Synthesis, crystal structures and luminescent properties of an isotypic series of rare-earths complexes with a dialdehyde ligand

Article abstract of DOI:10.1016/j.poly.2010.03.018

A series of mononuclear complexes based on lanthanide ions has been synthesized and X-ray characterized. The compounds [Ln^IIIL ₂(NO₃)₃(H₂O)₂] (Ln = La, Ce, Pr, Nd, Sm, Gd and Tm; L = 2, 6-bis(2-formylphenoxymethyl)pyridine) are found to be isomorphous and isostructural. Ligand L systematically coordinates through one carbonyl functionality, and the resulting complexes are placed on a twofold axis in crystals belonging to C2/c space-group. Emission spectra for Ln = La, Pr, Nd revealed a correlation between the Ln-O coordination bond length and the photoluminescent properties of the complexes, in line with a F?rster-Dexter mechanism for intramolecular energy transfer. Ligand L is therefore a suitable sensitizer for lanthanide ions.

Full text of DOI:10.1016/j.poly.2010.03.018

Polyhedron 29 (2010) 2048–2052
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structures and luminescent properties of an isotypic series
of rare-earths complexes with a dialdehyde ligand
*
Sara Luisa Rodríguez De Luna, Luis Ángel Garza, Sylvain Bernès, Perla Elizondo , Blanca Nájera, Nancy Pérez
DEP Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Guerrero y Progreso S/N, Col. Treviño, 64570 Monterrey, N.L., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of mononuclear complexes based on lanthanide ions has been synthesized and X-ray character-
ized. The compounds [LnIIIL2(NO3)3(H2O)2] (Ln = La, Ce, Pr, Nd, Sm, Gd and Tm; L = 2,6-bis(2-formylphen-
oxymethyl)pyridine) are found to be isomorphous and isostructural. Ligand L systematically coordinates
through one carbonyl functionality, and the resulting complexes are placed on a twofold axis in crystals
belonging to C2/c space-group. Emission spectra for Ln = La, Pr, Nd revealed a correlation between the
Ln–O coordination bond length and the photoluminescent properties of the complexes, in line with a För-
ster–Dexter mechanism for intramolecular energy transfer. Ligand L is therefore a suitable sensitizer for
lanthanide ions.
Received 22 December 2009
Accepted 23 March 2010
Available online 27 March 2010
Keywords:
Lanthanides
Coordination complexes
X-ray structures
Photoluminescence
Emission spectra
Ó 2010 Elsevier Ltd. All rights reserved.
2,6-Bis(2-formylphenoxymethyl)pyridine (L)
1. Introduction
[LnIIIL2(NO3)3(H2O)2], as well as the luminescent properties of
some of these complexes are discussed.
Coordination chemistry gives the opportunity to explore differ-
ent ways in order to synthesize new compounds with the hope to
obtain advanced materials having magnetic, optical, luminescent
or electrochemical properties [1–3]. On the other hand, it is well
known that dialdehydes and diamines derivatives are important
precursors in the synthesis of macrocyclic ligands. For instance,
2,6-bis(2-formylphenoxymethyl)pyridine (L hereafter), has been
used as a precursor to obtain macrocyclic systems including pen-
dant arms, which have the ability to complex metal ions [4,5]. A
number of related multidentate molecules also served as building
blocks for cryptands, for which host properties have been exten-
sively probed [6].
2. Experimental
2.1. Synthesis
All the reactants used to obtain the ligand and the lanthanide
complexes were analytic grade from Sigma–Aldrich Chemical
Company Inc., USA. IR spectra were measured on an IR-FT Nicolet
550 Model Magna-IR Spectrometer and ATR-FT Perkin Elmer Spec-
trum 1. ESI-TOF mass spectra were obtained with a Bruker Dalton-
ics Data Analysis 3.3, and elemental analyses (C, H, N) were
performed using
a Perkin Elmer Instruments Series II 2400
The aim of this research is to extend the study of L as a donor to
lanthanide ions to achieve materials with photoluminescent prop-
erties. We assume that these materials are potentially luminescent
CHNS-O. Single crystals diffraction data were collected on a Sie-
mens P4 diffractometer. The excitation and emission spectra were
obtained on a Luminescence Spectrometer Perkin Elmer LS55.
