Two 2-D Cd(II) complexes constructed from N-heterocyclic and aromatic polycarboxylate ligands

Article abstract of DOI:10.1016/j.molstruc.2010.09.040

Two new complexes, {[Cd(bimt)(bdc)(H₂O)]·2H ₂O}_n (1) and [Cd(bimt)(Hbtc)]_n (2) have been synthesized through the self-assembly of Cd(NO₃)₂· 4H₂O with unsymmetrical N-heterocyclic ligand 2-((benzoimidazol-yl) methyl)-1H-tetrazole (bimt) and aromatic polycarboxylate acid 1,3-benzenedicarboxylic acid (H₂bdc) or 1,3,5-benzenetricarboxylic acid (H₃btc). Single crystal X-ray diffraction determination shows that complex 1 possesses an infinite 2-D layered structure with parallelogram grid units. Complex 2 is a 2-D layered structure containing 1-D ladder-like chains linked by the Hbtc^2- anions in tridentate bridging fashion. Both complexes are stacked into complicated 3-D frameworks by coordinating bonds and hydrogen bonds in the solid state. In addition, the luminescent properties of both complexes have been investigated in the solid state at room temperature.

Full text of DOI:10.1016/j.molstruc.2010.09.040

Journal of Molecular Structure 984 (2010) 276–281
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Two 2-D Cd(II) complexes constructed from N-heterocyclic and aromatic
polycarboxylate ligands
a
Jun Zhang a, Baoding Li a, Xinjuan Wu a, Huaixia Yang b, Wan Zhou a, Xiangru Meng a, , Hongwei Hou
⇑
a Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
b Pharmacy College, Henan University of Traditional Chinese Medicine, Zhengzhou 450008, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new complexes, {[Cd(bimt)(bdc)(H2O)]Á2H2O}n (1) and [Cd(bimt)(Hbtc)]n (2) have been synthesized
through the self-assembly of Cd(NO3)2Á4H2O with unsymmetrical N-heterocyclic ligand 2-((ben-
zoimidazol-yl)methyl)-1H-tetrazole (bimt) and aromatic polycarboxylate acid 1,3-benzenedicarboxylic
acid (H2bdc) or 1,3,5-benzenetricarboxylic acid (H3btc). Single crystal X-ray diffraction determination
shows that complex 1 possesses an infinite 2-D layered structure with parallelogram grid units. Complex
2 is a 2-D layered structure containing 1-D ladder-like chains linked by the Hbtc2À anions in tridentate
bridging fashion. Both complexes are stacked into complicated 3-D frameworks by coordinating bonds
and hydrogen bonds in the solid state. In addition, the luminescent properties of both complexes have
been investigated in the solid state at room temperature.
Received 17 August 2010
Received in revised form 28 September
2010
Accepted 28 September 2010
Available online 25 October 2010
Keywords:
Cadmium complex
Crystal structure
Luminescent property
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
is able to coordinate simultaneously to both oxygen-containing
and nitrogen-containing ligands and the final products can exhibit
Although a variety of complexes with novel structure and supe-
rior functionality have been successfully synthesized, the rational
synthesis of these attractive MOFs is still facing many challenges
due to various uncontrollable factors dominating the formation
process of the final products [1,2]. Therefore, many new synthetic
strategies such as using secondary building units (SBUs) with pre-
organized geometry and connectivity, controlling the hydrolysis of
metal salts, or selecting flexible ligands and so on, have been
extensively applied, in which a mixed-ligand approach by the judi-
cious choice of the primary and auxiliary ligands has been proved
to be one of the most effective and important ways to construct the
novel MOFs [3–6].
promising luminescent properties [10,11]. The aforementioned
ideas prompt us to employ bimt and H2bdc (or H3btc) as ligands
(the Scheme with the chemical structure formulas are listed in
Scheme 1) and Cd(II) ion as the centers. In this contribution, we pres-
ent two new 2-D Cd(II) complexes, {[Cd(bimt)(bdc)(H2O)]Á2H2O}n
(1) and [Cd(bimt)(Hbtc)]n (2), constructed from N-heterocyclic
and aromatic polycarboxylate ligands and investigate their photolu-
minescence properties in the solid state at room temperature.
