Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole

May 10, 2011 - Kexuan Huang , Shannon A. Couchman , David J. D. Wilson , Jason L. Dutton ..... Holger Braunschweig , Ching-Wen Chiu , Alexander Damme ...
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Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole Holger Braunschweig,* Ching-Wen Chiu,† Alexander Damme, Katharina Ferkinghoff, Katharina Kraft, Krzysztof Radacki, and Johannes Wahler Institut f€ur Anorganische Chemie, Julius-Maximilians-Universit€at W€urzburg, Am Hubland, D-97074 W€urzburg, Germany

bS Supporting Information ABSTRACT: Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C4B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acidbase adducts of the title compound with 3,5-lutidine, PCy3, N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared.

’ INTRODUCTION The borole framework has become an important structural motif regarding the investigation of tangible antiaromatic compounds.1 According to H€uckel’s rules the elusive cyclopentadienyl cation (Cpþ) is one of the smallest possible antiaromatic systems, holding four π-electrons.2 By formal replacement of one CHþ unit in Cpþ by a BH fragment, the isoelectronic neutral borole system is attained. The work on isolated boroles was pioneered in 1969 by Eisch et al.3 and has been regaining attention only in the past few years.416 Typical features of free boroles include a strong absorption in the visible spectral region and a highly electron deficient boron center, making them potent Lewis acids and hence attractive for potential application in optoelectronics,17 molecular sensing,18 or catalysis.19 However, isolation of monomeric, nonannulated borole derivatives requires steric protection of the central C4B framework, which is commonly accomplished by aryl substitution around the core.1,3 The reason lies in antiaromatic destabilization of the system, which is substantially reflected in the reactivity of boroles targeting the breakup of antiaromaticity. One possibility is twoelectron reduction of the system, which is feasible due to the lowlying LUMO of the borole.5,20 The resulting product is an aromatic dianion (A, Chart 1) with six π-electrons and therefore isoelectronic to the cyclopentadienyl anion (Cp). Stabilization of the system can also be achieved in the ligand sphere of transition metals.20,21 Alternatively, conversion of boroles with dienophiles like diphenylacetylene yields products of [4 þ 2]cycloaddition reactions.3,22,23 The bicyclic derivatives (B) thus formed tend to undergo sigmatropic rearrangement toward the respective ring-extended borepin systems (C) under re-establishment of conjugation and gain of aromaticity.14,22,24 [4 þ 2]Cycloaddition also takes place when a sterically unhindered backbone (e.g., alkyl groups) is implemented at the borole r 2011 American Chemical Society

framework, so that the only isolable product is a nonconjugated DielsAlder dimer (D).25 Recently it was shown by Piers et al. that hydrogen activation is another suitable pathway out of antiaromaticity. Addition of H2 to pentaphenylborole gives the cis- and trans-isomers of 1,2,3,4,5-pentaphenyl-2,5-dihydroborole (E) bearing hydrogen atoms on the carbon atoms in R-position to the boron center.15 Furthermore, our group reported the dimerization/rearrangement reaction sequence of 1-chloro-2,3,4,5-tetraphenylborole (1) to yield an unprecedented bicyclic [1,6]diboraspiro[4.5]deca-3,7,9-triene derivative (2, Chart 1),12 which shows an interesting reduction chemistry.26 In the latter four cases the 1,3-butadiene moiety of the borole framework is effectively removed from conjugation. Moreover, reactions with Lewis bases produce stable Lewis acidbase adducts where the pz orbital of the borole center is excluded from conjugation with consequent loss of antiaromaticity (vide infra).1,6,10,16 The resulting four-coordinate boron species is considerably more stable than the parent borole. The chemistry of such systems, however, has not been extensively studied yet. It was demonstrated, for example, that the Lewis acidbase adduct of 1-chloro-2,3,4,5-tetraphenylborole (1) and an N-heterocyclic carbene [ClBC4Ph4(SIMes)] (3, NHC = SIMes [N,N0 -bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene]) allows formation of a novel type of borole-based π-boryl anion K[BC4Ph4(SIMes)], stabilized by the NHC.10 Furthermore, the Lewis acidbase adduct of pentaphenylborole and 2,6-lutidine [PhBC4Ph4(2,6-Me2C5H3)] shows a unique photochemical reactivity under migration of the Lewis base to the borole backbone, which is fully reversible upon thermal treatment.16

Received: April 13, 2011 Published: May 10, 2011 3210

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Organometallics Isolation of a 1-haloborole was first reported in 1978 devoid of detailed reactivity studies.27 Recently the spectrum has been extended by ClBC4Ph4 (1)6,12 and BrBC4Pf4 (4; Pf = C6F5)7 including solid-state structural analysis of 1. The boronhalide bond exhibits a broad range of reactivity such as substitution reactions,6,7 halide abstraction,6 oxidative addition,8 and reduction.10,12 This versatility allows preparation of borole derivatives that are not accessible via common synthesis methods (tin boron exchange reaction). In order to extend the range of 1-haloboroles, we report the synthesis of 1-bromo-2,3,4,5-tetraphenylborole (5), its dimerization/rearrangement reaction, and formation of Lewis acidbase adducts with various Lewis bases.

