Formation of cocrystals between alkali triazine tricarboxylates and cyanuric acid - Reactivity considerations and structural characterization of the adduct phases M3[C3N3(CO2) 3][C3N3O3H3] ·H2O (M = K, Rb)

Article abstract of DOI:10.1002/zaac.201100316

The reactivity of cyanuric acid towards alkali triazinetricarboxylates was investigated and the first triazine-triazine adduct phases comprising alkali metal ions were synthesized and characterized by single-crystal X-ray diffraction and thermal analysis. An investigation of the reaction between the alkali triazine tricarboxylates M₃[C₃N₃(CO ₂)₃]·xH₂O (M = Li, Na, K, Rb, Cs) and cyanuric acid showed that the degree of ion transfer from triazine tricarboxylate to cyanuric acid increases gradually from the lithium to the cesium salt reflecting an increasing basicity of the triazine tricarboxylates.The reaction of potassium and rubidium triazine tricarboxylate dihydrate with cyanuric yielded the novel co-crystalsK₃[C ₃N₃(CO₂)₃][C₃N ₃O₃H₃]·H₂O (3a) and Rb ₃[C₃N₃(CO₂)₃][C ₃N₃O₃H₃]·H₂O (3b). In comparison to metal free triazine-triazine adduct phases in these compounds the assembly of molecules in the crystal is mainly determined by Coulomb interactions and only to a certain degree by hydrogen bonds and dispersive interactions. In the crystal the s-triazine units exhibit a layered structure with triazine tricarboxylate and isocyanuric acid being arranged in zigzag strands within the layers and stacked in columns perpendicular to the layers. Thermal analysis revealed a quite weak cohesion between triazine tricarboxylate and cyanuric acid upon heating. Copyright

Full text of DOI:10.1002/zaac.201100316

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ARTICLE
DOI: 10.1002/zaac.201100316
Formation of Cocrystals between Alkali Triazine Tricarboxylates and
Cyanuric Acid – Reactivity Considerations and Structural Characterization
of the Adduct Phases M3[C3N3(CO2)3][C3N3O3H3]·H2O (M = K, Rb)
Sophia J. Makowski,[a] Evelyn Calta,[a] Monika Lacher,[a] and Wolfgang Schnick*[a]
Keywords: Alkali metals; Carbon nitrides; Cocrystals; Crystal structure; s-Triazine
Abstract. The reactivity of cyanuric acid towards alkali triazine K3[C3N3(CO2)3][C3N3O3H3]·H2O (3a) and Rb3[C3N3(CO2)3][C3N3O3H3]·
tricarboxylates was investigated and the first triazine-triazine adduct H2O (3b). In comparison to metal free triazine-triazine adduct phases
phases comprising alkali metal ions were synthesized and charac- in these compounds the assembly of molecules in the crystal is mainly
terized by single-crystal X-ray diffraction and thermal analysis. An determined by Coulomb interactions and only to a certain degree by
investigation of the reaction between the alkali triazine tricarboxylates
hydrogen bonds and dispersive interactions. In the crystal the s-triazine
M3[C3N3(CO2)3]·xH2O (M = Li, Na, K, Rb, Cs) and cyanuric acid
units exhibit a layered structure with triazine tricarboxylate and isocy-
showed that the degree of ion transfer from triazine tricarboxylate to anuric acid being arranged in zigzag strands within the layers and
cyanuric acid increases gradually from the lithium to the cesium salt stacked in columns perpendicular to the layers. Thermal analysis re-
reflecting an increasing basicity of the triazine tricarboxylates. vealed a quite weak cohesion between triazine tricarboxylate and cya-
The reaction of potassium and rubidium triazine tricarboxylate dihy-
drate with cyanuric yielded the novel co-crystals
nuric acid upon heating.
metric rosette-like arrangement.[4,6] Further adduct phases of
s-triazine (C3N3) and s-heptazine (C6N7) compounds were ob-
tained from solutions comprising both building blocks[5] or un-
der solvent free conditions as intermediates during the thermal
condensation of s-triazine precursors.[7] However, to the best
of our knowledge no structurally characterized compound is
known so far, in which two different s-triazine molecules are
present together with metal ions. In contrast to the above men-
tioned adduct phases in such systems not only hydrogen bonds
and dispersive interactions but also Coulomb and coordination
interactions will determine the assembly of molecules in the
crystal.
