Alcoholysis/hydrolysis of 1,1′-carbonyldiimidazole as a means of preparing unprecedented, imidazole-containing one-dimensional coordination polymers of copper(II)

Article abstract of DOI:10.1039/b900716d

The use of 1,1′-carbonyldiimidazole, (im)₂CO, for the synthesis of imidazolate (im^-) and/or imidazole (Him)-containing copper(II) coordination polymers is described. The [Cu₂(O ₂CMe)₄(H₂O)<

Full text of DOI:10.1039/b900716d

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Alcoholysis/hydrolysis of 1,1¢-carbonyldiimidazole as a means of preparing
unprecedented, imidazole-containing one-dimensional coordination polymers
of copper(II)†
Theocharis C. Stamatatos,a,b Spyros P. Perlepes,a Catherine P. Raptopoulou,c Aris Terzis,c
Costas S. Patrickios,b Anastasios J. Tasiopoulosb and Athanassios K. Boudalis*c
Received 14th January 2009, Accepted 18th February 2009
First published as an Advance Article on the web 16th March 2009
DOI: 10.1039/b900716d
The use of 1,1¢-carbonyldiimidazole, (im)2CO, for the synthesis of imidazolate (im-) and/or imidazole
(Him)-containing copper(II) coordination polymers is described. The [Cu2(O2CMe)4(H2O)2]/(im)2CO
reaction system in EtOH yields the new polymeric species [Cu(O2CMe)(im)(Him)(EtOH)]n (1) and the
known compound [Cu(im)2]n (2), depending on the reaction conditions. A mechanism for the
alcoholysis/hydrolysis of (im)2CO is proposed. Complex 1 comprises neutral, zigzag chains with the
1
1
h :h :m im- ligand bridging two neighbouring CuII atoms. Each square pyramidal metal centre is
coordinated to two imidazolate nitrogen atoms, the pyridine-type nitrogen atom of the terminal
imidazole ligand, one acetate oxygen atom and the ethanol oxygen atom. The dc magnetic susceptibility
data for 1 have been analysed according to the Bonner-Fisher model for an equally spaced S = 1/2
chain, revealing moderate antiferromagnetic CuII ◊ ◊ ◊ CuII exchange interactions (J = -33.5 cm-1 using
ꢀ
ˆ ˆ
the H = -2J SiSi+1 spin Hamiltonian). The reaction system Cu(NO3)2·3H2O/(im)2CO in EtOH
leading to the preparation of known trans-[Cu(NO3)2(Him)4] (3) is also described. With
terephthalate(-2) (tp2-), instead of MeCO2 , in MeOH/H2O the product is the new, 1D linear
-
coordination polymer [Cu(tp)(Him)2(H2O)]n·2nH2O (4·2nH2O). Adjacent square pyramidal CuII atoms
are singly bridged by the bis-monodentate tp2- ligand, while two monodentate Him groups and one
H2O molecule complete five-coordination at each metal centre. The chains of 1 and 4·2nH2O form
interesting, hydrogen-bonded 3D networks. The combined work demonstrates the usefulness of
(im)2CO in the preparation of interesting CuII coordination polymers which can not be obtained by the
use of Him.
The ultimate goal is the transformation of few coordination poly-
Introduction
mers to functional molecular materials. Coordination polymers
are also significant from a structural chemistry perspective with
new, intriguing molecular topologies being discovered, as well as
providing numerous examples of interesting phenomena such as
the interpenetration of networks.6
However, the factors influencing the synthesis of coordination
polymers are still not completely understood and inorganic
chemists are always looking for new methods that can lead
to predictable products. The two main trends in the field are
the ‘synthesis by design’ and the ‘non-programmed assembly’.
The former involves the use of rigid, often complicated organic
ligands that will ‘force’ or ‘drive’ the precipitation of the desired
product. The latter approach involves the use of more flexible
ligands and lacks control over the final product, but it has proven
to be successful in the synthesis of polymeric compounds with
interesting structures and properties.2,4
The utilisation of organic polymers, mainly composed of the
elements C, H, O, N and—to a lesser extent—some neighbouring
main group elements such as B, Si, P and S, in the 1960s
transformed the way in which we live. In the field of polymers, the
metallic elements, which comprise more than half of the elements
in the Periodic Table, had comparatively less to contribute until
the early 1990s.1 However, this situation has changed in the last
15 years; the reason is that metal ions have attractive features
for new generations of metalloorganic polymers, which are also
known2 as coordination polymers,3a metal-organic coordination
networks, metal-organic frameworks3b or organic-inorganic hy-
brid coordination polymers. To date, scientists have realised var-
ious applications of coordination polymers in catalysis, electrical
conductivity, luminescence, magnetism, non-linear optics, molec-
ular electronics, sensing, drug delivery and zeolitic behaviour.2,4,5
The p-excessive and strong s-donor ligand imidazole (Him)
has played a formative role in the development of coordination
chemistry.7 The imidazolate anion (im-, Scheme 1), obtained
by deprotonation of Him, is capable of acting as a biden-
tate bridging ligand. The 1,3-arrangement of the N atoms
imposes geometrical constraints such that only one imidazo-
late bridge is possible between two metal centres. Accordingly,
aDepartment of Chemistry, University of Patras, 265 04, Patras, Greece
bDepartment of Chemistry, University of Cyprus, 264 43, Nicosia, Cyprus
cInstitute of Materials Science, NCSR “Demokritos”, 153 10, Aghia
Paraskevi Attikis, Greece. E-mail: tbou@ims.demokritos.gr; Tel: +30 210
6503346
† CCDC reference numbers 715757 for 1 and 715758 for 4. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b900716d
3354 | Dalton Trans., 2009, 3354–3362
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Experimental
All manipulations were performed under aerobic conditions using
reagents and solvents as received.
