Organometallics 2010, 29, 5725–5727 DOI: 10.1021/om100386v
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Synthesis and Structure of a Heptacoordinate Trichlorostannane Bearing a Triarylmethyl-Type Tetradentate Ligand† Junji Kobayashi,* Kohei Iwanaga, and Takayuki Kawashima*,‡ Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. ‡ Present address: Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan. Received April 30, 2010 Summary: Trichloro[tris(3-tert-butyl-6-methoxyphenyl)methyl]stannane (1), containing a tetradentate ligand with three methoxy groups as coordinating sites, was synthesized by the reaction of the corresponding (triarylmethyl)lithium species with tin tetrachloride. In contrast to the cases of the previously reported silicon and germanium analogues, 1 decomposed during column chromatography on silica gel and was slightly unstable against moisture. X-ray crystallographic analysis of 1 showed a heptacoordinate structure with interatomic distances between the oxygen atom and the central tin atom shorter than the sum of van der Waals radii.
Figure 1. Heptacoordinate group 14 element compounds. Scheme 1. Synthetic Scheme of Trichlorostannane 1
Introduction Hypercoordinate main group element compounds are well studied because of their interesting structures and reactivities and also as models of the transition state or the intermediate of an SN2-type reaction.1 Among group 14 element compounds, penta- and hexacoordinate silicon compounds have been intensively and widely studied from a viewpoint of usefulness of their synthetic applications,2 while germanium and tin derivatives are rather less studied.3,4 In contrast to penta- and hexacoordinate compounds, heptacoordinate compounds are more rare. Although there have been several reports of the synthesis of neutral heptacoordinate compounds with four covalent bonds and three dative bonds,5-7 the systematic comparisons of these structural properties have been scant so far. We have reported the synthesis of a novel type of heptacoordinate trichlorosilane (2)5 and trichlorogermane (3),6 † Part of the Dietmar Seyferth Festschrift. *To whom correspondence should be addressed. E-mail: jkoba@ chem.s.u-tokyo.ac.jp;
[email protected]. (1) Chemistry of Hypervalent Compounds; Akiba, K.-y., Ed.; John Wiley & Sons: New York, 1999. (2) Kost, D.; Kalikhman, I. In The Chemistry of Organic Silicon Compounds; Wiley: New York, 1998; Vol. 2, p 1339. (3) (a) Breliere, C.; Carre, F.; Corriu, R. J. P.; Royo, G. Organometallics 1988, 7, 1006. (b) Breliere, C.; Carre, F.; Corriu, R. J. P.; Royo, G.; Man, M. W. C. Organometallics 1994, 13, 307. (c) Carre, F.; Chuit, C.; Corriu, R. J. P.; Fanta, A.; Mehdi, A.; Reye, C. Organometallics 1995, 14, 194. (d) Kano, N.; Nakagawa, N.; Kawashima, T. Angew. Chem., Int. Ed. 2001, 40, 3450. (4) (a) Kawachi, A.; Tanaka, Y.; Tamao, K. Organometallics 1997, 16, 5102. (b) Dostal, S.; Stoudt, S. J.; Fanwick, P.; Sereatan, W. F.; Kahr, B.; Jackson, J. E. Organometallics 1993, 12, 2284. (5) Kobayashi, J.; Ishida, K.; Kawashima, T. Silicon Chem. 2002, 1, 351. (6) (a) Iwanaga, K.; Kobayashi, J.; Kawashima, T.; Takagi, N.; Nagase, S. Organometallics 2006, 25, 3388. (b) Iwanaga, K.; Kobayashi, J.; Kawashima, T. Tetrahedron 2007, 63, 10127. (7) Takeuchi, Y.; Takase, Y. J. Organomet. Chem. 2004, 689, 3275.
r 2010 American Chemical Society
with a tetradentate ligand based on a tris(3-tert-butyl6-methoxyphenyl)methyl unit (Figure 1), and found that 2 is unusually stable to nucleophilic substitutions.5 Despite several reports on syntheses of heptacoordinate compounds bearing an intramolecularly coordinating tetradentate ligand,5-7 there have been few systematic studies on group 14 element compounds upon changing a central element using the same ligand. We report here the synthesis of a heptacoordinate trichlorostannane bearing a tris(3-tert-butyl-6-methoxyphenyl)methyl group, and we compare its structure and reactivities with those of silicon and germanium analogues systematically.
