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metal-organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoCRYSTALLOGRAPHIC
COMMUNICATIONS
ISSN: 2056-9890
Volume 67| Part 12| December 2011| Page m1857
https://doi.org/10.1107/S1600536811050276

Poly[(aqua­calcium)-μ4-pyrazine-2,3-di­carboxyl­ato]

Qing-Feng Yang,a* Yue-Ping Zhang,a Jing Lu,b Ping Xuea and Zheng Wanga

aKey Laboratory of Energy Resources and Chemical Engineering, Ningxia University, Yinchuan 750021, Ningxia, People's Republic of China, and bSchool of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, Shandong, People's Republic of China
*Correspondence e-mail: yangqf@nxu.edu.cn

(Received 17 October 2011; accepted 23 November 2011; online 30 November 2011)

The polymeric title compound, [Ca(C6H2N2O4)(H2O)]n, was synthesized from pyrazine-2,3-dicarb­oxy­lic acid and calcium dichloride under hydro­thermal conditions. The Ca2+ cation is seven-coordinated by five O atoms and one N atom of four pyrazine-2,3-dicarboxyl­ate anions, and one water mol­ecule. The complete deprotonated pyrazine-2,3-dicarboxyl­ate anion adopts a μ4-coordination mode, resulting in the formation of a three-dimensional structure.

Related literature

For transition and lanthanide metal complexes containing the pydc ligand (pydc = pyrazine-2,3-dicarboxyl­ate), see: Chen et al. (2008[Chen, L. F., Li, Z. J., Qin, Y. Y., Cheng, J. K. & Yao, Y. G. (2008). J. Mol. Struct. 892, 278-282.]); Hu et al. (2004[Hu, M.-L., Yuan, J.-X., Chen, F. & Shi, Q. (2004). Acta Cryst. C60, m186-m188.]); Kitaura et al. (2002[Kitaura, R., Fujimoto, K., Noro, S. I., Kondo, M. & Kitagawa, S. (2002). Angew. Chem. Int. Ed. 114, 141-143.]); Ma et al. (2006[Ma, Y., He, Y.-K. & Han, Z.-B. (2006). Acta Cryst. E62, m2528-m2529.]); Sakagami-Yoshida et al. (2000[Sakagami-Yoshida, N., Teramoto, M., Hioki, A., Fuyuhiro, A. & Kaizaki, S. (2000). Inorg. Chem. 39, 5717-5724.]); Yin (2009[Yin, H. (2009). Acta Cryst. E65, m971.]); Zou et al. (1999[Zou, J. Z., Xu, Z., Chen, W., Lo, K. M. & You, X. Z. (1999). Polyhedron, 18, 1507-1512.]).

[Scheme 1]

Experimental

Crystal data

  • [Ca(C6H2N2O4)(H2O)]

  • Mr = 224.19

  • Monoclinic, P 21 /n

  • a = 6.8109 (7) Å

  • b = 12.0469 (13) Å

  • c = 9.9191 (11) Å

  • β = 102.333 (1)°

  • V = 795.08 (15) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.79 mm−1

  • T = 298 K

  • 0.35 × 0.25 × 0.10 mm
Data collection

  • Bruker SMART APEX CCD diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker, 2005)[Bruker (2005). SADABS, SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.] Tmin = 0.770, Tmax = 0.926

  • 3904 measured reflections

  • 1405 independent reflections

  • 1210 reflections with I > 2σ(I)

  • Rint = 0.026
Refinement

  • R[F2 > 2σ(F2)] = 0.028

  • wR(F2) = 0.075

  • S = 1.06

  • 1405 reflections

  • 143 parameters

  • All H-atom parameters refined

  • Δρmax = 0.35 e Å−3

  • Δρmin = −0.34 e Å−3

Data collection: SMART (Bruker, 2005[Bruker (2005). SADABS, SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2005[Bruker (2005). SADABS, SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008)[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]; program(s) used to refine structure: SHELXL97 (Sheldrick, 2008)[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]; molecular graphics: SHELXTL (Sheldrick, 2008)[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]; software used to prepare material for publication: SHELXL97[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.].

