4-(Dimethyl­amino)­pyridinium trans-diaqua­bis­[oxalato(2−)-κ2O1,O2]chromate(III)

Nenwa, J.; Belombe, M.M.; Ngoune, J.; Fokwa, B.P.T.

doi:10.1107/S1600536810040353

metal-organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 66| Part 11| November 2010| Page m1410

https://doi.org/10.1107/S1600536810040353

Open

access

4-(Dimethylamino)pyridinium trans-diaquabis[oxalato(2−)-κ²O¹,O²]chromate(III)

Justin Nenwa,^a ^* Michel M. Belombe,^a Jean Ngoune ^b and Boniface P. T. Fokwa ^c

^aDepartment of Inorganic Chemistry, University of Yaounde I, POB 812 Yaounde, Cameroon, ^bDepartment of Chemistry, University of Dschang, POB 67, Dschang, Cameroon, and ^cInstitut für Anorganische Chemie, RWTH Aachen, D-52056 Aachen, Germany
^*Correspondence e-mail: jnenwa@yahoo.fr

(Received 28 September 2010; accepted 8 October 2010; online 13 October 2010)

In the title salt, (C₇H₁₁N₂)[Cr(C₂O₄)₂(H₂O)₂], the asymmetric unit contains one half-cation and one half-anion. The Cr atom, the C and N atoms involved in C— N(exocyclic) bonding and the N and H atoms of N—H groups lie on twofold rotation axis. The Cr^III atom of the complex anion is six-coordinated in a distorted (4 + 2) octahedral geometry with four equatorial O atoms of two nearly coplanar oxalate and two quasi-axial aqua O atoms. In the crystal, the protonated N atoms of the pyridine rings are hydrogen bonded to the carbonyl O atoms of the anions, forming chains along [010]. These chains are connected by lateral O—H⋯O hydrogen bonds, stabilizing the structure.

Related literature

For general background to the coordination chemistry of oxalate, see: Martin et al. (2007 ). For related structures, see: Bélombé et al. (2009 ); Ghouili et al. (2010 ).

Experimental

Crystal data

(C₇H₁₁N₂)[Cr(C₂O₄)₂(H₂O)₂]
M_r = 387.25
Monoclinic, C 2/c
a = 11.524 (4) Å
b = 20.372 (8) Å
c = 7.355 (2) Å
β = 120.626 (6)°
V = 1485.9 (9) Å³
Z = 4
Mo Kα radiation
μ = 0.83 mm⁻¹
T = 293 K
0.20 × 0.20 × 0.10 mm

Data collection

Bruker APEX CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2004 ) T_min = 0.895, T_max = 0.910
10203 measured reflections
1857 independent reflections
1739 reflections with I > 2σ(I)
R_int = 0.046

Refinement

R[F² > 2σ(F²)] = 0.033
wR(F²) = 0.089
S = 1.16
1857 reflections
127 parameters
2 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.37 e Å⁻³
Δρ_min = −0.38 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1A⋯O4ⁱ	0.82 (1)	1.91 (1)	2.719 (2)	171 (2)
O1—H1B⋯O5ⁱⁱ	0.80 (1)	1.91 (1)	2.680 (2)	160 (2)
N12—H12⋯O4ⁱⁱⁱ	0.86	2.19	2.906 (3)	141
N12—H12⋯O4^iv	0.86	2.19	2.906 (3)	141
Symmetry codes: (i) $[-x+{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]$ ; (ii) $[x, -y, z+{\script{1\over 2}}]$ ; (iii) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (iv) $[x+{\script{1\over 2}}, y-{\script{1\over 2}}, z]$ .

