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Reaction between tBuMgCl and SnCl4, Illustrating the Solvent-Dependent Predominant Formation of Cl(tBu2Sn)nCl (THF, n = 1; Toluene, n = 2; Hexane, n = 3) and the Subsequent Wavelength-Selective Photochemical Transformation of n = 3 f 2 f 1 Hemant K. Sharma, Alma Miramontes, Alejandro J. Metta-Maga~na, and Keith H. Pannell* Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968, United States
bS Supporting Information ABSTRACT: The addition of THF solutions of tBuMgCl to SnCl4 in varying solvents (S) results in a dramatic change of the predominant product obtained Cl(tBu2Sn)nCl (n = 13). For S = THF the product obtained was t Bu2SnCl2, S = toluene, benzene yields Cl(tBu2Sn)2Cl, while for S = hexane the tristannane Cl(tBu2Sn)3Cl was isolated in 45% yield. The last compound is photochemically sensitive when irradiated at 350 nm to form Cl(tBu2Sn)2Cl. Irradiation of this latter material with a 254 nm lamp results in the formation of tBu2SnCl2.
atenated group 14 compounds exhibit a σσ* delocalization along the elementelement backbone which imparts interesting electronic and optical properties with progressive red shifts in the UV/vis absorption maximum for increasing chain lengths.14 Silicon chains of 618 atoms5 and germanium analogues containing 46 atoms6 have been systematically designed, whereas the related tin chains have received less attention due to the lability of SnC and SnSn bonds (which can often lead to scrambling reactions) and a lack of simple synthetic procedures for SnSn bond formation. Saltelimination reactions have been used to synthesize chains up to 6 tin atoms;7,8 however, these reactions give side products due to cleavage of the SnSn bond, resulting in purification problems. Thermal treatment of monostannanes readily yield distannanes along with oligostannanes,9a and additionally ring-opening treatment of octa-tert-butylcyclotetrastannane (1) with I2 results in the formation of 1,4-diiodoocta-tert-butyltetrastannane.9b The Sita group designed an interesting but multistep methodology,10 and more relevant to this report, the Uhlig group reported the synthesis of 1,2-dichlorotetra-tert-butyldistannane (2) from the reaction between tBuMgCl and SnCl4 using THF as solvent (eq 1). The overall yield was 5% based upon starting SnCl4.11
C
t
THF
BuMgCl þ SnCl4 f cyclo-ðt Bu2 SnÞ4 1
þ Cl Bu2 SnSn Bu2 Cl þ t Bu2 SnCl2 t
t
2
ð1Þ
3
The limited formation of distannanes R3SnSnR3 from reactions between R3SnCl and tBuLi or tBuMgCl has been previously observed and tentatively explained via metalhalogen exchange reactions involving transient R3SnLi compounds.12 r 2011 American Chemical Society
Furthermore, there is some suggestion that increasing the solvent polarity can enhance SnSn bond formation; however, in these reports the reaction temperatures varied considerably.12b Our interest in the transition-metal-catalyzed dehydrogenative formation of SnSn bonds13 has led to a need for simple difunctional oligostannane precursors, e.g. HtBu2SnSntBu2H; thus, we have reinvestigated the Uhlig synthesis for the production of 2. We now report that a simple solvent change not only dramatically improves upon the 5% yields but can completely alter the product formation. The role of solvents in salt-elimination reactions has been studied a great deal,14 and we recently noted that for the chemistry of the transition-metal salt [(η5-Me3SiC5H4)Fe(CO)(PPh3)]Na+ changing from THF to hexane had a dramatic impact upon solution ion pairing and reaction product outcomes.15 Replacing THF as the solvent for SnCl4 with toluene in the Uhlig procedure resulted in a dramatic suppression of the formation of tBu2SnCl2 (3) and increased the isolated yield of 1,2-dichlorotetra-tert-butyldistannane (2) to >25%. There are still significant amounts of 1 (∼30%) formed; however, this material is very insoluble and precipitates out of solution, so that no separation problems are encountered. The 119 Sn NMR of the soluble crude reaction products is depicted in Figure 1a and illustrates the clean nature of the solution containing only 3 (55.2 ppm) and 2 (110.5 ppm). Replacement of toluene with benzene results in the crude product distribution noted in Figure 1b, illustrating two new lowintensity signals at 114.9 ppm and 0.8 ppm in addition to the major and minor signals at 110.5 and 55.2 ppm attributed to 2 and 3, respectively. This polar solvent clearly results in a further increase Received: July 5, 2011 Published: August 05, 2011 4501
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in yield of 2 at the expense of 3, but the two new 119Sn resonances at 114.9 and 0.8 ppm signify the formation of a new product. Decreasing the polarity of the solvent even further by use of hexane clarified the situation and resulted in the crude NMR spectrum illustrated in Figure 1c, exhibiting only the resonances associated with the minor product observed using benzene. Purification of the crude material depicted in Figure 1c via recrystallization from hexanes resulted in the isolation of 1,3-dichlorohexa-tertbutyltristannane (4) in 45% isolated yield (eq 2).16 No 2 and only trace amounts of 3 were formed in this reaction. The new results clearly demonstrate that reducing the solvent polarity enhances SnSn bond formation, a result directly opposite to that tentatively suggested by an earlier report noted above.12b t
hexane
BuMgCl þ SnCl4 f cyclo-ðt Bu2 SnÞ4 þ Clt Bu2 SnSnt Bu2 Snt Bu2 Cl
ð2Þ
4
Figure 1. 119Sn NMR spectra of the crude reaction mixture in (a) toluene, (b) benzene, and (c) hexane.