[Cu(O2CMe)(im)(Him)(EtOH)]n (1) and [Cu(im)2]n (2) in a
mixture
A
green-blue solution of [Cu2(O2CMe)4(H2O)2] (0.200 g,
0.50 mmol) in warm EtOH (15 mL) was added dropwise to a
colourless solution of (im)2CO (0.195 g, 1.20 mmol) in the same
solvent (10 mL). The resulting deep blue solution was refluxed
for 2 h, during which time a small amount of a purple solid (2)
precipitated. An intense effervescence was observed. The obtained
material was filtered off and dried in vacuo over silica gel. The
filtrate from this procedure was layered with n-hexane (50 mL).
After 5 days, blue crystals of 1 were collected by filtration, washed
with cold EtOH (2 ¥ 2 mL) and Et2O (2 ¥ 5 mL), and dried in
vacuo over silica gel; the yield was ~55%. Data for 1: Anal. Calc.
for C10H16CuN4O3: C, 39.53; H, 5.32; N, 18.44. Found: C, 39.34;
H, 5.41; N, 18.35%. IR (KBr, cm-1): 3410sb, 3145mb, 1583vs,
1556s, 1471s, 1412s, 1328m, 1276w, 1255m, 1228m, 1174m, 1140m,
1086vs, 1068s, 1047m, 945m, 828m, 755m, 654s, 618m. Data for 2:
Anal. Calc. for C6H6CuN4: C, 36.45; H, 3.07; N, 28.35. Found:
C, 36.18; H, 3.11; N, 28.29%. IR (KBr, cm-1): 2917w, 1598s,
1535m, 1464m, 1407m, 1382w, 1309m, 1234m, 1168w, 1085vs,
945m, 834m, 808m, 755s, 670s, 615m.
Scheme 1 Structural formulae and abbreviations of some of the ligands
discussed in the text.
imidazolate-bridged transition metal complexes show a high
degree of linearity. Restricting discussion to copper(II), only few
imidazolate-bridged CuII coordination polymers are known.8 The
usual methods for the preparation of such complexes involve direct
reactions of CuII sources and imidazole in the absence or presence
of ancillary ligands.
Our group has had a strong interest over many years in the
coordination chemistry of ligands of the general types X-CO-X
and X-CO-X-CO-X. Examples of two thoroughly studied
such ligands are di-2-pyridyl ketone,9,10 (py)2CO, and 2,6-di-(2-
pyridylcarbonyl)pyridine,11 (py)CO(py)CO(py) (Scheme 1), which
have led to a multitude of polynuclear complexes (clusters) with a
variety of metal ions and nuclearities reaching 26, some of them
with interesting spin topologies and exciting magnetic properties.
The particular interest on these two ligands stems from the
reactivity potential of the carbonyl function(s) which can undergo
metal ion-assisted hydrolysis or alcoholysis and subsequent de-
protonation of the hydroxyl group(s). The deprotonated forms are
coordinatively very versatile and can adopt a variety of bridging
modes. With this previous experience in mind we sought to modify
the possibilities provided by (py)2CO by studying the coordination
chemistry of X-CO-X molecules that would favour the isolation
of coordination polymers. The first ligand of our choice was
1,1¢-carbonyldiimidazole, (im)2CO (Scheme 1), a well known
reagent in organic synthesis.12
[Cu(O2CMe)(im)(Him)(EtOH)]n (1)
To a stirred green-blue solution of [Cu2(O2CMe)4(H2O)2] (0.200 g,
0.50 mmol) in EtOH (25 mL) was added a solution of (im)2CO
(0.243 g, 1.50 mmol) in the same solvent (10 mL). An intense
effervescence was observed. The resulting deep blue solution was
refluxed for 15 min and stored in the refrigerator (4–5 ◦C) for
one week. Blue crystals of 1 formed, which were collected by
filtration, washed with Et2O (2 ¥ 5 mL), and dried in vacuo
over silica gel; the yield was ~40%. The identity of the product
was confirmed by elemental analysis (Calc. for C10H16CuN4O3: C,
39.53; H, 5.32; N, 18.44. Found: C, 39.60; H, 5.21; N, 18.25%),
and IR spectroscopic comparison with the blue crystals from the
above mentioned procedure that gave a mixture of 1 and 2.
The free ligand exhibits very high reactivity toward nucleophilic
reagents, because of the electron attraction exerted from both
sides on the carbonyl group by the heterocycle. For example,
H2O hydrolyzes (im)2CO at room temperature within seconds,
with vigorous evolution of CO2 as shown in eqn (1).13
[Cu(im)2]n (2)
(im)2CO + H2O → 2 Him + CO2
(1)
To a stirred green-blue solution of [Cu2(O2CMe)4(H2O)2] (0.200 g,
0.50 mmol) in EtOH (25 mL) was added solid (im)2CO (0.130 g,
0.80 mmol). The solid dissolved immediately followed by a colour
change to purple and an effervescence. The solution was stirred
for a further 5 min and left undisturbed in a closed flask. After 2
days, X-ray quality purple crystals of the product were collected by
filtration and dried in air; the yield was ~35%. The identity of this
material was confirmed by comparison of its unit cell parameters
with those available in the literature,14a and elemental analysis
(Calc. for C6H6CuN4: C, 36.45; H, 3.07; N, 28.35. Found: C, 36.83;
H, 3.02; N, 28.55%). The IR spectrum of the crystals was identical
to that of the purple solid isolated from the above mentioned
procedure that resulted in the mixture of 1 and 2.
(im)2CO also reacts with alcohols forming diesters of car-
bonic acid or imidazole-N-carboxylic esters and yielding free
imidazole.13 Thus, at the outset of our efforts we were aware
that (im)2CO could not be incorporated in metal complexes in
aqueous or alcoholic reaction media. However, we hoped that such
reactions would likely give polymeric metal complexes containing
the bridging imidazolate(-1) ligand (im-), different from those
obtained with the use of Him as starting material. This has,
indeed, turned out to be the case. We herein report the initial
results of our approach in copper(II) chemistry, namely the use
of 1,1¢-carbonyldiimidazole, (im)2CO, as a reagent for synthesis of
interesting CuII complexes containing im- and/or Him ligands.
