The Sodium (Iso)Cyanurates Nax[H3–xC3N3O3]·yH2O (x = 1–3, y = 0, 1): A Key-Series for Understanding the Crystal Chemistry of Metal (Iso)Cyanurates

Article abstract of DOI:10.1002/zaac.201800438

The (iso)cyanurates Na[H₂C₃N₃O₃]·H₂O, Na₂[HC₃N₃O₃]·H₂O, Na₂[HC₃N₃O₃], and Na₃[C₃N₃O₃] were synthesized phase pure from Na₂CO₃·10H₂O, NaOH, and cyanuric acid, respectively, in aqueous solution by carefully adjusting the crystallization conditions. The crystal structures of all compounds were determined by single-crystal X-ray diffraction {Na₂[HC₃N₃O₃]·H₂O: P1, a = 3.51660(10) ?, b = 7.8300(3) ?, c = 11.3966(4) ?, α = 86.4400(10)°, β = 85.5350(10)°, γ = 85.0720(10)°, Z = 2, R₁ = 0.030, wR₂ = 0.078; Na₂[HC₃N₃O₃]: Pnma, a = 6.3409(6) ?, b = 12.2382(13) ?, c = 6.5919(7) ?, Z = 4, R₁ = 0.045, wR₂ = 0.079; Na₃[C₃N₃O₃]: R3c, a = 11.7459(3) ?, c = 6.5286(3) ?, Z = 3, R₁ = 0.039, wR₂ = 0.066}. The structures show ribbons (Na[H₂C₃N₃O₃]·H₂O), dimers (Na₂[HC₃N₃O₃]·H₂O), chains (Na₂[HC₃N₃O₃]), or columns (Na₃[C₃N₃O₃]) of hydrogen-bonded and parallel stacked (iso)cyanurate anions. These motifs are shown to be characteristic for certain degrees of protonation and hydration, and all (iso)cyanurate crystal structures found so far were classified accordingly. X-ray powder patterns, thermogravimetry curves, IR and UV/Vis spectra were measured for all compounds.

Full text of DOI:10.1002/zaac.201800438

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800438
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
The Sodium (Iso)Cyanurates Nax[H3–xC3N3O3]·yH2O
(x = 1–3, y = 0, 1): A Key-Series for Understanding the Crystal
Chemistry of Metal (Iso)Cyanurates
Peter Gross[a] and Henning A. Höppe*[a]
Dedicated to Professor Thomas Fässler on the Occasion of his 60th Birthday
¯
Abstract.
The
(iso)cyanurates
Na[H2C3N3O3]·H2O,
Na3[C3N3O3]: R3c, a = 11.7459(3) Å, c = 6.5286(3) Å, Z = 3, R1 = 0.039,
Na2[HC3N3O3]·H2O, Na2[HC3N3O3], and Na3[C3N3O3] were synthe-
wR2 = 0.066}. The structures show ribbons (Na[H2C3N3O3]·H2O), di-
sized phase pure from Na2CO3·10H2O, NaOH, and cyanuric acid, mers (Na2[HC3N3O3]·H2O), chains (Na2[HC3N3O3]), or columns
respectively, in aqueous solution by carefully adjusting the crystalli-
zation conditions. The crystal structures of all compounds were deter-
mined by single-crystal X-ray diffraction {Na2[HC3N3O3]·H2O:
(Na3[C3N3O3]) of hydrogen-bonded and parallel stacked (iso)cyan-
urate anions. These motifs are shown to be characteristic for certain
degrees of protonation and hydration, and all (iso)cyanurate crystal
structures found so far were classified accordingly. X-ray powder pat-
terns, thermogravimetry curves, IR and UV/Vis spectra were measured
for all compounds.
