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Cent. Eur. J. Chem. • 12(6) • 2014 • 652-658
DOI: 10.2478/s11532-014-0517-3
Central European Journal of Chemistry
Unusual potassium-oxalate coordination
in the two-dimensional trimetallic
[CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex
Short Communication
Grzegorz Wrzeszcz*, Andrzej Wojtczak, Magdalena Zawadzka
Faculty of Chemistry,
Nicolaus Copernicus University,
87-100 Toruń, Poland
Received 20 June 2013; Accepted 2 January 2014
Abstract: A new heterometallic compound, [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1), has been synthesized and characterized by elemental analysis, IR
and electronic spectra, thermal analysis, variable temperature magnetic susceptibility measurements, and single crystal X-ray diffraction.
Compound 1 consists of two-dimensional [{KCr(C2O4)3}n]2n− layers, [CoCl(NH3)5]2+ ions and water molecules. Within the 2-D layer,
three different types of oxalate coordination modes are present. Each K cation is coordinated by eight oxygen atoms from oxalate groups
and also weakly interacts with the ninth oxygen atom. The extensive network of hydrogen bond is formed between the [KCr(C2O4)3]2–
layer and the [CoCl(NH3)5]2+ ions. These interactions involve all hydrogen atoms of ammonia ligands and water molecule.
Keywords: Crystal structure • Oxalate bridge • Coordination mode • Spectroscopy • Hydrogen bonding
© Versita Sp. z o.o.
1. Introduction
The tris(oxalato) complexes [MIII(C2O4)3]3− (M = Cr, Fe,
Co, ...) have received considerable interest in the last
two decades. They have been used in the fields of
(i) molecular organic/inorganic hybrid materials with
organic π-donors [1-3], (ii) molecular based magnetic
materials as building block to prepare of poly- and
bimetallic compounds [4-6], (iii) magneto- and non-linear
optics [7,8], and (iv) microporous materials [9]. Special
attention is dedicated to polyfunctional oxalate bridged
materials [1-3,7,10].
The oxalate anion is a very versatile ligand that can
adopt many kinds of coordination modes: unidentate,
bidentate chelate to one metal centre and bridging
[11-13]. The remarkable ability of the oxalate as a
bridging ligand has played a key role in the development
of new coordination compounds, which have been of
high interest not only due to their properties and potential
applications but also due to their fascinating topologies
and intriguing structure features.
As a continuation of our studies on oxalate complexes
[14-16], in this paper, we describe the synthesis, X-ray
structure, IR and electronic spectra, thermal, and magnetic
characterization of new two-dimensional heterometallic
K(I)-Cr(III)-Co(III) complex derived from tris(oxalato)
chromate(III), namely [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O.
2. Experimental procedure
2.1. Materials
The compounds K3[Cr(C2O4)3]•3H2O [17] and [CoCl(NH3)5]
Cl2 [18] were prepared according to the literature
methods. Other reagents used in the syntheses were of
analytical grade and used without further purification.
2.2. Physical measurements
Elemental analyses (C, H, N) were carried out with a
Perkin Elmer Analyzer Model 240. Chromium content
was determined spectrophotometrically as CrO42− at
λ = 372 nm. IR spectra were recorded on a Perkin Elmer
FT-IR 2000 spectrophotometer in the 4000–400 cm−1
region using the KBr discs and from 700 to 30 cm−1
using polyethylene plates techniques. Electronic spectra
were measured on a SPECORD M-40 (Carl Zeiss,
* E-mail: [email protected]
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Table 1.
Crystal data and structure refinement parameters for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1).
