Field-Induced Slow Magnetic Relaxation and Gas Adsorption Properties of a Bifunctional Cobalt(II) Compound

Article abstract of DOI:10.1021/acs.inorgchem.5b02324

A new compound, {[Co(bmzbc)₂]·2DMF}_n (JXNU-1, JXNU denotes Jiangxi Normal University), based on the 4-(benzimidazole-1-yl)benzoate (bmzbc^-) ligand has been synthesized and structurally characterized. The Co(II) ions are br

Full text of DOI:10.1021/acs.inorgchem.5b02324

Article
pubs.acs.org/IC
Field-Induced Slow Magnetic Relaxation and Gas Adsorption
Properties of a Bifunctional Cobalt(II) Compound
Yu-Ling Wang,*,† Lin Chen,† Cai-Ming Liu,‡ Yi-Quan Zhang,§ Shun-Gao Yin,† and Qing-Yan Liu*,†
†College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China
‡Beijing National Laboratory for Molecular Sciences, Institution of Chemistry, Chinese Academy of Sciences, Center for Molecular
Sciences, Beijing 100190, P. R. China, and
§Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R.
China
S
* Supporting Information
ABSTRACT: A new compound, {[Co(bmzbc)2]·2DMF}n
(JXNU-1, JXNU denotes Jiangxi Normal University), based
on the 4-(benzimidazole-1-yl)benzoate (bmzbc−) ligand has
been synthesized and structurally characterized. The Co(II)
ions are bridged by the rod-like bmzbc− ligands to give a two-
dimensional (2D) sheet wherein the Co(II) ions are spatially
separated from each other by the long bmzbc− rods. The 2D
sheets are further stacked into a 3D framework with 1D
channels occluding the guest DMF molecules. Detailed
magnetic studies show that the individual octahedral Co(II)
ions in JXNU-1 exhibit field-induced slow magnetic relaxation,
which is characteristic behavior of single-ion magnets (SIMs).
The rarely observed positive value of zero-field splitting (ZFS)
parameter D for the Co(II) ion in JXNU-1 demonstrates that
JXNU-1 is a unique example of Co(II)-based SIMs with easy-plane anisotropy, which is also confirmed by the calculations. The
microporous nature of JXNU-1 was established by measuring CO2 sorption isotherms. The abrupt changes observed in the C3H8
and C2H6 adsorption isotherms indicate that a structural transformation occurred in the gas-loading process. The long
connection between the magnetic metal centers in JXNU-1 meets the requirements for construction of porosity and SIM in a
well-defined network, harmoniously providing a good candidate of functional molecular materials exhibiting SIM and porosity.
(Supporting Information). The Hbmzbc ligand is suitable as a
bridging ligand with diverse configurations, as a result of the
free rotation of the CN single bond between the benzene
and benzimidazole moieties. The carboxylate group and the N
donor atom are arranged at the head and tail of the ligand,
respectively, which can effectively separate the metal centers
from each other in a polymeric structure.
However, the porous coordination polymers or metal−
organic frameworks (MOFs) consist of metal ions and organic
linkers within the frameworks and have shown great promise as
functional solid materials for gas storage and separation,
catalysis, and sensing.7 The three-dimensional (3D) frame-
works have afforded the greatest diversity of the porous
coordination polymers due to their structural rigidity and
porous character. However, research on 2D porous coordina-
tion polymers has been significantly lagging behind that of 3D
porous coordination polymers. Indeed, the 2D structures can
be stacked into 3D dense frameworks with porosities exhibiting
adsorption properties.8
INTRODUCTION
■
Single-ion magnets (SIMs) with slow magnetic relaxation
stemming from a single paramagnetic ion are an important type
of single-molecule magnets (SMMs).1 Since the SIM behavior
has been observed first in the mononuclear Tb(III)Pc2
compound,2 the research of the lanthanide-based SIMs has
been extensively studied because these magnetic systems
provide elegant models to understand quantum phenomenon
at molecular level and have potential applications in quantum
computing and high-density information storage.3 Thus, many
lanthanide SIMs have been documented in the past ten years.4
Very recently, a few examples of SIM based on the 3d
transition-metal ions have been reported due to the single-ion
anisotropy.5 It has been reported that the single lanthanide ions
spatially separated in a coordination polymer can show slow
magnetic relaxation arising from the single-ion magnetic
anisotropy.6 Similarly, if the 3d transition-metal ions with
suitable local magnetic anisotropy in an appropriate ligand field
are magnetically isolated in a well-defined coordination
network, can it possess SIM behavior resulting from the
single-ion anisotropy? To realize this, a long spatial ligand of 4-
(benzimidazole-1-yl)benzoic acid (Hbmzbc) was synthesized
Received: August 27, 2015
© XXXX American Chemical Society
A
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the crystal parameters, data collection, and refinement are summarized
in Table 1. Important bond lengths are listed in Table S1. More details
on the crystallographic data are given in the X-ray crystallographic files
in CIF format.
