ORI GIN AL PA PER

Synthesis and Characterization of a NewNoncentrosymmetric Dihydrogenmonophosphate[C12H13N2O]H2PO4

Sami Soukrata • Mohamed Belhouchet • Tahar Mhiri

Received: 17 June 2014

� Springer Science+Business Media New York 2014

Abstract The organic–inorganic hybrid material [C12H13N2O]H2PO4 has been

synthesized at room temperature by slow evaporation. The compound crystallizes in

the noncentrosymmetric orthorhombic space group P212121 with the lattice

parameters a = 4.75 (5) A, b = 10.26 (5) A, c = 27.09 (3) A, Z = 4 and

V = 1323.3 (2) A3. The crystal structure has been determined and refined to

R1 = 0.047 and wR2 = 0.120 using 2870 independent reflections. The atomic

arrangement can be described by infinite anionic chains running parallel to the

a axis. The organic cations are linked to the inorganic chains by hydrogen bonds so

as to build a two-dimensional network. The infrared spectroscopy confirms the

presence of the organic group and the anionic entities. Concerning the differential

scanning calorimetry, it revealed one phase transition at 71 �C. Thermal analysis

was performed to study its thermal stability. Besides, the impedance spectroscopy

study, reported in the sample, reveals that the conduction in the material is due to a

Grotthus mechanism.

Keywords Inorganic compounds � Chemical synthesis � Infrared and NMR

spectroscopy � Thermal analysis � Electrical conductivity

Introduction

The prospect of creating new functional materials with tunable properties gives a

strong motivation on the research of organic–inorganic hybrid materials [1]. Interest

in these compounds with noncentrosymmetric structures has grown due to their

application in various fields such as e.g. quadratic non- linear optical research [2].

S. Soukrata (&) � M. Belhouchet � T. Mhiri

Laboratoire Physico-Chimie de l’Etat Solide. Departement de Chimie. Faculte des Sciences de Sfax,

Universite de Sfax, 3018 Sfax, Tunisie

e-mail: soukratasami@yahoo.fr

123

J Clust Sci

DOI 10.1007/s10876-014-0771-8

The synthesis of low-dimensional mixed organic–inorganic materials enables both

inorganic and organic components on the molecular scale to be optimized and thus

to exhibit specific properties, such as electronic, catalytic, optical, and second-order

non-linear optical to name just a few [3–6]. Among these hybrid compounds,

organic phosphates are particularly significant and their anions are interconnected

by strong hydrogen bonds so as to build infinite networks with various geometries

such as ribbons [6], chains [7], or layers [8, 9]. Considering the attractive properties

of organic phosphates and the new promising opportunities they may open with

regard to the development of useful organic–inorganic hybrids materials. In fact, the

current paper reports the synthesis, crystal structure, thermal behavior, vibrational

study and the dielectric measurements are combined to provide a good description

of this compound.

Experimental

Materials and Measurements

Infrared spectrum was diluted at room temperature on a Perkin-Elmer FT-IR

Spectrometer as KBr pellets in the region 4000–400 cm-1 region. All NMR spectra

were obtained on a Bruker DSX-300 spectrometer operating at 75.49 MHz for 13C

and 121.51 MHz for 31P, with classical 4-mm probehead allowing spinning rates up

to 10 kHz. 13C NMR chemical shifts are given relative to tetramethylsilane, while

the 31P ones are relative to 85 % H3PO4 (external reference precision 0.5 ppm). The

phosphorus spectrum was recorded under classical MAS conditions, while the

carbon ones was recorded by the use of crosspolarization from protons (contact time

5 ms) and MAS. In all cases, it was checked that there was a sufficient delay

between the scans, allowing a full relaxation of the nuclei. Simultaneous

thermogravimetry (TG) and differential thermal analyses (DTA) of a powdered

sample were performed in the temperature range of 25–450 �C, using a Setaram

TG–DTA92 thermo analyser, at a heating rate of 5 �C min-1. Differential scanning

calorimetry (DSC) measurements were measured in the temperature range of

25–300 �C using a Mettler Toledo DSC model DSC30 with samples placed inside

platinum crucibles at heating rate of 10 �C/min. Regarding the electrical conduc-

tivity, they were performed by means of a two-electrode configuration. Besides, the

polycrystalline [C12H13N2O]H2PO4 sample was pressed into pellets of 8 mm in

diameter and 1.1 mm in thickness using 3 tons/cm2 uniaxial pressures. The ac

conductivity measurements were performed with a Tegam 3550 impedance analyser

(209–4.63 MHz) which was also interfaced with a computer and a temperature

controller. Measurements were taken at temperatures from 291 to 393 K.

