A ladder-like one-dimensional chain based on a new biradical and cuII: Crystal structure and magnetic properties

Article abstract of DOI:10.1002/cjoc.201090270

A new biradical di(4-(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazo l-2'-yl)phenyl) ether (1) and its copper complex 2 were synthesized and characterized structurally. The X-ray structure determination revealed that, the complex is in

Full text of DOI:10.1002/cjoc.201090270

FULL PAPER
A Ladder-like One-dimensional Chain Based on a New
Biradical and CuII: Crystal Structure
and Magnetic Properties†
Wang, Chao(王超)
Ma, Yue(马越)
Yang, Xi(杨曦)
Yan, Shiping(阎世平)
Liao, Daizheng*(廖代正)
Department of Chemistry, Nankai University, Tianjin 300071, China
A new biradical di(4-(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazol-2'-yl)phenyl) ether (1)
and its copper complex 2 were synthesized and characterized structurally. The X-ray structure determination re-
vealed that, the complex is in the triclinic P-1 space group and displays a ladder-like structure. Each copper atom is
surrounded by four oxygen atoms from hexafluoroacetylacetone and two oxygen atoms from biradical. The mag-
netic measurements show that biradical 1 exhibits a weak intramolecular nitroxide-nitroxide antiferromagnetic cou-
pling and copper complex 2 possesses weak ferromagnetic coupling (J＝5.09 cm－ 1) between CuII and radical,
which attributed to interaction between copper spin and the axially coordinated nitronyl nitroxide spin.
Keywords copper, biradical, magnetic properties, crystal structures
Introduction
Experimental
Some of stable radicals,1 especially the Ullman’s
type, and their complexes2 exhibit unusual magnetic
properties due to their unpaired electron and versatile
coordination mode. In this Ullman’s radical, the coor-
dination of the two oxygens (O-N-C-N-O) with metals
can be used to form low dimensional structures.3-5
These low dimensional radical-metal complexes show a
wide variation of magnetic properties.6-29,46-48 On the
other hand, biradicals35-45 themselves have two unpaired
electrons and these electrons can interact with each
other, then metal complexes containing biradicals are
expected to show more manifold magnetic properties
than mono-radicals. However, the use of biradicals as
ligands has been somewhat limited due to the difficulty
in synthesis. Therefore, to explore further the research
area based on radicals, it seemed interesting to us to
design and synthesize new biradicals. With this aim, in
this paper, we first present the main structural features
and magnetic properties of a new biradical, di(4-
(1'-xyl-3'-oxido-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-
imidazol-2'-yl)phenyl) ether (1) (C26H32N4O5). We then
report its copper(II) complex 2 (C47H38Cl2Cu2F24N4O13),
in which the radical oxygen only acts as monodentate
ligand and each biradical molecule links four copper
atoms to form a ladder-like framework.
Material and general methods
All the commercial chemicals were used as received
without further purification. The solvents were purified
by standard methods. 4-(4-Formylphenoxy)benzalde-
hyde and 2,3-bis(hydroxylamino)-2,3-dimethylbutane
were synthesized by the procedure described by Dann30
and Ullman,34 respectively. Elemental analyses for C, H
and N were carried out on Perkin-Elmer elemental
analyzer (Model 240). Variable temperature magnetic
susceptibilities were measured on SQUID MPMS XL-7
magnetometer in the temperature range of 2.0 and 300
K at a magnetic field of 0.5 T using polycrystalline
samples. Diamagnetic corrections were estimated from
Pascal’s constants for all constituent atoms.
Single-crystal structure determination
The data were collected at 113(2) K on a Rigaku
MM-007 single-crystal diffractometer equipped with
CCD area detector with graphite-monochromatized Mo
Kα (λ＝ 0.71070 Å). A summary of crystallographic
data is given in Table 1. The empirical absorption cor-
rections by semi-empirical from equivalents were car-
ried out. The structure was solved with the SHELX
program,33 and refined by full-matrix least square
methods based on F2, with anisotropic thermal parame-
ters for the non-hydrogen atoms. The hydrogen atoms
*
E-mail: liaodz@nankai.edu.cn
Received July 12, 2010; revised August 16, 2010; accepted August 20, 2010.