2. Experimental
2.1. Materials and methods
Multifunctional N-heterocyclic ligand 2-((benzoimidazol-yl)-
methyl)-1H-tetrazole (bimt) has a flexible methylene group and
can freely rotate to meet the requirement of coordination geome-
tries of metal ions in the assembly process and thus produce unique
structural motifs with beautiful aesthetics and useful functional
properties. Meanwhile, aromatic polycarboxylic acids like 1,3-ben-
zenedicarboxylate (H2bdc), 1,3,5-benzenetricarboxylate (H3btc)
and 1,2,4,5-benzenetetracarboxylate (H4btec) and so on are widely
used as organic counterions because of their steric rigidity and
chemical robustness, in which multiple coordination fashions of
carboxyl such as monodentate, bidentate (bridging) and chelating,
can likely yield structural isomers [7–10]. In addition, Cd(II) cation
All of the chemicals are commercially available and used without
further purification. Elemental analyses were determined using
FLASH EA 1112 elemental analyzer. IR data were recorded on a BRU-
KER TENSOR 27 spectrophotometer with KBr pellets in the
400 cmÀ1–4000 cmÀ1 region. Steady state fluorescence measure-
ments were performed using a Fluoro Max-P spectrofluorimeter at
room temperature in the solid state. Ligand 2-((benzoimidazol-yl)-
methyl)-1H-tetrazole (bimt) was synthesized according to litera-
ture [12].
2.2. Preparation of {[Cd(bimt)(bdc)(H2O)]Á2H2O}n (1)
⇑
A water solution (3 mL) of H2bdc (16.6 mg, 0.100 mmol) was
added dropwise into a methanol solution (2 mL) of Cd(NO3)2Á4H2O
Corresponding author. Tel./fax: +86 371 67767164.
E-mail address: mxr@zzu.edu.cn (X. Meng).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.09.040

J. Zhang et al. / Journal of Molecular Structure 984 (2010) 276–281
277
H
N
COOH
HOOC
COOH
N
N
N
N
N
COOH
H2bdc
COOH
H3btc
bimt
Scheme 1. The ligands involved in this research.
(15.4 mg 0.0500 mmol), and then a methanol (3 mL) solution of
bimt (10.0 mg, 0.0500 mmol) was added dropwise into the above
mixture to give a clear solution. The reaction mixture was left to
stand at room temperature. The colorless block crystals suitable
for X-ray diffraction were obtained about a month later by slow
evaporation of the solvent. Crystals of 1 are stable in the air. Anal.
Calcd. For C17H18CdN6O7: C, 38.47; H, 3.42; N, 15.83. Found: C,
38.42; H, 3.46; N, 15.89%. IR (cmÀ1, KBr): 3417 m, 3153 w, 1605
s, 1554 s, 1478 w, 1446 m, 1375 s, 1313 s, 739 s, 723 m.
effects. The structures were solved by direct methods and refined
with full-matrix least-squares technique based on F2 with the
SHELXL-97 crystallographic software package [13]. All of the
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were assigned with common isotropic displacement factors
and included in the final refinement by using geometrical restrains.
Crystallographic crystal data and structure processing parameters
for complexes 1 and 2 are summarized in detail in Table 1. Selected
bond lengths and bond angles are listed in Table 2.
2.3. Preparation of [Cd(bimt)(Hbtc)]n (2)
3. Results and discussion
A mixture of Cd(NO3)2Á4H2O (15.4 mg, 0.0500 mmol), H3btc
(21.0 mg, 0.100 mmol), bimt (10.0 mg, 0.0500 mmol), and H2O
(10.0 mL) was placed in a 25.0 mL Teflon-lined stainless steel ves-
sel, and then the pH was adjusted to 7.0 by addition of CH3ONa
solution. The mixture was sealed and heated at 80 °C for 3 days,
and then the reaction system was cooled to room temperature.
Good colorless block crystals were obtained. Crystals of 2 are stable
in the air. Anal. Calcd. For C18H12CdN6O6: C, 41.52; H, 2.32; N,
16.14. Found: C, 41.60; H, 2.39; N, 16.22%. IR (cmÀ1, KBr):
3162 m, 2959 w, 1691 s, 1615 s, 1560 s, 1500 s, 1469 m, 1437 m,
1345 s, 850 m, 729 s.
