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Organometallics 2010, 29, 3862–3867 DOI: 10.1021/om100545b
Diorganostannide Dianions (R2Sn2-) as Reaction Intermediates Revisited: In Situ 119Sn NMR Studies in Liquid Ammonia Markus Trummer and Walter Caseri* Department of Materials, Eidgen€ ossische Technische Hochschule (ETH) Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland Received June 3, 2010
It has frequently been proposed that diorganostannide dianions, SnR22-, form during reactions of dihalodiorganostannanes or dihydrodiorganostannanes with sodium in liquid ammonia. The formation of this intermediate has been advanced to be an important step in the synthesis of a wide range of organostannanes. Here we report 119Sn NMR investigations in liquid NH3 of reaction intermediates that formed in situ during the conversions of dichlorodiphenylstannane, dihydrodiphenylstannane (diphenylstannane), dideuterodiphenylstannane, and dichlorodibutylstannane, respectively. This study revealed that the proposed SnR22- dianion was not present, but tetraorganodistannides, (R2Sn-SnR2)2-, and hydrodiorganostannides (tin hydrides), R2SnH- (or R2SnD-, respectively), were detected instead.
Introduction The in situ formation of organotin anions in liquid ammonia is a key step in the synthesis of a wide spectrum of organic tin compounds.1-7 In this context, diorganostannides R2Sn2have been advanced as intermediates for different organometallic syntheses, including deuteration of carbonyl compounds,8 preparation of asymmetric stannanes,9,10 and formation of polymers.11 However, conclusive spectroscopic evidence for the formation of diorganostannide dianions is ossbauer spectroscopy12 not available; for instance 119Sn-M€ did not lead to identification of specific compounds. Driven by our interest in the synthesis of macromolecular poly*Corresponding author. E-mail:
[email protected] (1) Chopa, A. B.; Lockhart, M. T.; Dorn, V. B. Organometallics 2002, 21, 1425–1429. (2) Dorn, V. B.; Badajoz, M. A.; Lockhart, M. T.; Chopa, A. B.; Pierini, A. B. J. Organomet. Chem. 2008, 693, 2458–2462. (3) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organometallics 2001, 20, 3358–3360. (4) Beermann, C.; Hartmann, C. Z. Anorg. Allg. Chem. 1954, 276, 20– 32. (5) Brown, T. L.; Morgan, G. L. Inorg. Chem. 1963, 2, 736–740. (6) Ma-kosza, M.; Grela, K. Synth. Commun. 1998, 28, 2697–2702. (7) Kraus, C. A.; Neal, A. M. J. Am. Chem. Soc. 1930, 52, 4426–4433. (8) K€ uhlein, K.; Neumann, W. P.; Mohring, H. Angew. Chem., Int. Ed. Engl. 1968, 7, 455–456. (9) Bullard, R. H.; Holden, F. R. J. Am. Chem. Soc. 1931, 53, 3150– 3153. (10) Ingham, R. K.; Rosenberg, S. D.; Gilman, H. Chem. Rev. 1960, 60, 459–539. (11) Ingham, R. K.; Gilman, H. Inorganic Polymers; Academic Press: New York, 1962. (12) Birchall, T.; Vetrone, J. Hyperfine Interact. 1988, 40, 291–294. (13) Choffat, F.; Smith, P.; Caseri, W. Adv. Mater. 2008, 20, 2225– 2229. (14) Choffat, F.; Wolfer, P.; Smith, P.; Caseri, W. Macromol. Mater. Eng. 2010, 295, 210–221. (15) de Haas, M. P.; Choffat, F.; Caseri, W.; Smith, P.; Warman, J. M. Adv. Mater. 2006, 18, 44–47. (16) Choffat, F.; Buchm€ uller, Y.; Mensing, C.; Smith, P.; Caseri, W. J. Inorg. Organomet. Polym. Mater. 2009, 19, 166–175. pubs.acs.org/Organometallics
Published on Web 08/19/2010
