New chiral pyridine-based Eu(III) complexes: Study of the relationship between the nature of the ligands and the 5D0 luminescence spectra

Article abstract of DOI:10.1016/j.ica.2011.12.042

In this paper a new family of imines [N,N′-bis(2-pyridylmethylidene)- 1,2-(R,S)-cyclohexanediamine (ligand 1); N,N′-bis(2-pyridylmethylidene)-1, 2-(R,S)-cyclohexanediamine (ligand 3)] and amines [N,N′-bis(2- pyridylmethylene)-1,2-(R,R + S,S)-cyclohexanedi

Full text of DOI:10.1016/j.ica.2011.12.042

Inorganica Chimica Acta 385 (2012) 65–72
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New chiral pyridine-based Eu(III) complexes: Study of the relationship between
5
the nature of the ligands and the D0 luminescence spectra
a
b
a
Fabio Piccinelli a, , Adolfo Speghini , Magda Monari , Marco Bettinelli
⇑
a Laboratorio di Chimica dello stato solido, DB, Univ. Verona and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy
b Dipartimento di Chimica ‘‘G. Ciamician’’, Universita‘ di Bologna, Via Selmi 2, 40126 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
In this paper a new family of imines [N,N0-bis(2-pyridylmethylidene)-1,2-(R,S)-cyclohexanediamine (ligand
1); N,N0-bis(2-pyridylmethylidene)-1,2-(R,S)-cyclohexanediamine (ligand 3)] and amines [N,N0-bis(2-pyr-
idylmethylene)-1,2-(R,R + S,S)-cyclohexanediamine (ligand 2); N,N0-bis(2-pyridylmethylene)-1,2-(R,S)-
cyclohexanediamine (ligand 4)] pyridine-based chiral ligands and their chiral nitrate Eu(III) complexes
is presented. Combination of structural and spectroscopic evidences in the solid state, reveals that the
Article history:
Received 1 August 2011
Received in revised form 16 December 2011
Accepted 21 December 2011
Available online 12 January 2012
Eu(III) ion environment is characterized by a low symmetry for all the complexes. The 5D0
?
7F0 Eu(III)
Keywords:
Europium
Luminescence
Crystal structure
Nitrogen ligands
Chirality
emission intensity reflects the degree of distortion of the metal surroundings, that is higher in the case
of cis isomer of amine-based complex. This new family of Eu(III) chiral complexes are promising candi-
dates for applications in the sensing field as probes in solution of the nitrate anion and of chiral
molecules.
Ó 2011 Elsevier B.V. All rights reserved.
Nitrate compounds
1. Introduction
this disadvantage can be overcome by indirect sensitization through
the absorption bands of the ligand molecules coordinated to the
Luminescent lanthanide complexes have interesting photo-
physical properties mainly due to the spectroscopic features of
the metal ion. In particular, the emission spectra of lanthanide ions
consist of sharp and narrow bands corresponding to the parity-for-
bidden f–f transitions that are characteristic of the metal ion.
Among the most emissive lanthanide ions we mention the euro-
pium(III) which possesses [Xe]4f6 electronic configuration, 7F0 as
a ground state and 5D0 long-lived excited state [1]. Some transi-
tions have variable intensities resulting from the sensitivity to
the structural details of the metal ion environment. It is well
known that the 5D0 ? 7F2 transition located around 615 nm is
highly sensitive to the chemical environment and for this reason
it is described as ‘‘hypersensitive’’. A particularly useful character-
istic of the Eu(III) ion is the emission features of the 5D0 ? 7F0 tran-
sition. Both the ground state and excited state are non-degenerate,
Ln(III) ion using UV light (antenna effect). Another advantage of
the bond between Ln(III) ions and organic ligands is to allow the
protection of the Ln(III) ions from external quenching processes
such as the intrusion of solvent molecules (i.e. H2O) into the inner
sphere coordination of the metal ion. These molecules are responsi-
ble of the non-radiative deactivation processes that occur upon
interaction with OH, NH, and CH oscillators present around the
lanthanide ion.
Concerning the applications in the biomedical field, Eu(III) com-
plexes exhibit some desirable and unusual characteristic when
compared with conventional organic fluorophores such as long
excited-state lifetime (in the ms-range), large shift between ab-
sorbed and emitted wavelength (in the case of ligand sensitization)
and line-like emission bands these features allowing the separa-
tion between Ln(III) luminescence and short-lived background
fluorescence [5]. Recently, the interest in Ln(III) compounds pos-
sessing chiral properties has started to increase because, in addi-
tion to the interesting spectral characteristics discussed above it
is possible to take advantage of chiral features introduced in the
design of the complex [6–9]. In particular, the availability of chiral
Ln(III) compounds, where the chirality is ‘‘introduced’’ through the
use of chiral ligands, has led to the development of new chiral
sensing/recognition applications, such as luminescence sensing of
anions [10,11] and of chiral biological substrates [11–13].
