On the Bonding in N-Heterocyclic Carbene Complexes of Germanium

Feb 15, 2010 - E-mail: [email protected]. ... over LGe–Cl and LGe(E)–Cl toward Me3SiX (L = Aminotroponiminate; NR2 = N(SiMe3)2/NC4H4; E = S/Se; X = ...
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Organometallics 2010, 29, 1362–1367 DOI: 10.1021/om900977g

On the Bonding in N-Heterocyclic Carbene Complexes of Germanium(II) Adam J. Ruddy, Paul A. Rupar, Kamila J. Bladek, Christopher J. Allan, Jessica C. Avery, and Kim M. Baines* Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7, Canada Received November 9, 2009

A series of N-heterocyclic carbene (NHC) complexes of Ge(II) were synthesized and structurally characterized. Unlike previous observations, the carbenic carbon-germanium bond length does not vary systematically with the electronic demands of the substituents on the germanium center. Computational analysis of the energetic and structural metrics of model compounds is consistent with a lack of a substituent effect on the carbenic carbon-germanium bond length.

Introduction Germylenes, divalent germanium(II) compounds, are isovalent with singlet carbenes.1 Like singlet carbenes, the frontier molecular orbitals of Ge(II) species consist of a lone pair of electrons and an empty p-orbital localized on germanium, which makes the center both Lewis acidic and Lewis basic. As a result of their amphoteric properties and the fact that they are in an intermediate oxidation state, simple Ge(II) compounds are, in general, very reactive. The donation of electron density into the empty p-orbital on germanium by the introduction of a Lewis base moderates the reactivity of germylenes. Most base-stabilized germylenes use an intramolecular donor.1 Although intermolecularly base-stabilized germylenes are less common, some examples, such as GeCl2 3 dioxane,2 represent important reagents in germanium chemistry. *Corresponding author. E-mail: [email protected]. (1) Reviews on germanium(II) chemistry: (a) Neumann, W. P. Chem. Rev. 1991, 91, 311. (b) Barrau, J.; Rima, G. Coord. Chem. Rev. 1998, 178-180, 593. (c) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (d) Satge, J. Chem. Heterocycl. Compd. 1999, 35, 1013. (e) Boganov, S. E.; Faustov, V. I.; Egorov, M. P.; Nefedov, O. M. Russ. Chem. Bull., Int. Ed. 2004, 53, 960. (f ) Zemlyanskii, N. N.; Borisova, I. V.; Nechaev, M. S.; Khrustalev, V. N.; Lunin, V. V.; Antipin, M. Y.; Ustynyuk, Y. A. Russ. Chem. Bull., Int. Ed. 2004, 53, 980. (g) K€uhl, O. Coord. Chem. Rev. 2004, 248, 411. (h) Leung, W. P.; Kan, K. W.; Chong, K. H. Coord. Chem. Rev. 2007, 251, 2253. (i) Saur, I.; Alonso, S. G.; Barrau, J. Appl. Organomet. Chem. 2005, 19, 414. ( j) Nagendran, S.; Roesky, H. W. Organometallics 2008, 27, 457. (k) Weinert, C. S. In Comprehensive Organometallic Chemistry III, Vol 3; Mingos, D. M. P., Crabtree, R. H., Housecroft, C. E., Eds.; Elsevier: Oxford, 2007; pp 699-808. (l) Mizuhata, Y.; Sasamon, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479. (2) Kulishov, V. I; Bokii, N. G.; Struchkov, Yu. T.; Nefedov, O. M.; Kolesnikov, S. P.; Perl’mutter, B. M. Zh. Strukt. Khim. 1970, 11, 71. (3) Selected recent examples: (a) Fillippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem., Int. Ed. 2009, 48, 5687. (b) Ghadwai, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 5683. (c) Quillian, B.; Wei, P. R.; Wannere, C. S.; Schleyer, P. V.; Robinson, G. H. J. Am. Chem. Soc. 2009, 131, 3168. (d) Wang, Y. Z.; Xie, Y. M.; Wei, P. R.; King, R. B.; Schaefer, H. F.; Schleyer, P. V.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 14970. (e) Wang, Y. Z.; Xie, Y. M.; Wei, P. R.; King, R. B.; Schaefer, H. F.; Schleyer, P. V.; Robinson, G. H. Science 2008, 321, 5892. (f ) Xiong, Y.; Yao, S. L.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562. (g) Dutton, J. L.; Tuononen, H. M.; Ragogna, P. J. Angew. Chem., Int. Ed. 2009, 48, 4409. (h) Ellis, B. D.; Dyker, C. A.; Decken, A.; Macdonald, C. L. B. Chem. Commun. 2005, 1965. pubs.acs.org/Organometallics

