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Coordination chemistry in the solid state
Guest Editor Russell E. Morris
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Published in Issue 14, Volume 41 of Dalton Transactions
Articles in this issue include:
Communications
Highly oriented surface-growth and covalent dye labeling of mesoporous metal–organic
frameworks
Florian M. Hinterholzinger, Stefan Wuttke, Pascal Roy, Thomas Preuße, Andreas Schaate,
Peter Behrens, Adelheid Godt and Thomas Bein
Papers
Supramolecular isomers of metal–organic frameworks: the role of a new mixed donor
imidazolate-carboxylate tetradentate ligand
Victoria J. Richards, Stephen P. Argent, Adam Kewley, Alexander J. Blake, William Lewis and
Neil R. Champness
Hydrogen adsorption in the metal–organic frameworks Fe2(dobdc) and Fe2(O2)(dobdc)
Wendy L. Queen, Eric D. Bloch, Craig M. Brown, Matthew R. Hudson, Jarad A. Mason, Leslie
J. Murray, Anibal Javier Ramirez-Cuesta, Vanessa K. Peterson and Jeffrey R. Long
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PAPER
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Systematic investigations on magneto-structural correlations of copper(II)
coordination polymers based on organic ligands with mixed carboxylic and
nitrogen-based moieties†
Mario Wriedt and Hong-Cai “Joe” Zhou*
Received 17th October 2011, Accepted 24th November 2011
DOI: 10.1039/c2dt11965j
Reaction of copper(II) tetrazolate-5-carboxylate with different neutral N-donor spacer ligands under
hydrothermal conditions leads to the formation of five new coordination polymers, [Cu(tzc)
( pyz)0.5(H2O)2]n·H2O (1), [Cu(tzc)( pyz)]n (2), [Cu(tzc)( pym)(H2O)]n (3), [Cu(tzc)(dpe)0.5(H2O)]n (4)
and [Cu(tzc)(azpy)0.5(H2O)]n (5) (tzc = tetrazolate-5-carboxylate, pyz = pyrazine, pym = pyrimidine, dpe
= 1,2-di(4-pyridyl)ethylene and azpy = 4,4′-azopyridine). All five structures were characterized by X-ray
single-crystal measurements and bulk material can be prepared phase pure in high yields. The crystal
structures of the hydrates 1, 3, 4 and 5 show dimeric [Cu2(Ntzc–Ntzc)2] building units formed by μ2-N1,
O1:N2 bridging tzc ligands as the characteristic structural motif. These six-membered entities in 1, 4 and
5 are connected by μ2-N,N′ bridging N-donor ligands into 1D chains and in 3 into 2D layers. In the
crystal structure of compound 2 adjacent Cu(II) cations are connected by μ2-N1,O1:N4,O2 bridging tzc
ligands into chains, which are further connected by μ2-N,N′ bridging pyz ligands forming 2D layers.
Extensive hydrogen bonds in all compounds play an important role in the construction of their
supramolecular networks. Investigations of their thermal properties reveal water release upon heating
according to the formation of anhydrates before starting decomposing above 220 °C. Furthermore, the
magnetic properties have been studied leading to consistent global antiferromagnetic exchange
interactions with coupling constants of J = 3 ± 1 cm−1 and long-range antiferromagnetic ordering states at
lower temperatures.
Introduction
The rational design and construction of coordination polymers
and metal–organic frameworks (MOFs) based on assembly of
metal cations and multifunctional organic ligands have gradually
attracted great attention in the fields of inorganic and material
chemistry, which mainly stems from their intriguing variety of
topologies and their potential application as functional materials
in e.g. catalysis,1–3 gas storage,4–8 and magnetism.9–14 In particular, the ability to adopt a variety of paramagnetic transitionmetal elements into their framework and by tuning the distance
between the magnetic ions via small bridging ligands may lead
to materials with interesting magnetic properties, for instance
Department of Chemistry, Texas A&M University, 3255 TAMU, College
Station, TX 77843-3255, USA. E-mail: zhou@chem.tamu.edu;
Fax: +1 (979) 845-1595; Tel: +1 (979) 422-6329
† Electronic supplementary information (ESI) available: Experimental
and calculated XRPD patterns for all compounds; IR spectroscopic data
for all compounds; Temperature ellipsoid plot for compound 5; TG,
DSC and MS-MID curves for compound 5; χM vs. T and χMT vs. T plots
for compounds 2–4; χM−1 vs. T plot for all compounds, Selected bond
lengths and angles as well as hydrogen bonding of all compounds.
CCDC reference numbers 847988–847992. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2dt11965j
This journal is © The Royal Society of Chemistry 2012
ferro-,10,15,16 meta-13,17 or single-chain18–20 magnetism. The
geometries of organic ligands have a great effect on the structural
architecture of these frameworks and thus, much effort has been
devoted to the design and modification of the organic ligands to
control the products. In this context, tetrazole and its 5-substitued derivates have been confirmed as excellent bridging ligands
for the formation of coordination compounds exhibiting great
structural diversity and interesting magnetic properties.21–24
In view of the above considerations we prepared a series of
new metal–organic framework materials based on the anionic
ligand tetrazolate-5-carboxylate (tzc2−); only recently a few compounds based on this ligand have been reported.25–29 With the
carboxylate and tetrazolate group linked directly, this ligand has
a large number of potential coordination modes, due to its abundance of N- and O-sites and their unique structural arrangement
(Scheme 1). Moreover, as a small and conjugated ligand,
it offers short potential magnetic exchange pathways to be an
excellent magnetic coupling mediator between paramagnetic
metal centers. In addition, it can be involved in inter- and intramolecular hydrogen bonding that can influence the molecular
and supramolecular structure. Taking all the above into account,
we have successfully synthesized five new coordination materials
based on copper(II) tetrazolate-5-carboxylate and different
neutral N donor ligands, pyrazine ( pyz), pyrimidine ( pym),
Dalton Trans., 2012, 41, 4207–4216 | 4207
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Synthesis of [Cu(tzc)( pyz)]n (2)
Scheme 1 Coordination modes of tetrazolate-5-carboxylate (tzc2−)
observed in the present compounds.