stannanes,13-19 we revisited the purported existence of diorganostannide dianions. Diorganostannides are commonly generated in situ by reaction of dihalodiorganostannanes with sodium in liquid ammonia; the intermediate products are further converted for synthetic purposes to tetraorganostannanes according to Scheme 1. Early reports from Kraus et al.20,21 propose the formation of trimethylstannide, SnMe3-, and dimethylstannide dianion, SnMe22-, by reaction of sodium with bromotrimethylstannane, Me3SnBr, or dibromodimethylstannane, Me2SnBr2, respectively. Depending on the applied ratio between the dibromodimethylstannane and sodium, also oligomeric stannides of the type (Me2Sn)x2- were postulated. Similar conclusions were drawn for the formation of monostannides and distannides upon conversion of the corresponding phenyl22 and ethyl halostannanes.23 More recently, synthesis of substituted diaryldimethylstannanes was proposed to proceed via the intermediate Me2Sn2-, but also formation of the dimeric species (Me2Sn)22was anticipated, since the dimer (PhMe2Sn)2 emerged among the reaction products.24 In-situ-formed tetraorganodistannides were also suggested to arise on the basis of crystallization of salts comprising the dimeric dianion (Ph2Sn)22- from lithium-treated dichlorodiphenylstannane in liquid ammonia in the presence of (17) Choffat, F.; Fornera, S.; Smith, P.; Caseri, W. R.; Breiby, D., W.; Andreasen, J. W.; Nielsen, M., M. Adv. Funct. Mater. 2008, 18, 2301– 2308. (18) Choffat, F.; Schmid, D.; Caseri, W.; Wolfer, P.; Smith, P. Macromolecules 2007, 40, 7878–7889. (19) Choffat, F.; Smith, P.; Caseri, W. J. Mater. Chem. 2005, 15, 1789–1792. (20) Kraus, C. A.; Sessions, W. V. J. Am. Chem. Soc. 1925, 47, 2361– 2368. (21) Kraus, C. A.; Greer, W. N. J. Am. Chem. Soc. 1925, 47, 2568– 2575. (22) Chambers, R. F.; Scherer, P. C. J. Am. Chem. Soc. 1926, 48, 1054–1062. (23) Harada, T. Sci. Papers Inst. Phys. Chem. Res. 1939, 35, 302–313. (24) Uberman, P. M.; Martin, S. E.; Rossi, R. A. J. Org. Chem. 2005, 70, 9063–9066. r 2010 American Chemical Society
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Scheme 1. Formation of the Previously Proposed Diorganostannide Dianions by Conversion of Diorganostannanes with Sodium in Liquid Ammonia and Subsequent Reaction with Haloalkanesa 8-10,21-24,30
a
Y = Cl, Br, H; X = Cl, Br, I.
tetraammine lithium25 or (18-crown-6)diammine potassium26 counterions. The dianion SnMe22- is believed to be formed also by reaction of dihydrodimethylstannane (dimethyltin dihydride, dimethylstannane), Me2SnH2, with sodium in liquid ammonia. Conductivity measurements27-29 and conductometric titration of the sodium with dihydrodimethylstannane30 (and also stannane, SnH431) were interpreted to indicate the formation of ionic products. A conductivity minimum was found at a Me2SnH2: Na ratio of ca. 0.5. Up to this ratio, development of one molar equivalent of hydrogen gas per dimethylstannane was reported, which might at first glance point to the formation of dimethylstannide dianions by replacement of the hydrogen atoms. At larger Me2SnH2:Na ratios, however, less than one equivalent of hydrogen per dimethylstannane evolved, and it was assumed; but not proven;that dimethylstannide reacted with dimethylstannane also to form tetramethyldistannide, (SnMe2)22-, or hydrodimethylstannide, Me2SnH-, respectively. In order to bring to light the nature of the intermediate form of stannides resulting from the reaction of dihalodiorganostannides or dihydrodiorganostannides with sodium, we employed 119Sn NMR spectroscopy in liquid ammonia to identify the in-situ-formed species ;previously claimed to be R2Sn2-;and examined their reactivity.