Supporting information

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S1600536811050276/ds2152sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536811050276/ds2152Isup2.hkl

checkCIF report


Comment top

Design and synthesis of metal-organic complexes have attracted much attention due to their intriguing molecular topologies and potentially useful properties, such as adsorption, catalytic, fluorescence, magnetic, and so on. Recently, we have learnt that pyrazine-2,3-dicarboxylic acid is an effective multifunctional bridging ligand to link the M ions with both N and O donor. A large number of transition and lanthanide metal complexes containing the pzdc ligand (pydc = pyrazine-2,3-dicarboxylate) have been reported, see: Zou et al.(1999); Sakagami-Yoshida et al.(2000); Kitaura et al.(2002); Hu et al.(2004); Ma et al.(2006); Chen et al.(2008); Yin et al.(2009). However, alkaline earth metal-containing metal-organic complexes with pydc ligand are less developed. In this paper, we report the synthesis and structure of a new Ca complex with the pzdc ligand.

The aim of the present study was to elucidate the crystal structure of the title compound, I. In I, The Ca center is seven-coordinated by five O atoms and one N atom of four deprotonated pyrazine-2,3-dicarboxylato ligands, and one water molecule (Fig. 1). The Ca—O bond lengths are between 2.3111 (14) and 2.5396 (14) Å, the Ca—N bond distance amount to 2.6159 (17) Å, (Table 1). The pyrazine-2,3-dicarboxylic acid is deprotonated completely and acted as µ4– ligand linking four Ca2+ cations. These CaO6N asymmetric units are connected via the anions into a three-dimensional network (Fig. 2).

Related literature top

For transition and lanthanide metal complexes containing the pzdc ligand (pydc = pyrazine-2,3-dicarboxylate), see: Chen et al. (2008); Hu et al. (2004); Kitaura et al. (2002); Ma et al. (2006); Sakagami-Yoshida et al. (2000); Yin (2009); Zou et al. (1999).

Experimental top

A mixture of pyrazine-2,3-dicarboxylic acid (0.17 g, 1.00 mmol) and Calcium dichloride (0.11 g, 1.00 mmol) in distilled water (15 ml) was stirred fully in air, and then sealed in 25 ml Teflon-lined stainless steel container, which was heated at 413 K for 3 days. The Colorless block-shaped product, I, was crystallized upon cooling to 243 K.

Refinement top

All H atoms were positioned geometrically and refined using the riding-model approximation with Uiso(H) = 1.5Ueq(O).

Structure description top

Design and synthesis of metal-organic complexes have attracted much attention due to their intriguing molecular topologies and potentially useful properties, such as adsorption, catalytic, fluorescence, magnetic, and so on. Recently, we have learnt that pyrazine-2,3-dicarboxylic acid is an effective multifunctional bridging ligand to link the M ions with both N and O donor. A large number of transition and lanthanide metal complexes containing the pzdc ligand (pydc = pyrazine-2,3-dicarboxylate) have been reported, see: Zou et al.(1999); Sakagami-Yoshida et al.(2000); Kitaura et al.(2002); Hu et al.(2004); Ma et al.(2006); Chen et al.(2008); Yin et al.(2009). However, alkaline earth metal-containing metal-organic complexes with pydc ligand are less developed. In this paper, we report the synthesis and structure of a new Ca complex with the pzdc ligand.

The aim of the present study was to elucidate the crystal structure of the title compound, I. In I, The Ca center is seven-coordinated by five O atoms and one N atom of four deprotonated pyrazine-2,3-dicarboxylato ligands, and one water molecule (Fig. 1). The Ca—O bond lengths are between 2.3111 (14) and 2.5396 (14) Å, the Ca—N bond distance amount to 2.6159 (17) Å, (Table 1). The pyrazine-2,3-dicarboxylic acid is deprotonated completely and acted as µ4– ligand linking four Ca2+ cations. These CaO6N asymmetric units are connected via the anions into a three-dimensional network (Fig. 2).

For transition and lanthanide metal complexes containing the pzdc ligand (pydc = pyrazine-2,3-dicarboxylate), see: Chen et al. (2008); Hu et al. (2004); Kitaura et al. (2002); Ma et al. (2006); Sakagami-Yoshida et al. (2000); Yin (2009); Zou et al. (1999).

