Electrical and magnetic properties of a radical-based Co(II) coordination complex with C–H?π and π?π supramolecular interactions

Article abstract of DOI:10.1016/j.inoche.2019.03.022

HAN^–[rad] radical based supramolecular Co(II) complex, Co-HAN, has been synthesized with reaction of large π-conjugated hexaazatrinaphthylene (HAN), cobalt salt and ethylenediamine reductant through one-pot solvothermal method. In aggregating crystal network of Co-HAN, two kinds of Zig-Zag supramolecular channels are formed by π?π stacking and C–H?π hydrogen-bond interactions, respectively. The supramolecular interactions, especially π?π stacking, help promote the charge transport, leading to an enhanced electrical conductivity (σ = 0.599 × 10^?7 S·cm^?1). In addition, electron paramagnetic resonance (EPR) measurements indicated of antiferromagnetic interactions between monoradical ligand and Co(II) ion. The temperature-dependent magnetic susceptibility and χ_mT value suggests that Co-HAN exhibits a normal paramagnetic behavior for an effective spin of 1/2 coming from the unquenched orbit magnetic moment of Co(II) ions.

Full text of DOI:10.1016/j.inoche.2019.03.022

Inorganic Chemistry Communications 103 (2019) 149–153
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Electrical and magnetic properties of a radical-based Co(II) coordination
complex with CeH⋯π and π⋯π supramolecular interactions
⁎
⁎
Wen-Hao Wu, Meng-Jiao Huang, Qi Zeng, Wan-Ru Xian, Wei-Ming Liao , Jun He
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
G R A P H I C A L A B S T R A C T
–
HAN % based Co(II) complex was synthesized through one-pot solvothermal method. The included supramolecular interactions of π⋯π and CeH⋯π can enhance
−
7
−1
electrical conductivity (σ = 0.599 × 10 S·cm ). EPR and magnetic research indicates that there is antiferromagnetic interaction between monoradical ligand and
Co(II) ion, while complex overall exhibits a normal paramagnetic behavior.
A R T I C L E I N F O
A B S T R A C T
–
Keywords:
HAN % radical based supramolecular Co(II) complex, Co-HAN, has been synthesized with reaction of large π-
Coordination complex
Supramolecular interaction
Conductivity
conjugated hexaazatrinaphthylene (HAN), cobalt salt and ethylenediamine reductant through one-pot sol-
vothermal method. In aggregating crystal network of Co-HAN, two kinds of Zig-Zag supramolecular channels are
formed by π⋯π stacking and CeH⋯π hydrogen-bond interactions, respectively. The supramolecular interac-
tions, especially π⋯π stacking, help promote the charge transport, leading to an enhanced electrical conductivity
Magnetic property
−7
−1
(
σ = 0.599 × 10 S·cm ). In addition, electron paramagnetic resonance (EPR) measurements indicated of
antiferromagnetic interactions between monoradical ligand and Co(II) ion. The temperature-dependent mag-
netic susceptibility and χ T value suggests that Co-HAN exhibits a normal paramagnetic behavior for an ef-
fective spin of 1/2 coming from the unquenched orbit magnetic moment of Co(II) ions.
m
1
. Introduction
However, It is still of high challenge to synthesize complexes presenting
both cooperative properties (electrical conductivity and magnetic
order), although some related materials have been investigated and
reported [1,2].
Coordination complexes are crystalline materials exhibiting wide
application in luminescent sensing, molecule conductor and magnet,
and photocatalysis for their novel optical, electrical and magnetic
properties. With adjustable metal ions and ligands, diverse structures of
complexes with expected function could be obtained by coordination
assembly. The materials with high electrical conductivity and good
long-range magnetic ordering simultaneously are quite important in
microwave absorber, electromagnetic shielding and stealth materials.
Supramolecular interactions such as hydrogen bonding and π⋯π
stacking often affect the crystal engineering and lead to various ag-
gregates [3–6]. Such non-covalent interactions are investigated and
applied in chemical and biological recognition, drug development and
catalysis, etc. [7,8]. Particularly, these weak interactions are found to
affect the electrical performance of complexes. For example, it helps
⁎
Corresponding authors.
