Article pubs.acs.org/Organometallics
Syntheses and Structures of Stable 1‑Aminoalumole Derivatives Tomohiro Agou, Tatsuya Wasano, Takahiro Sasamori, and Norihiro Tokitoh* Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *
ABSTRACT: Reaction of an isolable 1-bromoalumole with LiN(SiMe3)2 in toluene afforded a lithium 1,1-diaminodialumoluide, while in THF the corresponding 1-aminoalumole−THF complex was obtained. These amino-substituted alumole derivatives are characterized by X-ray crystallographic analysis and NMR spectroscopy. The lithium 1,1-diaminoalumoluide and the 1aminoalumole−THF complex were converted to each other by ligand exchange reactions.
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INTRODUCTION In contrast to the development of the chemistry of 1boracyclopentadienes (boroles),1 their heavier element analogues, such as 1-aluminacyclopentadienes (alumoles), have remained hitherto an underdeveloped class of compounds, probably because of their high reactivities and the lack of facile methods for their synthesis.2,3 Recently, we have reported the synthesis of stable alumole 14 by the reaction of 3,6-dilithio-3,5octadiene derivative 25 with Mes*AlCl26 (Scheme 1). Reaction
Scheme 2. Reactivities of 1-Bromoalumole 3 toward Nucleophiles
aminoalumole−THF complex 6, which are potential precursors for Lewis-base-free 1-aminoalumole 4.
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Scheme 1. Synthesis of Alumole 1
RESULTS AND DISCUSSION First, we have investigated the reaction of 1-bromoalumole 3 with LiN(SiMe3)2 in a noncoordinating solvent. Treatment of 3 with an equimolar amount of LiN(SiMe3)2 in toluene afforded not 1-aminoalumole 4 but a mixture of lithium 1,1diaminoalumoluide 5 (Scheme 3) with the remaining starting material 3, suggesting that LiN(SiMe3)2 reacts with the intermediate 4 faster than 3 under these conditions. Compound 5 was obtained alone by the treatment of 3 with 2 equiv of LiN(SiMe3)2 and isolated as a colorless crystalline solid in 89% yield. Lithium 1,1-diaminoalumoluide 5 was characterized by means of multinuclear NMR spectroscopy, elemental analysis, and X-ray crystallographic analysis (Figure 1a). In the crystalline state, 5 exhibited a contact ion-pair structure with the lithium cation coordinated by the alumole ring and by the N1 atom. The Al1−N1 bond length (1.9547(16) Å) is elongated compared to the Al1−N2 bond length (1.8759(15) Å), indicating that the Al1−N1 bond is weak and readily cleaved. The N1−Li1 distance (2.204(4) Å) is substantially elongated compared with those found in the crystal structures of LiN(SiMe3)2 (e.g., [LiN(SiMe3)2]3: Li−N 1.985(4)−
of 1 with lithium afforded the corresponding alumole dianion as its lithium salt, [Li+(thf)]2[12−], indicating the application of alumoles as a key framework of electron acceptors. However, examples of stable alumoles are still limited, and in particular isolable alumole derivatives bearing heteroatom substituents have not been reported.7 During the course of our investigation on the synthesis and properties of alumole derivatives, we have synthesized 1bromoalumole 3, the first example of a stable heteroatomsubstituted alumole.8 Treatment of 3 with Mes*Li9 afforded alumole 1 (Scheme 2), showing the utility of 3 as a precursor for functionalized alumoles that are otherwise difficult to synthesize.10 Given such reactivities of 1-bromoalumole 3 toward nucleophiles, we became interested in the synthesis of 1-aminoalumole 4 from the viewpoint of the delocalization of the nitrogen lone pair to the vacant 3p(Al) orbital, forming partial π(N−Al) bonding. Herein, we report on the syntheses and structures of lithium 1,1-diaminoalumoluide 5 and 1© XXXX American Chemical Society
Received: October 20, 2014
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the coordinated THF molecule from 6 and isolate 1aminoalumole 4 under various conditions have been unsuccessful so far.13 Single crystals of 6 suitable for X-ray diffraction analysis were obtained by recrystallization from hexane at −35 °C (Figure 1b). The crystal structure of 6 displays a nearly planar alumole ring (sum of the internal bond angles: 539°) with a distorted tetrahedral aluminum atom (Figure 1b). The Al1−C(butadiene) bond lengths of 6 (Al1−C1 1.9850(14), Al1−C4 1.9736(14) Å) lie between those of 1 (1.951(2)−1.955(2) Å) and 5 (Al1−C1 2.0200(18), Al1−C4 2.0590(18) Å), reflecting the change in the coordination environment of the aluminum centers from trigonal planar (1) to distorted tetrahedral (5 and 6). Furthermore, comparable Al−C(butadiene) bond lengths (1.965(6), 1.968(6) Å) were reported for the pentaphenylalumole−Et2O complex 7 (Figure 2).2b The C−C bond lengths in
Scheme 3. Syntheses of Lithium 1,1-Diaminoalumoluide 5 and 1-Aminoalumole−THF Complex 6
Figure 2. Pentaphenylalumole derivatives 7 and 8.