In order to obtain a system, in which the effect of various
types of non covalent interactions on the arrangement of
s-triazine molecules as well as the mutual influence of these
diverse types of interactions can be studied we tried to com-
bine s-triazine based salts with further s-triazine compounds
capable of metal coordination. For this purpose we used alkali
triazine tricarboxylates M3[C3N3(CO2)3] (M = Li, Na, K, Rb,
Cs), an interesting class of compounds that has been studied
in detail only recently,[8,9] as molecular s-triazine based salts
and cyanuric acid as further ligand (see Scheme 1). In this con-
tribution the reactivity of different alkali triazine tricarb-
oxylates (TTC) towards cyanuric acid is investigated and the
synthesis and crystal structure of the co-crystals
K3[C3N3(CO2)3][C3N3O3H3]·H2O (3a) and Rb3[C3N3(CO2)3]-
[C3N3O3H3]·H2O (3b) is presented. The influence of hydrogen
bonding and π–π stacking interactions in comparison to Cou-
lomb interactions on the arrangement of the s-triazine building
blocks in these adduct phases is discussed and the strength of
Introduction
In the last decades the concept of crystal engineering for the
rational design of new solid materials with desired physical
and chemical properties has attracted considerable interest.[1]
For the directed preparation of solids with specific architecture
and properties a detailed understanding of the factors that af-
fect the arrangement of molecular building blocks in the solid
state is an indispensable prerequisite. The assembly of mole-
cules and ions in the crystal is determined by several types of
non covalent interactions such as hydrogen bonding, coordina-
tion interactions, or π–π stacking interactions.[1,2] In this re-
gard we are especially interested in such non covalent interac-
tions between molecular carbon nitride compounds. Even
though recent research in the field of carbon nitrides is mainly
focused on synthesis, characterization, and application of co-
valently bound frameworks[3] also several adduct phases, in
which the cohesion between molecular carbon nitrides is solely
achieved by various types of non covalent interactions, were
investigated.[4–7] The most prominent example is probably the
adduct between the s-triazine compounds melamine and cya-
nuric acid, in which the self-assembly of two molecules with
a threefold symmetry leads to the formation of a highly sym-
* Prof. Dr. W. Schnick
Fax: +49-89-2180-77440
E-Mail: wolfgang.schnick@uni-muenchen.de
[a] Department Chemie
Lehrstuhl für Anorganische Festkörperchemie
Ludwig-Maximilians-Universität München
Butenandtstraße 5–13 (D)
81377 München, Germany
88
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Cocrystals between Alkali Triazine Tricarboxylates and Cyanuric Acid
the cohesion by non covalent interactions is further examined eventually completely transferred from triazine tricarboxylate
by thermal analysis.
to cyanuric acid. Aqueous solutions of 1d and 1e are basic
enough to deprotonate cyanuric acid resulting in the formation
of the alkali cyanurates M3[C3N3O3]·xH2O (M = Rb, Cs). This
observation is also in accordance with expectation due to the
HSAB concept. Metal ions are coordinated by triazine tricarb-
oxylate via two carboxylate groups and the intermediate nitro-
gen atom of the s-triazine ring. In contrast to these hard coordi-
nation sites the keto oxygen atoms of isocyanuric acid can be
regarded as rather soft. The binding strength between the hard
triazine tricarboxylate units and alkali metal ions decreases
from hard Li+ to soft Cs+, whereas the trend is just vice versa
for isocyanuric acid. The formation of co-crystals between tri-
azine tricarboxylate and cyanuric acid can only be achieved
for potassium and rubidium, where the coordination strength
of both ligands for the respective metal cation is comparable.
Scheme 1. Triazine tricarboxylate TTC (1), the enol form of cyanuric Crystal Structure
acid (2a) and its keto form isocyanuric acid 2b.