¯
P1, a = 3.51660(10) Å, b = 7.8300(3) Å, c = 11.3966(4) Å, α =
86.4400(10)°, 85.5350(10)°, 85.0720(10)°, 2,
β
=
γ
=
Z =
R1 = 0.030, wR2 = 0.078; Na2[HC3N3O3]: Pnma, a = 6.3409(6) Å,
b = 12.2382(13) Å, c = 6.5919(7) Å, Z = 4, R1 = 0.045, wR2 = 0.079;
Introduction
uniformly found for monobasic salts[16] to isocyanurate dimers
and chains for dibasic salts as well as columns for tribasic
salts, the latter of which are also known from various com-
pounds synthesized via solid state approaches.[18–28] In order
to shed light on the relationship between degree of deproton-
ation, crystal water content and formed structural motif, we
looked for a series of salts which does not incorporate other
chemical species except metal cation, (iso)cyanurate anion and
water, as it is well known that the structural pattern is mainly
governed by intermolecular hydrogen bonds,[30,31] and found
the Nax[H3–xC3N3O3]·yH2O series. Within this series, only
Na[H2C3N3O3]·H2O had been structurally characterized so far,
showing the familiar ribbon motif.[9]
Besides questions of structural chemistry, a thorough charac-
terization of sodium isocyanurates might also be of consider-
able interest to environmental chemists,[2,32] biochemists,[33,34]
and toxicologists,[35,36] as sodium isocyanurates are not only
the most common precursors[1] to, but also the main degrada-
tion products[37] of chlorinated isocyanuric acid derivatives
used as swimming pool disinfectants, as well as likely degrada-
tion products of atrazine and related s-triazine herbicides. The
first description of a member of the Nax[H3–xC3N3O3]·yH2O
series is given by Hofmann[38] with Na3[C3N3O3], including
synthesis instructions for phase pure products, common impu-
rities, description of crystal morphologies and chemical
analysis. Later, there have been some toxicological
studies[35–37,39–41] of presumably monosodium isocyanurate
(no analytical data regarding the probed substance were given)
as well as studies regarding infrared spectroscopy and thermal
behavior of Na[H2C3N3O3], which are summarized within an
extensive review by Seifer.[42]
Though cyanuric acid is a high volume industrial chemical
(worldwide production over 100 000 t/a[1]) and pivotal to a
whole spectrum of industries,[2–4] even its simple metal salts
are surprisingly sparsely explored by structure elucidation
techniques. A few years ago, only the crystal structures of two
alkaline earth,[5,6] a few alkali,[7–9] and some 3d-transition
metal[10–17] isocyanurates were reported, all except one[8] ex-
clusively containing monobasic isocyanurate anions and crys-
tallized from aqueous solution. Scientific interest in the topic
has considerably risen since the discovery of the remarkable
non-linear optical properties of Ca3[C3N3O3]2,[18] focussing
mainly on tribasic salts of divalent metal ions obtained via
solid state methods.[19–28] Recently, we accomplished to fuse
these two branches of metal (iso)cyanurate chemistry within a
broad structural and thermal study of alkali and alkaline earth
isocyanurates by successfully preparing the full range of
mono-, di- and tribasic (iso)cyanurate salts directly from aque-
ous solution.[29]
In that preceding study we encountered a variety of struc-
tural motifs, ranging from the well known isocyanurate ribbon
* Prof. Dr. H. A. Höppe
E-Mail: henning@ak-hoeppe.de
[a] Institut für Chemie
Universität Augsburg
Universitätsstr. 1
86159 Augsburg, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800438 or from the au-
thor.
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The second step consists of two substeps: one major step start-
Results
ing from about 340 °C with a mass loss of 39.6% and one
shoulder starting at about 410 °C and finished at 450 °C with
a mass loss of 13.2%. The third step consists of three substeps,
of which the first two are strongly overlapping and can only
be differentiated by a change of the slope. The first step ranges
form about 570 °C to about 665 °C with a mass loss of 7.6%,
the second from about 665 °C to about 765 °C with a mass
loss of 23.3%, and the third from about 765 °C to about
830 °C 4.3%. The overall mass loss accounts to 99.4%.
Na[H2C3N3O3]·H2O
We
redetermined
the
crystal
structure
of
Na[H2C3N3O3]·H2O and confirm the initial report by Zhu et
al.[9] and would therefore like to refer the reader to their publi-
cation for a general discussion of the structure. According to
the X-ray powder diffraction pattern (Figure S1, Supporting
Information), we were able to synthesize phase pure samples
of Na[H2C3N3O3]·H2O. The FT-IR spectrum is shown in Fig-
ure 1, the frequencies of all peaks are tabulated in Table S1
(Supporting Information) and assigned to vibrational modes to
the best of the authors’ knowledge, as well as compared to
frequencies of other monobasic isocyanurates and cyanuric
acid. The frequencies of all observed bands correspond well
with the expected frequencies of the assumed chemical spe-
cies.[42,43] The UV/Vis spectrum (Figure S2, Supporting Infor-
mation) shows a distinct bandgap absorption edge at about
245 nm, similar to other alkali (iso)cyanurates.[29]
Na2[HC3N3O3]·H2O
Na2[HC3N3O3]·H2O crystallizes in its own structure type in
¯
space group P1 (no. 2), its unit cell is depicted in Figure 3.
The crystal structure exhibits planar dibasic isocyanurate
anions [HC3N3O3]2– (Figure 4a) with typical bond lengths
(1.36 Å for C–N and 1.26 Å for C–O) and angles. These
anions form dimers via two strong hydrogen bonds between
the protonated nitrogen atoms and the carbonyl oxygen atom
of two neighboring anions (Figure 4b). The dimers are aligned
¯
¯
parallel to (31 2) and stacked parallel offset along (1 0 0), thus
forming columns with rhomboidal cross section (Figure 4c).
The columns are arranged in a simple tetragonal rod packing
(Figure 4d).
Figure 1. FT-IR spectrum of Na[H2C3N3O3]·H2O.
Subsequently we studied the thermal decomposition behav-
ior by thermogravimetry (Figure 2). Three major steps of mass
loss can be seen. The first, sharp and well defined step ranges
from 170 °C to 195 °C and the mass loss accounts to 10.7%.
Figure 3. The unit cell of Na2[HC3N3O3]·H2O viewed along [100].
(Na: grey spheres, C: black spheres, N: blue spheres, O: red spheres
and H: white spheres, covalent bonds: yellow sticks).
