supplementary materials


gk2586 scheme

Acta Cryst. (2013). E69, o1297    [ doi:10.1107/S1600536813019788 ]

N-tert-Butoxycarbonyl-[alpha]-(2-fluorobenzyl)-L-proline

P. Rajalakshmi, N. Srinivasan, R. V. Krishnakumar, I. A. Razak and M. M. Rosli

Abstract top

In the title compound, C17H22FNO4, the pyrrolidine ring adopts an envelope conformation with the disordered components of the methylene C atom, with site occupancies of 0.896 (7) and 0.104 (7), being the flap on either side of the mean plane involving the other atoms of the ring. The carboxylic acid group forms dihedral angles of 72.06 (11) and 45.44 (5)° with the N-tert-butoxycarbonyl group and the 2-fluorobenzyl group, respectively. In the crystal, two-dimensional layers of molecules parallel to (001) are built through an R44(23) motif generated via O-H...O, C-H...O and C-H...F interactions, and an R22(11) motif generated by C-H...O and C-H...F interactions.

Comment top

Modified amino acids are known to enhance the chemical, physical and biological properties of proteins (Anderson et al., 2004). Also, due to their structural diversity and functional versatility, they are widely used as chiral building blocks and molecular scaffolds in pharmaceutics (Taylor et al., 1998; Ryder et al., 2000; Jeng et al., 2002). N-Butoxycarbonyl-(S)-α-benzyl proline, a closely related analogue of the title compound N-tert-butoxycarbonyl-α-(2-fluorobenzyl)-L-proline, is a potential non-nucleoside reverse transcriptase inhibitor in anti-human-immunodeficiency virus type-1 (Tamazyan et al., 2004).

The present paper describes the crystal structure of the title compound (Fig. 1), which crystallizes in the orthorhombic space group P212121. It is a modified amino acid with the N-terminus protected by a tert-butyloxycarbonyl (Boc) group and the Cα (C2) H atom replaced by a 2-fluorobenzyl group. In the pyrrolidine ring, the C4 atom of the ring displays positional disorder with site-occupation factors of 0.896 (7) and 0.104 (7). The pyrrolidine ring (N1/C2/C3/C4A/C5) adopts the envelope conformation with the C4A atom deviating from the plane defined the remaining ring atoms by 0.5677 (4) Å. Puckering parameters calculated for this ring are of Q = 0.369 (3) Å, φ = 285.7 (3)° (Cremer & Pople, 1975). The dihedral angles between the mean plane of the carboxylic acid group, N-Boc and 2-methyl-2-fluorobenzene are 72.06 (11)° and 45.44 (5)°, respectively.

The molecules are linked by a combination of O—H···O, C—H···O and C—H···F hydrogen bonds. The carboxylic O1 acts as a donor to the carbonyl O3 at (-x + 1, y - 1/2, -z + 3/2) forming chains parallel to the b axis through C(7) motifs (Bernstein et al., 1995). These 21 screw-generated parallel chains are interconnected through C15—H15A···F1 (-x, y + 1/2, -z + 3/2) and C16—H16A···O1 (-x, y + 1/2, -z + 3/2) hydrogen bonds leading to a layer parallel to the ab plane. The characteristic building units of this layer are an R44(23) ring generated by all the hydrogen-bonds and an R22(11) generated exclusively by C—H···O and C—H···F hydrogen bonds (Fig. 2).

Related literature top

For general background, see: Taylor et al. (1998); Jeng et al. (2002); Anderson et al. (2004); Ryder et al. (2000). For biological activity of the title compound, see: Tamazyan et al. (2004). For graph-set notation of hydrogen bonding, see: Bernstein et al. (1995). For puckering parameters, see: Cremer & Pople (1975).