the alumole ring of 6 do not show any significant deviation from those of 1, 5, and 7. The Al1−N1 (1.8472(11) Å) and Al1−O1 (1.9246(10) Å) bond lengths are comparable to those of tetracoordinated aminoalumane−THF complexes (e.g., (2Ph-C6H4)2NAlMe2(thf): Al−N 1.856(2), Al−O 1.927(2) Å).14 Meanwhile, a slightly shorter Al−O bond length was observed for 7 (1.907(5) Å),2b suggesting the lower Lewis acidity of 4 relative to that of pentaphenylalumole 8 (Figure 2).15 Compounds 5 and 6 can be interconverted with each other by ligand exchange reactions (Scheme 4). At room temper-
Figure 1. Crystal structures of (a) 5 and (b) 6 with thermal ellipsoids set at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): 5: Al1−N1 1.9547(16), Al1−N2 1.8759(15), Al1−C1 2.0200(18), Al1−C4 2.0590(18), C1− C2 1.361(2), C2−C3 1.520(2), C3−C4 1.359(3), Li1−Al1 2.551(3), Li1−N1 2.204(4), Li1−C1 2.388(4), Li1−C2 2.471(4), Li1−C3 2.355(4), Li1−C4 2.388(4), C1−Al1−C4 86.67(7), C1−Al1−N1 105.33(7), C1−Al1−N2 115.63(7), C4−Al1−N1 100.55(7), C4− Al1−N2 123.46(7), N1−Al1−N2 119.45(7). 6: Al1−N1 1.8472(11), Al1−C1 1.9850(14), Al1−C4 1.9736(14), C1−C2 1.355(2), C2−C3 1.5222(19), C3−C4 1.3552(19), Al1−O1 1.9246(10), C1−Al1−C4 91.30(6), C1−Al1−O1 102.08(5), C1−Al1−N1 126.01(6), C4−Al1− O1 101.79(5), C4−Al1−N1 128.52(6), O1−Al1−N1 102.77(5).
Scheme 4. Interconversion between 5 and 6
2.033(4) Å),11 suggesting that the N1−Li1 bonding interaction in 5 is marginal. The 7Li and 27Al NMR chemical shifts of 5 in C6D6 [δLi = −1.1 ppm, δAl = 119 ppm (Δw1/2 = ca. 1500 Hz)] were in good accordance with the calculated values for the optimized structure (δLi = 0.2 ppm, δAl = 105 ppm), indicating that the structure in solution should closely resemble that observed in the crystalline state. When the reaction of 3 with LiN(SiMe3)2 was carried out in THF instead of toluene (Scheme 3), the 1H NMR spectrum of the crude mixture suggested quantitative formation of 1aminoalumole−THF complex 6. Complex 6 was isolated as a colorless solid in 69% yield and characterized by multinuclear NMR spectroscopy and elemental analysis. The 27Al NMR spectrum of 6 exhibited a considerably broadened signal at δ = 146 ppm (Δw1/2 = ca. 8500 Hz), indicating that the aluminum center adopts a tetracoordinated structure in solution (e.g., Ph3Al(thf): δAl = 148 ppm, Δw1/2 = 3600 Hz).12 Even in the presence of an excess amount of THF, only one set of 1H NMR signals corresponding to THF molecules was observed, suggesting the facile exchange of the coordinated and free THF molecules within an NMR time scale. Attempts to remove
ature, an excess amount of THF was added to the C6D6 solution of 5. Removal of excess THF and the liberated LiN(SiMe3)2 afforded 6 quantitatively. Conversely, 5 was quantitatively obtained by the treatment of 6 with LiN(SiMe3)2 in C6D6. The DFT calculations suggest comparable stability for compounds [5 + THF] and [6 + LiN(SiMe3)2] in the gas phase,16 corroborating the facile interconversion between 5 and 6.