Published on Web 02/15/2010

Increasingly, N-heterocyclic carbenes (NHCs) are being employed in p-block chemistry for the stabilization of unusual and reactive compounds.3,4 We have described the synthesis of a series of NHC complexes of Ge(II) (1-5, Chart 1) and showed that the coordination of NHCs to divalent germanium allows the isolation of otherwise transient germylenes (i.e., 4 and 5).5-12 These NHC-GeR2 complexes have proven to be useful reagents in the synthesis of cationic germanium(II) complexes,6,13 including an unprecedented cryptand-supported Ge2þ ion.14 Ol ah et al. have studied Lewis acid-base interactions between silicon(II) or germanium(II) compounds and the neutral donors NH3, PH3, and AsH3 computationally.15 In general, π-donating substituents on the heavy group 14 element reduce the interaction energy between the substituted germylene and a donor, presumably because of the transfer of electron density into the empty p-orbital. For germanium, interaction energies decrease in the following order: (forms energetically most favorable complex) GeH2 (4) Kuhn, N.; Al-Sheikh, A. Coord. Chem. Rev. 2005, 249, 829. (5) Rupar, P. A.; Jennings, M. C.; Baines, K. M. Organometallics 2008, 27, 5043. (6) Rupar, P. A.; Staroverov, V. N.; Ragogna, P. J.; Baines, K. M. J. Am. Chem. Soc. 2007, 129, 15138. (7) Rupar, P. A.; Jennings, M. C.; Ragogna, P. J.; Baines, K. M. Organometallics 2007, 26, 4109. (8) Examples of intrinsically stable germylenes coordinated by carbenes have reported by others. See: (a) Arduengo, A. J., III; Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Inorg. Chem. 1993, 32, 1541. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Dalton Trans. 2000, 3094. (9) An N-heterocyclic gallium anion can also stabilize a reactive germylene. See: Rupar, P. A.; Jennings, M. C.; Baines, K. M. Can. J. Chem. 2007, 85, 141. (10) Yao, S.; Xiong, Y.; Driess, M. Chem. Commun. 2009, 6466. (11) Thimer, K. C.; Al-Rafia, S. M. I.; Ferguson, M. J.; McDonald, R.; Rivard, E. Chem. Commun. 2009, 7119. (12) Digermanium(0) can also be stabilized by NHCs. See: Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Frenking, G. Angew. Chem., Int. Ed. 2009, 48, 9701. (13) Rupar, P. A.; Bandyopadhyay, R.; Cooper, B. F. T.; Stinchcombe, M. R.; Macdonald, C. L. B.; Ragogna, P. J.; Baines, K. M. Angew. Chem., Int. Ed. 2009, 48, 5155. (14) Rupar, P. A.; Staroverov, V. N.; Baines, K. M. Science 2008, 322, 5906. (15) Olah, J.; Proft, F. D.; Veszpremi, T.; Geerlings, P. J. Phys. Chem. A 2005, 109, 1608. r 2010 American Chemical Society

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Organometallics, Vol. 29, No. 6, 2010 Chart 1

