Metal-directed columnar phase formation in tetrahedral zinc(II) and manganese(II) metallomesogens

Article abstract of DOI:10.1039/b710745e

New, non-discoid M(II) complexes [MCl₂(L)] {M = Mn, Zn; L = 2,6-diformyl-4-methylphenolbis[3′,4′,5′-tris(hexadecyloxy) phenylimine]} have been prepared and shown to exhibit columnar mesophases, the nature of which was found to be metal-dependent. As analysed by DSC and small-angle X-ray diffraction experiments, the complex [ZnCl₂(L)] exhibits a single mesophase with rectangular symmetry (Col_r) between 46 and 145°C, whereas the Mn(ii) analogue [MnCl₂(L)] displays a purely hexagonal mesophase (Col_h) phase between 55 and 285°C. By combining these results with those obtained from dilatometry, a model for the molecular organisation within the columnar mesophases is proposed. The single-crystal X-ray structure of the related complex [Zn(NO₃) (L′)₂]NO₃·3MeOH {L′ = 2,6-diformyl-4-methylphenolbis[3′,4′,5′-tris(methoxy) phenylimine]} has been determined and supports the proposed mode of binding of the ligand to the metal via only one imine and a phenol. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b710745e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Metal-directed columnar phase formation in tetrahedral zinc(II) and
manganese(II) metallomesogensw
Francesca Morale,a Rachel L. Finn,a Simon R. Collinson,a Alexander J. Blake,a
Claire Wilson,a Duncan W. Bruce,*b Daniel Guillon,c Bertrand Donnio*c and
a
¨
Martin Schroder*
Received (in Durham, UK) 13th July 2007, Accepted 13th August 2007
First published as an Advance Article on the web 25th September 2007
DOI: 10.1039/b710745e
New, non-discoid M(II) complexes [MCl2(L)] {M = Mn, Zn; L = 2,6-diformyl-4-
methylphenolbis[30,40,50-tris(hexadecyloxy)phenylimine]} have been prepared and shown to exhibit
columnar mesophases, the nature of which was found to be metal-dependent. As analysed by
DSC and small-angle X-ray diffraction experiments, the complex [ZnCl2(L)] exhibits a single
mesophase with rectangular symmetry (Colr) between 46 and 145 1C, whereas the Mn(II) analogue
[MnCl2(L)] displays a purely hexagonal mesophase (Colh) phase between 55 and 285 1C. By
combining these results with those obtained from dilatometry, a model for the molecular
organisation within the columnar mesophases is proposed. The single-crystal X-ray structure of
the related complex [Zn(NO3)(L0)2]NO3 ꢀ 3MeOH {L0 = 2,6-diformyl-4-methylphenolbis[30,40,50-
tris(methoxy)phenylimine]} has been determined and supports the proposed mode of binding of
the ligand to the metal via only one imine and a phenol.
Of note here are the Robson-type Schiff-base compounds,13
Introduction
where imino derivatisation of the central phenol ring at the
two ortho positions results in typical ‘bent-core’ or ‘U-shaped’
chelate moieties able to bind two metals simultaneously in the
adjacent NO-‘compartments’ (Fig. 2a). The two, six-mem-
bered chelate rings formed by the compartmental 2,6-di-
iminophenol unit upon complexation to two metal ions share
the central O-donor atom, which acts as an endogeneous
bridge in its anionic form. The metal ions can be found either
lying in the NON-plane of co-ordination or displaced from it
and, in some binuclear species, both situations have been
observed simultaneously.11c The environments of the bound
metal ions can be extremely diverse as co-ordinative unsatura-
tion of the metal encourages the presence of additional donors,
often acting as inter-metallic bridges of the exogeneous type.
Moreover, these binuclear complexes often also exhibit what
has been termed ‘co-ordination number asymmetry’,14 where
inequivalence at the two central sites is the result of prefer-
ential metal-binding by the external ligating groups.
Metal complexes derived from Schiff-base ligands feature
amongst the earliest and most widely studied class of metallo-
mesogens.1 The advantages of incorporating the imine func-
tionality lie in the diverse range of potential structures and
their ease of preparation. Consequently, this versatile func-
tionality has produced metallomesogens of thermotropic
liquid crystals,2 surfactants,3 polycatenar mesogens,4 poly-
mers,5 luminescent materials,6 magnetic materials,7 ferrocene,8
chiral complexes9 and metallohelicates.10 Schiff-base ligands
have found widespread use11 and are often obtained by the
condensation of 2,6-dicarbonyl compounds with primary
amines. Of particular relevance here are the acyclic 2,6-phe-
noldiimines of the ‘end-off’ type,12 which are [1 + 2] Schiff-
base products (Fig. 1a), and which are readily derivatised
given the vast choice of amine starting materials available.
These commonly incorporate functional groups (designated as
Y in Fig. 1a). Related to this, reaction between monocarbonyl
compounds and primary diamines in 2 : 1 stoichiometric ratios
yields Schiff-base chelates of the ‘side-off’ type (Fig. 1b), while
macrocyclic products (Fig. 1c) are afforded by [2 + 2] Schiff-
base condensation with primary diamines.
