Organometallics 1996,14, 2133-2135
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Crystal Structure of a Carbanion-Amide Combination with Lithium and Sodium Cations? Sjoerd Harder,*>*Martin Lutz,* and Thomas Kremers Universitat Konstanz, Postfach 5560, M738, 78434 Konstanz, Germany, and Institiit f i r Organische Chemie, Henkestrasse 42, 91054 Erlangen, Germany Received February 21, 1995@ Summary: I n analogy with RLi IMOR' (M = Na, K, Rb, Cs) superbases, metal exchange in mixtures of (RLi)z and (MNR'dz to give (RM)z and ( L i N R d z is calculated to be exothermic. A small energy differencebetween complete metal exchange and the formation of mixed aggregates, RMILiNR'z, suggests a significant role of the latter. We prepared a mixed Li l N a carbanion-amide and studied the structure by single-crystal X-ray diffraction. The structure of this Li l N a carbanion-amide complex can be described as a tetrameric cluster from units i n which Li and N a cations bridge the carbanion and amide functionalities. The carbanion-amide bonding interactions with the Li cations are of major importance, whereas the N a cations are involved i n weaker bonding. The different influence of amide anions and alkoxide anions on the structure of mixed aggregates provoked a comparative study on the superbasic properties of such systems. Enhanced reactivity of organo alkali-metal species with mixed metals initiated the structural study of such systems.l While several X-ray structures of mixedmetal species have been determined,, only a few complexes are known in which both different metals and different anions O C C U ~ . We ~ reported the first structure of an organosodiudithium alkoxide compound, an intramolecular superbase model with both functionalities attached to the same molecule.3bSuperbasicity (i.e. enhanced deprotonating power)le,f of RLi/R'OM (M = Na, K, Rb, Cs) mixtures is generally ascribed to metal e x ~ h a n g e ,which ~ gives the more reactive RM/R'OLi combination. The driving force for this exothermic +Dedicated, with all best wishes, to Professor Paul von Rague Schleyer on the occasion of his 65th birthday. Universitat Konstanz. Instittit ftir Organische Chemie. @Abstractpublished in Aduance ACS Abstracts, May 1, 1995. (1) (a) Morton, A. A.; Claff, C. E., Jr.;J. Am. Chem. SOC. 1954, 76, 4935. (b) Wittig, G.; Ludwig, R.; Polster, R. Chem. Ber. 1955,88, 294. (c) Wittig, G.; Bickelhaupt, F. Chem. Ber. 1958, 91, 865. (d) Wittig, G.; Benz, E. Chem. Ber. 1958, 91, 873. (e) Lochmann, L.; Pospisil, J.; Lim, D. Tetrahedron Lett. 1966, 257. (0 Schlosser, M. J . Organomet. Chem. 1967,8, 9. (2) (a)Cambillau, C.; Bram, G.; Corst, J.; Riche, C. Nouu.J.Chin. 1979,3,9. (b) Weiss, E.; Sauermann, G.; Thirase, G . Chem. Ber. 1983, 116, 74. (c) Clegg, W.; Mulvey, R. E.; Snaith, R.; Toogood, G. E.; Wade, K. J. Chem. Soc., Chem. Commun. 1986, 1740. (d) Momany, C.; Hackert, M. L.; Sharma, J.; Poonia, N. S.J.Inclusion Phenom. 1987, 5, 3443. (e) Schuhmann, U.; Weiss, E. Angew. Chem. 1988,100, 573; Angew. Chem., Int. Ed. Engl. 1988, 27, 584. (0 Barr, D.; Clegg, W.; Mulvey, R. E.; Smith, R. J. Chem. SOC., Chem. Commun. 1989, 57. ( g ) Lorenzen, N. P.; Kopf, J.; Olbrich, F.; Schumann, U.; Weiss, E. Angew. Chem. 1990,102, 1481;Angew. Chem., Int. Ed. Engl. ISSO, 29, 1441. (h) Williard, P. G.; Hintze, M. J. J. Am. Chem. SOC.1990, 112, 8602. (i) Williard, P. G.; Nichols, M. A. J.Am. Chem. SOC. 1991,113, 9671. (j) Baker, D. R.; Mulvey, R. E.; Clegg, W.; ONeil, P. A. J. Am. Chem. SOC. 1993, 115, 6472. (k) Baker, D. R.; Clegg, W.; Horsburgh, L.; Mulvey, R. E. Organometallics 1994, 13, 4170 and references cited therein. (3) (a)Williard, P. G.; MacEwan, G. J. J.Am. Chem. SOC. 1989,111, 7671. (b) Harder, S.; Streitwieser, A. Angew. Chem. 1993, 105, 1108; Angew. Chem., Int. Ed. Engl. 1993,32, 1067. (4) MP4SDTQ/6-31+G*//6-3l+G* calculations.
