Hydrogen-bond-directing effect in the ionothermal synthesis of metal coordination polymers

Article abstract of DOI:10.1039/b802892c

Four new cobalt coordination polymers, (EMIm)[Co₂(TMA-H) ₂(44bpy)₃]Br 1, (EMIm)[Co(TMA-H)(44bpy) ₂](44bpy)Br 2, (EMIm)[Co(TMA)(Im-H)] 3 and (EMIm)₂[Co(TMA) ₂(TED-H₂)] 4, were prepared from 1-ethyl-3-methyl imidazolium bromide (EMIm-Br). All the compounds have similar two-dimensional cobalt trimesate (TMA) coordination layers but different three-dimensional supramolecular architectures that contain one of three potentially ditopic amines, 4,4′-bipyridine (44bpy), imidazole (Im-H) and triethylenediamine (TED). Two-fold interpenetration of hydrogen-bonding networks was found for 1, 2 and 4. The coordination layers of 1 and 2 are neutral while 3 and 4 have anionic molecular assemblies. The use of organic amines, that act as supramolecular bridging ligands, introduces hydrogen-bond-directing effects in the ionothermal synthesis of metal coordination polymers. Hydrogen bonding helps to align the packing between the coordination layers and control the formation of 3D supramolecular networks. In 1, hydrogen bonds between the ionic species within the channels direct the alignment of non-directional electrostatic interactions between EMIm⁺ and Br^- ions, which is a rare case of a hydrogen-bond-templating effect of ionic liquids in ionothermal synthesis.

Full text of DOI:10.1039/b802892c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Hydrogen-bond-directing effect in the ionothermal synthesis of metal
coordination polymers†
Zhuojia Lin,*‡ Yang Li, Alexandra M. Z. Slawin and Russell E. Morris
Received 20th February 2008, Accepted 2nd May 2008
First published as an Advance Article on the web 6th June 2008
DOI: 10.1039/b802892c
Four new cobalt coordination polymers, (EMIm)[Co2(TMA–H)2(44bpy)3]Br 1,
(EMIm)[Co(TMA–H)(44bpy)2](44bpy)Br 2, (EMIm)[Co(TMA)(Im–H)] 3 and
(EMIm)2[Co(TMA)2(TED–H2)] 4, were prepared from 1-ethyl-3-methyl imidazolium bromide
(EMIm–Br). All the compounds have similar two-dimensional cobalt trimesate (TMA) coordination
layers but different three-dimensional supramolecular architectures that contain one of three
potentially ditopic amines, 4,4ꢀ-bipyridine (44bpy), imidazole (Im–H) and triethylenediamine (TED).
Two-fold interpenetration of hydrogen-bonding networks was found for 1, 2 and 4. The coordination
layers of 1 and 2 are neutral while 3 and 4 have anionic molecular assemblies. The use of organic
amines, that act as supramolecular bridging ligands, introduces hydrogen-bond-directing effects in the
ionothermal synthesis of metal coordination polymers. Hydrogen bonding helps to align the packing
between the coordination layers and control the formation of 3D supramolecular networks. In 1,
hydrogen bonds between the ionic species within the channels direct the alignment of non-directional
electrostatic interactions between EMIm+ and Br− ions, which is a rare case of a
hydrogen-bond-templating effect of ionic liquids in ionothermal synthesis.
utilise different organic amines as ancillary ligands and hydrogen-
Introduction
bonding acceptors to study the hydrogen-bond-directing effect in
Metal coordination polymers and metal–organic frameworks
the ionothermal synthesis of metal coordination polymers.
