Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

doi:10.4236/jcpt.2013.34020

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Journal of Crystallization Process and Technology, 2013, 3, 123-129

http://dx.doi.org/10.4236/jcpt.2013.34020 Published Online October 2013 (http://www.scirp.org/journal/jcpt)

123

Synthesis, Growth, Crystal Structure and Characterization

of the o-Toluidinium Picrate

Kandasamy Mohana Priyadarshini1, Angannan Chandramohan1*, Thangarak Uma Devi2

1Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore, India; 2Department of

Physics, Government Arts College for Women, Pudukottai, India.

Email: *[email protected]

Received July 23rd, 2013; revised July August 23rd, 2013; accepted August 30th, 2013

tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

A new organic charge transfer molecular complex salt of o-toluidinium picrate (OTP) was synthesised and the single

crystals were grown by the slow solvent evaporation solution growth technique using methanol as a solvent at room

temperature. Formation of the new crystal has been confirmed by single crystal X-ray diffraction (XRD) and NMR

spectroscopic techniques. The crystal structure determined by single crystal X-ray diffraction indicates that both the

cation and the anion are interlinked to each other by three types of intermolecular hydrogen bonds, namely

N(4)-H(4A)···O(7), N(4)-H(4B)···O(5) and N(4)-H(4C)···O(7). The title compound (OTP) crystallizes in monoclinic

crystal system with the centrosymmetric space group P21/c. Fourier transform infrared (FT IR) spectral analysis was

used to confirm the presence of various functional groups in the grown crystal. The optical properties were analyzed by

the UV-Vis-NIR and fluorescence emission studies.

Keywords: Single Crystal; Organic Molecules; Solution Growth; X-Ray Diffraction; Characterization; Nonlinear

Optical

1. Introduction

The organic materials with aromatic ring, which are of

great interest for second and third-order nonlinear optical

applications due to their high nonlinearity, high optical

damage threshold and their ultrafast, almost purely elec-

tronic response. Based on the concepts of the molecular

and crystal engineering, the organic molecules offer

many possibilities to tailoring the substances with desired

properties through optimization of the microscopic hy-

perpolarizabilities and the incorporation of the molecules

in a crystalline lattice [1-4]. Mulliken suggested that the

charge transfer interactions from two aromatic molecules

can arise from the transfer of an electron from Lewis

base to Lewis acid and these complexes have attracted

great attention for nonlinear optical materials. Generally,

proton transfer interactions between electron donor and

electron acceptor molecules absorb radiation in the visi-

ble region leading to the formation of intensely colored

charge transfer complexes [5-10]. Picric acid forms crys-

talline picrates of various organic molecules through io-

nic and hydrogen bonding and π-π interactions and the

presence of phenolic OH in the picric acid favors the

formation of the salts with various organic bases [11].

The formation of charge transfer complex depending on

the nature of the donor-acceptor system and the orientation

of anionic and cationic species facilitates the formation

of expected N-H·····O hydrogen bonds between amino

hydrogen and phenolic oxygen [12]. It has been reported

that intramolecular hydrogen bonding interactions are

absent in most of the picrate salts [13] and picric acid

derivatives are interesting candidates, as the presence of

phenolic OH and electron withdrawing nitro groups fa-

vors the formation of salts with various organic bases

such as N,N-dimethylanilinium picrate [11], 3-Methyl

aniliniumpicrate [14], 2-Chloroanilinium picrate [15],

Anilinium picrate [16], p-toluidinium picrate [13], 8-

hydroxyquinolinium picrate [17], 1,3-Dimethylurea di-

methyl ammonium picrate [18], N,N-Dimethyl anilinium

picrate [19] have already been reported. The title salt

crystallizes in the monoclinic crystal system with cen-

trosymmetric space group P21/c, an analogue of p-Tolu-

idinium picrate. In the present work, we report the syn-

thesis, crystal growth, structural, spectral and optical stu-

dies of o-toluidinium picrate single crystal.

*Corresponding author.

Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

124

2. Experimental Procedure

2.1. Material Synthesis

Analar grade o-toluidine (1.07 g, 0.01 mol) and Picric

acid (2.29 g, 0.01 mol) were dissolved in pure methanol

separately in equimolar ratio and the two solutions were

mixed together. The solution was stirred well for about

one hour, when a yellow colored crystalline precipitate of

the charge transfer complex salt of o-toluidinium picrate

was obtained as a result of the acid-base reaction be-

tween picric acid and o-toluidine. The precipitate was

filtered off and recrystallised many times in methanol to

enhance the degree of purity of the product. The reaction

involved is illustrated in the Scheme 1.

2.2. Growth and Characterization of OTP Single

Crystals

A saturated methanolic solution of OTP was prepared,

stirred well for about five hours and filtered through a

quantitative whatmann 41 grade filter paper to eliminate

the unwanted suspended impurities present in the solu-

tion. The clear filtrate so obtained was kept aside unper-

turbed in a dust-free room for the growth of single crys-

tals. Well-defined, yellow colored crystals were collected

at the end of the 8th day. The photograph of as-grown

crystals of OTP is shown in Figure 1. The grown OTP

crystal was subjected to various characterization tech-

niques like 1H and 13C NMR spectral analyses, single

crystal X-ray diffraction studies, Fourier transform infra-

red (FT IR), UV-Vis-NIR spectral analysis and Fluores-

cence emission studies. The detailed results are presented

in the following sections.

o-Toluidine

Picric acido-To

crate

Scheme 1. Reaction mechanism of o-toluidinium picrate.

Figure 1. As-grown single crystals of OTP.

3. Results and Discussion

3.1. Nuclear Magnetic Resonance Studies

The 1H and 13C NMR spectra were recorded using the

BRUKER AVANCE III 500 MHz (AV 500) spectrome-

ter with TMS as the internal reference standard and

DMSO as the solvent.

The 1H NMR spectrum of the title crystal (Figure 2)

shows four proton signals indicating the presence of four

different proton environments in the OTP crystal. The

broad hump appearing at δ 9.66 ppm is assigned to the

highly deshielded +NH3 protons of o-toluidinium moiety.

The intense singlet signal appearing at δ 8.61 ppm has

been assigned to C3 and C5 aromatic protons of the same

kind in picrate moiety. The complex multiplet signal

centered at δ 7.32 ppm is arising due to the overlap be-

tween two triplets attributed to C4 and C5 aromatic pro-

tons and two doublets due to C3 and C6 aromatic protons

of o-toluidinium moiety in the salt. The triplet and dou-

blet signals coalesce into a multiplet due to the closeness

of the coupling constant values. The singlet signal at δ

2.32 ppm has been assigned to the methyl protons of o-

toluidinium moiety.

The 13C NMR spectrum of OTP is depicted in Figure

3. The appearance of eleven distinct peaks in the spec-

trum establishes the molecular structure of the OTP

complex salt. The weak carbon signal at δ 161.30 ppm

owes to the ipso carbon (C1) of picrate moiety. The C2

and C6 aromatic carbon atoms of the same kind in pi-

crate moiety appear at δ 142.28 ppm. The highly in-

tense peak at δ 125.70 ppm is due to C3 and C5 aromatic

carbon atoms of the same kind in picrate moiety. The

weak signal at δ 124.89 ppm is assigned to C4 carbon

atom of the same moiety in the complex salt. The peaks

appearing at δ 132.03, 131.83, 131.12, 128.66, 127.63

and 123.59 ppm have been assigned respectively to C2,

C3, C4, C5, C6 and C1 carbon atoms in o-toluidinium

moiety in the complex. The signal at δ 17.23 ppm is at-

tributed to the methyl carbon of o-toluidinium moiety.

Figure 2. 1H NMR spectrum of OTP.

Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

125

Figure 3. 13C NMR spectrum of OTP.

