Tetra­kis(di­propyl­ammonium) tetra­kis(oxa­lato-κ2O1,O2)stannate(IV) mono­hydrate: a complex with an eight-coordinate SnIV atom

Gueye, N.; Diop, L.; Stoeckli-Evans, H.

doi:10.1107/S160053681303496X

metal-organic compounds

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Volume 70| Part 2| February 2014| Pages m49-m50

doi:10.1107/S160053681303496X

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Tetrakis(dipropylammonium) tetrakis(oxalato-κ²O¹,O²)stannate(IV) monohydrate: a complex with an eight-coordinate Sn^IV atom

Ndongo Gueye,^a ^* Libasse Diop ^a and Helen Stoeckli-Evans ^b

^aLaboratoire de Chimie Minerale et Analytique, Departement de Chimie, Faculte des Sciences et Techniques, Universite Cheikh Anta Diop, Dakar, Senegal, and ^bInstitute of Physics, University of Neuchâtel, rue Emile-Argand 11, CH-2000 Neuchâtel, Switzerland
^*Correspondence e-mail: ndongo1982@gmail.com

(Received 10 December 2013; accepted 30 December 2013; online 18 January 2014)

In the title salt, [(CH₃CH₂CH₂)₂NH₂]₄[Sn(C₂O₄)₄]·H₂O, the Sn^IV atom of the stannate anion is located on a special position with -42m symmetry. It is eight-coordinated by four chelating oxalate anions. The dipropylammonium cation possesses mirror symmetry while the lattice water molecule is disordered about a position with -42m symmetry and has an occupancy of 0.25. In the crystal, the anions and cations are linked by N—H⋯O hydrogen bonds, forming a three-dimensional network. This network is futher stabilized by weak O—H⋯O hydrogen bonds involving the water molecules and oxalate O atoms. The crystal studied was refined as an inversion twin.

CCDC reference: 979175

Related literature

For the chemistry of organotin complexes, see: Evans & Karpel (1985 ). For examples of zirconate anions with eight-coordinate Zr^IV atoms, see: Fu et al. (2005 ); Imaz et al. (2007 ). For an example of a related oxalatostannate(IV) complex, see: Gueye et al. (2010 ).

Experimental

Crystal data

(C₆H₁₆N)₄[Sn(C₂O₄)₄]·H₂O
M_r = 897.57
Tetragonal, $[I \overline 42m ]$
a = 14.5996 (6) Å
c = 9.5718 (4) Å
V = 2040.21 (19) Å³
Z = 2
Mo Kα radiation
μ = 0.70 mm⁻¹
T = 173 K
0.45 × 0.30 × 0.22 mm

Data collection

STOE IPDS2 diffractometer
Absorption correction: multi-scan (MULscanABS in PLATON; Spek, 2009 )' T_min = 0.540, T_max = 1.000
9636 measured reflections
1030 independent reflections
1027 reflections with I > 2σ(I)
R_int = 0.071

Refinement

R[F² > 2σ(F²)] = 0.021
wR(F²) = 0.054
S = 1.08
1030 reflections
83 parameters
3 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.39 e Å⁻³
Δρ_min = −0.62 e Å⁻³
Absolute structure: Refined as an inversion twin
Absolute structure parameter: 0.45 (4)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1AN⋯O1ⁱ	0.88 (3)	2.50 (5)	3.091 (6)	125 (5)
N1—H1AN⋯O2ⁱ	0.88 (3)	2.12 (3)	2.997 (8)	179 (6)
N1—H1BN⋯O2	0.88 (3)	2.02 (4)	2.821 (7)	151 (5)
N1—H1BN⋯O4	0.88 (3)	2.44 (5)	3.105 (6)	133 (5)
O1W—H1WA⋯O3ⁱⁱ	0.85	2.27	3.125 (7)	179
Symmetry codes: (i) $[-x+{\script{1\over 2}}, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (ii) $[x-{\script{1\over 2}}, y-{\script{1\over 2}}, z+{\script{1\over 2}}]$ .

