Mixed Crystalline Lithium Organics and Interconversion in Solution

Communication pubs.acs.org/Organometallics

Mixed Crystalline Lithium Organics and Interconversion in Solution Ann-Christin Pöppler,† Margret M. Meinholz,† Hannes Faßhuber,‡ Adam Lange,‡ Michael John,† and Dietmar Stalke*,† †

Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany Max-Planck-Institut für biophysikalische Chemie, Am Faßberg 11, 37077 Göttingen, Germany

‡

S Supporting Information *

ABSTRACT: Scrambling, but not the Humpty Dumpty way! The crystallographically confirmed mixed lithium organic [ t BuLi] 4·4 [Me2NC6H4Li]4 (1) consists of independent alkyl- and aryllithium tetramers in a 1:4 ratio. It has been employed to study the interconversion in solution. 7Li-DOSY and 7Li-EXSY NMR spectroscopy was utilized to establish the nature of that exchange and to determine related rate constants along the equilibria to give [Me 2 NC 6 H 4 Li] 4 (A), [(Me 2 NC 6 H 4 Li) 3 ( t BuLi)] (B), [(Me 2 NC 6 H 4 Li) 2 ( t BuLi) 2 ] (C), [(Me2NC6H4Li)(tBuLi)3] (D), and [tBuLi]4 (E).

D

quite stable and could be handled without cooling. The solidstate structure is shown in Figure 1.

ue to their versatile application in synthetic chemistry, lithium organics1 have been subject of numerous investigations, both addressing their structure in the solidstate2 and their behavior in solution.3 The structure−reactivity relationship still is the Holy Grail to be found in this class of compounds, as it is commonly accepted that the metalated species determines the composition, yield, and stereochemistry of the product. The reactivity might be tuned by adding Lewis acidic donor bases4 or lithium halides.5 Current research has focused on multicomponent reagents, providing the opportunity to vary the properties of the reactant and product over a wide range. Selectivity and functional group tolerance can dramatically be increased, for example, by the turbo-Grignard reagents6 and the heterobimetallic amides in the alkali-metalmediated metalation, promoting e.g. deprotonation reactions considered to be impossible only a few years back.7 In comparison to these rapidly expanding areas, the chemistry of mixed lithium organics still is in its infancy.8 Little is known about the species that are present in solution, and only a few studies have been reported so far.9 Enhanced reactivity of the mixed lithium organics in comparison to that of the parent material is sometimes reported.10 This fueled the idea to study mixed lithium organics in more detail, and we set out to synthesize and characterize the starting material unambiguously from X-ray diffraction. As many of the species employed so far contain donating side arms, we decided to start with dimethylaniline. As the deprotonation would not occur at the methyl groups,11 the pendant dimethylamino group is a good option to tune the aggregation. We synthesized ortho-lithiated dimethylaniline in the reaction with tBuLi in pentane. When the yellow filtrate is stored at room temperature for 1 day, colorless needles crystallize. Surprisingly, they were © 2011 American Chemical Society

Figure 1. Crystal structure of [tBuLi]4·4[Me2NC6H4Li]4 (1). The [Me2NC6H4Li]4 moiety occupies a general position, while the [tBuLi]4 molecule is only present at a quarter in the asymmetric unit. Hydrogen atoms are omitted for clarity.

Interestingly, the unit cell of [tBuLi]4·4[Me2NC6H4Li]4 (1) really contains two different lithium organic molecules. One is the [Me2NC6H4Li]4 tetramer, and the second is a [tBuLi]4 molecule. Such separated lithium organic aggregates in the same single crystal are quite unique. To the best of our knowledge, there has been only one other example reported.12 Mitzel et al. crystallized 2-lithio-1,3-dimethyl-1,3-diazacyclohexane as a tetramer with one tBuLi moiety of a tetrahedron in the asymmetric unit. Their structural motif is therefore similar to that of 1. Both compounds crystallize in the tetragonal space group I4̅. The [tBuLi]4 units are located at four edges of the Received: November 2, 2011 Published: December 21, 2011 42