For porosity and long-range magnetic ordering, they are
inimical based on the fact that the former requires long
connections between the metal centers, while the latter needs
short ones.9 However, concerning the porosity and SIM in a
network, both of them require long connection between metal
centers. Following this approach, we choose the cobalt(II) ion
with large magnetic anisotropy as a metal center and the rod-
spaced Hbmzbc ligand as long connection for construction of
the multifunctional material exhibiting SIM and porosity. A
novel compound, {[Co(bmzbc)2]·2DMF}n (JXNU-1), is
reported in this contribution. The SIM-type slow magnetic
relaxation and interesting adsorption properties have been
observed in compound JXNU-1. Thus, it is a rare example of
functional molecular materials exhibiting SIM and porosity
harmoniously.
a
Table 1. Crystallographic Data for Compound JXNU-1
compound
formula
JXNU-1
C34H32N6O6Co
679.59
fw
temp (K)
cryst syst
space group
Z
100
monoclinic
C2/c
4
a (Å)
17.815(5)
15.672(4)
14.185(6)
90
b (Å)
c (Å)
EXPERIMENTAL SECTION
■
α (deg)
β (deg)
γ (deg)
V (Å3)
Dcalcd (g•cm−3
Chemicals. All chemicals were of reagent grade and used as
commercially obtained. The starting material 4-(benzimidazole-1-
yl)benzoic acid (Hbmzbc) was prepared following a modified known
procedure.10 It should be noted that our synthetic method for Hbmzbc
is different from that of the reported method.11
124.964(3)
90
2729.2(7)
1.391
)
μ (mm−1
)
0.583
Physical Measurements. Elemental analyses were carried out on
an Elementar Vario EL III analyzer and IR spectra (KBr pellets) were
recorded on PerkinElmer Spectrum One. Elemental analyses were
carried out on Elementar PerkinElmer 2400CHN microanalyzer.
Thermogravimetric analyses were carried out on a PE Diamond TG/
DTA unit at a heating rate of 10 °C/min under a nitrogen atmosphere.
Powder X-ray diffraction patterns were performed on a Rigaku
Miniflex Π powder diffractometer using Cu−Kα radiation (λ = 1.5418
Å). Magnetic measurements were carried out on crystalline samples
with a Quantum Design MPMS-XL5 SQUID magnetometer in the
temperature range of 2−300 K. Diamagnetic corrections were
estimated from Pascal’s constants for all constituent atoms.12 Gas
sorption isotherms were measured using a Micromeritics ASAP2020
gas adsorption instrument.
no. of reflns collected
independent reflns
obsd. reflns. (I > 2σ(I))
F(000)
9607
3729
2535
1412
GOF on F2
1.009
2θ range [°]
R[int]
1.98 to 27.62
0.0488
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
CCDC number
0.0708, 0.1878
0.1026, 0.2026
1420038
a
R1 = Σ∥Fo| − |Fc∥/Σ|Fo| and wR2 = {Σ[w(Fo2 − Fc2)2]/
Σ[w(Fo2)2]}1/2
.
