Aromatic SNF-Approach to Fluorinated Phenyl tert-Butyl Nitroxides

Article abstract of DOI:10.3390/molecules24244493

The interaction of octafluorotoluene (1a), as well as pentafluorobenzonitrile (1b) with tert-butylamine, followed by the oxidation of thus formed tert-butylanilines (2a,b) with meta-chloroperoxybenzoic acid led to functionalized perfluorinated phenyl tert-butyl nitroxides [namely, 4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a) and 4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile (3b)] with nearly quantitative total yields. The molecular and crystal structures of nitroxide 3a were proved by single crystal X-ray diffraction analysis. The radical nature of both nitroxides was confirmed by ESR data. The interaction of Cu(hfac)₂ with the obtained nitroxides 3a,b gave corresponding transbis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O⁰)bis{4-(N-tert-butyl(oxyl)amino)perfluoroareneκO}copper (II) complexes ([Cu(hfac)₂(3a)₂] and [Cu(hfac)₂(3b)₂]). X-ray crystal structure analysis showed square bipyramid coordination of a centrally symmetric Cu polyhedron with the axial positions occupied by oxygen atoms of the nitroxide groups. Magnetic measurements revealed intramolecular ferromagnetic exchange interactions between unpaired electrons of Cu(II) ions and paramagnetic ligands, with exchange interaction parameters J_Cu–R reaching 53 cm^?1

Full text of DOI:10.3390/molecules24244493

molecules
Article
F
Aromatic SN -Approach to Fluorinated Phenyl
tert-Butyl Nitroxides
Evgeny Tretyakov 1,2,
*
, Pavel Fedyushin 1, Elena Panteleeva 1,2, Larisa Gurskaya 1,
Tatyana Rybalova 1,2 , Artem Bogomyakov 2,3, Elena Zaytseva 1,2, Maxim Kazantsev 1
,
Inna Shundrina 1,2 and Victor Ovcharenko 3,
*
1
N. N. Vorozhtsov Institute of Organic Chemistry, 9 Ac. Lavrentiev Avenue, 630090 Novosibirsk, Russia;
feduyshin@nioch.nsc.ru (P.F.); pantel@nioch.nsc.ru (E.P.); gurlar82@nioch.nsc.ru (L.G.);
rybalova@nioch.nsc.ru (T.R.); elena@nioch.nsc.ru (E.Z.); kazancev@nioch.nsc.ru (M.K.);
ishund@nioch.nsc.ru (I.S.)
Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russia; bus@tomo.nsc.ru
International Tomography Center, 3a Institutskaya Str., 630090 Novosibirsk, Russia
Correspondence: tretyakov@nioch.nsc.ru (E.T.); ovchar@tomo.nsc.ru (V.O.)
2
3
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Fawaz Aldabbagh
Received: 17 November 2019; Accepted: 5 December 2019; Published: 8 December 2019
Abstract: The interaction of octafluorotoluene (1a), as well as pentafluorobenzonitrile (1b
)
with tert-butylamine, followed by the oxidation of thus formed tert-butylanilines (2a ) with
,b
meta-chloroperoxybenzoic acid led to functionalized perfluorinated phenyl tert-butyl nitroxides [namely,
4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a) and 4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile
(
3b)] with nearly quantitative total yields. The molecular and crystal structures of nitroxide 3a were
proved by single crystal X-ray diffraction analysis. The radical nature of both nitroxides was confirmed
by ESR data. The interaction of Cu(hfac)2 with the obtained nitroxides 3a gave corresponding trans-
bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-
2O,O0)bis{4-(N-tert-butyl(oxyl)amino)perfluoroarene-
O}copper (II) complexes ([Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]). X-ray crystal structure analysis
,
b
κ
κ
showed square bipyramid coordination of a centrally symmetric Cu polyhedron with the axial
positions occupied by oxygen atoms of the nitroxide groups. Magnetic measurements revealed
intramolecular ferromagnetic exchange interactions between unpaired electrons of Cu(II) ions and
paramagnetic ligands, with exchange interaction parameters JCu–R reaching 53 cm−1
.
Keywords: fluoroarenes; aromatic nucleophilic substitution; tert-butylanilines; nitroxides;
copper-nitroxide complexes; magneto-structural correlations
1. Introduction
The use of organofluorine compounds has had a huge impact on the main areas of modern
chemistry, such as the development of new functional materials and pharmaceuticals with unique
properties [
1
–
4
]. The ever-increasing importance of fluorine organic compounds brings the development
]. The chemistry of stable
of new directed and high-performance synthetic methods to the forefront [
5
organic radicals is among the scientific fields where the need for the creation of new methods for the
synthesis of fluorinated compounds is ripe. The number of studies here is extremely limited, although
interest in fluorinated organic radicals has been consistently strong [
6–
9]. In particular, this interest is due
to the unusual magnetic and functional properties that fluorinated paramagnets often manifest [10
–
13].
To devise new approaches to their synthesis, we applied nucleophilic substitution reactions of a fluorine
atom in a series of polyfluorinated aromatic compounds. This idea has been especially fruitful in
nitroxide chemistry. For example, the 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl
lithium derivative has been shown to react with polyfluoroaromatic compounds and give rise to
Molecules 2019, 24, 4493; doi:10.3390/molecules24244493
www.mdpi.com/journal/molecules

Molecules 2019, 24, 4493
2 of 14
new fluorinated nitronyl nitroxides [14
–
16]. It is noteworthy that according to quantum chemical
calculations, the reaction follows a concerted pathway without formation of an intermediate [14].
The reason is that at an early stage, the lithium ion facilitates fluoride elimination when the C–C bond
between nitroxide and aromatic carbon atoms is formed only partially.
tert-Butyl aryl nitroxides and polynitroxides represent an important class of paramagnets that have
been actively used as ligands for the construction of heterospin systems. The design of polyradicals
with >N–O groups at the meta position of the benzene ring, which favors intraligand ferromagnetic
coupling, and their complexation with M(hfac)2 have allowed Iwamura et al. to obtain a series
of molecular magnets capable of cooperative magnetic ordering at 3.4–46 K [17–20]. In addition,
nitroxides containing aryl fragment substituents able to form intermolecular H-bonds are attractive as
components of the controlled assembly of high-dimension systems [21–27].
A typical method for the preparation of tert-butyl aryl nitroxides involves oxidation of the
corresponding tert-butyl anilines or phenylhydroxylamines. As a rule, the general synthetic approach
starts with a reaction of an appropriate aromatic organometallic compound with tert-nitrosobutane, thus
providing aryl tert-butylhydroxylamine, followed by its oxidation to the target radical product [28,29].
The methods for preparation of radicals on the basis of UV irradiation of the corresponding iodo
arenes in the presence of tert-nitrosobutane or the interaction of nitro arenes with tert-butyl magnesium
chloride (Scheme 1) are also worthy of mention but of no practical value [30,31].
Scheme 1. Synthetic approaches to phenyl tert-butyl nitroxides.
In this study, we have successfully applied the early proposed concept based on nucleophilic
substitution of a fluorine atom [14] to the synthesis of fluorinated N-tert-butyl aryl nitroxides.
This approach was found to generate highly stable nitroxide radicals in almost quantitative yields.
An interaction of bis(hexafluoroacetylacetonato)copper (II) [abbreviated as Cu(hfac)2] with the
synthesized nitroxides led to formation of complexes of composition 1:2 containing heterospin
clusters (>N–O)2Cu.
2. Results and Discussion
2.1. Synthesis of Fluorinated Nitoxodes 3a,b
As an approach to phenyl tert-butyl nitroxides, we linked together two transformations: at the
first stage, the nucleophilic substitution of a fluorine atom in an activated arene under the action of
tert-butylamine, and at the second stage, oxidation of the obtained amine into the target paramagnetic
compound. As to the first transformation, it is known that the interaction of perfluorotoluene 1a with
methyl-, n-butyl-, tert-butylamine or perfluorobenzonitrile 1b with methylamine in isopropanol

