2,6-Diamino-4-chloro­pyrimidine–benzoic acid (1/1)

Thanigaimani, K.; Khalib, N.C.; Arshad, S.; Razak, I.A.

doi:10.1107/S160053681204768X

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 68| Part 12| December 2012| Pages o3442-o3443

doi:10.1107/S160053681204768X

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2,6-Diamino-4-chloropyrimidine–benzoic acid (1/1)

Kaliyaperumal Thanigaimani,^a Nuridayanti Che Khalib,^a Suhana Arshad ^a and Ibrahim Abdul Razak ^a ^*‡

^aSchool of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
^*Correspondence e-mail: arazaki@usm.my

(Received 12 November 2012; accepted 20 November 2012; online 24 November 2012)

The benzoic acid molecule of the title compound, C₄H₅ClN₄·C₇H₆O₂, is approximately planar, with a dihedral angle of 1.28 (9)° between the carboxy group and the benzene ring. In the crystal, two acid and two base molecules are linked through N—H⋯O and O—H⋯N hydrogen bonds, forming a centrosymmetric 2 + 2 unit with R₂²(8) and R₄²(8) motifs. These units are further linked through a pair of N—H⋯N hydrogen bonds into a tape structure along [1-20]. The crystal structure also features weak π–π [centroid–centroid distance = 3.5984 (11) Å] and C—H⋯π interactions.

Related literature

For the biological activity of pyrimidine and aminopyrimidine derivatives, see: Hunt et al. (1980 ); Baker & Santi (1965 ). For related structures, see: Schwalbe & Williams (1982 ); Hu et al. (2002 ); Chinnakali et al. (1999 ); Skovsgaard & Bond (2009 ). For hydrogen-bond motifs, see: Bernstein et al. (1995 ). For bond-length data, see: Allen et al. (1987 ). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986 ).

Experimental

Crystal data

C₄H₅ClN₄·C₇H₆O₂
M_r = 266.69
Monoclinic, P 2₁ /c
a = 8.7817 (17) Å
b = 5.7032 (12) Å
c = 24.026 (4) Å
β = 95.493 (4)°
V = 1197.8 (4) Å³
Z = 4
Mo Kα radiation
μ = 0.32 mm⁻¹
T = 100 K
0.36 × 0.30 × 0.16 mm

Data collection

Bruker SMART APEXII CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.895, T_max = 0.951
7539 measured reflections
2097 independent reflections
1891 reflections with I > 2σ(I)
R_int = 0.052

Refinement

R[F² > 2σ(F²)] = 0.036
wR(F²) = 0.101
S = 1.09
2097 reflections
183 parameters
1 restraint
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.25 e Å⁻³
Δρ_min = −0.24 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 is the centroid of the C5–C10 ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1O1⋯N2	0.87 (2)	1.74 (2)	2.5976 (18)	168 (3)
N4—H2N4⋯O2	0.88 (2)	2.03 (2)	2.894 (2)	171.2 (18)
N4—H1N4⋯O2ⁱ	0.88 (2)	2.07 (2)	2.902 (2)	158.2 (19)
N3—H1N3⋯N1ⁱⁱ	0.85 (2)	2.18 (2)	3.020 (2)	171 (2)
C9—H9A⋯Cg1ⁱⁱⁱ	0.95	2.99	3.6557 (19)	128
Symmetry codes: (i) -x+1, -y, -z+1; (ii) -x, -y+2, -z+1; (iii) $[-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ .

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/S160053681204768X/is5218sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681204768X/is5218Isup2.hkl

Supporting information file. DOI: 10.1107/S160053681204768X/is5218Isup3.cml

checkCIF report

Comment top

Pyrimidine and aminopyrimidine derivatives are biologically important compounds as they occur in nature as components of nucleic acids. Some aminopyrimidine derivatives are used as antifolate drugs (Hunt et al., 1980; Baker & Santi, 1965). The crystal structures of aminopyrimidine derivatives (Schwalbe & Williams, 1982), aminopyrimidine carboxylates (Hu et al., 2002) and co-crystal structures (Chinnakali et al., 1999; Skovsgaard & Bond, 2009) have ben reported. In the present study, hydrogen-bonding patterns in the 2,6-diamino-4-chloropyrimidine–benzoic acid (1/1) co-crystal are investigated.