The major advantage of introducing chirality into luminescent
Ln(III)-based complexes appears to be the enhancement of the
which results, in principle, in
a one-to-one correspondence
between the number of peaks in the emission spectrum and the
number of distinct Eu(III) ion environments [2–4]. Concerning
the transition energy of the 5D0 ? 7F0 emission, it is well known
that this energy is strongly connected to the total charge on ligands
coordinated to Eu(III) [2,4].
Although Ln(III) ions have weak absorption intensities, due to the
parity forbidden nature of the intraconfigurational f–f transitions,
⇑
Corresponding author.
E-mail address: fabio.piccinelli@univr.it (F. Piccinelli).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.042

66
F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
the Eu(III) ion (465 nm) were measured, and the crystal structure
of 5 and 6 (Figs. 1 and 2) were determined.
2. Experimental
We performed the synthesis of N,N0-bis(2-pyridylmethylidene)-
1,2-(R,R + S,S)-cyclohexanediamine (1) (racemic mixture) starting
from ( )-trans-1,2-cyclohexanediamine, following the same
synthetic protocol reported in the literature for the synthesis of
the enantiopure form [15]. The same protocol was also employed
for the synthesis of the ligand 3 (N,N0-bis(2-pyridylmethylidene)-
1,2-(R,S)-cyclohexanediamine), starting from cis-1,2-cyclohexane-
diamine.
Ligand 3 was purified by precipitation from a cold (0 °C) diethyl
ether solution. Chemical yield 72%, 1H NMR (CD3CN, 300 MHz): d
8.61 (d, J 4 Hz, 2H), 8.33 (s, 2H), 7.95 (d, J 8 Hz, 2H), 7.78 (t; J
8 Hz, 2H), 7.38 (dd, J 4, 8 Hz, 2H), 3.73 (m, 2H), 2.05 (m, 4H),
1.77 (m, 2H), 1.63 (m, 2H) IR (KBr pellet, cmꢀ1): 3086, 3057,
3010, 2925, 2856, 1649, 1585, 1564, 1468, 1444, 1435, 1318,
1070, 1041, 981, 978, 966, 875, 787, 769, 740.
Fig. 1. Synthetic protocol employed for the synthesis of chiral nitrate Eu(III)
Ligand 2 [N,N0-bis(2-pyridylmethylene)-1,2-(R,R + S,S)-cyclohex-
anediamine]: 250 mg (0.85 mmol) of the ligand 1 were dissolved
in 10 mL of methanol and at 0 °C, 155 mg (4.1 mmol) of NaBH4
were added. The reaction mixture was left to reach room temper-
ature and stirred for 3 h (the formation of the ligand 2 was checked
by TLC (Rf = 0.4, DCM/MeOH = 99.5/0.5). After that, 30 ml of dis-
tilled water were added to the reaction mixture and almost all
methanol was evaporated under vacuum. The aqueous layer was
extracted with 20 mL of DCM and the organic phase containing
the ligand was separated and dried under anhydrous Na2SO4. The
organic solvent was evaporated under vacuum and 2 (yield 92%)
was collected as a pale yellow oil. 1H NMR (CD3CN, 300 MHz): d
8.97 (d, J 4 Hz, 2H), 8.17 (dd, J 4, 8 Hz, 2H), 7.90 (d, J 8 Hz, 2H),
7.67 (t; J 8 Hz, 2H), 4.25 (d, J 12 Hz, 2H), 4.44 (d, J 12 Hz, 2H),
2.61 (m, 2H), 2.44 (m, 2H), 2.20 (m, 2H), 1.72 (m, 2H), 1.53 (m,
2H). IR (neat, cmꢀ1): 3292, 3062, 3008, 2923, 2852, 1591, 1570,
1473, 1448, 1433, 1354, 1255, 1146, 1119, 1047, 995, 858, 760.