Results and Discussion In a similar way previously reported by us for trichlorosilane 2 and trichlorogermane 3, trichlorostannane 1 was synthesized via the lithiation of tris(3-tert-butyl-6-methoxyphenyl)methane (4) with an excess amount of n-butyllithium and subsequent treatment of the resulting lithium reagent (5) with SnCl4 (Scheme 1). Improvement of the yield of 1 was achieved when 1,2-dimethoxyethane (DME) was used as a solvent in the presence of hexamethylphosphoric triamide Published on Web 07/29/2010
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Table 1. Selected Crystallographic Data and Collection Parameters for 1 3 (1/3CHCl3, 2/3n-hexane) formula fw cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚ Z dcalcd, Mg 3 m-3 absorp. coeff, mm-1 θ range reflns collected GOF R1[I>2σ(I)] wR2 (all data)
C36.35H50.03Cl4.02O3Sn 796.19 triclinic P1 10.755(5) 12.842(5) 15.346(6) 90.813(4) 98.633(6) 110.933(5) 1951.9(14) 2 1.355 0.961 3.08 to 25.00 11 590 1.109 0.0291 0.0697
(HMPA), which is well known to activate the lithium reagent by solvation to the lithium cation. Once isolated, 1 was relatively stable, in marked contrast to the usual trichlorostannanes, and could be stored in the air. However, 1 was slightly less stable than 3 and much more unstable than 2, as 1 decomposed during column chromatography on silica gel. The lesser stability of 1 resulted in a very low isolated yield (3%) despite the moderate yield of the crude product estimated by 1H NMR (62%). Such instability compared with trichlorosilane 2 seems to be caused by the relatively elongated bond lengths between the tin and the carbon atom and less hindrance around the central tin atom. Single crystals of 1 suitable for X-ray analysis were obtained by slow evaporation from a benzene/CHCl3 solution. The crystal data for 1 are summarized in Table 1, and an ORTEP drawing with selected bond lengths and angles is shown in Figure 2.8 1 has an approximate C3 symmetrical propeller-like structure, as had been found in the previously reported trichlorosilane 2 and trichlorogermane 3. The oxygen ligands adopt anti-positions to the chlorine atoms (O 3 3 3 Sn-Cl angles are 170-178°), while the three aryl rings have similar twists out of their common coordination plane with the tin atom. The interatomic distances between the oxygen atom and the tin atom are shorter than the sum of van der Waals radii,9 indicating the existence of an interatomic interaction between the oxygen atoms and the tin atom. These structural properties indicate that the tin atom of 1 has a heptacoordinate structure with “tricapped tetrahedral” geometry, by the coordination of three oxygen atoms from the opposite side of the tin-chlorine bonds, the same as in the cases of 2 and 3. A systematic tendency of these heptacoordinate species 1-3 was revealed by the comparison of the structure in detail. Selected interatomic distances, bond angles, and torsion angles are listed in Table 2. The ratio of O 3 3 3 E (E indicates group 14 element) distances to the sum of van der Waals radii gradually decreases as Lewis acidity of the central atom increases:10 1 (2.672 A˚; 75%) < 3 (2.769 A˚; 81%) < 2 (2.802 A˚; 83%). The average X-E-X angles also (8) One of the tert-butyl groups is disordered, and the unit cell contains solvent molecules (CHCl3/n-hexane, 2:1). (9) For the van der Waals radii of Ge: Bondi, A. J. Phys. Chem. 1964, 68, 441. (10) For the Lewis acidity of group 14 compounds: Huggett, P. G.; Manning, K.; Wade, K. J. Inorg. Nucl. Chem. 1980, 42, 665.
Figure 2. ORTEP drawing of trichlorostannane 1 (50% probability). Hydrogen atoms, disordered part of tert-butyl group, and solvent molecules are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Sn(1)-Cl(1) 2.347(1), Sn(1)Cl(2) 2.354 (1), Sn(1)-Cl(3) 2.364(1), Sn(1)-C(1) 2.215(3), Cl(1)-Sn(1)-Cl(2) 98.44(3), Cl(1)-Sn(1)-Cl(3) 99.12(4), Cl(2)Sn(1)-Cl(3) 96.97(3). Table 2. Selected Interatomic Distances, Bond Angles, and Torsion Angles of 1, 2, and 3a
1 2d 3e
E
rW/ A˚b
O 3 3 3 E/ A˚c
Cl-ECl/deg
O 3 3 3 ECl/deg
E-C(1)-C(ipso)C(OMe)/deg
Sn Si Ge
3.58 3.39 3.40
2.68 (75) f 2.80 (83)g 2.77(81)h
98.2 102.9 101.3
173 177 176
45.6 53.7 51.5
a Values are the average of the symmetric parts of the compounds. The sum of van der Waals radii of O 3 3 3 E. c Values in parentheses represent (O 3 3 3 E)/(sum of VDW radii) in percent. d Ref 5. e Ref 6. f Sn(1) 3 3 3 O(1), 2.693; Sn(1) 3 3 3 O(2), 2.679; Sn(1) 3 3 3 O(3), 2.644 A˚. g Si 3 3 3 O, 2.819, 2.843, 2.746 A˚. h Ge 3 3 3 O, 2.790, 2.723, 2.792 A˚. b
widen as 1 (98.2°) < 3 (101.3°) < 2 (102.9°), and the average torsion angle or E-C(1)-C(ipso)-C(OMe) also increases: 1 (45.6°) < 3 (51.5°) < 2 (53.7°). Since the NMR chemical shift of hypercoordinate atom nuclei is well known to shift upfield, NMR spectroscopy is known as a powerful tool to estimate the degree of hypercoordination in solution. The 119Sn NMR chemical shift of 1 is δSn -273, which is comparable to that of the heptacoordinate trichlorostannane reported by Kahr et al.4b (δSn -344) and upfield shifted compared to those of tetracoordinate trichlorostannanes, δSn 100 to -100.11 Although trichlorosilane 2 was quite stable and any derivatization except for the halogen exchange to a fluorine atom could not be performed,5 trichlorogermane 3 could be converted to trifluoro-, tribromo-, triiodo-, and trihydrogermanes by the reaction with AgF, BBr3, TMSBr, TMSI, and LiAlH4, respectively.6 While derivatization from trichlorostannane 1 could not be examined properly because of the low isolated yield and instability of 1, the reaction of 1 with iodotrimethylsilane was attempted, but it resulted in decomposition of the product. Judging from these results, 1 has enough reactivity similar to trichlorogermane 3,6 but the (11) Wrackmeyer, B. Ann. Rep. NMR 1999, 38, 203.
