Synthesis, structure, and magnetic properties of two dimer paddle-wheel CuII toluate complexes with nitronyl nitroxide radicals

Article abstract of DOI:10.1080/00958972.2013.800485

Two new paddle-wheel dimeric copper complexes, [Cu2(4-MePhCOO)4(NITmPy)2] (1) and [Cu2(3- MePhCOO)4(NITmPy)2] (2) (NITmPy = 2-(3-pyridyl)-4,4,5,5- tetramethyl-4,5-dihydro1H-imidazolyl- 1-oxyl-3-oxide), were synthesized by reaction of copper toluate and NITmPy. Single crystal X-ray analyses revealed that both complexes are symmetrical dimers. Cu with four deprotonated methylbenzoate bridging ligands form a paddle-wheel core and pyridyl nitrogen of radical ligands at the apical position. Based on the Cu-Cu axis of 1 and 2, we exploited the steric constraints of the methyl groups and induced the paddle-wheel. Two magnetic exchange pathways with strong antiferromagnetic interaction between dimeric CuII ions and weak antiferromagnetic interaction between NITmPy ligands exist in the complexes. IR and powder X-ray diffraction of the complexes were also studied.

Full text of DOI:10.1080/00958972.2013.800485

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Synthesis, structure, and magnetic
properties of two dimer paddle-wheel
CuII toluate complexes with nitronyl
nitroxide radicals
Jie Zhou a , Lin Du a , Zongze Li a , Yongfeng Qiao a , Jing Liu a ,
Minrong Zhu a , Peng Chen a & Qihua Zhao a
a Key Laboratory of Medicinal Chemistry for Natural Resource
Education Ministry, Department of Chemistry , Yunnan University ,
Kunming , China
Published online: 03 Jun 2013.
To cite this article: Jie Zhou , Lin Du , Zongze Li , Yongfeng Qiao , Jing Liu , Minrong Zhu , Peng
Chen & Qihua Zhao (2013) Synthesis, structure, and magnetic properties of two dimer paddle-wheel
CuII toluate complexes with nitronyl nitroxide radicals, Journal of Coordination Chemistry, 66:12,
2166-2177, DOI: 10.1080/00958972.2013.800485
To link to this article: http://dx.doi.org/10.1080/00958972.2013.800485
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 12, 2166–2177, http://dx.doi.org/10.1080/00958972.2013.800485
Synthesis, structure, and magnetic properties of two dimer
paddle-wheel CuII toluate complexes with nitronyl nitroxide
radicals
JIE ZHOU†, LIN DU†, ZONGZE LI, YONGFENG QIAO, JING LIU, MINRONG ZHU,
PENG CHEN and QIHUA ZHAO*
Key Laboratory of Medicinal Chemistry for Natural Resource Education Ministry,
Department of Chemistry, Yunnan University, Kunming, China
(Received 5 November 2012; in final form 8 March 2013)
Two new paddle-wheel dimeric copper complexes, [Cu2(4-MePhCOO)4(NITmPy)2] (1) and [Cu2(3-
MePhCOO)4(NITmPy)2] (2) (NITmPy = 2-(3-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro1H-imidazolyl-
1-oxyl-3-oxide), were synthesized by reaction of copper toluate and NITmPy. Single crystal X-ray
analyses revealed that both complexes are symmetrical dimers. Cu with four deprotonated methyl-
benzoate bridging ligands form a paddle-wheel core and pyridyl nitrogen of radical ligands at the
apical position. Based on the Cu–Cu axis of 1 and 2, we exploited the steric constraints of the
methyl groups and induced the paddle-wheel. Two magnetic exchange pathways with strong
antiferromagnetic interaction between dimeric CuII ions and weak antiferromagnetic interaction
between NITmPy ligands exist in the complexes. IR and powder X-ray diffraction of the complexes
were also studied.
Keywords: Magnetic properties; Nitronyl nitroxide radicals; Paddle-wheel complexes; Toluic
1. Introduction
Carboxylates are widely used as bridging ligands in design and synthesis of paddle-wheel
dimer metal carboxylate adducts, formulated as [M2(RCOO)4L2], where L usually repre-
sents the Lewis base located at the axial position and the carboxylates are bridging on the
equatorial position. By changing R and L, numerous zero-dimensional (0-D) to three-
dimensional (3-D) paddle-wheel compounds can be synthesized [1–7]. Design of esthetic
complexes and molecular assemblies that exhibit spontaneous magnetization are of interest
to the magneto structural correlations [8–10]. Much effort has been exerted to clarify mag-
netic exchange interaction that occurs in paddle-wheel complexes. Interaction between the
two metal ions of a paddle-wheel is generally antiferromagnetic regardless of the substitu-
ents R and apical ligands L; the antiferromagnetic interaction tends to increase as either L
or R becomes a stronger electron donor [11–13].
*Corresponding author. Email: qhzhao@ynu.edu.cn
†Contributed equally to this work.
Ó 2013 Taylor & Francis