dx.doi.org/10.1021/om2010639 | Organometallics 2012, 31, 42−45

Organometallics

Communication

unit cell as well as on two faces. Eight [Me2NC6H4Li]4 tetramers are located between. In both tetramers the lithium cations form triangles which are μ3-capped by the carbanionic Cα atom. This arrangement is a leading structure-building principle in lithium organic chemistry and can further be aggregated to build different lithium deltahedra. The Li4 tetrahedron is found in various lithium organic tetramers (for example [EtLi]4), while the Li6 octahedron is present in many hexamers ([(c-pentyl)Li]6 among others). 2b−d In the starting material a single dimethylamino group facilitates direct ortho metalation13 and the whole [Me2NC6H4Li]4 structure residue of 1 is reminiscent of the [Me2NCH2C6H4Li]4 tetramer.14 The Li−Cα bond lengths in 1 range from 2.207(4) Å (Li4−C2) to 2.293(4) Å (Li1−C12), compared to 2.295 Å in [Me2NCH2C6H4Li]4. The average Li−N bond lengths of 2.013 Å are slightly shorter in comparison to that of 1 (on average 2.04 Å). The average Li···Li distance in 1 is 2.562(6) Å. It is important to note that the [Me2NC6H4Li]4 tetrameric moiety is not of exact Td symmetry. On average the anilide anion coordinates the Ndonated lithium atom in the plane (Li−C = 2.27 Å) and bisects one Li···Li vector of the more closely bound lithium atoms (Li−C = 2.24 Å) The latter complex, only varying in an additional methylene bridge elongating the donating dimethylamino side arm, crystallizes also in the tetragonal space group I4̅. Furthermore, retention of the aggregation state in solution could be confirmed by NMR spectroscopic studies. In the lithiated aminoarenes the building principle of lithium organics of course competes with the ring laddering and stacking concept found for lithium amides.15 Already from this solidstate point of view we anticipated some interesting rearrangement to take place in solution. The [tBuLi]4 residue in 1 has the same structural features as the donor-free parent material in the solid state, strictly following the aggregation principle of lithium organics.16 The methyl groups are arranged eclipsed with respect to the lithium atoms of the capped metal triangle. The Li−Cα bond length of 2.267(5) Å is in the expected range and is slightly longer than the average in the [Me2NC6H4Li]4 residue (2.249(5) Å). The average Li···Cβ distance is 2.410(8) Å, while the Li···Li distance is 2.424(8) Å. The tBu group is rotationally disordered and was successfully refined to two positions. In accordance with the solid-state structure the 7Li NMR spectrum should show two signals representing the two different tetrameric homoleptic species [Me2NC6H4Li]4 and [tBuLi]4, respectively. However, the 7Li NMR spectrum in toluene-d8 is unexpectedly complicated and shows five relatively sharp distinguishable signals over a range of nearly 2.5 ppm among further unidentified broad peaks (Figure 2). The main peak (A) with a chemical shift of 3.5 ppm was tentatively assigned to the o-lithium anilide homotetramer [Me2NC6H4Li]4, while the peak at 1.4 ppm (E) shows 13C satellites with a coupling constant of 10.6 Hz, which is in agreement with a fluxional tBuLi homotetramer. Lowering the temperature to 223 K results in a static tetrameric structure with a coupling constant of 14.5 Hz. The corresponding 13C NMR spectrum at 223 K shows 10 lines, as would be expected for a coupling to three I = 3/2 nuclei (see the Supporting Information). This assignment could further be confirmed with a sample of tBuLi in toluene-d8. 13C satellites are also observed for the peak at 1.7 ppm (D), where the integral of the satellites indicates three instead of four tBuLi units. No satellites are

Figure 2. 7Li NMR spectrum of 1 in toluene-d8 (298 K), including integrals, 13C satellites, and assignments.

observed for the broader peaks A−C. Furthermore, the effect that only sterically hindered and less dynamic compounds such as tBuLi show satellites and reveal LiC couplings is well-known in the literature.17 To gain a further understanding of the 7Li NMR spectrum, we performed 7Li diffusion-ordered spectroscopy (DOSY) (Figure 3). The monotonically decreasing diffusion coefficient

Figure 3. 7Li DOSY spectrum of [Me2NC6H4Li]4tBuLi (1) in toluened8 (298 K).