Computing details top

Data collection: SMART (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Crystal structure of compound I with labelling and displacement ellipsoids drawn at the 30% probability level. Symmetry cods: (A) -x, -y, 1 - z; (B) 1/2 - x, -1/2 + y, 2/3 - z; (C) -x, -y, 1 - z.
[Figure 2] Fig. 2. A view of the packing of compound I along b axis.
Poly[(aquacalcium)-µ4-pyrazine-2,3-dicarboxylato] top
Crystal data top
[Ca(C6H2N2O4)(H2O)]F(000) = 456
Mr = 224.19Dx = 1.873 Mg m−3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 2583 reflections
a = 6.8109 (7) Åθ = 2.7–28.2°
b = 12.0469 (13) ŵ = 0.79 mm−1
c = 9.9191 (11) ÅT = 298 K
β = 102.333 (1)°Block, colorless
V = 795.08 (15) Å30.35 × 0.25 × 0.10 mm
Z = 4
Data collection top
Bruker SMART APEX CCD
diffractometer		1405 independent reflections
Radiation source: fine-focus sealed tube1210 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.026
ω scansθmax = 25.0°, θmin = 2.7°
Absorption correction: multi-scan
(SADABS; Bruker, 2005)		h = −8→8
Tmin = 0.770, Tmax = 0.926k = −12→14
3904 measured reflectionsl = −10→11
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)] = 0.028Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.075All H-atom parameters refined
S = 1.06 w = 1/[σ2(Fo2) + (0.0386P)2 + 0.4192P]
where P = (Fo2 + 2Fc2)/3
1405 reflections(Δ/σ)max < 0.001
143 parametersΔρmax = 0.35 e Å−3
0 restraintsΔρmin = −0.34 e Å−3
Crystal data top
[Ca(C6H2N2O4)(H2O)]V = 795.08 (15) Å3
Mr = 224.19Z = 4
Monoclinic, P21/nMo Kα radiation
a = 6.8109 (7) ŵ = 0.79 mm−1
b = 12.0469 (13) ÅT = 298 K
c = 9.9191 (11) Å0.35 × 0.25 × 0.10 mm
β = 102.333 (1)°
Data collection top
Bruker SMART APEX CCD
diffractometer		1405 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2005)		1210 reflections with I > 2σ(I)
Tmin = 0.770, Tmax = 0.926Rint = 0.026
3904 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0280 restraints
wR(F2) = 0.075All H-atom parameters refined
S = 1.06Δρmax = 0.35 e Å−3
1405 reflectionsΔρmin = −0.34 e Å−3
143 parameters
Special details top