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Scheme 1

Scheme 2

Chart 2

> GeHCH3 > GeCl2 ≈ GeF2 > Ge(OH)2 > Ge(NH2)2 (forms least energetically favorable complex).15 Upon examination of the structures of the NHC complexes of Ge(II) as determined by X-ray crystallography, a trend in the variation of the carbenic C-Ge bond length with respect to the π-donating ability of atoms located on germanium was observed. This is best illustrated by a comparison of the metrics of 16 (Cl-substituted) with 57 (Mes-substituted). On the basis of steric arguments and the electronegativity of the substituents, 1 may be expected to have the shorter C-Ge bond length since chlorine is more electron withdrawing than mesityl. However, 1 was observed to have a long carbenic C-Ge bond at 2.106(3) A˚, while the carbenic C-Ge bond in 5 is short at 2.078(3) A˚.6,7 These observations appeared consistent with Olah et al.’s findings:15 the lone pairs of electrons on chlorine donate electron density into the σ*-orbital of the carbenic carbon-germanium bond and, consequently, the bond length between C and Ge is elongated. In contrast, the π-electrons of the mesityl substituents are relatively poor electron donors, particularly to germanium, and the carbenic carbongermanium bond is one of the shortest in the series. In spite of the foregoing discussion, the trend observed for the Ge-carbenic carbon bond length in the NHC-GeR2 complexes may be a result of fortuitous crystal packing or other effects rather than electronic effects. Moreover, the computational analysis performed by Ol ah et al. did not specifically investigate the length of the bond between the donor atom and the germanium center. In this context, we now report on the structural characterization of a second series of NHC-GeR2 complexes and the results of a computational analysis of the bonding in model compounds. NHC 6 was selected for this study because it is structurally and electronically similar to NHC 7, which was used in our previous work (Chart 2).16 Furthermore, the replacement of the isopropyl substituents at nitrogen with methyl groups should minimize any effect of steric interactions on the structures of the complexes.

Results and Discussion Complexes 8-12 were synthesized in a manner similar to 1-5.5,6 The chloride derivative 8 was synthesized by the (16) Kuhn, N.; Kratz, T. Synthesis 1993, 561.

addition of NHC 6 to GeCl2 3 dioxane (Scheme 1). From complex 8, the dibromide 9 and diiodide 10 derivatives were formed by halide exchange using either Me3SiBr or Me3SiI, respectively. Both the tert-butoxy (11) and mesityl (12) substituted complexes were synthesized by the nucleophilic substitution of the chlorides in 8 with KOtBu or Mes2Mg, respectively (Scheme 2).17 The solid-state structures of 8-12 were determined by single-crystal X-ray diffraction (Figures 1-3, Table 1). Two crystal polymorphs of 8 were identified; one polymorph had an occluded C6H6 molecule in the unit cell, while the other polymorph was unsolvated. Two crystallographically distinct molecules were found in the unit cell of 10. The structural morphologies of 8-12 were similar to the previously reported structures of 1-5 (Tables 1 and 2).5-7 Specifically, the bond angles around germanium in the halogenated and tert-butoxy-substituted complexes are close to 90°. As a result of the increased steric bulk of the mesityl groups, the bond angles around germanium in 12 are more obtuse compared to 8-11; this was similar to what was observed with 5.7 As illustrated in Table 2, the trend observed with the carbenic C-Ge bond lengths in the analogous NHC-GeR2 complexes, 1-5, is not observed in the new series of compounds, 8-12. Within the experimental error of the measurements, there is little statistical difference in the carbenic C-Ge bond lengths in the MeNHC-germylene complexes 8-12. This observation suggests that the C-Ge bond lengths are not dependent on the π-donating ability of the ligands (17) The mesityl-substituted 12 can also be synthesized from the reaction of two equivalents of NHC 6 to tetramesityldigermene. See ref 7 and Hurni, K. L.; Rupar, P. A.; Payne, N. C.; Baines, K. M. Organometallics 2007, 26, 5569.

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Ruddy et al. Table 1. Structural Metrics of Compounds 8-12 compound NHC-GeR2

Figure 1. Thermal ellipsoid plot (30% probability surface) of 8. Hydrogen atoms are omitted for clarity. Compounds 9 and 10 are isomorphous to 8 and are not shown. See Table 1 for selected structural metrics.