D (vertical axis) identifies the five peaks A−E as the five conceivable tetramers [Me2NC6H4Li]4 (A), [(Me2NC6H4Li)3(tBuLi)] (B), [(Me2NC6H4Li)2(tBuLi)2] (C), [(Me2NC6H4Li)(tBuLi)3] (D), and [tBuLi]4 (E). Therefore, each of the narrow peaks in the 7Li NMR spectrum is due to a consecutive substitution of one o-lithium anilide by one t BuLi moiety in the tetrameric aggregate. The formation of such mixed aggregates has already been confirmed by X-ray crystallography for other, similar lithium organic compounds (e.g., Li 4 R 2 n Bu 2 with R = 2,6-(CH 2 NMe 2 ) 2 -3,5Me2C6HCH2).18 The presence of mixed aggregates is also in agreement with low-temperature 7Li NMR spectra. For example, lowering the temperature to 248 K leads to broadening and splitting of the 7 Li signal at 2.3 ppm (C) into two equally intense lines. This clearly indicates a deceleration of intraaggregate exchange in the two-by-two aggregate [(Me2NC6H4Li)2(tBuLi)2], resulting in 43

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Scheme 1. Exchange Rate Constants in s−1 between the Different Tetramers in a Saturated Solution of 1a

two inequivalent lithium sites. As the temperature is lowered further, signals that belong to mixed tetramers are broadened out completely, presumably due to low-barrier processes such as aryl rotations. The two remaining signals A and E show an integral ratio of almost 4:1, as would be required for the crystalline starting arrangement, which could also be verified by solid-state 13C NMR spectroscopy (see the Supporting Information). The 1H and 13C NMR spectra are consistent with the 7Li NMR data and show multiple signal sets due to the presence of mixed tetramers. By means of 1H,7Li-HOESY in combination with other standard two-dimensional NMR experiments the proton, carbon, and lithium chemical shifts of all five tetramers could be assigned unambiguously. A comparison of the integral ratio of the corresponding peaks in the 7Li and 1H NMR spectra shows good agreement (see Table 1 in the Supporting Information), as well as the diffusion coefficients determined by 1 H-DOSY experiments. The 7Li-EXSY spectrum recorded with a mixing time of 50 ms (Figure 4) demonstrates that aggregates A−D slowly

a

Rates were calculated from 7Li-EXSY integrals using EXSYcalc (Mestrelab Research, Santiago de Compostela). Certain exchange rates were too small to be determined. A second experiment with a mixing time of 5 ms was recorded as reference.