Empirical formula
Formula weight
C6H16N5O12.5ClCoCrK
543.72
2192
F(000)
0.32×0.27×0.14 mm
Crystal size
Theta range for data
collection
Temperature
293(2) K
Wavelength
0.71073 Å
Index ranges
Monoclinic
Reflections collected /
unique
Crystal system
Space group
C2/c
−33<=h<=34, −10<=k<=10, −28<=l<=23
Completeness to 2θ = 52
Unit cell dimensions
Absorption correction
A
23.9750(10) Å
Max. and min. transmission
B
7.4020(10) Å
Refinement method
C
20.0460(10) Å
Data / restraints / parameters
β
93.85(1) deg
Goodness-of-fit on F2
Volume
3549.4(5) Å
Z
Calculated density
Absorption coefficient
3
8
2.035 Mg m
–3
2.57 to 31.26 deg
16647 / 5399 [R(int) = 0.0418]
99.9%
Analytical
0.7665 and 0.5661
Full-matrix least-squares on F2
5399 / 0 / 257
1.083
Final R indices [I>2σ(I)]
R1 = 0.0393, wR2 = 0.0927
R indices (all data)
R1 = 0.0511, wR2 = 0.0983
Largest diff. peak and hole
0.725 and −0.661 e Å−3
2.006 mm−1
Jena) spectrophotometer. The magnetic susceptibility
measurements were performed between 78–300 K
by Faraday method on the balance constructed in
our laboratory at field strength of 1.0 T. The magnetic
field was calibrated with Hg[Co(NCS)4] [19]. The molar
susceptibilities were corrected for diamagnetism using
the Pascal’s constants (–215×10−6 cm3 mol−1) [20] and
temperature independent paramagnetism for Co(III)
(100×10−6 cm3 mol−1) [21]. The effective magnetic
moments were calculated from the equation: µeff =
2.828(χMcorr ·T)1/2.
2.3. [CoCl(NH
Synthesis
)]
) ][KCr(C2O4of
)3]•0.5H[CoCl(NH
O synthesis
3 5
3 5
2
[KCr(C
(1)2O4)3]•0.5H2O (1)
Pentaamminechlorocobalt(III)
chloride
(0.25
g,
1 mmol) was dissolved in water (60 mL). In another
beaker, potassium tris(oxalato)chromate(III) trihydrate
(0.49 g, 1 mmol) was dissolved in water (20 mL) at
room temperature. Both solutions were mixed, and the
resulting solution was acidified by adding a few drops of
1 M HCl and left in refrigerator. After several days the
dark red crystals suitable for the X-ray structure analysis
were obtained. The crystals were filtered, washed
with cold water and air-dried. Yield: 0.36 g (67%).
Anal. Calcd. for C6H16N5O12.5ClCoCrK (1): C, 13.25; H,
2.97; N, 12.88; Cl, 6,52; Cr, 9.56. Found: C, 13.20; H,
3.37; N, 12.82; Cl, 6.32; Cr, 9.27%. IR (cm−1): νas(OH)
3609m, νs(OH) 3538m, νas(NH3) 3299s, νs(NH3) 3205s,
νas(OCO) 1706s, 1685sh, 1659vs, 1626sh, νs(CO) +
ν(CC) 1396vs, δs(NH3) 1334s, νs(CO) + δ(OCO) 1263s,
ρr(NH3) 848m, ν(Cr-O) + δ(OCO) 800s, ν(Cr-O) + ν(CC)
542s, ring def. + δ(OCO) 484s, ν(Co-N) 470sh, ν(Cr-O)
+ ring def. 412s, δ(N-Co-N) 323s, ν(Co-Cl) 283s. λmax
(nm (ε) in water): 698 (6), 553 (118), 421 (110), 364sh.
2.4. X-ray crystallographic data
The diffraction data were measured for single crystal
of a complex [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1) on
an Oxford Sapphire CCD diffractometer using ω-2θ
method. The complex crystallized in the monoclinic
C2/c space group. The analytical absorption correction
was applied [22]. The structure was solved by direct
methods and refined with the full-matrix least-squares
procedure using SHELX-97 package [23]. The hydrogen
atoms were constrained during refinement. The details
of data collection and refinement are presented in
Table 1.