8, solvated polymorph 8, unsolvated polymorph, molecule 1 8, unsolvated polymorph, molecule 2 9 10, molecule 1 10, molecule 2 11 12 a

substituent (R)

carbenic C-Ge bond length (A˚)

R-Ge-R NHC-Ge-R bond bond angle angle (deg) (deg)a

Cl

2.088(4)

97.04(6)

93.67(13)

Cl

2.103(7)

96.05(7)

93.89(17)

Cl

2.098(7)

98.70(8)

92.74(18)

Br I I OtBu Mes

2.085(5) 2.099(7) 2.102(7) 2.110(5) 2.067(3)

97.63(3) 96.97(3) 95.37(3) 94.60(18) 106.60(12)

94.26(13) 95.95(2) 96.45(2) 90.31(19) 100.87(12)

Reported as an average of the two NHC-Ge-R angles.

Table 2. Comparison of Carbenic C-Ge Bond Lengths of the MeNHC- and iPrNHC-Substituted Germylenes substituent on Ge

MeNHC carbenic C-Ge bond length (A˚)

Cl Br I OtBu Mes

2.088(4), 2.103(7), 2.098(7) 2.085(5) 2.099(7), 2.102(7) 2.110(5) 2.067(3)

i

PrNHC carbenic C-Ge bond length (A˚)

2.106(3)6 2.089(5)5 2.086(3)6 av 2.1725 2.078(3)7

Chart 3

Figure 2. Thermal ellipsoid plot (30% probability surface) of 11. Hydrogen atoms are omitted for clarity. See Table 1 for selected structural metrics.

Figure 3. Thermal ellipsoid plot (30% probability surface) of 12. Hydrogen atoms are omitted for clarity. See Table 1 for selected structural metrics.

under study, a series of simplified compounds (13-20) were examined (Chart 3), where the vinylic methyl groups of the carbene were replaced with hydrogen atoms. Two different model chemistries were employed: MP2/6-311þG(d,p) and PBE1PBE/6-311þG(d,p).18,19 The ΔEcmplx for a given complex was determined in the following manner: the structures of the uncoordinated carbene (21) and uncoordinated germylene (GeH2, GeF2, etc.) were geometry optimized independently. The two species were then oriented into the positions observed in the experimentally determined structures of the complexes, and then the geometry of the complex was reoptimized. The optimized structures of model compounds 14, 18, 19, and 20 were in reasonable agreement with crystallographically determined

attached to Ge. Interestingly, the two polymorphs of 8 exhibit significantly different carbenic C-Ge bond lengths and bond angles. To increase our understanding of the experimental results, we examined computationally the energy of complexation (ΔEcmplx) and the structural metrics of model NHC complexes of GeR2. To reduce the complexity of the systems

(18) DFT has been endorsed as an excellent method for predicting bond lengths and bond dissociation energies of NHC complexes of metals. See: Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687. (19) The PBE1PBE function had been shown to be reasonable at predicting bond lengths. See: Staroverov, V. N.; Scuseria, G. E.; Tao, J. M.; Perdew, J. P. J. Chem. Phys. 2003, 119, 12129 and Staroverov, V. N.; Scuseria, G. E.; Tao, J. M.; Perdew, J. P. J. Chem. Phys. 2004, 121, 11507.

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Table 3. Complex Formation Energy (ΔEcmplx) and Bond Lengths of the Carbenic Carbon-Germanium Bond in Model NHC-GeR2 Complexes PBE1PBE/6-311þG(d,p) compound # (substitution)

ΔEcmplx (kJ/mol)

bond length (A˚)

13 (H) 14 (OH) 15 (NH2) 16 (CH3) 17 (F) 18 (Cl) 19 (Br) 20 (Ph)

-192.6 -108.2 -63.1 -133.4 -144.0 -154.7 -154.9 -147.8

2.021 2.107 2.114 2.047 2.150 2.129 2.126 2.060

MP2/6-311þG(d,p) ΔEcmplx (kJ/mol)

bond length (A˚)