’ RESULTS AND DISCUSSION Synthesis of 1-Bromo-2,3,4,5-tetraphenylborole. BrBC4Ph4 (5) was prepared via tinboron exchange reaction based on the procedure previously described for the synthesis of 1 (Scheme 1).6 Therefore, 1,1-dimethyl-2,3,4,5-tetraphenylstannole (6) was treated with 2.5 equiv of BBr3 in hexane at 0 °C, whereby an immediate color change from yellow to deep purple was observed. The product was obtained in yields of 71% as an analytically pure, deep purple solid after workup performed constantly at low temperatures. The identity of 5 was confirmed by means of NMR spectroscopy and elemental analysis. The 11B NMR resonance was observed as a broad signal (δ = 70.0 ppm) in a similar region to that found for 1 (δ = 66.4 ppm) and is consistent with a moderate downfield shift, which is commonly observed when chloro- and bromoboranes are compared.28 Similar to its chloro-substituted analogue (1), 5 is very sensitive toward air and moisture and readily decomposes in the solid state at room temperature (RT) within a few hours, but can be stored under an inert atmosphere at 30 °C over a period of several weeks. Due to an even lower stability in solution, both at RT and at low temperatures, all attempts to obtain single crystals suitable for X-ray diffraction and reliable UVvis data were unsuccessful. The 1H NMR spectrum

Chart 1. Possible Products of Reactions Induced by the Breakup of Antiaromaticity

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obtained from a concentrated solution of 5 in CDCl3 shows resonances in the typical range between δ = 6.78 and 7.23 ppm with the correct ratio of integration. 13C NMR spectroscopy displays the complete set of signals expected for 5, whereas the spectrum indicates the presence of minor impurities due to the aforementioned decomposition in solution during acquisition of the spectrum. This attribute of 5 stands in sharp contrast to the perfluorinated 1-bromo-2,3,4,5-tetraphenylborole (4), which is typically synthesized at 120 °C in good yields.7 Presumably, the presence of four C6F5 groups provides a significant stabilization to the system. Thermolysis. When 5 is exposed to elevated temperatures (55 °C) for 15 h, the 1,6-diboraspiro[4.5]deca-3,7,9-triene derivative 7 is obtained in analogy to the dimerization/rearrangement reaction of 1, previously reported in our group.12 The 11B NMR resonances of 7 are detected as broad signals at 59.9 and 76.5 ppm lying in a similar range as observed for the chlorosubstituted analogue (2, 59.0 and 74.4 ppm). The solid-state structure of 7 was determined by single-crystal X-ray diffraction. 7 crystallizes in the orthorhombic space group Pbca and shows an isostructural relation to 2. Hence, all BC and CC bond lengths and corresponding bond angles are in good agreement with 2, reflecting a boracyclohexadiene system directly fused to a borolene ring via a spiro carbon atom (C4). Both boron centers are constituted in a trigonal-planar geometry, which is shown by the sum of angles around the boron atoms (359.8° (B1) and 360.0° (B2)). Formation of this structural motif is limited to thermal treatment of 1 and 5 and has not been observed for the perfluorinated analogue 4 or any other borole system so far. Reactivity toward Lewis Bases. Stabilization of 5 was accomplished by coordination with a number of Lewis bases including 3,5-lutidine, PCy3 (Cy = cyclohexyl), N-heterocyclic carbene (NHC), cyclic (alkyl)(amino)carbene (CAAC), and THF. By addition of a Lewis base to the electron-deficient borole, the boron center rehybridizes from sp2 to sp3 with formation of Lewis acidbase adducts. This becomes evident by an immediate color change from deep purple to yellow upon addition of the base to the borole in solution. The obtained products (812, Chart 2) were characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction. 4-Picoline (4-MeC5H4N) has been used to prove the nature of ClBC4Ph4 (1) as its Lewis acidbase adduct [ClBC4Ph4(4MeC5H4N)] (13) prior to the elucidation of the solid-state structure of the free chloroborole (1).6,12 Likewise, the reaction of 5 with 3,5-lutidine (3,5-Me2C5H3N) affords the respective compound [BrBC4Ph4(3,5-Me2C5H3N)] (8, Figure 2a). To investigate the coordination behavior with a trialkylphosphine as a group 15 homologue, 5 was reacted with the Lewis base PCy3 to yield [BrBC4Ph4(PCy3)] (9, Figure 2b). Both compounds were obtained in good yields (82%) after recrystallization. The 11B NMR resonances are detected at δ = 4.4 ppm (8) and 6.4 ppm (9) in the typical range for four-coordinated boron centers at much higher

