Synthesis, crystal structure, magnetic, spectroscopic, and theoretical investigations of two new nitronyl-nitroxide complexes

Article abstract of DOI:10.1080/00958972.2021.1871900

Two mononuclear complexes [(Et₃NH)[M(hfac)₂L] (M = Ni, 1; Zn, 2) have been synthesized using a nitronyl-nitroxide radical substituted nitrophenol, that is 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, HL, as a proligand. The crystal structures of 1 and 2 have been solved and indicate an octahedral coordination geometry of the metal ions. The magnetic behavior for 1 is characterized by a strong antiferromagnetic metal–radical interaction (J = ?351 ± 1 cm^?1; H = ?JS _Ni S _Rad). This exchange interaction was rationalized by DFT calculations. The EPR spectra recorded in both solution and solid state at 120 K confirm the S = ? ground state for 1.

Full text of DOI:10.1080/00958972.2021.1871900

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis, crystal structure, magnetic,
spectroscopic, and theoretical investigations of
two new nitronyl-nitroxide complexes
Cristian Andrei Spinu, Céline Pichon, Gabriela Ionita, Teodora Mocanu,
Sergiu Calancea, Mihai Raduca, Jean-Pascal Sutter, Mihaela Hillebrand &
Marius Andruh
To cite this article: Cristian Andrei Spinu, Céline Pichon, Gabriela Ionita, Teodora Mocanu,
Sergiu Calancea, Mihai Raduca, Jean-Pascal Sutter, Mihaela Hillebrand & Marius Andruh
(2021) Synthesis, crystal structure, magnetic, spectroscopic, and theoretical investigations of
two new nitronyl-nitroxide complexes, Journal of Coordination Chemistry, 74:1-3, 279-293, DOI:
10.1080/00958972.2021.1871900
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NOS. 1–3, 279–293
https://doi.org/10.1080/00958972.2021.1871900
Synthesis, crystal structure, magnetic, spectroscopic,
and theoretical investigations of two new nitronyl-
nitroxide complexes
a
b
c
a,c
ꢀ
Cristian Andrei Spinu , Celine Pichon , Gabriela Ionita , Teodora Mocanu ,
Sergiu Calanceaa,d, Mihai Raducaa, Jean-Pascal Sutterb , Mihaela Hillebrande
and Marius Andruha
aInorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucharest, Bucharest, Romania;
b
ꢀ
Laboratoire de Chimie de Coordination du CNRS (LCC), Universite de Toulouse, CNRS, Toulouse,
c
France; “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania;
dFaculty of Chemistry and Chemical Technology, Moldova State University, Chisinau, Moldova;
ePhysical Chemistry Department, Faculty of Chemistry, University of Bucharest, Bucharest, Romania
ABSTRACT
ARTICLE HISTORY
Received 10 October 2020
Accepted 2 January 2021
Two mononuclear complexes [(Et3NH)[M(hfac)2L] (M ¼ Ni, 1; Zn, 2)
have been synthesized using a nitronyl-nitroxide radical substi-
tuted nitrophenol, that is 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-
4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, HL, as a
proligand. The crystal structures of 1 and 2 have been solved and
indicate an octahedral coordination geometry of the metal ions.
The magnetic behavior for 1 is characterized by a strong anti-
ferromagnetic metal–radical interaction (J ¼ ꢀ351 1 cmꢀ1; H ¼
ꢀJSNiSRad). This exchange interaction was rationalized by DFT cal-
culations. The EPR spectra recorded in both solution and solid
KEYWORDS
Nickel complexes; nitronyl-
nitroxide ligands; magnetic
properties; EPR spectra;
zinc complexes
state at 120K confirm the S ¼ 1= ground state for 1.
2
CONTACT Marius Andruh
Chemistry, University of Bucharest, Str. Dumbrava Rosie 23, Bucharest, 020464, Romania; Mihaela Hillebrand
mihaela.hillebrand@gmail.com Physical Chemistry Department, Faculty of Chemistry, University
of Bucharest, Bd. Elisabeta 4-12, 030018-Bucharest, Romania; Jean-Pascal Sutter sutter@lcc-toulouse.fr
marius.andruh@dnt.ro
Inorganic Chemistry Laboratory, Faculty of
ꢀ
Laboratoire de Chimie de Coordination du CNRS (LCC), Universite de Toulouse, CNRS, Toulouse, France
This article has been republished with minor changes. These changes do not impact the academic content of
the article.
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1871900
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