Figure 1. Electric conductivity of reaction mixtures upon addition of dichlorodiphenylstannane, Ph2SnCl2 (0), or dichlorodibutylstannane, Bu2SnCl2 (9), to a solution of sodium in liquid ammonia at 195 K (t = 0 indicates the moment of stannane addition; conductivity levels are normalized for comparison to 100% at t = 0). Inset: Schematic illustration of the experimental setup of the in situ conductivity measurements.
Results and Discussion In Situ Electric Conductivity. Reactions were carried out by treatment of dichlorodibutylstannane, Bu2SnCl2, and dichlorodiphenylstannane, Ph2SnCl2, with four molar equivalents of sodium in liquid ammonia. This stoichiometric ratio is just required for the formation of the hypothetical SnR22- under release of two equivalents of NaCl as byproduct (upon application of two molar equivalents of sodium, yellow precipitates emerged, apparently oligostannanes or polystannanes). In order to determine the time needed for completion of the reactions, its course was monitored in situ by recording the electric conductivities of the reacting solutions (Figure 1). The conductivity reached a plateau value within 30 min during the conversion of both stannanes with sodium, implying that the reaction had terminated. The conductivities at the plateau value corresponded to 30% and 20% of the initial value for dichlorodiphenylstannane and dichlorodibutylstannane, respectively. Qualitatively, a pronounced decrease in conductivity is expected indeed, as highly mobile and conductive solvated electrons, generated upon (25) Scotti, N.; Zachwieja, U.; Jacobs, H. Z. Anorg. Allg. Chem. 1997, 623, 1503–1505. (26) Wiesler, K.; Suchentrunk, C.; Korber, N. Helv. Chim. Acta 2006, 89, 1158–1168. (27) Kraus, C. A.; Kahler, W. H. J. Am. Chem. Soc. 1933, 55, 3537–3542. (28) Kraus, C. A.; Johnson, E. G. J. Am. Chem. Soc. 1933, 55, 3542–3547. (29) Kraus, C. A.; Bien, P. B. J. Am. Chem. Soc. 1933, 55, 3609–3614. (30) Kettle, S. F. A. J. Chem. Soc. 1959, 2936–2941. (31) Emeleus, H. J.; Kettle, S. F. A. J. Chem. Soc. 1958, 2444–2448. (32) Kraus, C. A. J. Am. Chem. Soc. 1921, 43, 749–770.
Figure 2. 119Sn NMR spectra recorded in situ in liquid ammonia (at 200 K) of reaction products of (a) Ph2SnCl2, (b) Ph2SnH2, and (c) Bu2SnCl2 formed during treatment with four molar equivalents of sodium.
dissolution of sodium in liquid ammonia,32 are consumed by the reaction with the stannanes to form negatively charged stannides. In Situ 119Sn NMR Spectroscopy. In order to characterize the in-situ-formed stannides, the solutions in liquid ammonia were transferred to NMR tubes and 119Sn NMR spectra were recorded at 200 and 220 K. The higher of these temperatures resulted in a significantly better resolution of 119Sn-1H and 119 Sn-2D couplings, but was relatively close to the boiling temperature of ammonia; therefore a temperature of 200 K was adopted for measurements that were unrelated to the aforementioned couplings. In particular at very long measurement times, often additional signals emerged, probably because slow diffusion of water or oxygen from the atmosphere through the cap of the NMR tube lead to subsequent reactions (Figure S1 in the Supporting Information). Reaction of Dichlorodiphenylstannane and Dihydrodiphenylstannane with Na in Liquid Ammonia. 119Sn NMR spectra of dichlorodiphenylstannane treated with sodium in liquid ammonia featured two strong signals at -132 and -197 ppm when measured at 200 K (Figure 2a), where the chemical shifts showed a pronounced temperature dependence (Δδ = 0.2 to 0.8 ppm K-1; Table 1, Figure S2 of the Supporting