3.2. FT-IR Spectroscopy

The characteristic vibrational frequencies of the func-

tional groups of OTP are identified from the fourier

transform infrared (FT-IR) spectrum recorded in the

range of 4000 - 400 cm−1 employing Perkin-Elmer FT-IR

spectrometer by using the KBr pellet technique. The

formation of charge transfer complex during the acid-

base interaction of o-toluidine with picric acid is strongly

evidenced through the realization of important bands of

donor and acceptor in the resultant spectrum of the com-

plex salt (Figure 4). The absorption at 3194 cm−1 is due

to the +N-H stretching vibration. The absorption band at

3060 cm−1 corresponds to aromatic C-H asymmetric

stretching vibration. The broad absorption bands in the

region 2940 to 2813 cm−1 are due to the overlapping of

C-H asymmetric and symmetric stretching vibration of

methyl group and aromatic C-H symmetric stretching

vibration. The absorptions at 1535 and 1359 cm−1 con-

firm the asymmetric and symmetric stretching vibrations

of NO2 group respectively. The C-O stretching vibration

is observed at 1192 cm−1. The band at 836 cm−1 is due to

C-N stretching vibration. The presence of C=C stretch-

ing vibration of aromatic ring is revealed from the ab-

sorption bands at 1606, 1570 and 1479 cm−1. The as-

signment is in very close agreement with data of the

complex salts reported already [7-9].

3.3. Single Crystal X-Ray Diffraction Studies

Single crystal XRD analysis for the grown o-toluidi-

nium picrate has been carried out to identify the unit

cell parameters and the crystal structure using “ENRAF

(BRUKER) NONIUS CAD4” diffractometer with graph-

ite monochromated MoKα radiation (λ = 0.71073 Å).

The structure was solved by direct methods procedure as

implemented in SHELXS 97 [20] program. Cell refine-

ment and data reduction were carried out using SAINT

program. All the hydrogen atoms were fixed geometri-

cally and allowed to ride on their parent atoms. All

non-hydrogen atoms were refined using anisotropic dis-

placement parameters.

The crystal structure analyses of OTP reveals that OTP

crystallized in a monoclinic crystal structure with cen-

trosymmetric space group P21/c, and the unit cell pa-

rameters are a = 11.6475(2) Å, b = 16.4763(5) Å, c =

7.5702(4) Å. Table 1 summarizes the crystal data, inten-

sity data collection and refinement details for the OTP

single crystals. The selected bond lengths and bond an-

gles of OTP charge transfer complex salt are given in

Tables 2 and 3 respectively. The protonation on the N1

site of the cation is confirmed from C-N bond distances

and C-N-C bond angles. All the bond distances and bond

angles of the two molecules in the asymmetric unit are

agreed with each other. The crystal structure of OTP

consists one molecule each of o-toluidinium cation and

picrate anion. The ORTEP of the charge transfer com-

plex OTP is clearly shown in the Figure 5 and the atom

numbering scheme adopted. A well-defined yellow col-

our single crystal of OTP with dimension 0.30 × 0.20 ×

0.20 mm was selected for diffraction analysis. A total of

12528 reflections (2541 Unique, R(int) = 0.0239) were

collected by using ω/2θ scan mode at 293(2) K in the

range of 2.15˚ < θ < 25˚ with the index ranges −13 <= h

= 13, −19 <= k <= 19, −7 <= 1 <= 9. The refinement

converged to final R-factor of 0.04%. The packing dia-

gram (Figure 6) indicates the existence of intermolecular

hydrogen bonds in the three dimensional network be-

tween the constituent molecules. The structure is based

on asymmetric part of o-Toluidinium picrate which con-

tains 2-methyl anilinium cation and picrate anion con-

nected by three intermolecular N-H····O hydrogen bonds

with a donor-acceptor distance of 2.7697(19), 2.853(2)

and 2.762(2) Å. The corresponding data for the H-bonds

are listed in Table 4. In the charge transfer complex of

OTP, the carbon skeleton of the anionic picrate species

Figure 4. FT-IR spectrum of OTP.

Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

126

Table 1. Crystallographic data for OTP.