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Table 1 Crystallographic data for complexes 1 and 4·2nH2O
trans-[Cu(NO3)2(Him)4] (3)
Method A. Treatment of
a
pale blue solution of
–
1
4·2nH2O
Cu(NO3)2·3H2O (0.121 g, 0.50 mmol) in EtOH (20 mL) with
solid (im)2CO (0.162 g, 1.00 mmol) resulted in a blue solution
which was stirred for a further 30 min without any noticeable
colour change. An effervescence was noticed during stirring.
The resulting solution was filtered and the filtrate layered with
n-hexane (40 mL). Slow mixing gave deep purple crystals of the
product which were collected by filtration, washed with cold Et2O
(2 ¥ 5 mL) and dried in vacuo over silica gel; the yield was
~85%. Determination of the unit cell parameters revealed that
this compound is the known complex trans-[Cu(NO3)2(Him)4].14b
Anal. Calc. for C12H16CuN10O6: C, 31.34; H, 3.51; N, 30.46. Found:
C, 31.24; H, 3.57; N, 30.36%. IR (KBr, cm-1): 3135s, 1753w, 1637m,
1538m, 1495m, 1451m, 1384vs, 1315m, 1259m, 1170m, 1134m,
1098s, 1068vs, 943m, 923w, 852m, 824m, 749s, 610s.
Formulaa
C10H16CuN4O3
303.81
Monoclinic
P21/n
11.686(5)
14.239(5)
8.467(3)
108.27(2)
1338.0(9)
4
C14H18CuN4O7
417.86
Monoclinic
C2/c
13.815(8)
5.931(3)
22.02(1)
101.81(2)
1766(2)
4
M/g mol-1
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
V/A
Z
3
˚
F(000)
T/K
628
298
860
298
l/A
1.54180b
1.508
1.54180b
1.572
˚
rc/g cm-3
m/mm-1
2.387b
2.165b
Meas./indep. refl. (Rint
Obs. refl. [I > 2s(I)]
wR2c
)
2162/2016 (0.0315)
1822
0.1426
0.0479
1.069
1317/1282 (0.1081)
1175
0.1281
0.0457
1.148
Method B. Solid Cu(NO3)2·3H2O (0.121 g, 0.50 mmol) and
(im)2CO (0.162 g, 1.00 mmol) were manually grinded in a Petri dish
for 5 min, during which time a vigorous effervescence occurred.
The resulting blue slurry was extracted with the minimum volume
of EtOH (6 mL) giving a deep blue solution. This solution was
allowed to slowly evaporate at room temperature. Deep purple
crystals of 3 formed over 2 days. The crystals were collected by
filtration, washed with cold Et2O (2 ¥ 5 mL) and dried in vacuo over
silica gel. The yield was low (~30%) but the reaction is reproducible.
The identity of this material was confirmed by elemental analysis
(Calc. for C12H16CuN10O6: C, 31.34; H, 3.51; N, 30.46. Found: C,
31.60; H, 3.40; N, 30.00%), and IR spectroscopic comparison with
material from method A.
R1d
Goodness of fit on F2
-3
˚
Drmax, min/e A
0.672/-0.637
0.563/-0.804
a Including solvate molecules. b Cu Ka radiation, graphite monochro-
ꢀ
ꢀ
ꢀ
mator. c wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}
.
d R1 =
ꢀFo| -
2
1/2
ꢀ
|Fcꢀ/ |Fo|.
C14H16CuN4O6: C, 42.05; H, 4.04; N, 14.01. Found: C, 42.20; H,
4.00; N, 13.82%), and IR spectroscopic comparison with authentic
material from method A.
X-Ray crystallography
Crystallographic data together with refinement details for the new
complexes reported in this work are summarized in Table 1. The
selected crystals of 1 (0.14 ¥ 0.20 ¥ 0.40 mm) and 4·2nH2O (0.05 ¥
0.15 ¥ 0.30 mm) were mounted in capillary, while those of 3 (0.08 ¥
0.18 ¥ 0.60 mm) were mounted in air. Data were collected using
a Crystal Logic Dual Goniometer diffractometer equipped with a
graphite monochromator utilising Mo Ka radiation (for complex
3) and on a P21 Nicolet diffractometer upgraded by Crystal Logic
using graphite monochromated Cu Ka radiation (for complexes
1 and 4·2nH2O). Unit-cell dimensions were determined and
refined by using the angular settings of 25 automatically centred
reflections in the range 22 < 2q < 54◦ (for complexes 1 and
4·2nH2O) and 11 < 2q < 23◦ (for complex 3). Intensity data
were recorded using a q-2q scan to a max 2q value of 122 (1)
and 118◦ (4·2nH2O). Three standard reflections monitored every
97 reflections showed less than 3% variation and no decay. Lorentz
and polarization corrections were applied using Crystal Logic
software.
[Cu(tp)(Him)2(H2O)]n·2nH2O (4·2nH2O)
Method A. To a stirred, colourless solution of H2tp (0.083 g,
0.50 mmol) and LiOH·H2O (0.042 g, 1.00 mmol) in H2O (10 mL)
was added a blue solution of Cu(NO3)2·3H2O (0.121 g, 0.50 mmol)
and (im)2CO (0.162 g, 1.00 mmol) in MeOH (15 mL). Efferves-
cence was observed. The resulting green-blue slurry was stirred
for a further 10 min, the insoluble material was filtered and the
filtrate layered with Me2CO (50 mL). After 2 days, blue crystals
of 4·2nH2O formed, which were collected by filtration, washed
with cold Me2CO (2 ¥ 5 mL) and Et2O (3 ¥ 5 mL), and dried
in vacuo over silica gel; the yield was ~60%. The dried solid
analysed as 4·nH2O. Anal. Calc. for C14H16CuN4O6: C, 42.05; H,
4.04; N, 14.01. Found: C, 41.94; H, 4.12; N, 13.96%. IR (KBr,
cm-1): 3419s, ~3200mb, 2930w, 1618s, 1540m, 1501m, 1471m,
1397s, 1367s, 1230m, 1069s, 947m, 855m, 818m, 739s, 658s, 615m,
566m.