Ligand 4 [N,N0-bis(2-pyridylmethylene)-1,2-(R,S)-cyclohexane-
diamine]: 205 mg (0.70 mmol) of the ligand 3 were dissolved in
10 mL of methanol and at 0 °C, 127 mg (3.37 mmol) of NaBH4 were
added. The reaction mixture was left to reach room temperature
and stirred for 3 h (the formation of 4 was checked by TLC
(Rf = 0.35, DCM/MeOH = 99.5/0.5)). After that, 30 ml of distilled
water were added to the reaction mixture and almost all methanol
was evaporated under vacuum. The aqueous layer was extracted
with 20 mL of DCM and the organic phase containing the ligand
was separated and dried under anhydrous Na2SO4. The organic
solvent was evaporated under vacuum and 4 (yield 81%) was
collected as a pale yellow oil. 1H NMR (CD3CN, 300 MHz): d 8.47
(d, J 4 Hz, 2H), 7.79 (pt, J 8 Hz, 2H), 7.51 (d, J 8 Hz, 2H), 7.30 (dd,
J 4, 8 Hz, 2H), 3.76 (d, J 16 Hz, 2H), 3.88 (d, J 16 Hz, 2H), 2.78 (m,
2H), 1.76 (m, 2H), 1.69 (m, 2H), 1.48 (m, 2H), 1.37 (m, 2H). IR (neat,
cmꢀ1): 3317, 3058, 3008, 2929, 2852, 1591, 1570, 1473, 1457,
1435, 1373, 1342, 1300, 1223, 1147, 1047, 995, 758.
complexes.
molecular recognition/sensing properties (i.e. sensitivity, specific-
ity, selectivity, etc.) towards biological materials. In fact, the lumi-
nescent chiral complexes are amenable to be analyzed by CPL
spectroscopy, that is strongly dependent on the chiral environment
around the metal. For this reason, this technique is particularly
suitable for investigating interactions with other chiral molecules
[14].
In the present contribution we show a convenient synthetic
protocol for the synthesis of a new family of tetradentate nitrogen
chiral ligands and their chiral nitrate Eu(III) complexes. We have
studied and analyzed the effects of the flexibility and stereochem-
istry of the ligand on the geometric environment of the Eu(III) ion,
establishing a relationship with the emission features of the metal
ion with the help, when possible, of the solid state molecular struc-
ture of the complexes. The emission spectra and the decay curves
of solids 5-8 complexes (Fig. 1) upon direct laser excitation of
The synthesis of Eu(III) complexes was performed in absolute
ethanol (20 mL), by adding 0.5 mmol of the ligand and 0.5 mmol
of Eu(NO3)3ꢁ5H2O (L:Eu = 1:1). The mixture was stirred for 3 h at
room temperature and in all cases white powder insoluble in eth-
anol was collected. All powders were characterized by elemental
analysis, IR spectroscopy and, when possible the molecular struc-
ture was determined by single-crystal X-ray diffraction.
Complex 5: Yield = 78%. Elemental Anal. Calc. for C20H23N8O9Eu
(MW 671.411): C, 35.78; H, 3.45; N, 16.69; O, 21.45. Found: C,
35.73; H, 3.40; N, 16.73; O, 21.48%. IR (KBr pellet, cmꢀ1): 3105,
3072, 2943, 2856, 2252, 1651, 1595, 1479, 1385, 1350, 1311,
Fig. 2. Perspective view of complex 5.

F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
67
1284, 1157, 1028, 1011, 814, 783, 741, 636. Single crystals suitable
for X-ray diffraction were obtained by slow evaporation of an ace-
tonitrile solution of 5.
Complex 6: Yield = 84%. Elemental Anal. Calc. for C18H24N7O9Eu
(MW 634.39): C, 34.08; H, 3.81; N, 15.46; O 22.70. Found: C,
34.06; H, 3.78; N, 15.49; O, 22.71%. IR (KBr pellet, cmꢀ1): 3257,
3093, 3068, 3035, 2945, 2926, 2862, 1608, 1572, 1520, 1462,
1371, 1319, 1302, 1161, 1038, 1014, 818, 769, 741. Single crystals
suitable for X-ray diffraction were obtained by slow diffusion of
diethyl ether in a solution of 6 in acetonitrile.
Complex 7: Yield = 85%. Elemental Anal. Calc. for C18H20N7O9Eu
(MW 630.358): C, 34.30; H, 3.20; N, 15.55; O 22.84. Found: C,
34.27; H, 3.16; N, 16.57; O, 22.86%. IR (KBr pellet, cmꢀ1): 3091,
3066, 2931, 2870, 1649, 1595, 1487, 1448, 1385, 1362, 1304,
1227, 1055, 1028, 1011, 989, 777, 739.
Complex 8: Yield = 92%. Elemental Anal. Calc. for C18H24N7O9Eu
(MW 634.39): C, 34.08; H, 3.81; N, 15.46; O, 22.70. Found: C,
34.05; H, 3.76; N, 15.47; O, 22.74%. IR (KBr pellet, cmꢀ1): 3284,
3107, 3070, 3037, 2976, 2945, 2924, 2858, 1608, 1572, 1495,
1435, 1385, 1317, 1290, 1155, 1045, 1034, 1003, 816, 764, 739.
Elemental analyses were carried out by using an EACE 1110
CHNOS analyzer.