(MOFs) have attracted constantly increasing interest in the last
Hydrogen bonding is a powerful organising force in the
two decades for their rich structural aesthetics and potential
synthesis of solids because of its directional and selective bond
applications in gas storage, separation and catalysis.1,2 More and
more efforts have been devoted to the exploration of MOFs with
different functionalities3 and the development of new synthetic
methods.4 Ionothermal synthesis5 has been developed into a ver-
satile and advantageous synthetic technique in materials science,
especially for the preparation of zeotypes6 and metal coordi-
nation polymers.7 Compared to traditional hydro(solvo)thermal
synthesis, the diverse and easily tuneable solvent properties for
ionic liquids allow the controlled introduction of new factors,
including structural directing,7a,d anion control7b and chiral induc-
tion effects,7c into the crystal engineering of metal coordination
polymers. In ionothermal synthesis, the ionic reaction medium
usually leads to the production of negatively charged coordination
frameworks. Strong electrostatic interactions between the ionic
liquid cations and the host networks are almost always found for
the ionothermal products.7 Hydrogen bonding, on the other hand,
plays a less prominent role in host–guest stabilisation and structure
arrangement than it often does in hydrothermal synthesis. Here we
formation.8 It can be compared to metal–ligand coordination
bonds in many respects and has been extensively used for con-
structing organic and metal–organic networks.9 In hydrothermal
systems proton transport is possible through a three-dimensional
(3D) hydrogen-bonding aqua network10 among the organic lig-
ands, hydrolysed metal ions and solvent molecules, which helps
to solvate the reactants and direct the formation of metal–
organic ligand coordination networks. Though more and more
studies suggest the existence of hydrogen bonds and other weak
supramolecular interactions in ionic liquids,10,11 the lack of effec-
tive hydrogen-bonding acceptors and donors potentially limits the
use of hydrogen bonding as a useful tool for the controlled synthe-
sis of metal coordination polymers under ionothermal conditions
unless ancillary ligands with stronger hydrogen-bonding potential
are added to the system.
In this article, we report four novel cobalt coordination
polymers with trimesic acid (TMA–H3) from 1-ethyl-3-methyl
imidazolium bromide (EMIm–Br). Three potentially ditopic
amines, 4,4ꢀ-bipyridine (44bpy), imidazole (Im–H) and triethylene-
diamine (TED), were used to help construct the frameworks
in ionic liquids. The resultant compounds, (EMIm)[Co2(TMA–
H)2(44bpy)3]Br 1, (EMIm)[Co(TMA–H)(44bpy)2](44bpy)Br 2,
(EMIm)[Co(TMA)(Im–H)] 3 and (EMIm)2[Co(TMA)2(TED–
H2)] 4, share characteristic two-dimensional coordination net-
works connected by hydrogen bonds forming 3D supramolecular
architectures.
EaStChem School of Chemistry, University of St Andrews, Purdie Building,
St Andrews, UK KY16 9ST. E-mail: zl10@st-andrews.ac.uk
† Electronic supplementary information (ESI) available: IR spectra and
PXRD patterns for 1, 2, 3 and 4. CCDC reference numbers 661275–
661278. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b802892c
‡ Current address: School of Chemistry and Chemical Engineering,
Sun Yat-Sen University, Guangzhou 510275, China. E-mail: linzhjia@
mail.sysu.edu.cn
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Crystallographic studies
Experimental
Single-crystal X-ray diffraction data for 1–4 were collected using
Materials and methods
˚
Mo Ka radiation (k = 0.71073 A) using a Rigaku rotating
Phase identification was accomplished using powder X-ray
anode single-crystal X-ray diffractometer at the University of
St Andrews. The structures were solved using standard direct
methods and refined using least-squares minimisation techniques
against F2. Anisotropic atomic displacement parameters were
applied to all non-hydrogen atoms. The organic hydrogen atoms
diffraction (Stoe STADIP diffractometer, Cu Ka radiation, k =
˚
1.54056 A) at ambient temperature. Simulated powder XRD
patterns of 1–4 were calculated with Mercury.12 The C, H, and N
microanalyses were carried out with Carlo Erba CHNS analyser.
The FT-IR spectra were recorded from KBr tablets in the region of
4000–400 cm−1 on a Perkin Elmer Spectrum GX IR spectrometer.
