Tris(2,4,6-triamino-1,3,5-triazin-1-ium) Dihydrogenphosphate Monohydrogenphosphate Tetrahydrate

Article abstract of DOI:10.1002/zaac.201400391

When "recrystallizing" the commercial flame retardant melaminium orthophosphate, surprisingly a hydrate with a diverging stoichiometry of melaminium (M) and phosphate (P) was obtained (M₃P₂ ·4H₂O). The structure of M₃P₂ ·4H₂O was solved by single-crystal X-ray diffraction analysis. The arrangement of positive melaminium and negative phosphate layers found in the crystal structure allows for both electrostatic attraction and hydrogen bonding networks. The melaminium layers are comprised of hydrogen-bonded ribbons of melaminium, which in turn are connected by π-π interactions into layers. The negatively charged phosphate layers are comprised of hydrogen bonded dihydrogenphosphate dimers and hydrogenphosphate dimers, which in turn are interconnected by direct and water-mediated hydrogen bonding into layers.

Full text of DOI:10.1002/zaac.201400391

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
ARTICLE
DOI: 10.1002/zaac.201400391
Tris(2,4,6-triamino-1,3,5-triazin-1-ium) Dihydrogenphosphate
Monohydrogenphosphate Tetrahydrate
[a]
[a]
[a]
[a]
Josef Hausner, Christian Butterhof, Thomas Martin, Wolfgang Milius, and
Josef Breu*[
a]
Dedicated to Professor Martin Jansen on the Occasion of His 70th Birthday
Keywords: Melaminium orthophosphate; Crystal structure; Flame retardant; X-ray diffraction; Hydrogen bonds
Abstract. When “recrystallizing” the commercial flame retardant mel-
aminium orthophosphate, surprisingly a hydrate with a diverging
stoichiometry of melaminium (M) and phosphate (P) was obtained
melaminium layers are comprised of hydrogen-bonded ribbons of mel-
aminium, which in turn are connected by π–π interactions into layers.
The negatively charged phosphate layers are comprised of hydrogen
bonded dihydrogenphosphate dimers and hydrogenphosphate dimers,
which in turn are interconnected by direct and water-mediated
hydrogen bonding into layers.
3 2 2 3 2 2
(M P ·4H O). The structure of M P ·4H O was solved by single-crys-
tal X-ray diffraction analysis. The arrangement of positive melamin-
ium and negative phosphate layers found in the crystal structure allows
for both electrostatic attraction and hydrogen bonding networks. The
(M
hydrogenphosphate) monohydrogenphosphate tetrahydrate
P ·3H
O)[12] and hexakis(melaminium)tetrakis (di-
4 3 2
Introduction
Assuring sufficient flame retardancy remains a major chal-
lenge in polymer science. On a quantitative base the market is
dominated by aluminum trihydroxide (ATH) and haloge-
nated flame retardants.[ Halogenated flame retardants are,
[13]
(
M P ·4H O).
6 5 2
Surprisingly, when trying to “recrystallize” MP·0H O we
[
1]
2
obtained yet another hydrate with a stoichiometry of
2]
M P ·4H O. The structure of M P ·4H O was solved by sin-
3
2
2
3
2
2
however, threatened to be banned due to environmental con-
gle-crystal X-ray diffraction analysis.
cerns,[
3,4]
in particular their toxic combustion products, like
[5,6]
halogenated dioxins and furans.
Among the more environ-
[7,8]
mentally friendly, halogen free additives, clay minerals,
or-
ganophosphor,[ or nitrogen based
9]
[10]
Results and Discussion
compounds are most
promising. In particular, melaminium (poly-)phosphates (e.g.
Melapur® family of compounds made by BASF SE) are able
to serve all potential mechanisms of flame retardancy
[14]
The acid constant of melamine (pK ≈ 5)
is very close to
a
the first equivalence point of the dissociation of phosphoric
acid. Slight deviations from the exact 1:1 stoichiometry of
melamine and phosphoric acid when preparing melaminium
phosphates therefore are likely to change the equilibrium be-
tween dihydrogenphosphate and hydrogenphosphate and con-
sequently the M:P ratio. When preparing melaminium di-
(intumescence, char formation, flame poisoning, cooling, and
oxygen dilution) concomitantly. Interestingly, the stoichio-
metry and the degree of hydration observed for crystalline
compounds of melaminium orthophosphates seems to be
largely independent from the stoichiometry applied in solution
but rather controlled by the conditions of the crystallization.
hydrogenphosphate (MP·0H O) according to de Ridder et
2
[15]
al.
typically the solution pH is slightly lower than the first
equilibrium point (pH ≈ 4) with H PO4 being the predominant
species. When recrystallizing this solid a slightly higher pH of
.8 is obtained shifting the equilibrium towards HPO4 and
different stoichiometry
M P ·4H O) and 1:1-ratio of H PO and HPO4 is obtained.