Molecules 2019, 24, 4493
3 of 14
◦
at 20–70 C for 20–90 h leads to selective para-fluorine substitution with the formation of the
corresponding N-alkyl anilines in 70–98% yields [32]. We carried out the reaction of 1a,b with
tert-butylamine in chloroform at room temperature for 72 and 1.5 h, respectively, and isolated
4-(tert-butylamino)heptafluorotoluene 2a or 4-(tert-butylamino)tetrafluorobenzonitrile 2b in almost
quantitative yields. The oxidation of anilines 2a,b with meta-chloroperoxybenzoic acid (m-CPBA) was
performed at room temperature and provided target nitroxides 3a
>90% (Scheme 2).
,b as viscous red liquids in yields
Scheme 2. Synthesis of fluorinated nitroxides 3a,b.
Newly obtained fluorinated nitroxides 3a,b were comprehensively studied both in solution and
in a condensed state.
2.2. ESR Measurement of Radicals 3a,b
The ESR spectra for diluted (~10−4 M) and oxygen-free chloroform solutions of radicals 3a
showed triplet patterns at g = 2.0057(2) owing to the hfs of an unpaired electron at the nitrogen
,b
nucleus (AN = 1.33 mT for 3a and 1.31 mT for 3b; Figure 1 and Figure S18). In the case of nitroxide 3a
a high-resolution ESR spectrum was recorded to obtain more complex splitting of each line of its triplet.
The spectrum was well reproduced, taking into account 9 hfs constants on the protons of tert-butyl
group (AH = 0.02 mT) and two pairs of hfs constants on the distant fluorine atoms (AFortho = 0.12 mT;
,
A
Fmeta = 0.06 mT).
(a)
(b)
348
349
350
351
352
350,0
350,2
350,4
350,6
350,8
Magnetic field/mT
Magnetic field/mT
Figure 1. Experimental (black curve) and simulated (red curve) ESR spectra of 3a: (
a) the whole
spectrum, and (b) only the central component.
2.3. Electrocemical Measurements
Electrochemical properties of the studied compounds were evaluated by cyclic voltammetry
measurements in a CH2Cl2 solution (Figure S21). Both radicals 3a and 3b demonstrated irreversible
oxidation waves with E1/2
corresponding oxoammonium cations. For comparison, 4-(N-tert-butyl-N-oxylamino)benzene (4a
≈
1 V, which were assigned to the oxidation of the nitroxide radicals to the
)
and 4-(N-tert-butyl-N-oxylamino)benzotrifluoride (4b) displayed the reversible redox potentials at
0.38 and 0.53 V, respectively (Table 1). The redox reversibility of 4a and 4b could be ascribed to the
higher stability of the corresponding oxoammonium cations generated at considerably less oxidative
potentials [33].

Molecules 2019, 24, 4493
4 of 14
Table 1. Half-wave potentials for the studied compounds.
E1/2ox, V
E1/2Red, V
Compound
3a *
3b *
1.04
1.00
1.04
1.06
0.38
0.53
−1.44
−1.14
−1.40
−1.20
−1.63
−1.37
[Cu(hfac)2(3a)2] *
[Cu(hfac)2(3b)2] *
4a **
4b **
* Data obtained in a CH2Cl2/0.1 M n-Bu4NPF6 solution at 298 K vs. Fc/Fc+. ** Data obtained in a MeCN/0.1 M
n-Bu4NBF4 solution [33].
On the cathodic side, both 3a and 3b exhibited an irreversible redox at
respectively. The non-substituted at the phenyl ring nitroxide 4a also reduced irreversibly, but at a
considerably higher potential of 1.63 V, which corresponds to the relative change in the SOMO energy
level (Table 1). It is interesting that the trifluoromethylphenyl nitroxide 4b exhibited redox at 1.37 V
−
1.44 and
−
1.14 V,
−
−
in a clear reversible manner that results from the stability of the corresponding aminoxy anion [33].
Therefore, the substitution of phenyl ring influences not only on the redox potentials, but also on the
stability of the corresponding anionic forms (oxoammonium cations and aminoxy anions).
2.4. Crystal Structure of Radical 3a
Even though both freshly prepared radicals 3a,b were red oils, repeated efforts were made to
obtain them in a crystalline form. Finally, by crystallization from a cold hexane solution, we managed
to isolate nitroxide 3a as high-quality crystals and determined its molecular and crystalline structure
by X-ray diffraction (XRD) analysis. The radical 3a crystallizes in the orthorhombic Pbca space group,
and bond lengths of the tert-butyl-nitroxide moiety are completely compatible with those of previously
described radicals of this family (see Table 2 for crystallographic data). The nitroxide group in radical
3a is twisted by a large angle (~68◦) relative to the aromatic ring (Figure 2), obviously owing to the
mutual effects of steric repulsion between the tert-butyl group and phenylene ortho-fluorines and of the
electrostatic repulsion of dipoles C–F and N–O. In this regard, it should be noted that a similar dihedral
angle in nonfluorinated tert-butyl phenyl nitroxides is twofold smaller and manifests an experimental
value of 23–32◦. This finding is consistent with the calculated minimum of the heat of formation (MO,
B3LYP/6-31G) for the model radical (N-tert-butyl(oxyl)amino)benzene at a 34◦ angle [34].
(b)
(a)
Figure 2. Molecular structure (
a
) and a fragment of the crystal structure (
b
) of nitroxide 3a (the thermal
ellipsoids are drawn at the 50% probability level).
Analysis of the crystal packing of nitroxide 3a revealed C–FCF3 . . . π interactions, with F . . . Cg
and Dpln distances equal to 3.069(2) and 3.049 Å, respectively, and ONO . . . C short contacts 3.101(3)
and 3.130(3) Å (Figure 2), that formally bind molecules into chains along the b axis. The chains in turn
are packed into layers parallel to plane (a, b) with contacts F . . . F equal to 2.650(2) Å, that is, shorter
than the sum of van der Waals radii of two fluorine atoms (2.92 Å) [35].