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CONCLUSIONS In summary, lithium 1,1-diaminoalumoluide 5 and 1-aminoalumole−THF complex 6 were synthesized by using 1bromoalumole 3 as an alumole source. Compounds 5 and 6 can be interconverted with each other by ligand exchange reactions at room temperature.
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EXPERIMENTAL SECTION
General Remarks. All the manipulations were performed under a dry argon atmosphere using standard Schlenk techniques or gloveboxes. LiN(SiMe3)2 was purchased from Aldrich Co., Ltd. and B
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used as received. Solvents were purified by the Ultimate Solvent System, Glass Contour Company,17 or by bulb-to-bulb distillation from a potassium mirror prior to use. 1H, 13C, 7Li, and 27Al NMR spectra were measured on a JEOL AL-300 spectrometer, a JEOL ECA600 spectrometer, or a Bruker Avance-600 spectrometer and referenced to SiMe4 (1H and 13C), LiBr/D2O (7Li), or AlNO3/D2O (27Al). Mass spectra were measured on a Bruker micrOTOF mass spectrometer equipped with an AMR DART-SVP ion source using He as an ionization gas. Melting points were determined on a Yanaco micro melting point apparatus and are uncorrected. Elemental analyses were carried out at the Microanalytical Laboratory, Institute for Chemical Research, Kyoto University. 1-Bromoalumole 3 was prepared according to the literature.8 Synthesis of Lithium 1,1-Diaminoalumoluide 5. To a toluene (3.0 mL) solution of 1-bromoalumole 3 (51.7 mg, 0.196 mmol) was added LiN(SiMe3)2 (64.1 mg, 0.383 mmol) at room temperature. The reaction mixture was stirred for 6 h at room temperature. The solvent was removed under reduced pressure, and the residue was dissolved in hexane and filtered. The filtrate was concentrated and stored at −35 °C to afford 5 as a colorless solid (88.9 mg, 0.171 mmol, 90%). Mp: 120 °C (dec). 1H NMR (600 MHz, C6D6, 25 °C): δ 0.20 (br, 18H Si(CH3)3), 0.46 (br, 18H, Si(CH3)3), 0.91 (t, J = 7.5 Hz, 6H, βCH2CH3), 1.31 (t, J = 7.5 Hz, 6H, α-CH2CH3), 2.21 (br, 4H, βCH2CH3), 2.48 (br, 4H, α-CH2CH3). 1H NMR (300 MHz, C6D6, 70 °C): δ 0.32 (s, 36H, Si(CH3)3), 0.92 (t, J = 7.5 Hz, 6H, β-CH2CH3), 1.26 (t, J = 7.5 Hz, 6H, α-CH2CH3), 2.25 (q, J = 7.5 Hz, 4H, βCH2CH3), 2.50 (q, J = 7.5 Hz, 4H, α-CH2CH3); 13C NMR (150 MHz, C6D6, 25 °C): δ 6.67 (Si(CH3)3), 15.26 (β-CH2CH3), 16.14 (αCH2CH3), 21.06 (β-CH2CH3), 26.99 (α-CH2CH3), 151.87 (Al−C C), 159.16 (Al−CC). 27Al NMR (156 MHz, C6D6, 25 °C): δ 119 (Δw1/2 = ca. 1500 Hz). 7Li NMR (44 MHz, C6D6, 25 °C): δ −1.1. Anal. Calcd (%) for 5: C, 55.54; H, 10.88; N, 5.40. Found: C, 55.63; H, 11.00; N, 5.33. Synthesis of 1-Aminoalumole−THF Complex 6. To a THF (3.0 mL) solution of 1-bromoalumole 3 (62.6 mg, 0.230 mmol) was added LiN(SiMe3)2 (39.4 mg, 0.235 mmol) at room temperature. The reaction mixture was stirred for 4 h at room temperature. The solvent was removed under reduced pressure, and the residue was dissolved in hexane and filtered. The filtrate was concentrated and stored at −35 °C to afford 6 as a colorless solid (67.3 mg, 0.159 mmol, 69%). Mp: 42 °C (dec). 