in its 73Ge NMR spectrum at δ -57 and -64 ppm, where the former resonance corresponds to the alkylsubstituted germanium atom and the latter corresponds to the triphenyl-substituted germanium atom. The spectrum of 8 exhibits a resonance for two of the three germanium atoms, with a feature for the terminal Ph3Ge- atom at δ -65 ppm and a resonance at δ -54 ppm that corresponds to the -GePh2CH2CH2OEt atom. This resonance is shifted upfield relative to triphenyl-substituted germanium atoms due to the presence of the ethoxyethyl substituent. (27) Zicmane, I.; Liepins, E.; Lukevics, E.; Gar, T. K. Zh. Obsch. Khim. 1982, 52, 896–899. (28) Wilkins, A. L.; Watkinson, P. J.; Mackay, K. M. J. Chem. Soc., Dalton Trans. 1987, 2365–2372. (29) Mackay, K. M.; Thomson, R. A. Main Group Met. Chem. 1987, 10, 83–108. (30) Liepins, E.; Zicmane, I.; Lukevics, E. J. Organomet. Chem. 1988, 341, 315–333. (31) Takeuchi, Y.; Takayama, T. Annu. Rep. NMR Spectrosc. 2005, 54, 155–200. (32) Hanson, P. J. Chem. Soc., Perkin Trans. 2 1984, 101–108.

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Organometallics, Vol. 28, No. 10, 2009 3071

Figure 3. Plots of the 73Ge NMR chemical shift (in ppm) of 1-4 versus the calculated inductive substituent constant (top) and polar substituent constant (bottom).

Figure 2. 73Ge NMR spectra of R3GeGePh3 (R ) Bun (1), Bus (2), or Et (3)).

The number of germanium-germanium bonds to each germanium atom can have a pronounced effect on the 73Ge chemical shift in these compounds. Both of the germanium atoms in the digermanes 1-6, as well as the terminal germanium atoms in the higher oligomers 7-14, are bound to only one other germanium atom. Resonances for these germanium atoms range from δ -33 ppm in 12 to δ -65 ppm in 8, which compare with the previously determined values of δ -35, -59, and -67 ppm for the compounds R3GeGePh3 (R ) Et,30 Me,18 Ph,18 respectively). Trends for the resonances corresponding to the terminal germanium centers in 7-14 are similar to those observed for the digermanes 1-6, in that phenyl substituents result in an upfield shift of the observed 73Ge NMR resonances. The trigermanes 7-10 and the branched species 14 each contain a germanium atom that is attached to two other germanium atoms, and for compounds 9 and 10 resonances corresponding to the central phenyl-substituted germanium atoms were observed at δ -111 and -121 ppm (respectively). Resonances for the germanium atoms bound to two other germanium centers in compounds 7, 8, and 14 were not observed. The upfield shift for these peaks in 9 and 10 versus those observed for germanium centers bound to only one germanium atom (Vide supra) is a result of the increased shielding resulting from attachment to

two germanium atoms. This effect is more evident in the data collected for the branched oligogermanes 11-14, where the central germanium atoms in these species are substantially more shielded due to their being connected to three other germanium atoms. Resonances for these atoms are shifted upfield and range from δ -195 to -209 ppm, and a similar chemical shift was reported for the central germanium atom of (Ph3Ge)3GeH at δ -314 ppm.18

Conclusions The 73Ge NMR resonances for oligogermanes can be correlated with the substitution pattern at germanium and also with the connectivity of the germanium centers. Germanium centers bound to only one additional germanium atom give rise to peaks in the approximate range δ -30 to -65 ppm, while the more shielded germanium centers which are connected to two or three additional germanium atoms exhibit resonances in the respective ranges of δ -100 to -120 and δ -195 to -210 ppm. In all cases, resonances for alkyl-substituted germanium atoms appear downfield from their phenyl-substituted analogues, which is consistent with previous findings for related systems. Due to the limitation that some substitution patterns do not allow an observable 73Ge NMR resonance, 73Ge NMR spectroscopy is not as versatile a technique as that for the corresponding group 14 elements carbon, silicon, tin, and lead. However, these investigations described here indicate that structural information

3072 Organometallics, Vol. 28, No. 10, 2009

Figure 4. 73Ge NMR spectra of Ph3GeGeBun2CH2CH2OEt (6) and Ph3GeGeBun2GePh2CH2CH2OEt (8).

can be obtained using this method, and the compilation of a database of 73Ge NMR spectral data is a worthwhile endeavor.