CCDC 739373 contains the supplementary
crystallographic data for [CoCl(NH3)5][KCr(C2O4)3]•0.5
H2O (1). These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected].
3. Results and discussion
3.1. Synthesis and properties
Pentaamminechlorocobalt(III) chloride and potassium
tris(oxalato)chromate(III) were reacted in 1:1 molar ratio
in aqueous medium forming a new two dimensional
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Unusual potassium-oxalate coordination in the two-dimensional
trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex
complex (1), [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O. Addition
of HCl to the reaction mixture is not crucial but increases
yield a little. Although, the complex is a stable crystalline
solid and can be stored in a dry atmosphere for extended
periods of time, water molecules can be lost.
Thermal decomposition of 1 was followed by
combined DTA-TG under an air atmosphere. The
liberation of ammonia molecules was accompanied by
endothermic effects on the DTA curve (min.) at 270°C.
Further heating above 300°C caused exothermic
decomposition in one step similarly as in the parent
complex, K3[Cr(C2O4)3] [24]. No attempt was made,
however, to study the details of the pyrolysis process.
Total decomposition occurs at ca. 450°C and leads to a
mixture of KCl, Co3O4 and Cr2O3.
3.2. Spectroscopy
In the electronic spectrum of 1, the strong absorption
maxima were observed at 553 and 421 nm showing d-d
transitions (4A2g→4T2g and 4A2g→4T1g) typical for Cr(III)
octahedral complexes [25]. Position, especially of the first
maximum is perturbed and molar absorption coefficients
are somewhat higher than in parent [Cr(C2O4)3]3− [26]. It
is an effect of superposition of Cr(III) and Co(III) bands.
The former have molar absorption coefficients ca. 50%
higher than the later, therefore dominate. Thus, 1A1→1E(a)
and 1A1→1A2 Co(III) transitions (C4v symmetry) [25,27]
overlap with 4A2g→4T2g Cr(III) transition and 1A1→1E(b)
Co(III) transition occurring at 364 nm as a shoulder. Low
intensity sharp band at 698 nm can be attributed to spin
forbidden Cr(III) transitions (4A2g→2T2g,2Eg).
The IR spectrum of 1 obviously present the
characteristic absorptions of the oxalato bridged
group, the coordinated ammonia, the metal-ligand,
and the lattice water. For the oxalate ligand, various
coordination modes via one, two, three or four oxygen
atoms have been previously characterized by IR method
[12,28,29]. The absorption bands which appear on the
IR spectrum of the complex 1 near 1650 cm−1 and at
1396, 1263, 800, and 484 cm−1 are consistent with the
presence of three different coordination modes of the
oxalate ligand, in agreement with the X-ray diffraction
study results (vide infra). For the coordinated ammonia,
the bands at 3299, 3205, 1334 and 848 cm−1 are
identified with antisymmetric and symmetric stretching,
symmetric bending, and rocking vibrations, respectively
[28]. The expected NH3 asymmetric deformation
vibration bands are coincided on strong bands from
oxalate ligand. It is worthy to notice that many other
absorption bands are observed in the 3100–2100 cm−1
region and could be assigned to the ν(NH), (N-H•••O)
vibrations.
Figure 1.
Asymmetric part of the structure of 1 with the atom
numbering scheme. Thermal ellipsoids are plotted at
30% probability level.
3.3. Description of the structure
The asymmetric unit of the structure contains two
complex counterions, potassium cation and a water
molecule positioned on the two-fold axis, therefore
exhibiting the partial occupancy of 0.50. Therefore,
only one water hydrogen atom was included in the
final model, while the second H is related by the twofold symmetry. The asymmetric part of the structure
with the atom numbering scheme is shown on Fig. 1.
The selected bond distances and angles are listed in
Table 2. The details of the H-bond network are presented
in Table 3.
