K3[C3N3(COO)3]·2H 2O - Crystal structure of a new alkali derivative of the multidentate ligand triazine tricarboxylate

Article abstract of DOI:10.1002/zaac.201000245

Potassium-1,3,5-triazine-2,4,6-tricarboxylate dihydrate K ₃[C₃N₃(COO)₃]·2H ₂O was obtained by saponification of the respective ethyl ester in aqueous solution under mild conditions and subsequent crystallization at 4 °C. The crystal structure of the molecular salt was elucidated by single-crystal X-ray diffraction [Pβar{1}$, a = 696.63(14), b = 1748.5(3), c = 1756.0(3) pm, α = 119.73(3), β = 91.96(3), Iγ = 93.84(3)°, V = 1847.6(6)·10⁶ pm³, Z = 6, T = 200 K]. Perpendicular to [100] the triazine tricarboxylate and potassium ions are arranged in layers alternating with layers of crystal water molecules. Two thirds of the triazine tricarboxylate units form hexagonal channels being filled with the remaining triazine tricarboxylate molecules. K₃[C ₃N₃(COO)₃]·2H₂O was additionally investigated by means of FTIR spectroscopy, TG and DTA measurements. Copyright

Full text of DOI:10.1002/zaac.201000245

ARTICLE
DOI: 10.1002/zaac.201000245
K [C N (COO) ]·2H O – Crystal Structure of a New Alkali Derivative of the
3
3
3
3
2
Multidentate Ligand Triazine Tricarboxylate
[a]
[a]
[a]
Sophia J. Makowski, Michael Hörmannsdorfer, and Wolfgang Schnick*
Keywords: s-Triazine; Potassium; Carboxylate ligands; Structure elucidation; Metal-organic frameworks
Abstract. Potassium-1,3,5-triazine-2,4,6-tricarboxylate
[C (COO) ]·2H O was obtained by saponification of the respec- layers alternating with layers of crystal water molecules. Two thirds of
tive ethyl ester in aqueous solution under mild conditions and subse- the triazine tricarboxylate units form hexagonal channels being filled
dihydrate to [100] the triazine tricarboxylate and potassium ions are arranged in
K
3
3
N
3
3
2
quent crystallization at 4 °C. The crystal structure of the molecular salt with
the
(COO)
remaining
triazine
tricarboxylate
molecules.
¯
was elucidated by single-crystal X-ray diffraction [P1, a = 696.63(14),
K
3
[C
3
N
3
3
]·2H O was additionally investigated by means of
2
b = 1748.5(3), c = 1756.0(3) pm, α = 119.73(3), β = 91.96(3), γ = FTIR spectroscopy, TG and DTA measurements.
6
3
9
3.84(3)°, V = 1847.6(6)·10 pm , Z = 6, T = 200 K]. Perpendicular
Introduction
cessitates an alternating sequence of carbon and nitrogen both
in the precursor and in the resulting polymeric network. Even
though such compounds are no suitable precursors for the syn-
thesis of covalently bound true carbon nitrides by condensation
reactions they can be regarded as promising linkers in coordi-
nation polymers. Metal-organic frameworks (MOFs) have
been extensively studied in the last years due to their potential
applications, e. g. as materials for ion exchange, separation,
catalysis, or gas storage [9, 10]. Whereas numerous MOFs are
known in which a hydrocarbon cycle functionalized with sev-
eral carboxylate groups, e. g. trimesic acid 3 (cf. Scheme 2)
Research in the field of carbon nitrides is mainly focused on
the synthesis of two- and three-dimensional networks, which
can serve as functional materials for a wide field of applica-
tions such as catalysis or optoelectronic uses [1–3]. A common
approach towards multidimensional networks is the pyrolysis
of s-triazine (C N ) or s-heptazine (C N ) based precursors to
3
3
6
7
induce condensation and thereby generating covalently bound
products. A variety of precursors with different functional
groups has been utilized in this regard (cf. Scheme 1, 1a–d
and 2a–d) [4–8].
[11–13], serves as linker between inorganic atoms or clusters,
the analog s-triazine or s-heptazine compounds 1f and 2f have
not been employed for this purpose so far.