The H2O molecules and the Na+ cations are located in the
voids between these columns, the latter forming layers. The
H2O molecules further connect [HC3N3O3]2– dimers from
neighboring layers of the same column by forming hydrogen
bonds towards the deprotonated N atoms of two stacked
[HC3N3O3]2– anions (Figure 4c). Na atoms are found on two
different crystallographic sites, Na1 and Na2, as shown in Fig-
ure 5. The coordination polyhedron of Na1 can be described
as a distorted octahedron (CN = 6), which is spanned by one
water molecule and five carbonyl oxygen atoms. The in-
teratomic distances between Na1 and the surrounding atoms
correspond on average (2.46 Å) sufficiently well with the sum
of the ionic radii (2.40 Å). The coordination polyhedron of
Figure 2. TG curve of Na[H2C3N3O3]·H2O.
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Figure 4. Ball-and-stick model of the [HC3N3O3]2
– anion of
Na2[HC3N3O3]·H2O (hydrogen bonds: broken violet lines); (a) atoms
shown as thermal anisotropic ellipsoids (50%).; (b) isocyanurate dimer
depicted from top; (c) column resulting from stacked isocyanurate di-
mers; (d) rod packing of columns (solid green lines indicate column
faces).
Figure 6. Polyhedral representation of cation environments in
Na2 can be either described as a distorted square pyramid, Na2[HC3N3O3]·H2O as light blue octahedra around Na1 and grey
octhaedra around Na2. The condensation pattern within the layer re-
sembles alternate slabs of Anatase type and ReO3 type.
spanned by one water molecule, three carbonyl oxygen atoms
and one deprotonated nitrogen atom, and capped on the square
face by one additional deprotonated nitrogen atom (CN = 5+1)
at a considerably longer distance (3.11 Å), or as heavily dis-
torted octahedron (CN = 6). The agreement between the
averaged interatomic distances between Na2 and the surround-
ing atoms (2.50 Å for CN = 6 and 2.38 Å for CN = 5) and the
sum of the respective ionic radii (2.43 Å for CN = 6 and
2.38 Å for CN = 5) rather indicates that coordination number
5+1 is the more adequate description. Nevertheless, both will
be considered as octahedra for exploring the cation substruc-
ture. Both types of octahedra are condensed via four common
edges to neighboring octahedra belonging to the same type and
via one corner to an octahedron belonging to the respective
other type, thus forming a layer consisting alternately of TiO2/
Anatase-type slabs and ReO3-type slabs (Figure 6).
pure samples of Na2[HC3N3O3]·H2O. The FT-IR spectrum is
shown in Figure 7, the frequencies of all peaks are tabulated in
Table S2 (Supporting Information) and assigned to vibrational
modes to the best of the authors’ knowledge, as well as com-
pared to frequencies of other dibasic isocyanurates and cyan-
uric acid. The frequencies of all observed bands correspond
well with the expected frequencies of the assumed chemical
species.[42,43] The UV/Vis spectrum (Figure S4, Supporting In-
formation) shows a distinct bandgap absorption edge at about
245 nm, similar to other alkali (iso)cyanurates.[29]
Figure 5. Coordination environment of the Na atoms in
Na2[HC3N3O3]·H2O, with atoms shown as thermal anisotropic ellip-
soids (50%) and interatomic distances represented by broken, light
blue (typical distances) and broken, grey (unusually long distances)
lines.
Figure 7. FT-IR spectrum of Na2[HC3N3O3]·H2O.
The thermal decomposition behavior was studied via ther-
mogravimetry (Figure 8). Four major steps of mass loss can be
seen. The first step ranges from about 100 °C to about 130 °C
According to the X-ray powder diffraction pattern (Figure with a mass loss of 9.6%. The second step ranges from about
S3, Supporting Information), we were able to synthesize phase 300 °C to about 450 °C with a mass loss of 23.7%. The third
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step ranges from about 475 °C to about 810 °C with a mass
loss of 15.1%. The fourth step ranges from about 810 °C to
about 1000 °C with a mass loss of 6.4%.
Figure 10. Ball-and-stick model of the [HC3N3O3]–2 anion of
Na2[HC3N3O3] (hydrogen bonds: broken violet lines); (a) atoms shown
as thermal anisotropic ellipsoids (50%); (b) isocyanurate chain de-
picted from top; (c) layers resulting from stacked isocyanurate chains.
Figure 8. TG curve of Na[H2C3N3O3]·H2O.
correspond on average (2.50 Å) reasonably well with the sum
of the ionic radii (2.43 Å).
Na2[HC3N3O3]
Na2[HC3N3O3] crystallizes in its own structure type in space
group Pnma (no. 62), its unit cell is depicted in Figure 9. The
crystal structure exhibits planar dibasic isocyanurate anions
[HC3N3O3]2– aligned parallel to (1 0 0) with typical bond
lengths (1.35 Å for C–N and 1.26 Å for C–O) and angles (Fig-
ure 10a). These anions form slightly undulated chains via
hydrogen bonds between a protonated nitrogen atom and a
carbonyl oxygen atom of a neighboring anion (Figure 10b).
The chains are stacked antiparallel offset along (1 0 0), thus
forming layers (Figure 10c).
Figure 11. Coordination environment of the Na atom in (a)
Na2[HC3N3O3] and (b) Na3[C3N3O3], with atoms shown as thermal
anisotropic ellipsoids (50%) and interatomic distances represented by
broken, light blue lines.
Figure 9. The unit cell of Na2[HC3N3O3] viewed along [100]. (Na:
grey spheres, C: black spheres, N: blue spheres, O: red spheres and H:
white spheres, covalent bonds: yellow sticks).