2.1. Spectroscopic measurements
Luminescence measurements were carried out using a tunable
dye laser pumped by a Nd:YAG laser. The emission signal was
analyzed by a half-meter monochromator (HR460, Jobin Yvon)
equipped with a 1200 lines/mm grating and detected with a CCD
detector (Spectrum One, Jobin Yvon) or with a photomultiplier.
The spectral resolution of the emission spectra is 0.1 nm. The emis-
sion decay curves were recorded upon pulsed laser excitation using
a water cooled GaAs photomultiplier (Hamamatsu) and a 500 MHz
digital oscilloscope (WaveRunner, LeCroy). The emission spectra of
5–8 complexes were recorded on powdered samples possessing
the same crystal structure shown in Figs. 1 and 2 in the case of
complex 5 and 6.
2.2. X-ray structural determinations
The X-ray intensity data were measured on a Bruker SMART
Apex II CCD area detector diffractometer for 5 and 6. The cell
dimensions and the orientation matrix were initially determined
from a least-squares refinement on reflections measured in three
sets of 20 exposures, collected in three different
eventually refined against all data. For all crystals, a full sphere
of reciprocal space was scanned by 0.3° steps. The software SMART
x regions, and
x
[16] was used for collecting frames of data, indexing reflections
and determination of lattice parameters. The collected frames were
then processed for integration by the SAINT [16] program, and an
empirical absorption correction was applied using SADABS [17].
The structures were solved by direct methods (SIR 97) [18] and
subsequent Fourier syntheses and refined by full-matrix least-
Fig. 3. (a) Dominant conformational isomer of complex 6 (occupation factor 0.66).
(b) Minor conformational isomer of the complex 6 (occupation factor 0.34).
squares on F2
(SHELXTL) [19], using anisotropic thermal parameters
of 49.8(5)° (please compare Fig. 3a and b). All hydrogen atoms,
except the ones bound to nitrogens that were refined isotropically,
were added in calculated positions, included in the final stage of
refinement with isotropic thermal parameters, U(H) = 1.2 Ueq(C)
and allowed to ride on their carrier carbons. Crystal data and de-
tails of the data collection for all structures are reported in
Table 1. Molecular graphics were generated using SCHAKAL [20].
for all non-hydrogen atoms. Crystals of complex 5 showed the
presence of one crystallization acetonitrile molecule in the asym-
metric unit. In complex 6 two nitrate ligands were affected by
disorder over two positions with occupation factors of 0.66 and
0.34 for the main and minor isomers, respectively. The kind of dis-
order is different in the two nitrates. In fact in one nitrate (N5) only
the two chelating oxygens (O1b and O2b) are disordered over two
sites and the second image results from a rotation of the planar
nitrate [O1b, O2b, N5, O3)] of 84.7(7)° with respect to the main
image [O1a, O1b, N5, O3] whereas for the second nitrate (N6) all
four atoms give a distinct second image [N6b, O4b,O5b, O6b]
which is rotated with respect to the first one [N6a, O4a, O5a,O6a]
3. Results and discussion
The synthetic protocol for the synthesis of the nitrate com-
plexes studied in this paper is shown in Fig. 1.

68
F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
Table 1
Crystal data and structure refinement for 5ꢁCH3CN and 6.
Compound
Formula
Formula weight
T (K)
5ꢁCH3CN
6
C
18H20EuN7O9ꢁCH3CN C18H24EuN7O9
671.42
293(2)
634.40
293(2)
k (Å)
0.71073
monoclinic
P21/c
9.7970(7)
16.2185(13)
16.5723(12)
90
0.71073
orthorhombic
Pbca
12.8584(17)
14.5569(19)
25.434(3)
90
Crystal symmetry
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
102.714(4)
90
2568.7(3)
4
1.736
2.506
90
90
c
(°)
Cell volume (Å3)
4760.7(10)
8
1.770
Z
Dcalc (Mg mꢀ3
)
l
(Mo K
a
) (mmꢀ1
)
2.697
F(000)
1336
2528
Crystal size (mm)
h limits (°)
Reflections collected
0.10 ꢂ 0.15 ꢂ 0.20
1.78–25.00
21881
0.20 ꢂ 0.25 ꢂ 0.30
2.25–28.69
48669
Fig. 4. Coordination polyhedron around the Eu(III) ion of complex 5.
Unique observed reflections
3661 [Rint = 0.0912]
5874
[F0 > 4
Goodness-of-fit (GOF) on F2
R1 (F)a, wR2 (F2) [I > 2 (I)]b
Largest difference in peak and
hole (e Åꢀ3
r
(F0)]
[Rint = 0.0682]
1.035
0.0478, 0.1244
1.307 and ꢀ1.742
0.979
0.0656, 0.1152
1.219 and ꢀ1.215
r
)
a
R1
wR2 = [
=
R
||F0| ꢀ |Fc||/
R|F0|.
