The [(DABCO)7·(LiCH2SiMe3)8] Octamer: More Aggregated than the

Article pubs.acs.org/Organometallics

The [(DABCO)7·(LiCH2SiMe3)8] Octamer: More Aggregated than the Parent Starting Material [LiCH2SiMe3]6 but Also Higher in Reactivity Tanja Tatić, Stefanie Hermann, and Dietmar Stalke* Institut für Anorganische Chemie der Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Herein, we report on the reaggregation of hexameric trimethylsilylmethyllithium [LiCH2SiMe3]6 with the donor base DABCO (1,4-diazabicyclo[2.2.2]octane) to give the unprecedented octamer [(DABCO)7·(LiCH2SiMe3)8] (1). The structure consists of four dimers, forming Li2C2 four-membered rings, connected to two [(DABCO)3·{(LiCH2SiMe3)2}2] chain fractions, interconnected by a single DABCO molecule. Interestingly, two different conformers of (LiCH2SiMe3)2 dimers are present, caused by different steric demand. Higher steric strain in the center of the molecule causes an ecliptic arrangement of the Me3Si group along the Si−Cα bond, while at the periphery the more relaxed staggered conformation is enabled. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. Although the aggregation of 1 is much higher than that of the parent [LiCH2SiMe3]6, the reactivity of the first is higher than that of the starting material, provided the octameric aggregation found in the solid state is maintained in nondonating solvents. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. The reaction was monitored by 1 H NMR spectroscopy. Revisiting that known structure with modern technology revealed that [(DABCO)·(LiCH2Ph)]∞ (2) crystallizes in the space group P21. 2 still is the only benzyllithium compound featuring the η3-coordination mode to the Cortho atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion.

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INTRODUCTION Organoalkali metal compounds have been studied in the solid state1 and in solution2 for over 50 years, and their synthetic applications have been increasing ever since.3 Basic organolithium reagents such as EtLi,4 MeLi,5 and t-BuLi6 form tetramers without coordinating donor bases, while c-HexLi,7 nBuLi,6 and i-PrLi8 form hexameric aggregates. Donor-free organolithiums hardly ever give separate monomers in solution or in the solid state, as instead they form coordination polymers of multiple cation/anion interactions to increase the lattice energy.9 However, most of the reactions employing organolithiums are accomplished in homogeneous solution, and for that reason the knowledge of the aggregation state of organolithium reagents in solution and the solid state is of inevitable importance for the deduction of structure/reactivity relationships.2f,i,10 For this endeavor we recently picked trimethylsilylmethyllithium because it can readily be purified by crystallization and shows a gentle reactivity compared to the other basic organolithiums. The parent donor-free [LiCH2SiMe3]6 is known to consist of hexamers in the solid state (a in Scheme 1).11 Unexpectedly the addition of simple ethers give asymmetrically coordinated tetramers in the solid state, staying intact in solution, proved by DOSY NMR experiments.12 Addition of chelating donor bases gives the expected dimers such as [(TMEDA)·LiCH2SiMe3]2 (b in Scheme 1) or [{(−)sparteine}·LiCH2SiMe3]2 (c in Scheme 1).13 In a toluene-mediated single-crystal-to-single-crystal transition the colorless dimer [{Me2N© 2012 American Chemical Society

Scheme 1. Coordination Motifs of Trimethylsilylmethyllithium

(CH2)2OMe}·(LiCH2SiMe3)]2 forms yellow crystals of the benzyllithium tetramer [{Me2N(CH2)2OMe}·(LiCH2C6H5)]4, while the more reactive monomer [(PMDETA)·LiCH2SiMe3] Received: June 25, 2012 Published: July 26, 2012 5615

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Figure 1. Molecular structure of [(DABCO)7(LiCH2SiMe3)8] (1) in the crystal. Anisotropic displacement parameters are depicted at the 50% probability level (constrained hydrogen atoms omitted for clarity).

