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Organometallics 2010, 29, 4746–4748 DOI: 10.1021/om1003804
A Chain Aggregate of Methyllithium Tetramers† Ina Kamps, Beate Neumann, Hans-Georg Stammler, and Norbert W. Mitzel* Lehrstuhl f€ ur Anorganische Chemie und Strukturchemie, Universit€ at Bielefeld, Universit€ atsstrasse 15, 33615 Bielefeld, Germany. Fax: (þ)49 521 106 6026. Received April 30, 2010 Summary: The reactions of 1,3,5-trimethyl-1,3,5-triazacyclohexane, TMTAC, with methyllithium (MeLi) in the absence or presence of the Lewis acid trimethylaluminum (AlMe3) give different kinds of adducts but do not lead to deprotonation of the heterocycle. With MeLi, an aggregation motif of endless linear chains of MeLi tetramers is obtained, in which the three nonconnected lithium atoms are saturated by binding to only one of the three nitrogen atoms of the tridentate terminal TMTAC ligands; in contrast, in the presence of AlMe3, methyllithium is completely disaggregated to give a [Li(TMTAC)2][AlMe4] salt with six-coordinated lithium atoms. The metalating activity of lithium alkyl bases depends not only on the nature of the formal carbanion but also on the grade of aggregation.1 This aggregation into oligomers generally reduces the reactivity, which can in turn be enhanced via disaggregation of the oligomers by addition of suitable (poly)donor molecules. The principles of this disaggregation have recently again been outlined by Dietmar Seyferth in two excellent reviews on alkyl and aryl derivatives of the alkali metals.2 The most simple organometallic is methyllithium (H3CLi), a threedimensional polymer in the solid state.3 Basic units are tetrameric heterocubane Li4(CH3)4 entities with alternating Liþ or CH3- ions. These are further linked by weaker interheterocubane Liþ 3 3 3 CH3- interactions in three dimensions. The addition of bases generally leads to disaggregation of the polymeric structure, often with conservation of the basic tetrameric units. Tetrahydrofuran leads to adducts of the compositions (MeLi 3 THF)4 with terminal THF ligands4 and tetramethylethylenediamine to a [(MeLi)4 3 (TMEDA)2]¥ adduct with (MeLi)4 tetramers linked in three dimensions by tmeda molecules.5 With diethoxymethane (DEM) the adduct [(MeLi)4 3 (DEM)1.5]¥ was isolated; its crystal structure comprises endless strands of C 3 3 3 Li-linked (MeLi)4 tetrahedra which are joined through their remaining three Li vertices by Li 3 3 3 O(Et)CH2(Et)O 3 3 3 Li linkages into a three-dimensional polymer.6 Few cases are † Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth, a Nestor of organometallic chemistry. *To whom correspondence should be addressed. E-mail: mitzel@ uni-bielefeld.de. (1) Schlosser, M., Ed. Organometallics in Synthesis-A Manual, 2nd ed., Wiley: Chichester, U.K., 2002. (2) (a) Seyferth, D. Organometallics 2006, 25, 2. (b) Seyferth, D. Organometallics 2009, 28, 2. (3) (a) Weiss, E.; Lucken, E. A. J. Organomet. Chem. 1964, 2, 197. (b) Weiss, E.; Hencken, G. J. Organomet. Chem. 1970, 21, 265. (4) (a) Ogle, C. A.; Huckabee, B. K.; Johnson, H. C., IV; Sims, P. F.; Winslow, S. D.; Pinkerton, A. A. Organometallics 1993, 12, 1960. A similar observation was deduced by NMR studies for the diethyl ether adducts: (b) West, P.; Waack, R. J. Am. Chem. Soc. 1967, 89, 4395. (c) McKeever, L. D.; Waack, R.; Doran, M. A.; Baker, E. B. J. Am. Chem. Soc. 1969, 91, 1057. (5) K€ oster, H.; Thoennes, D.; Weiss, E. J. Organomet. Chem. 1978, 160, 1. (6) Walfort, B.; Lameyer, L.; Weiss, W.; Herbst-Irmer, R.; Bertermann, R.; Rocha, J.; Stalke, D. Chem. Eur. J. 2001, 7, 1417.