The structures were solved by direct methods using SHELXS-
8615 and refined by full-matrix least-squares on F2 with SHELXL-
97.16 For 1 all H atoms were introduced at calculated positions as
riding on bonded atoms; for 4·2nH2O all H atoms were located by
difference maps and refined isotropically. All non-hydrogen atoms
of complexes 1 and 4·2nH2O were refined anisotropically.
Method B. To a stirred, colourless solution of H2tp (0.083 g,
0.50 mmol) and LiOH·H2O (0.042 g, 1.00 mmol) in H2O (10 mL)
was added a blue solution of complex 3 (0.230 g, 0.50 mmol) in
the same solvent (10 mL). The resulting green-blue solution was
stirred for a further 10 min at room temperature and layered with
Me2CO (40 mL). After few days, blue crystals of 4·2nH2O were
collected by filtration, washed with cold Me2CO (2 ¥ 3 mL) and
Et2O (2 ¥ 3 mL), and dried in vacuo over silica gel; the yield
was 35%. The identity of the product was confirmed by elemental
analysis (the material analysed satisfactorily as 4·nH2O; Calc. for
Physical studies
Elemental analyses (C, H, N) were performed by the in-house
facilities of the Chemistry Department at the University of Cyprus
3356 | Dalton Trans., 2009, 3354–3362
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using an Euro Vector EA elemental analyzer. Infrared spectra
were recorded in the solid state (KBr pellets) on a Perkin Elmer
PC 16 FTIR spectrometer in the 4000–500 cm-1 range. Magnetic
measurements of 1 were carried out on freshly isolated and dried
samples on a Quantum Design MPMS-5.5 SQUID susceptometer
under a field of 0.1 T, and between the 5–300 K temperature
range. Data were corrected with standard procedures for the
contribution of the sample holder and diamagnetism of the
sample. The magnetic susceptibility has been computed by an
analytical expression. Least-squares fittings were accomplished
im- has been reported by Ghosh and co-workers.19a The 1:3
reaction of Cu(ClO4)2·6H2O and (im)2CO in MeOH-H2O yielded
complex [Cu3(ClO4)4(im)2(Him)8]n containing both imidazolate
and perchlorate bridges; the authors avoided mechanistic discus-
sions. However, this complex had been prepared and structurally
characterized before from the use of Him as starting material.19b
With the identities of the purple powder and blue crystals
established by elemental analysis and single-crystal, X-ray crystal-
lography, respectively, convenient syntheses of pure materials were
sought under alternative reaction conditions. In both compounds,
the CuII to imidazole molar ratio is the same, i.e., 1:2. However,
the purple material seemed to contain only anionic imidazolate
ligands. Thus, attempts to obtain pure 1 and the purple powder
with an adapted version of the function-minimization program
ꢀ
MINUIT.17 The error factor R is defined as R =
[(cexp -
c
calc)2/Ncexp2], where N is the number of experimental points.
-
were made by decreasing and increasing, respectively, the MeCO2
-
to (im)2CO reaction ratio. Using the modified MeCO2 /(im)2CO
Results and discussion
ratios of 2:1.5 (~1.3) and 2:0.8 (2.5), we isolated 1 and the
-
Syntheses and IR spectra
purple powder, respectively. Note that the MeCO2 to (im)2CO
ratio is ~1.7 in the experiments that lead to the mixture of
products (see Experimental). We were lucky enough to isolate
X-ray quality crystals of the purple material. Examination of
the unit cell parameters revealed that the purple compound is
the known complex14 catena-bis(m-imidazolate-N,N¢)copper(II),
[Cu(im)2]n (2). The formation of the two coordination polymers is
summarized in eqn (5) and (6). In both of these balanced equations,
The original aim of this synthetic program was to explore the
use of (im)2CO for the assembly of coordination polymers of
3d-metals possessing bridging imidazolate (im-) ligands. One of
our routes takes advantage of the fact that the reactions between
metal carboxylates and (im)2CO in alcohols or/and H2O are
excellent springboards to a variety of such products. Obviously
-
the produced Him (see eqn (1)) can be deprotonated by the RCO2
groups in the presence of metal ions and we anticipated new types
-
the MeCO2 to (im)2CO ratio is the same, i.e., 2.0. The excess of
acetates in the preparation of pure 2 shifts the equilibrium (4)
to the right and this ensures the presence of all the amount of
imidazole in its deprotonated form. The excess of (im)2CO (and
hence of Him derived from the decomposition shown in eqn (1))
in the preparation of pure 1 seems necessary for the presence of
a considerable concentration of neutral imidazole in the reaction
solution.
of polymeric Mn+/im-/RCO2 species as long as the RCO2 to
Him ratio was high enough to leave an amount of nonprotonated
carboxylates in the reaction mixture, given the fact that both im-
and RCO2- can potentially adopt bridging modes. This idea proved
to be both successful and unsuccessful.
-
-
Our initial efforts involved the reaction of [Cu2(O2CMe)4(H2O)2]
with 2.4 equivs of (im)2CO, i.e., a potential CuII/MeCO2 /Him
-
reaction ratio of 1:2:2.4, in EtOH under reflux. A purple,
polycrystalline material, repeatedly analysed as Cu(im)2 and blue
crystals, crystallographically characterized as the 1D polymeric
compound [Cu(O2CMe)(im)(Him)(EtOH)]n (1), were isolated. In
eqn (2)–(4) we propose a simplistic reaction scheme that may
explain the observed (im)2CO to im- transformation. A detailed
mechanistic perspective is beyond our intention.
n[Cu2(O2CMe)4 (H2O)2 ] + 2n(im)2 CO + 2nEtOH æEætOHæÆ
T
(5)
2[Cu(O2CMe)4 (im)(Him)]n + 2nMeCO2H + 2nCO2≠
n[Cu2(O2CMe)4 (H2O)2 ] + 2n(im)2 CO + 2nEtOH æEætOHæÆ
20∞C
(6)
2[Cu(im)2 ]n + 4nMeCO2H + 2nCO2≠
The temperature is another synthetic parameter in the
[Cu2(O2CMe)4(H2O)2]/(im)2CO reaction system. We have ob-
served that the isolation of 1 is favoured if the reaction solution is
refluxed before the crystallisation procedure. For example, if the
reaction mixture that leads to pure 2 undergoes reflux, then the
product is contaminated with various amounts of 1 (analytical
and IR evidence). Thus, the reactions that give pure 1 and 2 take
place under reflux and at room temperature, respectively.
