Synthesis, spectroscopy and magnetism of novel metal complexes of 3-aminoflavone (3-af). X-ray crystal structure of 3-af and [Cu(3-af)2(NO3)2]

Article abstract of DOI:10.1016/j.ica.2008.04.014

The synthesis and characterization of four new complexes with the bioactive ligand 3-aminoflavone (3-af) are reported. The complexes of general formula [M(3-af)₂(H₂O)₂](NO₃)₂ · nH₂O], where

Full text of DOI:10.1016/j.ica.2008.04.014

Inorganica Chimica Acta 362 (2009) 739–744
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, spectroscopy and magnetism of novel metal complexes of
3-aminoflavone (3-af). X-ray crystal structure of 3-af and [Cu(3-af)2(NO3)2]
b
c
a
a
a
_
_
Bogumiła Zurowska , Andrea Erxleben , Łukasz Glinka , Małgorzata Łe˛czycka , Elzbieta Zyner ,
Justyn Ochocki a,
*
a Department of Bioinorganic Chemistry, Medical University, Muszynskiego 1, 90-151 Lodz, Poland
b Faculty of Chemistry, University of Wroclaw, Joliot-Curie, 50-383 Wroclaw, Poland
c School of Chemistry, National University of Ireland, Galway, University Road, Galway, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of four new complexes with the bioactive ligand 3-aminoflavone (3-
af) are reported. The complexes of general formula [M(3-af)2(H2O)2](NO3)2 ꢀ nH2O], where M = Co(II),
Ni(II), and Zn(II), and n = 0, 2, 0, respectively, and [Cu(3-af)2(NO3)2] compound were prepared and stud-
ied. In particular, to investigate the binding in detail, the crystal structures of the free ligand (3-af) and
[Cu(3-af)2(NO3)2] (1) were determined. The new coordination compounds were identified and character-
ized by elemental analysis, magnetic measurements, and infrared and ligand-field spectra. The crystal
structure of the Cu(II) complex reveals that the ligand acts as a N,O-bidentate chelate ligand forming a
five-membered ring with the copper(II) ion. The copper(II) ion is octahedrally surrounded by the two
amino nitrogens and two carbonyl oxygens from two chelating organic ligands in trans arrangement.
Two molecules of coordinated nitrate anions occupy axial positions. The spectral and magnetic properties
are in accordance with the structural data of the copper(II) compound. From X-ray powder-diffraction
patterns and IR spectra, the complexes of nickel(II) (2) and cobalt(II) (3) were found to be mutually iso-
morphous. The results of the spectroscopic studies suggest a mononuclear structure of 2 and 3 com-
Received 10 January 2008
Received in revised form 26 March 2008
Accepted 2 April 2008
Available online 14 April 2008
Dedicated to Bernhard Lippert.
Keywords:
Transition metals
Flavonoids
3-Aminoflavone
Complexes
X-ray crystallography
plexes. The variable-temperature (1.8–300 K) magnetic susceptibility data of
2 indicate a weak
ferromagnetic interaction. The magnetic behavior of complex 3 is characteristic of cobalt(II) systems with
an important orbital contribution via spin–orbit-coupling and also suggests a weak ferromagnetic
interaction.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
ment of leukemia L1210 cells in mice and cytotoxicity was tested
by measuring the inhibitory effect on L1210 cell proliferation
Flavonoids are a very important group of bioligands and their
activity is well recognized, e.g. as antiviral, antibacterial, cytostatic
and naturally occuring antioxidants. The flavonoid ligands them-
selves have anticancer activity as they are more toxic in cancer
cells than in normal ones [1,2]. In the course of search for new anti-
tumor agents the synthetic derivatives of flavone, the ring system
which belongs to the flavonoids, were intensively examined. In a
previous study, we have reported the results of the synthesis, spec-
troscopy and in vitro alkylating activity of the platinum(II) and pal-
ladium(II) complexes of oximes of flavanone and their O-methyl
ethers [3]. cis-Pt(II) complex of 3-aminoflavone (3-af), cis-bis(3-
aminoflavone)dichloridoplatinum(II) (cis-BAFDF) is structurally
related to cis-DDP, containing a flavone molecule instead of the
ammine with two cis bound labile chlorido ligands [4]. This com-
pound exhibited significant antitumor activity against the develop-
[5,6]. The new analogue induced apoptosis and necrosis and
showed a weaker genotoxicity than cisplatin [7,8]. It has been
proved that the new analogue induced a higher percentage of
P53 and BAX expressions in A549 cells in comparison with cis-
DDP [9]. Moreover, the cis-Pt(II) complex of 3-aminoflavone
showed a weaker genotoxic effect in normal lymphocytes in com-
parison with cis-DDP, which was found to be a stronger inducer of
apoptosis and necrosis [10].