The reagents for the syntheses were commercially available and
were used as purchased without further purification. The synthesis
procedures of the ionic liquid and 1–4 are described as follows.
˚
were generated geometrically (C–H 0.96 A); though the hydrogen-
bonded protons in 1, 2 and 3 were located from difference maps
˚
˚
firstly, they were refined geometrically (O–H 0.82 A, N–H 0.86 A
˚
and C–H 0.96 A) for consistency. In 4, the hydrogen atom involved
in the hydrogen bond between TMA and TED was found from
a difference map in the middle of the amine and the carboxylate
group and the hydrogen bonding is pretty strong (O · · · H · · · N =
Preparations
˚
2.575(11) A). It is less likely that the proton can be constrained to
1-Ethyl-3-methyl imidazolium bromide (EMIm–Br). Under
inert atmosphere conditions, degassed ethyl bromide (68.43 g,
0.582 mol, Avocado) was added to the redistilled N-methyl
imidazole (42.06 g, 0.511 mol, Aldrich) with constant stirring. This
was refluxed at 40 ◦C for 3 h and then cooled to room temperature.
Ethyl acetate was added, and the product crashed out of solution.
It was◦filtered, washed with ethyl acetate, and dried under vacuum
at 20 C for 10 h yielding a white solid. The product was stored
under an inert atmosphere. Yield: 94%. 1H NMR (D2O): d 1.41 (t,
3H, CH3, J = 7.3 Hz), 3.81 (s, 3H, NCH3), 4.15 (q, 2H, NCH2,
J = 7.3 Hz), 7.38 (d, 2H, NC(H)C(H)N, J = 21.0 Hz), 8.66 (s,
1H, NC(H)N). NMR is comparable with literature values.13
either the TMA or TED ligands. Crystallographic parameters and
datacollectionandrefinementinformationfor 1–4aresummarised
in Table 1. The refinement for 2 is relatively poor, which may
be due to the poor crystal quality and low X-ray diffraction
intensity. In compound 4, the disordered TED–H2 ligands at two
alternative sites on the supramolecular framework and the highly
disordered EMIm+ cations within the pores may also lead to the
poor refinement results.
Results
Crystal structure
The phase purity of 3 and 4 was identified by powder X-ray
diffraction (see, ESI†). The deviation between the calculated and
experimental CHN analysis results may be due to the slight
decomposition of the products in methanol during the process
of filtration, which also led to poor crystallisation of the bulk
samples.
In general, the structures of 1–4 all consist of cobalt trimesate
two-dimensional coordination layers. However, the introduction
of different amine ligands has dramatic effects on the interactions
between metal ions and the trimesic acid groups, which results in
significantly different three-dimensional architectures.
(EMIm)[Co2(TMA–H)2(44bpy)3]Br 1 and (EMIm)[Co(TMA–
H)(44bpy)2](44bpy)Br 2. Co(OAc)2·4H2O (250 mg, 1 mmol,
Fisons), trimesic acid (TMA–H3, 210 mg, 1 mmol, Avocado), 4,4ꢀ-
byridine (312 mg, 2 mmol, Avocado) and ∼10 mmol EMIm–Br
were sealed and heated in a 23 mL Teflon-lined stainless-steel
autoclave at 110 ◦C for 4 days. Pink block crystals of 1, purple
block crystal of 2 and a small amount of an unknown crystalline
impurity were obtained and washed with methanol.
(EMIm)[Co(TMA)(Im–H)] 3. Co(OAc)2·4H2O (250 mg,
1 mmol), TMA–H3 (210 mg, 1 mmol), imidazole (136 mg, 2 mmol,
Avocado) and ∼10 mmol EMIm–Br were mixed and heated
at 110 ◦C for 4 days. Purple block crystals were washed with
methanol, dried in air and recovered with ∼65% yield. Anal. Calcd
(Found) (%) C18H18N4O6Co: C 48.56 (49.71), H 4.08 (4.54), N
12.58 (10.18). IR (400–4000 cm−1): 3449 m, 3144 m, 3091 m, 2978
m, 2369 w, 2344 w, 1631 s, 1583 s, 1434 s, 1362 s, 1172 s, 1092 m,
1055 m, 936 w, 859 w, 768 m, 724 s, 620 m, 459 w.