EtOH + im-CO-im → Et-O-CO-im + Him
(2)
(3)
Et-O-CO-im + H2O → EtOH + Him + CO2 ↑
2+
Cu
-
Him + MeCO-2
im + MeCO H
(4)
ꢀꢀꢀꢀꢁ
ꢂꢀꢀꢀꢀ
2
According to the above equations, the first step of this transfor-
mation involves the ethanolysis of (im)2CO to the corresponding
imidazole-N-carboxylic ethyl ester, followed by the hydrolysis
(from the water present in the solvent and in the starting CuII
material) of the ester group to form a second equivalent of
Him. The third step involves the CuII-mediated deprotonation
of Him by the strongly basic acetate. It is difficult to decide if
the step represented by eqn (2) is CuII-assisted. Coordination of
the oxygen atom of the carbonyl group in solution is likely to
enhance the reactivity of that group.18 The metal centre polarises
the carbonyl group and EtOH can attack the carbonyl carbon to
generate the tetrahedral intermediate before the ester formation.
It is noteworthy that a similar transformation of (im)2CO to
The presence of both EtOH and Him ligands in 1 is manifested
by two broad IR bands at ~3400 and ~3150 cm-1, assigned to
20
19a
n(OH)EtOH and n(NH)Him
,
respectively. The spectra of 1 and 2
do not exhibit a band in the region for the carbonyl stretching
band as expected, with the bands at 1556 (1) and 1598 (2) cm-1
assigned to n(C N). The strong bands at 1583 and 1412 cm-1 in the
=
spectrum of 1 are assigned to nas(CO2) and ns(CO2), respectively.21
Having been encouraged by the above mentioned experiments
that (im)2CO in EtOH was indeed capable of giving clean reactions
and polymeric species containing im- ligands, our attention now
turned to the preparation of simple mononuclear complexes,
available by other methods, containing exclusively neutral Him
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ligands. Preliminary screening showed clean reactions could be
systems with CuII:H2tp:(im)2CO:OH- ratios of 1:1:1:3 or 4 (the
n
obtained by using a variety of weakly basic inorganic anions, e.g.,
cations of the bases were Me4N+, Et4N+ and Bu4 N+ to favour
Cl-, Br-, NO3- etc. in both MeOH and EtOH, but the NO3 /EtOH
formation of anionic polymeric species). However, these led to
product 4 in low (<20%) yields contaminated with amorphous
CuII hydroxo species. These combined results suggest complex
4 to be the thermodynamically favoured product under these
reaction conditions. This conclusion is further supported by the
observation that use of >2 equivs of (im)2CO and <1 equivs of
H2tp in the reactions (to see what type of compound might result
if the CuII to tp2- ratio in the product was 2:1), led only to the same
complex 4 being isolated.
-
combination was found to yield the most crystalline product. Thus,
treatment of a Cu(NO3)2·3H2O solution in EtOH with two equivs
of (im)2CO yielded a homogeneous, blue solution, from which the
known complex14b trans-[Cu(NO3)2(Him)4] (3) crystallised. The
formation of 3 is summarised in eqn (7).
Cu(NO3)2 ◊3H2O + 2(im)2CO æEætOHæÆ
(7)
trans-[Cu(NO3)2(Him)4 ] + H2O + CO2 ≠
H
O/MeOH
2
nCu(NO3)◊3H2O + nH2tp + n(im)2 CO + 2nLiOH◊H2O ææææÆ
The high yield of the preparation (>85% based on the available
CuII) strengthens our view that the decomposition of (im)2CO in
EtOH occurs in two steps, see eqn (2) and (3), producing two
equivalents of Him per (im)2CO. This is in accordance with the
observed effervescence during the reaction. This reaction does
not seem particularly alcohol dependent; the compound was
obtained cleanly in isolated yields of 40–70% from MeOH and
PrnOH. These yields are not optimized; no attempts to obtain
more product from the coloured mother liquors were made.
Complex 3 can also be prepared by a solid state-type reaction of
Cu(NO3)2·3H2O and (im)2CO in a 1:2 molar ratio and subsequent
isolation of the well-formed deep purple crystals from EtOH
(method B in Experimental).
The bands at 1451 and 1315 cm-1 in the IR spectrum of 3 are
assigned22 to the n5(B2) and n1(A1) (under C2V symmetry), respec-
tively, of the nitrato ligands; their separation is small (136 cm-1) in
accordance with the crystallographically established14b monoden-
tate character of the two nitrate groups.22 The two bands appear at
the same wavenumbers in the mull (nujol, hexachlorobutadiene)
spectra of 3.
[Cu(tp)(Him)2(H2O)]n + 2nHim + nH2O
(8)
Complex 4 can also be prepared by the 1:1 reaction between
the pre-isolated complex 3 and tp2- in H2O but in a lower yield
(~35%); the reaction is summarized in eqn (9).
H
O
2
ntrans-[Cu(NO3)2(Him)4 ] + nH2tp + 2nLiOH◊H2O æææÆ
[Cu(tp)(Him)2(H2O)]n + 2nLiNO3 + 2nHim + nH2O
(9)
The strong bands at 1618 and 1397 cm-1 in the IR spectrum
of 4 are assigned24 to nas(CO2) and ns(CO2), respectively. The
difference D (221 cm-1) suggests the presence of monodentate car-
boxylate groups21,24 in agreement with the structure of the complex
(vide infra).
Description of structures
Partially labeled representations of complexes 1 and 4·2nH2O are
shown in Fig. 1–4. Selected interatomic distances and angles are
listed in Tables 2 and 3. Hydrogen bonding details for complex
4·2nH2O are given in Table 4.