The Na+ cations are located between these anion layers on
a single crystallographic site, forming a heavily distorted,
puckered close-packed layer. The coordination polyhedron
around Na can be described as a distorted trigonal prism (CN
= 6), which is spanned by four carbonyl oxygen atoms and
two deprotonated nitrogen atoms (Figure 11a). These prisms
are condensed via three common edges and one common cor-
ner within the close-packed layers and via two common cor-
ners between the close-packed layers (Figure 12). The in-
Figure 12. Polyhedral representation of the cation environment as grey
trigonal prisms of Na2[HC3N3O3].
According to the X-ray powder diffraction pattern (Figure
teratomic distances between Na and the surrounding atoms S5, Supporting Information), we were able to synthesize phase
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pure samples of Na2[HC3N3O3]. The FT-IR spectrum is shown crystal structure exhibits tribasic cyanurate anions [C3N3O3]3–
in Figure 13, the frequencies of all peaks are tabulated in Table (Figure 16a) with typical bond lengths (1.35 Å for C–N and
S2 (Supporting Information) and assigned to vibrational modes 1.29 Å for C–O) and angles. The cyanurate anions are stacked
to the best of the authors’ knowledge, as well as compared to antiparallel face-centered along [0 0 1] (Figure 16b) to form
frequencies of other dibasic isocyanurates and cyanuric acid. columns with a hexagonal cross section. These columns are
The frequencies of all observed bands correspond well with arranged in a hexagonal closest rod packing (Figure 16c).
the expected frequencies of the assumed chemical spe-
cies.[42,43] The UV/Vis spectrum (Figure S6, Supporting Infor-
mation) shows a distinct bandgap absorption edge at about
245 nm, similar to other alkali (iso)cyanurates.[29]
Figure 15. The unit cell of Na3[C3N3O3] viewed along [100]. (Na:
grey spheres, C: black spheres, N: blue spheres and O: red spheres,
covalent bonds: yellow sticks).
Figure 13. FT-IR spectrum of Na2[HC3N3O3].
The thermal decomposition behavior was studied by thermo-
gravimetry (Figure 14). Two major steps of mass loss can be
seen. The first step ranges from about 340 °C to about 450 °C
with a mass loss of 23.7%. The second step consists of two
substeps, which are strongly overlapping and can only be dif-
ferentiated by a distinct change of the slope. The first substep
ranges from about 575 °C to about 845 °C with a mass loss of
31.0%, the second substep from about 845 °C to about 920 °C
with a mass loss of 33.2%.
Figure 16. Ball-and-stick model of the [C3N3O3]3– anion of
Na3[C3N3O3]; (a) atoms shown as thermal anisotropic ellipsoids
(50%); (b) column resulting from stacked cyanurate anions; (c) rod
packing of columns (solid green lines indicate column faces).
Na+ cations are located on a single crystallographic position
in the voids between the columns. They are coordinated by
two nitrogen and five carbonyl oxygen atoms forming heavily
distorted pentagonal bipyramids (Figure 11b). The interatomic
distances between Na and the surrounding atoms correspond
on average (2.52 Å) very well with the sum of the ionic radii
(2.52 Å). The distorted pentagonal bipyramids are condensed
Figure 14. TG curve of Na[H2C3N3O3].
via two common trigonal faces (Figure 17a), forming 31- or
¯
32-helices along the corresponding screw axis in R3c (Fig-
ure 17b). Further condensation via common edges and corners
leads to a three-dimensionally condensed framework with
Na3[C3N3O3]
Na3[C3N3O3] crystallizes in its own structure type in space hexagonal channels occupied by the mentioned columns of
group R3c (no. 167), its unit cell is depicted in Figure 15. The [C3N3O3]3– anions (Figure 17b).
¯
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step starts at about 990 °C and end at about 1090 °C with a
mass loss of 8.95%. The DSC curve comprises three major
features: a well defined, sharp endothermic signal with an on-
set at about 650 °C and a maximum at about to 705 °C. It
directly transitions into a second endothermic feature, which
consists of several subsignals that cannot be separated unam-
biguously and ranges to about 950 °C. The last signal is exo-
thermic and again broad and possibly consisting of several
overlapping subsignals. It ranges from about 1000 °C to about
1120 °C and peaks at 1100 °C.
Figure 17. Polyhedral representation of the cation environment in
Na3[C3N3O3] as pentagonal bipyramids. (a) Condensation via common
1
faces to form a helix (light blue bipyramids on c = /12, dark blue
5
9
bipyramids on c = /12, grey bipyramids on c = /12). (b) Further con-
densation of neighboring helices via common edges and corners (31
helices in yellow and 32 helices in violet).
According to the X-ray powder diffraction pattern (Figure
S7, Supporting Information), we were able to synthesize phase
pure samples of Na3[C3N3O3]. The FT-IR spectrum is shown
in Figure 18, the frequencies of all peaks are tabulated in Table
S3 (Supporting Information) and assigned to vibrational modes
to the best of the authors’ knowledge, as well as compared to
frequencies of other tribasic cyanurates and cyanuric acid. The
frequencies of all observed bands correspond well with the
expected frequencies of the assumed chemical species.[42,43]
The UV/Vis spectrum (Figure S8, Supporting Information)
shows a distinct bandgap absorption edge at about 245 nm,
similar to other alkali (iso)cyanurates.[29]
Figure 19. TG (black, left ordinate) and DSC curve (red, right ordi-
nate) of Na3[C3N3O3].