but that the mixed aggregates B−D are rather disfavored in comparison to the homoleptic tetramers A and E, which is consistent with the crystallized, thusunder the crystallization conditionsthermodynamically most stable structure in the solid state. We are currently investigating the possible synergy and trying to find out whether those disfavored mixed aggregates are more reactive to a benchmark system than the isolated parent lithium organics alone. Experimental Section. Synthesis of [ t BuLi] 4 ·4 [Me2NC6H4Li]4 (1). To Me2NPh (3.46 g, 30.0 mmol, 4.0 equiv) was added tBuLi in pentane (1.5 M, 25.0 mL, 37.5 mmol, 5.0 equiv) at room temperature. After it was stirred overnight, the yellow solution was reduced in volume under vacuum and stored at room temperature. After 1 day colorless crystals suitable for X-ray structure determination were obtained: yield 3.40 g, 5.94 mmol, 79%. Anal. Found (calcd): C, 75.25 (75.52); H, 8.99 (8.63); N, 9.86 (9.79) All manipulations were performed under an inert gas atmosphere of purified dry argon with standard Schlenk techniques and in an argon glovebox. The glassware was dried at 400 K, assembled hot, and cooled under vacuum. All solvents were dried over appropriate alkali metals, distilled, and degassed prior to use. tBuLi was supplied by Chemetall GmbH. NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer (Bruker Biospin, Rheinstetten, Germany) with a broadband-observe-probe, z-gradient, and temperature unit. The spectra were recorded at various temperatures in toluened8. All spectra were processed with Topspin 2.1 (Bruker Biospin, Rheinstetten, Germany) and further plotted with MestreNova, version 7.0 (Mestrelab Research, Santiago de Compostela, Spain). For detailed NMR assignments please see the Supporting Information. Single-Crystal Structural Analysis. The single crystal was mounted in inert oil under cryogenic conditions employing the X-Temp2 device.19 The X-ray data set was collected at 100(2) K on an INCOATEC microfocus source20 with mirrormonochromated Mo Kα radiation (λ = 0.710 73 Å) and equipped with a Bruker Smart Apex II detector. The structure was solved by direct methods with SHELXS and refined by fullmatrix least squares on F2 for all data with SHELXL.21 Nonhydrogen atoms were refined with anisotropic displacement parameters. H atoms were placed in calculated positions and refined using a riding-model. The crystallographically independent tBu group was rotationally disordered and modeled at two positions with a sofs of 0.52 and 0.48, respectively. 1: C144H196Li20N16, Mr = 2289.96, 0.3 × 0.04 × 0.04 mm crystal size, tetragonal, space group I4̅, a = 27.993(4), b = c = 8.8744(12) Å, V = 6954.3(16) Å3, ρ = 1.094 g/cm3, Z = 2,

Figure 4. 7Li-EXSY NMR spectrum of 1 (mixing time 0.05 s, 298 K).

equilibrate, whereas no exchange with tBuLi can be detected. Exchange cross-peaks are visible not only between direct neighbors (A−B, B−C, C−D) but also between the more remotely related species as well. Apparently, simultaneous substitution of more than one o-lithium anilide by one tBuLi moiety may occur. This becomes also evident from exchange rate constants (s−1) determined from a saturated solution of 1 in toluene-d8 (Scheme 1). We were interested whether the exchange rate constants displayed in Scheme 1 depend on the concentration of the sample and the sample stoichiometry. For this purpose, two samples with 20 and 50 mg of dimethylaniline and the corresponding 1.25 equiv of tBuLi were prepared in an NMR tube and monitored over 2 weeks. Not only the lithiation reaction but also lithium exchange at equilibrium is significantly slower for the more diluted sample. This points to an exchange mechanism implying collisions between tetrameric units rather than a breaking up and re-formation of the aggregates. Furthermore, we could observe that the peaks develop gradually in the order D, C, B, and A. In conclusion, starting from the firm ground of a determined solid-state structure of mixed lithium organics, we could prove that ab initio possible aggregates are not statistically distributed 44

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μ(Mo Kα) = 0.061 mm−1, F(000) = 2464, θ range 1.45−26.76°, T = 100(2) K, 40 145 reflections measured, 3964 unique reflections, Rint/Rσ = 0.0393/0.0402, 429 parameters refined, R1 (all data) = 0.0457, R1 (I > 2σ(I)) = 0.0379, wR2 (all data) = 0.0950, wR2 (I > 2σ(I)) = 0.0912, GOF = 1.080, largest difference peak and hole 0.258 and −0.183 e Å−3. More details about the crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data-request/cif deposited under the CCDC no. 829017.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and a CIF file giving experimental details, characterization data, and crystal structure data. 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].

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ACKNOWLEDGMENTS We kindly acknowledge funding from the DFG Priority Programme 1178, the DNRF funded Centre of Materials Crystallography, and the doctoral program Catalysis for Sustainable Synthesis, provided by the Land Niedersachsen. A.L. was supported by the DFG (Emmy Noether Fellowship). We appreciate chemical donations from CHEMETALL.

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REFERENCES

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