Project supported by the National Natural Science Foundation of China (Nos. 20631030 and 90922032), the National Basic Research Program of
China (973 Program, 2007CB815305), and Natural Science Foundation of Tianjin (No. 09JCYBJC05500).
† Dedicated to the 60th Anniversary of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.
Chin. J. Chem. 2010, 28, 1593— 1599
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Wang et al.
FULL PAPER
Preparation of the copper complex (2)
were located theoretically and refined isotropically. The
selected bond lengths and bond angles are listed in Ta-
ble 3.
51.4 mg (0.1 mmol) of Cu(hfac)2•2H2O was dis-
solved in 20 mL of boiling n-heptane and this light
blue-green transparent solution was refluxed for 2 h.
When the solution was cooled to about 60 ℃, 24.0 mg
(0.05 mmol) of 1 was added under stirring and followed
by 10 mL of CH2Cl2. After stirred for an additional 1 h,
the final dark green solution was allowed to cool, fil-
tered and was kept at 4 ℃ for 3 d to give dark green
crystals suitable for X-ray analysis. Yield: 19 mg (25%).
Elemental analysis calculated for C47H38Cl2Cu2F24N4-
O13(1520.81): C 37.12, H 2.52, N 3.68; found C 36.89,
H 2.66, N 3.81.
Synthesis of 1
2,3-Bis(hydroxylamino)-2,3-dimethylbutane (2.96 g,
20.0 mmol) and 4-(4-formylphenoxy)benzaldehyde
(2.26 g, 10.0 mmol) were dissolved in a minimum
amount of methanol. After stirring at room temperature
for 48 h, this slurry was filtered and washed with 5 mL
cold methanol twice. This off-white powder was sus-
pended in 150 mL CH2Cl2, and then 30 g (0.125 mol)
PbO2 was added to this suspension. The resulting mix-
ture was stirred vigorously for 30 min at room tempera-
ture and filtered, and then the dark blue solution was
obtained, dried over anhydrous MgSO4 overnight. After
filtration to remove any inorganic salts, the CH2Cl2 so-
lution was evaporated to a small amount under reduced
pressure. The residue was purified by column chroma-
tography on silica gel [V(hexane)∶ V(actone)＝ 3∶ 1 as
eluent]. The last violet blue fraction was collected,
evaporated to dryness, and recrystallized using
V(hexane)∶ V(actone)＝ 5∶ 1. The obtained precipitate
was filtered off, washed with cold hexane and dried in
air. Single-crystal suitable for X-ray analysis was ob-
tained by slow evaporation of a heptane-dichlor-
omethane solution. Yield 820 mg (17.1%). M.p. 189—
192 ℃. Elemental analysis calculated for C26H32N4O5
(480.56): C 64.92, H 6.71, N 11.66; found C 65.11, H
6.62, N 11.80.
Results and discussion
Crystal structure
Summary of crystallographic data is listed in Table 1.
Selected bond lengths and angles of 1 and 2 are summa-
rized in Tables 2 and 3. As shown in Figures 1 and 2,
this compound is a typical biradical with the N— O
bond length in the range of 1.2836— 1.2862 Å. The
molecule of 1 is not a centrosymmetric compound due
to the angle of C(11)-O(3)-C(14) to be 119.81°. The
shortest distance between two oxygen atoms from the
adjacent molecules is 4.089 Å, while the shortest dis-
tance between two oxygen atoms within the molecule is
9.231 Å.
