Article pubs.acs.org/Organometallics
Facile Synthesis of (3,5-(CF3)2C6H3)2BX (X = H, OMe, F, Cl, Br): Reagents for the Introduction of a Strong Boryl Acceptor Unit Kamil Samigullin, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner* Institut für Anorganische und Analytische Chemie, J. W. Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany S Supporting Information *
ABSTRACT: The reaction of (3,5-(CF3)2C6H3)Li ((Fxyl)Li) with BH3·SMe2 in Et2O furnishes Li[(Fxyl)BH3] in an essentially quantitative yield. Hydride abstraction with Me3SiCl followed by the addition of a second equivalent of (Fxyl)Li gives Li[(Fxyl)2BH2] in 71% yield. Treatment of Li[(Fxyl)2BH2] with 1 equiv of Me3SiCl and a subsequent targeted methanolysis provide access to the methoxyborane (Fxyl)2BOMe. The latter compound serves as starting material for the synthesis of the haloboranes (Fxyl)2BX (X = F, Cl, Br) through the reaction with KHF2/Me3SiCl, BCl3, and BBr3, respectively. All three haloboranes, as well as key synthesis intermediates, have been structurally characterized by X-ray crystallography.
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INTRODUCTION Highly Lewis acidic organoboranes are commonly used as catalysts in a variety of organic transformations or as cocatalysts in olefin polymerization reactions.1,2 Combined with sterically shielded Lewis bases, these Lewis acids compose “frustrated Lewis pairs” (FLPs). Being incapable of forming stable adducts, FLPs remain reactive toward small molecules, such as H2 or CO2, heterolytically cleaving element−element bonds and activating the substrates for various reactions.3,4 (C6F5)3B is the most widely used organoborane for these purposes, due to its high thermal and chemical stability and relatively strong Lewis acidity. The lower substituted borane (C6F5)2BH5,6 and its derivatives (C6F5)2BX (X = F, Cl, Br, OR, e.g.)7−9 are useful starting materials for syntheses of monomolecular FLPs, which have their acidic and basic centers connected by, e.g., alkylene or arylene links. The size and nature of the connecting units often influence the reactivity of such FLPs toward a given substrate, thereby allowing their adjustment for specific reactions.3,4 In this particular context, however, we recently encountered distinct shortcomings of the (C6F5)2B group, when we attempted to synthesize the geminal FLP (C6F5)2B− CH2−PtBu2 from (C6F5)2BOEt and LiCH2PtBu2: The formation of the methylene bridge was inevitably accompanied by a nucleophilic attack of the phosphane on one ortho-C−F bond, thereby furnishing the five-membered heterocycle I (Figure 1).10,11 Appropriate alternative substituents are larger perfluorinated aromatic systems, e.g., the 5,6,7,8-tetrafluoronaphthalen-2-yl group12 or CF3-substituted benzenes, such as the 2,4,6tris(trifluoromethyl)phenyl13−15 (fluoromesityl, Fmes) and 3,5-bis(trifluoromethyl)phenyl16 (fluoro-meta-xylyl, Fxyl) rings. Wang et al. investigated the reactivity of (Fmes)2BH as a strong, sterically encumbered Lewis acid, which activates H2 and CO2 even when combined with comparatively poorly © XXXX American Chemical Society
Figure 1. Five-membered heterocycle I formed in the reaction between (C6F5)2BOEt and LiCH2PtBu2.
shielded Lewis bases such as NEt3 or DABCO (1,4diazabicyclo[2.2.2]octane).14,15,17 Yet, the question emerges whether implementing the (Fmes)2B-group into monomolecular FLPs would still preserve the desired reactivity of the molecule, because attaching a third substituent larger than a hydrogen atom might well shut down any boron−substrate interaction for steric reasons. We note in this context that the (Fmes)2B unit is currently enjoying increasing attention as strong electron acceptor in luminescent dyes, where it does not even interact with water.18−20 Shifting the CF3 substituents from the ortho-positions to the meta-positions of the phenyl rings would open more space for substrate binding to the resulting (Fxyl)2B group at the likely price of a somewhat diminished Lewis acidity compared to the (Fmes)2B moiety. Due to the absence of ortho-fluorine atoms, the Fxyl substituent may be a viable alternative to the C6F5 group in the preparation of geminal FLPs. Yet, only few compounds with Fxylsubstituted boron atoms are described in the literature. The most prominent species are salts of the weakly coordinating [(Fxyl)4B]− anion, which is used, e.g., as the counterion for Received: May 5, 2014
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various cationic transition metal complexes,21−23 trityl24 and oxonium21,25 cations, in Fxyl transfer reactions,26,27 and for phase transfer catalysis.28,29 Among the tricoordinate boranes, only (Fxyl)3B has so far been synthesized on a preparative scale and fully characterized.16 (Fxyl)3B possesses a Lewis acidity slightly higher than that of (C6F5)3B, if evaluated with the Gutmann−Beckett method, but its Lewis acidity is lower according to the Childs method.16 For broadening the scope of Fxyl-substituted boranes as Lewis acids, and particularly for the synthesis of monomolecular FLPs, it is necessary to acquire convenient access to the key building blocks (Fxyl)2BH and (Fxyl)2BX (X = halogen, alkoxy, e.g.). Pfaltz et al. have used (Fxyl)2BCl in several reactions,30,31 but neither provided a synthesis protocol nor any spectroscopical data. Yamamoto et al. synthesized (Fxyl)2BOH from (Fxyl)MgBr and B(OMe)3, followed by aqueous workup; no yield was given. 32 (Fxyl)2BOH can be applied as a Lewis acidic catalyst for the dehydration of aldols.32 In another case, (Fxyl)2BOH was formed through the decomposition of the [(Fxyl)4B]− ion in the course of the synthesis of a rhenium catalyst under aqueous conditions.27 Herein, we disclose a first systematic synthesis approach to the reactive boranes (Fxyl)2BX (X = H, OMe, F, Cl, Br), which are ideally suited for implementing the (Fxyl)2B fragment into more sophisticated molecular scaffolds. Our overall strategy is based on the one-pot synthesis of the [(Fxyl) 2 BH 2 ] − hydridoborate and furnishes all target products in good yields.
