Effect of carboxylate spacers on the supramolecular self-assembly of dicopper(II) Schiff base complexes stabilizing water assemblies of different conformations

Article abstract of DOI:10.1039/b415945d

Dicopper(II) complexes, namely [Cu₂L(O₂C-CH=CH-C ₆H₄-p-OH)]·2H₂O (1·2H ₂O), [Cu₂L(O₂C-CH₂-C ₆H₄-p-OH)]·2H₂O (2·2H ₂O) and [Cu₂L(O₂C-CH₂CH ₂-C₆H₄-p-OH)]·0.5H₂O (3·0.5H₂O), having different carboxylate ligands with a p-hydroxyphenyl moiety and the pentadentate Schiff base N,N′-1, 3-diylbis(salicylaldimino)propan-2-ol (H₃L) in its trianionic form, were prepared and structurally characterized by X-ray crystallography. The complexes have a dicopper(II) unit with an alkoxo bridge from the Schiff base and the carboxylate, showing a three-atom bridging mode. The metal centres in a square planar CuNO₃ coordination geometry are antiferromagnetically coupled in the asymmetrically double-bridged dicopper(II) core. A significant effect of the -CH=CH-, -CH₂- and -CH₂CH₂- spacers of the carboxylate ligands on the formation of different supramolecular structures is observed. Complex [Cu₂L(O₂C-CH=CH-C ₆H₄-p-OH)], 1, forms a helical supramolecular structure due to hydrogen-bonding interactions involving the p-hydroxy group of the phenol from the carboxylate and one phenoxo oxygen atom from the Schiff base. The lattice waters form a helical one-dimensional chain, in which alternate water molecules are anchored to the supramolecular host and the chain propagates along the crystallographic 2₁ screw axis. Complex 2 forms water aggregates of quasi-linear and pseudo-hexameric cyclic chair conformations involving lattice water molecules, and the previously mentioned para OH group phenoxo oxygen atom. Complex 3·0.5H₂O shows the formation of a supramolecular one-dimensional chain structure due to hydrogen-bonding interactions between the p-OH group and the phenoxo oxygen atom. Two such supramolecular structures are linked by hydrogen-bonding interactions involving the lattice water. Differential scanning calorimetry (DSC) of 1·2H ₂O gives two endotherms at 61.5 and 88.5 °C for the loss of the free and the anchored water molecules, respectively. The overall change of enthalpy per water molecule is ～36 kJ mol^-1. Complex 2·2H₂O shows an endotherm at 131 °C with a shoulder at ～126 °C. The enthalpy change per water molecule is ～26 kJ mol^-1. The reversibility in loss or addition of lattice water molecules and the corresponding effect on the overall structure is probed by X-ray powder diffraction studies. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b415945d

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P A P E R
Effect of carboxylate spacers on the supramolecular self-assembly
of dicopper(II) Schiff base complexes stabilizing water assemblies
of different conformationsw z
Arindam Mukherjee, Manas K. Saha, Munirathinam Nethaji and Akhil R. Chakravarty*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore,
560012, India. E-mail: arc@ipc.iisc.ernet.in; Fax: þ91-80-23600683
Received (in Montpellier, France) 14th October 2004, Accepted 5th January 2005
First published as an Advance Article on the web 3rd March 2005
Dicopper(II) complexes, namely [Cu L(O C–CH CH–C H -p-OH)] ꢀ 2H O (1 ꢀ 2H O), [Cu L(O C–CH –
Q
2
2
6
4
2
2
2
2
2
C6H4-p-OH)] ꢀ 2H2O (2 ꢀ 2H2O) and [Cu2L(O2C–CH2CH2–C6H4-p-OH)] ꢀ 0.5H2O (3 ꢀ 0.5H2O), having
different carboxylate ligands with a p-hydroxyphenyl moiety and the pentadentate Schiff base N,N0-1,
3-diylbis(salicylaldimino)propan-2-ol (H3L) in its trianionic form, were prepared and structurally characterized
by X-ray crystallography. The complexes have a dicopper(II) unit with an alkoxo bridge from the Schiff base
and the carboxylate, showing a three-atom bridging mode. The metal centres in a square planar CuNO3
coordination geometry are antiferromagnetically coupled in the asymmetrically double-bridged dicopper(II)
core. A significant effect of the –CH CH–, –CH – and –CH CH – spacers of the carboxylate ligands on the
Q
formation of different supramolecular structures is observed. Complex [Cu L(O C–CH CH–C H -p-OH)],
2
2
2
Q
2
2
6
4
1, forms a helical supramolecular structure due to hydrogen-bonding interactions involving the p-hydroxy
group of the phenol from the carboxylate and one phenoxo oxygen atom from the Schiff base. The lattice
waters form a helical one-dimensional chain, in which alternate water molecules are anchored to the
supramolecular host and the chain propagates along the crystallographic 21 screw axis. Complex 2 forms
water aggregates of quasi-linear and pseudo-hexameric cyclic chair conformations involving lattice water
molecules, and the previously mentioned para OH group phenoxo oxygen atom. Complex 3 ꢀ 0.5H2O shows
the formation of a supramolecular one-dimensional chain structure due to hydrogen-bonding interactions
between the p-OH group and the phenoxo oxygen atom. Two such supramolecular structures are linked by
hydrogen-bonding interactions involving the lattice water. Differential scanning calorimetry (DSC) of
1 ꢀ 2H2O gives two endotherms at 61.5 and 88.5 1C for the loss of the ‘‘free’’ and the ‘‘anchored’’ water
molecules, respectively. The overall change of enthalpy per water molecule is B36 kJ molꢁ1. Complex
2 ꢀ 2H2O shows an endotherm at 131 1C with a shoulder at B126 1C. The enthalpy change per water
molecule is B26 kJ molꢁ1. The reversibility in loss or addition of lattice water molecules and the
corresponding effect on the overall structure is probed by X-ray powder diffraction studies.
