A unique mixed-bridged trinuclear Co (II) complex and its extended system: Structural and magnetic studies

Article abstract of DOI:10.1016/j.inoche.2011.08.014

Two novel complexes, [Co₃(N₃)₂(bca) ₄(phen)₂] (1, bca = benzenecarboxylic acid, phen = 1,10-Phenanthroline) and [Co₃(N₃)₂(bdc) ₂(phen)₂]_n (2, bdc = terephthalic acid), have been synthesized hydrothermally and characterized by single crystal X-ray diffraction (XRD), infrared spectra (IR) and element analysis (EA). Complex 1 features unprecedented discrete linear Co₃ clusters that formed by mixed (μ-EO-N₃)(μ-COO)₂ (EO = end-on) triple bridges whereas 2 presents an extended (2D) network based on the similar trinuclear units as 1. Magnetic studies of 1 and 2 reveal that the mixed (μ-EO-N ₃)(μ-COO)₂ triple bridges transmit ferromagnetic behavior.

Full text of DOI:10.1016/j.inoche.2011.08.014

Inorganic Chemistry Communications 14 (2011) 1802–1806
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A unique mixed-bridged trinuclear Co (II) complex and its extended system:
Structural and magnetic studies
⁎
Yu-Ting Yang, Fang-Hua Zhao, Yun-Xia Che, Ji-Min Zheng
Department of Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel complexes, [Co3(N3)2(bca)4(phen)2] (1, bca = benzenecarboxylic acid, phen = 1,10-Phenanthroline)
and [Co3(N3)2(bdc)2(phen)2]n (2, bdc = terephthalic acid), have been synthesized hydrothermally and character-
ized by single crystal X-ray diffraction (XRD), infrared spectra (IR) and element analysis (EA). Complex 1 features
unprecedented discrete linear Co3 clusters that formed by mixed (μ-EO-N3)(μ-COO)2 (EO = end-on) triple bridges
whereas 2 presents an extended (2D) network based on the similar trinuclear units as 1. Magnetic studies of 1 and
2 reveal that the mixed (μ-EO-N3)(μ-COO)2 triple bridges transmit ferromagnetic behavior.
© 2011 Elsevier B.V. All rights reserved.
Received 15 May 2011
Accepted 17 August 2011
Available online 26 August 2011
Keywords:
Cobalt
Azide
Carboxylate
Mixed-bridges
Magnetic property
Polynuclear-based magnetic complexes have attracted great at-
tention in the field of molecular magnetism for decades, because
they not only help to improve our understanding of mechanism of
magnetic coupling but also function as building blocks applicable in
molecular-based materials [1–3]. The most popular and effective
strategy to design such materials is connecting paramagnetic centers
by short bridging ligands such as oxo, hydroxide, alkoxo, phenolate,
carboxylate, or azide, in combination with different organic coligands
to adjust the structure and dimensionality [4–6]. Among these, azide
and carboxylate are the most extensively studied short bridges be-
cause of their multiple bridging modes and the diversity in the prop-
agation of magnetic behavior [7]. It is well-known that azide anion
can link metal ions in μ-1,1 (end-on, EO), μ-1,3 (end-to-end, EE),
μ-1,1,3, or still other modes, generating discrete, one-, two- and
three-dimensional species with various topologies [8]. In general,
the EO mode transmits ferromagnetic (FM) exchange interaction,
while the EE mode affords antiferromagnetic (AFM) exchange be-
tween the paramagnetic centers [9] and other mixed azide bridging
modes also give interesting magnetic properties [10]. An effective
synthetic approach for obtaining complexes with novel structures
and magnetic properties is to incorporate a second short bridge that
provides different pathways for magnetic exchange. Considering
that the carboxylate can also efficiently transmit magnetic interac-
tions, we pay particular attention to combining azide and carboxylate
into one system. However, turning the assumption into reality is still
a challenge in coordination chemistry, perhaps because of the
mismatch between azide and carboxylic oxygen ligand in the compe-
tition to bind metal ions [11]. In contrast to the extensive structural
and magnetic studies on compounds with azide or carboxylate brid-
ges, so far, only very few examples containing mixed azide and car-
boxylate bridges have been reported, and magnetic studies exhibit
that mixed bridges usually transmit FM interactions between neigh-
boring metal ions [11,12]. Herein, we report the synthesis, structure,
and magnetic properties of two cobalt complexes, namely [Co3(N3)2
(bca)4(phen)2] (1) and [Co3(N3)2(bdc)2(phen)2]n (2), which contain
rare [Co3(μ-EO-N3)2(μ-COO)4] molecular building blocks. To the best
of our knowledge, this is the first Co3 clusters bridged by mixed
(μ-EO-N3)(μ-COO)2 triple bridges [13].
Single crystal X-ray diffraction reveals that complex 1 crystallizes
in the triclinic space group P-1 [14]. The asymmetric unit consists of
two crystallographically independent Co (II) atoms (Co1 and Co2),
one azide anion, two bca ligands and one phen molecule. Complex 1
consists of linear trinuclear unit of Co (II) atoms, where the central
Co1 atom lies on a crystallographic inversion center and is linked by
double carboxyl groups of bca ligands and one end-on azide anion
to the terminal Co(2) atoms (Fig. 1.). The central Co1 atom in the
CoO4N2 coordination environment has slightly distorted octahedral
geometry with four carboxylic oxygen atoms from four different bca
ligands forming the equatorial plane and two end-on azide nitrogen
atoms occupying two apical positions. The angles around the Co1
atom are 87.96(18) – 92.04(18) °. The Co1–O distances are 2.070(4)
and 2.121(5) Å and Co1–N3 distance is 2.186(5) Å. The terminal
Co2 atom is coordinated by two carboxylic oxygen atoms from two
bca ligands, one azide nitrogen atom and two phen nitrogen atoms,
resulting the five-coordinate CoO2N3 coordination environment
with distorted trigonal-bipyramidal geometry. The Co2\O bond
⁎
Corresponding author. Tel.: +86 22 23507950; fax: +86 22 23508056.
E-mail address: jmzheng@nankai.edu.cn (J.-M. Zheng).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.08.014

