4-[(E)-2-Furylmethyl­eneamino]-3-phenyl-1H-1,2,4-triazole-5(4H)-thione

Fun, H.-K.; Jebas, S.R.; Sujith, K.V.; Patil, P.S.; Kalluraya, B.; Dharmaprakash, S.M.

doi:10.1107/S1600536808021806

organic compounds

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ISSN: 2056-9890

Volume 64| Part 8| August 2008| Pages o1528-o1529

doi:10.1107/S1600536808021806

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4-[(E)-2-Furylmethyleneamino]-3-phenyl-1H-1,2,4-triazole-5(4H)-thione

Hoong-Kun Fun,^a ^* Samuel Robinson Jebas,^a ‡ K. V. Sujith,^b P. S. Patil,^c B. Kalluraya ^b and S. M. Dharmaprakash ^c

^aX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, ^bDepartment of Studies in Chemistry, Mangalore University, Mangalagangotri, Mangalore 574 199, India, and ^cDepartment of Studies in Physics, Mangalore University, Mangalagangotri, Mangalore 574 199, India
^*Correspondence e-mail: hkfun@usm.my

(Received 11 July 2008; accepted 13 July 2008; online 19 July 2008)

In the title molecule, C₁₃H₁₀N₄OS, the triazole ring makes dihedral angles of 16.14 (9) and 58.51 (11)°, respectively, with the phenyl and furan rings. Intramolecular C—H⋯N hydrogen bonds generate S(5) and S(6) ring motifs. In the crystal structure, centrosymmetrically related molecules are linked via N—H⋯S hydrogen bonds to form dimeric pairs, which are interlinked via C—H⋯O and C—H⋯π interactions.

Related literature

For the biological activities of triazole derivatives, see: Clemons et al. (2004 ); Glerman et al. (1997 ); Holla et al. (2003 ); Johnston (2002 ); Kane et al. (1990 ); Kkgzel et al. (2004 ); Modzelewska & Kalabun (1999 ); Rollas et al. (1993 ); Shujuan et al. (2004 ); For bond-length data, see: Allen et al. (1987 ). For graph-set analysis of hydrogen bonding, see: Bernstein et al. (1995 ).

Experimental

Crystal data

C₁₃H₁₀N₄OS
M_r = 270.31
Orthorhombic, P b c n
a = 27.4006 (6) Å
b = 11.4940 (3) Å
c = 7.7886 (2) Å
V = 2452.96 (10) Å³
Z = 8
Mo Kα radiation
μ = 0.26 mm⁻¹
T = 100.0 (1) K
0.40 × 0.13 × 0.10 mm

Data collection

Bruker SMART APEXII CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2005 ) T_min = 0.829, T_max = 0.974
40042 measured reflections
3627 independent reflections
2573 reflections with I > 2σ(I)
R_int = 0.071

Refinement

R[F² > 2σ(F²)] = 0.043
wR(F²) = 0.125
S = 1.05
3627 reflections
176 parameters
1 restraint
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.26 e Å⁻³
Δρ_min = −0.32 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 is the centroid of the C3–C8 ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N1⋯S1ⁱ	0.85 (2)	2.42 (2)	3.265 (2)	169 (2)
C4—H4A⋯N2	0.93	2.55	2.859 (2)	100
C6—H6A⋯O1ⁱⁱ	0.93	2.59	3.347 (2)	139
C8—H8A⋯N4	0.93	2.29	2.942 (2)	126
C5—H5A⋯Cg1ⁱⁱⁱ	0.93	2.92	3.522 (2)	123
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) $[-x+{\script{1\over 2}}, -y+{\script{3\over 2}}, z+{\script{1\over 2}}]$ ; (iii) $[-x-{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ .

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2; data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2003 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808021806/ci2631sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808021806/ci2631Isup2.hkl

checkCIF report

Comment top

1,2,4-Triazoles and their derivatives are found to be associated with various biological activities such as anticonvulsant (Kane et al., 1990; Kkgzel et al., 2004), antifungal (Rollas et al., 1993), anticancer (Holla et al., 2003), anti-inflammatory (Modzelewska & Kalabun, 1999) and antibacterial properties (Glerman et al., 1997). Several compounds containing 1,2,4-triazole rings are well known as drugs. For example, fluconazole is used as an antimicrobial drug (Shujuan et al., 2004), while vorozole, letrozole and anastrozole are non-steroidal drugs used for the threatment of cancer (Clemons et al., 2004) and loreclezole is used as an anticonvulsant (Johnston, 2002) drug. In view of the above properties, we have synthesized the title compound and report here its crystal structure.

