Synthesis, crystal structures and magnetic properties of nitronyl nitroxide radical-coordinated copper(II) complexes

Article abstract of DOI:10.1007/s11243-019-00370-y

The coordination compound constructed for nitronyl nitroxide radical NIT-Ph-4-Br and Cu^II(hfac)₂(H₂O)₂ building blocks (NIT-Ph-4-Br = 2-(4-bromo-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hfac = hexafluoroacetylacetonato) was successfully synthesized. The single-crystal X-ray diffraction analyses indicated that the complexes {(NIT-Ph-4-Br)₂[Cu(hfac)₂]₃} have centrosymmetric five-spin structures consisting of three Cu(II) ions bridged by two nitroxide ligands and that they consist of two types of copper atoms, one with a heavily Jahn–Teller distorted (4 + 2) octahedral coordination (Cu^oct) and hfac in trans-positions and the other with square pyramidal five-coordinated (Cu^pyr) with three hfac oxygen atoms and N–O oxygen atom at the base and the one hfac oxygen atom at the apex. Different geometries of the copper ions are quite important for magnetochemistry. The magnetic susceptibility study of the coordination compound shows strong antiferromagnetic interactions between the metal center and the organic radical.

Full text of DOI:10.1007/s11243-019-00370-y

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00370-y
Synthesis, crystal structures and magnetic properties of nitronyl
nitroxide radical‑coordinated copper(II) complexes
Yan‑Li Gao1 · Katsuya Inoue2,3
Received: 8 October 2019 / Accepted: 23 November 2019
© Springer Nature Switzerland AG 2019
Abstract
The coordination compound constructed for nitronyl nitroxide radical NIT-Ph-4-Br and CuII(hfac)2(H2O)2 building blocks
(NIT-Ph-4-Br = 2-(4-bromo-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hfac= hexafuoroacetylacetonato)
was successfully synthesized. The single-crystal X-ray difraction analyses indicated that the complexes {(NIT-Ph-4-
Br)2[Cu(hfac)2]3} have centrosymmetric fve-spin structures consisting of three Cu(II) ions bridged by two nitroxide ligands
and that they consist of two types of copper atoms, one with a heavily Jahn–Teller distorted (4+2) octahedral coordination
(Cuoct) and hfac in trans-positions and the other with square pyramidal fve-coordinated (Cupyr) with three hfac oxygen
atoms and N–O oxygen atom at the base and the one hfac oxygen atom at the apex. Diferent geometries of the copper ions
are quite important for magnetochemistry. The magnetic susceptibility study of the coordination compound shows strong
antiferromagnetic interactions between the metal center and the organic radical.
Introduction
magnetism [7–15]. Among them, because of their rela-
tively stable and easy-to-obtain derivatives with substitu-
ents containing donor atoms, the nitronyl nitroxide (2-(R)-
4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide,
NITR) radicals have received more attention of researchers
[16–22]. Besides, nitronyl nitroxide family of radicals can
act as bidentate ligands with identical N–O coordination
groups. Compared with regular diamagnetic ligands, the
N–O groups of this kind of radical have weak basicity, so
strong acidic partners were needed for ligation to the radi-
cal. Thus, it is necessary to utilize the strong electron-with-
drawing coligands such as hexafuoroacetylacetonate (hfac)
and trifuoroacetylacetonate (tfac) in the metal to improve
the weak coordination ability. However, the steric demand of
hfac or tfac ancillary ligand can restrict the dimensionality
of the resulting metal–radical complex.
In the past two decades, molecular magnetic materials have
attracted scientists’ attention due to their potential appli-
cations in high-density magnetic memories, quantum com-
puting devices and molecular spintronics [1–6]. Among
the variety of molecular magnetic materials with diferent
structures and dimensionalities, the strategy of metal–radi-
cal, which is formed by matching paramagnetic organic mol-
ecules and transition metal complexes, is still the key way
to synthesize and design magnets. Up to now, a variety of
organic paramagnetic molecules such as aminoxyl, verda-
zyl, thiazyl, triazinyl, nindigo, semiquinone and nitroxide
radicals have been widely studied in the feld of molecular
* Yan-Li Gao
Furthermore, copper(II)–nitronyl nitroxide coordination
complexes are the best documented and characterized on the
basis of their coordination geometry toward development of
gaoyanli8503@126.com
* Katsuya Inoue
kxi@hiroshima-u.ac.jp
magnetic materials [23, 24]. The copper(II) ion has a well-
22
1
defned magnetic orbital of 3d ; in parallel, the N–O group
School of Chemistry and Chemical Engineering, Yulin
x-y
University, Yulin 719000, China
has an unpaired electron in the molecular π* orbital. The
magnetic coupling parameter J is typically determined as a
singlet–triplet energy gap of 2 J at a>NO–Cu unit. Gener-
ally, an equatorial coordination results in a strong antiferro-
magnetic metal–radical exchange coupling (J<0), while an
2
Department of Chemistry, Hiroshima University, 1-3-1,
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526,
Japan
3
Center for Chiral Science, Hiroshima University, 1-3-1,
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526,
Japan
Vol.:(0123456789)
1 3

