Synthesis and characterization of a new monophosphate [2,5-(CH 3)2C6H3NH3]H 2PO4

Article abstract of DOI:10.1016/j.jpcs.2004.04.003

Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric organic cation monophosphate [2,5-(CH₃)₂C₆H ₃NH₃]H₂PO₄. This compound is orthorhombic P2₁2₁2₁ with the following unit-cell parameters: a = 5872(4)b = 20984(3)c = 8465(1) ?, Z = 4V = 10430(5) ?³ and D_x = 1396 g cm^-3. Crystal structure has been solved and refined to R = 0048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to (ab) planes between which organic groups [2,5-(CH₃)₂C ₆H₃NH₃]⁺ are located. Multiple hydrogen bonds connecting the different entities of compound thrust upon three-dimensional network a noncentrosymmetric configuration.

Full text of DOI:10.1016/j.jpcs.2004.04.003

Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
www.elsevier.com/locate/jpcs
Synthesis and characterization of a new monophosphate
[2,5-(CH3)2C6H3NH3]H2PO4
*
K. Kaabi, C. Ben Nasr, M. Rzaigui
´ ´
Laboratoire de Chimie des Materiaux, Faculte des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia
Received 25 November 2003; revised 20 February 2004; accepted 12 April 2004
Abstract
Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric
organic cation monophosphate [2,5-(CH3)2C6H3NH3]H2PO4. This compound is orthorhombic P212121 with the following unit-cell
3
parameters: a ¼ 5:872ð4Þ; b ¼ 20:984ð3Þ; c ¼ 8:465ð1Þ A, Z ¼ 4; V ¼ 1043:0ð5Þ A and Dx ¼ 1:396 g cm23. Crystal structure has been
solved and refined to R ¼ 0:048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to ða; bÞ planes
between which organic groups [2,5-(CH3)2C6H3NH3]þ are located. Multiple hydrogen bonds connecting the different entities of compound
thrust upon three-dimensional network a noncentrosymmetric configuration.
˚
˚
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials; B. Chemical synthesis; C. X-ray diffraction; C. Infrared spectroscopy; C. Thermogravimetric analysis (TGA); D. Crystal
structure
1. Introduction
2. Experimental
2.1. Chemical preparation
Crystal engineering of non-linear optical crystals [1] is
supported by some observations: (i) the anchorage of
organic cations onto inorganic subnetworks through
multiple and short hydrogen bonds providing the packing
with the cohesion observed in the ionic inorganic
crystals; (ii) a blue shift of the crystal transparency
resulting from the less polarizable nature of the proto-
nated 2,5-dimethylphenylaminium cation as compared to
an equivalent purely organic crystal; (iii) the building of
acentric framework without using polarisable chiral
entities. In this context, we report the synthesis and the
crystal structure of a new organic cation monophosphate:
[2,5-(CH3)2C6H3NH3]H2PO4. This compound was syn-
thesized within a systematic search on new materials
resulting from the association of organic and inorganic
entities, which could be of particular interest in
non-linear optics.
Crystals of the title compound [2,5-(CH3)2C6H3NH3]-
H2PO4 were prepared by slowly adding, at room tempera-
ture, 6.8 cm3 of H3PO4 (85%, d ¼ 1:7) to an alcoholic
solution containing 12.6 cm3 of 2,5-dimethylaniline (98%,
d ¼ 0:98). A crystalline precipitate was formed resulting
from the following reaction:
2; 5-ðCH3Þ2C6H3NH2 þ H3PO4
! ½2; 5-ðCH3Þ2C6H3NH3ꢀH2PO4
After dissolution by adding H2O the solution is slowly
evaporated at room temperature during several days until
the formation of transparent prismatic crystals with suitable
dimensions for crystallographic study. The crystals are
stable for months in normal conditions of temperature and
humidity.
2.2. Investigation techniques
The title compound has been studied by various physico-
chemical methods.
*
Corresponding author. Tel.: þ216-72-591-906; fax: þ216-72-590-566.
E-mail address: mohamed.rzaigui@fsb.rnu.tn (M. Rzaigui).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.04.003

