Diorganostannide dianions (R2Sn2-) as reaction intermediates revisited: In situ 119Sn NMR studies in liquid ammonia

Article abstract of DOI:10.1021/om100545b

It has frequently been proposed that diorganostannide dianions, SnR ₂^2-, 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 SnR₂^2- dianion was not present, but tetraorganodistannides, (R₂Sn-SnR₂)^2-, and hydrodiorganostannides (tin hydrides), R₂SnH^- (or R ₂SnD^-, respectively), were detected instead.

Full text of DOI:10.1021/om100545b

3862 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, Eidgenossische 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 (diphenyl-
stannane), dideuterodiphenylstannane, and dichlorodibutylstannane, respectively. This study revealed
2-
that the proposed SnR2 dianion was not present, but tetraorganodistannides, (R2Sn-SnR2)2-, and
hydrodiorganostannides (tin hydrides), R2SnH- (or R2SnD-, respectively), were detected instead.
Introduction
stannanes,13-19 we revisited the purported existence of di-
organostannide dianions.
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 R2Sn2-
have been advanced as intermediates for different organo-
metallic syntheses, including deuteration of carbonyl com-
pounds,8 preparation of asymmetric stannanes,9,10 and
formation of polymers.11 However, conclusive spectroscopic
Diorganostannides are commonly generated in situ by reac-
tion of dihalodiorganostannanes with sodium in liquid ammo-
nia; the intermediate products are further converted for syn-
thetic 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, respec-
tively. Depending on the applied ratio between the dibromodi-
methylstannane and sodium, also oligomeric stannides of the
evidence for the formation of diorganostannide dianions is
not available; for instance 119Sn-Mossbauer spectroscopy
12
€
did not lead to identification of specific compounds. Driven
by our interest in the synthesis of macromolecular poly-
2-
type (Me2Sn)x were postulated. Similar conclusions were
drawn for the formation of monostannides and distannides
upon conversion of the corresponding phenyl22 and ethyl halo-
stannanes.23 More recently, synthesis of substituted diaryldi-
methylstannanes was proposed to proceed via the intermediate
*Corresponding author. E-mail: wcaseri@mat.ethz.ch
(1) Chopa, A. B.; Lockhart, M. T.; Dorn, V. B. Organometallics 2002,
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Pierini, A. B. J. Organomet. Chem. 2008, 693, 2458–2462.
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(7) Kraus, C. A.; Neal, A. M. J. Am. Chem. Soc. 1930, 52, 4426–4433.
2-
Me2Sn2-, but also formation of the dimeric species (Me2Sn)2
was 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
€
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r
pubs.acs.org/Organometallics
Published on Web 08/19/2010
2010 American Chemical Society

Article
Scheme 1. Formation of the Previously Proposed Diorgano-
Organometallics, Vol. 29, No. 17, 2010 3863
stannide 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.
2-
The dianion SnMe2 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 stan-
nane, 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 dimethyl-
stannide 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 dimethyl-
stannane also to form tetramethyldistannide, (SnMe2)22-, or
hydrodimethylstannide, Me2SnH-, respectively.
Figure 1. Electric conductivity of reaction mixtures upon addi-
tion of dichlorodiphenylstannane, Ph2SnCl2 (0), or dichlorodi-
butylstannane, 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.
In order to bring to light the nature of the intermediate
form of stannides resulting from the reaction of dihalodi-
organostannides 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.
Results and Discussion
In Situ Electric Conductivity. Reactions were carried out by
treatment of dichlorodibutylstannane, Bu2SnCl2, and dichloro-
diphenylstannane, Ph2SnCl2, with four molar equivalents of
sodium in liquid ammonia. This stoichiometric ratio is just
Figure 2. 119Sn NMR spectra recorded in situ in liquid ammo-
nia (at 200 K) of reaction products of (a) Ph2SnCl2, (b) Ph2SnH2,
and (c) Bu2SnCl2 formed during treatment with four molar
equivalents of sodium.
2-
required for the formation of the hypothetical SnR2 under
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
119Sn-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 mea-
surement times, often additional signals emerged, probably
because slow diffusion of water or oxygen from the atmo-
sphere through the cap of the NMR tube lead to subsequent
reactions (Figure S1 in the Supporting Information).
Reaction of Dichlorodiphenylstannane and Dihydrodiphenyl-
stannane 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
release of two equivalents of NaCl as byproduct (upon applica-
tion 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 conductiv-
ities 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 termi-
nated. The conductivities at the plateau value corresponded to
30% and 20% of the initial value for dichlorodiphenylstannane
and dichlorodibutylstannane, respectively. Qualitatively, a pro-
nounced 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.

