ARTICLE pubs.acs.org/Organometallics
Synthesis and Characterization of Bulky Cationic Arylalkylaluminum Compounds Tomasz Klis,† Douglas R. Powell,‡ Lukasz Wojtas,§ and Rudolf J. Wehmschulte*,† †
Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251, United States § Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, CHE205, Tampa, Florida 33620-5250, United States ‡
bS Supporting Information ABSTRACT: The cationic m-terphenyl-substituted organoaluminum compounds [Dipp*AlEt][CH6B11Cl6] (3; Dipp* = 2,6-(2,6-i-Pr2C6H3)2C6H3), [DcpAlEt][CH6B11Cl6] (4; Dcp = 2,6-(2,6-Cl2C6H3)2C6H3), [Dipp*AlEt][CH6B11I6] (5), and [DcpAlEt][CH6B11I6] (6) have been obtained by ethide elimination through trityl or preferably silylium salts from the precursors Dipp*AlEt2 (1) and DcpAlEt2 (2). The crystal structures of compounds 35 reveal the presence of cationanion adducts involving two halides of the carborane anions, and the NMR spectroscopic data and solubility properties indicate such an interaction for 6. The interactions of the hexaiodocarborane anion with the [Dipp*AlEt]þ cation are stronger than those of the hexachlorocarborane anion. Interestingly, cation 3 3 3 anion contacts are preferred to intramolecular Al 3 3 3 Cl interactions in the Dcp species 4. Compound 4 forms the bisamine adduct [DcpAlEt(NH2t-Bu)2][CH6B11I6] (8) upon exposure to tBuNH2, and compounds 3 and 4 slowly catalyze the alkylation of benzene with 1-hexene. The compounds have been characterized by 1H, 13C, and 11B NMR spectroscopy and by X-ray crystallography in the case of 15, 7, and 8.
’ INTRODUCTION The relatively short history of cationic organoaluminum compounds began in 1987 with the synthesis of donor-stabilized dialkylaluminum complexes such as [AlMe2(18-crown-6)]þ, [AlMe2 (15-crown-5)]þ,1 and [Al{2-(Me3Si)2C}C5H4N]þ.2 The two closely related cationic dichloroaluminum compounds [AlCl2(12crown-4)]þ and [AlCl2(18-crown-6)]þ were reported 3 years earlier.3 The first example of a practical application of these compounds was presented by Bochmann,4 who synthesized [AlCp2]þ and demonstrated its reactivity as an initiator for the polymerization of isobutene. Two recent applied examples involve the hydrodefluorination of nonactivated CF bonds with the simple dialkylaluminum cations [R2Al]þ (R = Et, iBu).5,6 An excellent overview of these and related cationic species of group 13 is given in a review by Dagorne and Atwood.7 The coordinative unsaturation of the cationic aluminum centers in these compounds is crucial for the catalytic activity and the high level of Lewis acidity, which can be achieved by separation of cation and anion and the avoidance of donor solvents. The application of so-called weakly coordinating anions (WCA’s) such as the fluorinated tetraphenylborates [B(C6F5)4] and [B{3,5-(CF3)2C6H3}4], carborane anions [CH6B11X6] (X = Cl, Br), and fluorinated tetraalkoxyalanates [Al{OC(CF3)3}4] 8 facilitates cationanion separation, but additional measures often need to be undertaken. Despite their superior properties as almost “noncoordinating” anions the tetraphenylborates r 2011 American Chemical Society
and tetraalkoxyalanates are sensitive to strong Lewis acids in that they suffer from facile phenyl or alkoxide migration to the Lewis acid,8 and the residual but still significant coordinating properties of the carborane anions were demonstrated by the cationanion adducts [Et2Al][commo-3,30 -Al(3,1,2-A1C2B9H11)2]9 and [Et2Al][CH6B11X6].10 We have shown previously that full cationanion separation can be achieved by steric protection through two bulky m-terphenyl substituents in the compounds [(2,6-Mes2C6H3)2M]þ (M = Al,11 Ga;12 Mes = 2,4,6-Me3C6H2). As this led to low catalytic activities, we have prepared the less protected and hence more reactive cationic mono-m-terphenyl-substituted gallium species [2,6-Mes2C6H3Ga(nBu)]þ and [2,6-Dipp2C6H3Ga(n-Bu)]þ (Dipp = 2,6-iPr2C6H3).13 In combination with the anion [CHB11Cl11] these compounds were thermally stable to at least 70 °C and alkylated benzene with 1-octene at room temperature. Here, we report the extension of this work to the more Lewis acidic and more reactive cationic mono-m-terphenyl-substituted aluminum analogues.