-190.8 -114.3 -72.2 -149.3 -148.0 -174.7 -179.9 not calculated

2.037 2.114 2.123 2.061 2.149 2.116 2.111

structures of compounds 11, 8, 9, and 12, respectively.20 The ΔEcmplx was determined by the difference between the total energy of the uncoordinated species and the total energy of the complex (Table 3). Included within the calculation of ΔEcmplx are corrections for zero-point energy (ZPE).21 The two different model chemistries employed (PBE1PBE/6311þG(d,p) and MP2/6-311þG(d,p)) resulted in similar complexation energies and bond lengths (Table 3). For simplicity, only the results from the PBE1PBE calculations will be discussed. As expected, the complexation energies (Table 3) show similar trends to those observed by Ol ah et al.15 Specifically, the ΔEcmplx’s of the model complexes in which the substituents on germanium are strongly π-donating (i.e., R = NH2 or R = OH) are less than those in which the substituents are not (i.e., R = H or Ph). The length of the Ge-C(1) bond, however, does not appear to related to ΔEcmplx (Table 3). As shown in Figure 4, in which the carbenic C-Ge bond length is plotted against ΔEcmplx, there is no apparent correlation between the complexation energy and the carbenic carbon-germanium bond length for the model compounds 13-20. The lack of correlation between bond length and ΔEcmplx (and by extension, the π-donating ability of the substituent)15 is in agreement with our experimental observations. Furthermore, relaxed potential energy surface (PES) scans of the Ge-C(1) bond in model compounds 13-20 were performed; the PES scan for 13 is shown in Figure 5. Notably, stretching the Ge-C(1) bond by 0.18 A˚ requires only 11.22 kJ/mol.22 The results for model compounds 14-20 are summarized in Table 4. The shallow PES for the stretching of the Ge-C(1) bond suggests that the length of the bond may be easily influenced by intermolecular forces and may explain the poor correlation between the Ge-C(1) bond length and the complexation energy. Therefore, it seems reasonable that the Ge-C(1) bond length variations observed in compounds 1-5 and 13-20 are (20) The orientation of the GeR2 fragment in model compounds 13, 15, and 16 is twisted approximately 90° along the carbene C-Ge bond from what is observed in the experimental structures. This was found to have minimal effect on the ΔEcmplx of the systems under study; see Supporting Information for details. (21) The basis set superposition errors (BSSEs) were calculated but not included in the final results, as they were not found to affect the overall results. Using PBE1PBE/6-311þG(d,p) model chemistry, for R = H, BSSE = 2.3 kJ/mol; R = CH3, BSSE = 3.2 kJ/mol; R = NH2, BSSE = 5.9 kJ/mol; R = OH, BSSE = 8.2 kJ/mol; R = F, BSSE = 6.3 kJ/mol; R = Cl, BSSE = 6.7 kJ/mol; R = Br, BSSE = 29.4 kJ/mol; R = Ph, BSSE = 5.6 kJ/mol. (22) For comparison, the corresponding value for the Ge-C bond in MeGeH3 is 19.6 kJ/mol and for the C-C bond in ethane is 31.3 kJ/mol using PBE1PBE/6-311þG(d,p) model chemistry. See the Supporting Information for further details.

Figure 4. Plot of complex formation energy (ΔEcmplx) against the carbenic carbon-Ge bond length in compounds 13-20.

Figure 5. Plot of a relaxed PES scan of the Ge-C(1) bond in compound 13. Table 4. Increase in Energy for Complexes 13-20 Caused by a Stretch of 0.18 A˚ along the Ge-C(1) Bond compound # (substitution)

kJ/mol

13 (H) 14 (OH) 15 (NH2) 16 (CH3) 17 (F) 18 (Cl) 19 (Br) 20 (Ph)

11.23 9.45 7.62 8.78 7.67 8.76 8.93 9.27

influenced more by crystal packing forces23 than by intramolecular electronic effects. Thus, our previous conclusion that the Ge-C(1) bond length in compounds 1-5 was influenced by the nature of the substituent on Ge was likely incorrect.5

Conclusions The synthesis of a series of NHC-germylene complexes, compounds 8-12, was achieved. The molecular structures of (23) (a) Tiekink, E. R. T. Rigaku J. 2002, 19, 14. (b) Steed, J. W. In Frontiers in Crystal Engineering; Tiekink, E. R. T., Vittal, J. J., Eds.; John Wiley & Sons, Ltd: Chichester, England, 2006; pp 68-90.

