Ammelinium Sulfate Monohydrate and Ammelinium Sulfate Cyanuric Acid – Synthesis and Structural Characterization

Article abstract of DOI:10.1002/zaac.201900042

Two ionic carbon nitride type compounds containing the ammelinium cation, ammelinium sulfate cyanuric acid (6C₃N₅H₆O⁺·3SO₄ ^2–·1?C₃N₃H₃O₃·H₂O) (1) and ammelinium sulfate monohydrate (2C₃N₅H₆O⁺·SO₄ ^2–·H₂O) (2) were synthesized through hydrolysis of melam (C₆N₁₁H₉) in diluted sulfuric acid. 1 crystallizes in hexagonal space group P6₃ (no. 173) with lattice parameters of a = 14.642(3), c = 13.113(4), and Z = 2. The structure is comprised of protonated ammelinium ions and neutral cyanuric acid molecules, which form a layered structure, as well as sulfate ions that span through these layers. 2 crystallizes in the triclinic space group P1 with lattice parameters of a = 7.404(3), b = 9.673(4), c = 10.040(4), α = 91.098(15), β = 109.884(10), γ = 92.567(13), and Z = 2. As for 1, the ammelinium rings form layers with the sulfate ions located in between. In both structures, no extended hydrogen bond networks between the respective triazine-based molecules are formed. Instead, single molecules or small building blocks occur isolated and interact primarily with sulfate anions. Compound 1, which was obtained phase pure, was further investigated by FTIR spectroscopy, solid-state NMR spectroscopy and powder X-ray diffractometry.

Full text of DOI:10.1002/zaac.201900042

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201900042
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Ammelinium Sulfate Monohydrate and Ammelinium Sulfate
Cyanuric Acid – Synthesis and Structural Characterization
Fabian K. Kessler[a] and Wolfgang Schnick*[a]
Abstract. Two ionic carbon nitride type compounds containing the
ammelinium cation, ammelinium sulfate cyanuric acid 91.098(15), β = 109.884(10), γ = 92.567(13), and Z = 2. As for 1, the
·H O) (1) and ammelinium sulfate ammelinium rings form layers with the sulfate ions located in between.
O) (2) were synthesized through In both structures, no extended hydrogen bond networks between the
) in diluted sulfuric acid. 1 crystallizes respective triazine-based molecules are formed. Instead, single mol-
(no. 173) with lattice parameters of a = ecules or small building blocks occur isolated and interact primarily
lattice parameters of a = 7.404(3), b = 9.673(4), c = 10.040(4), α =
+
2–
2
(
6C
3
N
5
H
6
O ·3SO
4
·1 ͞
3
C
3
N
3
H
3
2–
O
3
2
+
monohydrate (2C
3
5 6 4 2
N H O ·SO ·H
hydrolysis of melam (C
in hexagonal space group P6
4.642(3), c = 13.113(4), and Z = 2. The structure is comprised of
6 11 9
N H
3
1
with sulfate anions. Compound 1, which was obtained phase pure, was
further investigated by FTIR spectroscopy, solid-state NMR spec-
troscopy and powder X-ray diffractometry.
protonated ammelinium ions and neutral cyanuric acid molecules,
which form a layered structure, as well as sulfate ions that span
¯
through these layers. 2 crystallizes in the triclinic space group P1 with
Introduction
insoluble compounds, primarily adducts of melamine and
cyanuric acid, which lead to renal failure by blocking of kid-
In 1834, Liebig laid the foundations of carbon nitride chem-
istry with his extensive work on certain compounds that later
proofed to be based on triazine and heptazine building units,
respectively.[ Some of these compounds – such as melamine,
which finds various applications in the form of melamine-
[19]
ney tubules.
These melamine cyanurate (1:1) complexes
have become famous before as an example of noncovalent syn-
thesis of nanostructured materials via self-assembly and a
prototype for the arrays of hydrogen bonds that can be found
1]
[20–22]
between the base pairs in DNA and RNA.
[2–5]
formaldehyde resins,
intensively investigated as a photocatalyst material
or melon, which recently has been
[6–12]
– rose
to great industrial and scientific relevance. Almost two centu-
ries after their initial discovery, these compounds and their de-
rivatives are still subject to scientific investigations and con-
tinue to raise interest.
Less attention has been shed on ammeline and ammelide. Scheme 1. Triazine compounds, from left to right: melamine, amme-
line, ammelide, cyanuric acid.
These triazine based compounds with compositions of
(
C N )(NH ) (OH) and (C N )(NH )(OH) represent the par-
3 3 2 2 3 3 2 2
In the 2008 incident, more than 50.000 infants were affec-
ted, with six confirmed deaths caused by contaminated milk
powder. As a result, the majority of recent research concerned
with ammeline, ammelide and cyanuric acid revolves around
the simultaneous detection and quantification of these mol-
ecules through a plethora of analytical methods.[ However,
very little research is conducted towards their properties, char-
acterization, and reactivity.
tial hydrolysis products of melamine, whose final hydrolysis
product would be cyanuric acid; or the partial ammonolysis
products of cyanuric acid, vice versa (Scheme 1).[
13–15]
All
four of these closely related molecules came to unfortunate
fame through two major food adulteration scandals in 2007
and 2008, in which melamine and cyanuric acid were added
23]
to infant formula and pet food to increase the apparent protein
contents.[
16,17]
Since protein content in foods is often indirectly
In recent years, the deamination of ammeline has been
measured by the Kjeldahl method, which unspecifically quan-
tifies nitrogen, certain manufacturers added inexpensive nitro-
gen-rich materials to increase their economic profits.
studied in detail by means of DFT calculations.[
24,25]
Under-
[18]
standing of this reaction is of great interest in view of the
aforementioned hydrolysis of melamine, which is thought to
be the cause for the formation of melamine cyanurate com-
plexes from melamine contaminated foodstuff.
