Toward lanthanide-containing metallomesogens with tridentate ligands

Article abstract of DOI:10.1039/a705776h

Tridentate aromatic receptors L¹-L⁴ containing the 2,6-bis-(benzimidazol-2-yl)pyridine chelating unit are designed to exhibit thermotropic calamitic liquid-crystalline phases; the mesomorphic behaviour is essentially retained in the

Full text of DOI:10.1039/a705776h

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Toward lanthanide-containing metallomesogens with tridentate ligands
Homayoun Nozary,a Claude Piguet,*a Paul Tissot,a Ge´rald Bernardinelli,b Robert Deschenaux*c and
Maria-Teresa Vilchesc
a Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4,
Switzerland
b Laboratory of X-ray Crystallography, University of Geneva, 24 quai E. Ansermet, CH-1211 Geneva 4, Switzerland
c Institute of Chemistry, University of Neuchaˆtel, Av. de Bellevaux 51, CH-2000 Neuchaˆtel, Switzerland
Tridentate aromatic receptors L1–L4 containing the 2,6-bis-
(benzimidazol-2-yl)pyridine chelating unit are designed to
exhibit thermotropic calamitic liquid-crystalline phases; the
mesomorphic behaviour is essentially retained in the com-
plex [LuL2(NO3)3].
thermodynamic, structural, electronic and magnetic properties
of the resulting mononuclear 1:1 and 1:3 complexes.2–4 The
incorporation of these building blocks into extended organized
polynuclear architectures has been demonstrated,4–6 but this
approach is severely limited by the large amount of structural
information required by segmental ligands for the selective
recognition of 4f-block metal ions.5,6 An alternative approach
could take advantage of the molecular order associated with
liquid-crystalline phases. Only a few lanthanide-containing
metallomesogens have been prepared7 and none with tridentate
ligands, most likely as a consequence of their bent arrangement
which results upon coordination.
We have recently launched into a research program aimed at
developing lanthanide building blocks with tailored tridentate
binding units.1 It is now possible to finely control and tune the
R
R
We report here on the design of semi-rigid tridentate
receptors tailored for the preparation of calamitic lanthanide-
containing liquid crystals. Substituted 2,6-bis(1A-ethylbenz-
imidazol-2A-yl)pyridine binding units (L1–L4) have been selec-
ted for their peculiar electronic and structural properties which
fit the LnIII stereochemical preferences, leading to stable
complexes with predetermined properties.3,8 Semi-rigid lipo-
philic substituents have been connected to the 5-positions of the
benzimidazole rings in order (i) to extend the rigid core and the
axial anisotropy, (ii) to improve intermolecular interactions and
(iii) to limit the stability of the crystalline phase owing to
flexible lipophilic chains. The spacers between the tridentate
unit and the para-substituted phenyl side-arms are crucial since
they must be flexible enough to allow the ligands to adopt
suitable conformations in the free form (trans–trans)5 and in the
complexes (cis–cis), and rigid enough to ensure a rod-like shape
compatible with the formation of calamitic mesophases. The
synthons 1 and 2 are obtained in satisfying yields (28 and 38%,
respectively) according to a multistep strategy based on the
recently developed cyclisation of unsymmetrical benzimidazole
rings from N-(2-nitroaryl)arenecarboxamide (Scheme 1).9 Nu-
cleophilic substitutions using 1 as electrophile give L1, L2 and
L5. Deprotonation of 2 with NBu4OH followed by alkylation or
acylation provides the ligands L3 and L4.
O2N
N
N
N
N
O
O
R
i, ii
iii, iv
N
N
Cl
NO2
N
N
R = CH2OMe
R = OMe
O2N
R
R
R = CH2OMe
R = OMe
R = CH2Br :
R = OH :
1
2
1
–
X
v
+
NBu4
Y
Y
Y
N
N
O
O
N
N
N
O
O
O
C
O
X =
; Y = OC12H25 : L1
1H NMR nuclear Overhauser enhancement effects (NOE) in
solution suggest that the tridentate units adopt the expected
trans–trans conformation in all free ligands L1–L5, which is
confirmed in the solid state by the crystal structure of the non-
mesogenic model compound L5 [N(4) is trans to N(3) and N(1)
is trans to N(3), Fig. 1].† All C–C, C–N and C–O bonds are
standard10 and the Car–C–O–Car atoms of the two spacers adopt
all-trans conformations leading to an approximate rod-like
shape of the receptor (Fig. 1). Reaction of L2 with stoichio-
metric amounts of Ln(NO3)3 (Ln = La, Lu) in acetonitrile
produces quantitatively the complexes [LnL2(NO3)3]·nH2O
(Ln = La, n = 3; Ln = Lu, n = 1) where LnIII is nine-
coordinated by the tridentate aromatic unit which adopts the
cis–cis conformation (NOE effects observed between H38–H17)
and three bidentate nitrate anions as similarly reported for the
analogous complexes with unsubstituted 2,6-bis(1A-alkylbenz-
imidazol-2A-yl)pyridine ligands.8,11
N
N
Y
X
X
Y
N
N
N
X = O; Y = OC12H25 : L2 X = O; Y = OMe : L5
2
Y
X
vi
Cl
O
O
N
N
Y
X
X
Y
N
N
N
O
C
X = ; Y = OC12H25 : L3 X = CH2; Y = OC12H25 : L4
Scheme 1 Reagents: i, EtNH2, EtOH; ii, pyridine-2,6-dicarbonyldichloride,
NEt3, CH2Cl2; iii, Fe, HCl–EtOH–H2O; iv, BBr3, CH2Cl2; v, CH2Cl2; vi,
NBu4OH, CH2Cl2
The thermal and liquid-crystal properties were investigated
by a combination of differential scanning calorimetry (DSC)
Chem. Commun., 1997
2101