Data collection: X-AREA (Stoe & Cie, 2009 ); cell refinement: X-AREA (Stoe & Cie, 2009); data reduction: X-RED32 (Stoe & Cie, 2009); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2006 ); software used to prepare material for publication: SHELXL2013 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 979175

Crystal structure: contains datablocks I, global. DOI: 10.1107/S160053681303496X/wm2795sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681303496X/wm2795Isup2.hkl

checkCIF report

Comment top

The chemistry and applications of organotin(IV) complexes have been extensively discussed by Evans & Karpel (1985). Continuing our interest in Sn^IV oxalate complexes (Gueye et al., 2010), we studied the reaction of dipropylammonium oxalate with SnBr₂, and report herein on the crystal structure of the title compound, ((CH₃CH₂CH₂)₂NH₂)₄[Sn(C₂O₄)₄]^.H₂O.

The molecular structure of the title salt is illustrated in Fig. 1. The Sn^IV atom of the stannate anion is located on a special position with 42m symmetry. It is chelated by four bidentate oxalate ions, each lying in a mirror plane, and hence has a coordination number of eight. This coordination number has also been reported for some zirconium complexes, viz. bis(4,4'-bipyridinium) tetrakis(oxalato-κ²O,O')zirconate(IV) (Fu et al., 2005), or several salts with general composition [(H₂amine)₂Zr(C₂O₄)₄] (Imaz et al., 2007).

The Sn1—O1 and Sn1—O3 bond lengths in the anion of the title compound are 2.142 (3) and 2.226 (3) Å, respectively. These values are similar to the bond lengths [2.189 (2) and 2.229 (2) Å] observed for another tin(IV)-oxalato complex, bis(dicyclohexylammonium)µ-oxalato-κ⁴O1,O2:O1',O2'-bis[aqua(oxalato -κ²O1,O2)diphenylstannate(IV)] (Gueye et al., 2010).

In the crystal of the title salt, the stannate(IV) anions are linked via N—H···O hydrogen bonds to the [(CH₃CH₂CH₂)₂NH₂]⁺ cations (which have mirror symmetry), forming a three-dimensional network. The water molecule (disordered about a position with 42m symmetry), is also involved in weak O—H···O hydrogen bonds with the stannate(IV) anions, hence futher stabilizing the three-dimensional network (Fig. 2).

Related literature top

For the chemistry of organotin complexes, see: Evans & Karpel (1985). For examples of zirconate anions with eight-coordinate Zr^IV atoms, see: Fu et al. (2005); Imaz et al. (2007). For an example of a related oxalatostannate(IV) complex, see: Gueye et al. (2010).

Experimental top

The title compound was prepared by reacting in a 1:1 molar ratio of SnBr₂ and (Pr₂NH₂)₂·C₂O₄ in methanol. The solution was allowed to stand and yielded colourless block-like crystals of the title compound. The proof of the presence of the tin atom was confirmed by the structure analysis and by electron dispersive X-ray (EDX) analysis.

Structure description top

For the chemistry of organotin complexes, see: Evans & Karpel (1985). For examples of zirconate anions with eight-coordinate Zr^IV atoms, see: Fu et al. (2005); Imaz et al. (2007). For an example of a related oxalatostannate(IV) complex, see: Gueye et al. (2010).

Computing details top

Data collection: X-AREA (Stoe & Cie, 2009); cell refinement: X-AREA (Stoe & Cie, 2009); data reduction: X-RED32 (Stoe & Cie, 2009); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL2013 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Figures top

	Fig. 1. A view of the molecular components of the title salt. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A view approximately along the c axis of the crystal packing of the title compound. The hydrogen bonds are shown as dashed lines (see Table 1 for details; C-bound H atoms have been omitted for clarity).

Tetrakis(dipropylammonium) tetrakis(oxalato-κ²O¹,O²)stannate(IV) top

Crystal data top

(C₆H₁₆N)₄[Sn(C₂O₄)₄]·H₂O	D_x = 1.461 Mg m⁻³
M_r = 897.57	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I42m	Cell parameters from 16495 reflections
a = 14.5996 (6) Å	θ = 2.0–26.1°
c = 9.5718 (4) Å	µ = 0.70 mm⁻¹
V = 2040.21 (19) Å³	T = 173 K
Z = 2	Rod, colourless
F(000) = 944	0.45 × 0.30 × 0.22 mm

Data collection top

STOE IPDS2 diffractometer	1030 independent reflections
Radiation source: fine-focus sealed tube	1027 reflections with I > 2σ(I)
Plane graphite monochromator	R_int = 0.071
φ + ω scans	θ_max = 25.6°, θ_min = 2.0°
Absorption correction: multi-scan (MULscanABS in PLATON; Spek, 2009)'	h = −17→17
T_min = 0.540, T_max = 1.000	k = −17→17
9636 measured reflections	l = −10→11