A peculiarity of this NON-ligand motif is that mononuclear
species will be formed should the central hydroxyl proton be
retained upon complexation (Fig. 2b). In the cases where
mononuclear products are observed,15 the neutral chelate
system HL binds one metal via one NO-donor set, while the
a School of Chemistry, University of Nottingham, University Park,
Nottingham, UK NG7 2RD. E-mail:
M.Schroder@Nottingham.ac.uk; Fax: (+44) 115 9513563
b Department of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: db519@york.ac.uk; Fax: (+44) 1904 432516
c Institut de Physique et Chimie des Mate´riaux de Strasbourg
(IPCMS), UMR 7504 (CNRS-ULP), 23 rue du Loess, BP 43,
F-67034 Strasbourg Cedex 2, France. E-mail: bdonnio@ipcms.
u-strasbg.fr; Fax: (+ 33) 388 10 72 46
w Electronic supplementary information (ESI) available: Schematic
representation of the various parameters defining the columnar
packing. See DOI: 10.1039/b710745e
Fig. 1 Phenol-based acyclic and macrocyclic Schiff-base ligands; (a)
‘end-off’ ligands, (b) ‘side-off’ ligands and (c) macrocyclic ligands.
Adjacent metal-binding compartments (NO/YO) share the central
bridging donor atom (O). Y is a functional co-ordinating group.
ꢁc
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studied as a model species with the aim of producing com-
plexes suitable for study by single crystal X-ray diffraction,
and this was achieved for the complex [Zn(NO3)-
(L0)2]NO3 ꢀ 3MeOH.
Fig. 2 (a) A 2,6-functionalised phenol-based diimine displays a ‘bent-
core’ upon co-ordination to two metal ions. The planar NON-donor
set features distinctive chelation modes, forming two adjacent, six-
membered ring compartments with a communal endogenous O-bridge
in binuclear compounds. (b) The mononucleating behaviour of neutral
2,6-di-iminophenol chelates (HL) with associated intramolecular
H-bonding taking place in one of the NO-cavities.
Results and discussion
Synthesis
The complexes [MCl2(L)] (M = Mn, Zn) were prepared by
template [1 + 2] Schiff-base condensation of 2,6-diformyl-4-
methylphenol17 with two molar equivalents of 3,4,5-tris(hexa-
decyloxy)aniline18 in the presence of one molar equivalent of
MCl2 (Scheme 1a). The mixture was heated under reflux
overnight and then left to stand at room temperature. The
final products were isolated in high yield following slow
concentration of the reaction mixture; the precipitates thus
obtained did not require further purification. The brightly
coloured complexes appeared as waxy solids, most probably
due to the presence of the long hydrocarbon chains. The free
ligands L and L0 were also prepared in order to test their
thermal stability and behaviour.
non-metal binding NO-donor set has an intramolecular hy-
drogen bond between the phenolate oxygen and the other
protonated imine nitrogen.
With the aim of developing novel mesomorphic materials,
the present work focuses on a phenol-based 2,6-diimine ligand
(L) of the ‘end-off’ type, bearing six alkoxy chains (R =
C16H33) attached to the two lateral phenyl substituents. The
first-row transition metal ions chosen for this investigation
were Mn(II) (d5) and Zn(II) (d10), and it was anticipated that
such angular complexes with asymmetric occupancy of the
phenolic-imine compartments might lead to interesting liquid-
crystalline properties via assembly and packing of molecules in
the mesophase (Fig. 3).16 Additionally, in our design we
targeted mononuclear complexes incorporating intramolecu-
lar hydrogen bonding between one imine and the phenol. We
have reported previously lanthanide iminophenolate metallo-
mesogens where the metal cation is bound to the phenolic
oxygen and the imine nitrogen is protonated.2e–g To the best of
our knowledge, however, liquid crystals derived from 2,6-
diiminophenols have not yet been reported in the literature.
We report herein the synthesis and characterisation of new
metallomesogens [MCl2(L)] {M = Mn, Zn; L = 2,6-diformyl-
4-methylphenolbis[30,40,50-tris(hexadecyloxy)phenylimine]}.
The mesomorphic behaviour of these complexes was evaluated
by a combination of complementary techniques including
polarised optical microscopy, differential scanning calorimetry
(DSC), small-angle X-ray diffraction and dilatometry. Com-
plexation of the analogous ligand L0 {L0 = 2,6-diformyl-4-
methylphenolbis[30,40,50-tris(methoxy)phenylimine]} was also
Given the possibility of obtaining chemical species of vari-
able stoichiometric composition, metal chloride salts were
favoured in order to generate neutral 1 : 1 metal–ligand
complexes of the type [MCl2(L)] (Fig. 4), intended for com-
parison with the pyridyl analogues.16a It was envisaged that
introducing chloride co-ordinating counterions will discourage
the formation of cationic 1 : 2 metal–ligand complexes as was
found for the product [Zn(NO3)(L0)2]+ formed by reaction of
Zn(NO3)2 with L0 (Scheme 1b). Furthermore, the low basicity
of the chloride ions will help maintain a neutral phenol system
and thus avoid the formation of 2 : 1 metal–ligand species. It
was also anticipated that compounds of 2 : 2 metal–ligand
stoichiometry of type [M2(L)2]n+, observed only in cases of
Fig. 3 View of the mononuclear complex of neutral 2,6-diformyl-4-
methylphenolbis[30,40,50-tris(hexadecyloxy)phenylimine] (L), adopting
a ‘U-shape’ upon complexation and by virtue of intramolecular
H-bonding.
Scheme 1 Scheme for the preparation of the manganese(II) and
zinc(II) complexes of 2,6-diformyl-4-methylphenolbis[30,40,50-tris(alk-
oxy)phenylimine] [R = C16H33 (L); R = CH3 (L0)] via a template
[1 + 2] Schiff-base condensation.
