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Journal of Coordination Chemistry
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Tris(oxalato)chromate(III) hybrid salts templated
by pyridinium and mixed pyridinium-ammonium
cations: synthesis, structures and magnetism
Coustel M. N. Choubeu, Bridget N. Ndosiri, Hervé Vezin, Claire Minaud,
James B. Orton, Simon J. Coles & Justin Nenwa
To cite this article: Coustel M. N. Choubeu, Bridget N. Ndosiri, Hervé Vezin, Claire Minaud, James
B. Orton, Simon J. Coles & Justin Nenwa (2021): Tris(oxalato)chromate(III) hybrid salts templated
by pyridinium and mixed pyridinium-ammonium cations: synthesis, structures and magnetism,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2021.1890048
To link to this article: https://doi.org/10.1080/00958972.2021.1890048
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Tris(oxalato)chromate(III) hybrid salts templated
by pyridinium and mixed pyridinium-ammonium
cations: synthesis, structures and magnetism
Coustel M. N. Choubeu
a
, Bridget N. Ndosiri
a
, Herv
e Vezin
b
, Claire Minaud
c
,
James B. Orton
d
, Simon J. Coles
d
and Justin Nenwa
a
a
Inorganic Chemistry Department, University of Yaounde 1, Yaounde, Cameroon;
b
Universit
e de Lille,
LASIRE CNRS UMR 8516, Villeneuve d’Ascq, Lille, France;
c
Institut Chevreul CNRS FR2638, Universit
e
de Lille, Villeneuve d’Ascq, Lille, France;
d
School of Chemistry, Faculty of Engineering and Physical
Sciences, University of Southampton, Southampton, SO17 1BJ, UK
ABSTRACT
By modifying the stoichiometric ratio of starting materials, two
tris(oxalato)chromate(III) salts, (C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1) and (C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O(2)f(C
7
H
11
N
2
)
þ
¼2-amino-4,6-dime-
thylpyridinium, (C
5
H
8
N
3
)
þ
¼2,6-diaminopyridiniumg, were synthe-
sized and characterized by elemental and thermal analyses,
single-crystal X-ray diffraction, IR and UV Vis spectroscopies, EPR
and SQUID measurements. Salt 1exhibits a 3-D supramolecular
framework based on [Cr(C
2
O
4
)
3
]
3-
and 2-amino-4,6-dimethylpyridi-
nim cations, (C
7
H
11
N
2
)
þ
, via N–HO hydrogen bonds.
Interestingly, p–pstacking interactions between pyridine rings
contribute to the stabilization of the crystal packing. In contrast
to salt 1,nop–pstacking interactions are observed in the
mixed-cation salt 2and its crystal packing is consolidated by
N–HO and O HO hydrogen bonds. EPR spectra of 1and 2
are consistent with the oxidation state þ3 of the chromium cen-
ter in an octahedral environment. Temperature-dependence of
the magnetic susceptibility data investigated from 2 to 300 K
revealed the existence of zero-field splitting effects (ZFS) for
Cr(III) ions in both compounds.
ARTICLE HISTORY
Received 8 July 2020
Accepted 27 December 2020
KEYWORDS
Hybrid salts; oxalate
chromium(III) complex;
stoichiometry; crystal
structures; magnetism
CONTACT Justin Nenwa jnenwa@yahoo.fr Inorganic Chemistry Department, University of Yaounde 1, P.O.
Box 812, Yaounde, Cameroon
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1890048.
ß2021 Informa UK Limited, trading as Taylor & Francis Group
JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2021.1890048
1. Introduction
Organic inorganic hybrid salts (OIHSs) currently represent a topic of intense research
in crystal engineering [1,2]. They provide the possibility of combining suitable organic
cations and inorganic counter-parts within a single crystal, leading to unique elec-
tronic [3], magnetic [4,5], catalytic [6], metallic conductivity [7] and optical [8,9] prop-
erties. In general, transition metal complexes provide useful anionic building blocks for
the construction of OIHSs. In this respect, among the tris(oxalato)metalates
[M(C
2
O
4
)
3
]
3-
(M ¼Fe
III
,Co
III
,Cr
III
), the [Cr(C
2
O
4
)
3
]
3-
unit is the most intensively investi-
gated, given the stable electronic configuration (t
2g
)
3
(e
g
)
0
of its central metal ion
[1013]. However, control over the formation of these OIHSs resulting from the self-
assembly of [M(C
2
O
4
)
3
]
3-
units and organic cations remains a challenging task.