Bond lengths and angles in the title molecule (Fig. 1) are found to have normal values (Allen et al., 1987). The furan ring is planar to within ±0.002 (2) Å and the triazole ring is also planar with a maximum deviation of 0.016 (2) Å for atom C1. The triazole and phenyl rings are twisted away from each other by an angle of 16.14 (9)°. The dihedral angle between the furan and triazole rings is 58.51 (11)°. Intramolecular C—H···N hydrogen bonds generate S(5) and S(6) ring motifs (Bernstein et al., 1995).

The crystal structure is stabilized by intermolecular C—H···O and N—H···S hydrogen bonds together with C—H···π interactions involving the phenyl ring. The centrosymmetrically related molecules are linked by N—H···S hydrogen bonds to form a dimeric pair (Fig. 2) which are interlinked via C—H···O hydrogen bonds.

Related literature top

For the biological activities of triazole derivatives, see: Clemons et al. (2004); Glerman et al. (1997); Holla et al. (2003); Johnston (2002); Kane et al. (1990); Kkgzel et al. (2004); Modzelewska & Kalabun (1999); Rollas et al. (1993); Shujuan et al. (2004); For bond-length data, see: Allen et al. (1987). For graph-set analysis of hydrogen bonding, see: Bernstein et al. (1995). Cg1 is the centroid of the C3–C8 ring.

Experimental top

The title Schiff base compound was obtained by refluxing a mixture of 4-amino-5-methyl-2,4-dihydro-3H-1,2,4-triazole-3-thione (0.01 mol), furfural (0.01 mol) in ethanol (30 ml) and 2 drops of concentrated H₂SO₄ for 3 h. The solid product obtained was collected by filtration, washed with ethanol and dried. Single crystals suitable for X-ray analysis were obtained from acetone-N,N-dimethylformamide (DMF) (1:2) solution by slow evaporation (yield 63%; m.p. 451–453 K). Analysis for C₁₃H₁₀N₄SO, found (calculated) in %: C 57.63 (57.77), H 3.62 (3.7), N 20.6 (20.74), S 11.79 (11.85).

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2 (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures top

	Fig. 1. The molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme.
	Fig. 2. The crystal packing of the title compound, viewed along the b axis. Hydrogen bonds are shown as dashed lines.

4-[(E)-2-Furylmethyleneamino]-3-phenyl-1H-1,2,4- triazole-5(4H)-thione top

Crystal data top

C₁₃H₁₀N₄OS	F(000) = 1120
M_r = 270.31	D_x = 1.464 Mg m⁻³
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2ab	Cell parameters from 5448 reflections
a = 27.4006 (6) Å	θ = 2.9–27.9°
b = 11.4940 (3) Å	µ = 0.26 mm⁻¹
c = 7.7886 (2) Å	T = 100 K
V = 2452.96 (10) Å³	Block, orange
Z = 8	0.40 × 0.13 × 0.10 mm

Data collection top

Bruker SMART APEXII CCD area-detector diffractometer	3627 independent reflections
Radiation source: fine-focus sealed tube	2573 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.072
ϕ and ω scans	θ_max = 30.2°, θ_min = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2005)	h = −38→38
T_min = 0.829, T_max = 0.974	k = −16→16
40042 measured reflections	l = −10→10

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.125	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ²(F_o²) + (0.0649P)² + 0.3858P] where P = (F_o² + 2F_c²)/3
3627 reflections	(Δ/σ)_max = 0.001
176 parameters	Δρ_max = 0.26 e Å⁻³
1 restraint	Δρ_min = −0.32 e Å⁻³

Crystal data top

C₁₃H₁₀N₄OS	V = 2452.96 (10) Å³
M_r = 270.31	Z = 8
Orthorhombic, Pbcn	Mo Kα radiation
a = 27.4006 (6) Å	µ = 0.26 mm⁻¹
b = 11.4940 (3) Å	T = 100 K
c = 7.7886 (2) Å	0.40 × 0.13 × 0.10 mm