1H NMR (600 MHz, C6D6, 25 °C): δ 0.38 (s, 18H, Si(CH3)3), 1.08−1.05 (m, 4H, OCH2CH2), 1.11 (t, J = 7.4 Hz, 6H, βCH2CH3), 1.17 (t, J = 7.4 Hz, 6H, α-CH2CH3), 2.45 (q, J = 7.4 Hz, 4H, β-CH2CH3), 2.56−2.44 (br, 4H, α-CH2CH3), 3.65−3.63 (m, 4H, OCH2CH2). 13C NMR (150 MHz, C6D6, 25 °C): δ 5.52 (Si(CH3)3), 15.76 (β-CH2CH3) 17.01 (α-CH2CH3), 21.33 (β-CH2CH3), 24.86 (OCH2CH2), 25.27 (α-CH2CH3), 70.59 (OCH2CH2), 143.02 (Al− CC), 155.64 (Al−CC). 27Al NMR (156 MHz, C6D6, 25 °C): δ 146 (Δw1/2 = ca. 8500 Hz). Anal. Calcd (%) for 6: C, 62.36; H, 10.94; N, 3.31. Found: C, 62.12; H, 11.17; N, 3.32. Reaction of Lithium 1,1-Diaminoalumoluide 5 with THF. To a C6D6 (0.5 mL) solution of 5 (10.5 mg, 0.0202 mmol) was added THF (0.1 mL, ca. 1 mmol) at room temperature. The reaction was monitored by 1H NMR spectroscopy, showing the quantitative formation of LiN(SiMe3)2. After removal of the solvents under reduced pressure, the residue was dissolved in hexane and filtered. The filtrate was concentrated to afford 1-aminoalumole−THF complex 6 (8.5 mg, 0.020 mmol, 99%). Reaction of 1-Aminoalumole−THF Complex 6 with LiN(SiMe3)2. To a C6D6 (0.5 mL) solution of 6 (15.9 mg, 0.0376 mmol) was added LiN(SiMe3)2 (7.6 mg, 0.045 mmol) at room temperature. After removal of the solvent under reduced pressure, the residue was dissolved in hexane and filtered. The filtrate was concentrated to afford a mixture of lithium 1,1-diaminoalumoluide 5 and LiN(SiMe3)2. X-ray Crystallographic Analysis. Single crystals of lithium 1,1diaminoalumoluide 5 and 1-aminoalumole−THF complex 6 suitable for X-ray crystallographic analysis were obtained by recrystallization from hexane at −35 °C. The intensity data were collected at −170 °C on a Rigaku Mercury CCD diffractometer (5) or on a Rigaku Saturn CCD diffractometer equipped with VariMax Mo Optic System (6) by
using Mo Kα X-ray radiation (λ = 0.710 75 Å). The reflection data were integrated, scaled, and averaged by using the HKL-2000 program package.18 Semiempirical absorption correction was applied using the program MULABS.19 The structures were solved by a direct method (SIR2004)20 and refined by the full-matrix least-square method on F2 for all reflections (5: SHELXL-97, 6: SHELXL-2013).21 Crystal data for 5: C24H56AlLiN2Si4, fw 518.99, colorless block, 0.35 × 0.30 × 0.17 mm3, −170 °C, orthorhombic, Pna21 (#33), a = 17.2815(8) Å, b = 9.8343(4) Å, c = 18.6058(8) Å, V = 3162.1(2) Å3, Z = 4, Dcalcd = 1.090 g cm−3, μ = 0.230 mm−1, 2.34° < θ < 25.50°, reflections collected 25 697, independent reflections 5864 (Rint = 0.0299), completeness to θmax 100.0%, data/restraints/parameters 5864/1/327, R1(I > 2σ(I)) = 0.0273, wR2(all data) = 0.0679, GOF = 1.090, absolute structure parameter −0.01(7), largest diff peak and hole 0.224 and −0.170 e Å−3. CCDC-995226. Crystal data for 6: C22H46AlNOSi2, fw 423.76, colorless block, 0.10 × 0.08 × 0.07 mm3, −170 °C, monoclinic, P21/n (#14), a = 10.6677(2) Å, b = 16.6123(3) Å, c = 15.1293(3) Å, β = 99.8430(13)°, V = 2641.67(9) Å3, Z = 