Aside from the commercial non-hydrate (MP·0H O) of 1:1
–
2
2
stoichiometry, three hydrated compounds of melamine ortho-
phosphate with diverging stoichiometry are hitherto known
2–
4
(
Table 1): Bis(2,4,6-triamino-1,3,5-triazin-1-ium) hydrogen-
consequently
a
salt with
a
[11]
phosphate trihydrate (M P·3H O),
tetrakis(melaminium)-
–
2–
2
2
(
3
2
2
2
4
bis(dihydrogenphosphate) monohydrogen phosphate trihydrate
*
Prof. Dr. J. Breu
Fax: +49-921-55-2788
Single Crystal Structure of Melaminium Orthophosphate
E-Mail: josef.breu@uni-bayreuth.de
[
a] Department of Inorganic Chemistry I
Crystallographic data and details of the refinement are sum-
marized in Table 2. An ORTEP plot of the asymmetric unit of
M P ·4H O is shown in Figure 1.
University of Bayreuth
Universitätsstr. 30
95440 Bayreuth, Germany
3
2
2
Z. Anorg. Allg. Chem. 0000, ᭿,(᭿), 0–0
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
J. Breu et al.
ARTICLE
Table 1. Listing of melaminium phosphate structures (hydrated and non-hydrated).
CSD entry code
Formula moiety
HUGQAC
2(C
+)
MOJDIZ
4(C
XORAYE
6(C
+)
YADNAT
+
3
H
7
N
6
3
H
7
N
6
+)
3
H
7
N
6
3 7 6
C H N
H O P
2 4
–
–
P 2–
–
P2–
–
HO
(H
P1
6.767(4)
4
P
2(H
3(H
P1
2
O
O)
4
P ), HO
4
4(H
4(H
2
O
O)
4
P ),HO
4
3
2
O)
2
2
¯
¯
¯
P1
Space group
a /Å
b /Å
c /Å
α /°
P2
1
8.9867(1)
20.8097(3)
8.9843(1)
98.1691(5)
93.4080(5)
93.1424(9)
11.341(2)
20.965(4)
12.334(2)
9.3639 (1)
10.2418 (1)
4.5778(1)
91.7587(2)
94.7710(2)
83.5385(2)
10.548(7)
12.497(8)
91.865(11)
105.609(10)
108.020(9)
β /°
γ /°
117.80(3)
Stoichiometry (M:acid) meta phosphoric acid (1:1)
phosphoric acid (85%)
1:0.77)
cooling from 75 °C
excess 20 wt% phosphoric
acid
slow evaporation at RT
phosphoric acid (85%)
(1:0.85)
(
Crystallization
hot solution crystallization
at RT
2
evaporation of H O at
80 °C
Table 2. Crystallographic data and refinement details for the melamin-
ium orthophosphate.
Empirical formula
9 32 18 12 2
C H N O P
Formula weight
Temperature /K
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
646.47
173(2)
triclinic
¯
P1
11.232(2)
11.833(2)
11.984(2)
66.91(3)
71.51(3)
64.28(3)
1299.1(4)
2
γ /°
V /Å3
Z
ρ
calc /mg·m–3
1.65
0.3
μ (Mo-K
α
) /mm–1
R
int
0.0454
0.0282
7008
R
σ
No. reflns.
Unique reflns. [I Ն 2σ (I)]
6305
1.089
0.0413
0.1055
0.0462
0.1120
2
GOOF (F )
a)
R
wR
1
[I Ն 2σ (I)]
Figure 1. ORTEP plot and crystallographic numbering schemes of the
asymmetric unit.
b)
2
[I Ն 2σ (I)]
(all data)
(all data)
R
1
wR
2
= [Σ[w(F 2
– F
2)2]/Σ[w(F
2 2
) ]]1/2.
Crosschecking by the valence bond method confirmed the
same hydrogen positions.