E-mail addresses: wmliao@gdut.edu.cn (W.-M. Liao), junhe@gdut.edu.cn (J. He).
https://doi.org/10.1016/j.inoche.2019.03.022
Received 11 March 2019; Received in revised form 23 March 2019; Accepted 26 March 2019
Available online 27 March 2019
1387-7003/ © 2019 Elsevier B.V. All rights reserved.

W.-H. Wu, et al.
Inorganic Chemistry Communications 103 (2019) 149–153
form extended charge transport pathway resulting from orbital overlap
of ligands, which could enhance the electrical conductivity [9,10]. π⋯π
interactions as well as hydrogen bonds have also been proved to play an
important role in tuning the magnetic exchange interactions and in-
tensity [11–13] Therefore, it is necessary to investigate the supramo-
lecular interactions for developing the multi-functional complexes with
attractive electrical and magnetic properties. Large π-conjugated com-
pounds are favorable to form supramolecular interactions of π⋯π
stacking by intermolecular aggregates, which could help enhance the
electron transport [14,15].
powder X-ray diffraction (PXRD) data was recorded on a Bruker D8
Advance diffractometer at 40 kV, 40 mA with a Cu-target tube and a
1
graphite monochromator. H NMR spectra were recorded on a Bruker
AVANCE III 400 MHz NMR spectrometer. Diffuse reflectance spectrum
was achieved from a Lambda 950 UV–Vis-near-infrared spectro-
photometer. FT-IR spectra were obtained by using a Nicolet Avatar 360
spectrophotometer. Thermogravimetric analyses (TG) were carried out
in a nitrogen stream using PerkinElmer Thermal analysis equipment
(TA-Q600) with a heating rate of 5 °C/min. The resistance of complex
was measured with a solid pellet achieved from pressing in a micro-tube
(r = 0.9 mm) on a source meter equipment (Keithley 2400), then
electrical conductivity was determined by the function of σ = L / (R·S),
Radical-based ligands possess unique spin single electron and have
been used to synthesize coordination complexes for potential material
as magnetic switch [16], single-molecule magnet [17–19] and super-
conductor [20], etc. Hexaazatrinaphthylene (HAN) is a large π-con-
jugated ligand with six nitrogen atoms. Interestingly, it can acts as a
2
2
in which R = 65.1 MΩ, L = 0.99 mm, and S = πr = 2.5434 mm . All
of the magnetic data was obtained from a physical property measure-
ment system (PPMS, DynaCool, Quantum Design Inc.)
–
redox-active linker and could be reduced to HAN % in the process of
self-assembly by reductant, such as K [21], KC [22] and Mg(I) reagent
8
3
. Experimental sections
[
23]. This stable radical exhibits novel magnetic exchange coupling
property by coordination with metal ions, and pave a route to new
magnetic materials [21]. In the other hand, these graphene-like ligands
with large π-conjugated system have been importantly investigated in
conductive complex materials and proved to be of high conductivity
Synthesis of HAN ligand: A mixture of hexaketocyclohexane octa-
hydrate (0.78 g, 2.50 mmol) and 1,2‑phenylenediamine (0.89 g,
8
.25 mmol) in anhydrous methanol (50 mL) were added into a round
bottom flask (250 mL) settled on a magnetic stir rotor. A solution was
obtained and subsequently refluxed for 4 h. The achieved yellow-green
suspension was cooled to room temperature naturally. Then 0.90 g
[
9,10]. In 2012, Yaghi et al. [24] synthesized the first two-dimensional
2+
complex, Cu-CAT-1, with hexahydroxytriphenylene (HHTP) and Cu
.