4, Dcalcd = 1.065 g cm−3, μ = 0.179 mm−1, 1.84° < θ < 25.50°, reflections collected 22 717, independent reflections 4908 (Rint = 0.0267), completeness to θmax 99.8%, data/restraints/parameters 4908/0/254, R1(I > 2σ(I)) = 0.0309, wR2(all data) = 0.0811, GOF = 1.055, largest diff peak and hole 0.305 and −0.221 e Å−3. CCDC-1011984. DFT Calculations. All calculations were performed with the Gaussian 09 (revision C.01) program package22 at the DFT-B3PW91 level employing the 6-31G(d) basis set. Frequency calculations confirmed that all the optimized structures correspond to the energy minima. NMR chemical shifts were calculated by using the gaugeindependent atomic orbital (GIAO) method at the DFT-B3PW91/6311+G(2df) level of theory. SiMe4 (1H, 13C), Al2Me6 (27Al, 153 ppm),23 and [Li(OH2)4]+ (7Li) were used as the chemical shift references.
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ASSOCIATED CONTENT
S Supporting Information *
NMR spectra, a CIF file giving full crystallographic data for compounds 5 and 6, a text file of all computed molecule Cartesian coordinates in a format for convenient visualization, and complete citation for ref 22. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +81-774-38-3200. Fax: +81-774-38-3209. E-mail:
[email protected]. Web: http://boc.kuicr.kyotou.ac.jp/www/index-e.html. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Nos. 24550048, 24109013, and 26620028), MEXT Project of Integrated Research on Chemical Synthesis from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, the “Molecular Systems Research” project of RIKEN Advanced Science Institute, and the Ube Industries Award in Synthetic Organic Chemistry, Japan. T.A. is thankful to the Kyoto Technoscience Center and the Research Institute for Production Development for their financial support. Computational time was provided by the Super Computer Laboratory, Institute for Chemical Research, Kyoto University. We are grateful to Ms. K. Omine (JURC in the Institute for Chemical Research, Kyoto University) for the NMR measurements. The manuscript was written at Rheinische Friedrich-WhilhelmsUniversität Bonn during the tenure of a Alexander von C
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(18) Otwinoski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (19) Blessing, R. H. Acta Crystallogr., Sect. A 1995, A51, 33. (20) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2005, 38, 381. (21) Sheldrick, G. Acta Crystallogr., Sect. A 2008, A64, 112. (22) Frisch, M. J.; et al. Gaussian 09 (Revision C.01); Gaussian, Inc.: Wallingford, CT, 2010. Complete citation is included in the Supporting Information. (23) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH, 2006.
Humboldt Award (N.T.) and a Friedrich Wilhelm Bessel Research Award (T.S.) of two of the authors, who are grateful to the Alexander von Humboldt Stiftung for their generosity and to Prof. R. Streubel and his group for their warm hospitality.
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REFERENCES
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