Another subject that has been intensely discussed is
tautomerism of ammeline and the preference of either the keto
or hydroxy form of the ring bound oxygen atom (Scheme 2).
How-
ever, partial hydrolysis of melamine led to the formation of
*
Prof. Dr.·W. Schnick
E-Mail: wolfgang.schnick@uni-muenchen.de
[
a] Department of Chemistry
Chair in Inorganic Solid-State Chemistry
University of Munich (LMU)
Butenandtstraße 5–13
Ab initio calculations predicted the hydroxy tautomer to be the
8
1377 Munich, Germany
[26]
most stable form in the gas phase.
However, the keto form
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201900042 or from the au-
thor.
appears to be more prevalent in solid-state structures, in which
keto dimers are formed. Hydrated keto molecules are predicted
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Journal of Inorganic and General Chemistry
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to be the most stable species in aqueous solution.[27] Indepen- dissolving melam, [C N (NH ) ]NH, in diluted mineral acids,
3
3
2 2
dent of state, the quinoid carbonyl tautomer is predicted to be however, provided little characterization and no structural
the least stable form and no experimental evidence of this tau- data.[ Later, Lotsch et al. described the crystal structure of
30]
tomer has been reported.
melamium perchlorate hydrate that has been obtained by the
same synthesis route.[
31]
Trying to reproduce the synthesis of melamium sulfate
yielded no product at all. We thus varied the concentration of
sulfuric acid as well as the reaction and crystallization times.
Given sufficient time to crystallize, a large volume of colorless
crystals was obtained. However, none of the prepared samples
proved to be the target compound. Instead, a combined ap-
proach of elemental analysis, single-crystal XRD, FTIR spec-
troscopy, and solid-state NMR spectroscopy identified two dif-
ferent yet undescribed ammelinium salts.
Scheme 2. Different tautomers of ammeline, from left to right: hy-
droxy form, keto form, quinoid carbonyl form.
As in pure ammeline, the keto form is present in various
ammelinium salts. Ammelinium chloride, bromide, and nitrate
[28]
The ammelinium sulfate cyanuric acid adduct (1) was ini-
can be obtained from acidic hydrolysis of melam, C N H .
6
11 9
+
2–
tially assumed to possess the sum formula 6C N H O ·3SO ·
The structure of all three salts was established from single-
crystal X-ray diffractometry. C–O bond lengths of 1.22 Å are
indicative for carbonyl C=O bonds. A range of further ammeli-
3
5
6
4
2
C N H O based on elemental analysis and single crystal
3 3 3 3
XRD. However, one cyanuric acid molecule exhibits only
nium salts, namely the nitrate, sulfate, perchlorate, and partial site occupation, resulting in the sum formula
+
2–
2
5
-tetrazolate have been discussed as insensitive energetic ma- 6C
terials.
3
N
5
H
6
O ·3SO
4
·1 ͞
3 3 3 3 3 2
C N H O ·H O instead. The com-
[29]
C–O bond lengths in these compounds are compar- pound was obtained as either tapering needle-shaped crystals,
able to those in the abovementioned salts, so a trend is evident. with bushels of smaller needles often growing on larger ones
However, ammelinium salts are structurally interesting beyond (Figure S1a, Supporting Information); or in the form of large,
the preference for keto, hydroxy, or quinoid carbonyl tauto- hexagonal pyramids that showed a high degree of intergrowth
mers. As is not uncommon for crystalline triazine or heptazine (Figure S1b, Supporting Information). However, neither sin-
compounds, none of their structures are isotypic. Crystal struc- gle-crystal nor powder X-ray diffraction data shows any differ-
tures are composed of different arrays of strands or layers, ence between the different appearances. It is thus assumed that
which can be stacked in several different patterns. Hydrogen these differences do not result from different polymorphs, but
bridge networks between the ammelinium rings themselves as are due to external influences. Since the reaction conditions
well as their respective anions appear to be the dominating for both samples were chosen identically, however, we are not
structure directing factor, leading to a vast range of possible able to name the respective parameter.
bonding motifs.
A less well-defined sample could be obtained by the same
Herein, we expand the group of ammelinium salts by two synthesis route. Optical microscopy showed spheroid agglom-
new representatives, both containing the sulfate anion. Ammel- erates of colorless, polycrystalline material that showed no re-
inium sulfate monohydrate is a sulfate salt, which, however, is semblance to the characteristically shaped single-crystals of 1.
structurally independent from the known ammelinium sulf- However, a single-crystalline side phase of translucent plate-
ate.[ Ammelinium sulfate cyanuric acid is an adduct between lets could be detected within this sample. A specimen was iso-
ammelinium sulfate and non-protonated cyanuric acid that em- lated and identified as an unknown polymorph of ammelinium
29]
+
2–
phasizes the partial hydrolysis process of melamine and its de- sulfate monohydrate 2C N H O ·SO ·H O (2).