280
C. A. SPINU ET AL.
1. Introduction
A large family of molecular magnetic materials consists of heterospin complexes gen-
erated by d or f metal ions and paramagnetic organic radicals as ligands [1]. The most
popular radicals are the nitronyl-nitroxides, which are readily obtained, starting from
aldehydes, following or adapting Ullman’s synthetic approach [2]. The rich diversity of
nitronyl-nitroxide ligands arises from the availability of a very large number of mono-
and polyaldehydes, as well as from the possibility to decorate the nitronyl-nitroxide
platform with other coordinating groups (pre-existing in the starting aldehydes), which
play an important role on the nuclearities and spin topologies of the resulting systems
[3]. The family of magnetic materials constructed using paramagnetic organic ligands
has been recently extended with polynuclear complexes containing two different
metal ions, both paramagnetic [4]. Most of these heterotrispin complexes contain 3d
and 4f metal ions.
The exchange interactions between 2p radicals and paramagnetic metal ions vary
between large limits, from strong antiferromagnetic to strong ferromagnetic couplings
[1, 3], and usually the 2p-nd couplings are stronger than those involving lanthanides.
Magnetostructural studies highlight the factors influencing the nature and strength of
the exchange interactions [1]. The geometric parameters, at the level of aminoxyl–me-
ꢁ
tal bond, influence the orientation of the magnetic orbitals, p (radical) and d or f,
and, consequently, their overlap or orthogonality.
Surprisingly, in spite of their simplicity and popularity in coordination chemistry, o-
vanillin [5] and its substituted derivatives have rarely been employed as precursors for
nitronyl-nitroxide ligands. To the best of our knowledge, there is only one report on

JOURNAL OF COORDINATION CHEMISTRY
281
Scheme 1. Synthesis of the paramagnetic ligand.
the synthesis of the nitronyl-nitroxide obtained from o-vanillin [6], but this radical has
never been employed as a ligand. This might be related to the poor yield of the syn-
thesis of the radical derived from o-vanillin. A more convenient method consists in
employment of derivatives that contain a substituent in para position with respect to
the phenolic group, preventing the formation of by-products during the oxidation
step with sodium periodate. The nitro-derivative can be easily synthesized from o-van-
illin and nitric acid and is subsequently transformed into the desired radical, HL
(Scheme 1) [7]. In this article, we report the synthesis, crystal structure, and physical
properties of two new mononuclear compounds, which contain L- as a ligand,
(Et3NH)[M(hfac)2L] (M ¼ Ni, Zn).
2. Experimental
2.1. General procedures
Syntheses of 2,3-dimethyl-2,3-dinitrobutane and 2-hydroxy-3-methoxy-5-nitrobenzalde-
hyde were performed according to reported procedures [8, 9]. All other reagents and
solvents were commercially purchased and used without further purification, if not
stated otherwise. IR spectra were recorded on a FTIR Bruker Tensor V-37 spectropho-
tometer (KBr pellets) from 4000 to 400 cmꢀ1. UV–Vis diffuse reflectance spectra were
recorded on a JASCO V-670 spectrophotometer on undiluted samples from 200 to
1800 nm. Elemental analyses (C, H, and N) were performed on a EuroEA Elemental
Analyzer. All nuclear magnetic resonance (1H and 13C NMR) measurements were
recorded on a Bruker 500 MHz spectrometer at 25 ꢂC in CDCl3. X-ray powder diffraction
measurements were carried out on a Proto AXRD Benchtop using Cu-Ka radiation with
a wavelength of 1.54059 Å in the range 5–35ꢂ (2h). The metal ratio in the compound
[Ni0.2Zn0.8] was determined using element energy dispersive spectroscopy (EDS) (Smart
Insight AMETEK), coupled with a Nova NanoSEM 630 Scanning Electron Microscope
(FEI Company, Hillsboro, OR). The EDAX spectrum was acquired at an acceleration volt-
age of 18 kV, with a working distance of 5 mm and 30,000ꢃ magnification.
EPR spectra were recorded with a JEOL FA 100 spectrometer equipped with a VT
controller using the following setting: microwave power 1 mW, frequency 100 kHz,
sweep field 2000 G, center field 3221.8 G, sweep time 1800s, and modulation
width 2 G.
The magnetic field calibration was performed with a DPPH (diphenylpicrylhydrazyl)
standard marker, exhibiting a narrow EPR line at g ¼ 2.0036. EPR spectra of zinc and
nickel complexes and their mixture (molar ratio Ni/Zn 2:8) were recorded by filling a

