Angewandte
Chemie
(1mL), yielding a yellow solution. After 30 min at ꢀ508C, the
ligands to become more covalent, with significantly shorter
significant formation of red selenium indicated decomposition.
In another experiment, a sample of SeF4 (0.80 mmol) was
condensed into a teflon–FEP ampule with subsequent addition of
SO2 (2 mL) and Me3SiN3 (4.00 mmol) by condensation in vacuo at
ꢀ1968C. The mixture was warmed to ꢀ648C. Within minutes, the
mixture turned yellow, the color intensified, and a lemon-yellow solid
precipitated while the reaction proceeded. Keeping the reaction
mixture for about 15 min at ꢀ648C resulted in a violent explosion that
destroyed the sample container and the surrounding stainless-steel
Dewar flask.
ꢀ
Se N bonds and smaller negative charges.
In all three selenium azide species, the azido groups have
strong covalent character, which is exemplified by typical N-
ꢀ
N-N bond angles of 175–1778, the longer Na Nb bonds
ꢀ
(1.20 in 5) and the shorter terminal Nb Ng bonds (1.125–
1.140 in 5). The calculated Mulliken partial charges (see the
Supporting Information) on the selenium atoms are all close
to unity; the additional negative charges in the anions are
spread over the azido ligands.
In summary, we were able to show that the binary
selenium azide Se(N3)4 and its anions [Se(N3)5]ꢀ and
[Se(N3)6]2ꢀ exist. The neutral azide is thermally unstable
and explosive, but the anions, particularly when combined
with large inert counterions, are more manageable.
[PNP][SeF5]: Silver fluoride (1.77 mmol) was added at ambient
temperature to a solution of SeF4 (1.77 mmol) in CH3CN (4 mL). The
resulting solution was stirred for 2 h, and then [PNP]Cl (1.77 mmol)
was added. After additional stirring for 30 min, the pale yellow
solution was decanted from a gray precipitate, and all volatile
material was removed from the filtrate in vacuo, yielding a colorless
solid.[25]
2: A solution of [PNP][SeF5] (0.26 mmol) in CD2Cl2 (0.6 mL) was
treated with Me3SiN3 (1.4 mmol) at ꢀ508C. After a few minutes, a
yellow solution had formed, which was analyzed by NMR spectros-
copy. After 1h at ꢀ508C, the significant formation of red selenium
indicated decomposition.
[PNP]2[SeF6]: Silver fluoride (3.55 mmol) was added at ambient
temperature to a solution of SeF4 (1.77 mmol) in CH3CN (6 mL). The
resulting solution was stirred for 2 h, and then [PNP]Cl (3.55 mmol)
was added. After additional stirring for 30 min, the pale yellow
solution was decanted from a gray precipitate, and all volatile
material was removed from the filtrate in vacuo, yielding a colorless
solid.[25]
4: A solution of [PNP]2[SeF6] (0.17 mmol) in CD2Cl2 (0.6 mL)
was treated with Me3SiN3 (1.1 mmol) at ꢀ508C. After a few minutes,
a pale yellow solution was formed and analyzed by NMR spectros-
copy. After 1h at ꢀ508C, decomposition was evident from the
observation of significant amounts of red selenium.
Experimental Section
CAUTION! Binary selenium azides are unstable, hazardous, and
moisture-sensitive materials. Se(N3)4 is extremely sensitive and, even as
a suspension in SO2 solution, has exploded violently at low temper-
atures without any provocation. All compounds should be handled
only on a scale of less than 2 mmol with appropriate safety precautions
(safety shields, safety glasses, face shields, leather suits, gloves, and ear
plugs). Teflon containers and stainless steel Dewar flasks should be
used whenever possible to avoid hazardous shrapnel formation and to
contain explosions, respectively. The use of chlorinated solvents is not
recommended when working with azides, owing to facile chloride–
azide exchange reactions, which can result in the formation of explosive
alkylazides.[22] However, during our studies at LMU, such hazardous
byproducts were never observed. Ignoring these safety precautions
can result in serious injury.