R
w(F0 ꢀ Fc2)2/
R
w(F02)2]1/2 where w = 1/[
r
2(F02) + (aP)2 + bP] where
b
2
P = (F02 + Fc2)/3.
As shown in this figure the 1 and 2 carbon stereocenters were
systematically varied as well as the flexibility of the ligand struc-
ture. In fact, it is possible to increase the structural flexibility
employing diamine-like ligands (2 and 4) instead of imine-like
ones (1 and 3).
3.1. Structural studies of 5 and 6
Complex 5 crystallizes in the centric space group P21/c and
therefore the racemic mixture is present. In the asymmetric unit
of the crystal of complex 5 one complex and one crystallization
acetonitrile molecule are present. The Eu(III) ion has a 10-fold
Table 2
Selected bond lengths (Å) and angles (°) for complexes 5 and 6.
Bond lengths for complex 5
Bond lengths for complex 6
Eu(1)–N(1)
Eu(1)–N(2)
Eu(1)–N(3)
Eu(1)–N(4)
Eu(1)–O(1)
Eu(1)–O(3)
Eu(1)–O(4)
Eu(1)–O(5)
Eu(1)–O(7)
Eu(1)–O(8)
2.561(9)
2.53(1)
2.63(1)
2.622(9)
2.549(8)
2.539(9)
2.488(8)
2.496(8)
2.485(8)
2.465(8)
Eu(1)–N(1)
Eu(1)–N(2)
Eu(1)–N(3)
Eu(1)–N(4)
Eu(1)–O(1A)
Eu(1)–O(1B)
Eu(1)–O(2A)
Eu(1)–O(2B)
Eu(1)–O(4A)
Eu(1)–O(4B)
Eu(1)–O(5A)
Eu(1)–O(5B)
Eu(1)–O(7)
Eu(1)–O(8)
2.575(4)
2.544(4)
2.584(5)
2.560(4)
2.644(4)
2.60(2)
2.506(8)
2.624(5)
2.464(6)
2.56(1)
2.516(7)
2.57(1)
2.470(4)
2.531(4)
Bond angles for complex 5
N(1)–Eu(1)–N(2)
N(1)–Eu(1)–N(3)
N(1)–Eu(1)–N(4)
N(2)–Eu(1)–N(3)
N(2)–Eu(1)–N(4)
N(3)–Eu(1)–N(4)
Bond angles for complex 6
N(1)–Eu(1)–N(2)
N(1)–Eu(1)–N(3)
N(1)–Eu(1)–N(4)
N(2)–Eu(1)–N(3)
N(2)–Eu(1)–N(4)
N(3)–Eu(1)–N(4)
63.4(3)
123.2(3)
63.3(3)
64.2(3)
126.7(3)
158.5(3)
66.7(1)
128.5(1)
64.8(1)
63.2(2)
129.7(1)
152.6(2)
Fig. 5. Coordination polyhedra around the Eu(III) ion of the dominant (a) and the
minor (b) conformational isomers of complex 6.
Average bond lengths: for complex 5 Eu–N(av) 2.59 Å, Eu–O(av) 2.50 Å; for complex
6 Eu–N(av) 2.57 Å, Eu–O 2.52 Å (dominant conformational isomer) 2.56 Å (minor
conformational isomer). Average N–Eu–N bite angles: for complex 5 63.5°, for the
two isomers of complex 6 64.9°.
coordination, being coordinated by one tetradentate ligand 1 and
three bidentate nitrates (Fig. 2, selected bond lengths and angles
in Table 2). The observed coordination polyhedron is a distorted

F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
69
Fig. 6. Detail of the observed structural motif in the crystal packing of complex 6 (fragment of chain built by intermolecular hydrogen bonds). [N(1)–H(1N), 0.891(3) Å, N(1)–
H(1N)ꢁ ꢁ ꢁO90, 142.40(7)°; H(1N)ꢁ ꢁ ꢁO90, 2.259(4) Å, N(1). . .O(90), 3.015(8) Å and N(2)–H(2N), 0.861(4) Å, N(2)–H(2N)ꢁ ꢁ ꢁO6a00, 139.58(8)°; H(2N)ꢁ ꢁ ꢁO6a00, 2.390(6) Å;
N(2). . .O(6A00), 3.096(4) Å]. [The primed atoms are generated by the symmetry operation: (0) ꢀx + 3/2, y + 1/2, z; (00) ꢀx + 3/2, y ꢀ 1/2, z] (All hydrogens not involved in H
bonds are omitted for clarity).
sphenocorona (Fig. 4). The N atoms of the tetradentate ligand devi-
ate from an ideal square planar geometry [N(1): +0.20(1) Å. N(2):
ꢀ0.20(1) Å, N(3): 0.102(5) Å, N(4): ꢀ0.101(6) Å], and the distance
of the Eu ion from the coordinating N atoms of the N4 least square
plane is 0.41(1) Å.