of composition (H2O)n (n 4 2) can assume, the cyclic and cage
Introduction
structures are rather common and are known to be more stable
than a linear chain structure.1,4 While the cage structures of
water molecules in crystal hosts provide models for bulk water,
single file linear chain structures of water aggregates are likely
to model systems such as the biological membrane water
channel in aquaporin-1 or the ion channel of gramicidin A.19–22
Water molecules play an important role in the formation of
supramolecular structures involving hydrogen bonds of varied
strengths and become an integral part of such self-assembled
structures.12,23 Alternatively, a supramolecular host could lead
to the formation of unusual water aggregates that are more
dynamic in nature, such as found in biological systems. The
present work stems from our interest to synthesize alkoxo-
carboxylato double-bridged dicopper(II) complexes as host
structures using carboxylate ligands with pendant hydroxyl
groups that could participate in hydrogen-bonding interactions
between the ligand and lattice solvent molecule(s) to form new
supramolecular structures. We have shown in a preliminary
communication that the Schiff base dicopper(II) complex [Cu2L
The structural and dynamical aspects of water molecules in
confined or constricted spaces, such as in the cavities or
channels of supramolecular structures that are largely preva-
lent in biological systems and in inorganic solids, are of
considerable current importance to understand the behaviour
of such water molecules in a variety of hydrophilic and/or
hydrophobic environments.1–5 The properties of water mole-
cules in the aggregate form showing different morphologies
often vary significantly from those of bulk water in the solid
(i.e., ice) or liquid phases. While water molecules are ubiqui-
tous as the solvent of crystallization in crystal lattices, observa-
tion of water aggregates having structures similar to those
found in bulk water or in biological systems are limited6–18 and
such water assemblies could be used as potential models to gain
a better insight into the nature of hydrogen bonding, its
morphology and the dynamical aspects of these water mole-
cules. Among the different morphologies that water aggregates
(O C–CH CH–C H -p-OH)] ꢀ 2H O (1 ꢀ 2H O), where L is
Q
2
6
4
2
2
w Dedicated to Prof. Animesh Chakravorty on his 70th birthday.
z Electronic supplementary information (ESI) available: variable tem-
perature magnetic susceptibility data for the complexes (Tables S1–S3),
ORTEP diagrams of the complexes 1 ꢀ 2H2O and 3 ꢀ 0.5H2O (Figs. S1,
S2). See http://www.rsc.org/suppdata/nj/b4/b415945d/
the trianionic form of N,N0-1,3-diylbis(salicylaldimino)pro-
pan-2-ol (H3L), forms an unprecedented type of one-dimen-
sional helical chains of hydrogen-bonded water molecules,
anchored onto a helical supramolecular host resulting from
T h i s j o u r n a l i s & 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 a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
596
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 9 6 – 6 0 3

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the self-assembly of the dicopper(II) complex.24 It was deter-
mined that the dihedral angle between the –CH CH–C H –
Syntheses
[Cu L(O C–CH CH–C H -p-OH)] (1). [Cu L(O CCH )]
Q
6
4
Q
2
2
6
4
2
2
3
(styrene) unit of the p-hydroxycinnamate ligand and the basal
coordination plane of {Cu2L}1 was a crucial parameter in the
formation of a helical structure. We have since investigated in
detail the role of the spacer in the carboxylate ligand on the
supramolecular structure by using two additional carboxylates,
from p-hydroxyphenylacetic acid (–CH2–) and p-hydroxyphe-
nylpropionic acid (–CH2CH2–, Scheme 1). The corresponding
complexes [Cu2L(O2C–CH2–C6H4-p-OH)] ꢀ 2H2O (2 ꢀ 2H2O)
and [Cu2L(O2C–CH2CH2–C6H4-p-OH)] ꢀ 0.5H2O (3 ꢀ 0.5H2O)
have been prepared and their crystal structures determined.
Herein, we present the supramolecular chemistry of these three
complexes, showing the significant geometrical control exerted
by the carboxylate spacers on the formation of water aggre-
gates. The ability of the complexes to reversibly dehydrate is
investigated by powder X-ray diffraction methods.
(0.65 g, 1.35 mmol) in MeOH (50 ml) was reacted with the
sodium salt of p-hydroxycinnamic acid (0.25 g, 1.35 mmol)
under refluxing conditions for 30 min. The solution was cooled
to ambient temperature and filtered. The product was isolated
in good yield (0.5 g, B65%) as a green solid by slow concen-
tration of the filtrate. Anal. calcd for C26H22Cu2N2O6 (M
585.54): C 53.33, H 3.79, N 4.78%; found: C 53.04, H 3.92,
N 5.06%; UV-Vis in THF [lmax/nm (e/dm3 molꢁ1 cmꢁ1)]: 639
(350), 377 (9100), 277 (31 000), 247 (33 500), 227 (47 600); IR
(KBr, cmꢁ1; br, broad; w, weak; m, medium; s, strong): 3368
br, 3059 w, 3020 w, 2931 w, 2904 w, 1635 s, 1617 m, 1534 s,
1508 s, 1451 s, 1397 s, 1296 m, 1243 m, 1198 m, 1159 m, 1129 w,
987 w, 897 w, 840 m, 760 m, 703 m, 566 w, 456 w, 429 w; meff
(per copper)/mB: 1.67 at 300 K; 0.36 at 18 K; 2J ¼ ꢁ116 cmꢁ1
from theoretical fitting with g ¼ 2.01, r ¼ 0.015, g1 ¼ 2.2, y ¼
P
1.5 K, R ¼ 7.2 ꢂ 10ꢁ3 (where R ¼ i{[wobs(Ti) ꢁ wcalcd(Ti)]2/
w
obs(Ti)2}).
Experimental
Materials and instruments
[Cu2L(O2CCH2C6H4-p-OH)] (2). [Cu2L(O2CCH3)] (0.65 g,
All chemicals were purchased from commercial sources and
used as received. The Schiff base N,N0-1,3-diylbis(salicylaldi-
mino)propan-2-ol (H3L) and the dicopper(II) precursor com-
plex [Cu2L(O2CCH3)] were prepared by a literature method.25
Elemental analyses were done using a Thermo Finnigan
FLASH EA 1112 CHNS analyzer. UV-Vis and infrared spec-
tra were recorded from Hitachi U-3400 and Bruker Equinox 55
spectrometers, respectively. X-Ray powder diffraction data
were recorded on a Philips X’Pert diffractometer. TGA ana-
lyses were performed on a Mettler Toledo Star thermal analy-
zer. Differential scan calorimetric (DSC) data were obtained
from a Rheometric Scientific DSC Plus instrument.