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known where the tetramers are broken into dimeric or trimeric units; these include the adducts [MeLi 3 ((-)-spartein)]27 and [MeLi 3 ((R,R)-TMCDA)]2.8 Only one monomeric adduct of MeLi is known; it is the adduct [(thf)3Li3Me{(NtBu)3S}] containing a one triangular Li3Me unit cut out from (MeLi)4 which is stabilized by the tripodal triazasulfite and three thf ligands.6 The situation with the higher lithium alkyls is related,9,10 but there are many more aggregation motifs, including monomers such as the (-)-spartein adduct of tert-butyllithium, recently isolated by Strohmann and co-workers.11 It is easy to imagine that the disaggregation of lithium alkyls occurs stepwise and that the described examples represent a kind of snapshot of adducts isolated in the solid state, whereas the degradation process in solution is richer in structural motifs and intermediates and, moreover, might be dynamic. We have now found12 that the reaction of MeLi with the triaminal 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) allows for the isolation of an “earlier snapshot” in this “disaggregation movie” of methyllithium. This interest in the interactions of TMTAC with lithium alkyls stems from our observation that it can be deprotonated at the seemingly doubly deactivated aminal position NCH2N by n- or tert-butyllithium.13 The synthetic potential of such diaminosubstituted carbanion reagents is their ability to replace the lithiated dithianes for a heavy-metal-free Corey-Seebach analogous Umpolung protocol. The interest in such reactions and the unexpected ease of direct R-lithiation of amines resulted not only in the isolation of reaction intermediates of tBuLi with TMTAC14 but also in a combined experimental-theoretical approach to learn more about the different regioselectivities for deprotonation of open-chain aminals (e.g., LiCH2N(Me)CH2N(Me)CH2Li) and ring aminals which are always deprotonated at the NCH2N position.15 In contrast to the butyllithium bases, MeLi is not able to deprotonate TMTAC. However, mixing the two components TMTAC and a MeLi solution in diethyl ether in the solvent hexane led to precipitation of an adduct (1) which (7) Strohmann, C.; Strohfeldt, K.; Schildbach, D.; McGrath, M. J.; O’Brien, P. Organometallics 2004, 23, 5389. (8) Strohmann, C.; Gessner, V. Angew. Chem. 2007, 119, 8429; Angew. Chem., Int. Ed., 2007, 46, 8281. (9) Dimeric alkyllithiums: (a) Nichols, M. A.; Williard, P. G. J. Am. Chem. Soc. 1993, 126, 1568. (b) Barnett, N. D. R.; Mulvey, R. E.; Clegg, W.; O'Neil, P. A. J. Am. Chem. Soc. 1993, 115, 1573. (c) Strohmann, C.; Strohfeldt, K.; Schildbach, D. J. Am. Chem. Soc. 2003, 125, 13672. (d) Strohmann, C.; Dilsky, S.; Strohfeldt, K. Organometallics 2006, 25, 41. (10) Monomeric alkyllithiums: (a) Sch€ umann, U.; Weiss, J. Angew. Chem. 1985, 3, 222; Angew. Chem., Int. Ed. Engl. 1985, 24, 215, (b) Arnold, J.; Knapp, V.; Schmidt, J. A. R.; Shafir, A. Dalton Trans. 2002, 3273. (c) Zarges, W.; Marsch, M.; Harms, K.; Boche, G. Chem. Ber. 1989, 122, 2303. (d) Strohmann, C.; Seidel, T.; Schildbach, D. J. Am. Chem. Soc. 2004, 126, 9876. (11) Strohmann, C.; Seibel, T.; Strohfeldt, K. Angew. Chem. 2003, 115, 4669; Angew. Chem., Int. Ed. 2003, 42, 4531. r 2010 American Chemical Society
Communication