1,2-di(4-pyridyl)ethylene (dpe) and 4,4′-azopyridine (azpy).
Depending on the nature of the organic ligands as well as the
content of coordinating or non-coordinating solvent molecules,
compounds with different coordination topologies are obtained.
Here we report on their structural, thermal and magnetic
properties.
Single crystals suitable for X-ray structure determination were
prepared by reaction of Cu(CH3COO)2 (36.3 mg, 0.2 mmol), tzc
ethyl ester sodium salt (32.8 mg, 0.2 mmol) and pyrazine
(96.1 mg, 1.2 mmol) in 3 mL of water in a closed snap vial
without stirring at 100 °C. Blue block shaped single crystals of
compound 2 were obtained after one day as a major phase. Bulk
material was prepared by decanting the mother-liquor from the
vial, and washing the crystals several times with water until only
blue crystals of compound 2 remain. They were filtered off and
washed with ethanol and diethyl ether and dried in air. The
purity was checked by X-ray powder diffraction (Fig. S2, ESI†).
Yield based on the metal salt: 47 mg (92%). Calc. for
C6H4CuN6O2 (255.68): C 28.19, H 1.58, N 32.87; found: C
28.09, H 1.64, N 32.75%. IR: ν̃ = 3121 (w), 1607 (vs), 1487 (s),
1439 (m), 1418 (vs), 1325 (vs), 1231 (w), 1191 (w), 1163 (m),
1126 (m), 1115 (s), 1069 (s), 1022 (w), 831 (s), 797 (vs), 671 (s)
cm−1 (Fig. S7, ESI†).
Synthesis of [Cu(tzc)( pym)(H2O)]n (3)
Experimental
General information
Commercially available reagents were used as received without
further purification. Tetrazolate-5-carboxylate (tzc) was produced
in situ by hydrothermal treatment of the 1H-tetrazole-5carboxylic acid ethyl ester sodium salt. The purity of all compounds was checked by elemental analysis and X-ray powder
diffraction (XRPD, Fig. S1–S5, ESI†).
CAUTION! Although not encountered in our experiments,
metal complexes of tetrazolate compounds are potentially explosive. Only a small amount of the materials should be prepared
and handled with care.
Synthesis of [Cu(tzc)( pyz)0.5(H2O)2]n·H2O (1)
Bulk material was prepared by reaction of Cu(CH3COO)2
(36.3 mg, 0.2 mmol), tzc ethyl ester sodium salt (32.8 mg,
0.2 mmol) and pyrazine (16.0 mg, 0.2 mmol) stirring in 3 mL of
water in a closed snap vial on a heating plate at 100 °C. A blue
polycrystalline precipitate was obtained after three days. The
residue was filtered off and washed with water, ethanol and
diethyl ether and dried in air. The purity was checked by X-ray
powder diffraction (Fig. S1, ESI†). Yield based on the metal
salt: 50 mg (98%). Calc. for C4H8CuN5O5 (269.69): C 17.81, H
2.99, N 25.97; found: C 17.89, H 3.00, N 25.95%. IR: ν̃ = 3576
(w), 3271 (br), 1657 (vs), 1649 (vs), 1643 (vs), 1634 (vs), 1497
(s), 1423 (s), 1335 (vs), 1227 (m), 1180 (w), 1163 (w), 1124
(m), 1096 (w), 1088 (w), 1072 (s), 1051 (w), 997 (w), 845 (s),
820 (vs), 669 (s), 622 (s) cm−1 (Fig. S6, ESI†).
Single crystals suitable for X-ray structure determination were
prepared by reaction of Cu(CH3COO)2 (18.2 mg, 0.1 mmol), tzc
ethyl ester sodium salt (16.4 mg, 0.1 mmol) and pyrazine
(8.0 mg, 0.1 mmol) in 1 mL of water without stirring in a closed
snap vial at 85 °C. On cooling blue block shaped single crystals
of compound 1 were obtained as a major phase.
4208 | Dalton Trans., 2012, 41, 4207–4216
Single crystals suitable for X-ray structure determination were
prepared by reaction of Cu(CH3COO)2 (36.3 mg, 0.2 mmol), tzc
ethyl ester sodium salt (32.8 mg, 0.2 mmol) and pyrimidine
(16.0 mg, 0.2 mmol) in 2 mL of water in a closed snap vial
without stirring at 100 °C. Blue block shaped single crystals of
compound 3 were obtained after one day as a major phase. Bulk
material was prepared by decanting the mother-liquor from the
vial, and washing the crystals several times with water until only
blue crystals of compound 3 remain. They were filtered off and
washed with ethanol and diethyl ether and dried in air. The
purity was checked by X-ray powder diffraction (Fig. S3, ESI†).