Data collection top

Bruker SMART APEXII CCD area-detector diffractometer	3627 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2005)	2573 reflections with I > 2σ(I)
T_min = 0.829, T_max = 0.974	R_int = 0.072
40042 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.043	1 restraint
wR(F²) = 0.125	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	Δρ_max = 0.26 e Å⁻³
3627 reflections	Δρ_min = −0.32 e Å⁻³
176 parameters

Special details top

Experimental. The data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
S1	0.494570 (15)	0.69781 (4)	0.50297 (6)	0.02259 (13)
O1	0.36669 (4)	1.02453 (10)	0.58350 (18)	0.0258 (3)
N1	0.44316 (5)	0.52097 (12)	0.6503 (2)	0.0225 (3)
N2	0.39895 (5)	0.49428 (12)	0.7222 (2)	0.0228 (3)
N3	0.40473 (5)	0.68093 (11)	0.66047 (19)	0.0190 (3)
N4	0.38912 (5)	0.79579 (12)	0.6319 (2)	0.0206 (3)
C1	0.44803 (6)	0.63262 (15)	0.6058 (2)	0.0203 (3)
C2	0.37554 (6)	0.59359 (14)	0.7269 (2)	0.0204 (4)
C3	0.32509 (6)	0.60514 (14)	0.7901 (2)	0.0198 (3)
C4	0.30539 (6)	0.51246 (15)	0.8824 (2)	0.0244 (4)
H4A	0.3247	0.4484	0.9086	0.029*
C5	0.25719 (6)	0.51543 (16)	0.9351 (3)	0.0267 (4)
H5A	0.2444	0.4533	0.9971	0.032*
C6	0.22781 (6)	0.60973 (16)	0.8967 (2)	0.0259 (4)
H6A	0.1954	0.6111	0.9320	0.031*
C7	0.24716 (6)	0.70213 (16)	0.8051 (3)	0.0255 (4)
H7A	0.2275	0.7656	0.7783	0.031*
C8	0.29558 (6)	0.70074 (15)	0.7530 (2)	0.0229 (4)
H8A	0.3084	0.7637	0.6931	0.028*
C9	0.41795 (6)	0.87250 (15)	0.6957 (2)	0.0212 (4)
H9A	0.4458	0.8494	0.7548	0.025*
C10	0.40718 (6)	0.99364 (15)	0.6757 (2)	0.0214 (4)
C11	0.42966 (7)	1.08948 (16)	0.7401 (3)	0.0268 (4)
H11A	0.4578	1.0913	0.8067	0.032*
C12	0.40148 (7)	1.18664 (15)	0.6854 (3)	0.0282 (4)
H12A	0.4075	1.2646	0.7095	0.034*
C13	0.36459 (7)	1.14332 (15)	0.5921 (3)	0.0289 (4)
H13A	0.3406	1.1883	0.5399	0.035*
H1N1	0.4628 (6)	0.4674 (14)	0.619 (3)	0.034 (6)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.0170 (2)	0.0210 (2)	0.0298 (3)	0.00050 (15)	0.00408 (17)	0.00147 (18)
O1	0.0210 (6)	0.0219 (6)	0.0344 (8)	0.0019 (5)	−0.0013 (5)	0.0001 (6)
N1	0.0169 (7)	0.0191 (7)	0.0316 (9)	0.0034 (5)	0.0022 (6)	0.0014 (6)
N2	0.0161 (7)	0.0206 (7)	0.0317 (9)	0.0010 (5)	0.0025 (6)	0.0013 (6)
N3	0.0153 (6)	0.0172 (7)	0.0244 (8)	0.0004 (5)	−0.0002 (6)	0.0008 (6)
N4	0.0185 (6)	0.0173 (7)	0.0260 (8)	0.0016 (5)	0.0010 (6)	0.0015 (6)
C1	0.0170 (7)	0.0208 (8)	0.0232 (9)	0.0014 (6)	−0.0020 (6)	−0.0018 (7)
C2	0.0182 (8)	0.0182 (8)	0.0247 (9)	−0.0013 (6)	−0.0014 (7)	0.0010 (7)
C3	0.0167 (7)	0.0215 (8)	0.0212 (8)	−0.0018 (6)	−0.0011 (6)	−0.0011 (7)
C4	0.0214 (8)	0.0256 (9)	0.0262 (9)	0.0001 (7)	0.0001 (7)	0.0040 (7)
C5	0.0240 (8)	0.0306 (10)	0.0255 (9)	−0.0038 (7)	0.0025 (7)	0.0058 (8)
C6	0.0199 (8)	0.0320 (10)	0.0259 (9)	−0.0012 (7)	0.0031 (7)	−0.0031 (8)
C7	0.0180 (8)	0.0231 (9)	0.0353 (10)	0.0026 (7)	0.0003 (7)	−0.0044 (8)
C8	0.0206 (8)	0.0204 (8)	0.0279 (10)	−0.0013 (6)	0.0008 (7)	0.0008 (7)
C9	0.0161 (7)	0.0240 (8)	0.0236 (9)	0.0006 (6)	0.0024 (7)	0.0003 (7)
C10	0.0169 (7)	0.0235 (9)	0.0238 (9)	−0.0008 (6)	0.0028 (7)	−0.0001 (7)
C11	0.0214 (8)	0.0254 (9)	0.0334 (10)	−0.0042 (7)	0.0033 (7)	−0.0027 (8)
C12	0.0280 (9)	0.0199 (9)	0.0366 (11)	−0.0031 (7)	0.0106 (8)	−0.0019 (8)
C13	0.0280 (9)	0.0216 (9)	0.0372 (11)	0.0054 (7)	0.0072 (8)	0.0050 (8)