Experimental Section All manipulations were carried out using standard Schlenk, syringe, and glovebox techniques.33 The reagents BusMgCl (2.0 M in Et2O solution), LiNMe2, and Ph3GeH were purchased from Aldrich, while GeCl4 and PhMe2GeCl were purchased from Gelest, and were used as received. Solvents were purified using a Glass Contour solvent purification system. 1H (400 MHz) and 13C (100.5 MHz) NMR spectra were recorded on a Varian INOVA 400 MHz spectrometer and were referenced to the benzene-d6 solvent. Germanium-73 NMR spectra of the products (50 mg/mL in benzene-d6) were recorded on a Varian INOVA 500 MHz spectrometer using a 10 mm low gamma broadband probe at 17.43 MHz with the Carr-Purcell-Maiboom-Gill (CPMG) pulse sequence34,35 to reduce baseline roll. The following parameters were used with proton decoupling during acquisition: spectral width ) 100 000 Hz, acquisition time ) 0.01 s (except for compound 2), delay time ) 0 s, line broadening factor ) 20, number of transients ) 1 × 106 to 1 × 107. This pulse sequence was found to give peak widths within ca. 5% of those obtained using the standard pulse sequence up to peak widths of ca. 800 Hz. Based on multiple runs of the same sample, the error in the chemical shifts is estimated to be (3 ppm. The error in helf-height linewidths is ca. 10%. No correction was applied to the measurement of overlapping peaks. The spectra were referenced to external GeMe4 by substitution. Elemental analyses were obtained from Midwest Microlab, LLC (Indianapolis, IN). Preparation of Bus3GeCl. A solution of BusMgCl (44.0 mL, 2.0 M in Et2O, 88.0 mmol) was added to a solution of GeCl4 (6.25 g, 29.1 mmol) in Et2O (140 mL) at 0 °C over 30 min. The reaction (33) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air SensitiVe Compounds, 2nd ed.; John Wiley and Sons: New York, 1986. (34) Carr, H. Y.; Purcell, E. M. Phys. ReV. 1954, 94, 630–638. (35) Meiboom, S.; Gill, D. ReV. Sci. Instrum. 1958, 29, 688–691.

Amadoruge et al.

Figure 5. 73Ge NMR spectra of PhGe(GeBun2CH2CH2OEt)3 (13) and PhGe(GeBun2Gen2CH2CH2OEt)3 (14). Table 4. Crystallographic Data for 5 formula fw (g mol-1) cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalc (g cm-3) absorp coeff (mm-1) F(000) θ range (deg) index ranges reflns collected indep reflns completeness to θ ) 25.00° absorp corr max. and min. transmn refinement method data/restraints/params goodness-of-fit on F2 temperature (K) radiation wavelength (Å) R Rw largest peak and hole (e Å-3)

C26H26Ge2 483.65 0.07 × 0.07 × 0.02 orthorhombic Pna2 24.7570(3) 7.7560(5) 11.701(1) 90 90 90 2246.8(3) 4 1.430 2.684 984 1.65 to 28.20 -30 e h e 31, -10 e k e 10, -15 e l e 15 29 898 5198 (Rint ) 0.0567) 100.0% semiempirical from equivalents 0.9483 and 0.8344 full-matrix least-squares on F2 5198/1/255 1.048 100(2) Mo KR 0.71073 0.0332 0.0655 0.952 and -0.324

mixture was stirred at 0 °C for 1 h, and the volatiles were removed in Vacuo. The resulting material was dissolved in hexane (100 mL) and treated with 50 mL of 1.0 M HCl. The organic layer was separated and the aqueous layer was extracted with hexane (3 × 15 mL). The combined organic layer and extracts were dried over