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Table 1. Chemical Shifts (δ), Coupling Constants (J) and Full-Widths at Half-Maximum (fwhm) in Proton-Decoupled and Proton-Coupled 119 Sn NMR Spectra of Different Diorganostannanes Exposed to Four Molar Equivalents of Sodium in Liquid Ammonia proton decoupled measurement temp, K Ph2SnCl2
200 220
Ph2SnH2
200 220
Ph2SnD2
220
Bu2SnCl2
200
220
a
δ119Sn, ppm -131.7 -197.2 -115.9 -192.7 -132.2 -197.2 -116.1 -192.8 -116.4 -192.9 -196.8 -136.3 -161.1 -212.0 -228.0 -136.7 -143.0 -207.9 -219.5
fwhm, Hz s s s s s s s s s s m s s s s s s s s
100 58 38 30 102 75 44 28 18 9 9 92 120 57 118 67 75 60 120
proton coupled 1
δ119Sn, ppm
J, Hz
2027
(Sn,Sn)
2029
(Sn,Sn)
2023
(Sn,Sn)
24
(Sn,D)
fwhm, Hz
-131.7 -197.1 -116.2 -191.8 -131.5 -197.1
s d s d s d
124 111 45 35 130 90
-116.5 -192.9 -196.8 -136.3 -161.1 -212.0 -228.0 -137.4 -141.6 -208.1 -218.6
s d m s s m m s s m m
26 22 26 104 125 155 220 80 86 144 195
1
J, Hz
147 2018 145
(Sn,H) (Sn,Sn) (Sn,H)
148
(Sn,H)
2023 144 23
(Sn,Sn) (Sn,H)a (Sn,D)b
96
(Sn,H)b
b
Hydrostannide formed by D-H exchange (see text). Determined by peak deconvolution.
Information). Notably, the same signals were found when dihydrodiphenylstannane, Ph2SnH2, was exposed to four molar equivalents of sodium (Figure 2b), i.e., under conditions where the formation of diorganostannide dianions was expected,30 indicating that in the cases of dichlorodiphenylstannane and dihydrodiphenylstannane the same products are formed, although possibly via different processes. The signal at -132 ppm detected at 200 K (-116 ppm at 220 K) is accompanied by those of satellites of tin (totally ∼8% of the integrated intensity of the main signal, corresponding to the natural abundance of 117Sn) with a coupling constant 1 119 J( Sn,117Sn) = 2020-2030 Hz (at 220 K) (Figure 3a). This indicates the presence of a binuclear species in the solution, with the coupling constant being in the common range of that of distannanes.33 Hence, we attribute this signal;which remained unaffected in hydrogen-coupled 119Sn NMR spectra (Figure 3a);to tetraphenyldistannide, (Ph2Sn)22-.34 Remarkably, the signal at -193 ppm (measured at 220 K) split into a well-defined doublet in the hydrogen-coupled spectra, indicating the presence of a monohydrostannide (Figure 3a; the coupling typically poorly resolved at 200 K, cf. Figure S3 of the Supporting Information); the coupling constant 1J(Sn,H) = 145 Hz is in good agreement with the coupling constant for hydrostannides reported earlier.35 Therefore, we assign this signal to hydrodiphenylstannide, Ph2SnH-. Obviously, we failed to detect any evidence for the presence of diphenylstannide, Ph2Sn2-, in surprising variance with the literature. Reaction of Dideuterodiphenylstannane with Na in Liquid Ammonia. In order to elucidate if ammonia is the respective hydrogen source for the hydrodiphenylstannide resulting from conversion of dihydrodiphenylstannane, or if the corresponding hydrogen atom remained from the starting compound, dideuterodiphenylstannane, Ph2SnD2, was exposed (33) Wrackmeyer, B.; Webb, G. A. Annu. Rep. NMR Spectrosc. 1985, 16, 73–186. (34) NB: uncharged phenyl- or butylstannanes were not soluble in liquid ammonia. (35) Wasylishen, R. E.; Burford, N. J. Chem. Soc., Chem. Commun. 1987, 1414–1415.
Figure 3. (a) Proton-coupled 119Sn NMR spectrum of Ph2SnCl2/Na in liquid ammonia (at 220 K). The signal at -116 ppm shows a 119Sn-117Sn coupling with a coupling constant of 2018 Hz, and the signal at -192 ppm a 119Sn-1H coupling with a coupling constant of 145 Hz. (b) Proton-coupled 119 Sn NMR spectrum showing the hydride region of Ph2SnD2/ Na in liquid ammonia after 60 min (cf. also Figure S4 of the Supporting Information) and (c) the corresponding protondecoupled spectrum.