Refinement top

Refinement on F²	Secondary atom site location: inferred from neighbouring sites
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.054	w = 1/[σ²(F_o²) + (0.030P)² + 1.5457P] where P = (F_o² + 2F_c²)/3
S = 1.08	(Δ/σ)_max < 0.001
1030 reflections	Δρ_max = 0.39 e Å⁻³
83 parameters	Δρ_min = −0.62 e Å⁻³
3 restraints	Absolute structure: Refined as an inversion twin
Primary atom site location: difference Fourier map	Absolute structure parameter: 0.45 (4)

Crystal data top

(C₆H₁₆N)₄[Sn(C₂O₄)₄]·H₂O	Z = 2
M_r = 897.57	Mo Kα radiation
Tetragonal, I42m	µ = 0.70 mm⁻¹
a = 14.5996 (6) Å	T = 173 K
c = 9.5718 (4) Å	0.45 × 0.30 × 0.22 mm
V = 2040.21 (19) Å³

Data collection top

STOE IPDS2 diffractometer	1030 independent reflections
Absorption correction: multi-scan (MULscanABS in PLATON; Spek, 2009)'	1027 reflections with I > 2σ(I)
T_min = 0.540, T_max = 1.000	R_int = 0.071
9636 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.054	Δρ_max = 0.39 e Å⁻³
S = 1.08	Δρ_min = −0.62 e Å⁻³
1030 reflections	Absolute structure: Refined as an inversion twin
83 parameters	Absolute structure parameter: 0.45 (4)
3 restraints

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. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Sn1	0.5000	0.5000	0.5000	0.01816 (16)
O1	0.40111 (16)	0.40111 (16)	0.5675 (4)	0.0298 (8)
O2	0.29743 (14)	0.29743 (14)	0.5079 (8)	0.0356 (7)
O3	0.43891 (16)	0.43891 (16)	0.3084 (3)	0.0281 (7)
O4	0.3366 (2)	0.3366 (2)	0.2305 (4)	0.0436 (10)
C1	0.3559 (2)	0.3559 (2)	0.4778 (6)	0.0276 (13)
C2	0.3776 (2)	0.3776 (2)	0.3231 (5)	0.0249 (9)
N1	0.1921 (2)	0.1921 (2)	0.3202 (5)	0.0289 (9)
H1AN	0.195 (3)	0.195 (3)	0.228 (3)	0.035*
H1BN	0.2322 (15)	0.2322 (15)	0.350 (6)	0.035*
C3	0.0985 (3)	0.2174 (3)	0.3686 (4)	0.0369 (8)
H3A	0.0947	0.2087	0.4711	0.044*
H3B	0.0532	0.1760	0.3245	0.044*
C4	0.0741 (3)	0.3159 (3)	0.3335 (4)	0.0475 (10)
H4A	0.0160	0.3315	0.3811	0.057*
H4B	0.1223	0.3564	0.3720	0.057*
C5	0.0637 (3)	0.3361 (3)	0.1801 (4)	0.0497 (10)
H5A	0.1232	0.3296	0.1337	0.075*
H5B	0.0413	0.3989	0.1679	0.075*
H5C	0.0199	0.2931	0.1388	0.075*
O1W	0.0000	−0.026 (3)	0.5000	0.091 (15)	0.25
H1WA	−0.0167	−0.0355	0.5840	0.136*	0.25

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Sn1	0.02109 (19)	0.02109 (19)	0.0123 (2)	0.000	0.000	0.000
O1	0.0352 (12)	0.0352 (12)	0.0190 (15)	−0.0117 (16)	−0.0020 (10)	−0.0020 (10)
O2	0.0417 (10)	0.0417 (10)	0.0234 (17)	−0.0187 (13)	−0.0001 (18)	−0.0001 (18)
O3	0.0346 (11)	0.0346 (11)	0.0151 (14)	−0.0089 (14)	−0.0026 (9)	−0.0026 (9)
O4	0.0548 (17)	0.0548 (17)	0.0211 (19)	−0.026 (2)	−0.0044 (12)	−0.0044 (12)
C1	0.0298 (13)	0.0298 (13)	0.023 (4)	−0.0010 (17)	−0.0020 (15)	−0.0020 (15)
C2	0.0273 (14)	0.0273 (14)	0.020 (2)	−0.0032 (18)	−0.0006 (13)	−0.0006 (13)
N1	0.0323 (14)	0.0323 (14)	0.022 (2)	−0.0083 (18)	−0.0029 (13)	−0.0029 (13)
C3	0.0356 (19)	0.049 (2)	0.0262 (17)	0.0009 (16)	0.0026 (14)	0.0014 (16)
C4	0.056 (3)	0.054 (3)	0.033 (2)	0.015 (2)	−0.0008 (19)	−0.0046 (18)
C5	0.064 (3)	0.051 (3)	0.035 (2)	0.002 (2)	0.000 (2)	0.0054 (18)
O1W	0.15 (5)	0.065 (15)	0.057 (7)	0.000	0.01 (3)	0.000