Paddle-wheel CuII
2167
Stable organic radicals that can act as paramagnetic carriers and magnetic couplers have
been widely used as molecular units in the metal-radical approach to molecular magnetic
complexes [14–17]. Nitronyl nitroxide radicals can easily be chemically modified by vary-
ing R substituents which are capable of coordinating with the metal ions; many functional-
ized nitronyl nitroxide radicals have been synthesized such as pyridyl-, imidazolyl-, and
carboxyl-substituted nitronyl nitroxides [18–22]. Thus, coordination abilities of the radicals
which are limited by the weak basic nature of the nitroxide are increased [23]. Among the
functionalized nitroxide radical ligands, pyridyl-substituted nitroxide radicals are the most
extensively used to induce coupling due to their donor nitrogen [13, 24]. Copper(II)
complexes are significant in the study of molecular magnets [25, 26].
Based on the above premises, we reacted copper p-toluate and copper m-toluate with
the nitronyl nitroxide radicals in methanol and dichloromethane solutions and obtained
two dimeric copper complexes, [Cu2(4-MePhCOO)4(NITmPy)2] (1) and [Cu2(3-MePh-
COO)4(NITmPy)2] (2). In the complexes, two copper ions are bridged by four toluate
ligands. Two radical molecules are coordinated to copper through pyridine N on the apical
position. Based on the Cu–Cu axis of 1 and 2, we exploited the steric constraints of the
methyl groups and induced the paddle-wheel to run. To the best of our knowledge, such
observation on copper paddle-wheel complexes with radicals has not been previously
reported. In this paper, we report the crystal structure and magnetic properties of 1 and 2.
2. Experimental
2.1. General
All chemicals used were of reagent grade and used without purification. The carbon,
hydrogen, and nitrogen contents were acquired with a Vario-EL III element analyzer. Infra-
red spectra were recorded with a SHIMADZU IR prestige-21 FTIR-8400S spectrometer
from 4000–400 cmꢀ1 as KBr pellets. The phase purities of the samples were investigated
by powder X-ray diffraction (PXRD) measurements on a Bruker D8-Advance diffractome-
ter equipped with Cu-Kα radiation (λ = 1.5406 Å) at a scan speed of 1 ̊/min. Simulation of
the PXRD patterns were obtained using single-crystal data and were processed by the
Mercury v1.4 program available free of charge through the Internet at http://www.iucr.org.
The crystal structures were determined by a Rigaku SCXmini diffractometer and the
SHELXL-97 crystallographic software package. The temperature dependent magnetizations
(M vs. T) (2–300 K) of 1 and 2 were measured by using a Quantum Design vibrating
sample magnetometer in Physical Property Measurement System.
2.2. Preparation of 1 and 2
2-(3-Pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITmPy) [14, 15] and copper
p-toluate [27] were prepared according to methods described previously. Complex 1 was
synthesized by adding a solution of NITmPy (0.1 mmol) in CH2Cl2 (10 mL) to a solution
of copper p-toluate (0.1 mmol) in methanol (10 mL) dropwise. The mixture was stirred for
1 h at ambient temperature and then filtered. The clear blue filtrate was kept in the dark
and was slowly evaporated at room temperature. A few weeks later, dark-blue crystals
suitable for X-ray analysis were obtained. C56H60Cu2N6O12 Calcd: C 59.20, H 5.32,