3
5
6
4
2
rivatives. Both of these salts have been structurally elucidated
The composition of 1 was investigated by combustion
by single-crystal X-ray diffractometry. Further spectroscopic analysis. Results are listed in Table 1. It can be seen that ob-
characterization was performed for the adduct compound.
served values fit reasonably well to the calculated sum for-
mula. Deviations are attributed to disorder phenomena, which
are discussed below.
Results and Discussion
The identified composition is in good agreement with the
The initial scope of this work was not the synthesis of crystal structure as established from single-crystal XRD data
ammelinium salts, but to synthesize and structurally elucidate (see below). It assumes the coexistence of ammeline and cyan-
a
“
related
triazine-based
salt,
melamium
sulfate, uric acid within the structure. The formation of these from
(C N H ) ·H SO ”. Gavrilova et al. reported on the synthe- melam is readily explained. As has been shown for ammelin-
6
11 9 2
2
4
sis of this compound and a range of other melamium salts by ium chloride, bromide, and nitrate, strong mineral acids cata-
Table 1. Elemental composition of 1. All values in w%. For observed values, Δ is the difference to 100%, which is supposed to be oxygen.
N
C
H
S
Δ / O
1
6
6
(obs.)
37.94
38.35
21.10
21.92
20.89
3.47
3.22
3.51
7.63
7.32
7.60
29.86
29.20
30.35
+
2–
2–
C
C
3
N
N
5
H
H
6
O ·3SO
4
4
·2C
·1 ͞
3
N
3
H
3
O
3
(calcd.)
2
·H O (calcd.) 37.65
+
2
3
5
6
O ·3SO
3
C
3
N
3
H
3
O
3
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+
2–
2
lyze the hydrolysis of melam to ammelinium (Scheme 3). In with two formula units of 6C N H O ·3SO ·1 ͞ C N H O ·
3
5
6
4
3 3 3 3 3
contrast, weak acids such as perchloric acid only protonate the H O in the unit cell. This results in a total of 16 triazine deriva-
2
molecule.[
28,31]
Since sulfuric acid is a strong and oxidizing tives per unit cell. These ring molecules form layers stacked
acid, even a diluted solution appears to be able to not only along c, which are very slightly corrugated along a as well as
hydrolyze melam to ammelinium, but to continue the hydroly- b axes (Figure 1).
sis along the path described in Scheme 1. This is not surpris-
ing, considering that this reaction is well known. However, it
is remarkable to find ammeline next to cyanuric acid with no
traces of the intermediate ammelide. We propose two possible
explanations for this: either, the hydrolysis of ammeline is sig-
nificantly slower than the hydrolysis of ammelide, resulting in
all the formed ammelide to hydrolyze further to the final hy-
drolysis product; or, the solubility of ammelide in the chosen
reaction medium significantly exceeds that of ammeline and
cyanuric acid, so that ammelide remains in solution while am-
meline and cyanuric acid precipitate via adduct formation.
Scheme 3. Hydrolysis of melam towards ammelinium.
Figure 1. Crystallographic structure of 1, viewing direction along b.
C atoms in black, N atoms in blue, H atoms in gray, O atoms in red,
S atoms in yellow. Disorder for sulfate anions and cyanuric acid mol-
ecules omitted for reasons of visibility.
Single-crystal X-ray diffraction played a major part in not
only structure elucidation of 1 but also in the determination
of its composition. Data were obtained from a needle-shaped
specimen, which was refined as a merohedral twin, although
Each unit cell contains four independent layers that show no
with a rather small twin population. Details of measurement particular stacking order towards each other. However, the first
as well as structure solution and refinement can be found in and third layers as well as the second and forth layer are con-
Table 2. 1 crystallizes in hexagonal space group P6 (no. 173) nected by a twofold screw axis, respectively, resulting in two
3
Table 2. Crystallographic data for ammelinium sulfate cyanuric acid (1) and ammelinium sulfate monohydrate (2).
Ammelinium sulfate cyanuric acid (1)
Ammelinium sulfate monohydrate (2)
Empirical formula
C
22
N
34
H
44
S
3
O
24
6 10 7
C N H14SO
370.30
Formula weight /g·mol–1
1265.09
hexagonal
P6 (no. 173)
Mo-K , 0.71073
Crystal system
Space group
Radiation, λ /ppm
triclinic
¯
3
P1 (no. 2)
Mo-K , 0.71073
α
α
a /Å
b /Å
c /Å
α /°
14.642(3)
14.642
13.113(4)
90
90
120
2434.6(13)
2
1.726
100(2)
0.273
2.234 Յ θ Յ 27.220
1308
7.404(3)
9.673(4)
10.040(4)
91.098(15)
109.884(10)
92.567(13)
675.0(4)
2
1.822
100(2)
0.306
2.930 Յ θ Յ 25.000
384
β /°
γ /° 3
V /Å
Z
Density (calcd.) /g·cm–1
Temperature /K
Absorption coefficient /mm–1
Diffraction range /°
F(000)
Index range
–18 Յ h Յ 18
–8 Յ h Յ 8
–11 Յ k Յ 11
–11 Յ l Յ 11
multi-scan
288
–
–
18 Յ k Յ 18
16 Յ l Յ 16
Absorption correction
No. of parameters
multi-scan
398
No. of independent reflections
No. of independent reflections (I Յ 2σ)
3622
3410
2383
2013
R
GooF
σ
, Rint
0.0134, 0.0323
1.105
0.408, 0.405
1.146
R indices (I Յ 2σ)
R indices (all data)
R
1
= 0.0522, wR
= 0.0560, wR
2
= 0.1523
= 0.1551
R
1
= 0.0472, wR
= 0.0590, wR
2
= 0.1108
= 0.1148
R
1
2
R
1
2
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different types of layers. Exemplarily, a layer of type 2 is de- donor-acceptor distances of 2.67 to 2.88 Å were found. In type
picted in Figure 2. The two different types of layers are more 1 layers, the tips of these tetrahedra are oriented in the same
or less identical apart from a shift of origin, however, type 1 – direction as for the tetrahedra whose tips occupy the other half
represented by the first and third layer of each unit cell, respec- of the cavities. In type 2 layers, however, sulfur and tip oxygen
tively – is affected by structural disorder, which is discussed atom of this sulfate tetrahedron can be oriented above as well
below. A comparison of the different types of layers can be as below the layer plane – i.e. pointing in either direction –
found in Figure S2 (Supporting Information).