(EMIm)[Co2(TMA–H)2(44bpy)3]Br 1. The coordination layer
in 1 consists of two crystallographically independent cobalt atoms,
two TMA–H (i.e. singly protonated TMA) ligands in syn-anti
and chelating binding modes and three 44bpy units, one bridging
and two monodentate. Both cobalt centres have almost the same
coordination environments in octahedral geometry enclosed by
three TMA–H groups, one bridging and one monodentate 44bpy
(Fig. 1a). Two syn-anti carboxylate moieties from two TMA–H
groups bridge the cobalt atoms forming a Co2(COO)2 dimer,
which is further linked through the chelating carboxylates and
forms a step-ladder chain running along the a-axis. The bridging
44bpy ligands tie the Co2(TMA–H)2 chains in the ac plane,
forming a 2D layer of Co8(TMA–H)4(bridging-44bpy)2 grids with
(EMIm)2[Co(TMA)2(TED–H2)] 4. Co(OAc)2·4H2O (250 mg,
1 mmol), TMA–H3 (210 mg, 1 mmol), triethylenediamine (TED,
112 mg, 1 mmol, Avocado) and ∼10 mmol EMIm–Br were mixed
and heated at 110 ◦C for 5 days. Purple block crystals were filtered,
washed with MeOH and recovered with ∼ 50% yield. Anal. Calcd
(Found) (%) C36H20N6O12Co: C 54.86 (53.64), H 2.54 (3.26), N
10.67 (11.57). IR (400–4000 cm−1): 3425 s, 3141 s, 3038 s, 2855 m,
2587 m, 1945 w, 1677 s, 1612 s, 1563 s, 1434 s, 1363 s, 1329 s, 1170 s,
1092 m, 1066 m, 934 m, 880 m, 834 m, 756 s, 711 m, 683 m, 663
m, 622 m, 538 w, 441 w.
˚
dimensions of ∼8 × 9 A (Fig. 2a). The monodentate 44bpy,
which is cis to the bridging-44bpy in the coordination sphere
of cobalt, and the protonated TMA–H carboxylic group extend
above and below the layer, respectively. Adjacent nets are at-
tached by hydrogen bonds between the terminal 44bpy and
3990 | Dalton Trans., 2008, 3989–3994
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Table 1 Crystallographic data and structure refinement results for 1–4
1
2
3
4
Chemical formula
Formula weight
Crystal size
C54H43N8O12Co2Br
1193.73
0.10 × 0.06 × 0.03
Monoclinic
Cc
9.967(1)
35.170(3)
15.151(3)
90.00
107.658(2)
90.00
C45H39N8O6CoBr
926.68
0.15 × 0.15 × 0.05
Monoclinic
P21/c
11.355(2)
9.9917(17)
37.908(7)
90.00
94.025(5)
90.00
C18H18N4O6Co
445.29
0.15 × 0.10 × 0.10
Monoclinic
P21
7.6770(14)
16.387(2)
8.2916(15)
90.00
116.458(3)
90.00
C36H20N6O12Co
787.51
0.20 × 0.10 × 0.07
Orthorhombic
Fddd
11.3058(18)
17.284(3)
37.082(6)
90.00
90.00
90.00
7246.0(19)
8
1.444
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
5060.6(12)
4
1.567
4290.2(14)
4
1.435
933.8(3)
2
1.584
Z
D
calcd/g cm−3
l/mm−1
1.519
1.390
0.964
0.546
Reflections collected
Unique reflections (Rint
Parameters
17244
8425(0.0462)
702
—
0.917
0.0503, 0.0790
0.0735, 0.0874
23100
7648(0.1774)
554
—
0.960
0.0863, 0.1742
0.1700, 0.2219
6330
3363(0.0344)
262
11563
1772(0.0613)
156
—
1.067
0.1065, 0.2777
0.1170, 0.2873
)
Flack parameter
−0.02(2)
S on F2
1.061
b
R1a, wR2 (I > 2r(I))
0.0470, 0.0860
0.0595, 0.0967
R1a, wR2 (all data)
b
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢁF0|–|Fcꢁ/ |Fo| b wR2 = [ w(F02–Fc2)2/ w(F02)2]1/2
.