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Table 5. Crystallographic Parameters of Compounds 8-12

empirical formula fw cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) b (deg) γ (deg) volume (A˚3) Z data/restraints/params goodness-of-fit R[I > 2σ(I)] wR2(all data) largest diff peak and hole (e A˚-3)

8

8 3 1/2(C6H6)

9 3 1/2(C6H6)

10

11

12

C7H12Cl2GeN2 267.68 orthorhombic Pna21 30.2766(10) 7.5605(2) 9.3169(3) 90 90 90 2132.70(11) 8 4356/1/226 1.037 0.0498 0.1242 0.589, -0.880

C10H15Cl2GeN2 306.73 monoclinic P21/n 9.1392(18) 10.396(2) 13.647(3) 90 90.52(3) 90 1296.6(4) 4 3469/0/141 1.115 0.0638 0.1925 1.018, -1.044

C10H15Br2GeN2 395.65 monoclinic P21/c 8.9596(18) 8.1184(16) 19.024(4) 90 100.39(3) 90 1361.1(5) 4 3250/0/141 0.999 0.0484 0.1310 0.805, -0.965

C7H12GeI2N2 450.58 orthorhombic Pca21 18.963(4) 8.4176(17) 15.353(3) 90 90 90 2450.6(9) 8 5163/1/226 1.051 0.0360 0.0931 0.899, -1.045

C15H30GeN2O2 343.02 orthorhombic Pbca 16.623(3) 9.0607(18) 24.332(5) 90 90 90 3664.8(12) 8 4204/0/192 0.959 0.0819 0.2604 3.892, -0.890

C25H34GeN2 435.13 orthorhombic Pbca 14.350(3) 16.739(3) 19.434(4) 90 90 90 4668.3(16) 8 5345/0/263 1.033 0.0503 0.1367 0.493, -0.753

these complexes were analyzed, and no trend was observed in the carbenic C-Ge bond lengths. According to computational models, it appears that although the substituents on germanium strongly affect the formation energy of the NHC-germylene complex, there is no correlation with the carbenic carbon-germanium bond length. These results lead to the conclusion that the elongation of the C-Ge bond seen in the previously reported structures of 1-55 with the increasing π-donating ability of the substituent on Ge was, most likely, fortuitous. Our results are consistent with work performed by Olah et al. on Lewis acid-base interactions between germanium(II) compounds and neutral donors.15

Experimental Section Reactions were performed under an inert atmosphere of nitrogen using standard techniques. Solvents were purified according to literature procedures24 and stored over 4 A˚ molecular sieves under N2. All NMR spectra were acquired using C6D6 as the solvent. 1H NMR spectra were referenced to residual C6D5H (7.15 ppm). Melting points were determined under an N2 atmosphere and are uncorrected. FT-Raman spectra were acquired on bulk samples sealed in a melting point tube under nitrogen. Mes2Mg was prepared using a modified literature procedure.25 1H NMR spectra of compounds 8-12 are included in the Supporting Information. Single-Crystal X-ray Diffraction. Data were collected at low temperature (-123 °C) on a Nonius Kappa-CCD area detector diffractometer with COLLECT. The unit cell parameters were calculated and refined from the full data set. Crystal cell refinement and data reduction were carried out using HKL2000 DENZO-SMN.26 Absorption corrections were applied using HKL2000 DENZO-SMN (SCALEPACK). The SHELXTL/PC V6.14 suite of programs was used to solve the structures by direct methods.27 Subsequent difference Fourier syntheses allowed the remaining atoms to be located. All of the non-hydrogen atoms were refined with anisotropic thermal (24) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (25) For the preparation of the mesityl Grignard reagent, the Schlenk equilibrium was driven toward Mes2Mg by the precipitation of MgBr2 3 dioxane. The addition of this Grignard reagent to 8 resulted in a higher yield for the formation of 12 in comparison to the addition of MesMgBr. See: Cannon, K. C.; Krow, G. R. In Handbook of Grignard Reagents; Silverman, G. S., Rakita, P. E., Eds.; Marcel Dekker: New York, 1996. (26) Otwinowski, Z; Minor, W. In Methods in Enzymology. Vol. 276: Macromolecular Crystallography, Part A; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York. 1997; p 307. (27) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

