Mesomorphism of a tetrahedral zinc complex

Article abstract of DOI:10.1039/b407652d

Thermotropic smectic phases have been observed for the first time in a zinc coordination complex with tetrahedral geometry; this complex, which contains the pyrazole dimer bis[3,5-bis(p-decyloxyphenyl)pyrazolyl]ethane as the ligand, exhibits fluorescence.

Full text of DOI:10.1039/b407652d

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Mesomorphism of a tetrahedral zinc complex{
Raquel Gime´nez,a Ana Bele´n Manrique,a Santiago Uriel,b Joaqu´ın Barbera´a and Jose´ Luis Serrano*a
a Dpto Qu´ımica Orga´nica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago´n, Universidad
de Zaragoza-CSIC, Pedro Cerbuna, 12, 50009 Zaragoza, Spain. E-mail: joseluis@unizar.es;
Fax: 34 976761209; Tel: 34 976761209
b Dpto Qu´ımica Orga´nica, Centro Polite´cnico Superior-Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, Mar´ıa de Luna, 3, 50015 Zaragoza, Spain
Received (in Cambridge, UK) 20th May 2004, Accepted 24th June 2004
First published as an Advance Article on the web 13th August 2004
Thermotropic smectic phases have been observed for the first
time in zinc coordination complex with tetrahedral
to its twin by a spacer (methylene or ethylene group in this work)
and prepare the corresponding zinc complexes.
a
The synthesis of the novel ligands L2m and L2e was approached
by solid–liquid phase-transfer catalysis by adapting a method
described by Diez Barra et al.9 The zinc complexes were prepared
using anhydrous zinc(II) chloride in a stoichiometric amount in
THF.10 Polarising optical microscopy and DSC studies (Table 1)
reveal that compound L is mesomorphic, displaying SmC and
SmA phases, but its zinc complex [ZnL2Cl2] is not liquid crystalline.
The structural analysis of a similar compound [Zn(3,5-dimethyl-
pyrazole)2Cl2]11 shows that the zinc atom in the complex is in a
tetrahedral geometry, which introduces an unfavourable angle
between the mesogens at the N1 position and precludes meso-
morphism. Similar behaviour is observed for the ligand L2m with a
tetrahedral methylene bridge, although it has the same melting
point as L. In the complex [Zn(L2m)Cl2] the pyrazole dimer L2m
coordinates to the metal through its two pyridinic nitrogen atoms
and generates a compound that is not mesomorphic. The absence
of mesomorphism is probably due to the non-planar shape of the
central metallacycle, a situation previously described in similar
derivatives [Zn[2,2-bis(pyrazolyl)propane]Cl2]12 or [Zn[bis(3,4,5-
trimethylpyrazolyl)methane]Br2],13 where the six-membered
C(NN)2Zn ring adopts a boat form with a fold angle of 140u
and 126u, respectively. Ligand L2e displays a monotropic smectic A
phase, probably due to the increased degree of freedom introduced
by the ethylene spacer with respect to the methylene one. However,
the short range of the mesophase indicates that the L units are
still far from being independent. In contrast, the zinc complex
[Zn(L2e)Cl2] shows enantiotropic smectic C and A phases, indicat-
ing that the liquid crystalline phases are significantly favoured by
complexation. The combination of the metal centre, which closes a
seven-membered ring, and the flexibility of the ethylene spacer are
crucial to minimise the steric demands of the tetrahedral centre and
allow the organisation of the molecules in layers.
geometry; this complex, which contains the pyrazole dimer
bis[3,5-bis(p-decyloxyphenyl)pyrazolyl]ethane as the ligand,
exhibits fluorescence.
Zinc(II) can coordinate with a vast number of N and N,O donor
ligands that display interesting luminescent properties for LED
devices and sensors.1 In most of these compounds, the zinc atom
adopts a tetrahedral geometry, which has been proven to be a
major drawback for mesomorphism.2 Indeed, in the liquid
crystalline zinc complexes described to date the metal centre is in
a different geometry, e.g. forced into a planar coordination (with
porphyrin ligands3 and extended analogues) or pentacoordinated
in a trigonal bipyramidal geometry (with dithiobenzoates4 or
tridentate pyridines5). Only one report concerning columnar phases
in tetrahedral 2,2’-bipyridine zinc complexes has been commu-
nicated and this only appeared very recently.6 By means of ligand
design it is possible to obtain controlled supramolecular assemblies
with coordination metal complexes.7 This idea has been exploited
in the work described here to generate smectic order in a fluorescent
tetrahedral zinc complex. The combination of fluidity and
orientational ability of liquid crystals with the particular properties
of the complexes may lead to new candidates for multifunctional
materials.
We prepared several compounds derived from 3,5-bis(p-
decyloxyphenyl)pyrazole (L, Scheme 1). This ligand was chosen
for several reasons: it can be coordinated to zinc through the
pyridine-type nitrogen, it is liquid crystalline8 and it can be
modified through chemistry on the N1 nitrogen. Therefore, it is
possible to synthesize pyrazole dimers in which a pyrazole is joined