The asymmetric unit (Fig. 1) contains one 2,6-diamino-4-chloropyrimidine molecule and one benzoic acid molecule. The 2,6-diamino-4-chloropyrimidine molecule is essentially planar, with a maximum deviation of 0.009 (2) Å for atom C4. The carboxyl group of the benzoic acid molecule is twisted slightly from the ring with a dihedral angle between C5–C10 ring and O1/O2/C10/C11 plane being 1.28 (9)°. The bond lengths (Allen et al., 1987) and angles are normal.

In the crystal packing (Fig. 2), the 2,6-diamino-4-chloropyrimidine molecules interact with the carboxylic group of the respective benzoic acid molecules through N4—H2N4···O2 and O1—H1O1···N2 hydrogen bonds, forming a cyclic hydrogen-bonded motif of R₂²(8) (Bernstein et al., 1995). These motifs are centrosymmetrically paired via N4—H1N4···O2ⁱ hydrogen bonds, resulting in a DADA array (Where D is a hydrogen-bond donor and A is a hydrogen-bond acceptor) of quadruple hydrogen bonds (symmetry code in Table 1); this can be represented by the graph-set notations of R₂²(8) and R₄²(8). The quadruple hydrogen-bonding motifs are further extended through a couple of N3—H1N3···N1ⁱⁱ hydrogen bonds (symmetry code in Table 1), leading to the formation of hydrogen-bonded supramolecular tape. The crystal structure is further stabilized by π–π interactions between the pyrimidine (Cg2; N1/N2/C1–C4) rings [Cg2···Cg2 = 3.5984 (11) Å; -x, 1 - y, 1 - z] and C—H···π interactions (Table 1) involving the centroid of the C5–C10 (centroid Cg1) ring.

Related literature top

For the biological activity of pyrimidine and aminopyrimidine derivatives, see: Hunt et al. (1980); Baker & Santi (1965). For related structures, see: Schwalbe & Williams (1982); Hu et al. (2002); Chinnakali et al. (1999); Skovsgaard & Bond (2009). For hydrogen-bond motifs, see: Bernstein et al. (1995). For bond-length data, see: Allen et al. (1987). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986).

Experimental top

A hot methanol solutions (20 ml) of 2,6-diamino-4-chloropyrimidine (36 mg, Aldrich) and benzoic acid (30 mg, Merck) were mixed and warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title compound (I) appeared 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 atom labels with 50% probability displacement ellipsoids. Dashed lines indicate the hydrogen bonds.
	Fig. 2. The crystal packing of the title compound. H atoms not involved in the intermolecular interactions (dashed lines) have been omitted for clarity.

2,6-Diamino-4-chloropyrimidine–benzoic acid (1/1) top

Crystal data top

C₄H₅ClN₄·C₇H₆O₂	F(000) = 552
M_r = 266.69	D_x = 1.479 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 5646 reflections
a = 8.7817 (17) Å	θ = 3.0–30.0°
b = 5.7032 (12) Å	µ = 0.32 mm⁻¹
c = 24.026 (4) Å	T = 100 K
β = 95.493 (4)°	Block, colourless
V = 1197.8 (4) Å³	0.36 × 0.30 × 0.16 mm
Z = 4

Data collection top

Bruker SMART APEXII CCD area-detector diffractometer	2097 independent reflections
Radiation source: fine-focus sealed tube	1891 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.052
ϕ and ω scans	θ_max = 25.0°, θ_min = 2.3°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −10→10
T_min = 0.895, T_max = 0.951	k = −6→6
7539 measured reflections	l = −28→28

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)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.101	H atoms treated by a mixture of independent and constrained refinement
S = 1.09	w = 1/[σ²(F_o²) + (0.0471P)² + 0.4196P] where P = (F_o² + 2F_c²)/3
2097 reflections	(Δ/σ)_max < 0.001
183 parameters	Δρ_max = 0.25 e Å⁻³
1 restraint	Δρ_min = −0.24 e Å⁻³