Article
to four equivalents of sodium in liquid ammonia. Water and other highly active hydrogen-donating impurities can be most likely excluded as hydrogen source, since they are expected to react rapidly with sodium in liquid ammonia before the addition of the stannanes. After exposure of dideuterodiphenylstannane to four equivalents of sodium in liquid ammonia for 60 min, the 119Sn NMR spectrum showed, as expected, on one hand the signal at -116 ppm of tetraphenyldistannide at the measurement temperature of 220 K (see above) and, on the other hand, two signals in the hydride region (Figure 3c). A small signal, which appeared as a singlet at -192.9 ppm in the proton-broadband-decoupled spectrum, split into a doublet in the proton-coupled spectrum (Figure 3b; a series of additional spectra recorded at reaction times between 30 min and 10 h are displayed in Figures S4 and S5 of the Supporting Information), which is indicative of the abovementioned hydrodiphenylstannide. Further, a pronounced threeline feature with 1J(Sn,D) = 25 Hz at -196.8 ppm (center peak) is indicative of the presence of deuterodiphenylstannide, Ph2SnD- (NB: the three signals in the proton-coupled spectra are poorly resolved due to additional couplings with the phenyl protons). The intensity of the Ph2SnH- signal observed after one hour is by far too strong to be due to residual hydrogen atoms in the starting compound Ph2SnD2 (as evident from analysis of 1H and 119Sn NMR spectra of Ph2SnD2 in organic solvents). Subsequently, the ratio between hydrodiphenylstannide and deuterodiphenylstannide increased steadily during a period of more than one week, indicating that further H-D exchange occurred very slowly (∼25% exchange after 12 h at 220 K and 45% after one week, monitored directly in the NMR test tube). It appears, therefore, that at least two processes contribute to the H-D exchange: one largely advancing within one hour or less, probably along the reaction path from Ph2SnD2 to Ph2SnD-, and another one lasting for days or weeks. Since Ph2SnD2 itself is essentially insoluble in liquid ammonia (it dissolves only upon treatment with sodium), it seems unlikely that the faster of the two processes is due to a H-D exchange of Ph2SnD2 with the solvent (ammonia). Besides, it is worth noting that the percentage of tetraphenyldistannide compared to the sum of the two monostannides remained constant over time, within experimental precision. Since the hydrogen atoms of hydrodiphenylstannide extracted from dichlorodiphenylstannane or dihydrodiphenylstannane at least partially stem from different sources, hydrodiphenylstannide is formed by different processes. This might be due to the higher reactivity of sodium to Sn-Cl than Sn-H groups at initial stages of the reaction. Dichlorodiphenylstannane may initially lose both chlorine atoms and quickly react with hydrogen atoms from the solvent (liquid NH3) to form the hydrodiphenylstannide. Dihydrodiphenylstannane initially loses preferentially only one hydrogen atom, while the other remains bound to the tin atom for a long time. The exchange of the remaining hydrogen (deuterium) atom with hydrogen atoms from the solvent is a second reaction step proceeding at a different time scale and with complex reaction order. The initial reactions that lead to the distannide and the hydrostannide (or deuterostannide) are completed within less than 60 min, whereas the second step takes days. Reaction of Dichlorodibutylstannane with Na in Liquid Ammonia. 119Sn NMR spectra of dichlorodibutylstannane exposed to sodium in liquid ammonia were more complex than those of the diphenylstannanes. In the case of dichlorodibutylstannane, four major signals emerged (Figure 2c,
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additional spectra in Figure S6 in the Supporting Information), i.e., two additional signals featured when compared to those resulting from experiments with dichlorodiphenylstannane (occasionally, a minor signal at 27 ppm was also present, probably as a result of a reaction with traces of oxygen). The two main signals (with respect to the integrated intensities) were linked to the binuclear species tetrabutyldistannide, (Bu4Sn)22- (-161 ppm at 200 K), and hydrodibutylstannide, Bu2SnH- (-228 ppm at 200 K). Accordingly, the latter signal splits into a doublet in proton-coupled 119Sn NMR spectra (1J(119Sn-1H) = 96 Hz at 220 K), but the resolution is relatively low due to the coupling with the methylene protons of the