Molecules 2019, 24, 4493
5 of 14
Table 2. XRD data on radical 3a and complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2].
Compound
3a
[Cu(hfac)2(3a)2]
[Cu(hfac)2(3b)2]
Empirical formula
Formula weight
C11H9F7NO
304.19
C32H20CuF26N2O6
1086.04
C32H20CuF20N4O6
1000.06
Crystal system
Space group
Orthorhombic
Pbca
Triclinic
P-1
Monoclinic
P21/c
11.6131(7)
10.9047(7)
19.7695(12)
90.00
90.00
90.00
2503.6(3)
8
1.614
10.0680(3)
13.8773(5)
16.1651(6)
96.289(2)
102.367(2)
104.872(2)
2099.6(1)
11.3678(8)
8.6913(7)
20.219(1)
90.00
90.158(3)
90.00
1997.6(2)
2
1.663
a, Å
b
c
α, o
β, o
γ, o
Volume, Å3
Z
2
Density (calcd.), mg·m−3
Abs. coefficient, mm−1
F(000)
1.718
0.680
1074
0.174
1224
0.687
994
Crystal size, mm3
Θ range for data collection, ◦
0.40 × 0.70 × 1.00
2.76–26.07
−14 ≤ h ≤ 14,
−13 ≤ k ≤ 13,
−24 ≤ l ≤ 24
34,642
0.30 × 0.40 × 0.50
1.31–26.43
−12 ≤ h ≤ 12,
−17 ≤ k ≤ 17,
−20 ≤ l ≤ 20
64,762
0.10 × 0.12 × 0.80
1.79–25.05
−13 ≤ h ≤ 13,
−10 ≤ k ≤ 10,
−24 ≤ l ≤ 24
26,173
3530 R(int) 0.069
99.7
3530/48/376
Index ranges
Reflections collected
Independent reflections
Completeness to θ 50º, %
Data/restraints/parameters
Goodness-of-fit on F2
2473 R(int) 0.0266
99.6
8633 R(int) 0.052
99.7
2473/0/184
8633/0/613
1.044
1.007
1.03
Reflections with I > 2σ(I)
Final R indices at I > 2σ(I)
Final R indices (all data)
Largest diff. peak/hole, e·Å−3
2057
5535
2587
R1 0.0555, wR2 0.1399
R1 0.680, wR2 0.1554
0.59/−0.27
R1 0.0630, wR2 0.1764
R1 0.1034, wR2 0.2165
0.73/−0.47
R1 0.0530, wR2 0.1466
R1 0.0791, wR2 0.1716
0.58/−0.38
2.5. Synthesis and Crystal Structure of Complexes [Cu(hfac)2(3a)2], [Cu(hfac)2(3b)2] and [Cu(hfac)2(2b)2]
Radicals 3a underwent complexation with half equivalents of Cu(hfac)2 in CHCl3 to form 1:2
,b
complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2] (Scheme 3), which were recrystallized from n-hexane
to obtain crystals suitable for XRD analysis.
F
F
X
F
N
O
F
O
CF3
F3C
N
X
O
O
F
F
F
Cu(hfac)2
CHCl3
Cu
O
O
F
F
CF3
F3C
F
O
N
3a,b
a
b
: X = CF3; : X = CN
[Cu(hfac)2(3a)2]
3b
[Cu(hfac)2( )2]
F
X
F
Scheme 3. Complexation of Cu(hfac)2 with nitroxides 3a,b.
Figure 3 illustrates the molecular structures of [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2] (see Table 2
for crystallographic data). The asymmetric part of [Cu(hfac)2(3a)2] includes two halves of independent
units located on the centers of symmetry. In the [Cu(hfac)2(3b)2] complex, CF3 groups and tert-butyl
groups are disordered. In the distorted octahedron around the copper ion, two ONO atoms occupy

Molecules 2019, 24, 4493
6 of 14
axial positions (dCu–O = 2.398(4), 2.411(5) Å in [Cu(hfac)2(3a)2], and 2.452(4) Å in [Cu(hfac)2(3b)2]).
Equatorial Cu–Ohfac distances in both complexes do not exceed 1.946(3) Å (Table 3).
(b)
(a)
Figure 3. XRD structures of [Cu(hfac)2(3a)2], one of two independent molecules (a), and [Cu(hfac)2(3b)2]
with omitted minor positions of CF3 and tert-butyl groups (b).
Table 3. Selected geometrical parameters of complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2].
[Cu(hfac)2(3a)2] 1
Bond Lengths, Å
[Cu(hfac)2(3b)2]
Parameter
Cu1–O1
Cu1–O2
Cu1–O3
2.411(5)
1.942(3)
1.935(3)
2.398(4)
1.946(3)
1.935(3)
2.452(4)
1.944(2)
1.937(2)
Bond angles, ◦
O1–Cu1–O2
O1–Cu1–O3
O2–Cu1–O3
89.0(1)
84.1(1)
92.4(1)
88.1(1)
82.3(1)
92.4(1)
86.6(1)
83.9(1)
92.7(1)
1
For two independent moieties.
Polyfluorinated compounds, including complexes of transition metal ions, are known to have
high volatility. Our experiments showed that at reduced pressure (~1 Torr) and a temperature of
~95 ◦C, at which according to thermal analysis data (Figures S19 and S20) complexes [Cu(hfac)2(3a)2]
and [Cu(hfac)2(3b)2] are stable, they almost quantitatively collected on a sublimator finger with the
formation of crystalline phases. The infrared (IR) spectra of these phases are fully identical to those
of the initial complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]. Moreover, according to the results
of the XRD analysis, the structure of the sublimed complexes coincides with that of the starting
compounds [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]. This finding indicates that the formation constants
of these complexes are quite high, and these compounds are transferred to the gas phase in the form of
molecules of coordination compounds with subsequent precipitation.
Of note, the interaction of Cu(hfac)2 with radical 3b leads to coordination of the nitroxide group,
not the nitrile one. Driven by curiosity, we carried out the reaction of acceptor matrix Cu(hfac)2 with
amine 2b (Figure S22). In this case, the nitrile group underwent coordination with the formation of a
centrosymmetrical mononuclear complex in which two NCN atoms occupy axial positions [dCu–N
=
2.534(3) Å] (Figure 4). This means that the nitrile group in 3b may be also involved in the coordination,
as may the nitroxide group, thus affording complexes of chain-polymeric structure.