Synthesis of {[Co(bmzbc)2]·2DMF}n (JXNU-1). A mixture of
Co(NO3)2·6H2O (0.0291 g, 0.1 mmol), 4-(benzimidazole −1-
yl)benzoic acid (0.0714 g, 0.3 mmol), methylamine (0.05 mL) and
DMF (10 mL) was sealed in a Teflon-lined stainless steel autoclave,
which was heated at 120 °C for 72 h under autogenous pressure. The
resulting mixture was cooled naturally to obtain the purple crystals
(yield: 0.018 g, 62% on the basis of Co). Anal. Calcd. for
C34H32N6O6Co (679.59): C, 60.09; H, 4.75; N, 12.37%. Found: C,
60.13; H, 4.79; N, 12.48%. Main IR features (KBr pellet, cm−1): 3290
(m), 3130 (w), 2965 (w), 1660 (s), 1604 (s), 1540 (s), 1517 (m),
1502 (w), 1478 (w), 1460 (m), 1425 (s), 1408 (w), 1385 (m), 1323
(m), 1298 (m), 1236 (s), 1176 (w), 1103 (w), 1061 (w), 1012 (m),
987 (m), 940 (w), 87 (w), 859 (m),789 (s), 778 (w), 747 (m), 736
(w), 704 (m), 652 (w), 622 (w), 583 (w), 531 (w), 493 (m), 429 (w).
X-ray Crystallography. X-ray diffraction data were collected on a
Bruker Apex II CCD diffractometer equipped with a graphite-
monochromated Mo−Ka radiation (λ = 0.710 73 Å) at 100 K. Data
reduction was performed using SAINT and corrected for Lorentz and
polarization effects. Adsorption corrections were applied using the
SADABS routine.13 The structure was solved by the direct methods
and successive Fourier difference syntheses, and refined by the full-
matrix least-squares method on F2 (SHELXTL Version 5.1).14 All non-
hydrogen atoms are refined with anisotropic thermal parameters.
Hydrogen atoms were assigned to calculated positions. The DMF
solvents were highly disordered, and attempts to locate and refine the
solvent peaks were unsuccessful. The diffused electron densities
resulting from these residual solvents were removed from the data
using the SQUEEZE routine of PLATON15 and refined further using
the data generated. Two guest DMF molecules per formula unit are
observed in the channels, as established by elemental and
thermogravimetric analyses. The R1 values are defined as R1 = Σ∥F0|
Theoretical Calculations. Complete active space second-order
perturbation theory (CASPT2) considering the effect of the dynamical
electronic correlation based on complete-active-space self-consistent
field (CASSCF) using the MOLCAS 7.8 program package16 was
performed on the model structure of compound JXNU-1 to obtain the
parameters D and E. For the first CASSCF calculation, the basis sets
for all atoms are atomic natural orbitals from the MOLCAS ANO-
RCC library: ANO-RCC-VTZP for magnetic center ion Co(II); VTZ
for close O and N; VDZ for distant atoms. The calculations employed
the second order Douglas−Kroll−Hess Hamiltonian, where scalar
relativistic contractions were taken into account in the basis set. After
that, the effect of the dynamical electronic correlation was applied
using CASPT2. And then, the spin−orbit coupling was handled
separately in the restricted active space state interaction (RASSI-SO)
procedure. The active electrons in 10 active spaces include all seven 3d
electrons, and the mixed spin-free states are 50 (all from 10
quadruplets; all from 40 doublets).
RESULTS AND DISCUSSION
■
Solvothermal reaction of Co(NO3)2, Hbmzbc, and methyl-
amine in DMF solvent afforded purple crystals of {[Co-
(bmzbc)2]·2DMF}n (JXNU-1) (Table 1). Half a Co(II) ion
and one bmzbc− ligand, together with an additional disordered
DMF molecule are in the asymmetric unit. The Co atom is
located on a 2-fold rotation axis and bonded to four carboxylate
O atoms from two bmzbc− ligands and two benzimidazole N
atoms from another two bmzbc− ligands in an octahedral
coordination geometry (Figure 1a). Two N atoms and two O
atoms (O1 and O1A) form the equatorial plane and the
2
− |Fc∥/Σ|F0| and wR2 = {Σ[w(F02 − Fc2)2]/Σ[w(F0 )2]}1/2. Details of
B
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Figure 1. (a) Coordination environment of Co(II) center and (b) 2D sheet of JXNU-1.
remaining two O atoms take up the axial positions. The axial
CoO bond distances [2.109(2) Å] are slightly shorter than
the equatorial CoO/N bond distances [2.185(3) and
2.115(3) Å] (Table S1), indicating a slightly compressed
[CoN2O4] octahedron. These bond distances are normal and
comparable to those in other Co(II) compounds.17 Addition-
ally, the equatorial O/NCoO/N bond angles varying from
87.09(15) to 95.07(10)o and the axial OCoO bond angle
window size of about 7.1 × 7.1 Å2, wherein the guest DMF
molecules reside in.