Organic (melaminium) and inorganic (dihydrogenphosphate,
a) R
1
= Σ||F | – |F
o
c
||/Σ|F
o
|. b) wR
2
o
c
o
As pointed out by Janczak et al.[13] the geometry of all three
crystallographically independent melaminium rings deviate hydrogenphosphate, water) moieties segregate into layers
significantly from an ideal hexagon. The mean C–N–C angle stacked alternating along the c* axis in the crystal structure
at the protonated nitrogen atoms (N2, N8, N14) is significantly (Figure 2). In addition to Coulomb forces, hydrogen bonding
larger than the mean value of the C–N–C angle at non-proton- motives are the main interaction forces as can be seen from a
ated nitrogen atoms (N1, N3, N7, N9, N13, N15) with values side view along the ab plane in Figure 2(b). Per unit cell
of 118.9° and 115.9°, respectively. Looking at the N–C–N eleven hydrogen bonds are formed between the melaminium
angles the ones with a protonated nitrogen atom involved (N1, moieties and phosphate or water molecules. In all eleven of
N2; N2, N3; N7, N8; N8, N9; N13, N14; N14, N15) have a these bonds the melaminium is acting as an hydrogen donor
mean angle value significantly smaller (121.8°) than the ones molecule. The melaminium ring I is involved in five of the
involving a non-protonated nitrogen atom (N1, N3; N7, N9; eleven hydrogen bonds. Thereby N–H···O hydrogen bonds are
N13, N15) (125.5°). These variations were explained by the formed between N2 and O5 and N5 and O8 of the hydrogen-
steric demand of a lone-pair electron.[ All hydrogen atoms phosphate, and between N4 and O2 of the dihydrogenphos-
of melaminium cations were added at geometrically ideal posi- phate, whereas N6 is involved in two hydrogen bonds to two
tions and refined isotropically. The hydrogen atoms of the different water molecules, O10 and O12, respectively. Melamin-
hydrogenphosphate and dihydrogenphosphate were located ium ring II is involved in three N–H···O hydrogen bonds, one
from the electron density and refined using a riding model. between N8 and O4 of the dihydrogenphosphate and two be-
16]
2
www.zaac.wiley-vch.de
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 0000, 0–0

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
Tris(2,4,6-triamino-1,3,5-triazin-1-ium) Dihydrogenphosphate Monohydrogenphosphate Tetrahydrate
tween N10 and O7 and N12 and O7 of the hydrogenphosphate. bonds, too. Thereby two N–H···O hydrogen bonds are formed
Melaminium ring III is involved in three N–H···O hydrogen between N14 and O9 and N16 and O10 of two water
molecules and one is built up between N18 and O2 of the
dihydrogenphosphate.
As already pointed out by Brodski et al.[12] the crystal struc-
tures of all known melaminium phosphates have a similar
structural feature, which is confirmed for M P ·4H O: Ribbons
3
2
2
of melaminium rings arranged alternating with the protonated
nitrogen atoms up and down. The melaminium layers, as
shown in Figure 2 (a, c, d), consist of three ribbons in the unit
cell, which are aligned parallel and shifted relative to each
other. Two ribbons are connected by the inversion center and
are made up by rings I and III [rb I in Figure 2 (c)] with the
protonated nitrogen atom of ring I pointing up, while the one
of ring III points down and vice versa. The N–H···N hydrogen
bonds are formed between N4 and N13, N5 and N15, N16 and
N1, N17 and N3, wherein the nitrogen atoms N4, N5, N16 and
N17 act as the donor atoms. The ribbons are almost flat. Only
small torsions were observed between the adjacent rings
[5.2(1)° and –3.9(1)°]. The third ribbon, rb II, is made up by
ring II [Figure 2 (d)] with the protonated nitrogen atom of the
ring alternating up and down with an inversion center between
the rings in the ribbon. The N–H···N hydrogen bonds are
formed between N10 and N7 and N11 and N9, wherein the
nitrogen atoms N10 and N11 act as the donor atoms. This rib-
bon is completely flat. The main interaction forces between
the adjacent ribbons in a layer are shifted π–π stacking interac-
tions. The π–π interaction distances between adjacent melamin-
ium rings (I-II, I-III, and II-III) are 4.442 Å, 3.538 Å, and
4.057 Å. The angles between the two adjacent ring planes
(I-II, I-III, and II-III) are 38.9°, 23.5° and 39.1°, respectively.
The negatively charged phosphate layers (Figure 3) are com-
prised of hydrogen bonded dihydrogenphosphate dimers and
hydrogenphosphate dimers, which in turn are connected by
hydrogen bonding to zigzag chains. The chains are intercon-
nected by water-mediated hydrogen bonding into layers.