Tristannane 4 was characterized by elemental analysis, NMR spectroscopy, and single crystal X-ray diffraction.17 The 119Sn NMR spectrum of 4 exhibited the two signals noted in Figure 1c at 114.9 and 0.8 ppm assigned to the terminal (119Sn119/117Sn, 1J = 1514/1472 Hz) and central tin atoms, respectively. The low-field signal has a chemical shift value comparable to the tin signal observed for 2 at 110.5 ppm.11 The 1H NMR spectrum of 4 exhibited two singlets at 1.36 and 1.44 ppm due to the methyl protons of the tBu groups of the terminal and internal tins, respectively. In the 13C NMR spectrum two distinct resonances were observed at 31.7 and 34.8 ppm for the methyl carbons of the tert-butyl groups attached to the terminal and internal tins and resonances at 35.6 and 39.1 ppm were assigned to ipso carbons of the tert-butyl groups. The structure of 4 is illustrated in Figure 2. Selected bond lengths and angles are given in Table 1. In the unit cell the two terminal tin atoms are related by a C2 axis of rotation and all tin atoms exhibit distorted-tetrahedral geometry with the SnSnSn angle being the largest. The SnSn distance of 2.8681(6) Å is slightly longer than the SnSn distance of 2.829(1) Å observed in 211 but significantly shorter than the SnSn bond distance of 2.966(3)Å in octa-tert-butyltristannane, tBu3SnSntBu2SntBu3, due to the substitution of two tBu groups by chlorine atoms.18 Overall the SnSn bond distance is longer than the literature median value of 2.805 Å,7,19,20 and the SnCl bond distance of 2.4005(8)Å is also slightly longer than SnCl distance of 2.393(1) Å observed in 2.11 The two chlorine atoms are oriented in a gauche conformation with a torsion angle of 107°. No inter- or intramolecular interactions between Sn and Cl atoms were observed. The new tristannane 4 is stable at ambient temperature in the solid state; however, it is very susceptible to photochemical irradiation in solution. A typical irradiation using a 350 W medium-pressure Hg lamp (wavelength output range 700200 nm) to irradiate a benzene solution in a quartz tube was monitored by 119Sn NMR and illustrated a clean removal of 4 along with concomitant formation of 2, which finally also disappeared with final formation of 3 (Figure A in the Supporting Information). Independent photolysis of 2 under the same conditions led, as expected, to the formation of 3 (eq 3). hν
Clt Bu2 SnSnt Bu2 Snt Bu2 Cl f Figure 2. Molecular structure of ClSn Bu2Sn Bu2Sn Bu2Cl. The ellipsoids are drawn at the 35% probability level. t
t
t
hν
Clt Bu2 SnSnt Bu2 Cl f Clt Bu2 SnCl
ð3Þ
Table 1. Select Bond Lengths (Å), Angles (deg), and Torsion Angles (deg) for ClSntBu2SntBu2SntBu2Cla Sn(1)Sn(2)
2.8681(6)
Sn(1)C(5)
2.202(2)
Sn(2)Sn(1)#1 Sn(1)Cl(1)
2.8681(6) 2.4005(8)
Sn(2)C(9) Sn(2)C(9)#1
2.225(2) 2.225(2)
Sn(1)C(1)
2.223(2)
Cl(1)Sn(1)C(5)
100.47(7)
C(9)#1Sn(2)C(9)
113.66(14)
Cl(1)Sn(1)C(1) Cl(1)Sn(1)Sn(2)
99.64(7) 106.408(18)
C(9)#1Sn(2)Sn(1) C(9)Sn(2)Sn(1)
105.89(7) 107.47(7)
Sn(1)Sn(2)Sn(1)#1
116.71(2)
C(1)Sn(1)Sn(2)
119.35(7)
C(5)Sn(1)Sn(2)
113.50(6)
C(5)Sn(1)C(1)
114.07(9) 57.204(19)
Cl(1)Sn(1)Sn(2)Sn(1)#1 a
Symmetry transformations used to generate equivalent atoms: (#1) x, y, z + 1/2. 4502
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of the 285 nm band of 4 (105 M/L) during irradiation (Figure B), and a CIF file giving crystallographic data for 4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT This work was supported by the Welch Foundation, Houston, TX (Grant AH-0546), and the NIH-MARC U-STAR program. ’ REFERENCES
Figure 3. Photchemical irradiation of (a) 4 for ∼10 min time periods using a 350 nm lamp in a quartz cell and (b) 3 for ∼10 min time periods using a 254 nm lamp in a quartz cell.