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
xyzUiso*/Ueq
O50.2040 (3)−0.20134 (16)0.40189 (19)0.0390 (5)
O20.2816 (2)0.22697 (11)0.83932 (14)0.0279 (4)
O40.4176 (2)0.09317 (12)0.37610 (18)0.0343 (4)
N20.2587 (3)0.37894 (14)0.64192 (17)0.0218 (4)
C40.2491 (4)0.45523 (18)0.5424 (2)0.0268 (5)
Ca10.23561 (6)−0.08343 (3)0.59754 (4)0.01641 (15)
O30.1145 (2)0.06496 (11)0.41707 (14)0.0207 (3)
O10.3116 (2)0.08934 (10)0.69619 (14)0.0256 (4)
C60.2677 (3)0.12378 (16)0.41813 (19)0.0170 (4)
C10.2679 (3)0.27200 (15)0.60607 (19)0.0173 (4)
C50.2868 (3)0.18869 (15)0.72322 (19)0.0181 (4)
C20.2662 (3)0.24255 (16)0.46930 (19)0.0171 (4)
N10.2632 (3)0.31989 (14)0.37173 (17)0.0238 (4)
C30.2526 (4)0.42579 (17)0.4091 (2)0.0284 (5)
H20.242 (3)0.526 (2)0.564 (2)0.024 (6)*
H10.245 (3)0.479 (2)0.336 (2)0.029 (6)*
H30.212 (5)−0.198 (3)0.319 (4)0.072 (11)*
H40.207 (5)−0.260 (3)0.426 (4)0.073 (12)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O50.0702 (13)0.0292 (10)0.0187 (9)−0.0027 (9)0.0122 (8)−0.0006 (7)
O20.0484 (10)0.0199 (8)0.0161 (7)0.0005 (7)0.0082 (7)−0.0016 (6)
O40.0211 (9)0.0356 (9)0.0487 (10)−0.0014 (7)0.0130 (7)−0.0179 (7)
N20.0281 (10)0.0171 (8)0.0193 (9)0.0012 (7)0.0034 (7)−0.0005 (7)
C40.0397 (14)0.0160 (10)0.0243 (11)0.0017 (9)0.0062 (10)0.0023 (9)
Ca10.0167 (2)0.0162 (2)0.0158 (2)−0.00003 (14)0.00219 (16)−0.00004 (14)
O30.0178 (7)0.0215 (7)0.0225 (7)−0.0037 (6)0.0035 (6)−0.0001 (6)
O10.0408 (9)0.0156 (7)0.0190 (8)0.0019 (6)0.0031 (7)−0.0007 (5)
C60.0179 (11)0.0208 (10)0.0110 (9)0.0009 (8)0.0002 (8)0.0008 (8)
C10.0176 (10)0.0175 (10)0.0164 (10)0.0002 (8)0.0028 (8)−0.0012 (8)
C50.0192 (10)0.0171 (10)0.0170 (10)−0.0008 (8)0.0016 (8)−0.0019 (8)
C20.0144 (10)0.0202 (10)0.0162 (10)−0.0013 (8)0.0023 (8)0.0003 (8)
N10.0292 (10)0.0233 (9)0.0184 (8)−0.0005 (7)0.0043 (7)0.0016 (7)
C30.0422 (14)0.0208 (11)0.0215 (12)−0.0003 (9)0.0055 (10)0.0059 (9)
Geometric parameters (Å, º) top
O5—Ca12.3774 (17)Ca1—O2iv2.3779 (14)
O5—H30.83 (4)Ca1—O32.5396 (14)
O5—H40.74 (4)Ca1—N2iv2.6159 (17)
O2—C51.248 (2)Ca1—Ca1iii3.9284 (8)
O2—Ca1i2.3779 (14)Ca1—H42.70 (4)
O4—C61.238 (2)O3—C61.260 (2)
O4—Ca1ii2.3233 (15)O3—Ca1iii2.3682 (14)
N2—C41.340 (3)O1—C51.246 (2)
N2—C11.341 (3)C6—C21.519 (3)
N2—Ca1i2.6159 (17)C1—C21.400 (3)
C4—C31.374 (3)C1—C51.520 (3)
C4—H20.88 (2)C2—N11.340 (2)
Ca1—O12.3111 (14)N1—C31.335 (3)
Ca1—O4ii2.3233 (15)C3—H10.96 (2)
Ca1—O3iii2.3682 (14)
Ca1—O5—H3139 (2)O3iii—Ca1—Ca1iii38.35 (3)
Ca1—O5—H4108 (3)O5—Ca1—Ca1iii88.95 (5)
H3—O5—H4112 (3)O2iv—Ca1—Ca1iii122.70 (4)
C5—O2—Ca1i127.64 (12)O3—Ca1—Ca1iii35.36 (3)
C6—O4—Ca1ii150.28 (14)N2iv—Ca1—Ca1iii114.42 (4)
C4—N2—C1117.46 (18)O1—Ca1—H4162.6 (8)
C4—N2—Ca1i126.65 (14)O4ii—Ca1—H488.1 (7)
C1—N2—Ca1i115.89 (12)O3iii—Ca1—H495.7 (7)
N2—C4—C3121.6 (2)O5—Ca1—H415.2 (8)
N2—C4—H2118.6 (14)O2iv—Ca1—H453.8 (8)
C3—C4—H2119.8 (14)O3—Ca1—H498.1 (8)
O1—Ca1—O4ii82.39 (5)N2iv—Ca1—H4117.9 (8)
O1—Ca1—O3iii94.17 (5)Ca1iii—Ca1—H498.7 (8)
O4ii—Ca1—O3iii176.20 (5)C6—O3—Ca1iii140.29 (13)
O1—Ca1—O5148.96 (6)C6—O3—Ca1104.26 (11)
O4ii—Ca1—O588.53 (7)Ca1iii—O3—Ca1106.29 (5)
O3iii—Ca1—O595.26 (6)C5—O1—Ca1158.12 (14)
O1—Ca1—O2iv140.60 (5)O4—C6—O3124.80 (18)
O4ii—Ca1—O2iv91.60 (6)O4—C6—C2117.25 (17)
O3iii—Ca1—O2iv90.05 (5)O3—C6—C2117.90 (16)
O5—Ca1—O2iv68.91 (6)N2—C1—C2120.60 (17)
O1—Ca1—O370.98 (5)N2—C1—C5115.63 (16)
O4ii—Ca1—O3106.51 (5)C2—C1—C5123.74 (17)
O3iii—Ca1—O373.71 (5)O1—C5—O2125.83 (18)
O5—Ca1—O383.45 (6)O1—C5—C1117.58 (17)
O2iv—Ca1—O3146.61 (5)O2—C5—C1116.55 (16)
O1—Ca1—N2iv77.47 (5)N1—C2—C1121.28 (18)
O4ii—Ca1—N2iv94.59 (6)N1—C2—C6114.43 (16)
O3iii—Ca1—N2iv83.05 (5)C1—C2—C6124.29 (17)
O5—Ca1—N2iv133.04 (6)C3—N1—C2117.23 (17)
O2iv—Ca1—N2iv64.18 (5)N1—C3—C4121.8 (2)
O3—Ca1—N2iv138.80 (5)N1—C3—H1115.3 (14)
O1—Ca1—Ca1iii80.40 (4)C4—C3—H1122.9 (14)
O4ii—Ca1—Ca1iii141.75 (5)
Symmetry codes: (i) −x+1/2, y+1/2, −z+3/2; (ii) −x+1, −y, −z+1; (iii) −x, −y, −z+1; (iv) −x+1/2, y−1/2, −z+3/2.