Yield based on the metal salt: 54 mg (93%). Calc. for
C6H6CuN6O3 (273.71): C 26.33, H 2.21, N 30.71; found: C
26.40, H 2.19, N 30.61%. IR: ν̃ = 3393 (w), 1643 (s), 1630 (vs),
1601 (s), 1572 (s), 1487 (s), 1479 (s), 1416 (vs), 1323 (vs),
1238 (w), 1219 (m), 1173 (m), 1117 (w), 1080 (m), 1024 (m),
952 (w), 839 (s), 815 (s), 700 (vs), 692 (vs), 680 (vs), 654 (vs)
cm−1 (Fig. S8, ESI†).
Synthesis of [Cu(tzc)(dpe)0.5(H2O)]n (4)
Bulk material was prepared by reaction of Cu(CH3COO)2
(36.3 mg, 0.2 mmol), tzc ethyl ester sodium salt (32.8 mg,
0.2 mmol) and 1,2-di(4-pyridyl)ethylen (18.2 mg, 0.1 mmol)
stirring in 2 mL of water in a closed snap vial on a heating plate
at 100 °C. A blue polycrystalline precipitate was obtained after
three days. The residue was filtered off and washed with water,
ethanol and diethyl ether and dried in air. The purity was
checked by X-ray powder diffraction (Fig. S4, ESI†). Yield
based on the metal salt: 53 mg (93%). Calc. for C8H7CuN5O3
(284.73): C 33.75, H 2.48, N 24.60; found: C 33.99, H 2.46, N
24.42%. IR: ν̃ = 3470 (w), 1668 (vs), 1612 (s), 1557 (w), 1508
(w), 1479 (s), 1433 (m), 1420 (s), 1352 (w), 1312 (vs), 1254
(w), 1233 (m), 1206 (m), 1184 (m), 1130 (w), 1088 (m), 1070
(m), 1036 (m), 974 (m), 959 (s), 968 (w), 835 (vs), 818 (s), 746
(w), 679 (s), 604 (s) cm−1 (Fig. S9, ESI†).
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Table 1
Selected crystal data and details on the structure determinations from single-crystal data for compounds 1–5
Compound
1
2
3
4
5
Formula
Mr
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å3
T/K
Z
Dc/g cm−3
μ/mm−1
Min./max. transmission
θmax/°
Measured reflections
Unique reflections
Reflections [F0 > 4σ(F0)]
Parameters
Rint
R1 [F0 > 4σ(F0)]
wR2 (all data)
GOF
Δρmax/min/e Å−3
C4H8CuN5O5
269.69
Triclinic
P1ˉ
7.639(2)
8.088(2)
8.169(2)
67.860(3)
71.560(3)
84.827(3)
443.3(2)
120(2)
2
2.021
2.478
0.726/0.798
26.73
3633
1861
1733
136
0.0145
0.0253
0.0681
1.083
0.500/−0.619
C6H4CuN6O2
255.69
Monoclinic
P21/m
5.606(3)
6.830(4)
11.891(7)
90
99.379(7)
90
449.2(5)
120(2)
2
1.891
2.420
0.690/0.827
28.51
4077
1215
1024
82
0.0422
0.0442
0.1044
1.097
1.692/−0.671
C6H6CuN6O3
273.71
Monoclinic
P21/n
9.272(3)
6.864(3)
14.484(5)
90
94.052(4)
90
919.4(6)
120(2)
4
1.977
2.379
0.715/0.767
25.99
6881
1817
1663
145
0.0224
0.0201
0.0549
1.072
0.358/−0.333
C8H7CuN5O3
284.73
Monoclinic
P21/c
7.5502(19)
12.884(3)
10.036(3)
90
98.038(3)
90
966.7(4)
120(2)
4
1.956
2.264
0.727/0.812
28.50
11532
2414
2071
154
0.0372
0.0271
0.0701
1.049
0.459/−0.310
C7H6CuN6O3
285.72
Monoclinic
P21/c
7.633(6)
12.718(9)
10.105(7)
90
101.016(8)
90
963.0(12)
120(2)
4
1.971
2.276
0.890/0.923
28.40
10832
2371
1815
154
0.0463
0.0375
0.1024
1.050
0.643/−0.661
Single crystals suitable for X-ray structure determination were
prepared by reaction of Cu(CH3COO)2 (36.3 mg, 0.2 mmol), tzc
ethyl ester sodium salt (32.8 mg, 0.2 mmol) and 1,2-di
(4-pyridyl)ethylen (18.2 mg, 0.1 mmol) in 3 mL of water
without stirring in a closed snap vial at 100 °C. On cooling blue
rod shaped single crystals of compound 4 were obtained as a
major phase.
Synthesis of [Cu(tzc)(azpy)0.5(H2O)]n (5)
Bulk material was prepared by reaction of Cu(CH3COO)2
(36.3 mg, 0.2 mmol), tzc ethyl ester sodium salt (32.8 mg,
0.2 mmol) and 4,4′-azopyridine (18.4 mg, 0.1 mmol) stirring in
2 mL of water in a closed snap vial on a heating plate at 100 °C.
A blue polycrystalline precipitate was obtained after three days.
The residue was filtered off and washed with water, ethanol and
diethyl ether and dried in air. The purity was checked by X-ray
powder diffraction (Fig. S5, ESI†). Yield based on the metal
salt: 54 mg (95%). Calc. for C7H6CuN6O3 (285.72): C 29.43, H
2.12, N 22.24; found: C 29.49, H 2.14, N 22.36%. IR: ν̃ = 3453
(w), 1668 (vs), 1630 (m), 1603 (m), 1568 (w), 1485 (s), 1423
(s), 1315 (vs), 1250 (w), 1227 (m), 1184 (m), 1092 (m), 1055
(m), 1036 (m), 874 (m), 856 (vs), 835 (s), 818 (s), 746 (w), 679
(s), 621 (m), 611 (m) cm−1 (Fig. S10, ESI†).