The direct current (dc) magnetic susceptibility of JXNU-1
was collected in an applied magnetic field of 1 kOe. The plot of
χMT (χM is the molar magnetic susceptibility per Co(II)) versus
T is depicted in Figure 3. Above 110 K, the χMT value (2.88
o
of 159.97(14) (Table S1), deviate from the corresponding
angles of 90 and 180°, respectively, of an ideal octahedron. All
these structural geometric parameters indicate an overall
rhombic (C2v) distortion of the octahedral metal environment
for Co atom.
Each Co(II) ion is surrounded by four bmzbc− ligands, which
link the Co(II) ions to give a 2D sheet running along ab plane
(Figure 1b). The 2D sheet features the metal−organic squares
with the Co···Co separation of 11.86 Å. The 2D sheets are
stacked along c axis in a repeated[A−B]staggered
configuration forming a 3D framework (Figure 2). Careful
examination of the crystal packing indicates the shortest
interlayered Co···Co separation of 8.32 Å (Figure S1),
indicative of an isolated magnetic environment for the Co(II)
ions. The 3D packing possesses square 1D open channels with
Figure 3. χMT versus T plot for JXNU-1. Solid line is the best fit
obtained with the PHI program. Inset: Field dependence of the
magnetization measured at different temperatures and calculated
energy splitting of d orbits for the Co(II) ion.
cm3 K mol−1) is a constant and much higher than the
theoretical spin-only value of 1.87 cm3 K mol−1 for a Co(II) ion
(S = 3/2 and g0 = 2.0). However, comparable large room
temperature χMT values have been reported for other Co(II)
compounds.18 An effective geff tensor of value of 2.47 can be
deduced from the room temperature χMT value, which is larger
than the spin-only value of g0, indicating the considerable
orbital contributions to the spin-only value. χMT value sharply
decreases upon cooling below 100 K to a value of 2.17 cm3 K
mol−1 at 2 K, indicative of a significant zero-field splitting
(ZFS).
2
2
2
̂
̂
̂
̂
̂
H = D[Sz − S(S + 1)/3] + E(Sx − Sy ) + μBgSB
(1)
In order to describe the magnetic anisotropy qualitatively,
the spin Hamiltonian of eq 1 (where D is the axial ZFS
parameter, E is the rhombic or transverse ZFS parameter, S is
the spin operator, μB is the Bohr magneton, and B is the
Figure 2. View of the 3D framework of JXNU-1 along the c axis
showing the 1D open channels.
C
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magnetic field vector, respectively), which takes into account
the axial (D) and rhombic (E) magnetic anisotropies, was used.
Some simplification of the structure (Figure S2) was performed
and calculations were performed with the MOLCAS 7.8
program.16 The calculation results gave D = 62.6 cm−1, E =
13.4 cm−1, gx = 2.055, gy = 2.362, and gz = 2.849 (Figure S3).
According to the calculation results, the energy splitting of d
orbits (Figure 3 inset) and easy axis for the Co(II) ion were
successfully obtained. The direction of magnetic axis is
approximately perpendicular to the C2-axis that passed through
the Co atom (Figure 4).
in a metallamacrocycle compound wherein the Co(II) ions are
alternately separated by two Li+ cations that bridged by the γ-
cyclodextrins,21 but smaller than that of 91.0 cm−1 in the 1D
cobalt(II) compound.22
The alternating-current (ac) magnetic susceptibility at
various frequencies and temperatures were collected at a zero
dc field with an ac field of 2.5 Oe. No out-of-phase (χ″) signal
can be observed for JXNU-1 (Figure S5), suggesting a fast
quantum tunneling of the magnetization (QTM). Both in-
phase (χ′) and out-of-phase (χ″) susceptibilities show
significant frequency dependence below 9 K, when a 2 kOe
dc field was applied (Figure 5). The χ″ peaks can be observed
in the frequency region of 50−1399 Hz. Arrhenius analysis of
the χ″ peaks from the high temperature regime of the relaxation
gave the anisotropy energy barrier Ueff of 11.8 K (8.7 cm−1)
with a pre-exponential factor value τ0 of 1.3 × 10−5 s (Figure
S6). The Ueff value is higher than that of (5.1 cm−1)
dicyanamide-bridged 2D coblat(II) compound under 1 kOe
dc field.23
Interestingly, the Cole−Cole plots measured at 2 and 3 K in
the 2 kOe dc field exhibit obvious two-step magnetic relaxation
properties (Figure 6): the left and right parts of the Cole−Cole
Figure 4. Scheme of the local hard axis and the easy plane of the
ground state on Co(II) in JXNU-1.