3864 Organometallics, Vol. 29, No. 17, 2010
Trummer and Caseri
Table 1. Chemical Shifts (δ), Coupling Constants (J) and Full-Widths at Half-Maximum (fwhm) in Proton-Decoupled and Proton-Coupled
119Sn NMR Spectra of Different Diorganostannanes Exposed to Four Molar Equivalents of Sodium in Liquid Ammonia
proton decoupled
proton coupled
measurement
temp, K
δ119Sn, ppm
fwhm, Hz
1J, Hz
δ119Sn, ppm
fwhm, Hz
1J, Hz
Ph2SnCl2
Ph2SnH2
200
220
200
220
220
-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
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
-131.7
-197.1
-116.2
-191.8
-131.5
-197.1
s
d
s
d
s
d
124
111
45
35
130
90
147
2018
145
(Sn,H)
(Sn,Sn)
(Sn,H)
2027
(Sn,Sn)
148
(Sn,H)
2029
2023
24
(Sn,Sn)
(Sn,Sn)
(Sn,D)
Ph2SnD2
Bu2SnCl2
-116.5
-192.9
-196.8
-136.3
-161.1
-212.0
-228.0
-137.4
-141.6
-208.1
-218.6
s
26
22
26
104
125
155
220
80
2023
144
23
(Sn,Sn)
(Sn,H)a
(Sn,D)b
d
m
s
9
200
220
92
120
57
118
67
75
60
120
s
m
m
s
s
m
m
86
144
195
96
(Sn,H)b
a Hydrostannide formed by D-H exchange (see text). b Determined by peak deconvolution.
Information). Notably, the same signals were found when
dihydrodiphenylstannane, Ph2SnH2, was exposed to four mo-
lar equivalents of sodium (Figure 2b), i.e., under conditions
where the formation of diorganostannide dianions was
expected,30 indicating that in the cases of dichlorodiphenyl-
stannane 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
1J(119Sn,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
2- 34
(Figure 3a);to tetraphenyldistannide, (Ph2Sn)2
.
Remarkably, the signal at -193 ppm (measured at 220 K)
split into a well-defined doublet in the hydrogen-coupled spec-
tra, 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 sig-
nal 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 cor-
responding hydrogen atom remained from the starting com-
pound, dideuterodiphenylstannane, Ph2SnD2, was exposed
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
119Sn 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 proton-
decoupled spectrum.
(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.

Article
Organometallics, Vol. 29, No. 17, 2010 3865
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.
additional spectra in Figure S6 in the Supporting Informa-
tion), i.e., two additional signals featured when compared to
those resulting from experiments with dichlorodiphenylstan-
nane (occasionally, a minor signal at 27 ppm was also pre-
sent, probably as a result of a reaction with traces of oxygen).
The two main signals (with respect to the integrated inten-
sities) were linked to the binuclear species tetrabutyldistannide,
After exposure of dideuterodiphenylstannane to four equiva-
lents 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 above-
mentioned hydrodiphenylstannide. Further, a pronounced three-
line 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).
2-
(Bu4Sn)2 (-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 tri-
butylstannide, SnBu3-, since the reaction mixture of chlorotri-
butylstannane and two equivalents of sodium in liquid ammo-
nia resulted in a single peak at the same chemical shift.
The intensity of the Ph2SnH- signal observed after one hour
is by far too strong to be due to residual hydrogen atoms in the
1
starting compound Ph2SnD2 (as evident from analysis of H
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 ex-
tracted from dichlorodiphenylstannane or dihydrodiphenyl-
stannane at least partially stem from different sources, hydro-
diphenylstannide 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. Dichlorodiphenyl-
stannane 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.
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 signifi-
cantly stronger, and therefore, the line broadening increases
additionally in species with nonresolved Sn-H bonds. The full-
width 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 15-
20% at 220 K and even less at 200 K. Thus, the signal at -212
ppm may represent, for instance, a mononuclear dihydrostan-
nide 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 inter-
mediate diorganostannide dianion can be trapped by reac-
tion 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 dichlorodibutyl-
stannane with bromoethane yielded two additional products
compared to the analogous conversion with dichlorodiphenyl-
stannane. 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
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 dichloro-
dibutylstannane, four major signals emerged (Figure 2c,