2,6-Bis(2-formylphenoxymethyl)pyridine (L). The ligand was pre-
pared via Williamson condensation between the sodium salt of sal-
icylaldehyde and 2,6-bis(bromomethyl)pyridine in a 2:1 ratio,
under N2 atmosphere, by using a modified literature method [10]
(the detailed preparation is transferred to the supplementary
material). Colorless crystals, mp. 141–142 °C; Micro Anal. Calc.
for C21H17NO4: C, 72.61; H, 4.94; N, 4.03. Found: C, 72.25; H,
because L contains both chromophore and aryl groups with
p elec-
trons, which are required for the effective intramolecular transfer
of excitation energy from the resonance levels of the chromophore
groups to Ln3+ (antenna effect) [7]. The expected function of the
ligand is to increase the luminescent properties toward visible
and infrared ranges [8,9]. We report here on the synthesis of an
isotypic series of new complexes where ligand L coordinates to
lanthanides: LaIII, CeIII, PrIII, NdIII, SmIII, GdIII and TmIII. The crystal-
lographic characterization of the free ligand L and complexes
4.67; N, 4.23%. IR (KBr pellets, cmꢀ1): 1683 (
mC@O), 1591 (mC@N)
and 1237 (mC–O–C).
[LnIIIL2(NO3)3(H2O)2]. The complexes were synthesized by the
reaction of the appropriate LnIII salt (Ln = La, Ce, Pr, Nd, Sm, Gd
and Tm, 0.5 mmol) and L (1 mmol), in refluxing acetonitrile
* Corresponding author. Tel.: +52 81 83760570.
E-mail address: perlaelizondomx@hotmail.com (P. Elizondo).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.03.018

S.L. Rodríguez De Luna et al. / Polyhedron 29 (2010) 2048–2052
2049
(50 mL) for 5 h. The mixture was cooled and the volume reduced
by evaporation under reduced pressure, to give the expected com-
plex as product. The crude solid was recrystallized from acetoni-
trile, affording air-stable crystals suitable for X-ray structural
analysis.
tics in line with a non-centrosymmetric space-group, Cc. However,
the final structures clearly display a twofold symmetry (see infra),
consistent with space-group C2/c, which was eventually retained.
The main disadvantage of such a choice is that the nitrate ion
placed on the twofold axis presents high thermal displacements.
Attempts to resolve disordered sites (i.e. considering that this ion
belongs to Cc rather than C2/c) were unsuccessful, and the final
refinements [13] were carried out with restraints applied on aniso-
tropic displacement parameters for N3, O9 and O10 sites (see
deposited CIF). C-bonded H atoms were placed in idealized posi-
tions and refined as riding to their carrier C atoms. H atoms for
the coordinated water molecule were found in a difference map,
and refined freely, although the geometry was regularized through
soft restraints: O–H = 0.85(1) and HꢁꢁꢁH = 1.34(2) Å.
[LaL2(NO3)3(H2O)2] orange crystals; mp. 153–154 °C (dec.);
Anal. Calc. for C42H38N5O19La: C, 47.78; H, 3.63; N, 6.63. Found:
C, 47.87; H, 3.11; N, 6.93%. IR (KBr pellets): 3457 (
mO–H), 1661 (mC@O
free), 1642 ( C@O–La), 1481 ( .
m
m
NO3), 1296, 1038, 818, 730 cmꢀ1
[CeL2(NO3)3(H2O)2] yellow crystals; mp. 156 °C; Anal. Calc. for
C42H38N5O19Ce: C, 47.73; H, 3.62; N, 6.63. Found: C, 47.5; H,
3.51; N, 7.01%. IR (KBr pellets): 3458 (
mO–H), 1665 (mC@O free),
1643 ( C@O–Ce), 1461 ( .
m
m
NO3), 1301, 1057, 818, 733 cmꢀ1
[PrL2(NO3)3(H2O)2] yellow crystals; mp. 163–164 °C; Anal. Calc.
for C42H38N5O19Pr: C, 47.64; H, 3.63; N, 6.61. Found: C, 47.72; H,
3.11; N, 6.91%. IR (KBr pellets): 3457 (
mO–H), 1661 (mC@O free),
2.3. Preliminary study of the photoluminescent properties of the
complexes
1642 ( C@O–Pr), 1459 ( .
m
m
NO3), 1296, 1056, 818, 760 cmꢀ1
[NdL2(NO3)3(H2O)2] pink crystals; mp. 157–158 °C (dec.); Anal.