Transition Metal Chemistry
axial bonding results in a moderate ferromagnetic coupling
(J>0) [25–36].
cm−1): 3399(s), 1644(s), 1561(s), 1497(s), 1 210(s), 1151(s),
800(s), 766(w), 743(w), 668(s), 590(s).
Inspired by these ideas, we synthesized the nitronyl
nitroxide ligands (Scheme 1) which can provide an efcient
way for magnetic spin exchange, following a detailed study
of the structures and magnetism of the metal–radical com-
pounds. In this contribution, we report the crystal structures
and magnetic properties of {(NIT-Ph-4-Br)2[Cu(hfac)2]3}
obtained from the nitronyl nitroxide radicals and CuII(hfac)2
building blocks.
Preparation of {[(NIT‑Ph‑4‑Br)2[Cu(hfac)2]3}
CuII(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. NIT-Ph-4-Br (0.05 mmol, 15 mg), dissolved
in 2 mL of CHCl3, was added to it with constant stirring. The
mixed solution was stirred for ca. 3 min and then cooled to
room temperature. The fltrate was kept under N2 stream
at room temperature until solid appeared. A few drops of
CHCl3 were added until the solid disappears, and then,
the fask was sealed and placed in the refrigerator. After
2–3 days, block-shaped dark blue crystals were obtained,
yield 81%. Chemical analyses for C56H38Br2Cu3N4O16
are: calculated (%) C: 32.67, H: 1.85, N: 2.72. Found (%):
C: 32.84, H: 1.69, N:2.57. FTIR (KBr, cm −1): 1658(s),
1559(w), 1534(s), 1479(s), 1356(w), 1 221(s), 1145(s),
1084(s), 862(s), 798(w), 663(s), 614(s).
Experimental
Materials and methods
All reagents were obtained from commercial sources and
used without further purifcation.
2-(4-Bromo-phenyl)-4,4,5,5-tetramethylimidazoline-
1-oxyl-3-oxide radical (NIT-Ph-4-Br) was prepared
as reported previously with only minor modifications
(Scheme 2) [37, 38].
General characterizations
Preparation of Cu(hfac)2·2H2O
Infrared spectra were recorded on a JASCO FT/IR-660 Plus
spectrometer by transmission through KBr pellets in the
range 400–4000 cm−1. Elemental analyses for C, H, and
N were carried out using a PerkinElmer series II CHNS/O
Analyzer 2400. The magnetization measurements on the
crystalline solids were taken using Quantum Design MPMS-
5S and MPMS-2 SQUID magnetometers. The magnetic feld
was varied from − 50 to 50 kOe and the temperature from 2
to 300 K. The data were corrected for the sample diamagnet-
ism using Pascal’s constants [39, 40].
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 under stirring. After 10 min, a light blue precipi-
tate was obtained and washed three times with n-hexane.
Then, the light blue solid was dried at 50 °C for three days,
and Cu(hfac)2·2H2O (0.74 g) was obtained. FTIR (KBr,
O
N
F3C
CF3
Single‑Crystal X‑ray difraction studies
O
O
O
O
Cu
Br
Their X-ray difraction intensities were collected for the
selected single crystals individually mounted on glass
fbers at diferent temperature using a Bruker difractom-
eterSMART APEX II with a CCD and D8-QUEST with a
CMOS area detector. Both employed graphite-monochro-
mated Mo Kα (λ = 0.71073 Å) radiation. Data reduction
N
O
CF3
F3C
Scheme 1 Organic radicals NIT-Ph-4-Br and inorganic complexes:
CuII(hfac)2
Br
OHC
OH
N
Zn/NH4Cl
THF/H2O
N
O
N
NHOH
NHOH
HO
NO2
NO2
N
O
NaIO4
CH2Cl2/H2O
methanol, CHCl3
Br
Br
Scheme 2 Synthesis pathway of NIT-Ph-4-Br radical
1 3