Having obtained and identified complex 1, another question
addressed was in what manner changes in the carboxylate anion
might affect the product identity. Since 1 contains a bridging
-
im- ligand and a terminal MeCO2 group, we were interested in
preparing polymeric CuII species possessing both imidazolate and
carboxylate groups as bridging ligands. An obvious choice was a
dicarboxylate ligand. Preliminary experiments were highly encour-
aging using the benzene-1,4-dicarboxylate(-2) (terephthalate; tp2-,
Scheme 1) ligand. The tp2- ion is a versatile ligand as exemplified
by the formation of polymeric structures.23 Terepththalato(-2)-
bridged complexes have been actively studied to elucidate the
limiting distance of magnetic exchange between paramagnetic
metal centres. The tp2- bridges typically give rise to M ◊ ◊ ◊ M sepa-
˚
rations of ~11 A, which generally lead to weak antiferromagnetic
Fig. 1 Partially labelled plot of a hydrogen-bonded pair of portions of
1D chains in 1. For clarity only H(O3) and H(N4) hydrogen atoms are
shown. Weak Cu ◊ ◊ ◊ O(2) interactions are shown as black dashed lines, and
H(O3) ◊ ◊ ◊ O(2) hydrogen bonds are shown as grey dashed lines. Primed and
unprimed atoms are symmetry-related (¢ = 0.5 + x, 1.5 - y, 0.5 - z).
interactions for M = MnII, CoII, NiII and CuII.24 The 1:1:2 reaction
between Cu(NO3)2·3H2O, H2tp and (im)2CO in H2O/MeOH in
the presence of two equivs of OH- produced the 1D coordination
polymer [Cu(tp)(Him)2(H2O)]n·2nH2O (4·2nH2O). The prepara-
tion of 4 is summarized in eqn (8). Although we were rather
disappointed to see (vide infra) that the complex contains neutral,
terminal imidazole ligands, we proceeded with its characterization
because it is a rare example of a CuII terephthalate(-2) complex
containing monodentate ancillary ligands. The excess (im)2CO
used in the preparation of 4 is beneficial to the yield; when the
stoichiometric amount (CuII:H2tp:(im)2CO:OH- = 1:1:1:2) was
used, the yield was lower than 40%. In an attempt to prepare a
CuII polymer with both tp2- and im- ligands, we explored reaction
Complex 1 (Fig. 1 and 2) crystallises in the monoclinic space
group P21/n. The complex is a 1D coordination polymer com-
prising neutral, zigzag [Cu(O2CMe)(im)(Him)(EtOH)]n chains.
1
1
The h :h :m im- ligand bridges two neighbouring CuII atoms,
thus promoting the formation of the chain. The CuII centre is
coordinated to two nitrogen atoms, N(1) and N(2), of the bridging
imidazolate anion in a trans manner, one nitrogen atom, N(3),
of the terminal Him ligand, one oxygen atom, O(1), of the
3358 | Dalton Trans., 2009, 3354–3362
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◦
a
˚
Table 2 Selected interatomic distances (A) and angles ( ) for complex 1
Cu–O(1)
Cu–O(3)
Cu–N(1)
Cu–N(2)
Cu–N(3)
Cu ◊ ◊ ◊ Cu¢
Cu ◊ ◊ ◊ O(2)
O(1)–Cu–O(3)
O(1)–Cu–N(1)
O(1)–Cu–N(2)
O(1)–Cu–N(3)
O(3)–Cu–N(1)
O(3)–Cu–N(2)
O(3)–Cu–N(3)
N(1)–Cu–N(2)
N(1)–Cu–N(3)
N(2)–Cu–N(3)
2.039(2)
2.385(2)
1.989(3)
1.984(3)
2.030(3)
6.047(8)
2.764(2)
99.0(1)
90.8(1)
87.6(1)
164.7(1)
92.4(1)
88.4(1)
96.2(1)
178.4(1)
89.8(1)
Fig. 2 Partially labelled plot showing the packing of the chains in 1
91.5(1)
through hydrogen bonds.
a Symmetry code (¢): 0.5+x, 1.5-y, 0.5+z.
◦
˚
Table 3 Selected interatomic distances (A) and angles ( ) for complex
4·2nH2Oa
Cu–O(1)
Cu–O(1¢)
Cu–OW(1)
Cu–N(1)
Cu–N(1¢)
Cu ◊ ◊ ◊ Cu#
O(1)–Cu–O(1¢)
O(1)–Cu–OW(1)
O(1)–Cu–N(1)
O(1)–Cu–N(1¢)
OW(1)–Cu–N(1)
N(1)–Cu–N(1¢)
1.967(3)
1.967(3)
2.278(4)
1.982(3)
1.982(3)
11.025(3)
176.3(1)
88.1(7)
89.6(1)
90.9(1)
97.8(9)
164.4(2)
Fig. 3 Partially labelled plot of a portion of the 1D chain in 4·2nH2O.
For clarity only H(N1) and H(N2) hydrogen atoms are shown. Primed and
unprimed atoms are symmetry-related (¢ = 0.5 + x, 1.5 - y, 0.5 + z).
-
monodentate MeCO2 group and the oxygen atom, O(3), of the
terminal EtOH molecule.
The metal coordination geometry is well described as distorted
square pyramidal with the coordinated EtOH molecule occupying
the apical position. Analysis of the shape-determining angles
using the approach of Reedijk, Addison and co-workers25 yields a
trigonality index, t, value of 0.23 for Cu (t = 0 and 1 for perfect
a Symmetry code (¢): -x, y, -z + 1/2; (#): x, -y, z + 1/2.
˚
0.129(1) A by N(2)) towards O(3). As expected, the axial Cu–O(3)
˚
˚
sp and tbp geometries, respectively). Cu lies 0.142(1) A above
bond is the longest (2.385(3) A). The trans coordination angles
the N(1), N(2), N(3), O(1) least-squares plane (max deviation
are 164.7(1) (O(1)–Cu–N(3)) and 178.4(1)◦ (N(1)–Cu–N(2)). The
Fig. 4 Hydrogen bonding between the chains in 4·2nH2O. Primes, double primes and asterisks correspond to the symmetry operations described in
Table 4. The lattice H2O molecules are not shown.
