Low-dimensional compounds containing cyano groups. XVII. Crystal structure, spectroscopic, thermal and magnetic properties of [Cu(bmen)2][Pt(CN)4] (bmen=N,N′-dimethylethylenediamine)

Article abstract of DOI:10.1016/j.jssc.2008.10.017

The synthesis, structural analysis, spectroscopic studies, susceptibility and specific-heat measurements of {[Cu(bmen)₂][Pt(CN)₄]}_n (bmen=N,N′-dimethylethylenediamine) are presented. X-ray crystal-structure analysis reveal

Full text of DOI:10.1016/j.jssc.2008.10.017

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Journal of Solid State Chemistry 182 (2009) 196–202
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Journal of Solid State Chemistry
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Low-dimensional compounds containing cyano groups. XVII. Crystal
structure, spectroscopic, thermal and magnetic properties of
[Cu(bmen)2][Pt(CN)4] (bmen ¼ N,N0-dimethylethylenediamine)
b
a,
Ã
a
b,c
b
ˇ
ˇ ˇ ´
ˇ
´
ˇ
´
´
´
Ivan Potocnak , Martin Vavra , Erik Cizmar , Marcela Kajnakova , Alena Radvakova ,
Dirk Steinborn d, Sergei A. Zvyagin c, Jochen Wosnitza c, Alexander Feher b
a
ˇ
´
ˇ
Department of Inorganic Chemistry, Institute of Chemistry, P.J. Safarik University, Moyzesova 11, SK-04154 Kosice, Slovakia
b
ˇ
´
ˇ
Centre of Low Temperature Physics of the Faculty of Science of P.J. Safarik University and IEP SAS, Park Angelinum 9, SK-04154 Kosice, Slovakia
c Dresden High Magnetic Field Laboratory (HLD), Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany
d Institute of Inorganic Chemistry, Martin-Luther-University, Halle-Wittenberg, Kurt-Mothes-Str. 2, D-06120 Halle, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, structural analysis, spectroscopic studies, susceptibility and specific-heat measurements
of {[Cu(bmen)2][Pt(CN)4]}n (bmen ¼ N,N0-dimethylethylenediamine) are presented. X-ray crystal-
structure analysis revealed that the [Pt(CN)4]2ꢀ building blocks are combined with [Cu(bmen)2]2+ units
to form a chain-like structure along the a axis. The Cu(II) atoms are hexacoordinated by four nitrogen
atoms in the equatorial plane belonging to two molecules of bidentate bmen ligands with average Cu–N
Received 23 June 2008
Received in revised form
1 October 2008
Accepted 12 October 2008
Available online 25 October 2008
˚
distance of 2.043(18) A. The axial positions are occupied by two nitrogen atoms from bridging
[Pt(CN)4]2ꢀ anions at a longer axial Cu–N distance of 2.490(4) A. The compound is characterized by the
Keywords:
˚
1D crystal structure
[Pt(CN)4]2ꢀ anion
Hydrogen bonds
presence of a weak antiferromagnetic exchange coupling J/kB ¼ 0.6 K. Despite the one-dimensional (1D)
character of the structure, the analysis of the magnetic properties and specific heat at very low
temperatures shows that [Cu(bmen)2][Pt(CN)4] behaves as a two-dimensional (2D) square-lattice
Heisenberg magnet with weak interlayer coupling.
Thermal studies
Spectroscopic studies
Heisenberg square lattice
& 2008 Elsevier Inc. All rights reserved.
1. Introduction
function, also exhibits an important electronic function: it forms
an exchange path mediating the interaction among spins localized
on paramagnetic centers. The dimensionality of the magnetic
subsystem is fundamental for cooperative processes, but it does
not mean that the magnetic and structural dimensionality must
be the same. The increase of magnetic dimensionality can be
The field of transition metal cyanide chemistry has a remark-
able history that spans almost three centuries, dating back to the
earlier eighteenth century. The wide availability of cyano com-
plexes together with their diverse bonding and structural
chemistry has led to their widespread applications in the field
of materials chemistry [1]. Several detailed reviews on metal
cyanides describing their structures, reactivity and physical
properties have been published over the years [2–4]. A review
on the coordination modes of the [Pt(CN)4]2ꢀ anion has been
presented in our previous paper [5].