Calc. for C42H38N5O19Nd: C, 47.55; H, 3.61; N, 6.60. Found: C,
The study of the photoluminescent properties was realized by
irradiating at 254 and 366 nm the ligand L, lanthanides salts used
as starting materials, and the synthesized complexes. Ligand and
complexes were studied both in the solid-state and as DMF solu-
tions, at 4.5 ꢂ 10ꢀ3 M concentration. Afterward, their UV–Vis spec-
tra were recorded, in order to know the wavelength of maximum
absorption, and the emission and excitation spectra were eventu-
ally measured for solid and solution samples.
47.63; H, 3.10; N, 6.89%. IR (KBr pellets): 3457 (
mO–H), 1664 (mC@O
free), 1634 ( C@O–Nd), 1464 ( .
m
m
NO3), 1302, 1054, 822, 761 cmꢀ1
[SmL2(NO3)3(H2O)2] colorless crystals; mp. 159 °C (dec.); Anal.
Calc. for C42H38N5O19Sm: C, 47.73; H, 3.62; N, 6.63. Found: C,
47.50; H, 3.51; N, 7.01%. IR (KBr pellets): 3461 (
mO–H), 1665 (mC@O
free), 1634 ( C@O–Sm), 1487 ( .
m
m
NO3), 1305, 1057, 817, 761 cmꢀ1
[GdL2(NO3)3(H2O)2] yellow crystals; mp. 156 °C; Anal. Calc. for
C42H38N5O19Gd: C, 46.97; H, 3.57; N, 6.52. Found: C, 47.03; H,
4.11; N, 5.96%. IR (KBr pellets): 3462 (
mO–H), 1662 (mC@O free),
NO3), 1309, 1021, 817 cmꢀ1
3. Results and discussion
1646 ( C@O–Gd), 1446 ( .
m
m
[TmL2(NO3)3(H2O)2] yellow crystals; mp. 158 °C; Anal. Calc. for
C42H38N5O19Tm: C, 46.64; H, 3.53; N, 6.45. Found: C, 45.24; H,
3.1. Synthesis
3.51; N, 6.69%. IR (KBr pellets): 3424 (
mO–H), 1664 (mC@O free),
Complexes [LnIIIL2(NO3)3(H2O)2] were successfully prepared
(Scheme 1) and spectroscopic studies are consistent with the pro-
posed formula. Similar IR spectra are obtained along the series, fea-
turing two carbonyl vibrations, for example at 1661 and 1642 cmꢀ1
in the case LnIII = PrIII (Fig. S1, Supplementary material), suggesting
that one carbonyl group coordinates the metal while the other re-
mains free. Strong IR vibrations at 1459 and 1296 cmꢀ1 (LnIII = PrIII)
are assigned to coordinated nitrate ions, and the stretching signal
at 3457 cmꢀ1 accounts for water molecules included in the coordi-
nation sphere. Elemental analyses agree with the 1:2 ratio for Ln:L,
and the formula was finally confirmed by mass spectrometry. All
ESI-TOF mass spectra exhibit a base peak (100%) corresponding
to the free ligand (LH+, m/z 348) and peaks of smaller intensity
for LnLH+ and LnL(NO3)2H+ species.
1634 ( C@O–Tm), 1448 ( .
m
m
NO3), 1291, 1028, 811, 748, 721 cmꢀ1
2.2. X-ray diffraction
Pertinent crystal data and other crystallographic parameters are
listed in Table 1. Diffraction data were collected at room tempera-
ture (294–298 K) using the Mo K
through standard procedures [11]. Raw data were corrected for
absorption effects, either using suitable -scans data [12], or a
a radiation (k = 0.71073 Å),
W
Gaussian face-indexed correction, if available [13]. The refinement
of the organic ligand did not present special difficulties. In contrast,
space-group determination for the lanthanide complexes is not so
straight. Diffraction patterns systematically afford intensity statis-
Table 1
X-ray parameters.