Moreover, the experimental χMT versus T plot was fit using
the PHI program,19 giving D = 53.2 cm−1, E = 7.5 cm−1, g =
2.46 (Figure 3). The field dependence of the magnetization
reveals a relatively rapid increase of the magnetization at low
field followed by a slow linear increase at high field in the low
temperatures (Figure 3 inset). The experimental field depend-
ence of magnetization data were analyzed using PHI program
adopting the spin Hamiltonian of eq 1, affording D = 60.4
cm−1, E = 8.0 cm−1, and g = 2.42 (Figure S4). The fitting D and
E values are comparable and slightly smaller than the above
calculated values. The cobalt(II)-based SMMs with a positive D
value, which are different the traditional SMMs with a negative
D valule, are rarely documented.20 The D value fitted from the
experimental χMT versus T plot is larger than that of 27.9 cm−1
Figure 6. Cole−Cole plots at 2 and 3 K under 2 kOe for JXNU-1
(The solid lines represent the best fitting with the sum of two modified
Debye functions).
curve correspond to the fast relaxation phase (FR) and the slow
relaxation phase (SR), respectively. The two steps of magnetic
relaxation processes can be fitted with the sum of two modified
Debye functions (Equation S1).24 The fitting results are
depicted as Figures 6 and S7, and summarized in Table S2. The
α1 values (0.375 and 0.210 for 2 and 3 K, respectively) are
Figure 5. Temperature dependence of the in-phase (a) and out-of phase (b) ac susceptibility signals under 2 kOe dc field with an ac field of 2.5 Oe.
D
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Figure 7. (a) CO2 sorption isotherms at 195, 273, and 298 K (solid symbols: adsorption, open symbols: desorption) and (b) the isosteric heats of
adsorption of CO2.
Figure 8. CH4, C2H6, and C3H8 sorption isotherms at 273 K (a) and 298 K (b) (solid symbols: adsorption, open symbols: desorption).
obviously larger than the coresponding α2 values (0.149 and
0.155 for 2 and 3 K, respectively), suggesting that the SR phase
has a relatively narrower distribution of the relaxation time with
respect to the FR phase. As mentioned above, there is only one
kind of Co(II) ion in JXNU-1, so the field-inducing role is
ascribed to the two-step magnetic relaxation.25
The phase purity of JXNU-1 was confirmed by elemental
analysis and powder X-ray diffraction (PXRD) (Figure S8).
Solvent accessible volume, calculated using the PLATON
routine,26 is 37.1% of the total crystal volume. The
thermogravimetric profile of JXNU-1 showed that the loss of
DMF solvents occurs in two steps (Figure S9). The release of
DMF molecules commences at 40 °C and is completed by 205
°C (observed 21.11%, calculated 21.14%). After removal of the
guest DMF molecules, no obvious weight loss occurred below
330 °C.