due to disorder affecting these sulfate anions.
Diffraction images of 1 show very weak additional reflec-
tions that indicate a threefold superstructure along c (Figure
S3, Supporting Information). However, it proved impossible to
incorporate these in the refinement through use of a unit cell
with tripled c axis, possibly due to the very low intensities of
the superstructure reflections. Within the structure, this disor-
der is manifested in the cyanuric acid molecules of type 1 lay-
ers. Initially, disorder was only assumed for the sulfate tetrahe-
dra located between these molecules and cyanuric acid itself
was refined as fully occupied. However, this yielded aniso-
tropic displacement ellipsoids that were rather inconsistent
with the rest of the structure. A more consistent model could
be obtained by partially replacing cyanuric acid molecules by
three water molecules located in the sites of cyanuric acid keto
oxygen atoms. This not only significantly improved refinement
parameters but also removed an improbably short distance be-
Figure 2. Single layer (type 2) of 1, viewing direction along c. C atoms tween a sulfate tetrahedron tip and the cyanuric acid triazine
in black, N atoms in blue, H atoms in gray, O atoms in red, S atoms nucleus for the case of the tetrahedron pointing towards this
in yellow. Dashed black bonds indicate hydrogen bridge interactions.
molecule. Freely refining the occupancy of cyanuric acid
Half of the sulfate anions is located above the layer plane. Disorder
against water, coupled to the respective orientation of the sulf-
for sulfate anions omitted for reasons of visibility.
ate tetrahedron, yields a ratio close to 1:1. However, the dif-
fraction pattern shows no indication of such a twofold super-
In each of these layers, the structure is comprised of triangu-
structure. Neither would the resulting sum formula agree with
lar motifs, in which three ammelinium units are arranged
the experimentally observed elemental composition. The occu-
around one cyanuric acid molecule. In this, the protonation
pancy was thus restrained to 0.6667 for cyanuric acid atoms
and 0.3333 for water to better fit those observations, which
yields only marginally poorer figures of merit. Structurally,
this translates to the cyanuric acid molecule being replaced by
three water molecules in every third layer of type 1. The disor-
dered sulfate tetrahedra between the cyanuric acid units point
away from the ring when the site is occupied by cyanuric acid
to thus avoid short interatomic separations and point towards
the respective site when it is instead occupied by water mol-
ecules. Hydrogen atoms found for these water molecules as
established from residual electron density are oriented to allow
hydrogen bridge interactions towards these sulfate oxygen
pattern allows for the formation of three hydrogen bridges per
ammelinium unit: twice between amino N–H groups and cyan-
uric acid keto O atoms and once between protonated cyanuric
acid ring N atoms and non-protonated ring N atoms of ammeli-
nium. Donor-acceptor distances range from 2.89 to 2.98 Å for
the former and 2.79 to 2.88 Å for the latter. Every ammelinium
unit is exclusive to one such triangular motif. These groups
are isolated from each other with no hydrogen bridges or other
interactions between them. However, ammelinium units from
three adjacent triangular motifs form cavities that accommo-
date sulfate anions in two different fashions.
For half of the cavities, the tip of a sulfate tetrahedron is
oriented towards its center. In type 1 layers, this oxygen atom atoms.
is located within the layer plane. This allows for the formation
It has to be noted, however, that other interpretations of the
of three additional hydrogen bridges towards protonated ring observed disorder phenomena are conceivable as well. The
N atoms. Here, donor-acceptor distances of 2.93 Å were found. threefold superstructure might exclusively result from partial
In contrast, in type 2 layers the respective oxygen atom is ordering of the mentioned sulfate group, while the deviations
found considerably above the layer plane, resulting in donor- in elemental composition might be explained by coexistence
acceptor distances of 2.98 Å for potential hydrogen bridges, as of crystals that do or do not show disorder of the cyanuric
well as hydrogen atoms forming a rather steep 20.3° angle acid / water molecules. We propose the herein described disor-
towards the triazine ring plane. This orientation makes any der model since it deems us to best explain the entirety of
kind of strong interactions unlikely. In the other half of the observations.
cavities, three oxygen atoms forming one face of a sulfate tet-
Cyanuric acid molecules in the adduct structure show rather
rahedron are located in a fashion that allows for the formation uniform C–N bond lengths of 1.36 to 1.38 Å, which are well
of six hydrogen bridges – three towards protonated ring N in line with expected distances for triazine derivatives. The C–
atoms and towards amino groups, respectively. Rather short O bond lengths of 1.20 to 1.23 Å clearly indicate a double
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bond and thus a keto tautomer, as is expected for cyanuric
acid. Triazine rings moderately deviate from ideal arrangement
as the angles range from 115.4 to 124.6°. In ammelinium, a
larger range of C–N bond lengths within the rings was ob-
served, ranging from 1.32 to 1.37 Å. C–N bond lengths
towards the amino groups are a bit shorter (1.29 to 1.31 Å).