Potassium triazine tricarboxylate isocyanuric acid hydrate
K3[C3N3(CO2)3][C3N3O3H3]·H2O (3a) crystallizes in the tri-
¯
clinic space group P1 and rubidium triazine tricarboxylate iso-
Results and Discussion
cyanuric acid hydrate Rb3[C3N3(CO2)3][C3N3O3H3]·H2O (3b)
in the monoclinic space group P21/c. Crystallographic data and
details of the structure refinements are summarized in Table 1.
Reactivity
The alkali triazine tricarboxylates M3[C3N3(CO2)3]·xH2O In both compounds the alkali ions are coordinated by triazine
(M = Li, Na, K, Rb, Cs) (1a–e) were combined with cyanuric tricarboxylate and cyanuric acid simultaneously. Cyanuric acid
acid C3N3O3H3 (2) in an attempt to prepare metal comprising is present in its keto form isocyanuric acid thereby providing
s-triazine adduct phases. Both starting materials represent three carbonyl groups as coordination sites, whereas the nitro-
s-triazine-based tridentate ligands with threefold symmetry and gen atoms of the triazine ring are protonated and hence do not
sufficient solubility in water. Aqueous solutions of the respec- participate in metal coordination. Each triazine tricarboxylate
tive alkali triazine tricarboxylate and cyanuric acid were mixed unit coordinates three K+ or Rb+ atoms, respectively, via two
and hold at room temperature until the evaporation of solvent carboxylate groups and the intermediate s-triazine nitrogen
caused the precipitation of the product. A characterization of atom as in the respective triazine tricarboxylate salts. The coor-
the products by X-ray diffraction revealed a dependency of the dination sphere is completed by the carboxyl groups of other
reactivity on the alkali ions. For the combination of lithium triazine tricarboxylates and the oxygen atoms of isocyanuric
and sodium triazine tricarboxylate with cyanuric acid no co- acid and water molecules amounting to coordination numbers
crystallization occurred but the starting materials crystallized of 5–8 for 3a and 7–9 for 3b. The M–O distances (K: 258–
separately. In the case of potassium and rubidium triazine tri- 295 pm, Rb: 276–364 pm) are comparable with the distances
carboxylate co-crystals with cyanuric acid were obtained but observed in the respective triazine tricarboxylate salts[9] and
only the combination of potassium triazine tricarboxylate and are in accordance with the ionic radii sums.[10] On contrast
cyanuric acid yielded a phase pure sample of the adduct the M–N distances (K: 281–290 pm, Rb: 301–303 pm) are on
K3[C3N3(CO2)3][C3N3O3H3]·H2O (K-TTC-CA·H2O) (3a). average 5 pm longer than in the triazine tricarboxylate salts,
The reaction of rubidium triazine tricarboxylate with cyanuric indicating a less tighter binding of the alkali ions to the triazine
acid resulted in a reaction mixture comprising single-crystals core as can be expected due to the additional coordination by
of the adduct Rb3[C3N3(CO2)3][C3N3O3H3]·H2O (Rb-TTC- isocyanuric acid. Both adduct phases form layered structures
CA·H2O) (3b), single-crystals of rubidium cyanurate and fur- typically for s-triazine systems (cf. Figure 1). The interlayer
ther unidentified phases. From the reaction product of cesium distances amount to 355 pm in 3a and 360 pm in 3b and hence
triazine tricarboxylate and cyanuric acid only cesium cyanurate are slightly larger than in the respective triazine tricarboxylate
could be isolated.
salts. Hydrogen bonds are mainly present within the layers and
Hence from 1a to 1e the tendency of a transfer of the alkali especially in 3b only to a small amount also between the lay-
ions from triazine tricarboxylate to cyanuric acid increases ers. As illustrated in Figure 2 each two isocyanuric acid (CA)
gradually. The lithium and sodium ions remain bound to tri- units are interconnected by a double N–H···O hydrogen bridge.