{ Electronic supplementary information (ESI) available: characterization
data for the novel compounds; microphotographs of the mesophase
textures of compound [Zn(L2e)Cl2] and absorption and emission spectra
for L2e and [Zn(L2e)Cl2]. See http://www.rsc.org/suppdata/cc/b4/b407652d/
Examples of similar structures have not been described pre-
viously so, in an attempt to confirm our hypotheses, we attempted
to grow crystals of [Zn(L2e)Cl2]. Unfortunately, all efforts to
Scheme 1 Reagents and conditions: (i) ClCH2CH2Cl or CH2Br2, K2CO3, NBu4Br, 90 uC, 64–70%; (ii) ZnCl2, THF, r.t., 60–70%.
2 0 6 4
C h e m . C o m m u n . , 2 0 0 4 , 2 0 6 4 – 2 0 6 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Transition temperatures and thermodynamic dataa
Compound
Tonset/uC (DH/kJ mol21
)
L [Ref. 8]
L2m
L2e
Cr 79 (31.8) Cr’ 130 (13.1) SmC 180b SmA 183 (6.1) I
Cr 129 (59.7) I
Cr [122 (7.2) SmA]c 124 (56.1) I
Cr 108 (95.0) Cr’ 178 (53.2) I
[ZnL2Cl2]
[Zn(L2m)Cl2] Cr 97 (16.0) Cr’ 164 (27.2) I
[Zn(L2e)Cl2] Cr 95 (10.6) SmC 134 (13.7d ) SmA 154b
I
a Cr, Cr’: crystal phases, I: isotropic liquid, SmA: smectic A, SmC:
smectic C. b Optical microscopy data. c Monotropic transition.
d Transitions SmC–SmA and SmA–I appear by DSC as a single
peak.
Fig. 2 Absorption and emission spectra for compound L2e (dotted line) and
[Zn(L2e)Cl2] (solid line).
study new multifunctional materials based on zinc-containing
mesogens.
We thank the Diputacio´n General de Arago´n, the CICYT-
FEDER projects MAT2002-04118-CO2-02 and MAT2003-07806-
CO2-01, and the ‘‘Programa Ramo´n y Cajal’’ from the Ministerio
de Ciencia y Tecnolog´ıa (Spain) for financial support.
Notes and references
{ Crystal data for [Zn(bmpze)Cl2]: colourless crystals suitable for crystal-
lographic analysis were obtained by slow diffusion of hexane into a
solution of the compound in dichloromethane. C24H36Cl4N8Zn2, M ~
˚
709.15, monoclinic, a ~ 14.976(3), b ~ 11.164(3), c ~ 18.877(3) A, b ~
3
˚
101.590(9)u, V ~ 3091.8(10) A , T ~ 293 K, space group P21/c, Z ~ 4,
Fig. 1 (a) ORTEP representation of [Zn(bmpze)Cl2] (50% probability
ellipsoids) with atom numbering scheme. (b) Packing diagram showing two
kinds of planes parallel to bc.
m(Mo-Ka) ~ 1.926 mm21. The structure, refined on F2, converged for
5434 unique reflections (Rint ~ 0.0376) to give R1 ~ 0.0569, wR2 ~ 0.1270
[I w 2s(I)] and R1 ~ 0.1112, wR2 ~ 0.1536 (all data) and a goodness-of-
fit
~ 1.033. CCDC 239724. See http://www.rsc.org/suppdata/cc/b4/
achieve this goal proved unsuccessful. However, we were able to
solve the structure of [Zn(bmpze)Cl2] [bmpze: 1,2-bis(3,5-dimethyl-
pyrazolyl)ethane], in which the metal is in a similar environment,
and use this as a model for comparison.{ The molecular structure is
shown in Fig. 1a. The zinc atom is in a tetrahedral environment
and the planes containing the pyrazole rings are arranged with
respect to one another at an angle of only 16u, indicating that the
molecule is more planar than the ones with a methylene spacer.
From the packing study (Fig. 1b) it appears that the chloro ligands
are disposed in pseudoaxial and equatorial positions with respect to
the main molecular plane and the molecules are arranged in layers
in an antiparallel fashion.
b407652d/ for crystallographic data in .cif or other electronic format.
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The mesophase assignment was confirmed by powder X-ray
diffraction14 at different temperatures. In the smectic C phase layer
˚
˚
spacings of 29 A (108 uC) and 30 A (118 uC) were measured. The
˚
smectic A phase was found to have a layer spacing of 32 A at
different temperatures (140, 145 and 150 uC). These values are in
accordance with a packing model similar to the mesophases of L
˚
˚
(measured layer spacing of 34.7 A in the SmA and 32–33 A for the
SmC)8 where the molecules are arranged with their long axes
perpendicular (SmA) or tilted (SmC) to the layer. It seems reason-
able that [Zn(L2e)Cl2] can lead to a smaller layer spacing as the
dimeric nature of the compound may leave more free volume to
accommodate the conformationally disordered decyloxy chains in
the layer.
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14 Powder XRD patterns were obtained using a pinhole camera (Anton-
Paar) operating with a point-focused Ni-filtered Cu-Ka beam and the
diffraction pattern was collected on flat photographic film. The samples
were held in Lindemann glass capillaries (1 mm diameter).
A preliminary study of the optical properties in dichloromethane
solutions showed that the ligand L2e exhibits fluorescence at 338 nm
upon excitation at the absorption maximum (263 nm). The zinc
complex [Zn(L2e)Cl2] fluoresces under similar conditions at 365 nm
with a higher intensity. Thus, on complexation both a red-shift and
a chelation-enhanced fluorescence, estimated to be multiplied by a
factor of three, are observed (Fig. 2).
Following this work we will apply the ‘‘flexible ligand’’ approach
to other structures in order to generate new phases as a way to
C h e m . C o m m u n . , 2 0 0 4 , 2 0 6 4 – 2 0 6 5
2 0 6 5