Tris(oxalato)metalate(III) hybrid salts can be prepared following a handful of strategies,
one of which is the salt metathesis approach. This strategy can formally generate the
target compound if appropriate precursor salts are chosen. The outcome often is influ-
enced by the pH of the medium, the synthesis method and, more importantly, the
stoichiometric ratio of the starting materials [14,15].
Recent reports by our research group on a series of OIHSs with general formula
(Org-H)
3
[M
III
(C
2
O
4
)
3
]nH
2
O (Org-H
þ
¼iminium cations) clearly demonstrated the crucial
role of the stoichiometric ratio of the starting materials in organizing individual enti-
ties into supramolecular three-dimensional frameworks [16,17]. Hence, interesting
architectural features and magnetic properties of these OIHSs and the relatively unex-
plored structural chemistry of their Cr(III) complexes prompted us to undertake the
present study. Herein, we report the crystal structures, thermal analyses and magnetic
properties of two Cr(III) hybrid salts, (C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1) and (C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O(2)f(C
7
H
11
N
2
)
þ
¼2-amino-4,6-dimethylpyridinium, (C
5
H
8
N
3
)
þ
¼
2,6-diaminopyridiniumg. The overall magnetic behavior of 1and 2suggests the pres-
ence of the zero-field splitting effects (ZFS) for Cr(III) ions.
2 C. M. N. CHOUBEU ET AL.
2. Experimental
2.1. Materials and physical measurements
All chemicals used in the synthesis were reagent grade, commercially available (Aldrich,
Prolabo and Merck), and were used without purification. The precursor salt
(NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O was synthesized based on literature method [18]. Elemental analy-
ses (CHN) were performed on a Perkin–Elmer 240C analyser. Infrared spectra were
recorded from 4000 to 400 cm
1
on a Perkin-Elmer 2000 spectrometer. Electronic spectra
were measured in aqueous solutions using a UV Vis HACH/Series DR 3900 spectropho-
tometer. Thermogravimetric analysis (TGA) was carried out on a LINSEIS STA PT-1000
thermal analyser. The powdered sample (ca. 20 mg) was heated in air from 25 to 500Cat
a scan rate of 10 C min
1
. Powder X-ray diffraction experiments were carried out on a
Bruker D8 diffractometer using Cu Karadiation. The sample was ground to a fine powder
and loaded into an aluminium tray with the conditions: angular range: 5 80; step: 0.02;
integration time: 0.5 s. Where available these spectra were compared with those deter-
mined from the single crystal structures. Electron paramagnetic resonance (EPR) measure-
ments were carried out with a Bruker ELEXYS E500 spectrometer operating at 9 GHz. The
samples, each containing 30 mg of loose powder, were placed into 4mm diameter quartz
tubes. The measurements were performed at room temperature with a microwave power
of 0.3 mW and an amplitude modulation of 3G. Variable temperature magnetic suscepti-
bilities were measured with a Quantum Design PPMS Dynacool 9T magnetometer.
Measurements were done at 0.1 T down to 2 K on polycrystalline samples. Diamagnetic
corrections were made with Pascal’s constants for all the constituent atoms [19].
2.2. Syntheses
2.2.1. Synthesis of (C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
] (1)
To a solution of oxalic acid (0.63 g, 5.0 mmol, slight excess) in water (50 mL), 2-amino-4,6-
dimethylpyridine (1.23 g, 10 mmol) was added. The mixture was stirred for 1 h at 50 C.