Discussion
Table 1 gives a short overview of the packing motifs and
crystal structures containing only metal cations, [H3–xC3N3O3]
and H2–zO species known so far, including the members of the
Nax[H3–xC3N3O3]·yH2O series. For merely all monobasic salts,
a ribbon motif of hydrogen-bonded [H2C3N3O3]– is found,
with each isocyanurate anion having two N atoms acting as
protondonortotwocarbonylgroupsoftwodifferent,adjacentiso-
cyanurate anions and two carbonyl groups acting as acceptors
for hydrogen bonds to the same two adjacent isocyanurate
anions. This pattern also extends to structures containing ad-
ditionally fully protonated cyanuric acid or dibasic isocyanur-
ate anions.
A notable exception from this pattern is
Pb3O2[H2C3N3O3]2,[22] which shows a strongly undulated
chain. Pb3O2[H2C3N3O3]2 fits for several reasons rather poorly
in this series: it is the only p-block metal salt, its cation coordi-
nation environment shows strong sterically active lone-pair ef-
Figure 18. FT-IR spectrum of Na3[C3N3O3].
The thermal decomposition behavior was studied by thermo- fects, and it is the only (iso)cyanurate structure obtained from
gravimetry and differential scanning calorimetry (Figure 19). powder X-ray data. More crystal structures of p-block metal
The TG curve shows one step of decomposition, which con- isocyanurates should be elucidated, before Pb3O2[H2C3N3O3]2
sists of three overlaying substeps. The first substep sets in at can be unambiguously classified. Regarding the cation sub-
about 680 °C and blends into the second substep at about structure, a prevalence of isolated coordination polyhedra and
795 °C with a mass loss of 7.6% up to this point. The second chains of edge-sharing coordination polyhedra is observed,
substep ranges from 795 °C to 990 °C and accounts for the though the trend is much less clear than for the anion substruc-
largest part of the mass loss of about 73.63%. The third sub- ture.
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Table 1. Overview of crystal structures and structural motifs of the not seem to depend on the charge of the cation, as it is ob-
served for both Na+ and Sr2+, but might still be a size-specific
packing effect, as both cations have somewhat similar ionic
radii.[44] For the cation substructures, layers as well as frame-
works of condensed coordination polyhedra are found.
anion and cation substructure of compounds containing only metal cat-
ions, [H3– xC3N3O3] and H2– zO species (C3N3O3 = cya).
Chemical formula
Anion topology
Cation topology
Monobasic salts
Ba[H2cya]2·2H2O
[5]
Crystal structures containing tribasic isocyanurate anions
typically show parallel stacking of the anions to infinite col-
umns, with varying degree of displacement and rotation.
Ba3[C3N3O3]2 is an exception so far, as only two stapled cyan-
urate anions form a sandwich configuration, which is sur-
rounded in all directions by cations. This may be attributed to
the unusual trigonal prismatic coordination polyhedra around
some of the Ba atoms yielding a framework topology that – in
stark contrast to all other cation substructures found in this
group – contains no hexagonal channels in which columns of
[C3N3O3]3– anions might be situated.
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
ribbon
chain
chain
layer
[29]
Ba3[H2cya]2[Hcya]2·4H2O
[6]
Ca[H2cya]·7H2O
Co[H2cya]·7H2O
Cs[H2cya]·0.5H2O
isolated
isolated
layer
isolated
layer
chain
isolated
chain
[10,11,14]
[29]
[17]
Fe[H2cya]·7H2O
[8]
K[H2cya]·H2O
[8]
K3[D2cya]3[D3cya]·4D2O
[11,14]
Mn[H2cya]·H2O
Na[H2cya]·H2O
[9]
[12,16]
Na2Cu[H2cya]4·6H2O
chain
[11,14]
Ni[H2cya]·7H2O
Ni[H2cya]·4H2O
Pb3O2[H2cya]2
isolated
isolated
chain
chain
layer
[15]
[22]
The cation substructure correlates less strictly with the de-
gree of deprotonation, presumably because it is overshadowed
by cation size effects on one hand and simple packing effects
on the other. Larger cations tend to form condensed coordina-
tion polyhedra more readily, as their larger coordination
number and longer interatomic distances account for less re-
pulsive Coulomb interaction between them. The ratio of the
different constituent chemical species determined by the com-
position can also force the cation substructure to adopt a dif-
ferent degree of condensation due to simple packing argu-
ments, so that crystal structures with higher metal to crystal
water or metal to cyanuric acid ratio, respectively, favor a
higher degree of condensation in the cation substructure. This
is usually accompanied by an increase in the coordination
number of the metal atoms as well as the number of nitrogen
atoms in the metal atoms’ first coordination sphere. For exam-
ple, from Na[H2C3N3O3]·H2O via Na2[HC3N3O3]·H2O and
Na2[HC3N3O3] to Na3[C3N3O3] the number of coordinating
nitrogen atoms (from zero to two) and the coordination number
(from six to seven) rise simultaneously with the degree of con-
densation in the cation substructure, going from chains via lay-
ers to a framework motif. This phenomenological approach
of classifying metal (iso)cyanurates according to their anion
topology can be justified also by considerations of chemical
bonding. Though energetically the lattice energy will be domi-
nated by ionic interactions, their isotropic character allows the
(iso)cyanurate anions – regarded as rigid bodies hold together
by anisotropic covalent bonds – to rotate freely to energetically
favorable positions with regard to hydrogen bonds and stack-
ing, leading to characteristic topologies for specific degrees of
deprotonation.