Transition Metal Chemistry
was made using SAINT, and intensities were corrected for
absorption by SADABS [41, 42]. The structures were solved
by direct methods and refned by full-matrix least squares
against F2 using ShelXL [43]. A part of the hydrogen atoms
were located in the diference Fourier maps, and those not
found were added at theoretical positions using the riding
model. The details can be obtained from the cif fles depos-
ited at the Cambridge Crystallographic Data Centre. The
crystals and refnements data are summarized in Table 1.
As shown in Fig. 1a, the asymmetric unit of complex
{(NIT-Ph-4-Br)2[Cu(hfac)2]3} consists of two types of
copper atoms, one with a heavily Jahn–Teller distorted
(4 + 2) octahedral coordination (Cuoct) and hfac in trans-
positions which are occupied by four oxygen atoms (O7,
O8) from two hfac units, two oxygen atoms (O1) from
two nitroxide ligands, and the other with nearly square
pyramidal fve-coordinated (Cupyr) with three hfac oxygen
atoms (O3, O5, O6) and the N–O (O2) at the base and one
hfac oxygen atom (O4) at the apex. The asymmetric unit
of {(NIT-Ph-4-Br)2[Cu(hfac)2]3} consists of one radical,
half a Cuoct (Cu1) and one Cupyr (Cu2). The whole mol-
ecule has fve localized S = 1/2 spins, which form a zigzag
linear spin system along the copper–radical–copper–radi-
cal–copper array. The molecular structure consists of alter-
nating metal–radical units. The radical plays a key role in
establishing this zero-dimensional structure, acting as a
bidentate ligand to connect the metal centers. In addition,
there are two slightly longer intermolecular interactions
N2–O2···O2–N2 = 3.540(1) Å and Br1···Br1 = 3.333(1) Å,
which bridge the molecular into a layer structure as shown
in Fig. 2.
Results and discussion
Crystal structure of complex {(NIT‑Ph‑4‑Br)2[Cu(h
fac)2]3}
The molecular structures {(NIT-Ph-4-Br)2[Cu(hfac)2]3}
were successfully determined by means of X-ray crys-
tallographic analysis at diferent temperatures. (Table 1
shows selected crystallographic data.) {(NIT-Ph-4-
Br)2[Cu(hfac)2]3} crystallizes in the triclinic space group
P − 1 with Z = 1. The molecular structure is shown in
Fig. 1, and it forms a centrosymmetric dimer (Fig. 1b).
Selected bond lengths and angles are listed in Table 2.
For the six-coordinated octahedral copper(II) ions, the
apical Cu1–O1 bond lengths are 2.572(1)Å, while the
equatorial Cu–O bonds are much shorter than the apical
bonds (the Cu1–O distances are ~ 1.92 Å), as shown in
Figure 3a. Furthermore, the Cu1–O1 distance was longer
compared with typical copper(II)–radical bond lengths
(> 2.3 Å) [44–47]. For the square pyramidal fve-coordi-
nated copper(II) ions, the apical Cu2–O4 bond lengths are
2.202(1) Å, while the equatorial Cu2–O bond distances
range from 1.923 to 1.961 Å, as shown in Figure 3b. Fur-
thermore, Cu2–O2 bond distance is 1.951(1)Å, which is a
typical length for a copper(II)–nitroxide equatorial coor-
dinate bond (1.93–2.03Å) [46–49]. The bond distances
of uncoordinated N–O(1.266(2) Å) are shorter than those
of the coordinated ones (1.306(2) Å). In addition, the
angles of N1–O1–Cu1 and N2–O2–Cu2 are 161.56(1)°
and 118.14(9)°, respectively. The O1–N1–C3–N2–O2 is
almost planar. The dihedral angle between nitronyl nitrox-
ide and the benzene ring is 38.7°. The molecular arrange-
ment of complex {(NIT-Ph-4-Br)2[Cu(hfac)2]3} is shown
in Fig. 2, the nearest intramolecular metal–metal contacts
give a Cu···Cu distance of 7.759(1) Å, while the intermo-
lecular Cu···Cu distance is 6.658(1) Å. The distance of
the two corresponding radicals from adjacent molecules
is 3.54 Å. No meaningful interdimer interaction, such as a
π–π stacking or an interaction through counteranion, was
observed in the crystal structures. Thus, the interdimer
magnetic interaction is negligible for magnetic analysis.
Table 1 Crystallographic data and refnement parameters for com-
plexes at 173 K and 90 K
173(2) K
90(2) K
Formula
fw
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
C56H38Br2Cu3F36N4O16 C56H38Br2Cu3F36N4O16
2057.35
Triclinic
P−1
2057.35
Triclinic
P−1
11.7115(6)
12.5627(6)
13.0878(6)
75.3240(10)
81.6560(10)
89.501(2)
1842.25(15)
1.854
11.6325(6)
12.5658(6)
12.9706(6)
75.2860(10)
82.2800(10)
89.2400(10)
1816.71(15)
1.880
α (˚)
β (˚)
γ (˚)
V (Å3)
Dc/(g cm−3
Z
)
1
1
Refs. total
Unique
Param.
Rint
R1/wR2 [I>2σ(I)] 0.0302
0.0779
58,631
9179
641
0.0225
47,815
9039
616
0.0216
0.0267
0.0685
R1/wR2 (all data) 0.0347
0.0816
0.0289
0.0699
1.033
0.998
GoF
1.031
0.995
28.35
Completeness
Theta (°)
28.31
1 3