butyl groups. The resolution decreased even more upon very long periods of measurement (>7 h), which is probably a result of the limits in temperature control: the chemical shift strongly depends on the temperature (Δδ = 0.5 ppm K-1; cf. Table 1; compare also to the fluctuations of chemical shifts of phenylstannides during NMR measurements, as displayed in Figures S4 and S5 in the Supporting Information). The additional signals are presumably due to butyl group migration (see also below, reactions with bromoethane section). The signal at -136 ppm (at 200 K) obviously represents tributylstannide, SnBu3-, since the reaction mixture of chlorotributylstannane and two equivalents of sodium in liquid ammonia resulted in a single peak at the same chemical shift. The signal at -212 ppm showed pronounced broadening in the proton-coupled 119Sn NMR spectra, at least partially due to nonresolved couplings to protons of the butyl groups. Yet the coupling of 119Sn nuclei to protons of Sn-H bonds is significantly stronger, and therefore, the line broadening increases additionally in species with nonresolved Sn-H bonds. The fullwidth at half-maximum (fwhm) of the peak at -212 ppm (-208 ppm at 220 K) extended in the proton-coupled spectra by 140% (170% at 220 K) compared to the decoupled spectra (cf. Table 1). This is even more than the signal of the hydrostannide at -228 ppm (-218 ppm at 220 K), which was broadened by about 80% (60% at 220 K). For comparison, the fwhm of the two peaks at -136 and -161 ppm (-137 and -142 ppm at 220 K), which represent species without Sn-H bonds, was only little influenced when proton-coupled and proton-decoupled spectra are compared; the broadening amounted to only 1520% at 220 K and even less at 200 K. Thus, the signal at -212 ppm may represent, for instance, a mononuclear dihydrostannide or, since the signal intensity was not sufficiently high to allow detection of tin satellites, a binuclear hydrostannide. Reaction of the Stannide Intermediates with Bromoethane. As mentioned above, it has been postulated that the intermediate diorganostannide dianion can be trapped by reaction with organohalides (Scheme 1).20-23 Accordingly, we transferred solutions with the in-situ-prepared stannides in the final state into a large excess of precooled bromoethane. In the case of the intermediates resulting from conversion of dichlorodiphenylstannane, only diethyldiphenylstannane, Et2Ph2Sn, was found after the reaction (Figure 4a, for chemical shifts see Experimental Section). The stannides resulting from conversion of dichlorodibutylstannane with bromoethane yielded two additional products compared to the analogous conversion with dichlorodiphenylstannane. Note that in the former case also two additional reaction intermediates were detected (see above). Besides the expected main product, dibutyldiethylstannane, Bu2Et2Sn, also tributylethylstannane, Bu3EtSn, and butyltriethylstannane, BuEt3Sn, were found by 119Sn NMR analysis (Figure 4b; for chemical shifts see Experimental Section), in line with the alkyl
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Figure 4. 119Sn NMR spectra of (a) dichlorodiphenylstannane and (b) dichlorodibutylstannane converted with 4 molar equiv of sodium in liquid ammonia and subsequently reacted with an excess of bromoethane.
group migration implied by the reaction intermediates. Consequently, reaction experiments with 1-bromobutane yielded only one product, i.e., tetrabutylstannane. Thus, the two different stannides (R2Sn)22- and HR2Snreact with bromoethane to form the same product (Et2R2Sn), which would be expected from a reaction of R2Sn2- with bromoethane. These findings show that the reaction products of the intermediates in liquid ammonia with haloalkanes allow only limited conclusions on the composition of the tin species present in liquid ammonia, although starting from Bu2SnCl2 they appear to reflect at least the in situ migration of organic groups. As a final remark, note that the quantity of sodium in the system corresponds to the stoichiometry of the overall reaction according to Scheme 1 (Na:R2SnCl2 = 4:1); that is, some sodium is also involved in the reaction of the stannides with bromoethane, since sodium is only partially consumed upon formation of tetraorganodistannide and hydrodiorganostannide. In the case of R2SnH2, there is sufficient sodium for a reduction under formation of NaH (it is not evident if H2 formed; in this case sodium would be present in excess).