At LMU, all manipulations of air- and moisture-sensitive
materials were performed under an inert atmosphere of dry argon
using flame-dried glass vessels or oven-dried plastic equipment and
Schlenk techniques.[23] The selenium fluorides were handled in
perfluoroalkoxy copolymer (PFA) vessels owing to their moisture-
sensitivity. For the NMR spectroscopic measurements, 4-mm PFA
tubes were used, which were placed into standard 5-mm NMR glass
tubes. Selenium tetrafluoride (Galaxy Chemicals), silver fluoride
3 and 5: Under a stream of dry dinitrogen gas, a stochiometric
amount of [Ph4P]N3 was added to a frozen solution of SeF4
(0.43 mmol) in CH3CN (1.5 mL) at ꢀ1968C. The reactor was
evacuated, and CH3CN (0.3 mL) and Me3SiN3 (3.16 mmol) were
condensed in. The reaction mixture was warmed to ꢀ408C, and the
reactor was gently agitated. After 30 minutes, an orange-red solution
with either an orange or a red precipitate was obtained. All volatile
material was removed in vacuo at ꢀ358C. The solids 3 and 5 were
characterized by low-temperature Raman spectroscopy, and 5 was
also characterized by its crystal structure. 3: orange, temperature-
sensitive solid (0.30 g, weight calculated for 0.43 mmol: 0.27 g). 5: red,
temperature-sensitive solid (0.45 g, weight calculated for 0.43 mmol:
0.43 g). Single crystals were grown from a solution in CH3CN by slow
evaporation of the solvent in a dynamic vacuum at ꢀ358C.[26]
Thermal decomposition of 5: A solution of 5 (0.4 mmol) in
CH3CN (4 mL) at ꢀ408C was warmed to ambient temperature over a
period of 6 h. A light yellow solution and a maroon precipitate
formed. The reaction mixture was cooled to ꢀ1968C and inspected
for dinitrogen (noncondensible compounds). P,V,T measurements
indicated that 2.1mmol dinitrogen had formed. The reaction mixture
was then warmed to ambient temperature and the precipitate filtered
off. The precipitate was identified as amorphous selenium by its
Raman spectrum, which showed a single, very intense line at
(ABCR),
bis(triphenylphosphoranylidene)ammonium
chloride
([PNP]Cl, Aldrich), and trimethylsilyl azide (Aldrich) were used as
received. The solvents dichloromethane and acetonitrile were dried
by standard methods and freshly distilled prior to use. NMR spectra
were recorded on a JEOL Eclipse 400 instrument, and chemical shifts
are reported with respect to MeNO2 (14N, 28.9 MHz) and Me2Se (77Se,
76.3 MHz).
At USC, all reactions were carried out in teflon–FEP ampules
that were closed by stainless steel valves. Volatile materials were
handled in a pyrex glass vacuum line. Nonvolatile materials were
handled in the dry argon atmosphere of a glove box. Raman spectra
were recorded at ꢀ808C in the range 4000–80 cmꢀ1 on a Bruker
Equinox 55 FT-RA spectrometer using a Nd-YAG laser at 1064 nm
with power levels of less than 100 mW. Teflon–FEP tubes with
stainless steel valves were used as sample containers. The starting
materials SeCl4, SeO2, and [Ph4P]Cl (all from Aldrich) were used
without further purification. Me3SiN3 (Aldrich) was purified by
fractional condensation prior to use. Solvents were dried by standard
methods and freshly distilled prior to use. [Ph4P]N3 was prepared
from [Ph4P]Cl and NaN3 by ion exchange.[24] SeF4 was prepared from
SeCl4 or SeO2 and ClF3 in HF solution.
252 cmꢀ1 [27]
Volatile components were removed from the light
.
yellow solution in vacuo, leaving behind a pale yellow solid that was
identified by Raman spectroscopy as [Ph4P]N3.[24]
The structure and frequency calculations were performed at the
B3LYP level of theory using a D95 V basis set for N, while the core
electrons of Se were treated with an ECP28MWB pseudopotential;
1: A solution of SeF4 (0.18 mmol) in CD2Cl2 (1mL) was treated
with Me3SiN3 (0.79 mmol) at ꢀ508C. After 30 min stirring, a pale
yellow precipitate was obtained and dissolved in additional CD2Cl2
Angew. Chem. Int. Ed. 2007, 46, 8686 –8690
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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