In known s-triazine and s-heptazine based linkers such as s-
triazine-2,4,6-tris(4-pyridyl) (4) [14, 15], s-triazine-2,4,6-triyl-
tribenzoic acid (5) [16–20] or s-heptazine-2,5,8-triyl-tribenzoic
acid (6) [20, 21] (cf. Scheme 2), the s-triazine or s-heptazine
cores, respectively, only provide the threefold symmetry but
do not contribute to metal coordination. In contrast, in com-
pounds 1f and 2f both the carboxylate groups and the nitrogen
atoms of the ring can be regarded as coordination positions.
The s-heptazine compound 2f has not been mentioned in the
literature so far, whereas the synthesis of 1f by a multistep
reaction via the respective ethyl ester and potassium salt has
already been described over 50 years ago [22]. However, no
structural characterization of the product or any intermediate
was performed and since then only very little research regard-
ing triazine tricarbonic acid or its salts has been carried out
until 2002 when Dunbar et al. reported the crystal structure
and magnetic properties of K{Fe(1,3,5-triazine-2,4,6-
tricarboxylate)(H O) }·2H O [23].
Scheme 1. Molecular precursors for carbon nitride networks compris-
ing the s-triazine (1) or s-heptazine (2) core.
In general, molecules containing residues that are bound to
the s-triazine or s-heptazine core through C–C bonds (cf.
Scheme 1, 1e–f and 2e–f) are adverse in this regard. This is
due to the strength of the C–C bonds impeding a successful
condensation. Furthermore, the synthesis of carbon nitrides ne-
*
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)
2
2
2
To the best of our knowledge no crystal structure of any
other salt of triazine tricarbonic acid has been described so far.
81377 München, Germany
2
584
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 636, 2584–2588

3 3 3 3 2
K [C N (COO) ]·2H O – An Alkali Derivative of Triazine Tricarboxylate
Table 1. Crystallographic data and details of the structure refinement
for K [C (COO) ]·2H O.
3
3
N
3
3
2
molar mass /g·mol–1
crystal system
space group
T /K
363.41
triclinic
¯
P1 (no. 2)
200
diffractometer
radiation, λ /pm
a /pm
Nonius Kappa-CCD
α
Mo-K , 71.073
696.63(14)
1748.5(3)
1756.0(3)
119.73(3)
91.96(3)
b /pm
c /pm
α /°
β /°
γ /°
93.84(3)
6
3
V /10 ·pm
1847.6(8)
6
Z
–3
calculated density /g·cm
1.960
crystal size /mm
0.70 × 0.07 × 0.05
1.150
–
1
absorption coefficient /mm
diffraction range
3.26° ≤ θ ≤ 27.54°
–8 ≤ h ≤ 9
index range
Scheme 2. Multidentate ligands, which are commonly employed as
–
22 ≤ k ≤ 22
22 ≤ l ≤ 22
linkers in metal-organic frameworks.
–
parameters / restraints
total no. of reflections
577 / 16
14291
no. of independent reflections 8452
However, regarding the potential application of triazine tricar-
no. of observed reflections
5617
boxylate as a multidentate ligand, a deeper understanding of min./max. residual electron den- –0.679/ +0.984
6
3
sity / 10 ·pm
its coordination behavior towards various metal ions is of great
interest. In this contribution we report on synthesis and crystal
structure of potassium triazine tricarboxylate dihydrate
K [C N (COO) ]·2H O as a further example of a structurally
GooF
1.047
a)
a)
final R indices [I > 2σ(I)]
final R indices (all data)
R
R
1
= 0.0513, wR
= 0.0868, wR
–1
2
2
= 0.1068
= 0.1228
1
3
3
3
3
2
2
2
2
2+2F 2)/3.
a) w = [σ (F
o
)+(0.0389P) +1.6956P] , with P = (F
0
c
characterized triazine tricarboxylate and thereby providing fur-
ther insight into the coordination properties of this linker.
tricarboxylate ions are not coplanar but are slightly tilted out
of the bc plane. Perpendicular to [100] the triazine tricarboxyl-
ate units form two different kinds of columns. One third of the
columns consist solely of molecules B (cf. Figure 1) with the
Results and Discussion
Crystal Structure
Potassium triazine tricarboxylate crystallizes as a dihydrate molecules of two adjacent layers A and A' being rotated by
¯
in the triclinic space group P1 with six formula units in the 60° within the plane. In the remaining columns molecules A
unit cell. Crystallographic data and details of the structure re- and C alternate and are rotated by 30°. These columns form
finement are summarized in Table 1. The crystal structure hexagonal channels around the B columns (cf. Figure 3).