Figure 2. 11B NMR spectra (a,b: CDCl3; c,d: C6D6) of the crude reaction mixtures after adding (Fxyl)MgBr (black) or (Fxyl)Li (red) to BH3·SMe2 (a,c) and after hydride abstraction with Me3SiCl, followed by a second addition of the nucleophiles (b,d).
as a result of LiF elimination and tetrafluorobenzyne formation,5 the use of 1 does not impose such hazards. Compound 1 was prepared at −78 °C by lithium−bromine exchange between n-BuLi and (Fxyl)Br in Et 2 O and subsequently treated with 1 equiv of BH3·SMe2. This time, the reaction proceeded cleanly upon warming of the mixture to room temperature to selectively furnish the aimed-for aryl(trihydrido)borate Li[2] (11B NMR spectroscopic control; Figure 2c). The reaction outcome remained the same, irrespective of whether BH3·SMe2 was added before or after the n-BuLi, since the lithium−bromine exchange was much faster than a conceivable nucleophilic attack of n-BuLi on BH3· SMe2 at −78 °C. To introduce the second Fxyl group, hydride abstraction with Me3SiCl was performed on Li[2] at room temperature. The resulting suspension (LiCl) was cooled to −78 °C, and (Fxyl)Br and nBuLi were added to obtain Li[4] in high purity upon letting the reaction warm to room temperature (Figure 2d). Unlike Grignard reagents, organolithium compounds readily react with Me3SiCl in Et2O.34 Thus, a one-step synthesis33 as realized for [(C6F5)2BH2]− is not possible for [4]−. The lithium aryl(trihydrido)borate Li[2] is characterized by a quartet at −26.0 ppm in the 11B NMR spectrum (1JBH = 76 Hz; C6D6); both the chemical shift value and the coupling constant agree nicely with those of other lithium aryl(trihydrido)borates.35−37 The proton resonance of the BH3 moiety is split into a 1:1:1:1 quartet (δ(1H) = 1.31). The Fxyl substituent gives rise to one signal at −62.4 ppm in the 19F NMR spectrum. In the 13C{1H} NMR spectrum, fluorine coupling is resolved for the CF3 carbon atoms (q, 272 Hz), the meta-carbon atoms (q, 32 Hz), and the para-carbon atom (spt, 4 Hz). The resonance of the boron-bonded carbon atom appears at 153.3 ppm as a 1:1:1:1 quartet (58 Hz). Given that the key NMR features of the Fxyl substituent remain very similar within the series of compounds Li[2]−10, we will not discuss these data any further in the paragraphs to follow. For the lithium diaryl(dihydrido)borate Li[4], we observe a triplet at δ(11B) = −14.0 (1JBH = 74 Hz) and a 1:1:1:1 quartet at δ(1H) = 2.19 (BH2). We also recorded NMR spectra (C6D6) of the borane· SMe2 intermediates 3 and 5 (Scheme 1), generated through hydride abstraction from Li[2] and Li[4] in the presence of SMe2 remaining from the BH3·SMe2 starting material.38 The adducts 3 and 5 give rise to a triplet resonance at δ(11B) = −9.5 ppm (1JBH = 101 Hz) and a broad, unresolved signal at δ(11B) = −2.0 ppm, respectively. The corresponding BH2 and BH
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RESULTS AND DISCUSSION Synthesis and NMR Spectroscopic Characterization of (Fxyl)borates and (Fxyl)boranes. In previous work, we described the synthesis of the [(C6F5)2BH2]− ion from BH3· SMe2 through a three-step procedure. In the first step, (C6F5)MgBr reacts with BH3·SMe2 in Et2O to furnish a mixture of the three hydridoborates [BH4]−, [(C6F5)BH3]−, and [(C6F5)2BH2]−. Hydride abstraction with Me3SiCl, followed by the addition of a second equivalent of (C6F5)MgBr, resulted in the exclusive formation of [(C6F5)2BH2]−.33 The synthesis protocol could even be developed further to a onestep procedure, in which all starting materials were combined at the very beginning,33 because (C6F5)MgBr does not react with Me3SiCl in Et2O.34 We emphasize at this point that the formation of a single anion (i.e., [(C6F5)2BH2]−) from a mixture of three different hydridoborates requires extensive substituent redistribution (probably via hydride transfer) as a key feature of the overall reaction scenario. In our early attempts at the synthesis of [(Fxyl)2BH2]−, we employed reagents similar to those that had been successfully used for the preparation of [(C6F5)2BH2]− also for the introduction of the Fxyl substituents. As to be expected in view of the above-mentioned results, the reaction between (Fxyl)MgBr and BH3·SMe2 produced a mixture of the hydridoborates [BH 4 ] − , [(Fxyl)BH 3 ] − ([2] − ), and [(Fxyl)2BH2]− ([4]−; Figure 2a). In contrast to the case of C6F5, however, the later hydride-abstraction/(Fxyl)MgBraddition sequence did not furnish a uniform product [4]−, but the reaction solution contained an even more complex mixture of products than after the first step (Figure 2b). Both the three-step and the one-step protocol failed equally for the synthesis of [4]−. Given this background, we next switched from the Grignard reagent (Fxyl)MgBr to the organolithium compound (Fxyl)Li (1; Scheme 1). Contrary to (C6F5)Li, which tends to explode violently at temperatures above −30 °C B