’ EXPERIMENTAL SECTION General Procedures. All work was performed under anaerobic and anhydrous conditions by using either modified Schlenk techniques Received: February 17, 2011 Published: April 01, 2011 2563
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Organometallics or a Vacuum Atmospheres drybox. Solvents were freshly distilled under N2 from sodium, potassium, or sodium/potassium alloy and degassed twice prior to use or they were dispensed from a commercial solvent purification system. The compounds 2,6-(2,6-i-Pr2C6H3)2C6H3Li (Dipp*Li),14 2,6-(2,6-Cl2C6H3)2C6H3I (DcpI),15 [Ph3C][CH6B11Cl6],16 [Ph3C][CH6B11I6],16 and [Et2Al][CH6B11Cl6]10 were prepared according to literature procedures. All other reagents were obtained from commercial suppliers and used as received. NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. 1H NMR chemical shift values were determined relative to the residual protons in C6D6 or CDCl3 as internal reference (δ 7.15 or 7.26 ppm). 13C NMR spectra were referenced to the solvent signal (δ 128.0 or 77.0 ppm) and 11 B NMR spectra to a solution of F3B 3 OEt2 in C6D6 as external standard (δ 0 ppm). Melting points were determined in Pyrex capillary tubes sealed under nitrogen with a Mel-Temp apparatus and are uncorrected. Elemental analyses for the compounds 3 3 C6H6 and 6 3 0.25C6H14 were performed by Columbia Analytical Services in Tucson, AZ. However, the results using well-shaped crystals were not consistent for 6 3 0.25C6H14 and were variable in repeat analyses, due to the incomplete loss of solvent from the crystals during handling. NMR spectra for all compounds are given in the Supporting Information. GC/MS analysis was performed using a Hewlett-Packard G1800A GCD system. Dipp*AlEt2 (1). A solution of Et2AlCl (0.66 g, 5.5 mmol) in hexanes (10 mL) was added dropwise to a suspension of Dipp*Li (2.20 g, 5 mmol) in hexanes (20 mL) at 78 °C. The reaction mixture was stirred at 78 °C for 2 h and then warmed to room temperature and stirred overnight. The light yellow solution was transferred from the colorless precipitate to another flask by cannula. Concentration of the hexane solution to ca. 10 mL under vacuum and subsequent cooling to 30 °C for 2 days afforded colorless crystals of Dipp*AlEt2. Yield: 48%, 1.06 g. Mp: 103105 °C. 1H NMR (400.13 MHz, C6D6): δ 7.29 (A-part of AB2 system, p-H, 3JHH = 7.7 Hz, 1H), 7.24 (A-part of AB2 system, p-H(Dipp), 3JHH = 8.0 Hz, 2H), 7.23 (B-part of AB2 system, m-H, 3JHH = 7.7 Hz, 2H), 7.18 (B-part of AB2 system, m-H(Dipp), 3JHH = 8.0 Hz, 4H), 3.06 (sept, CH(CH3)2, 3JHH = 6.8 Hz, 4H), 1.26 (d, CH(CH3)2, 3 JHH = 6.8 Hz, 12H), 1.05 (d, CH(CH3)2, 3JHH = 6.8 Hz, 12H), 0.78 (t, CH2CH3, 3JHH = 8.2 Hz, 6H), 0.07 (q, CH2CH3, 3JHH = 8.2 Hz, 4H). 13C{1H} NMR (100.61 MHz, C6D6) δ 152.66 (i-C), 147.29, 147.21 (o-C(Dipp)), 142.27, 128.99 (p-C(Dipp)), 127.50 (p-C), 127.15 (m-C), 123.61 (m-C(Dipp)), 30.66 (CH(CH3)2), 26.32 (CH(CH3)2), 22.43 (CH(CH3)2), 7.83 (AlCH2CH3), 3.54 (AlCH2CH3). 2,6-(2,6-Cl2C6H3)2C6H3Li (DcpLi). A suspension of DcpI (2.57 g, 5.2 mmol) in hexanes (50 mL) was treated dropwise with a solution of nBuLi (3.6 mL, 5.7 mmol, 1.6 M) in hexanes at 0 °C, and after addition the cooling bath was removed. After 510 min a fine colorless precipitate formed. After the mixture was warmed to room temperature (1 h), the solid was collected on a sintered-glass frit and dried under vacuum to give crude DcpLi (1.68 g, 86%). According to 1H NMR spectroscopy it was contaminated with some alkyl derivatives, including mainly nhexane and 2-methylpentane (ca. 1 equiv of hexanes per DcpLi), from incomplete drying but was used without further purification. 