Single crystals suitable for X-ray structure determination were
prepared by reaction of Cu(CH3COO)2 (9.1 mg, 0.05 mmol), tzc
ethyl ester sodium salt (8.2 mg, 0.05 mmol) and 4,4′-azopyridine (9.2 mg, 0.05 mmol) in 2 mL of water without stirring in a
closed snap vial at 100 °C. On cooling blue rod shaped single
crystals of compound 5 were obtained as a major phase.
This journal is © The Royal Society of Chemistry 2012
Single-crystal structure analysis
Single-crystal X-ray data of all compounds were collected on a
Bruker AXS Smart III X-ray diffractometer outfitted with a Mo
X-ray source (λ = 0.71073 Å) and an APEX II CCD detector
equipped with an Oxford Cryostream low-temperature device.
The APEX-II software suite was used for data collection, cell
refinement, reduction and absorption correction. The structure
solutions were performed with direct methods using
SHELXS-97, and structure refinements were performed against
|F|2 using SHELXL-97.30 All non-hydrogen atoms were refined
with anisotropic displacement parameters. The O–H-hydrogen
atoms in compounds 1, 3, 4 and 5 were located in the difference
map, where the bond lengths set to ideal values and were refined
using a riding model. In compound 1 the O–H hydrogen atoms
were disordered over three positions and therefore refined using
a splitting model with site occupancy factors of ⅔. The C–H
atoms were positioned with idealized geometry and were refined
with fixed isotropic displacement parameters [Ueq(H) =
1.2Ueq(C)] using a riding model with dC–H = 0.95 Å. Details of
the structure determination are given in Table 1.
CCDC 847988 (1), 847989 (2), 847990 (3), 847991 (4), and
847992 (5) contain the supplementary crystallographic data for
this paper.
X-Ray powder diffraction (XRPD)
X-Ray powder diffraction patterns were recorded on a Bruker
D8 Discover diffractometer equipped with a Cu sealed tube (λ =
1.54178 Å). Powder samples were dispersed on low-background
quartz discs (G. M. Associates, Inc., Oakland, California) for
Dalton Trans., 2012, 41, 4207–4216 | 4209
analyses. Simulation of the XRPD data was carried out by using
the single-crystal data and the Theoretical Pattern module of the
WinXPOW Software package.31
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Thermogravimetry and differential scanning calorimetry
coupled to mass spectroscopy (TG-DSC-MS)
forming an almost planar six-membered ring with a torsion
angle of Cu1–N1–N2–Cu1A being 0.77(3)° and with a Cu1–
Cu1A distance in this dimeric Cu2(N–N)2 moiety of 4.080(1) Å.
These units are μ2-N,N′ bridged by the pyz ligands into chains
forming a dihedral angle between the Cu2(N–N)2 and pyz plane
of 37.55(1)° (Fig. 2: top). Interchain hydrogen-bonding
TG-DSC data were recorded using a TGA/DSC 1 STAR System
from Mettler Toledo, which is coupled to a mass spectrometer
PrismaPlus QMG 220 M with a C-SEM-detector from Pfeiffer.
All measurements were performed using Al2O3 crucibles in a
dynamic argon atmosphere and a heating rate of 3 °C min−1.
The instrument was corrected for buoyancy and current effects,
and was calibrated using standard reference materials.
Magnetic measurements
Magnetic measurements were performed on crushed polycrystalline samples with a Quantum Design SQUID magnetometer
MPMS-XL. The dc magnetic susceptibility measurements were
carried out in an applied field of 1000 Oe over the temperature
range of 300–1.8 K. The ac magnetic susceptibility measurements were performed in a 4 Oe ac field with a frequency of 100
Hz. Magnetization data were measured at 1.8 K with the magnetic field varying from 0 to 70 kOe. The data were corrected for
diamagnetic contributions calculated from the Pascal
constants.32
Elemental analysis
Elemental analyses (C, H, and N) were obtained by Atlantic
Microlab, Inc.
Fig. 1 Crystal structure of [Cu(tzc)( pyz)0.5(H2O)2]n·H2O (1) with a
view of the coordination sphere of the copper(II) cation with displacement ellipsoids drawn at the 50% probability level. The water hydrogen
atoms are disordered over three positions. Selected atoms are labeled.
Symmetry codes: A = −x + 1, −y + 1, −z + 1; B = −x + 2, −y + 2, −z.
Spectroscopy
FT-IR data were recorded on an IRAffinity-1 instrument from
SHIMADZU.
Results and discussion
Crystal structures
Compound [Cu(tzc)( pyz)0.5(H2O)2]n·H2O (1) crystallizes in the
centrosymmetric triclinic space group P1ˉ with two formula units
in the unit cell (Table 1). The asymmetric unit consists of one
Cu(II) cation, one pyz and tzc ligand and three water molecules.
