Mononuclear single helices of lanthanum(III) and europium(III). Metal dependent diastereomeric excess

Article abstract of DOI:10.1016/j.ica.2010.05.016

Using the 1:2 condensate of benzildihydrazone and 2-acetylpyridine as a tetradentate N donor ligand L, LaL(NO₃)₃ (1) and EuL(NO₃)₃ (2), which are pale yellow in colour, are synthesized. While single crystals of 1 could not be obtained, 2 crystallises as a monodichloromethane solvate, 2·CH₂Cl₂ in the space group Cc with a = 11.7099(5) , b = 16.4872(5) , c = 17.9224(6) and β = 104.048(4)°. From the X-ray crystal structure, 2 is found to be a rare example of monohelical complex of Eu(III). Complex 1 is diamagnetic. The magnetic moment of 2 at room temperature is 3.32 BM. Comparing the FT-IR spectra of 1 and 2, it is concluded that 1 also is a mononuclear single helix. ¹H NMR reveals that both 1 and 2 are mixtures of two diastereomers. In the case of the La(III) complex (1), the diastereomeric excess is only 10% but in the Eu(III) complex 2 it is 80%. The occurrence of diastereomerism is explained by the chiralities of the helical motif and the type of pentakis chelates present in 1 and 2.

Full text of DOI:10.1016/j.ica.2010.05.016

Inorganica Chimica Acta 363 (2010) 4123–4126
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Note
Mononuclear single helices of lanthanum(III) and europium(III). Metal
dependent diastereomeric excess
Senjuti De a, Michael G.B. Drew b, Dipankar Datta a,
*
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India
b School of Chemistry, The University, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Using the 1:2 condensate of benzildihydrazone and 2-acetylpyridine as a tetradentate N donor ligand L,
LaL(NO3)3 (1) and EuL(NO3)3 (2), which are pale yellow in colour, are synthesized. While single crystals of
1 could not be obtained, 2 crystallises as a monodichloromethane solvate, 2ÁCH2Cl2 in the space group Cc
with a = 11.7099(5) Å, b = 16.4872(5) Å, c = 17.9224(6) Å and b = 104.048(4)°. From the X-ray crystal
structure, 2 is found to be a rare example of monohelical complex of Eu(III). Complex 1 is diamagnetic.
The magnetic moment of 2 at room temperature is 3.32 BM. Comparing the FT-IR spectra of 1 and 2, it
is concluded that 1 also is a mononuclear single helix. 1H NMR reveals that both 1 and 2 are mixtures
of two diastereomers. In the case of the La(III) complex (1), the diastereomeric excess is only 10% but
in the Eu(III) complex 2 it is 80%. The occurrence of diastereomerism is explained by the chiralities of
the helical motif and the type of pentakis chelates present in 1 and 2.
Received 30 January 2010
Received in revised form 27 April 2010
Accepted 7 May 2010
Available online 19 May 2010
Keywords:
Lanthanum(III)
Europium(III)
Helicates
Diastereomer
Diastereomeric excess
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
There are ample examples of helical motifs in biology. These are
self-assembled. These prompted the syntheses of artificial helices
initially. But now synthetic helical molecules find applications in
different fields like stereoselective catalysis, chemical sensing,
DNA binding etc. [1–5]. Syntheses of purely organic helical mole-
cules are more difficult than metallo-organic helices. This is be-
cause the directive coordination behaviour of the metal ions
facilitates generation of metallo helices. A variety of inorganic sin-
gle, double and triple stranded helices have been constructed in the
past using non-helical organic ligands [6–8]. For the past few years
we have been engaged in synthesizing metal complexes of ligands
which themselves are helical. One such ligand is L. Our synthetic
strategy has generated a number of helical and möbius molecules
[9–15]. Metals used are Ni, Cu, Ag, Zn, Cd and Hg. In the present re-
port, the metal ions involved are the lanthanides La(III) and Eu(III).
2. Results and discussion
The ligand L is the 1:2 condensate of benzildihydrazone and 2-
acetylpyridine [12]. The N@C(Ph)–C(Ph)@N torsion angle of
85.9(5)° brings about its helicity.
Reaction of La(NO3)3ÁxH2O and Eu(NO3)3ÁxH2O with L in equi-
molar ratio in acetonitrile at room temperature gives LaL(NO3)3
(1) and EuL(NO3)3 (2) respectively in moderate yield. We have
not been able to grow single crystals of 1. But direct diffusion of
n-hexane into a dichloromethane solution of 2 yields single crys-
tals of its mono dichloromethane solvate.
The X-ray crystal structure of 2ÁCH2Cl2 contains the metal com-
plex EuL(NO3)3 together with solvent dichloromethane. The com-
plex is shown in Fig. 1 along with the numbering scheme in the
metal coordination sphere. Selected bond distances and angles
are given in Table 1. The metal complex adopts an approximate
C2 conformation as is apparent from Fig. 1. The europium atom
is ten-coordinate. All the three nitrate ligands are bonded to the
metal centre in a bidentate fashion with Eu–O distances ranging
from 2.463(7) to 2.523(6) Å. The four nitrogen atoms of the ligand
L are bonded to the metal with distances to the imino nitrogen
atoms N(48), N(53) of 2.566(7), 2.589(7) Å and to the pyridine
nitrogen atoms N(60) and N(41) of 2.623(7), 2.679(7) Å. The ligand
adopts a conformation in which the central torsion angles are
C(54)–N(53)–N(52)–C(51) 112.8(3), N(53)–N(52)–C(51)–C(50)
À5.1(3), N(52)–C(51)–C(50)–N(49) 76.4(3), C(51)–C(50)–N(49)–
Me
N
Ph
N
Ph
N
Me
N
N
N
ligand L
* Corresponding author. Tel.: +91 33 24735374; fax: +91 33 24732805.
E-mail address: icdd@iacs.res.in (D. Datta).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.016