Molecules 2019, 24, 4493
7 of 14
Figure 4. XRD structure of [Cu(hfac)2(2b)2].
2.6. Magnetic Measurements of Complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]
Temperature dependences of the effective magnetic moment (µeff) for complexes [Cu(hfac)2(3a)2]
and [Cu(hfac)2(3b)2] are presented in Figure 5. At 300 K, the µeff values are 3.28 and 3.34 µB for
[Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2], respectively, and are in agreement with a theoretical spin-only
value of 3 µB for three paramagnetic centers: one Cu(II) ion and two nitroxides. The increase in µeff
with lowering temperature indicates domination of ferromagnetic exchange interactions between
spins of paramagnetic centers. Analysis of experimental µeff(T) dependences using a model of a
three-spin exchange cluster [spin Hamiltonian H =
allowed us to estimate exchange interaction parameters. The best-fit values of gCu and exchange
interaction parameters JCu–R and JR–R are 2.26, 53.1 cm−1, and 17 cm−1 for [Cu(hfac)2(3a)2] and 2.27,
27.0 cm−1, and 0.4 cm−1 for [Cu(hfac)2(3b)2]. The g-factors for nitroxides were fixed at gR = 2 to
−
2JCu–R
×
(SR1SCu + SCuSR2
)
−
2JR–R
×
SR1SR2]
−
−
avoid overparametrization.
μeff, μB
μeff, μB
4.0
3.5
3.0
4.0
3.5
3.0
0
100
200
300
0
100
200
300
T, K
T, K
(a)
(b)
Figure 5. The µeff(T) dependences for complexes [Cu(hfac)2(3a)2] (
a) and [Cu(hfac)2(3b)2] (
b
). Solid
lines are theoretical curves.
The observed ferromagnetic exchange interactions between the unpaired electrons in
[Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2] are consistent with the XRD data on the axial coordination
of the nitroxide groups with the Cu2+ ion. According to the results of experimental [36
–
39] and
theoretical studies [40
,41], such coordination provides the orthogonal arrangement of the spins in the

Molecules 2019, 24, 4493
8 of 14
exchange cluster with JCu–R values, which in some cases exceed 50 cm−1. With regard to complexes
[Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2], such a strong ferromagnetic coupling leads to that the half-wave
potentials for electrochemical oxidation and reduction are almost the same as those for the free radicals
(Table 1).
3. Conclusions
Using octafluorotoluene 1a and pentafluorobenzonitrile 1b as examples, we demonstrated a
new synthetic approach for obtaining functionalized fluorinated phenyl tert-butyl nitroxides by
sequential substitution of a fluorine atom in polyfluorinated arenes with tert-butylamine and oxidation
of resultant tert-butylaniline with m-CPBA. This convenient approach can be useful in the design of
nitroxide structures as precursors of new complexes with desired magnetic characteristics. For instance,
the interaction of Cu(hfac)2 with newly synthesized nitroxides 3a and 3b yielded isomorphic three-spin
complexes [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]. They are centrosymmetrical square bipyramids with
two axial positions occupied by oxygen atoms of the nitroxide groups; this arrangement gives rise to
ferromagnetic intramolecular exchange interactions between Cu(II) and radical spins. In complexes with
axial coordination of the nitroxyl group observed in compounds [Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2],
thermo- or photo-induced spin transitions may take place [42
necessary to modify the structure of tert-butyl nitroxides of type
–
47]. To obtain such complexes, it is
in an appropriate way. We believe
3
that this may be achieved by a decrease in the number of fluorine atoms on the aromatic ring of
paramagnetic ligands 3.
4. Materials and Methods
4.1. Reagents and General Methods
Bis(hexafluoroacetylacetonato)copper-(II) [abbreviated as Cu(hfac)2] was prepared and purified by
previously described procedures [48]. Other chemicals were of the highest purity commercially available
and were used as received. The progress of reactions was monitored by thin-layer chromatography (TLC)
on Silica gel 60 F254 aluminum sheets with hexane or CHCl3 as the eluent. Column chromatography
was carried out on silica gel (0.063–0.200 mm). NMR spectra were recorded for 2a
on Bruker Avance-300 (300.13 MHz for 1H, 282.25 MHz for 19F) and Avance-400 (400.13 MHz for 1H,
100.62 MHz for 13C) spectrometers; chemical shifts ( ) of 1H and 13C{1H} are given in ppm, with the
solvent signals serving as the internal standard ( H = 7.24 ppm, C = 76.9 ppm); the internal standard
for 19F spectra was C6F6 (
162.9 ppm). Fourier transform infrared (FT-IR) spectra were acquired in
,b solutions in CDCl3
δ
δ
δ
δ
=
−
KBr pellets on a Bruker Vector-22 spectrometer. UV-vis spectra were registered on an HP Agilent 8453
spectrophotometer (in 10−5–10−4 M solutions in EtOH). Masses of molecular ions were determined
by high-resolution mass spectrometry (HRMS) by means of a DFS Thermo Scientific instrument (EI,
70 eV). Melting points were recorded on a Melter-Toledo FP81 Thermosystem apparatus. Elemental
analyses were performed using a Euro EA 3000 elemental analyzer.
4.2. Synthesis of 4-(tert-butylamino)arenes 2a,b (General Procedure)
A mixture of tert-butyl amine (5.0 mmol, 365 mg) and perfluoroarene 1a or 1b (1.0 mmol) in CHCl3
(5 mL) was stirred at room temperature until the starting substrate was completely consumed: for
72 and 1.5 h for 1a and 1b, respectively. Flash chromatography (SiO2, column 3
eluent) followed by solvent removal under reduced pressure yielded colorless crystals of pure product
×
4 cm, CHCl3 as an
2a or 2b.
1
4-(tert-Butylamino)heptafluorotoluene (2a) [32]: Colorless crystals; yield 99% (286 mg). H NMR
(300.13 MHz, CDCl3):
δ δ 144.94 (dm,
4.07 (s, 1H), 1.37 (s, 9H). 13C NMR (125.75 MHz, CDCl3):
J = 254.7 Hz, 2C) 137.86 (dm, J = 239.3 Hz, 2C), 130.22 (tt, J1 = 12.5 Hz, J2 = 3.2 Hz, 1C) 121.52 (q, J =
272.4 Hz, 1C), 97.04–98.08 (m, 1C), 53.76 (s, 1C), 30.46 (t, J = 3.7 Hz, 3C). 19F NMR (282.37 MHz, CDCl3):
δ
109.58 (t, J = 20.6 Hz, 3F), 21.09–21.48 (m, 2F), 10.67 (d, J = 15.9 Hz, 2F). IR (KBr, cm−1) 3433, 2978,