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J. Zhou et al.
N 7.40; found: C 59.02, H 5.21, N 7.52. IR (KBr, cmꢀ1): 2986.27, 2928.24, 1622.13,
1563.78, 1404.07, 1323.20, 1209.73, 1169.95, 766.93, 697.38, 631.56, 461.29.
Complex 2 was synthesized in a manner similar to that of 1 replacing copper p-toluate
with copper m-toluate. C56H60Cu2N6O12 Calcd: C 59.20, H 5.32, N 7.40; found: C 59.11,
H 5.41, N 7.26. IR data (KBr, cmꢀ1): 2986.27, 2923.15, 1625.72, 1587.71, 1400.45,
1321.00, 1212.37, 1131.99, 755.66, 687.87, 487.77.
2.3. Crystallographic data collection and refinement
Single crystal X-ray diffraction data were collected at 293 K on a Bruker Smart AXS CCD
diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Absorption
correction was performed by the multi-scan method. Empirical absorption corrections were
carried out using SADABS. The structures were solved by direct methods and refined on
F2 by full-matrix least-squares using SHELXL-97. SHELXS-97 and SHELXL-97 pro-
grams were used for structure solution and refinement [28, 29]. All nonhydrogen atoms
were refined with anisotropic displacement parameters. For 2, the highest peak was 0.40
(0.4819, 0.0650, and 0.3507) [0.97 Å from O2] and the deepest hole was ꢀ1.05 (0.4423,
0.0000, and 0.2409) [1.02 Å from Cu1]. The collection parameters are shown in table 1.
Selected bond distances and angles of 1 and 2 are listed in tables 2 and 3, respectively.
Table 1. Crystallographic data and structure refinement parameters for 1and 2.
Complex
1
2
Empirical formula
Crystal system
Space group
Formula weight
Wavelength
C56H60Cu2N6O12
Triclinic
P ī
1136.20
C56H60Cu2N6O12
Monoclinic
C2/m
1136.18
0.71073 Å
0.71073 Å
Crystal size
Unit cell dimensions
0.23 ꢁ 0.17 ꢁ 0.12 mm
a = 10.7932(9) Å
b = 13.3891(11) Å
c = 19.5436(16) Å
α = 94.8570(10)°
β = 95.5170(10)°
γ = 95.3170(10)°
2786.6(4) Å3
0.26 ꢁ 0.17 ꢁ 0.12 mm
a = 21.130(5) Å
b = 15.890(4) Å
c = 8.1389(18) Å
α = 90°
β = 96.861(3)°
γ = 90°
Volume
Z
Calculated density
Absorption coefficient
h range for data collection
Limiting indices
2713.1(11) Å3
2
2
1.354 g cmꢀ3
1.391 g cmꢀ3
0.829 mmꢀ1
0.851 mmꢀ1
1.53–25.10°
1.61–25.09°
ꢀ12 6 h 6 12; ꢀ15 6 k 6 157;
ꢀ25 6 h 6 24; ꢀ18 6 k 6 18;
ꢀ23 6 l 6 23
ꢀ6 6 l 6 9
Data/restraints/parameters
Refinement method
Reflections collected/unique 22,440/9889 [R (int) = 0.0387]
Completeness to
9889/48/685
2511/0/190
Full-matrix least-squares on F2
Full-matrix least-squares on F2
7782/2511[R (int) = 0.0904]
99.9%
99.7%
theta = 25.00
Goodness-of-fit on F2
Final R1, indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
1.025
1.032
R1 = 0.0504, wR2 = 0.1175
R1 = 0.0936, wR2 = 0.1394
0.397 and ꢀ0.493 eÅꢀ3
R1 = 0.0694, wR2 = 0.1534
R1 = 0.1189, wR2 = 0.1844
0.395 and ꢀ1.049 eÅꢀ3