Data collection: SMART (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2010 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S1600536810040353/bx2312sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536810040353/bx2312Isup2.hkl

checkCIF report

Comment top

The coordination chemistry of oxalate (C₂O₄^2-) continues receiving considerable attention, due to the ability of this ion to act as a remarkably flexible ligand system in complexations with a wide range of metal ions [Martin et al.,2007]. Recently, we published the structure of an organic-inorganic hybrid salt involving the quinolinium cation, [C₉H₈N]⁺and complex anion, [Cr(H₂O)₂(C₂O₄)₂]^-(Bélombé et al., 2009). We report here the crystal structure of a homologous salt, with 4-Dimethylaminopyridinium as the organic cation. The Fig. 1 shows the 4-dimethylaminopyridinium cation, [C₇H₁₁N₂]⁺, and the complex anion, [Cr(H₂O)₂(C₂O₄)₂]^-.The asymmetric unit is formed by one-half cation and one-half anion. The geometrical parameters of the [C₇H₁₁N₂]⁺ cation are in agreement with those found in salts with the same cationic entity (Ghouili et al., 2010). The Cr^III ion of the complex anion adopts a distorted (4 + 2) octahedral coordination involving four equatorial O atoms (O2, O2ⁱ, O3, O3ⁱ) of two nearly coplanar oxalate and two quasi axial O atoms (O1, O1ⁱ) of water ligands (Fig. 1). The equatorial Cr–O distances are 1.9706 (13) Å (Cr–O(2), Cr–O(2ⁱ)) and 1.9468 (13) Å (Cr–O(3), Cr–O(3ⁱ)) respectively, and are significantly shorter than the axial Cr–O distance of 2.0055 (14)Å (Cr–O(1), Cr–O(1ⁱ)). The bond distances in the complex anion are comparable with those reported for the quinolinium compound (Bélombé et al., 2009).In the crystal structure, intermolecular N–H···O(carbonyl) hydrogen bonds connect the ionic entities, generating layers parallel to [010]. These layers are further connected by lateral O–H···O hydrogen bonds,stabilizing the structure (Table 1, Fig. 2)

Related literature top

For general background to the coordination chemistry of oxalate, see: Martin et al. (2007). For related structures, see: Bélombé et al. (2009); Ghouili et al. (2010).

Experimental top

A mixture of 4-dimethylaminopyridine (1 mmol, 122.2 mg) and oxalic acid (2.2 mmol, 277.2 mg) was dissolved in 30 ml of water. The filtered solution was stirred at 328 K and an aqueous solution (20 ml) of CrCl₃.6H₂O (1 mmol, 266.5 mg) was added in successive small portions and stirred for 2 h continuously. The final red-violet solution obtained was left at room temperature and brown plate-like crystals suitable for X-ray diffraction were obtained after a few days.

Structure description top

For general background to the coordination chemistry of oxalate, see: Martin et al. (2007). For related structures, see: Bélombé et al. (2009); Ghouili et al. (2010).

Computing details top

Data collection: SMART (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top

	Fig. 1. A view of the title compound with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Unlabelled atoms are related two labeled atoms by the symmetry code for cation and anion are: 1-x,y,1/2-z; -x,y,1/2-z
	Fig. 2. Packing diagram of the title compound. Dotted lines show hydrogen bonding.

4-(Dimethylamino)pyridinium trans-diaquabis[oxalato(2-)-κ²O¹,O²]chromate(III) top

Crystal data top

(C₇H₁₁N₂)[Cr(C₂O₄)₂(H₂O)₂]	F(000) = 796
M_r = 387.25	D_x = 1.731 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1857 reflections
a = 11.524 (4) Å	θ = 2.0–28.3°
b = 20.372 (8) Å	µ = 0.83 mm⁻¹
c = 7.355 (2) Å	T = 293 K
β = 120.626 (6)°	Prism, dark-violet
V = 1485.9 (9) Å³	0.20 × 0.20 × 0.10 mm
Z = 4

Data collection top

Bruker APEX CCD area-detector diffractometer	1857 independent reflections
Radiation source: fine-focus sealed tube	1739 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.046
integration method scans	θ_max = 28.3°, θ_min = 2.0°
Absorption correction: multi-scan (SADABS; Bruker, 2004)	h = −15→15
T_min = 0.895, T_max = 0.910	k = −27→27
10203 measured reflections	l = −9→9

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.033	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.089	w = 1/[σ²(F_o²) + (0.0395P)² + 1.1284P] where P = (F_o² + 2F_c²)/3
S = 1.16	(Δ/σ)_max < 0.001
1857 reflections	Δρ_max = 0.37 e Å⁻³
127 parameters	Δρ_min = −0.38 e Å⁻³
2 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0065 (6)