−
1
The complex exhibited good electrical conductivity (0.21 S cm ) due
(
yield, 94%, based on hexaketocyclohexane octahydrate) yellow-green
to the large π-conjugated plane Cu
port. Feng and Cánovas et al. [25] synthesized a π-d conjugated two-
dimensional semiconductor, Fe (THT) with hexamethylene-
3
(HHTP) promoting charge trans-
2
flake solids could be separated from filtration by washing with me-
1
thanol (3 × 15 mL) followed by drying under vacuum for 10 h. H NMR
3
2
,
(
400 MHz, CDCl ): 8.05 (s, 6H). 8.70 (d, 6H).
3
2
+
triphenylene (THT) and Fe , showing a high charge mobility
Synthesis of Co-HAN complex: The mixture of Co(NO
3
)
2
·6H O
2
2
−1 −1
(
~220 cm V
s
). To date, Zhu and Xu's group [26] keep the record
(BHT)
(
(
(
90.0 mg, 0.309 mmol) in ethylenediamine (2 mL) and HAN
40.0 mg,0.104 mmol) in DMA (10 mL) were sealed in a Schlenk tube
25 mL) and the tube was heated in an oven at 130 °C for 72 h. After
of highest conductive coordination complex of Cu
3
2
−
1
(
1580 S cm ) achieving from hexamethylenebenzene (BHT) and
2+
Cu . With good electrical conductivity and structural stability, these
complexes are potential for battery, supercapacitor, sensor and field-
effect transistor, etc. [27]. However, no HAN-based complexes for
conductivity studies have been reported so far.
cooling to room temperature within 12 h, the solid was separated and
washed with DMA and methanol solvents. Purple-black rhombic crys-
tals could be obtained after being under vacuum for 10 h. Elemental
analysis of the product, C48
H
32
N
12
4
O Co, calculated: C 64.07, H 3.58, N
Herein, we prepared a radical-based HAN ligand and synthesized
Co-HAN complexes with Co(II) salt by solvothermal method. The
1
8.68%; found: C 63.68, H 3.55, N 18.82%.
–
HAN % included complex exhibits moderate electrical conductivity and
4
. Results and discussions
magnetic coupling interaction which are importantly related to the
supramolecular interactions, providing a potential strategy to design
and synthesize multi-functional materials.
Through solvothermal reaction of Co(II) ion and π-conjugated HAN
ligand (Fig. 1a), Co-HAN complex was achieved and appropriate single
crystal was picked for structure determination. It indicates that Co-
2
. Materials and general methods
HAN crystallizes in I4 /a space group according to crystal refinement
1
results (Table S1). The ORTEP view shows that Co-HAN is a dimer
complex with spindle-like sandwich structure (Fig. 1b). Metal center
Co1 possesses a tetrahedron configuration and is coordinated with
chelate nitrogen atoms from two different HAN ligands, which is similar
All the materials, regents and solvents were purchased from com-
mercial sources and used without further purification. Single-crystal X-
ray diffraction (SCXRD) data was collected on a Bruker D8 Venture X-
ray diffractometer equipped with Cu-Kα radiation (λ = 1.54178 Å). The
to the trimetallic complex [(HAN){Co(N″)
2
} ] reported by Layfield
3
Fig. 1. The molecule structure of HAN ligand (a), and
the ORTEP views of Co-HAN complex along b (b) and
c (c) axis, respectively. Thermal ellipsoids are at 50%
probability. Atoms in color: cobalt, turquoise; ni-
trogen, blue; carbon, grey; hydrogen, green. -X,1/2-
Y,+Z for N1′, C1′, C2′ and C3′. The simulated and as-
synthesized PXRD patterns of Co-HAN complex (d).
(
For interpretation of the references to color in this
figure legend, the reader is referred to the web ver-
sion of this article.)
150

W.-H. Wu, et al.
Inorganic Chemistry Communications 103 (2019) 149–153
[
21]. The bond length of CoeN1 is 1.950 Å which is quite close to the
conjugated ligands, and the geometrical parameters are achieved from
PLATON program [33] (Table S3). The distance from one centroid of
aromatic plane to partner is 3.57, 3.71 and 3.72 Å for A to B, A to C and
D to E, respectively (Fig. 3a). The dihedral angle between these π⋯π
partner planes is quite small which varies from 0.00 to 5.13°. These
results indicate of moderate face-to-face π-stacking interactions be-
tween two close dimer complex molecules [34,35]. CeH⋯π interaction
was also found between two almost vertical aromatic rings in the
stacking network of Co-HAN (Fig. 3b). According to the geometrical
parameters shown in Table S4, the distance of H11 and plane F is small
to 2.54 Å, indicating of strong CeH⋯π interaction in comparison to
some other complexes [36–38]. The CeHeCg (Cg is the center of
gravity of aromatic ring) angle is 168°, and the angle of the C11eH11
bond with the plane F is 80°. Some other distance (such as, 3.48, 3.55
and 3.65 Å) of hydrogen atom and aromatic centroid is too large to
form CeH⋯π interaction (Fig. 3b). Interestingly, with the supramole-
cular interactions between the close complex molecules, two kinds of
Zig-Zag chains go through the network structure, one is from in-
tramolecular covalent bonds accompanied with CeH⋯π interactions
while the other is with π⋯π interactions (Fig. 3c).