The ability of the cyano group to link various metal ions leads
to a wide diversity of structural architectures ranging from
discrete oligonuclear complexes to various more-dimensional
networks. Recently, cyano complexes with various degrees of
structural dimensionality have often been the subject of magnetic
studies. In this case, the cyano group or cyano complex anion
(with a diamagnetic central atom), in addition to its structural
caused by the influence of hydrogen bonds (HBs),
dipole–dipole interactions [6,7].
p–p and also
Previous studies of [Cu(L)2][M(CN)4] (MQNi, Pd and Pt; L is
ethylenediamine ¼ en; N-methylethylenediamine ¼ men; asym-
metric N,N-dimethylethylenediamine ¼ dmen; symmetric N,N0-
dimethylethylenediamine ¼ bmen) revealed that despite the 1D
character of their structure they behave as 2D Heisenberg
antiferromagnets (HAF) [5,8–11]. The 2D character of the
magnetic subsystem can be explained by the formation of HBs
linking the neighboring chains and thus serving as exchange paths
for magnetic interactions. In the isotropic case of the 2D HAF
model, thermal fluctuations destroy magnetic long-range order at
any finite temperature as known from the Mermin–Wagner
theorem [12]. However, the presence of even very low uniaxial
anisotropy of Ising type, induces a finite-temperature phase
Ã
´
transition into the Neel ordered state [13]. An anisotropy of the
Corresponding author. Fax: +42155 62 22124.
ˇ ˇ ´
E-mail address: ivan.potocnak@upjs.sk (I. Potocnak).
XY type leads to a Berezinski–Kosterlitz–Thouless (BKT) transition
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.10.017

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197
[14–17] at finite temperature TBKT. Nevertheless, it is very difficult
Table 1
Crystal data and structure refinement of 1.
to observe a BKT transition in real magnets due to the presence of
interlayer coupling, which induces a second-order phase transi-
tion into a state with 3D magnetic order. In this paper, we present
the results of infrared and UV–VIS spectroscopy, thermal and
X-ray crystal-structure analyses, electron spin resonance,
magnetic and specific-heat measurements of the new complex
[Cu(bmen)2][Pt(CN)4] (1), which was synthesized with the aim to
tune the ratio of interlayer and intralayer exchange couplings by
suppressing some of the HBs in comparison to materials studied
in our previous work [5].
Empirical formula
Formula weight
Temperature
C12H24N8CuPt
539.01
293(2) K
˚
Wavelength
0.71073 A
Orthorhombic
Pbca
Crystal system
Space group
˚
Unit cell dimensions
10.2938(4) A
˚
12.5923(5) A
˚
13.5164(5) A
3
˚
Volume
1752.03(12) A
Z; density (calculated)
Absorption coefficient
F(000)
4; 2.043 g cmꢀ3
9.198 mmꢀ1
1036
Crystal shape, color
Crystal size
Needle, blue
2. Experimental
0.43 ꢂ 0.15 ꢂ 0.11 mm
2.971–25.501
ꢀ12php11, ꢀ6pkp15, ꢀ16plp16
Analytical
y
range for data collection
2.1. Materials
Index ranges
Absorption correction
Max. and min. transmission
Reflections collected/unique
Refinement method
0.396 and 0.214
5131/1638 [R(int) ¼ 0.0166]
Full-matrix least-squares on F2
1638/0/105
Copper chloride dihydrate (CuCl2 ꢁ 2H2O) and bmen (C4H12N2)
were of Merck quality and used as received. K2[Pt(CN)4] ꢁ
3H2O was prepared according to the literature [18] using
H2[PtCl6] ꢁ 6H2O from Chempur as starting platinum-containing
material.