ꢁc
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Fig. 4 Left: view of the cation in the molecular structure of [Zn(NO3)(L0)2]NO3 ꢀ 3MeOH. Two ligands bind the Zn ion asymmetrically via only
one NO-compartment and a nitrate ion completes the distorted octahedral co-ordination environment. The other nitrate anion, solvent molecules
and hydrogen atoms have been omitted for clarity. Right: a view of the complex showing the torsion of the lateral phenyl rings minimising steric
crowding.
non-bulky imino substituents present on the ligand frame-
work,19 were unlikely to form due to the presence of the
voluminous alkyl chains on L.
and aromatic regions recorded for metal-free L are doubled
for [ZnCl2(L)] and are consistent with the complex losing the
symmetry of the ligand, L. All assignments by 13C NMR
spectroscopy were made using carbon–hydrogen shift
correlation experiments. Microanalytical data and mass spec-
trometry are also in agreement with a formulation of the
products as 1 : 1 metal–ligand species with two chloride
ligands completing the metal co-ordination environment.
Tetrahedral geometries are assigned to the complexes
[MnCl2(L)] and [ZnCl2(L)] by comparison with related litera-
ture examples.20 The binding mode of the ligand is further
supported by the crystal and molecular structure of
Characterisation
The ligands L, L0 and their corresponding complexes were
characterised by infrared spectroscopy, mass spectrometry and
elemental analysis, with the additional use of 1H and 13C
NMR spectroscopy for the free ligands and the diamagnetic
Zn(II) complexes.
The infrared spectra of the products do not contain bands
assignable to the amine and carbonyl absorptions associated
with the starting aniline and dialdehyde materials, respec-
tively, with the imine stretching vibrations occurring as two
bands centred around 1620–47 and 1590 cmꢂ1. In the infrared
spectra of the ligands, the higher frequency imine absorption is
of weak intensity, whereas for [MnCl2(L)], [ZnCl2(L0)] and
[Zn(NO3)(L0)2][NO3], this stretch appears as a band of inten-
sity comparable to the lower frequency imine stretch. More-
over, both signals undergo blue shifts of between 7 and
21 cmꢂ1 upon complexation. This is particularly accentuated
for the high frequency band, which shifts by between 10 and
21 cmꢂ1. The fact that it is one particular absorption band that
is affected predominantly by metal ion complexation, is con-
sistent with the formation of an unsymmetric complex. The
high frequency imine stretch is, therefore, assigned to the
metal-bound imine.
1H NMR spectroscopy confirms the unsymmetrical nature
of [ZnCl2(L)] by revealing doubling of many signals found in
the spectrum of the ligand L. Thus, the metal-free ligand
shows two equivalent imine protons as a singlet at d = 8.8,
while the aromatic region shows two other singlets arising
from the meta-protons of the phenol ring and from the ortho-
protons of the lateral phenyl substituents (around d 7.3 and
6.5, respectively). In the aliphatic region, the first methylene
groups of the alkoxy chains give rise to overlapping triplets at
around d 4.0. On complexation, doubling of the above reso-
nance in the 1H NMR spectrum is particularly evident for the
imine and aromatic protons, and six singlets from these groups
can now be identified in the region d 9.4 to 6.6 for [ZnCl2(L)].
Likewise, resonances in the 13C NMR spectrum for the imine
[Zn(NO3)(L0)2]NO3 (L0
bis[30,40,50-tris(methoxy)phenylimine]).
=
2,6-diformyl-4-methylphenol-
Single-crystal X-ray structure of [Zn(NO3)(L0)2]NO3
{L0 = 2,6-diformyl-4-methylphenolbis[30,40,50-
tris(methoxy)phenylimine]}
The complexes [ZnCl2(L0)] and [Zn(NO3)(L0)2]NO3 were pre-
pared according to the same procedure as that above for the
long-chain homologous complexes. Orange needles of the
[ZnCl2(L0)] were formed by allowing the ligand in CH2Cl2
and ZnCl2 in MeCN to diffuse and react together slowly over a
number of weeks. Unfortunately, due to the poor quality of
the crystals, a full structure determination could not be
achieved. However, the structure of a closely-related mono-
nuclear ZnCl2 complex of a diiminophenol ligand has recently
been reported,20a and this confirms the nature of our com-
plexes prepared herein. The reported complex shows one
Zn(II) ion co-ordinated in a tetrahedral geometry to a NOCl2
donor set, the second imine nitrogen donor being protonated
and involved in hydrogen bonding with the phenolate oxygen.
Single
red
cube-like
crystals
of
[Zn(NO3)-
(L0)2]NO3 ꢀ 3MeOH were obtained via vapour diffusion of
diethyl ether into the original reaction solution in CHCl3–
MeOH. The structure of the complex shows the Zn atom lying
on a 2-fold rotation axis with the asymmetric unit consisting of
0.5 Zn, one ligand L0, half a bound nitrate (where the second
ligand and remainder of the Zn and nitrate are symmetry-
generated) and a half-occupied unbound nitrate (a symmetry-
generated equivalent is adjacent to this site) and 1.5 MeOH.
ꢁc
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Table
1
Selected
crystallographic
data
for
[Zn(NO3)-
Table
2
Selected bond distances and angles for [Zn(NO3)-
(L0)2]NO3 ꢀ 3MeOH
(L0)2]NO3 ꢀ 3MeOH
Chemical formula
C57H72N6O23Zn
Atoms
Distance/A
Atoms
Angle/1
Formula weight/g molꢂ1
Crystal system
Space group
Crystal colour
Crystal dimensions/mm
a/A
b/A
c/A
1274.58
Monoclinic
C2/c
Dark red
0.35 ꢃ 0.32 ꢃ 0.30
17.358(2)
14.1481(14)
24.213(2)
90.00
Zn–O1
Zn–N2
Zn–O1N
2.0221(19)
2.114(2)
2.226(2)
O1–Zn–O1
O1–Zn–N2
O1–Zn–N2
O1–Zn–O1N
O1–Zn–O1N
N2–Zn–O1N
N2–Zn–N2
N2–Zn–O1N
177.25(11)
85.81(8)
96.04(8)
83.18(8)
94.40(8)
104.46(9)
95.64(12)
157.95(9)
a/1
b/1
92.636(2)
90.00
5940.0(10)
150(2)
4
1.425
g/1
V/A3
T/K
Z
and indicated that the complexes were thermally stable on the
time scale of the experiment with very good reproducibility of
their thermal behaviour. The DSC traces of [ZnCl2(L)] showed
two endotherms, a large one at 46 1C (DH = 128 kJ molꢂ1),
corresponding to the melting point, followed by a smaller one
at 143 1C (DH = 2 kJ molꢂ1), assigned to the clearing process.