Conclusions 119
Sn NMR measurements in liquid ammonia showed that, in contrast to the generally accepted view, diorganostannide dianions are not formed significantly by exposure of dichlorodiorganostannanes or dihydrodiorganostannanes with four equivalents of sodium in liquid ammonia, as deduced previously from indirect experiments. The species that are present in the reacting medium, as a matter of fact, are tetraorganodistannide, (R2Sn-SnR2)2-, and hydrodiorganostannide, HR2Sn-. The latter slowly exchanges hydrogen atoms with the solvent. In addition, butyl group migration takes place in liquid ammonia, while significant phenyl group migration does not occur under the applied reaction conditions. All experiments showed that, in contrast to previous assumptions, reaction products of the in-situ-generated diorganostannides with haloalkanes do not represent the chemical nature of the intermediates in liquid ammonia.
Experimental Section Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland, 99.999%), dichlorodibutylstannane from ABCR GmbH (Karlsruhe, Germany), and dichlorodiphenylstannane from Sigma Aldrich (Buchs, Switzerland). Both solid substances were recrystallized twice by dissolving in boiling pentane and subsequent precipitation of the product at 250 K. Deuterated dichloromethane (99.9% D) was purchased from Cambridge Isotope Laboratories (ReseaChem GmbH, Burgdorf, Switzerland), and organic solvents were from Fluka (Buchs, Switzerland).
Trummer and Caseri Conductivity Measurements. Electrical conductivity measurements in liquid ammonia solutions were performed with a TetraCon 325/Pt electrode from WTW (Weilheim, Germany) in combination with a WTW MultiLab 540 instrument. In a typical reaction, 150 mL of ammonia was condensed with a coldfinger condenser in a 200 mL flame-dried, three-neck, flatbottomed flask under nitrogen atmosphere. The conductivity cell was immersed into the cold solution (195 K), and 8 mmol of sodium was introduced in a nitrogen counterflow. The reaction mixture was stirred until the conductivity level was constant, which took about 15 min. Subsequently 2 mmol of the respective dichlorodiorganostannane was added at 195 K, and the electrical conductivity was monitored as a function of time. NMR Spectroscopy. 119Sn NMR spectra were recorded with a Bruker UltraShield 300 MHz/54 mm Fourier-transform spectrometer at a frequency of 112 MHz either with inverse-gated decoupling or without decoupling, as indicated in the text. In both cases a delay time of 0.5 s, an acquisition time of 0.1 s, and a pulse angle of 3 μs (90°) was applied. The sweep width was 700 ppm with a 16k data point acquisition range, resulting in a digital resolution of 4.78 Hz. Chemical shifts (δ) are reported in ppm referenced to tetramethylstannane (δ(Me4Sn) = 0 ppm). Syntheses of Dihydrodiphenylstannane and Dideuterodiphenylstannane. These compounds were synthesized according to the literature36 but with LiAlD4 instead of LiAlH4 for the deuterated compound. NMR analysis (in CD2Cl2, room temperature, chemical shifts in ppm, coupling constants in Hz, q: quintet, m: multiplet): 1 H: δ 8.07 (m, 2 H), 7.81 (m, 3 H); 13C: δ 129.3 (J(C,117Sn/119Sn) 51.7/54.2), 129.6, 129.6 (J(C,Sn) 11.8), 138.2 (J(C,117Sn/119Sn) 39.3/ 40.7); 119Sn: δ -233.6 (q, 1J(119Sn,D) 296.6); proton-coupled 119Sn: -233.6 (qq, 3J(119Sn,H) 53.9, 4J(119Sn,H) 11.4). Reactions in Liquid Ammonia. The conversion of dichlorodiorganostannanes or dihydrodiorganostannanes with sodium in liquid ammonia was conducted in a flame-dried three-neck flask with a flat bottom under nitrogen atmosphere where ca. 100 mL of ammonia was condensed with a coldfinger condenser. A quantity of 8 mmol of sodium was introduced in a nitrogen counterflow and dissolved by stirring with a magnetic glass stirring bar to result in a homogeneous blue solution (15 min). Thereafter, 2 mmol of