Molecules 2019, 24, 4493
9 of 14
2945, 2918, 2879, 1659, 1516, 1470, 1433, 1400, 1371, 1335, 1240, 1203, 1180, 1138, 1043, 993, 930, 872, 804,
714, 422. HRMS calcd for C11H10F7N: 289.0696. Found 289.0698.
1
4-(tert-Butylamino)tetrafluorobenzonitrile (2b): Colorless crystals; yield 99% (244 mg). H NMR
(400.13 MHz, CDCl3):
δ δ 147.98 (dm,
4.46 (s, 1H), 1.40 (s, 9H). 13C NMR (100.62 MHz, CDCl3):
J = 256.1 Hz, 2C), 136.13 (dm, J = 240.0 Hz, 2C), 132.22 (tt, J1 = 11.9 Hz, J2 = 3.6 Hz, 2C), 108.76 (t,
J = 3.7 Hz, 2C), 79.76 (tt, J1 = 18.0 Hz, J2 = 1.8 Hz, 1C), 53.85 (s, 1C) 30.60 (s, 1C). 19F NMR (282.37 MHz,
CDCl3):
δ
26.34 (m, 2F), 7.68 (d, J = 17.4 Hz, 2F). IR (KBr, cm−1) 3427, 3020, 2978, 2943, 2881, 2617, 2420,
2233, 1655, 1525, 1510, 1477, 1435, 1400, 1373, 1313, 1302, 1336, 1201, 1176, 1126, 1047, 993, 978, 889, 806,
721, 661, 634, 523, 494. HRMS calcd for C11H10F4N2: 246.0775. Found 246.0771. The molecular and
crystal structures of 2b were refined from single-crystal XRD data (see SI).
4.3. Synthesis of 4-(N-tert-butyl(oxyl)amino)arenes 3a,b (General Procedure)
A solution of 4-(tert-butylamino)perfluoroarene 2a,b (1 mmol) and m-CPBA (1.2 mmol, 208 mg) in
CHCl3 (10 mL) was stirred at room temperature for 48 h. Column chromatography (SiO2, column
3 × 20 cm, CHCl3 as an eluent) afforded a red fraction of radical 3a
reduced pressure at room temperature to obtain radicals 3a,b.
,b. The solvent was removed under
4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a): Red crystals; yield 96% (292 mg). IR (KBr,
cm−1) 2987, 2945, 1790, 1766, 1655, 1606, 1504, 1469, 1417, 1358, 1336, 1257, 1232, 1190, 1153, 1197, 991,
904, 831, 791, 715, 588. UV-Vis (EtOH)
(3.93). HRMS calcd for C11H9O1N1F7: 304.0567. Found 304.0564. Crystals of 3a were grown from
hexane at
15 ◦C. The molecular and crystal structures of 3a were refined from single-crystal XRD data
(see main text).
4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile (3b): Red oil; yield 95% (248 mg). IR (neat,
cm−1) 3440, 2987, 2945, 1790, 1766, 1655, 1606, 1504, 1469, 1417, 1358, 1336, 1257, 1232, 1190, 1153, 1107,
991, 904, 831, 791, 715, 588. UV-Vis (EtOH) max/nm (lg ): 291 (4.34), 220 (4.02), 204 (4.06). HRMS
calcd for C11H9O1N2F4: 261.0646. Found 261.0643.
λmax/nm (lg ε): 383 (2.49), 301 (3.30), 271 (3.51), 221 (3.73), 203
−
λ
ε
4.4. Synthesis of Complexes [Cu(hfac)2(3a)2], [Cu(hfac)2(3b)2] and [Cu(hfac)2(3b)2] (General Procedure)
Cu(hfac)2 H2O (248 mg, 0.5 mmol) was added to a solution of radical 3a or amine 2b (1.0 mmol)
in CHCl3 (10 mL). The reaction mixture was stirred for 30 min and then was incubated at
15 ◦C for
10 h. The solution was filtered and evaporated. The residue was dissolved in n-hexane (5 mL); the
·
,b
−
solution was filtered and incubated at
air dried.
−
15 ◦C for 10 h to prepare crystals that were filtered off and
trans-Bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-
heptafluorotoluene-
O}copper (II) ([Cu(hfac)2(3a)2]): Brown crystals; yield 30% (163 mg). Mp 123.6 ◦C
κ
2O,O0)bis{4-(N-tert-butyl(oxyl)amino)
κ
(DSC), mp 137.8–138.6 ◦C (Melting Point Apparatus). IR (KBr, cm−1) 3140, 2997, 1641, 1604, 1558, 1529,
1504, 1485, 1406, 1362, 1336, 1263, 1211, 1173, 1147, 1107, 1030, 989, 903, 827, 800, 746, 715, 683, 596, 530,
453. Anal. Calcd for C32H20CuF26N2O6: C, 35.39; H, 1.86; F, 45.48; N, 2.58. Found C, 35.07; H, 2.04; F,
45.54; N, 2.45. The molecular and crystal structures of [Cu(hfac)2(3a)2] were refined from single-crystal
XRD data (see main text).
trans-Bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-
tetrafluorobenzonitrile- O}copper (II) ([Cu(hfac)2(3b)2]): Brown crystals; yield 30% (151 mg). Mp 68.8
κ
2O,O0)bis{4-(N-tert-butyl(oxyl)amino)
κ
◦C (DSC), mp 70.6 ◦C with decomposition (Melting Point Apparatus). IR (KBr, cm−1) 3431, 3149, 2993,
2927, 2249, 1643, 1606, 1560, 1531, 1500, 1487, 1402, 1354, 1311, 1257, 1213, 1151, 1109, 993, 976, 831, 804,
746, 681, 596, 530, 513. Anal. Calcd for C32H20CuF20N4O6: C, 38.43; H, 2.02; F, 38.00; N, 5.60. Found C,
38.87; H, 2.03; F, 38.05; N, 5.60. The molecular and crystal structures of [Cu(hfac)2(3b)2] were refined
from single-crystal XRD data (see main text).
trans-Bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-
tetrafluorobenzonitrile- N}copper (II) [Cu(hfac)2(2b)2]: Green crystals; yield 89% (431 mg). Mp 120.7
◦C (Melting Point Apparatus). IR (KBr, cm−1) 3427, 31612980, 2623, 2239, 1657, 1643, 1612, 1562, 1533,
κ
2O,O0)bis{4-(N-tert-butyl(oxyl)amino)
κ