For [MnCl2(L)], melting was observed at a slightly higher
temperature (55 1C, DH = 94 kJ molꢂ1), followed by a
surprisingly high clearing point detected at 285 1C (DH =
2 kJ molꢂ1). Thus, a great enhancement of the mesophase
stability was achieved by simply changing the metal ion from
Zn(II) to Mn(II). Nevertheless, [ZnCl2(L)] is of special interest
as it is only the second example of a mesomorphic Zn(II)
complex with simple, tetrahedral geometry. The first, reported
Dc/g cmꢂ3
Selected crystallographic data are provided in Table 1 and
selected bond angles and bond lengths in Table 2.z
The Zn(II) ion in [Zn(NO3)(L0)2]+ is bound by a bidentate
nitrato group and the NO-donor sets from two bidentate
ligand molecules to give a distorted octahedral geometry
(Fig. 4, left). The remaining imine N-centre of the ligand is
protonated and forms an intramolecular hydrogen bond
(Oꢀ ꢀ ꢀH 1.88 A) to the phenolate oxygen,21 supporting a
binding mode for the ligand L0 similar to that suggested20a
for the complex [ZnCl2(L0)]. The two phenolate oxygen groups
are in mutually trans positions with a ligand bite angle of
85.81; the ligand is twisted at the bound imine N-centres with
torsion angles of ꢂ1.71 and 14.91 for C1–C2–C8–N1 and
C1–C6–C9–N2, respectively. The lateral trimethoxyphenyl
rings attached to the free imininium N-centre are almost
coplanar with the central phenolate ring (interplanar angle
of 4.51), whereas the trimethoxyphenyl ring of the bound imine
nitrogen is at an angle of 25.91 to the phenolate ring and 21.61
to the other trimethoxyphenyl ring. However, it is much closer
to coplanarity to the phenolate ring of the other ligand L0,
with an angle of 10.61 between these planes (Fig. 4, right).
by Gimenez et al., is based on co-ordination of ZnCl2 to a
´
pyrazole dimer,22 while other, unsuccessful attempts have also
been reported by Pucci et al. using 4,40-bipyridine diesters.23
Small-angle powder X-ray diffraction allowed the unequi-
vocal assignment of the mesophases. The experiments were
carried out from the crystalline to the isotropic state with an
X-ray pattern being recorded every 20 1C for each compound.
The X-ray diffraction experiment data support the results
obtained from optical microscopy and DSC for which a single
mesophase was displayed by each of the complexes in the
defined temperature ranges.
For [ZnCl2(L)], the X-ray pattern exhibited an intense,
small-angle Bragg reflection, followed by a series of smaller
reflections at mid-angles. In the wider angle region, a diffuse,
broad-scattering was observed, centred around 4.5 to 4.6 A,
corresponding to the liquid-like order (short-range correla-
tions) of the aliphatic chains and confirming the fluid-like
nature of the mesophases (Fig. 5). Detailed viewing of the
single reflection in the small-angle region reveals a shoulder
that corresponds to an additional reflection; the proximity of
both fundamental reflections explains the relative broadness of
the base of the Bragg peak. These two reflections can be
indexed as the 11 and 20 reflections of a rectangular lattice.
The position of the other reflections allowed for an indexation
compatible with such a rectangular symmetry. Moreover,
since we do not observe the presence of any hk pairs with
h + k = 2n + 1, the symmetry of the lattice can be further
assigned as belonging to a c2mm plane group. The resulting
calculated lattice parameters a and b, and columnar cross-
section S are given in Table 3. X-Ray patterns recorded at
other temperatures gave identical data. As a representative
example, the diffraction pattern of the Colr phase for
[ZnCl2(L)] recorded at 100 1C is shown in Fig. 5.
Mesomorphic behaviour
The metal-free ligand with six hexadecyloxy chains (L) was
found to show no mesomorphism, as anticipated given its
expected bent molecular structure and small molecular aniso-
tropy. However, on co-ordination to MCl2 fragments meso-
morphism
was
induced,
reflecting
conformational
rigidification of the ligand on complexation. Significantly,
the mesophases exhibited by these materials exist over large
temperature ranges, illustrating mesophase induction and a
stabilisation effect upon complexation.1 Moreover, the choice
of the metal ion seems to affect the observed mesomorphism
quite radically.
The complexes [MCl2(L)] were investigated by polarised
optical microscopy, but this technique was not sufficient to
allow the phases to be assigned unequivocally. Thus, the
materials were further studied by X-ray diffraction (vide infra).
DSC experiments confirmed the presence of phase transitions,
z CCDC reference number 656905. For crystallographic data in CIF
format see DOI: 10.1039/b710745e
ꢁc
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Fig. 5 X-Ray diffraction pattern of the Colr of [ZnCl2(L)] {L = 2,6-
diformyl-4-methylphenolbis[30,40,50-tris(hexadecyloxy)phenylimine]}
at 100 1C.