Y.-T. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1802–1806
1803
Fig. 1. View of the local coordination environment for Co atoms in 1 (symmetry codes A: −x+2, −y, −z+1). Hydrogen atoms are omitted for clarity.
Fig. 2. (a) 2D layer structure in 2. (b) Schematic description of the 4-connected (4,4) net.

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Y.-T. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1802–1806
mode. The basic building block of 2 is linear trinuclear [Co3(μ-EO-N3)2
(μ-COO)4] unit which is bridged by two nitrogen atoms of azide anion
and four carboxyl oxygen atoms of bdc ligands [Co1–O=2.0027(16)–
2.0466(16) Å, Co1–N=2.080(2)–2.0297(19) Å, Co1–N3–Co2=103.07
(8)°]. This unit is similar to complex 1. It should be noted that the bond
angle Co1–N3–Co2 (103.07(8)°) in 2 is larger than the corresponding
one (94.0(2)°) in 1, which suggests that it may affect the magnetic prop-
erties of molecules [12]. The adjacent Co1…Co2 and Co1…Co1 distances
are 3.230 Å and 6.460 Å in Co3 unit. Each trimer is connected to four ad-
jacent ones by four bdc ligands to a 2D layer structure (Fig. 2a). The near-
est interlayer Co…Co distance is 10.616 Å. Besides, a (4, 4) 2D net is
constructed by making the [Co3(μ-EO-N3)2(μ-COO)4] unit as 4-connected
node (Fig. 2b).
Further analysis of the crystal packing reveals that the phen planes
of the neighboring layers are obviously parallel with each other, and
the phenylphen–phenylphen rings stack in an offset face-to-face orien-
tation (π–π stacking offset distance is 3.310 Å), connecting neighbor-
ing layers into 3D supramolecular network (Fig. S1).
Temperature-dependent magnetic susceptibility measurements of
complexes 1 and 2 were performed under a field of 1000 Oe in the
temperature range of 2–300 K. 1 and 2 show similar magnetic behav-
iors (Fig. 3). The χMT values of 1 and 2 are 7.94 and 7.64 cm3 K mol−1
at 300 K, respectively. These two values are larger than the sum of the
expected value (5.62 cm3 mol−1 K, g=2.0, S=3/2 [1]) for three
uncoupled high-spin Co (II) ions, because of the significant contribu-
tion from the unquenched orbital momentum in the octahedral field
4
(the T1g state). As the temperature is lowered, the χMT values first
increase smoothly and then rise abruptly to the maxima values of
13.33 cm3 mol−1 K at 7.5 K for 1, and 14.65 cm3 mol−1 K at 5.0 K
for 2. Upon further cooling, χMT curves sharply decrease to minimum
values of 10.68 cm3 mol−1 K and 13.09 cm3 mol−1 K at 2 K for 1 and
2, respectively.
Temperature-dependent magnetic susceptibilities of complexes 1
and 2 indicate significant ferromagnetic (FM) behavior. This is consis-
tent with that observed in other Co-based complexes with the mixed
(μ-EO-N3)(μ-COO)2 triple bridges motif [12]. The decrease of χMT is
possibly due to ZFS (zero field splitting) and/or inter-trimer antiferro-
magnetic interaction [1].
Magnetic susceptibility data per Co3 unit for 1 and 2 can be fitted
to the Curie–Weiss law (χM =C/(T−θ)), giving a Curie constant
C=7.96 cm3 mol−1 K, and Weiss constant θ=+6.0 K at 2–300 K
for 1, C=7.37 cm3 mol−1 K, and θ=+11.79 K at 90–300 K for 2, re-
spectively. The positive θ values indicate the domination of FM inter-
action between the adjacent Co1 and Co2 atoms through the mixed
(μ-EO-N3)(μ-COO)2 triple bridges.
Fig. 3. Temperature dependence of χMT vs. T plot for 1 (a) and 2 (b) at 1000 Oe. Inset: χM vs.
T plot, and the red solid line represent the simulation results.
distances fall in the range between 2.022(5) and 2.111(5) Å, and
Co2–N distances range from 2.089(6) to 2.128(5) Å. The angles around
the terminal Co2 atom are 78.9(2) – 139.90(19) °. The Co\O/N bond
lengths and angles are all consistent with corresponding ones found
in other Co-based complexes [15].
The completely deprotonated bca anion acts as a bidentate bridg-
ing spacer linking two Co (II) atoms, and the carboxyl group show the
μ2: η1, η1 coordinated mode. While the azide anion shows the EO
(end-on) bridging mode, acts as a bidentate bridging spacer linking
two Co (II) atoms. Each Co1 atom connects to two Co2 atoms through
mixed (μ-EO-N3)(μ-COO)2 triple bridges, generating a linear trimeric
Co3 cluster along the a-axis direction [Co1–N3–Co2=94.0(2)°]. To
our knowledge, such a mixed bridging mode for Co3 is unprecedented
in the literature. The Co1⋯ Co2 distance spanned by the (μ1,1-N3)
(μ-COO)2 triple bridges is 3.12 Å, and the Co1–N3–Co2 angle is 94.0
(2)°, comparable to the reported (μ-EO-N3)(μ-COO)2-bridged Co (II)
complexes [12].
Furthermore, we chose bdc that contains two carboxylate groups
in opposite position to replace bca in 1, and isolated a 2D layer frame-
work. Complex 2 crystallizes in the monoclinic space group P21/n. The
asymmetric unit consists of two crystallographically independent Co
(II) atoms (Co1 and Co2), one azide anion, two bdc ligands and one
phen molecule. The azide anion adopts μ-1,1 (EO) coordination
mode, while bdc ligand shows the μ4: η1, η1, η1, η1 coordination
Fig. 4. Field dependence of magnetization for 1 and 2 at 2 K.