Crystal data top

C₄H₅ClN₄·C₇H₆O₂	V = 1197.8 (4) Å³
M_r = 266.69	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 8.7817 (17) Å	µ = 0.32 mm⁻¹
b = 5.7032 (12) Å	T = 100 K
c = 24.026 (4) Å	0.36 × 0.30 × 0.16 mm
β = 95.493 (4)°

Data collection top

Bruker SMART APEXII CCD area-detector diffractometer	2097 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	1891 reflections with I > 2σ(I)
T_min = 0.895, T_max = 0.951	R_int = 0.052
7539 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.036	1 restraint
wR(F²) = 0.101	H atoms treated by a mixture of independent and constrained refinement
S = 1.09	Δρ_max = 0.25 e Å⁻³
2097 reflections	Δρ_min = −0.24 e Å⁻³
183 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 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
Cl1	−0.05557 (4)	0.64565 (9)	0.367228 (16)	0.03348 (18)
O1	0.43197 (13)	0.5175 (2)	0.61239 (5)	0.0312 (3)
O2	0.52962 (16)	0.1720 (2)	0.59015 (5)	0.0372 (3)
N1	0.07033 (15)	0.7543 (3)	0.46563 (5)	0.0274 (4)
N2	0.27154 (14)	0.5362 (3)	0.51588 (5)	0.0266 (4)
N3	0.16941 (19)	0.8669 (3)	0.55310 (6)	0.0313 (4)
N4	0.37676 (16)	0.2112 (3)	0.47835 (7)	0.0325 (4)
C1	0.07703 (17)	0.5991 (3)	0.42471 (7)	0.0272 (4)
C2	0.17389 (17)	0.4136 (4)	0.42399 (7)	0.0287 (4)
H2A	0.1734	0.3108	0.3929	0.034*
C3	0.27541 (17)	0.3846 (3)	0.47284 (7)	0.0272 (4)
C4	0.17144 (17)	0.7163 (3)	0.51057 (6)	0.0265 (4)
C5	0.71603 (19)	0.1587 (3)	0.69200 (7)	0.0264 (4)
H5A	0.7220	0.0302	0.6672	0.032*
C6	0.80722 (19)	0.1626 (3)	0.74236 (7)	0.0276 (4)
H6A	0.8760	0.0372	0.7519	0.033*
C7	0.79790 (17)	0.3497 (3)	0.77877 (7)	0.0251 (4)
H7A	0.8608	0.3526	0.8132	0.030*
C8	0.69723 (17)	0.5321 (3)	0.76507 (7)	0.0258 (4)
H8A	0.6908	0.6594	0.7902	0.031*
C9	0.60544 (17)	0.5293 (3)	0.71455 (7)	0.0241 (4)
H9A	0.5361	0.6542	0.7052	0.029*
C10	0.61545 (17)	0.3430 (3)	0.67768 (6)	0.0214 (4)
C11	0.52134 (18)	0.3366 (3)	0.62273 (7)	0.0249 (4)
H2N3	0.233 (3)	0.851 (4)	0.5785 (10)	0.039 (6)*
H2N4	0.432 (2)	0.196 (4)	0.5105 (10)	0.045 (6)*
H1N4	0.388 (2)	0.113 (4)	0.4508 (9)	0.033 (5)*
H1N3	0.109 (2)	0.983 (4)	0.5499 (9)	0.037 (6)*
H1O1	0.386 (3)	0.508 (7)	0.5789 (7)	0.113 (13)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0286 (3)	0.0508 (4)	0.0183 (2)	−0.00328 (18)	−0.01233 (16)	0.00310 (18)
O1	0.0260 (6)	0.0437 (8)	0.0220 (6)	0.0042 (6)	−0.0080 (5)	0.0035 (6)
O2	0.0521 (8)	0.0321 (8)	0.0240 (6)	−0.0019 (6)	−0.0147 (6)	−0.0010 (6)
N1	0.0230 (6)	0.0382 (9)	0.0190 (7)	−0.0093 (6)	−0.0076 (5)	0.0077 (7)
N2	0.0203 (6)	0.0395 (9)	0.0183 (7)	−0.0085 (6)	−0.0059 (5)	0.0068 (6)
N3	0.0318 (8)	0.0370 (10)	0.0217 (8)	−0.0044 (7)	−0.0144 (6)	0.0040 (7)
N4	0.0262 (7)	0.0503 (11)	0.0195 (7)	−0.0020 (7)	−0.0058 (6)	0.0009 (7)
C1	0.0202 (7)	0.0439 (11)	0.0159 (8)	−0.0113 (7)	−0.0071 (6)	0.0080 (7)
C2	0.0218 (8)	0.0457 (12)	0.0177 (8)	−0.0074 (8)	−0.0032 (6)	0.0029 (8)
C3	0.0178 (7)	0.0447 (12)	0.0181 (8)	−0.0098 (7)	−0.0029 (6)	0.0071 (7)
C4	0.0214 (7)	0.0387 (11)	0.0178 (8)	−0.0124 (7)	−0.0062 (6)	0.0083 (7)
C5	0.0325 (9)	0.0257 (10)	0.0201 (8)	0.0009 (7)	−0.0015 (7)	−0.0003 (7)
C6	0.0283 (8)	0.0292 (10)	0.0244 (8)	0.0070 (7)	−0.0030 (7)	0.0050 (7)
C7	0.0220 (8)	0.0322 (10)	0.0196 (8)	−0.0019 (7)	−0.0053 (6)	0.0024 (7)
C8	0.0249 (8)	0.0268 (10)	0.0244 (8)	−0.0006 (7)	−0.0038 (6)	−0.0034 (7)
C9	0.0199 (7)	0.0248 (10)	0.0267 (8)	0.0005 (7)	−0.0027 (6)	0.0028 (7)
C10	0.0192 (7)	0.0263 (9)	0.0181 (8)	−0.0046 (6)	−0.0014 (6)	0.0039 (6)
C11	0.0243 (8)	0.0291 (10)	0.0205 (8)	−0.0063 (7)	−0.0023 (6)	0.0043 (7)