membered rings of trimethylsilylmethyllithium to give [(DABCO)7·(LiCH2SiMe3)8] (1). The structural motif of these dimeric rings is formed by lithium atoms and carbanionic (Cα) atoms, which bond to SiMe3 groups. It was suggested that a carbanion, which is bound in a α-position to a silicon atom, is stabilized by negative hyperconjugation. This interaction is explained by an overlap of an occupied p-orbital of a carbon atom in trans position to a nonoccupied, vacant σ*-orbital of a silicon atom.16 This should result in a weakened bond in trans position to the lone pair orientation, reflected by elongated silicon carbon bond lengths at the SiMe3 groups. An earlier charge density investigation showed that the bonding was not exclusively controlled by orbital interactions but was strongly influenced by the charge localized at the carbanion Cα.17 Knowing the structural criteria indicating negative hyperconjugation, potential Li−H agostic interactions, and the α-effect18 in the Si−C bond from highresolution experimental and theoretical charge density investigations19 in a related molecule,20 we concentrated on the bonding situation in the dimeric (Li2C2) rings of trimethylsilylmethyllithium in the crystal structure of [(DABCO)7·(LiCH2SiMe3)8] (1) to judge whether or not the effects are reliably detectable in the standard structure determination. Unfortunately, the crystal structure of [(DABCO) 7 ·(LiCH 2 SiMe 3 ) 8 ] (1) is disordered at the DABCO positions, preventing a multipole refinement from high-resolution data. The occupancy factors for one DABCO molecule (N3, N4) refine to 61.8%:38.2%. The DABCO molecule (N7/N7A) lies on a special position (inversion center), and the second site of each atom is directly determined by the positions of the atoms of the first component. The DABCO molecules N1/N2 and N5/N6 show no disorder, and the Me3Si groups are not disordered. All hydrogen atom positions at the Cα-positions could be subtracted from the difference Fourier map and refined freely, although the Cα−H distances were restrained to be equal. One octamer of 1 contains four dimeric (Li2C2) rings, two of them symmetry independent and each in a different conformation. They differ significantly with respect to the arrangement of the SiMe3 groups relative to the carbanionic

(d in Scheme 1) immediately gives [(PMDETA)·(LiCH2 C6H5)]. Hence a straightforward access to commercially interesting pure crystalline benzyllithium is provided.14 This approach is hampered by the employment of potentially toxic and harmful chelating amines, unwanted in any pharmaceutical process. With this in mind we decided to investigate 1,4diazabicyclo[2.2.2]octane (DABCO), as it is a celebrated substitute for chelating amines. Although it is classified as harmful, it is not as toxic as other amines, e.g., N,N,N′,N′,N″pentamethyldiethylenetriamine (PMDETA), and is employed as a catalyst in polyurethane production.15 As DABCO is a bidentate donor base with two nitrogen atoms arranged at opposite sites, providing a 180° linker, it should give rise to unprecedented aggregation. The opposite coordination sites enable the ligand to coordinate two metal atoms straight via the two nitrogen atoms. This should either evoke infinite coordination polymers or at least higher aggregates than tetra- or hexamers.

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RESULTS AND DISCUSSION [(DABCO)7·(LiCH2SiMe3)8] (1). DABCO at room temperature is crystalline and was kept under an argon atmosphere due to its hygroscopic character. Crystals of DABCO were dissolved in n-heptane, and trimethylsilylmethyllithium, dissolved in n-hexane, was added dropwise at room temperature, yielding a colorless clear solution. Within hours colorless crystals start to grow, which finally were suitable for X-ray structure analysis. The crystal structure consists of the complex [(DABCO)7·(LiCH2SiMe3)8] (1), which crystallizes in the triclinic space group P1̅. The asymmetric unit contains half of the octamer, and the other half is generated by an inversion center. The structure of 1 is remarkable because at first sight the molecule has characteristics of a coordination polymer consisting of the infinite translational repetition of a DABCO coordinated to (LiCH2SiMe3)2 four-membered rings. Further inspection of the structure however reveals that it consists only of two [(DABCO)3·(LiCH2SiMe3)4] strands linked via the two nitrogen atoms of a central DABCO molecule between them (Figure 1). The [(DABCO)3·(LiCH2SiMe3)4] strands are made up of DABCO molecules coordinating dimeric (Li2C2) four5616