73

Ge NMR Spectral InVestigations of Oligogermanes

anhydrous MgSO4 and filtered, and the volatiles were removed in Vacuo to yield a colorless oil. Vacuum distillation (47 °C, 0.25 Torr) yielded Bus3GeCl (2.20 g, 27%) as a colorless oil. 1H NMR (C6D6, 25 °C): δ 1.77-1.66 (m, 3H, CH3CHCH2CH3), 1.38 - 1.31 (m, 6H, CH3CHCH2CH3), 1.12-1.07 (m, 9H, CH3CHCH2CH3), 0.89 (t, J ) 9.6 Hz, 9H, CH3CHCH2CH3) ppm. 13C NMR (C6D6, 25 °C): δ 27.00, 26.97, 26.94 (CH3CHCH2CH3), 25.95, 25.32 (CH3CHCH2CH3), 15.01, 14.96 (CH3CHCH2CH3), 13.4 (CH3CHCH2CH3) ppm. Anal. Calcd for C12H27ClGe: C, 51.59; H, 9.74. Found: C, 51.69; H, 9.50. Preparation of Bus3GeNMe2. A flask was charged with 0.274 g (0.981 mmol) of Bus3GeCl dissolved in benzene (10 mL). To this was added LiNMe2 (0.057 g, 1.12 mmol) in benzene (10 mL). The resulting suspension was stirred 36 h and then filtered through Celite to yield a clear solution. The volatiles were removed in Vacuo to yield Bus3GeNMe2 (0.221 g, 78%) as a slightly turbid colorless oil. 1H NMR (C6D6, 25 °C): δ 2.67, 2.66, 2.65 (s, 6H, -N(CH3)2), 1.78-1.70 (m, 3H, CH3CHCH2CH3), 1.36-1.30 (m, 6H, CH3CHCH2CH3), 1.16-1.11 (m, 9H, CH3CHCH2CH3), 0.89 (t, J ) 7.2 Hz, 9H, CH3CHCH2CH3) ppm. 13C NMR (C6D6, 25 °C): δ 42.44, 42.11 (-N(CH3)2), 27.03, 26.98 (CH3CHCH2CH3), 25.93 (CH3CHCH2CH3), 15.04, 14.86 (CH3CHCH2CH3), 13.61, 13.44, 13.27 (CH3CHCH2CH3) ppm. Anal. Calcd for C14H33GeN: C, 58.38; H, 11.55. Found: C, 58.32; H, 11.24. Preparation of PhMe2GeNMe2. To a solution of PhMe2GeCl (1.000 g, 4.646 mmol) in Et2O (15 mL) was added a solution of LiNMe2 (0.249 g, 4.88 mmol) in Et2O (10 mL). The reaction mixture was stirred for 18 h, and the ether was removed to yield a yellow oil, which was dissolved in hexane and filtered through Celite. The volatiles were removed from the filtrate to yield PhMe2GeNMe2 (0.775 g, 75%) as a colorless oil. 1H NMR (C6D6, 25 °C): δ 7.61 (d, J ) 6.4 Hz, 2H, m-H), 7.37-7.32 (m, 3H, o-H and p-H), 2.64 (s, 6H, -N(CH3)2), 0.49 (s, 6H, -CH3) ppm. 13C NMR (C6D6, 25 °C): δ 133.79 (ipso-C), 129.20 (ortho-C), 128.40 (meta-C), 128.08 (para-C), 40.94 (-N(CH3)2), - 4.33 (-CH3) ppm. Anal. Calcd for C10H17GeN: C, 53.66; H, 7.65. Found: C, 53.36; H, 7.41. Preparation of Bus3GeGePh3 (2). To a solution of Bus3GeNMe2 (0.201 g, 0.698 mmol) in acetonitrile (5 mL) was added a solution of Ph3GeH (0.220 g, 0.721 mmol) in acetonitrile (10 mL). The reaction mixture was sealed in a Schlenk tube and heated at 90 °C for 72 h. The volatiles were removed in Vacuo to yield a viscous yellow oil, which was Kugelrohr distilled (135 °C, 0.05 Torr) to yield Bus3GeGePh3 (0.308 g, 81%) as a colorless oil. 1H NMR (C6D6, 25 °C): δ 7.68 (t, J ) 7.2 Hz, 3H, para-H), 7.63 (d, J ) 7.2 Hz, 6H, ortho-H), 7.13-7.08 (m, 6H, meta-H), 1.78-1.73 (m, 3H, CH3CHCH2CH3), 1.46-1.23 (m, 6H, CH3CHCH2CH3), 1.13-1.04 (m, 9H, CH3CHCH2CH3), 0.77 (t, J ) 7.6 Hz, 9H, CH3CHCH2CH3) ppm. 13C NMR (C6D6, 25 °C): δ 138.91 (ipso-C), 135.33 (orthoC), 128.83 (meta-C), 128.44 (para-C), 28.80, 28.64 (CH3CHCH2CH3), 26.83 (CH3CHCH2CH3), 14.49, 14.46 (CH3CHCH2CH3), 13.78 (CH3CHCH2CH3) ppm. Anal. Calcd for C30H42Ge2: C, 65.77; H, 7.73. Found: C, 65.72; H, 7.88. Synthesis of PhMe2GeGePh3 (5). To a solution of PhMe2GeNMe2 (0.220 g, 0.983 mmol) in acetonitrile (10 mL) was added