Paddle-wheel CuII
Table 2. Selected bond lengths (Å) and angles (°) for 1.
2169
Cu(1)–O(3)
Cu(1)–O(5)
Cu(1)–N(1)
O(1)–N(2)
Cu(2)–O(15)
Cu(2)–O(17)
Cu(2)–N(7)
O(13)–N(8)
O(5)–Cu(1)–O(4)
O(4)–Cu(1)–O(6)
O(4)–Cu(1)–O(3)
O(5)–Cu(1)–N(1)
O(3)–Cu(1)–N(1)
O(5)–Cu(1)–Cu(1)a
O(3)–Cu(1)–Cu(1)a
N(1)–Cu(1)–Cu(1)a
C(13)a–O(4)–Cu(1)
C(21)a–O(6)–Cu(1)
O(18)–Cu(2)–O(15)
O(18)–Cu(2)–O(16)
O(15)–Cu(2)–O(16)
O(17)–Cu(2)–N(7)
O(16)–Cu(2)–N(7)
O(17)–Cu(2)–Cu(2)b
O(15)–Cu(2)–Cu(2)b
N(7)–Cu(2)–Cu(2)b
1.974(2)
1.965(3)
2.197(3)
1.267(5)
1.973(2)
1.956(3)
2.150(3)
1.259(6)
90.6(1)
Cu(1)–O(4)
Cu(1)–O(6)
Cu(1)–Cu(1)a
O(2)–N(3)
Cu(2)–O(16)
Cu(2)–O(18)
Cu(2)–Cu(3)b
O(14)–N(9)
O(5)–Cu(1)–O(3)
O(3)–Cu(1)–O(6)
O(5)–Cu(1)–O(6)
O(4)–Cu(1)–N(1)
O(6)–Cu(1)–N(1)
O(4)–Cu(1)–Cu(1)a
O(6)–Cu(1)–Cu(1)a
C(13)–O(3)–Cu(1)
C(21)–O(5)–Cu(1)
O(18)–Cu(2)–O(17)
O(17)–Cu(2)–O(15)
O(17)–Cu(2)–O(16)
O(18)–Cu(2)–N(7)
O(15)–Cu(2)–N(7)
O(18)–Cu(2)–Cu(2)b
O(17)–Cu(2)–Cu(2)b
O(16)–Cu(2)–Cu(2)b
C(69)–O(15)–Cu(2)
1.970(2)
1.967(3)
2.6464(9)
1.276(4)
1.985(2)
1.962(3)
2.6213(9)
1.250(5)
88.7(1)
87.1(1)
168.3(1)
97.6(1)
91.0(1)
167.9(1)
98.1(1)
93.4(1)
94.4(1)
85.50(8)
80.84(7)
173.43(8)
119.3(2)
124.9(3)
89.8(1)
87.52(8)
82.56(8)
127.0(2)
121.0(3)
168.7(1)
87.8(1)
89.7(1)
89.2(1)
96.8(1)
81.33(8)
87.55(8)
81.29(8)
119.9(2)
90.3(1)
168.5(1)
101.9(1)
94.6(1)
81.33(8)
87.41(8)
169.69(9)
Symmetry codes: a: ꢀx + 2, ꢀy + 2, ꢀz; b: ꢀx + 1, ꢀy, ꢀz + 1.
Table 3. Selected bond lengths (Å) and angles (°) for 2.
Cu(1)–O(1)
Cu(1)–O(2)b
Cu(1)–N(1)
O(5)–N(2)
O(2)a–Cu(1)–O(2)b
O(2)b–Cu(1)–O(1)c
O(1)c–Cu(1)–O(1)
O(1)c–Cu(1)–N(1)
O(1)c–Cu(1)–Cu(1)a
C(13)–O(1)–Cu(1)
1.967(3)
1.953(3)
2.185(5)
1.260(8)
87.4(2)
167.5(1)
88.9(2)
90.8(1)
Cu(1)–O(2)a
Cu(1)–O(1)c
Cu(1)–Cu(1)a
O(6)–N(3)
O(2)a–Cu(1)–O(1)c
O(2)a–Cu(1)–O(1)
O(2)a–Cu(1)–N(1)
O(2)a–Cu(1)–Cu(1)a
N(1)–Cu(1)–Cu(1)a
C(13)–O(2)–Cu(1)a
1.953(3)
1.967(3)
2.670(1)
1.22(1)
90.4(1)
167.5(1)
101.6(1)
86.9(9)
168.0(1)
120.6(3)
80.67(9)
126.7(3)
Symmetry codes: a: ꢀx + 1, ꢀy, ꢀz + 1; b: ꢀx + 1, y, ꢀz + 1; c: x, ꢀy, z.
3. Results and discussion
3.1. Crystal structure analysis
Single crystal X-ray analysis revealed that 1 crystallizes in the triclinic space group P ī,
whereas 2 has higher symmetry crystallizing in the monoclinic space group C2/m. The
two complexes have the same stoichiometry and connectivity consisting of centrosymmet-
ric dinuclear units. Each CuII has four oxygens from four different carboxylates in the
equatorial position and two pyridyl nitrogens of the radicals apical. In a five-coordinate
copper system, the geometry around CuII may be square-pyramidal or trigonal-bipyramidal
depending on the geometric parameter τ (τ = (β–α)/60, where β is defined as the greater of