Empirical formula C13H12N4O7

Formula weight 336.27

Temperature 293(2) k

Wavelength 0.71073 Å

Crystal system, space group Monoclinic, P21/c

Unit cell dimensions

a = 11.6475(2) Å, α = 90˚ b =

16.4763(5) Å, β = 94.1980(10)˚

c = 7.5702(4) Å, γ = 90˚

Volume 1448.88(9) Å3

Z, Calculated density 4, 1.542 Mg/m3

Absorption coefficient 0.128 mm−1

F(000) 696

Crystal size 0.30 × 0.20 × 0.20 mm

Theta range for data collection 2.15 to 25.00 deg

Limiting indices −13 <= h <= 13, −19 <= k <= 19,

−7 <= 1 <= 9

Reflections collected/unique 12528/2541 [R(int) = 0.0239]

Completeness to theta = 25.00 99.9%

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 0.9924 and 0.9245

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 2541/0/220

Goodness-of-fit on F2 1.046

Final R indices [I > 2 sigma(I)] R1 = 0.0402, wR2 = 0.1084

R indices (all data) R1 = 0.0468, wR2 = 0.1152

Extinction coefficient 0.0042(11)

Largest diff. peak and hole 0.382 and −0.264 e.Å−3

Figure 5. ORTEP diagram of OTP.

Table 2. Selected bond lengths in OTP (Å).

C(1)-C(6) 1.374(2)

C(1)-C(2) 1.432(2)

C(1)-N(1) 1.461(2)

C(2)-O(7) 1.270(2)

C(2)-C(3) 1.429(2)

C(3)-C(4) 1.365(2)

C(3)-N(2) 1.462(2)

C(4)-C(5) 1.386(3)

C(5)-C(6) 1.380(3)

C(5)-N(3) 1.447(2)

C(7)-C(12) 1.380(2)

C(7)-C(8) 1.386(2)

C(7)-N(4) 1.470(2)

C(8)-C(9) 1.391(3)

C(8)-C(13) 1.503(3)

C(9)-C(10) 1.376(3)

C(10)-C(11) 1.375(3)

C(11)-C(12) 1.380(3)

C(13)-H(13A) 0.9600

N(1)-O(1) 1.216(2)

N(1)-O(2) 1.221(2)

N(2)-O(4) 1.209(2)

N(2)-O(3) 1.213(2)

N(3)-O(6) 1.215(2)

N(3)-O(5) 1.228(2)

N(4)-H(4A) 0.8900

Figure 6. Packing arrangement of molecules showing in-

termolecular N-H···O hydrogen bonding interactions and

π-π interactions viewed down in the c-axis.

Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

127

Table 3. Selected bond angles in OTP (˚).

C(6)-C(1)-C(2) 124.17(16)

C(6)-C(1)-N(1) 116.84(16)

C(2)-C(1)-N(1) 118.88(15)

O(7)-C(2)-C(3) 122.26(15)

O(7)-C(2)-C(1) 125.48(16)

C(3)-C(2)-C(1) 112.14(14)

C(4)-C(3)-C(2) 125.28(15)

C(4)-C(3)-N(2) 117.71(15)

C(2)-C(3)-N(2) 116.99(14)

C(3)-C(4)-C(5) 118.03(16)

C(6)-C(5)-C(4) 121.50(16)

C(6)-C(5)-N(3) 119.09(16)

C(4)-C(5)-N(3) 119.37(16)

C(1)-C(6)-C(5) 118.77(16)

C(12)-C(7)-C(8) 122.89(16)

C(12)-C(7)-N(4) 118.12(15)

C(8)-C(7)-N(4) 118.98(15)

C(7)-C(8)-C(9) 116.38(17)

C(7)-C(8)-C(13) 122.73(16)

C(9)-C(8)-C(13) 120.89(17)

C(10)-C(9)-C(8) 121.68(18)

C(11)-C(10)-C(9) 120.34(18)

C(10)-C(11)-C(12) 119.74(18)

C(11)-C(12)-C(7) 118.96(17)

C(8)-C(13)-H(13A) 109.5

H(13A)-C(13)-H(13B) 109.5

O(1)-N(1)-O(2) 123.16(18)

O(1)-N(1)-C(1) 118.20(17)

O(2)-N(1)-C(1) 118.65(16)

O(4)-N(2)-O(3) 123.06(17)

O(4)-N(2)-C(3) 118.66(16)

O(3)-N(2)-C(3) 118.24(16)

O(6)-N(3)-O(5) 123.45(16)

O(6)-N(3)-C(5) 119.62(17)

O(5)-N(3)-C(5) 116.93(17)

Table 4. Hydrogen bond parameters in OTP.