Chemical Preparation

Transparent plates of compound were synthesized in good yields by an alcoholic

reaction of phosphoric acid (0.098 g, 1 mmol Aldrich 85 %) with 2-amino-3-

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123

benzyloxy pyridine (0.2 g, 0.99 mmol Aldrich 99 %) followed by slow evaporation

at room temperature.

X-ray Crystallography

Intensity data of the compound were collected using a Kappa CCD diffractometer

(Bruker-Nonius) using graphite monochromated MoKa radiation (k = 0.71073 A).

The structure was solved with a direct method from the SHELXS-97 program

[11].Which permitted the location of the PO4 groups. The remaining non-hydrogen

atoms, were deduced from difference Fourier maps during the refinement. All

hydrogen atoms were geometrically fixed by appropriate instructions of the

SHELXL-97 program [11] and held in the riding mode. The successive refinements

lead to a reliability factors of R1 = 0.047 and wR2 = 0.120. The crystal data and

details of data collection and refinement are summarized in Table 1.

Results and Discussion

Structure Description

The X-ray analysis of the title compound revealed that the asymmetric unit is

composed of one [C12H13N2O]? cation and one [H2PO4]- anionic unit (Fig. 1).

Figure 2 shows that the [H2PO4]- entities are connected by strong hydrogen

bonds, O(1)–H(O1)…O(4) and O(3)–H(O3)…O(2), to form double infinite chains

running parallel to the a axis. The detailed geometry of the H2PO4- groups,

gathered in Table 2, shows two types of P–O distances depending on whether the

oxygen atoms are hydrogen donors or acceptors. As expected, the P–OH distances

ranging from 1.566 (2) A to 1.568 (2) A, is significantly longer than the P–O

distances ranging from 1.501 (2) A to 1.508 (2) A. The average values of the P–O

distances and the O–P–O angles are 1.535 (2) A and 109.37 (2)�, respectively.

These values are generally in excellent agreement with other phosphates [13–15]. It

is worth noting that the O…O distances involved in the hydrogen bonds [2.587 (3)

and 2.591 (3) A] are of the same order of magnitude as in the H2PO4 tetrahedron

[between 2.446 (4) and 2.536 (4) A]. Such distances along with the short P…P

distance of 4.449 (2) A allow us to consider the [H4P2O8]n2n- network as a

polyanion. This result has also been noticed in other crystal structures [16–18]. With

regards to the organic-cation arrangement, the main features measured are similar to

intramolecular bond distances and angles usually reported for such species [18, 19].

The calculated average values of distortion indices [20], correspond to the different

angles and distances in the PO4 tetrahedra [DI(OPO) = 0.0206, DI(PO) = 0.0203,

DI(OO) = 0.0078] exhibit a pronounced distortion of the O–P–O angles and P–O

distances if compared to O–O distances. So, the PO4 group can be considered as a

rigid regular arrangement of oxygen atoms, with the phosphorus atom slightly

displaced from the gravity center. The perspective view display in the Fig. 3 shows

that the organic entities, are lodged between the anionic chains through N–H…O

hydrogen bonds to form a two dimensional network. An examination of the organic

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

moiety geometrical features shows that the dihedral angle between the pyridine ring

and benzene plane is 80.17�. Furthermore, the mean value of the C–C bond lengths

is equal to 1.391 (5) A, being the distance between single bond and double bond,

which agrees well with that in benzene [21]. Besides, the C–N, C–C bond lengths

vary from 1.314 (4) to 1.506 (4) A, the C–C–N, C–C–C angles are included between

Table 1 Crystal data and structure refinement of the title compound

Empirical formula [C12H13N2O]H2PO4

Formula weight 298.23

Crystal system / space group Orthorhombic / P212121

a /A 4.75 (5)

b /A 10.26 (5)

c /A 27.09 (3)

V /A3 1323.3 (2)

Z 4

D calc (g/cm3) 1.497

l (mm-1) 0.23

Crystal size (mm) 0.3 9 0.2 9 0.1

Color / shape Transparent / plate

Temp (K) 293

Theta range for collection 1.5–27

Reflections collected 9068

Independent reflections 2870

Data/restraints/parameters 2466/0/168

Goodness of fit on F2 1.00

Final R indices [I [ 2r(I)] 0.047

R indices (all data) 0.120

Largest difference peak/hole -0.34 / 0.66

Fig. 1 Asymmetric unit of [C12H13N2O]H2PO4

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123

117.7 (3) and 123.5 (3)�, and C–C–O, C–O–C angles spread from 112.2 (3) to 128.0

(3)� Table 2. The crystal structure exhibits two types of hydrogen bonds, the first of

which, O–H…O, involve one short contact with H…A distances ranging from 1.78

to 1.79 A. The second one, N–H…O bonds, with H…A distances ranging from 1.76

to 2.05 A with D–H…A angles spread from 169 to 176� establishes the contact of

organic cation with the anionic chains (Table 3). This atomic arrangement includes

four hydrogen bond donors (two N and two O atoms) and two hydrogen bond

acceptors (O(2) and O(4) atoms).