Figure 2. Molecular packing of the MP crystal structure. (a) Layer by
layer assembely of the melaminium and phosphate layers parallel to
the a-b-plane. (b) Hydrogen bonding motif between the melaminium
layer and the phosphate layer looking along the ab plane. (c) Hydrogen
bonding motif of ribbon I formed by ring I and III. (d) Hydrogen
bonding motif of ribbon II made up by ring II.
Figure 3. Hydrogen bonding motif of the phosphate layer looking
along the c axis.
Z. Anorg. Allg. Chem. 0000, 0–0
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
3

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
J. Breu et al.
ARTICLE
The angles formed in the zigzag pattern are 74.928(14)° 320 °C (5.2% and 5.3%, respectively) amounting to a total
P1–P1–P2) and 161.628(18)° (P1–P2–P2). The phosphate mass loss of 10.5%. The mass loss corresponds well with the
(
–
2–
species, H PO and HPO
4
deviate from ideal tetrahedral release of two water molecules in each step suggesting that a
water free intermediate may be formed (expected weight loss
2
4
symmetry as expected.[
13]
11.1%). Further loss of water detected in the QMID above
3
50 °C may be attributed to the condensation to pyrophos-
Powder Diffraction Data of Melaminium Orthophosphate
phates.
To check for phase purity of the crystallized M P ·4H O,
3
2
2
additionally a Rietveld refinement was performed. The Riet-
veld plot is shown in Figure 4 and the results of the refinement
are summed up in Table 3. The quality of the Rietveld fit is
high indicating a phase pure material.
Figure 5. TGA measurement of M
of water (m/z = 18) (dashed line).
3 2 2
P ·4H O (solid line) and the QMID
Conclusions
[
14]
The acid constant of melamine (pK ≈ 5)
by chance hap-
a
Figure 4. Rietveld refinement (applying TOPAS Academic) of the sin-
gle crystal of M P ·4H O. The refinement is showing the good fit
3 2 2
pens to be near the first equivalence point of the dissociation
between the observed and calculated data. Shown are the observed of phosphoric acid. Minor variations of the pH consequently
pattern (O), the best Rietveld-fit profile (–), the tick marks (|) indicate change the equilibrium between dihydrogenphosphate and
3 2 2
the positions of the Bragg reflections of the unit cell of M P ·4H O.
Δ means the difference.
hydrogenphosphate and consequently the M:P ratio giving rise
to a surprisingly rich chemistry of this technically important
flame retardant. Simply by “recrystallization” of the commer-
Table 3. Rietveld refinement details for the laboratory PXRD data ob- cial melaminium dihydrogenphosphate (MP·0H O), the equi-
2
tained.
2–
librium is shifted sufficiently towards HPO4 to obtain yet
another stoichiometry (M ·4H O). It would be interesting to
Refinement
3
P
2
2
explore the varying M:P stoichiometries in respect to their per-
formance as flame retardants.
Temperature /K
173(2)
11.249(1)
11.849(1)
11.983(1)
66.95(1)
71.55(1)
64.23(1)
1302.8(1)
2
a /Å
b /Å
c /Å
α /°
β /°
γ /° 3
V /Å
Z
Experimental Section
Melamine (purity Ն 99%) was purchased from Merck, phosphoric
acid (85%) from Sigma Aldrich. All chemicals were used without fur-
ther purification. Distilled water was used.
Z’
1
R
R
R
1
(all data)
0.0243
0.0177
0.0252
X-ray powder diffraction traces were recorded with a STOE STADI P
(Cu-Kα1 radiation, transmission geometry) diffractometer. The samples
were filled in capillaries (diameter 0.5 mm). Single crystal X-ray dif-
fraction data were collected with a STOE IPDS 2 instrument (Mo-
P
wp
α
K radiation) equipped with an Oxford cryostream system. Selected
crystallographic data are listed in Table 2. The crystal structure was
solved and refined using SHELXTL 5.1 (Bruker AXS). All non-
hydrogen atoms were refined anisotropically with the hydrogen atoms
added to their geometrically ideal positions and refined isotropically.
The hydrogen atoms of the phosphoric groups were located from the
TGA Measurement
The loss of water of the new hydrate was followed by TGA
with concomitant mass spectroscopy (Figure 5). Quasi Mul-
tiple Ion Detection (QMID) at m/z = 18 identifies two well electron density and refined using a riding model. All figures were
separated steps of water release at temperatures of 272 °C and drawn with the DIAMOND program.