parameters. The hydrogen atom positions were calculated geometrically and were included as riding on their respective carbon atoms. The crystallographic parameters of 8-12 are listed in Table 5. Preparation of Compound 8. A mixture of compound 6 (1.03 g, 8.29 mmol) and GeCl2 3 dioxane (1.92 g, 8.29 mmol) was dissolved in THF (20 mL); the solution was then left to stir at room temperature. After 1 h, hexanes (20 mL) were added to the reaction mixture. After allowing the solution to stir for 5 min, a light brown-orange solid precipitated. The supernatant was removed by pipet, and the solid was washed with hexanes (5 mL  2) and Et2O (5 mL). The solid was dried under vacuum to give 1.56 g (69%) of a light brown-orange solid identified as 8. Crystals suitable for single-crystal X-ray diffraction analysis were obtained by slow diffusion of pentane into a saturated C6H6 solution. Mp: 98-104 °C. 1H NMR (C6D6): δ 0.97 (s, 6H C-CH3), 3.11 (s, 6H N-CH3). FT-Raman (cm-1) (rank intensity): 84 (1), 107 (5), 139 (9), 174 (10), 221 (14), 277 (6), 302 (3), 323 (7), 568 (17), 597 (12), 740 (21), 1095 (20), 1344 (16), 1386 (15), 1405 (8), 1439 (19), 1465 (18), 1642 (11), 2923 (2), 2956 (4), 2990 (13). Preparation of Compound 9. Compound 8 (0.1 g, 0.37 mmol) was dissolved in benzene (5 mL). Excess bromotrimethylsilane (0.56 g, 0.47 mL, 3.7 mmol) was added to this mixture, which was then allowed to stir for 18 h at room temperature. After 18 h, hexanes (5 mL) were added to the reaction mixture; a beige precipitate formed. The solvent was removed by decantation. The solid was washed with hexanes (3 mL  2) and then dried under vacuum to give 0.1 g (76%) of a beige solid identified as compound 9. Crystals suitable for single-crystal X-ray diffraction analysis were obtained by slow diffusion of pentane into a saturated C6H6 solution. Mp: 104-106 °C. 1H NMR (C6D6): δ 0.95 (s, 6H C-CH3), 3.08 (s, 6H N-CH3). FT-Raman (cm-1) (rank intensity): 103 (14), 150 (6), 186 (13), 200 (10), 212 (4), 229 (1), 331 (15), 567 (20), 599 (7), 649 (17), 743 (24), 988 (3), 1098 (21), 1171 (22), 1350 (16), 1381 (23), 1405 (5), 1438 (18), 1471 (19), 1649 (12), 2859 (25), 2921 (2), 2948 (8), 2985 (9), 3050 (11). Preparation of Compound 10. Compound 8 (0.1 g, 0.37 mmol) was dissolved in benzene (5 mL). Iodotrimethylsilane (0.32 g, 0.23 mL, 1.61 mmol) was added to the solution, which was then allowed to stir for 2 h at room temperature, during which time a precipitate formed. Hexanes (6 mL) were added to the reaction mixture, and the solvent was removed by pipet. The solid was then washed with hexanes (3 mL  3) and dried under vacuum to give 0.12 g (68%) of a light brown-orange solid identified as 10. Crystals suitable for single-crystal X-ray diffraction analysis were obtained by slow diffusion of pentane into a saturated C6H6 solution. Mp: 110-114 °C. 1H NMR (C6D6): δ 0.92 (s, 6H C-CH3), 3.03 (s, 6H N-CH3). FT-Raman (cm-1) (rank intensity): 85 (4), 131 (2), 161 (3), 200 (1), 324 (11), 597 (10), 649 (16), 738 (19), 1088 (14), 1344 (12), 1377 (17), 1400 (7),