Data/restraints/parameters
Goodness-of-fit on F2
0.982
Final R factors [I42
R factors (all data)
Hydrogen atoms
s(I)]
R1 ¼ 0.0151, wR2 ¼ 0.0381
R1 ¼ 0.0527, wR2 ¼ 0.0430
Constrained
Weighting scheme
w ¼ 1/[
s
2(Fo2)+(0.0180P)2] where P ¼ (Fo2+2F2c)/3
2.2. Synthesis
–3
˚
Largest diff. peak and hole
0.290 and ꢀ0.488 eA
Into stirring 10 ml water–methanol solution, (1:1) of CuCl2
(0.5 mmol; 0.085 g of CuCl2 ꢁ 2H2O), bmen (2 mmol; 0.22 ml
of bmen) was added and finally 10 ml aqueous solution of
K2[Pt(CN)4] (0.5 mmol; 0.216 g of K2[Pt(CN)4] ꢁ 3H2O) was added
after 30 min. As a result a blue precipitate was immediately
formed. This precipitate was dissolved by the addition of 20 ml
of concentrated ammonia solution to the reaction mixture.
After 3 days, blue crystals of 1 were filtered off and dried on air.
Yield—86%. Anal. Calc. for C12H24N8CuPt—C: 26.74; H: 4.49; N:
20.78. Found—C: 26.44; H: 4.96; N: 20.43%.
2.4. X-ray data collection and structure refinement
A summary of crystal data and structure refinement for 1 is
presented in Table 1. The crystal structure of 1 was determined
using an Oxford Diffraction Xcalibur2 diffractometer equipped
with a Sapphire2 CCD detector. Crysalis CCD [20] was used for
data collection while Crysalis RED [20] was used for the cell
refinement, data reduction and absorption correction. The
structure was solved by the direct method with SHELXS97 [21]
and subsequent Fourier syntheses using SHELXL97 [21]. The
anisotropic displacement parameters were refined for all non-H
atoms. The H atoms were placed in calculated positions and
refined riding on their parent C or N atoms with C–H distances of
2.3. Physical measurements
Elemental analysis for C, H and N was carried out using an
LECO CHNS-932. The IR spectrum was recorded on an Avatar 330
FT-IR Spectrometer from ThermoNicolet by the method of KBr
pellets in the range from 4000 to 400 cmꢀ1. Reflectance UV–VIS
spectrum was measured with Perkin–Elmer UV/VIS/NIR Spectro-
meter Lambda 19 in the range from 200 to 1100 nm. The thermal
investigation was performed using NETZSCH STA 409 PC/PG
thermal analyzer under dynamic conditions in nitrogen atmo-
sphere with a heating rate of 6 1C/min to 700 1C using 30.094 mg
of the sample and Al2O3 crucibles. ESR data were collected at low
temperatures in an X-band Bruker ELEXSYS E500 spectrometer.
Measurements of the magnetic susceptibility have been carried
out using a commercial SQUID magnetometer in the temperature
range from 2 to 300 K. The background contribution from the
gelatine capsule to the magnetic moment of the sample has been
subtracted. The specific heat of [Cu(bmen)2][Pt(CN)4] was studied
in the temperature range from 125 mK to 3 K in zero magnetic
field using a dual-slope technique [19] in a dilution refrigerator
made by Oxford Instruments (model TLE 2000). For monitoring
the temperature of the sample a RuO2 resistor of Dale type with a
˚
0.97 and 0.96 A for methylene and methyl H atoms, respectively,
˚
and with N–H distances of 0.91 A for amino H atoms, and
Uiso(H) ¼ 1.2Ueq (C or N). A geometric analysis was performed
using SHELXL97 and PARST [22]. DIAMOND [23] was used for
molecular graphics.
3. Results and discussion
3.1. Infrared spectroscopy
Several significant bands appear in the IR spectrum of 1
(Table 2), including the
n
(N–H),
t
n
(C–H),
n
(CSN), (–CH3),
d
o
(–CH2–),
t
(–CH2–),
t
(–CH3), (–NHR),
n
(C–N),
n(C–C), s(–CH2–)
and
n
(Pt–C) vibrations which confirm the presence of bmen and
the [Pt(CN)4]2ꢀ anion. Individual bands were assigned according
to [24,25]. Special attention is paid to the
n(CSN) stretching
vibrations which prove the presence of [Pt(CN)4]2ꢀ in the
prepared compound and whose positions are an important tool
to distinguish between terminal and bridging character of cyano
groups. This position is observed at 2080 cmꢀ1 in ionic KCN as
a single absorption band [25]. Upon coordination to a metal,
nominal value of 1 kO calibrated against a commercial Lake Shore
thermometer GR200A-30 was used. The experimental inaccuracy
of the specific-heat measurements is less then 4%. The ESR and
magnetic measurements have been carried out on powdered
samples, while for the thermodynamic experiments pelletized
powder samples have been used.