Crystal data top

(C₇H₁₁N₂)[Cr(C₂O₄)₂(H₂O)₂]	V = 1485.9 (9) Å³
M_r = 387.25	Z = 4
Monoclinic, C2/c	Mo Kα radiation
a = 11.524 (4) Å	µ = 0.83 mm⁻¹
b = 20.372 (8) Å	T = 293 K
c = 7.355 (2) Å	0.20 × 0.20 × 0.10 mm
β = 120.626 (6)°

Data collection top

Bruker APEX CCD area-detector diffractometer	1857 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2004)	1739 reflections with I > 2σ(I)
T_min = 0.895, T_max = 0.910	R_int = 0.046
10203 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.033	2 restraints
wR(F²) = 0.089	H atoms treated by a mixture of independent and constrained refinement
S = 1.16	Δρ_max = 0.37 e Å⁻³
1857 reflections	Δρ_min = −0.38 e Å⁻³
127 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

	x	y	z	U_iso*/U_eq
Cr1	0.0000	0.148676 (17)	0.2500	0.02141 (14)
O1	0.10432 (13)	0.15066 (6)	0.5670 (2)	0.0278 (3)
H1A	0.1841 (11)	0.1603 (11)	0.618 (4)	0.038 (6)*
H1B	0.097 (2)	0.1194 (8)	0.626 (3)	0.039 (6)*
O2	0.11598 (12)	0.22143 (6)	0.2606 (2)	0.0273 (3)
O3	0.10971 (12)	0.07688 (6)	0.2448 (2)	0.0275 (3)
O4	0.12757 (13)	0.33040 (6)	0.2756 (2)	0.0324 (3)
O5	0.11585 (14)	−0.03196 (7)	0.2324 (2)	0.0394 (4)
C1	0.06468 (17)	0.01913 (8)	0.2430 (3)	0.0259 (4)
C2	0.07080 (16)	0.27821 (8)	0.2608 (3)	0.0236 (3)
N11	0.5000	0.15711 (11)	0.2500	0.0374 (5)
N12	0.5000	−0.04417 (12)	0.2500	0.0393 (6)
H12	0.5000	−0.0864	0.2500	0.096 (17)*
C11	0.3808 (3)	0.19340 (12)	0.2074 (5)	0.0594 (7)
H11C	0.3998	0.2396	0.2191	0.121 (15)*
H11D	0.3548	0.1811	0.3077	0.088 (11)*
H11E	0.3087	0.1836	0.0671	0.077 (10)*
C12	0.5000	0.09124 (12)	0.2500	0.0263 (5)
C13	0.38704 (18)	0.05453 (10)	0.2186 (3)	0.0341 (4)
H13	0.3097	0.0760	0.1967	0.049 (7)*
C14	0.3915 (2)	−0.01145 (10)	0.2204 (3)	0.0394 (5)
H14	0.3167	−0.0349	0.2005	0.070 (9)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cr1	0.0191 (2)	0.0153 (2)	0.0318 (2)	0.000	0.01434 (16)	0.000
O1	0.0272 (7)	0.0222 (6)	0.0332 (7)	−0.0025 (5)	0.0147 (6)	0.0010 (5)
O2	0.0237 (6)	0.0200 (6)	0.0417 (7)	−0.0003 (4)	0.0192 (6)	0.0007 (5)
O3	0.0236 (6)	0.0222 (6)	0.0400 (7)	0.0021 (5)	0.0186 (5)	−0.0004 (5)
O4	0.0275 (6)	0.0205 (6)	0.0477 (8)	−0.0031 (5)	0.0180 (6)	0.0019 (5)
O5	0.0379 (8)	0.0254 (7)	0.0459 (8)	0.0093 (5)	0.0148 (6)	−0.0056 (6)
C1	0.0242 (8)	0.0219 (8)	0.0252 (8)	0.0029 (6)	0.0080 (7)	−0.0011 (6)
C2	0.0216 (8)	0.0216 (8)	0.0258 (8)	0.0007 (6)	0.0110 (6)	0.0014 (6)
N11	0.0452 (14)	0.0244 (11)	0.0443 (14)	0.000	0.0240 (12)	0.000
N12	0.0497 (15)	0.0244 (12)	0.0397 (13)	0.000	0.0198 (11)	0.000
C11	0.0716 (18)	0.0393 (13)	0.0646 (16)	0.0220 (12)	0.0327 (15)	−0.0016 (11)
C12	0.0265 (12)	0.0271 (12)	0.0259 (11)	0.000	0.0138 (10)	0.000
C13	0.0248 (9)	0.0395 (10)	0.0398 (10)	−0.0006 (7)	0.0178 (8)	−0.0021 (8)
C14	0.0376 (11)	0.0398 (11)	0.0399 (11)	−0.0139 (8)	0.0190 (9)	−0.0035 (8)