The UV/vis spectrum of 4 in CH3OH exhibits two major absorptions at 284 (ε = 1.0 105 M1 cm1) and 223 (ε = 1.4 105 M1 cm1) nm, respectively, and that of the distannane 2 exhibits a single absorbance at 232 nm. Taking advantage of this data, if the photochemical irradiation of 4 is performed using a 350 nm wavelength lamp, only the formation of 2 is observed (Figure 3a). Changing the irradiation lamp to a 254 nm source results in the continuation of the overall process observed with the mercury lamp and the transformation 2 f 3 may be observed (Figure 3b). Continued irradiation of 3 leads, inter alia, to the formation of isobutylene, CH2dCMe2, and SnCl2. A further important point of interest with respect to the solvent variation for the reaction of SnCl4 and tBuMgCl is that the photochemical transformation 4 f 2 is much more rapid in THF than in methanol (Figure B in the Supporting Information), and indeed ordinary laboratory light can effectively perform the transformation in Pyrex glassware, thus presenting a secondary rationale for the absence of 4 in the THF reaction products; however, in a single experiment we could not observe the formation of 4 when the reaction between tBuMgCl and SnCl4 (in only THF) was performed in the dark. Studies on the photochemistry of Cl(tBu2Sn)nCl and related compounds, including trapping of the expected stannylene, [tBu2Sn], are continuing.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text giving experimental procedures, figures detailing the photolysis of 4 in a quartz tube monitored by 119Sn NMR spectra (Figure A) and disappearance
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M.; Rosenblum, M. J. Organomet. Chem. 1977, 136, C23. (d) Chen, Y.-S.; Ellis, J. J. Am. Chem. Soc. 1982, 104, 1141. (15) Munguia, T.; Bakir, Z. A.; Cervantes-Lee, F.; Metta-Maga~ na, A.; Pannell, K. H. Organometallics 2009, 28, 5777. (16) For the synthesis and spectral data of 4, see the Supporting Information. This unreported material has also been noted by the Uhlig group in Graz using a very different chemical route, and we thank Professor Uhlig for this information. (17) X-ray single crystal diffraction data of 4 was collected with APEX2 on a Bruker APEX CCD diffractometer (Mo KR, λ = 0.710 73 Å). Crystal data for 4: C24H54Cl2Sn3, Mr = 769.64, space group C2/c, monoclinic, a = 22.234(6) Å, b = 9.352(2) Å, c = 17.041(4) Å, R = 90°, β = 112.376(3)°, γ = 90°, V = 3276.7(14) Å3, T = 296(2) K, Z = 4, 4766 unique reflections collected (Rint = 0.0367), 100% completeness to 2θmax = 60°, R1 (I > 2σ(I)) = 0.0212. CCDC 817128 contains supplementary crystallographic data for 4. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (18) Puff, H.; Breuer, B.; Gehrke-Brinkmann, G.; Kind, P.; Reuter, H.; Schuh, W.; Wald, W.; Weidenbr€uck, G. J. Organomet. Chem. 1986, 363, 265. (19) (a) Braunschweig, H.; D€orfler, R.; Mager, J.; Radacki, K.; Seeler, F. J. Organomet. Chem. 2009, 694, 1134. (b) Bera, H.; Braunschweig, H.; D€orfler, R.; Hammond, K.; Oechsner, A.; Radacki, K.; Uttinger, K. Chem. Eur. J. 2009, 15, 12092. (c) Braunschweig, H.; D€orfler, R.; Gruss, K.; K€ohler, J.; Radacki, K. Organometallics 2011, 30, 305. (d) Sharma, H. K. ; Metta-Maga~na, A. J.; Pannell, K. H.; Uhlig, F.; Zarl, E. Manuscript in preparation. (20) Ruhlandt-Senge, K.; Uhlig, F. J. Organomet. Chem. 2000, 613, 139.
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