the
n(CSN) mode shifts to higher frequencies and the range of
n(CSN) for cyanoplatinates(II) with terminal cyano ligands

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198
Table 2
IR spectrum of 1 (cmꢀ1).
n
(N–H)
(C–H)
3301 vs
3206 vs
3014 w
2967 m
2940 s
n
2884 m
n
(CSN)
2132 s
2122 vs
1481 s
1467 m
1453 s
1435 m
1424 sh
1372 m
1279 m
1148 m
1084 s
1036 s
1024 s
966 vs
861 m
827 w
499 m
451 w
d
(–CH3)
o
(–CH2–)
t(–CH2–)
t(–CH3)
t(–NHR)
n(C–N)
250
300
350
400
450
500
550
Wavelength [nm]
n(C–C)
Fig. 1. Reflectance UV–VIS spectrum of 1.
s
(–CH2–)
n(Pt–C)
397 m
lapped by the vibrations of terminal cyano groups. Therefore, only
one (CSN) absorption band in their IR spectra has been
observed.
n
n—stretching,
d—scissoring, o—wagging, t—twisting, s—rocking vibrations,
vs—very strong, s—strong, m—middle, w—weak vibrations, sh—shoulder.
3.2. UV–VIS spectroscopy
extends from 2120 to 2140 cmꢀ1 [26]. Generally, if the M–CSN
group of an [M(CN)4]2ꢀ anion forms an M–CSN–M0-type bridge,
The reflectance UV–VIS spectrum of 1 (Fig. 1) was taken
as described earlier. A broad almost symmetric band with a
maximum at 397 nm is observed in the spectrum. This could be
the
single
n
(CSN) band shifts to even higher frequency [25], and a
(CSN) band can be split into two bands due to the
n
assigned to the 2B1g-2Eg transition suggesting
a deformed
presence of the terminal and bridging cyano groups in the
octahedral coordination of the CuN6 chromophoric group.
complex [27,28]. According to this, the bands recorded at 2122
and 2132 cmꢀ1 in 1, can be assigned to
n(CSN) stretching
vibrations of terminal and bridging cyano groups, respectively.
The intensity of both bands is almost the same, in agreement with
the same number of terminal and bridging cyano groups in the
3.3. Thermal studies
structure of 1. Two
n
(CSN) vibrations have also been observed in
The usual way of thermal decomposition of cyano complexes is
characterized by liberation of N-donor ligands followed by
separated decomposition of all cyano groups in one step. In
accordance with this and with the results of thermal decomposi-
tions of previously studied similar compounds [5], the thermal
decomposition of 1 is a multistage process. The compound 1 is
stable up to 202 1C. In the temperature range 202–300 1C, a weight
loss of 17.0% corresponding to the release of one bmen molecule
(calc. 16.4%) is observed during the overlapping of slightly
endothermic and exothermic stages as evidenced by a minimum
and a maximum in the differential thermal analysis (DTA) at 233
and 248 1C, respectively. Within 300–488 1C the decomposition
continues with liberating the second bmen molecule, reflected as
very small endothermic effect followed by a slight exothermic
process (minimum and maximum in DTA at 350 and 460 1C,
respectively). The release of the second bmen molecule is over-
lapping with a strongly exothermic decomposition of the cyanides
completed at 670 1C. The total weight loss of these stages is 29.0%
that corresponds to the decomposition of one bmen molecule and,
contrary to the previous decompositions of similar compounds,
only three of four cyano groups (calc. 14.5%). Their decomposition
includes the oxidation of cyano groups to form one and a half
cyanogen molecules during the reduction of one Pt(II) to a Pt atom
and one Cu(II) to a Cu(I) atom. The final thermal decomposition
product is a mixture of CuCN and metallic Pt (solid residue 54.0%;
calculated 52.8%). The most probable thermal decomposition
other [Cu(bmen)2][M(CN)4] (MQNi and Pd) [10,11]) compounds.