2170
J. Zhou et al.
the basal angles and α is the remaining angle) [30]. For a perfectly square-pyramidal
geometry, τ is equal to 0. However, the value becomes unity for ideal trigonal-bipyramidal
geometry. The τ value is 0.007 for 1 and 0 for 2, respectively, which indicates that the
coordination geometry of the two complexes is square-pyramidal and thus, different from
the trigonal-bipyramidal dimer (figures 1(a) and 1(b)) [10]. In 1, the dimers have two
different orientations in the crystal (figure 1(c)). The Cu–Cu distances are 2.6464(9) and
2.6213(9) Å, Cu–O distances range from 1.962(3) to 1.985(2) Å, and Cu–N distances are
2.150(3) and 2.197(3) Å. All values are comparable to those in the previously reported
dimeric CuII complexes with nitronyl nitroxides [31–34]. In 2, the Cu–Cu distance is
2.670(1) Å and Cu–N is 2.180(5) Å. Both values are larger than those of 1, ascribed to
the presence of m-methyl that increases the steric effect of m-toluic with NITmPy ligands.
As a result, the average Cu–O distance (1.960 Å) is relatively shorter than that of 1
(1.969 Å).
Within the coordinated NITmPy of 1, the imidazole is nearly planar and forms a
dihedral angle of 14.36(4)° with the pyridine ring. In 2, the imidazole and pyridine rings
are nearly in the same plane, and the other distances and angles within the nitroxide group
of both complexes are similar to those previously reported [31–34]. Both complexes have
relatively short contacts between O of N–O of neighboring dimer molecules. The distances
are 3.68 and 6.32 Å for 1 and 4.99 Å for 2 (figures 1(c) and 1(d)). These values were
considered in the treatment of the experimental dependence magnetic susceptibility.
In 1, based on the Cu–Cu axis (figure 2(a)), the four p-toluic acids that bridge two cop-
per ions are approximately perpendicular. Moreover, two apical NITmPy ligands are nearly
in one plane with two p-toluic acids. However, in 2, four m-toluic acids are vertical to
each other and NITmPy ligands split the angle made of m-toluic acids and form a dihedral
angle of 45° and 42° (figure 2(b)). This arrangement reduces the energy of the system and
enhances the stability of the molecules. Based on the Cu–Cu axis of 1 and 2, we exploited
the steric constraints of the methyl groups and induced the paddle-wheel to run. To the
best of our knowledge, such observation on copper paddle-wheel complex with radicals
has not been previously reported.
Figure 1(a). ORTEP drawing of [Cu2(4-MePhCOO)4(NITmPy)2] (1) with selected atom-labeling and 30%
thermal ellipsoids. All hydrogens are omitted for clarity (symmetry codes: a: –x + 2, –y + 2, –z).

Paddle-wheel CuII
2171
Figure 1(b). ORTEP drawing of [Cu2(3-MePhCOO)4(NITmPy)2] (2) with selected atom-labeling and 30%
thermal ellipsoids. All hydrogens are omitted for clarity (symmetry codes: a: –x + 1, –y, –z + 1; b: –x + 1, y, – z
+ 1; c: x, –y, z).
Figure 1(c). The packing plot of 1, viewed from the a axis, two different orientations in the crystal and the
adjacent distance (Å) of the NITmPy O are shown. For clarity, all hydrogens and methyls of the NITmPy are
omitted.