1.35 mmol) in MeOH (20 ml) was reacted with the sodium salt
of p-hydroxyphenylacetic acid, prepared in situ from the acid
(0.21 g, 1.35 mmol) and NaOH (0.05 g, 1.3 mmol) in ethanol
(5 ml). The mixture was refluxed for 30 min, followed by
cooling to 25 1C, and was then filtered. The filtrate was slowly
concentrated to give 2 as a green solid (0.55 g, yield: B70%).
Anal. calcd for C25H22Cu2N2O6 (M 573.53): C 52.35, H 3.87,
N 4.88%; found: C 52.58, H 4.03, N 4.76%; UV-Vis in THF
[lmax/nm (e/dm3 molꢁ1 cmꢁ1)]: 640 (330), 376 (9200), 272
(29 100), 242 (40 000); IR (KBr, cmꢁ1): 3387 br, 2931 w,
1634 s, 1602 m, 1573 s, 1539 m, 1516 m, 1470 m, 1450 s,
1404 s, 1343 m, 1304 m, 1249 w, 1198 m, 1154 m, 1131 m, 1032
w, 975 w, 897 w, 859 w, 794 w, 762 s, 742 w, 702 m, 650 w, 600
m, 555 m, 466 w; meff (per copper)/mB: 1.59 at 300 K; 0.19 at
18 K; 2J ¼ ꢁ176 cmꢁ1 from theoretical fitting with g ¼ 2.06,
Magnetic measurements
r ¼ 0.009, g1 ¼ 2.15, y ¼ 2 K, R ¼ 7.7 ꢂ 10ꢁ3
.
Variable temperature magnetic susceptibility data for the
polycrystalline samples were obtained in the temperature range
18–300 K using a model 300 Lewis coil force magnetometer
from George Associates, Inc. (Berkeley, USA) equipped with a
Cahn balance and a closed-cycle cryostat (Air Products).
Hg[Co(NCS)4] was used as a standard. Experimental suscept-
ibility data were corrected for diamagnetic contributions and
for the temperature-independent paramagnetism (Na ¼ 60 ꢂ
10ꢁ6 cm3 molꢁ1 per copper). The molar magnetic susceptibil-
ities were fitted by the modified Bleaney–Bowers expression by
means of a least-squares computer program.26,27 The Hamil-
[Cu2L(O2CCH2CH2C6H4-p-OH)] (3). 3 was prepared by a
procedure similar to that of complex 2 with p-hydroxyphenyl-
propionic acid instead of p-hydroxyphenylacetic acid. Yield:
B61%; anal. calcd for C26H24Cu2N2O6 (M 587.55): C 53.15, H
4.12, N 4.76%; found: C 53.07, H 4.20, N 4.89%; UV-Vis in
THF [lmax/nm (e/dm3 molꢁ1 cmꢁ1)]: 640 (370), 376 (9300), 272
(30 000), 247 (41 500); IR (KBr, cmꢁ1): 3350 br, 3021 br, 2908
m, 1637 s, 1601 m, 1513 s, 1472 m, 1447 s, 1397 m, 1342 m,
1314 s, 1197 s, 1156 m, 1130 m, 1103 w, 1056 w, 971 w, 909 w,
895 w, 855 w, 874 w, 829 m, 794 m, 756 s, 740 m, 703 s, 607 m,
572 m, 490 w, 464 w; meff (per copper)/mB: 1.75 at 300 K; 0.22 at
18 K; 2J ¼ ꢁ140 cmꢁ1 from theoretical fitting with g ¼ 2.18,
ˆ
ˆ
tonian and susceptibility equation used were: g ¼ ꢁ2J(S1S2),
where S1 ¼ S2 ¼ 1/2 for a d9-d9 dicopper(II) core, and wCu
¼
2
[Ng2b2/k(Tꢁ y)][3 þ exp(ꢁ2J/k(Tꢁ y)]ꢁ1(1 ꢁ r)þ (Ng1 b2/4kT)r
þ Na, where r is the fraction of monomeric impurity and 2J is
the singlet–triplet separation energy. The magnetic moments
were calculated in mB unit.
r ¼ 0.013, g1 ¼ 2.2, R ¼ 5.9 ꢂ 10ꢁ3
.
X-Ray crystallography
Single crystals of the complexes were obtained from slow
concentration of the complex solutions in aqueous methanol.
A green crystal of 1 ꢀ 2H2O of size 0.2 ꢂ 0.1 ꢂ 0.05 mm3 was
mounted on a glass fibre with epoxy cement and all geometric
and intensity data were collected using an automated Enraf–
Nonius CAD4 diffractometer fitted with MoKa radiation.
Intensity data, collected using the o scan technique for 2545
reflections in the range 1.621 r y r 24.991, were corrected for
Lorentz polarization effects and for absorption.28 Of 2545
unique data, 1799 with I Z 2s(I) were used for structure
determination involving 363 parameters, giving a goodness-
of-fit value of 0.977 and highest shift/e.s.d. value of 0.003. All
nonhydrogen atoms were refined anisotropically. The hydro-
gen atoms attached to the oxygen atoms were located from the
difference Fourier map and refined isotropically using geome-
Scheme 1 Synthetic scheme for the preparation of asymmetrically
double-bridged dicopper(II) complexes 1–3, with different X spacers in
the carboxylate ligand.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 9 6 – 6 0 3
597

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trical restraints, while those attached to the carbon atoms were
generated and refined using a riding model with fixed thermal
parameters.