Article 1439 (13), 1464 (15), 1644 (9), 2857 (18), 2918 (5), 2944 (6), 2983 (8). Preparation of Compound 11. Compound 8 (0.1 g, 0.37 mmol) was dissolved in THF (4 mL). Potassium tert-butoxide (0.08 g, 0.74 mmol) was added to the solution, which was then allowed to stir for 18 h at room temperature. After 18 h, the solvent was removed under vacuum. The residue was suspended in hexanes (5 mL) and then centrifuged. The supernatant was transferred to another flask, and the solid, presumed to be KCl, was discarded. The solvent was removed from the supernatant to yield 0.08 g (63%) of a flaky white-beige solid identified as compound 11. Crystals suitable for single-crystal X-ray diffraction were grown by storing a saturated solution of 11 in hexanes at -20 °C for one week. Mp: 98-103 °C. 1H NMR (C6D6): δ 1.11 (s, 6H C-CH3), 1.70 (s, 18H OC(CH3)3), 3.53 (s, 6H N-CH3). FTRaman (cm-1) (rank intensity): 137 (12), 159 (13), 190 (16), 232 (17), 289 (14), 320 (15), 458 (11), 557 (19), 571 (18), 600 (4), 739 (22), 764 (5), 900 (20), 939 (23), 1100 (24), 1189 (25), 1233 (9), 1352 (10), 1406 (7), 1450 (3), 1648 (6), 2685 (21), 2865 (8), 2913 (2), 2962 (1). Preparation of Compound 12. Compound 8 (0.1 g, 0.37 mmol) was dissolved in THF (4 mL) and dioxane (1 mL). Mes2Mg (0.098 g, 0.37 mmol) was added to this solution, which was then allowed to stir for 48 h. After 48 h, the suspension was centrifuged to give a clear brown supernatant and a brown solid, presumed to be MgCl2, which was discarded. The solvent from the supernatant was then removed under vacuum to give a flaky brown solid identified as compound 12. Crystals suitable for single-crystal X-ray diffraction analysis were obtained by slow diffusion of pentane into a saturated C6H6 solution. Mp: (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

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170 °C (dec). 1H NMR (C6D6): δ 1.11 (s, 6H C-CH3), 2.30 (s, 6H Mes p-CH3), 2.63 (s, 12H Mes o-CH3), 3.04 (s, 6H N-CH3), 6.97 (s, 4H Mes-CH). FT-Raman (cm-1) (rank intensity): 1014 (7), 1087 (5), 1285 (3), 1376 (1), 1599 (2), 1658 (6), 2919 (4). Computational Details. Calculations were performed using either Gaussian0328 or Gaussian09.29 The optimized geometries did not have any imaginary frequencies and, therefore, are minima on the potential energy surface. For the DFT calculations, tight convergence criteria for the self-consistent field (SCF=Tight) and an ultrafine integration grid (Int=Grid= Ultrafine) were used during the calculations. For the MP2 calculations, tight convergence criteria for the self-consistent field (SCF=Tight) were used during the calculations. The basis set superposition error was calculated using the Counterpoise keyword in Gaussian03. PBE1PBE/6-311þG(d,p)-optimized geometries are listed in the Supporting Information.

Acknowledgment. We thank the NSERC and the University of Western Ontario for funding. We thank Prof. Viktor N. Staroverov for helpful discussions. We also thank Teck Cominco Ltd. for a generous gift of GeCl4. Computational resources were provided by the facilities of the Shared Hierarchical Academic Research Computing Network (www.sharcnet.ca). Supporting Information Available: Crystallographic data in .cif format, 1H NMR spectra, and optimized geometries. This material is available free of charge via the Internet at http:// pubs.acs.org. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.