The Co(III) coordination sphere in the complex
cation is a slightly distorted octahedron with a chloride
ion and five ammonia molecules. The Co-Cl distance is
2.2645(7) and the Co-N distances vary from 1.949(2)
to 1.968(2) Å. The bond angles within the CoClN5
coordination sphere vary from 88.18(6) to 93.20(8) and
from 177.77(11) to 178.44(7)o.
Three oxalate ligands form an octahedral
coordination sphere of Cr(III) in the complex anion. The
Cr-O bond distances vary from 1.960(2) Å for Cr2-O21
to 1.980(2) Å for Cr2-O11. The O-Cr-O angles vary in a
relatively broad range between 82.47(7) and 95.66(7)o
while those between the oxygen atoms occupying the
opposite corners of the polyhedron are from 172.44(7)
to 173.98(7)o. The deviations from the expected 90/180
degrees reflect the tight five-membered chelate ring
formed by each of the oxalate ligands. All three chelate
rings are planar, the absolute values for the torsion
angles varying between 0.1(2) and 0.6(3), 0.2(3) and
4.0(3), as well as 2.8(2) and 12.3(3)o for Cr2-O11-C1C2-O21, Cr2-O31-C3-C4-O41 and Cr2-O51-C5-C6O61 rings, respectively. The largest deviations from
the ring planarity are detected for the ring formed by
the C5-C6 oxalate ligand involved in five interactions to
surrounding K1 ions.
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Table 2. Selected bond lengths and distances [Å], and angles [deg]
for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1).
Co1-Cl1
2.2645(7)
K1-O51
2.743(2)
Co1-N2
1.968(2)
K1-O22#2
2.751(2)
Co1-N3
1.955(2)
K1-O11#4
2.804(2)
Co1-N4
1.965(2)
K1-O52#5
2.965(2)
Co1-N5
1.951(2)
K1-O12#2
3.015(2)
Co1-N6
1.949(2)
K1-O61#4
3.133(2)
K1-O62#5
3.182(2)
Cr2-O11
1.980(2)
K1-O31#4
3.370(2)
Cr2-O21
1.960(2)
K1-O52
3.187(2)
Cr2-O31
1.977(2)
Cr2-O41
1.969(2)
Cr2-O11-K1#1
Cr2-O51
1.970(2)
Cr2-O31-K1#1
Cr2-O61
1.979(2)
Cr2-O51-K1
140.64(8)
K1#3-O52-K1
149.97(7)
100.84(6)
84.01(6)
N6-Co1-N5
89.64(11)
N6-Co1-N3
90.69(11)
O51-K1-O22#2
73.81(6)
N5-Co1-N3
88.91(9)
O51-K1-O11#4
160.51(5)
N6-Co1-N4
177.77(11)
O22#2-K1-O11#4
103.23(5)
N5-Co1-N4
89.28(9)
O51-K1-O52#5
123.71(6)
N3-Co1-N4
91.24(10)
O22#2-K1-O52#5
110.27(6)
N6-Co1-N2
88.65(10)
O11#4-K1-O52#5
N5-Co1-N2
93.20(8)
O51-K1-O12#2
N3-Co1-N2
177.78(8)
O22#2-K1-O12#2
N4-Co1-N2
89.47(9)
O11#4-K1-O12#2
63.77(5)
N6-Co1-Cl1
89.67(9)
O52#5-K1-O12#2
130.43(6)
N5-Co1-Cl1
178.44(7)
O51-K1-O61#4
124.15(5)
N3-Co1-Cl1
89.70(7)
O22#2-K1-O61#4
160.43(5)
N4-Co1-Cl1
91.45(7)
O11#4-K1-O61#4
57.21(4)
N2-Co1-Cl1
88.18(6)
O52#5-K1-O61#4
68.02(5)
O12#2-K1-O61#4
107.85(5)
75.63(5)
100.03(5)
57.40(5)
O21-Cr2-O41
92.49(7)
O51-K1-O62#5
O21-Cr2-O51
94.28(7)
O22#2-K1-O62#5
60.85(5)
O41-Cr2-O51
95.66(7)
O11#4-K1-O62#5
106.22(5)
O21-Cr2-O31
92.06(8)
O52#5-K1-O62#5
53.80(5)
O41-Cr2-O31
82.61(7)
O12#2-K1-O62#5
111.11(5)
O51-Cr2-O31
173.49(7)
O61#4-K1-O62#5
121.75(5)
O21-Cr2-O61
173.98(7)
O51-K1-O52
O41-Cr2-O61
92.94(8)
O22#2-K1-O52