[29]
Rb[H2cya]·H2O-I
ribbon
ribbon
ribbon
ribbon
[29]
Rb[H2cya]·H2O-II
[8]
Rb3[H2cya]3[H3cya]·4H2O
layer
isolated
[29]
Sr[H2cya]2·4H2O
Dibasic salts
[29]
Ca[Hcya]3.5H2O
dimer
chain
chain
chain
dimer
chain
dimer
layer
[29]
Cs2[Hcya]·2H2O
framework
framework
framework
layer
layer
layer
[8]
K2[Hcya]
Na2[Hcya]
Na2[Hcya]·H2O
[29]
Sr[HC3N3O3]·2H2O
Sr[HC3N3O3]·3H2O
[29]
Tribasic salts
[20]
Ba3[cya]2
sandwich
column
column
column
column
column
column
framework
framework
framework
framework
framework
framework
framework
[18]
Ca3[cya]2
Eu3[cya]2
[21]
Na3[cya]
Sr2[cya]OH
α–Sr3[cya]2
β–Sr3[cya]2
[29]
[26]
[21]
For dibasic salts, two structural motifs are found: either the
anions form dimers via two hydrogen bonds between the same
two isocyanurate anions (involving one N atom as proton do-
nor and the carbonyl group in ortho position as acceptor for
each of the two isocyanurate species) or the anions adopt a
chain motif via two hydrogen bonds between one central and
two adjacent isocyanurate anions (involving one N atom as
proton donor and the carbonyl group in para position as ac-
ceptor). So far, apparently structures with a lower content of
crystal water tend to form chains, whereas structures with a
higher content of water of crystallization tend to form dimers.
This can be understood, as the chain motif allows a farther
spatial separation of species that are likely to coordinate a
metal cation, namely deprotonated N atoms and carbonyl
groups not involved in hydrogen bonding, than the dimer mo-
tif, so less crystal water is needed to complete the first coordi-
nation sphere around the cation. Vice versa, the dimer motif
provides more deprotonated N atoms which are too close to
coordinating carbonyl groups in order to contribute to the first
As more nitrogen atoms of isocyanuric acid get deproton-
ated, delocalization of the π-electrons is enhanced and the aro-
maticity increases, as the keto–enol tautomerism between iso-
cyanurate and cyanurate anions shifts towards the latter. This
can be seen in the stepwise shortening of the averaged C–N
distances from 1.37 Å in H3C3N3O3 to 1.35 Å in Na3[C3N3O3]
accompanied by an elongation of the corresponding C–O dis-
tances from 1.22 Å, as expected for a typical C=O bond, to
coordination sphere themselves but still can act as proton ac- 1.29 Å, which is between the typical length of a C=O bond
ceptors for additional crystal water molecules. This trend does and 1.36 Å, the C–O distance encountered for carboxylate
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
ARTICLE
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groups. The IR frequencies associated with ring deformation as the diffractometric, spectroscopic and thermal characteriza-
[δ(NCO), δ(CNC), δ(C N), ν(C–N), δ(C=O, π(CO)] are corre- tion of Na[H2C3N3O3]·H2O, Na2[HC3N3O3]·H2O,
spondingly shifted to higher frequencies, whereas the ν(C=O) Na2[HC3N3O3], and Na3[C3N3O3]. The compounds were clas-
modes are shifted to lower energies mirroring the longer bond sified by the topology of their anion substructures, which are
length. The increased tendency for especially tribasic salts to formed by hydrogen bonding between and stacking of (iso)
stack parallel face-centered might be also attributed to the cyanurate anions: ribbons for Na[H2C3N3O3]·H2O, dimers for
lesser degree of localization and therefore weaker polar elec- Na2[HC3N3O3]·H2O, chains for Na2[HC3N3O3], and columns
trostatic interactions, the latter usually leading to a parallel off- for Na3[C3N3O3]. These packing motifs are shown to be char-
set arrangement, which is encountered more often for isocyan- acteristic for certain degrees of deprotonation and hydration
urates with a lower degree of deprotonation.
throughout nearly all metal (iso)cyanurate crystal structures
The thermal decomposition of alkali (iso)cyanurates hy- determined up to now. We further showed that bond lengths
drates is believed[42] to follow the following reaction determined by X-ray diffraction and vibrational frequencies
pattern:[42]
determined by IR spectroscopy both show a step wise shift
from a typical isocyanurate species towards a bond situation
between isocyanurate and cyanurate anion within the keto–
enol equilibrium. The thermal decomposition was shown to be
much more complicated than believed throughout the litera-
ture, and heavily dependant on even very small impurities.