282
C. A. SPINU ET AL.
Table 1. Crystallographic data, details of data collection, and structure refinement parameters for
1 and 2.
Complex
1
2
Empirical formula
Formula weight
C30H35N4NiO10F12
898.33
C30H35N4ZnO10F12
904.99
Temperature (K)
293.15
293.15
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
a (Å)
b (Å)
20.2955(15)
9.6043(6)
21.5013(15)
90
20.4327(7)
9.5457(2)
21.5624(7)
90
c (Å)
a (ꢂ)
b (ꢂ)
110.349(5)
111.461(4)
c (ꢂ)
90
3929.6(5)
4
1.518
0.607
90
3914.0(2)
4
1.536
0.741
Volume (Å3)
Z
Dc (g/cm3)
Absorption coefficient (mmꢀ1
)
F(000)
1836.04
1844.0
Ɵ range for data collection (ꢂ)
1.93 to 24.999
ꢀ24ꢄh ꢄ 24,
ꢀ11ꢄk ꢄ 11,
ꢀ25ꢄl ꢄ 25
40888
2.03 to 30.783
ꢀ25ꢄh ꢄ 26,
ꢀ10ꢄk ꢄ 13,
ꢀ26ꢄl ꢄ 27
39190
Index ranges
Reflections collected
Independent reflections [Rint
Completeness to Ɵ full (%)
Data / restraints / parameters
Goodness of fit on F2
R1, wR2 [I > 2r I]
]
6936 [Rint ¼ 0.1320]
9656 [Rint ¼ 0.0337]
100.0
100.0
6936 / 18 / 642
1.039
0.0596, 0.1108
0.1414, 0.1476
0.23/ꢀ0.34
9656 / 25 / 654
1.015
0.0378, 0.0984
0.0680, 0.1111
0.28/ꢀ0.24
R1, wR2 (all data)
Largest diff. peak/hole (Å3)
glass capillary that was placed in an EPR tube. For solution samples, the complexes or
nitronyl-nitroxide radicals were dissolved in CH2Cl2; oxygen has been removed from
solution by bubbling Ar gas. Frozen spectra were recorded at 120 K. The aN values of
nitronyl-nitroxide radical were obtained by simulation of experimental spectra using
the Winsim program.
Magnetic measurements for 1 were carried out with a Quantum Design MPMS 5S
SQUID magnetometer from 2 to 300 K. The crystalline powder of the complex was
mixed with grease in a gelatin capsule. The temperature dependence of the magnet-
ization was collected in an applied field of 1 kOe, and the isothermal field dependence
of the magnetizations was collected up to 5 T. The molar susceptibility (vM) was cor-
rected for sample holder, grease, and for the diamagnetic contribution of all the atoms
by using Pascal’s tables. Possible slow relaxation of the magnetization was examined
by AC susceptibility collected in zero field and with applied fields.
2.2. X-Ray data collection and crystal structure refinement
Crystallographic data were collected on a STOE IPDS II diffractometer for 1 and on a
Rigaku XtaLAB Synergy, single source at offset/far, HyPix diffractometer for 2 using a
graphite-monochromated Mo Ka radiation source (k ¼ 0.71073 Å). The structures were
solved by direct methods and refined by full-matrix least squares techniques based on
F2. The non-H atoms were refined with anisotropic displacement parameters.
Hydrogens were placed in fixed, idealized positions, and refined as rigidly bonded to