91.46(6)
O51-Cr2-O61
82.59(7)
O11#4-K1-O52
155.20(5)
O31-Cr2-O61
91.21(7)
O52#5-K1-O52
80.60(3)
O21-Cr2-O11
82.47(7)
O12#2-K1-O52
140.25(5)
O41-Cr2-O11
172.44(7)
O61#4-K1-O52
107.10(5)
O51-Cr2-O11
90.35(7)
O62#5-K1-O52
63.64(5)
O31-Cr2-O11
91.91(7)
O51-K1-O31#4
109.88(5)
118.61(5)
O61-Cr2-O11
89.36(5)
43.32(5)
92.37(7)
O22#2-K1-O31#4
O21-Cr2-K1#1
120.06(5)
O11#4-K1-O31#4
53.93(4)
O41-Cr2-K1#1
132.51(6)
O52#5-K1-O31#4
114.76(5)
O51-Cr2-K1#1
113.51(5)
O12#2-K1-O31#4
61.87(4)
O31-Cr2-K1#1
64.13(5)
O61#4-K1-O31#4
51.38(4)
O61-Cr2-K1#1
57.25(5)
O62#5-K1-O31#4
160.13(5)
O11-Cr2-K1#1
47.69(5)
O52-K1-O31#4
134.43(5)
Symmetry transformations used to generate equivalent atoms:
#1 x,y-1,z #2 -x+1/2,-y+1/2,-z #3 -x+1/2,y-1/2,-z+1/2 #4 x,y+1,z
#5 -x+1/2,y+1/2,-z+1/2
Figure 2.
The nine-coordination environment of potassium in
complex 1. Atoms are displayed as gray, red, pink
and blue ellipsoids for carbon, oxygen, chromium and
potassium, respectively.
The potassium K1 cation is found 3.7243(8) Å from
Cr2[x,y+1,z] and interacts with nine oxygen atoms of
neighbouring oxalates (Fig. 2, Table 2), the distances
ranging from K1-O51 2.743(2) to K1-O52 3.187(2) Å and
also contact K1-O31[x,y+1,z] of 3.370(2) Å. The latest is
close to the K-O distance of 3.5120(3) Å as reported
by Nelyubina et al. [30]. Analysis of the electron-density
distribution allowed the authors to conclude that such
interaction is similar to the shorter bonds in that the
unshared electron pairs of oxygen pointed towards the
metal centre. Therefore the contact K1-O31[x,y+1,z]
found in the structure reported here might be considered
as an additional K-O bond, what results in the ninecoordination environment of potassium.
There are numerous literature reports on the
polyhedral sphere of potassium cations, some of
them formed by oxalate ligands. Among those, most
have eight K-O bonds. The polyhedron with eight K-O
interactions formed by oxalate ions and water molecules
was reported for the complexes containing yttrium
and terbium [31] with the K-O distances ranging from
2.844 to 2.941 Å, while K-O(oxalate) bonds are 2.844
and 2.858 Å, for Y and Tb complex, respectively. The
coordination sphere formed by oxalate ligands and
water was reported [32] with nine K-O bonds ranging
from 2.756 to 3.108 Å. In that structure the K-O bonds
involving oxalate O are from 2.768 to 3.108 Å. The
search with CDS [33] has revealed 58 structures with
oxalate ligands forming K-O interactions, and the bond
lengths ranging from 2.374 to 3.301 Å. Therefore, the
structure reported here seems to be unique in that the
oxalates are the only ligands bound to potassium, they
as many as nine K-O interactions, and some of them are
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Unusual potassium-oxalate coordination in the two-dimensional
trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex
Figure 3.