3
–
comprises triazine tricarboxylate [C N (COO) ] and potas-
The interlayer distance of 347 pm between two triazine tri-
carboxylate layers is in the expected range for molecular s-
3
3
3
sium ions and crystal water molecules (cf. Figure 1).
The triazine tricarboxylate ions are nearly planar with only triazine derivatives [1, 24, 25]. Between the layers the triazine
the carboxylate groups being slightly rotated out of plane so tricarboxylate units are interconnected by a network of me-
that the anions exhibit molecular symmetry C . The substitu- dium strong [26] hydrogen bonds (cf. Table 3) by the carbox-
3
ents cause an increase of the adjacent angles (N1–C1–N3 = ylate groups. All crystal water molecules are part of the hydro-
1
24.5°), whereas the remaining angles of the s-triazine core gen bonding network although due to the strong disorder of
are accordingly reduced (C1–N1–C2 = 115.5°). In accordance one oxygen atom (O24) no hydrogen bond can be clearly as-
with expectation, the bond lengths of both the C–N bonds signed to the linked hydrogen atom H11.
within the ring (133–134 pm) and the C–O bonds of the car-
The potassium ions are coordinated in the form of irregular
boxylate groups (124–125 pm) correspond to conjugated dou- polyhedrons by four to six oxygen atoms of water molecules
ble bonds, whereas the C–C bond lengths of 153–154 pm re- and carboxylate groups and by one nitrogen atom of the tria-
semble those of single bonds (cf. Table 2).
zine rings. The triazine tricarboxylate units serve as tridentate
+
The triazine tricarboxylate ions, the potassium atoms, and ligands and coordinate the K atoms through the O–N–O enti-
+
water molecules are arranged in an AA'BB' type stacking pat- ties (cf. Figure 1) with the K atom being slightly shifted to-
tern perpendicular to [100] (cf. Figure 2) with the A and A' wards one of the oxygen atoms. The coordination sphere is
layers consisting of the triazine tricarboxylate and potassium completed by the oxygen atoms of further triazine tricarboxyl-
ions and the crystal water molecules being located in the inter- ate units and water molecules. The K–O distances (273–
mediate B and B' layers. Within the A and A' layers the triazine 297 pm) are in accordance with the sum of the ionic radii [27]
Z. Anorg. Allg. Chem. 2010, 2584–2588
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
2585

S. J. Makowski, M. Hörmannsdorfer, W. Schnick
ARTICLE
Figure 1. Representation of one formula unit of K
3
[C
3
N
3
(COO)
3
]·2H
2
O. The three triazine tricarboxylate entities present in the crystal structure
are labeled A–C. Thermal ellipsoids (except for hydrogen) are drawn at the 50 % probability level.
3 3 3 3 2
Table 2. Selected bond lengths /pm and angles /° in K [C N (COO) ]·2H O, standard deviations in parentheses.