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Scheme 1. Synthesis Route for (Fxyl)2BH·SMe2 (5)
In the case of the Et2O solvate, the problem is aggravated by disordered ethyl groups. We will therefore discuss only the solid-state structures of Li(OEt2)2[2], K[7], and 10; the other crystal structure analyses are provided in the Supporting Information, merely as an additional proof of the compounds’ constitutions. Li(OEt2)2[2] establishes centrosymmetric dimers in the crystal lattice (Figure 3). Each Li+ ion coordinates two Et2O
proton resonances appear at 2.82 and 3.24 ppm. Even though we started with pure Li[2] and Li[4], the generated samples of 3 and 5 always contained about 10% of 5 and 3, respectively. This observation indicates a certain degree of substituent scrambling at the borane-adduct stage. It is therefore remarkable that the conversion of Li[2] to Li[4] finally proceeds without the formation of significant amounts of byproducts such as [(Fxyl)3BH]−. The methanolysis of 5 (+ 10% 3) yields (Fxyl)2BOMe (6) contaminated with approximately 10% of (Fxyl)B(OMe)2. The target compound 6 can readily be separated from the side product through crystallization (overall yield with respect to BH3·SMe2: 90%; δ(11B) = 43.0). The success of substitution reactions between (aryl)2BX and organometallic nucleophiles for the introduction of (aryl)2B groups often depends strongly on the choice of the leaving group X. In some cases, alkoxy groups (as in 6) are preferable, whereas in others, better nucleofuges such as the bromide ion are ideally suited. Other examples do exist, in which satisfactory results were obtained only with X = F.39 Starting from 6, we therefore decided to prepare the three boron halides (Fxyl)2BX (X = F (8), Cl (9), Br (10); Scheme 2). Treatment of 6 with 1
Figure 3. Molecular structure of Li(OEt2)2[2] in the solid state. The ethyl groups of coordinated Et2O are omitted for clarity; displacement ellipsoids are drawn at the 50% probability level. Selected bond length [Å], atom···atom distances [Å], and bond angles [deg]: B(1)−C(1) = 1.616(4); B(1)···Li(1) = 2.502(7), B(1)···Li(1A) = 2.488(7), Li(1)··· Li(1A) = 3.364(12), B(1)···B(1A) = 3.687(7); B(1)···Li(1)···B(1A) = 95.2(2), Li(1)···B(1)···Li(1A) = 84.8(2). Symmetry transformation used to generate equivalent atoms is A: −x + 1, −y + 1, −z.
Scheme 2. Syntheses of the Borinic Acid Methyl Ester (Fxyl)2BOMe (6) and the Halides (Fxyl)2BX (X = F (8), Cl (9), Br (10)), Starting from (Fxyl)2BH·SMe2 (5)
ligands and two [2]− anions with Li···B distances of 2.502(7) Å and 2.488(7) Å. The resulting RBH3−η2−Li coordination mode35,40,41 requires that one hydrogen atom of each BH3 fragment bridges two Li+ ions. Compound K[7] crystallizes from Et2O without coordinated solvent molecules. The K+ ions are therefore exclusively surrounded by fluorine atoms, such that an intricate coordination polymer network results. The asymmetric unit of the crystal nevertheless contains only one K+ and one [7]− ion. Five molecules of [7]− contribute to the ligand environment of each K+ ion: two of them by BF2−η2−K and three by CF3−η2−K contacts (Figure 4). The latter interaction probably restricts the motion of the CF3 groups in the solid state, thereby reducing their disorder and significantly improving the quality of the X-ray diffraction data. Considering the bond angles about the tetracoordinate boron center, the F(1)−B(1)−F(2) angle is slightly compressed to 103.8(2)°, whereas the C(1B)−B(1B)−C(11B) angle is expanded to 115.7(2)°. The X-ray crystal structure analysis of 10 fully confirms its proposed structure (Figure 5). The boron atom is located in a trigonal-planar environment (sum of bond angles about boron: 360.0°). Both Fxyl substituents include a dihedral angle of 53.4(1)°. All other structural parameters are unremarkable and therefore do not merit a detailed discussion.