Molecules 2019, 24, 4493
10 of 14
1512, 1487, 1443, 1402, 1373, 1356, 1306, 1259, 1217, 1200, 1153, 1111, 1047, 997, 982, 806, 681, 596, 528,
490. Anal. Calcd for C33H22CuF20N4O4: C, 39.62; H, 2.29; F, 39.17; N, 5.57. Found C, 39.55; H, 2.30; F,
39.11; N, 5.75. The molecular and crystal structures of [Cu(hfac)2(2b)2] were refined from single-crystal
XRD data (see main text and SI).
4.5. X-Band ESR Measurements
ESR spectra were acquired in diluted and oxygen-free chloroform solutions at 295 K at the
concentrations of ~10−4 M by means of the commercial Bruker X Band (9 GHz) spectrometer Elexsys E
540 (Bruker Corporation, Billerica, MA, USA). For determining the isotropic g-factors (giso), we recorded
X-band continuous-wave ESR spectra of a mixture of the investigated radical with the Finland trityl.
Then, the known giso of the Finland trityl was used for the spectrum simulation, and the target giso
value was excluded. Simulations of the solution ESR lines were carried out in software Easy Spin,
which is available at http://www.easypin.org.
4.6. Cyclic Voltammetry Measurements
The analysis of electrochemical behavior of 3a,b, [Cu(hfac)2(3a)2], and [Cu(hfac)2(3b)2] was
performed in a CH2Cl2 solution by computer-controlled P-8 nano potentiostat (Elins, Chernogolovka,
Russia) in combination with a three-electrode cell (Gamry, Warminster, PA, USA); 0.1 M
tetrabutylammonium hexafluorophosphate served as a supporting electrolyte. Pt, a Pt wire, and
Ag/AgCl were used as working, counter, and reference electrodes, respectively. The reference electrode
was calibrated by measuring the redox potential of ferrocene. The scan rate was 100 mV/s.
4.7. Crystallographic Analysis
XRD data were collected at 200(2) K for 2b and 3a, and at room temperature for [Cu(hfac)2(2b)2],
[Cu(hfac)2(3a)2], and [Cu(hfac)2(3b)2] on a Bruker Kappa Apex II CCD diffractometer using
ϕ, ω scans
of narrow frames with Mo K radiation ( = 0.71073 Å) and a graphite monochromator. Absorption
α
λ
corrections were applied empirically using SADABS programs [49]. The structures were solved by direct
methods and refined by full-matrix least-squares method against all F2 in anisotropic approximation
(beside the atoms H) using the SHELX-97 programs set [50]. The H atoms positions, except the
amino hydrogen in 2b, were treated with the riding model. The amino hydrogen position of 2b was
localized from difference map and refined independently as well as thermal parameter. The analysis
of the geometry and the intermolecular interactions for nitroxide 3a and complexes [Cu(hfac)2(3a)2],
[Cu(hfac)2(3b)2] was performed using PLATON program. [51,52] CCDC 1962404–1962408 contains
the supplementary crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, or from the Cambridge Crystallographic Data Centre,
12 Union Road, CambridgeCB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
4.8. Thermal Analysis
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements
were performed on a NETZSCH STA 409 instrument at a heating rate of 10 K/min under He flow
30 mL/min. The onset temperature of decomposition (T0), the melting temperature (Tm) were
determined using NETZSCH Proteus Thermal Analysis software.
4.9. Magnetic Measurements
Magnetic susceptibility of the polycrystalline samples of complexes [Cu(hfac)2(3a)2] and
[Cu(hfac)2(3b)2] was measured with a Quantum Design MPMSXL SQUID magnetometer in the
temperature range 2 to 300 K with a magnetic field of up to 5 kOe. Diamagnetic corrections were made
2
via the Pascal constants. The effective magnetic moment was calculated as µeff(T) = [(3k/NAµB
)χ
T]1/2
≈ (8χT)1/2
.

Molecules 2019, 24, 4493
11 of 14
Supplementary Materials: The following are available online, Figure S1: 1H NMR spectrum of 2a, Figure S2: 13C
NMR spectrum of 2a, Figure S3: 19F NMR spectrum of 2a, Figure S4: IR spectrum of 2a, Figure S5: 1H NMR
spectrum of 2b, Figure S6: 13C NMR spectrum of 2b, Figure S7: 19F NMR spectrum of 2b, Figure S8: IR spectrum
of 2b, Figure S9: IR spectrum of 3a, Figure S10: UV spectrum of 3a, Figure S11: IR spectrum of 3b, Figure S12:
UV spectrum of 3b, Figure S13: IR spectrum of [Cu(hfac)2(3a)2], Figure S14: IR spectrum of [Cu(hfac)2(3a)2]
after sublimation, Figure S15: IR spectrum of [Cu(hfac)2(3b)2], Figure S16: IR spectrum of [Cu(hfac)2(3a)2] after
sublimation, Figure S17: IR spectrum of [Cu(hfac)2(2b)2], Figure S18: Experimental and simulated ESR spectrum
for 3b, Figure S19: DSC and TG curves for [Cu(hfac)2(3a)2], Figure S20: DSC and TG curves for [Cu(hfac)2(3b)2],
Figure S21: Cyclic voltammograms of nitroxides 3a,b and complexes [Cu(hfac)2(3a)2], [Cu(hfac)2(3b)2] in CH2Cl2
solution, S22: Crystallographic data for amine 2b and complex [Cu(hfac)2(2b)2], Figure S22: The molecular
structure and atom-labelling, the fragment of crystal packing of compound 2b.
Author Contributions: Conceptualization, E.T. and V.O.; methodology, E.P. and L.G.; validation, P.F. and T.R.;
formal analysis, A.B., T.R., and E.Z.; investigation, P.F., T.R., E.Z., M.K., and I.S.; writing—original draft preparation,
E.T., E.P., T.R., A.B., and E.Z.; supervision, E.T. and V.O.; funding acquisition, E.T.
Funding: This research was funded by the Ministry of Science and Higher Education of the Russian Federation
(the Affiliate Program of Hubert Curien–A.N. Kolmogorov, Identifier RFMEFI61619X0116).
Acknowledgments: The authors would like to acknowledge the Multi-Access Chemical Research Center SB RAS
for spectral and analytical measurements.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
References
1.
Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications, 2nd Completely Revised and
Enlarged ed.; Wiley-VCH: Weinheim, Germany, 2013.
2.
3.
Nenajdenko, V. (Ed.) Fluorine in Heterocyclic Chemistry; Springer: New York, NY, USA, 2014.
Gakh, A.A.; Kirk, K.L. (Eds.) Fluorinated Heterocycles; ACS Symposium Series 1003; American Chemical
Society: Washington, DC, USA, 2009.
4.
5.
Reddy, V.P. Organofluorine Compounds in Biology and Medicine, 1st ed.; Elsevier: Amsterdam, The Netherlands,
2015; pp. 1–23.
Politanskaya, L.V.; Selivanova, G.A.; Panteleeva, E.V.; Tretyakov, E.V.; Platonov, V.E.; Nikul’shin, P.V.;
Vinogradov, A.S.; Zonov, Ya.V.; Karpov, V.M.; Mezhenkova, T.V.; et al. Organofluorine chemistry: Promising
growth areas and challenges. Russ. Chem. Rev. 2019, 88, 425–569. [CrossRef]
Bobko, A.A.; Bagryanskaya, E.G.; Reznikov, V.A.; Kolosova, N.G.; Clanton, T.L.; Khramtsov, V.V.
6.
7.
Redox-sensitive mechanism of NO scavenging by nitronyl nitroxides. Free Radical Bio. Med. 2004
36, 248–258. [CrossRef] [PubMed]
,
Murata, H.; Miyazaki, Y.; Inaba, A.; Paduan-Filho, A.; Bindilatti, V.; Oliveira, N.F., Jr.; Delen, Z.;
Lahti, P.M. 2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazole-3-oxide-
1-oxyl, a hydrogen-bonded organic quasi-1D ferromagnet. J. Am. Chem. Soc. 2008, 130, 186–194. [CrossRef]
[PubMed]
8.
9.
Delen, Z.; Lahti, P.M. Crystallography and magnetism of 1-(4-[N-tert-butylaminoxyl]-2,3,5,6-
tetrafluorophenyl)pyrrole. Polyhedron 2007, 26, 2031–2036. [CrossRef]
Mei, H.; Han, J.; White, S.; Butler, G.; Soloshonok, V.A. Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent
radical: A new reagent for direct, metal-free radical trifluoromethylation and polymer initiation. J. Fluor.
Chem. 2019, 227, 109370. [CrossRef]
10. Bannister, A.J.; Bricklebank, N.; Lavender, I.; Rawson, J.M.; Gregory, C.I.; Tanner, B.K.; Clegg, W.;
Elsegood, M.R.J.; Palacio, F. Spontaneous magnetization in a sulfur-nitrogen radical at 36 K. Angew. Chem.
Int. Ed. Engl. 1996, 35, 2533–2535. [CrossRef]
11. Palacio, F.; Antorrena, G.; Castro, M.; Burriel, R.; Rawson, J.; Smith, J.N.B.; Bricklebank, N.; Novoa, J.; Ritter, C.
High-Temperature Magnetic Ordering in a New Organic Magnet. Phys. Rev. Lett. 1997, 79, 2336–2339.
[CrossRef]

