A new heterospin complex from oxamide-bridged Cu(II) binuclear units and carboxy-phenyl-substituted nitronyl nitroxide

Article abstract of DOI:10.1016/j.inoche.2005.02.015

A new heterospin complex 1, [Cu₂(oxpn)(NITPBA)₂], derived from trans-oxamido-bridged copper(II) binuclear units [Cu ₂(oxpn), oxpn = N,N′-bis(2-aminopropyl)oxamide] and carboxyl-substituted nitronyl nitroxide, (NITPBAH = 2-(4-carboxy-phenyl)-4,4,5, 5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide), has been synthesized and characterized structurally and magnetically. The complex 1 is a four-spin system. The magnetic susceptibility of 1 was measured over the range 2-300 K, which shows that there are unusually moderate antiferromagnetic-interactions between two copper(II) ions bridged by the oxamato bridge.

Full text of DOI:10.1016/j.inoche.2005.02.015

Inorganic Chemistry Communications 8 (2005) 564–567
www.elsevier.com/locate/inoche
A new heterospin complex from oxamide-bridged Cu(II)
binuclear units and carboxy-phenyl-substituted nitronyl nitroxide
a
a,b,
a
Jing Chen a,1, Dai-Zheng Liao , Zong-Hui Jiang
*, Shi-Ping Yan
a
Department of Chemistry, Nankai University, Weijin Road, Tianjin 300071, PR China
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
Received 12 January 2005; accepted 23 February 2005
Available online 5 May 2005
Abstract
A new heterospin complex 1, [Cu2(oxpn)(NITPBA)2], derived from trans-oxamido-bridged copper(II) binuclear units
[Cu2(oxpn), oxpn = N,N0-bis(2-aminopropyl)oxamide] and carboxyl-substituted nitronyl nitroxide, (NITPBAH = 2-(4-carboxy-
phenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide), has been synthesized and characterized structurally and mag-
netically. The complex 1 is a four-spin system. The magnetic susceptibility of 1 was measured over the range 2–300 K, which shows
that there are unusually moderate antiferromagnetic-interactions between two copper(II) ions bridged by the oxamato bridge.
ꢀ 2005 Published by Elsevier B.V.
Keywords: Copper(II) complex; Heterospin; Trans-oxamido-bridged; Crystal structure; Antiferromagnetism
The synthesis and magnetic characterization of poly-
nuclear heterospin complexes derived from paramag-
netic metal ions and organic radical molecules have
attracted much attention in recent years [1–4]. One of
the best strategies to design and synthesize polynuclear
species is the ꢀcomplex as ligandꢁ approach. For the
mononuclear Cu(II) complexes of N,N0-bis(coordinat-
ing-group)-substituted oxamides [5], such as [Cu(oxpn)]
(Scheme 1), one of the most outstanding characters is
the easy transformation of cis–trans conformations
upon coordination to metal ions (Scheme 2), which
makes it practical to design tunable molecular materials
with extended structures and desired properties [6,7].
Meanwhile, stable organic radicals, especially, intro-
duction of potential donor atoms into the substituent
group of the nitronyl nitroxide, have been found wide-
spread used as ligands in the metal-radical approach
to molecular magnetic materials [8–10]. Investigation
on the magnetic behavior of the salts of cationic nitronyl
nitroxides have been performed and fascinating results
have been obtained [11,12]. The use of anionic deriva-
tives will probably offer more possibilities to synthesize
extended systems.
Thus, it would be interesting to obtain oxamido-
bridged dimetallic complex with carboxyl-substituted
nitronyl nitroxide radical. Herein, we reported the syn-
thesis, crystal structure and magnetic properties of a
novel heterospin complex from trans-oxamido-bridged
copper(II) binuclear units and carboxyl-substituted
nitronyl nitroxide radical ligands.
The carboxyl-substituted nitronyl nitroxide radical li-
gands HL (p-NITPBAH) (Scheme 3) was prepared by
the literature method [13]. Cu(oxpn) was synthesized
according to the reported procedure [5].
The title complex 1 was prepared by the following
method: To
(20 mL in 1:1 ratio) containing Cu(ClO4)2 Æ 6H2O
(2 mmol) and Cu(oxpn) (2 mmol) was added p-NITP-
BAH, dissolved in 10 mL of methanol. The mixture
a green aqueous methanol solution
*
Corresponding author. Fax: +86 22 23502779.
E-mail addresses: chenjing64@mail.nankai.edu.cn (J. Chen), coord
@nankai.edu.cn (Z.-H. Jiang).
1
Tel.: +86 22 23509957; fax: +86 22 23502779.
1387-7003/$ - see front matter ꢀ 2005 Published by Elsevier B.V.
doi:10.1016/j.inoche.2005.02.015