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K. Kaabi et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
2.2.1. X-ray diffraction
2.2.4. NMR spectroscopy
All NMR spectra were recorded on a bruker DSX-300
spectrometer operating at 75.49 MHz for 13C and
121.51 MHz for 31P with a classical 4 mm probehead
allowing spinning rates up to 10 kHz. 13C NMR chemical
shifts are given relative to tetramethylsiline and 31P ones
relative to 85% H3PO4 (external references, precision
0.5 ppm). Phosphorous spectra were recorded under classi-
cal MAS conditions while the carbon ones were recorded by
use of cross-polarization from protons (contact time 5 ms).
Analysis of MAS-NMR spectrum was carried out by using
the Bruker program WINTFIT [5]. To assign NMR com-
ponents to the various carbons of the organic groups, ab
initio calculations were performed with the GAUSSIAN 98
software [6].
The intensity data collection was performed using a
MACH3 Enraf–Nonius diffractometer. The experimental
conditions of data collection, the strategy followed for the
structure determination and its final results are given in
Table 1.
2.2.2. Thermal behavior
Thermal analysis was performed using the ‘multimodule
92 Setaram’ analyzer operating from room temperature up
to 450 8C at an average heating rate of 5 8C/min.
2.2.3. Infrared spectroscopy
Spectra were recorded in the range 4000–200 cm21 with
a ‘Perkin-Elmer FTIR’ spectrophotometer 1000 using
samples dispersed in spectroscopically pure KBr pellets.
3. Results and discussion
Table 1
3.1. Structure description
Crystal data, experimental parameters used for the intensity collection,
strategy and final results of the structure determination
Structural determination shows that the title compound
crystallizes in the orthorhombic system with the noncen-
trosymmetric space group, P212121, which is confirmed by a
positive second harmonic generation powder test observed
on a sample illuminated by YAG Nd3þ laser radiation at
1.06 mm.
The final atomic coordinates of all non-hydrogen atoms
of [2,5-(CH3)2C6H3NH3]H2PO4 and their Beq are given in
Table 2. Those of hydrogen atoms, determined by difference
Fourier maps and not refined, are not given in order to
shorten the table. The main geometrical features of different
entities are reported in Table 3.
I-Crystal data
Formula:
Fw ¼ 219:18
[2,5-(CH3)2C6H3NH3]H2PO4
System: orthorhombic
a ¼ 5:872ð4Þ; b ¼ 20:984ð3Þ;
Space group: P212121
3
V ¼ 1143:0ð1Þ A , Z ¼ 4
˚
˚
c ¼ 8:465ð1Þ A
Refinement of unit cell
parameters with 25
reflections ð108 , u , 128Þ
rcal: ¼ 1:396 g. cm23
Morphology: prism
Fð000Þ ¼ 464
Crystal size (mm): 0.40 £ 0.35 £ 0.30
Linear absorption factor:
m (Mo Ka) ¼ 2.525 cm21
II-Intensity measurements
Diffractometer: Enraf–
A perspective view of the asymmetric unit of the
structure is depicted in Fig. 1, while the complete atomic
arrangement is shown in Fig. 2. This latter shows that the
H2PO24 inorganic entities have a layered organization
around the planes y ¼ 1=4 and 3/4.
˚
Wavelength: Ag Ka (0.5608 A)
¯
Nonius MACH3 (296 K)
Scan mode: v2u
Theta range: 2–308
hmax: ¼ 10; kmax: ¼ 37; lmax: ¼ 15
Nb. of independent reflections: 3528
Measurement area: h; k; l
Nb. of scanned reflections: 3528
Two intensity and orientation
control reflections: 0 7 4 and
Table 2
Final atomic coordinates and Beq for the non-hydrogen atoms
¯
0 7 4, no variation, every 400
2
Beq (A )
˚
Atoms
xðsÞ
yðsÞ
0.22775(1)
zðsÞ
III-Structure determination
Lorentz and polarization
corrections
Absorption correction: empirical
(Tmax ¼ 1:000; Tmin ¼ 0:738)
Program used: teXsan [3]
P
1.38335(5)
1.2354(2)
1.6284(2)
1.3883(2)
1.3038(2)
0.9415(2)
0.9893(2)
1.1786(3)
1.2176(3)
1.0719(4)
0.8806(4)
0.8424(3)
1.3319(4)
0.7194(5)
1.33199(4)
1.4021(2)
1.31809(13)
1.44338(13)
1.16788(13)
1.0537(1)
1.0089(2)
0.9152(2)
0.8787(3)
0.9303(3)
1.0194(2)
1.0606(2)
0.8514(3)
1.0702(4)
1.973(4)
3.01(2)
2.89(2)
2.53(2)
3.04(2)
2.23(2)
2.25(2)
2.78(2)
3.69(3)
3.88(3)
3.31(3)
2.72(2)
3.90(3)
4.88(5)
Determination: a direct
method SIR92 [2]
O(1)
O(2)
O(3)
O(4)
N
0.17284(5)
0.19800(5)
0.28370(4)
0.24202(5)
0.17362(5)
0.10787(6)
0.09513(7)
0.03097(8)
0.01672(7)
20.00300(7)
0.06069(7)
0.14601(10)
0.05418(9)
Thermal displacement parameters:
isotropic for H atoms, anisotropic
for non-H atoms
H atoms position were located
by Fourier difference syntheses
Unique reflections included:
2694 with I . 2sðIÞ
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
Weighting scheme: sigma
Refined parameters: 183
Largest shift/error: 0.8
Agreement factors
R : 0.038, Rw : 0.055
Residual Fourier density:
23
˚
20:25 , r , 0:25 e A
Drawings made with Diamond [4] Esd: 1.00
Note. Esd are given in parentheses.