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S. De et al. / Inorganica Chimica Acta 363 (2010) 4123–4126
Fig. 2. A space-filling model of 2 showing its helical nature. Colour code: Eu, white;
N, black; C, dark gray; O, light gray.
Fig. 1. The structure of 2 in 2ÁCH2Cl2 with ellipsoids at 25% probability.
1
1
N(48) À2.0(3), C(50)–N(49)–N(48)–C(47) 118.5(3)°. The angle be-
tween the planes containing pyridine rings is 77.36°.
2
2
The topology of 2 is helical. This is evident from its space-filling
model shown in Fig. 2. Complex 2 is a mononuclear single helix.
Though there are many literature reports of double and triple heli-
ces [16,17] but till date only two examples of mononuclear single
helix of Eu(III) are known [18,19]. The FT-IR spectra of 1 and 2 are
virtually identical. Further, as revealed in molar conductance mea-
surements, complex 1 is non-electrolyte like 2 in methanol indicat-
ing that the nitrate groups are bound to the metal in 1 as well.
Hence it is reasonable to assume that our La(III) complex also
has a structure similar to that of the Eu(III) complex 2.
3
3
4
4
5
5
I
II
Complexes 1 and 2, being single helical, can have two optical
isomers – P (plus; right handed) and M (minus; left handed)
[20]. The configuration shown in Figs. 1 and 2 is M. Now the sort
Fig. 3. Schematic representation of the type of pentakis chelate observed in Fig. 2 (I)
and its non-superimposable mirror image (II).
of pentakis chelate encountered in 2 (and in 1) is schematically
shown in Fig. 3. It can have two absolute configurations. This type
of chelate is identified for the first time. So a convention has to be
developed to determine their absolute configurations. In structure I
of Fig. 3, if one traces the chelate rings from the top in a sequence
indicated in the figure (1 ? 2 ? 3 ? 4 ? 5), then a left handed
screw is generated. In keeping with the convention used for octa-
Table 1
Selected bond distances (Å) and angles (°) in 2ÁCH2Cl2. For atom labeling scheme, see
Fig. 1.
Eu(1)–O(11)
Eu(1)–O(21)
Eu(1)–O(31)
Eu(1)–N(48)
2.463(7)
2.480(6)
2.513(6)
2.566(7)
2.623(7)
77.9(2)
Eu(1)–O(12)
Eu(1)–O(23)
Eu(1)–O(32)
Eu(1)–N(53)
2.523(6)
2.440(6)
2.503(6)
2.589(7)
2.679(7)
51.6(2)
169.0(2)
132.7(2)
75.1(2)
51.0(2)
51.5(2)
72.7(2)
73.8(2)
73.8(2)
139.9(2)
95.4(2)
137.9(2)
76.4(2)
71.8(2)
73.0(2)
110.4(2)
116.5(2)
60.5(2)
114.5(2)
66.4(2)