Transition Metal Chemistry
Fig. 1 Molecular structure
of a asymmetric unit and b
trinuclear unit of {(NIT-Ph-4-
Br)2[Cu(hfac)2]3} with ellip-
soids at 50% probability level.
Fluorine and hydrogen atoms
are omitted for clarity
Fig. 2 Crystal packing
of complex {(NIT-Ph-4-
Br)2[Cu(hfac)2]3} showing the
O···O (blue) and Br···Br (black)
interchain short contacts. Fluo-
rine and hydrogen atoms are
omitted for clarity
1 3

Transition Metal Chemistry
Table 2 Selected bond lengths [Å] and angles [°] for {(NIT-Ph-4-
Br)2[Cu(hfac)2]3} at 173 K
Bond lengths [Å]
N1–O1 1.266(2)
N2–O2 1.306(2)
Cu1–O1 2.572(1)
Cu2–O2 1.951(1)
Cu2–O3 1.936(2)
Cu2–O4 2.202(1)
Cu2–O5 1.961(1)
Cu2–O6 1.923(1)
Cu2–O7 1.926(1)
Cu2–O8 1.916(1)
Cu2–O6 1.923(1)
C7–Br1 1.930(3)
Angles [°]
N1 O1 Cu1 161.56(1)
N2 O2 Cu2 118.14(9)
O2 Cu2 O5 167.36(6)
O2 Cu2 O4 98.25(5)
O3 Cu2 O2 93.28(5)
O3 Cu2 O5 85.62(5)
O3 Cu2 O4 89.67(5)
O6 Cu2 O2 87.61(5)
O6 Cu2 O3 175.41(5)
O6 Cu2 O5 92.53(5)
O6 Cu2 O4 94.67(5)
O5 Cu2 O4 94.33(6)
O8 Cu1 O7 92.87(5)
O2 N2 C3 125.39(13)
O5 Cu2 O4 94.33(6)
O8 Cu1 O7 92.87(5)
C15 O3 Cu2 121.40(11)
O1–Cu1–O7 89.39(1)
O1–Cu1–O8 99.27(1)
O1 N1 C3 125.49(15)
O1 N1 C1 121.73(14)
O2 N2 C2 120.98(12)
O2 N2 C3 125.39(13)
N2 C3 N1 107.90(14)
C22 O6 Cu2 123.89(10)
C27 O8 Cu1 124.57(11)
C25 O7 Cu1 124.50(11)
C20 O5 Cu2 124.33(11)
C17 O4 Cu2 118.79(12)
C15 O3 Cu2 121.40(11)
C13–O4–Cu1 124.1(2)
C16–O5–Cu1 123.47(19)
C18–O6–Cu1 124.7(2)
Fig. 4 Temperature dependence of χm (circular) and χmT (square) for
complex {(NIT-Ph-4-Br)2[Cu(hfac)2]3}
value of χmT (square), at room temperature the value is
0.71 cm3 K mol−1, which is obviously lower than the theo-
retical value (1.875 cm3 K mol−1) for an uncoupled system
of three copper(II) (S=1/2, χmT=0.375 cm3 K mol−1) plus
two radicals (S=1/2, χmT=0.375 cm3 K mol−1), assuming
g=2, indicating the existence of strong antiferromagnetic
interactions within the compound [48, 49]. With reduction
in temperature, χmT sharply decreases to reach a value of
0.40 cm3 K mol−1, which is close to a spin value for S=1/2