was only sparingly soluble in hexane but was soluble enough to allow crystallization from this solvent at -26 C to afford crystals suitable for X-ray diffraction.12 Adduct 1 is slightly more soluble in benzene and could thus be characterized by NMR spectroscopy. The 1H NMR spectrum shows a broad singlet for the carbanionic methyl groups at 0.5 ppm, a singlet at 2.14 ppm for the N-bound methyl groups of TMTAC, and another broad singlet at 3.06 ppm for its methylene units. Corresponding observations are made in the 13C NMR spectrum, where resonances at 5.0, 40.3, and 77.6 ppm can be assigned to these units. With the composition [(MeLi)4(TMTAC)3], 1 comprises four lithium acceptor units but nine N-donor sites. The fact that only one resonance is found for each of the H3CN and NCH2N units indicates rapid exchange in solution. The crystal structure of 1 shows a chain adduct of (MeLi)4 tetramers (Figure 1). This structure thus represents an intermediate aggregation mode between the 3D-polymeric network of pure crystalline methyllithium and the solvated heterocubane tetramers: (MeLi 3 donor)4. In this sense it is related to the aforementioned adduct [(MeLi)4 3 (DEM)1.5]¥,6 but in contrast to the coordination polymer linked in three dimensions with the difunctional DME ligands linking the [(MeLi)4]¥ chains, the chains in compound 1 bear terminal TMTAC ligands and are not interwoven by coordinative bonds. Due to the fact that (12) (a) Synthesis of [(MeLi)4(TMTAC)3] (1): a solution of TMTAC (3.30 mL, 3.00 g, 23.2 mmol) in 10 mL of hexane was added dropwise to a solution of methyllithium in diethyl ether (1.6 M, 14.5 mL, 23.2 mmol) at -78 C. The stirred mixture was warmed to room temperature over 12 h, while a white pyrophoric product precipitated, which was filtered, washed with hexane (2 10 mL), and dried under vacuum. The filtrate contained further small amounts of dissolved product, which were crystallized by slow cooling to -26 C. Yield: 3.34 g (22.1 mmol, 95%). 1H NMR (500 MHz, C6D6): δ 0.50 (br, 3H, LiCH3), 2.14 (s, 9H, NCH3), 3.06 ppm (br, 6H, NCH2N). 13C{1H} NMR (125 MHz, C6D6): δ 5.0 (LiCH3), 40.3 (NCH3), 77.6 ppm (NCH2N). Anal. Calcd for C7H18N3Li (151.17): C, 55.57; H, 12.08; N, 26.51. Found: C, 54.79; H, 11.83; N, 26.14. (b) Synthesis of [Li(TMTAC)2][AlMe4] (2): a solution of TMTAC (0.59 mL, 540 mg, 4.2 mmol) in 5 mL of hexane was slowly dropped into a mixture of solvent-free methyllithium (93 mg, 4.2 mmol) and AlMe3 (300 mg, 4.2 mmol) in 10 mL of hexane at -78 C. The stirred mixture was warmed to room temperature over 12 h and refluxed at 80 C for 2 h, and the resulting precipitate was filtered, washed with hexane (2 10 mL), and dried under vacuum. The filtrate contained further small amounts of dissolved product, which crystallized over 2 days at 4 C. Yield: 590 mg (43%, 1.81 mmol). 1H NMR (500 MHz, C6D6): δ -0.33 (s, 12H, AlMe4-), 1.65 (s, 9H, NCH3), 1.69 and 2.93 (2 d, 2 6H, NCH2N, 2JH,H = 6.5 Hz). 13C{1H} NMR (125 MHz, C6D6): δ -5.9 (AlMe4-), 38.3 (NCH3), 76.8 ppm (NCH2N). 