Fig. 6 X-Ray diffraction pattern of the Colh of [MnCl2(L)] {L = 2,6-
diformyl-4-methylphenolbis[30,40,50-tris(hexadecyloxy)phenylimine]}
at 100 1C.
For [MnCl2(L)], a series of three sharp reflections were
observed in the small-angle region with reciprocal spacing
ratio 1 : O3 : O4, characteristic of a two-dimensional hexago-
nal lattice and corresponding to the indexation (hk) = (10),
(11) and (20); additionally, a broad diffuse scattering was
observed at 4.5 to 4.6 A, confirming the liquid crystal nature
of the mesophase (Fig. 6; Table 3). The X-ray pattern
remained unchanged over the whole mesomorphic tempera-
ture range, with almost no variation within the intercolumnar
distance. As a representative example, the diffraction pattern
of the Colh phase for [MnCl2(L)] recorded at 100 1C is shown
in Fig. 6.
In the Colh phase, the cylindrical columns are organised
onto a hexagonal lattice with the columns located at the nodes
of a hexagonal cell (one column per cell) and the columnar
axes perpendicular to this lattice plane (Fig. 7). The symmetry
of the mesophase further requires that the molecules project a
circular cross-section onto this lattice plane. In the case of the
Colr phase, however, the columns display an elliptical cross
section and are disposed into a rectangular cell (two columns
per lattice). Such elliptical cross-sections may be generated
either by hindered molecular rotation of the non-discoid
molecules around the columnar axis, or by the tilt of the
molecular discs perpendicular to the columnar axis,24 the
ellipses being oriented along one main direction along the
parameter a. The reasons for the formation of different
mesophases upon changing the metal ion, and the greatly
enhanced mesomorphic range for the Mn(II) complex, are
not clear at this stage, the major difference between the two
complexes being the smaller ionic radius for the Zn(II), some
10% lower than that for Mn(II).
Table 3 Detailed indexation of the liquid-crystalline phases observed
at 100 1C for the complexes [ZnCl2(L)] and [MnCl2(L)]
M
dexp/Aa Intensityb hkc dtheor/Ad Mesophase and parameterse
Zn 24.6
23.3
VS
VS
S
M
M
S
11 24.6
20 23.3
02 14.5
31 13.7
22 12.3
40 11.6
hch
Cr 46 [128] Colr 143 [2] I
Colr ꢂ c2mm
14.35
13.5
12.2
11.5
4.6
a = 46.6 A
b = 29.0 A
Scol = 675.7 A2
h = 4.7 A
br
Mn 25.45 VS
10 25.45
11 14.7
20 12.7
hch
Cr 55 [94] Colh 285 [2] I
Colh ꢂ p6mm
Scol = 748.5 A2
h = 4.25 A
14.7
12.7
4.6
M
M
br
The variation in the molecular volume of [ZnCl2(L)] was
followed as a function of temperature (Fig. 8). On increasing
the temperature, a discrete discontinuity in the molecular
volume was found in the vicinity of the melting temperature,
indicating a significant structural change due to the melting of
the numerous aliphatic chains. Above the melting point, the
a = 29.4 A
a
dexp and dtheor are the experimentally measured and theoretical
b
diffraction spacings at 100 1C. Intensity of the reflections: VS: very
strong, S: strong, M: medium; br stands for broad (diffuse)
c
reflections. [hk] are the indexation of the reflections, and hch are
the short range periodicities determined by XRD corresponding to the
d
liquid-like order of the molten chains. dtheor and the lattice para-
meters a and b are deduced from the following mathematical expres-
sions: for the Colh, hd10i = 1/Nhk[Shkdhk(h2 + k2 + hk)1/2] where Nhk
is the number of hk reflections and a = 2hd10i/O3, and for the Colr, a
and b are deduced from the following mathematical expressions: a =
e
2d20, and 1/dhk = O(h2/a2 + k2/b2). Colh: hexagonal columnar
phase, Colr: rectangular columnar phase, Cr: crystalline solid, I:
isotropic liquid. Scol is the columnar cross-section area: a2O3/2 (for
Colh) and 1/2ab (for Colr). Vm is the molecular volume at 100 1C
deduced from the equation in Fig. 8 (Vm = 3178.2 A3). h is the
intracolumnar repeating distance, deduced directly from the measured
molecular volume and the columnar cross-section according to h =
NVm/Scol, with N the number of complexes with this portion of
column (hScol).
Fig. 7 Representation of the 2D lattice of the Colh (left) and Colr
(right) mesophases formed by [MnCl2(L)] and [ZnCl2(L)], respectively.
ꢁc
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Fig. 9 Molecular arrangement forming columnar mesophases in
time-averaged disc-like structures from the non-discoid metallomeso-
gens.
also provides for an anti-parallel arrangement of the dipole
arising from the ZnCl2 fragment, which is also favourable
energetically. In this way, each pair of molecules completes a
discoid-like shell of chains surrounding the rigid part of the
complex (Fig. 9).
Fig.
8 Temperature-dependence of the molecular volume of
[Zn(L)Cl2].
volume was found to vary almost linearly with T in the
temperature range of the mesophase (the quadratic term being
small compared to the linear term). The slope is in fairly good
agreement with the volume variation in the methylene groups
in liquid alkanes,25 an indication that the effective volume of
In conclusion, we have confirmed that the M(II) complexes
[MCl2(L)] {M = Mn, Zn; L = 2,6-diformyl-4-methyl-
phenolbis[30,40,50-tris(hexadecyloxy)phenylimine]} exhibit co-
lumnar mesophases, the nature and stability of which depend
intimately upon the choice of metal ion. Thus, [ZnCl2(L)]
exhibits a single mesophase with rectangular symmetry (Colr)
between 46 and 145 1C, whereas the Mn(II) analogue
[MnCl2(L)] displays a purely hexagonal mesophase (Colh)
phase between 55 and 285 1C. Current work seeks to rationa-
lise these observations via the study of related metallo–ligand
systems and their analogues.
the aromatic part, VAr,
26 is almost invariant with temperature
in the mesophase. Indeed, the dilatation of the aromatic part
can be considered negligible compared to that of the aliphatic
chains which are in the molten state. Moreover, the transition
to the isotropic liquid is evidenced by a small change in the
molecular volume, and of the slope of the curve (Vm in
isotropic liquid = 2956.5 + 2.27 T).
