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Organometallics 2010, 29, 5762–5765 DOI: 10.1021/om100825m
Dilithio 9,10-Diborataanthracene: Molecular Structure and 1,4-Addition Reactions Andreas Lorbach, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner* Institut f€ ur Anorganische und Analytische Chemie, J. W. Goethe-Universit€ at Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany Received August 24, 2010 Summary: 9,10-Dihydro-9,10-diboraanthracene ([1]n) and its SMe2 adduct 1(SMe2)2 are readily reduced with lithium in THF to the dianionic 9,10-diborataanthracene Li2[1]. An X-ray crystal structure analysis of (Li(thf )2)2[1] revealed monomeric inverse sandwich complexes, each of them containing two Li(thf )2 moieties coordinated to both sides of the central B2C4 ring. Compared to 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene, the four B-CAr bonds of (Li(thf )2)2[1] are shorter by 0.046(4) A˚, thereby indicating an increased degree of BdCAr double-bond character. Consequently, (Li(thf )2)2[1] reacts with 4,40 -dimethylbenzophenone as a BdCAr-CArdB diene and undergoes a [4þ2] cycloaddition reaction with formation of a bicyclic product. In contrast, tert-butylacetylene reacts with (Li(thf )2)2[1] under formal 1,4-addition of its methinic C-H group instead of its CtC triple bond to the two boron atoms.
9,10-Dihydro-9,10-diboraanthracene [1]n (Scheme 1) has proven to be a highly useful building block for the preparation of photoluminescent boron-doped π-electron systems via the hydroboration of arylalkynes.1,2 [1]n possesses a remarkable polymeric structure in the solid state, established by B-H-B two-electron three-center bonds.1 We have recently shown that the addition of SMe2 to [1]n gives the diadduct 1(SMe2)2 (Scheme 1), which is soluble in common noncoordinating solvents, but at the same time easy to crystallize and purify.3 *Corresponding author. Fax: þ49 69 798 29260. E-mail: Matthias.
[email protected]. (1) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H.-W.; Holthausen, M. C.; J€ akle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584–4588. (2) Chai, J.; Wang, C.; Jia, L.; Pang, Y.; Graham, M.; Cheng, S. Z. D. Synth. Met. 2009, 159, 1443–1449. (3) Lorbach, A.; Bolte, M.; Lerner, H.-W.; Wagner, M. Chem. Commun. 2010, 46, 3592–3594. (4) Synthesis of Li2[1]. A lithium granule (7 mg, 1 mmol) was added to a clear, colorless solution of 1(SMe2)2 (120 mg, 0.400 mmol) in dry THF (5.5 mL) under an atmosphere of argon, and the mixture was stirred for 24 h at rt, whereupon its color changed to red. Excess lithium and minor amounts of a red precipitate were removed by filtration, and the filtrate was evaporated to dryness in vacuo. The dark red residue was dissolved in dry Et2O (3.0 mL), and the solution was stored for 24 h at -78 °C to obtain dark red crystals. The mother liquor was removed via syringe at -78 °C, and the crystalline solid was dried in vacuo to obtain (Li2(Et2O)(thf))[1]. Yield: 71 mg (53%). Single crystals of (Li(thf)2)2[1] suitable for X-ray diffraction were isolated by storing a saturated THF solution of Li2[1] at -78 °C for 1 day. 1H NMR (400.1 MHz, THF-d8): δ 5.54 (h1/2 = 170 Hz, 2H; BH), 6.58 (m, 4H; C6H4), 8.02 ppm (m, 4H; C6H4). 11B NMR (128.4 MHz, THF-d8): δ 22.0 ppm (h1/2 = 370 Hz). 13C{1H} NMR (75.5 MHz, THF-d8): δ 118.4 (C6H4), 139.5 ppm (C6H4), n.o. (BC). Anal. Calcd for C12H10B2Li2 [189.71] 3 C4H10O [74.12] 3 C4H8O [72.11]: C, 71.51; H, 8.40. Found: C, 70.96; H, 8.33. Note: For the following reasons, it is difficult to obtain a reliable combustion analysis of (Li2(Et2O)(thf))[1]: (i) the compound is extremely sensitive even to traces of oxygen and water; (ii) the THF and Et2O content of the sample can vary and should therefore be controlled by NMR spectroscopy. pubs.acs.org/Organometallics
Published on Web 10/05/2010