equiv of KHF2 in MeOH gave the air- and water-stable potassium difluoroborate K[(Fxyl)2BF2] (K[7]) in 98% yield. The salt is characterized by a triplet resonance in the 11B NMR spectrum (δ = 5.7), which transforms into a singlet upon 19F decoupling; two signals are observable in the 19F NMR spectrum (−161.9 ppm (br m; BF2), −63.0 ppm (s; CF3)). The conversion of K[7] into 8 was achieved via fluoride abstraction with excess Me3SiCl in toluene (yield: 67%; δ(11B) = 46.1; δ(19F) = −54.1, −63.1). A direct transformation of 6 into 9 (96%; δ(11B) = 61.9) or 10 (92%; δ(11B) = 66.1) is possible through the addition of 1 equiv of BCl3 or BBr3 in n-hexane, respectively. X-ray Crystal Structure Analyses of Li(OEt2)2[2], K[7], and 10. Single crystals suitable for X-ray crystallography were obtained for Li(OEt2)2[2], K[7], 8, 9, and 10. The quality of some data sets is affected by severe disorder of the CF3 groups. C
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different boron atoms plays a decisive role in the ligand scrambling scenario. This hydride transfer could either be mediated directly by Lewis acidic metal ions (e.g., Li+, Mg2+) or by catalytic amounts of free boranes.
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EXPERIMENTAL SECTION
General Considerations. All reactions were performed under a nitrogen atmosphere using Schlenk tube techniques. All solvents except for CD 3 CN and MeOH were dried with sodium/ benzophenone and distilled prior to use. Me3SiCl was stirred with CaH2 to remove HCl and then left standing until the solid sedimented. MeOH was dried with Mg turnings and distilled prior to use. BBr3 (Sigma-Aldrich) was stored over Hg to remove traces of Br2. BH3· SMe2 (Sigma-Aldrich), (Fxyl)Br (Apollo Scientific), n-BuLi (1.6 M in n-hexane, Rockwood Lithium), BCl3 (1.0 M in n-hexane, Acros), and KHF2 (Fluka) were used as received. NMR spectra were recorded on Bruker Avance II 300, Avance 400, and Avance III HD 500 spectrometers. Chemical shifts are referenced to (residual) solvent signals (1H and 13C{1H}; C6D6: δ = 7.16/128.39; CD3CN: δ = 1.94/ 1.39), or BF3·OEt2 (11B) and CFCl3 (19F) as external standards. Abbreviations: s = singlet, t = triplet, q = quartet, spt = septet, m = multiplet, br = broad, nr = not resolved. Combustion analyses were performed by the Microanalytical Laboratory of the Goethe University Frankfurt. Synthesis of Li(OEt2)2[2]. A stirred solution of (Fxyl)Br (0.36 mL, 0.61 g, 2.1 mmol) in Et2O (10 mL) was cooled to −78 °C. n-BuLi in n-hexane (1.60 M, 1.3 mL, 2.1 mmol) was added rapidly via syringe, whereupon the initially colorless solution turned pale yellow and became turbid (it is important to use clear n-BuLi solutions to avoid LiH contamination of Li[2]). After the addition of neat BH3·SMe2 (0.20 mL, 0.16 g, 2.1 mmol), the cooling bath was removed, and the reaction mixture was allowed to warm to room temperature (20 min), during which time it turned colorless and clear. Large crystals of Li(OEt2)2[2] were obtained by vapor diffusion of the solvent into paraffin beads. Outside an Et2O vapor-containing atmosphere, the crystals rapidly lose coordinated Et2O ligands at room temperature and turn into a viscous oil. Crystals used for X-ray crystallography had therefore to be isolated at low temperature; the remaining sample was dried to constant mass under vacuum to furnish a colorless oil of the composition Li(OEt2)[2]. Yield: 0.63 g (2.0 mmol, 95%). 1H NMR (300.0 MHz, C6D6): δ 8.10 (br, 2H; H-o), 7.79 (s, 1H; H-p), 2.95 (q, 3 JHH = 7.1 Hz, 4H; CH2) 1.31 (q, 1JHB = 76 Hz, 3H; BH3), 0.78 (t, 3 JHH = 7.1 Hz, 6H; CH3). 11B NMR (96.3 MHz, C6D6): δ −26.0 (q, 1 JBH = 76 Hz). 13C{1H} NMR (75.4 MHz, C6D6): δ 153.3 (q, 1JCB = 58 Hz; C-i), 135.3 (br s; C-o), 130.5 (q, 2JCF = 32 Hz; C-m), 125.5 (q, 1 JCF = 272 Hz; CF3), 118.8 (spt, 3JCF = 4 Hz; C-p), 66.6 (s; CH2), 14.5 (s; CH3). 19F NMR (282.3 MHz, C6D6): δ −62.4 (s). MS (ESI−): m/z (%) 227.0 (100). Synthesis of 3. A stirred solution of (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) in Et2O (10 mL) was cooled to −78 °C. n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) was added rapidly via syringe, whereupon the initially colorless solution turned pale yellow and became turbid. After the addition of neat BH3·SMe2 (0.40 mL, 0.32 g, 4.2 mmol), the cooling bath was removed, and the reaction mixture was allowed to warm to room temperature (20 min), during which time it turned colorless and clear. The solution was treated with neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) and stirred for 30 min; a colorless solid (LiCl) precipitated. The product could not be isolated from the solution without partial decomposition and was therefore characterized only by NMR spectroscopy. 