(E)-4-Hy­droxy-2-[(2-hy­droxy­phen­yl)iminiometh­yl]phenolate

Eltayeb, N.E.; Teoh, S.G.; Fun, H.-K.; Chantrapromma, S.

doi:10.1107/S1600536810020295

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 66| Part 7| July 2010| Pages o1536-o1537

doi:10.1107/S1600536810020295

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(E)-4-Hydroxy-2-[(2-hydroxyphenyl)iminiomethyl]phenolate

Naser Eltaher Eltayeb,^a ‡ Siang Guan Teoh,^a Hoong-Kun Fun ^b ^*§ and Suchada Chantrapromma ^c ¶

^aSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, ^bX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, and ^cCrystal Materials Research Unit, Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand
^*Correspondence e-mail: hkfun@usm.my

(Received 17 May 2010; accepted 28 May 2010; online 5 June 2010)

The title compound, C₁₃H₁₁NO₃, crystallizes in a zwitterionic form and has a trans configuration about the C=N bond. The molecule is almost planar, the dihedral angle between the two benzene rings being 4.32 (8)°. The two hydroxy substituents are coplanar with each of their attached benzene rings [r.m.s. deviations of 0.0053 (2) and 0.0052 (2) Å]. An intramolecular N—H⋯O hydrogen bond formed between the iminium N and the phenolate O atom generates an S(6) ring motif. In the crystal, the molecules are linked through O—H⋯O hydrogen bonds into chains along [110]. Two neighbouring chains are further connected through O—H⋯O hydrogen bonds in an antiparallel manner. π–π interactions are also observed, with centroid–centroid distances of 3.7115 (19) and 3.743 (2) Å.

Related literature

For background to Schiff bases and their applications, see: Dao et al. (2000 ); Kagkelari et al. (2009 ); Karthikeyan et al. (2006 ); Sriram et al. (2006 ). For related structures, see: Eltayeb et al. (2009 , 2010a ,b ); Tan & Liu (2009 ). For the stability of the temperature controller used in the data collection, see Cosier & Glazer, (1986 ). For bond-length data, see: Allen et al. (1987 ). For hydrogen-bond motifs, see: Bernstein et al. (1995 ).

Experimental

Crystal data

C₁₃H₁₁NO₃
M_r = 229.23
Monoclinic, C 2/c
a = 11.048 (5) Å
b = 8.187 (3) Å
c = 22.858 (10) Å
β = 102.242 (13)°
V = 2020.5 (15) Å³
Z = 8
Mo Kα radiation
μ = 0.11 mm⁻¹
T = 100 K
0.35 × 0.12 × 0.04 mm

Data collection

Bruker APEXII DUO CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.963, T_max = 0.996
15107 measured reflections
2967 independent reflections
2122 reflections with I > 2σ(I)
R_int = 0.087

Refinement

R[F² > 2σ(F²)] = 0.054
wR(F²) = 0.173
S = 1.06
2967 reflections
198 parameters
All H-atom parameters refined
Δρ_max = 0.48 e Å⁻³
Δρ_min = −0.28 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1O1⋯O2ⁱ	0.90 (3)	1.74 (3)	2.625 (2)	166 (3)
O3—H1O3⋯O2ⁱⁱ	0.95 (3)	1.69 (3)	2.633 (2)	173 (2)
N1—H1N1⋯O2	0.89 (3)	1.83 (3)	2.581 (2)	141 (2)
C13—H13⋯O1ⁱⁱ	1.00 (2)	2.60 (2)	3.411 (3)	137.8 (18)
Symmetry codes: (i) $[-x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ ; (ii) $[x-{\script{1\over 2}}, y-{\script{1\over 2}}, z]$ .

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; 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, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536810020295/rz2453sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536810020295/rz2453Isup2.hkl

checkCIF report

Comment top

Schiff base ligands and their complexes have varieties of biological activities and applications such as antibacterial and antifungal (Karthikeyan et al., 2006), anticancer (Dao et al., 2000), anti-HIV (Sriram et al., 2006) properties as well as being used in coordination chemistry (Kagkelari et al., 2009). Our on going research on Schiff base ligands and their complexes (Eltayeb et al., 2009; 2010a,b) has lead us to synthesize the title Schiff base ligand (I) and its crystal structure is reported herein.