In most cases, we therefore found it convenient to use 1(SMe2)2 instead of [1]n as starting material for further derivatization of the 9,10-dihydro-9,10-diboraanthracene framework. The fact that the polymer backbone of [1]n is disrupted by σ-donors led us to investigate whether depolymerization can also be achieved by two-electron reduction of 1/n [1]n to the 9,10-diborataanthracene state [1]2-. Apart from a resulting Coulomb repulsion between the individual monomer units, the increase of charge density in the π-electron system should weaken the B-H-B bridges and in turn favor the formation of low molecular weight species. Indeed, treatment of [1]n with excess lithium in THF leads to a red solution from which (Li(thf)2)2[1] was isolated in single crystalline form (Scheme 1). The same compound can be obtained by reduction of 1(SMe2)2 instead of [1]n.4 The monolithio salt Li[1] is readily accessible by comproportionation of Li2[1] and 1(SMe2)2 in THF; addition of excess lithium to the solution of Li[1] quantitatively regenerates Li2[1] (Scheme 1; see the SI for the UV/vis spectra of Li[1] and Li2[1]). The crystal lattice of (Li(thf)2)2[1] contains discrete centrosymmetric units in which Li(thf)2þ ions are η6-coordinated to both sides of the B2C4 ring (Figure 1; Li 3 3 3 COG = 1.960 A˚; COG = centroid of the B2C4 ring).5 Compared to the most closely related monomeric neutral species 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene 2,6 the average B-CAr bond in (Li(thf)2)2[1] is shorter by Δ = -0.046(4) A˚ (Table 1). Characteristic changes are also observed for the aryl C-C bonds a (Δ = 0.041(4) A˚; Table 1), b (Δ=0.045(4) A˚), c (Δ=-0.019(5) A˚), and d (Δ=0.038(5) A˚). The corresponding bond lengths of the monoanion in (K(18crown-6)(thf)2)[2]6 are intermediate between the values of 2 and (Li(thf)2)2[1] (Table 1). We note in this context that a DFT study on anthracene and [anthracene]þ revealed differences in the C-C bond lengths between both species that are in good qualitative agreement (5) Compound (Li(thf)2)2[1] crystallizes with two crystallographically independent half-molecules in the asymmetric unit; structural parameters discussed in this paper are averaged values over all chemically equivalent bonds. An X-ray crystal structure analysis of the compound (K2(tmeda)(thf)2)[2] has been described by Siebert et al. (cf. ref 6). However, the structure solution suffers from large error margins, so that a meaningful interpretation of subtle structural changes with respect to 2 is not possible. Crystal data of (Li(thf )2)2[1]: C28H42B2Li2O4, M = 478.12 g mol-1, triclinic, P1, a = 8.5547(13) A˚, b = 10.8865(15) A˚, c = 16.236(2) A˚, R = 101.971(11)°, β=104.514(11)°, γ=100.804(11)°, V=1385.5(3) A˚3, Z=2, T =173(2) K, μ(Mo KR) =0.072 mm-1, 20 027 reflections measured, 5203 unique (Rint =0.1216), which were used in all calculations. The final wR(F2) was 0.1628 (all data), min./max. residual electron density -0.288/0.558 e A˚-3. The H-atom positions were taken from a difference Fourier synthesis and refined using a riding model. CCDC reference number 777558. (6) M€ uller, P.; Huck, S.; K€ oppel, H.; Pritzkow, H.; Siebert, W. Z. Naturforsch. B 1995, 50, 1476–1484. r 2010 American Chemical Society
Communication
Organometallics, Vol. 29, No. 22, 2010
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Table 1. Key Structure Parameters of the Neutral 9,10-Dihydro9,10-diboraanthracene Framework and Structural Changes after One- and Two-Electron Reduction
bond lengtha [A˚]
2 n = 0, R = Me
[2]n = 1, R = Me
[1]2n = 2, R = H
B-CAr a b c d
1.565(4) 1.423(4) 1.398(4) 1.379(5) 1.371(5)
1.525(8) 1.426(7) 1.412(8) 1.36(1) 1.37(1)
1.519(4) 1.464(3) 1.443(4) 1.360(5) 1.409(4)
a Averaged structural parameters of the neutral species, the mono- and the dianion are taken from 2,6 (K(18-crown-6)(thf)2)[2],6 and (Li(thf)2)2[1], respectively.
Figure 1. Crystal structure of compound (Li(thf)2)2[1] (only one of the two crystallographically independent molecules in the asymmetric unit is shown); displacement ellipsoids are drawn at the 50% probability level. Scheme 1. Synthesis of Li2[1] and Li[1]a
a
All reactions were performed at rt in THF.
with the trends shown in Table 1 for their isoelectronic congeners [1]2- and [2]-.7 The NMR spectra of Li2[1] in THF-d8 are best compared to those of 9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene,6,8 because in this reference system, unwanted boron coordination of the donor solvent can safely be excluded due to the steric demand of the mesityl substituents. Two-electron reduction leads to an upfield shift of the 11B{1H} NMR resonance from 72.3 ppm in the borane to 22.0 ppm in Li2[1]. One of the hydrogen-bearing phenylene carbon atoms is also much (7) Kukhta, A. V.; Kukhta, I. N.; Kukhta, N. A.; Neyra, O. L.; Meza, E. J. Phys. B 2008, 41, 205701–205707. (8) Agou, T.; Sekine, M.; Kawashima, T. Tetrahedron Lett. 2010, 51, 5013–5015.