2172
J. Zhou et al.
Figure 1(d). The packing plot of 2, viewed from the b axis, the adjacent distance (Å) of the NITmPy O are
shown. For clarity, all hydrogens and methyls of the NITmPy are omitted.
Figure 2. Wires drawing for 1 (a) and 2 (b), viewed from the Cu–Cu axis; NITmPy ligands are shown in blue
and fixed on the equatorial position, p-toluic acid and m-toluic acid are shown in red (see http://dx.doi.org/
10.1080/00206814.2013.800485 for color version).
3.2. PXRD and IR spectra
Purities of samples were confirmed by comparison of experimental and simulated PXRDs
(figure S1). The IR spectra of 1 and 2 (figure S2) shows the absorptions of carboxylate

Paddle-wheel CuII
2173
stretch at 1620 cmꢀ1 for asymmetric stretch (νas) and 1400 cmꢀ1 for symmetric stretch (νs)
[35]. The absence of strong absorption around 1250 cmꢀ1 indicates that all carboxyl
groups (ꢀCOOH) of toluic ligands are deprotonated [36]. Moreover, bands at 2986 cmꢀ1
can be attributed to C–H stretching vibrations. The characteristic absorptions of N–O of
the radical should appear at 1360 cmꢀ1, but was covered by the absorption of the strong
carboxylate stretch [37].
3.3. Magnetic properties
To analyze exchange coupling in the four spin systems, variable-temperature magnetic-
susceptibility measurements were performed on powdered crystalline samples of 1 and 2
in a 1000 Oe field from 2 K–300 K. The obtained data for 1 anꢀd1 2 are shown as χM vs.
T and χMT vs. T plots in figures 3 and 4, respectively. The χM vs. T plots are shown
in supplementary material (figure S3).
ꢀ1
According to the data for 1 and 2 of χM vs. T, the magnetic behavior of both com-
plexes follow the Curie–Weiss law χM = C/(T–h) from approximately 150 to 300 K, with a
Curie constant C value of 2.41 cm3 K mol–1 and a Weiss temperature h of ꢀ219 K for 1,
and C value of 2.25 cm3 K mol–1 and a Weiss temperature h of ꢀ245 K for 2. This behav-
ior is the characteristics of a system with strong antiferromagnetic interactions and mag-
netic exchange coupling interaction. The χM vs. T plots of 1 and 2 continuously increase,
but no maximum is observed.
For the χMT vs. T curve of 1 (figure 3), χMT is 1.40 cm3 mol–1 K at 300 K, lower than
the value (1.50 cm3 mol–1 K) expected for four uncoupled spins of S = 1/2 for two nitronyl
nitroxide radical ligands and S = 1/2 for two copper atoms. Overall, χMT decreases with
decreasing temperature, which indicates the existence of antiferromagnetic spin exchange
interactions in the dimer. However, when χMT reaches 0.63 cm3 mol–1 K at 24 K, it begins
to increase sharply and reaches ca. 0.90 cm3 mol–1 K at 8 K. Then, χMT quickly decreases,
finally reaching ca. 0.65 cm3 mol–1 K at 2 K. This behavior indicates the existence of weak
ferromagnetic spin exchange interactions in 1. As shown in figure 4, the χMT of 2 is
Figure 3. Plots of the temperature dependence of χM and χMT product at 1 kOe for 1. The solid lines
correspond to theoretical fits.