69.7(2)
111.9(2)
Eu(1)–N(60)
Eu(1)–N(41)
hedral tris chelates [20], we assign
Fig. 3 can then be assigned as . Consequently we can have four
enantiomers for each of 1 and 2 – P , M , P and M , i.e. two dia-
K to I. Configuration II in
O(21)–Eu(1)–O(11)
O(11)–Eu(1)–O(21)
O(11)–Eu(1)–O(32)
O(21)–Eu(1)–O(31)
O(21)–Eu(1)–O(31)
O(21)–Eu(1)–O(12)
O(21)–Eu(1)–O(12)
O(31)–Eu(1)–O(12)
O(11)–Eu(1)–N(48)
O(32)–Eu(1)–N(48)
O(12)–Eu(1)–N(48)
O(11)–Eu(1)–N(53)
O(32)–Eu(1)–N(53)
O(12)–Eu(1)–N(53)
O(21)–Eu(1)–N(60)
O(21)–Eu(1)–N(60)
O(31)–Eu(1)–N(60)
N(48)–Eu(1)–N(60)
O(21)–Eu(1)–N(41)
O(21)–Eu(1)–N(41)
O(31)–Eu(1)–N(41)
N(48)–Eu(1)–N(41)
N(60)–Eu(1)–N(41)
O(22)–Eu(1)–O(21)
O(21)–Eu(1)–O(32)
O(21)–Eu(1)–O(32)
O(11)–Eu(1)–O(31)
O(32)–Eu(1)–O(31)
O(11)–Eu(1)–O(12)
O(32)–Eu(1)–O(12)
O(21)–Eu(1)–N(48)
O(21)–Eu(1)–N(48)
O(31)–Eu(1)–N(48)
O(21)–Eu(1)–N(53)
O(21)–Eu(1)–N(53)
O(31)–Eu(1)–N(53)
N(48)–Eu(1)–N(53)
O(11)–Eu(1)–N(60)
O(32)–Eu(1)–N(60)
O(12)–Eu(1)–N(60)
N(53)–Eu(1)–N(60)
O(11)–Eu(1)–N(41)
O(32)–Eu(1)–N(41)
O(12)–Eu(1)–N(41)
N(53)–Eu(1)–N(41)
D
72.5(2)
K
K
D
D
112.8(2)
133.7(2)
144.3(2)
117.8(2)
77.7(2)
70.5(2)
144.9(2)
97.1(2)
128.5(2)
132.0(2)
75.5(2)
143.9(2)
69.2(2)
115.8(2)
67.2(2)
114.4(2)
112.6(2)
69.2(3)
stereomers are possible for 1 and 2. In the chemistry of helicates
diastereomeric selectivity/excess has often been observed [21–
25]. In order to find out whether there is any diastereomeric selec-
tivity in our case, we have examined the 1H NMR spectra of 1 and 2
in CDCl3 (Fig. 4). Since complexes 1 and 2 have a C2 axis (revealed
from Fig. 1), the two methyl groups in them are expected to give
rise to only one singlet in the absence of any diastereomeric prob-
lem. In reality, both for 1 and 2, two methyl signals are observed
with different intensity. In the case of 1, the methyl signals appear
at 2.28 and 2.40 ppm (reference: TMS) with an intensity ratio of
0.83:1 indicative of 45:55% population of the two diastereomers
[26]. In the case of 2, the proton NMR spectrum is paramagnetically
shifted. The two methyl signals appear at 7.48 and 8.87 ppm (ref-
erence: TMS) with an intensity ratio of 1:0.24 which means that in
2 the population of the two diastereomers is 10:90% [26]. Thus in 1
and 2 we have metal dependent diastereomeric excess. Earlier
Perrett-Aebi et al. [24] have observed such metal dependent
112.6(2)
60.3(2)
172.3(2)