Metal ions relevant in biology have been chosen for the present
investigation of 3-af. Copper and zinc compounds are widely used
in biochemistry (enzymes) and medicine; nickel is an essential
trace element present in many hydrogenases and cobalt as constit-
uent of vitamin B12 (which is necessary for the synthesis of nucleic
acids) are essential in living systems. Besides, the copper ion as a
component of novel organic complexes was found as a promising
metal in relation to their antitumor activity [11,12]. Recently, cop-
per complexes of 2-furaldehyde oxime have been found to exhibit
higher cytotoxic activity against murin and inhibit L1210 lymphoid
leukemia DNA in comparison to etoposide [13]. Other research has
* Corresponding author. Tel.: +48 42 677 92 16; fax: +48 426779220.
_
E-mail addresses: zurowska@chem.uni.wroc.pl (B. Zurowska), andrea.erxle-
ben@nuigalway.ie (A. Erxleben), ochocki@ich.pharm.am.lodz.pl (J. Ochocki).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.04.014

_
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B. Zurowska et al. / Inorganica Chimica Acta 362 (2009) 739–744
2.2. Crystal structure determination of 3-af and [Cu(3-af)2(NO3)2] (1)
Crystal data of 3-af and [Cu(3-af)2(NO3)2] (1) were collected at
room temperature on an Enraf-Nonius-KappaCCD diffractometer
using graphite-monochromated Mo Ka radiation (k = 0.71069 Å).
For data reduction and cell refinement the programs DENZO and
SCALEPACK were used [21]. The structure was solved by conventional
Patterson methods for (1) or direct methods for (3-af) and subse-
quent Fourier syntheses and refined by full-matrix least squares
Fig. 1. Lewis formula of the ligand 3-aminoflavone.
on F2 using the SHELXTL
ing factors were those given in the SHELXTL
-PLUS and SHELXL-93 programs [22]. The scatter-
-
PLUS program. All non-
shown that a copper complex with tetrabenzotetraazacyclohexade-
cine ligand is a promising compound against the melanoma B16
and leukemia P388 [14,15]. There are some copper compounds, that
exhibit antiproliferative activity by themselves [16,17].
hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms in 1 were located in the Fourier-dif-
ference map and refined isotropically. In the structure of free 3-
af ligand hydrogen atoms were generated geometrically and given
fixed isotropic thermal parameters in order to save parameters.
Crystallographic data and details of refinement for 3-af and Cu(3-
af)2(NO3)2 (1) are reported in Table 1.
3-Aminoflavone (3-af) (Fig. 1) as a synthetic bioligand contains
both O (carbonyl) and N (amine) sites to form chelate complexes as
resulted from X-ray crystal structure of [Cu(3-af)2(ClO4)2] as was
presented by us earlier [18]. Here, we are interested in the synthe-
sis of nitrate transition metal complexes (M = Cu(II), Ni(II), Co(II),
Zn(II) ions) of 3-af. In this paper, we present the synthesis, spectro-
scopic and magnetic properties (for paramagnetic ions) of [Cu(3-
af)2(NO3)2] (1), [Ni(3-af)2(H2O)2](NO3)2 ꢀ 2H2O (2), [Co(3-af)2(-
H2O)2](NO3)2 (3) and [Zn(3-af)2(H2O)2](NO3)2 (4) compounds.