Experimental top

A mixture of 2-(2-fluorobenzyl)-L-proline (1.0 mmol) and tetramethylammonium hydroxide pentahydrate (1.2 mmol) in acetonitrile (10 ml) was stirred for 30 min. After 30 min, Boc2O (2.0 mmol) was added and stirred continuously for 2 d. The acetonitrile was removed in vacuo and residue was partitioned between ether (20 ml) and water (10 ml). The aqueous layer was washed with ether (10 ml) and acidified with 10% aqueous citric acid to pH 3–4. The aqueous layer was extracted with ethyl acetate (3 × 10 ml) and combined organic extracts were washed with brine solution (1 × 10 ml), dried over Na2SO4, and concentrated to yield N-tert-butoxycarbonyl-α-(2-fluorobenzyl)-L-proline (m.p. 430-433 K) as a white solid. Crystals were grown from ethanolic solution by slow evaporation at room temperature.

Refinement top

All H atoms, except hydroxy H1 atom, were placed at geometrically calculated positions (0.99 Å for methylene C—H, 0.98 Å for methyl C—H and 0.95 Å for aromatic C—H) and refined using a riding model. The Uiso values of all H atoms were constrained to 1.2Ueq (1.5 times for hydroxyl and methyl H atoms) of the respective atom to which the H atom bonds. The hydroxy H1 atom was freely refined. In the pyrrolidine ring, the C4 atom exhibits disorder (resolved into C4A and C4B) and the same was modelled using SIMU and SADI restraints leading to site-occupancies of 0.896 (7) and 0.104 (7). In the absence of significant anomalous scattering effects 1492 Friedel pairs were merged. The enantiomer has been assigned by reference to an unchanging chiral centre in the synthetic procedure.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: APEX2 (Bruker, 2009); data reduction: APEX2 (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLUTON (Spek, 2009); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Molecular structure of the title compound showing the major component of the disorder. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level.
[Figure 2] Fig. 2. Part of the crystal structure of the title compound, showing R22(11) and R44(23) motifs through a combination of O—H···O, C—H···O and C—H···F hydrogen bonds and the formation of a two dimensional layer parallel to the ab plane. For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted.
N-tert-Butoxycarbonyl-α-(2-fluorobenzyl)-L-proline top
Crystal data top
C17H22FNO4F(000) = 688
Mr = 323.36Dx = 1.254 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 3426 reflections
a = 10.4777 (1) Åθ = 2.3–30.0°
b = 12.4283 (2) ŵ = 0.10 mm1
c = 13.1550 (2) ÅT = 100 K
V = 1713.04 (4) Å3Block, colourless
Z = 40.54 × 0.34 × 0.24 mm
Data collection top
Bruker Kappa APEXII
diffractometer
3426 independent reflections
Radiation source: fine-focus sealed tube2712 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.031
φ and ω scansθmax = 32.6°, θmin = 2.3°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2008)
h = 1315
Tmin = 0.962, Tmax = 0.977k = 1818
14001 measured reflectionsl = 1818
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.050Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.114H atoms treated by a mixture of independent and constrained refinement
S = 1.06 w = 1/[σ2(Fo2) + (0.043P)2 + 0.5231P]
where P = (Fo2 + 2Fc2)/3
3426 reflections(Δ/σ)max < 0.001
225 parametersΔρmax = 0.37 e Å3
8 restraintsΔρmin = 0.32 e Å3
Crystal data top
C17H22FNO4V = 1713.04 (4) Å3
Mr = 323.36Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 10.4777 (1) ŵ = 0.10 mm1
b = 12.4283 (2) ÅT = 100 K
c = 13.1550 (2) Å0.54 × 0.34 × 0.24 mm
Data collection top
Bruker Kappa APEXII
diffractometer
2712 reflections with I > 2σ(I)
Absorption correction: multi-scan
(SADABS; Sheldrick, 2008)
Rint = 0.031
Tmin = 0.962, Tmax = 0.977θmax = 32.6°
14001 measured reflectionsStandard reflections: 0
3426 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.050H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.114Δρmax = 0.37 e Å3
S = 1.06Δρmin = 0.32 e Å3
3426 reflectionsAbsolute structure: ?
225 parametersAbsolute structure parameter: ?
8 restraintsRogers parameter: ?
Special details top