Eu(1)–N(3): 2.584(5) Å [being N(2) an sp3 nitrogen and the pyrid-
inic N(3) an sp2 nitrogen, respectively]. This behavior is in contrast
with the general trend, predicting that the metal–N(sp2) bond is
shorter than metal–N(sp3) one, as observed by Gregolinski et al.
[22] who presented a study of the crystal structures of some rare
earth ion complexes with ligands very similar to our tetradentate
molecules. Interestingly, the disordered nitrate ions with the N5
nitrogen atom are differently bound to the europium ion in the
two conformational isomers. In particular, while in the minor spe-
cies the nitrate is weakly bonded [i.e. Eu(1)–O(1b) 2.60(2) Å and
Eu(1)–O(2b) 2.624(5) Å] in a bidentate coordination mode, in the
major one it is more strongly connected but in an asymmetric
coordination mode [i.e. Eu(1)–O(1a) 2.644(4) Å and Eu(1)–O(2a)
2.506(8) Å].
Finally, a close inspection of the crystal packing reveals the
presence of intermolecular N–H. . .O hydrogen bonds running in a
direction almost parallel to the crystallographic b axis. These
hydrogen bonds of moderate strength, involving the amine hydro-
gens and nitrate oxygens join together the complex molecules,
forming parallel chains (Fig. 6).
A close inspection of the Eu–O and Eu–N bond lengths as well
as of the N–Eu–N bite angles for complex 5 and the dominant
conformer of complex 6, reveals that for the former the two
Eu–N (py) bond lengths are almost identical and the distribution
of both the Eu–O bond lengths and N–Eu–N bite angles fall in a
narrower range (Table 2). Also in the case of the minor
conformer observed in the crystal of complex 6 we observe a
narrower range of Eu–O bond lengths than that in the main con-
former. From these evidences we can conclude that the more
distorted (less symmetric) environment around the Eu(III) ion
was observed in the main conformer of complex 6. This fact is
in agreement with the major conformational flexibility of the
amine ligand 2 that generates a less symmetric (or more dis-
torted) coordination environment.
The crystal structure of complex 6 consists of discrete mole-
cules of [Eu(2)(NO3)3]. Complex 6 crystallizes in the centric space
group Pbca and therefore the racemic mixture is present. The
Eu(III) ion is coordinated by one tetradentate ligand 2 and analo-
gously to complex 5 three chelating nitrates give rise to 10-fold
coordination at the metal ion (Fig. 3a and b). Two out of the three
coordinated nitrates (N5 and N6) show disorder (see Section 2)
only of the chelating oxygen atoms over two positions (N5) or an
independent second image (N6) thus generating two conforma-
tional isomers characterized by different occupation factors
(Fig. 3a and b) and generating different coordination polyhedra.
In particular, we observe a sphenocorona and a bicapped square
antiprism polyhedra [21], for the major and the minor conforma-
tional isomers, respectively (Fig. 5a and b). In both cases, due to
the very low site symmetry (C1) of the Eu(III) ion, the polyhedra
are significantly distorted compared with the ideal geometry. It
is worth to mention the presence of an axial distorsion in the metal
coordination environment of the major isomer of complex 6. In
fact, in comparison with the averaged Eu–O bond length (2.52 Å,
Table 2), we observe two short Eu–O bonds for the almost antipo-
dal O4a and O7 ligand atoms (i.e. Eu(1)–O(4a) 2.464(4) Å, Eu(1)–
O(7) 2.470(4) Å, O(4a)–Eu(1)–O(7) 142.85(8)°, Table
1 and
Fig. 5a). The distance of the Eu ion from the least square plane
N4 defined by the coordinating N atoms is 0.456(3) Å, and the devi-
ations of the nitrogen atoms from the N4 least square plane are less
pronounced in comparison with those found in complex 5.
Interestingly, the shortest Eu–N bond is Eu–N(2): 2.543(4) Å
presumably as a consequence either of packing effects or steric
hindrance of the tetradentate ligand, while the longest one is

70
F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
a
a
5D0->
5D0-> 7F0
5D0->
7F2
7F2
576
578
580
582
584
7F1
7F4
7F0
7F4
7F1
7F3
7F3
7F0
580
600
620
640
660
680
700
580
600
620
640
660
680
700
Wavelength (nm)
wavelength (nm)
b
5D0->
7F2
b
5D0->
7F2
7F0
7F4
7F1
7F4
7F1
7F3
7F0
7F3
580
600
620
640
660
680
700
580
600
620
640
660
680
700
wavelength (nm)
wavelength (nm)
Fig. 8. (a) Emission spectrum of the complex 6 kexc = 465 nm and (b) emission
spectrum of the complex 8 kexc = 465 nm.