Complex
L
La
Ce
Pr
Nd
Sm
Gd
Tm
Chemical formula
Fw
Color
Space-group
a (Å)
C21H17NO4
347.36
pale yellow
P21/n
8.710(4)
18.002(7)
11.038(5)
95.89(2)
1721.8(14)
4
C42H38LaN5O19
1055.68
orange
C42H38CeN5O19
1056.89
yellow
C42H38N5O19Pr
1057.68
yellow
C42H38N5NdO19
1061.01
pale pink
C2/c
20.613(6)
9.205(3)
22.581(6)
103.822(12)
4161(2)
4
C42H38N5O19Sm
1067.12
colorless
C2/c
20.676(6)
9.223(3)
22.603(7)
103.766(18)
4186(2)
4
C42H38GdN5O19
1074.02
yellow
C42H38N5O19Tm
1085.70
yellow
C2/c
C2/c
C2/c
C2/c
C2/c
20.802(3)
9.2209(12)
22.875(2)
103.683(8)
4263.2(9)
4
20.724(6)
9.214(3)
22.747(7)
103.772(14)
4219(2)
4
20.682(4)
9.222(2)
22.712(6)
103.72(2)
4208.2(17)
4
20.646(5)
9.242(3)
22.545(6)
103.825(14)
4177(2)
4
20.606(6)
9.277(4)
22.546(10)
103.86(3)
4185(3)
4
b (Å)
c (Å)
b (Å)
V (Å3)
Z
2h Range (°)
3–55
0.093
0.048, 0.121
0.084, 0.156
1.340
4–60
1.090
0.029, 0.076
0.033, 0.076
1.645
4–55
1.168
0.033, 0.083
0.040, 0.090
1.664
4–52.5
1.247
0.027, 0.066
0.034, 0.072
1.669
4–60
1.338
0.037, 0.093
0.048, 0.101
1.694
4–55
1.493
0.041, 0.104
0.050, 0.114
1.693
4–55
1.678
0.036, 0.079
0.042, 0.086
1.708
4–55
2.210
0.040, 0.102
0.053, 0.113
1.723
l
(mmꢀ1
)
R Indices [I > 2
R indices (all data)a
Dcalc. (g cmꢀ3
r
(I)]a
)
Data/parameters
3948/236
1.054
6184/310
1.066
4822/310
1.047
4261/310
1.085
6058/310
1.082
4790/310
1.050
4792/310
1.081
4765/310
1.076
Sa
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
P
P
2
2
jjFo jꢀjFc
wðF2o ꢀF2c
Þ
wðF2o ꢀF2c
Þ
jj ; wR2
¼
; S ¼
.
a
P
P
R1
¼
2
wðF2o
Þ
dataꢀparameters
jFo
j

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S.L. Rodríguez De Luna et al. / Polyhedron 29 (2010) 2048–2052
Scheme 1. Synthetic route for lanthanide complexes.
Fig. 1. Molecular structure of ligand L. Displacement ellipsoids are shown at the
30% probability level.
ORTEP-style views are given for ligand L and complex [LaL2(-
NO3)3(H2O)2] in Figs. 1 and 2, respectively. Complete geometric
parameters may be consulted from the archived CIF file, and dif-
fraction data are available on request to authors.
The free ligand is stabilized in the solid-state in a nearly planar
arrangement (Fig. 1), characterized by dihedral angles between the
central pyridine ring (N1/C9ꢁꢁꢁC13) and benzene rings (C2ꢁꢁꢁC7 and
C15ꢁꢁꢁC20) of 5.75(9)° and 26.19(8)°. The molecule thus approxi-
mates C2v point symmetry, with the carbonyl functional groups
oriented in the same direction with respect the core pyridine ring.