All atoms are located in general positions. In the crystal structure
each Cu(II) cation is coordinated by two symmetry-related tzc
ligands, one pyz ligand and two water molecules in an octahedral geometry (Fig. 1). The CuN2O4 octahedron is markedly
stretched with two long Cu–Owater distances of 2.387(2) and
2.421(2) Å and four short Cu–Otzc/Ntzc/Npyz distances in the
range of 1.980(2)–2.032(2) Å, whereas the angles around the
metal cations range between 81.39(7)–98.99(8) and 167.40(7)–
175.77(5)° (Table S1, ESI†). Each tzc ligand connects two metal
ions through a carboxylate oxygen atom and two adjacent nitrogen atoms to give a μ2-N1,O1:N2 bridging mode. Double N–N
bridges of two opposite tzc ligands link adjacent Cu(II) cations
4210 | Dalton Trans., 2012, 41, 4207–4216
Fig. 2 Crystal structure of [Cu(tzc)( pyz)0.5(H2O)2]n·H2O (1) with a
view onto a single chain approximately along the crystallographic a-axis
(top) and its stacking form approximately along the crystallographic
c-axis (bottom). Dashed orange lines indicate interchain hydrogen
bonding; the disorder of the water hydrogen atoms is not shown for
clarity.
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interactions, i.e. O1W/O2W–H⋯O2 with 2.757/2.731 Å and
173.04/163.50° (Table S2, ESI†), connect the [Cu(tzc)
( pyz)0.5(H2O)2]n chains to give a 3D supramolecular arrangement. Non-coordinating water molecules occupy the channels
formed by these interactions (Fig. 2: bottom) and have further
hydrogen bonding interactions with the coordinating water molecules and the tzc ligand (Table S2, ESI†). The shortest interlayer separation of two adjacent metal cations amounts to 6.152
(1) Å.
Compound [Cu(tzc)( pyz)]n (2) crystallizes in the centrosymmetric monoclinic space group P21/m with two formula units in
the unit cell (Table 1). The asymmetric unit consists of one
Cu(II) cation and one tzc ligand located on a mirror plane and
one pyz ligand in general position. In the crystal structure each
Cu(II) cation is coordinated by two symmetry-related tzc and two
symmetry related pyz ligands in a distorted octahedral geometry
(Fig. 3) with two long Cu–Otzc/Ntzc distances of 2.485(3) and
2.364(4) Å and four short Cu–Otzc/Ntzc/Npyz distances in the
range of 1.977(4)–2.034(3) Å, whereas the angles around the
metal cation range between 81.73(15)–104.59(16) and 173.68
(15)–177.38(17)° (Table S3, ESI†). The tzc ligands connect
adjacent Cu(II) cations in a μ2-N1,O1:N4,O2 bridging mode into
chains, which elongate in the direction of the crystallographic aaxis. These chains are further connected by μ2-N,N′ bridging
pyz ligands forming layers in the ab plane with all bridging
ligands coplanar (Fig. 4: top). The metal–metal separation
through the tzc and pyz ligands amounts to 5.606(3) and 6.830
(4) Å, respectively. Taking interlayer weak hydrogen bonding
with C12–H12⋯O2 = 2.659(3) Å and 120.16(1)° into account,
double layers are formed which are stacked in the direction of
the crystallographic c-axis (Fig. 4: bottom). The smallest interlayer separation of two adjacent Cu(II) cations amounts to 6.975
(3) Å.
Compound [Cu(tzc)( pym)(H2O)]n (3) crystallizes in the centrosymmetric monoclinic space group P21/n with four formula
units in the unit cell (Table 1). The asymmetric unit consists of
one Cu(II), one tzc ligand and one pym ligand in general positions. In the crystal structure each Cu(II) cation is coordinated
by two symmetry related tzc and two symmetry-related pym
ligands as well as one water molecule in a distorted octahedral
geometry (Fig. 5). Two long Cu–Owater/Npym distances of 2.408
(2) and 2.377(2) Å and four short Cu–Otzc/Ntzc/Npym distances
in the range of 2.004(2)–2.035(2) Å with angles around the
metal cation in the range between 81.70(6)–97.47(7) and 168.19
(6)–175.14(6)° are found in this markedly stretched CuN4O2
octahedron (Table S4, ESI†). The Cu2(tzc)2 moiety with μ2-N1,
O1:N2 bridging tzc ligands is the same as found in compound 1,
forming a six-membered ring with a torsion angle Cu1–N1–N2–
Cu1A of 8.3(3)° and with a Cu1–Cu1A distance being 4.079(2)
Å. Each of these dimeric units are μ2-N,N′ bridged by four pym
ligands into layers (Fig. 6: top), which are further connected by
interlayer hydrogen bonding with O1W–H1/H2⋯O2 = 2.817/
2.808 Å and 171.98/169.71° (Table S5, ESI†) into a 3D network
(Fig. 6: bottom) with a smallest interlayer separation of two adjacent Cu(II) cations being 7.116(2) Å.
Compounds [Cu(tzc)(L)(H2O)]n (L = dpe 4, azpy 5) crystallize in the centrosymmetric monoclinic space group P21/c with
Fig. 3 Crystal structure of [Cu(tzc)( pyz)]n (2) with a view of the
coordination sphere of the copper(II) cation with displacement ellipsoids
drawn at the 50% probability level. Selected atoms are labeled. Symmetry codes: A = x + 1, y, z; B = x, −y + 1/2, z; C = x, −y + 3/2, z.
Fig. 4 Crystal structure of [Cu(tzc)( pyz)]n (2) with a view onto a
single layer approximately along the crystallographic c-axis (top) and its
stacking form approximately along the crystallographic a-axis (bottom).
Dashed orange lines indicate interlayer hydrogen bonding.