To this solution, a freshly prepared (NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O salt (1.06 g, 2.5 mmol) was
added in successive small portions and a purple solution was formed. After 30 min of stir-
ring at 50 C, the mixture was cooled to room temperature and filtered. The filtrate was
allowed to evaporate in a hood at room temperature. Two weeks later, purple block crys-
tals suitable for X-ray diffraction were harvested. Yield: 1.5 g (87.7% based on
(NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O). Anal. Calcd for C
27
H
33
CrN
6
O
12
(1) (%): C, 47.25; H, 4.81; N, 12.25.
Found (%): C, 47.04; H, 4.80; N, 12.19. IR data (cm
1
): 3327 (w), 3168 (w), 3085 (w), 1703
(s), 1633 (s); 1386 (s), 1254 (s), 538 (s); 512 (s), 472 (s) (Supporting Information Figure S1).
UV–Vis (H
2
O solution, nm): 419, 571 (Supporting Information Figure S2).
2.2.2. Synthesis of (C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O (2)
2,6-Diaminopyridine (0.83 g, 7.5 mmol) was added to an aqueous solution (50 mL) of
oxalic acid (0.48 g, 3.75 mmol). The mixture was stirred for 1 h at 50 C. A freshly pre-
pared (NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O salt (1.06 g, 2.5 mmol) was added in successive small por-
tions to the previous solution. After 30 min of stirring at 50 C, the mixture was cooled
to room temperature and filtered. The filtrate was allowed to evaporate in a hood at
JOURNAL OF COORDINATION CHEMISTRY 3
room temperature. Two weeks later, dark-orange block crystals suitable for X-ray dif-
fraction were obtained. Yield: 1.30 g (88.13% based on (NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O). Anal.
Calcd for C
16
H
24
CrN
7
O
14
(2) (%): C, 32.51; H, 4.06; N, 16.59. Found (%): C, 32.72; H,
4.07; N, 16.77. IR data (cm
1
): 3559 (w), 3424 (w), 3326 (w), 3164 (w), 1702 (s), 1680 (s),
1369 (s), 1255 (s), 534 (s), 466 (s), 403 (s) (Supporting Information Figure S3). UV–Vis
(H
2
O solution, nm): 418, 569 (Supporting Information Figure S4).
2.3. Crystal structure determination
The data for 1and 2were collected at 100(2) K on a Rigaku FRE þdiffractometer using
graphite-monochromated Mo Karadiation (k¼0.71075 Å). The dataset coverage and col-
lection strategy were based on the strategy calculation in the program CrysAlisPro
1.171.40.47a [20]. The structures were solved with the SHELXT program [21] using the
intrinsic phasing solution method. The model was refined with SHELXL-2018/3 [22] using
least squares minimization. All non-hydrogen atoms were refined anisotropically and all
N–H and O–H atoms were located from the electron density difference map and refined
with their thermal parameters linked to their parent atoms. The positions of the remain-
ing C–H atoms were calculated geometrically and refined using riding models. The mate-
rials used for publication were prepared within the Olex2 software package [23]. The
crystallographic data and structure refinement details are summarized in Table 1.
Table 1. Crystal data and structure refinements for 1 and 2.
Compound 1 2
Empirical formula C
27
H
33
CrN
6
O
12
C
16
H
24
CrN
7
O
14
Formula weight 685.59 590.42
Temperature (K) 100(2) 100(2)
Wavelength (Å) 0.71075 0.71075
Crystal system Triclinic Monoclinic
Space group P
1C2/c
a(Å) 11.0141(2) 10.0801(1)
b(Å) 11.3782(3) 19.8755(2)
c(Å) 12.6695(3) 12.3914(2)
a() 93.3946(19) 90
b() 92.3602(19) 108.8250(10)
c() 96.5092(18) 90
V(Å
3
) 1572.91(6) 2349.78(5)
Z24
D
calc
(g/cm
3
) 1.448 1.669
m(mm
1
) 0.435 0.574
F(000) 714 1220
Crystal size (mm) 0.260 0.160 0.080 0.349 0.180 0.120
ƟRange for data collection () 1.9 27.5 2.1 27.5
Index ranges 14h13,
14k14,
16l16
13 h13,
25 k25,
16 l16
Total reflections 35,161 25,716
Unique reflections (R
int
) 7222 [R(int) ¼0.0452] 2697 [R(int) ¼0.0357]