dichlorodibutylstannane, dichlorodiphenylstannane, or dihydrodiphenylstannane was dissolved in 1 mL of THF (dried over molecular sieves) and slowly added to the sodium/ammonia solution with a syringe through a septum (to examine the influence of the THF in the reaction mixture, some reactions were conducted by directly adding solid dichlorodiorganostannane under a nitrogen counterflow, which led to the same results). The color of the solution changed from clear blue to dark red. The reaction mixture was stirred for 30 min in order to complete the reaction (as previously determined with conductivity measurements). Syntheses of phenylstannides from dideuterodiphenylstannane were performed in the same way. For in situ investigations with low-temperature NMR spectroscopy in liquid ammonia, the reaction mixture containing the final stannides was transferred via a bent glass tube into a flame-dried and precooled NMR tube (195 K, type 5UP 5 178 mm; ARMAR AG, D€ ottingen Switzerland) equipped with a sealed capillary with deuterated dichloromethane. The NMR tube was stored under argon atmosphere in a 250 mL Schlenk tube, and the transfer of the reaction mixture was performed with nitrogen overpressure by carefully excluding oxygen (argon counterflow from the Schlenk tube). The first 5-10 mL was poured into the Schlenk tube before about 0.5 mL was added into the NMR tube. The filled NMR tube was flushed with argon and stored at 195 K before it was inserted into the precooled NMR spectrometer. Conversion of the in-Situ-Prepared Stannides with Bromoethane. For the reactions of the in-situ-prepared stannides with bromoethane, 5-10 mL of the ammonia solutions containing the final (36) Imori, T.; Lu, V.; Cai, H.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 9931–9940.
Article stannides was transferred at 195 K to 20 mL of precooled bromoethane (195 K) in a 100 mL two-neck, round-bottomed flask via a bent glass tube with nitrogen overpressure. The ammonia was evaporated by warming the flask to room temperature in a N2 stream. The remaining products were dried under vacuum (ca. 0.1 mbar) for 24 h, and the solids thus obtained were dissolved in deuterated dichloromethane for analysis with NMR spectroscopy. 119Sn NMR analysis (CD2Cl2, Me4Sn), chemical shifts δ in ppm (discussion of the products see text): Bu4Sn δ -11.8, Bu3EtSn δ -7.9, Bu2Et2Sn δ -4.1, BuEt3Sn δ -0.5, Et2Ph2Sn δ -65. Selected literature values for comparison: Bu4Sn δ -11.5,33 Et2Ph2Sn δ -66;37 since we did not find chemical shifts of Bu2Et2Sn, EtBu3Sn, and Et3BuSn, we also quote the value of Et4Sn (δ 1.437), which discloses that the chemical shifts of the stannanes comprising mixed alkyl groups are located between the chemical shifts of Bu4Sn and Et4Sn. Further, the chemical shifts reported for Et3PhSn δ -3437 and EtPh3Sn δ -9837 reveal that these products did not appear in the spectra obtained by conversion of the related phenylstannides with bromoethane. (37) McFarlane, W.; Maire, J. C.; Delmas, M. J. Chem. Soc., Dalton Trans. 1972, 1862–1865.
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The same reaction procedure with 1-bromobutane gave only one product, Bu4Sn.
Acknowledgment. We are highly indebted to Paul Smith (ETH Z€ urich) for his helpful comments and encouragement throughout this work. We also thank Frank Uhlig (TU Graz) for the fruitful discussions and input concerning the chemistry of tin, and Aitor Moreno and Heinz Ruegger (ETH Z€ urich) for assistance with NMR spectrometry. The Swiss National Science Foundation is gratefully acknowledged for financial support. Note Added after ASAP Publication. This paper was published on the Web on August 19, 2010, with an error in the Conclusions section in the formula describing hydrodiorganostannide. The corrected version was reposted on September 7, 2010. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