Fig. 1 Coordination environments of cobalt atoms in (a) 1, (b) 2, (c) 3
and (d) 4: Co = cyan; C = grey; O = red; N = blue; H = white.
uncoordinated TMA–H acid groups from different layers (Fig. 2b)
to form a 3D hydrogen-bonding framework with 3D channels
Fig. 2 Crystal structure of 1: (a) 2D Co8(TMA-H)4(bridging-44bpy)2
coordination layer in the ac plane; (b) hydrogen-bond motifs of 44bpy
and TMA–H between the coordination layers; (c) 3D hydrogen-bonding
structure with 3D channels viewed along the b-axis; (d) 2-fold interpen-
etration of the supramolecular networks with guest ions occupied within
the channels.
˚
of ∼11 × 13 A (Fig. 2c). Such a large void space within a
framework often leads to molecular entanglement and two-fold
interpenetration is observed for 1 as the terminal 44bpys from the
second 3D supramolecular architecture thread oppositely through
the open area of the rectangular Co8(TMA–H)4(bridging-44bpy)2
˚
layer (Fig. 2d). One-dimensional channels of ∼5 × 12 A are still
non-directional electrostatic interactions between EMIm+ and Br−
ions.
found along the c-axis even after the interpenetration, inside which
the guest EMIm+ and Br− are situated. Notably 1 has a neutral
coordination network with both components of ionic liquids in
the pores, compared to the characteristic charged frameworks
from ionothermal synthesis.7 The included Br− guest ions have
weak hydrogen-bonding interactions with the bridging 44bpy on
the mother framework and the hydrogen atoms on the 2- and 4-
positions of the imidazolium ring of EMIm+ cations (Br · · · H =
(EMIm)[Co(TMA–H)(44bpy)2](44bpy)Br 2. Compound 2,
which was always co-existent with 1, has an isomorphous co-
ordination layer but a distinctive 3D supramolecular packing
assembled by different hydrogen-bonding synthons between 44bpy
and trimesic acid. The cobalt atom in 2 adopts octahedral
geometry, surrounded by two trimesates, one terminal and
two bis-coordinated 44bpy ligands (Fig. 1b). The chelated and
˚
2.708, 2.714 and 2.726 A), which directs the alignment of
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monodentate carboxylates of TMA–H link the cobalt atoms into
a Co(TMA–H) chain along the b-axis, which are further con-
nected by the bridging 44bpys into a 2D rectangular Co4(TMA–
H)2(bridging-44bpy)2 grid (Fig. 3a). Surprisingly, it is the unco-
ordinated 44bpy that acts as the supramolecular bridging ligands
that attach the pendant carboxylic groups of TMA–Hs to adjacent
layers (Fig. 3b) and forms a 3D hydrogen-bonding framework
with 3D channels (Fig. 3c). The terminal 44bpys, arranged in
parallel on the same side of the coordination planes have weak
p · · · p interactions with the unsupported 44bpy ligands and a
˚
closest distance of 3.751 A between the centroid of two aromatic
rings. The packing of the supramolecular frameworks is further
stabilised by the two-fold interpenetration and the extensive
p · · · p interactions among uncoordinated/terminal 44bpy units
and TMA–H (Fig. 3d). Although the 1D channels formed
after interpenetration along the a-axis are occupied with both
components of the ionic liquid and the coordination assembly are
again neutral, no significant hydrogen-bonding interactions are
found around the guest EMIm+ and bromide ions.