This journal is © The Royal Society of Chemistry 2012
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Fig. 5 Crystal structure of [Cu(tzc)( pym)(H2O)]n (3) with a view of
the coordination sphere of the copper(II) cation with displacement ellipsoids drawn at the 50% probability level. Selected atoms are labeled.
Symmetry codes: A = −x + 1, −y + 1, −z + 1; B = −x + 1/2, y − 1/2, −z
+ 1/2.
four formula units in the unit cell (Table 1). The asymmetric unit
consists of one Cu(II) cation, one tzc ligand, one dpe (or azpy)
ligand and one water molecule in general positions. In the crystal
structure each Cu(II) cation is pentacoordinated by one dpe (or
azpy) ligand, one water molecule and two symmetry-related
tzc ligands in a quadratic-pyramidal geometry (Fig. 7). This
Fig. 6 Crystal structure of [Cu(tzc)( pym)(H2O)]n (3) with a view onto
a single layer (top) and its stacking form approximately along the crystallographic b-axis (bottom). Dashed orange lines indicate interlayer
hydrogen bonding.
4212 | Dalton Trans., 2012, 41, 4207–4216
CuN3O2 pentagon is formed by four approximately equal Cu–
Otzc/Ntzc/Ndpe/azpy distances in the range of 1.979(2)–2.000(2) Å
for 4 and 1.996(3)–2.003(3) Å for 5, and one longer Cu–Owater
distance of 2.218(2) Å for 4 and 2.202(3) Å for 5 with angles
found around the metal cation in the range between 81.31(6)–
100.61(6) and 165.50(7)–172.63(7)° for 4 as well as 81.54(10)–
98.72(11) and 165.70(11)–172.81(11)° for 5 (Table S6, ESI†).
On viewing their extended crystal structures, the same dimeric
Cu2(tzc)2 moieties with μ2-N1,O1:N2 bridging tzc ligands are
found as described in compound 1 and 3, which are μ2-N,N′
bridged by the dpe and azpy ligands, respectively, into chains
forming a dihedral angle of 49.30(1)° for 4 and 50.92(6)° for 5
(Fig. 8: top). Characteristic torsion angles of Cu1–N1–N2–Cu1A
in these dimeric moieties amount to 5.36(3)° for 4 and 4.9(5)°
for 5 with intramolecular Cu1–Cu1A distances of 4.033(1) Å for
4 and 4.061(2) Å for 5. Taking interchain hydrogen bonding
with O1W–H1⋯N1 and O1W–H⋯O2 = 2.905/2.795 Å and
152.45/163.06° for 4 as well as 2.879/2.748 Å and 154.18/
163.54° for 5 into account (Table S7 and S8, ESI†), the overall
crystal packing can be regarded as a 3D supramolecular coordination network with a smallest interchain separation of two adjacent Cu(II) cations being 7.113(2) Å for 4 and 7.041(4) Å for 5
(Fig. 8: bottom).
Thermal decomposition reactions
On heating the trihydrate [Cu(tzc)( pyz)0.5(H2O)2]n·H2O (1),
three mass steps are observed in the TG curve that are
accompanied with two endothermic and one exothermic event in
the DSC curve (Fig. 9: top left). From the MS trend scan curve,
it is proven that only water (m/z = 18) is emitted in the first and
second mass step as well as pyrazine (m/z = 80) and nitrogen (m/
z = 28, decomposition product of tzc) in the third step. All TG
steps are well resolved and separated. The experimental mass
losses of Δmexp(1st step) = 13.3% and Δmexp(2nd step) = 6.1%
are in good agreement with that calculated for the release of two
Fig. 7 Crystal structure of [Cu(tzc)(dpe)0.5(H2O)]n as a representative
structure model for [Cu(tzc)(dpe)0.5(H2O)]n (4) and [Cu(tzc)
(azpy)0.5(H2O)]n (5) with a view of the coordination sphere of the
copper(II) cation with displacement ellipsoids drawn at the 50% probability level. Selected atoms are labeled. Symmetry codes: A = −x, −y +
1, −z + 1; B = −x + 2, −y + 1, −z + 2. For ellipsoid plot of compound 5
see Fig. S11, ESI†.
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Fig. 8 Crystal structure of [Cu(tzc)(dpe)0.5(H2O)]n as a representative
structure model for [Cu(tzc)(dpe)0.5(H2O)]n (4) and [Cu(tzc)
(azpy)0.5(H2O)]n (5) with a view onto a single chain (top) and its stacking form approximately along the crystallographic a-axis (bottom).
Dashed orange lines indicate interchain hydrogen bonding.
and one water molecule, respectively [Δmcalc(water) = 6.7%].
The final mass loss is imprecise because of the explosive
decomposition of the tzc ligand and thus, it cannot be calculated.
On the basis of the experimental mass losses, it can be assumed
that in the first step the monohydrate [Cu(tzc)( pyz)0.5]n·H2O is
formed leading to the formation of the anhydrate [Cu(tzc)
( pyz)0.5)]n upon further heating before it starts decomposing at
approx. 230 °C. Please note that it is highly likely that the two
coordinating water molecules are released first, followed by the
non-coordinating water molecule, because of its hydrogen
bonding to the framework (Table S2, ESI†).
On heating [Cu(tzc)( pyz)]n (2), one well resolved mass step is
observed in the TG curve that is accompanied by an exothermic
event in the DSC curve indicating explosive decomposition at
ca. 250 °C (Fig. 9: top right). MS trend scan curves support this
behavior with signals of pyrazine (m/z = 80) and nitrogen (m/z =
28, decomposition product of tzc) around this step.