reported value in other Co(II) complexes. The Co(II)eN bond length in
Singh, Cano, and Mukherjee's work is 2.041(3), 1.981(6) and 1.944(3)
Å, respectively, indicating of a moderate value [28–30]. Viewing along
c axis, two HAN ligands exhibits a crossing shape, suggesting that they
are nearly vertical (Fig. 1c). On the basis of the charge balance in this
complex, the oxidation state of each anion moiety should be −2, in-
–
dicating of −1 for each HAN % ligand. HAN molecules were reduced to
–
HAN % radicals in the process of self-assembly by ethylenediamine re-
–
ductant. The CeC distances of HAN % ligand are in the range of
1
1
.371(2)–1.439(2) Å, while the CeN distances vary from 1.3534(18) to
.3804(19) Å. The radical electron might locate close to C N-moiety
2
because it usually leads to shorter bond length due to the electrostatic
interaction comparing with other reported samples (Table S2) [22].
Powder X-ray diffraction (PXRD) data of as-synthesized Co-HAN com-
plex was recorded and compared to simulated pattern (Fig. 1d). The
peak positions of as-synthesized sample are highly consistent with si-
mulated one though with slightly different peak intensity which results
from crystal anisotropy, which indicates of successful synthesis and
phase purity of Co-HAN complex.
Thermogravimetric curve was also recorded and shown in Fig. S1.
About 2% weight loss is found before 400 °C which should be ascribed
to few solvent molecules or some water from the air. Another large
weight loss appears after 400 °C indicates of destruction of coordination
and covalent bonds. With the coordination interactions of Co(II) and
nitrogen atoms, IR spectrum of Co-HAN exhibits much difference in
peak intensity and position comparing to HAN ligand (Fig. S2). The
The novel supramolecular interactions and radical π-conjugated
structure in Co-HAN might provide a potential pathway of charge
transport and lead to attractive electrical properties. Solid-state ab-
sorption spectrum of Co-HAN has been first measured by diffuse re-
flection method and it is further transformed to Kubelka-Munk curve.
Then the band gap could be subsequently determined to be 1.74 eV by
tangent method (Fig. S4), indicating of semiconductive nature for Co-
HAN sample. To investigate the electron conductive property, the re-
sistance of Co-HAN was measured with solid pressing pellets and
−1
C]C and C]N stretching vibration peaks at 1611 and 1571 cm
in
−
−
1
1
HAN have shifted to 1598 and 1558 cm
tively. The peaks of 1471 and 1404 cm
after coordination, respec-
corresponding to the scis-
−7
−1
electrical conductivity was calculated to be σ = 0.599 × 10 S·cm
.
soring and rocking vibration of aromatic CeH groups, have shifted to
Complex Co-HAN possesses a low band gap value and exhibits mod-
erate electrical conductivity comparing to some semiconductive tran-
sition metal complexes [9,39–41].
−
1
1
460 and 1392 cm , respectively. Some absorption peaks those are
not easily assigned in the fingerprint region also exhibit slight shift and
intensity changes. All of these changes in IR spectrum should be as-
cribed to the formation of coordination bonds and indicates of suc-
cessful synthesis of coordination complex.