J. Chen et al. / Inorganic Chemistry Communications 8 (2005) 564–567
565
O
N
O
HO
N
O
O
O
NH2
N
HL ( = NITPBAH)
Cu
Scheme 3.
N
NH2
˚
1.980(4) A. Both copper atoms are displaced towards
the apices, which are hypothetically perpendicular the
mean plane (N3, N3A, O5, O5A), by 0.0634 and
˚
[Cu(oxpn)]
ꢀ0.0634 A, respectively. The N3A–Cu1–N4A angle is
96.30(15)ꢁ, and the N3A–Cu1–O1 is 169.36(14)ꢁ. The
angle of O1–Cu1–O5 is 88.52(13)ꢁ. Dihedral angle be-
tween the planes of the nitronyl (O3, N1, C8, N2, O4)
and the phenyl ring is 35.3ꢁ, while the plane (O5, O1,
N3A, N4A) shows an angle of 47.4ꢁ with the phenyl
ring. The mean plane (N3, N3A, O5, O5A) shows an an-
gle of 26.6ꢁ with the nitronyl nitroxide moiety (O1, N2,
C6, N3). The separation between the neighboring Cu(II)
Scheme 1.
solution was further stirred for 1 h at room temperature,
and filtered off. The filtrate was kept at room tempera-
ture within a couple of days to grow well-formed
dark-blue crystals of the complex 1 [Cu2(oxpn)(p-NIT-
PBAꢀ)2], suitable for an X-ray structure determination.
Yield: 34%. Anal. Calcd. for Elem. C, 49.10; H, 5.50; N,
12.73. Found: C, 48.78; H, 5.35; N, 12.69%.
The IR spectra of the complex 1 show the N–O
stretching vibration of the NITPBAꢀ at ca. 1360 cmꢀ1
as well as the antisymmetric m(C–O) and the symmetric
m(C–O) stretching mode of the carboxyl group at 1605
and 1428 cmꢀ1, respectively, in accordance with a biden-
tate-bridging mode [14,15].
˚
ions through the oxamide bridge is 5.226 A.
The molecular units are interconnected to form a 1-D
chain along the a-axis via O (2 ꢀ x, 2 ꢀ y, ꢀz)–Cu1 (+x,
+y, +z) weak coordinated bond, as illustrated in Fig. 2.
The separation of the nearest Cu(II) ions through the
oxygen bridge is 3.267 A. The shortest distant of O–O
of carboxyl-substituted nitronyl nitroxide radical from
˚
˚
two different molecular units is 3.621 A.
The crystal structure was determined by single crystal
X-ray analysis [16,17] (Fig. 1). Complex 1 is a binuclear
centro-symmetric unit. Only one oxygen atom from one
p-NITPBAꢀ anion is coordinated to each Cu(II) of the
trans-oxamido-bridged copper(II) binuclear unit,
respectively, to form a new four-spin complex. The coor-
dination geometry around each Cu(II) ion is the slightly
distorted square plane, composed of one carboxylic oxy-
gen from p-NITPBAꢀ radical, one carbonyl oxygen, and
two nitrogen atoms from the amide and amine of the
oxamide ligand. The Cu–O bonds are 1.983(3) (Cu1–
The magnetic susceptibilities, vM, of 1 were measured
between 2 and 300 K at 2000 Oe, and Fig. 3 shows the
plots of vM and vMT versus T. The vMT value at room
temperature is 1.58 cm3 K molꢀ1, close to the value ex-
pected for non-coupling two SCu = 1/2 spin and two
SRad = 1/2 spins (1.50 cm3 K molꢀ1). When the tempera-
ture is lowered, the value of vMT decreases, indicating
that the spins are antiferromagnetically coupled in the
complex 1.
In complex 1, there are four kinds of magnetic inter-
actions, namely: (i) Cu(II) interacting with the coordi-
nated radical ligand, (ii) Cu(II)–Cu(II) interaction via
the oxamato bridge, (iii) a magnetic coupling between
˚
O1), 1.995(3) A (Cu1–O5), respectively. The bond
lengths of the Cu–N are in the range of 1.946(3)–
NH2
NH2
N
N
O
O
NH2
N
O
Ligands
O
Cu
Cu
Cu
NH2
Ligands
N
Scheme 2.