Geometric and intensity data for 2 ꢀ 2H2O (crystal size:
0.36 ꢂ 0.08 ꢂ 0.02 mm3) and 3 ꢀ 0.5H2O (crystal size: 0.28ꢂ
0.19 ꢂ 0.02 mm3) were collected using a Bruker SMART
APEX CCD diffractometer having a fine focus and a 1.75
kW sealed tube MoKa X-ray source with increasing o (width
of 0.31 frameꢁ1) at a scan speed of 12 s frameꢁ1. The data were
corrected for absorption.29 A total of 8510 reflections were
collected for 2 ꢀ 2H2O in the range 1.261 r y r 24.711 of which
5567 reflections with I Z 2s(I) were used for structure solution
using 689 parameters, giving a goodness-of-fit value of 1.143
and highest shift/e.s.d. value of 0.001. All nonhydrogen atoms,
except for two positionally disordered oxygen atoms, were
refined anisotropically for 2 ꢀ 2H2O. The oxygen atoms of
two lattice water molecules showed positional disorder and
two sets of atoms, namely O(15), O(16) and O(15A), O(16A),
were refined with site occupancies of 0.7 and 0.3, respectively.
The hydrogen atoms attached to the carbon atoms were fixed
in their calculated positions and refined using a riding model.
The hydrogen atoms of the p-hydroxy groups and the non-
disordered water molecules were located from the difference
Fourier map and refined with an O–H bond distance constraint
of 0.96 A. There were two dimeric complexes and four water
molecules (in total) in the crystallographic asymmetric unit of
Fig. 1
theoretical fit to the experimental data (&: 1; J: 2; n: 3).
w
MT vs. T plots for complexes 1–3; the solid lines show the
energy (2J) in the range from ꢁ116 to ꢁ176 cmꢁ1 (with the
singlet as the ground state; Fig. 1). The magnitude of J in such
{Cu2(m-OR)(m-O2CR)} cores is generally small due to the
noncomplementary nature of the super-exchange pathways
involving symmetric (jsym) and antisymmetric (jasym) combi-
nations of magnetic orbitals.24,33,34 The 2J values observed
for 1–3 compare well to that of the precursor complex
(ꢁ170 cmꢁ1).25
ꢀ
2 ꢀ 2H2O in the triclinic space group P1, possibly due to the
presence of two different water aggregates in the structure. For
3 ꢀ 0.5H2O, a total of 4128 reflections were collected in the
range 1.74 r y r 24.71 of which 3534 reflections with I Z
2s(I) were used for structure solution using 409 parameters
giving a goodness-of-fit value of 1.075 and highest shift/e.s.d.
value of 0.002. All nonhydrogen atoms were refined anisotro-
pically, except for O(7), which was modelled as a disordered
lattice water molecule with a site occupancy of 0.5. Hydrogen
atoms attached to carbon atoms were generated and refined
using a riding model.
Crystal structures
All three complexes are structurally characterized by single
crystal X-ray crystallography. Selected crystallographic data
for 1–3 are given in Table 1.y Relevant bond distances and
angles are given in Tables 2 and 3. Selected hydrogen-bonding
parameters are presented in Table 4. Complex 1 ꢀ 2H2O crystal-
lizes in the orthorhombic space group P21nb with four dicop-
per(II) units in the unit cell. The complex self-assembles into a
helical supramolecular structure through hydrogen-bonding
interactions involving the pendant hydroxy group of the
carboxylate and one phenoxo oxygen atom of the Schiff base
L (Fig. 2). The cinnamate ligand forms an angle of B301 with
the plane of the {Cu2L}1 unit and such an inclination could be
responsible for the formation of the helical structure. The
supramolecular host forms a pore in which the lattice water
molecules accumulate, forming an unprecedented helical one-
dimensional chain, stabilized by anchoring to the host struc-
ture through hydrogen-bonding interactions involving alter-
nating water molecules of the chain and the phenoxo oxygen
atoms of the Schiff base. The hydrogen bonds, showing Oꢀ ꢀ ꢀO
distances of B3.0 A along the chain, are weak. Both water
molecules are expected to be labile with the ‘‘free’’ water
molecule being itself relatively more labile than the ‘‘host-
bound’’ one. Although dynamic water molecules assuming a
single file chain structure are known19–22 in biological mem-
brane proteins like aquaporin-1 and gramicidin A, an analo-
gous one-dimensional chain in a crystal host is a rare
occurrence. Buchanan and co-workers have reported one-
dimensional water chains stabilized by imidazole channels in
which all the water molecules are bonded to the organic host in
a ‘‘zig-zag’’ motif.8 Another report on self-assembled chains of
tetrameric water aggregates in a crystal host comprised of
1,4,7,10-tetraazacyclodecane has been made recently by Sa-
manta and co-workers.9 While these water chains could have
relevance to biological systems of water or ion transport, the
The structure solution and refinement were made using
SHELX programs.30 Perspective views of the molecules were
obtained by ORTEP.31
Results and discussion
Synthesis and general aspects
Dicopper(II) complexes 1–3 were prepared in good yields from
a general reaction involving substitution of the bridging acetate
in [Cu2L(O2CCH3)] by the carboxylate ligand HO-p-C6H4–X–
CO2ꢁ, where X is the spacer (–CH CH–, –CH – or –CH
Q
CH2–, respectively; Scheme 1). The complexes crystallize with
2
2
lattice water molecules. We have shown in a previous study
that
a similar reaction with p-hydroxybenzoate (HO–
p-C6H4CO2ꢁ), which lacks the spacer X, leads to the formation
of a discrete tetranuclear copper(II) complex [Cu4L2(O2C–
C6H4–p-OH)2] in which two {Cu2L(O2C–C6H4-p-OH)} units
are covalently linked through the phenoxo oxygen atom of the
Schiff base ligand, showing axial/equatorial bonding modes.32
This tetranuclear complex does not form any supramolecular
structure: it crystallizes with one lattice isopropanol molecule
(and not with water from the aqueous alcohol), which is
involved in hydrogen-bonding interactions with the p-hydroxy
group of the carboxylate, resulting in the discrete nature of the
complex. However, with the inclusion of spacers, complexes
1–3 are dimeric in nature and form a variety of supramolecular
structures, involving the lattice water molecules.
The complexes are soluble in polar organic solvents and
display a d-d band at 640 nm in THF. Having an asymme-
trically double-bridged dicopper(II) core with a syn,syn-bonded
carboxylato and a monoatomic alkoxo bridges, they show
antiferromagnetic properties with a singlet–triplet separation
y CCDC reference numbers: 222969 for 1 ꢀ 2H2O, 224514 for 2 ꢀ 2H2O
and 260832 for 3 ꢀ 0.5H2O. See http://www.rsc.org/suppdata/nj/b4/
b415945d/ for crystallographic data in .cif or other electronic format.