3.1. IR spectroscopy
The IR spectra of complexes 1 and 2 show that the absorption
band at 3417 cmÀ1 for 1 can be attributed to the stretching vibra-
tions of the hydroxyl groups. The absorption bands at 3153 cmÀ1
for 1 and 3162 cmÀ1 for 2 originate from Ar–H stretching vibrations.
In addition, the separations (D) between ma(COO) and ms(COO) are
different for the unidentate, chelating (bidentate) and bridging com-
plexes. In complex 1, unidentate and bidentate carboxylate groups
are mixed; the unidentate ligand exhibits the
at 1605 and 1375 cmÀ1 = 230 cmÀ1), whereas the bidentate li-
a(COO) and
s(COO) at 1554 and 1478 cmÀ1
ma(COO) and ms(COO)
(D
gand exhibits the
m
m
2.4. Crystal structure determination
Table 2
The data of the two complexes were collected on a Rigaku
Selected bond lengths (Å) and bond angles (deg) for complexes 1 and 2.
Saturn 724 CCD diffractomer (Mo Ka, k = 0.71073 Å) at tempera-
Complex 1
Cd(1)–O(4)#1
Cd(1)–O(2)
ture of 20 °C. The empirical absorption corrections were applied
to them. The data were corrected for Lorentz and polarization
2.204(2)
2.298(2)
2.442(3)
141.53(9)
85.91(9)
92.38(9)
84.03(10)
90.83(10)
80.39(8)
52.19(8)
83.03(10)
Cd(1)–N(1)
Cd(1)–O(5)
Cd(1)–O(1)
O(4)#1–Cd(1)–O(2)
O(4)#1–Cd(1)–O(5)
O(2)–Cd(1)–O(5)
N(1)–Cd(1)–N(6)#2
O(5)–Cd(1)–N(6)#2
N(1)–Cd(1)–O(1)
O(5)–Cd(1)–O(1)
2.252(2)
2.346(3)
2.628(3)
132.56(8)
86.99(9)
92.64(9)
96.85(10)
170.35(9)
138.02(8)
91.99(9)
Cd(1)–N(6)#2
O(4)#1–Cd(1)–N(1)
N(1)–Cd(1)–O(2)
N(1)–Cd(1)–O(5)
O(4)#1–Cd(1)–N(6)#2
O(2)–Cd(1)–N(6)#2
O(4)#1–Cd(1)–O(1)
O(2)–Cd(1)–O(1)
N(6)#2–Cd(1)–O(1)
Table 1
Crystal data and structure refinement for complexes 1 and 2.
Complexes
1
2
Formula
Mr
Crystal system
Space group
a (Å)
C17H18CdN6O7
530.77
Triclinic
P À 1
C18H12CdN6O6
520.74
Triclinic
P À 1
Complex 2
Cd(1)–N(1)#1
Cd(1)–O(4)#2
Cd(1)–O(3)#2
8.7990(18)
10.147(2)
13.038(3)
95.95(3)
106.93(3)
113.27(3)
991.0(3)
2
8.3570(17)
9.801(2)
11.071(2)
95.85(3)
90.50(3)
95.64(3)
897.6(3)
2
2.2547(18)
2.3464(17)
2.5173(18)
2.628(2)
96.01(7)
121.45(7)
98.52(7)
89.89(7)
53.46(6)
83.91(6)
77.45(6)
91.94(7)
52.64(7)
79.06(7)
109.41(7)
Cd(1)–O(2)
Cd(1)–N(6)
Cd(1)–O(4)#3
2.2640(18)
2.3765(19)
2.5463(18)
b (Å)
c (Å)
a
(°)
Cd(1)–O(1)
b (°)
N(1)#1–Cd(1)–O(2)
O(2)–Cd(1)–O(4)#2
O(2)–Cd(1)–N(6)
N(1)#1–Cd(1)–O(3)#2
O(4)#2–Cd(1)–O(3)#2
N(1)#1–Cd(1)–O(4)#3
O(4)#2–Cd(1)–O(4)#3
O(3)#2–Cd(1)–O(4)#3
O(2)–Cd(1)–O(1)
N(6)–Cd(1)–O(1)
O(4)#3–Cd(1)–O(1)
N(1)#1–Cd(1)–O(4)#2
N(1)#1–Cd(1)–N(6)
O(4)#2–Cd(1)–N(6)
O(2)–Cd(1)–O(3)#2
N(6)–Cd(1)–O(3)#2
O(2)–Cd(1)–O(4)#3
N(6)–Cd(1)–O(4)#3
N(1)#1–Cd(1)–O(1)
O(4)#2–Cd(1)–O(1)
O(3)#2–Cd(1)–O(1)
137.45(6)
91.08(7)
101.49(7)
174.02(6)
80.34(7)
89.66(7)
170.83(6)
144.26(6)
78.29(6)
121.45(6)
c
(°)
V (Å3)
Z
Dc (Mg/m3)
1.779
1.157
12,188/4698
[R(int) = 0.0269]
4698/0/280
1.927
1.272
10,977/4225
[R(int) = 0.0213]
4225/0/281
l
/mmÀ1
Reflections collected/
unique
Data/restraints/
parameters
GOF
1.033
1.021