Scheme 1. Synthesis of 5 and 7

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Organometallics field in comparison to the free borole (5, 70.0 ppm). 31P{1H} NMR spectroscopy of 9 reveals a broad signal at δ = 2.3 ppm, which is downfield shifted compared to uncoordinated PCy3 at δ = 9.8 ppm.29 Single crystals of 9 were obtained in two different space groups depending on the solvent employed. The discussion of 9 is focused on structural data of single crystals obtained from slow evaporation of a benzene solution, due to a high degree of disorder in the alternative morphology acquired from crystallization of 9 in hexane (see Supporting Information). Suitable single crystals of 8 showing two independent molecules in the asymmetric unit were obtained from a concentrated chloroform solution. Due to similar bond parameters the discussion is limited to one selected molecule of 8. The solid-state structures of 8 and 9 display an almost planar central C4B moiety with a distorted tetrahedral environment around the boron center. The observed bond lengths within the C4B ring (Table 1) correspond to those of a typical diene system such as cis,cis-1,2,3,4-tetraphenylbuta-1,3-diene (CC single bond: 1.484 Å; CdC double bond: 1.356 Å).30 The peripheral phenyl groups in 8 are situated in the characteristic propeller-like arrangement, whereas in 9 the phenyl groups at C2 and C3 are directly facing each other. Likewise, the phenyl groups bound to C1 and C4 show the same alignment, resulting in a butterfly-like structure (Figure 2b). This rather untypical situation might be a

Figure 1. Molecular structure of 7 in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å] and angles [deg]: B1Br1 1.909(3), B2Br2 1.923(3), B1C1 1.587(4), B1C4 1.589(4), C1C2 1.546(4), C2C3 1.347(4), C3C4 1.537(4), B2C4 1.582(4), B2C8 1.533(4), C4C5 1.515(4), C5C6 1.359(4), C6C7 1.476(4), C7C8 1.376(4); C1B1C4 112.1(2), C1B1Br1 124.4(2), C4B1Br1 123.3(2), C4B2C8 120.1(2), C4B2Br2 120.5(2), C8B2Br2 119.4(2).

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consequence of the steric demand of the phosphine. Despite the increased steric congestion of the borole periphery, the B1P1 bond length of 9 (2.016(2) Å) is comparable to those observed in [Cy3PBBr3] (2.008(11) Å) and [Cy3PBBr2(o-Tol)] (2.030(2) Å).31 The B1N1 bond distance in 8 (1.596(4) Å) resembles that in [ClBC4Ph4(4-MeC5H4N)] (13, 1.602(3) Å) and [C5H5NBBr3] (1.59(2) Å), respecively.32 Furthermore, the B1Br1 bond distances of 8 (2.042(3) Å) and 9 (2.068(2) Å) are similar to those reported for [C5H5NBBr3] (1.99(2) Å) and [Cy3PBBr3] (2.010(4) Å), respectively. In our previous work, we were able to stabilize a π-boryl anion by coordination of a N-heterocyclic carbene (NHC = SIMes [N,N0 bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene]) to the boron center via reduction of the respective Lewis acidbase adduct [(ClBC4Ph4 (SIMes)] (14).10 The corresponding compound [BrBC4Ph4(SIMes)] (10, Figure 3a) was obtained in 74% yield by reaction of 5 with SIMes in benzene solution. As an extension to Lewis acidbase adducts between boroles and carbenes, a cyclic (amino)(alkyl)carbene (CAAC [N-(2,6-diisopropylphenyl)-2,2,4,4tetramethylpyrrolidine-5-ylidene]) was reacted with 5 to obtain [BrBC4Ph4(CAAC)] (11, Figure 3b). 11B NMR spectroscopy shows resonances at δ = 6.2 ppm (10) and δ = 4.8 ppm (11) in a range similar to 14 (δ = 3.3 ppm) and 9 (δ = 6.4 ppm). The broad 13C NMR resonances of the carbene nuclei are detected at δ = 187 ppm (10) and δ = 226 ppm (11) by means of HMBC (heteronuclear multiple-bond correlation) methods. The B1C5 bond lengths of 10 (1.655(3) Å) and 11 (1.628(2) Å) show no significant deviation. Moreover, the data are in good agreement with the BCcarbene single bond observed in [MeBC4Ph4(SIMes)] (15, 1.647(2) Å).10 In 10 the phenyl group bound to C1 is in almost coplanar arrangement with the central C4B ring showing dihedral angles of 3.4(3)° (C2C1C11C12) and 3.5(3)° (B1C1 C11C13), presumably induced by the steric repulsion of the NHC periphery. The C1C11 bond length (1.481(3) (Å)) is not significantly affected by this geometry, which might have been expected as a consequence of a potentially improved ππ overlap in coplanar π-systems. Structurally characterized Lewis acidbase adducts of haloboranes and ether molecules are rare and limited to a few examples of fluoro- and chloroboranes.3337 Studies of the respective bromoborane ether adducts have been possible only at low temperatures employing NMR spectroscopy38 or matrix isolation techniques.39 Moreover, the Lewis acidity of dialkylbromoboranes is commonly exploited in organic synthesis to induce cleavage reactions of cyclic and noncyclic ethers under mild conditions.40 This useful application demonstrates the

Chart 2. Lewis AcidBase Adducts of BrBC4Ph4 (5) and Various Donor Molecules

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Figure 3. Molecular structures of 10 (a) and 11 (b) in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability.