The molecule of (I) (Fig. 1), C₁₃H₁₁NO₃, crystallizes in a zwitterionic form with cationic iminium and anionic enolate. The molecule exists in a trans configuration about the C═N bond [1.302 (2)Å] as indicated by the torsion angle C1–N1–C7–C8 of 178.69 (15)°. The molecule is essentially planar with the dihedral angle of 4.32 (8)° between the two benzene rings. The two hydroxy groups are co-planar with each of their attached benzene rings with the r.m.s. of 0.0053 (2) and 0.0052 (2) Å for the seven non hydrogen atoms of C1–C6/O1 and C8–C13/O3, respectively. An intramolecular N—H···O hydrogen bond (Fig. 1; Table 1) between the NH⁺ with the phenolate O^- atom generate an S(6) ring motif (Bernstein et al., 1995) which stabilizes the planarity of the molecule. The bond distances are in normal ranges (Allen et al., 1987) and comparable with those of related structures (Eltayeb et al., 2009; 2010a,b; Tan & Liu, 2009).

In the crystal packing (Fig. 2), the zwitterionic molecules are linked through O3—H1O3···O2 hydrogen bonds into chains along the [110] direction and two neighboring chains are further connected to each other by O1—H1O1···O2 hydrogen bonds in an antiparallel manner (Table 1). The crystal is stabilized by intermolecular O—H···O hydrogen bonds and weak C—H···O interactions (Table 1). π–π interactions with centroid···centroid distances of Cg₁···Cg₂ⁱⁱⁱ = 3.7115 (19) Å and Cg₁···Cg₂^iv = 3.743 (2) Å were observed (symmetry codes (iii) = -x, 1-y, 1-z and (iv) = 1/2-x, 1/2-y, 1-z); Cg₁ and Cg₂ are the centroids of C1–C6 and C8–C13 benzene rings, respectively.

Related literature top

For background to Schiff bases and their applications, see: Dao et al. (2000); Kagkelari et al. (2009); Karthikeyan et al. (2006); Sriram et al. (2006). For related structures, see: Eltayeb et al. (2009, 2010a,b); Tan & Liu (2009). For the stability of the temperature controller used in the data collection, see Cosier & Glazer, (1986). For bond-length data, see: Allen et al. (1987). For hydrogen-bond motifs, see: Bernstein et al. (1995).

Experimental top

The title compound was synthesized by adding 2,5-dihydroxybenzaldehyde (0.276 g, 2 mmol) to a solution of 2-aminophenol (0.218 g, 2 mmol) in ethanol (30 ml). The mixture was refluxed with stirring for half an hour. The resultant red solution was filtered and the filtrate was evaporated to give a red powder product. Red plate-shaped single crystals of the title compound suitable for x-ray structure determination were obtained from acetone by slow evaporation in the refrigerator after a few days.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); 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, 2009).

Figures top

	Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme. Hydrogen bond is shown as dashed line.
	Fig. 2. The crystal packing of the title compound viewed down the c axis. Hydrogen bonds are shown as dashed lines.

(E)-4-Hydroxy-2-[(2-hydroxyphenyl)iminiomethyl]phenolate top

Crystal data top

C₁₃H₁₁NO₃	F(000) = 960
M_r = 229.23	D_x = 1.507 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2967 reflections
a = 11.048 (5) Å	θ = 1.8–30.1°
b = 8.187 (3) Å	µ = 0.11 mm⁻¹
c = 22.858 (10) Å	T = 100 K
β = 102.242 (13)°	Plate, red
V = 2020.5 (15) Å³	0.35 × 0.12 × 0.04 mm
Z = 8

Data collection top

Bruker APEXII DUO CCD area-detector diffractometer	2967 independent reflections
Radiation source: sealed tube	2122 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.087
ϕ and ω scans	θ_max = 30.1°, θ_min = 1.8°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −15→15
T_min = 0.963, T_max = 0.996	k = −11→11
15107 measured reflections	l = −32→32

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)] = 0.054	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.173	All H-atom parameters refined
S = 1.06	w = 1/[σ²(F_o²) + (0.1024P)² + 0.2914P] where P = (F_o² + 2F_c²)/3
2967 reflections	(Δ/σ)_max = 0.001
198 parameters	Δρ_max = 0.48 e Å⁻³
0 restraints	Δρ_min = −0.28 e Å⁻³