Table 4 Dimensions of the unique hydrogen bonds (distances in A and angles in ◦) for complex 4·2nH2O (A = Acceptor; D = Donor)
˚
H Bond
D ◊ ◊ ◊ A
H ◊ ◊ ◊ A
D–H ◊ ◊ ◊ A
Symmetry operator
OW1–(OW1) ◊ ◊ ◊ O2¢
OW1–(OW1) ◊ ◊ ◊ O2¢¢
N2-H(N2) ◊ ◊ ◊ O1*
2.799(3)
2.799(3)
2.962(2)
2.006(3)
2.006(3)
2.082(2)
162.7(1)
162.7(1)
162.7(1)
¢ = x, 1 + y, z
¢¢ = -x, 1 + y, 0.5 - z
* = 0.5 + x, -0.5 + y, z
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carboxylate group of the MeCO2- ligand can be alternatively con-
The chains of 4·2nH2O form a hydrogen-bonded 3D network
1
1
sidered as asymmetric chelating (h :h ) as evidenced by the Cu–O
(Fig. 4), through a group of interactions listed in Table 4. The
˚
˚
distances of 2.039(2) and 2.764(2) A. Adopting this alternative
interchain Cu ◊ ◊ ◊ Cu separations are 5.931(3) A along the b axis
˚
description of the acetate coordination mode, the Cu coordination
geometry may be described as very distorted octahedral with the
weakly interacting O(2) atom occupying the second axial position
at the metal; this view is emphasized in Fig. 1.
Within the imidazolate-bridged chains, the distance between ad-
jacent CuII centres is 6.047(8) A. There are two types of Cu–N
˚
and 7.517(4) A along the a axis. The packing of the polymeric
chains generates microchannels parallel to the c axis. The resulting
cavities are filled by the solvate H2O molecules.
Complex 4·2nH2O joins a small family of structurally char-
acterized, tp2--bridged polymeric CuII complexes also containing
monodentate,23,27 bidentate28 or tridentate29 terminal N-donors as
ancillary ligands. Complex 4·2nH2O is unique in the group of
polymeric CuII complexes with monodentate N-donors because
it has three monodentate ligands per metal centre; all the other
bonds: the Cu–N(3) bond to the neutral imidazole is slightly
˚
elongated at a distance of 2.030(3) A, whereas the two other
bonds to the bridging imidazolate ligand are shorter (average
-
distance 1.987(3) A). The latter two Cu–N(im ) bond lengths
complexes23,27 have two. From that group, only the 1D complex
˚
27a
˚
are in the reported range of 1.9–2.0 A for other imidazolate-
[Cu(tp)(NH3)2]n
is structurally related to 4 in that it also
bridged complexes containing square pyramidal CuII atoms.8a,26
The bridging im- ring is planar. This ring is flattened with the
C–N–C angles (104.8(3) and 104.8(3)◦) smaller, and the N–C–N
(113.4(8)◦) and C–C–N (108.4(3) and 108.5(3)◦) angles larger than
108◦. This type of flattening is typical for bridging im- ligands.26
The 1D chains of 1 (Fig. 2) are hydrogen-bonded into 3D
has a bis-monodentate bridging tp2- ligand. However, the CuII
coordination geometry in the ammine complex is square planar
(and not square pyramidal as in 4), a consequence of the presence
of two ammine ligands per metal ion.
Magnetic susceptibility studies on complex 1
frameworks through OMeCO2- ◊ ◊ ◊ H(OEtOH) and OMeCO2- ◊ ◊ ◊ H(NHim
)
Due to the large Cu ◊ ◊ ◊ Cu distances in complex 2 and the probably
weak exchange couplings that may result, we focused on the
magnetic study of complex 1. Variable-temperature dc magnetic
susceptibility studies were performed on a powdered sample of
1 in the 5.0–300 K range. Samples of 1 suffered from ageing
effects and the magnetic properties of aged samples could not be
interpreted by assuming a simple 1D Heisenberg chain. Magnetic
measurements were therefore carried out on freshly prepared and
dried samples.
The cMT product for 1 (Fig. 5) at 300 K is 0.35 cm3 mol-1 K,
slightly lower than the spin-only value of an isolated S = 1/2 ion
(0.38 cm3 mol-1 K, g = 2.0). This shows a continuous drop upon
cooling, consistent with the interplay of antiferromagnetic inter-
actions. The magnetic susceptibility (cM) shows a smooth increase
upon cooling until a broad maximum around 55 K, dropping
to a minimum at 26 K, before increasing upon further cooling.
The whole behaviour is typical of dominant antiferromagnetic
interactions, while the increase at low temperatures is indicative of
a paramagnetic impurity.
˚
interactions. The dimensions are: O(3¢) ◊ ◊ ◊ O(2) 2.735(3) A, O(3¢)-
H(O3¢) ◊ ◊ ◊ O(2) 169.1(1)◦ (symmetry code: ¢ = 0.5 - x, -0.5 +
˚
y, 0.5 - z) and N(4¢¢) ◊ ◊ ◊ O(1) 2.828 A, N(4¢¢)-H(N4¢¢)–O(1) =
162.8(1)◦ (symmetry code: ¢¢ = 0.5 + x, 1.5 - y, -0.5 + z); the
˚
closest interchain Cu ◊ ◊ ◊ Cu separations are 7.185 and 8.221 A,
respectively. Closer inspection of the structure reveals that the very
˚
weak bonding Cu ◊ ◊ ◊ O(2) interaction (2.764(2) A) may be due to
the involvement of O(2) in the interchain hydrogen bond with the
coordinated EtOH oxygen atom; this hydrogen bond separates
further O(2) from Cu.
Complex 1 joins a small family of imidazolate-bridged CuII
coordination polymers.8,14,19 It is only the third example of a
polymeric CuII compound containing both Him and im- lig-
8b
ands, the two previous examples being [CuCl(im)(Him)2]n and
[Cu3(ClO4)4(im)2(Him)8]n19; in the latter, however, the bridging of
the trinuclear repeating units to form the chain is achieved by
perchlorate groups.