K1– O4
K1– O5
292.6(3) K3– O16 281.0(3)
281.7(3) K3– O16 283.7(3)
K5– N4 278.2(3)
K6– O2 278.6(3)
K6– O6 287.2(3)
K6– O18 280.0(3)
K6– O18 293.0(3)
K6– O20 285.4(4)
K6– O24 284.7(4)
K6– N3 279.3(3)
K7– O6 277.8(3)
K7– O10 276.2(3)
K7– O12 291.8(3)
K7– O19 295.5(3)
K7– O23 285.2(3)
K7– N6 279.1(3)
K8– O4 278.7(3)
K8– O8 284.0(3) N3– C3
K8– O9 296.2(3) C4– O1
K8– O22 278.2(3) C4– O2
K8– N5 279.7(3) C1– C4
K9– O7 274.4(3) C2– C5
K9– O13 274.0(3) C3– C6
132.9(4) N2– C2– C5 117.9(3)
123.9(4) N2– C3– C6 118.2(3)
125.0(4) N3– C3– C6 117.3(3)
153.0(4) O1– C4– O2 127.5(3)
154.2(5) O4– C5– O3 127.8(3)
153.0(5) O6– C6– O5 128.5(4)
K1– O14 279.1(3) K3– O21 282.2(3)
K1– O14 284.6(3) K3– N1 279.7(3)
K1– O19 280.3(3) K4– O1 287.5(3)
K1– N2
K2– O4
278.6(3) K4– O9 276.9(3)
286.3(3) K4– O16 290.3(3)
K9– O18 297.1(3) C2– N1– C1 115.4(3) O1– C4– C1 117.5(3)
K9– O20 275.7(3) C2– N2– C3 115.4(3) O2– C4– C1 115.0(3)
K9– O24 274.0(4) C3– N3– C1 115.7(3) O3– C5– C2 115.2(3)
K9– N7 281.8(3) N3– C1– N1 124.3(3) O4– C5– C2 117.0(3)
N1– C1 133.8(4) N2– C2– N1 124.7(3) O5– C6– C3 115.3(3)
N1– C2 133.4(4) N3– C3– N2 124.5(3) O6– C6– C3 116.1(3)
N2– C2 133.1(4) N1– C1– C4 117.9(3)
K2– O12 281.2(3) K4– O17 286.5(3)
K2– O14 284.4(3) K4– O21 273.4(3)
K2– O15 283.0(3) K4– O22 279.3(3)
K2– O19 275.7(3) K4– N8 281.8(3)
K2– O23 273.4(3) K5– O1 280.5(3)
K2– N9
K3– O1
K3– O3
280.7(3) K5– O7 291.3(3)
287.6(3) K5– O11 277.1(3)
281.1(3) K5– O24 280.0(5)
N2– C3 133.9(4) N3– C1– C4 117.8(3)
N3– C1 133.6(4) N1– C2– C5 117.4(3)
core to metal coordination is observed so that the triazine tri-
carboxylate unit can be regarded as an interesting linker for
coordination networks potentially allowing other linkage mo-
tifs than those observed in MOFs, which contain trimesic acid
as the organic linker.
Vibrational Spectroscopy
3 3 3 3 2
Figure 2. Crystal structure of K [C N (COO) ]·2H O representing the
layered structure perpendicular to [100]. Thermal ellipsoids (except for
hydrogen) are drawn at the 50 % probability level.
Potassium triazine tricarboxylate dihydrate was analyzed by
FTIR spectroscopy. All signals in the spectrum can be attrib-
uted to vibrations of the triazine tricarboxylate entities and wa-
ter molecules, respectively (cf. Figure 4). The bending and
whereas the K–N distances (278–282 pm) are relatively short stretching vibrations of the s-triazine ring lead to a characteris-
–1
compared to the potassium salts of other s-triazine or s-hepta- tic absorption band at 740 cm and a group of signals at
–1
zine compounds [28–30]. In K{Fe(1,3,5-triazine-2,4,6- 1300–1660 cm , respectively. In the range between 1300 and
–1
tricarboxylate)(H O) }·2H O, the iron atoms are coordinated 1660 cm , the stretching vibrations of the carboxylate groups
2
2
2
by an O–N–O unit in a similar way as in the title compound can be observed as well. The broad band at 3000–3600 cm–1
with a short Fe–N distance of 212 pm, whereas the potassium can be attributed to the OH stretching vibrations of the crystal
ions are coordinated solely by oxygen atoms and the K–O dis- water molecules and in accordance with the crystal structure
tances are comparable in both compounds. Hence, in both tria- the redshift of the ν(OH) signals corresponds to medium strong
zine tricarboxylate salts a strong contribution of the s-triazine hydrogen bonds [26].
2
586
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 2584–2588

3 3 3 3 2
K [C N (COO) ]·2H O – An Alkali Derivative of Triazine Tricarboxylate
between 65 and 150 °C, the crystal water molecules are re-
leased (mass loss observed: 10.9 %, calculated: 9.9 %) and the
so obtained dehydrated compound is thermally stable up to
3
25 °C. The decomposition of the sample starts with a mass
loss of 17 % accompanied by a sharp endothermic signal at
25–390 °C, probably due to a partial decarboxylation, and
3
proceeds with a slow and steady mass loss at higher tempera-
tures.