Refinement method Full-matrix least squares
on F
2
Full-matrix least squares
on F
2
Data / restraints / parameters 7222 / 0 / 448 2697 / 0 / 205
Goodness-of-fit on F
2
1.04 1.03
Final Rindices [I >2r(I)] R
1
¼0.0375, wR
2
¼0.0845 R
1
¼0.0253, wR
2
¼0.0691
Rindices (all data) R
1
¼0.0483, wR
2
¼0.0909 R
1
¼0.0275, wR
2
¼0.0704
Largest diff. peak and hole (e/Å
3
) 0.551 and 0.439 0.441 and 0.373
4 C. M. N. CHOUBEU ET AL.
3. Results and discussion
3.1. Formation of 1 and 2
In order to obtain (C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1) with the expected general formula (Org-
H)
3
[M
III
(C
2
O
4
)
3
]nH
2
O (Org-H
þ
¼iminium cations), ion-exchange strategy was carried
out by reacting (NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O with (C
7
H
11
N
2
)
2
C
2
O
4
in 1:2 instead of the nor-
mal 1:1.5 molar ratio in water. The slight excess of the precursor salt (C
7
H
11
N
2
)
2
C
2
O
4
played a key role in formation of the target salt 1. By contrast, combining
(NH
4
)
3
[Cr(C
2
O
4
)
3
]3H
2
O with bis(2,6-diaminopyridinium) oxalate, (C
5
H
8
N
3
)
2
C
2
O
4
, in nor-
mal 1:1.5 molar ratio in water resulted in the partial exchange of ammonium cations
by 2,6-diaminopyridinium cations to yield not the expected salt, but rather the
mixed 2,6-diaminopyridinium-ammonium salt (C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O(2). It is
worth noting at this stage that for successful preparation of both materials to pre-
vail, care must also be taken to work in slightly acidic medium (pH 3–4) in order
to avoid destruction of the anionic building block [Cr
III
(C
2
O
4
)
3
]
3-
. If the organic mole-
cules (Org) are protonated using a strong acid like (H
3
O
þ
Cl
-
) (pH 2), the reaction
is bound to fail, leading to a diversity of unexpected structures [14,15,17]. This
result shows that the outcome of these ion-exchange reactions is strongly influenced
by both the stoichiometric ratio of the starting precursor salts and the pH of the
medium. Elemental analyses (CHN) nicely agree with the chemical compositions of 1
and 2.
3.2. Structural description of 1
Salt 1crystallizes in the triclinic P1 space group. As shown in Figure 1(a), the asym-
metric unit consists of one tris(oxalato)chromate(III) anion [Cr(C
2
O
4
)
3
]
3-
and three
2-amino-4,6-dimethylpyridinium cations (C
7
H
11
N
2
)
þ
. Selected bond lengths and angles
are listed in Table 2. Salt (C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1) and its homologue (C
7
H
11
N
2
)
3
[Fe(C
2
O
4
)
3
][17] are isostructural. The pseudo-octahedral coordination in
[Cr(C
2
O
4
)
3
]
3-
is similar to the chiral geometries. The Cr O distances range from
1.9568(12) to 1.9810(12) Å and the O–Cr–O angles from 82.18(5) to 173.96(5). These
geometric parameters compare within experimental error with those reported for simi-
lar structures [16,17,24]. The bulk structure of 1is consolidated by a 3-D network of
intermolecular N–HO [2.7243(19) to 3.034(2) Å] hydrogen bonds linking (C
7
H
11
N
2
)
þ
cations and [Cr(C
2
O
4
)
3
]
3-
anions (Figure 2(a),Table 3).