Fig. 7. (a) Emission spectrum of the complex 5 kexc = 465 nm and (b) emission
spectrum of the complex 7 kexc = 465 nm.
3.2. Optical spectroscopy
taking into account the higher flexibility of the amine ligands com-
pared to the imine ones, allowing for a possible stronger distortion
of the first coordination sphere of Eu(III). We also note that for
complex (6) two features are present in the region of the 0–0 tran-
sition, therefore indicating the presence of two Eu(III) emitting
centres (see inset of Fig. 8a).
This observation is consistent with the presence of disorder in
the nitrate ligands, giving rise to two different conformational iso-
mers in the crystal structure (see above).
The room temperature luminescence spectra of the complexes
containing the imine ligands, obtained upon excitation at 465 nm
(i.e. in the 5D2 level) are shown in Fig. 7. The spectra are very sim-
ilar and show the typical profile of the emission from the 5D0 level
of the Eu(III) ion in a strongly distorted environment, with a dom-
inant 5D0 ? 7F2 hypersensitive band. The Stark splittings of the ter-
minal 7FJ (J = 1–4) levels are not fully resolved and therefore are not
informative. However, the relative intensity of the 5D0 ? 7F0
(briefly, 0–0) band is extremely low, indicating that the effective
local symmetry at the metal ion is not strongly axial.
On the other hand, the emission spectra of the complexes con-
taining the amine ligands are characterized by different features
(Fig. 8). In fact, they present again a dominant hypersensitive tran-
sition, characteristic of generally low symmetry at the Eu(III) ion,
but in both cases the 0–0 band is far stronger than for the com-
plexes discussed above. This indicates that the symmetry around
the metal ion is lower, and is compatible of an axial character typ-
ical of the Cn and Cnv point groups which have been shown to give
rise to strong 0–0 transitions [23,24]. This feature seems to be
detectable from the crystal structures of the compound 6. The low-
er effective local symmetry at the Eu(III) ion, according to the lumi-
nescence spectra, for the complexes (6) and (8) can be explained
The room temperature decay curves of the 5D0 level of Eu(III) in
the various complexes are almost perfectly exponential, as shown
in the Fig. 9, even in the case of complex 6 where two emitting con-
formational isomers, characterized by different coordination envi-
ronment for Eu(III) ion, are present. Presumably, these two species
possess very similar decay kinetics of the 5D0 level yielding no
detectable deviations from a single exponential decay. The life-
times of the emitting level extracted from the experimental curves
are reported in Table 3, whose inspection clearly shows a large dif-
ference between the imine and amine complexes. In the former
case, the lifetimes are close to 1 ms, therefore indicating that mul-
tiphonon relaxation due to high frequency vibrations is not opera-
tive, as expected from the absence of N–H groups in the organic
ligand. On the other hand, for complexes (6) and (8) containing

F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
71
1
0.1
complex 5
0.01
1E-3
1
complex 6
0.1
complex 7
0.01
1E-3
complex 8
0
2
4
6
8
10
time (ms)
Fig. 9. Decay curves of 5–8 complexes.
Table 3
the stereogenic carbons on the cyclohexyl backbone seems to mar-
ginally affect the spectroscopic features of the Eu(III) ion emission
in the case of imine-based complexes, while for the amine-based
complexes the high intensity of the 5D0 ? 7F0 Eu(III) emission
probably indicates a more axially distorted crystal field around
the metal ion. In addition, due to the high affinity for the nitrate
ion of these new chiral pyridine-based Eu(III) moiety, some exper-
iments concerning the luminescence sensing of nitrate anion in
solution are currently in progress. Finally, we believe that this
new family of chiral Eu(III) complexes can be promising candidates
also for the sensing in solution of chiral substrates.
5D0 lifetime (
sobs) of the complexes under investigation.
Crystalline compound
5D0 excited state sobs (ms)
5
6
7
8
0.94(1)
0.22(1)
1.07(1)
0.26(1)
an amine ligand, the presence of two N–H amino groups in close
proximity to the metal ion induces the presence of a non-radiative
relaxation mechanism resulting in short lifetimes just above
0.2 ms.
In the case of amine complexes the lifetimes were measured
also at 77 K and they were found to be equal to the ones recorded
at room temperature. Since the N–H multiphonon relaxation pro-
cess is known to be in practice temperature independent, it ap-
pears to be the only active non-radiative relaxation channel.