NMR Spectroscopy

The solid state 31P MAS-NMR spectrum of the title compound is show on Fig. 4.

The spectrum exhibits a single resonance peak at 0.98 ppm. This value agrees with

those corresponding to monophosphate (between -10 and 5 ppm) [21–23] and is in

agreement with only one phosphorus crystallographic site in the structure. The 13C

CP-MAS NMR spectrum is show on Fig. 5. It displays ten different signals,

corresponding to twelve carbon atoms of the organic cation. The first one, whose

chemical shift peak is 68.5 ppm, is attributed to the methylene group substituted

found in the compound under study. The most shifted NMR components, whose

chemical shifts range from 111.8 to 147.3 ppm, are attributed to the aromatic carbon

atoms. To assign NMR components to different carbon atoms, the chemical shifts

have been calculated by means of the Chem Draw Ultra 6.0 software. The carbon

atoms are labeled in Fig. 1. The obtained results are gathered in Table 4, prove the

presence of only one organic moiety in the asymmetric unit of the compound which

agrees with the X-ray diffraction data.

IR Spectroscopy

To gain more information on the crystal structure, we have undertaken a vibrational

study using infrared spectroscopy. The infrared spectrum of the studied compound

at room temperature is show in Fig. 6. The assignments of the observed bands are

realized by comparison with similar compounds [19, 24]. Besides, the wavenumbers

of the observed peaks are quoted in Table 5. The isolated [PO4]3- tetrahedron with

an ideal Td symmetry has four vibrational modes; two stretching modes, m3 and m1

and two deformation modes, m4 and m2. These modes are observed at 1017, 938, 567

and 420 cm-1, respectively [25]. The three strong bands at 1090, 1062 and

1038 cm-1 are assigned to m3[PO4]3-. The stretching mode m1 appears as one strong

band at 980 cm-1. The bending vibrations arising from the m4[PO4]3-of the

phosphate anions are observed as three bands at 622, 580, and 496 cm-1. Next, the

medium band observed at 412 cm-1 is proposed as originating from m2[PO4]3-

bending vibrations. The broad bands with frequencies in the range of 3500 and

2700 cm-1 are attributed to the stretching of the organic and hydroxyl groups

(m(N–H), m(C–H)), and m(O–H) of P-OH groups. The ABC type bands characteristic

of the OH modes in the H2PO4 groups are barely perceived between 2500 and

1700 cm-1. The observed bands at 1628, 1584 and 1526 cm-1 are assigned to the

bending modes of the NH2 moiety (das(NH2) and ds(NH2), respectively). The

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

intense band around 1386 cm-1 is assigned to the m(C = C) vibration mode. The

bands which appear from 1494 to 1412 cm-1 are attributed to the bending mode

d(CH2). The observed bands at 1284-1158 cm-1 are assigned to m(C–N), m(C–C),

d(C–H), d(N–H) and d(OH).

Thermal Analysis

The thermal behavior of [C12H13N2O]H2PO4 has been studied by TG-DTA and

DSC from room temperature to 400 �C. Figure 7 shows the TG and DTA

thermograms of the anhydrous compound in which thermal behavior occurs in three

stages. The first one represented by an endothermic peak at 73 �C corresponds to the

phase transition. The second stage related to the endothermic peak at 168 �C is

attributed to the anhydrous compound. The third stage representing a set of

endothermic peaks in the range of 170–350 �C is assigned to the degradation of the

compound. This phase transition is confirmed by differential scanning calorimetry

(DSC) at 71 �C (Fig. 8).