0276-7333/95/2314-2133$09.00/0
exchange reaction (note the ab initio results of eq is the greater electrostatic attraction between cations and anions: the larger cation (M+) prefers to be associated with the larger anion (C-) and the smaller cation (Li+) with the smaller anion (O-).3b~5Indeed, the structure of the only known mixed organosodiumkthium alkoxide compound shows a strong preference for Li-0 bonding.3b (MeLi),
+ (NaOH), - (MeNa), + (LiOH), AE = -8.3
(MeLi),
kcallmol
+ (NaNH,), - (MeNa), + (LiNH,), AE = -7.0
(MeLi),
(1)
(2)
kcallmol
+ (NaOH,), - 2(MeNa/LiNH,)
(3)
(AE)= -5.3 kcallmol In analogy with RLilMOR superbases, mixtures of RLi and MNR2 (M = Na, K, Rb, Cs) also should exchange metals t o form RM/LiNR2, in which the smaller lithium cation interacts with the smaller amide anion. Ab initio calculations on dimeric species (eq 2) show this metal exchange to be exothermic by 7.0 kcaY mol.4 The smaller exothermicity, compared t o the Li/ Na exchange in eq 1,is due t o the larger radius of the amide anion compared to that of the alkoxide anion. Since formation of a mixed aggregate (MeNa/LiNHz)is only 1.7 kcaYmol less favorable than complete transmetalation (eq 31, mixed aggregates may well be significant components under experimental conditions. Equilibria between many different species (and crystallization only of the least soluble species) has hindered a structural study of a mixed alkali-metal carbanionamide system. Therefore, we chose to study an intramolecular system in which carbanion and amide functionalities are incorporated in the same molecule. We determined the X-ray structure of the crystalline product (2otmeda) obtained in the reaction of sodium methyl(4-methy1benzyl)amide(1)with n-butyllithiuml N,N,N',N'-tetramethylethylenediamine (tmeda).6 A structure in which Li and Na are doubly bridging the carbanion and amide units (as in 2) resembles a mixed aggregate, whereas a compound in which distinct N-Li and C-Na bonds can be recognized (e.g. 3) resembles a situation of complete metal exchange. (5) Grovenstein, E., Jr. In Recent Advances in Anionic Polymerization; Hogen-Esch, T. E., Smid, J., Eds., Elsevier:Amsterdam, 1987, p 3.
0 1995 American Chemical Society
2134 Organometallics, Vol. 14, No. 5, 1995
A single-crystal X-ray structure analysis7 of 2.tmeda reveals a centrosymmetric tetrameric aggregate which consists of four distinct monomeric units of 2 in which both Li and Na bridge the same carbanion and amide functionalities (this LYNa double bridging within these units is indicated by darkened bonds in Figure 1). The core of the aggregate is formed by the four Li cations, which are arranged on the corners of a planar (centrosymmetric) rhomboid. The negatively charged carbon atoms C11 and C11' bridge the two available triangular Li3 faces on opposite sides of the rhomboid. The other two negatively charged carbon atoms bridge opposing Liz sides of the rhomboid. All C-Li distances are in the narrow range of 2.26(1)-2.37(1) A. The remaining coordination sites on the Li cations are occupied by the negatively charged amide functionalities, so that all the Li cations have a distortedtetrahedral coordination environment. The Na cations are more weakly bound and are positioned in the outer regions of the tetrameric cluster. Each Na cation bridges one amide and two aryl functionalities and is additionally solvated by tmeda to complete the coordination sphere. The C(ipso)-Na bond distances range widely from 2.660(6)-3.049(6) A. For comparison, the C(ipso)-Na distances in phenylsodium