S6N2O15—A Nitrogen-Poor Sulfur Nitride Oxide, and the Anhydride of Nitrido-tris-Sulfuric Acid

Article abstract of DOI:10.1002/anie.202005056

The reaction of hexachlorophosphazene, P₃N₃Cl₆, with SO₃ leads to the new sulfur nitride oxide S₆N₂O₁₅. The compound displays an extraordinarily low nitrogen content and exhibits a bicyclic cage structure according to the formulation N{S(O)₂O(O)₂S}₃N, with both nitrogen atoms in trigonal planar coordination of sulfur atoms. Interestingly, the new nitride oxide can be also seen as the anhydride of nitrido-tris-sulfuric acid, N(SO₃H)₃.

Full text of DOI:10.1002/anie.202005056

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Accepted Article
Title: S6N2O15 – a nitrogen-poor sulfur nitride-oxide, and the
anhydride of nitrido-tris-sulfuric acid
Authors: Mathias Wickleder and David van Gerven
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S6N2O15 – a nitrogen-poor sulfur nitride-oxide, and the anhydride
of nitrido-tris-sulfuric acid
David van Gerven,[a] and Mathias S. Wickleder*[a]
[a]
M. Sc. D. van Gerven, Prof. M. S. Wickleder
University of Cologne
Institute of Inorganic Chemistry
Greinstr. 6, 50939 Cologne (Germany)
E-mail: mathias.wickleder@uni-koeln.de
Supporting information for this article is available on the WWW under http://www.angewandte.org or from the
author.
Abstract: The reaction of hexachlorophosphazene, P3N3Cl6, with SO3 leads to the new sulfur nitride-oxide
S6N2O15. The compound displays an extraordinarily low nitrogen content and exhibits a bicyclic cage structure
according to the formulation N{S(O)2O(O)2S}3N, with both nitrogen atoms in trigonal planar coordination of
sulfur atoms. Interestingly, the new nitride-oxide can be also seen as the anhydride of nitrido-tris-sulfuric acid,
N(SO3H)3.
Sulfur trioxide, SO3, is an interesting reagent in chemical reactions. On one hand, it is a very strong oxidizer and
on the other hand, it can act as a typical Lewis base. We have used the oxidation strength of SO3, especially under
harsh conditions, for the oxidation of noble metals and noble metal compounds, respectively. The formation of
two modifications of Pd(S2O7) by the reaction of elemental palladium with SO3 is a nice example of these
efforts.[1,2] On the other hand, SO3 is a strong Lewis acid and forms readily adducts with several Lewis bases. Well
known examples are the complexes with dioxane and pyridine (py).[3,4] The latter, SO3·py, is even a commercial
product that serves as a save SO3 source for many reactions. In fact, Lewis acid/base adducts with N-donor
molecules and SO3 have been studied quite extensively starting already in the 1950s.[5] Actually, very spectacular
compounds have been prepared at that time, for example the adducts S4N4·xSO3 (x = 1-4), for which S4N4·SO3 has
been structurally characterized later.[6] Another potential base that has been considered for SO3 interaction was
hexachlorophosphazene, P3N3Cl6.[5] It has formally three available nitrogen atoms bearing free electron pairs.
Thus, the composition P3N3Cl6·3SO3 of the reported complex it is very reasonable, even if structural information
is still lacking. We came across that compound for two reasons: On one hand, we are interested in Lewis acid/base
2–
complexes of SO3 since we discovered that the rarely known polysulfates [SnO3n+1
]
have to be described as
adducts according to [SnO3n+1]2–·SO3, at least for larger numbers of n.[7,8] In the hexasulfate Rb2[S6O19] (n = 6), the
distance of the sulfur atom of SO3 to the next oxygen atom is already as long as 231 pm.[8] For a detailed
investigation of bond lengths within Lewis acid/base complexes structure elucidations of complexes with different
bases are desirable. On the other hand, we have recently started a research project aiming at a detailed
understanding of nitrogen-based derivatives of sulfuric acid. These are for example the slightly acidic sulfimide,
1
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10.1002/anie.202005056
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SO2(NH2)2,[9] and its cyclic condensation products S3O6(NH)3 and S4O8(NH)4,[10,11] for which a limited number of
salts are known.[12-14] However, the more prominent of these derivatives are amidosulfuric acid, imido-bis-sulfuric
acid, and nitrido-tris-sulfuric acid (Fig. 1). Even if all of these acids are textbook examples, our knowledge is still
quite limited. Only amidosulfuric acid, in its zwitterionic ground state a Lewis acid/base complex of SO3 and NH3,
and amidosulfates have been frequently reported.[15] For all of the other anions depicted in figure 1 a very limited
number of salts is known.[16] Especially for the nitrido-tris-sulfuric acid, N(SO3H)3, which is not known in a pure
form, there is just one report of respective salt, namely K3[N(SO3)3]·2H2O.[17] The acid and their salts are prone to
hydrolysis, what is certainly a drawback for synthesis, especially from aqueous solution. In this case,
hexachlorophosphazene might be a suitable nitrogen source for the preparation of N-based sulfuric acids under
anhydrous conditions.