Additionally, the coordination compounds have been characterized
by powder diffractometry. For [Cu(3-af)2(NO3)2] (1) compound, the
X-ray crystal structure has been determined.
2.3. Synthesis of 3-af and complexes 1, 2 and 3
The starting compound flavanone was purchased from Sigma–
Aldrich CO (St. Louis, MO, USA). 3Z-Hydroxyiminoflavanone was
prepared according to the procedure described elsewhere [23,24].
Infrared spectra of KBr pellets were measured with an ANTI MATT-
SOM IFS FT spectrometer.
2.3.1. 3-Aminoflavone (3-af)
2. Experimental
To the solution of 3Z-hydroxyiminoflavanone (3.0 g, 1.2 mmol)
in a glacial acetic acid (72 mL) with concentrated hydrochloric acid
(12 mL), SnCl2 (6.0 g, 3.2 mmol) was added. The mixture was vigor-
ously stirred at room temperature for 6 h. After this time the pre-
cipitate was filtered off, washed with water to pH about 7 and
recrystallized from methanol to give 3-aminoflavone (3-af) as yel-
low needles (2.3 g, yield 78% m.p. 138–139 °C) [24]. Calc. for
C15H11NO2 (m.mol. 237.25): C, 75.94; H, 4.67; N, 5.90. Found: C,
75.78; H, 4.51; N, 5.85%.
2.1. Reagents and physical measurements
Melting points were determined with a Beathius apparatus and
have not been uncorrected. Elemental analyses were carried out
using Perkin Elmer PE 2040. Infrared spectra were recorded on a
Bruker IFS 113v spectrometer and Infinity MI 60 series FTIR using
KBr pellets and solid-state electronic spectra (28000–4000 cmꢁ1
)
on a Cary 500 spectrophotometer. 1H NMR spectra were recorded
with Tesla BS-567A spectrometer (300 MHz), chemical shifts (d)
are reported in ppm in respect to TMS as an internal standard.
EPR spectra of the polycrystalline samples were recorded at X-
band at room temperature and 77 K on an ESP 300E Bruker spec-
trometer with an ER 035M Bruker NMR Gaussmeter and a Hewlett
Packard microwave frequency counter.
Table 1
Crystal data and structure refinement for 3-af and Cu(3-af)2(NO3)2 (1)
3-af
1
Formula
Fw
C15H11NO2
237.25
C30H22CuN4O10
662.06
Magnetic susceptibility measurements were carried out on so-
lid polycrystalline samples with a Quantum Design SQUID magne-
tometer (type MPMSXL-5). Measurements were recorded at a
magnetic field of 0.5 T in the temperature range 1.8–300 K. Correc-
tions were based on subtracting the sample holder signal and esti-
mating the contribution vD from the Pascal constants [19]. The
Crystal habit
Crystal dimensions (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
colorless needle
0.10 ꢂ 0.10 ꢂ 0.3
monoclinic
P21/n
3.887(1)
22.007(1)
13.199(1)
green block
0.12 ꢂ 0.15 ꢂ 0.21
triclinic
ꢀ
P1
5.450(1)
11.125(1)
12.092(1)
102.43(1)
100.35(1)
98.64(1)
690.6(2)
1
effective
magnetic
moments
were
calculated
from
leff = 2.83(vMT)1/2 using temperature-independent paramagnetism
(TIP) of 60 ꢂ 10ꢁ6 cm3 molꢁ1, 220 ꢂ 10ꢁ6 cm3 molꢁ1 and 150 ꢂ
10ꢁ6 cm3 molꢁ1 for Cu(II), Ni(II) and Co(II), respectively [20].
X-ray powder-diffraction patterns were collected at room tem-
perature using PANalytical X’Pert Pro MPD diffractometer with the
copper long line fine focus X-ray tube and the PANanalitical X’Cel-
erator detector based on the real time multiple strip technology.
The latter is capable of simultaneously measuring the intensities
within the O range of 2.122°. Divergent optics in a Bragg–Brentano
(flat-plate sample) geometry with fixed divergence and antiscatter
slits was applied. Additionally incident and receiving 0.04 rad Sol-
ler slits were used to limit axial divergence and a nickel filter on
the receiving side was applied to eliminate Cu Kb radiation. Data
were collected in the range 4° < O < 35° with the scan width
0.0167° and scan exposition 20 s.
97.32(1)
c (°)
V (Å3)
Z
1119.9(3)
4
496
0.71069
1.407
0.094
6492/2204 (0.066)
980
163
0.042
F(000)
k (Å)
339
0.71069
1.592
qcalc. (g cmꢁ3
)
l(Mo Ka) (mmꢁ1
)
0.86
Reflections collected/unique (Rint
Observed reflectionsa
Parameters
)
6340/1646 (0.045)
1445
249
0.037
0.099
a,b
R1
a,c
wR2
0.087
0.240/ꢁ0.173
(D/q) maximum/minimum (e Åꢁ3
)
0.280/ꢁ0.431
a
Observation criterion: I > 2r(I).
P
P
b
c
R1
=
jjFoj ꢁ jFcjj/ jFoj.
h
.
i
1=2
P
P
2
2
wR2
¼
wðF2o ꢁ F2c Þ
wðF2oÞ
.

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B. Zurowska et al. / Inorganica Chimica Acta 362 (2009) 739–744
741
2.3.2. [Cu(3-af)2(NO3)2] (1)
To the solution of 3-af (237 mg, 1 mmol) in methanol (20 mL),
copper nitrate (148 mg, 0.5 mmol) was added. The mixture was
vigorously stirred at room temperature for 4 h. The microcrystal-
line powder of complex was filtered off and recrystallized from
methanol giving a dark-green complex (m.p. 179–181 °C prisms).
Yield: 280 mg (85%). Selected IR data (KBr, cmꢁ1) m: 3424 (H2O);
3062, 3024 (NH2); 1621 (CO); 1408, 1298 (NO3). Calc. for
C30H22CuN4O10 (m.mol. 661.8): C, 54.44; H, 3.32; N, 8.46. Found:
C, 54.18; H, 3.41; N, 8.40%.
2.3.3. [Ni(3-af)2(H2O)2](NO3)2 ꢀ 2H2O (2)
To the solution of 3-af (237 mg, 1 mmol) in dry ethyl acetate
(50 mL), nickel nitrate (146 mg, 0.5 mmol) was added. The reaction
mixture was stirred under reflux for 6 h. The microcrystalline pow-
der of complex was filtered off and washed with dry diethyl ether
(m.p. 273–276 °C). Yield: 260 mg (71%). Selected IR data (KBr,
cmꢁ1) m: 3424 (H2O); 3295, 3233 (NH2); 1621 (CO); 1408, 1384,
1317 (NO3). Calc. for C30H30NiN4O14 (m.mol. 729.0): C, 49.42; H,
4.11; N, 7.68. Found: C, 49.06; H, 3.88; N, 7.91%.
Fig. 2. X-ray powder diffractograms of 3-af complexes.
The crystal packing of the free ligand is stabilized by one weak
intermolecular hydrogen bond C(4)–H(4)ꢀ ꢀ ꢀO(2) (ꢁ1/2 + x, 1/2 ꢁ y,
1/2 + 2) [C(4)ꢀ ꢀ ꢀO(2) 3.229(3) Å]. Additionally, aromatic p–p stack-
ing interactions between molecules along x-axis are observed. The
distance between respective ring centroids is 3.586(2) Å (Fig. 5).
In 1 the 3-aminoflavone (3-af) ligand chelates the copper atom,
forming a five-membered ring. The intraligand geometry is normal
in this structure and compares well with that of the free ligand. The
copper atom is surrounded by six symmetrically related atoms,
namely two carbonyl oxygens and two amino nitrogens from
two 3-af ligands in the basal plane in trans conformation, and, at
longer distances, two oxygen atoms from two nitrate anions in ax-
ial positions. The length of the Cu–O (from nitrate anions) bond,
being 2.427(3) Å is significantly longer compared with those be-
tween Cu and the ligand donor atoms, which are 1.973(2) (Cu–O)
and 1.986(3) Å (Cu–N), respectively, which causes a significant dis-
tortion of the coordination polyhedron around the Cu(II) ion.
Therefore, the polyhedron can be described as a tetragonally dis-
torted octahedron symmetrically elongated along Cu–O (from
coordinated nitrate groups) direction. The values observed for the
Cu–N and Cu–O distances are within reasonable bonding distances
and comparable with similar CuN2O4 chromophores [25–28]. The
C@O bond lengths in 1 and in the free ligand are equal within
the experimental error (1.254(4) Å in 1, 1.244(2) Å in AF) and con-
sistent with a C@O double bond. However, Cu coordination moves
the exocyclic oxygen slightly out of the ring plane (distance to the
best-weighted plane 0.224(4) Å in 1 versus 0.004(2) Å in 3-af). The
exocyclic nitrogen is coplanar with the ring plane (distance to the
best-weighted plane 0.022(5) Å for 1, 0.034(2) Å for AF). Binding of
Cu leads to an increase in the C–N bond length of about 0.04 Å
1.437(4) Å in 1, 1.396(2) in free 3-af. In 1, a twist angle of
39.2(1)° is observed between the phenyl ring and the heterocyclic
ring which compares well with that found in the free ligand
(40.66(4)°). The nitrate anion has its expected planar trigonal
geometry. The nitrogen–oxygen bond lengths and intraanion bond
angles are average (1.220–1.268 Å) and (119.1–121.1°). The amino
group of 3-af forms an intramolecular hydrogen bond with nitrate
oxygen (N(1)ꢀ ꢀ ꢀO(5a) 2.875(4), N(1)ꢀ ꢀ ꢀHꢀ ꢀ ꢀO(5a), 167(4)°, ꢁx, ꢁy,
ꢁz). In the crystal packing, molecules stack along the x-axis with
stacking distances between 3-aminoflavone moieties being 3.8 Å
2.3.4. [Co(3-af)2(H2O)2](NO3)2 (3)
To the solution of 3-af (237 mg, 1 mmol) in dry ethyl acetate
(50 mL), cobalt nitrate (146 mg, 0.5 mmol) was added. The reaction
mixture was stirred under reflux for 6 h. The microcrystalline pow-
der of complex was filtered off and washed with dry diethyl ether
(m.p. 222–226 °C). Yield: 270 mg (78 %). Selected IR data (KBr,
cmꢁ1) m: 3424 (H2O); 3306, 3236 (NH2); 1621 (CO); 1408, 1384,
1317 (NO3). Calc. for C30H26CoN4O12 (m.mol. 693.3): C, 51.97; H,
3.75; N, 8.08. Found: C, 51.77; H, 3.49; N, 8.19%.
2.3.5. Zn[(3-af)2(H2O)2](NO3)2 (4)
To the solution of 3-af (237 mg, 1 mmol) in dry ethyl acetate
(50 mL), zinc nitrate (149 mg, 0.5 mmol) was added. The reaction
mixture was stirred under reflux for 6 h. The microcrystalline pow-
der of complex was filtered off and washed with dry diethyl ether
(m.p. 225–229°C). Yield: 230 mg (67%). Selected IR data (KBr,
cmꢁ1) m: 3433 (H2O); 3206, 3059 (NH2); 1621 (CO); 1406, 1384,
1293 (NO3). Calc. for C30H26ZnN4O12 (FW 699.9): C, 51.48; H,
3.74; N, 8.12. Found: C, 51.39; H, 3.66; N, 8.16%. 1H NMR (DMSO-
d6) d: 4.73 (s, 4H, NH2, disappear in D2O), 7.41–7.79 (m, 12H,
arom.), 7.97 (d, 4H, arom.), 8.10 (dd, 2H–C5).
The attempts of the preparation of Co(II), Ni(II) and Zn(II) com-
pounds were unsuccessful so far.
3. Results and discussion
We have synthesized a series of M(II) nitrate complexes with 3-
aminoflavone (3-af) ligand. The stoichiometry of the investigated
complexes was established from the elemental analysis. The ana-
lytical results demonstrated that the 3-af ligand forms coordina-
tion compounds of formula [Cu(3-af)2(NO3)2] (1), [Ni(3-af)2-
(H2O)2](NO3)2 ꢀ 2H2O (2), [Co(3-af)2(H2O)2](NO3)2 (3) and
Zn[(3-af)2(H2O)2](NO3)2 (4). Single X-ray crystal studies of [Cu(3-
af)2(NO3)2] (1) compound were undertaken to elucidate the coordi-
nation sphere of copper(II). The nickel, cobalt and zinc compounds
were found as structurally almost isomorphous, because their
X-ray powder patterns are identical (Fig. 2).
Table 2
Selected bond lengths (Å) and angles (°) in [Cu(3-af)2(NO3)2] (1)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(1)–O(3)
N(1)–Cu(1)–O(2)
O(2)–Cu(1)–O(3)
N(1)–Cu(1)–O(2)a
1.986(3)
1.973(2)
2.427(3)
85.0(1)
95.3(1)
95.0(1)
3.1. Description of the structure of AF and [Cu(3-af)2(NO3)2] (1)
Crystal data of 3-af and copper(II) compound (1) are summa-
rized in Table 2, relevant bond lengths and angles for 1 are listed
in Table 1. The molecular structures of the free 3-af ligand and of
[Cu(3-af)2(NO3)2] are depicted in Figs. 3 and 4.
a
Symmetry operation: ꢁx, ꢁy, ꢁz.

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B. Zurowska et al. / Inorganica Chimica Acta 362 (2009) 739–744
Fig. 3. Molecular structure and labeling scheme of 3-af.
Fig. 6. Crystal packing of [Cu(3-af)2(NO3)2] (1) viewed along the x-axis.
reported isostructural trans-Cu(3-af)2(ClO4)2 complex [18] in
which central copper atom is significantly deformated towards a
tetragonal bipyramid in which perchlorate oxygen atoms occupy
apical positions.
3.2. Spectroscopic properties
The infrared spectrum of 1 shows NO3 bands at 1299 and
1408 cmꢁ1 typical for unidentate coordination of the nitrate ion,
which is confirmed by the structure determination [29]. The infra-
red spectra of 2, 3 and 4 show a very strong band at about
1384 cmꢁ1, characteristic of an ionic nitrate [29]. Compounds 2,
3 and 4 exhibit in the O–H stretching region, a strong and sharp
absorption at about 3430 cmꢁ1 due to coordinated water molecule
involved in the Oꢀ ꢀ ꢀH hydrogen bond, while the IR spectrum of 1
shows the absence of all vibrations that might be attributed to
the presence water. Compounds 2, 3 and 4 exhibit two sharp
absorptions, respectively, associated [30] with the NH2 groups (2:
3295, 3233 cmꢁ1, 3: 3306, 3236 cmꢁ1, 4: 3206, 3059 cmꢁ1). For 1
mNH is at 3020sh, 3063 cmꢁ1. The ligand exhibits two mNH stretching
frequencies (3314, 3404 cmꢁ1) associated with NH2 groups in 3-af.
The absorption band at 1621 cmꢁ1, which corresponds to C@O
stretching and these associated with NH2 group of the free ligand
are shifted towards lower frequencies, suggesting their participa-
tion in coordination of the metal ions.
Fig. 4. The molecular structure and labeling scheme of [Cu(3-af)2(NO3)2] (1). Hy-
drogen atoms are omitted for clarity.
The ligand-field spectrum of 1 shows a broad asymmetric band
with maximum centered at 15720 cmꢁ1 indicating an elongated
octahedral geometry for the copper ion which is confirmed by
the structure determination. The ligand-field spectra of 2 (16610,
10470 cmꢁ1) and 3 (20570, 9470 cmꢁ1) are typical for octahedral
high-spin complexes of these divalent metal ions [31]. The spectro-
chemical parameters calculated are 10Dq = 10470 cmꢁ1 for 2 and
10Dq = 10350 cmꢁ1, B = 820 cmꢁ1 for 3 [32,33]. The values of Dq
determined for Ni(II) and Co(II) complexes are in good agreement
with those expected for isomorphous nickel and cobalt compounds
and with MN2O4 chromophore [32,33]. The rather high value of B
for 3 may suggest a distorted octahedral geometry [31]. For 2 the
B value cannot be calculated because the highest energy triplet–
triplet transition is not observed, masked by strong UV absorption.
The 10Dq values for both complexes show that the nearest coordi-
Fig. 5. Crystal packing of 3-aminoflavone. Hydrogen bonds are shown with dashed
line.
(Fig. 6). Intermolecular hydrogen bonds are formed between the
amino group of 3-af and nitrate oxygen (N(1)ꢀ ꢀ ꢀO(3), 2.865(5) Å,
1 ꢁ x, ꢁy, ꢁz). Quite similar arrangement has been found in already

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B. Zurowska et al. / Inorganica Chimica Acta 362 (2009) 739–744
743
nation sphere produces a strong ligand field. Asymmetric bands in
the near-IR region of 2 and 3 show that the nickel(II) and cobalt(II)
ions are in a tetragonally distorted N2O4 environment.
lated for a spin quartet (vMT = 1.87 cm3 molꢁ1 K, 3.87lB, for S = 3/2
with g = 2.0), but it agrees with the values observed for octahedral
cobalt(II) complexes with a significant first-order orbital contribu-
4
The X-band EPR spectrum of the polycrystalline sample of
tion to the magnetic moment typical for the T1g ground state
[Cu(3-af)2(NO3)2] (1) is an axial type showing g = 2.375 and
[33,19]. The vMT plot (Fig. 7) gradually decreases with temperature
and reaches the minimum value of 1.63 cm3 K molꢁ1 (leff = 3.61 lB)
at 1.8 K; therefore Co(II) ion remains totally high-spin down to
1.8 K. The vMT values follow the Curie–Weiss law from room tem-
perature to 150 K with C = 2.99 cm3 K molꢁ1 and H = ꢁ1.7 K. The
value of ꢁ20 K is lower that expected for an isolated Co(II) with
a spin–orbit constant of ꢁ170 cmꢁ1 [19].
k
g\ = 2.040 at r.t., and g = 2.379 and g\ = 2.046 at 77 K. The hyper-
k
fine splittings are observed in room temperature and 77 K. The
hyperfine splitting of the parallel component is well resolved with
A = 160 G. The observed g is a high for 3-af chelating ligand. This
k
k
fact seems to indicate that the powder spectrum does not reflect
molecular g value. There are no substantial differences between
the spectra at the two temperatures. [Ni(3-af)2(H2O)2](-
NO3)2 ꢀ 2H2O (2) compound does not exhibit any signal and
[Co(3-af)2(H2O)2](NO3)2 (3) shows no lines at room temperature
and only broad single line at 77 K at g = 4.57.
Generally, the negative value of H for an isolated cobalt(II) ion is
due to the overall effect of low-symmetry crystal-field components
and spin–orbit coupling, resulting in a gradual transformation of an
isotropic S = 3/2 at high temperature to an anisotropic effective
S = 1/2 at low temperature. The calculated very small negative va-
lue of H (ꢁ1.7 K), which is lower than the expected value for an
isolated Co(II), suggests an overall effect of the weak ferromagnetic
interaction producing an increase of vMT values and of small anti-
ferromagnetic exchange in the lowest temperature range, produc-
ing a decrease of vMT values. Because both 2 and 3 complexes have
been found to be mutually isomorphous by X-ray powder determi-
nation, a ferromagnetic exchange interaction observed for 2 cannot
be excluded between Co(II) centers in 2.
3.3. Magnetic properties
Variable-temperature (1.8–300 K) magnetic susceptibility data
were collected for 1, 2 and 3. The magnetic behavior of complexes
1, 2 and 3 is depicted in Fig. 7 in the form of vMT versus T ,
respectively.
The effective magnetic moment (leff = 1.84 lB) at room temper-
ature for Cu(II) complex (1) is within usually observed range of
experimental values for the copper(II) complexes in octahedral
configuration [34]. The values of vMT are slightly dependent on
the temperature and decreases only at the lowest temperature re-
gion (Fig. 7). The negative Weiss constant h values of ꢁ0.62 K, ob-
tained from the Curie–Weiss law [35] within the measured
temperature region, suggest the possibility of a very weak antifer-
romagnetic interaction between magnetic centers inside crystal
lattice in lower temperatures (see Section 3.1).
Finally, on the basis of the spectroscopic properties of 2 and 3
compounds mononuclear structure is proposed. Unusual magnetic
properties observed for these compounds clearly indicate magnetic
exchange between metal centers. Probably these interactions occur
within crystal lattice through H-bonds.
4. Conclusions
The value of vMT of 2 at room temperature is 1.56 cm3 K molꢁ1
(leff = 3.53 lB), as compared to the expected spin-only value of
1.21 cm3 K molꢁ1 for S = 1 with g = 2.20. The vMT value follows the
quasi-Curie law from room temperature to 100 K: H = 8.3 K and
C = 1.51 cm3 K molꢁ1. At lower temperature it increases sharply to
reach a maximum of 2.21 cm3 K molꢁ1 at 12 K (leff = 4.21 lB) indicat-
ing weak ferromagnetic interaction. After the maximum, the vMT va-
lue decreases to 0.92 cm3 K molꢁ1 at 1.8 K (leff = 2.73 lB) due to zero
field splitting and/or weak antiferromagnetic coupling (Fig. 7).
The bidentate 3-aminoflavone (3-af) ligand has been synthe-
sized and its crystal structure has been determined. Attempts to
prepare divalent copper, nickel, cobalt and zinc complexes were
successful. In the mononuclear copper(II) complex the ligand forms
a five-membered chelate ring with Cu, as inferred from the crystal
structure of [Cu(3-af)2(NO3)2] (1). The Cu(II) ion has a tetragonally
distorted coordination sphere formed by two N,O-chelate ligands
in trans arrangement and two nitrate ions.
So far it has not been possible to obtain single crystals suitable
for X-ray measurement of 2, 3 and 4 compounds, but the spectro-
scopic data indicate mutually similar distorted octahedral sur-
roundings for the metal ions and monomeric structure. The
unusual magnetic properties observed for 2 and 3 will deal with
subsequent studies.
Compound
(leff = 4.77 lB) at room temperature which is larger than that calcu-
3
exhibits the vMT value of 2.85 cm3 K molꢁ1
Acknowledgments
We thank Prof. J. Reedijk (Leiden) for helpful discussions and
´
reading the manuscript. We also thank Prof. W. Wolf (Łódz) for col-
lecting X-ray powder diffractograms. This study was supported by
the Grant 4T09A 11523 of the Polish Ministry of Science and Com-
puterization (J.M.), and by the Grant Nos. 502-13-777 and 503-
´
3016-2 from the Medical University of Łódz (J.O.). J.O. acknowl-
edges gratefully a fellowship from the Deutscher Akademischer
Austauschdienst (DAAD).
Appendix A. Supplementary material
CCDC 637639 and 637638 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
Fig. 7. Plot of vM
NO3)2 ꢀ 2H2O (2) and (ꢃ) [Co(3-af)2(H2O)2](NO3)2 (3).
T vs. T for (M) Cu(3-af)2(NO3)2 (1), (s) [Ni(3-af)2(H2O)2](-

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B. Zurowska et al. / Inorganica Chimica Acta 362 (2009) 739–744
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