JOURNAL OF COORDINATION CHEMISTRY
283
the corresponding atoms. Calculations were performed using SHELXT and SHELXL-
2015 crystallographic software packages [10]. A summary of the crystallographic data
and the structure refinement is given in Tables 1 and S1. CCDC reference numbers:
2032386 (1); 2032384 (2); 2032385 (Ni0.2Zn0.8).
2.3. Synthesis of 2,3-bis(hydroxylamino)-2,3-dimethylbutan
The synthesis of this ligand was adapted from a reported procedure [11]. A solution of
HgCl2 (1.028 g, 3.78 mmol) in 50 mL of water was added over strips of aluminum foil
(1.534 g, 57.82 mmol). The mixture was stirred for 2 min after which the liquid was
removed with a pippette. The amalgamated aluminum was cooled keeping it in a bea-
ker immersed in an ice bath, and then a solution of 2,3-dimethyl-2,3-dinitrobutane
(2 g, 11.36 mmol) in 60 mL of THF and 6 mL of H2O was added sequentially. The reac-
tion was left to evolve for 1 h, vigorously stirred at low temperature, followed by filtra-
tion on a pad of Celite which was washed with another 60 mL of THF. The solvents
from the filtered solution were removed under vacuum to obtain a white precipitate.
The precipitate was dissolved in 25 mL of CHCl3 over which 25 mL of diethyl ether was
added. The solution was left overnight in a freezer to form the product as a white
crystalline powder, 0.555 g, yield 33%. The product was stored dried in a refrigerator.
Anal. Calcd for C6H16N2O2 (%): C, 48.63; H, 10.88; N, 18.90. Found: C, 48.68; H, 10.90; N,
18.96. Selected IR peaks (cmꢀ1): 3368 (s), 3287 (s), 3252 (vs), 2988 (vs), 2943 (s), 1479
(m), 1452 (m), 1425 (m), 1404 (s), 1387 (s), 1375 (s), 1358 (m), 1261 (m), 1177 (m), 1146
(s), 1080 (m), 1036 (m), 989 (w), 951 (m), 932 (w), 905 (s), 853 (w), 791 (w), 690 (m),
1
609 (w), 494 (w), 441(w). H-NMR (500.13 MHz, CDCl3, d ppm): 1.19 (s, 12 H, CH3-); 13C-
NMR (125.77 MHz, CDCl3, d ppm): 62.97 (CH3-C-CH3), 20.81(CH3-).
2.4. Synthesis of 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-
4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (HL)
The synthesis of this ligand was devised and adapted from reported procedures [2, 7].
To a solution of 2,3-bis(hydroxylamino)-2,3-dimethylbutane (1.523 g, 10.288 mmol) in
50 mL of MeOH, another solution of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde
(1.843 g, 9.353 mmol) in 75 mL of MeOH was added. The mixture was refluxed for 4 h
followed by evaporation of the solvent under vacuum. Over the yellow intermediate,
100 mL of CHCl3 was added and the solution was ice-cooled, and then, a solution of
NaIO4 (2.002 g, 9.353 mmol) in 100 mL of water was added. The obtained biphasic sys-
tem was vigorously stirred and the reaction left to evolve at 0 ꢂC for 15 min and then
at 25 ꢂC for 45 min. The organic phase was isolated using a separatory funnel and
dried with MgSO4. The solvent was evaporated under vacuum to obtain the crude
blue product which was recrystallized four times from a mixture of CH2Cl2:diethyl
ether, 1:1 volumetric ratio, to obtain the pure blue product, 1.391 g, yield 46%. Anal.
Calcd for C14H18N3O6 (%): C, 51.85; H, 5.59; N, 12.97. Found: C, 52.27; H, 5.41; N, 12.75.
Selected IR peaks (cmꢀ1): 1581 (w), 1530 (m), 1462 (m), 1429 (m), 1394 (m), 1373 (w),
1350 (vs), 1258 (m), 1201 (w), 1163 (m), 1138 (w), 1105 (w), 1069 (w), 923 (w), 885 (w),
739 (m), 457 (m). UV–Vis (nm): 362, 571.