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steric considerations and is not caused by negative hyperconjugation. The angles between Li···Cα···Li for both possible arrangements of the dimeric rings are averaged between 64.4(2)° and 65.9(2)° and for Cα−Li−Cα between 114.1(2)° and 115.1(2)°, yielding a rhombic arrangement. The sum of the angles adds up to 360.0(2)° and 358.9(2)°, illustrating the planar arrangements of the four-membered rings. All Li−N bond lengths are in an expected range compared to all Li−N bond lengths of any structures deposited in the CCDC,22 but show an obvious trend. Short Li−N distances (Table 1, Li4−N5 206.5(2) and Li3−N4 207.1(2) pm) are found to the DABCO molecules bound to the (Li2C2) dimers in the periphery of the [(DABCO)7·(LiCH2SiMe3)8] octamer (highlighted in green, Figure 2). The lithium atoms Li3 and Li4, located on the periphery, are only 3-fold coordinated, and the sterical strain is released by the positions of the Me3Si groups, providing more room for the DABCO molecules. This results in shorter Li−N distances. The lithium atom Li1, closer to the center of the bridging moieties, is also 3-fold coordinated, but the coordinated DABCO molecule together with the nearby bridging DABCO determines the ecliptic conformation of the Me3Si groups and steric strain because the two methyl groups are arranged opposite the terminal DABCO molecule, elongating the Li1−N2 distance to 212.2(2) pm. The central lithium atom Li2 is 4-fold coordinated by the nitrogen atoms N3 and N7 and the carbanionic atoms C4 and C7. The steric demand at Li2 is higher than anywhere in the octamer, mirrored by the longest N···Li distances in the molecule (Li2− N3 214.1(2) and Li2−N7 218.9(2) pm). The distances of the lithium atom to the carbanionic atoms are also elongated to 227.1(2) (Li2−C4) and 223.0(2) pm (Li2−C7). Reactivity Studies. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. To a solution of parent trimethylsilylmethyllithium in n-hexane was added toluene at room temperature (Scheme 2). The reaction mixture was stored at room temperature for 72 h. During this time no deprotonation of toluene to give benzyllithium could be observed. Presumably, this is a consequence of the octahedral arrangement of [LiCH2SiMe3]6 in hydrocarbon solutions, which shields the reactive carbanions in the center of the molecule. In a second experiment trimethylsilylmethyllithium coordinated to DABCO was tested. DABCO crystals were dissolved in toluene, and trimethylsilylmethyllithium in n-hexane was added dropwise at room temperature. The flask was stored at −3 °C, and after a short period of time the colorless mother liquor turned slightly yellow and colorless crystals started to grow. These colorless crystals turned out to be [(DABCO)7·(LiCH2SiMe3)8] (1). The yellow color of the mother liquor became more and more intense, and after 24 h additionally yellow crystals formed, next to the already existing colorless crystals. Interestingly, the yellow crystals grew at the expense of the colorless crystals in a single-crystal-to-singlecrystal transition (Figure 2). Compared to the colorless crystals the yellow crystals adopt a defined cubic shape of approximately 0.1 × 0.1 × 0.1 mm in size (Figure 2c). After some days only yellow crystals remain in the reaction flask. The X-ray structure analysis revealed that the yellow crystals consisted of deprotonated toluene coordinated by DABCO molecules to give the coordination polymer [(DABCO)(LiCH2Ph)]∞ (2, Scheme 2).