Table 1 Summary of crystallographic data for 1 and 2
1
C26H32N4O5
480.56
2
C47H38Cl2Cu2F24N4O13
2242.36
Empirical formula
Formula weight
Temperature
113(2) K
0.71070 Å
Monoclinic
P2(1)/c
113(2) K
Wavelength
0.71070 Å
Triclinic
Crystal system
Space group
P-1
Unit cell dimensions
a
10.202(2) Å
22.707(5) Å
11.601(2) Å
90°
10.550(6) Å
16.045(11) Å
18.554(13) Å
107.492(13)°
92.251(9)°
90.234(11)°
2993(3) Å3
2
b
c
α
β
109.20(3)°
90°
2538.1(9) Å3
γ
Volume
Z
4
Calculated density
Absorption coefficient
F(000)
1.258 Mg/m3
1.688 Mg/m3
0.088 mm－
0.937 mm－
1
1
1024
1520
Crystal size
0.30 mm×0.24 mm×0.10 mm
0.80 mm×0.74 mm×0.56 mm
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Chin. J. Chem. 2010, 28, 1593— 1599

A Ladder-like One-dimensional Chain Based on a New Biradical and CuII
Continued
1
2
θ range for data collection
1.859° to 27.857°
－12≤h≤12
－21≤k≤27
－13≤l≤13
1.475° to 27.104°
－12≤h≤12
－18≤k≤19
－22≤l≤21
Limiting indices
Reflections collected/unique
Completeness to theta
Absorption correction
Max and min transmission
Refinement method
14532/4464 [R(int)＝0.0609]
99.7%
22549/10574 [R(int)＝0.0758]
99.2%
Semi-empirical from equivalents
Semi-empirical from equivalents
0.9912 and 0.9740
0.5768 and 0.3157
Full-matrix least-squares on F2
4464/0/325
Full-matrix least-squares on F2
10574/0/856
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
1.052
0.957
R1＝0.0470, wR2＝0.1151
R1＝0.0598, wR2＝0.1234
R1＝0.0729, wR2＝0.1715
R1＝0.1041, wR2＝0.1880
Table 2 Selected bond lengths (Å) and angles (°) for 1
Bond length
1.2858(18)
1.2836(18)
1.357(2)
1.360(2)
1.355(2)
1.356(2)
O(2)— N(2)
O(5)— N(4)
N(1)— C(2)
N(2)— C(5)
N(3)— C(21)
N(4)— C(24)
O(1)— N(1)
1.2862(19)
1.2857(18)
1.503(2)
1.500(2)
1.504(2)
1.504(2)
O(4)— N(3)
N(1)— C(1)
N(2)— C(1)
N(3)— C(20)
N(4)— C(20)
Bond angle
119.81(13)
120.07(13)
127.16(15)
112.02(14)
121.08(13)
126.94(15)
111.95(13)
107.65(15)
O(1)-N(1)-C(1)
C(1)-N(1)-C(2)
O(2)-N(2)-C(5)
O(4)-N(3)-C(20)
C(20)-N(3)-C(21)
O(5)-N(4)-C(24)
N(1)-C(1)-N(2)
C(11)-O(3)-C(14)
O(1)-N(1)-C(2)
O(2)-N(2)-C(1)
C(1)-N(2)-C(5)
O(4)-N(3)-C(21)
O(5)-N(4)-C(20)
C(20)-N(4)-C(24)
N(3)-C(20)-N(4)
127.22(14)
112.20(14)
120.19(14)
126.75(14)
111.86(13)
120.80(13)
106.95(15)
Table 3 Selected bond lengths (Å) and angles (°) for 2
Bond length
1.938(4)
Cu(1)— O(1)
Cu(1)— O(3)
Cu(1)— O(11)
Cu(2)— O(6)
Cu(2)— O(7)
Cu(2)— O(9)
N(2)— O(10)
N(4)— O(12)
Cu(1)— O(4)
Cu(1)— O(2)
Cu(1)— O(10)
Cu(2)— O(8)
Cu(2)— O(5)
Cu(2)— O(12)
N(1)— O(9)
1.939(4)
1.950(4)
2.501(4)
1.953(5)
1.959(5)
2.457(4)
1.290(6)
1.291(6)
1.947(4)
2.321(4)
1.950(4)
1.957(4)
2.331(4)
1.275(6)
1.285(6)
N(3)— O(11)
Bond angle
120.2(5)
O(4)-Cu(1)-O(1)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-O(3)
O(4)-Cu(1)-O(10)
C(21)-O(13)-C(34)
O(4)-Cu(1)-O(2)
O(4)-Cu(1)-O(3)
O(2)-Cu(1)-O(3)
178.06(16)
91.88(18)
86.51(17)
84.89(15)
88.69(18)
93.42(17)
165.01(17)
Chin. J. Chem. 2010, 28, 1593— 1599
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Wang et al.