1H{11B} NMR (400.1 MHz, C6D6): δ 8.00 (s, 2H; H-o), 7.82 (s, 1H; H-p), 2.82 (s, 2H; BH2), 1.02 (s, 6H; SMe2). 11 B NMR (160.5 MHz, C6D6): δ −9.5 (br t, 1JBH = 101 Hz). 13C{1H} NMR (100.6 MHz, C6D6): δ 148.8 (br; C-i), 136.3 (br s; C-o), 130.9 (q, 2JCF = 32 Hz; C-m), 125.2 (q, 1JCF = 272 Hz; CF3), 120.6 (spt, 3JCF = 4 Hz; C-p), 22.2 (s; SMe2). 19F NMR (470.6 MHz, C6D6): δ −62.3 (s). Synthesis of Li(OEt2)2[4]. A stirred solution of (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) in Et2O (10 mL) was cooled to −78 °C. n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) was added rapidly via syringe,
Figure 4. Molecular structure of K[7] in the solid state. For clarity, only one [7]− anion and the full coordination sphere of one K+ ion are shown; displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å], atom···atom distances [Å], and bond angles [deg]: B(1B)−C(1B) = 1.612(3), B(1B)−C(11B) = 1.616(3), B(1)−F(1) = 1.441(3), B(1)−F(2) = 1.453(3); B(1)···K(1) = 3.264(3), B(1)···K(1A) = 3.297(3), K(1)···F(1) = 2.952(2), K(1)··· F(1A) = 3.221(2), K(1)···F(2) = 2.637(2), K(1)···F(2A) = 2.681(2), K(1)···F(71B) = 2.987(2), K(1)···F(72B) = 3.211(2), K(1)···F(171C) = 3.467(8), K(1)···F(173C) = 2.877(6), K(1)···F(181D) = 3.374(2), K(1)···F(183D) = 2.973(2); F(1)−B(1)−F(2) = 103.8(2), C(1B)− B(1B)−C(11B) = 115.7(2). Symmetry transformations used to generate equivalent atoms are A: −x, −y, −z + 1; B: x − 1, y, z; C: −x + 1, −y, −z + 1; D: x, −y + 1/2, z + 1/2.
Figure 5. Molecular structure of 10 in the solid state; displacement ellipsoids are drawn at the 50% probability level. The disordered, isotropically refined CF3 groups are displayed in only one of two/three positions. Selected bond lengths [Å], bond angles [deg], and dihedral angle [deg]: B(1)−C(1) = 1.554(5), B(1)−C(11) = 1.558(5), B(1)− Br(1) = 1.922(4); C(1)−B(1)−C(11) = 123.7(3), Br(1)−B(1)−C(1) = 118.1(2), Br(1)−B(1)−C(11) = 118.2(2); Fxyl(C(1))//Fxyl(C(11)) = 53.4(1).
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CONCLUSION Adduct formation between (3,5-(CF3)2C6H3)Li ((Fxyl)Li) and BH3·SMe2 in Et2O proceeds with almost perfect selectivity to form Li[(Fxyl)BH3]. The higher substituted derivative Li[(Fxyl)2BH2] can be generated from Li[(Fxyl)BH3] through a hydride-abstraction/(Fxyl)Li-addition sequence. However, if (Fxyl)MgBr is used instead of (Fxyl)Li in any of the two synthesis steps, complex product mixtures are obtained, which contain (among other species) the anions [(Fxyl)nBH4−n]− (n = 0−3). Thus, the degree of substituent redistribution in aryl(hydrido)borates strongly depends on the nature of the applied counterion. We assume that hydride transfer between D
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Organometallics
Article
mmol, 90%). 1H NMR (500.2 MHz, C6D6): δ 7.85 (s, 2H; H-p), 7.74 (s, 4H; H-o), 3.07 (s, 3H; OMe). 11B NMR (160.5 MHz, C6D6): δ 43.0 (s). 13C{1H} NMR (75.4 MHz, C6D6): δ 138.2 (br; C-i), 134.2 (br m; C-o), 131.8 (q, 2JCF = 33 Hz; C-m), 124.9 (spt, 3JCF = 4 Hz; Cp), 124.3 (q, 1JCF = 273 Hz; CF3), 56.1 (s; OMe) 19F NMR (470.6 MHz, C6D6): δ −62.9 (s). Anal. Calcd for C17H9BF12O [468.05]: C, 43.62; H, 1.94. Found: C, 43.39; H, 1.98. Synthesis of K[7]. A solid mixture of 6 (2.0 g, 4.3 mmol) and KHF2 (0.34 g, 4.3 mmol) was dispersed in MeOH (35 mL; p.a. grade, undried) at room temperature, and the dispersion was stirred until KHF2 was completely dissolved (ca. 2 h). All volatiles were removed in vacuo at 40 °C using a rotary evaporator to obtain colorless solid K[7]. Yield: 2.18 g (4.24 mmol, 98%). X-ray quality crystals were grown by dissolving the product in Et2O (3 mL) and slow evaporation of the solvent (3 d). 1H NMR (500.2 MHz, CD3CN): δ 7.98 (s, 4H; H-o), 7.69 (s, 2H; H-p). 11B NMR (160.5 MHz, CD3CN): δ 5.7 (br t, 1JBF = ca. 60 Hz). 13C{1H} NMR (125.8 MHz, CD3CN): δ 158.4 (nr; C-i), 132.3 (br m; C-o), 130.1 (q, 2JCF = 32 Hz; C-m), 125.7 (q, 1JCF = 272 Hz; CF3), 119.9 (spt, 3JCF = 4 Hz; C-p). 