Crystal data top

C₁₃H₁₁NO₃	V = 2020.5 (15) Å³
M_r = 229.23	Z = 8
Monoclinic, C2/c	Mo Kα radiation
a = 11.048 (5) Å	µ = 0.11 mm⁻¹
b = 8.187 (3) Å	T = 100 K
c = 22.858 (10) Å	0.35 × 0.12 × 0.04 mm
β = 102.242 (13)°

Data collection top

Bruker APEXII DUO CCD area-detector diffractometer	2967 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	2122 reflections with I > 2σ(I)
T_min = 0.963, T_max = 0.996	R_int = 0.087
15107 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.054	0 restraints
wR(F²) = 0.173	All H-atom parameters refined
S = 1.06	Δρ_max = 0.48 e Å⁻³
2967 reflections	Δρ_min = −0.28 e Å⁻³
198 parameters

Special details top

Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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
O1	0.33647 (12)	0.68246 (15)	0.56400 (5)	0.0181 (3)
H1O1	0.339 (3)	0.792 (4)	0.5686 (14)	0.049 (8)*
O2	0.19184 (11)	0.49929 (14)	0.43502 (5)	0.0172 (3)
O3	−0.15299 (12)	0.01393 (15)	0.36222 (6)	0.0195 (3)
H1O3	−0.212 (3)	0.001 (3)	0.3867 (14)	0.045 (8)*
N1	0.18126 (13)	0.43402 (17)	0.54431 (6)	0.0136 (3)
H1N1	0.213 (2)	0.484 (3)	0.5165 (12)	0.034 (7)*
C1	0.22799 (14)	0.47819 (19)	0.60450 (7)	0.0137 (3)
C2	0.30862 (15)	0.6115 (2)	0.61334 (7)	0.0147 (3)
C3	0.35926 (16)	0.6624 (2)	0.67122 (8)	0.0167 (3)
H3	0.413 (2)	0.755 (3)	0.6766 (10)	0.024 (6)*
C4	0.32876 (16)	0.5824 (2)	0.71942 (8)	0.0177 (3)
H4	0.363 (2)	0.614 (3)	0.7600 (11)	0.026 (6)*
C5	0.24766 (16)	0.4510 (2)	0.71042 (8)	0.0178 (3)
H5	0.227 (2)	0.393 (3)	0.7447 (11)	0.028 (6)*
C6	0.19743 (16)	0.3972 (2)	0.65277 (7)	0.0161 (3)
H6	0.1437 (19)	0.309 (3)	0.6478 (9)	0.016 (5)*
C7	0.09838 (15)	0.32381 (19)	0.52344 (7)	0.0142 (3)
H7	0.0643 (18)	0.254 (2)	0.5512 (9)	0.013 (5)*
C8	0.05807 (15)	0.29461 (19)	0.46077 (7)	0.0139 (3)
C9	0.10800 (15)	0.38498 (19)	0.41820 (7)	0.0139 (3)
C10	0.06509 (16)	0.3443 (2)	0.35726 (7)	0.0160 (3)
H10	0.101 (2)	0.405 (3)	0.3290 (11)	0.027 (6)*
C11	−0.02034 (15)	0.2222 (2)	0.34016 (7)	0.0156 (3)
H11	−0.049 (2)	0.192 (3)	0.2956 (11)	0.025 (6)*
C12	−0.06925 (15)	0.1342 (2)	0.38267 (7)	0.0144 (3)
C13	−0.03095 (15)	0.17081 (19)	0.44220 (7)	0.0146 (3)
H13	−0.066 (2)	0.111 (3)	0.4732 (10)	0.024 (6)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0240 (7)	0.0158 (6)	0.0154 (6)	−0.0033 (5)	0.0063 (5)	0.0012 (4)
O2	0.0194 (6)	0.0153 (6)	0.0168 (6)	−0.0061 (5)	0.0040 (5)	−0.0009 (4)
O3	0.0194 (6)	0.0202 (6)	0.0198 (6)	−0.0099 (5)	0.0062 (5)	−0.0055 (5)
N1	0.0137 (6)	0.0136 (6)	0.0131 (6)	−0.0001 (5)	0.0018 (5)	0.0008 (5)
C1	0.0141 (7)	0.0136 (7)	0.0128 (7)	0.0030 (6)	0.0014 (6)	−0.0005 (5)
C2	0.0157 (7)	0.0140 (7)	0.0146 (7)	0.0013 (6)	0.0037 (6)	0.0021 (6)
C3	0.0169 (8)	0.0139 (7)	0.0182 (8)	−0.0009 (6)	0.0014 (6)	−0.0019 (6)
C4	0.0205 (8)	0.0174 (8)	0.0139 (7)	0.0026 (6)	0.0011 (6)	−0.0008 (6)
C5	0.0203 (8)	0.0185 (8)	0.0150 (7)	0.0031 (7)	0.0045 (6)	0.0021 (6)
C6	0.0168 (8)	0.0151 (8)	0.0163 (8)	−0.0005 (6)	0.0036 (6)	0.0016 (6)
C7	0.0134 (7)	0.0131 (7)	0.0164 (7)	0.0011 (6)	0.0037 (6)	0.0015 (6)
C8	0.0133 (7)	0.0130 (7)	0.0152 (7)	−0.0002 (6)	0.0025 (6)	−0.0010 (6)
C9	0.0119 (7)	0.0127 (7)	0.0172 (7)	0.0002 (6)	0.0032 (6)	−0.0001 (6)
C10	0.0176 (8)	0.0159 (7)	0.0153 (7)	−0.0014 (6)	0.0049 (6)	0.0010 (6)
C11	0.0143 (7)	0.0175 (7)	0.0151 (7)	0.0004 (6)	0.0035 (6)	−0.0011 (6)
C12	0.0125 (7)	0.0123 (7)	0.0183 (8)	−0.0021 (6)	0.0031 (6)	−0.0016 (6)
C13	0.0155 (7)	0.0134 (7)	0.0158 (7)	−0.0012 (6)	0.0052 (6)	0.0010 (6)