On the other hand, in the spectra of [Cu(en)2][Pt(CN)4] [5] and
[Cu(dmen)2][M(CN)4] compounds (MQNi, Pd and Pt) [5,10,29]
only one
n(CSN) absorption band has been observed. The
discrepancy in the number of observed vibrations may be
explained by the different local symmetry of [Pt(CN)4]2ꢀ anions
in the discussed compounds, whereas the CSN bonds in
[Cu(en)2][Pt(CN)4] and [Cu(dmen)2][M(CN)4] compounds differ
˚
˚
only by 0.002 A, corresponding difference in 1 is 0.016 A. Thus,
the local symmetry of [Pt(CN)4]2ꢀ anions in [Cu(en)2][Pt(CN)4]
and [Cu(dmen)2][M(CN)4] compounds is D4h with only one Eu IR
active vibration and the local symmetry of [Pt(CN)4]2ꢀ anion in 1
is D2h with B2u and B3u IR active vibrations. The different local
symmetry is also reflected by the different Cu–N(cyano) bond
lengths in 1 and [Cu(bmen)2][M(CN)4] compounds on the one
hand, and [Cu(en)2][Pt(CN)4] and [Cu(dmen)2][M(CN)4] com-
pounds on the other hand. The corresponding Cu–N(cyano)
˚
bond length in 1 is 2.490(4) A and analogous distances in
other [Cu(bmen)2][M(CN)4] compounds are only a little shorter.
However, these distances in the [Cu(en)2][Pt(CN)4] and
[Cu(dmen)2][M(CN)4] compounds are within the range of
˚
2.537(1)–2.600(5) A and, therefore, the bridging cyano groups
adopt partially ionic character and their behavior is rather close to
the terminal one. In accordance, the wavenumbers of the
corresponding vibrations decrease and eventually may be over-

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199
scheme for 1 could be given as:
The Cu–N2 bonds in the axial positions are slightly tilted from the
normal to the CuN4 plane (the tilt angle between the apical axis
and the normal to the plane is 4.401), forming a slightly distorted
and tetragonally elongated octahedral geometry.
ꢄC
ꢄ
½CuðbmenÞ2ꢃ½PtðCNÞ4ꢃ 202ꢀ2!300 ½CuðbmenÞPtðCNÞ4ꢃ 300ꢀ2!488 C CuPtðCNÞ4
ꢀbmen
ꢀbmen
The geometry of the [Pt(CN)4]2ꢀ anion is square planar, with
C1–Pt–C2 bond angle of 90.81(17). The Pt–C and CSN bond
distances (Table 3) are in an expected range, with regard to both
bridging and terminal modes, with only small differences to the
other structures [5,30]. The Pt–CSN bond angles are almost
ꢄ
CuPtðCNÞ4 488ꢀ2!670 C CuCN þ Pt
ꢀ1:5ðCNÞ2
3.4. X-ray crystallography
linear what is also in
compounds [5,30].
a very good agreement with other
X-ray analysis has revealed that the crystal structure of 1
consists of 1D zigzag chains running along the [100] direction. An
ORTEP view of this compound is depicted in Fig. 2 with the atomic
numbering scheme; selected bond lengths and bond angles of 1
are given in Table 3. The structure of 1 is similar to the structures
of compounds with the general formula [Cu(L)2][M(CN)4] men-
tioned above. In 1, both Cu(II) and Pt(II) ions sit on inversion
centers. The Cu(II) ion has a six-fold coordination, consisting of
the four nitrogen atoms of two bmen ligands in the equatorial
plane and two nitrogen atoms from different bridging cyano
groups in the axial positions. The Cu–N bond distances in the
The structure of 1 is stabilized by a pair of weak HBs shown in
Fig. 3. One of them, the interchain HB N10–H10?N1, with an
˚
N10?N1 length of 3.117(5) A (Table 4), connects the chains into
2D sheets in the ab plane; a similar situation was observed in the
structure of [Cu(dmen)2][Pt(CN)4] too, which contains an asym-
metric N,N-dimethylethylenediamine. In comparison to that
˚
structure, the N20?N1 distance of 3.247(5) A (Table 4) indicates
an N20–H20?N1 intrachain HB interaction in 1 which was
not observed in the structure containing dmen. We can conclude
that due to the presence of one hydrogen atom on each nitrogen
donor atom of bmen there is one additional intrachain HB
interaction. Nevertheless, the HB system in 1 is strongly reduced
in comparison to the similar compound with en [5].