Figure 2. Molecular structures of 8 (a) and 9 (b) in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability.

Table 1. 11B NMR Chemical Shifts δ [ppm], Bond Lengths [Å], and Angles [deg] of 812 8

9

10

11

12

δ ( B) B1Br1

4.4 2.042(3)

6.4 2.068(2)

6.2 2.092(2)

4.8 2.071(2)

10.3 2.047(2)

B1C1

1.604(4)

1.617(2)

1.626(3)

1.637(2)

1.588(3)

B1C4

1.617(4)

1.614(2)

1.624(3)

1.634(2)

1.610(2)

C1C2

1.347(4)

1.358(2)

1.361(3)

1.355(2)

1.362(2)

C2C3

1.507(4)

1.497(2)

1.491(3)

1.492(2)

1.512(2)

C3C4

1.352(4)

1.362(2)

1.361(3)

1.358(2)

1.355(2)

BEb

1.596(4)

2.016(2)

1.655(3)

1.628(2)

1.572(2)

C1B1C4 B1C1C2

101.5(2) 108.0(2)

101.9(1) 107.4(1)

101.7(2) 107.2(2)

99.6(1) 108.1(1)

103.3(1) 107.1(1)

C1C2C3

111.4(2)

111.6(1)

111.8(2)

111.9(1)

111.4(2)

C2C3C4

111.4(2)

112.1(1)

112.6(2)

111.6(1)

111.6(1)

C3C4B1

107.3(2)

107.0(1)

106.7(2)

108.4(1)

106.6(1)

C1B1Br1

112.1(2)

108.8(1)

103.9(1)

105.4(1)

110.0(1)

C4B1Br1

108.3(2)

109.9(1)

105.9(1)

106.8(1)

114.0(1)

C1B1Eb

110.6(2)

111.3(1)

113.9(2)

110.2(1)

111.3(1)

C4B1Eb BrB1Eb

113.8(2) 110.2(2)

112.6(1) 111.8(1)

117.9(2) 112.2(1)

119.2(1) 114.0(1)

113.3(1) 105.1(1)

11

a

a Referenced against external BF3 3 Et2O. b E defines the respective donor atom of the Lewis base (8: N1; 9: P1; 10: C5; 11: C5; 12: O1).

Figure 4. Molecular structure of 12 in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability.

reactivity of the BBr bond and the pronounced oxophilicity of the boron center. Surprisingly, the reaction of 5 with THF gives the isolable adduct [BrBC4Ph4(THF)] (12) despite the strong Lewis acidity of the borole moiety. After recrystallization from benzene/hexane at ambient temperature, pure 12 is obtained as pale yellow crystals suitable for X-ray crystallography. The 11B NMR resonance of 12 (δ = 10.3 ppm) is found in the expected area, and quite surprisingly, 12 was found to be stable in benzene solution for at least 15 h at room temperature as deduced from 1 H NMR spectroscopy. Warming of the benzene solution to 65 °C for about six days leads to a color change from yellow to dark orange and ring-opening of the coordinated THF molecule. The product shows a 11B NMR resonance at δ = 46.0 ppm, which is assigned to formation of the respective 1-alkoxy-2,3,4,5-tetraphenylborole. The solid-state structure of 12 shows that the B1O1 bond length (1.572(2) Å) and BC distances (B1C1: 1.588(3) Å; B1C4: 1.610(2) Å) are comparable to those observed in [Ph2BCl(THF)] (16, BO: 1.569(2) Å, BC: 1.599(2) and 1.602(2) Å).36 The bond angles around the boron center of 12 (103.3(1)° (C1B1C4) to 114.0(1)° 3213

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Organometallics (C4B1Br1)) display a higher degree of distortion of the tetrahedral coordination in comparison to 16 (104.1(1)° to 111.7(1)°), which is presumably induced by the ring geometry of the borole C4B framework.