Geometric parameters (Å, º) top

Sn1—O1ⁱ	2.142 (3)	N1—H1AN	0.88 (3)
Sn1—O1ⁱⁱ	2.142 (3)	N1—H1BN	0.88 (3)
Sn1—O1ⁱⁱⁱ	2.142 (3)	C3—C4	1.519 (6)
Sn1—O1	2.142 (3)	C3—H3A	0.9900
Sn1—O3	2.226 (3)	C3—H3B	0.9900
Sn1—O3ⁱⁱ	2.226 (3)	C4—C5	1.506 (5)
Sn1—O3ⁱⁱⁱ	2.226 (3)	C4—H4A	0.9900
Sn1—O3ⁱ	2.226 (3)	C4—H4B	0.9900
O1—C1	1.269 (6)	C5—H5A	0.9800
O2—C1	1.240 (6)	C5—H5B	0.9800
O3—C2	1.274 (6)	C5—H5C	0.9800
O4—C2	1.225 (6)	O1W—O1W^v	0.55 (6)
C1—C2	1.547 (7)	O1W—O1W^vi	0.55 (6)
N1—C3	1.490 (4)	O1W—O1W^vii	0.77 (8)
N1—C3^iv	1.490 (4)	O1W—H1WA	0.8505

O1ⁱ—Sn1—O1ⁱⁱ	95.23 (5)	O4—C2—O3	127.3 (4)
O1ⁱ—Sn1—O1ⁱⁱⁱ	95.23 (5)	O4—C2—C1	119.5 (4)
O1ⁱⁱ—Sn1—O1ⁱⁱⁱ	144.86 (18)	O3—C2—C1	113.2 (4)
O1ⁱ—Sn1—O1	144.86 (18)	C3—N1—C3^iv	111.0 (4)
O1ⁱⁱ—Sn1—O1	95.23 (5)	C3—N1—H1AN	110 (2)
O1ⁱⁱⁱ—Sn1—O1	95.23 (5)	C3^iv—N1—H1AN	110 (2)
O1ⁱ—Sn1—O3	142.09 (12)	C3—N1—H1BN	110.1 (19)
O1ⁱⁱ—Sn1—O3	75.60 (8)	C3^iv—N1—H1BN	110.1 (19)
O1ⁱⁱⁱ—Sn1—O3	75.60 (8)	H1AN—N1—H1BN	106 (6)
O1—Sn1—O3	73.05 (13)	N1—C3—C4	112.4 (3)
O1ⁱ—Sn1—O3ⁱⁱ	75.60 (8)	N1—C3—H3A	109.1
O1ⁱⁱ—Sn1—O3ⁱⁱ	73.05 (13)	C4—C3—H3A	109.1
O1ⁱⁱⁱ—Sn1—O3ⁱⁱ	142.09 (12)	N1—C3—H3B	109.1
O1—Sn1—O3ⁱⁱ	75.60 (8)	C4—C3—H3B	109.1
O3—Sn1—O3ⁱⁱ	132.75 (11)	H3A—C3—H3B	107.9
O1ⁱ—Sn1—O3ⁱⁱⁱ	75.60 (8)	C5—C4—C3	115.2 (4)
O1ⁱⁱ—Sn1—O3ⁱⁱⁱ	142.09 (12)	C5—C4—H4A	108.5
O1ⁱⁱⁱ—Sn1—O3ⁱⁱⁱ	73.05 (13)	C3—C4—H4A	108.5
O1—Sn1—O3ⁱⁱⁱ	75.60 (8)	C5—C4—H4B	108.5
O3—Sn1—O3ⁱⁱⁱ	132.75 (11)	C3—C4—H4B	108.5
O3ⁱⁱ—Sn1—O3ⁱⁱⁱ	69.05 (17)	H4A—C4—H4B	107.5
O1ⁱ—Sn1—O3ⁱ	73.05 (13)	C4—C5—H5A	109.5
O1ⁱⁱ—Sn1—O3ⁱ	75.60 (8)	C4—C5—H5B	109.5
O1ⁱⁱⁱ—Sn1—O3ⁱ	75.60 (8)	H5A—C5—H5B	109.5
O1—Sn1—O3ⁱ	142.09 (12)	C4—C5—H5C	109.5
O3—Sn1—O3ⁱ	69.05 (17)	H5A—C5—H5C	109.5
O3ⁱⁱ—Sn1—O3ⁱ	132.75 (11)	H5B—C5—H5C	109.5
O3ⁱⁱⁱ—Sn1—O3ⁱ	132.75 (11)	O1W^v—O1W—O1W^vi	90.004 (7)
C1—O1—Sn1	119.8 (3)	O1W^v—O1W—O1W^vii	45.002 (4)
C2—O3—Sn1	118.2 (3)	O1W^vi—O1W—O1W^vii	45.002 (4)
O2—C1—O1	124.0 (6)	O1W^v—O1W—H1WA	108.2
O2—C1—C2	120.3 (5)	O1W^vi—O1W—H1WA	84.7
O1—C1—C2	115.7 (4)	O1W^vii—O1W—H1WA	98.9