K. Kaabi et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
1761
Table 3
˚
Main interatomic distances (A) and angles (8) in [2,5-(CH3)2C6H3NH3]H2PO4 atomic arrangement
The PO4 tetrahedron
P
O(1)
1.560(1)
O(2)
2.471(2)
O(3)
O(4)
O(1)
O(2)
O(3)
O(4)
2.518(2)
2.519(2)
1.506(1)
2.490(2)
2.471(3)
2.540(2)
1.496(1)
104.15(8)
110.39(7)
109.02(8)
1.573(2)
109.78(7)
107.21(9)
115.56 (1)
O(1)–H(1)
0.81(2)
115(2)
O(2)–H(2)
0.79(3)
117(2)
P–O(1)–H(1)
P–O(2)–H(2)
[2,5–(CH3)2C6H3NH3]þgroup
C(1)–C(2)
C(2)–C(3)
C(2)–C(7)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(5)–C(8)
C(6)–C(1)
N–C(1)
1.391(2)
1.400(2)
1.498(3)
1.387(3)
1.384(3)
1.399(2)
1.494(3)
1.384(2)
1.458(2)
0.98(3)
C(1)–C(2)–C(3)
C(1)–C(2)–C(7)
C(3)–C(2)–C(7)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
C(4)–C(5)–C(8)
C(6)–C(5)–C(8)
C(5)–C(6)–C(1)
C(6)–C(1)–C(2)
N–C(1)–C(2)
116.2(2)
123.3(1)
120.5(2)
121.6(2)
121.5(2)
117.7(2)
121.4(2)
120.9(2)
120.3(2)
122.7(1)
118.9(1)
118.3(1)
N–H(3)
N–H(4)
0.95(3)
N–H(5)
0.95(3)
N–C(1)–C(6)
C(1)–N–H(3)
113(5)
C(1)–N–H(4)
115(1)
110(1)
C(1)–N–H(5)
The hydrogen bonds
O(N)–H· · ·O
O(N)–H
0.81(2)
0.79(3)
0.98(3)
0.95(3)
0.95(3)
H· · ·O
O(N)· · ·O
O(N)–H· · ·O
174(2)
O(1)–H(1)· · ·O(3)
O(2)–H(2)· · ·O(3)
N–H(3)· · ·O(4)
N–H(4)· · ·O(2)
N–H(5).· · ·O(4)
1.78(2)
1.78(3)
1.73(2)
2.00(3)
1.80(3)
2.588(2)
2.560(2)
2.703(2)
2.941(2)
2.742(2)
167(2)
170(2)
173(2)
170(2)
Note. Esd are given in parentheses.
Fig. 3 represents a projection of such a layer located in
the plane y ¼ 3=4: It shows that the H2PO24 groups are
connected by strong hydrogen bonds to form infinite
corrugated chains in the c-direction of composition
(H2PO4)nn2. These chains are themselves interconnected
by means of N–H· · ·O hydrogen bonds originating from the
NHþ3 group, so as to build inorganic layers formed by
H2PO24 and NHþ3 groups. The P–P distance between H2PO4
tetrahedra: 4.194(3) is slightly shorter than that observed in
NH3(CH2)4NH3HPO4·H2O [7]. This is probably due to the
presence of two acidic hydrogen atoms on the PO4, which is
favorable for the formation of strong hydrogen bonds. The
detailed geometry of H2PO24 groups shows that the P–O
The 2,5-dimethylphenylammonium [2,5-(CH3)2C6H3-
NH3]þ groups are anchored onto successive inorganic layers
through hydrogen bonds involving the hydrogen atoms of
the NH3 groups with H(N)· · ·O distances varying between
˚
1.73(2) and 2.00(3) A. N–C and C–C distances and C–C–N
and C–C–C angles in this group are reported in Table 3.
The 2,5-(CH3)2C6H3NH2 organic molecule, containing
delocated and asymmetric P-bonds, is highly polarizable
entity in which transparency could be controlled. This
property favours the formation of noncentrosymmetric
materials as found with benzylamine C6H5CH2NH2 and
paraphenolamine 1,4-HOC6H4NH2 which reacts with
H3PO4 to form (C6H5CH2NH2)H2PO4 [9] and (1,4-HOC6-
H4NH2)H2PO4 [10] salts for Second Harmonic Generation.
˚
bonds are significantly shorter (1.469(1), 1.506(1) A) than
˚
the P–OH bonds (1.560(1), 1.573(1) A). This is in
agreement with the data relative to the protonated oxoanions
3.2. Thermal analysis
˚
[8]. Relatively short distances, from 1.73(2) to 2.00(3) A,
characterize all H· · ·O bonds, which maintain the cohesion
of this arrangement. It is worth noting that the O· · ·O
distances involved in the hydrogen bonds (2.560(2)–
The two curves corresponding to DTA and TGA analysis
in argon are given in Fig. 4. The DTA curve shows a series
of weak peaks in a wide temperature range (190–475 8C),
the most important one appears at about 203 8C. The TGA
curve shows a continuous weight loss in all this temperature
area. So, the corresponding phenomena could be interpreted
by H2PO24 condensation and a [2,5-(CH3)2C6H3NH3]þ
˚
2.588(2) A) are of the same order of magnitude as the
˚
O–O in the PO4 tetrahedron (2.471(2)–2.540(2) A), this
should allow us to consider the [H2PO4]nn2 subnetwork as
a polyanion.