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 > 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
xyzUiso*/UeqOcc. (<1)
F10.00102 (13)0.97131 (13)0.77647 (14)0.0493 (4)
O10.29559 (14)0.71232 (12)0.76893 (13)0.0272 (3)
H10.320 (3)0.645 (3)0.756 (3)0.055 (9)*
O20.47731 (16)0.74954 (12)0.68734 (14)0.0346 (4)
O30.65381 (14)1.01134 (12)0.77232 (14)0.0296 (4)
O40.53366 (13)0.89798 (11)0.86819 (11)0.0227 (3)
N10.45214 (15)0.96664 (13)0.72598 (13)0.0191 (3)
C10.38068 (19)0.77841 (16)0.72823 (16)0.0205 (4)
C20.33880 (18)0.89677 (15)0.73257 (15)0.0177 (4)
C30.2696 (2)0.92068 (18)0.63034 (17)0.0272 (5)
H3A0.29180.86600.57860.033*0.896 (7)
H3B0.17590.92100.63980.033*0.896 (7)
H3C0.23260.85390.60170.033*0.104 (7)
H3D0.20010.97350.64070.033*0.104 (7)
C4A0.3162 (2)1.0299 (2)0.59876 (18)0.0271 (7)0.896 (7)
H4A0.26801.08750.63370.032*0.896 (7)
H4B0.30841.03980.52440.032*0.896 (7)
C4B0.3694 (15)0.9656 (18)0.5602 (4)0.058 (7)0.104 (7)
H4C0.33061.01310.50830.070*0.104 (7)
H4D0.41690.90720.52570.070*0.104 (7)
C50.4573 (2)1.02965 (19)0.63191 (17)0.0299 (5)
H5A0.51220.99470.58040.036*0.896 (7)
H5B0.48871.10360.64470.036*0.896 (7)
H5C0.54531.03340.60490.036*0.104 (7)
H5D0.42481.10360.64270.036*0.104 (7)
C60.55438 (19)0.96175 (15)0.78805 (16)0.0212 (4)
C70.6244 (2)0.89395 (17)0.95460 (17)0.0247 (4)
C80.6362 (3)1.0048 (2)1.0019 (2)0.0452 (7)
H8A0.68011.05310.95450.068*
H8B0.68520.99981.06510.068*
H8C0.55091.03321.01660.068*
C90.7514 (2)0.8487 (2)0.9205 (2)0.0371 (6)
H9A0.79440.90090.87640.056*
H9B0.73730.78170.88280.056*
H9C0.80480.83410.98010.056*
C100.5574 (3)0.8169 (2)1.02650 (18)0.0337 (5)
H10A0.54440.74760.99240.050*
H10B0.47450.84701.04620.050*
H10C0.60990.80631.08730.050*
C110.25451 (19)0.92051 (15)0.82578 (15)0.0186 (4)
H11A0.18250.86900.82630.022*
H7B0.30540.90770.88800.022*
C120.20116 (18)1.03351 (16)0.82987 (14)0.0182 (4)
C130.2744 (2)1.12161 (15)0.85860 (16)0.0212 (4)
H130.36131.11090.87640.025*
C140.2240 (2)1.22470 (17)0.86194 (18)0.0292 (5)
H14A0.27651.28370.88090.035*
C150.0967 (2)1.24157 (19)0.83757 (19)0.0342 (6)
H15A0.06191.31210.84030.041*
C160.0208 (2)1.1563 (2)0.80940 (19)0.0357 (6)