Finally, the high affinity for the nitrate ion of these new family
of pyridine-based Eu(III) moieties could be exploited in order to
Acknowledgments
The authors gratefully thank Erica Viviani for expert technical.
M.M. wish to thank the University of Bologna for financial support.
Appendix A. Supplementary material
ꢀ
detect the presence of NO3 in solution. In fact, molecules similar
CCDC 837304 and 837305 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.12.042.
to 5–8 complexes, containing a weak coordinating anion instead
of nitrate can be easily prepared. The anion substitution in the first
coordination sphere of the weakly coordinating anion with nitrate,
should experience a change of the geometrical environment and
therefore of the spectroscopic behavior of the Eu(III) ion. The same
idea could be successfully applied in order to detect chiral mole-
cules, such as biological substrates.
References
4. Conclusions
[1] J.-C.G. Bünzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048.
[2] M. Albin, W.de W. Horrocks Jr., Inorg. Chem. 24 (1985) 895.
[3] G.R. Choppin, D.R. Peterman, Coord. Chem. Rev. 174 (1998) 283.
[4] M. Albin, R.R. Whittle, W.de W. Horrocks Jr., Inorg. Chem. 24 (1985) 4591.
[5] J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729.
[6] G. Muller, Dalton Trans. (2009) 9692.
[7] P. Gawryszewska, J. Legendziewicz, Z. Ciunik, N. Esfandiari, G. Muller, C. Piguet,
M. Cantuel, J.P. Riehl, Chirality 18 (6) (2006) 406.
[8] B.S. Murray, D. Parker, C.M.G. dos Santos, R.D. Peacock, Eur. J. Inorg. Chem.
(2010) 2663.
[9] H.C. Aspinall, Chem. Rev. 2002 (1807) 102.
[10] T. Yamada, S. Shinoda, H. Tsukube, Chem. Commun. (2002) 1218.
[11] J. Lisowski, S. Ripoli, L. Di Bari, Inorg. Chem. 43 (2004) 1388.
[12] S.C.J. Meskers, H.P.J.M. Dekkers, J. Phys. Chem. A 105 (2001) 4589.
[13] G.-L. Law, C. Man, D. Parker, J.W. Walton, Chem. Commun. 46 (2010) 2391.
In the present paper we presented an easy synthetic protocol,
free from time-consuming purification procedures, which allows
obtaining in a high chemical yield a new family of pyridine-based
chiral ligands and their chiral nitrate Eu(III) complexes. From struc-
tural and spectroscopic evidences in the solid state, we observed
an Eu(III) ion environment characterized by low symmetry for all
the complexes. However, the lower 5D0 ? 7F0 Eu(III) emission
intensity observed in the case of the imine-based complexes re-
flects a less distorted metal surroundings due to the more con-
formationally rigid imine ligands 1 and 3. The configuration of

72
F. Piccinelli et al. / Inorganica Chimica Acta 385 (2012) 65–72
[14] C.P. Montgomery, E.J. New, D. Parker, R.D. Peacock, Chem. Commun. (2008)
[20] E. Keller, SCHAKAL A Computer Program for the Graphic Representation of
4261.
Molecular and Crystallographic Models, Institute for Crystallography of the
University of Freiburg, Freiburg (Germany), 1997.
[21] N.G. Connelly, T. Damhus, R.M. Hartshorn, A.T. Hutton, IUPAC. Nomenclature
of Inorganic Chemistry, Recommendations 2005, RSC Publishing, Cambridge,
2005.
[15] S. Kano, H. Nakano, M. Kojima, N. Baba, K. Nakajima, Inorg. Chim. Acta 349
(2003) 6.
[16] SMART
Version), Bruker Analytical X-ray Instruments Inc., Madison, Wi, 1998.
[17] G.M. Sheldrick, SADABS, program for empirical absorption correction, University
of Göttingen, Germany, 1996.
& SAINT Software Reference Manuals, version 5.051 (Windows NT
[22] J. Gregolinski, P. Starynowicz, K.T. Hua, J.L. Lunkley, G. Muller, J. Lisowski, J.
Am. Chem. Soc. 130 (2008) 17761.
[18] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A.
Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32
(1999) 115.
[23] B. Piriou, D. Falimi, J. Dexpert-Ghys, A. Taitai, J.L. Lacout, J. Lumin. 39
(1987) 97.
[24] R.D. Peacock, Struct. Bond. 22 (1975) 83.
[19] G.M. Sheldrick, SHELXTLplus (Windows NT Version) Structure Determination
Package; Version 5.1, Bruker Analytical X-ray Instruments Inc., Madison, WI,
USA, 1998.