’ CONCLUSION In this contribution we present the synthesis and characterization of 1-bromo-2,3,4,5-tetraphenylborole (5) by tinboron exchange reaction. Thermal treatment of 5 results in breakup of antiaromaticity by reaction of two molecules of 5 to the chiral spirobicyclic product 7. Structural characterization of 5 was accomplished as its Lewis-base adducts [BrBC4Ph4(3,5-Me2C5H3N)] (8), [BrBC4Ph4(PCy3)] (9), [(BrBC4Ph4(SIMes)] (10), and [BrBC4Ph4(CAAC)] (11). Additionally the ether adduct of 5 with THF ([BrBC4Ph4(THF)] (12)) was obtained. To the best of our knowledge this constitutes an unprecedented isolable adduct of a bromoborane species with an ether molecule, which is stable at ambient temperature in solution and in the solid state. The solidstate structures of 812 feature similar bond parameters with no striking differences among each other and lie in the anticipated range. The title compound 5 is another potential candidate for functionalization reactions at the boron center of the borole. Reactivity studies of the BBr bond are currently under investigation. ’ EXPERIMENTAL SECTION General Considerations. All syntheses were carried out under an argon atmosphere with standard Schlenk and glovebox techniques. Me2SnC4Ph4,1 PCy3,41 SIMes,42 and CAAC43 were prepared according to published procedures. 3,5-Lutidine was dried over CaH2 and distilled prior to use. Hexane, benzene, THF, and DCM were dried by distillation over Na/K alloy (hexane, benzene, THF) or phosphorus pentoxide (DCM) under argon and stored over molecular sieves. CDCl3 was dried over CaH2 and distilled under argon. CD2Cl2 and C6D6 were degassed with three freezepumpthaw cycles and stored over molecular sieves. Elemental analyses were obtained from an Elementar Vario MICRO cube instrument. NMR spectra were recorded on a Bruker Avance 500 NMR spectrometer (500 MHz for 1H, 160 MHz for 11B{1H}, 126 MHz for 13C{1H}, 202 MHz for 31P{1H}). Chemical shifts are given in ppm and are referenced against external Me4Si (1H, 13C), BF3 3 Et2O (11B), and H3PO4 (85%, 31P). Synthesis of BrBC4Ph4 (5). BBr3 (0.5 mL, 1.32 g, 5.27 mmol) was added dropwise to a cooled (0 °C) suspension of Me2SnC4Ph4 (1.04 g, 2.06 mmol) in hexane (25 mL), resulting in an immediate color change from pale yellow to deep purple. The mixture was stirred for 4 h at 0 °C, giving a purple precipitate, which was filtered, washed with cold hexane (2  2.5 mL), and dried under vacuum. Me2SnBr2 was removed by sublimation (7 h, 0 °C, 2  106 mbar) to yield BrBC4Ph4 (660 mg, 1.48 mmol, 71%) as a dark purple solid. 1H NMR (500 MHz, CDCl3): δ 6.786.80 (m, 4H, C6H5), 7.057.08 (m, 4H, C6H5), 7.127.18 (m, 8H, C6H5), 7.207.23 ppm (m, 4H, C6H5). 11B{1H} NMR (160 MHz, CDCl3): δ 70.0 ppm (br). 13C{1H} NMR (126 MHz, CDCl3): δ 126.51, 127.63, 127.89, 128.12, 129.41, 129.89 (CH), 135.26, 135.57 (br), 136.36, 162.80 ppm (C). Anal. Calcd (%) for C28H20BBr: C 75.21; H 4.51. Found: C 74.82; H 4.63. Synthesis of Br2B2C8Ph8 (7). BBr3 (0.5 mL) was added dropwise to a cooled (0 °C) suspension of Me2SnC4Ph4 (507 mg, 1.00 mmol) in hexane (5 mL), resulting in an immediate color change from pale yellow to deep purple. The mixture was stirred for 4 h at 0 °C, and all volatiles were removed under vacuum. Hexane (5 mL) was added and the mixture was stirred for 15 h at 55 °C to give a yellow precipitate, which was filtered, washed with hexane (2  1.5 mL), and dried under vacuum.