atom (Figure 1; different arrangements highlighted in red and green; for space-filling model see Supporting Information). The (Li2C2) dimers in the peripheries (Figure 1, highlighted in green) adopt a staggered confirmation of the methyl groups relative to the freely refined hydrogen atom positions at Cα. As a result, one methyl group at the silicon atom is oriented trans to the position of a virtual lone pair at Cα (Figure 1, left). This is the conformation that would be expected from considerations concerning negative hyperconjugation. In contrast, the (Li2C2) dimers in the center of the double chain show a different arrangement. The methyl groups bound to the silicon atoms adopt an ecliptical conformation relative to the hydrogen atoms bound to Cα (Figure 1, highlighted in red). This results in a cis conformation of one methyl group relative to the virtual lone pair position. A space-filling model of [(DABCO)7·(LiCH2SiMe3)8] (1) provides insight into the reasons for such different conformers (see Supporting Information). The space available for the Me3Si groups bound to the (Li2C2) four-membered rings at the periphery of the octamer (Figure 1, highlighted in green) is considerably wider than that close to the center, bridging the two moieties (Figure 1, highlighted in red). Three of the four metal atoms are threecoordinated, and it is only Li2 that is coordinated to two DABCO molecules. The hydrogen atoms of that additional bridging DABCO molecule provide the steric strain to the adjacent Me3Si groups, inhibiting them from orienting in the staggered orientation. The Si−Cα bond lengths in both arrangements show a significant contraction (av 183.01(12) pm) compared to a standard silicon carbon single bond (187 pm).21 The Si−CH3 bond lengths within both conformationsstaggered and eclipticshow no significant differences within their estimated standard deviations (Table 1). The weakening, i.e., the elongation of the Si−CH3trans bond, due to negative hyperconjugation could not be detected. Hence the arrangement of the methyl groups at the silicon atom in the more favorable trans position seems to be triggered only by Table 1. Selected Bond Lengths [pm] and Angles [deg] of 1

Si−Cα Si−CH3 Li···N

Li···Cα

Li···Li Li···Cα···Li Cα···Li···Cα

183.02(12) Si−CH3 trans: 189.09(13) other Si−CH3: 188.51(13) Li3···N4 207.1(2) Li4···N5 206.5(2) Li3···C12 215.2(2) Li3···C15 215.2(2) Li4···C12 219.1(2) Li4···C15 219.5(2) 232.1(3) 64.5(2) and 64.3(2) 111.2(2) and 117.0(2)

183.00(11) Si−CH3 cis: 188.93(14) other Si−CH3: 188.23(15) Li1···N2 212.2(2) Li2···N7 218.9(4) Li2···N3 214.1(2) Li1···C4 219.3(2) Li1···C7 216.3(2) Li2···C4 227.1(2) Li2···C7 223.0(2) 240.8(3) 66.5(2) and 65.3(2) 113.8(2) and 116.3(2) 5617

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Figure 2. Dominating colorless crystals of [(DABCO)7·(LiCH2SiMe3)8] (1) under the microscope (a), dominating yellow crystals of [(DABCO)·(LiCH2Ph)]∞ (b), and finally exclusively yellow crystals of [(DABCO)·(LiCH2Ph)]∞ (2) (c).

on rotational disorder or rigid body rotation of the DABCO ligand, but a refinement of the disordered model was not attempted at that time due to a limited amount of data. [(DABCO)(LiCH2Ph)]∞ (2). The present structure is solved in the monoclinic space group P21 with cell dimensions of a = 1176.7(2), b = 1605.7(2), c = 1252.2(2) pm and a monoclinic angle of β = 90.028(2)°, hence very close to 90°. This monoclinic angle mimics metric orthorhombic symmetry to a good approximation and causes pseudomerohedral twinning. A closer look at the cell dimensions showed that the bisection of the c-axis would lead to a smaller unit cell, which was reported previously by Stucky et al. The reflections requiring the longer c-axis are weak on average, and in 1970 they were overlooked, most likely due to limited technical facilities. The structure, solved in the space group P21, shows no signs of disorder, in contrast to a refinement in P212121. All this indicates that the valid space group of [(DABCO)(LiCH2Ph)]∞ (2) is P21 (see Supporting Information). Due to missing heavy atoms, the correct absolute structure could not be determined successfully.24 2 consists of infinite polymeric chains of benzyllithium molecules linked by DABCO ligands (Figure 3). Every lithium atom is 4-fold coordinated by nitrogen atoms of the ligands and the Cα and Cipso carbon atoms, respectively. The asymmetric unit of [(DABCO)(LiCH2Ph)]∞ (2) contains two fragments of infinite chains, each made up of two independent benzyllithium