1H NMR (400.13 MHz, C6D6): δ 7.20 (t, J = 7.6 Hz, p-H, 1H), 6.92 (d, partially obscured, m-H, 2H), 6.90 (d, J = 8.1 Hz, m-H(C6H3Cl2), 4H), 6.50 (t, J = 8.1 Hz, p-H(C6H3Cl2), 2H). DcpAlEt2 (2). A solution of Et2AlCl (0.54 g, 4.5 mmol) in hexanes (10 mL) was added dropwise to a suspension of DcpLi (1.68 g, 4.5 mmol) in hexanes (40 mL) at 78 °C. The reaction mixture was stirred at 78 °C for 1/2 h and then warmed to room temperature and stirred overnight, followed by additional stirring for 2 days at 4345 °C. The light yellow cloudy mixture was filtered through a medium-porosity sintered-glass frit. After the mixture was cooled to room temperature, colorless needles of 2 separated (0.38 g). A second batch (1.00 g) was obtained after concentration to ca. 3040 mL and slow crystallization at room temperature for 6 days. Yield 68%, 1.38 g. Mp: 104106 °C. 1H NMR (400 MHz,
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C6D6): δ 7.32 (A-part of A2B system, m-H, J = 8.0 Hz, 2H), 7.28 (B-part of A2B system, p-H, J = 8.0 Hz, 1H), 6.97 (d, J = 8.1 Hz, m-H(C6H3Cl2), 4H), 6.47 (t, J = 8.1 Hz, p-H(C6H3Cl2), 2H), 1.00 (t, AlCH2CH3, J = 8.1 Hz, 6H), 0.12 (q, AlCH2CH3, J = 8.1 Hz, 4H). 13C{1H} NMR (100.61 MHz, C6D6): 152.28 (i-C), 142.71 (o-C or i-C(C6H3Cl2)), 141.47 (o-C or iC(C6H3Cl2)), 134.40 (o-C(C6H3Cl2)), 130.41 (m-C), 129.21 (m-C(Cl2C6H3)), 128.95 (p-C(Cl2C6H3)), 127.59 (p-C), 9.09 (AlCH2CH3), 0.90 (s, broad, w1/2= 7.7 Hz, AlCH2CH3). [Dipp*AlEt][CH6B11Cl6] (3). A small grease-free Schlenk flask was charged with [Ph3C][CH6B11Cl6] (0.13 g, 0.22 mmol), benzene (3 drops), hexanes (1.5 mL), and Et3SiH (0.22 g, 1.9 mmol), and the resulting mixture was stirred at room temperature for 4 days. The mother liquor was pipetted off, and the remaining fine beige solid was washed with hexanes (3 1.5 mL). A solution of Dipp*AlEt2 (0.091 g, 0.19 mmol) in hexanes (1.5 mL) was added at room temperature, and the mixture was stirred at room temperature for 24 h. As no visible change was observed, C6D6 (0.5 mL) was added, and the mixture clarified within minutes, leaving a small amount of brownish sticky solid undissolved. The clear colorless supernatant liquid was transferred into another Schlenk flask, concentrated to ca. 0.5 mL and left standing for 2 days at room temperature for crystallization. The resulting bundle of small colorless crystals was isolated, washed with hexanes (1 mL), and dried under vacuum. Yield: 0.122 g, 69%. Mp: softens and turns pale pink at 110110 °C, melts at 220225 °C. The compound contains approximately 20% of the anion [CH5B11Cl7] according to the X-ray crystal structure and the negative ion MALDI spectrum. 1H NMR (C6D6, 400.13 MHz): 7.21 (m, 3H), 7.16 (m, partially obscured by C6D5H), 2.88 (sept, J = 6.7 Hz, CH(CH3)2, 4H), 2.0 (s, br, w1/2 = 400 Hz, BH, 5H), 1.37 (d, CH(CH3)2, J = 6.7 Hz, 12H), 1.12 (s, CHB11, 1H), 0.92 (d, CH(CH3)2, J = 6.7 Hz, 12H), 0.86 (t, AlCH2CH3, J = 8.0 Hz, 3H), 0.41 (q, AlCH2CH3, J = 8.0 Hz, 2H). 