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Continued
93.19(16)
96.87(16)
104.83(16)
87.49(16)
87.19(19)
92.31(19)
163.35(16)
84.89(16)
91.94(16)
81.91(16)
101.36(16)
165.75(15)
O(2)-Cu(1)-O(10)
O(4)-Cu(1)-O(11)
O(2)-Cu(1)-O(11)
O(10)-Cu(1)-O(11)
O(8)-Cu(2)-O(5)
O(8)-Cu(2)-O(7)
O(5-)Cu(2)-O(7)
O(6)-Cu(2)-O(12)
O(7)-Cu(2)-O(12)
O(6)-Cu(2)-O(9)
O(7)-Cu(2)-O(9)
O(1)-Cu(1)-O(10)
O(3)-Cu(1)-O(10)
O(1)-Cu(1)-O(11)
O(3)-Cu(1)-O(11)
O(8)-Cu(2)-O(6)
O(6)-Cu(2)-O(5)
O(6)-Cu(2)-O(7)
O(8)-Cu(2)-O(12)
O(5)-Cu(2)-O(12)
O(8)-Cu(2)-O(9)
O(5)-Cu(2)-O(9)
O(12)-Cu(2)-O(9)
98.10(16)
77.10(16)
78.51(16)
161.70(15)
176.58(15)
93.73(19)
87.71(19)
103.83(16)
92.81(16)
80.93(16)
82.75(16)
Figure 1 Chemical structure of biradical (1).
Figure 3 The coordination environments of two nonequivalent
copper atoms in 2.
Figure 2 View of the molecular structure showing the distance
of the oxygen atoms.
Figure 4 The rectangle unit of ladder-like framework. The fluo-
rine atoms, hydrogen atoms and the disordered dichloromethane
are omitted for clarity.
Single-crystal X-ray analysis revealed triclinic P-1
space group for 2. This complex is formed by infinite
chains, in which the [Cu(hfac)2] fragments are con-
nected via the O-N-C-N-O atom groups from nitronyl
nitroxide units. Figures 3 and 4 show a part of a one
dimensional chain structure of 2, in this complex, the
copper atoms are all in severely distorted coordination
environments and coordinated by four oxygen atoms
coming from two hfacs occupying the equatorial posi-
tions and two oxygen atoms from two different nitronyl
nitroxide units in the axial positions. As shown in Figure
3, the coordination environments of two copper atoms
[Cu(1) and Cu(2)] are not completely equivalent but
exceedingly similar. In the octahedral coordination en-
vironment of Cu(1) center, the two radical oxygen at-
oms [O(11) and O(12)] have different bond lengths with
Cu(1) [Cu(1)— O(11) of 2.501 Å, Cu(1)— O(10) of
2.321 Å]. Both of them are much longer than the equa-
torial ones (1.938— 1.950 Å). The bond angles of
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Chin. J. Chem. 2010, 28, 1593— 1599

A Ladder-like One-dimensional Chain Based on a New Biradical and CuII
Cu(1)-O(10)-N(2) and Cu(1)-O(11)-N(3) are 118.59°
and 150.93°, respectively. This bond angles are in good
agreements with the previously reported.6,9,17,26,32
Because one biradical has two radical unit, and the
four coordination sites of this biradical are all employed
to construct this one-dimensional chain, thus, not a sim-
ple one dimensional chain but a ladder-like structure
(Figure 5) consisting of two parallel alternating
Cu-R-Cu-R chains is formed with the adjacent stairs
distance of 8.023 Å. The shortest interchain Cu— Cu
distance is 8.286 Å, while the Cu— Cu distance of two
parallel alternating chains is 11.552 Å, which is longer
than 7.864 Å, the shortest intra-chain Cu— Cu distance
along the Cu-R-Cu-R chains.