284
C. A. SPINU ET AL.
2.5. Synthesis of (Et3NH)[NiL(hfac)2], 1
Ni(hfac)2ꢅ2H2O (0.060 g, 0.1852 mmol) was dissolved in 15 mL of heptane and refluxed
for 30 min. Then, after cooling the solution, another 15 mL of CH2Cl2 solution contain-
ing HL (0.094 g, 0.1852 mmol) and 1 drop of triethylamine was added. The solution
was refluxed another 30 min, cooled and filtered. After allowing the solvent to slowly
evaporate for three days, needle-shaped violet crystals of the product were obtained,
0.125 g, yield 75%. Anal. Calcd for C30H35F12N4O10Ni (%): C, 40.11; H, 3.93; N, 6.24.
Found: C, 39.61; H, 4.13; N, 5.87. Selected IR peaks (cmꢀ1): 1657 (s), 1645 (s), 1600 (w),
1555 (m), 1524 (s), 1502 (s), 1396 (w), 1364 (m), 1310 (s), 1256 (vs), 1234 (m), 1204 (vs)
1148 (vs), 1097 (m), 1070 (w), 870 (w), 793 (m), 673 (m), 586 (w). UV–Vis (nm): 413,
571, 751, 1142.
2.6. Synthesis of (Et3NH)[ZnL(hfac)2], 2
Zn(hfac)2ꢅ2H2O (0.047 g, 0.0926 mmol) was dissolved in 7 mL of heptane and refluxed
for 30 min. Then, after cooling the solution, another 7 mL of CH2Cl2 solution containing
HL (0.030 g, 0.0926 mmol) and 1 drop of triethylamine was added. The solution was
refluxed another 30 min, cooled and filtered. After allowing the solvent to slowly evap-
orate for three days, needle-shaped violet crystals of the product were obtained,
0.046 g, yield 55%. Anal. Calcd for C30H35F12N4O10Zn (%): C, 39.82; H, 3.90; N, 6.19.
Found: C, 40.33; H, 3.80; N, 6.06. Selected IR peaks (cmꢀ1): 1663 (m), 1649 (s), 1601
(m), 1557 (m), 1528 (s), 1504 (s), 1399 (w), 1366 (m), 1331 (m), 1310 (s), 1286 (m), 1256
(vs), 1234 (m), 1204 (s), 1144 (vs), 1096 (m), 1071 (w), 869 (w), 795 (m), 765 (w), 746
(w), 669 (m), 583 (w). UV–Vis (nm): 424, 584.
The molecular alloy, [Ni0.2Zn0.8(L(hfac)2], has been obtained by reacting the zinc and
nickel salts in 1:4 molar ratio in the same experimental conditions described above for
1 and 2. The powder X-ray diffractogram is similar to those recorded for 1 and 2
(Figure S3). The Ni:Zn molar ratio in the resulting alloy was confirmed by EDX
measurements.
2.7. Computational details
The exchange coupling interaction was calculated by the DFT method using the
Gaussian 09 program [12], considering two spin states, a low (S ¼ 1/2 resulting from
SNi ¼ 1 and SRad ¼ ꢀ1/2 spin states) and a high spin HS (S ¼ 3/2 resulting from SNi
¼
1 and SRad ¼ 1= spin states) one. The calculations were performed on the crystal
2
geometry without further optimization. The energy of the low spin state, hereafter
labeled as the broken symmetry (BS) state, was calculated by the fragment procedure
as implemented in the Gaussian09 program and further checked for its stability. The
calculations were performed using the basis set, TZVP [13], and three functionals,
uB3LYP [14], uB3PW91 [15], and uM06 [16]. The EPR spectra of the Ni and Zn com-
plexes were first calculated using the same theoretical model and the crystal geom-
etry, but considering literature data [17], we have also used a new basis set
ꢁ
recommended for EPR, TZVP/EPR-III, and a very simple one, 6-31 g . Taking into
account the results obtained by these models, the DCM solution spectra were

