Synthesis, crystal structure, and magnetic properties of a nitronyl nitroxide biradical-coordinated copper(II) complex

Article abstract of DOI:10.1177/1747519820982539

A coordination compound constructed from a nitronyl nitroxide biradical NITPh(3-NIT) and Cu^II(hfac)₂(H₂O)₂ building blocks [NITPh(3-NIT) = 1,3-bis(1′-oxyl-3′-oxido-4′,4′, 5′, 5′-tetramethyl-4,5-dihydro-1H-imidazol-2-y1)-benzene, hfac = hexafluoroacetylacetonato] is successfully synthesized. The crystal structure is determined by X-ray single-crystal diffraction. The asymmetric complex {[(NITPh(3-NIT)]Cu(hfac)₂} consists of one Cu(II) ion and two >N–O^? groups and adopts a distorted triangular bipyramid with a penta-coordinated central copper(II) atom and three hfac oxygen atoms at the base and a >N–O^? oxygen atom and one hfac oxygen atom at the apices. Intramolecular O..O bonding and π–π stacking interactions are observed between molecules. A magnetic susceptibility study of the coordination compound shows antiferromagnetic interactions between Cu(II) ions and >N–O^? groups and very weak ferromagnetic interactions between Cu(II) ions and the free >N–O^? group through O..O bonding between the nitroxide group oxygen atom and the oxygen atom of hfac.

Full text of DOI:10.1177/1747519820982539

982539CHL0010.1177/1747519820982539Journal of Chemical ResearchGao et al.
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Synthesis, crystal structure, and magnetic
properties of a nitronyl nitroxide biradical-
coordinated copper(II) complex
https://doi.org/10.1177/1747519820982539
DOI: 10.1177/1747519820982539
journals.sagepub.com/home/chl
Yan-Li Gao , Pingqiang Gao, Ying Gong and Guoyu Ren
Abstract
A coordination compound constructed from a nitronyl nitroxide biradical NITPh(3-NIT) and CuII(hfac)2(H2O)2 building
blocks [NITPh(3-NIT) = 1,3-bis(1′-oxyl-3′-oxido-4′,4′, 5′, 5′-tetramethyl-4,5-dihydro-1H-imidazol-2-y1)-benzene, hfac
= hexafluoroacetylacetonato] is successfully synthesized. The crystal structure is determined by X-ray single-crystal
diffraction. The asymmetric complex {[(NITPh(3-NIT)]Cu(hfac)2} consists of one Cu(II) ion and two >N–O• groups and
adopts a distorted triangular bipyramid with a penta-coordinated central copper(II) atom and three hfac oxygen atoms
at the base and a >N–O• oxygen atom and one hfac oxygen atom at the apices. Intramolecular O. . .O bonding and π–π
stacking interactions are observed between molecules. A magnetic susceptibility study of the coordination compound
shows antiferromagnetic interactions between Cu(II) ions and >N–O• groups and very weak ferromagnetic interactions
between Cu(II) ions and the free >N–O• group through O. . .O bonding between the nitroxide group oxygen atom and
the oxygen atom of hfac.
Keywords
CuII(hfac)2 complexes, magnetic property, nitronyl nitroxide biradical, X-ray crystal structure
Date received: 11 September 2020; accepted: 2 December 2020
three-dimensional structures and heterospin exchange
Introduction
clusters with different structures.1–3 Hence, favorable
Cu(hfac)2 is widely used to prepare molecular magnets
(hfac = hexafluoroacetylacetonate). On one hand, it is a
strong acceptor readily forming complexes with a coordi-
nated nitroxyl group because of the presence of the strong
opportunities are available for sequential definition of
magnetostructural correlations through modification of the
electron-withdrawing hfac co-ligands. On the other hand,
the interactions of heterospin systems with nitroxides yield
magnetic properties that are convenient for subsequent
theoretical analysis. In recent years, multifunctional deriv-
atives of stable nitroxides have been used for syntheses of
School of Chemistry and Chemical Engineering, Yulin University, Yulin,
P.R. China
Corresponding author:
Yan-Li Gao, School of Chemistry and Chemical Engineering, Yulin
University, Yulin 719000, P.R. China.
heterospin systems that are increasingly complicated with Email: gaoyanli8503@126.com
Creative Commons Non Commercial CC BY-NC: This article is distributed under the terms of the Creative Commons
Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use,
reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and
Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).