better shielded in Li2[1] (δ(13C) = 118.4, 139.5 ppm) than in 9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene (δ(13C) = 134.3, 139.5 ppm). Thus, negative charge density is obviously accumulated both in the boron p-orbitals and in the phenylene π-systems. This suggests an increase in BdCAr double-bond character and is in line with the contraction of the corresponding bond length in (Li(thf)2)2[1] with respect to 2. X-ray crystallography and NMR spectroscopy on (Li(thf)2)2[1] indicate that the boron atoms of this compound are less electron deficient than those of 2 (and as they would be in the hypothetical monomer 1), which explains why (Li(thf)2)2[1] has no tendency to oligomerize via B-H-B bonds. By the same token, 9,10-diborataanthracene is unlikely to undergo hydroboration reactions, but, due to its formally anthracenelike electronic structure, may well act as a diene in [4þ2] cycloaddition reactions. To test this hypothesis, we next treated Li2[1] in an NMR tube with 1 equiv of 4,40 -dimethylbenzophenone ((p-Tol)2CO, THF-d8, rt; Scheme 2).9 The reaction mixture immediately adopted the deep blue color characteristic of the ketyl radical. The NMR tube was flame-sealed and the sample investigated by NMR spectroscopy. We obtained well-resolved signals, even though the solution still had a blue tint. (9) Synthesis of Li2[3]. A solution of (p-Tol)2CO (3 mg, 0.015 mmol) in dry THF-d8 (0.5 mL) was transferred under argon via pipet to solid (Li2(Et2O)(thf))[1] (5 mg, 0.015 mmol) in an NMR tube. The resulting blue solution was frozen at -196 °C; the NMR tube was evacuated and flame-sealed. Note: The solution requires 10 days to lose its color completely; this color change is not accompanied by any detectable changes in the NMR spectra of the sample. 1H NMR (400.1 MHz, THF-d8): δ 2.05 (s, 6H; CH3), 2.98 (q, 1JBH = 86 Hz, 1H; BH), 6.51 (vtd, 3JHH = 6.8 Hz, 4JHH = 1.3 Hz, 2H; C6H4), 6.53 (d, 3JHH = 8.0 Hz, 4H; p-Tol), 6.59 (vtd, 3JHH =6.8 Hz, 4JHH =1.2 Hz, 2H; C6H4), 6.99 (d, 3JHH =8.0 Hz, 4H; p-Tol), 7.09 (d, 3JHH =6.8 Hz, 2H; C6H4), 7.19 ppm (d, 3JHH =6.8 Hz, 2H; C6H4), n.o. (1 BH; this resonance was detected at 3.53 ppm in the 1 H{11B} NMR spectrum). 11B NMR (128.4 MHz, THF-d8): δ -7.9 (d, 1JBH = 86 Hz), 0.7 ppm (h1/2 = 320 Hz). 13C{1H} NMR (62.9 MHz, THF-d8): δ 21.2 (CH3), 122.8 (C6H4), 122.9 (C6H4), 126.9 (p-Tol), 128.8 (p-Tol), 129.4 (C6H4), 130.4 (p-TolC-4), 131.9 (C6H4), 156.0 ppm (p-TolC-1), n.o. (BC). ESI-MS m/z: 387.5 [3þH]-, 393.7 [3þLi]-. The THF-d8 solution of Li2[3] was evaporated to dryness under vacuum; the remaining solid was dissolved in dry Et2O (2 mL) and treated with excess 12crown-4 at rt. An off-white precipitate formed immediately, which was separated by centrifugation, washed with dry Et2O (2 2 mL), and dried in vacuo to yield (Li2(12-crown-4)3)[3]. Anal. Calcd for C51H72B2Li2O13 [928.61]: C, 65.96; H, 7.82. Found: C, 65.60; H, 7.45.
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Organometallics, Vol. 29, No. 22, 2010 Scheme 2. Synthesis of Li2[3], Li2[4], and Li2[5]a
a
All reactions were performed at rt in THF.