7Li NMR (194 MHz, C6D6): δ 0.18 ppm. 27Al NMR (130 MHz, C6D6): δ 154 ppm (ν1/2 = 140 Hz). Anal. Calcd for C16H42AlLiN6 (325.34): C, 54.52; H, 12.01; N, 23.84. Found: C, 53.97; H, 11.78; N, 23.11. (c) Crystal data for 1: C22H57Li4N9, hexagonal, R3, a = 21.3969(4) A˚, c = 12.3807(7) A˚, Z = 6, λ = 1.54178 A˚, θmax = 72.45, T = 153(2) K, 9771 measured and 2039 independent reflections (Rint = 0.072), 249 parameters, 33 restraints, R1 = 0.045 for 1641 reflections with Fo > 4σ(Fo) and wR2 = 0.117 for all 2035 data. Crystal data for 2: C12H30LiN6 3 C4H12Al, monoclinic, P21/c, a = 8.3824(2) A˚, b = 15.4957(4) A˚, c = 18.4008(5) A˚, β = 99.7841(14), Z = 4, λ = 0.710 73 A˚, θmax = 27.5, T = 100(2) K, 31 138 measured and 5369 independent reflections (Rint = 0.035), 227 parameters, R1 = 0.037 for 4370 reflections with Fo > 4σ(Fo) and wR2 = 0.105 for all 5369 data. Structure solution and refinement followed standard procedures.18 Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as Supplementary Publication Nos. CCDC775103 (1) and CCDC-775104 (2). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax (þ44)1223-336-033; e-mail
[email protected]). (13) Bojer, D.; Kamps, I.; Tian, X.; Hepp, A.; Pape, T.; Fr€ ohlich, R.; Mitzel, N. W. Angew. Chem., Int. Ed. 2007, 46, 4176. (14) Strohmann, C.; Gessner, V. H. Chem. Asian J. 2008, 3, 1929. (15) (a) Kamps, I.; Bojer, D.; Hayes, S. A.; Berger, R. J. F.; Neumann, B.; Mitzel, N. W. Chem. Eur. J. 2009, 15, 11123. (b) Kamps, I.; Mix, A.; Berger, R. J. F.; Neumann, B.; Stammler, H.-G.; Mitzel, N. W. Chem. Commun. 2009, 5558.
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Figure 1. Molecular structure of the monomers of [(MeLi)4(TMTAC)3]¥. Selected bond lengths (A˚) and angles (deg): Li(1) 3 3 3 Li(2)=2.527(8), Li(2) 3 3 3 Li(2a)=2.551(7), Li(1)-C(22)=2.260(4), Li(2)-C(21) = 2.272(5), Li(2)-C(22) = 2.258(6), N(1)-Li(2) = 2.102(5), N(5)-Li(4) = 2.135(5); Li(2)-C(22)-Li(1) = 68.0(2), Li(2)-C(21)-Li(2a) = 68.3(2), Li(2)-C(22)-Li(1a) = 68.5(5), C(22)-Li(2)-C(21) = 107.0(2), C(1)-N(1)-Li(2) = 112.2(2), C(3)-N(1)-Li(2) = 107.0(2), N(1)-Li(2)-C(22) = 111.0(2), N(1)-Li(2)-C(22a) = 110.7(2).
Figure 2. Arrangement of the monomeric units shown in Figure 1 into endless chain aggregates of [(MeLi)4(TMTAC)3]¥ by Li-C contacts (Li(1)-C(23)=2.388(9) A˚). For clarity, no nitrogen atoms (except the binding nitrogen atoms of the TMTAC ligands) or hydrogen atoms are shown.
the employed ethereal methyllithium solution in the preparation was already disaggregated into tetrameric units with four terminal ether ligands, this structure demonstrates that, despite the presence of a large excess of base functionalities and contrary to expectation, no degradation reactions of the lithium alkyl oligomers occur; rather, reaggregation processes are taking place, as the starting material in the synthesis was the ethereal solution containing (MeLi 3 OEt2)4 heterocubane aggregates. In the present case these lead to a head-to-tail linkage of (MeLi)4 heterocubane units. It is the lithium atom on one side and the methyl anion on the other side of one heterocubane which form links to the next (MeLi)4 cubes by Li 3 3 3 C contacts (Figure 2). These have a length of 2.388(9) A˚, which is much longer than the intracubane Li 3 3 3 C distances; these fall in a range between 2.272(5) and 2.232(6) A˚. This compares well
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Organometallics, Vol. 29, No. 21, 2010
Figure 3. Molecular structure of the monomers of [Li(TMTAC)2][AlMe4] (2). Selected bond lengths (A˚) and angles (deg): N(1)-C(1) = 1.461(2), N(1)-Li(1) = 2.305(2), N(2)-Li(1) = 2.183(2), N(3)-Li(1) = 2.311(2), N(4)-Li(1) = 2.245(2), N(5)Li(1)= 2.350(2), N(6)-Li(1) = 2.189(2), C(13)-Al(1) = 2.007(1); N(1)-Li(1)-N(2) = 63.1(1).