azine tricarboxylate, the potassium ions are coordinated by tri- Additionally, each CA entity forms a strong hydrogen bond to
azine tricarboxylate and cyanuric acid simultaneously, the ru- a carboxylate group of triazine tricarboxylate (TTC). Further
bidium ions are partially shared between both ligands and are hydrogen bonding interactions connect crystal water molecules
partially transferred to cyanuric acid and the cesium ions are with both CA and TTC units with all hydrogen atoms of H2O
Z. Anorg. Allg. Chem. 2012, 88–93
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S. J. Makowski, E. Calta, M. Lacher, W. Schnick
ARTICLE
Table 1. Crystallographic data and details of the structure refinement for K3[C3N3(CO2)3][C3N3O3H3]·H2O (3a) and Rb3[C3N3(CO2)3]-
[C3N3O3H3]·H2O (3b).
3a
3b
Molar mass /g·mol–1
Crystal system
Space group
474.49
613.58
triclinic
monoclinic
P21/c (no. 14)
293
¯
P1 (no. 2)
T /K
293
Diffractometer
Radiation, λ /pm
a /pm
b /pm
c /pm
Nonius Kappa-CCD
Mo-Kα, 71.073
709.66(14)
964.97(19)
1279.1(3)
75.83(3)
75.38(3)
70.88(3)
788.1(3)
2
Nonius Kappa-CCD
Mo-Kα, 71.073
720.01(14)
3756.4(8)
1256.9(3)
90
101.81(3)
90
3327.5(11)
4
α /°
β /°
γ /°
V /106·pm3
Z
Calculated density /g·cm–3
Crystal size /mm
Absorption coefficient /mm–1
F(000)
1.911
0.33ϫ0.23ϫ0.20
0.930
2.450
0.27ϫ0.13ϫ0.11
8.859
450
2336
Absorption correction
Min. / max. transmission
Diffraction range
index range
none
multi-scan
0.1889 / 0.3084
3.18° Յ θ Յ 26.42°
3.23° Յ θ Յ 34.41°
–11 Յ h Յ 11, –15 Յ k Յ 15, –20 Յ l Յ 20 –7 Յ h Յ 8, –46 Յ k Յ 45, –15 Յ l Յ 15
Parameters / restraints
Total no. of reflections
No. of independent reflections
No. of observed reflections
Min. / max. residual electron density /
e·10–6·pm–3
271 / 5
11999
6608
5053
–0.472 / +0.563
517 / 4
44111
6796
5128
–2.026 / +0.849
GooF
0.935
1.055
Final R indices [I Ͼ 2σ(I)]
Final R indices (all data)
R1 = 0.0407, wR2 = 0.1008a)
R1 = 0.0570, wR2 = 0.1123a)
R1 = 0.0473, wR2 = 0.1195b)
R1 = 0.0712, wR2 = 0.1318b)
2
2
2
2
2
2
a) w = [σ2(Fo ) + (0.0483P)2 + 0.5488P]–1, with P = (F0 + 2 Fc )/3. b) w = [σ2(Fo ) + (0.0606P)2 + 14.1010P]–1, with P = (F0 + 2 Fc )/3.