The 2D supramolecular structure of the layer formed by
[Cr(C2O4)3]3− anions and K+ cations, which are parallel to
the Y and Z crystallographic axes. Atoms are displayed
as gray, red, pink and blue ellipsoids for carbon, oxygen,
chromium and potassium, respectively. Metal centres are
labelled for clarity.
Figure 5.
Figure 4.
A sketch of the unit cell of complex 1. Atoms are colourcoded: potassium ions are larger blue spheres, cobalt
ions are medium size blue spheres, chromium is
displayed as pink spheres, and chloride ions are green.
Carbon, nitrogen and oxygen atoms are displayed gray,
small blue and red spheres, respectively. Hydrogen
atoms are omitted for clarity.
among the longest K-O(ox) reported. The CSD search
revealed also that some compounds with different
ligands might have a potassium coordination number as
large as eleven or twelve [34,35].
The K-O series of interactions, involving also oxygen
atoms coordinated to Cr(2) central ion, results in the
layer of [Cr(C2O4)3]3− complex ions bridged by K+ cations
and parallel to the Y and Z crystallographic axes (Fig. 3).
These layers are bridged by the complex [CoCl(NH3)5]2+
cations and water molecules. The coordination of a
Temperature dependencies of corrected molar
susceptibility (top) and reciprocal susceptibility (bottom)
for 1.
potassium cation to the oxalato oxygens results in short
contacts K1•••C2[-x+1/2,-y+1/2,-z] of 3.514(2) Å and
K1•••C5 of 3.326(2) Å. The structure analysis revealed
that three oxalate anions have a different coordination
mode. One oxalate (C1C2O42−), coordinates the metal
centres via all four oxygen atoms, and three of them are
involved in K binding. Only two oxygen atoms of other
oxalate (C3C4O42−), are involved in coordination bonds
to Cr and one of them forms an additional contact to
K, while each of remaining two oxygens participate in
three hydrogen bonds to the ammonia ligands. For the
third oxalate (C5C6O42−), all oxygen atoms are involved
in coordination of surrounding K ions while two of them
form a chelate ring with Cr.
The structure contains an extensive network of
hydrogen bonds (Table 3, Fig. 4). All N-H group of
ammonia ligands act as donors in these interactions,
while acceptors are the oxalate oxygen atoms. The
N•••O distances vary from N5•••O21[x,-y+1,z+1/2]
2.918 Å to 3.380 Å for N5•••O31[x,-y,z+1/2]. The single
N3-H32•••Cl1[-x,y,-z+1/2] interaction is formed. The
N•••Cl distance being 3.374 Å. Among these H-bonds,
three bifurcated interactions are formed by N2-H23,
N3-H33 and N5-H52 groups. Water molecule is involved
in a pair of two-fold related O•••Cl H-bonds. The
O3•••Cl1[x,y+1,z] distance being 3.473 Å.
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G. Wrzeszcz, A. Wojtczak, M. Zawadzka
Table 3. Hydrogen bonds for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1).