This phenomenon has been reported for ammelinium chloride,
bromide, and nitrate as well[ and can be explained as a result
of the mixed functionalization of the triazine ring. The electro-
negative oxygen atom of the keto group possesses an electron
withdrawing inductive effect on the conjugated aromatic sys-
tem of the triazine nucleus. This electron depletion increases
the pull on electrons located at the amino groups, resulting in
a partial double-bond character for the C–Namino bond under
participation of the nitrogen free electron pair and thus reduc-
28]
Figure 3. Crystallographic structure of 2, viewing direction along a.
C atoms in black, N atoms in blue, H atoms in gray, O atoms in red,
ing the distance. Calculations of molecular orbitals for amme- S atoms in yellow, sites with mixed O/N occupation in purple.
line or ammelinium might be a suitable way to test this hypoth-
esis but are beyond the scope of this work.
As for cyanuric acid, C–O distances of 1.20 to 1.21 Å indi-
cate the keto tautomer. Deviations from ideal hexagonal sym-
metry in ammelinium can be explained by the mixed substitu-
tion of the terminal groups. N–C–N angles found at the O atom
range from 113.4 to 113.8°, while remaining N–C–N angles
span 120.6 to 121.8°. Contrary, C–N–C angles between 122.5
and 123.9° were found, except for those opposite the keto
group, which showed angles of only 117.5 to 117.6°. All of
these values are well in line with related triazine-based com-
pounds and thus meet the expectations. Atomic coordinates as
Figure 4. Complete repetition unit of a layer of 2, viewing direction
perpendicular to the layer plane. C atoms in black, N atoms in blue,
H atoms in gray, O atoms in red, S atoms in yellow, sites with mixed
well as further bond lengths can be found in Tables S1 and S2
(Supporting Information).
All three independent sulfate anions are located on threefold O/N occupation in purple. 2ϫ3 unit cells plotted.
screw axes. Tetrahedra are thus generated by cell symmetry.
However, it was shown that better refinement parameters as
well as more fitting hydrogen bridge donor acceptor distances
of 0.59 to 0.41, which is attributed to these hydrogen bridge
interactions. The local arrangement necessitates an amino do-
nor group opposite to a keto acceptor group, elsewise, how-
ever, the two respective sites are absolutely equivalent, thus
allowing for both functionalities. Furthermore, the ring N atom
of one ammelinium unit is required to be protonated while the
other must be un-protonated. To achieve this pattern, every
second ammelinium unit has to be protonated not in the pre-
ferred ortho position, but instead in the usually un-protonated
para position.
were obtained when modeling a complete tetrahedron through
1
restraints and refining it with occupancy of ͞ .
3
+
2–
For 2C N H O ·SO ·H O a crystal structure has already
3
5
6
4
2
been published.[
29]
However, the structure established from
single-crystal measurement of 2 is completely unrelated and
appears to represent a different polymorph of this compound.
¯
2
crystallizes in triclinic space group P1 with two formula
units per unit cell, whereas for the polymorph established in
literature monoclinic space group P2 /c has been reported. A
No network formation could be observed for the remaining
1
summary of crystallographic data is presented in Table 2. The ammelinium units. Instead, the ammelinium ions interact with
structure is layered, with ammelinium ions forming planar sulfate groups and water located in two different types of cavi-
sheets and sulfate tetrahedra located between these (Figure 3). ties. Type one is formed by three ammelinium ions and con-
Layers are arranged in an abc-stacking pattern. Consequently, tains three oxygen atoms forming the face of a sulfate tetrahe-
the repetition unit within one layer equals three unit cells (Fig- dron in an analogous way to the one described for 1. Six
ure 4).