Figure 5 The ladder-like chain structure of the complex 2. The
copper atoms are displayed as octahedrons, hexfluoroacetylacet-
one, hydrogen atoms and the disordered dichloromethane are
omitted for clarity.
Figure 6 The magnetic behavior of the biradical. The solid line
of the later figure is the simulation by Curie-Weiss law with the
parameters: T＝－3.03 K, C＝0.778 emu•K•mol－
.
1
Magnetic properties
The magnetic behavior of the biradical is shown in
Figure 6. The data of variable temperature magnetic
susceptibility were collected in the range of 2.0— 300 K
at 5000 Oe field. The χMT value at room temperature is
tween the two radical unit within one molecule. The best
fitting parameters were obtained as, g＝ 2.01, J＝ － 2.12
cm . Below 5 K, these theoretical analysis did not give
－1
us a satisfactory fit with the experimental data, possibly
due to the complicated intermolecular interactions.
The magnetic properties of 2 in the form of χMT ver-
sus T plots are presented in Figu－re 7. At room tempera-
－1
0.765 cm3•K•mol , which is slightly higher than the
expected value for uncoupled system of two radicals
－1
(0.750 cm3•K•mol ) with g value of 2.00. The χMT
1
ture, the χMT is 0.870 cm3•mol •K, which is slightly
value decreases gradually on cooling. Below 20 K, the
χMT value decreases sharply reaching 0.323 cm3•K•
－1
higher than the expected value (0.750 cm3•mol •K) for
－1
mol at 2.0 K, suggesting that there are antiferromag-
an uncoupled system with one Cu(II) ion (S＝ 1/2,
－1
χMT＝ 0.375 cm3•mol •K) and one radical unit (S＝ 1/2,
netic interactions between the biradical molecules.
These antiferromagnetic interactions are further demon-
strated by negative θ value (T＝ － 3.03 K) when the Cu-
χMT＝ 0.375 cm3•mol－ 1•K). As the temperature de-
creases, χMT value increases gradually below 80 K, then
increases sharply on cooling and reaches a peak of 2.025
－
rie-Weiss law is used to simulate the χM 1— T linear,
－1
－1
cm3•mol •K at 3.5 K. Finally, χMT decreases slowly as
while a rational C＝ 0.778 emu•K•mol is obtained. To
evaluate the exchange coupling constants within this
the temperature descended. The magnetic behavior sug-
gests ferromagnetic interaction is dominant within 2.
The decrease of χMT values in the low temperature re-
gion is most likely due to very weak inter-chain interac-
tion.
The experimental data of 2 were analyzed by using
an S＝ 1/2 ferromagnetic chain model31 with the Hamil-
tonian H＝ － 2JΣSi•Si＋1, where all symbols have their
usual meaning (Eq. 2).
biradical at the temperature range of 5— 300 K, Eq. (1)
ˆ ˆ
ˆ
derived from the spin Hamiltonian H＝－2JSR SR' is
applied.