D-H···A d(D-H) d(H···A) d(D···A) <(DHA)

N(4)-H(4A)···O(7)#1 0.89 1.89 2.7697(19) 172.0

N(4)-H(4B)···O(5)#2 0.89 2.14 2.853(2) 136.9

N(4)-H(4C)···O(7) 0.89 1.88 2.762(2) 172.5

#1 x, −y + 1/2, z + 1/2; #2 –x + 1, −y, −z + 1.

and cationic o-Toluidinium species is non-planar and is

shown in the torsion angle of N(1)-C(1)-C(2)-C(3),

N(2)-C(3)-C(4)-C(5), N(3)-C(5)-C(6)-C(1), N(4)-C(7)-

C(8)-C(13), C(13)-C(8)-C(9)-C(10), C(10)-C(11)-C(12)-

C(7) which are 179.28(16)˚, −178.89(16)˚, 178.76(16)˚,

−0.8(3)˚, 178.8(2)˚, −0.6(3)˚ respectively.

3.4. UV-Vis-NIR Transmission Studies

The optical transmission spectrum of OTP crystals was

recorded in the region 200 - 1100 nm employing a Shi-

madzu UV-1061 UV-Vis spectrophotometer in solution

using DMSO as the solvent. The recorded transmission

spectrum of OTP is shown in Figure 7. The lower cutoff

wavelength of the OTP crystal was around 488 nm. The

attained percentage of transmittance was around 97 for

OTP complex in the region between 500 and 1100 nm.

Hence, this crystal can be used for the suitable optical

applications due to its wide transparency range in the part

of visible region above 488 nm and in the entire near

infrared region.

3.5. Fluorescence Emission Studies

Fluorescence may be expected generally in molecules

that are aromatic or contain multiple conjugated double

bonds with a high degree of resonance stability [21].The

fluorescence emission spectrum of OTP was recorded

using HORIBA JASCO V-670 FLUOROLOG 3 spectro-

fluorometer. The fluorescence emission spectrum was

recorded in the range from 500 to 900 nm and depicted in

Figure 8. Two peaks at 555 and 607 nm observed in the

emission spectrum indicates that OTP crystal has a green-

orange fluorescence emission.

4. Conclusion

The organic molecular charge transfer complex salt OTP

Figure 7. Optical transmission spectrum of OTP.

Synthesis, Growth, Crystal Structure and Characterization of the o-Toluidinium Picrate

128

Figure 8. Fluorescence emission spectrum of OTP.

was synthesized and the single crystals of it were grown

by slow evaporation solution growth technique using

methanol as the solvent. FT-IR, 1H and 13C NMR spec-

tral techniques confirm the molecular structure of OTP

and also bring forth the evidence for the prevalent charge

transfer activity in the complex salt. The single crystal

XRD study reveals that OTP crystallizes in monoclinic

crystal system with P21/c space group. UV-Vis-NIR

transmittance study shows that the attained percentage of

transmission was around 97% for OTP complex in the

region between 500 - 1100 nm. Hence the title crystal is

a good candidate for suitable optical applications. Fluo-

rescence emission study shows that OTP crystal has a

green-orange fluorescence emission.

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worth Publishing Company, Belmont, 1986.

Supplementary Material

CCDC 894814 contains the supplementary crystallo-gra-

phic data for this paper. These data can be obtained free

of charge via www.ccdc.cam.ac.uk/data-request/cif, by

e-mailing data-request @ccdc.cam.ac.uk or by contact-

ing The Cambridge Crystallographic Data Centre, 12

Union Road, Cambridge CB21 EZ, UK; Fax: + 44 1223

336033.