As is now well-described, columnar phases may be char-
acterised by the columnar cross-section, S, and the stacking
periodicity along the columnar axis, h.27 Knowledge of these
two structural parameters permits interpretation of the mole-
cular packing inside the columns and a better understanding of
the influence of this packing on the two-dimensional arrange-
ment of the columns. The periodicity, h, the columnar cross-
section, Scol, and the molecular volume, Vm, are linked
analytically through the now well-established relationship:27
Experimental
Materials
All chemicals were purchased from commercial suppliers and
were used without further purification. The EtOH used in the
preparation of the anilines was dried over Mg turnings and
freshly distilled prior to use. 2,6-Diformyl-4-methylphenol17
and the 3,4,5-trialkoxyanilines18 were prepared according to
published methods.
hScol = NVm
(1)
where N is the number of molecules within a volume fraction
of column. Assuming that both complexes have about the
same volume (in both case, the main contribution to the
volume is the ligand) at the same temperature (ca. 3200 A3
at 100 1C), it is found that one complex (N = 1) occupies a
slice of column on average 4.7 and 4.25 A thick for [ZnCl2(L)]
and [MnCl2(L)], respectively, in agreement with the observed
XRD patterns. In order to propose a slightly more detailed
model for the packing, it is necessary to take account of the
overall shape of the complex as extrapolated from the single
crystal structure. Thus, the complex can regarded as angular in
nature defining an open V-shape, with the terminal chains at
the two extremities of the V. Given the steric demands of these
chains, it would then be expected that the next complex in the
stack would fit approximately ‘back-to-back’ to minimise
unfavourable interactions between these aliphatic areas. Thus,
the complexes adopt an anti-parallel arrangement of the type
postulated by Trzaska and Swager28 and others16a,29 in some
structurally-related materials. Fortuitously, this arrangement
Instrumentation
NMR spectra were recorded on a Bruker DPX300 FT-NMR
spectrometer operating at 300.13 MHz for 1H and 75.48 MHz
for 13C. Chemical shifts are referenced with respect to residual
proton and carbon solvent (with dH = 7.26 ppm and dC
=
77.0 ppm for CDCl3). IR spectra were obtained on a Nicolet
AVATAR 360 FT-IR spectrometer as KBr pellets. UV-Vis
spectra were measured on a Philips PU 8720 spectrophoto-
meter. FAB mass spectra were obtained on a Finnigan MAT
TSQ-700 at the University of Wales, Swansea, with 3-nitro-
benzyl alcohol (NOBA) as matrix. Elemental analyses (C, H,
N) were carried out by the Analytical Department of the
University of Nottingham. The transition temperatures and
enthalpies were measured by differential scanning calorimetry
with a Perkin-Elmer DSC-7 instrument operated at a scanning
rate of 10 1C minꢂ1 on heating. The apparatus was calibrated
with indium (156.6 1C; 28.4 J gꢂ1) as the standard. For the
dilatometry technique, the measurements were performed with
ꢁc
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302 | New J. Chem., 2008, 32, 297–305

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a high precision home-built apparatus, automatically compu-
ter-controlled, including data acquisition and temperature
control within ꢄ0.03 1C. The XRD patterns were obtained
with two different experimental set-ups (I, II). In all cases, a
linear monochromatic Cu-Ka1 beam (l = 1.5405 A) was
obtained using a sealed-tube generator (900 W) equipped with
a bent quartz monochromator. In the first set, the transmission
Guinier geometry (I) was used, whereas Debye–Scherrer-like
geometry (II) was used in the second experimental set-up. In
all cases, the crude powder was loaded into Lindemann
capillaries of 1 mm diameter and 10 mm wall thickness. The
initial set of diffraction patterns was recorded on an image
plate; periodicities up to 80 A can be measured, and the sample
temperature controlled to within ꢄ0.3 1C in the 20 to 350 1C
temperature range. The second set of diffraction patterns (II)
was recorded with a curved Inel CPS120 counter gas-filled
detector linked to a data acquisition computer (periodicities
up to 60 A) or can be measured on image plates (periodicities
up to 100 A); the sample temperature controlled to within
ꢄ0.05 1C in the 20 to 200 1C temperature range. In each case,
exposure times were varied from 1 to 24 h.
methylphenol (0.328 g, 2.00 mmol) and 3,4,5-tri(methoxy)ani-
line (0.732 g, 4.00 mmol) was heated at reflux in MeOH (150
cm3) for 1 h, resulting in a red solution, which was left to
evaporate at room temperature. The precipitate that formed
after 24 h was collected by filtration and washed with a little
cold MeOH, affording an orange-red crystalline solid in 89%
yield (0.879 g). 1H NMR (d6-DMSO): dH 8.99 (2H, s,
HCQN), 7.78 (2H, s, phenol-Hmeta), 6.72 (4H, s, Ar-H),
3.83 (12H, s, m-OCH3), 3.68 (6H, s, p-OCH3), 2.35 (3H, s,
phenol-CH3). 13C NMR (d6-DMSO): dC 158.17 (HCQN),
153.32 (Cq), 145.08 (Cq), 136.44 (Cq), 133.35 (phenol-Cmeta),
127.66 (Cq), 121.25 (Cq), 98.85 (Ar-CH), 60.08 (p-OCH3),
55.94 (m-OCH3), 19.85 (phenol-CH3). IR (cmꢂ1): u(CQN)
1626w and 1584s; u(C–O) 1126. FAB-MS: m/z 494 ([M]+).