19F NMR (470.6 MHz, CD3CN): δ −63.0 (s, 12 F; CF3), −161.9 (br m, 2 F; BF2). MS (ESI−): m/z (%) 475.0 (100). Anal. Calcd for C16H6BF14K [514.12]: C, 37.38; H, 1.18. Found: C, 37.23; H, 1.37. Synthesis of 8. Neat Me3SiCl (0.25 mL, 0.21 g, 2.0 mmol) was added to a suspension of K[7] (0.51 g, 0.99 mmol) in toluene (5 mL). The reaction mixture was stirred for 16 h at room temperature, briefly heated to 110 °C, and filtered while still hot. The filter cake was extracted with hot toluene (2 × 5 mL). The combined toluene solutions were evaporated to dryness in vacuo to yield a yellow solid. The crude product was recrystallized from hot toluene (0.2 mL), and the resulting colorless crystals were washed with n-hexane (2 × 1 mL). Yield: 0.30 g (0.66 mmol, 67%). 1H NMR (500.2 MHz, C6D6): δ 7.97 (s, 4H; H-o), 7.89 (s, 2H; H-p). 11B NMR (160.5 MHz, C6D6): δ 46.1 (nr). 13C{1H} NMR (125.8 MHz, C6D6): δ 149.0 (C-i; detected via a 1 H−13C-HMBC cross-peak), 136.1 (m; C-o), 132.2 (q, 2JCF = 33 Hz; C-m), 127.1 (spt, 3JCF = 4 Hz; C-p), 124.0 (q, 1JCF = 273 Hz; CF3). 19F NMR (470.6 MHz, C6D6): δ −54.1 (br s, 1 F; BF), −63.1 (s, 12 F; CF3). Anal. Calcd for C16H6BF13 [456.02]: C, 42.14; H, 1.33. Found: C, 41.86; H, 1.45. Synthesis of 9. Compound 6 (1.07 g, 2.29 mmol) was dissolved in n-hexane (5 mL). BCl3 in n-heptane (1.0 M, 2.3 mL, 2.3 mmol) was added, and the reaction mixture was stirred for 1 h. All volatiles were removed in vacuo to give a colorless solid. Yield: 1.04 g (2.2 mmol, 96%). Crystals were obtained by recrystallization of the product from hot C6D6 in a sealed NMR tube. 1H NMR (500.2 MHz, C6D6): δ 7.96 (s, 4H; H-o), 7.81 (s, 2H; H-p). 11B NMR (160.5 MHz, C6D6): δ 61.9 (h1/2 = ca. 750 Hz). 13C{1H} NMR (125.8 MHz, C6D6): δ 138.6 (nr; C-i), 136.7 (br m; C-o), 132.1 (q, 2JCF = 33 Hz; C-m), 127.3 (spt, 3JCF = 4 Hz; C-p), 123.9 (q, 1JCF = 273 Hz; CF3). 19F NMR (470.6 MHz, C6D6): δ −62.9 (s). Anal. Calcd for C16H6BClF12 [472.47]: C, 40.67; H, 1.28. Found: C, 40.72; H, 1.51. Synthesis of 10. Compound 6 (1.88 g, 4.02 mmol) was dissolved in n-hexane (10 mL). Neat BBr3 (0.39 mL, 1.01 g, 4.03 mmol) was added at room temperature, and the reaction mixture was stirred for 20 min. All volatiles were removed in vacuo to give a colorless solid. Yield: 1.90 g (3.68 mmol, 92%). Crystals were obtained from a solution of 10 in n-hexane by gas-phase diffusion of the solvent under reduced pressure into a vessel containing toluene. 1H NMR (500.2 MHz, C6D6): δ 7.98 (s, 4H; H-o), 7.81 ppm (s, 2H; H-p). 11B NMR (160.5 MHz, C6D6): δ 66.1 (h1/2 = ca. 750 Hz). 13C{1H} NMR (125.8 MHz, C6D6): δ 140.7 (br; C-i), 136.9 (br m; C-o), 132.1 (q, 2JCF = 33 Hz; C-m), 127.3 (spt, 3JCF = 4 Hz; C-p), 123.8 (q, 1JCF = 273 Hz; CF3). 19F NMR (470.6 MHz, C6D6): δ −62.9 (s). Anal. Calcd for C16H6BBrF12 [516.93]: C, 37.18; H, 1.17. Found: C, 37.26; H, 1.42. X-ray Crystal Structure Determinations. Data for all structures were collected on a STOE IPDS II two-circle diffractometer with a Genix Microfocus tube with mirror optics using Mo Kα radiation (λ = 0.71073 Å) and were scaled using the frame-scaling procedure in the X-AREA program system.43 All structures were solved by direct methods using the program SHELXS44 and refined against F2 with fullmatrix least-squares techniques using the program SHELXL-97.44
whereupon the initially colorless solution turned pale yellow and became turbid. After the addition of neat BH3·SMe2 (0.40 mL, 0.32 g, 4.2 mmol), the cooling bath was removed, and the reaction mixture was allowed to warm to room temperature (20 min), during which time it turned colorless and clear. The solution was treated with neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) and stirred for 30 min; a colorless solid (LiCl) precipitated. The resulting slurry was cooled to −78 °C, and neat (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) and n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) were added. The reaction mixture was allowed to warm to room temperature (20 min), all volatiles were removed in vacuo, and the residue was dispersed in C6H6 (10 mL). The slurry was filtered, the filter cake was extracted with C6H6 (2 × 10 mL), and the combined C6H6 solutions were freeze-dried in vacuo to yield a colorless solid. The crude product was dissolved in Et2O (1 mL), and the solvent was removed again to obtain a colorless oil. One droplet of the oil was cooled with dry ice, until it solidified and could subsequently be used as a seed crystal. Placement of the seed crystal into the oil induced rapid crystallization of Li(OEt2)2[4] in the form of colorless rods.42 Yield: 1.78 g (3.00 mmol, 71%) 1H NMR (300.0 MHz, C6D6): δ 8.13 (br s, 4H; H-o), 7.76 (s, 2H; H-p), 2.80 (q, 3JHH = 7.1 Hz, 8H; CH2), 2.19 (q, 1JHB = 74 Hz, 2H; BH2), 0.64 (t, 3JHH = 7.1 Hz, 12H; CH3). 