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Br2B2C8Ph8 (354 mg, 396 μmol, 79%) was obtained as a yellow solid. Single crystals suitable for X-ray crystallography were obtained by recrystallization from THF/hexane. 1H NMR (500 MHz, CD2Cl2): δ 5.785.79 (m, 1H, C6H5), 6.036.05 (m, 1H, C6H5), 6.356.38 (m, 1H, C6H5), 6.466.48 (m, 1H, C6H5), 6.636.71 (m, 6H, C6H5), 6.806.87 (m, 3H, C6H5), 6.917.06 (m, 16H, C6H5), 7.097.27 ppm (m, 11H, C6H5). 11B{1H} NMR (160 MHz, CD2Cl2): δ 59.9 (br), 76.5 ppm (br). 13C{1H} NMR (126 MHz, CD2Cl2): δ 125.42, 125.88, 126.25, 126.41, 126.46, 126.49, 126.62, 126.81 (br), 126.87, 127.01, 127.09 (br), 127.59, 127.68, 127.74, 127.77, 128.19, 128.42, 128.47, 129.57 (br), 129.64 (br) 129.95, 129.99, 130.21 (br), 130.64, 131.02 (br), 131.28, 131.56, 131.84, 133.19 (CH), 72.28, 87.28, 138.46, 139.78, 139.84, 140.87, 141.37, 141,43, 143.01, 143.36, 143.61, 145.96, 146.13, 147.96 (br) 154.64, 164.32 (C) ppm. Anal. Calcd (%) for C56H40B2Br2: C 75.21; H 4.51. Found: C 74.84; H 4.60. Synthesis of [BrBC4Ph4(3,5-Me2C5H3N)] (8). A solution of BrBC4Ph4 (102 mg, 228 μmol) in benzene (2 mL) was titrated with a solution of 3,5-Me2C5H3N (0.08 mL) in benzene (2 mL) until the color changed from deep purple to yellow. The mixture was stirred at ambient temperature for 10 min, and the solvent was removed under vacuum. The yellow residue was dissolved in DCM (0.5 mL), and hexane (3 mL) was diffused into the mixture at 0 °C. The formed precipitate was filtered, washed with hexane (3  0.5 mL), and dried under vacuum to yield [BrBC4Ph4(3,5-Me2C5H3)] (104 mg, 188 μmol, 82%) as a yellow solid. Single crystals suitable for X-ray crystallography were obtained by recrystallization from chloroform. 1H NMR (500 MHz, C6D6): δ 1.30 (s, 6H, m-CH3-3,5-Lut), 6.19 (s, 1H, p-H-3,5-Lut), 6.876.95 (m, 8H, C6H5), 7.057.08 (m, 4H, C6H5), 7.237.25 (m, 4H, C6H5), 7.467.48 (m, 4H, C6H5), 8.66 (s, 2H, o-H-3,5-Lut). 11B{1H} NMR (160 MHz, C6D6): δ 4.4 ppm. 13C{1H} NMR (126 MHz, C6D6): δ 17.50 (CH3), 125.57, 126.39, 127.84, 128.08, 129.80, 130.74, 142.67, 142.95 (CH), 135.68, 139.21, 141.82, 150.17, 152.27 (br) ppm (C). Anal. Calcd (%) for C35H29BBrN: C 75.84; H 5.27; N 2.53. Found: C 75.98; H 5.59; N 2.69. Synthesis of [BrBC4Ph4(PCy3)] (9). Benzene (1.5 mL) was added to a stirred mixture of BrBC4Ph4 (40.9 mg, 91.5 μmol) and PCy3 (25.3 mg, 90.2 μmol) at 10 °C, resulting in formation of an orange solution. The mixture was stirred at ambient temperature for 1.5 h, and the solution was concentrated to 0.5 mL. Hexane (0.3 mL) was added to form pale yellow crystals after 2 d, which were filtered, washed with hexane (2  0.3 mL), and dried under vacuum. [BrBC4Ph4(PCy3)] (54.0 mg, 74.2 μmol, 82%) was obtained as a pale yellow solid. Single crystals suitable for X-ray crystallography were obtained by recrystallization from benzene. Single crystals with a different morphology were obtained by crystallization from hexane (90 ; Supporting Information). 1 H NMR (500 MHz, C6D6): δ 0.841.03 (m, 9H, Cy), 1.351.57 (m, 15H, Cy), 2.072.09 (m, 6H, Cy), 2.292.36 (m, 3H, Cy), 6.746.77 (m, 2H, C6H5), 6.896.97 (m, 6H, C6H5), 7.097.13 (m, 4H; C6H5), 7.187.20 (m, 4H, C6H5), 7.967.98 (m, 4H, C6H5). 11B{1H} NMR (160 MHz, C6D6): δ 6.4 ppm. 13C{1H} NMR (126 MHz, C6D6): δ 26.34, 27.53 (d, 2J(C,P) = 10.23 Hz), 29.03 (d, 3J(C,P) = 3.73 Hz) (CH2), 33.09 (d, 1J(C,P) = 26.68 Hz), 125.39, 125.99, 127.47, 127.69, 130.53 (d, 4J(C,P) = 1.10 Hz), 130.71 (CH), 139.99, 144.24, 151.27 (br), 153.23 (d, 3J(C,P) = 9.59 Hz) ppm (C). 31P{1H} NMR (202 MHz, C6D6): δ 2.3 ppm (br). Anal. Calcd (%) for C46H53BBrP: C 75.93; H 7.34. Found: C 75.60; H 7.44. Synthesis of [BrBC4Ph4(SIMes)] (10). Benzene (2 mL) was added to a stirred mixture of BrBC4Ph4 (40.0 mg, 89.5 μmol) and SIMes (28.0 mg, 91.4 μmol), resulting in formation of a yellow suspension. The mixture was stirred at ambient temperature for 10 min, and the solvent was removed under vacuum. The residue was washed with hexane (2  2 mL) and recrystallized from THF/hexane (1:3) at 0 °C. The obtained solid was filtered, washed with hexane (2  0.3 mL), and dried under vacuum. [BrBC4Ph4(SIMes)] (50.0 mg, 66.3 μmol, 74%) was obtained 3214