Figure 1 Nitrogen-based sulfuric acid derivatives
With respect to the two above-mentioned issues, i.e. hexachlorophosphazene as Lewis base and as starting material
for the synthesis of N-based sulfuric acid derivates, we have investigated the reaction of SO3 and P3N3Cl6 under
various conditions in more detail. According to the findings of Goehring et al., at low temperature (≈ 40 °C) a
reaction is observed, however without gaining crystalline material. Only if the temperature is raised up to 80 °C a
huge number of single crystals growths from excess SO3 in a short period of time (Fig. 2). Structure elucidation
revealed that the anhydride of nitrido-tris-sulfuric acid had formed, namely S6N2O15. With respect to the amount
of the gained product, the reaction is almost quantitative, so that the reaction could be written as 2 P3N3Cl6 + 18
SO3 → 3 S6N2O15 + 4 POCl3 + ½ P4O10. We have not identified the by-products unambiguously up to now,
however, we do not observe elemental chlorine, which is, according to [5], a reaction product at higher
temperature. A very likely product is phosphoryl chloride, POCl3. The presence of POCl3 would also explain that
the sulfur trioxide which is used in excess in the reaction stays liquid, even if the ampoules are stored in a
refrigerator. In similar reactions, we usually observe the formation of asbestos type sulphur trioxide (-SO3) at
lower temperature, visible by large needle shaped crystals growing in the ampoule. Compounds like SO2Cl2 or
POCl3 are well-known stabilizers that are used to keep sulphur trioxide liquid below 30 °C by supressing the
polymerisation of SO3 molecules.[18,19] Attempts to separate the obtained by-product from SO3 failed up to now.
2
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Figure 2 Single crystals of S6N2O15
The molecular compound has a unique structure with two three-coordinate nitrogen atoms connected by three
[S2O5] groups (Fig. 3), as it would be emphasized by the more descriptive formula N{S(O)2O(O)2S}3N. The
distances S-N fall is a narrow range between 170.6 and 171.6 pm, and the surrounding of the nitrogen atoms is
almost perfectly planar. Thus, no activity of the lone electron pair is observable, obviously due to significant -
bonding to the sulfur atoms. The observation is in line with the reported findings for the anion [N(SO3)3]3–.[17] The
nitrogen atoms are connected by three nearly identical S-O-S bridges, displaying distances 161.8 and 163.8 pm
and angles S-O-S of about 125°. These are the typical values that are, for example, observed for the disulfate
ion, S2O72–. The distances and angles within the S6N2O15 molecule are well reflected by quantum mechanical
calculations (cf. caption figure 3 and supplement). As expected, the calculations result in C3h symmetry for the
molecule, while in the solid state (space group C2/c) only C1 symmetry is found.
Figure 3 Structure and labelling of the S6N2O15 molecule viewed in different directions. The middle picture shows
the molecule viewed along an axis through the nitrogen atoms, emphasizing their almost perfect trigonal planar
coordination by sulphur atom. At right, the molecule bicyclo[3.3.3]undecane is depicted which represents the
[S6O3N2] cage of S6N2O15 (emphasized by black bonds). Selected distances (in pm) compared to theoretical values
(in italics): S(1-6)–Oterminal (O11, O12; O21, O22; O31, O32; O41, O42; O51, O52; O61, O62) ≈ 140.5(2)/141.75,
S1–O121 161.8(1)/164.12, S2–O121 163.8(2)/164.2, S3–O341 163.6(2)/164.14, S4–O341 162.3(2)/164.12, S5–
O561 163.1(2)/164.13, S6–O561 162.8(2)/164.13, N1–S1 170.6(2)/172.64; N1–S3 171.6(2)/172.67, N1–S5
171.6(2)/172.67, N2–S2 171.5(2)/172.67, N2–S4 171.6(2)/172.69, N2–S6 171.6(2)/172.69.
The core cage of the S6N2O15 molecule (emphasized by black bonds in figure 3) has the shape of the bicyclic
organic molecule bicyclo[3.3.3]undecane. Such a cage has not been observed before in the chemistry of sulphur
nitride-oxides, although a significant number of compounds has been observed in the system S/N/O (Fig. 4).[20]
[21]
With respect to the structural characterizations these compounds show chain structures like S2(NSO)2
or
S3N2O2,[22,23] cyclic molecules like S3N2O5,[24,25] S7N6O8,[26] and S4N4O2,[27] as well as ionic species like
3
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(NO)2[S4O13].[28] The most unique compound among the molecular sulfur nitride-oxides is probably the adduct
S4N4·SO3 which has already been mentioned in the introduction.[6] Another outstanding molecule is sulfuryl azide,
SO2(N3)2,[29] which presents the nitrogen richest molecule in the S/N/O system. The new compound S6N2O15 is up
to now the sulfur nitride-oxide with the highest oxygen and the lowest nitrogen content.