This conformation is reminiscent of that previously described, for
instance, for 2,6-bis(phenoxymethyl)pyridine [14]. The observed
arrangement for heteroatoms in L seems to be unfavorable for its
use as a chelating ligand. However, the possibility for different con-
formations in solution should not be ruled out, as rotations about
methoxy C–O bonds are not sterically impeded. Indeed, a very dif-
ferent conformation was described for closely related ligands,
where aldehyde groups are replaced by dimethoxymethyl or
hydroxylmethyl substituents [15]. In these cases, bent conforma-
tions are stabilized, with a benzene ring almost perpendicular to
the rest of the molecule. In the same way, formal substitution of
aldehyde functional groups in L by hydroxyl groups affords an ac-
tual podant ligand, stabilized by soft intramolecular hydrogen
bonds. This molecule has been shown to have complexing ability
towards Na+, involving all heteroatoms in coordination bonds [16].
Lanthanide complexes are all isostructural and isomorphous
(Table 1). A previous report about the Ce(III) complex was pub-
lished [17]. We however included this complex in this work, be-
cause the former refinement was of limited resolution and was
not deposited with the CCDC. On the other hand, for comparison
purposes, it is preferable to have all diffraction patterns measured
in the same conditions and refinements carried out with identical
models.
Fig. 2. Molecular structure of complex [LaL2(NO3)3(H2O)2]. Displacement ellipsoids
are shown at the 30% probability level. The inset represents the coordination
polyhedron around the lanthanide center.
Pr, Nd, Sm, Gd, Tm; following geometric parameters are given for
Ln = La). The molecule lies on a twofold axis, with atoms Ln, N3
and O9 placed in special positions. As mentioned in Section 2,
nitrate anion N3 is probably disordered across the C2 axis, as
reflected in the prolate thermal ellipsoid of O9. The nitrate ions
behave as bidentate ligands, as frequently observed in lanthanide
complexes [18,19]. Ligand
L is monodentate, using a single
carbonyl group for coordination. The conformation of this ligand
is indeed close to that found in the free molecule: the ligand is quite
planar, the dihedral angle between the pyridine and benzene rings
being 11.04(14)° and 8.36(15)°. As a consequence of the molecular
symmetry, both ligands L in the molecule are almost perpendicular,
the angle between corresponding mean planes being 86.35(2)°. The
coordination environment is completed by two water molecules,
giving a 10-coordinated metal center. The coordination geometry
(Fig. 2, inset) is best described as a distorted bicapped square antip-
rism [20], approaching D4d symmetry [21].
Complexes are formulated [LnIIIL2(NO3)3(H2O)2], where L is the
ligand above described, and LnIII is a lanthanide ion (Ln = La, Ce,

S.L. Rodríguez De Luna et al. / Polyhedron 29 (2010) 2048–2052
2051
eral trend is observed for unit cell volumes and coordination bond
lengths.
3.2. Luminescence study of LaIII, PrIII and NdIII complexes
Under UV irradiation, above described LaIII, PrIII and NdIII com-
plexes emit light, either in the solid-state or in solution, at different
wavelengths compared to emission produced in absence of irradi-
ation. The La-based complex emits in the yellow range, more inten-
sively if irradiation is applied. The NdIII complex emits in the
purple region if not irradiated and shifts emission to green under
irradiation. Finally, the PrIII complex behaves similarly to the La
complex, emitting a bright yellow light under irradiation.
Fig. 3 (upper panel) shows the emission spectra of L in the solid-
state and dissolved in DMF. After excitation at 350 nm, a Stokes
displacement toward the visible range is observed: kem = 588 nm
and kem = 446 nm in the solid and solution state respectively, indi-
cating that L is a good candidate for sensitizing lanthanides. The
smallest Stokes shift observed in solution compared to solid-state
should be related to the involvement of DMF, which activate vibra-
tion modes using UV photons, at expense of the
in solvated L.
p
p
* transitions
For LnIII complexes (Ln = La, Pr, Nd), the emission is found in the
visible range, regardless of the lanthanide used. Excitation in the
UV region affords emission at 455 nm (La), 760 nm (Pr) and
864 nm (Nd) in the solid-state. Similarly to the free ligand L, emis-
sion intensity decreases for DMF solutions: kem = 453, 757 and
862 nm for La, Pr and Nd, respectively. Spectra for Ln = Pr are dis-
played in Fig. 3 (lower panel).