hydrogen bridges per cavity are formed, with donor-acceptor
Each layer is constructed from strands of ammelinium ions, distances ranging from 2.75 to 2.85 Å. Each repetition unit
which are mostly isolated. In only one single pair of adjacent contains two of these cavities with the sulfate tetrahedron
strands, molecules are arranged in a way to allow the forma- pointing upwards in one and downwards in the other. The tip
tion of hydrogen bridges. In these, two crystallographically oxygen atoms of these tetrahedra are located in the second
equivalent molecules form three hydrogen bridges with donor- type of cavity. These are formed by four ammelinium ions and
acceptor distances of 2.88 and 2.93 Å, respectively. The par- contain two oxygen atoms from sulfate anions (one pointing
ticipating keto and amino group in this ammelinium unit are upwards and one downwards, respectively) as well as two
disordered and share a crystallographic site with a distribution water molecules. Two ammelinium amino groups form
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
hydrogen bridges towards each sulfate oxygen atom with do- been reported, which are blue-shifted compared to pure amme-
–1
nor-acceptor distances of 2.88 and 2.90 Å. However, the ac- line (1718 cm ). The observed values appear to be well in line
–1
ceptor is located slightly above the layer plane, resulting in O– with both of these. We attribute the band at 1722 cm to cyan-
–1
N–H angles of 16.3 to 19.1°, thus weakening the hydrogen uric acid, for which a vibration band at 1720 cm has been
bonds. Furthermore, two hydrogen bridges are formed towards reported.[ The ammelinium keto band would thus be shifted
each water molecule with ammelinium acting as donor in one in respect to ammeline and well in line with other ammelinium
case and as acceptor in the other. Here, distances of 2.73 and salts.[28,29] Likewise, the band attributed to triazine ring
32]
–1
2
.90 Å were found, indicating medium to strong bonds. No stretching vibrations by Lotsch et al. is found at 1661 cm ,
hydrogen bonds appear to occur between water and sulfate shifted in respect to ammeline. The characteristic triazine out-
–1
since neither of the water protons is oriented towards the sulf- of-plane breathing mode, typically located around 800 cm , is
–1
ate ion. These intense interactions between ammelinium and found at 764 cm . This red-shift is typical for protonated tri-
sulfate ions are in stark contrast to the monoclinic polymorph, azine rings and vibration bands for other ammelinium salts
–1 [28,29]
in which significantly less hydrogen bridges are formed be- occur between 780 and 765 cm .
Again, the respective
tween different ions and whose structure appears to be much band of cyanuric acid would be expected in a comparable spec-
–1
more dominated by ammelinium-ammelinium interactions.
Hydrogen bridge interactions in 2 are depicted in Figure S4 possible.[ We attribute the intense band at 1080 cm to sulf-
Supporting Information). ate S–O vibrations. A similar band is found in 2,4-diamino-
The two ammelinium ions making up the structure differ 1,3,5-triazinate-6-one sulfate centered at 1105 cm–1 and vari-
slightly. Molecule 1 shows bond lengths and angles that are ous inorganic sulfates show vibrations in a comparable
tral region (772 cm ), so an unambiguous assignment is not
32]
–1
(
very characteristic for ammelinium. Ring C–N bonds range in range.[
29,33]
The spectrum appears consistent with the structure
their lengths from 1.33 to 1.38 Å with the longest bonds adja- established from single-crystal XRD.
cent to the keto group and the shortest bonds opposite this
group. Amino N atoms show bond lengths of 1.31 to 1.32 Å
towards the ring and are thus slightly shorter than the ring
bonds themselves, as already discussed for 1. The C–O bond
length was found to be 1.21 Å, unambiguously identifying a
keto group. The ring is slightly compressed with angles of
113.7 and 116.7° at the keto group and the opposite N atom
and angles of 121.5 to 123.5° for all other angles with no sig-
nificant differentiation between C–N–C and N–C–N angles.
All of these values are well in line with those found in other
ammelinium compounds – including 1 and the monoclinic
polymorph of ammelinium sulfate monohydrate – which show
the same trends and peculiarities. Molecule 2, as already men-
tioned above, shows disorder in two of its three functionalities.
The respective sites could be refined as partially occupied by
a keto as well as an amino group. Accordingly, the remaining
structure is most likely an average of two molecules in these
two orientations. The triazine ring is an almost perfect hexagon
with rather even ring C–N bond lengths of 1.35 to 1.36 Å.
Likewise, all angles are found in the narrow range between
Figure 5. FT-IR spectrum of 1.
The sample, from which 2 was obtained was investigated
by FT-IR spectroscopy and powder X-ray diffractometry. The
polycrystalline material making up the bulk of the sample was
shown to differ from 2 but could not be identified from its
diffractogram. Spectroscopic measurements yielded only very
119.2 and 120.9°. For the fully occupied amino group position,
a C–N bond length of 1.30 Å was observed, while for sites of
mixed occupancy, lengths of 1.31 Å were found. A C–O bond limited information about its nature. However, two IR bands
length of 1.24 Å was obtained for the keto groups. This is at 1767 and 1728 cm–1 were both assigned to ν(C=O) vi-
slightly larger than expected, however, might be interpreted as brations of keto groups deviating in their structural environ-
an artefact of the refinement of the mixed occupancy. Keto and ment and a very intense band originating from sulfate groups
–1
amino groups, respectively, show no significant divergences could be found at 1089 cm . We thus assume the unknown
for the two sites. Atomic coordinates and bond lengths for 2 bulk phase to be similar in composition to 1 and 2. However,
can be found in Tables S3 and S4 (Supporting Information).
FT-IR spectroscopy of 1 was performed in reflection geome- minor side phase 2.
Solid-state MAS NMR spectra of the nuclei H, 13C and 15
try using the pure substance (Figure 5). The spectrum pos-
available methods allowed no further characterization of the
1
N
sesses certain noteworthy characteristics that have also been were recorded for compound 1 to verify the parallel presence
reported for ammelinium chloride, bromide, and nitrate and of ammelinium and cyanuric acid within the structure. Spectra
[
28]
13
15
1
thus seem to be common to salts of protonated ammeline.
for C and N NMR are depicted in Figure 6. The H NMR
–1
ν(C=O) vibration bands are found at 1754 and 1722 cm . For spectrum shows a rather broad main signal at δ = 12.21 ppm
ammelinium salts, values between 1737 and 1757 cm–1 have as well as a smaller signal at δ = 6.17 ppm that is partially
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
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Zeitschrift für anorganische und allgemeine Chemie
obscured by the main resonance (Figure S5, Supporting Infor- rings. We assign these to ammelinium and cyanuric acid rings,
mation). The main signal is attributed to protonated triazine respectively. However, no reference data could be obtained,
ring N atoms of ammelinium as well as cyanuric acid. The thus making separate assignments difficult. No signals were
signal at a chemical shift of 6.17 ppm represents amino groups found in the spectral region around –200 ppm in which tertiary
of ammelinium.