Scheme 2. Reactivity of Trimethylsilylmethyllithium toward Toluene

A crystal structure of [(DABCO)·(LiCH2Ph)]∞ has already been published by Stucky et al. in 1970.23,10 Their crystals were prepared by deprotonation of toluene with n-BuLi in the presence of DABCO. The compound is published in the orthorhombic space group P212121 with cell dimensions of a = 1623.1(8), b = 625.5(3), c = 1180.0(6) pm. They also reported

Figure 3. Fragment of the coordination polymer of infinite chains of [(DABCO)(LiCH2Ph)]∞ (2) in the solid state. Anisotropic displacement parameters are depicted at the 50% probability level (constrained hydrogen atoms omitted for clarity) (left) and superposition plot of all independent benzyllithium molecules in 2 (view along the Cα−Cipso bond) (right). 5618

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sequently the reactivity in solution.28 Hence we picked toluened8, and other deuterated solvents such as THF-d8 were avoided. First [(DABCO)7·(LiCH2SiMe3)8] (1) was crystallized and dried in vacuo. A 27 mg amount of 1 were transferred in an argon drybox into a NMR tube, and a mixture of toluene/ toluene-d8 was added. The NMR spectra were recorded every 24 h over a period of one week. A mixture of toluene/toluened8 was used because toluene was operating as a reactive agent and the original reaction conditions should be maintained during the experiment. Toluene-d8 was used to avoid signal overlapping in the NMR spectra, and the addition had no effect on the reaction process. The decrease of the signals of 1 and the simultaneous increase of the signals of 2 were monitored. Interestingly, the signal intensity of 2 increased during the experiment, reached a maximum after 72 h, and disappeared again (Figure 4, left). This clearly was due to the low solubility of 2 and the beginning crystallization process in the NMR tube (Figure 4, right).

molecules. A superposition plot of the four symmetryindependent benzyllithium molecules in 2 shows that all lithium atoms are clearly η3-coordinated at a favored position and shifted to the same specific Cortho atom. A closer look at the Cortho−Cipso−Cα−Li torsion angles indicates a tendency of decreasing angles from 64.5° to 56.4°, correlated with increasing Li−Cortho bond lengths (Table 2). The Li−Cα Table 2. Selected Bond Lengths [pm] and Angles [deg] of 2, Sorted by Decreasing Torsion Angles Cortho−Cipso−Cα−Li Li−Cα Li−Cipso Li−Cortho Cα−Cipso Cipso−Cα−Li (Li−N)av

Li4

Li2

Li1

Li3

64.5° 220.0(3) 238.3(3) 265.9(3) 141.5(2) 65.1(2) 209.4(3)

62.2° 218.9(3) 235.0(3) 256.4(3) 142.3(2) 78.0(2) 210.1(3)

56.6° 221.5(3) 236.4(3) 250.0(3) 141.3(2) 66.4(2) 209.0(3)

56.4° 216.4(3) 235.2(3) 249.6(3) 142.0(2) 79.1(2) 211.0(3)

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CONCLUSION In conclusion, we present the structure of [(DABCO)7·(LiCH2SiMe3)8] (1). It is remarkable because at first sight the molecule has characteristics of a coordination polymer consisting of the infinite translational repetition of a DABCO coordinated to (LiCH2SiMe3)2 four-membered rings. It contains two different conformers of (LiCH2SiMe3)2 dimers, caused by various steric demand. The standard structure determination does not furnish any proof for negative hyperconjugation nor provide special characteristics of an uncommon Si−C bonding situation. Although the aggregation of 1 is much higher than that of the parent [LiCH2SiMe3]6, the reactivity of the first is higher than that of the starting material. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. As the aggregation in the solid state has not yet been confirmed in nonpolar solvents by, for example, HOESY or DOSY NMR experiments, it remains an open question whether this putative higher reactivity contradicts the general statement that organolithiums are more reactive the less aggregated they are. Although the reaction could be monitored by 1H NMR spectroscopy the reactive species might still be the dimer. Revisiting the known structure of [(DABCO)·(LiCH2Ph)]∞ with new technology revealed that 2 crystallizes in the space