Geometric parameters (Å, º) top

Cl1—C1	1.7395 (15)	C2—C3	1.414 (2)
O1—C11	1.306 (2)	C2—H2A	0.9500
O1—H1O1	0.866 (10)	C5—C6	1.386 (2)
O2—C11	1.229 (2)	C5—C10	1.395 (2)
N1—C1	1.328 (2)	C5—H5A	0.9500
N1—C4	1.348 (2)	C6—C7	1.387 (3)
N2—C4	1.350 (2)	C6—H6A	0.9500
N2—C3	1.351 (2)	C7—C8	1.384 (2)
N3—C4	1.336 (2)	C7—H7A	0.9500
N3—H2N3	0.79 (2)	C8—C9	1.392 (2)
N3—H1N3	0.85 (2)	C8—H8A	0.9500
N4—C3	1.328 (3)	C9—C10	1.391 (2)
N4—H2N4	0.88 (2)	C9—H9A	0.9500
N4—H1N4	0.88 (2)	C10—C11	1.490 (2)
C1—C2	1.358 (3)

C11—O1—H1O1	110 (2)	C6—C5—C10	120.18 (16)
C1—N1—C4	114.40 (16)	C6—C5—H5A	119.9
C4—N2—C3	118.60 (14)	C10—C5—H5A	119.9
C4—N3—H2N3	117.1 (16)	C5—C6—C7	119.92 (16)
C4—N3—H1N3	119.2 (15)	C5—C6—H6A	120.0
H2N3—N3—H1N3	123 (2)	C7—C6—H6A	120.0
C3—N4—H2N4	118.1 (15)	C8—C7—C6	120.20 (15)
C3—N4—H1N4	121.4 (13)	C8—C7—H7A	119.9
H2N4—N4—H1N4	121 (2)	C6—C7—H7A	119.9
N1—C1—C2	126.90 (15)	C7—C8—C9	120.15 (16)
N1—C1—Cl1	114.35 (13)	C7—C8—H8A	119.9
C2—C1—Cl1	118.76 (14)	C9—C8—H8A	119.9
C1—C2—C3	115.25 (17)	C10—C9—C8	119.83 (15)
C1—C2—H2A	122.4	C10—C9—H9A	120.1
C3—C2—H2A	122.4	C8—C9—H9A	120.1
N4—C3—N2	117.76 (15)	C9—C10—C5	119.71 (15)
N4—C3—C2	122.23 (17)	C9—C10—C11	121.29 (15)
N2—C3—C2	120.01 (17)	C5—C10—C11	118.99 (15)
N3—C4—N1	116.94 (17)	O2—C11—O1	123.62 (15)
N3—C4—N2	118.24 (15)	O2—C11—C10	121.44 (16)
N1—C4—N2	124.81 (16)	O1—C11—C10	114.94 (15)