Molecules 2019, 24, 4493
12 of 14
12. Constantinides, C.P.; Berezin, A.A.; Zissimou, G.A.; Manoli, M.; Leitus, G.M.; Bendikov, M.; Probert, M.R.;
Rawson, J.M.; Koutentis, P.A. A Magnetostructural investigation of an abrupt spin transition for
1-Phenyl-3-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl. J. Am. Chem. Soc. 2014, 136, 11906–11909.
[CrossRef]
13. Zakrassov, A.; Shteiman, V.; Sheynin, Y.; Tumanskii, B.; Botoshansky, M.; Kapon, M.; Keren, A.; Kaftory, M.;
Vos, T.E.; Miller, J.S. Synthesis, structural, and magnetic characterization of substituted benzimidazole-1-yl
N,N0-dioxides. J. Mater. Chem. 2004, 14, 1827–1837. [CrossRef]
14. Tretyakov, E.V.; Fedyushin, P.A.; Panteleeva, E.V.; Stass, D.V.; Bagryanskaya, I.Y.; Beregovaya, I.V.;
Bogomyakov, A.S. Substitution of a Fluorine Atom in Perfluorobenzonitrile by a Lithiated Nitronyl Nitroxide.
J. Org. Chem. 2017, 82, 4179–4185. [CrossRef]
15. Fedyushin, P.; Panteleeva, E.; Bagryanskaya, I.; Maryunina, K.; Inoue, K.; Stass, D.; Tretyakov, E. An approach
to fluorinated phthalonitriles containing a nitronyl nitroxide or iminonitroxide moiety. J. Fluor. Chem. 2019
,
217, 1–7. [CrossRef]
16. Fedyushin, P.; Gurskaya, L.; Panteleeva, E.; Koshcheev, B.; Maksimov, A.; Rybalova, T.V.; Zaytseva, E.;
Tretyakov, E. Exploration of SNF-Approach toward Functionalized Nitronyl Nitroxides. Fluor. Notes 2019
123, 7–8. [CrossRef]
,
17. Iwamura, H.; Inoue, K.; Hayamizu, T. High-spin polynitroxide radicals as versatile bridging ligands for
transition metal complexes with high ferri/ferromagnetic TC. Pure Appl. Chem. 1996, 68, 243–252. [CrossRef]
18. Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. Assemblage and Alignment of the Spins of
the Organic Trinitroxide Radical with a Quartet Ground State by Means of Complexation with Magnetic
Metal Ions. A Molecule-Based Magnet with Three-Dimensional Structure and High TC of 46 K. J. Am.
Chem. Soc. 1996, 118, 1803–1804. [CrossRef]
19. Itoh, T.; Matsuda, K.; Iwamura, H.; Hori, K. Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine, -methyl, and
-borane Have Doublet, Triplet, and Doublet Ground States, Respectively. J. Am. Chem. Soc. 2000, 122,
2567–2576. [CrossRef]
20. Inoue, K.; Iwahori, F.; Markosyan, A.S.; Iwamura, H. Synthesis and magnetic properties of one-dimensional
ferro- and ferrimagnetic chains made up of an alternating array of 1,3-bis(N-tert-butyl-N-oxyamino)benzene
derivatives and Mn(II)(hfac)2. Coord. Chem. Rev. 2000, 198, 219–229. [CrossRef]
21. Ferrer, J.R.; Lahti, P.M.; George, C.; Antorrena, G.; Palacio, F. Synthesis, Crystallography, and Magnetic
Properties of 2-tert-Butylaminoxylbenzimidazole. Chem. Mater. 1999, 11, 2205–2210. [CrossRef]
22. Lahti, P.M.; Ferrer, J.R.; George, C.; Oliete, P.; Julier, M.; Palacio, F. Hydrogen-bonded benzimidazole-based
tert-butylnitroxides. Polyhedron 2001, 20, 1465–1473. [CrossRef]
23. Ferrer, J.R.; Lahti, P.M.; George, C.; Oliete, P.; Julier, M.; Palacio, F. Role of Hydrogen Bonds in
Benzimidazole-Based Organic Magnetic Materials: Crystal Scaffolding or Exchange Linkers. Chem. Mater.
2001, 13, 2447–2454. [CrossRef]
24. Field, L.M.; Lahti, P.M. Coordination complexes of 1-(4-[N-tert-butyl-N-aminoxyl]phenyl)-1H-1,2,4-triazole
with paramagnetic transition metal dications. Inorg. Chem. 2003, 42, 7447–7454. [CrossRef]
25. Maspoch, D.; Catala, L.; Gerbier, P.; Ruiz-Molina, D.; Vidal-Gancedo, J.; Wurst, K.; Rovira, C.; Veciana, J.
Radical para-benzoic acid derivatives: Transmission of ferromagnetic interactions through hydrogen bonds
at long distances. Chem.–Eur. J. 2002, 8, 3635–3645. [CrossRef]
26. Field, L.M.; Lahti, P.M. Magnetism of conjugated organic nitroxides: Structural scaffolding and exchange
pathways. Chem. Mater. 2003, 15, 2861–2863. [CrossRef]
27. Nagao, O.; Kozaki, M.; Shiomi, D.; Sato, K.; Takui, T.; Okada, K. Magnetic behavior of a manganese(II)
complex of nitroxide-substituted 1,3-di(4-pyridyl)benzene. Polyhedron 2001, 20, 1653–1658. [CrossRef]
28. Hicks, R.G. (Ed.) Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; John Wiley and
Sons: Chichester, UK, 2010.
29. Tretyakov, E.V.; Ovcharenko, V.I. The chemistry of nitroxide radicals in the molecular design of magnets.
Russ. Chem. Rev. 2009, 78, 971–1012. [CrossRef]
30. Goldman, J.; Petersen, T.E.; Torssell, K.; Becher, J. Fluorine-19 and proton NMR and ESR investigations of
aryl tert-butyl nitroxides and nitronylnitroxides. Tetrahedron 1973, 29, 3833–3843. [CrossRef]