With the existence of special spinning single electrons in Co(II) ion
and radical ligand, Co-HAN complex might possess interesting and
novel magnetic properties. The temperature-dependent magnetic sus-
ceptibility were recorded under an applied magnetic field of 2000 Oe
–
In order to verify the existence of HAN % ligand, the electron
paramagnetic resonance (EPR) measurements were recorded at 298 and
from 1.9 to 300 K (Fig. 4a). The χ T value obtained by applying
m
3
−1
1
00 K, respectively (Fig. 2). A near-symmetric EPR signal can be ob-
2000 Oe at 300 K is 0.47 cm K mol , which is much less than the
3 −1
served around 3360 G (g = 2.020, Fig. S3) at 298 K, indicating of an
expected spin-only value (2.625 cm K mol ) for uncoupled one high-
–
organic monoradical for HAN % ligand [31]. When the temperature is
spin Co(II) ion (S = 3/2) and two radical ligand (S = 1/2) but close to
3
−1
low to 100 K, some new signals are found. These splitting signals are
ascribed to the antiferromagnetic interactions between monoradical
ligand and orbital energy levels of high-spin Co(II) ion [32]. All of these
the spin-only value (0.375 cm K mol ) for a spin of 1/2. Regarding
the
fact
that
a
very-strong
antiferromagnetic
exchange
−
1
(J ≈ −290 cm
in −2 J formalism) could be existed between Co(II)
–
–
results support the conclusion of existent Co(II) and HAN % in Co-HAN
ion and HAN % ligand in a trimetallic radical-bridged cobalt(II) complex
complex.
[21], the χ T value at 300 K indicated Co-HAN reveal an S = 1/2 spin
m
The supramolecular assembly in Co-HAN could be readily analyzed
in terms of a dimeric substructures unit as the building block. Crystal
packing structure is stabilized by a combination result of intermolecular
π⋯π stacking and CeH⋯π interactions (Fig. 3). Three kinds of π⋯π
interactions could be observed between two almost parallel π-
system owing the very-strong cobalt-ligand antiferromagnetic interac-
tions. The estimated g factor for this effective 1/2 spin is about 2.0,
owing to the unquenched orbit magnetic moment of Co(II) ions, which
is commonly observed for Co(II) compounds. With lowing temperature,
the χ T value gradually and slightly increase to about
m
Fig. 2. Solid-state EPR spectra of Co-HAN at 298 K (a) and 100 K (b), respectively.
151

W.-H. Wu, et al.
Inorganic Chemistry Communications 103 (2019) 149–153
Fig. 3. The π⋯π (a) and CeH⋯π (b) distances in Co-
HAN complex. The Zig-Zag supramolecular channels
(
c) resulting from π⋯π and CeH⋯π interactions.
Some atoms of HAN ligands have been omitted for
clarity. The red ball is nothing, it represents the
center of gravity of the aromatic ring. Atoms in color:
cobalt, turquoise; nitrogen, blue; carbon, grey; hy-
drogen, green. (For interpretation of the references to
color in this figure legend, the reader is referred to
the web version of this article.)
Fig. 4. Temperature-dependent magnetic susceptibility (a) at 2000 Oe varying from 1.9 to 300 K and magnetization curves at 2 K (b).
3
−1
0
.69 cm K mol at 2.0 K, suggesting a normal paramagnetic behaviors
Acknowledgements
for an effective spin of 1/2. The magnetization curves are also measured
with magnetic field varying from −70–70 kOe at 2 K (Fig. 4b). At 2 K,
no obvious hysteresis loop was found, and the magnetization is 0.91 Nβ
at 70 kOe but still not saturated, well confirming a paramagnetic be-
havior of an effective 1/2 spin for Co-HAN.
This work is supported by Science and Technology Planning Project
of Guangdong Province (2017A050506051), and Science and
Technology Program of Guangzhou (201807010026).
Appendix A. Supplementary material
5
. Conclusions
CCDC 1893420 contains the supplementary crystallographic data
for Co-HAN. Supplementary data to this article can be found online at
https://doi.org/10.1016/j.inoche.2019.03.022.
A large π-conjugated supramolecular complex, Co-HAN, was syn-
–
thesized by reduced state radical ligand (HAN %) and cobalt ion
through solution self-assembly. Electron paramagnetic resonance
measurements indicated of antiferromagnetic interactions between
monoradical ligand and Co(II) ion. In this supramolecular system, the
intermolecular interactions of π⋯π stacking and CeH⋯π hydrogen-
bond construct the aggregated network and help enhance the electrical
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