Geometric parameters (Å, º) top

S1—C1	1.6820 (17)	C5—C6	1.383 (3)
O1—C13	1.368 (2)	C5—H5A	0.93
O1—C10	1.368 (2)	C6—C7	1.385 (3)
N1—C1	1.336 (2)	C6—H6A	0.93
N1—N2	1.369 (2)	C7—C8	1.388 (2)
N1—H1N1	0.853 (9)	C7—H7A	0.93
N2—C2	1.310 (2)	C8—H8A	0.93
N3—C1	1.377 (2)	C9—C10	1.432 (2)
N3—C2	1.384 (2)	C9—H9A	0.93
N3—N4	1.4054 (18)	C10—C11	1.358 (2)
N4—C9	1.284 (2)	C11—C12	1.423 (3)
C2—C3	1.474 (2)	C11—H11A	0.93
C3—C4	1.394 (2)	C12—C13	1.341 (3)
C3—C8	1.395 (2)	C12—H12A	0.93
C4—C5	1.384 (2)	C13—H13A	0.93
C4—H4A	0.93

C13—O1—C10	105.50 (14)	C5—C6—C7	119.33 (16)
C1—N1—N2	114.19 (14)	C5—C6—H6A	120.3
C1—N1—H1N1	123.8 (15)	C7—C6—H6A	120.3
N2—N1—H1N1	120.9 (15)	C6—C7—C8	120.52 (17)
C2—N2—N1	104.44 (13)	C6—C7—H7A	119.7
C1—N3—C2	108.72 (13)	C8—C7—H7A	119.7
C1—N3—N4	126.29 (13)	C7—C8—C3	120.18 (16)
C2—N3—N4	124.37 (13)	C7—C8—H8A	119.9
C9—N4—N3	113.36 (14)	C3—C8—H8A	119.9
N1—C1—N3	102.77 (14)	N4—C9—C10	119.92 (16)
N1—C1—S1	128.84 (13)	N4—C9—H9A	120.0
N3—C1—S1	128.36 (13)	C10—C9—H9A	120.0
N2—C2—N3	109.79 (14)	C11—C10—O1	110.56 (15)
N2—C2—C3	123.18 (15)	C11—C10—C9	130.92 (17)
N3—C2—C3	127.01 (15)	O1—C10—C9	118.46 (15)
C4—C3—C8	119.00 (15)	C10—C11—C12	106.24 (17)
C4—C3—C2	117.83 (15)	C10—C11—H11A	126.9
C8—C3—C2	123.05 (15)	C12—C11—H11A	126.9
C5—C4—C3	120.26 (16)	C13—C12—C11	106.25 (16)
C5—C4—H4A	119.9	C13—C12—H12A	126.9
C3—C4—H4A	119.9	C11—C12—H12A	126.9
C6—C5—C4	120.71 (17)	C12—C13—O1	111.45 (16)
C6—C5—H5A	119.6	C12—C13—H13A	124.3
C4—C5—H5A	119.6	O1—C13—H13A	124.3