Geometric parameters (Å, º) top

O1—C2	1.3608 (19)	C5—C6	1.389 (2)
O1—H1O1	0.91 (3)	C5—H5	0.98 (2)
O2—C9	1.316 (2)	C6—H6	0.93 (2)
O3—C12	1.364 (2)	C7—C8	1.427 (2)
O3—H1O3	0.95 (3)	C7—H7	0.99 (2)
N1—C7	1.302 (2)	C8—C13	1.413 (2)
N1—C1	1.410 (2)	C8—C9	1.423 (2)
N1—H1N1	0.89 (3)	C9—C10	1.413 (2)
C1—C6	1.389 (2)	C10—C11	1.373 (2)
C1—C2	1.396 (2)	C10—H10	0.97 (2)
C2—C3	1.387 (2)	C11—C12	1.406 (2)
C3—C4	1.383 (2)	C11—H11	1.03 (2)
C3—H3	0.96 (2)	C12—C13	1.370 (2)
C4—C5	1.387 (3)	C13—H13	1.00 (2)
C4—H4	0.96 (2)

C2—O1—H1O1	109.4 (19)	C1—C6—H6	122.1 (13)
C12—O3—H1O3	112.0 (17)	N1—C7—C8	121.96 (15)
C7—N1—C1	128.22 (15)	N1—C7—H7	120.0 (12)
C7—N1—H1N1	114.2 (17)	C8—C7—H7	118.0 (12)
C1—N1—H1N1	117.6 (17)	C13—C8—C9	120.89 (14)
C6—C1—C2	120.88 (14)	C13—C8—C7	118.07 (14)
C6—C1—N1	123.52 (15)	C9—C8—C7	121.02 (15)
C2—C1—N1	115.60 (14)	O2—C9—C10	121.61 (14)
O1—C2—C3	123.09 (15)	O2—C9—C8	121.39 (14)
O1—C2—C1	117.61 (14)	C10—C9—C8	116.99 (15)
C3—C2—C1	119.28 (15)	C11—C10—C9	121.25 (15)
C4—C3—C2	120.03 (16)	C11—C10—H10	122.8 (14)
C4—C3—H3	121.6 (14)	C9—C10—H10	116.0 (14)
C2—C3—H3	118.3 (14)	C10—C11—C12	121.19 (15)
C3—C4—C5	120.51 (15)	C10—C11—H11	120.5 (12)
C3—C4—H4	122.0 (14)	C12—C11—H11	118.3 (12)
C5—C4—H4	117.5 (14)	O3—C12—C13	122.86 (14)
C4—C5—C6	120.17 (16)	O3—C12—C11	117.72 (15)
C4—C5—H5	120.3 (14)	C13—C12—C11	119.42 (15)
C6—C5—H5	119.5 (14)	C12—C13—C8	120.25 (14)
C5—C6—C1	119.12 (16)	C12—C13—H13	120.8 (13)
C5—C6—H6	118.8 (13)	C8—C13—H13	119.0 (13)