The 11B NMR spectrum was characterized by an unresolved signal at 0.7 ppm (h1/2 = 320 Hz) and a doublet resonance at -7.9 ppm (1JBH =86 Hz). In the 1H NMR spectrum, a 1:1:1:1 quartet (1H; 1JBH = 86 Hz) appeared at 2.98 ppm; upon 11 B-decoupling, a further resonance emerged at 3.53 ppm (1H). From these data it becomes evident that the product contains two magnetically inequivalent, tetracoordinate, hydrogenbearing boron atoms. The 1H NMR spectrum further revealed two doublets (2 2H) and two virtual triplets of doublets (vtd, 2 2H) assignable to the 1,2-diborylated fragments; each of these resonances showed cross-peaks to two of the other signals in the 1H-1H-COSY spectrum. Thus, these two phenylene rings of the product are symmetry-related, but all four protons within each ring are unique. Moreover, we observed one singlet at 2.05 ppm (6H) and two doublets in the aromatic region of the spectrum (2 4H), which are assignable to two equivalent 4-methylphenyl moieties. All these NMR features are in full agreement with the structure proposed for the [4þ2] cycloadduct Li2[3] (Scheme 2). In line (10) Wood, T. K.; Piers, W. E.; Keay, B. A.; Parvez, M. Org. Lett. 2006, 8, 2875–2878. (11) Reaction of Li2[1] with tBuCCH. (Li2(Et2O)(thf))[1] (5 mg, 0.015 mmol) was treated with excess tBuCCH (15 mg, 0.183 mmol) in dry THF-d8 (0.5 mL) at rt in an NMR tube under argon. The red solution was frozen at -196 °C; the NMR tube was evacuated and flame-sealed. After 30 min at rt, NMR spectroscopy indicated no conversion. After 3 days, the now pale red mixture contained (apart from tBuCCH) unreacted Li2[1] (10%; estimated from proton resonance integrals), Li2[4] (80%), and Li2[5] (10%). After 21 days at rt a colorless, clear solution was obtained that contained Li2[4] (50%) and Li2[5] (50%) together with H2 (δ(1H) = 4.55 ppm) as the only reaction products. Li2[4]: 1H NMR (300.0 MHz, THF-d8): δ 1.30 (s, 9H; CH3), 6.66 (vtd, 3JHH = 7.0 Hz, 4JHH = 1.6 Hz, 2H; C6H4), 6.75 (vtd, 3 JHH = 7.0 Hz, 4JHH = 1.5 Hz, 2H; C6H4), 7.27 (d, 3JHH = 7.0 Hz, 2H; C6H4), 7.82 ppm (d, 3JHH = 7.0 Hz, 2H; C6H4), n.o. (BH). 11B NMR (96.3 MHz, THF-d8): δ -18.4 (d, 1JBH =67 Hz; BH), -16.8 ppm (t, 1JBH = 76 Hz; BH2). 13C{1H} NMR (75.5 MHz, THF-d8): δ 33.7 (CH3), 122.4 (C6H4), 123.1 (C6H4), 133.3 (C6H4), 133.7 ppm (C6H4), n.o. (BC, CCCH3). Li2[5]: 1H NMR (300.0 MHz, THF-d8): δ 1.32 (s, 18H; CH3), 6.71 (m, 4H; C6H4), 7.75 ppm (m, 4H; C6H4), n.o. (BH). 11B NMR (96.3 MHz, THF-d8): δ -18.4 ppm (d, 1JBH = 67 Hz). 13C{1H} NMR (75.5 MHz, THF-d8): δ 33.7 (CH3), 122.3 (C6H4), 131.9 ppm (C6H4), n.o. (BC, CCCH3).
Lorbach et al.
with that, peaks at m/z=387.5 ([3þH]-) and 393.7 ([3þLi]-) have been detected by ESI(-) mass spectrometry. Transformations like the one just described involving aromatic boron-containing heterocycles are very rare. The most closely related example known to date stems from the group of Piers, who found the cycloaddition of borabenzene-pyridine with dimethyl acetylenedicarboxylate or benzyne to afford substituted borabarrelene and borabenzobarrelene, respectively.10 In order to find out how Li2[1] behaves toward alkynes, we treated the compound with excess tBuCCH at rt in THF-d8 (Scheme 2). The reaction was carried out in a sealed NMR tube and continuously monitored by NMR spectroscopy.11 After 3 days, most of the starting material had been consumed and one major product had been formed, which gave rise to one triplet (-16.8 ppm; 1JBH =76 Hz) and one doublet (-18.4 ppm; 1JBH = 67 Hz) in the tetracoordinate region of the 11B NMR spectrum. The phenylene proton resonance pattern resembled the one observed for Li2[3], thereby suggesting a Cs-symmetric compound. Finally, one singlet at 1.30 ppm (9H) testified to the presence of one tertbutyl substituent in the molecule. These NMR data led to the conclusion that compound Li2[4] has been formed, which is a formal 1,4-addition product of the methinic C-H bond across the B2C4 ring. Upon further storage of the NMR tube at rt, the signals of the initially minor product (