Final R indices
[I > 2 sigma (I)]
R indices (all data)
R1 = 0.0349,
wR2 = 0.0881
R1 = 0.0390,
wR2 = 0.0945
R1 = 0.0245,
wR2 = 0.0569
R1 = 0.0263,
wR2 = 0.0580
Symmetry transformations used to generate equivalent atoms:
Complex 1: #1 x, y À 1, z #2 x + 1, y, z.
Complex 2: #1 x À 1, y, z #2 x, y + 1, z #3 Àx À 1, Ày + 1, Àz + 2.

278
J. Zhang et al. / Journal of Molecular Structure 984 (2010) 276–281
Fig. 1a. The coordination environment of the Cd(II) center in complex 1 with the atom numbering scheme, showing 30% thermal ellipsoids. Hydrogen atoms and water
molecules are omitted for clarity.
Fig. 1b. Two-dimensional layered structure of complex 1 with parallelogram grid units.
(
m
D
= 76 cmÀ1) [14]. In complex 2, the separations (
a(COO) and
s(COO) are 123 cmÀ1 (1560 cmÀ1, 1437 cmÀ1) and
D
) between
tively. These Cd–O/N distances are in the normal range of those ob-
served in other Cd(II)-complexes [15].
m
31 cmÀ1 (1500 cmÀ1, 1469 cmÀ1), respectively. Therefore, the car-
boxylate groups in 2 coordinate to the central metal ions with bridg-
ing and chelating modes. Furthermore, the very strong absorption
band observed at 1691 cmÀ1 in 2 can be attributed to the presence
of COOH unit. The above analyses are confirmed by the results of
the X-ray diffraction.
The neighboring Cd(II) ions are connected by the bidentate
bridging bimt ligands and chelating/monodentate bdc2À anions
forming a parallelogram grid unit. Each grid is constructed from
two bimt ligands and two bdc2À anion ligands acting as the four
edges and four Cd(II) ions acting as the four vertexes. The lengths
of the opposite edges equal at 10.147 and 8.799 Å and the interior
angles are 66.7 and 113.3°, respectively. Along one direction, Cd(II)
ions are connected by bimt ligands generating a linear chain ÁÁÁCd–
bimt–Cd–bimt–Cd–bimt–CdÁÁÁ. Along the other direction, Cd(II)
ions are connected by chelating/monodentate bdc2À anions leading
to another linear chainÁÁÁCd–bdc–Cd–bdc–Cd–bdc–CdÁÁÁ. Linear
chains along the two directions are interconnected each other to
finish the two dimensional layered framework as depicted in
Fig. 1b. As shown in Table 3, there are one kind of hydrogen bond
between benzoimidazol and uncoordinated water molecule, two
kinds of hydrogen bonds between uncoordinated water molecules
and carboxylates, two kinds of hydrogen bonds between coordi-
nated water molecules and carboxylates, and two kinds of hydro-
3.2. Description of crystal structures
Single crystal X-ray diffraction reveals that complex 1 possesses
an infinite 2-D layered structure with parallelogram grid units. As
illustrated in Fig. 1a, each Cd(II) ion is coordinated by four oxygen
atoms belonging to one chelating and one monodentate carboxyl-
ate groups of two 1,3-benzenedicarboxylate groups, one coordi-
nated water molecule, and two nitrogen atoms from two bimt
ligands, forming a distorted [CdN2O4] coordination octahedron.