Y.-T. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1802–1806
1805
Fig. 5. Hysteresis loop at 2 K for 1 and 2. The insets give a blown-up view of the hysteresis loop below 200 Oe.
According to the structural data, both 1 and 2 can magnetically be
Acknowledgments
treated as trinuclear complexes in which magnetic coupling is medi-
ated through the triple bridge (μ-EO-N3)(μ-COO)2. To simulate the
experimental magnetic behavior, the data of 1 and 2 are approxi-
mately fitted by the isotropic linear trinuclear model that is based
on spin Hamiltonian:
This work was funded by the National Natural Science Foundation
of China under Project (50872057).
Appendix A. Supplementary data
ꢀ
ꢁ
χtrimer
1− Ng χtrimer
2β2
ˆ
ˆ
ˆ
ˆ
ˆ
H ¼ −2 J S1·S2 þ S1A· S2 and χ ¼
2
CCDC-820575 (1) and CCDC-820576 (2) contain the supplemen-
tary crystallographic 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. or e-mail: deposit@ccdc.cam.
ac.uk. Additional figures, selected bond lengths and angles are avail-
able as electronic supplementary information in the online version,
at doi:10.1016/j.inoche.2011.08.014.
where S1 =S1A =S2 =3/2. The best fit gives the J=4.15 cm−1
,
g=2.4 for 1 (Fig. 3a) and J=7.85 cm−1, g=2.25 for 2 (Fig. 3b).
These results are also consistent with the experimental research.
The M vs. H measurement at 2 K among 0–50 kOe is given in Fig. 4.
The curves raise first sharply till saturation and then gradually to 8.46
Nβ for 1 and 7.86 Nβ for 2 at 50 kOe. These findings are close to the
expected value (Ms=9 Nβ) for g=2 and S=3/2, confirming the
expected FM coupling in 1 and 2. Isothermal magnetization experi-
ments performed at 2 K exhibit a hysteresis with small coercive
field (Hc) and remnant magnetization (Mr) of 12 Oe and 0.014 Nβ
for 1, 15 Oe and 0.015 Nβ for 2 (Fig. 5), typical of soft ferromagnetic
behavior.
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[13] CAUTION! Although not encountered in our experiments, azide compounds of
metal ions are potentially explosive. The materials should be handled in small
amounts and with care. Synthesis of 1:
A mixture of CoCl2·6H2O (0.36 g,
1.5 mmol), NaN3 (0.065 g, 1.0 mmol), bca (0.24 g, 2.0 mmol), and phen (0.20 g,
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