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Organometallics as a yellow solid. Single crystals suitable for X-ray crystallography were obtained by recrystallization from benzene. 1H NMR (500 MHz, C6D6): δ 2.04 (s, 12H, o-CH3-Mes), 2.13 (s, 6H, p-CH3-Mes), 3.01 (s, 4H, NCH2CH2N), 6.73 (s, 4H, Mes), 6.796.90 (m, 10H, C6H5), 7.027.05 (m, 2H, C6H5), 7.097.12 (m, 4H, C6H5), 7.687.69 (m, 4H, C6H5). 11B{1H} NMR (160 MHz, C6D6): δ 6.2 ppm. 13C{1H} NMR (126 MHz, C6D6): δ 19.64, 20.89 (CH3), 51.26 (br, CH2), 124.97, 125.46, 127.05, 127.34, 129.91, 130.81, 131.77 (CH), 136.24 (br), 136.37, 138.70, 140.88, 143.16, 150.87, 152.07 (br), 187.35 (br) ppm (C). Anal. Calcd (%) for C49H46BBrN2: C 78.09; H 6.15; N 3.72. Found: C 77.74; H 6.38; N 3.58. Synthesis of [BrBC4Ph4(CAAC)] (11). Benzene (2 mL) was added to a stirred mixture of BrBC4Ph4 (40.3 mg, 90.1 μmol) and CAAC (26.1 mg, 91.4 μmol), resulting in formation of a brown suspension. The mixture was stirred at ambient temperature for 2 h, and the solvent was removed under vacuum. The residue was washed with hexane (3  1.5 mL) and recrystallized from THF/hexane (1:2) at RT. The obtained solid was filtered, washed with hexane (2  0.4 mL), and dried under vacuum. [BrBC4Ph4(CAAC)] (24.0 mg, 32.8 μmol, 26%) was obtained as a yellow solid. Single crystals suitable for X-ray crystallography were obtained by crystallization from THF/hexane. 1H NMR (500 MHz, C6D6): δ 0.81 (s, 6H, CH3), 0.98 (d, 2J(H,H) = 6.44 Hz, 6H, CH3-iPr), 1.10 (d, 2J(H,H) = 6.36 Hz, 6H, CH3-iPr), 1.63 (s, 2H, CH2), 1.75 (s, 6H, CH3), 2.452.53 (m, 2H, H-iPr), 6.786.81 (m, 2H, C6H5), 6.846.87 (m, 4H, C6H5), 6.956.97 (m, 8H, m-H-Ar and C6H5), 7.037.09 (m, 5H, p-H-Ar and C6H5), 7.537.55 (m, 4H, C6H5). 11 B{1H} NMR (160 MHz, C6D6): δ 4.8 ppm. 13C{1H} NMR (126 MHz, C6D6): δ 24.25, 27.43, 29.02, 29.95 (CH3), 50.80 (CH2), 29.53, 125.12, 125.23, 125.58, 127.30, 127.33, 129.37, 130.78, 131.47 (CH), 55.48, 79.95, 134.37, 140.23, 144.78, 146.03, 152.24, 156.95 (br), 226.16 (br) ppm (C). Anal. Calcd (%) for C48H51BBrN: C 78.69; H 7.02; N 1.91. Found: C 78.87; H; 7.06 N; 1.93. Synthesis of [BrBC4Ph4(THF)] (12). A solution of THF (0.05 mL) in benzene (1.8 mL) was added dropwise to a solution of BrBC4Ph4 (40.0 mg, 89.0 μmol) in benzene (1.5 mL), resulting in a color change from deep purple to orange. The mixture was stirred at ambient temperature for 20 min and concentrated. Hexane (1 mL) was added to give [BrBC4Ph4(THF)] (23.0 mg, 44.0 μmol, 49%) as pale yellow crystals, which were dried under vacuum. 1H NMR (500 MHz, C6D6): δ 0.780.81 (m, 4H, CH2), 3.843.87 (m, 4H, CH2), 6.846.88 (m, 2H, C6H5), 6.906.93 (m, 4H, C6H5), 6.977.01 (m, 2H, C6H5), 7.087.16 (m, 8H, C6H5), 7.647.67 ppm (m, 4H, C6H5). 11B{1H} NMR (160 MHz, C6D6): δ 10.3 ppm. 13C{1H} NMR (126 MHz, C6D6): δ 24.53, 75.69 (CH2), 125.73, 126.47, 127.80, 128.13, 129.98, 130.50 (CH), 138.88, 142.06, 145.05 (br), 151.19 ppm (C). Anal. Calcd (%) for C32H28BBrO: C 74.01; H 5.43. Found: C 74.05; H 5.75.