Fig. 4 Supramolecular framework of 3 connected by weak hydrogen
bonds of imidazoles and coordinated carboxylate groups between layers
viewed along the c-axis.
(EMIm)[Co(TMA)(Im–H)] 3. The structure of 3 is rather
straightforward. It is composed of one cobalt atom, one TMA, one
terminal imidazole and one EMIm+ cation per asymmetric unit.
The cobalt centre is connected to three TMAs and one imida-
zole ligand in a highly distorted trigonal bipyramidal geometry
(Fig. 1c) while the TMA links three cobalt atoms via one chelating
and two monodentate modes. If the cobalt is considered as a 3-
connected node and the bridging ligand as a trigonal spacer, the
2D Co-TMA coordination molecular assembly can be described as
a fluctuated (6,3)-honeycomb net on the crystallographic bc plane
(Fig. 4). The terminal imidazoles stand alternatively on both sides
of the layer and form weak hydrogen bonds with the coordinated
carboxylate oxygen atoms on neighbouring networks (O · · · H–
˚
N = 2.921(6) A). The anionic network is charge-compensated
with the EMIm+ cations between the layers.
(EMIm)2[Co(TMA)2(TED–H2)] 4. Similar to 3, compound 4
has two-dimensional cobalt trimesate layers. Nevertheless, the
cobalt atom in 4 is surrounded by four TMAs in a tetrahedral
geometry (Fig. 1d) while the acid ligand links only two metal
centres in a monodentate fashion. The TMA ligands are related
Fig. 3 Crystal structure of 2: (a) 2D Co4(TMA–H)2(bridging-44bpy)2 coordination layer in the ab plane; (b) hydrogen-bond motif of 44bpy and TMA–H
between the coordination layers; (c) 3D hydrogen-bonding framework viewed along the b-axis; (d) 2-fold interpenetration of the supramolecular networks
with guest ions occupied within the channels.
3992 | Dalton Trans., 2008, 3989–3994
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˚
to each other around the metal ions by rotation symmetry and
Table 2 Carboxylate C–O bond lengths (A) for 1–4
˚
form a rhombic Co4(TMA)4 grid of ∼9 × 9 A, which spreads out
1
2
3
in the ab plane into a (4,4)-connected net (Fig. 5a). Interestingly
the TED–H2 in 4 is a hydrogen-bonding bridging ligand and,
analogous to a metal node in a linear coordination geometry, links
the pendant carboxylate groups of TMA on the layers (Fig. 5b).
The 3D supramolecular architecture is therefore formed with the
C7–O1
C7–O2
C8–O3
C8–O4
C17–O11
C17–O12
C18–O13
C18–O14
C9–O5
C9–O6
C19–O15
C19–O16
1.273(7)
1.271(7)
1.267(8)
1.281(7)
1.253(7)
1.268(7)
1.269(8)
1.275(7)
1.320(8)
1.218(8)
1.336(8)
1.216(7)
C7–O1
C7–O2
C8–O3
C8–O4
1.282(8)
1.234(9)
1.271(8)
1.264(7)
C7–O1
C7–O2
C8–O3
C8–O4
C9–O5
C9–O6
1.265(6)
1.260(5)
1.234(5)
1.279(5)
1.271(5)
1.253(5)
˚
˚
largest channels of ∼6 × 16 A along the a-axis and ∼8 × 16 A along
the (110) direction (Fig. 5c). Two-fold interpenetration is also
found for 4, where the bridging TED–H2 of one supramolecular
framework threads through the Co4(TMA)4 grids of the second
network and divides the largest channels along the (110) axis into
C9–O5
C9–O6
1.307(9)
1.220(8)
4
C7–O1
C7–O2
C8–O3
1.263(7)
1.268(8)
1.242(6)
˚
˚
three smaller pores, one of ∼7 × 8 A and two of ∼3 × 7 A (Fig. 5d).