D-H
d(D-H)
d(H×××A)
<DHA
d(D×××A)
A
N2-H23
0.890
2.454
136.22
3.157
O52
N2-H23
0.890
2.491
127.28
3.112
O11
[ -x+1/2, y+1/2, -z+1/2 ]
N2-H22
0.890
2.124
164.51
2.991
O22
[ x, -y+1, z+1/2 ]
N2-H21
0.890
2.484
123.43
3.066
O62
[ x, y+1, z ]
N3-H33
0.890
2.200
175.70
3.088
O42
[ -x, y, -z+1/2 ]
N3-H33
0.890
2.548
125.37
3.148
O41
[ -x, y, -z+1/2 ]
N3-H32
0.890
2.516
162.31
3.374
Cl1
[ -x, y, -z+1/2 ]
N3-H31
0.890
2.088
163.04
2.951
O32
[ -x, y+1, -z+1/2 ]
N4-H43
0.890
2.253
155.65
3.086
O12
[ -x+1/2, y+1/2, -z+1/2 ]
N4-H42
0.890
2.239
154.62
3.067
O62
N4-H41
0.890
2.283
170.61
3.164
O42
[ -x, y, -z+1/2 ]
N5-H53
0.890
2.192
138.41
2.918
O21
[ x, -y+1, z+1/2 ]
N5-H52
0.890
2.430
129.59
3.075
O32
[ x, -y, z+1/2 ]
N5-H52
0.890
2.491
176.66
3.380
O31
[ x, -y, z+1/2 ]
N5-H51
0.890
2.278
152.99
3.097
O32
[ -x, y+1, -z+1/2 ]
N6-H63
0.890
2.155
154.98
2.985
O42
[ -x, y+1, -z+1/2 ]
N6-H62
0.890
2.545
115.86
3.041
O3
N6-H61
0.890
2.201
170.65
3.082
O62
[ x, y+1, z ]
O3-H1O3
0.919
2.591
160.95
3.473
Cl1
[ x, y+1, z ]
3.4. Magnetic properties of the complex
Magnetic properties of complex 1 were studied
within the range 78-300 K (Fig. 5). The temperature
dependencies of magnetic susceptibilities obey the
Curie-Weiss law, i.e., χMcorr = C/(T−θ). The best fit
values of the Curie (C) and Weiss constant (θ) are
1.77 cm3 K mol−1 and −0.1 K, respectively. Although
complex 1 shows a small negative Weiss constant,
the magnetic moment is nearly constant down to
liquid nitrogen temperature (3.79±0.05 B.M.), and from
mathematical point of view (the same agreement factor),
the complex equally obeys the Curie law with the same
C parameter. It indicates that magnetic interactions even
if operate are negligible, and complex 1 is essentially
paramagnetic. The observed magnetic moment of 1 has
approximately the expected spin-only value for isolated
Cr(III) (S = 3/2), which is 3.87 B.M. In spite of the
oxalate bridge ability to transmit exchange interactions
effectively [36,37], any interaction between Cr(III) ions
through the O-K-O, O-K-O-C-O and O-C-O-K-O-C-O
paths has been observed, due to long K-O distances
and presumably purely electrostatic K-O interactions
within [{KCr(C2O4)3}n]2n− layers.
Symmetry code
4. Conclusions
A
novel
tris(oxalato)chromate(III)-based
twodimensional
three-metal
complex
[CoCl(NH3)5]
[KCr(C2O4)3]•0.5 H2O (1) has been synthesized and
structurally characterized. An interesting aspect to this
structure is the simultaneous presence of three different
coordination modes of the oxalate ligands to both,
potassium and chromium, metal centres: tetradentate
(C1C2O42−: µ3-ox and µ2-O11), bidentate (C3C4O42−:
µ2-ox and µ2-O31) and tetradentate (C5C6O42−: µ4-ox
and µ2-O51, µ2-O52, µ2-O61). A 2D structure is formed
due to the charge incompatibility of initial complexes.
The crystallisation of 1 depends on the stabilizing
effect of the potassium cations through multiple K-O
interactions. The structure of 1 seems to be unique
in that the oxalates are the only ligands bound to
potassium, they as many as nine K-O interactions. On
the other hand, magnetic susceptibility measurements
show no exchange interaction between Cr(III) ions.
657
Unauthenticated
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Unusual potassium-oxalate coordination in the two-dimensional
trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex
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