JOURNAL OF COORDINATION CHEMISTRY
285
Figure 1. Molecular structure of the complex anion 1, along with the atom numbering scheme.
Hydrogen and fluorine atoms have been omitted for clarity.
ꢁ
calculated using only the TZVP and the 6-31 g basis sets after the geometry optimiza-
tion in the solvent.
3. Results and discussion
3.1. Description of the crystal structures
The new compounds, (Et3NH)[M(hfac)2L] (M ¼ Ni 1, Zn 2), have been synthesized by
reacting the metal precursors, [M(hfac)2(H2O)2], with the paramagnetic proligand, HL,
in the presence of triethylamine, which was added for deprotonation of the phenolic
group. The PXRD patterns for 1 and 2 confirm the purity of the crystalline phases
(Figures S1 and S2). Since the two compounds are isostructural, only the crystal struc-
ture of 1 will be described in detail. Its crystal structure consists of anionic complex,
[Ni(hfac)2L]- (Figure 1) and organic cation, Et3NHþ. The nickel ion has an octahedral
geometry, coordinated by four oxygen atoms from the hfac- ligands and two others
from L- (one phenoxido and one aminoxyl oxygen). The Ni–O oxygen distances vary
between 2.008(4) and 2.072(4) Å. The N–O bond within the aminoxyl group coordi-
nated to NiII (1.305(5) Å) is longer than the one within the uncoordinated NO group
(1.266(5) Å). The complex is chiral and both enantiomers cocrystallize within the same
crystal (Figure 2). The crystal structure of the complex anion in 2 is illustrated in
Figure S4. Selected bond distances and angles for 1 and 2 are collected in Table 2.
The diffuse reflectance spectra of the ligand and the two complexes are displayed in

286
C. A. SPINU ET AL.
Figure 2. The packing diagram of 1 showing the closest distances between nickel metal ions and
delta (D) or lambda (K) configurations of the metal centers.
Figure 3. Diffuse reflectance spectra for HL, 1, and 2.

JOURNAL OF COORDINATION CHEMISTRY
287
Figure 4. Magnetic behavior for 1: experimental (O) and calculated (2) (a) temperature depend-
ence of the vMT and (b) magnetization vs. field.
Table 2. Selected bond distances (Å) and angles (ꢂ) for 1 and 2.
1
2
Distances
Ni1-O1
Ni1-O3
Ni1-O7
Ni1-O8
Ni1-O9
Ni1-O10
Distances
Zn1-O1
Zn1-O3
Zn1-O7
Zn1-O8
Zn1-O9
Zn1-O10
2.072(4)
2.035(3)
2.046(4)
2.029(4)
2.008(4)
2.026(4)
2.204(2)
2.035(1)
2.095(2)
2.089(2)
2.051(2)
2.090(2)
Angles
Angles
O1-Ni1-O3
O1-Ni1-O7
O1-Ni1-O8
O1-Ni1-O9
O1-Ni1-O10
O3-Ni1-O7
O3-Ni1-O8
O3-Ni1-O9
O3-Ni1-O10
O7-Ni1-O8
O7-Ni1-O9
O7-Ni1-O10
O8-Ni1-O9
O8-Ni1-O10
O9-Ni1-O10
89.53(13)
89.67(14)
173.15(16)
82.83(15)
94.70(15)
91.75(14)
97.21(15)
172.16(15)
87.92(15)
88.88(15)
90.00(15)
175.62(15)
90.47(16)
86.82(15)
90.91(15)
O1-Zn1-O3
O1-Zn1-O7
O1-Zn1-O8
O1-Zn1-O9
O1-Zn1-O10
O3-Zn1-O7
O3-Zn1-O8
O3-Zn1-O9
O3-Zn1-O10
O7-Zn1-O8
O7-Zn1-O9
O7-Zn1-O10
O8-Zn1-O9
O8-Zn1-O10
O9-Zn1-O10
85.93(5)
90.65(5)
172.31(6)
79.49(6)
95.85(6)
93.83(6)
101.23(6)
164.47(6)
87.86(6)
86.10(6)
91.67(6)
173.39(6)
93.62(7)
87.30(6)
88.34(6)
Figure 3. Compound 1 shows, apart from the bands arising from the organic
3
3
ligands, two other bands which are due to the d–d transitions: A2 ! T2 (1122 nm)
3
3
and A2 ! T1 (751 nm), assuming the Oh point group.
One single crystal from the [Ni0.2Zn0.8] sample was measured, and the structure was
refined with occupation factors 0.2 for Ni and 0.8 for Zn (Table S1), in agreement with
the EDX data. The powder X-ray diffractogram for the alloy (Figure S3) is identical with
those of 1 and 2.