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Journal of Chemical Research 00(0)
Scheme 1. Organic biradicals NITPh(3-NIT) and inorganic complexes: CuII(hfac)2.
Scheme 2. Synthesis of the NITPh(3-NIT) biradical.
paramagnetic ligand molecule, which can change the magnetic spin exchange. In this contribution, we report the
structure and the magnetic behavior of the exchange crystal structure and magnetic properties of {[(NITPh(3-
cluster.4–6
NIT)]Cu(hfac)2} obtained from NIT biradicals and
The use of organic monoradicals is well documented, CuII(hfac)2 building blocks.
with many interesting examples of the creation of new
molecular magnetic compounds in the literature. Such
complexes display several architectures and different
behaviors.7–14 Organic biradicals have also attracted con-
siderable interest as building blocks for molecular magnetic
materials.15 This is because it is possible to vary the ground
spin state by choosing an appropriate conjugated spacer
between groups with unpaired spin. The m-phenylene ring
is one of the most frequently used spacers due to its non-
disjointed connectivity which induces an S = 1 spin ground
state for many biradicals.16–22 Also, heteroatom substituents
or the inclusion of functional group substituents on this ring
allows modification of the overall magnetic properties.23,24
In addition, much attention has been devoted to nitronyl
Results and discussion
Preparation of the biradical NITPh(3-NIT)
The biradical NITPh(3-NIT) was prepared as reported pre-
viously with only minor modifications;26,27 the synthetic
route is shown in Scheme 2.
The preparation of the {[NITPh(3-NIT)]Cu(hfac)2}
complex is described in the experimental.
Crystal structure of the {[NITPh(3-NIT)]
Cu(hfac)2} complex
nitroxide (NIT) biradicals in the field of molecular magnet- The molecular structure of {[NITPh(3-NIT)]Cu(hfac)2}
ism. They represent useful building blocks for heteromo- was successfully determined by means of X-ray crystallo-
lecular complexes in the so-called metal-radical approach. graphic analysis. The crystal and refinement data are sum-
Hence, many possibilities remain unexplored for coordina- marized in Table 1. Table 2 shows selected crystallographic
tion using these spin carriers to tune structural and mag- data. {[NITPh(3-NIT)]Cu(hfac)2} crystallizes in the tri-
netic behavior by using different spacer groups to connect clinic space group P-1 with Z = 2. The molecular structure
the radical units.25
is shown in Figure 1(a), revealing that the complex forms a
Inspired by these ideas, we synthesized an NIT biradical centrosymmetric dimer. The asymmetric unit of the com-
(Scheme 1), which can provide an efficient route to plex consists of one crystallographic-dependent copper