The Cd–O bond lengths are in the range of 2.204(2)–2.628(3) Å,
and the Cd–N bond distances are 2.252(2) and 2.442(3), respec-

J. Zhang et al. / Journal of Molecular Structure 984 (2010) 276–281
279
Table 3
Hydrogen bonds of complexes 1 and 2.
D–HÁ Á ÁA
d(D–H) (Å)
d(HÁ Á ÁA) (Å)
d(DÁ Á ÁA) (Å)
<(DHA) (deg)
Complex 1
N(2)–H(2B)Á Á ÁO(6)
O(7)–H(5W)Á Á ÁO(2)#1
O(5)–H(1W)Á Á ÁO(1)#5
O(5)–H(2W)Á Á ÁO(4)#6
O(6)–H(3W)Á Á ÁO(3)#7
O(6)–H(4W)Á Á ÁO(7)#8
O(7)–H(6W)Á Á ÁO(7)#9
0.86
0.72
0.85
0.85
0.85
0.85
0.72
1.92
2.12
1.96
1.89
1.98
1.95
2.51
2.736(4)
2.818(4)
2.745(3)
2.702(3)
2.819(4)
2.734(4)
3.182(6)
159.0
163.8
153.4
160.3
167.6
152.9
156.3
Complex 2
N(2)–H(2B)Á Á ÁN(5)#6
0.86
0.82
2.14
1.94
2.950(3)
2.752(3)
156.0
169.9
O(6)–H(6)Á Á ÁO(5)#7
Symmetry transformations used to generate equivalent atoms:
For 1: #1 x, y À 1, z #5 Àx + 1, Ày + 1, Àz + 1 #6 Àx + 1, Ày + 2, Àz + 1 #7 Àx + 1, Ày + 2, Àz + 2 #8 Àx + 1, Ày + 1,
Àz + 2 #9 Àx + 2, Ày, Àz + 2.
For 2: #6 Àx, Ày + 2, Àz + 1 #7 Àx, Ày + 1, Àz + 3.
gen bonds between uncoordinated water molecules. The adjacent
layers are connected through the above seven kinds of hydrogen
bonds resulting in a three-dimensional packing structure in solid
state with the small solvent-accessible void space occupied by
uncoordinated water molecules (91.2 Å3, equal to 9.2% of the cell
volume).
In addition, it is essential to analyze the conformation of bimt
ligand since the flexible it can adopt different conformations in
the process of obtaining the complexes. Values in the ranges 0–
( )90° can be indicative of cis or syn conformation, while values
in the range ( )90–180° can be indicative of trans or anti conforma-
tion [16,17]. In the crystal structure of complex 1, all of the bimt
moieties are equivalent and the torsion angle of N1–C7–N3–N6 is
126.7°. Thus, bimt adopts trans conformation in complex 1.
If H3btc is substituted for H2bdc in the formation of complex, a
new complex 2 with distinct structure from 1 is obtained. X-ray
structural analysis reveals that complex 2 crystallizes in the tri-
clinic system with space group P À 1. As shown in Fig. 2a, Cd(II)
ion shows a seven-coordinated geometry which is different from
that in complex 1 and is coordinated by five oxygen atoms from
three Hbtc2À anions and two nitrogen atoms from two bimt li-
gands. The Cd–O bond distances of 2.2640(18)–2.628(2) Å and
Cd–N bond lengths of 2.2547(18) and 2.3765(19) Å are close to
those observed in complex 1.
Notably, in complex 2 only two of the carboxylate groups of
each H3btc coordinate to the central Cd(II) ion in bidentate chelate
or tridentate-bridging mode and the third one, which is not depro-
tonated, remains uncoordination. Each Hbtc2À ligand coordinates
to three Cd(II) centers to form a 1-D ladder-like chain (Fig. 2b).