’ ASSOCIATED CONTENT

bS

Supporting Information. Further details, including tables, figures, and crystallographic information files (CIF) of 7, 8, 9, 10, 11, and 12. This information is available free of charge via the Internet at http://pubs.acs.org. CCDC-821482 (7), CCDC-821483 (8), CCDC-821484 (9), CCDC-821485 (90 ), CCDC-821486 (10), CCDC-821487 (11), and CCDC-821488 (12) also contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (þ49) 931- 31-85260. Fax: (þ49) 931-31-84623. E-mail: [email protected].

ARTICLE

Present Addresses †

Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617.

’ ACKNOWLEDGMENT We thank the German Science Foundation (DFG) for financial support. C.-W.C. is grateful for a postdoctoral fellowship from the Alexander von Humboldt Foundation. ’ REFERENCES (1) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986, 108, 379. (2) (a) H€uckel, E. Grundz€uge der Theorie Unges€attigter und Aromatischer Verbindungen; Verlag Chemie: Berlin, 1938. (b) Breslow, R. Chem. Eng. News. 1965, 43, 90. (c) Breslow, R. Acc. Chem. Res. 1973, 6, 393. (3) Eisch, J. J.; Hota, N. K.; Kozima, S. J. Am. Chem. Soc. 1969, 91, 4575. (4) Braunschweig, H.; Fernandez, I.; Frenking, G.; Kupfer, T. Angew. Chem., Int. Ed. 2008, 47, 1951. (5) So, C. W.; Watanabe, D.; Wakamiya, A.; Yamaguchi, S. Organometallics 2008, 27, 3496. (6) Braunschweig, H.; Kupfer, T. Chem. Commun. 2008, 4487. (7) Fan, C.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed. 2009, 48, 2955. (8) Braunschweig, H.; Chiu, C.-W.; Radacki, K.; Brenner, P. Chem. Commun. 2010, 46, 916. (9) Braunschweig, H.; Breher, F.; Chiu, C.-W.; Gamon, D.; Nied, D.; Radacki, K. Angew. Chem., Int. Ed. 2010, 49, 8975. (10) Braunschweig, H.; Chiu, C.-W.; Radacki, K.; Kupfer, T. Angew. Chem., Int. Ed. 2010, 49, 2041. (11) Steffen, A.; Ward, R. M.; Jones, W. D.; Marder, T. B. Coord. Chem. Rev. 2010, 254, 1950. (12) Braunschweig, H.; Chiu, C.-W.; Wahler, J.; Radacki, K.; Kupfer, T. Chem.Eur. J. 2010, 16, 12229. (13) K€ ohler, J.; Lindenmeier, S.; Fischer, I.; Braunschweig, H.; Kupfer, T.; Gamon, D.; Chiu, C.-W. J. Raman Spectrosc. 2010, 41, 636. (14) Fan, C.; Piers, W. E.; Parvez, M.; McDonald, R. Organometallics 2010, 29, 5132. (15) Fan, C.; Mercier, L. G.; Piers, W. E.; Tuononen, H. M.; Parvez, M. J. Am. Chem. Soc. 2010, 132, 9604. (16) Ansorg, K.; Braunschweig, H.; Chiu, C.-W.; Engels, B.; Gamon, D.; H€ugel, M.; Kupfer, T.; Radacki, K. Angew. Chem., Int. Ed. 2011, 50, 2833. (17) (a) Cao, H.; Ma, J.; Zhang, G.; Jiang, Y. Macromolecules 2005, 38, 1123. (b) Yuan, Z.; Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams, I. D.; Cheng, L. T. J. Organomet. Chem. 1993, 449, 27. (c) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.; Brockhinke, A.; Stammler, H. G.; Neumann, B. Dalton Trans. 2009, 1339. (d) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H. W.; Holthausen, M. C.; Jaekle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584. (e) Sundararaman, A.; Victor, M.; Varughese, R.; Jaekle, F. J. Am. Chem. Soc. 2005, 127, 13748. (f) Kim, S.; Song, K. H.; Kang, S. O.; Ko, J. Chem. Commun. 2004, 68. (g) Wakamiya, A.; Mishima, K.; Ekawa, K.; Yamaguchi, S. Chem. Commun. 2008, 579. (h) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2002, 124, 8816. (18) (a) Wade, C. R.; Gabbai, F. P. Inorg. Chem. 2010, 49, 714. (b) Wade, C. R.; Gabbai, F. P. Dalton Trans. 2009, 9169. (c) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001, 123, 11372. (19) (a) Chase, P. A.; Piers, W. E.; Patrick, B. O. J. Am. Chem. Soc. 2000, 122, 12911. (b) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391. (c) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527. (20) Herberich, G. E.; Buller, B.; Hessner, B.; Oschmann, W. J. Organomet. Chem. 1980, 195, 253. 3215

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