The crystal packing of 1illustrating columns of cations and anions is depicted in
Figure 3(a). The stabilizing effect of the long-range hydrogen bonding is reinforced by
p–pstacking interactions between pyridine rings, the centroid-to-centroid distances
ranging from 3.54(3) to 3.66(3) Å (Supporting Information Figure S5). Similar p–pinter-
actions with a centroid-centroid distance of 3.66(2) Å have been found in some salts
containing iminium cations [16,17,25]. The great steric hindrance provided by the
2-amino-4,6-dimethylpyridinium cation in comparison with the small 2,6-diaminopyri-
dium cation is likely to justify the absence of water molecules of crystallization in the
structure of 1[17].
JOURNAL OF COORDINATION CHEMISTRY 5
3.3. Structural description of 2
Salt 2crystallizes in the monoclinic C2/c space group. As shown in Figure 1(b), the asym-
metric unit contains a half tris(oxalato)chromate(III) anion [Cr(C
2
O
4
)
3
]
3-
, one 2,6-diamino-
pyridinium cation (C
5
H
8
N
3
)
þ
, one ammonium cation (NH
4
)
þ
and one crystal water.
Selected bond lengths and angles are listed in Table 2. As in salt 1, the Cr(III) atom has a
distorted octahedral coordination geometry defined by six O atoms from three chelating
oxalato(2–) ligands. The Cr O distances are 1.9646(9) to 1.9870(9) Å and the chelate
O–Cr–O angles from 82.66(4) to 174.18(5). These geometric parameters are in agree-
ment with those found in the Refs. [16,17,24]. Hydrogen bond lengths (Å) and angles ()
in 2are listed in Table 3. The 3-D framework is stabilized by intermolecular N–HO
[2.88(2) to 2.98(1) Å] and O–HO [2.86(1) to 2.94(1) Å] hydrogen bonds linking the
coordination sphere, the cationic species and crystal water molecules (Figure 2(b)).
A projection of the unit cell of 2on the bc plane is highlighted in Figure 3(b).In
contrast to salt 1, adjacent pyridine rings in 2are 5.47 Å apart and are shifted from
one another. As a result, no p–pstacking interactions are observed in 2.
A partial view of the structure of 2highlighting the 50:50 disorder of the ammo-
nium ion over a twofold rotation axis is highlighted in Supporting Information Figure
S6 and atomic occupancies for atoms that are not fully occupied are listed in Table 4.
Figure 1. Asymmetric units of 1(a) and 2(b); thermal ellipsoids drawn at the 50% probabil-
ity level.
6 C. M. N. CHOUBEU ET AL.
Table 2. Selected bond lengths (Å) and angles () within the coordination spheres around the
metal centers in 1 and 2.
(C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1)
Cr1–O1 1.9789(12) O5–Cr1–O1 94.12(5)
Cr1–O3 1.9726(12) O5–Cr1–O3 93.75(5)
Cr1–O5 1.9627(12) O5–Cr1–O9 94.01(5)
Cr1–O7 1.9568(12) O5–Cr1–O11 173.04(5)
Cr1–O9 1.9795(12) O7–Cr1–O1 173.96(5)
Cr1–O11 1.9810(12) O7–Cr1–O3 92.70(5)
O1–Cr1–O9 90.10(5) O7–Cr1–O5 82.97(5)
O1–Cr1–O11 91.76(5) O7–Cr1–O9 95.37(5)
O3–Cr1–O1 82.18(5) O7–Cr1–O11 91.50(5)
O3–Cr1–O9 169.42(5) O9–Cr1–O11 82.26(5)
O3–Cr1–O11 90.72(5)
(C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O(2)
Cr1–O1
1
1.9646(9) O1
1
–Cr1–O5 92.33(4)
Cr1–O1 1.9646(9) O1–Cr1–O5
1
92.33(4)
Cr1–O3 1.9648(9) O1
1
–Cr1–O5
1
92.03(4)
Cr1–O3
1
1.9648(9) O1–Cr1–O5 92.02(4)
Cr1–O5
1
1.9870(9) O3–Cr1–O3
1
94.27(5)
Cr1–O5 1.9869(9) O3
1
–Cr1–O5
1
172.30(4)
O1
1
–Cr1–O1 174.18(5) O3–Cr1–O5
1
91.60(4)
O1
1
–Cr1–O3
1
82.66(4) O3–Cr1–O5 172.30(4)
O1–Cr1–O3
1
93.36(4) O3
1
–Cr1–O5 91.60(4)
O1
1
–Cr1–O3 93.36(4) O5–Cr1–O5
1
83.01(5)
O1–Cr1–O3 82.67(4)
Symmetry transformations used to generate atoms for 2:
1
1–x,þy,3/2 –z.