4
www.zaac.wiley-vch.de
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 0000, 0–0

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
Tris(2,4,6-triamino-1,3,5-triazin-1-ium) Dihydrogenphosphate Monohydrogenphosphate Tetrahydrate
Further details of the crystal structure investigations may be
obtained from the Fachinformationszentrum Karlsruhe, 76344 Egg-
[2] D. W. Edward, V. L. Sergei, in Flame Retardants for Plastics and
Textiles, Carl Hanser Verlag GmbH & Co. KG, 2009, p. 3–34.
enstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: [3] L. S. Birnbaum, D. F. Staskal, Environ. Health Perspect. 2004,
crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request for de-
posited data.html) on quoting the depository number CSD-428282
112, 9–17.
[4] R. J. Law, D. Herzke, S. Harrad, S. Morris, P. Bersuder, C. R.
Allchin, Chemosphere 2008, 73, 223–241.
[5] J. de Boer, P. G. Wester, H. J. C. Klammer, W. E. Lewis, J. P.
Boon, Nature 1998, 394, 28–29.
Melaminium Orthophosphate: The synthesis of melaminium di-
2
hydrogenphosphate (MP·0H O) was done according to de Ridder et
al.[ A mixture of melamine (1.5 g, 0.0119 mol) with distilled water [6] A. Shanley, Chem. Eng. 1998, 105, 61.
15]
(
500 mL) was heated to 75 °C. After melamine was completely diluted [7] M. R. Schütz, H. Kalo, T. Lunkenbein, J. Breu, C. A. Wilkie,
the phosphoric acid (1.37 g of phosphoric acid, 85%, 0.0119 mol) was
Polymer 2011, 52, 3288–3294.
added to the mixture (pH = 4) under vigorous stirring and the solution [8] M. R. Schütz, H. Kalo, T. Lunkenbein, A. H. Groeschel, A. H.
was kept 2 h at 75 °C whilst stirring. Afterwards the mixture was
cooled down to room temperature smoothly while a white solid of
non-hydrated MP (X-ray diffractogram not shown) precipitated. The
white solid was filtered and dried at 80 °C in an oven. When the ob-
Mueller, C. A. Wilkie, J. Breu, J. Mater. Chem. 2011, 21, 12110–
12116.
[9] T. Orhan, N. A. Isitman, J. Hacaloglu, C. Kaynak, Polym. Degrad.
Stabil. 2011, 96, 1780–1787.
2
tained powder of MP·0H O, 50 mg was “recrystallized” by dissolving [10] D. Guo, Q. Wang, S. Bai, Polym. Adv. Technol. 2013, 24, 339–
it in distilled water (10 mL, pH = 4.8) at 75 °C for 30 minutes followed
347.
by slow cooling (6–8 h) to room temperature, a phase pure compound
with a stoichiometry diverging (M P ·4H O) from that of the starting
3 2 2
[11] X. M. Li, S. S. Feng, F. Wang, Q. Ma, M. L. Zhu, Acta Crys-
tallogr., Sect. E 2010, 66, 239–240.
compound was obtained. Appropriate colorless crystals were picked [12] V. Brodski, R. Peschar, H. Schenk, A. Brinkmann, E. R. H.
for data collection.
van Eck, A. P. M. Kentgens, J. Phys. Chem. C 2008, 112, 12515–
2523.
13] J. Janczak, G. J. Perpetuo, Acta Crystallogr., Sect. C 2002, 58,
55–459.
1
[
[
[
[
Acknowledgements
4
14] Y. H. Jang, S. Hwang, S. B. Chang, J. Ku, D. S. Chung, J. Phys.
This work was supported by the German Science Foundation (Grant
SFB 840).
Chem. A 2009, 113, 13036–13040.
15] D. J. A. De Ridder, K. Goubitz, V. Brodski, R. Peschar, H.
Schenk, Helv. Chim. Acta 2004, 87, 1894–1905.
16] R. J. Gillespie, J. Chem. Educ. 1963, 40, 295.
References
Received: August 13, 2014
[
1] A. Witkowski, A. A. Stec, T. Hull, Polym. Degrad. Stabil. 2012,
Published Online: ᭿
97, 2231–2240.
Z. Anorg. Allg. Chem. 0000, 0–0
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
5

Job/Unit: Z14391
/KAP1
Date: 08-10-14 17:43:22
Pages: 6
z201400391e1.xml
J. Breu et al.
ARTICLE
J. Hausner, C. Butterhof, T. Martin, W. Milius,
J. Breu* ............................................................................. 1–6
Tris(2,4,6-triamino-1,3,5-triazin-1-ium) Dihydrogenphosphate
Monohydrogenphosphate Tetrahydrate
6
www.zaac.wiley-vch.de
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 0000, 0–0