11B NMR (96.3 MHz, C6D6): δ −14.0 (t, 1JBH = 74 Hz). 13C{1H} NMR (75.4 MHz, C6D6): δ 157.7 (q, 1JCB = 55 Hz; C-i), 135.0 (br s; C-o), 130.5 (q, 2JCF = 32 Hz; C-m), 125.5 (q, 1JCF = 273 Hz; CF3), 118.8 (spt, 3JCF = 4 Hz; C-p), 66.4 (s; CH2), 14.4 (s, CH3). 19F NMR (282.3 MHz, C6D6): δ −62.3 (s). MS (ESI−): m/z (%) 439.0 (100). Synthesis of 5. A stirred solution of (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) in Et2O (10 mL) was cooled to −78 °C. n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) was added rapidly via syringe, whereupon the initially colorless solution turned pale yellow and became turbid. After the addition of neat BH3·SMe2 (0.40 mL, 0.32 g, 4.2 mmol), the cooling bath was removed, and the reaction mixture was allowed to warm to room temperature (20 min), during which time it turned colorless and clear. The solution was treated with neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) and stirred for 30 min; a colorless solid (LiCl) precipitated. The resulting slurry was cooled to −78 °C, and neat (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) and n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) were added. The reaction mixture was allowed to warm to room temperature (20 min), neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) was added, and stirring was continued for 30 min. The product could not be isolated from the solution without partial decomposition and was therefore characterized only by NMR spectroscopy. 1H{11B} NMR (500.2 MHz, C6D6): δ 7.93 (s, 4H; Ho), 7.78 (s, 2H; H-p), 3.24 (s, 1H; BH), 0.79 (s, 6H; SMe2). 11B NMR (160.5 MHz, C6D6): δ −2.0 (nr). 13C{1H} NMR (125.8 MHz, C6D6): δ 148.4 (nr; C-i), 134.5 (br m; C-o), 131.5 (q, 2JCF = 32 Hz; C-m), 124.9 (q, 1JCF = 273 Hz; CF3), 121.4 (spt, 3JCF = 4 Hz; C-p), 20.8 (s; SMe2). 19F NMR (470.6 MHz, C6D6): δ −62.6 (s). Synthesis of 6. A stirred solution of (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) in Et2O (10 mL) was cooled to −78 °C. n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) was added rapidly via syringe, whereupon the initially colorless solution turned pale yellow and became turbid. After the addition of neat BH3·SMe2 (0.40 mL, 0.32 g, 4.2 mmol), the cooling bath was removed and the reaction mixture was allowed to warm to room temperature (20 min), during which time it turned colorless and clear. The solution was treated with neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) and stirred for 30 min; a colorless solid (LiCl) precipitated. The resulting slurry was cooled to −78 °C and neat (Fxyl)Br (0.73 mL, 1.24 g, 4.2 mmol) and n-BuLi in n-hexane (1.60 M, 2.6 mL, 4.2 mmol) were added. The reaction mixture was allowed to warm to room temperature (20 min), neat Me3SiCl (0.54 mL, 0.46 g, 4.3 mmol) was added, and stirring was continued for 30 min. Neat MeOH (0.34 mL, 0.27 g 8.4 mmol) was carefully added to the reaction mixture via syringe, accompanied by the vigorous evolution of H2. Stirring was continued for 60 min, all volatiles were removed in vacuo, and the residue was dispersed in C6H6 (10 mL). The dispersion was filtered, and the filter cake was extracted with C6H6 (2 × 10 mL). The combined C6H6 solutions were evaporated to dryness in vacuo to form a colorless oil, which crystallized overnight.42 Yield: 1.78 g (3.80 E
dx.doi.org/10.1021/om500476d | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