These 1D chains are further connected by bidentate bridging bimt
spacers with CdÁÁÁCd distance of 8.357 Å, and thus giving rise to the
formation of 2-D layered structure (Fig. 2c). In addition, there are
two kinds of hydrogen bonds in complex 2. One kind of hydrogen
bonds is N–HÁÁÁN (between benzoimidazol and tetrazole) with the
bond length of 2.950(3) Å and bond angle of 156.0°. The other kind
of hydrogen bonds is O–HÁÁÁO (between uncoordinated carboxyl
groups) with the bond length of 2.752(3) Å and bond angle of
169.9°. The adjacent layers are linked through these two kinds of
Fig. 2a. The coordination environment of the Cd(II) center in complex 2 with atom numbering scheme, showing 30% thermal ellipsoids. Hydrogen atoms are omitted for
clarity.

280
J. Zhang et al. / Journal of Molecular Structure 984 (2010) 276–281
Fig. 2b. One-dimensional ladder-like chain constructed from Cd(II) centers and Hbtc2À anions.
Fig. 2c. The 2-D layered structure of complex 2.
hydrogen bonds leading to the three-dimensional packing struc-
ture in solid state. Unlike complex 1, all of the bimt moieties are
equivalent and adopt cis conformation in complex 2. The torsion
angle of N1–C7–N3–N6 is À76.8°.
7000000
6000000
5000000
4000000
3000000
2000000
1000000
0
complex 1
3.3. Luminescent properties
complex 2
Taking into account of the excellent luminescent properties of
d10 metal complexes, the luminescent properties of complexes 1
and 2 were investigated in the solid state at room temperature.
As shown in Fig. 3, complex 1 displays an intense emission band
at about 352 nm when excited at 299 nm, while complex 2 gives
an emission band at about 443 nm (kex = 295 nm). To understand
the nature of the emission bands, the luminescent properties of
the ligands bimt, H2bdc and H3btc were also measured in the solid
state. It was found that the free bimt shows an emission band
located at 302 nm (kex = 291 nm), H2bdc and H3btc molecules give
300
350
400
450
500
550
Wavelength/nm
Fig. 3. Emission spectra of complexes
temperature.
1 and 2 in the solid state at room
emission bands at 355 nm (kex = 280 nm) and 395 nm (kex
=
320 nm), respectively. Obviously, the emissions of the two com-
plexes are not involved with contributions from the bimt ligand.
The emission band of complex 1 may be attributable to intraligand
n ? pà transitions within 1,3-benzenedicarboxylate ligands [7]. In
comparison with the band for uncoordinated H3btc, the emission

J. Zhang et al. / Journal of Molecular Structure 984 (2010) 276–281
281
spectra for complex 2 appear bathochromic shift. This indicates
that the emission in this complex includes ligand-to-metal charge
transfer (LMCT) character [18–20]. The interesting results indicate
that the organic counterions and the structures of the complexes
can affect their fluorescence emission features.
publication. CCD numbers are 762713 and 762714. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.htm (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.09.040.
4. Conclusion
References
In this work, we have studied the system of Cd(II)-bimt-poly-
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2-((benzoimidazol-yl)methyl)-1H-tetrazole as ligand and 1,3-ben-
zenedicarboxylate or 1,3,5-benzenetricarboxylate as organic coun-
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structure connected by coordinating bonds and hydrogen bonds.
Furthermore research indicates that the change of organic counte-
rions can influence the coordination number of the central Cd(II)
ion, and thus influence the detailed architectures of the complexes,
and finally lead to the different fluorescent properties of them. To
some extent, this work will enrich the field of complexes.
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Acknowledgments
We gratefully acknowledge the financial support by the Science
and Technology Department of Henan Province (082102330003),
the National Natural Science Foundation of China (J0830412),
National Undergraduates Innovative Experimentation Project
(091045901), and the Education Department of He’nan Province
(2009A150029).
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Appendix A. Supplementary material
Crystallographic data reported in this paper have been deposited
with the Cambridge Crystallographic Data Center as supplementary