598
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 9 6 – 6 0 3

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Table
1
Crystallographic data for [Cu L(O C–CH CH–
Q
Table 3 Selected bond lengths (A) and angles (1) for 2 ꢀ 2H2O
2
2
C6H4-p-OH)]ꢀ 2H2O (1 ꢀ 2H2O), [Cu2L(O2C–CH2–C6H4-p-OH)] ꢀ 2H2O
Molecule A
Molecule B
(2ꢀ 2H2O) and [Cu2L(O2C–CH2CH2–C6H4-p-OH)]ꢀ 0.5H2O (3ꢀ 0.5H2O).
Cu(1)ꢀ ꢀ ꢀCu(2)
Cu(1)–O(1)
Cu(1)–O(2)
3.500(2)
1.893(5)
1.903(5)
1.930(6)
1.911(7)
1.917(5)
1.904(5)
1.936(6)
1.914(7)
Cu(3)ꢀ ꢀ ꢀCu(4)
Cu(3)–O(7)
Cu(3)–O(8)
3.503(1)
1.905(5)
1.906(5)
1.931(6)
1.913(6)
1.915(5)
1.893(5)
1.929(6)
1.909(7)
1 ꢀ 2H2O
2 ꢀ 2H2O
3 ꢀ 0.5H2O
C26H26Cu2N2O8 C25H26Cu2N2O8 C26H25Cu2N2O6.5
Formula
FW
Crystal system Orthorhombic Triclinic
621.57
609.56
596.56
Monoclinic
P21/c (no. 14)
7.8190(10)
22.534(5)
13.699(2)
90
Cu(1)–O(5)
Cu(1)–N(1)
Cu(3)–O(11)
Cu(3)–N(3)
ꢀ
P21nb (no. 33) P1 (no. 2)
Space group
Cu(2)–O(2)
Cu(2)–O(3)
Cu(4)–O(8)
Cu(4)–O(9)
a/A
b/A
c/A
a/1
8.106(3)
13.166(3)
25.160(4)
90
10.861(2)
14.419(3)
16.634(3)
80.332(4)
80.068(4)
80.199(4)
2502.5(8)
4
Cu(2)–O(4)
Cu(2)–N(2)
Cu(4)–O(10)
Cu(4)–N(4)
Cu(1)–O(2)–Cu(2)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–O(5)
O(5)–Cu(1)–N(1)
O(2)–Cu(2)–O(3)
O(2)–Cu(2)–O(4)
O(3)–Cu(2)–O(4)
O(2)–Cu(2)–N(2)
O(3)–Cu(2)–N(2)
O(4)–Cu(2)–N(2)
132.8(3)
Cu(3)–O(8)–Cu(4)
O(7)–Cu(3)–O(8)
O(7)–Cu(3)–O(11)
O(8)–Cu(3)–O(11)
O(7)–Cu(3)–N(3)
O(8)–Cu(3)–N(3)
O(11)–Cu(3)–N(3)
O(8)–Cu(4)–O(9)
O(8)–Cu(4)–O(10)
O(9)–Cu(4)–O(10)
O(8)–Cu(4)–N(4)
O(9)–Cu(4)–N(4)
O(10)–Cu(4)–N(4)
132.9(3)
b/1
90
90
94.203(5)
90
177.2(2)
86.8(2)
93.2(3)
85.2(3)
94.7(2)
177.8(3)
175.4(2)
94.0(2)
88.2(2)
84.7(3)
93.6(3)
174.1(3)
177.9(2)
87.6(2)
94.4(2)
93.8(3)
84.2(2)
176.2(3)
178.3(2)
93.9(2)
87.2(2)
84.0(3)
94.9(3)
175.9(3)
g/1
U/A3
Z
2685.2(12)
4
1.538
2407.2(7)
4
1.646
r
calcd/g cmꢁ3
1.618
17.52
m(MoKa)/cmꢁ1 16.35
18.15
293(2)
T/K
Rint
293(2)
0.0
293(2)
0.0985
0.0282
0.0370
0.0463
a
R1
R1
0.0546
0.0931
0.0891
0.1429
(all data)
b
wR2
wR2
0.1399
0.1669
0.1635
0.1842
0.0906
0.0954
(all data)
P
P
P
P
of B3.5 A and B1331, respectively. What makes the structure
of complex 2 significantly different from that of 1 is the angle of
B721 between the –CH2–C6H4-p-OH group and the Cu2N2O5
plane. Such a stereochemical arrangement possibly prevents
the phenoxo oxygen atoms in L from axial bonding to the
copper centres. Complex 2 thus self-assembles into a different
supramolecular structure, involving the pendant hydroxyl
group of the carboxylate, the lattice water molecules and the
phenoxo oxygen atom of the Schiff base. The lattice water
molecules show two types of water aggregates: pseudo-hex-
americ cyclic chairs and quasi-linear forms. Unlike 1 ꢀ 2H2O,
the supramolecular structure in 2 ꢀ 2H2O includes the lattice
water molecules as an integral part, thus making them rela-
tively more stable in comparison to the helical water chain
(Fig. 4). Fig. 5 shows the hydrogen-bonding network in the
water aggregates in 2 ꢀ 2H2O. Water oxygen atoms O(13)#1,
O(13)#2, O(14), O(14)#3 and hydroxy group atoms O(6), and
O(6)#3 form together a cyclic hexameric structure, giving
Oꢀ ꢀ ꢀO distances in the range 2.74(9)–2.78(1) A. In addition,
the water molecules are also hydrogen-bonded to the phenoxo
oxygen atoms of the Schiff base (Oꢀ ꢀ ꢀO distance B2.84 A).