H16A0.06651.16700.79280.043*
C170.0743 (2)1.05449 (18)0.80576 (18)0.0272 (5)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F10.0207 (6)0.0479 (9)0.0792 (12)0.0004 (6)0.0048 (8)0.0278 (9)
O10.0206 (7)0.0158 (6)0.0451 (9)0.0020 (5)0.0062 (7)0.0057 (7)
O20.0263 (8)0.0254 (7)0.0522 (10)0.0054 (6)0.0151 (8)0.0038 (8)
O30.0184 (7)0.0204 (7)0.0501 (10)0.0006 (5)0.0024 (7)0.0084 (7)
O40.0185 (7)0.0237 (7)0.0259 (7)0.0023 (6)0.0066 (6)0.0078 (6)
N10.0188 (8)0.0191 (7)0.0193 (7)0.0021 (6)0.0023 (7)0.0041 (7)
C10.0184 (9)0.0200 (8)0.0232 (9)0.0035 (7)0.0021 (8)0.0037 (8)
C20.0177 (9)0.0168 (8)0.0185 (8)0.0039 (7)0.0000 (7)0.0016 (7)
C30.0272 (11)0.0329 (11)0.0214 (10)0.0072 (9)0.0055 (9)0.0034 (9)
C4A0.0359 (14)0.0264 (12)0.0189 (11)0.0043 (10)0.0072 (10)0.0021 (10)
C4B0.094 (16)0.047 (13)0.034 (12)0.037 (13)0.001 (12)0.011 (11)
C50.0379 (13)0.0284 (11)0.0233 (10)0.0024 (10)0.0057 (10)0.0093 (9)
C60.0192 (9)0.0134 (8)0.0310 (11)0.0031 (7)0.0015 (8)0.0030 (8)
C70.0222 (10)0.0232 (9)0.0288 (11)0.0031 (8)0.0111 (9)0.0023 (9)
C80.0569 (17)0.0303 (12)0.0485 (16)0.0053 (12)0.0199 (14)0.0135 (12)
C90.0251 (12)0.0356 (12)0.0506 (16)0.0086 (10)0.0058 (11)0.0071 (12)
C100.0347 (13)0.0404 (13)0.0258 (11)0.0022 (11)0.0086 (10)0.0069 (10)
C110.0175 (9)0.0159 (8)0.0226 (10)0.0030 (7)0.0039 (7)0.0000 (8)
C120.0175 (9)0.0189 (8)0.0180 (9)0.0044 (7)0.0027 (7)0.0007 (8)
C130.0195 (9)0.0205 (9)0.0236 (10)0.0018 (7)0.0026 (8)0.0019 (8)
C140.0350 (12)0.0199 (9)0.0329 (12)0.0015 (9)0.0066 (10)0.0026 (9)
C150.0430 (14)0.0242 (10)0.0354 (12)0.0173 (10)0.0045 (11)0.0014 (10)
C160.0260 (12)0.0402 (13)0.0407 (13)0.0178 (10)0.0068 (10)0.0068 (11)
C170.0184 (10)0.0304 (11)0.0326 (12)0.0034 (8)0.0004 (9)0.0091 (10)
Geometric parameters (Å, º) top
F1—C171.356 (3)C5—H5C0.9900
O1—C11.325 (2)C5—H5D0.9900
O1—H10.89 (3)C7—C91.513 (3)
O2—C11.201 (2)C7—C81.517 (3)
O3—C61.228 (2)C7—C101.518 (3)
O4—C61.337 (2)C8—H8A0.9800
O4—C71.483 (2)C8—H8B0.9800
N1—C61.348 (3)C8—H8C0.9800
N1—C51.465 (3)C9—H9A0.9800
N1—C21.474 (3)C9—H9B0.9800
C1—C21.536 (3)C9—H9C0.9800
C2—C111.540 (3)C10—H10A0.9800
C2—C31.557 (3)C10—H10B0.9800