with the corresponding distances in pure MeLi (2.279 A˚)3 and also with those in the thf solvate [(MeLi)4 3 (THF)4] (2.24 A˚).4 The remaining three lithium atoms of a (MeLi)4 cube, which are not involved in the links between the heterocubane units, are coordinated to the nitrogen atoms of TMTAC molecules with Li-N distances between 2.102(5) and 2.135(5) A˚. The TMTAC molecules adopt a conformation with the Li-coordinated N atom bearing an axial methyl group, while the other two are in equatorial positions. This parallels the situation in AlMe3 adducts of TMTAC.16 One could expect that the free nitrogen donor functionalities in 1 could be complexed by Lewis acids. However, in contrast to the described aggregation of (MeLi)4 cubes into chains, complete disaggregation takes place under similar conditions in the presence of the Lewis acid trimethylaluminum. TMTAC reacts with a mixture of MeLi and AlMe3 in hexane to afford a compound (2) with the lithium cations [Li(TMTAC)2]þ hexacoordinated by two TMTAC ligands and well-separated tetramethylaluminate anions [AlMe4]-.12 This means that in this case the TMTAC molecules make use of all of their three nitrogen donor sites for binding to the lithium atom and AlMe3 abstracts the (16) Venugopal, A.; Kamps, I.; Bojer, D.; Berger, R. J. F.; Mix, A.; Willner, A.; Neumann, B.; Stammler, H.-G.; Mitzel, N. W. Dalton Trans. 2009, 5755.
Kamps et al.
CH3- anion completely from lithium rather than binding to the free donor sites of aggregate 1. Although they are prepared in a different way, tetraalkylaluminates with [Li(TMTAC)2]þ counterions have been described earlier.17 Compound 2 was characterized by 1H, 7Li, 13C, and 27Al NMR of a C6D6 solution by elemental analysis and determination of its crystal structure.12 Interestingly, the cations differ in symmetry from those centrosymmetric ones found in earlier reports,17 due to the fact that they are of approximate D3h symmetry: i.e., their TMTAC ligands adopt an eclipsed conformation relative to one another (Figure 3). Both TMTAC ligands are also bound slightly asymmetrically to the lithium atoms with two longer (N(1)-Li(1) = 2.305(2) A˚, N(3)-Li(1) = 2.311(2) A˚) and one shorter Li-N distance (N(2)-Li(1) = 2.183(2) A˚). This work demonstrates that Lewis bases-even if the donor functionalities are present in excess-not only can lead to disaggregation as observed before but also can induce the aggregation of lithium alkyl units. The example of 1 shows nicely the arrangement of the formerly separated (MeLi)4 tetramers into endless chain aggregates. However, in the presence of the additional Lewis acidic agent AlMe3 completely different reaggregation processes are occurring: namely, a complete disaggregation of MeLi oligomers to afford hexacoordinate lithium cations with the methyl anions being completely abstracted from lithium. It is thus the competition for the strongest donor-acceptor interactions which determines the products in such systems.
Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft for financial support, to Dr. Andreas Mix and Peter Mester for NMR measurements, and to Brigitte Michel for elemental analyses. Supporting Information Available: Tables and CIF files giving crystallographic details for compounds 1 and 2 and figures giving 1 H, 13C, 7Li, and 27Al NMR spectra of compound 2. This material is available free of charge via the Internet at http://pubs.acs.org. (17) (a) Uhl, W.; Koch, M.; Pohl, S.; Saak, W. Z. Anorg. Allg. Chem. 1994, 620, 1619. (b) Uhl, W.; Sch€utz, U.; Kaim, W.; Waldh€or, E. J. Organomet. Chem. 1995, 501, 79. (c) Uhl, W.; Koch, M.; Wagner, J. Z. Anorg. Allg. Chem. 1995, 621, 249. (d) Uhl, W.; Gerding, R.; Hannemann, F. Z. Anorg. Allg. Chem. 1998, 624, 937. (18) (a) SHELXTL 6.10; Bruker-AXS X-Ray Instrumentation Inc., Madison, WI, 2000; (b) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Solution and Refinement; Universit€at G€ottingen, G€ottingen, Germany, 1997.