N atoms would be expected. This, however, does not indicate
a complete protonation of the triazine rings, but can rather be
explained as an artefact of the applied cross-polarization pulse
sequence, which amplifies signals according to their proximity
to protons, resulting in poor intensities for tertiary N atoms, so
that potential signals might simply be too weak to be resolved.
Rietveld refinement for 1 was performed to verify phase
purity and establish room temperature cell parameters. The re-
sults are summarized in Figure 7 and Table S5 (Supporting
Information). No reflections that could not be explained by
Bragg reflections of ammelinium sulfate cyanuric acid as es-
tablished from single-crystal XRD could be observed in the
powder X-ray diffractogram. We thus conclude that the sample
is phase pure and no other compounds – such as compound
2
– formed during the precipitation of the adduct. However, as
seen in Figure 7, not all reflections could be refined with
proper intensities. We attribute this to the superstructure, which
has not been taken into account, as well as the difference in
temperature between the single-crystal and powder XRD mea-
surement.
Figure 6. 13C (top) and 15N (bottom) solid-state MAS NMR spectra
of 1.
Four different signals could be observed in the 13C NMR
spectrum, the most intense one at δ = 157.4 ppm. Two ad-
ditional signals of lower intensity were found at δ = 153.9 and
1
47.2 ppm and a small shoulder at δ = 148.6 ppm. A database
Figure 7. Rietveld plot of 1. Measured data points are depicted as
search yielded reported shifts of 154.5 and 159.5 ppm for am- black circles, the calculated model is depicted in red, the difference
meline and of 150 ppm for cyanuric acid. We thus interpret the
signals at δ = 157.4 and 153.9 ppm as belonging to ammeline
plot Yobs–Ycalc in gray. Blue tick marks indicate theoretical Bragg re-
flection positions.
and the two close signals at δ = 148.6 and 147.2 ppm as re-
Room temperature cell parameters have been determined to
sulting from two cyanuric acid molecules of different chemical a = 14.64540(39) Å and c = 13.12999(35) Å. As is often the
environment. These different environments have been found in case for layered carbon nitride type compounds,[ the elong-
the crystal structure. Furthermore, the observed chemical shifts ation along the stacking direction – here c – is significantly
clearly identify the triazine derivatives that make up 1 as am- larger than along the other directions. This can be explained
melinium and cyanuric acid since neither melamine nor as an expansion of the interlayer distance, which is determined
ammelide could explain the number and shifts of the observed by van der Waals interactions, whereas the distances within
34]
resonances.
the network layers (determined by hydrogen bonds) hardly
Three signals were observed in the 15N spectrum of 1, show- elongate at all.
ing chemical shifts of –243.45, –253.81, and –287.09 ppm,
respectively. The signal at –287.09 ppm, which is also the most
intense, thus indicating the vicinity of several protons, is inter-
Conclusions
preted as ammelinium amino groups. The shifts of the two
The ammelinium salts ammelinium sulfate cyanuric acid (1)
remaining signals indicate monoprotonated N atoms of triazine and ammelinium sulfate monohydrate (2) were synthesized
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
Zeitschrift für anorganische und allgemeine Chemie
thesis of melamium sulfate.[
ARTICLE
through a synthesis route that has been described for the syn- stirring melamium bromide, which was prepared according to litera-
[
34]
30]
ture,
in a 25% aqueous solution of ammonia for 2 h, filtrating and
As has been reported before
for nitric acid, hydrochloric acid and hydrobromic acid,[28] we
observed the hydrolysis of melam in the presence of sulfuric
acid, resulting in the formation of two structurally interesting
species. 1 represents the first adduct compound between am-
meline and cyanuric acid. While adduct formation between
drying the residue at 105 °C for 12 h.
Melam (250 mg) was suspended in 12.5 mL of deionized water in a
two-neck flask equipped with a reflux condenser and heated to boiling
SO (25 mL of 10%) was added dropwise. Heating
2 4
was continued until the suspended colorless solid completely dis-
of the solvent. H
melamine and cyanuric acid is well known, no such behavior solved. The flask was closed with a stopper and placed in a refrigerator
has been reported for the partial hydrolysis/ammonolysis prod- for 15 d. The colorless precipitate was isolated by filtration.
ucts ammeline and ammelide.[
20,21]
In the structure of 1, am-
Single-Crystal X-ray Diffraction: Single-crystal X-ray diffraction
data sets were measured with either a Bruker D8 Venture or a Bruker
D8 Quest diffractometer (both Bruker, Billerica MA, USA). Measure-
meline occurs as a protonated species, while cyanuric acid re-
mains uncharged. Single-crystal X-ray diffractometry eluci-
dated a layered structure, in which isolated triangular motifs
of three ammelinium ions surrounding one cyanuric acid mol-
ecule form cavities that contain the sulfate anions. A super-
ments were performed at 100 K applying Mo-K
α
radiation (λ =
0
.71073 Å). For absorption correction, raw data integration and struc-
[35]
ture solution the program APEX 3 was used.