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K. Kaabi et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
Fig. 3. Projection along the b-axis of the inorganic layer in [2,5(CH3)2-
C6H3NH3]H2PO4 structure. Note: organic radical are omitted for the clarity
of the figure.
an isolated PO4 in its ideal Td local symmetry to the nine
normal modes fundamentals of vibrations. The normal
modes are given by the representation:
Gint ¼ A1 þ E2 þ 2F2
Four are stretching modes, one is symmetric nsðA1Þ and
Fig. 1. Asymmetric unit of [2,5-(CH3)2C6H3NH3]H2PO4. Thermal
ellipsoids are shown at 40% probability.
three are asymmetric nasðF2Þ :
Gstr ¼ A1 þ F2
degradation leading to viscous matter of polyphosphoric
acids with a carbon black residue.
Five are bending modes, two are symmetric dsðE2Þ and
three are asymmetric dsðF2Þ :
3.3. Infrared spectroscopy
Gben ¼ E2 þ F2
Among these, only the F2 is active in IR and E2; A1
and F2 are active in Raman. Nevertheless, crystals of the
latter compound belong to the P212121 space group and
there are four units PO4 per cell, at a site of C1 symmetry.
According to the literature and to the theoretical group
analysis of the crystal of the title compound, there are 36
normal modes classified in this point group:
The monophosphate vibrations have been investigated
obviously [11,12]. The theoretical group analysis applied to
Gint ¼ 9A þ 9B1 þ 9B2 þ 9B3
In Table 4 are attributed the remaining observed bands in
the spectrum in Fig. 5 to the stretching and bending modes
corresponding to the different organic and inorganic groups.
Fig. 2. Arrangement of [2,5-(CH3O)2C6H3NH3]H2PO4 in projection along
the c-axis. PO4 is given in the tetrahedral representation. Hydrogen bonds
are denoted by dotted lines.
Fig. 4. DTA and TGA curves of [2,5-(CH3)2C6H3NH3]H2PO4 at rising
temperature.