Figure 2. Hydrogen bonding (dashed lines) within 1(a) and 2(b).
JOURNAL OF COORDINATION CHEMISTRY 7
Structural differences between 1and 2ought to be linked essentially to two facts:
(a) charge balancing organic cations are identical in 1, but are different in 2; (b) water
of crystallization is absent in the former with the sterically demanding organic cations,
but is present in the latter salt.
3.4. PXRD patterns and thermal behavior of 1 and 2
From the PXRD patterns of 1and 2(Supporting Information Figure S7), the peak posi-
tions agree well with their simulated ones, which indicates that the products have
been obtained as pure crystalline phases.
Thermal stability of powder samples of 1and 2were studied by thermogravimetric
analysis (TGA) from 20 to 700 C under a heating rate of 10 Cmin
1
(Figure 4). The
TGA curve of 1shows no obvious weight loss from room temperature to 280 C, thus,
confirming the absence of water molecules of crystallization in 1. Upon further heat-
ing, a rapid and significant weight loss of 68.2% is observed from 280 to 350 C, which
corresponds to the decomposition of the framework. In the case of salt 2, the TGA
curve shows from 100 to 150 C a weight loss of 6.2% corresponding to the release of
two water molecules of crystallization (calcd 6.1%) to give the anhydrous derivative
(C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]. Upon further heating, the anhydrous derivative undergoes
two continuous weight loss processes between 250–300 C and 325–375 C, respect-
ively, leading to the decomposition of the framework.
3.5. Magnetic properties of 1 and 2
EPR spectra of powdered samples of the two salts measured at room temperature are
similar (Figure 5). In both cases, the anisotropy of gparameters is present along the
three principal axes. The spectra of 1and 2display an anisotropy of g factors (g
x
¼
3.69, g
y
¼2.89 and g
z
¼2.03) and (g
x
¼3.55, g
y
¼2.80 and g
z
¼2.15), respectively.
Table 3. Hydrogen bond lengths (Å) and angles () for 1 and 2.
D–HA d(D–H) d(HA) d(DA) <(D–HA)
(C
7
H
11
N
2
)
3
[Cr(C
2
O
4
)
3
](1)
N1–H1O2 0.84(2) 1.97(2) 2.779(2) 160(2)
N2–H2AO4 0.89(2) 2.03(2) 2.906(2) 169(2)
N2–H2BO11
1
0.91(2) 2.01(2) 2.913(2) 176(2)
N3–H3O8
2
0.84(2) 1.92(2) 2.724(2) 161(2)
N4–H4AO6
2
0.87(2) 2.05(2) 2.898(2) 166(2)
N4–H4BO9
3
0.85(2) 2.21(2) 3.034(2) 164(2)
N5–H5O12 0.84(2) 2.03(2) 2.827(2) 160(2)
N6–H6AO10 0.86(2) 2.21(2) 2.979(2) 148(2)
N6–H6BO1
3
0.88(2) 2.06(2) 2.926(2) 168(2)
(C
5
H
8
N
3
)
2
(NH
4
)[Cr(C
2
O
4
)
3
]2H
2
O(2)
N1–H1O5 0.84(1) 2.15(1) 2.984(1) 169(2)
N2–H2AO6 0.84(2) 2.04(2) 2.883(1) 175(2)
N2–H2BO7 0.85(1) 2.03(2) 2.881(1) 172(2)
N3–H3AO3
1
0.80(2) 2.26(2) 2.978(1) 149(2)
N3–H3BO2
2
0.83(2) 2.17(1) 2.891(1) 145(2)
O7–H7AO1
3
0.80(2) 2.14(2) 2.941(1) 179(2)
O7–H7BO3
4
0.84(2) 2.06(2) 2.857(1) 158(2)
Symmetry transformations used to generate equivalent atoms (D, donor; A, acceptor):
1
1–x,1 –y,1 –z;
2
1þx,1 þ
y,þz;
3
1–x,1 –y,–zfor 1 and
1
1–x,þy,3/2 –z;
2
1–x,1 –y,1 –z;
3
3/2 –x,3/2 –y,1 –z;
4
3/2 –x,1/2 þy,3/2 –z
for 2.