C1—N1—N2—C2	−1.5 (2)	C8—C3—C4—C5	0.4 (3)
C1—N3—N4—C9	−60.1 (2)	C2—C3—C4—C5	−175.78 (17)
C2—N3—N4—C9	129.83 (18)	C3—C4—C5—C6	0.3 (3)
N2—N1—C1—N3	2.8 (2)	C4—C5—C6—C7	−0.3 (3)
N2—N1—C1—S1	−175.45 (13)	C5—C6—C7—C8	−0.3 (3)
C2—N3—C1—N1	−2.91 (19)	C6—C7—C8—C3	1.0 (3)
N4—N3—C1—N1	−174.24 (15)	C4—C3—C8—C7	−1.0 (3)
C2—N3—C1—S1	175.32 (14)	C2—C3—C8—C7	174.91 (17)
N4—N3—C1—S1	4.0 (3)	N3—N4—C9—C10	179.42 (14)
N1—N2—C2—N3	−0.50 (19)	C13—O1—C10—C11	0.1 (2)
N1—N2—C2—C3	177.88 (17)	C13—O1—C10—C9	177.54 (16)
C1—N3—C2—N2	2.2 (2)	N4—C9—C10—C11	174.88 (19)
N4—N3—C2—N2	173.76 (15)	N4—C9—C10—O1	−2.0 (3)
C1—N3—C2—C3	−176.07 (17)	O1—C10—C11—C12	0.1 (2)
N4—N3—C2—C3	−4.5 (3)	C9—C10—C11—C12	−176.88 (18)
N2—C2—C3—C4	14.2 (3)	C10—C11—C12—C13	−0.3 (2)
N3—C2—C3—C4	−167.70 (17)	C11—C12—C13—O1	0.4 (2)
N2—C2—C3—C8	−161.76 (17)	C10—O1—C13—C12	−0.3 (2)
N3—C2—C3—C8	16.3 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···S1ⁱ	0.85 (2)	2.42 (2)	3.265 (2)	169 (2)
C4—H4A···N2	0.93	2.55	2.859 (2)	100
C6—H6A···O1ⁱⁱ	0.93	2.59	3.347 (2)	139
C8—H8A···N4	0.93	2.29	2.942 (2)	126
C5—H5A···Cg1ⁱⁱⁱ	0.93	2.92	3.522 (2)	123

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+1/2, −y+3/2, z+1/2; (iii) −x−1/2, y+1/2, z.

Experimental details

Crystal data
Chemical formula	C₁₃H₁₀N₄OS
M_r	270.31
Crystal system, space group	Orthorhombic, Pbcn
Temperature (K)	100
a, b, c (Å)	27.4006 (6), 11.4940 (3), 7.7886 (2)
V (Å³)	2452.96 (10)
Z	8
Radiation type	Mo Kα
µ (mm⁻¹)	0.26
Crystal size (mm)	0.40 × 0.13 × 0.10

Data collection
Diffractometer	Bruker SMART APEXII CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2005)
T_min, T_max	0.829, 0.974
No. of measured, independent and observed [I > 2σ(I)] reflections	40042, 3627, 2573
R_int	0.072
(sin θ/λ)_max (Å⁻¹)	0.708

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.043, 0.125, 1.06
No. of reflections	3627
No. of parameters	176
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.26, −0.32

Computer programs: APEX2 (Bruker, 2005), SAINT (Bruker, 2005), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2003).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···S1ⁱ	0.85 (2)	2.42 (2)	3.265 (2)	169 (2)
C4—H4A···N2	0.93	2.55	2.859 (2)	100
C6—H6A···O1ⁱⁱ	0.93	2.59	3.347 (2)	139
C8—H8A···N4	0.93	2.29	2.942 (2)	126
C5—H5A···Cg1ⁱⁱⁱ	0.93	2.92	3.522 (2)	123

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+1/2, −y+3/2, z+1/2; (iii) −x−1/2, y+1/2, z.

Footnotes

‡Permanent address: Department of Physics, Karunya University, Karunya Nagar, Coimbatore 641 114, India.

Acknowledgements

HKF and SRJ thank the Malaysian Government and Universiti Sains Malaysia for the Science Fund grant No. 305/PFIZIK/613312. SRJ thanks the Universiti Sains Malaysia for a postdoctoral research fellowship.

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