288
C. A. SPINU ET AL.
Table 3. Calculated J values (cmꢀ1) for 1.
Functional
Equation (1)
uB3LYP
uB3PW91
uM06
ꢀ400.53
ꢀ398.23
ꢀ408.51
3.2. Magnetic properties of (Et3NH)[NiL(hfac)2]
The temperature dependence of the molar magnetic susceptibility, vM, for 1 has been
investigated from 2 to 300 K, and the field dependence of the magnetization was
recorded at 2 K. The vMT versus T and M versus H plots are presented in Figure 4. The
value of vMT at 300 K is 0.70 cm3molꢀ1K, much smaller than the expected
1.375 cm3molꢀ1K for the uncoupled S ¼ 1 (NiII) and S ¼ 1/2 (Rad). Moreover, reducing
the temperature, vMT decreases to 0.475 cm3molꢀ1K at 120 K and below remains con-
stant to 2 K. Such a behavior is indicative for a strong antiferromagnetic radical–Ni(II)
interaction. The plateau reached below 150 K is the signature of a spin S ¼ 1= , result-
2
ing from the antiferromagnetic interaction between the spins S ¼ 1= and 1. The result-
2
ing S ¼ 1= ground state is further confirmed by the field dependence of the
2
magnetization recorded at 2 K that tends to 1 mB at 5 T. These magnetic data have
been analyzed using the HDvV Hamiltonian: H ¼ -JSNiSRad. In order to avoid overpara-
meterization, only a mean g parameter was considered (the local g parameters were
accurately determined by EPR spectroscopy, vide infra). The best fit yielded J ¼
ꢀ351 1 cmꢀ1 and g ¼ 2.185 0.001.
The nature of the exchange interactions (antiferro- and ferromagnetic) between
nickel(II) and the aminoxyl group was subject of several magnetostructural correlation
studies [18]. We recall here the most important conclusions, resulting from both
experimental and theoretical investigations: (i) the coplanarity between the radical
ring and the equatorial plane of the octahedral NiII ion favors a ferromagnetic cou-
ꢁ
pling (the dx2-y2 and p radical orbital are orthogonal, while the overlap with the dz2
orbital is symmetry forbidden); (ii) the Ni-O-N-C dihedral angle as well as the Ni-O-N
angle play a crucial role in overlap of the magnetic orbitals; (iii) large overlaps
between the magnetic orbitals, and consequently, strong antiferromagnetic interac-
tions are favored by M-O-N-C dihedral angles close to 90ꢂ and by Ni-O-N angles close
to 120ꢂ. These geometrical parameters for 1 (Ni1-O1-N1 ¼ 126.1(3)ꢂ, Ni1-O1-N1-
C1 ¼ 58.6(6)ꢂ) suggest that the coupling between the nickel(II) ion and the radical
must be antiferromagnetic and quite strong, in line with the experimental value.
DFT calculations undertaken for 1 confirm the strong antiferromagnetic interaction
between the paramagnetic centers, the ground state of the complex being the low
spin state, that is the broken symmetry one.
Starting from the spin Hamiltonian previously mentioned, H ¼ -JSNiSRad, the J value
was obtained using Equation (1) [19]:
J ¼ ðEBSꢀEHSÞ=ð2SNiSRad þ SRad
Þ
(1)
The results presented in Table 3 are in agreement with the experimental value, J ¼
ꢀ351 1 (cmꢀ1). It can also be remarked that the three functionals used lead to quite
similar results. As shown in Figure 5(a,b), in both states, BS and HS, the spin densities