8 C. M. N. CHOUBEU ET AL.
No additional EPR line is observed for 1and 2, and this supports the absence of the
hyperfine interaction of chromium(III) ions [26]. The g anisotropy with three different
Eigen values indicates an octahedral distortion around the chromium(III) complex in
both salts. Similar EPR spectra were observed for SrK
0.5
Ag
0.5
[Cr(C
2
O
4
)
3
] with g factor
(g
x
¼3.70, g
y
¼3.01 and g
z
¼2.18) at room temperature [27].
Magnetic susceptibilities of both 1and 2were measured down to 2 K, and the v
M
T
versus temperature plots, where v
M
is the molar magnetic susceptibility, are depicted
in Figure 6. The two compounds show similar magnetic behavior with a slight increase
of the v
M
Tvalues upon cooling to reach a maximum at 20 K and then decrease at
low temperatures (T<20 K). Increase of v
M
Tupon cooling is often associated with
ferromagnetic interactions [2830] and its decrease upon cooling with antiferromag-
netic interactions [3134]. But here, the shortest CrCr distances in 1and 2are 9.298
Figure 3. Packing diagrams of 1(a) and 2(b).
JOURNAL OF COORDINATION CHEMISTRY 9
and 7.483 Å, respectively. Thus, the Cr(III) spin carriers in 1and 2are relatively far
away and the interaction should be very weak and could not lead to the increase or
to the decrease of v
M
T. Therefore, the temperature dependence of v
M
Ton the overall
temperature domain (increase–decrease) can be attributed to the effect of zero-field
splitting (ZFS) of the Cr(III) ions [35], rather than intermolecular interactions due to
long distances between the Cr(III) ions.
4. Conclusion
Using a stoichiometry-controlled strategy, we have synthesized two
tris(oxalato)chromate(III) salts templated by pyridinium and mixed pyridinium-ammo-
nium cations. Thermal behavior investigations indicate that both salts possess good
Figure 4. TGA curves for 1(blue) and 2(red).
Figure 5. EPR spectra for 1(a) and 2(b).
10 C. M. N. CHOUBEU ET AL.
thermal stabilities. EPR results confirmed the oxidation state þ3 of the chromium cen-
ter in an octahedral environment. Magnetic susceptibility measurements revealed the
existence of the zero-field splitting effects (ZFS) for Cr(III) ions in both compounds.
The isolation of these compounds not only expands the relatively small family of
tris(oxalato)chromate(III) hybrid salts involving iminium cations, but also confirms that
the structural architecture of the target products is strongly dependent on the stoi-
chiometric ratio of starting materials. The synthetic procedure used in the present
work may be extended so as to access a wide range of highly interesting homologous
materials, especially with respect to their solid state magnetic properties.
Supplementary material
CCDC 1937022 and 1937023 contain supplementary crystallographic data for 1and 2.
These data can be obtained free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (þ44) 1223
336 033; or E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
The authors thank Prof. Augustin Ephraim Nkengfack (Organic Chemistry Department, University
of Yaounde I, Cameroon) for the donation of organic reagents.
Table 4. Atom occupancies for all atoms that are not fully occupied in 2.
Atoms N4 H4A H4B H4C H4D
Occupancies 0.5 0.5 0.5 0.5 0.5
Figure 6. Temperature dependences of the v
M
Tproduct for 1and 2.
JOURNAL OF COORDINATION CHEMISTRY 11
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was financially supported by the UK Engineering and Physical Sciences Council fund-
ing for the UK National Crystallography Service.
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