The disordered EMIm+ ions occupy the void space of the small
channels and are charge-balanced with anionic frameworks.
cations are captured within the cages of the framework by strong
electrostatic forces and no significant hydrogen bonds can be
observed. The linear amines, such as 44bpy and TED, increase
the void space of the frameworks and therefore result in two-fold
interpenetration, as shown in 1, 2 and 4.
The protons involved in hydrogen bonds are tentatively as-
signed from the difference Fourier electron density maps to be
‘TMA–H · · · amine’ for 1 and 2, ‘TMA · · · H–amine’ for 3 and
‘TMA · · · H · · · amine’ for 4. Moreover, the carboxylate groups in
1–4 are delocalised with bond lengths in the range of 1.234(6)
˚
and 1.282(8) A while the carboxylic groups in 1 and 2 have
distinguishable single and double bonds (Table 2). In general,
there seems to be a correlation between the types of interactions
of hydrogen bond and the pKa values of the organic amines
(pKa = 5.14 for pyridinium16a and 8.47 for TED–H216b in aqueous
solution). However, other more subtle effects undoubtedly play a
role. For example, the proton transfer in ionic liquids may be less
efficient as the hydrogen atoms of imidazole (pKa = 6.9516c) in 3 are
found intact and the hydrogen bonds that form are rather weak.
The EMIm+ and Br− ions captured in the channels by hydrogen
bonds in 1 suggest an elegant example of hydrogen-bond-directing
and templating effects of ionic liquids in the process of forming
metal coordination polymers using ionothermal synthesis.
Fig. 5 Crystal structure of 4: (a) Co4(TMA)4 (4,4)-connected coordi-
nation net in the ab plane; (b) hydrogen-bond motifs of TED bridges
and TMA between layers (the TED are disordered in two equilibrium
positions to form hydrogen bonds with both carboxylate oxygen atoms in
50% possibility); (c) 3D supramolecular structure linked by TED ligands
viewed along the a-axis; (d) 2-fold interpenetration of the supramolecular
networks with EMIm+ ions within the channels viewed along the (110)
direction.
Conclusion
In summary, we have synthesised four new cobalt trimesate
three-dimensional supramolecular architectures. Hydrogen bonds
between the acids and amines apparently have a significant effect
on the control and construction of supramolecular frameworks for
all the compounds. Since the competition of hydrogen-bonding
interactions between the ligands and the solvent molecules, e.g.
water or other small organic molecules in hydrothermal synthesis,
is minimised in ionic liquids, ionothermal synthesis may have more
control over hydrogen bonding and act as a powerful tool in the
prediction and controlled synthesis of materials.
Discussion
Trimesic acid is a versatile ligand that has been extensively used for
the preparation of microporous metal–organic frameworks includ-
ing [Cu3(TMA)2(H2O)3] (HKUST-1)14 and [Co3(TMA)(TMA–
H)(TMA–H2)(py)2]15. In aqueous solution, the interactions among
metal ions, organic ligands and solvent molecules are competitive
and very complicated. In such circumstances, it is difficult to attain
a significant degree of control and predictability on the resultant
structures. In 1–4, strong hydrogen bonds are very efficient in the
arrangement of supramolecular packing and can only been found
between trimesic acid and ancillary amines.
Acknowledgements
The addition of organic amines has great influence on the pack-
ing of the metal–organic frameworks. Without the ancillary lig-
ands, a 3D coordination framework (EMIm)2[Co3(TMA)2(OAc)2]
was obtained.7b This compound has a (3,6)-connected net con-
structed by octahedral Co3(COO)8 secondary building units
(SBUs) and topologically 3-connected TMA ligands. The EMIm+
The authors thank the EPSRC (UK) for funding.
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