Gao et al.
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Table 1. Crystallographic data and refinement parameters of the complex.
Crystallographic data and refinement parameters of the {[NITPh(3-NIT)]Cu(hfac)2} complex
Formula
Formula wight
Temperature (K)
Wavelength (Å)
crystal system
Space group
Size (mm)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C30H30CuF12N4O8
866.12
296(2)
0.71073
Triclinic
Total reflections
Unique
Parameters
Index ranges
45257
9158
504
−13 ⩽ h ⩽ 13, −16 ⩽
k ⩽ 16, −20 ⩽ l ⩽ 20
P-1
Refinement method
Full-matrix least-
squares on F2
0.20 × 0.22 × 0.36
10.1239(19)
12.521(2)
15.611(3)
99.284(6)
108.370(5)
96.398(5)
1825.3(6)
1.576
θ range (°3)
F(000)
Rint
R1[I > 2σ(I)]
wR2[I > 2σ(I)]
R1 (all data)
wR2 (all data)
GoF
2.811–28.458
878
0.0443
0.0726
0.1948
0.1250
0.2300
1.024
γ (°)
V (Å3)
Dc / (g cm–3)
Z
2
Completeness
0.998
Table 2. Selected bond lengths (Å) and angles (°) for
{[NITPh(3-NIT)]Cu(hfac)2} at 296 K.
the equatorial Cu–O(hfac) bonds, Cu–O5, Cu–O7 and Cu–O8,
are 1.920(3) Å, 2.001(3) Å, and 2.157(3) Å, respectively. The
bond angles of N1–O1–Cu and O1–Cu–O6 are 125.2(2)° and
175.4(1)°, respectively. In the biradical, the O1–N1–C8–
N2–O2 and O3–N3–C7–N4–O4 angles are almost planar, and
the conjugation plane O1–N1–C8–N2–O2 and the benzene
plane have a dihedral angle of 41.44°. The plane of the ben-
zene ring makes an angle of about 20.71° with the O3–N3–
C7–N4–O4 plane. In addition, there is one slightly longer
intramolecular O. . .O interaction (O3⋯O5 = 3.157(6) Å)
(Figure 1(c)) and two intermolecular F. . .F interactions
(F9. . .F11 = 2.972(6) Å and F3. . .F12 = 3.08(1) Å), which
contribute to the layered structure as shown in Figure 2.
Moreover, interlayer π···π stacking interactions with a distance
of 3.539 Å make the adjacent coordination layers more stable
(Figure 1(d)).
Bond lengths (Å)
N1–O1
N2–O2
N3–O3
N4–O4
Cu1–O1
1.309(2)
1.265(2)
1.272(5)
1.280(5)
1.937(2)
Cu1–O5
Cu1–O6
Cu1–O7
Cu1–O8
1.920(3)
1.960(3)
2.001(3)
2.157(3)
Angles (°)
N1–O1–Cu1
O1–Cu1–O7
O1–Cu1–O8
O5–Cu1–O1
O5–Cu1–O7
O5–Cu1–O6
O5–Cu1–O8
O7–Cu1–O8
O6–Cu1–O7
O6–Cu1–O8
C27–O7–Cu1
C22–O5–Cu1
C29–O8–Cu1
C24–O6–Cu1
125.2(2)
88.08(11)
90.53(12)
91.79(11)
147.36(12)
92.08(12)
123.03(11)
89.61(11)
87.27(14)
89.52(14)
125.7(3)
O1–N1–C8
O1–N1–C16
C8–N1–C16
C8–N2–C15
O4–N4–C7
O4–N4–C9
C7–N4–C9
N1–C8–N2
O3–N3–C7
O3–N3–C10
C7–N3–C10
N3–C7–N4
O2–N2–C8
O1–Cu1–O6
124.2(3)
120.8(3)
113.6(3)
113.2(3)
126.1(4)
120.6(3)
113.0(3)
108.4(3)
126.7(3)
120.2(4)
112.9(3)
108.0(4)
125.1(3)
175.4(1)
Magnetic properties
The temperature-dependent magnetic susceptibilities of the
complex were measured in the temperature range of 2–300
K in an external magnetic field of 20 kOe, and the data
were corrected for the diamagnetism of the components.
The magnetic data for the {[(NITPh(3-NIT)]Cu(hfac)2}
complex are shown in the plots in Figure 3 as χm versus T
and χmT versus T. For χmT (square), at room temperature,
the value is 0.378 cm3 K mol−1, which is equal to the theo-
retical value for an uncoupled system of one radical (S =
1/2, χmT = 0.375 cm3 K mol−1), assuming g = 2, indicating
the existence of strong antiferromagnetic interactions
between copper and the coordinating N–O• group. At a
lower temperature, χmT increased to reach a value of 0.419
cm3 K mol−1, which is greater than the spin value for S = 1/2
with g = 2. The Curie and Weiss constants were 0.378 cm3
K mol−1 and 3.01 K, respectively, from the Curie–Weiss fit
of the reciprocal susceptibility versus the temperature curve
for the complex (Figure 3, red solid line). The ferromagnetic
interactions among the spin carriers are weak. In fact, the
intramolecular distances between the nitroxide group
124.3(3)
122.0(3)
124.9(3)
center coordinated to one NITPh(3-NIT) biradical ligand.
The CuII atom is penta-coordinated by one radical and two
hfac ligands, leading to a compressed triangular bipyramid
with three hfac oxygen atoms (O5, O7, O8) at the base and
with N–O (O1) and an hfac oxygen atom (O6) at the apex
as shown in Figure 1(b). The packing arrangement of the
{[NITPh(3-NIT)]Cu(hfac)2} complex shows that the near-
est intermolecular metal–metal contacts give a Cu. . .Cu
distance of 7.610(1) Å, while the intercell Cu. . .Cu dis-
tance is 10.272(2) Å.
The apical Cu–O1 and Cu–O6 bond lengths are 1.937(2)
and 1.960(3) Å, respectively, which are typical for copper(II)–
nitroxide equatorial coordinated bonds (1.93–2.03) Å, while

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Journal of Chemical Research 00(0)
Figure 1. (a) Molecular structure of the asymmetric unit of {[(NITPh(3-NIT)]Cu(hfac)2} with ellipsoids at 50% probability level,
(b) the bipyramid structure with a penta-coordinated central copper(II) atom, (c) intramolecular O. . .O contacts, and (d) π. . .π
stacking interactions. Fluorine and hydrogen atoms are omitted for clarity.
Figure 2. Crystal packing of the {[(NITPh(3-NIT)]Cu(hfac)2} complex showing the F. . .F (blue) intramolecular short contacts.
Hydrogen atoms are omitted for clarity.