Experimental details

Crystal data
Chemical formula[Ca(C6H2N2O4)(H2O)]
Mr224.19
Crystal system, space groupMonoclinic, P21/n
Temperature (K)298
a, b, c (Å)6.8109 (7), 12.0469 (13), 9.9191 (11)
β (°) 102.333 (1)
V (Å3)795.08 (15)
Z4
Radiation typeMo Kα
µ (mm−1)0.79
Crystal size (mm)0.35 × 0.25 × 0.10
Data collection
DiffractometerBruker SMART APEX CCD
Absorption correctionMulti-scan
(SADABS; Bruker, 2005)
Tmin, Tmax0.770, 0.926
No. of measured, independent and
observed [I > 2σ(I)] reflections		3904, 1405, 1210
Rint0.026
(sin θ/λ)max (Å−1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.028, 0.075, 1.06
No. of reflections1405
No. of parameters143
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å−3)0.35, −0.34

Computer programs: SMART (Bruker, 2005), SAINT (Bruker, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

 

Acknowledgements

This work was supported by the Scientific Research Foundation of Ningxia University (No. (E)-nzdr09–5), the Natural Science Foundation of Ningxia Hui Autonomous Region (No. NZ1150) and the Special Program for Key Basic Research of the Ministry of Science and Technology, China (No. 2010CB534916).

References

First citationBruker (2005). SADABS, SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
 First citationChen, L. F., Li, Z. J., Qin, Y. Y., Cheng, J. K. & Yao, Y. G. (2008). J. Mol. Struct. 892, 278–282.  CSD CrossRef CAS Google Scholar
 First citationHu, M.-L., Yuan, J.-X., Chen, F. & Shi, Q. (2004). Acta Cryst. C60, m186–m188.  Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
 First citationKitaura, R., Fujimoto, K., Noro, S. I., Kondo, M. & Kitagawa, S. (2002). Angew. Chem. Int. Ed. 114, 141–143.  CrossRef Google Scholar
 First citationMa, Y., He, Y.-K. & Han, Z.-B. (2006). Acta Cryst. E62, m2528–m2529.  Web of Science CSD CrossRef IUCr Journals Google Scholar
 First citationSakagami-Yoshida, N., Teramoto, M., Hioki, A., Fuyuhiro, A. & Kaizaki, S. (2000). Inorg. Chem. 39, 5717–5724.  Web of Science PubMed CAS Google Scholar
 First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals Google Scholar
 First citationYin, H. (2009). Acta Cryst. E65, m971.  Web of Science CSD CrossRef IUCr Journals Google Scholar
 First citationZou, J. Z., Xu, Z., Chen, W., Lo, K. M. & You, X. Z. (1999). Polyhedron, 18, 1507–1512.  Web of Science CSD CrossRef CAS Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.

Journal logoCRYSTALLOGRAPHIC
COMMUNICATIONS
ISSN: 2056-9890
Volume 67| Part 12| December 2011| Page m1857
https://doi.org/10.1107/S1600536811050276
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