6 A[H2C3N3O3·xH2O] Ǟ 6 A[H2C3N3O3] + 6x H2O
6 A[H2C3N3O3] Ǟ 3 A2[HC3N3O3] + 9 HNCO
3 A2[HC3N3O3] Ǟ 2 A3[C3N3O3] + 3 HNCO
2 A3[C3N3O3] Ǟ 6 ANCO
(1)
(2)
(3)
(4)
While this holds reasonably true for Na[H2C3N3O3]·H2O
(measured/theoretical mass loss for the corresponding
decomposition steps: (1) 10.7%/9.8%, (2) 39.6%/38.2%, (3)
13.2%/12.7% (4) 35.9%/39.3%), the decomposition of
Na2[HC3N3O3]·H2O, Na2[HC3N3O3], and Na3[C3N3O3] can-
not be understood as easily. In the latter cases, there is signifi-
cant mass left even at temperatures as high as 1200 °C (pure
NaNCO should have completely evaporated), similar to what
Experimental Section
Synthesis: All compounds were synthesized from Na2CO3·10H2O
(p.a., Merck) respectively NaOH (99%, VWR Chemicals), and cyan-
uric acid (98%, Merck), which were all used without further purifica-
tion and handled in air. The specific experimental conditions were
found empirically, as stoichiometric mixtures of dissolved starting ma-
terials tend to form mixtures of several species of cyanuric acid depro-
tonated to different degrees. Na3[C3N3O3] showed inverse solubility
has been found for rubidium and cesium isocyanurates.[29] As behavior as already mentioned by Hofmann.[38] While Na2[HC3N3O3]
could be obtained from solutions near to the boiling point,
Na2[HC3N3O3]·H2O was crystallized at room temperature.
it is well known for gaseous molecular compounds and cyanate
melts,[45,46] more complex condensation reactions might hap-
pen, which might lead to carbonitridic species of higher ther-
mal stability. The elucidation of possible reaction pathways
and identification of the products is still subject of our current
research and will be treated in a subsequent study. Heating a
Na3[C3N3O3] sample to 650 °C in a tube furnace in a nitrogen
stream showed no sign of decomposition, whereas heating it
to 700 °C yielded a solidified melt of NaOCN. We therefore
conclude that the endothermic signal at 705 °C in the DSC
curve of Na3[C3N3O3] corresponds to the monomerization of
the cyanurate ring accompanied by melting of the resulting
Na[H2C3N3O3]·H2O was synthesized by dissolving Na2CO3·10H2O
(1 mmol, 286.2 mg) and H3C3N3O (1 mmol, 129.1 mg) in 10 mL of
demineralized water whilst stirring and heating at the boiling point.
The solution was concentrated to about 5 mL, causing the product to
crystallize. The product was separated from the mother liquor by hot
vacuum filtration and the filter cake was dried overnight in air at room
temperature. The yield was 27.1%.
Na2[HC3N3O3]·H2O was synthesized by dissolving NaOH
(14.75 mmol, 590 mg) and H3C3N3O (5.44 mmol, 702.5 mg) in 20 mL
of demineralized water whilst stirring and heating at the boiling point.
sodium cyanate and the second endothermic feature corre- The solution was concentrated to about 5 mL, during which
Na3[C3N3O3] crystallized. Na3[C3N3O3] was removed from the mother
liquor by hot vacuum filtration and the product was crystallized by
slow evaporation of the obtained filtrate. As this sample contained a
small amount of Na2CO3, which was probably formed via absorption
of CO2 from air, it was dispersed in 2 mL of demineralized H2O via
stirring for 30 min, vacuum filtered and the filter cake dried in air at
room temperature. The overall yield was 20.4%.
sponds to evaporation of NaOCN. The much lower decomposi-
[42]
tion point stated by Seifer
(about 430 °C) can be easily
explained by one or several unnoticed side phases of less de-
protonated cyanurate species: in our experiments, even very
small amounts of Na2[C3N3O3] present have significantly de-
creased the decomposition temperature of the sample, in some
cases down to 550 °C. The nature of the exothermic feature at
1100 °C remains unclear, but is probably connected to a more
complex condensation reaction to a presumably carbontridic
species, as it coincides with the third substep of weight loss
seen in the TG curve of Na3[C3N3O3].
Na2[HC3N3O3] was synthesized by dissolving NaOH (14.33 mmol,
573.2 mg) and H3C3N3O (6.23 mmol, 804.2 mg) in 15 mL of deminer-
alized water whilst stirring and heating at the boiling point. The solu-
tion was concentrated to about 5 mL, and at the start of the crystalli-
zation of the product the stirring was turned off. The obtained suspen-
sion was cooled in an ice bath for 15 min and the product was sepa-
rated from the mother liquor by cold vacuum filtration. The filter cake
was dried overnight in air at room temperature and after removal from
the filter for another 15 min at 100 °C in a compartment dryer. The
Conclusions
In this article we described the syntheses, the crystal struc-
tures as determined by single-crystal X-ray diffraction, as well yield was 22.4%.
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Table 2. Crystal data and structure refinement of Na2[HC3N3O3]·H2O, Na2[HC3N3O3], and Na3[C3N3O3].