Ng2β 2
KT
2
χM＝
(
)
(1)
－2J
3＋exp(
)
KT
where J corresponds to the magnetic interactions be-
2
Ng2β 2 1＋5.7979916Y＋16.902653Y 2＋29.376885Y3＋29.832959Y 4＋14.036918Y5
3
(2)
χchain
(
)
＝
1＋2.7979916Y＋7.0086780Y 2＋8.6538644Y3＋4.5743114Y 4
2KT
Chin. J. Chem. 2010, 28, 1593— 1599
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Wang et al.
FULL PAPER
where Y＝ J/KT.
ments show that biradical 1 exhibits a weak in-
tramolecular nitroxide-nitroxide antiferromagnetic cou-
pling and copper complex 2 exhibits a weak ferromag-
netic coupling between CuII and radical.
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Figure 7 Plots of χMT versus T of complex 2. The solid line
represents the best theoretical fitting with the parameter: g＝2.13,
J＝5.09 cm－ 1 and zJ'＝－0.38 cm－
.
1
A molecular field correction has been taken into ac-
count in the mean-field approximation with the parame-
ter zJ' (Eq. 3) as the magnetic interaction between
chains.49
χchain
2zJ '
Ng2β 2
χM
＝
2
3
4
5
6
7
8
9
⎛
⎜
⎝
⎞
⎟
⎠
(3)
1－
χchain
The best fit parameters for the data are g＝ 2.13, J＝
5.09 cm－ and zJ'＝ － 0.38 cm . This obtained pa-
rameter value are comparatively close to the previously
reported for a similar compound with different sub-
stituents attached to the radical ring, and these parame-
ters are listed in the Table 4. This ferromagnetic cou-
pling between the radical and the Cu(hfac)2 probably
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－
1
1
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2
2
nitrogen is nearly orthogonal to the magnetic 3dx - y
orbital of the copper atom.17
Table 4 One dimensional ferromagnetic chain compounds with
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10 Musin, R. N.; Schastnev, P. V.; Malinovskaya, S. A. Inorg.
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1
1
R
g factor J/cm－ zJ'/cm－ Hamiltonian
Me (low temperature)6
2.08 －30.8 0.046
Jij∑ijSiSj
11 Caneschi, A.; Chiesi, P.; David, L.; Ferraro, F.; Gatteschi,
D.; Sessoli, R. Inorg. Chem. 1993, 32, 1445.
i-Pr (low temperature)9
n-Pr32
2.13 －24.5 －0.002 Jij∑ijSiSj
2.10 －32.1 0.038
Jij∑ijSiSj
Ph-C≡C-SiMe3-p17
2.1
2.03
2.13
6
6.7
N/A －2Jij∑ijSiSj
－0.72 －2Jij∑ijSiSj
12 Fokin, S.; Ovcharenko, V.; Romanenko, G.; Ikorskii, V.;
Inorg. Chem. 2004, 43, 969.
Ph-C≡N-p26
13 Tretyakov, E.; Fokin, S.; Romanenko, G.; Ikorskii, V.; Va-
silevsky, S.; Ovcharenko, V. Inorg. Chem. 2006, 45, 3671.
14 Hirel, C.; Li, L. C.; Brough, P.; Vostrikova, K.; Pecaut, J.;
Mehdaoui, B.; Bernard, M.; Turek, P.; Rey, P. Inorg. Chem.
2007, 46, 7545.
15 Fedin, M.; Veber, S.; Gromov, I.; Maryunina, K.; Fokin, S.;
Romanenko, G.; Sagdeev, R.; Ovcharenko, V.; Bagryan-
skaya, E. Inorg. Chem. 2007, 46, 11405.
This work
5.09 －0.38 －2Jij∑ijSiSj
Conclusion
In conclusion, we have characterized a new biradical
1 and its copper complex 2 structurally and magnetically.
The complex 2 is in the triclinic P-1 space group and
displays a ladder-like structure. The magnetic measure-
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Chin. J. Chem. 2010, 28, 1593— 1599

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www.cjc.wiley-vch.de
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