Geometric parameters (Å, º) top

Cr1—O3ⁱ	1.9466 (13)	N11—C12	1.342 (3)
Cr1—O3	1.9466 (13)	N11—C11ⁱⁱ	1.447 (3)
Cr1—O2	1.9706 (13)	N11—C11	1.447 (3)
Cr1—O2ⁱ	1.9706 (13)	N12—C14ⁱⁱ	1.333 (3)
Cr1—O1ⁱ	2.0067 (14)	N12—C14	1.333 (3)
Cr1—O1	2.0067 (14)	N12—H12	0.8600
O1—H1A	0.821 (10)	C11—H11C	0.9600
O1—H1B	0.802 (10)	C11—H11D	0.9600
O2—C2	1.269 (2)	C11—H11E	0.9600
O3—C1	1.283 (2)	C12—C13	1.414 (2)
O4—C2	1.224 (2)	C12—C13ⁱⁱ	1.414 (2)
O5—C1	1.218 (2)	C13—C14	1.345 (3)
C1—C1ⁱ	1.546 (4)	C13—H13	0.9300
C2—C2ⁱ	1.557 (3)	C14—H14	0.9300

O3ⁱ—Cr1—O3	82.57 (8)	O4—C2—C2ⁱ	119.61 (10)
O3ⁱ—Cr1—O2	177.08 (5)	O2—C2—C2ⁱ	114.16 (9)
O3—Cr1—O2	97.57 (6)	C12—N11—C11ⁱⁱ	120.73 (15)
O3ⁱ—Cr1—O2ⁱ	97.57 (6)	C12—N11—C11	120.73 (15)
O3—Cr1—O2ⁱ	177.08 (5)	C11ⁱⁱ—N11—C11	118.5 (3)
O2—Cr1—O2ⁱ	82.45 (7)	C14ⁱⁱ—N12—C14	120.0 (3)
O3ⁱ—Cr1—O1ⁱ	91.57 (5)	C14ⁱⁱ—N12—H12	120.0
O3—Cr1—O1ⁱ	90.16 (5)	C14—N12—H12	120.0
O2—Cr1—O1ⁱ	91.35 (5)	N11—C11—H11C	109.5
O2ⁱ—Cr1—O1ⁱ	86.91 (5)	N11—C11—H11D	109.5
O3ⁱ—Cr1—O1	90.16 (5)	H11C—C11—H11D	109.5
O3—Cr1—O1	91.57 (5)	N11—C11—H11E	109.5
O2—Cr1—O1	86.91 (5)	H11C—C11—H11E	109.5
O2ⁱ—Cr1—O1	91.35 (5)	H11D—C11—H11E	109.5
O1ⁱ—Cr1—O1	177.70 (7)	N11—C12—C13	121.92 (12)
Cr1—O1—H1A	114.1 (17)	N11—C12—C13ⁱⁱ	121.92 (12)
Cr1—O1—H1B	116.7 (17)	C13—C12—C13ⁱⁱ	116.2 (2)
H1A—O1—H1B	110 (2)	C14—C13—C12	119.97 (19)
C2—O2—Cr1	114.53 (11)	C14—C13—H13	120.0
C1—O3—Cr1	115.20 (11)	C12—C13—H13	120.0
O5—C1—O3	125.28 (17)	N12—C14—C13	121.95 (19)
O5—C1—C1ⁱ	121.26 (11)	N12—C14—H14	119.0
O3—C1—C1ⁱ	113.46 (9)	C13—C14—H14	119.0
O4—C2—O2	126.23 (15)