The cyclic structure resembles cyclohexane in its chair con-
figuration. Since there are only four water molecules instead of
six, this aggregate structure can be best described as pseudo-
hexameric. The cyclic structure of (H2O)6 in a chair conforma-
tion is a rare occurrence. It has been previously reported in
liquid helium droplets or in organic supramolecular net-
works.35 Studies on such water assemblies have shown that a
cyclic structure is energetically less favorable in comparison to
prism, cage, book and boat conformations.4 The average
Oꢀ ꢀ ꢀOꢀ ꢀ ꢀO angle of B116.51 in the cyclic structure shows a
minor deviation from the C–C–C bond angle of 111.051 in the
chair conformation of cyclohexane. This angle is, however, in
very good agreement with the 116.51 observed36 in liquid water
hexamer, but deviates considerably from the angle of 109.31 in
hexagonal ice.37 The pseudo-hexameric moiety does have an
inversion center and three C2 axes bisecting the three pairs of
oxygen atoms O(13)#1, O(13)#2; O(14), O(14)#3; O(6), O(6)#3
as in cyclohexane, but, unlike cyclohexane, it does not possess
a C3 or S6 axis. Among the structurally characterized cyclic
hexamers, the one reported by Custelcean and co-workers has
an ice-like chair conformation of six water molecules trapped
in an organic host lattice derived from 2,4-dimethyl-5-amino-
benzo-[b]-1,8-naphthyridine.10 A planar high energy cyclic
a
b
2
R1 ¼ 8Fo| ꢁ |Fc8/ |Fo|. wR2 ¼ { [w(Fo ꢁ Fc2)2/ [w(Fo2)2]}1/2
;
w ¼ 1/[s2(Fo2)þ (AP)2 þ BP] where P ¼ [max(Fo2,0) þ 2Fc2]/3 and
A
1–3, respectively.
¼
0.1108, 0.0595, 0.0609 and
B
¼
0.0; 2.3093; 1.3998 for
helical water chain in a helical supramolecular host, as ob-
served in 1 ꢀ 2H2O, is a good potential model, considering the
helical nature of membrane proteins.
ꢀ
Complex 2 ꢀ 2H2O crystallizes in the triclinic space group P1
with two dicopper(II) complexes in the asymmetric unit (Z ¼
4). The dinuclear complex with a {Cu2(m-OR)(m-O2CR)} core
has a similar structure to that of 1, where the monoatomic
alkoxo bridge is derived from the pentadentate Schiff base and
p-hydroxyphenylacetate is a three-atom bridging carboxylate
(Fig. 3). Each copper has a planar CuNO3 coordination
geometry, giving a Cuꢀ ꢀ ꢀCu distance and Cu–O(R)–Cu angle
Table 2 Selected bond distances (A) and angles (1) in 1 ꢀ 2H2O and
3 ꢀ 0.5H2O
1 ꢀ 2H2O
3.476(2)
3 ꢀ 0.5H2O
Cu(1)ꢀ ꢀ ꢀCu(2)
Cu(1)–O(1)
Cu(1)–O(2)
3.464(2)
1.879(2)
1.904(2)
1.905(7)
1.911(7)
1.914(6)
1.935(7)
1.926(7)
1.901(7)
1.939(7)
1.932(8)
Cu(1)–O(5)
Cu(1)–N(1)
1.932(2)
1.925(3)
Cu(2)–O(2)
Cu(2)–O(3)
1.894(2)
1.894(2)
Cu(2)–O(4)
Cu(2)–N(2)
1.904(2)
1.902(3)
Cu(1)–O(2)–Cu(2)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(2)–Cu(1)–O(5)
O(2)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
O(2)–Cu(2)–O(3)
O(2)–Cu(2)–O(4)
O(2)–Cu(2)–N(2)
O(3)–Cu(2)–O(4)
O(3)–Cu(2)–N(2)
O(4)–Cu(2)–N(2)
129.8(3)
131.63(11)
176.86(10)
87.45(10)
93.67(11)
94.37(10)
84.36(15)
176.22(11)
176.68(10)
95.64(9)
175.8(3)
86.5(3)
93.3(3)
96.6(3)
83.9(3)
174.8(4)
177.3(3)
95.9(3)
83.8(3)
86.5(3)
94.0(4)
175.8(4)
85.15(10)
85.36(10)
94.22(10)
173.57(12)
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 9 6 – 6 0 3
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Table 4 Selected hydrogen-bonding parameters in the supramolecular structures of 1 ꢀ 2H2O, 2 ꢀ 2H2O and 3 ꢀ 0.5H2O
O–Hꢀ ꢀ ꢀO
d(Oꢀ ꢀ ꢀO)/A
d(Hꢀ ꢀ ꢀO)/A
+O–Hꢀ ꢀ ꢀO/1
Oꢀ ꢀ ꢀOꢀ ꢀ ꢀO
+Oꢀ ꢀ ꢀOꢀ ꢀ ꢀO/1
a
.
1 2H2O
O(8)–Hꢀ ꢀ ꢀO(7)
2.92(3)
2.98(3)
2.67(1)
2.78(2)
2.73(1)
2.85(1)
2.78(1)
2.87(1)
2.77(1)
2.63(1)
2.72(2)
2.84(1)
2.74(1)
2.94(1)
2.92(1)
2.81(1)
2.01
2.23
1.84
1.86
2.00
1.93
1.86
2.28
2.04
1.68
—
159.7
135.5
140.5
163.0
131.5
160.1
161.4
117.9
130.3
159.3
—
O(3)#3ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(8)
95.3
98.9
114.3
116.0
111.7
130.8
107.1
—
O(7)–Hꢀ ꢀ ꢀO(8)#2
O(6)#3–Hꢀ ꢀ ꢀO(1)d
O(7)–Hꢀ ꢀ ꢀO(3)#3
O(14)–Hꢀ ꢀ ꢀO(13)#2
O(13)#1–Hꢀ ꢀ ꢀO(1)#1
O(13)#1–Hꢀ ꢀ ꢀO(6)
O(14)–Hꢀ ꢀ ꢀO(9)#3
O(6)–Hꢀ ꢀ ꢀO(14)
O(3)#3ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(8)#2
O(7)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(7)#1
O(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(8)#2
b
.