Complex 4·2nH2O (Fig. 3 and 4) crystallises in the mon-
oclinic space group C2/c. Its structure consists of neutral
[Cu(tp)(Him)2(H2O)]n chains and lattice H2O molecules. There is
a crystallographic inversion centre which is located at the middle
of the benzene ring of the tp2- ligand. Each chain consists of
a linear array of CuII atoms parallel to the c axis. Adjacent
1
1
metal ions are singly bridged by one bis-monodentate h :h :m
tp2- ligand, coordinated in a monodentate fashion through each
of its carboxylate groups. Two monodentate Him ligands and
the H2O molecule complete five-coordination at each metal
centre.
The coordination geometry about the CuII centre is well
described as square pyramidal (t = 0.20) with the aquo ligand
occupying the axial position. The basal plane is defined by atoms
O(1) and O(1¢) from two tp2- ligands, and the pyridine-type atoms
N(1) and N(1¢) from two Him ligands. As expected, the Cu–OW(1)
˚
bond is the longest (2.278(4) A). The copper(II)–oxygen (carboxy-
Fig. 5 cM vs. T and cMT vs. T data (open circles) and best-fit curves
according to the above-mentioned model.
˚
late) bond distance (1.967(3) A) and the intrachain intermetallic
˚
separation (11.025(3) A) are in the corresponding ranges reported
for bis-monodentate tp2--bridged copper(II) complexes containing
5-coordinate metal ions.24
The magnetic properties were analyzed according to the Bonner-
Fisher model,30 for an equally spaced S = 1/2 chain. At a
3360 | Dalton Trans., 2009, 3354–3362
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first approximation interchain interactions were disregarded as
too weak to be reliably accounted for by magnetic susceptibility
measurements. The Hamiltonian considered was:
couplings are maximized when the CuII–Nim- bonds are parallel to
the imidazolate carbon–carbon bond, i.e. for CuII–Nim- ◊ ◊ ◊ Nim-
angles close to 180◦. Thus, the moderate coupling observed
in 1 can be partly interpreted by the average CuII–Nim- ◊ ◊ ◊ Nim-
angle value of 163.83◦, which deviates significantly from 180◦.
However, since the experimental exchange coupling is a function of
several structural and electronic factors,35 any magnetostructural
discussion cannot be reliable beyond this point.
ˆ
ˆ ˆ
SiSi +1
H = -2J
(10)
Â
i
A rough estimate of ca. -30 cm-1 can be obtained for the
exchange coupling based on the position of the cM vs. T curve
maximum, according to the relation kTmax/|J| = 1.282, proposed
by Bonner and Fisher (-2JSiSj Hamiltonian formalism).
For the fitting of the data the empirical eqn (11) derived by Hall
was employed:31
Conclusions
The present work emphasises the ability of (im)2CO to form new
coordination polymers. The use of (im)2CO in reactions with CuII
and carboxylates in alcohols has provided access to two neutral,
1D polymeric species, one containing the bridging imidazolate(-1)
ligand (1), and one containing the bridging terephthalate(-2)
ligand and terminal imidazole groups (4·2nH2O). It is noteworthy
that we could not prepare these two compounds from conventional
reactions employing Him as starting material. We have no reasons
to believe that this research area is exhausted of new results.
Indeed, ongoing studies are producing additional and interesting
products, and our belief is that we have scratched only the
surface of metal polymer chemistry based on (im)2CO. As far
as future perspectives are concerned, ‘analogues’ of 1 and 4 with
other divalent 3d-metals are not known to date, and preliminary
work shows that the preparation and stability of such polymers
are dependent on the particular nature of the metal ion. In
Ng2 b2
kT
A + Bx + Cx2
È
Í
Î
˘
˙
˚
cchain
=
(11)
1 + Dx + Ex2 + Fx3
with A = 0.25, B = 0.14995, C = 0.30094, D = 1.9862, E =
0.68854, F = 6.0626 and x = |J|/kT for the above mentioned
Hamiltonian formalism. The final expression used, accounts for a
small fraction r of paramagnetic impurity (S = 1/2) obeying the
Curie law:
Ng2 b2
4kT
(12)
c = (1-r)cchain + r
According to this model, the best-fit parameters are: J =
-33.5 cm-1, g = 2.105, r = 2.8% with R = 2.3 ¥ 10-5. The high qual-
ity of the fit does not allow us to consider an interchain interaction
without risk of overparametrizing the problem. The value of the
derived J coupling falls within the range of 0 to -88 cm-1 previously
reported26,32 for imidazolato-bridged copper(II) complexes, and
can be characterized as moderate.
With one exception,33 the magnetically coupled M–im–M(M¢)
systems explored to date have displayed antiferromagnetic cou-
pling, and the focus of study has been in establishing its magnitude
and mechanism.7b, 26,33,34 Considering imidazolate-bridged copper
systems, the interaction between the CuII centres can be explained
in terms of a s- and/or a p-exchange pathway. In general,
there is agreement26,35 that the p orbitals of the im- ligand are
not involved in the coupling. Since the magnetic orbitals are
s antibonding for square planar and square pyramidal CuII
environments, the relevant exchange pathway through the im-
bridge is of the s type. The imidazolate orbitals that are responsible
for this kind of interaction are of the type sketched in Scheme 2;
-
addition, the terminal monodentate ligands present in 1 (MeCO2 ,
EtOH) and 4 (H2O) could have future utility as sites for facile
incorporation of neutral or anionic bis(monodentate) bridging
ligands, e.g. azides, 4,4¢-bipyridine etc., as a means of accessing
higher-dimensionality copper(II) polymers.
Acknowledgements
Th. C. S., C. S. P. and A. J. T. thank the European Commission
for providing a Marie-Curie Grant (HPMT-CT-2001-00421) that
enabled the stay of Th. C. S. at the University of Cyprus. S. P. P.
thanks the European Social Fund (ESF), Operational Program
for Educational and Vocational Training II (EPEAEK II), and
particularly the Program PYTHAGORAS (Grant b.365.037), for
funding the above work.
=
these are essentially parallel to the N–N¢ (or C C) direction.
References
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shown that magnetic exchange is more sensitive to the latter rather
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3362 | Dalton Trans., 2009, 3354–3362
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