Molecules 2019, 24, 4493
13 of 14
31. Pedersen, J.A.; Torssell, K. Electron spin resonance and nuclear magnetic resonance spectra of sterically
hindered aromatic nitroxide radicals. Synthesis of stable nitroxide radicals. Acta Chem. Scand. 1971, 25,
3151–3162. [CrossRef]
32. Platonov, V.E.; Haas, A.; Schelvis, M.; Lieb, M.; Dvornikova, K.V.; Osina, O.I.; Gatilov, Yu.V. Polyfluorinated
arylnitramines. J. Fluor. Chem. 2001, 109, 131–139. [CrossRef]
33. Suga, T.; Pu, Y.-J.; Kasatori, S.; Nishide, H. Cathode- and Anode-Active Poly(nitroxylstyrene)s for Rechargeable
Batteries: P- and n-Type Redox Switching via Substituent Effects. Macromolecules 2007, 40, 3167–3173.
[CrossRef]
34. Ohshita, J.; Iida, T.; Ohta, N.; Komaguchi, K.; Shiotani, M.; Kunai, A. Synthesis of Phenylnitroxides Bridged
by an sp3-Linkage. Org. Lett. 2002, 4, 403–406. [CrossRef]
35. Rowland, R.S.; Taylor, R. Intermolecular Nonbonded Contact Distances in Organic Crystal Structures:
Comparison with Distances Expected from van der Waals Radii. J. Phys. Chem. 1996, 100, 7384–7391.
[CrossRef]
36. Caneschi, A.; Gatteschi, D.; Grand, A.; Laugier, J.; Pardi, L.; Rey, P. Moderate Ferromagnetic Exchange
between Copper(II) and a Nitronyl Nitroxide in a Square-Pyramidal Adduct. MO Interpretation of the
Mechanism of Exchange in Copper(II)–Nitroxide Complexes. Inorg. Chem. 1988, 27, 1031–1035. [CrossRef]
37. Iwahori, F.; Markosyan, A.S.; Inoue, K. Structures and Magnetic Properties of the Complexes Made up by
Cu(hfac)2 and Bisnitroxide Radical Derivatives. Mol. Cryst. Liq. Cryst. 2002, 376, 449–454. [CrossRef]
38. Ishimaru, Y.; Kitano, M.; Kumada, H.; Koga, N.; Iwamura, H. Regiospecificity in the Exchange Coupling of
the Spins of Copper(II) Ion Coordinated with the Ring Nitrogen Atoms and N-tert-Butylaminoxyl Radical
Attached as a Substituent on the Pyridine and N-Phenylimidazole Rings. Inorg. Chem. 1998, 37, 2273–2280.
[CrossRef] [PubMed]
39. Field, L.M.; Lahti, P.M.; Palacio, F.; Paduan-Filho, A. Manganese(II) and Copper(II) Hexafluoroacetylacetonate
1:1 Complexes with 5-(4-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine: Regiochemical Parity Analysis for
Exchange Behavior of Complexes between Radicals and Paramagnetic Cations. J. Am. Chem. Soc. 2003, 125,
10110–10118. [CrossRef]
40. Lanfranc de Panthou, F.; Luneau, D.; Musin, R.; Oehrstroem, L.; Grand, A.; Turek, P.; Rey, P. Spin-Transition
and Ferromagnetic Interactions in Copper(II) Complexes of 2-(3-Pyridyl)imidazole N-Oxide. Dependence of
the Magnetic Properties upon Crystal Packing. Inorg. Chem. 1996, 35, 3484–3491. [CrossRef]
41. Musin, R.N.; Schastnev, P.V.; Malinovskaya, S.A. Delocalization mechanism of ferromagnetic exchange
interactions in complexes of copper(II) with nitroxyl radicals. Inorg. Chem. 1992, 31, 4118–4121. [CrossRef]
42. Ovcharenko, V.I.; Fokin, S.V.; Romanenko, G.V.; Shvedenkov, Yu.G.; Ikorskii, V.N.; Tretyakov, E.V.;
Vasilevskii, S.F. Nonclassical Spin Transitions. J. Struct. Chem. 2002, 43, 153–167. [CrossRef]
43. Ovcharenko, V.; Bagryanskaya, E. Breathing Crystals from Copper Nitroxyl Complexes in Spin-Crossover Materials:
Properties and Applications; Halcrow, M.A., Ed.; John Wiley & Sons Ltd.: Chichester, UK, 2013; pp. 239–280.
44. Drozdyuk, I.Yu.; Tolstikov, S.E.; Tretyakov, E.V.; Veber, S.L.; Ovcharenko, V.I.; Sagdeev, R.Z.;
Bagryanskaya, E.G.; Fedin, M.V. Light-Induced Magnetostructural Anomalies in a Polymer Chain Complex
of Cu(hfac)2 with tert-Butylpyrazolylnitroxides. J. Phys. Chem. A 2013, 117, 6483–6488. [CrossRef]
45. Kaszub, W.; Marino, A.; Lorenc, M.; Collet, E.; Bagryanskaya, E.G.; Tretyakov, E.V.; Ovcharenko, V.I.;
Fedin, M.V. Ultrafast Photoswitching in a Copper-Nitroxide-Based Molecular Magnet. Angew. Chem. Int. Ed.
2014, 53, 10636–10640. [CrossRef]
46. Barskaya, I.Yu.; Tretyakov, E.V.; Sagdeev, R.Z.; Ovcharenko, V.I.; Bagryanskaya, E.G.; Maryunina, K.Y.;
Takui, T.; Sato, K.; Fedin, M.V. Photoswitching of a Thermally Unswitchable Molecular Magnet Cu(hfac)2Li-Pr
Evidenced by Steady-State and Time-Resolved Electron Paramagnetic Resonance. J. Am. Chem. Soc. 2014
136, 10132–10138. [CrossRef]
,
47. Barskaya, I.Yu.; Veber, S.L.; Fokin, S.V.; Tretyakov, E.V.; Bagryanskaya, E.G.; Ovcharenko, V.I.; Fedin, M.V.
Structural Specifics of Light-Induced Metastable States in Copper(II)-Nitroxide Molecular Magnets. Dalton
Trans. 2015, 44, 20883–20888. [CrossRef] [PubMed]
48. Wang, X.-L.; Li, Y.-X.; Yang, S.-L.; Zhang, C.-X.; Wang, Q.-L. Two copper complexes based on nitronyl
nitroxide with different halides: Structures and magnetic properties. J. Coord. Chem. 2017, 70, 487–496.
49. SADABS, v. 2008-1; Bruker AXS: Madison, WI, USA, 2008.
50. Sheldrick, G.M. A short history of SHELX. Acta Crystallogr. Sect. A 2008, 64, 112–122. [CrossRef] [PubMed]

Molecules 2019, 24, 4493
14 of 14
51. Spek, A.L. PLATON, A Multipurpose Crystallographic Tool (Version, 10M); Utrecht University: Utrecht, The
Netherlands, 2003.
52. Spek, A.L. Single-crystal structure validation with the program PLATON. J. Appl. Cryst. 2003, 36, 7–13.
[CrossRef]
Sample Availability: Samples of the compounds 3a,b, [Cu(hfac)2(3a)2], and [Cu(hfac)2(3b)2] are available from
the authors.
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).