2 2H2O
O(6)ꢀ ꢀ ꢀO(13)#1ꢀ ꢀ ꢀO(14)#3
O(13)#1ꢀ ꢀ ꢀO(6)ꢀ ꢀ ꢀO(14)
O(6)ꢀ ꢀ ꢀO(14)ꢀ ꢀ ꢀO(13)#2
—
—
—
—
—
—
—
—
—
—
—
O(12)–Hꢀ ꢀ ꢀO(16)#2
O(15)#2ꢀ ꢀ ꢀO(16)#2
O(15)#2ꢀ ꢀ ꢀO(7)#2
O(16)#2ꢀ ꢀ ꢀO(3)#2
O(7)#2ꢀ ꢀ ꢀO(1)
—
—
—
—
—
—
—
—
c
.
3 0.5H2O
—
—
—
—
O(7)#3ꢀ ꢀ ꢀO(1)
—
—
O(6)–Hꢀ ꢀ ꢀO(3)#1e
2.06
164.6
a
b
Symmetry codes: #1: x ꢁ1/2, ꢁy, ꢁz þ 1; #2: x þ 1/2, ꢁy, ꢁz þ 1; #3: x þ 1/2, ꢁy þ 1, ꢁz þ 1. Symmetry codes: #1: ꢁx þ 2, ꢁy þ 1, ꢁz þ 1;
c
d
#2: x þ 1, y, z; #3: ꢁx þ 3, ꢁy, ꢁz. Symmetry codes: #1: x þ 1, þy, þz; #2 ꢁx, þy ꢁ 1/2, ꢁz þ 1/2 þ 1; #3 x, ꢁy þ 1/2 þ 1, þz þ 1/2. O(6)#3–H,
e
0.98 A. O(6)–H, 0.78 A.
form of water aggregates trapped in an organic supramolecular
host based on bismesityl-3,30-dicarboxylic acid has been
reported by Venugopalan and co-workers.11 Gibson and
co-workers have reported a supramolecular cryptand structure
presenting a pseudo-hexameric cyclic chair form of water
molecules.12 Cyclic hexameric water aggregates in chair con-
formation are also known to form in 3D metal-organic frame-
work structures of Ce(III) and Pr(III) with pyridine-2,6-
dicarboxylic acid.15
The quasi-linear arrangement of water molecules in 2 ꢀ 2H2O
involves two positionally disordered water molecules [O(15)
and O(16) as major (site occupancy 0.7) and O(15A), O(16A)
as minor (site occupancy 0.3) components], along with the
pendant p-hydroxy group of the carboxylate (Fig. 5). A
distance of 2.72(2) A between O(15)#2 and O(16)#2 belonging
to two water molecules suggests moderately strong hydrogen-
bonding interactions. The terminal O(16) water is relatively
strongly hydrogen-bonded to the host structure, involving the
pendant p-hydroxy group of the carboxylate and the phenoxo
atom O(3) and giving Oꢀ ꢀ ꢀO distances of B2.6 and B2.7 A,
respectively. The central water molecule is relatively unsup-
ported, showing a weakly hydrogen-bonding interaction with
the phenoxo oxygen atom [O(7)#2ꢀ ꢀ ꢀO(15)#2, 2.84(1) A]. The
relatively short distance of 2.32(3) A between O(15A)#2 and
O(15A)#4 could be related to the positional disorder of the
central two water molecules among four sites.
Complex 3, as a hemi-hydrate, crystallizes in the monoclinic
space group P21/c. The structural features of the dicopper(II)
core are similar to the other two complexes, having a {Cu2
(m-OR)(m-O2CR)} core. The Cuꢀ ꢀ ꢀCu distance is 3.464(2) A.
The {Cu2L}1 unit has a planar structure. The carboxylate has a
–CH2CH2– spacer with two sp3-hybridized carbon atoms; such
a conformation leads to the formation of a one-dimensional
chain-like supramolecular structure due to hydrogen-bonding
interactions involving the pendant p-hydroxy group of the
carboxylate and one phenoxo oxygen atom of the Schiff base
[Fig. 6(a)]. The p-hydroxy group in 3 ꢀ 0.5H2O makes an angle
of B651 with the Cu2N2O5 plane. The angle between the –
CH2CH2– spacer and the aromatic ring of the carboxylate is
B1291. The positionally disordered lattice water molecule acts
as a linker between two supramolecular 1D chains [Fig. 6(b)].
Thermal and powder XRD studies
The thermal stability of the water molecules in complexes
1 ꢀ 2H2O and 2 ꢀ 2H2O has been studied by thermogravimetric
Fig. 3 ORTEP views of the two molecules of [Cu2L(O2C–CH2–C6H4-
p-OH)] ꢀ 2H2O (2 ꢀ 2H2O) in the crystallographic asymmetric unit,
showing 50% probability thermal ellipsoids and the atom numbering
scheme for the metal and the heteroatoms. Atoms O(15), O(16) and
O(15A), O(16A) are positionally disordered, having site occupancies
of 0.7 and 0.3, respectively.
Fig. 2 (a) Perspective view of the self-assembled helical supramole-
cular host structure of [Cu L(O C–CH CH–C H -p-OH)] ꢀ 2H O
Q
2
2
6
4
2
(1 ꢀ 2H2O), showing ‘‘free’’ and ‘‘host-bound’’ water molecules in the
water chain. (b) Hydrogen-bonding network in the helical water chain.
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Fig. 6 (a) Perspective view of the 1D chain supramolecular host
structure in [Cu2L(O2C–CH2CH2–C6H4-p-OH)] ꢀ 0.5H2O, (3ꢀ 0.5H2O),
showing atom labelling for the metal and the heteroatoms. (b) Hydro-
gen-bond linkage of two chains by lattice water.
Fig. 4 A perspective view of the self-assembled supramolecular
structure in [Cu2L(O2C–CH2–C6H4-p-OH)] ꢀ 2H2O (2 ꢀ 2H2O).
analysis (TGA) and differential scanning calorimetry (DSC).