(17) Ye, H.; Lu, Z.; You, D.; Chen, Z.; Li, Z. H.; Wang, H. Angew. Chem., Int. Ed. 2012, 51, 12047−12050. (18) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.; Brockhinke, A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009, 1339−1351. (19) Wang, J.; Wang, Y.; Taniguchi, T.; Yamaguchi, S.; Irle, S. J. Phys. Chem. A 2012, 116, 1151−1158. (20) Taniguchi, T.; Wang, J.; Irle, S.; Yamaguchi, S. Dalton Trans. 2013, 42, 620−624. (21) Hauptman, E.; Brookhart, M.; Fagan, P. J.; Calabrese, J. C. Organometallics 1994, 13, 774−780. (22) Rifat, A.; Kociok-Köhn, G.; Steed, J. W.; Weller, A. S. Organometallics 2004, 23, 428−432. (23) Geiger, W. E.; Barrière, F. Acc. Chem. Res. 2010, 43, 1030−1039. (24) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545−5547. (25) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920−3922. (26) Konze, W. V.; Scott, B. L.; Kubas, G. J. Chem. Commun. 1999, 1807−1808. (27) Lai, Y.-Y.; Bornand, M.; Chen, P. Organometallics 2012, 31, 7558−7565. (28) Kobayashi, H.; Sonoda, T.; Iwamoto, H.; Yoshimura, M. Chem. Lett. 1981, 10, 579−580. (29) Kobayashi, H.; Sonoda, T.; Iwamoto, H. Chem. Lett. 1982, 11, 1185−1186. (30) Mazet, C.; Köhler, V.; Pfaltz, A. Angew. Chem., Int. Ed. 2005, 44, 4888−4891. (31) Köhler, V.; Mazet, C.; Toussaint, A.; Kulicke, K.; Häussinger, D.; Neuburger, M.; Schaffner, S.; Kaiser, S.; Pfaltz, A. Chem.Eur. J. 2008, 14, 8530−8539. (32) Ishihara, K.; Kurihara, H.; Yamamoto, H. Synlett 1997, 1997, 597−599. (33) Schnurr, A.; Samigullin, K.; Breunig, J. M.; Bolte, M.; Lerner, H.W.; Wagner, M. Organometallics 2011, 30, 2838−2843. (34) Edmondson, R. C.; Jukes, A. E.; Gilman, H. J. Organomet. Chem. 1970, 25, 273−276. (35) Franz, D.; Bolte, M.; Lerner, H.-W.; Wagner, M. Dalton Trans. 2011, 40, 2433−2440. (36) Seven, Ö .; Qu, Z.-W.; Zhu, H.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Chem.Eur. J. 2012, 18, 11284− 11295. (37) Seven, Ö .; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2014, 33, 1291−1299. (38) Hydride abstraction with Me3SiCl from [4]− or [(C6F5)2BH2]− (cf. ref 33) in the presence of excess Et3PO furnished the adducts Et3PO−B(H)(Fxyl)2 or Et3PO−B(H)(C6F5)2. The corresponding 31 1 P{ H} NMR resonances appeared at 46.2 (free Et3PO), 86.1, and 87.3 ppm (C6D6), respectively. In line with ref 16, these data point toward a comparable Lewis acidity of the B(Fxyl)2 and B(C6F5)2 groups. (39) Heilmann, J. B.; Scheibitz, M.; Qin, Y.; Sundararaman, A.; Jäkle, F.; Kretz, T.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Angew. Chem., Int. Ed. 2006, 45, 920−925. (40) Scheibitz, M.; Li, H.; Schnorr, J.; Sánchez Perucha, A.; Bolte, M.; Lerner, H.-W.; Jäkle, F.; Wagner, M. J. Am. Chem. Soc. 2009, 131, 16319−16329. (41) Edelstein, N. Inorg. Chem. 1981, 20, 297−299. (42) The connectivities of Li(OEt2)2[4] and 6 are supported by Xray diffraction studies, the quality of which prevents their publication. CIF files available from: Kamil Samigullin, Michael Bolte, HansWolfram Lerner, Matthias Wagner (2014) Private communication to the Cambridge Structural Database; deposition numbers CCDC 999004, 999005. (43) X-AREA. Diffractometer control program system; Stoe & Cie GmbH: Darmstadt, Germany, 2002. (44) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
In Li(OEt2)2[2], one ethyl chain of an Et2O molecule is disordered over two positions with a site occupation factor of 0.70(1) for the major occupied site. The disordered atoms were isotropically refined. The H atoms bonded to B were isotropically refined. In K[7], one CF3 group is disordered over two positions with a site occupation factor of 0.61(1) for the major occupied site. The displacement ellipsoids of the disordered atoms were restrained to an isotropic behavior. In 10, one CF3 group is disordered over two positions with a site occupation factor of 0.55(1) for the major occupied site, and one CF3 group is disordered over three positions with site occupation factors of 0.393(3), 0.281(3), and 0.326(3). The disordered atoms were isotropically refined. CCDC reference numbers: CCDC 999001 (Li(OEt2)2[2]), 999002 (K[7]), 999003 (10), 999006 (8), and 999007 (9).
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ASSOCIATED CONTENT
S Supporting Information *
Details of the X-ray crystal structure analyses of Li(OEt2)2[2], K[7], 10, 8, and 9; 1H and 13C{1H} NMR spectra of Li(OEt2)[2], 3, Li(OEt2)2[4], and 5; crystallographic data of Li(OEt2)2[2], K[7], 10, 8, and 9 in crystallographic information file (CIF) format. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +49 69 798 29260. E-mail: Matthias.Wagner@chemie. uni-frankfurt.de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.S. wishes to thank the Fonds der Chemischen Industrie for a Chemiefonds Fellowship.
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REFERENCES
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dx.doi.org/10.1021/om500476d | Organometallics XXXX, XXX, XXX−XXX