C7—N1—C1—C6	5.5 (3)	N1—C7—C8—C9	0.4 (2)
C7—N1—C1—C2	−174.41 (15)	C13—C8—C9—O2	−179.28 (14)
C6—C1—C2—O1	178.56 (15)	C7—C8—C9—O2	−0.7 (2)
N1—C1—C2—O1	−1.6 (2)	C13—C8—C9—C10	−0.4 (2)
C6—C1—C2—C3	0.5 (2)	C7—C8—C9—C10	178.20 (15)
N1—C1—C2—C3	−179.68 (14)	O2—C9—C10—C11	178.48 (15)
O1—C2—C3—C4	−178.66 (15)	C8—C9—C10—C11	−0.4 (2)
C1—C2—C3—C4	−0.7 (2)	C9—C10—C11—C12	0.6 (3)
C2—C3—C4—C5	0.0 (3)	C10—C11—C12—O3	−179.51 (15)
C3—C4—C5—C6	0.8 (3)	C10—C11—C12—C13	0.0 (2)
C4—C5—C6—C1	−1.0 (3)	O3—C12—C13—C8	178.68 (14)
C2—C1—C6—C5	0.4 (2)	C11—C12—C13—C8	−0.8 (2)
N1—C1—C6—C5	−179.48 (15)	C9—C8—C13—C12	1.0 (2)
C1—N1—C7—C8	178.69 (15)	C7—C8—C13—C12	−177.65 (15)
N1—C7—C8—C13	179.06 (14)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O1···O2ⁱ	0.90 (3)	1.74 (3)	2.625 (2)	166 (3)
O3—H1O3···O2ⁱⁱ	0.95 (3)	1.69 (3)	2.633 (2)	173 (2)
N1—H1N1···O2	0.89 (3)	1.83 (3)	2.581 (2)	141 (2)
C13—H13···O1ⁱⁱ	1.00 (2)	2.60 (2)	3.411 (3)	137.8 (18)

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

Experimental details

Crystal data
Chemical formula	C₁₃H₁₁NO₃
M_r	229.23
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	11.048 (5), 8.187 (3), 22.858 (10)
β (°)	102.242 (13)
V (Å³)	2020.5 (15)
Z	8
Radiation type	Mo Kα
µ (mm⁻¹)	0.11
Crystal size (mm)	0.35 × 0.12 × 0.04

Data collection
Diffractometer	Bruker APEXII DUO CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2009)
T_min, T_max	0.963, 0.996
No. of measured, independent and observed [I > 2σ(I)] reflections	15107, 2967, 2122
R_int	0.087
(sin θ/λ)_max (Å⁻¹)	0.705

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.054, 0.173, 1.06
No. of reflections	2967
No. of parameters	198
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.48, −0.28

Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O1···O2ⁱ	0.90 (3)	1.74 (3)	2.625 (2)	166 (3)
O3—H1O3···O2ⁱⁱ	0.95 (3)	1.69 (3)	2.633 (2)	173 (2)
N1—H1N1···O2	0.89 (3)	1.83 (3)	2.581 (2)	141 (2)
C13—H13···O1ⁱⁱ	1.00 (2)	2.60 (2)	3.411 (3)	137.8 (18)

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

Footnotes

‡On study leave from Department of Chemistry International University of Africa, Khartoum, Sudan; e-mail: nasertaha90@hotmail.com.

§Thomson Reuters ResearcherID: A-3561-2009.

¶Thomson Reuters ResearcherID: A-5085-2009.

Acknowledgements

The authors thank the Malaysian Government and Universiti Sains Malaysia for the RU research grant (PKIMIA/815002). NEE thanks Universiti Sains Malaysia for a post-doctoral fellowship and the International University of Africa (Sudan) for providing study leave. The authors thank the Malaysian Government and Universiti Sains Malaysia for the Research University Golden Goose grant No. 1001/PFIZIK/811012.

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