bond lengths decrease in the same way, except for Li1−Cα. The Li−Cortho distances range from 249.6(4) pm for Li3 to 265.9(3) pm for Li4; hence are longer than a typical bond.1d,22 However, Figure 3 clearly shows that the lithium atoms are attracted by the same Cortho ring atom in all independent benzyllithium molecules. If they were merely η2-bound, there was no reason that the phenyl rings should be tilted toward the metal atom, causing a η3-coordination. In the cyclic tetramer [{Me2N(CH2)2OMe}·(LiCH2C6H5)]4 this additional Cortho coordination is not present.14 Solid-state structures of [(donor)·MCH2Ph]n (M = s-block metal) have been reported previously25 and were studied theoretically,26 but the η3coordination mode for benzyllithium was found only in [(DABCO)(LiCH2Ph)]∞ (2) (av 255.5(3) pm, Table 2) and confirmed theoretically by IGLO calculations (259 pm).25k,26 Only the much softer metals potassium and rubidium readily move to the η3- and even η6-position in coordination polymers.25k Introduction of a heteroatom in the aromatic perimeter would have the same effect.27 NMR spectroscopic experiments are hampered by the low solubility of 2 in common deuterated, nondonating solvents. NMR monitoring needs to be primed under the same conditions that the genuine reaction was performed. Of course, donating solvents would change the aggregation and con-

Figure 4. 1H NMR spectra of [(DABCO)·(LiCH2Ph)]∞ (2, para-proton region, right) and crystals of 2 in the NMR tube after several days. Photo by M. Eng. A. Loh. 5619

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empirical absorption correction (SADABS)32 was applied. The structures were solved by direct methods (SHELXS-97)33a and refined by full-matrix least-squares methods against F2 (SHELXL-97)33b,c within the SHELXLE GUI.33d All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically on calculated positions using a riding model with their Uiso values constrained to equal 1.5 times the Ueq of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms. Disordered moieties were refined using bond length restraints and isotropic displacement parameter restraints. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. The CCDC numbers, crystal data, and experimental details for the X-ray measurements are listed in the Supporting Information. Copies of the data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif or from the corresponding author.

group P21. The new refinement affords more resilient structural parameters, matching theoretical investigations. 2 is the only benzyllithium compound featuring the η3-coordination mode to the Cortho atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion.