K. Kaabi et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
1763
Table 4
IR frequencies and assignments in the stretching and bending domain of PO34- in [2,5-(CH3O)2C6H3NH3]H2PO4
Note: vs, very strong; s, strong; m, middle; w, weak; vw, very weak.
Frequencies in the range 4000–1400 cm21 are attributed to
C(N)–H stretching and bending modes [13]. Absorption
bands in the range 1400–1200 cm21, corresponding to
stretching vibrations of C–N bonds and O–H groups [14].
The two stretching vibration bands characteristics of a PO4
tetrahedron ns and nas are observed about 1200–1000
and 1000–850 cm21, those ranging from 650–500 to
500–300 cm21 attributed to bending modes ds and das of
PO4 [11,12]. We note that the supplementary frequencies in
the ns(PO4) domain are attributed to the bending modes
dðCaryl 2 HÞip [15].
However, the experimental and theoretical results are not
compatible, this can be explained by the interactions
between the different PO432 ions, because the unit cell of
this compound contains four anions.
3.4. NMR spectroscopy
Proton decoupled 31P MAS-NMR spectrum of crystalline
monophosphate [2,5-(CH3)2C6H3NH3]H2PO4 is given in
Fig. 6. It exhibits only one peak with two corresponding
satellite spinning side bands spaced at equal intervals
(spinning rate of the sample expressed in ppm). The
corresponding chemical shift value 20.02 ppm was
Fig. 6. Theoretical and experimental 31P MAS-NMR spectrum of
crystalline dihydrogenomonophosphate [2,5-(CH3)2C6H3NH3]H2PO4. *
Spinning side bands.
Fig. 5. IR-spectra of [2,5-(CH3)2C6H3NH3]H2PO4.

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K. Kaabi et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1759–1764
Table 5
Calculated ðdisoÞ and experimental ðdexpÞ chemical shifts of the organic
groups carbon atoms
Carbon atoms C1
C2
C3
C4
C5
C6
C7
C8
diso (ppm)
dexp (ppm)
60.6 57.1 31.5 39.7 42.4 52.0 166.3 159.1
122.9 128.3 138.3 133.2 131.3 130.1 17.4 19.8
the eight carbon atoms were calculated and the results are
regrouped in Table 5.
diso being the absolute chemical shift and the relative
chemical shifts, such as those measured experimentally,
correspond to the difference
Fig. 7. 13C CP-MAS-NMR spectrum of crystalline dihydrogenomonopho-
sphate [2,5-(CH3)2C6H3NH3]H2PO4. * Spinning side bands.
dexp ¼ dref 2 diso
thus, we can propose the attribution gathered in Table 5.
recorded with respect to 85% H3PO4 (negative chemical
shifts are towards higher fields in spectrum). This chemical
shift value agrees with those of monophosphates (between
210 and þ5 ppm), depending on the compound [16–18].
The existence of a single peak in the region analyzed
indicates the presence of only one crystallographic site in
the unit cell of this monophosphate, which agrees with the
X-ray results.
Acknowledgements
We would like to thank the Secretary of State for Scientific
Research and Technology for their support, in this work.
References
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asymmetric parameter equal to zero suggests the existence
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