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C(18)
C(17)
C(19)
C(21)
N(1)
N(4)
C(16)
C(3)
C(2)
C(22)
C(23)
C(24)
C(4)
C(5)
C(14)
C(8)
C(34)
C(29)
C(20)
C(1)
C(13)
C(11)
N(3)
C(28)
C(8)
C(33)
C(32)
C(7)
O(4)
N(5)
O(2)
N(2)
C(12)
C(27)
C(36)
O(1)
C(38)
O(3)
C(6)
C(26)
C(25)
C(30)
C(35)
C(10)
C(31)
C(15)
C(37)
C(39)
Fig. 1 ORTEP15 view of the ligand L5 with ellipsoids represented at 50% probability level
Table 1 Phase-transition temperatures and enthalpy and entropy changes for
ligands L1–L4 and complex [LuL2(NO3)3]·H2O
Footnotes and References
* E-mail: claude.piguet@chiam.unige.ch
¯
† Crystal data for L5: C39H37N5O4, M = 639.8, triclinic, space group P1,
a = 10.315(1), b = 11.8767(7), c = 14.170(1) Å, a = 105.076(5), b
= 108.972(6), g = 95.109(5)°, U = 1556.2(2) Å3 (by least-squares
DH/kJ
DS/J
Compound
Transitiona
T/°C
mol21
mol21 K21
refinement of 24 reflections, 57 @ 2q @ 76°), Z = 2, Dc = 1.37 g cm23
,
L1
L2
K–I
107
132
98
80
50
—
211
122
—
37
86
—
—
14e
66
—
39
176
—
50
F(000) = 676. Yellow prisms. Crystal dimensions 0.15 3 0.20 3 0.37
mm, m(Cu-Ka) = 0.721 mm21. Data Collection and Processing: Stoe
STADI4 diffractometer, T = 200 K, w–2q scan, scan width = 1.05 + 0.35
tanq, scan speed 0.06° s21, Cu-Ka radiation (l = 1.5418 Å); 3741
reflections measured (3 @ 2q @ 105°, 210 < h < 10, 212 < k < 11, 0
< l < 14), 3561 unique reflections (Rint for equivalent reflections = 0.017)
of which 3091 were observable [ıFoı > 4s(Fo)]. Two reference reflexions
were measured every 45 min and showed no variation in intensity. Structure
analysis and refinement: data were corrected for Lorentz, polarization and
absorption effects12 (A*min = 1.102, A*max = 1.186). The structure was
solved by direct methods using MULTAN 87;13 all other calculations used
XTAL14 system and ORTEP II15 programs. Full-matrix least-squares
refinements (on F) using weights of w = 1/[s2(Fo) + 0.0001(Fo2)] gave
final values R = 0.044, Rw = 0.049, for 434 variables and 3091 contributing
reflections. All non-H atoms were refined with anisotropic displacement
parameters. H-atoms were placed in calculated positions and contributed to
Fc calculations. CCDC 182/599.
K–SA
(SC–SA)b
SA–I
K–SC
SC–SA
SA–N
N–I
K–SA
(SC–SA)b
SA–I
K–SA
(SC–SA)b
SA–I
c
188
131
217
223
226
144
107f
193
133
98
17
35
—
L3
c
d
—
7e
28
—
18
72
L4
[LuL2(NO3)3]
c
—
188
23
a K = crystal, SC = smectic C phase, SA = smectic A phase, N = nematic
phase, I = isotropic fluid; temperatures are given as the onset of the peak
(Seiko DSC 220C differential scanning calorimeter, 5 °C min21, under N2);
the liquid crystalline phases were identified from their optical textures:
SC = broken focal-conic fan and schlieren textures; SA = focal-conic fan
texture and homeotropic zones; N = schlieren and marbled textures.
b Monotropic transition. c Second-order transition determined by polarized
optical microscopy. d Masked by isotropization. e Cumulated enthalpies and
entropies. f Approximate value: the SC phase formed during the crystalliza-
tion process.
1 For reviews see: C. Piguet and J.-C. G. Bu¨nzli, Eur. J. Solid State Inorg.
Chem., 1996, 33, 165; C. Piguet, J.-C. G.Bu¨nzli, G. Bernardinelli,
G. Hopfgartner and A. F. Williams, J. Alloys Compd., 1995, 225, 324;
C. Piguet, Chimia, 1996, 50, 144.
2 S. Petoud, J.-C. G. Bu¨nzli, F. Renaud and C. Piguet, J. Alloys Compd.,
1997, 249, 14.
3 C. Piguet, A. F. Williams, G. Bernardinelli and J.-C. G. Bu¨nzli, Inorg.