Anal. Calcd for C27H30N2O7: C, 65.57; H, 6.11; N, 5.66.
Found: C, 65.40; H, 6.20; N, 5.57%.
Synthesis of 2,6-diformyl-4-methylphenolbis[30,40,50-tris-
(hexadecyloxy)phenylimine] (L). A mixture of 2,6-diformyl-4-
methylphenol (0.0355 g, 0.216 mmol) and 3,4,5-tris(hexadecyl-
oxy)aniline (0.352 g, 0.432 mmol) was heated at reflux in
CHCl3–MeOH (20 : 50 cm3) for 2 h, resulting in a red solution,
which was left to evaporate at room temperature. The pre-
cipitate which formed after 24 h was filtered off and washed
with a little cold MeOH, affording an orange solid in 93%
yield (0.260 g). 1H NMR (CDCl3): dH 8.80 (2H, br-s, HCQN),
7.26 (2H, s, phenol-Hmeta), 6.51 (4H, s, Ar-H), 3.97 (12H, m,
OCH2), 2.37 (3H, s, phenol-CH3), 1.88–1.26 (168H, m, CH2),
0.88 (18H, 2 overlapping triplets, CH3). 13C NMR (d6-
DMSO): dC 158.17 (HCQN), 153.32 (Cq), 145.08 (Cq),
136.44 (Cq), 133.35 (phenol-Cmeta), 127.66 (Cq), 121.25 (Cq),
98.85 (Ar-CH), 73.99, 73.60, 69.68, 69.28 (OCH2), 31.94,
30.43, 29.77, 29.58, 29.51, 29.38, 29.14, 26.12, 22.69 (CH2),
19.85 (phenol-CH3). IR (KBr, cmꢂ1): u(C–H) 2907vs and
2847vs; u(CQN) 1626w and 1582m; u(C–O) 1119s. FAB-
MS: m/z 1756 ([M]+). Anal. Calcd for C117H210N2O7: C,
79.98; H, 12.05; N, 1.59. Found: C, 80.03; H, 11.89, N, 1.54%.
X-Ray crystallographic data
Data collection and processing. Single-crystal diffraction
data were collected on a Bruker SMART1000 CCD area
detector diffractometer,30 equipped with an Oxford Cryosys-
tem open-flow nitrogen cryostat,31 using graphite-monochro-
mated Mo-Ka radiation (l = 0.71073 A): 33) 995 data
collected (2ymax = 581, ꢂ23 r h r 23, ꢂ19 r k r 18, ꢂ32
r l r 32), 7432 unique data (Rint = 0.036) giving 5572 with
F 4 2s(F). Integrated intensities, corrected for Lorentz and
polarisation effects, were obtained using the Bruker SAINT
package,32 as were the cell parameters. Multi-scan absorption
corrections were applied, Tmin and Tmax of 0.876 and 1.000.
Structure solution and refinement. The crystal structure was
solved by direct methods33 and refined on F2 using SHELXL-
97.34 All non-hydrogen atoms were refined with anisotropic
displacement parameters except those of the MeOH and the
partially occupied unbound nitrate. Additionally, the metha-
nol was modelled as one fully occupied O atom with two half-
occupied C sites and a second half-occupied MeOH group.
Hydrogen atoms were placed in calculated positions and
refined as part of a riding model except the methyl C(7) and
hydrogen atoms on the phenol and the imine N(1)–hydrogen,
which were located from DF syntheses. The N(1)–hydrogen
was then refined with idealised geometry and included in a
riding model and the C(7)–hydrogen atoms were refined as a
rigid rotating group. The hydrogen atoms were not located for
the solvate units but included in the formula for the unit cell
contents and all quantities derived from them.
Synthesis of dichloro-2,6-diformyl-4-methylphenolbis[30,40,50-
tris(methoxy)phenylimine]zinc(II) ([ZnCl2(L0)]). A solution of
2,6-diformyl-4-methylphenol (0.0126 g, 0.0768 mmol) and
3,4,5-tri(methoxy)aniline (0.0282 g, 0.154 mmol) in CHCl3
(20 cm3) was stirred for 10 min, turning yellow, before adding
a solution of ZnCl2 (0.0105 g, 0.0770 mmol) in MeOH (7.5
cm3). The orange precipitate which had formed upon addition
of the metal salt dissolved once the solution was heated at
reflux over 20 h. The resulting orange solution was left to
stand and the orange needles which formed upon cooling were
collected by filtration and washed with a little cold Et2O, with
a yield of 0.0412 g (85%). 1H NMR (d6-DMSO): dH 9.00 (2H,
s, HCQN), 7.78 (2H, s, phenol-Hmeta), 6.72 (4H, s, Ar-H),
3.83 (12H, s, m-OCH3), 3.67 (6H, s, p-OCH3), 2.35 (3H, s,
phenol-CH3). IR (cmꢂ1): u(CQN) 1639m and 1595s; u(C–O)
1128vs. FAB-MS: m/z 593 ([M ꢂ Cl]+) and 630 ([M]+). Anal.
Calcd for C27Cl2H30N2O7Zn ꢀ H2O: C, 49.98; H, 4.97; N, 4.32.