with g=2. The efective expression for calculating the sus-
ceptibility of the fve-spin system has not been found [50,
51]. And, based on the crystal data and the comparison with
magnetic properties of other well-understood copper–radical
complexes [50–55], the χmT − T curves for this complex
can be rationalized qualitatively in term of an approximate
model shown as follows:
Cu2 − R1 − Cu1 − R2 − Cu2
In this model, we presume that the interactions between
Cu1 and R1 and Cu1 and R2 are very small in this complex,
while the interactions of Cu2–R1 and Cu2–R2 are strongly
antiferromagnetic, because it has been found that two spins
will have diferent coupling phenomena in the coordina-
tion environment of copper(II): When the N–O occupies
an equatorial position, the spins are strongly coupled in an
antiferromagnetic way with J > 500 cm−1 (Ĥ = JŜiŜj) [49,
53]; when the nitroxide occupies an axial position, a weak
coupling is found [49]. Therefore, the monotonous decrease
in χmT with the decrease in temperature should be ascribed
to the very strong antiferromagnetic interaction between
the Cu2–R1 and the Cu2–R2. The minimum of χmT of
0.40 cm3 K mol−1 at 2.0 K is close to an uncoupled Cu(II)
ion (S=1/2, 0.375 cm3 K mol−1), which may be due to the
very strong antiferromagnetic interactions between Cu2
and radicals, which leads to complete spin cancellation. It
can also be proved by ftting the susceptibilities (circular)
by the Curie–Weiss law (red curve). In the Curie-Weiss ft
Fig. 3 Octahedral structure of Cu1 (a) and square pyramidal structure
of Cu2 (b)
Magnetic Properties
The temperature-dependent magnetic susceptibilities of
{(NIT-Ph-4-Br)2[Cu(hfac)2]3} were measured in the tem-
perature range 2–300 K in an external magnetic feld of 10
kOe, and the data were corrected for the diamagnetism of
the components.
Magnetic data for {(NIT-Ph-4-Br)2[Cu(hfac)2]3} are
shown in Fig. 4 as χm versus T and χmT versus T. For the
1 3

Transition Metal Chemistry
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