566
J. Chen et al. / Inorganic Chemistry Communications 8 (2005) 564–567
˚
Fig. 1. ORTEP drawing of the complex 1; thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (A) and bond angles (ꢁ):
Cu(1)–N(3)#1 1.946(3), Cu(1)–N(4)#1 1.980(4), Cu(1)–O(1) 1.983(3), Cu(1)–O(5) 1.995(3), O(1)–C(1) 1.281(6), O(2)–C(1) 1.226(6), O(3)–N(1)
1.282(5), O(4)–N(2) 1.269(5), O(5)–C(15) 1.278(5) and N(3)#1–Cu(1)–N(4)#1 96.30(15), N(3)#1–Cu(1)–O(1) 169.36(14), N(4)#1–Cu(1)–O(1)
91.47(15), N(3)#1–Cu(1)–O(5) 84.81(14), N(4)#1–Cu(1)–O(5) 171.88(16), O(1)–Cu(1)–O(5) 88.52(13), O(2)–C(1)–O(1) 124.4(5), N(3)#1–Cu(1)–
O(1)–C(1) 25.9(9), O(5)–Cu(1)–O(1)–C(1) 77.1(3), N(3)#1–Cu(1)–O(5)–C(15) 2.0(3).
˚
Fig. 2. Showing 1-D chain parallel to a-axis and the Oꢁ ꢁ ꢁCu interactions in the crystal packing. (The distance of Oꢁ ꢁ ꢁCu is 2.592 A.)
0
50
100
150
200
250
2.0
0.025
0.020
0.015
0.010
0.005
Scheme 4. Simplified illustration of the spin–spin interaction in the
unit of the complex 1.
1.5
1.0
0.5
0.0
Due to the shortest intermolecular Oꢁ ꢁ ꢁO (from NIT-
ꢀ
˚
PBA ) distance of more than 3.5A, the intrachain radi-
cal spin–spin interaction from two adjacent units can be
neglected in order to avoid the complicated analysis.
Density functional calculation of the spin density in
Lꢀ shows the unpaired electron to be localized on the
NO-groups with little contributions from the phenyl
ring due to spin polarization, and the spin density on
the carboxylate group to be negligible [18]. The spin
density on the carboxylate group is vanishing small in
this case, which explains the lack of exchange interaction
between the NIT-carboxylate and coordinated cop-
per(II) ion. The intramolecular exchange interaction be-
tween the radical ligand and Cu(II) ion is negligible. So,
χ
χ
0
50
100
T /K
150
200
250
Fig. 3. The plots of vM (h) and vMT (s) versus T for the complex, the
solid line showing the best fit to the value.
ꢀ
˚
neighboring NO groups of two NITPBA (3.621 A), (iv)
˚
a interaction between neighboring Cuꢁ ꢁ ꢁCu (3.267 A)
through the oxygen bridge space.

J. Chen et al. / Inorganic Chemistry Communications 8 (2005) 564–567
567
the spin–spin interaction of the complex 1 can be simpli-
fied as shown as Scheme 4.
The Hamiltonian for the complex 1 can be written as
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (No. 20331010) and Tian-
jin Science Foundation (No. 33602011).
^
^
^
^
^
^
^
H ¼ ꢀJðSCu1SCu2 þ SCu3SCu4Þ ꢀ jSCu2SCu3. To simplify
the question, when gCu = gR = g is assumed, based on
the above isotropic Hamiltonian, the temperature depen-
dence of the magnetic susceptibility can be expressed as:
Ng2b2 A
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