being involved in the hydrogen bonding network. Hydrogen
The ligands triazine tricarboxylate and isocyanuric acid are
bonds in 3a and 3b can be classified as medium strong with stacked in columns perpendicular to [100] (cf. Figure 3). In 3b
donor-acceptor distances of 267–299 pm (3a) and 264–301 pm these columns form single zigzag strands along c, whereas the
(3b) and donor-hydrogen-acceptor angles of 121–166° (3a) crystal structure of 3b comprises double strands along c which
and 113–173° (3b) (see Table 2).[11]
surround hexangular voids that are filled with crystal water
molecules and rubidium ions. The remaining rubidium ions in
3b and all potassium ions in 3a are located between the zigzag
strands of ligands forming corrugated bands. The composition
of the columns is different for both adduct phases. In 3a only
one type of columns is present, in which triazine tricarboxylate
TTC and isocyanuric acid CA units are alternately stacked
with a partial overlap of the s-triazine cores. In contrast in 3b
the columns comprise either TTC or CA and show a complete
overlap of the s-triazine cores (cf. Figure 4). Hence in 3b the
two ligands TTC and CA are merely connected via shared
metal coordination and hydrogen bonds, whereas in 3a ad-
ditionally weak dispersive interactions between the s-triazine
cores of both ligands are present. Interestingly, in the metal
free adduct melamine·cyanuric acid M·CA an alternative as-
sembly M–CA–M–CA is the most stable stacking arrange-
ment,[6] whereas in 3a and 3b a homogeneous CA–CA and
TTC–TTC stacking is preferred to an alternating TTC–CA–
TTC–CA stacking as the varying degrees of overlap show. The
observation that homogeneous CA–CA stacks are favored over
CA–TTC stacks in 3a and 3b but not over CA–M stacks in
M·CA might be explained by the influence of the rather strong
Figure 1. Layered structures of K-TTC-CA·H2O (3a) (top) and Rb-
TTC-CA·H2O (3b) (bottom). Dashed lines represent hydrogen bonds.
Thermal ellipsoids (except for hydrogen) are drawn at the 50% prob-
ability level.
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Cocrystals between Alkali Triazine Tricarboxylates and Cyanuric Acid
Figure 2. Hydrogen bonding interactions in K-TTC-CA·H2O (3a) (top) and Rb-TTC-CA·H2O (3b) (bottom). Thermal ellipsoids (except for
hydrogen) are drawn at the 50% probability level.
Table 2. Donor-acceptor distances and donor-hydrogen-acceptor hydrogen bonds – are essentially determining the arrangement
angles /pm, ° for the hydrogen bonding network in K-TTC-CA·H2O
(3a) and Rb-TTC-CA·H2O (3b).
of molecules in the crystal these interactions become less im-
portant in the presence of strong Coulomb interactions.
D–H···A
D···A
ϽD–H···A
K-TTC-CA·H2O (3a)
N1–H1···O3
N2–H2···O1
N3–H3···O10
N3–H3···O9
O10–H4···O6
O10–H5···O3
Rb-TTC-CA·H2O (3b)
N7–H7···O8
N8–H8···O12
Thermal Behavior
264.6
266.9
294.3
298.5
276.7
288.9
165.11
165.61
151.06
120.55
151.79
153.75
The thermal behavior of the adduct phase K-TTC-CA·H2O
(3a) was compared with that of the starting materials K-
TTC·2H2O (1c) and cyanuric acid (2) (see Figure 5). The po-
tassium salt 1c is dehydrated below 200 °C and starts to de-
compose around 350 °C. Cyanuric acid 2 completely trans-
forms into gaseous isocyanic acid HNCO around 400 °C.[12]
TG and DTA curves of the adduct phase 3a show a release
of crystal water at low temperatures and subsequent a slow
decomposition of the dehydrated compound starting around
200 °C. Hence the adduct phase 3a exhibits a lower thermal
stability than its components. The main mass loss occurs step-
wise between 320 and 420 °C accompanied by three exother-
mic signals in the DTA curve. Two of these signals closely
match the decomposition temperatures of K-TTC·2H2O (1c)
and cyanuric acid (2). This indicates that the cohesion between
264.1
266.2
279.2
265.5
266.4
282.0
286.1
300.2
286.1
268.7
300.6
167.17
161.79
172.98
164.76
170.48
172.96
124.91
120.89
118.41
140.79
112.73
N9–H9···O14
N10–H10···O1
N11–H11···O5
N12–H12···O17
O19–H19A···O18
O19–H19A···O20
O19–H19B···O18
O20–H20A···O7
O20–H20B···O20
Coulomb interactions in 3a and 3b impeding a complete over- the ligands TTC and CA seems to be broken up while heating,
lap of TTC and CA. In contrast to metal free triazine–triazine so that the thermal decomposition of the two individual com-
adduct phases, where π–π stacking interactions – next to strong pounds TTC and CA can be observed.
Z. Anorg. Allg. Chem. 2012, 88–93
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