13C{1H} NMR (100.61 MHz, C6D6) δ 148.55 (i-C(Dipp) or o-C), 147.87 (o-C), 140.71 (i-C(Dipp) or o-C), 130.24 (p-C(Dipp) or m-C), 130.05 (pC(Dipp) or m-C), 128.75 (p-C), 123.98 (m-C(Dipp)), 32.82 (CH6B11Cl6), 31.01 (CH(CH3)2), 26.72 (CH(CH3)2), 22.35 (CH(CH3)2), 9.19 (AlCH2CH3), 5.11 (AlCH2CH3). 11B NMR (C6D6, 128.38 MHz): 1.2 (s, BCl, 1B), 6.3 (s, BCl, 5B), 23.5 (s, broad, w1/2 = 460 Hz, BH, 5B). [DcpAlEt][CH6B11Cl6] (4). Et3SiH (3.50 g, 30 mmol) was added to a suspension of [Ph3C][CH6B11Cl6] (0.91 g, 1.5 mmol) in hexanes (20 mL), and the resulting reaction mixture was stirred at room temperature for 1 week, during which time the suspended solid changed from yellow to almost colorless. The mother liquor was siphoned off, and the solid was washed with hexanes (2 20 mL) and dried under vacuum. This solid, [Et3Si][CH6B11Cl6] (0.62 g, 1.3 mmol), was partially dissolved in benzene (5 mL), and the mixture was added to a solution of 2 (0.60 g, 1.3 mmol) in benzene (1 mL) at room temperature. After the mixture was stirred for 12 h, the resulting small colorless crystals were isolated, washed with benzene (1 mL), and dried inside the drybox. Yield: 0.43 g, 42%. Mp 140 °C dec. 1H NMR (400 MHz, C6D6): δ 7.17 (t, J = 7.7 Hz, p-H, 1H), 6.95 (d, J = 7.7 Hz, m-H, 2H), 6.91 (d, J = 8.1 Hz, m-H(Cl2C6H3), 4H), 6.49 (t, J = 8.1 Hz, p-H(Cl2C6H3), 2H), 2.2 (“d”, broad, Δν = 194 Hz, w1/2 = 480 Hz, BH, 5H), 1.24 (s, CHB11, 1H), 1.02 (t, J = 8.0 Hz, AlCH2CH3, 3H), 0.64 (q, J = 8.0 Hz, AlCH2CH3, 2H). 11B NMR (C6D6, 128.38 MHz): 0.9 (s, BCl, 1B), 6.3 (s, BCl, 5B), 23.4 (d, J = 128 Hz, BH, 5B). 13C{1H} NMR (100.61 MHz, C6D6): 145.05, 144.96 (o-C or i-C(Cl2C6H3)), 140.31 (i-C), 140.10 (o-C or i-C(Cl2C6H3)), 135.86 (o-C(Cl2C6H3)), 130.82 (p-C(Cl2C6H3)), 130.76 (p-C), 130.33 (m-C), 129.19 (m-C(Cl2C6H3)), 33.01 (CH6B11Cl6), 8.89 (AlCH2CH3), 8.09 (br, w1/2 = 8 Hz, AlCH2CH3). [Dipp*AlEt][CH6B11I6] (5). A small Schlenk flask was charged with [Ph3C][CH6B11I6] (0.21 g, 0.18 mmol), benzene (5 drops), hexanes (3 mL), and Et3SiH (0.20 g, 1.7 mmol), and the resulting mixture was stirred at room temperature for 3 days. The mother liquor was 2564
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Organometallics pipetted off, and the resulting fine beige solid was washed with hexanes (3 1.5 mL). Hexanes (3 mL) and Dipp*AlEt2 (0.091 g, 0.19 mmol) were added, and the mixture was stirred at room temperature for 24 h. The fine beige precipitate was collected on a glass frit inside a drybox, washed with hexanes (3 mL), and dried under vacuum. It was identified as mainly [Et3Si][CH6B11I6] by 1H NMR spectroscopy. This solid (0.17 g, 0.17 mmol if assumed to be pure silylium salt) was combined with the NMR sample, and benzene (2 mL) and Dipp*AlEt2 (0.08 g, 0.16 mmol) were added. After 4 h of stirring most of the solid had dissolved. Stirring was continued for another 16 h, followed by removal of the volatiles under vacuum to give a colorless solid. Crystallization from benzene (0.5 mL) at room temperature for 1 week afforded 5 as well-shaped colorless crystals. Yield: 0.05 g, 22%. Mp: darkens at 145 °C, partially melts at 230 °C, does not melt completely below 260 °C. 1H NMR (C6D6, 400.13 MHz): 7.15 (t, J = 7.6 Hz, partially obscured by C6D5H, p-H(Dipp)), 7.09 (m, 7 H), 3.1 (s, br, w1/2 = 413 Hz, BH, 5H), 2.81 (sept, J = 6.7 Hz, CH(CH3)2, 4H), 2.06 (s, CHB11, 1H), 1.36 (d, CH(CH3)2, J = 6.7 Hz, 12H), 1.05 (t, AlCH2CH3, J = 8.0 Hz, 3H), 0.91 (d, CH(CH3)2, J = 6.7 Hz, 12H), 0.68 (q, AlCH2CH3, J = 8.0 Hz, 2H). 