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higher instability of the tin analogue resulted in failure in the isolation of products. In summary, a heptacoordinate trichlorostannane bearing a triarylmethyl-type tetradentate ligand was synthesized. X-ray crystallographic analysis revealed that the trichlorostannane has a heptacoordinate structure with “tricapped tetrahedral” geometry. Systematic correlation was observed between the ratio of O 3 3 3 E interatomic distances to the sum of van der Waals radii and the Lewis acidity of the central atom. This tendency indicates the existence of the interatomic interactions in the crystalline state.
Experimental Section General Procedures. All solvents used in the reactions were purified by the reported methods or by MBRAUN MB SPS-800 purification equipment. All reactions were carried out under an argon atmosphere unless otherwise noted. 1H NMR (400 MHz), 13 C NMR (126 MHz), and 119Sn NMR (187 MHz) were measured in CDCl3 with a JEOL AL-400, Bruker DRX-500, or JEOL ECA-500 spectrometer using residual nondeuterated solvent as an internal standard for 1H NMR, deuterated solvent as an internal standard for 13C NMR, and tetramethyltin as an external standard for 119Sn NMR. Fast atom bombardment mass spectrometry (FAB-MS) was done with a JEOL JMX-SX 102 mass spectrometer. Melting points were determined on a Yanaco micro melting point apparatus. All melting points were uncorrected. Synthesis of Trichloro[tris(3-tert-butyl-6-methoxyphenyl) methyl]stannane, 1. To a solution of tris(3-tert-butyl-6-methoxyphenyl)methane (4) (2.00 g, 3.98 mmol) in benzene (100 mL) was added n-BuLi (1.67 M hexane solution, 20 mmol) at 50 °C. The mixture was stirred at 50 °C for 19 h. After removal of the solvent under reduced pressure, DME (50 mL) and HMPA
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(2.50 mL, 14.4 mmol) were added to the residue. To the mixture was added tin tetrachloride (4.6 mL, 40 mmol) at 0 °C. The mixture was stirred at room temperature for 17 h. After removal of the solvent, the residue was treated with 100 mL of EtOH and then 50 mL of H2O. The mixture was extracted with CHCl3. After removal of the solvent under reduced pressure, the residue was dissolved in ether and washed with 2% aqueous NaOH. The extracts were combined and dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. The residue was subjected to column chromatography on silica gel. Elution with hexane/AcOEt (15:1) gave 1, which was further purified by recrystallization from hexane to afford pure product (334 mg, 3%). 1: colorless crystal, mp 254.4-254.7 °C (dec). 1H NMR (400 MHz, CDCl3, 27 °C): δ 1.11 (s, 27H), 3.76 (s, 9H), 6.39 (d, 3H, J = 2.4 Hz, 4JHSn = 28, 32 Hz), 6.86 (d, 3H, J = 8.8 Hz), 7.33 (dd, 3H, J = 2.4, 8.8 Hz). 13C NMR (126 MHz, CDCl3, 27 °C): δ 31.2 (q), 34.0 (s), 54.7 (q), 65.6 (s, 1JCSn = 901, 943 Hz), 109.2 (d), 125.3 (d), 126.8 (d, JCSn = 94 Hz), 127.8 (s, JCSn = 66 Hz), 143.8 (s, JCSn = 18 Hz), 152.5 (s, JCSn = 61 Hz). 119Sn NMR (187 MHz, CDCl3, 27 °C): δ -273.7. HRMS-FAB (m/z): [M - Me]+ calcd for C33H42Cl3O3Sn 711.1222, found 711.1196.
Acknowledgment. This study was supported by a Grant- in-Aid for The 21st Century COE Program for Frontiers in Fundamental Chemistry and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We also thank Tosoh Finechem Co., Ltd., for the generous gifts of alkyllithiums. Supporting Information Available: 1H NMR spectra and a CIF file giving crystallographic data for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