2174
J. Zhou et al.
Figure 4. Plots of the temperature dependence of χM and χMT product at 1 kOe for 2. The solid lines
correspond to theoretical fits.
1.24 cm3 mol–1 K at 300 K, lower than the expected value (1.50 cm3 mol–1 K, similar to 1).
As the system cools from 300 K to 2 K, χMT decreases to ca. 0.45 cm3 mol–1 K.
The data of 1 and 2 were fitted in several ways. First, we considered the dimer as a
centrosymmetric linear tetramer of four S = 1/2 spins, S1–S2–S3–S4, with the exchange
Hamiltonian of H = –J1(S1S2 + S3S4)–J2S2S3–J3(S1S3 + S2S4)–J4S1S4. Four exchange cou-
pling constants are involved, where S2 and S3 refer to CuII ions, and S1 and S4 refer to the
nitroxide sites of the NITmPy radicals. However, fitting the magnetic data for the two
complexes with this model yielded poor results. Most of the intramolecular exchange path
lengths between oxygens of NO groups and CuII are long, and the spin couplings between
the radicals and the CuII ions are weak. Second, as reported, we considered the system as
a strongly coupled dimeric copper unit. The radicals were neglected and the fitting equa-
ꢀ
ꢁ
tion vM ¼ 2Ng2b2=kT ½3 þ expðꢀ2J=kTÞꢂꢀ1 was used, but we still obtained poor results.
We found that the two complexes have relatively a short contact between the oxygens
of N–O (NITmPy) of neighboring dimers. The distances are 3.68 and 6.32 Å for 1 and
4.99 Å for 2, which indicate that the intermolecular magnetic exchange interaction between
NITmPy cannot be neglected. Therefore, the magnetic interaction of the two complexes
can be simplified as the exchange coupled Cu–Cu dimer and the radical exchange chain.
The model of magnetic interaction in the complexes is shown in scheme 1. The suscepti-
bility data were fitted by the following approximate equations [38, 39]:
vM ¼ vdimer þ vchain
vdimer
¼
2 ꢁ
2Ng2cub
e2J=kT
1 þ 3e2J=kT
kT
NgR2b
0:25 þ 0:074975x þ 0:075235x2
vchain
¼
2 ꢁ
kT
1 þ 0:9931x þ 0:172135x2 þ 0:757825x3
where J is the exchange constant for two CuII sites, x ¼ jJ1j = kT, J1 is the exchange
constant for the radicals, and the other symbols have their typical meanings. The best-fit
for 1 is obtained as the solid line (24–300 K) in figure 3. The values of the parameters

Paddle-wheel CuII
2175
Scheme 1. Model of magnetic interaction in the complexes.
are J = ꢀ146.26 cmꢀ1
,
gCu = 2.35, J1 = ꢀ8.71 cmꢀ1
,
gR = 2.94, and R = 5.0 ꢁ 10ꢀ5
ꢀ
ꢀ
ꢁ ꢀ
ꢁ ꢁ
P
2
2
R ¼
xcMalcd ꢀ xoMbsd xMobsd . The best-fit for 2 is obtained as the solid line (24–
300 K) in figure 4. The values of the parameters are J = ꢀ147.20 cmꢀ1, gCu = 2.26,
J1 = ꢀ13.12 cmꢀ1, gR = 2.77, and R = 5.6 ꢁ 10ꢀ5. The relatively large gCu values (2.35,
2.26) are similar to the previously reported dimer CuII carboxylates [32,35,40]. The large
negative J of the two complexes indicates strong antiferromagnetic interaction between
CuII ions, and the small negative J1 suggests weak antiferromagnetic interaction between
NITmPy ligands.
In these copper paddle-wheel complexes, nitrogens are located at the apical position in
most cases, except for solvent molecules such as water, methanol, or ethanol [41].
Selvakumar et al. investigated the strong Cu–Cu interaction and the dinuclear assembly in
detail [42]. The χMT vs. T plots of 1 are similar to that of [Cu2(C9N5H9)2(C6H5CO2)4] [43].
The copper paddle-wheel complexes possess chelate rings and Cu(II) active sites for
potential application in host–guest chemistry and catalysis [42]. Moreover, the short
distance between the uncoordinated N–O moieties cause more significant intermolecular
interactions similar to the lanthanide–radical complexes [Tb(hfac)3(EtVNIT)2] and
[Dy(hfac)3(EtVNIT)2], and might exclude the possibility of single-molecule magnet [44].
4. Conclusion
Two new paddle-wheel dimer copper complexes were obtained, and they were structurally
and magnetically characterized. CuII ions are square-pyramidal and short contacts exist
between O of the N–O groups of neighboring dimers. Based on the Cu–Cu axis of 1 and
2, we exploited the steric constraints of the methyl groups and induced the paddle-wheel
to run. Two magnetic exchange pathways with strong antiferromagnetic interaction
between dimeric Cu ions and weak antiferromagnetic interaction between NITmPy ligands
exist in the complexes.
Supplementary material
ꢀ1
PXRD graphics, solid state IR spectra, and plots of temperature dependence of χM of
the complexes are available as supplementary material. Crystallographic data for the
structural analysis have been deposited with the Cambridge Crystallographic Data Center,

2176
J. Zhou et al.
CCDC No. 882654 for 1 and 882653 for 2. Copies of this information can be obtained
free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(Fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Project
21061016) and Yunnan Provincial Department of Education Research Fund (Project
2011Y111).
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