Figure 4 Molecular compounds in the system S/N/O according to the atomic ratios
The successful synthesis of S6N2O15 by the reaction of P3N3Cl6 and SO3 leads to several new directions that are
worthwhile to be followed. On one hand, the reaction might be also suitable for the preparation of the rarely seen
nitrido-tris-sulfates, if suitable cations are added to the reaction mixture. On the other hand, variation of the
reaction conditions may lead to other species is thinkable, for example the [N(SO3)2]3– ion mentioned in the
introduction (cf. Fig. 1). Moreover, even anions with both, tri- and bi-coordinate nitrogen atoms come into sight,
for example the hypothetical anion [S6N3O12]3–. Finally, it is worthwhile to remember that there is no nitrido-
sulfate ion, [SN4]6–, known up to now, also not in form of condensed species. This finding for sulfur is in strong
contrast to the findings for the neighbor elements silicon and phosphorous. Only in organic derivatives, like the
famous [S(NtBu)4]2– ion, a complete nitrogen coordination is possible so far.[30]
Experimental Section
Synthesis
SO3 was obtained in a specially designed apparatus for the generation, distillation and the subsequent transfer into glass
ampoules under nitrogen gas. For this purpose, fuming sulfuric acid (5 mL, 65% SO3, used as received, Merck, Darmstadt,
Germany) was slowly added via a dropping funnel into a 500 mL flask with P4O10 (250 g, > 97%, Merck, Darmstadt). At the
same time, the flask was heated at 130 °C and the generated SO3 distilled into a connected burette body (scaling 0.01 ml ±
0.01). A connected glass ampoule (l = 200 mm, ø = 20 mm, thickness of the tube wall = 2 mm) containing P3N3Cl6 (150 mg,
0.431 mmol) was then filled with 1.92 g (24.0 mmol) of freshly distilled SO3. The ampoule was torch sealed and heated to
80 °C in a tube furnace. This temperature was maintained for 48 h and the reaction mixture was then allowed to cool down to
room temperature within 100 h. S6N2O15 could be obtained in form of transparent highly moisture sensitive crystals, which
have to be handled under inert gas conditions.
4
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Caution! SO3 is a strong oxidizer which needs careful handling. During the reaction and even after cooling down to room
temperature the glass tube might be under pressure. The tube should be cooled with liquid nitrogen before opened.
Structure determination
Crystal structure determination has been performed at the P24.1 beamline of the PETRA III facility at German Electron
Synchrotron (DESY) Hamburg (Germany). Therefore, single crystals of S6N2O15 were prepared under inert oil and selected
with the aid of a polarization filter of a light microscope. Attached to a micromount, one single crystal was positioned in the
cold nitrogen gas stream (100.0(2) K) of the single crystal diffractometer (Huber 4-circle Kappa, P24 Beamline, Petra III) and
intensity data was collected. Both the collect intensity data were reduced, and a cell refinement was carried out.[1] The structure
solution under the assumption of the respective space group was successful by intrinsic phasing (SHELXT).[2] Finally,
anisotropic displacement parameters were introduced and a multi-scan absorption correction was applied to the reflection data.
Atomic positions and further details of the crystal structures can be obtained from the joint CCDC/FIZ Karlsruhe deposition
service on quoting the deposition number 1901458.
IR analysis
IR spectroscopic data were collected with a Bruker Alpha Platinum spectrometer, using the ATR method (attenuated total
reflection) in transmission mode. The spectra were measured in the bulk phase on several collected crystals in the range from
4000 to 570 cm–1. The IR spectroscopic data were processed and corrected for atmospheric influence by using the OPUS 7.5
software. Important IR bands in cm–1 (exptl./calcd.): 1512/1535, 1478/1487, 1232/1253, 889/893, 850/831, 699/708, 531/560,
414/403.
Calculation method
A full geometry optimization of the trimer of S6N2O15 was performed within density functional theory (DFT) using the PBE0
exchange-correlation functional and a cc-pVTZ basis set. The calculations were also used for assigning the IR frequencies.
Throughout the study the Turbomole program package was used.
Acknowledgement
We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental
facilities. Parts of this research were carried out at PETRA III and we would like to thank Carsten Paulmann and Heiko Schulz-
Ritter for assistance in using the P24 EH1 X-ray Diffraction Kappa-diffractometer. Furthermore, we are indebted to André
Santos Martins for technical assistance.
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