compounds range from 2.566 to 2.756 A.2e,8 This sug(6) All experiments were carried out under an inert argon atmosphere using Schlenk techniques and syringes. Solvents were freshly destilled from sodiumhenzophenone prior to use. n-Butyllithium (1.6 M in hexane, 2.8 mL, 4.58 mmol) is added to a solution of l-tmeda (1.0 g, 3.66 mmol) in 80 mL of hexane and 3.0 mL of tmeda. Air-sensitive orange crystals of 2.tmeda suitable for X-ray diffraction are formed overnight at room temperature (0.53 g; yield 52%). Crystals dissolved in toluene-& show 'H NMR spectra with many overlapping, extremely broadened signals (temperature range -50 to +BO "C) which are due to slow exchange processes. Extreme line broadening prohibited the recording of 13CNMR spectra. Dissolving and quenching the crystals in methanol-& give clean spectra of the deuterated product, 4-Me-2D(CeH&HzN(D)Me, and tmeda in a 1/1ratio. 'H NMR (250 MHz, methanol-&, 25 "C, TMS): 6 2.24 (s, 12 H, tmeda Me), 2.30 (a, 3 H, NMe), 2.33 (s, 3H, Me), 2.45 (s, 4H, tmeda CHz), 3.62 (s, 2H, NCHz), 7.05 (d, V(H,H) = 7.2 Hz, l H , arom), 7.06 (8, lH, arom), 7.14 (d, V(H,H) = 7.2 Hz, 2H, arom). 13CNMR: 6 21.2 (Me), 35.5 (NMe), 45.9 (tmeda Me), 56.1 (NCHz), 57.9 (tmeda CHz), 129.3 (t, 'J(C,D) = 23.6 Hz), 129.6, 130.0, 130.1, 137.3, 137.9 (arom). (7) Crystal structure determination of 2.tmeda: the crystal was covered with high-grade parafin oil and mounted on a glass fiber in a cold Nz stream; a = 13.752(3)A, b = 21.940(4)A, c = 12.188(3)A, p = 107.12(1)", V = 3514(1) A3, space group P21/c, formula C9HllNLiNaCGHla2, M , = 279.33, 2 = 8, ecdcd = 1.056, p(Mo Ka)= 0.79 cm-'; 7974 unique reflections were measured on a Enraf-Nonius CAD4 diffractometer (Mo Ka radiation, graphite monochromator, T = -75 "C); solution by direct methods with SHELXS-86,ll refinement with 361 parameters and 2767 observed reflections (F > 2.0u(F '9) to R,( F 2 )= 0.179 and Rl(F3 = 0.078; non-hydrogen atoms anisotropic, hydrogen atoms located in difference Fourier maps and included with fixed parameters during refinement; refinement with SHELXL-9312 and plots with the EUCLID package.I3
Communications
Figure 1. Crystal structure of (2.tmedah. Nitrogens are black, and lithiums are speckled. Hydrogens have been omitted; atoms marked with a prime are symmetry-related by a center of inversion. Selected distances (A): Lil-Cll, 2.36(1); Lil-C12, 2.26(1); Lil-Cll', 2.27(1); Lil-Nll, 2.09(1);LiP-Cl1,2.34(1); Li2-C12,2.37(1); Li2-N12,1.95(1); Li2-Nll', 2.04(1); Nal-C11, 3.025(6); Nal-C12', 2.660(6); Nal-N11, 2.594(5); Nal-N13, 2.583(6); NalN23, 2.550(6); Na2-C11, 2.904(6); Na2-C12, 3.049(6); Na2-N12,2.380(5); Na2-N14,2.669(6); Na2-N24,2.512(6); Lil-Li2, 2.49(1);Lil-Lil', 2.45(2);Lil'-Li2, 2.59(1). gests a weak electrostatic C-Na bonding in the structure of 2.tmeda, which is due to the enormous crowding of Li and Na cations around the negatively charged C(ipso) centers (C11 and C12), as can be seen in Figure 2. Repulsive interactions between Li2-Na2 and Li1'N a l bends the Na cations away from o-electron density on C(ipso) and weakens the electrostatic C-/Na+ interaction. Less bending of the Na cations is observed in the metal coordination geometry for C12, which only binds two Li cations. Even though the electrostatic C-Na bonding is weakened due to crowding, the seuencoordinate carbons C11 and C11' are remarkable. The short distances of the Na cations to other carbon atoms in the aryl rings implies an interaction of Na with the aryl n-system which is especially important for Na2. Such bonding plays