3 3 3 3 2
Figure 3. Crystal structure of K [C N (COO) ]·2H O representing the
stacked arrangement of the triazine tricarboxylate entities. Crystal wa-
ter molecules are omitted for clarity. The labels A–C refer to Figure 1,
indicating, which triazine tricarboxylate unit lies on top of the respec-
tive column. Thermal ellipsoids are drawn at the 50 % probability
level.
Figure 5.
TG
3
(solid)
2
and
DTA
(dotted)
curves
of
–1
K
3
[C (COO)
3
N
3
]·2H
O, recorded with a heating rate of 5 °C min .
Table 3. Donor-acceptor distances /pm and donor-hydrogen-acceptor
angles /° for the hydrogen bonding network in K
3
[C
3 3 3 2
N (COO) ]·2H O.
D–H···A
D···A <D–H···A D–H···A
D···A <D–H···A
Conclusions
O19–H1···O8 283.9 154.79
O19–H2···O5 272.0 159.35
O20–H3···O2 277.5 152.52
O20–H4···O10 279.6 166.63
O21–H5···O3 270.6 160.13
O21–H6···O11 281.4 169.61
O22–H7···O15 266.5 173.09
O22–H8···O11 292.8 176.50
O23–H9···O13 269.0 157.25
O23–H10···O8 290.1 167.27
O24–H12···O17 266.6 155.49
We reported on the crystal structure and thermal behavior of
potassium triazine tricarboxylate dihydrate. Triazine tricarbox-
ylate represents a multidentate ligand, which can be regarded
as the s-triazine analog of trimesic acid and hence as a promis-
ing linker for metal-organic frameworks but only one triazine
tricarboxylate salt has been structurally characterized so far.
This study has expanded the investigations of this class of
compounds to a new salt and thereby provides further insight
into the coordination properties of triazine tricarboxylate. The
presented crystal structure shows that the nitrogen atoms of the
s-triazine cycle exhibit a strong interaction with the potassium
ions and that the coordination of metal ions through the
O–N–O units of the triazine tricarboxylate entities is a recur-
ring binding motif for this ligand. Because of the differing
coordination properties compared to trimesic acid, triazine tri-
carboxylate seems to be an interesting linker for coordination
networks and with the observed high thermal stability of
K [C N (COO) ] an important condition for syntheses of
3
3
3
3
three-dimensional networks at elevated temperatures is ful-
filled.
3 3 3 3 2
Figure 4. FTIR spectrum of K [C N (COO) ]·2H O.
Experimental Section
Syntheses
Thermal Behavior
Potassium triazine tricarboxylate dihydrate was synthesized according
TG and DTA curves of potassium triazine tricarboxylate di- to the procedure described in [22, 31] by saponification of the respec-
hydrate are displayed in Figure 5. In the temperature range tive triethyl ester. To prepare the triethyl ester, dry HCl was conducted
Z. Anorg. Allg. Chem. 2010, 2584–2588
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
2587

S. J. Makowski, M. Hörmannsdorfer, W. Schnick
ARTICLE
through ethyl cyanoformate (20.0 g, 202 mmol, Aldrich, 99 %) under
argon until the trimerization product started to precipitate. Afterwards,
the conversion was completed by cooling the reaction mixture to 4 °C
overnight. The product was filtered, washed with cold water and dried
at 75 °C to yield 7.15 g (24.1 mmol, 36 %) triethyl-s-triazine-2,4,6-
tricarboxylate as colorless needles. Elemental analysis: N 14.22 (calcd.
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3 3 3 3 2
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Acknowledgement
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
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(
project SCHN 377/12) and the Fonds der Chemischen Industrie (FCI)
for financial support and Dr. Peter Mayer (Department Chemie, LMU
München) for the single-crystal data collection.
Received: June 18, 2010
Published Online: August 20, 2010
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www.zaac.wiley-vch.de
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 2584–2588