As verified by PXRD (Figure S8), the activated sample
retained the crystallinity after being evacuated under high
vacuum for 10 h at 150 °C. The permanent porosity of JXNU-1
was established by measuring CO2 sorption isotherms (Figure
7a). The desolvated JXNU-1a shows a significant CO2
adsorption at 195 K, and the isotherm shows a very sharp
uptake at P/P0 < 0.15, confirming the presence of permanent
microporosity. The CO2 absorption exhibits a type-I iso-
therm,27 with a measured saturated adsorption amount of 124
cm3(STP) g−1 (5.5 mmol g−1) at 195 K. The pore volume is
0.31 cm3 g−1 calculated by assuming liquid filling of CO2 at
saturated state, which matches well with the value of 0.34 cm3
g−1 estimated from the crystal data. Meanwhile, CO2 uptakes at
273 and 298 K are 91 and 42 cm3 g−1, respectively,
demonstrating JXNU-1a as a solid material for potential CO2
capture. The existence of extensive aromatic rings on the pore
surfaces would largely account for the high CO2 uptake. The
coverage-dependent adsorption enthalpy of JXNU-1a for CO2
was calculated based on the viral method from fits of their
adsorption isotherms at 273 and 298 K (Figure S10).28 The
isosteric heat of adsorption Qst at zero loading was calculated to
be 35.4 kJ mol−1 (Figure 7b), which is comparable to those
observed in other compounds, such as HKUST-1 (35 kJ
mol−1)29 and MOF-5 (34 kJ mol−1),30 but is larger than those
of NOTT-140 (25 kJ mol−1)31 and Tripp-1-Co (25.6 kJ
mol−1).32 In contrast, the N2 uptake of JXNU-1a at 77 K is very
low with only 15.5 cm3 g−1 at P/P0 = 0.90 (Figure S11),
indicating that N2 molecules are unable to interact with the
pores.
Adsorption isotherms for C1 to C3 paraffins were measured
at 273 and 298 K (Figure 8). As shown in Figure 8a, JXNU-1a
takes up less CH4 (23 cm3 g−1, 273 K) at 1 atm, but adsorbs
significant amounts of C3H8 and C2H6, with the uptakes of 118
and 73 cm3 g−1 (273 K) at 1 atm. The C3H8 uptake is higher
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than that of the recently reported Zn-pyrazole-adenine
framework (88 cm3 g−1 at 273 K).33 It is interesting that the
C3H8 and C2H6 isotherms at 273 K exhibit an abrupt increase
at P/P0 = 0.51 and 0.47, respectively. Additionally, large
hysteretic desorption behavior is observed in the isotherms of
C3H8 and C2H6 (Figure 8a), suggesting the framework
flexibility and the existence of molecular gates.34 Such behavior
can be attributed to the association with a structural
transformation from the guest-free JXNU-1a to gas-loaded
JXNU-1g. The less porous JXNU-1a can be expanded to more
porous JXNU-1g once gas molecules have been gradually
loaded into the pores. However, this adsorption behavior is
different from that of the reported interdigitated 2D framework
wherein a structural transformation from a nonporous to a
microporous phase is observed.35 With increasing temperature,
the adsorptions of C3H8 and C2H6 gradually decrease but
become more reversible (Figure 8b). It should be noted that
the isotherms for C3H8 and C2H6 at 298 K and for CO2 and
CH4 at all testing temperatures exhibit no hysteretic profile
(Figure 7a and Figure 8b). As is well-known, the adsorption
properties are related to the interactions between gas molecules
with the frameworks, and the gate-opening pressure is sensitive
to the property of the gas adsorbed. The larger gas molecules
(such as C3H8 and C2H6) commonly have strong interactions
with the host frameworks, thus the hysteresis will be obvious,
whereas at higher temperatures such phenomena will not be
very obvious or not be observed at all due to the lower host−
guest interactions and much higher gate-opening pressure.
ACKNOWLEDGMENTS
■
This work was supported by the NNSF of China (Grants
21361011, 21101081, and 21561015), the NSF of Jiangxi
(Grant 20151BAB203002), the Young Scientist Training
Project of Jiangxi Province (Grant 20153BCB23017), and the
project of Education Department of Jiangxi (Grant GJJ14235).
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CONCLUSIONS
■
A novel cobalt(II) compound, {[Co(bmzbc)2]·2DMF}n
(JXNU-1), displays field-induced slow magnetic relaxation
resulting from the anisotropic Co(II) ions which behave as SIM
units. The rod-like bmzbc− ligand spatially separates the
magnetic ions in a well-defined network, which provides an
approach to obtaining independent single-ion magnets. More-
over, the 3D porous packing framework shows abrupt changes
in its adsorption isotherms with guest-dependent gate-opening
pressure. This work demonstrates a rare example of bifunc-
tional materials, which opens up new opportunities in the
preparation of the multifunctional molecular materials with
SIM and porosity.
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ASSOCIATED CONTENT
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ylwangchem@gmail.com (Y.-L.W.).
*Fax: +86-791-88336372. E-mail: qyliuchem@hotmail.com
(Q.-Y.L.).
Notes
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.inorgchem.5b02324
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