˚
equatorial plane have an average value of 2.043(18) A, while weak
axial interactions, characteristic of d9 systems, give rise to
˚
a
Cu–N(cyano) bond length of 2.490(4) A. Cu–N(amine)
bond lengths are in comparison with other [Cu(L)2][Pt(CN)4]
compounds either equal (LQen) or different (LQdmen) [5].
N10i
N1i
N20
N1ii
Pt
N2
Cu
H20
N20
C
H
Cu
N
Pt
C2
N20i
N2i
H10
N10
N2ii
C1
N1
N1iii
b
N10
a
c
Fig. 2. Structure of 1. Displacement ellipsoids are plotted at the 50% probability
level and H atoms are shown as small spheres of arbitrary radii. (Symmetry codes:
(i) 1ꢀx, ꢀy, ꢀz; (ii) ꢀx, ꢀy, ꢀz.)
Fig. 3. HB system in 1 viewed along the chain direction. The interchain HBs
connecting chains into sheets are represented by black dashed lines, while
intrachain HBs are represented by empty dashed lines. Only hydrogen atoms of
amino groups are shown for clarity. (Symmetry codes: (i) 1ꢀx, ꢀy, ꢀz; (iii) 1/2+x,
ꢀ1/2ꢀy, ꢀz.)
Table 3
˚
Selected bond lengths (A) and bond angles (1) of 1.
Cu–N2
2.490 (4)
2.056 (4)
2.030 (4)
1.995 (5)
1.993 (5)
1.122 (5)
1.138 (5)
89.19 (17)
135.6 (4)
178.6 (4)
175.1 (4)
84.63 (15)
94.40 (13)
90.83 (13)
Cu–N10
Cu–N20
Pt–C1
Pt–C2
Table 4
C1–N1
˚
Hydrogen bonds for 1 [A and 1].
C2–N2
C1–Pt–C2
Cu–N2–C2
N1–C1–Pt
N2–C2–Pt
N20–Cu–N10
N20–Cu–N2
N10–Cu–N2
D–H?A
D–H
H?A
D?A
D–H?A
N20–H20?N1i
N10–H10?N1iii
0.91
0.91
2.48
2.26
3.247 (5)
3.117 (5)
142.7
157.3
Symmetry transformations used to generate equivalent atoms: (i) 1ꢀx, ꢀy, ꢀz; (iii)
1/2+x, ꢀ1/2ꢀy, ꢀz.

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2.0
1
0
25
1.5
g = 2.08 0.01
= - 0.87 K 0.1
20
ꢀ
B
15
μ
-1
-2
-3
/
1.0
ef
10
5
μ
experiment
simulation
0.5
0.0
0
0
20
40
60
T (K)
80
100
250
300
350
400
0
50
100
150
T (K)
200
250
300
Field (mT)
Fig. 4. Comparison of the experimental ESR spectrum of [Cu(bmen)2][Pt(CN)4]
measured at 9.4 GHz and the EasySpin [31] simulation for gx ¼ 2.0570.005,
Fig. 5. Effective magnetic moment of [Cu(bmen)2][Pt(CN)4]. The inset shows the
temperature dependence of the inverse susceptibility (circles) with a fit to the
Curie–Weiss law (solid line).
gy ¼ 2.0970.005, gz ¼ 2.1870.005 and
broadening parameters
DB ¼ 3.570.2 mT including the anisotropic
D
Bx ¼ 1.870.2 mT, DBy ¼ 2.870.2 mT and DBz ¼ 6.870.2
mT due to unresolved hyperfine coupling.
4
3.5. Magnetic properties
[Cu(bmen)2][Pt(CN)4]
The ESR spectra of 1 obtained at 2 K on powdered samples
(Fig. 4) have been analyzed using the ESR spectra simulation
package EasySpin [31]. The best fit to the experimental data using
linear chain,J/kB = - 0.77 K
3
square lattice,J/kB = - 0.6 K
a
least-squares method (Fig. 4) has been obtained for the
quasi-2D lattice
J/kB = - 0.6 K,J⊥/J = 2-6
following set of parameters: gx ¼ 2.0570.005, gy ¼ 2.0970.005,
gz ¼ 2.1870.005 and simple isotropic linewidth B ¼ 3.570.2 mT.