C3—C4A1.501 (3)C10—H10C0.9800
C3—C4B1.502 (4)C11—C121.513 (3)
C3—H3A0.9900C11—H11A0.9900
C3—H3B0.9900C11—H7B0.9900
C3—H3C0.9900C12—C171.391 (3)
C3—H3D0.9900C12—C131.389 (3)
C4A—C51.541 (4)C13—C141.386 (3)
C4A—H4A0.9900C13—H130.9500
C4A—H4B0.9900C14—C151.388 (4)
C4B—C51.540 (4)C14—H14A0.9500
C4B—H4C0.9900C15—C161.376 (4)
C4B—H4D0.9900C15—H15A0.9500
C5—H5A0.9900C16—C171.385 (3)
C5—H5B0.9900C16—H16A0.9500
C1—O1—H1108 (2)N1—C5—H5D111.2
C6—O4—C7121.37 (16)C4B—C5—H5D111.2
C6—N1—C5120.42 (17)C4A—C5—H5D73.1
C6—N1—C2125.31 (16)H5A—C5—H5D134.9
C5—N1—C2113.21 (17)H5C—C5—H5D109.1
O2—C1—O1124.25 (19)O3—C6—O4124.63 (19)
O2—C1—C2122.92 (19)O3—C6—N1123.33 (19)
O1—C1—C2112.72 (16)O4—C6—N1112.04 (17)
N1—C2—C1109.38 (15)O4—C7—C9110.42 (18)
N1—C2—C11113.33 (16)O4—C7—C8109.64 (18)
C1—C2—C11112.15 (16)C9—C7—C8112.8 (2)
N1—C2—C3102.24 (16)O4—C7—C10101.69 (17)
C1—C2—C3106.49 (16)C9—C7—C10110.92 (19)
C11—C2—C3112.60 (15)C8—C7—C10110.8 (2)
C4A—C3—C2105.08 (18)C7—C8—H8A109.5
C4B—C3—C2106.1 (6)C7—C8—H8B109.5
C4A—C3—H3A110.7H8A—C8—H8B109.5
C4B—C3—H3A70.7C7—C8—H8C109.5
C2—C3—H3A110.7H8A—C8—H8C109.5
C4A—C3—H3B110.7H8B—C8—H8C109.5
C4B—C3—H3B140.0C7—C9—H9A109.5
C2—C3—H3B110.7C7—C9—H9B109.5
H3A—C3—H3B108.8H9A—C9—H9B109.5
C4A—C3—H3C141.3C7—C9—H9C109.5
C4B—C3—H3C110.5H9A—C9—H9C109.5
C2—C3—H3C110.5H9B—C9—H9C109.5
H3B—C3—H3C70.3C7—C10—H10A109.5
C4A—C3—H3D71.2C7—C10—H10B109.5
C4B—C3—H3D110.5H10A—C10—H10B109.5
C2—C3—H3D110.5C7—C10—H10C109.5
H3A—C3—H3D136.3H10A—C10—H10C109.5
H3C—C3—H3D108.7H10B—C10—H10C109.5
C3—C4A—C5103.42 (19)C12—C11—C2114.72 (16)
C3—C4A—H4A111.1C12—C11—H11A108.6
C5—C4A—H4A111.1C2—C11—H11A108.6
C3—C4A—H4B111.1C12—C11—H7B108.6
C5—C4A—H4B111.1C2—C11—H7B108.6
H4A—C4A—H4B109.0H11A—C11—H7B107.6
C3—C4B—C5103.4 (2)C17—C12—C13116.22 (19)
C3—C4B—H4C111.1C17—C12—C11121.26 (19)
C5—C4B—H4C111.1C13—C12—C11122.51 (17)
C3—C4B—H4D111.1C14—C13—C12121.8 (2)
C5—C4B—H4D111.1C14—C13—H13119.1
H4C—C4B—H4D109.0C12—C13—H13119.1
N1—C5—C4B102.7 (6)C15—C14—C13119.9 (2)
N1—C5—C4A101.83 (18)C15—C14—H14A120.1
N1—C5—H5A111.4C13—C14—H14A120.1
C4B—C5—H5A72.6C16—C15—C14120.1 (2)
C4A—C5—H5A111.4C16—C15—H15A120.0
N1—C5—H5B111.4C14—C15—H15A120.0