Chem., 1993, 32, 4139; C. Piguet, J.-C. G. Bu¨nzli, G. Bernardinelli,
C. G. Bochet and P. Froidevaux, J. Chem. Soc., Dalton Trans., 1995,
83.
and polarized optical microscopy (Table 1). The ligand L1
(three-atom spacers) does not show mesogenic behaviour.
However, the ligands L2–L4, which possess two-atom spacers,
display interesting mesomorphism: they all present disordered
smectic C and smectic A phases. An additional nematic phase is
obtained for L3. Interestingly, the mesomorphic behaviour of L2
is essentially retained in the lanthanide complexes. In [LaL2-
(NO3)3]·3H2O, the melting process (130 °C) is associated with
a fast decomposition which prevents an unambigous character-
ization of the mesophase(s), but the analogous complex
[LuL2(NO3)3]·H2O melts at 133 °C to give a stable SA phase
which isotropizes at 188 °C. Although the temperatures of the
K ? SA and SA ? I transitions are similar for L2 and its
complex [LuL2(NO3)3]·H2O, the enthalpy and entropy changes
are significantly larger for the latter compound.
4 C. Piguet, E. Rivara-Minten, G. Bernardinelli, J.-C. G. Bu¨nzli and
G. Hopfgartner, J. Chem. Soc., Dalton Trans., 1997, 421.
5 C. Piguet, J.-C. G. Bu¨nzli, G. Bernardinelli, G. Hopfgartner, S. Petoud
and O. Schaad, J. Am. Chem. Soc., 1996, 118, 6681.
6 C. Piguet, J.-C. G. Bu¨nzli, G. Bernardinelli, G. Hopfgartner and
A. F. Williams, J. Am. Chem. Soc., 1993, 115, 8197.
7 Z. K. Wang, C. H. Huang and F. Q. Zhou, Solid State Commun., 1995,
95, 223; Y. Galymetdinov, A. M. Athanassopoulou, O. Kharitonova,
E. A. Soto-Bustamante, I. K. Ovchinnikov and W. Hasse, Chem. Mater.,
1996, 8, 922; I. Bikhantaev, Y. Galymetdinov, O. Kharitonova,
I. K. Ovchinnikov, D. W. Bruce, D. A. Dunmur, D. Guilter and
B. Heinrich, Liq. Cryst., 1996, 20, 489.
8 C. Piguet, A. F. Williams, G. Bernardinelli, E. Moret and J.-C. G.
Bu¨nzli, Helv. Chim. Acta, 1992, 75, 1697.
9 C. Piguet, B. Bocquet and G. Hopfgartner, Helv. Chim. Acta, 1994, 77,
931.
10 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. Taylor, J. Chem. Soc., Dalton Trans., 1987, S1.
11 S. Petoud, J.-C. G. Bu¨nzli, K. J. Schenk and C. Piguet, Inorg. Chem.,
1997, 36, 1345.
12 E. Blanc, D. Schwarzenbach and H. D. Flack, J. Appl. Crystallogr.,
1991, 24, 1035.
13 P. Main, S. J. Fiske, S. E. Hull, L. Lessinger, D. Germain, J. P. Declercq
and M. M. Woolfson, MULTAN 87; Universities of York, England, and
Louvain-La-Neuve, Belgium, 1987.
14 XTAL 3.2 User’s Manual, ed. S. R. Hall and J. M. Stewart, Universities
of Western Australia and Maryland, 1992.
15 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
In conclusion, two-atom spacers as in L2–L4 are suitable for
the incorporation of bent tridentate units into rod-like shape
receptors exhibiting thermotropic calamitic behaviours. A fine
tuning of the thermal properties results from the nature of the
spacer, while metallic size effects are responsible for the larger
stability of the Lu complex as previously reported for other
lanthanide-containing metallomesogens.7 The introduction of
spectroscopically and magnetically active LnIII into smectic
phases is currently under investigation.
We gratefully thank Mrs He´le`ne Lartigue, Dr J.-P. Rivera and
C. Fouillet for their technical assistance. C. P. thanks the
Werner Foundation for a fellowship. This work is supported
through grants from the Swiss National Science Foundation.
Received in Cambridge, UK, 7th August 1997; 7/05776H
2102
Chem. Commun., 1997