a dominant role in anionic benzylic systems (particularly those of the higher alkali metal^)^ in which the benzylic negative charge is strongly delocalized into the n-system. The negative charge in 2 will be mainly localized on C(ipso), which therefore is of major significance in bonding of the Na cations. The amide functionalities either bridge two metal cations or bridge triangular arrangements of three alkali-metal cations. The Li-NR2 and Na-NR2 bond distances are in the normal range for lithium and sodium amides.1° (8)Schiimann, U.; Behrens, U.; Weiss, E. Angew. Chem. 1989,101, 481; Angew. Chem., Int. Ed. Engl. 1989,28, 476. (9) Hoffmann, D.; Bauer, W.; Hampel, F.; van Eikema Hommes, N. J. R.; Schleyer, P. v. R.; Otto, P.; Pieper, U.; Stalke, D.; Wright, D. S.; Snaith, R. J . Am. Chem. SOC.1994, 116, 528. (10)(a) Lappert, M. F.; Slade, M. J.; Singh, A.; Atwood, J. L.; Rogers, R. 0.;Shakir, R. J . Am. Chem. SOC.1983, 105, 302. (b) Haase, M.; Sheldrick, G . M. Acta Crystallogr., Sect. C 1986,42,1009. ( c ) Williard, P. G.; Salvino, J. M. J. Org. Chem. 1993,58,1.(d) Barr, D.; Clegg, W.; Mulvey, R. E.; Snaith, R.; Wright, D. S. J. Chem. SOC.,Chem. Commun. 1987, 716.
Communications
Organometallics, Vol. 14,No.5, 1995 2135 aggregates (2)in which carbanion-amide bonding to the Li cations is of major importance and weaker bonding interactions are observed with the Na cations. In contrast with the structure of the organosodidithium alkoxide ~ o m b i n a t i o nthere , ~ ~ seems to be no particular preference for interaction of Li with the smaller anion (Le. the RzN- functionality) in the structure of S-tmeda. To our knowledge, no research results on the use of BuLi/MNRz mixtures as superbases have been described in the literature. The structurally different behavior between amide anions and alkoxide anions has provoked a study of the possible superbase potential of BuLi/MNR2 (M = Na, K) and is currently under investigation.
Acknowledgment. S.H. thanks the Alexander von Humboldt Foundation for the award of a fellowship. Professors H.-H. Brintzinger, G. Muller, and P. v. R. Schleyer are kindly acknowledged for discussions and for providing laboratory facilities. Supplementary Material Available: For a-tmeda,tables of crystal and structure refinement data, coordinates, bond
Figure 2. 2. Partial structures showing the complete metal surroundings for the two different Ar-)CHZN(-)Me dianions (views are perpendicular to the aryl ring plane). Short Na-C distances to the aryl system (upper part; A): Nal-C11, 3.025(6);Nal-C21, 2.768(6);Na2-C11, 2.904(6); NaZ-CZl, 2.889(6); Na2-C31, 3.119(6); Na2-C41, 3.315(6); Na2-C51, 3.338(6); Na2-C61, 3.089(6). Short Na-C distances to the aryl system (lower part; A): Na1’C12, 2.660(6); Na1’-C22, 3.142(6); Na2-Cl2, 3.049(6); Na2-C22, 3.015(6). In summary, the Li/Na carbanion-amide compound can be described as a cluster of intramolecular mixed
lengths and angles, anisotropic displacement parameters, and hydrogen atom coordinates and U(eq) values and an ORTEP plot (12 pages). Ordering information is given on any current masthead page. OM950137B (11)Sheldrick, G. M. SHELXS-86. In Crystallographic Computing; Sheldrick, G. M., Kriiger, C., Goddard, R., Eds.; Oxford University Press: Oxford, U.K., 1985; Vol. 3, p 175. (12) Sheldrick, G. M. SHELXL-93: Program for the Refinement of Crystal Structures; Institute ftir Anorganische Chemie, Gottingen, Germany, 1993. (13) Spek, A. L. EUCLID Package. In Computational Crystallography; Sayre, D., Ed.; Clarendon Press: Oxford, U.K., 1982.