13C{1H} NMR (100.61 MHz, C6D6): δ 148.34 (i-C(Dipp) or o-C), 147.87 (o-C), 140.87 (i-C(Dipp) or o-C), 140.23 (i-C), 130.42 (p-C(Dipp) or m-C), 130.40 (p-C(Dipp) or m-C), 128.35 (p-C), 124.49 (m-C(Dipp)), 55.47 (CH6B11I6), 30.99 (CH(CH3)2), 26.29 (CH(CH3)2), 23.12 (CH(CH3)2), 11.48 (AlCH2CH3), 9.44 (AlCH2CH3). 11 B NMR (C6D6, 128.38 MHz): 6.7 (s, BI, 1B), 15.6 (s, broad, w1/2 = 513 Hz, BH, 5B), 18.3 (s, BI, 5B). [DcpAlEt][CH6B11I6] (6). A small Schlenk flask was charged with [Ph3C][CH6B11I6] (0.150 g, 0.13 mmol), benzene (10 drops), hexanes (3 mL), and Et3SiH (0.23 g, 2.0 mmol), and the resulting mixture was stirred at room temperature at high speed for 27 h. The mother liquor was pipetted off, and the resulting fine beige solid was washed with hexanes (3 1.5 mL). A solution of 2 (0.059 g, 0.13 mmol) in C6D6 (1.5 mL) was added at room temperature, and the reaction mixture was stirred at room temperature for 2 h to give an almost clear colorless solution. After an additional 3 h a microcrystalline colorless precipitate had formed. The volatiles were removed under vacuum, and the resulting colorless solid was redissolved in C6D6 (1.5 mL) with brief heating with a heat gun. As no crystals formed upon standing at room temperature or at 4 °C for several days, hexanes (1.5 mL) was added to give a milky mixture. A few colorless crystalline aggregates formed after 5 days at room temperature, and more hexanes (1.5 mL) was added to increase the yield. The aggregates were isolated and dried under vacuum. Yield: 0.047 g, 26%. The crystals contain 0.25 equiv of cocrystallized hexanes. Mp: 150155 °C dec (turns black). 1H NMR (C6D6, 400.13 MHz): 7.14 (t, J = 7.7 Hz, p-H, 1H), 6.87 (d, J = 8.1 Hz, m-H(Cl2C6H3), 4H), 6.84 (d, J = 7.7 Hz, m-H, 2H), 6.48 (t, J = 8.1 Hz, p-H(Cl2C6H3), 2H), 3.1 (“d”, broad, Δν = 192 Hz, w1/2 = 460 Hz, BH, 5H), 2.10 (s, CHB11, 1H), 1.09 (t, J = 8.0 Hz, AlCH2CH3, 3H), 0.76 (q, J = 8.0 Hz, AlCH2CH3, 2H). 13C{1H} NMR (C6D6, 100.61 MHz): 145.41 (o-C or i-C(Cl2C6H3)), 140.69 (o-C or i-C(Cl2C6H3)), 139.12 (i-C), 136.51 (o-C(Cl2C6H3)), 130.90 (p-C(Cl2C6H3)), 130.79 (p-C), 130.52 (m-C), 129.37 (m-C(Cl2C6H3)), 55.83 (CH6B11I6), 11.64 (AlCH2CH3), 10.83 (AlCH2CH3). 11B NMR (C6D6, 128.38 MHz): 6.2 (s, BI, 1B), 15.1 (s, broad, w1/2 = 380 Hz, BH, 5B), 18.4 (s, BI, 5B). {Dipp*AlEt(μ-OH)}2 (7). During one of the attempted syntheses of 3 a small amount of a colorless crystalline solid was isolated from benzene solution and identified as {Dipp*AlEt(μ-OH)}2 (7) by X-ray diffraction. [DcpAlEt(NH2t-Bu)2][CH6B11I6] (8). t-BuNH2 (1.3 μL, 0.87 mg, 12 μmol) was added to a solution of 6 (19 mg, 13 μmol) in C6D6 (0.6 mL) in a J. Young NMR tube at room temperature, resulting in no visible change. The 1H NMR spectrum showed unreacted starting material (ca. 60%) and a new set of signals belonging to [DcpAlEt(NH2t-Bu)2][CH6B11I6] (ca. 40%). Addition of another 1 equiv of t-BuNH2 (1.3 μL, 0.87 mg, 12 μmol) resulted in a mixture of [DcpAlEt][CH6B11I6] (13%),