The reversibility of the dehydration process was probed by
powder X-ray diffraction studies. The TGA of 1 ꢀ 2H2O gives a
weight loss of B4.5% in the temperature range 25–140 1C due
to loss of two water molecules (Fig. 7). The lability of the water
molecules is evidenced from significant loss of water even at
25 1C. The experimental TGA data show a sharp weight loss
from the beginning of the experiment, hence rendering the
observed water loss percentage (4.6%) less than the theoretical
value of 5.8%. The DSC experiments reveal two endotherms at
61.5 and 88.5 1C, corresponding to the loss of ‘‘free’’ and
‘‘host-bound’’ water molecules, respectively, with the process
of water loss beginning below 25 1C (Fig. 8). The overall
change of enthalpy per water molecule is B36 kJ molꢁ1. The
TGA of 2 ꢀ 2H2O shows a weight loss of 5.85% (calculated
weight loss 5.9%) within 100–150 1C, corresponding to the loss
of both water molecules (Fig. 7). The DSC plot of 2 ꢀ 2H2O
displays one endotherm at 131 1C with a shoulder at B126 1C,
due to successive loss of both water molecules (Fig. 8). The
Fig. 7 Thermogravimetric plots of (a) 1 ꢀ 2H2O and (b) 2 ꢀ 2H2O,
showing weight loss of the sample with increasing temperature.
overall change in enthalpy per water molecule is B26 kJ molꢁ1
.
Fig. 5 The hydrogen-bonding networks in the cyclic pseudo-hexame-
ric and the quasi-linear water aggregates in [Cu2L(O2C–CH2–C6H4-
p-OH)] ꢀ 2H2O (2 ꢀ 2H2O).
Fig. 8 Differential scanning calorimetric plots of the complexes (a) 1 ꢀ
2H2O and (b) 2 ꢀ 2H2O, showing endotherms due to loss of water
molecules.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 5 9 6 – 6 0 3
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Fig. 11 Powder XRD data for (a) 3 ꢀ 0.5H2O, (b) on removal of lattice
Fig. 9 Powder XRD plots for (a) 1 ꢀ 2H2O, (b) on dehydrating the
water and (c) on subsequent rehydration.
sample and (c) after rehydration.
mate dicopper(II) Schiff base complex. The powder XRD data
for 2 ꢀ 2H2O, however, reveal significant structural changes
upon loss of the lattice water molecules (Fig. 10). The sample
on subsequent rehydration recovers the original XRD pattern
with some variations. Structural change upon loss of water is
expected for this complex as the water molecules are an integral
part of the supramolecular structure (Fig. 4). The powder
XRD patterns of 3 ꢀ 0.5H2O upon dehydration and subsequent
rehydration are similar to the original pattern, suggesting
reversibility of the water loss and gain processes (Fig. 11).
The water aggregates in 2 ꢀ 2H2O have a significantly higher
thermal stability than those in 1 ꢀ 2H2O.
The reversibility of the processes involving water loss and
subsequent addition of lattice water molecules, along with their
effect on the supramolecular structure, have been studied by
the powder XRD method. Complex 1 ꢀ 2H2O, upon dehydra-
tion at 60 1C under vacuum, shows XRD features that closely
resemble the original sample with minor variations but sig-
nificant broadening of the pattern (Fig. 9). The sample on
subsequent rehydration gives an XRD pattern that is similar to
that of the original sample. The results indicate reversible loss
and formation of the helical water chain in the pore of the self-
assembled supramolecular structure of the p-hydroxycinna-
Conclusion
We have synthesized three new asymmetrically double-bridged
dicopper(II) complexes [Cu2L(O2C–X–C6H4-p-OH)] (1–3) con-
taining pentadentate trianionic Schiff base (L) and carboxylate
ligands. The carboxylate ligands, having different spacers,
–CH CH–, –CH – and –CH CH –, show noticeable stereo-
2
Q
2
2
control effects on the formation of a variety of self-assembled
supramolecular structures, involving the pendant hydroxy
groups of the carboxylates and the phenoxo oxygen atoms of
the Schiff bases.
The complexes crystallize with lattice water molecules that
arrange in the form of linear chains or cyclic structures during
the self-assembly processes. Complex 1 ꢀ 2H2O, with the
p-hydroxycinnamate ligand, forms a helical supramolecular host
structure that stabilizes a helical chain of water molecules, with
reversible loss and gain of the water chain during dehydration
and subsequent hydration processes. Complex 1 ꢀ 2H2O exem-
plifies a potential model for the single file water arrangements
in helical membrane proteins with aqua pores for transmem-
brane water permeation or proton transport. Complex
2 ꢀ 2H2O presents a supramolecular structure that stabilizes a
rare pseudo-hexameric cyclic chair conformation of the water
aggregate and a quasi-linear arrangement of water molecules.
The water loss from the structure and subsequent hydration
take place essentially in a reversible fashion although the
dehydrated sample displays significant change of the host
structure for 2 ꢀ 2H2O. The –CH2CH2– spacer of the carboxy-
late gives a new 1D chain supramolecular structure in
3 ꢀ 0.5H2O and the lattice water molecule acts as a linker of
two such chains.
Fig. 10 Powder XRD plots for (a) 2 ꢀ 2H2O, (b) on loss of water and
(c) on subsequent rehydration.
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This work demonstrates the stereocontrol effect of the
carboxylate spacers in the formation of a variety of supra-
molecular structures that stabilize water aggregates of novel
conformations.
13 S. Supriya and S. K. Das, New J. Chem., 2003, 27, 1568.
14 C. Foces-Foces, F. H. Cano, M. Martinez-Ripoll, R. Faure, C.
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15 S. K. Ghosh and P. K. Bharadwaj, Inorg. Chem., 2003, 42, 8250.
16 K. M. Park, R. Kuroda and T. Iwamoto, Angew. Chem., Int. Ed.
Engl., 1993, 32, 884.
Acknowledgements
17 (a) R. J. Doedens, E. Yohannes and I. M. Khan, Chem. Commun.,
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The authors are thankful to the Department of Science and
Technology, Government of India, for financial support
(Grant SP/S1/F-01/2000) and for the CCD diffractometer
facility. Thanks are due to Prof. S. Ramakrishnan of our
department for the thermal analysis data, Prof. T. N. Guru
Row of the Solid State and Structural Chemistry Unit for the
powder XRD data, and the Convener of Bioinformatics Center
of our Institute for database search.
21 R. Pomes and B. Roux, Biophys. J., 1996, 71, 19.
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