On heating [Cu(tzc)( pym)(H2O)]n (3) and [Cu(tzc)(L)(H2O)]n
(L = dpe 4, azpy 5), two well resolved mass steps can be found
in their TG curves. The first step of each compound is
accompanied by an endothermic event in the DSC curve and the
second step by an exothermic event corresponding to explosive
decomposition. The experimental mass losses of around 6.5% in
the first TG steps are in good agreement with that calculated for
the release of the water molecules [Δmcalc(water) = 6.6% for 3
and 6.3% for 4 and 5]. Based on these calculations it can be
assumed that anhydrates are formed in these steps.
Investigations by ex situ XRPD experiments of all anhydrates
formed as intermediates in the thermal decomposition reactions
show that the crystallinity upon dehydration is not retained. The
formation of amorphous intermediates is observed.
This journal is © The Royal Society of Chemistry 2012
Fig. 9 TG, DSC and MS trend scan curves for compounds 1 (top left),
2 (top right), 3 (bottom left) and 4 (bottom right). Heating rate = 3 °C
min−1; m/z = 18 (water), 28 (N2, decomposition product of tzc), 80 ( pyz
or pym), 182 (dpe); given are the peak temperatures TP/°C. For thermal
properties of compound 5 see Fig. S12, ESI†.
Magnetic investigations
From a magnetic point of view, compounds 1, 3, 4 and 5 endow
interesting magnetic properties due to their superexchange
capacities of the N–N bridges in the binuclear Cu2(N–N)2
entities..33–39 In this context, some recently reported polynuclear
copper(II) compounds based on pyrazole or 1,2,4-triazole ligands
with N–N bridges have shown that these exchange interactions
are mostly antiferromagnetic.33,34,36–38,40 To investigate the
herein reported compounds for their magnetic properties, the
temperature dependence of their dc and ac susceptibility was
measured (for details see Experimental section).
The global magnetic behavior of compounds 1, 4 and 5 with
their dimeric Cu(II) units are very similar. The χMT and χM vs. T
plots per Cu(II) cation of compound 1 are shown as representative in Fig. 10 (for plots of compounds 4 and 5 see Fig. S13 and
S14, ESI†). On cooling, the value of χM increases smoothly to a
maximum at 6.4 K followed by steeply decreasing values
(Fig. 10: top). This sharp maximum in the χM vs. T curve indicates a long-range antiferromagnetic ordering around TN = 6.4 K
for 1, 3.1 K for 4 and 3.2 K for 5 (Table 3). Further temperature
dependent ac susceptibility measurements with a maximum of
their in-phase part χM′ around the same temperature are in agreement with these findings (Fig. 11) Above this ordering
Dalton Trans., 2012, 41, 4207–4216 | 4213
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Table 2 Fitting parameters for the magnetic susceptibilities according
to different fitting models for compounds 1, 2, 4 and 5
Fig. 10 Temperature dependence (from 300 to 1.8 K) of the magnetic
susceptibility (top) and the product of the magnetic susceptibility and
temperature (bottom) for compound 1. The red lines correspond to the
best fit of a magnetic dimer model.
temperature the χM−1 vs. T curve is essentially linear (Fig. S17,
S20 and S21, ESI†) following the Curie–Weiss law with a Curie
constant of ca. 0.44 cm3 K mol−1 (Table 3). The negative Weiss
constant θ = −4.28 K for 1 and 4 and −1.99 K for 5 (Table 3)
and the continuous decrease of the χMT vs. T values upon
cooling (Fig. 10: bottom) confirms these global antiferromagnetic interactions between the Cu(II) centers. The effective magnetic moments μeff are slightly higher than the spin-only values
of 1.73 μB for a Cu(II) cation (S = ½ and g = 2.00), but it is well
documented that a significant spin–orbit coupling/zero-field
Table 3
Compound
Fitting model
J/cm−1
g
1
2
4
5
Dimer
1D chain
Dimer
Dimer
−3.55
−1.92
−2.00
−2.00
2.15
2.12
2.09
2.09
splitting with g values deviating from 2.00 yields higher effective magnetic moments (Table 3).41 Because well-isolated Cu(II)
dimers are the dominant magnetic units in these compounds, the
magnetic data were fitted by a simple Bleaney–Bowers dimer
model (the Hamiltonian is written as −2JijSiSj ).42 This model
produced an excellent fit to the experimental data over the whole
temperature range (Fig. 10). The magnetic parameters obtained
from the best fit were J = −3.55 cm−1 and g = 2.15 for 1 and J =
−2.00 cm−1 and g = 2.09 for 4 and 5 (Table 2). The negative
J values are consistent with the above mentioned global antiferromagnetic properties. Due to this perfect fit by a dimer model, a
magnetic exchange pathway via the aromatic system of the pyz
ligands in 1 and the dpe and azpy ligand respectively in 4 and 5
can be excluded.
The curve shape of the magnetic susceptibility measurements
of compound 2 are very similar to those described above for 1, 4
and 5. A maximum in its χM (Fig. S15, ESI†) and χM′ (Fig. 11)
vs. T curve defines a long-range antiferromagnetic ordering state
at TN = 4.0 K (Table 3). A negative Weiss constant of
θ = −4.41 K (Table 3) and continuous decreasing χMT values on
cooling (Fig. S15, ESI†) confirm the global antiferromagnetic
behavior. The effective magnetic moment is in the same range as
described above (Table 3), but the effective magnetic exchange
pathway in 2 is different: two potential pathways either via the
μ2-N1,O1:N4,O2 bridging tzc ligands or via the μ-N,N′ bridging
pyz ligands are possible. Regarding the metal–metal separation
through the tzc and pyz ligand, respectively, of 5.606(3) vs.