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[DcpAlEt(NH2t-Bu)2][CH6B11I6] (72%), and a new set of signals (14%). After 5 days at room temperature the starting material was consumed, and the 1H NMR spectrum showed a mixture of [DcpAlEt(NH2t-Bu)2][CH6B11I6] (61%) and the unknown compound (39%). Heating at 90 °C for 4 h did not cause any change. Additional t-BuNH2 (0.9 μL, 0.63 mg, 9 μmol) was added, and the 1H NMR spectrum of the resulting mixture only contained signals of [DcpAlEt(NH2t-Bu)2][CH6B11I6] and unreacted t-BuNH2. Storage of the solution at room temperature for 6 weeks resulted in the separation of a bundle of colorless crystals of sufficient quality for X-ray diffraction in approximately 77% yield (as judged from the 1H NMR spectrum of the mother liquor). 1H NMR (400.13 MHz, C6D6): 7.09 (d, J = 8.1 Hz, m-H(C6H3Cl2), 4H), 7.08 (t, partially obscured, J = 7.7 Hz, p-H, 1H), 6.88 (t, J = 8.1 Hz, p-H(C6H3Cl2), 2H), 6.62 (d, J = 7.7 Hz, m-H, 2H), 3.43 (s, broad, w1/2 = 440 Hz, BH, 5H), 2.87 (d, J = 13.4 Hz, NH, 2H), 2.71 (d, J = 13.4 Hz, NH, 2H), 2.54 (s, CHB11, 1H), 1.04 (s, C(CH3)3, 18H), 0.64 (t, J = 7.8 Hz, AlCH2CH3, 3H), 0.61 (q, J = 7.8 Hz, AlCH2CH3, 2H). 11B NMR (C6D6, 128.38 MHz): 5.2 (s, BI, 1B), 14.1 (s, broad, w1/2 = 400 Hz, BH, 5B), 18.2 (s, BI, 5B). 13C{1H} NMR (C6D6, 100.61 MHz): 146.19 (o-C or i-C(C6H3Cl2)), 142.38 (o-C or iC(C6H3Cl2)), 135.05 (o-C(C6H3Cl2)), 131.75 (p-C(Cl2C6H3)), 130.53 (m-C), 130.45 (p-C), 129.89 (m-C(Cl2C6H3)), 56.66 (CH6B11I6), 56.49 (C(CH3)3), 30.57 (C(CH3)3), 9.78 (AlCH2CH3), 3.02 (AlCH2CH3). On the basis of the 1H NMR spectrum of the mixture of 8 and the unknown compound, the identity of this compound is possibly the monoamine adduct [DcpAlEt(NH2t-Bu)][CH6B11I6] (9): 7.20 (d, J = 8.2 Hz, m-H(C6H3Cl2), 4H), 6.96 (d, J = 7.8 Hz, m-H, 2H), 6.83 (t, J = 8.1 Hz, p-H(C6H3Cl2), 2H), 3.03 (s, NH, 2H), 2.50 (s, CHB11, 1H), 0.86 (s, C(CH3)3, 9H), 0.84 (t, J = 8.2 Hz, AlCH2CH3, 3H), 0.15 (q, J = 8.2 Hz, AlCH2CH3, 2H). Reaction of 3 with 1-Hexene. A solution of 3 (22 mg, 29 μmol) in C6D6 (0.6 mL) was treated with 1-hexene (0.29 mmol, 36 μL) at room temperature. The reaction was monitored by 1H NMR spectroscopy. After the free olefin had been consumed (3 days), additional olefin was added at various intervals, starting with 20 μL (0.16 mmol), and the reaction mixture was heated to 80 °C. The next olefin additions came after 1 day (20 μL, 0.16 mmol), 2 days (40 μL, 0.32 mmol), 4 days (250 μL, 2.0 mmol) and finally 6 days (40 μL, 0.32 mmol). The final olefin addition was consumed after 8 days more at 80 °C. The reaction mixture was poured into a vial in air, the solvent was allowed to evaporate, and the remaining oil was investigated by GC/MS: tR = 9.06 min (19%), calcd for C12H12D6 (3-phenylhexane) 168, found 168; tR = 9.42 min (60%), calcd for C12H12D6 (2-phenylhexane) 168, found 168; tR = 15.35 min (6%), calcd for C18H24D6 (di-2-hexylbenzene) 252, found 252; tR = 15.79 min (4%), calcd for C18H24D6 (di-2hexylbenzene) 252, found 252; tR = 18.85 min (11%), calcd for C24H36D6 (tri-2-hexylbenzene) 336, found 336. Assignments were made on the basis of the fragmentation patterns. Reaction of 4 with 1-Hexene. A solution of 4 (86 mg, 0.11 mmol) in C6D6CD3 (2 mL) was treated with 1-hexene (3 g, 36 mmol) at room temperature. The reaction mixture was kept at 80 °C over 10 days and next distilled under vacuum to give three fractions which were investigated by GC-MS: first fraction (0.57 g, 38 °C) tR = 10.36 (5%), 10.56 (18%), 10.58 min (8%), calcd for C13H12D8 (p-, m- and o-3-hexytoluene) 176, found 176; tR = 10.75 (13%), 10.90 (36%), 10.96 min (20%), calcd for C13H12D8 (p-, m-, and o-2-hexytoluene) 176, found 176; second fraction (2 g, 70125 °C) tR = 15.61 (12%), 15.90 (21%), 16.38 min (47%), calcd for C19H24D8 (mixture of di-2-hexyltoluenes) 260, found 260; tR = 15.99 (30%), 16.24 min (34%), calcd for C19H24D8 (mixture of di-3hexyltoluenes) 260, found 260; third fraction (0.81 g, 130160 °C) tR = 19.02, 19.33, 19.40, 19.47, 19.52, 19.58, 19.66, 19.71, 19.77 min, calcd for C25H36D8 (mixture of trihexylated toluenes) 344, found 344. Reaction of [Et2Al][CH6B11Cl6] with 1-Hexene. A solution of [Et2Al][CH6B11Cl6] (6 mg, 14 μmol) in C6D6 (0.6 mL) was treated with 1-hexene (1.7 μL, 14 μmol) at room temperature, and the progress 2565
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Organometallics
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Table 1. Crystallographic Data for 15, 7, and 8 1
a
2
3 3 C6H6
4 3 C6H6
5
7 3 4C6H6
8 C29H42AlB11Cl4I6N2
chem formula
C34H47Al
C22H19AlCl4
C39H53.80AlB11Cl6.20
C27H26AlB11Cl10
C33H48AlB11I6
C88H110Al2O2
formula wt
482.70
452.15
888.30
850.87
1352.00
1253.72
1467.74
space group
C2/c
P21/n
P21/c
P21/n
P21/n
P21/c
P21/c
a (Å)
38.802(16)
10.119(2)
13.4689(2)
12.1316(14)
13.736(10)
13.723(4)
9.967(4)
b (Å)
9.746(4)
7.5504(14)
17.0328(2)
12.1249(14)
20.352(15)
19.109(6)
31.025(12)
c (Å)
18.301(8)
27.389(6)
23.6118(3)
25.840(3)
17.040(12)
15.786(5)
15.405(6)
β (deg)
117.007(9)
92.576(8)
119.9490(10)
96.978(2)
107.880(16)
115.330(12)
94.07(6)
V (Å3) Z
6166(5) Å3 8
2090.5(7) 4
4693.5 (1) 4
3772.8(7) 4
4534(6) 4
3742(2) 2
4752(3) 4
T (K)
100(2)
100(2)
100(2)
100(2)
100(2)
100(2)
100(2)
λ (Å)
0.71073
0.71073
1.54178
0.71073
0.71073
0.71073
0.71073
Dcalcd (Mg/m3)
1.040
1.437
1.257
1.498
1.981
1.113
2.052 4.192
μ (mm1)
0.084
0.613
3.827
0.785
4.155
0.086
R1a (I > 2σ(I))
0.0417
0.0806
0.0343
0.0341
0.0510
0.0603
0.0718
wR2b
0.1271
0.1756
0.0914
0.0874
0.0986
0.1514
0.1339
R1 = ∑||Fo| |Fc||/∑|Fo|. b wR2 = {∑[w(Fo2 Fc2)2]/∑[w(Fo2)2]}1/2.
Scheme 1
of the reaction was monitored by 1H NMR spectroscopy. Isomerization was apparent (multiplets at 5.4 ppm belonging to 2- and 3-hexene) after 6 h, and all the 1-hexene was consumed after 1 week. Addition of 1-hexene (6 μL, 48 μmol) and heating at 80 °C for 4 h resulted in the consumption of 77% of 1-hexene. The reaction was complete after an additional 16 h at room temperature. The 1H NMR spectrum of this mixture resembles those of the reactions of 3 and 4 with 1-hexene. X-ray Crystallography. Intensity data for compounds 15, 7, and 8 were collected using diffractometers with Bruker APEX ccd area detectors and graphite-monochromated Mo KR radiation (λ = 0.710 73 Å) or Cu KR radiation (λ = 1.541 78 Å) (3) at 100(2) K. The data were corrected for absorption by the semiempirical method.17 The structures were solved by direct methods and refined by full-matrix least-squares methods on F2.18 Hydrogen atom positions were initially determined by geometry and refined by a riding model. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atom displacement parameters were set to 1.2 (1.5 for methyl) times the displacement parameters of the bonded atoms. For 3, the positions of atoms Cl7 and H1b have 20 and 80% occupancy, respectively. The intensity data for compounds 7 and 8 were truncated to 0.92 and 0.8 Å resolution, because data in higher resolution shells all had I/σ < 2. Some details of the data collections and refinements are given in Table 1, and selected bond distances and angles are given in the figure legends.