Na2[HC3N3O3]·H2O
Na2[HC3N3O3]
Na3[C3N3O3]
Formula
Na2H3C3N3O4
191.05
0.05ϫ0.09ϫ0.14
triclinic
¯
P1
3.51660(10)
7.8300(3)
11.3966(4)
86.4400(10)
85.5350(10)
85.0720(10)
311.211(18)
2
Na2HC3N3O
173.04
0.01ϫ0.02ϫ0.24
orthorhombic
Pnma
6.3409(6)
12.2382(13)
6.5919(7)
90
90
90
511.54(9)
4
2.247
Na3C3N3O
195.02
0.04ϫ0.05ϫ0.17
trigonal
¯
R3
11.7459(3)
11.7459(3)
6.5286(3)
90
90
120
780.05(5)
3
2.247
0.414
576
Mr /g·mol–1
Crystal size /mm3
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
β /°
V /Å3
Z
Dcalcd /g·cm–3
μ(Mo–Kα) /cm–1
F(000) / e
2.039
0.294
192
0.331
344
hkl range
Ϯ4, Ϯ9, Ϯ14
0.62
12230
1244
0.029
Ϯ7, Ϯ15, Ϯ8
0.62
4192
516
0.039
Ϯ14, Ϯ14, Ϯ8
0.62
7985
174
0.077
[(sinθ)/λ]max /Å3
Reflections measured
Reflections unique
R(int)
Parameter refined
R1(F)/wR2 (F2) (all reflections)
GoF (F2)
118
0.030/0.078
1.08
56
21
0.045/0.079
1.06
0.22/–0.23
0.039/0.066
1.23
0.24/–0.19
Δρfin (max./min.) /e·Å–3
0.33/–0.26
Na3[C3N3O3] was synthesized by dissolving NaOH (10 mmol,
400 mg) and cyanuric acid (2 mmol, 258.2 mg) in 10 mL of demineral-
ized water whilst stirring and heating at the boiling point. The solution
was concentrated to about 4 mL, causing the product to crystallize.
The product was separated from the mother liquor by hot vacuum
filtration and the filter cake was dried together with the filter paper for
15 min at 80 °C in a compartment dryer. The yield was 27.4%.
crement: 0.02°, 40 scans per data point, integration time: 100 s per
degree).
IR spectroscopy: The infrared spectrum was recorded at room tem-
perature with a Bruker EQUINOX 55 T-R spectrometer using a Platin-
num ATR device (scan range: 400–4000 cm–1, resolution: 4 cm–1,
32 scans per sample).
UV/Vis Spectroscopy: The UV/Vis spectra were recorded as diffuse
reflection spectra at room temperature with a Varian Cary 300 Scan
UV/Vis spectrophotometer using an Ulbricht sphere detector and a
deuterium lamp / mercury lamp light source (scan range: 200–800 nm,
increment 1 nm, scan-rate: 600 nm·min–1).
X-ray Structure Determination: Suited single crystals were selected
for single-crystal XRD under a polarized-light microscope. Diffraction
data were collected with a Bruker D8 Venture diffractometer using
Mo-Kα radiation (λ = 0.7093 Å). Absorption correction was performed
by the multiscan-method with the program SAINT within the Bruker
APEXIII software suite.[47] The structures were solved by direct meth-
ods and refined by full-matrix least-squares technique with the
SHELXTL crystallographic software package.[48] The Na, C, N, and
O atoms could be directly located, and the hydrogen atoms were either
calculated using the AFIX 43 instruction (for some the nitrogen atoms)
or assigned from the difference Fourier map and refined with restraints
for the hydrogen to donor atom distance using the DFIX instruction
(0.99Ϯ0.01 Å). Relevant crystallographic data and further details of
the structure determinations are summarized in Table 2.
Thermogravimetry and DSC: The TG analyses were performed with
a NETZSCH STA/TG 409 PC Luxx thermobalance in a nitrogen atmo-
sphere (flow rate: 70 ml·min–1) in corundum crucibles (heating rate:
5 K·min–1). The DSC analysis were performed with a NETZSCH STA/
TG-DSC 409 PC Luxx thermobalance in a nitrogen atmosphere (flow
rate: 70 ml·min–1) in lidded platinum crucibles with a corundum inlay
(heating rate: 5 K·min–1).
Supporting Information (see footnote on the first page of this article):
Further figures illustrating the crystal structures, powder diffraction
patterns, UV/Vis spectra, and tables comparing vibrational frequencies.
Further details of the crystal structures investigations may be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail:
crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request for de-
posited data.html) on quoting the depository numbers CSD-1871546
(Na[H2C3N3O3]·H2O), CSD-1871545 (Na2[HC3N3O3]·H2O), CSD-
1871544 (Na2[HC3N3O3]) and CSD-1871543 (Na3[C3N3O3]).
Keywords: Cyanurate; Isocyanurate structure elucidation,
sodium, x-ray diffraction, nitrogen heterocycles
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METEOR 1D line detector, and a Ni-filter to suppress Kβ radiation
(X-ray tube operated at 40 kV and 40 mA, scan range: 5–80°, in-
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Journal of Inorganic and General Chemistry
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Published Online: ᭿
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Journal of Inorganic and General Chemistry
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P. Gross, H. A. Höppe* ................................................... 1–11
The Sodium (Iso)Cyanurates Nax[H3–xC3N3O3]·yH2O (x = 1–
3, y = 0, 1): A Key-Series for Understanding the Crystal
Chemistry of Metal (Iso)Cyanurates
Z. Anorg. Allg. Chem. 0000, 0–0
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© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim