Two novel cyclic dimer M2L2 complexes constructed from 2-[(4-carboxyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide: Crystal structure and magnetic properties

Article abstract of DOI:10.1016/j.poly.2012.06.076

Two novel complexes derived from 2-[(4-carboxyl)phenyl]-4,4,5,5- tetramethylimidazoline-1-oxy-3-oxide (NITPBAH), [Mn(hfac)₂(NITPBAH)] ₂ (1) and [Co(hfac)₂(NITPBAH)]₂ (2) (hfac = hexafluoroacetylacetonate), have been synthesized as well as characterized structurally and magnetically. The X-ray crystal structure analyses show that the structures of the two compounds are isomorphic and both consist of cyclic dimer units bridged by NITPBAH ligands, in which metal ion Mn(II) or Co(II) is coordinated by four oxygen atoms from two hfac ligands, one oxygen atom from nitronyl nitroxide and one oxygen atom from carboxyl group of another radical ligand. The magnetic studies show that the manganese(II) or cobalt(II) ion strong antiferromagnetically interacts with the directly coordinated nitroxide group. There exists a weak antiferromagnetic coupling between the metal ion and the nitroxide through phenyl ring and carboxyl group of the radical ligand, which is agreement with spin polarization mechanism.

Full text of DOI:10.1016/j.poly.2012.06.076

Polyhedron 52 (2013) 1053–1058
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Two novel cyclic dimer M2L2 complexes constructed from
2-[(4-carboxyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide: Crystal
structure and magnetic properties
⇑
Haixia Tian, Yungai Li, Congming Zhang, Xuelan Mei, Peng Hu, Licun Li
Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry, Nankai University, Tianjin 300071, China
Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 10 July 2012
Two novel complexes derived from 2-[(4-carboxyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxy-3-
oxide (NITPBAH), [Mn(hfac)2(NITPBAH)]2 (1) and [Co(hfac)2(NITPBAH)]2 (2) (hfac = hexafluoroacetylace-
tonate), have been synthesized as well as characterized structurally and magnetically. The X-ray crystal
structure analyses show that the structures of the two compounds are isomorphic and both consist of
cyclic dimer units bridged by NITPBAH ligands, in which metal ion Mn(II) or Co(II) is coordinated by four
oxygen atoms from two hfac ligands, one oxygen atom from nitronyl nitroxide and one oxygen atom from
carboxyl group of another radical ligand. The magnetic studies show that the manganese(II) or cobalt(II)
ion strong antiferromagnetically interacts with the directly coordinated nitroxide group. There exists a
weak antiferromagnetic coupling between the metal ion and the nitroxide through phenyl ring and car-
boxyl group of the radical ligand, which is agreement with spin polarization mechanism.
Ó 2012 Elsevier Ltd. All rights reserved.
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel prize in Chemistry
in 1913.
Keywords:
Nitronyl nitroxide
Crystal structure
Cyclic dimmer
Magnetism
1. Introduction
oxides substituted by a nitrogen-containing group, which can not
function as a chelating ligand such as 3-pyridyl [27] and phenylpy-
The increasing attention has been focused on the coordination
complexes of paramagnetic metal ions with organic radical ligands
in order to exploit new molecular magnetic materials involving
practical magnetic properties and diversified topological structures
[1–5]. Among the organic paramagnetic molecules, nitroxide [6],
verdazyl [7], semiquinone [8] and thiazyl [9] radical ligands have
been used extensively. In particular, nitronyl nitroxide derivatives
are one of the most explored in this strategy [10]. So far, a large
quantity of complexes based on diverse nitroxyl nitroxides with
various topological structures and magnetic behaviors have been
prepared and characterized [11–20]. Recently, a few interesting
cyclic metal–radical complexes by the means of nitroxide radical
ligands have been described [21–26]. Among these cyclic com-
plexes, some show particularly interesting temperature-dependent
spin-transition-like behavior [17,18]. Such cyclic complexes are
important. They are good candidates not only for fundamental
studies of magnetic-structural correlations, especially how the
structural factors affect the interaction between metal and radical,
but also for the development of novel molecular magnetic materi-
als. One method for obtaining such cyclic complexes is to use nitr-
rimidyl groups [28]. Along this line, we use 2-[(4-carboxyl)phenyl]-
4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide (NITPBAH) with
non-chelating carboxyl group to build new cyclic metal-radical
complexes. In this paper, we report on the synthesis, crystallogra-
phy and magnetic characterization of two new cyclic complexes
[M(hfac)2(NITPBAH)]2 (M = Mn(II) 1, Co(II) 2).
2. Experimental
2.1. Materials and physical measurements
All reagents and solvents were purchased from commercial
sources and used as received. NITPBAH radical ligand was prepared
from 4-carboxybenzaldehyde by the method similar to a literature
[29]. Elemental analysis for carbon, hydrogen and nitrogen were
carried out on a Perkin–Elmer elemental analyzer model 240.
Infrared spectra were taken on a Bruker Tensor 27 Fourier trans-
form infrared spectroscopy in the region 4000–400 cmꢀ1, using
KBr pellets. Temperature dependence of magnetic susceptibilities
was performed on a Quantum Design SQUID magnetometer work-
ing at 1000 G field strength in the 2–300 K temperature range. Dia-
magnetic corrections were made with Pascal’s constants for all the
constituent atoms.
⇑
Corresponding author at: Department of Chemistry, Key Laboratory of
Advanced Energy Materials Chemistry, Nankai University, Tianjin 300071, China.
E-mail address: llicun@nankai.edu.cn (L. Li).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.076

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H. Tian et al. / Polyhedron 52 (2013) 1053–1058
2.2. Synthesis of complexes
2.2.1. Synthesis of [Mn(hfac)2(NITPBAH)]2
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1 and 2.
1
Complex 1
Complex 2
A solution of Mn(hfac)2ꢁ2H2O(20 mg, 0.04 mmol) in 20 mL dry
heptane was refluxed for 3 h to azeotropically remove hydration
water molecules. After that, the solution was cooled to 60 °C, and
then a solution of NITPBAH (11 mg, 0.04 mmol) in 4 mL dry CH2Cl2
was added. The resulting dark blue solution was stirred for 10 min
at this temperature, cooled to room temperature and filtered. The
filtrate was allowed to stand at room temperature for slow evapo-
ration. After one week, the dark blue crystals suitable for single
crystal diffraction were collected. Yield: 63%. Anal. Calc. for
Mn2C48H38N4O16F24: C, 38.62; N, 3.75; H, 2.57. Found: C, 38.32;
N, 3.79; H, 2.72%. FT-IR (KBr, cmꢀ1): 1647(w), 1617(m), 1505(s),
1254(s), 1196(s), 1142(s), 792(m), 663(m).
Mn(1)–O(7)
Mn(1)–O(4)
Mn(1)–O(3)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–O(5)
O(8)–N(2)
O(7)–N(1)
N(1)–O(7)–Mn(1)
O(7)–Mn(1)–O(4)
O(7)–Mn(1)–O(3)
O(4)–Mn(1)–O(3)
O(7)–Mn(1)–O(1)
O(4)–Mn(1)–O(1)
O(3)–Mn(1)–O(1)
O(7)–Mn(1)–O(2)
O(4)–Mn(1)–O(2)
O(3)–Mn(1)–O(2)
O(1)–Mn(1)–O(2)
O(7)–Mn(1)–O(5)
O(4)–Mn(1)–O(5)
O(3)–Mn(1)–O(5)
O(1)–Mn(1)–O(5)
O(2)–Mn(1)–O(5)
2.118(2)
2.126(2)
2.131(2)
2.165(2)
2.189(2)
2.226(2)
1.273(3)
1.303(3)
126.65(18)
89.68(8)
172.06(9)
84.62(8)
89.73(8)
173.82(10)
96.52(8)
96.14(9)
92.91(10)
89.68(9)
81.03(8)
92.07(8)
102.16(10)
83.76(9)
84.01(8)
162.88(8)
Co(1)–O(2)
Co(1)–O(1)
Co(1)–O(4)
Co(1)–O(3)
Co(1)–O(5)
Co(1)–O(6)
O(1)–N(2)
O(8)–N(1)
N(2)–O(1)–Co(1)
O(2)–Co(1)–O(1)
O(2)–Co(1)–O(4)
O(1)–Co(1)–O(4)
O(2)–Co(1)–O(3)
O(1)–Co(1)–O(3)
O(4)–Co(1)–O(3)
O(2)–Co(1)–O(5)
O(1)–Co(1)–O(5)
O(4)–Co(1)–O(5)
O(3)–Co(1)–O(5)
O(2)–Co(1)–O(6)
O(1)–Co(1)–O(6)
O(4)–Co(1)–O(6)
O(3)–Co(1)–O(6)
O(5)–Co(1)–O(6)
2.019(2)
2.037(3)
2.040(2)
2.043(3)
2.072(2)
2.142(2)
1.295(4)
1.270(4)
124.1(2)
90.39(10)
176.82(10)
87.80(10)
90.35(10)
173.53(9)
91.74(10)
89.68(10)
98.35(10)
88.00(9)
88.09(10)
94.69(10)
90.00(10)
87.92(9)
83.53(10)
170.55(10)
2.2.2. Synthesis of [Co(hfac)2(NITPBAH)]2
2
Complex 2 was synthesized using the same procedure for com-
plex 1 but with Co(hfac)2ꢁ2H2O instead of Mn(hfac)2ꢁ2H2O. Yield:
50%. Anal. Calc. for Co2C48H38N4O16F24: C, 38.42; N, 3.73; H, 2.55.
Found: C, 38.16; N, 3.69; H, 2.57%. FT-IR (KBr,cmꢀ1): 1644(w),
1616(m), 1524(s), 1256(s), 1197(s), 1143(s),791(m), 667(m).
2.3. X-ray crystal structure determinations
Diffraction intensity data of the single crystals of complexes 1
and 2 were collected on a Rigaku Saturn CCD diffractometer at
3. Results and discussion
3.1. Crystal structure
113 K employing graphite-monochromated Mo
Ka radiation
(k = 0.71073 Å). The structure was solved by direct methods by
using the program SHELXS-97 [30] and refined by full matrix least-
squares methods on F2 with the use of the SHELXL-97 [31] program
package. Anisotropic thermal parameters were assigned to all non-
hydrogen atoms. The hydrogen atoms were set in calculated posi-
tions and refined as riding atoms with a common fixed isotropic
thermal parameter. Disorders were observed in complexes 1 and
2 for some fluorine atoms. The pertinent crystallographic data
and structure refinement parameters for two complexes were
listed in Table 1. Selected bond lengths and angles of the com-
plexes 1 and 2 were presented in Table 2.
3.1.1. [Mn(hfac)2(NITPBAH)]2
1
The X-ray crystallographic analysis reveals that 1 crystallizes in
ꢀ
the triclinic space group P1. The molecular structure of 1 is shown
in Fig. 1. Two MnII ions are bridged by two NITPBA radicals to result
in a four-spin centrosymmetric cyclic dimer. Each Mn(II) ion lies in
a distorted octahedron environment, coordinated by four oxygen
atoms from two hfac ligands, one oxygen atom from the nitronyl
nitroxide unit and one oxygen atom from the carboxyl group of
the other radical ligand. The bond lengths of MnII–O(hfac) are in
the range of 2.126(2)–2.189(2) Å. The bond lengths of MnII–O(car-
boxyl) and MnII–O(nitroxide) are 2.226(2) and 2.118(2) Å, respec-
tively. These bond lengths are comparable to those of the
reported cyclic Mn-nitroxide dimers [21,24,26]. In NITPBAH radical
ligand, the O–N–C–N–O group containing the unpair electron
makes an angle of 30.16° with the phenyl ring. In the dimer unit,
two phenyl rings are parallel with a distance of 2.916 Å. The dis-
tance of two MnII ions is 9.413 Å. Intramolecular hydrogen bonding
interaction exists between oxygen atom(O1) from hfac group and
another oxygen atom(O6) from carboxyl group with a distance of
2.600 Å. In the packing arrangement of complex 1 (Fig. 2), the
shortest distance between Mn ions in different dimer units is
9.441 Å, while the shortest contact between the uncoordinated
N–O groups is 5.047 Å.
Table 1
Crystallographic and refinement data for complexes 1 and 2.
1
2
Formula
Mr
Mn2C48H38N4O16F24
1492.70
Co2C48H38N4O16F24
1500.68
T (K)
113(2)
113(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
triclinic
P1
ꢀ
ꢀ
9.441(3)
12.170(4)
13.425(4)
85.248(9)
87.057(7)
89.562(10)
1535.2(8)
1
9.139(4)
12.062(6)
13.982(6)
83.096(11)
87.295(16)
89.196(15)
1528.4(12)
1
a
(°)
b (°)
c
(°)
V (Å3)
Z
3.1.2. [Co(hfac)2(NITPBAH)]2
2
Dcalc (g cmꢀ3
)
1.615
0.550
1.630
0.682
The labeling diagram for the crystal structure of complex 2 is
depicted in Fig. 3. Complex 2 crystallizes in the triclinic P1 space
l
(mmꢀ1
)
ꢀ
h range (°)
Limiting indices
1.52–26.01
ꢀ11 6 h 6 10
ꢀ15 6 k 6 12
ꢀ16 6 l 6 16
5995
1.47–27.32
ꢀ10 6 h 6 11
ꢀ15 6 k 6 15
ꢀ17 6 l 6 18
6737
group, which consists of neutral dimer unit with inversion center.
Two NITPBAH ligands link two CoII ions to form a four-spin cyclic
complex. The CoII ions of the cyclic dimer exhibit a slightly dis-
torted octahedral coordination sphere with four oxygen atoms
from two hfac ligands, one oxygen from the nitronyl nitroxide unit
and one oxygen atom from the carboxyl group of the other radical.
The bond lengths of CoII–O(hfac) are in the range of 2.019(2)–
2.072(2) Å, while the bond lengths of Co(II)–O(carboxyl) and
Co(II)–O(nitroxide group) are 2.142(2) and 2.037(3) Å, respec-
Unique reflections (Rint
)
0.0406
0.0434
Goodness-of-fit (GOF)
1.027
0.991
R1[I > 2r(I)]
0.0564
0.0569
wR2 [I > 2r(I)]
0.1147
0.1496
R1(all data)
0.0816
0.0798
wR2(all data)
0.1273
0.1613

H. Tian et al. / Polyhedron 52 (2013) 1053–1058
Fig. 1. The cyclic dimer structure of complex 1. Hydrogen and fluorine atoms are omitted for clarity.
Fig. 2. The packing arrangement of complex 1.
1055
Fig. 3. The cyclic dimer structure of complex 2. Hydrogen and fluorine atoms are omitted for clarity.
tively. The dihedral angle between phenyl and the plane containing
3.2. Magnetic properties
The temperature dependence of magnetic susceptibility of com-
of O–N–C–N–O group is 37.04°. The distance of two metal ions is
9.305 Å in the dimer unit, in which the two phenyl planes are par-
allel with an interplanar distance of 2.947 Å. The shortest distance
between Co ions in different dimer units is 8.584 Å, while the
shortest contact between uncoordinated nitroxide groups is
4.004 Å (See Fig. 4).
plex 1 is plotted in Fig. 5. At room temperature the value of vMT is
6.63 emu K molꢀ1, which is much lower than spin-only value
(9.50 emu K molꢀ1) anticipated for the magnetically isolated two
S = 1/2 and two S = 5/2 spins, but close to spin-only value

1056
H. Tian et al. / Polyhedron 52 (2013) 1053–1058
Fig. 4. The packing arrangement of complex 2.
(6.00 cm3 K molꢀ1) expected for two isolated S = 2 spins. As shown
in Fig. 5, the reciprocal susceptibility varied as the temperature fol-
The magnetic data (300–50 K) were analyzed and a good fit of
the experimental data was obtained with g = 2.03,
lows the Curie–Weiss law, with
a
negative Weiss constant
J1 = ꢀ196.25 cmꢀ1 with R = 1.55 ꢂ 10ꢀ5. The strong antiferromag-
netic interaction is probably due to the smaller angle of Mn–O–N
(126.60(16)°), which results in the effective overlap of the mag-
netic orbitals. The determined J value is comparable to the reported
analogous literature values [24,32,33].
h = ꢀ2.64 K and Currie constant C = 6.48 cm3 K molꢀ1.Upon cool-
ing, the value ꢀo1f
vMT gradually decreases to reach a value of
at 50 K, and then decreases rapidly to
6.09 cm3 K mol
4.44 cm3 K molꢀ1 at 2.0 K. These results show there exists strong
antiferromagnetic coupling between the Mn(II) ion and the directly
coordinated nitroxide group.
Consider that the strong antiferromagnetic interaction between
the directly coordinated nitroxide group and the Mn(II) ion results
in one net spin with S = 2 below 50 K, so the magnetic data (2–
50 K) were fitted to Eq. (2) for pairing of S = 2 and the best fitness
to the magnetic data (2–50 K) produces g = 2.03, J2 = ꢀ0.20 cmꢀ1
and R = 1.10 ꢂ 10ꢀ5, indicating that the magnetic coupling be-
tween two Mn(II)–nitroxide units is very weak. The observed weak
magnetic coupling is due to the small spin density on the oxygen
atom of the carboxyl group (Chart 1).
Based on the analysis of crystal structure, there are mainly two
kinds of magnetic interactions for the present four-spin magnetic
system, i.e. the magnetic interaction between Mn(II) ion and the
directly coordinated nitroxide group (J1), and the magnetic cou-
pling between Mn(II) ion and nitroxide group through phenyl ring
and carboxyl group (J2).The magnetic interaction transmitted by
phenyl ring and carboxyl group should be weak, so the magnetic
behavior of complex 1 can be treated as two Mn-Rad magnetic
units, and the magnetic data were analyzed by a theoretical
expression (1) deduced from the spin Hamiltonian
Ng2b2
60 þ 28 expðꢀ4J2=kTÞ þ 10 expðꢀ7J2=kTÞ þ 2 expðꢀ9J2=kTÞ
kT 9 þ 7 expðꢀ4J2=kTÞ þ 5 expðꢀ7J2=kTÞ þ 3 expðꢀ9J2=kTÞ þ expðꢀ10J2=kTÞ
vM
¼
ð2Þ
^
^
^
:
H ¼ ꢀJ1SRadSMn
Recently, two isostructrual complexes Mn(NITPBA)2(H2O)2 [29]
and Co(NITPBA)2(H2O)2 [34] have been obtained by the reaction of
Ng2b2 10 expðꢀ3J1=kTÞ þ 28
the NITPBAH radical with MnII or CoII ion without hfac coligand. It
vM
¼
ð1Þ
kT
5 expðꢀ3J1=kTÞ þ 7
is interesting to compare complex
1 with complex Mn
(NITPBA)2(H2O)2: complex 1 is cyclic dimmer, while Mn(NIT-
50
40
30
20
10
0
7.0
6.5
6.0
5.5
5.0
4.5
Mn
N+
N
O
O
O
N
O
O
O
O
O
N+
Mn
0
50
100
150
T/K
200
250
300
Fig. 5.
fit.
v
MT and 1/vM vs. T plots for complex 1 and the solid line represents the best
Chart 1. Spin polarization mechanism for intramolecular magnetic coupling in
cyclic dimer derived from complex 1.

H. Tian et al. / Polyhedron 52 (2013) 1053–1058
1057
PBA)2(H2O)2 shows one-dimensional ladder structure. But their
magnetic behaviors are similar: the strong antiferromagnetic cou-
pling was observed between the directly coordinated NO group
and Mn(II) ion (ꢀ196 cmꢀ1 for complex 1 and ꢀ193 cmꢀ1 for
Mn(NITPBA)2(H2O)2) for both complexes and the magnetic interac-
tion mediated by NITPBA or NITBAH ligand through the phenyl ring
and carboxyl group is weak. These can be attributed to the same
coordination mode of NITPBA and NITPBAH radical ligands in both
complexes in which the NO groups of two radical ligands are di-
rectly coordinated to metal ions, resulting in strong antiferromag-
netic interaction.
unit through the phenyl ring and carboxyl group. A spin polariza-
tion mechanism can be used to explain the magnetic coupling in
the complex.
Acknowledgment
This work was supported by the National Science Foundation of
China (Nos. 20971072, 91122013, 90922032).
Appendix A. Supplementary material
The magnetic susceptibility of complex 2 in form of
M versus T plots are given in Fig. 6. Above 40 K, The reciprocal sus-
ceptibility varied as the temperature obeys the Curie–Weiss law
vMT and 1/
CCDC 870396 and 870397 contains the supplementary crystal-
lographic data for 1 and 2, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
v
with negative Weiss constant h = ꢀ32.1 K and Curie constant
C = 5.78 cm3 K molꢀ1
, indicating the global antiferromagnetic
interaction in complex 2. Using Eq. (3) with grad = 2.0, this leads
to gCo = 2.32, which is a reasonable value for octahedral high-spin
cobalt(II) complexes [35].
References
Nb2
3k
Â
Ã
C ¼
2g2radSradðSrad þ 1Þ þ 2gC2oSCoðSCo þ 1Þ
ð3Þ
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v
MT is 5.30 cm3 K molꢀ1
.
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MT is about 3.2 cm3 K molꢀ1 [35], so the value
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v
v
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We have successfully obtained two new cyclic complexes by
using the carboxyl substituted nitronyl nitroxide radical ligand
with M(hfac)2 (M = Mn(II), Co(II)). The nitronyl nitroxide radical
acts as bridge ligand linking two metal ions through the oxygen
atoms of the N–O and carboxyl groups. In complexes 1 and 2, the
magnetic coupling between the metal ion and the directly coordi-
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