C4B—C5—H5B141.5C15—C16—C17118.6 (2)
C4A—C5—H5B111.4C15—C16—H16A120.7
H5A—C5—H5B109.3C17—C16—H16A120.7
N1—C5—H5C111.2F1—C17—C16118.07 (19)
C4B—C5—H5C111.2F1—C17—C12118.55 (19)
C4A—C5—H5C142.4C16—C17—C12123.4 (2)
H5B—C5—H5C73.0
C6—N1—C2—C155.2 (2)C3—C4A—C5—N135.4 (2)
C5—N1—C2—C1113.02 (19)C3—C4A—C5—C4B60.5 (8)
C6—N1—C2—C1170.8 (2)C7—O4—C6—O39.2 (3)
C5—N1—C2—C11121.03 (18)C7—O4—C6—N1170.18 (16)
C6—N1—C2—C3167.78 (18)C5—N1—C6—O32.7 (3)
C5—N1—C2—C30.4 (2)C2—N1—C6—O3170.11 (19)
O2—C1—C2—N126.3 (3)C5—N1—C6—O4177.96 (18)
O1—C1—C2—N1157.34 (17)C2—N1—C6—O410.5 (3)
O2—C1—C2—C11152.9 (2)C6—O4—C7—C965.0 (2)
O1—C1—C2—C1130.7 (2)C6—O4—C7—C859.9 (3)
O2—C1—C2—C383.5 (2)C6—O4—C7—C10177.21 (18)
O1—C1—C2—C392.9 (2)N1—C2—C11—C1260.3 (2)
N1—C2—C3—C4A23.3 (2)C1—C2—C11—C12175.22 (17)
C1—C2—C3—C4A138.06 (18)C3—C2—C11—C1255.1 (2)
C11—C2—C3—C4A98.6 (2)C2—C11—C12—C17103.8 (2)
N1—C2—C3—C4B21.3 (9)C2—C11—C12—C1376.9 (2)
C1—C2—C3—C4B93.5 (9)C17—C12—C13—C140.7 (3)
C11—C2—C3—C4B143.2 (9)C11—C12—C13—C14180.0 (2)
C4B—C3—C4A—C560.8 (8)C12—C13—C14—C150.9 (3)
C2—C3—C4A—C536.8 (2)C13—C14—C15—C160.5 (4)
C4A—C3—C4B—C560.9 (8)C14—C15—C16—C170.2 (4)
C2—C3—C4B—C534.1 (16)C15—C16—C17—F1179.1 (2)
C6—N1—C5—C4B147.8 (9)C15—C16—C17—C120.5 (4)
C2—N1—C5—C4B21.1 (9)C13—C12—C17—F1179.50 (19)
C6—N1—C5—C4A169.45 (18)C11—C12—C17—F11.2 (3)
C2—N1—C5—C4A21.7 (2)C13—C12—C17—C160.0 (3)
C3—C4B—C5—N133.3 (16)C11—C12—C17—C16179.3 (2)
C3—C4B—C5—C4A60.4 (8)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C15—H15A···F1i0.952.593.378 (3)141
C16—H16A···O1i0.952.603.541 (3)173
O1—H1···O3ii0.89 (3)1.73 (3)2.611 (2)173 (3)
Symmetry codes: (i) x, y+1/2, z+3/2; (ii) x+1, y1/2, z+3/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C15—H15A···F1i0.952.593.378 (3)141
C16—H16A···O1i0.952.603.541 (3)173
O1—H1···O3ii0.89 (3)1.73 (3)2.611 (2)173 (3)
Symmetry codes: (i) x, y+1/2, z+3/2; (ii) x+1, y1/2, z+3/2.
Acknowledgements top

The authors thank Dr Mutharasu Devarajan, Associate Professor, and the staff of the X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, for their help in the data collection.

references
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