3866 Organometallics, Vol. 29, No. 17, 2010
Trummer and Caseri
Conductivity Measurements. Electrical conductivity measure-
ments 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, flat-
bottomed 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 elec-
trical conductivity was monitored as a function of time.
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.
NMR Spectroscopy. 119Sn NMR spectra were recorded with a
Bruker UltraShield 300 MHz/54 mm Fourier-transform spec-
trometer 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 Dideuterodiphenyl-
stannane. These compounds were synthesized according to the
literature36 but with LiAlD4 instead of LiAlH4 for the deuterated
compound. NMR analysis (in CD2Cl2, room temperature, chemi-
cal shifts in ppm, coupling constants in Hz, q: quintet, m: multiplet):
1H: 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).
group migration implied by the reaction intermediates. Conse-
quently, reaction experiments with 1-bromobutane yielded only
one product, i.e., tetrabutylstannane.
2-
Thus, the two different stannides (R2Sn)2 and HR2Sn-
react 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 hydrodiorgano-
stannide. 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).
Reactions in Liquid Ammonia. The conversion of dichloro-
diorganostannanes 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, dichlorodiphe-
nylstannane, 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 dichlorodi-
organostannane 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 con-
ductivity measurements). Syntheses of phenylstannides from
dideuterodiphenylstannane were performed in the same way.
For in situ investigations with low-temperature NMR spectro-
scopy 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 Â 178 mm; ARMAR
Conclusions
119Sn NMR measurements in liquid ammonia showed
that, in contrast to the generally accepted view, diorganostan-
nide dianions are not formed significantly by exposure of
dichlorodiorganostannanes or dihydrodiorganostannanes with
four equivalents of sodium in liquid ammonia, as deduced pre-
viously from indirect experiments. The species that are present in
the reacting medium, as a matter of fact, are tetraorganodi-
stannide, (R2Sn-SnR2)2-, and hydrodiorganostannide, HR2-
Sn-. 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 di-
organostannides with haloalkanes do not represent the
chemical nature of the intermediates in liquid ammonia.
€
AG, Dottingen 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.
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).
Conversion of the in-Situ-Prepared Stannides with Bromoethane.
For the reactions of the in-situ-prepared stannides with bromo-
ethane, 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
Organometallics, Vol. 29, No. 17, 2010 3867
stannides was transferred at 195 K to 20 mL of precooled bromo-
ethane (195 K) in a 100 mL two-neck, round-bottomed flask via a
bent glass tube with nitrogen overpressure. The ammonia was eva-
porated 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 chem-
ical 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.
The same reaction procedure with 1-bromobutane gave only
one product, Bu4Sn.
Acknowledgment. We are highly indebted to Paul
€
Smith (ETH Zurich) for his helpful comments and en-
couragement 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 Zurich) for assistance with NMR
spectrometry. The Swiss National Science Foundation is
gratefully acknowledged for financial support.
Note Added after ASAP Publication. This paper was pub-
lished on the Web on August 19, 2010, with an error in the
Conclusions section in the formula describing hydrodiorgano-
stannide. 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.
(37) McFarlane, W.; Maire, J. C.; Delmas, M. J. Chem. Soc., Dalton
Trans. 1972, 1862–1865.