Organometallics, Vol. 28, No. 10, 2009 3073 a solution of Ph3GeH (0.302 g, 0.990 mmol) in acetonitrile (10 mL). The reaction mixture was stirred at 85 °C for 96 h, and the volatiles were removed in Vacuo to yield a yellow viscous oil. The crude product was vacuum distilled (120 °C, 0.05 Torr) to yield 5 (0.360 g, 76%) as a colorless solid. 1H NMR (C6D6, 25 °C): δ 7.58-7.55 (m, 6H, m-(C6H5)3), 7.44-7.41 (m, 2H, m-C6H5), 7.17-7.14 (m, 9H, o-(C6H5)3 and p-(C6H5)3), 7.12-7.09 (m, 3H, o-C6H5 and p-C6H5), 0.64 (s, 6H, -CH3) ppm. 13C NMR (C6D6, 25 °C): δ 135.80 (ipso-C), 134.19 (ipso-C), 129.00 (ortho-C), 128.72 (ortho-C), 128.39 (meta-C), 128.22 (meta-C), 128.76 (paraC), 128.68 (para-C), 1.94 (-CH3) ppm. Anal. Calcd for C26H26Ge2: C, 64.57; H, 5.42. Found: C, 64.28; H, 5.64. Synthesis of PhGe(GeBun3)3 (12). To a solution of Bun3GeNMe2 (0.819 g, 2.84 mmol) in acetonitrile (15 mL) was added a solution of PhGeH3 (0.132 g, 0.864 mmol) in acetonitrile (5 mL). The reaction mixture was stirred at 85 °C for 72 h, and the volatiles were removed in Vacuo to yield a colorless oil. The crude product mixture was vacuum distilled at 140 °C (0.01 Torr) to yield 0.747 g (89%) of 12 as a colorless oil. 1H NMR (C6D6, 25 °C): δ 7.77 (d, 1H, J ) 7.8 Hz, para-H), 7.68 (d, 2H, J ) 7.8 Hz, ortho-H), 7.08 (t, 2H, J ) 7.5 Hz, meta-H), 1.54-1.24 (m, 36H, -CH2CH2CH2CH3), 1.11-1.06 (m, 18H, -CH2CH2CH2CH3), 0.97-0.82 (m, 27H, -CH2CH2CH2CH3) ppm. 13C NMR (C6D6, 25 °C): δ 136.22 (ipso-C), 128.41 (ortho-C), 128.19 (meta-C), 127.38 (para-C), 28.92 (-CH2CH2CH2CH3), 27.00 (-CH2CH2CH2CH3), 15.31 (-CH2CH2CH2CH3), 13.94 (-CH2CH2CH2CH3) ppm. UV/vis (hexane): λmax 233 nm (ε 1.8 × 195 L mol-1 cm-1). Anal. Calcd for C42H86Ge4: C, 57.23; H, 9.83. Found: C, 56.77; H, 9.44. X-ray Crystal Structure of PhMe2GeGePh3 (5). Diffraction intensity data were collected with a Siemens P4/CCD diffractometer. Crystallographic data for the X-ray analysis of 5 are collected in Table 4. Crystal-to-detector distance was 60 mm and exposure time was 20 s per frame using a scan width of 0.5°. Data collection was 100.0% complete to 25.00° in θ. The data were integrated using the Bruker SAINT software program and scaled using the SADABS software program. Solution by direct methods (SIR-2004) produced a complete heavy-atom phasing model consistent with the proposed structure. 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.

Acknowledgment. Funding for this work by the Department of Chemistry at Oklahoma State University and a CAREER grant from the National Science Foundation (CHE0844758) is gratefully acknowledged. Supporting Information Available: Crystallographic data for 5 in.cif format and 73Ge NMR spectra for compounds 4, 5, 7, and 9-12. This material is available free of charge via the Internet at http://pubs.acs.org. OM900035R