Structure refinement
structure caused by partial occupation of cyanuric acid sites, was performed using Shelxl-97.[36,37] Diamond 3 was used for visual-
[
38]
shared with three water molecules, as well as different orienta- ization of single-crystal structures.
tions of one sulfate tetrahedron could be observed, however,
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting the de-
pository numbers CCDC-1898048 for 1 and CCDC-1898049 for 2
not refined from the available data.
The compound, whose phase purity was shown by powder
X-ray diffractometry and Rietveld refinement, was further
characterized spectroscopically. FT-IR spectroscopy showed
bands shifted in a characteristic way that has also been de-
scribed for other ammelinium salts.[ Two independent keto
vibration bands could be assigned to ammelinium and cyanuric
(
Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
28]
www.ccdc.cam.ac.uk).
1
3
acid, respectively. A C NMR spectrum showed four different Powder X-ray Diffraction: Powder X-ray diffraction measurements
were performed with a Stoe Stadi-P powder diffractometer (Stoe,
resonances, two for ammelinium and cyanuric acid each.
Darmstadt, Germany) equipped with a Mythen 1 K silicon strip detec-
Chemical shifts are well in line with related compounds. The
1
5
tor (Dectris, Baden, Switzerland) in Debye–Scherrer set-up. Mono-
chromated Cu-Kα1 radiation (λ = 1.54056 Å) was applied. Samples
were prepared in sealed glass capillaries (inner diameter 0.5 mm, wall
N spectrum consists of one resonance attributed to terminal
amino groups as well as two signals in a shift range expected
for NH groups. We assign these to protonated ring N atoms of
ammelinium and cyanuric acid.
Compound 2 was obtained as a minor side phase to a poly-
crystalline, as yet unidentified material. A crystal structure for
thickness 0.01 mm). For data processing Stoe WinXPOW Vers. 3.06
[39]
was used.
Rietveld refinement was performed using TOPAS-Aca-
[40]
demic V4.1.
ammelinium sulfate monohydrate has already been published General Techniques: FT-IR spectra of all samples were recorded with
by Liu et al.,[29]
a Perkin–Elmer BX II spectrometer equipped with a DuraSampler Dia-
mond ATR (attenuated total reflection) unit at room temperature. Ele-
mental analyses of C, N, H and S were conducted by combustion
analysis with a Vario EL elemental analyzer (Elementar Analysensys-
teme GmbH) at the microanalytic laboratory of the Department of
Chemistry, LMU Munich. Solid-state MAS NMR spectra were re-
corded with a Bruker Avance III-500 spectrometer with an external
magnetic field of 11.74 T. Measurements were performed at room tem-
however, 2 appears to represent a different
polymorph. The structure obtained from single-crystal X-ray
diffractometry is layered with an abc stacking order. Mostly
isolated ammelinium ions form cavities, in which sulfate and
water are located. Hydrogen bridges occur primarily between
ammelinium and sulfate ions. One of two crystallographically
independent ammelinium ions shows structural disorder over
two of three functional groups, which are partially occupied perature. Samples were measured at rotary frequencies of 10 kHz in
1
3
15
by an amino as well as a keto group. This disorder is caused 4 mm double resonance probe heads. C and N spectra were re-
by hydrogen bridge interactions, which only find fitting donors corded applying cross polarization (CP) pulse sequences to amplify
1
signal intensities and H decoupled. 0.1% TMS in CDCl
a reference for all nuclei.
3
was used as
and acceptors through mixed occupation of these sites. This
might indicate a lower symmetry than found from single-crys-
tal X-ray diffractometry. Both compounds show several com-
mon structural features and appear to be closely related. We
were, however, not able to determine the factors leading to the
formation of one or the other.
Supporting Information (see footnote on the first page of this article):
The Supporting Information comprises optical images of the crystal
habitus, additional structure images of both compounds, atomic coordi-
nates and bond lengths obtained from single crystal structure refine-
ment as well as details of the Rietveld refinement.
Experimental Section
Acknowledgements
Synthesis of Ammelinium Sulfate Hydrate and Ammelinium Sulf-
ate Cyanuric Acid: Syntheses for both compounds were identical.
Melam was used as a carbon nitride precursor. It was obtained from
The authors wish to express their gratitude to Arthur Haffner and
Dr. Peter Mayer (both at Department of Chemistry, LMU Munich) for
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
single-crystal X-ray diffraction measurements. Furthermore, the au-
[19] R. L. M. Dobson, S. Motlagh, M. Quijano, R. T. Cambron, T. R.
Baker, A. M. Pullen, B. T. Regg, A. S. Bigalow-Kern, T. Vennard,
A. Fix, R. Reimschuessel, G. Overmann, Y. Shan, G. P. Daston,
Toxicol. Sci. 2008, 106, 251–262.
thors are very grateful for valuable input on crystallographic problems
during the revision process. Financial support granted by the Fonds der
Chemischen Industrie (FCI) and the Deutsche Forschungsgemeinschaft
[
20] G. Whitesides, J. Mathias, C. Seto, Science 1991, 254, 1312–
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
F. K. Kessler, W. Schnick* .............................................. 1–10
Ammelinium Sulfate Monohydrate and Ammelinium Sulfate
Cyanuric Acid – Synthesis and Structural Characterization
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