Electrical Conductivity

The study of the dielectric properties is an important source for valuable

information about conduction processes [26]. Some complex impedance diagrams

Z00 versus Z0 cole–cole plots recorded at various temperatures are given in Fig. 9a

and b. The difference between the cole–cole and Debye law is determined by a (p/2)

Fig. 2 Projection along the b-axis of the inorganic entities of the title compound

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dispersion angle when a = 0.122. The intercept on the real axis of the zero phases

angle extrapolation of the highest-frequency curve determines the bulk ohmic

resistance relative to experimental temperature and it is used to show the evolution

of the conductivity versus inverse temperature. The curves show the temperature

dependence on the resistance proving the protonic-conduction properties. Thermal

evolution of conductivity for the title compound is presented in Fig. 10. The plot in

this figure shows the evolution of the conductivity Log (rT) versus 103/T for the

compound under study. The conductivity to this compound is significant and

increases from r = 2.38 10-8 X-1 cm-1 at 291 K to r = 6.26 10-4 X-1 cm-1 at

393 K. We point out that the break in this curve observed around 348 K,

is accompanied by an increase of the activation energy (Ea1 = 0.2 eV,

Ea2 = 1.306 eV). It can be noted that this compound presents in the range of

temperature from 291 K to 338 K a behavior capacitive. Also, this compound

presents from the temperature 348 K a behavior resistive. This important evolution

Table 2 Interatomic distances (A) and angles (�) of [C12H13N2O]H2PO4

Tetrahedron PO4

P O1 O2 O3 O4

O1 1.566 (2) 110.03 (2) 102.6 (2) 109.6 (2)

O2 2.518 (3) 1.508 (2) 109.8 (2) 112.8 (2)

O3 2.446 (4) 2.517 (3) 1.568 (2) 111.4 (2)

O4 2.507 (4) 2.506 (3) 2.536 (4) 1.501 (2)

O1—H(O1) = 0.820 P—O1—H(O1) = 109.5

O3—H(O3) = 0.820 P—O3—H(O3) = 109.5

Organic cation

C1—N1 1.314 (4) O5—C6 1.450 (4)

C1—N2 1.332 (4) C6—C7 1.506 (4)

C1—C5 1.427 (4) C7—C8 1.378 (4)

N2—C2 1.361 (4) C7—C12 1.390 (4)

C2—C3 1.359 (4) C8—C9 1.361 (5)

C3—C4 1.403 (4) C9—C10 1.342 (5)

C4—C5 1.371 (4) C10—C11 1.356 (5)

C5—O5 1.361 (4) C11—C12 1.409 (5)

N1—C1—N2 120.3 (3) C5—O5—C6 118.8 (3)

N1—C1—C5 121.9 (3) O5—C6—C7 112.2 (3)

N2—C1—C5 117.7 (3) C8—C7—C12 118.3 (3)

C1—N2—C2 123.5 (3) C8—C7—C6 120.8 (3)

C3—C2—N2 120.1 (3) C12—C7—C6 120.9 (3)

C2—C3—C4 119.0 (3) C7—C8—C9 121.4 (3)

C5—C4—C3 120.0 (3) C10—C9—C8 120.6 (4)

O5—C5—C4 128.0 (3) C9—C10—C11 120.6 (4)

O5—C5—C1 112.4 (3) C10—C11—C12 120.1 (4)

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

confirms the protonic conductor character of our material. The sudden variation of

the conductivity at 348 K marks the transition already observed by the DSC analysis

at 346 K. This phase transition can be explained by a more disordered state, which

might be correlated with changes in the orientation of molecular entities as

[C12H13N2O]? or [H2PO4]- and to the high dynamical disorder of NH2 groups in

the hydrogen N–H…O bonds. Dielectric relaxation studies have been undertaken at

temperature between 348 and 393 K, in the formalism of the complex electric

modulus M*. For a given temperature and frequency, the real part M0 and the

imaginary part M00 of the M* complex modulus (M* = M0 ? j M00) have been

calculated from the complex data (Z* = Z0 - jZ00) by the relations M0 = xC0Z00

and M00 = xC0Z0. The plots of log M0 and the normalized M00/M00max imaginary part

of the complex modulus of [C12H13N2O]H2PO4 versus log (f) are given in Figs. 11

and 12 at various temperatures. Whatever the temperature, the value of M0 reaches,

at high frequencies, a constant value (M0 = 1/e?) at high frequencies and at low

Fig. 3 Perspective view of [C12H13N2O]H2PO4 compound. (H-bonds are represented by dashed lines)

Table 3 Hydrogen bond geometry of [C12H13N2O]H2PO4

D—H���A D—H H���A D���A D—H���A

O1—H(O1)���O4i 0.82 1.79 2.591 (4) 165

O3—H(O3)���O2ii 0.82 1.78 2.587 (3) 167

N2—H2A���O2 0.86 1.76 2.609 (3) 169

N1—H1A���O4 0.86 2.05 2.910 (4) 176

Symmetry codes: (i) x-1, y, z; (ii) x?1, y, z?1; (iii) -x?1, y-1/2, -z?1

S. Soukrata

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frequencies it approaches zero, which indicates that the electrode polarization

phenomenon make a negligible contribution to M* and maybe ignored when the

electric data are analyzed in this form [27]. The M00/M00max spectrum relative to a