C4—N1—C1—C2	−0.5 (2)	C10—C5—C6—C7	0.3 (3)
C4—N1—C1—Cl1	179.02 (11)	C5—C6—C7—C8	0.3 (3)
N1—C1—C2—C3	1.1 (3)	C6—C7—C8—C9	−0.4 (2)
Cl1—C1—C2—C3	−178.45 (11)	C7—C8—C9—C10	−0.2 (2)
C4—N2—C3—N4	178.65 (15)	C8—C9—C10—C5	0.9 (2)
C4—N2—C3—C2	−1.2 (2)	C8—C9—C10—C11	−178.71 (14)
C1—C2—C3—N4	179.98 (15)	C6—C5—C10—C9	−0.9 (2)
C1—C2—C3—N2	−0.1 (2)	C6—C5—C10—C11	178.67 (15)
C1—N1—C4—N3	−179.82 (14)	C9—C10—C11—O2	−179.79 (15)
C1—N1—C4—N2	−1.0 (2)	C5—C10—C11—O2	0.6 (2)
C3—N2—C4—N3	−179.31 (15)	C9—C10—C11—O1	0.5 (2)
C3—N2—C4—N1	1.9 (2)	C5—C10—C11—O1	−179.08 (14)

Hydrogen-bond geometry (Å, º) top

Cg1 is the centroid of the C5–C10 ring.

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O1···N2	0.87 (2)	1.74 (2)	2.5976 (18)	168 (3)
N4—H2N4···O2	0.88 (2)	2.03 (2)	2.894 (2)	171.2 (18)
N4—H1N4···O2ⁱ	0.88 (2)	2.07 (2)	2.902 (2)	158.2 (19)
N3—H1N3···N1ⁱⁱ	0.85 (2)	2.18 (2)	3.020 (2)	171 (2)
C9—H9A···Cg1ⁱⁱⁱ	0.95	2.99	3.6557 (19)	128

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

Experimental details

Crystal data
Chemical formula	C₄H₅ClN₄·C₇H₆O₂
M_r	266.69
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	8.7817 (17), 5.7032 (12), 24.026 (4)
β (°)	95.493 (4)
V (Å³)	1197.8 (4)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.32
Crystal size (mm)	0.36 × 0.30 × 0.16

Data collection
Diffractometer	Bruker SMART APEXII CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2009)
T_min, T_max	0.895, 0.951
No. of measured, independent and observed [I > 2σ(I)] reflections	7539, 2097, 1891
R_int	0.052
(sin θ/λ)_max (Å⁻¹)	0.594

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.036, 0.101, 1.09
No. of reflections	2097
No. of parameters	183
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)	0.25, −0.24

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

Hydrogen-bond geometry (Å, º) top

Cg1 is the centroid of the C5–C10 ring.

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O1···N2	0.867 (18)	1.74 (2)	2.5976 (18)	168 (3)
N4—H2N4···O2	0.88 (2)	2.03 (2)	2.894 (2)	171.2 (18)
N4—H1N4···O2ⁱ	0.88 (2)	2.07 (2)	2.902 (2)	158.2 (19)
N3—H1N3···N1ⁱⁱ	0.85 (2)	2.18 (2)	3.020 (2)	171 (2)
C9—H9A···Cg1ⁱⁱⁱ	0.95	2.99	3.6557 (19)	128

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

Footnotes

‡Thomson Reuters ResearcherID: A-5599-2009.

Acknowledgements

The authors thank the Malaysian Government and Universiti Sains Malaysia (USM) for the research facilities and Fundamental Research Grant Scheme (FRGS) No. 203/PFIZIK/6711171 to conduct this work. KT thanks The Academy of Sciences for the Developing World and USM for a TWAS–USM fellowship.

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