As expected, the LaIII emission yield is only slightly enhanced in
the antenna-containing complex compared with the starting mate-
rial La(NO3)2.6 H2O, since the 4f shell of LaIII is empty. For such salts
and complexes [23], specific luminescence mechanisms have been
invoked [24], like self-trapped exciton luminescence [25]. In con-
trast, the lanthanide–chromophore interaction affords very effi-
cient energy transfer in PrIII and NdIII complexes (see inset for
LnIII = PrIII). Regarding the energy transfer, although a limited series
of complexes has been studied, a clear structure–property correla-
tion is observed: the more the Ln–O coordination bond length is
short, the more luminescent properties are improved. The För-
ster–Dexter theory [26] seems thus to be applicable to the present
series of complexes, although it would be difficult to resolve unam-
biguously if the actual mechanism for energy transfer is by ex-
change interaction (Dexter transfer [27]) or rather through a
dipole–dipole interaction mechanism (Förster model [28]).
Fig. 3. Normalized emission spectra for free L (top) and Pr complex (bottom). For
ligand L, kexc = 350 nm and emission is observed in DMF solution (red line) and in
the solid-state (blue line). For the Pr complex, kexc = 290 nm and emission is
observed in DMF solution (red line) and in the solid-state (blue line). The insert
compares emissions for Pr(NO3)3ꢁ6H2O (green line) and [PrL2(NO3)3(H2O)2] (blue
line). (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)
Coordinated water molecules also serve as donor for intermo-
lecular hydrogen bonds with the free aldehyde group and the
pyridine N atom of two symmetry-related molecules. The result-
ing 3D supramolecular network, together with the planar geome-
try of the main ligand, allows the building of an efficient packing
structure in the crystal. Two molecules related by an inversion
center have two ligands L arranged in a slightly shifted face-to-
4. Conclusions
We have synthesized and characterized an isotypic series of
air-stable monometallic lanthanide complexes formulated [LnL2-
(NO3)3(H2O)2] using a dialdehyde ligand, L. This ligand clearly
behaves as an efficient sensitizer for the lanthanide photolumines-
cence. The antenna effect is probably limited to the benzaldehyde
group coordinated to the metal center. Since a site of coordination
remains free in the ligand, the reported complexes could be used as
building blocks for the preparation of more sophisticated binuclear
devices [29], allowing the design of sensors with tunable photolu-
minescent properties. In a different point of view, these new
complexes also deserve further physical characterization. For
instance, emission spectra at variable temperature could deter-
mine the exact mechanism for energy transfer, since the Dexter
mechanism is known to be a temperature-dependent process,
while in the Förster model, assuming Coulombic interactions, the
energy depends only on the separation between the sensitizer
and activator.
face configuration, which generates
p p interactions between
ꢁꢁꢁ
aromatic rings. These stabilizing contacts may be characterized
by the distance separating two L mean planes interacting in the
crystal, 3.30 Å, shorter than the distance between planes in
graphite, 3.35 Å. The resulting crystal structure displays a high
packing index, 0.72 [22], and no residual voids are available for
lattice solvent inclusion.
Since an efficient stacking of molecules in the solid-state is
facilitated by the molecular symmetry, it is not surprising that all
synthesized complexes are isomorphous. As expected, the density
is increased from 57La (1.645 g/cm3) to 69Tm (1.723 g/cm3), reflect-
ing the ion contraction along the lanthanide series. A similar gen-

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S.L. Rodríguez De Luna et al. / Polyhedron 29 (2010) 2048–2052
[9] A.M. Klonkowski, S. Lis, M. Pietraszkiewicz, Z. Hnatejko, K. Czarnobaj, M.
Acknowledgment
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[10] S. Rodríguez, Undergraduate Thesis, Universidad Autónoma de Nuevo León,
2007.
The authors acknowledge the Facultad de Ciencias Químicas,
UANL, and PAYCyT for financial support (project number
CA1260–06).
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Appendix A. Supplementary material
CCDC 755323, 755324, 755325, 755326, 755327, 755328,
755329 and 755330 contain the supplementary crystallographic
data for ligand L, Ln = La, Ln = Ce, Ln = Pr, Ln = Nd, Ln = Sm, Ln = Gd
and Ln = Tm. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2010.03.018.
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