Fig. 4 31P MAS-NMRspectrum of [C12H13N2O]H2PO4

Fig. 5 13C MAS-NMR spectrum of [C12H13N2O]H2PO4 compound

Table 4 Calculated and experimental chemical shifts of the carbon atoms of the organic entity

Carbon atoms C6 C3 C4 C8 and

C12

C10 C9 and

C11

C2 C7 C5 C1

dcal. (ppm) 70.9 113.5 122.3 127.2 127.7 129.0 139.8 141.2 141.6 148.6

dexp. (ppm) 68.5 111.5 119.6 126.4 130.3 133.9 136.2 141.1 147.2 147.8

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

given temperature shows an asymmetrical peak. The modulus peak maximum shifts

to higher frequencies as temperature increases. The region to the left of the peak is

where the H? protons are mobile over long distances, whereas the region to the right

Fig. 6 IR spectrum of [C12H13N2O]H2PO4

Table 5 Spectral data and band assignments of the title compound

IR wavenumbers (cm-1) Assignment

3470 vs m(OH)

3288 vs ms(NH)

3144-2690 s mas(NH) ? m(C–H)

2440 -1970 m m(OH) ‘‘A ? B ? C’’

1628 vs das(NH2)

1584 m -1526 s ds(NH2)

1494 vs -1412 vs d(CH2)

1386 s m(C=C)

1346 s w(CH2)

1284 vs -1158 m m(C–N) ?m(C–C) ? d(C–H) ? d(N–H) and d(OH)

1090 s -1038 s m3(PO4)

980 s m1(PO4)

882 m

812 s

746 vs -700 s r(CH2)

622 m

580 m

540 s -496 s m4(PO4)

412 m m2(PO4)

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is where the ions are spatially confined to their potential wells. The frequency range

where the peak occurs is indicative of the transition from short-range to long-range

mobility at decreasing frequency and is defined by the condition xsr = 1, where

sr is the most probable proton relaxation time [28]. In summary, the electric

properties of this hybrid compound may be interpreted by the following way: The

rise of temperature can favor the vibration of the inorganic chains, which induce a

rapid reorientation H2PO4- and fast H? moving [30]. The fact that the OH groups

Fig. 7 TG-DTA thermogram of [C12H13N2O]H2PO4 compound

Fig. 8 DSC curve of [C12H13N2O]H2PO4

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

Fig. 9 a Complex impedance diagrams (-Z00 versus Z0) for [C12H13N2O]H2PO4 at T = [291–363 K],b Complex impedance diagrams (-Z00 versus Z0) for [C12H13N2O]H2PO4 at T = [368–393 K]

S. Soukrata

123

belonging to the phosphate anions form infinite chains is strongly in favor of a

protonic mobility between the oxygens of the phosphate anions. Indeed, the H2PO4-

anion performs both as proton donor and acceptor, by its four oxygen atoms, thus

producing an extended intermolecular H- bonds network, through which structural

migration of proton may occur via Grotthus mechanism [30]. The literature provides

many examples of this type of situation [14, 15].

Fig. 10 Arrhenius plot of the electrical conductivity of the compound

Fig. 11 A plot of log M0 versus log (f) at various temperature for [C12H13N2O]H2PO4

Noncentrosymmetric Dihydrogenmonophosphate [C12H13N2O]H2PO4

123

Conclusion

The present study reports on the synthesis and subsequent characterization of a

novel hybrid compound [C12H13N2O]H2PO4. This compound has been character-

ized by various physico-chemical methods. On the structural level, the atomic

arrangement of the title compound can be described by infinite anionic chains

running parallel to the a axis. The organic cations are linked to the inorganic chains

by hydrogen bonds so as to build a two-dimensional network. The number of solid-

state 13C and 31P MAS NMR components agrees perfectly with the ones of

crystallographically in dependent sites. Indeed, the thermal analysis results allow us

to show clearly that the compound presents an anhydrous character. The vibrational

properties of this compound were studied by Raman scattering and infrared

spectroscopy. The assignment of the vibrational bands was based on comparisons

with vibrational mode frequencies of homologous compounds. Finally, the

impedance spectroscopy study, reported in the sample, reveals that the conduction

in the material is due to a Grotthus mechanism.

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