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EXPERIMENTAL SECTION

General Procedure. All experiments were carried out in an atmosphere of purified, dry argon by using modified Schlenk techniques or in an argon drybox. The glassware was dried for several hours at 150 °C, assembled hot, and cooled under vacuum. The solvents were freshly distilled from drying agents and degassed before use. The organometallic starting materials were supplied by Chemetall GmbH, Frankfurt and Langelsheim. Elemental analyses were performed by the Mikroanalytisches Labor des Instituts fü r Anorganische Chemie der Universität Göttingen with an Elementar Vario EL3 apparatus. The determined values deviated somewhat more than usual from the calculated ones, as the substances were highly sensitive to oxygen and moisture. The inclusion of argon, from canning in an argon drybox, led to systematic errors. NMR spectra were recorded on a Bruker AVANCE III 400 MHz spectrometer (Bruker Biospin, Rheinstetten) with a broadband observe probe, zgradient, and temperature unit. The spectra were recorded at various temperatures in toluene-d8. All spectra were processed with Topspin 2.1 (Bruker Biospin, Rheinstetten) and further plotted with MestReNova, Version 7.0 (Mestrelab Research, Santiago de Compostela, Spain). [(DABCO)7·(LiCH2SiMe3)8] (1). Colorless DABCO crystals (0.16 g, 1.44 mmol, 1 equiv) were dissolved in heptane (10 mL) in a Schlenk flask, yielding a clear, colorless solution. Trimethylsilylmethyllithium in n-hexane (1.75 mL, 1.44 mmol, 1 equiv) was added dropwise. After a short period of time at room temperature colorless crystallites of [DABCO·(LiCH2SiMe3)2] (1) could be observed. At the end of the crystallization the mother liquor was removed by a syringe and the crystals were dried under vacuum. (C14H34Li2N2Si2)∞: M = 300.26 g/ mol, yield 0.027 g (0.09 mmol after approximately 1 h, 6.24% applied to [LiCH2SiMe3]6). 1H NMR (300 MHz, tol-d8): δ −1.92 (s, 2 H, CH2Li), 0.32 (s, 9 H, SiMe3), 2.45 (s, 12 H, DABCO). 13C NMR (75 MHz, tol-d8): δ −5.9 (CH2Li), 5.2 (SiMe3), 47.4 (CH2, DABCO). 7Li NMR (194 MHz, tol-d8): δ 2.22 (s). Anal. Found (calcd) [%]: 57.05 (55.96), 10.87 (11.40), 13.0 (9.32). [(DABCO) (LiCH2Ph)]∞ (2). Colorless DABCO crystals (0.16 g, 1.44 mmol, 1 equiv) were dissolved in toluene (3 mL) in a NMR tube, yielding a clear, colorless solution. Trimethylsilylmethyllithium in nhexane (1.75 mL, 1.44 mmol, 1 equiv) was added dropwise. After the addition the solution changed color to slightly yellow. After one day small intensely yellow colored crystals of [DABCO·LiCH2C6H5]∞ (2) could be observed. (C13H19LiN2)∞: M = 210.17 g/mol, yield 0.13 g (0.62 mmol, 43%, applied to [LiCH2SiMe3]6). 1H NMR (300 MHz, tol-d8): δ 1.62 (s, 2 H, CH2Li), 2.45 (s, 12 H, DABCO), 6.75 (m, H, Hp), 6.78 (m, 2 H, Ho), 6.87 (m, 2 H, Hm). 13C NMR (75 MHz, told8): δ 29.1 (CH2Li), 47.4 (CH2,DABCO) 120.3 (Cp), 155.5 (Ci). 7Li NMR (194 MHz, tol-d8): δ 0.86 (s). Anal. Found (calcd) [%]: 71.09 (74.27), 8.96 (9.11), 12.56 (13.32). Single-Crystal Structural Analyses. Single crystals were selected from a Schlenk flask under an argon atmosphere and covered with perfluorinated polyether oil on a microscope slide, which was cooled with a nitrogen gas flow using the X-TEMP2 device.29 An appropriate crystal was selected using a polarized microscope, applied to the tip of a MITEGENMicroMount or glass fiber, fixed to a goniometer head, and shock cooled by a crystal cooling device. The data for 1 and 2 were collected from shock-cooled crystals at 100(2) K. The data of 1 were collected on a INCOATEC Mo Microsource with mirror optics and APEX II detector with a D8 goniometer.30 The data of 2 were measured on a Bruker TXS-Mo rotating anode with mirror optics and an APEX II detector with a D8 goniometer. Both diffractometers were equipped with a low-temperature device and used Mo Kα radiation, λ = 71.073 pm. The data were integrated with SAINT,31 and an

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ASSOCIATED CONTENT

* Supporting Information S

Further experimental details of 1 and 2 can be found here. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: int. (+49) 551-39-3459. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Danish National Research Foundation (DNRF)-funded Center for Materials Crystallography (CMC) for support and the Land Niedersachsen for providing a fellowship in the Catalysis for Sustainable Synthesis (CaSuS) Ph.D. program. We thank the Chemetall GmbH for the donation of chemicals.

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REFERENCES

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