O3ⁱ—Cr1—O2—C2	−89.0 (10)	Cr1—O3—C1—C1ⁱ	−3.0 (2)
O3—Cr1—O2—C2	178.46 (12)	Cr1—O2—C2—O4	176.72 (14)
O2ⁱ—Cr1—O2—C2	1.40 (9)	Cr1—O2—C2—C2ⁱ	−3.4 (2)
O1ⁱ—Cr1—O2—C2	88.11 (12)	C11ⁱⁱ—N11—C12—C13	177.00 (16)
O1—Cr1—O2—C2	−90.37 (12)	C11—N11—C12—C13	−3.00 (16)
O3ⁱ—Cr1—O3—C1	1.22 (9)	C11ⁱⁱ—N11—C12—C13ⁱⁱ	−3.00 (16)
O2—Cr1—O3—C1	178.27 (12)	C11—N11—C12—C13ⁱⁱ	177.00 (16)
O2ⁱ—Cr1—O3—C1	−91.7 (10)	N11—C12—C13—C14	−179.79 (14)
O1ⁱ—Cr1—O3—C1	−90.34 (12)	C13ⁱⁱ—C12—C13—C14	0.21 (14)
O1—Cr1—O3—C1	91.18 (12)	C14ⁱⁱ—N12—C14—C13	0.22 (15)
Cr1—O3—C1—O5	177.34 (15)	C12—C13—C14—N12	−0.4 (3)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O4ⁱⁱⁱ	0.82 (1)	1.91 (1)	2.719 (2)	171 (2)
O1—H1B···O5^iv	0.80 (1)	1.91 (1)	2.680 (2)	160 (2)
N12—H12···O4^v	0.86	2.19	2.906 (3)	141
N12—H12···O4^vi	0.86	2.19	2.906 (3)	141

Symmetry codes: (iii) −x+1/2, −y+1/2, −z+1; (iv) x, −y, z+1/2; (v) −x+1/2, y−1/2, −z+1/2; (vi) x+1/2, y−1/2, z.

Experimental details

Crystal data
Chemical formula	(C₇H₁₁N₂)[Cr(C₂O₄)₂(H₂O)₂]
M_r	387.25
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	293
a, b, c (Å)	11.524 (4), 20.372 (8), 7.355 (2)
β (°)	120.626 (6)
V (Å³)	1485.9 (9)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.83
Crystal size (mm)	0.20 × 0.20 × 0.10

Data collection
Diffractometer	Bruker APEX CCD area-detector
Absorption correction	Multi-scan (SADABS; Bruker, 2004)
T_min, T_max	0.895, 0.910
No. of measured, independent and observed [I > 2σ(I)] reflections	10203, 1857, 1739
R_int	0.046
(sin θ/λ)_max (Å⁻¹)	0.668

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.033, 0.089, 1.16
No. of reflections	1857
No. of parameters	127
No. of restraints	2
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.37, −0.38

Computer programs: SMART (Bruker, 2004), SAINT (Bruker, 2004), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2010), WinGX (Farrugia, 1999).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O4ⁱ	0.821 (10)	1.905 (11)	2.719 (2)	171 (2)
O1—H1B···O5ⁱⁱ	0.802 (10)	1.912 (12)	2.680 (2)	160 (2)
N12—H12···O4ⁱⁱⁱ	0.86	2.19	2.906 (3)	140.8
N12—H12···O4^iv	0.86	2.19	2.906 (3)	140.8

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

Acknowledgements

The authors thank Pr. Barthelemy Nyasse (Organic Chemistry Department, University of Yaounde I) for the donation of 4-dimethylaminopyridine and Klaus Kruse (RWTH Aachen) for his technical support during the X-ray experiments.

References

Bélombé, M. M., Nenwa, J. & Emmerling, F. (2009). Z. Kristallogr. New Cryst. Struct. 224, 239–240. Google Scholar
Brandenburg, K. (2010). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2004). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Ghouili, A., Chaari, N. & Zouari, F. (2010). X-ray Struct. Anal. Online, 26, x21–x22. CSD CrossRef Google Scholar
Martin, L., Day, P., Clegg, W., Harrington, R. W., Horton, P. N., Bingham, A., Hursthouse, M. B., McMillan, P. & Firth, S. (2007). J. Mater. Chem. 17, 3324–3329. Web of Science CSD CrossRef CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals 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.