D
2
The hyperfine structure expected for Cu(II) atoms could not be
resolved in the spectrum suggesting the existence of exchange
couplings between magnetic centers [32], but its influence is
taken into account using the anisotropic broadening parameters
1
0
D
Bx ¼ 1.870.2 mT,
DBy ¼ 2.870.2 mT and
DBz ¼ 6.870.2 mT.
The anisotropic g-factors are consistent with the axial type
of anisotropy due to the Jahn–Teller effect typical for spin
S ¼ 1/2 Cu(II) ions including the influence of a weak rhombic
distortion of the coordination octahedron. The deviations between
the fit and data might be attributed to the low-dimensional
character of the magnetic subsystem. The results confirm that
the electronic ground state of the Cu(II) ion is described by
a wave function of dz2 symmetry and the exchange paths will
propagate along the directions determined by the lobes of the
dx2ꢀy2 orbital.
0
1
2
3
T (K)
Fig. 6. Magnetic specific heat of [Cu(bmen)2][Pt(CN)4] compared to the HAF linear-
chain model with J/kB ¼ ꢀ0.77 K (dotted line), while the dashed line represents the
HAF square-lattice model with J/kB ¼ ꢀ0.6 K. The solid line represents the quasi-2D
HAF model with intralayer exchange interaction J/kB ¼ ꢀ0.6 K and an interlayer
exchange interaction given by J /J ¼ 2ꢀ6ffi0.0156.
?
The magnetic susceptibility of a powdered sample measured at
0.1 T was corrected for the diamagnetic contribution of the
material which was estimated using Pascal’s constants [33] to
be wDIA ¼ ꢀ2.9586 ꢂ10ꢀ9 m3 molꢀ1. From the susceptibility at
T ¼ 300 K, the effective magnetic moment may be quantified
and yields a value typical for a Cu(II) cation with d9 configuration,
The specific heat of 1 has been investigated in zero magnetic
field in the temperature range 125 mK–3 K with the aim to clarify
the magnetic structure and dimensionality of the systems. Since
the compound is a magnetic insulator, only the lattice contribu-
tion to the total specific heat was subtracted after fitting the high-
temperature region to the equation C ¼ aTꢀ2+bT3, where aTꢀ2
describes the high-temperature expansion of the magnetic
specific heat, while bT3 represents the low-temperature descrip-
tion of the lattice contribution in the Debye approximation. The
best fit was obtained for a ¼ 1.101 J Kmolꢀ1 and b ¼ 0.0211 J Kꢀ4
molꢀ1. The temperature dependence of the magnetic specific heat
namely meff mB ¼ 1.85 (see Fig. 5). No difference between the
/
susceptibility of the zero-field cooled and field-cooled sample has
been observed down to 2 K, which suggests that no long-range
order appears above 2 K. The temperature dependence of the
susceptibility is characterized by a Curie-like behavior without
the short-range-order maximum expected at low temperatures
for the low-dimensional system with antiferromagnetic exchange
coupling. Consequently, only an estimate of the effective strength
of the exchange coupling zJ/kB, where z is the number of the
nearest neighbors, by fitting the Curie–Weiss law to the experi-
mental data in the temperature range 2–100 K is obtained. The fit
yields the values zJ/kB ¼ ꢀ3.4870.05 K and g ¼ 2.0870.01
(inset in Fig. 5).
displays a round maximum at Tmax ¼ 0.37 K and a
l-like anomaly,
associated with long-range ordering, at T ¼ 0.22 K (Fig. 6). After
l
subtraction of the lattice contribution, the magnetic entropy has
been calculated from the experimental data with simple extra-
polations used for T-0 and T-N. The calculated magnetic
entropy reaches the theoretically predicted value Stheor ¼ R

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201
ln(2S+1) ¼ 5.76 J Kꢀ1 molꢀ1 for magnetic systems with spin
unit. The UV–VIS spectrum of 1 indicates the presence of six-
coordinated Cu(II) atoms in the form of deformed octahedrons
what was definitely confirmed by an X-ray analysis. The structure
is formed by infinite zigzag covalent chains, parallel to the a axis,
which are bound into sheets, lying in ab plane, by relatively weak
hydrogen bonds of N–H?N type.