S. De et al. / Inorganica Chimica Acta 363 (2010) 4123–4126
4125
Fig. 4. 300 MHz 1H NMR spectra of 1 (a) and 2 (b) in CDCl3 where TMS is the reference. The sharp peaks at 7.26 ppm in (a) and at 7.69 ppm in (b) are due to the residual
protons in the solvent. The peaks at 1.60 ppm in (a) and at 3.19 ppm in (b) are due to water present in the solvent.
diastereomeric excess in the helicates of Cu(I) and Ag(I) of the
same ligand.
Anal. Calc. for C28H24N9EuO9: C, 42.98; H, 3.09; N, 16.11. Found: C,
42.80; H, 3.04; N, 16.29%. Main FT-IR bands (KBr)
/cmÀ1: 3398
m
(br,s), 1612 (w), 1591 (m), 1568 (w), 1527 (sh), 1500 (m), 1471
(sh), 1445 (sh), 1385 (vs), 1279 (m), 1030 (m), 1009 (w), 785
3. Experimental
(m), 741 (m) and 692 (m). KM/mho cm2 molÀ1: 21.
l/l
B: 3.32
(at 297 K). UV–Vis: kmax/nm (
e
/dm3 molÀ1 cmÀ1): 263 (4 040),
3.1. Materials and physical measurements
298 (4 840).
The ligand was prepared by a procedure reported earlier by us
[12]. Micro-analyses were performed by a Perkin–Elmer 2400II ele-
mental analyser. Molar conductances were measured in methanol
by a Systronics (India) conductivity bridge (model 306). FT-IR spec-
tra (KBr) were recorded on a Shimadzu FT-IR-8400S spectropho-
3.4. X-ray crystallography
Crystal data for 2ÁCH2Cl2 are given in Table 2. These were col-
lected at 150(2) K with Mo K radiation using the Oxford Diffrac-
a
tometer, UV–Vis spectra in acetonitrile on
a Perkin–Elmer
tion X-Calibur CCD System. The crystal was positioned at 50 mm
from the CCD. Three hundred and twenty one frames were mea-
sured with a counting time of 2 s. Data analysis was carried out
with the CRYSALIS program [27]. The structure was solved using di-
rect methods with the SHELX97 program [28]. The non-hydrogen
atoms were refined with anisotropic thermal parameters. The
hydrogen atoms bonded to carbon were included in geometric
Lambda 950 spectrophotometer and 300 MHz 1H NMR spectra in
CDCl3 on a Bruker DPX300 spectrometer. Magnetic moments were
measured at room temperature by a magnetic susceptibility bal-
ance procured from Sherwood Scientific, UK. Pascal’s constants
were used for diamagnetic corrections.
3.2. Syntheses of LaL(NO3)3 (1)
Table 2
L (0.089 g, 0.2 mmol) dissolved in 10 ml of acetonitrile was
added to an acetonitrile solution (10 ml) of La(NO3)3ÁxH2O
(0.065 g) dropwise with stirring. The yellow colour of the solution
faded and compound started appearing within 5 min of stirring.
After stirring for 1 h the precipitated compound was filtered off,
washed with few drops of petroleum ether and dried in air. Yield:
0.100 g (65%). Anal. Calc. for C28H24N9LaO9: C, 43.71; H, 3.14; N,
16.38. Found: C, 43.59; H, 3.15; N, 16.29%. Main FT-IR bands
Crystallographic data for EuL(NO3)3ÁCH2Cl2 (2ÁCH2Cl2).
Formula
M
Space group
Cell dimensions (Å, °)
C29H26N9EuCl2O9
867.45
monoclinic, Cc
a
b
c
b
11.7099(5)
16.4872(5)
17.9224(6)
104.048(4)
3356.7(2)
4, 1.716
2.094
1728
4864
(KBr)
(sh), 1497 (sh), 1466 (m), 1441 (sh), 1385 (vs), 1275 (m), 1028
(w), 1005 (w), 785 (m), 739 (m) and 692 (m). KM/mho cm2 molÀ1
17. UV–Vis kmax/nm (
/dm3 molÀ1 cmÀ1): 263 (4 200), 298(5 300).
m
/cmÀ1: 3404 (br,s), 1614 (m), 1591 (w), 1566 (w), 1517
U (Å3)
Z, Dcalc (g cmÀ3
)
l
(mmÀ1
F(0 0 0)
Number of reflections unique
)
:
e
Observed [I > 2
r(I)]
4649
3.3. Synthesis of EuL(NO3)3 (2)
Parameters
R indices (observed data)
453
R1, 0.0518; wR2, 0.1358
R1, 0.0534; wR2, 0.1378
2.375 and 2.506
R indices (all data)
It was prepared exactly the same way as 1 by starting with Eu(-
NO3)3ÁxH2O (0.068 g) and L (0.089 g, 0.2 mmol). Yield: 78 mg (50%).
Largest difference in peak and hole (e ÅÀ3
)

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S. De et al. / Inorganica Chimica Acta 363 (2010) 4123–4126
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those of the atom to which they were attached. An absorption cor-
rection was carried out using the ABSPACK program [29]. The struc-
ture was refined on F2 using SHELX97 [28].
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Acknowledgments
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We thank EPSRC and the University of Reading for funds for the
X-Calibur system. Discussions with Dr. T.K. Paine are gratefully
acknowledged.
Appendix A. Supplementary material
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CCDC 706293 contains the supplementary crystallographic 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 associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2010.
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[27] CRYSALIS Program, Version 1, Oxford Diffraction, 2005.
[28] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University
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