Found: C, 50.28; H, 4.80; N, 4.26%. Orange columnar crystals
of the complex were grown for single-crystal X-ray analysis by
allowing the ligand in CH2Cl2 and ZnCl2 in MeCN to diffuse
and react together slowly over a number of weeks.
At final convergence R[F Z 4s(F)] = 0.058, wR[F2, all
data] = 0.173, GoF = 1.02 for 386 parameters. The final
difference Fourier synthesis showed a maximum peak of
+1.07 and maximum trough of ꢂ0.61 eAꢂ3
.
Synthetic procedures
Synthesis of 2,6-diformyl-4-methylphenolbis[30,40,50-tris-
(methoxy)phenylimine] (L0). A mixture of 2,6-diformyl-4-
ꢁc
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Synthesis of bis{2,6-diformyl-4-methylphenolbis[30,40,50-tris-
(methoxy)phenylimine]}nitratozinc(II) [Zn(L0)(NO3)]NO3.
154.07 (Cq), 153.85 (Cq), 146.80 (phenol-Cmeta), 142.68 (Cq),
141.04 (phenol-Cmeta), 139.15 (Cq), 138.40 (Cq), 130.66 (Cq),
126.77 (Cq), 121.94 (Cq), 116.92 (Cq), 100.31 (Ar-CH), 97.93
(Ar-CH), 73.99, 73.60, 69.68, 69.28 (OCH2), 31.94, 30.43,
29.77, 29.58, 29.51, 29.38, 29.14, 26.12, 22.69 (CH2), 19.68
(phenol-CH3), 14.09 (CH3). IR (KBr, cmꢂ1): u(C–H) 2919vs
and 2850vs; u(CQN) 1647m and 1592m; u(C–O) 1119s. FAB-
MS: m/z 1851 ([M ꢂ Cl]+) and 1892 ([M]+). Anal. Calcd for
A
solution of 2,6-diformyl-4-methylphenol (0.011 g, 0.067 mmol)
and 3,4,5-tri(methoxy)aniline (0.0246 g, 0.134 mmol) in CHCl3
(20 cm3) was stirred for 10 min, turning yellow, before adding
a solution of Zn(NO3)2 ꢀ 6H2O (0.0199 g, 0.0669 mmol) in
MeOH (7.5 cm3). The resulting orange solution was heated at
reflux for 20 h and then its volume was reduced to ca. 10 cm3.
The orange precipitate which had formed upon addition of the
metal salt dissolved once the solution was heated at reflux over
20 h. Vapour diffusion of diethyl ether into this solution
produced red cube-like crystals, suitable for study by X-ray
diffraction, with a yield of 0.173 g (44%). 1H NMR (d6-
DMSO): dH 8.99 (2H, s, HCQN), 7.78 (2H, s, phenol-Hmeta),
6.72 (4H, s, Ar-H), 3.83 (12H, s, m-OCH3), 3.68 (6H, s, p-
OCH3), 2.35 (3H, s, phenol-CH3). 13C NMR (d6-DMSO): dC
158.17 (HCQN), 153.32 (Cq), 145.08 (Cq), 136.44 (Cq), 133.35
(phenol-Cmeta), 127.66 (Cq), 121.25 (Cq), 98.85 (Ar-H), 60.08
(p-OCH3), 55.94 (m-OCH3), 19.85 (phenol-CH3). IR (cmꢂ1):
u(CQN) 1638m and 1593s; u(N–O) 1384vs; u(C–O) 1128vs.
C
117Cl2H210N2O7Zn ꢀ H2O: C, 73.53; H, 11.18; N, 1.47.
Found: C, 73.21; H, 11.12; N, 1.67%.
Acknowledgements
We thank the EPSRC and University of Nottingham for
support and the EPSRC National Mass Spectrometry Service
at the University of Wales, Swansea (UK) for mass spectra.
MS gratefully acknowledges receipt of a Royal Society Wolf-
son Merit Award and of a Royal Society Leverhulme Trust
Senior Research Fellowship. BD and DG greatly acknowledge
the CNRS-ULP (UMR7504) for financial support, and
Dr B. Heinrich for the dilatometry experiments and his
constructive and helpful comments.
FAB-MS: m/z 1118 ([M
ꢂ
NO3]+). Anal. Calcd for
C54H60N6O20Zn ꢀ H2O: C, 54.21; H, 5.22; N, 7.02. Found: C,
53.80; H, 4.97; N, 6.74%.
Synthesis of dichloro-2,6-diformyl-4-methylphenolbis[30,40,50-
tris(hexadecyloxy)phenylimine]manganese(II) ([MnCl2(L)]). A
mixture of 2,6-diformyl-4-methylphenol (0.0131 g, 0.0798
mmol) and 3,4,5-tris(hexadecyloxy)aniline (0.130 g, 0.160
mmol) in CHCl3 (20 cm3) was stirred at room temperature
for 10 min, turning to a yellow-orange solution. A solution of
MnCl2 ꢀ 4H2O (0.0158 g, 0.0798 mmol) in EtOH (10 cm3) was
then added and the solution heated at reflux overnight. The
resulting bright-orange solution was left to evaporate at room
temperature and the precipitate that formed after 2 d was
collected by filtration and washed with a small amount of cold
EtOH, yielding 0.117 g (78%) of product as a bright-orange
solid. IR (KBr, cmꢂ1): u(C–H) 2919vs and 2850vs; u(CQN)
1636m and 1589m; u(C–O) 1117s. FAB-MS: m/z 1845 ([M ꢂ
Cl]+). Anal. Calcd for C117Cl2H210MnN2O7 ꢀ H2O: C, 73.93;
H, 11.24; N, 1.47. Found: C, 73.92; H, 11.23; N, 1.53%.
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ꢁc
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