Sn1—O1—C1—O2	180.000 (1)	O1—C1—C2—O4	180.000 (1)
Sn1—O1—C1—C2	0.000 (1)	O2—C1—C2—O3	180.000 (1)
Sn1—O3—C2—O4	180.000 (1)	O1—C1—C2—O3	0.000 (1)
Sn1—O3—C2—C1	0.000 (1)	C3^iv—N1—C3—C4	174.3 (3)
O2—C1—C2—O4	0.000 (1)	N1—C3—C4—C5	67.2 (5)

Symmetry codes: (i) −x+1, −y+1, z; (ii) −x+1, y, −z+1; (iii) x, −y+1, −z+1; (iv) y, x, z; (v) −y, −x, z; (vi) y, −x, −z+1; (vii) x, −y, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1AN···O1^viii	0.88 (3)	2.50 (5)	3.091 (6)	125 (5)
N1—H1AN···O2^viii	0.88 (3)	2.12 (3)	2.997 (8)	179 (6)
N1—H1BN···O2	0.88 (3)	2.02 (4)	2.821 (7)	151 (5)
N1—H1BN···O4	0.88 (3)	2.44 (5)	3.105 (6)	133 (5)
O1W—H1WA···O3^ix	0.85	2.27	3.125 (7)	179

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1AN···O1ⁱ	0.88 (3)	2.50 (5)	3.091 (6)	125 (5)
N1—H1AN···O2ⁱ	0.88 (3)	2.12 (3)	2.997 (8)	179 (6)
N1—H1BN···O2	0.88 (3)	2.02 (4)	2.821 (7)	151 (5)
N1—H1BN···O4	0.88 (3)	2.44 (5)	3.105 (6)	133 (5)
O1W—H1WA···O3ⁱⁱ	0.85	2.27	3.125 (7)	179

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

Acknowledgements

HSE thanks the XRD Application Laboratory, CSEM, Neuchâtel for access to the X-ray diffraction equipment.

References

Evans, C. J. & Karpel, S. (1985). Organotin Compounds in Modern Technology, J. Organomet. Chem. Library, Vol. 16. Amsterdam: Elsevier. Google Scholar
Fu, Y.-L., Ren, J.-L., Xu, Z.-W. & Ng, S. W. (2005). Acta Cryst. E61, m2397–m2399. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Gueye, N., Diop, L., Molloy, K. C. K. & Kociok-Köhn, G. (2010). Acta Cryst. E66, m1645–m1646. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Imaz, I., Thillet, A. & Sutter, J.-P. (2007). Cryst. Growth Des. 7, 1753–1761. Web of Science CSD CrossRef CAS Google Scholar
Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Stoe & Cie. (2009). X-AREA and X-RED32. Stoe & Cie GmbH, Darmstadt, Germany. Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar

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