Gao et al.
5
The field dependence of the magnetization has been
determined at 2 K in the 0–50 kOe field range (Figure 4).
Upon increasing the applied field, M increases up to 0.96
NµB at 50 kOe, which is near saturation.
Conclusion
A centrosymmetric {[(NITPh(3-NIT)]Cu(hfac)2} complex
based on copper(II) and NIT biradicals has successfully
been prepared. The single-crystal X-ray structure shows
that one of the >N–O• groups is linked to the Cu(II) ion to
form a distorted triangular bipyramid with a penta-coordi-
nated central copper(II) atom. The magnetic measurements
indicate that the complex exhibits antiferromagnetic inter-
actions between Cu(II) ions and the linking >N–O• group,
but weak ferromagnetic interactions with the free >N–O•
group through the O. . .O contacts. The magnetostructural
correlations are explained based on the crystal structure.
Figure 3. Temperature dependence of χm and χmT versus
T for the {[NITPh(3-NIT)]Cu(hfac)2} complex. The solid lines
show the best fit.
1.0
0.8
Experimental
All reagents were obtained from commercial sources and
used without further purification.
0.6
µ
Preparation of Cu(hfac)2·2H2O
0.4
0.2
0.0
Hfac (1 g, 7.1 mmol) was added to a suspension of
Cu(CH3COO)2·H2O (0.71 g, 3.55 mmol) in H2O (17 mL) at
room temperature and then NaHCO3 (0.071 g, 0.8 mmol)
was added with stirring. After 10 min, a light blue precipi-
tate was obtained, which was washed three times with
n-hexane. The light blue solid was dried at 50 °C for 3 days
to afford Cu(hfac)2·2H2O (0.74 g). FTIR (KBr, cm−1):
3396(s), 1647(s), 1551(s), 1493(s), 1216(s), 1157(s),
808(s), 769(w), 745(w), 669(s), 591(s).
0
10
20
30
40
50
H / kOe
Figure 4. Isothermal magnetization at 2 K of {[NITPh(3-NIT)]
Cu(hfac)2}.
Preparation of {[NITPh(3-NIT)]Cu(hfac)2}
oxygen atom and the CuII center are large, being 7.610 Å. A
possible path way for the magnetic interactions would be
through intramolecular O. . .O bonding between the nitrox-
ide (O3) and the hfac oxygen atom (O5). A careful analysis
of this intramolecular contact showed that the distance
between the magnetic centers was shorter than the intramo-
lecular distance (O3. . .Cu = 3.477 Å).
In order to account for the magnetic behavior of the
complex, a model with three exchange coupling con-
stants was used (Figure 3), where J1, J2, and J3 represent
the intermolecular magnetic interactions between radi-
cal–radical, metal–radical and metal–uncoordinated radi-
cal, respectively. The magnetic data are the result of the
three interactions, but the magnetic interaction between
the metal and uncoordinated radical, J3, made the most
important contribution. It is important to stress that a
simplified model was used to describe the magnetic
behavior. The blue solid line in Figure 3 shows the best fit
achieved with grad = 2.00 (fixed) and J = 3.47 cm−1.
These values indicate that the magnetic coupling between
the Cu(II) atom and the uncoordinated nitroxide radical is
weakly ferromagnetic.
Cu(hfac)2(H2O)2 (1 mmol, 51.5 mg) was dissolved in 10
mL of hot n-heptane and then the solution was cooled to
about 50 °C. NITPh(3-NIT) (1 mmol, 39.0 mg), dissolved
in 2 mL of CHCl3, was added with constant stirring. The
mixed solution was stirred for ca. 3 min and then cooled
to room temperature. The filtrate was kept under an N2
stream at room temperature until a solid appeared. A few
drops of CHCl3 were added until the solid disappeared
and then the flask was sealed and placed in a dark room.
After 5–6 days, block-shaped, dark blue crystals were
obtained. Yield: 72%. Anal. calcd for C30H30F12CuN4O8:
C, 41.57; H, 3.46; N, 6.47; found: C, 41.74; H, 3.69; N,
6.32. FTIR (KBr, cm−1): 1662(s), 1554(w), 1531(s),
1477(s), 1358(w), 1223(s), 1147(s), 1087(s), 866(s),
800(w), 661(s), 604(s).
General characterization
Infrared spectra were recorded on a JASCO FT/IR-660
PLUS spectrometer by transmission through KBr pellets in
the range of 400–4000 cm−1. Elemental analyses for C, H,
and N were recorded using a PerkinElmer series II CHNS/O

6
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Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article.
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Funding
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The author(s) disclosed receipt of the following financial support for
the research, authorship, and/or publication of this article: This work
was supported by the Doctoral Scientific Research Foundation of
Yulin University (No. 18GK24), the Science and Technology
Program of Shaanxi Province (No. 2017GY-131), a Project of
Production and Study of Yulin City (No. 2016CXY-01), and the
National Nature Science Foundation of China (No. 21968036).
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ORCID iD
Yan-Li Gao
https://orcid.org/0000-0001-7830-6016
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Supplemental material
CCDC 1970512 contains the supplementary crystallographic data
for this paper. The data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/structures.
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