’ RESULTS AND DISCUSSION The ionic compound [(2,6-Mes2C6H3)2Al]þ[B(C6F5)4] features separated anions and cations in the solid state and an
intramolecular stabilization of the Lewis acidic aluminum center via close secondary Al 3 3 3 C contacts (A in Scheme 1).11 The ionic monoterphenyl butylgallium species13 [2,6-Mes2C6H3Ga(n-Bu)]þ and [2,6-Dipp2C6H3Ga(n-Bu)]þ form solvent-separated ions as well. On the basis of these results we hypothesized that the analogous monoterphenyl alkylaluminum cations should be accessible as well. We assumed that the large Dipp* substituent would provide sufficient steric protection and electronic stabilization to obtain isolated (B in Scheme 1) or at least only weakly bound ion pairs. In addition to the predominantly steric protection, we investigated the application of the tetrachlorinated m-terphenyl substituent 2,6-(2,6-Cl2C6H3)2C6H3- (Dcp, C in Scheme 1), which had been introduced by Protasiewicz for the stabilization of diphosphenes and phosphinidenes.15 We expected that weak intramolecular Al 3 3 3 Cl contacts would stabilize the Lewis acidic center and facilitate cationanion separation. Furthermore, the distortion at the ipso carbon, which was observed for A11 and is expected for B, should be minimized for C, leading to a strengthening of the AlC bond. The precursors Dipp*AlEt2 (1) and DcpAlEt2 (2) were obtained via standard procedures (eq 1)19 in 48% and 47% yield as colorless crystalline solids.
Compounds 1 and 2 are rare examples of mixed arylalkyl aluminum species with three-coordinate aluminum centers, and therefore their crystal structures were determined by X-ray crystallography (Figures 1 and 2). Both structures feature an unsymmetrically coordinated trigonal-planar three-coordinate aluminum center (∑(angles at Al) = 360.0° in 1, 359.8° in 2) with the smallest angle (115.14(8) and 116.0(3)°) between the ethyl substituents. The AlC distances are at the lower end of the scale observed for related compounds with d(AlCTerph) = 1.9679(17), 1.985(6) Å and d(AlCethyl) = 1.973 (av), 1.976 (av) Å. For comparison see Mes3Al (d(AlC) = 1.995(8)Å),20 (2,6-Mes2C6H2)2AlH (d(AlC) = 1.990(av) Å),13 and (Mes*AlCH2N(t-Bu)}2 (Mes*=2,4,6-t-Bu3C6H2, d(AlCaryl = 1.986(2) Å, 2566
dx.doi.org/10.1021/om200120y |Organometallics 2011, 30, 2563–2570
Organometallics
Figure 1. Thermal ellipsoid plot (50% probability) of 1. H atoms are omitted for clarity. Selected distances (Å) and angles (deg): Al(1)C(1) = 1.9679(17), Al(1)C(31) = 1.9686(17), Al(1)C(33) = 1.9778(17); C(1)Al(1)C(31) = 119.88(7), C(1)Al(1)C(33) = 124.94(7), C(31)Al(1)C(33) = 115.14(8), C(6)C(1)C(2) = 118.70(13).
ARTICLE
Figure 3. Thermal ellipsoid plot (50% probability) of 3. H atoms are omitted for clarity. Selected distances (Å) and angles (deg): Al(1)C(12) = 1.9597(18), Al(1)C(31) = 1.9477(19), Al(1) 3 3 3 Cl(1) = 2.3953(6), Al(1) 3 3 3 Cl(2) = 2.4151(6); C(12)Al(1)C(31) = 132.86(8), C(12)Al(1)Cl(1) = 113.67(6), C(31)Al(1)Cl(1) = 99.78(6), C(12)Al(1)Cl(2) = 104.90(5), C(31)Al(1)Cl(2) = 106.66(6), Cl(1)Al(1)Cl(2) = 91.17(2).
reaction rate is mainly due to the limited solubility of the trityl salt.
Compounds 3 and 4 are soluble in benzene, suggesting the formation of cationanion adducts similar to Reed’s [Et2Al] [CH6B11Cl6]. As our attempts to obtain crystals of these compounds were unsuccessful (only Ph3CH was obtained), we investigated the use of the silylium ion [Et3Si][CH6B11Cl6] as the ethide abstracting agent. In this case, the expected side products Et3SiH and Et4Si are volatile and should not interfere with the crystallization process. The reactions of 1 and 2 with [Et3Si][CH6B11Cl6] in benzene at room temperature proceeded quickly (