S ¼ 1/2. The magnetic entropy removed above T represents 86%
l
of the total magnetic entropy suggesting a low-dimensional
character of the studied systems.
Considering the topology of covalent and possible HBs, the
magnetic specific heat of [Cu(bmen)2][Pt(CN)4] has been com-
pared to the theoretical models for a HAF linear-chain [34], an
isotropic HAF square-lattice [35,36] and quasi-2D HAF square-
lattice magnet [37] (Fig. 6). The exchange coupling parameters
obtained from the comparison are J/kB ¼ ꢀ0.77 and ꢀ0.6 K for the
HAF linear-chain model and the HAF square-lattice model,
respectively. Despite the chain-like crystal structure of the title
compound better agreement is obtained for the HAF square lattice
models, suggesting the dominant role of HBs mediating the
exchange interaction in the directions of the magnetic dx2ꢀy2
orbitals oriented within the equatorial plane of the local
octahedron of the Cu(II) atoms. The values of the exchange
couplings are in good agreement with the estimate of the effective
strength of the exchange coupling from the Curie constant.
As mentioned above, no magnetic long-range order at any
finite temperature should be observed for an isotropic 2D HAF
The analysis of the magnetic properties of 1 suggests that
although having a chain-like crystal structure, the title compound
exhibits
a 2D magnetic structure with exchange coupling
J/kB ¼ ꢀ0.6 K, including the influence of very weak interlayer
exchange coupling. The 2D magnetic structure can be explained
by the fact that the magnetic dx2ꢀy2 orbital is oriented within the
equatorial plane of the local octahedron due to the Jahn-Teller
distortion and is not involved in the covalent bonds. Consequently,
the exchange path between the Cu(II) atoms is preferred through
hydrogen bonds creating a square network and not through the
covalent bonds within the chains.
Supplementary material
CCDC 692343 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033;
e-mail: deposit@ccdc.cam.ac.uk).
model [12] and the origin of the
l-like anomaly should be
discussed. First, we estimate the value of a possible exchange
anisotropy
D
/J ¼ ((gJꢀg )/g)2 ¼ 0.0027, where J? ¼ (1+
D)J, in-
?
duced by the anisotropy of the g-factor [38]. Even such a weak
easy-axis exchange anisotropy leads to a 2D long-range order of
Ising type [13]. Nevertheless, the estimated value of the exchange
coupling J/kB ¼ ꢀ0.6 K for the square-lattice model is about 2/3 of
the expected value for a 2D Ising transition to appear at 0.22 K
[13]. In addition, the effect of a very weak interlayer interaction in
a 2D HAF is to induce a transition to a 3D long-range ordered state
Acknowledgments
This work was supported by the Grants of the Slovak Grant
Agency VEGA nos. 1/0079/08 and 1/3027/06, by the Research and
Development Support Agency APVV nos. 0006-07 and 0058-07.
Part of this work was supported by the DFG. One of the authors
(M.V.) thanks DAAD for financial support and hospitality of
Martin-Luther-University. The financial support of US Steel—DZ
at finite temperature with a tiny
work of Sengupta [37]. The height of the maximum at Tmax ¼ 0.37
K and the position of the
-like anomaly at T ¼ 0.22 K suggests
l-like anomaly as shown in the
l
l
the influence of an interlayer coupling with a strength less then
J
¼ 10 mK (J /J ¼ 2ꢀ6). In the range 0.001pJ /Jp1 we can
?
?
?
ˇ
Energetika Kosice is acknowledged.
estimate the interlayer coupling from the critical temperature
using the formula T ¼ 4prS/[2.43ꢀln(J /J)], where rS ¼ 0.183J is
l
?
the spin stiffness [39], resulting in J ¼ 6.5 mK. This value is
?
Appendix A. Supplementary material
slightly higher than the maximum magnitude of the dipolar
interaction between the layers, Jdip ¼ 2.5 mK, estimated using a
simple approach [40].
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2008.10.017.
To elucidate the exact role of the anisotropy and the interlayer
interaction in the formation of the magnetic long-range order at T
l
it would be necessary to study the critical coefficients in the
vicinity of the critical temperature, which was not possible from
the measurements on the powdered sample due to the rounding
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