6.830(4) Å (for details see the Crystal structures section), the
former is the most likely exchange pathway. Taking this thinking
into account, the magnetic data were fitted by a simple Fisher
chain model (the Hamiltonian is written as H = −2JiJi−1).43 This
model produced a very good fit of the experimental data over the
whole temperature range leading to best fit parameters of
J = −1.92 cm−1 and g = 2.12 (Table 2).
For compound 3 the magnetic properties are completely
different: only Curie–Weiss paramagnetism is found (Fig. S16,
ESI†) although powerful magnetic exchange pathways with
either μ2-N1,O1:N2 bridging tzc ligands or μ2-N,N′ bridging
pym ligands are present. The χM−1 vs. T curve is essentially
Fitting parameters for the magnetic susceptibilities according to the Curie–Weiss law for compounds 1–5
Compound
C/cm3 K mol−1
θ/K
μeff (exptl.)/μB
μeff (calc.)41/μB
TN/K
Fit area/K
1
2
3
4
5
0.44
0.44
0.43
0.43
0.42
−4.28
−4.41
−0.25
−4.28
−1.99
1.88
1.87
1.86
1.86
1.83
1.73
1.73
1.73
1.73
1.73
6.40
4.00
—
3.10
3.20
20–300
40–300
1.8–300
20–300
20–300
4214 | Dalton Trans., 2012, 41, 4207–4216
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Fig. 11 Temperature dependence (from 20 to 1.8 K) of the in-phase ac
magnetic susceptibility χM′ for compound 1 (cyan), 2 (blue), 4 (red) and
5 (black).
chains bridged by pyrazine forming 2D layers in compound 2.
Magnetic investigations show that the dimers 1, 3, 4 and 5
exhibit globally consistent antiferromagnetic exchange interactions mediated by the double N–N bridges of the μ2-N1,O1:
N2 bridging tzc ligands. However, in 2 the global magnetic
behavior is different: antiferromagnetic exchange interactions
mediated by the μ2-N1,O1:N4,O2 bridging tzc ligands are cancelled by equivalent ferromagnetic exchange interactions
mediated by the pyrimidine ligands, leading to only quasiCurie–Weiss behavior. Emphasizing the wide ranging coordination diversity and good ability to act as a magnetic mediator of
the tzc ligand, more systematic studies are needed to gain more
detailed structure–property relationships. Thus, investigations on
the influence of different paramagnetic transition metal ions in
similar tzc containing metal–organic magnetic materials will be
part of future studies.
Acknowledgements
linear (Fig. S19, ESI†) and a fit according to the Curie–Weiss
law yields a Weiss constant around zero (Table 3). On cooling,
the χMT values are temperature independent up to 10 K followed
by very steeply decreasing values on further cooling (Fig. S16,
ESI†). The latter behavior can be traced back to antiferromagnetic interlayer dipole–dipole exchange interactions between
adjacent Cu(II) centers and the former quasi Curie behavior can
be explained by assuming that the antiferromagnetic interaction
mediated by the μ2-N1,O1:N2 bridging tzc ligands are cancelled
by equivalent ferromagnetic interactions mediated by the μ2-N,
N′ bridging pym ligands.15,44–46
Systematic comparisons with literature known magnetic
copper(II) materials based on tzc are difficult to perform. Only
one compound, [Cu2(tzc)2(H2O)6]·2H2O, containing similar
dimeric Cu(II) units with μ2-N1,O1:N2 bridging tzc ligands is
characterized, with its magnetic properties showing similar magnetic behavior as described above for 1, 4 and 5.28 However, it is
reported that for this kind of dinuclear copper(II) complexes, the
difference between Cu–N1–N2 and Cu–N2–N1 angles (δ) is
related with the value of J, such that J significantly decreases
with increasing the asymmetry δ.47 According to significant δ
values of 21.5° in 1, 20.5° in 4 and 20.7° in 5, the drastic
reduction in the antiferromagnetic interaction with small negative
J values can be explained. Same findings are described for
[Cu2(tzc)2(H2O)6]·2H2O.28
Conclusion
In this contribution five new metal–organic magnetic materials
based on copper(II) tetrazolate-5-carboxylate with different
neutral N-donor spacer ligands have been investigated for their
structural properties, thermal reactivity and magnetic behavior.
Depending on the reaction conditions and the nature of the
N-donor spacer ligands, two different coordination modes of the
tzc ligand lead to the formation of two main structural building
units: dimeric [Cu2(Ntzc–Ntzc)2] entities bridged by various
spacer ligands forming 1D chains in compounds 1, 4 and 5 and
2D layers, respectively, in compound 3, and 1D [Cu–tzc–Cu]n
This journal is © The Royal Society of Chemistry 2012
Funding was provided by the Welch Foundation (A-1725).
Dr Mario Wriedt thanks the Postdoc-Programme of the German
Academic Exchange Service (DAAD) for his financial support.
We thank Professor Dr Karen L. Wooly and Professor Dr Kim
Dunbar for the access to their experimental facilities and in this
context special thanks to Kevin Pollack for his support in the
TG-DSC-MS measurements and to Dr Andrey Prosvirin for his
support in the magnetic measurements. The magnetic measurements were conducted in the Department of Chemistry SQUID
Facility and the magnetometer was purchased with a grant from
the National Science Foundation (CHE-9974899).
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