JOURNAL OF COORDINATION CHEMISTRY
289
Figure 5. Spin density (0.04) isosurfaces (0.04 e-/(a.u.)3): (a) BS state; (b) HS state.
Figure 6. The EPR spectra in DCM recorded at 293 and 120 K: (a) 1 at 293 K; (b) 1 at 120 K; (c) 2
at 293 K; (d) 2 at 120 K; (e) Ni0.2Zn0.8 at 293 K; (f) Ni02Zn0.8 at 120 K.
are localized on the nickel ion and on the NO groups of the nitronyl-nitroxide radical,
explaining the EPR spectra, vide infra.
3.3. EPR spectra
The EPR spectra of the three paramagnetic compounds, complexes 1 and 2 and the
nitronyl-nitroxide radical, were recorded at room temperature, 293 K, and 120 K. The
spectrum of HL in CH2Cl2 at room temperature is displayed in Figure S5 and presents
the known features of nitronyl-nitroxide radicals with a g factor of 2.0053 and a five-

290
C. A. SPINU ET AL.
Figure 7. Solid state EPR spectra of: (a) 1 at 293 K; (b) 1 at 120K; (c) 2 at 293 K; (d) 2 at 120 K; (e)
Ni0.2Zn0.8 at 293 K; (f) Ni0.2Zn0.8 at 120 K.
Table 4. Calculated isotropic hyperfine splitting constants (G) for the three paramagnetic species
ꢁ
optimized in DCM, using the uB3LYP functional and two basis sets, TZVP, and 6-31 g .
HL
1
2
ꢁ
ꢁ
ꢁ
6-31g
7.46
7.79
jaNj (exp)
7.40
7.55
TZVP
6-31g
TZVP
6-31g
TZVP
aN1
aN2
5.33
5.06
7.68
7.59
ꢀ4.98
ꢀ4.95
ꢀ7.22
5.12
5.23
ꢀ7.40
line spectrum due to the hyperfine splitting (hfs) of two quasi equivalent nitrogen
atoms; the values obtained by simulation are aN1 ¼ 7.40 G and aN2 ¼ 7.55 G. The EPR
spectrum recorded at 120 K indicates a restricted motion (Figure S5c).
The EPR spectra of the two complexes and their mixture, recorded in CH2Cl2, are plotted
in Figure 6. At room temperature, the spectrum of the nickel complex, 1 (Figure 6(a)),
presents only the signal of the organic radical (g¼ 2.0053); at 120 K, the spectrum consists
of two signals, a strong one characterized by g¼ 2.2751 and a very weak signal at
g¼ 2.0058; the first signal is characteristic to a paramagnetic species with the electron
localized on the nickel center [20], while the weak one is due to a tiny fraction of the
organic radical that can result from the dissociation of the complex in solution. As
expected, at both temperatures the spectrum of 2 presents only a single signal belonging
to the organic radical; the spectrum is unresolved at low temperature due to a strong
broadening of the lines, but the g value is not influenced by temperature (293 K,
g¼ 2.0055; 120 K, g¼ 2.0051). The isodensity spin surface displayed in Figure S6 reflects
this behavior, and the spin density localized only on the NO groups. The assignment of the
EPR spectra of 1 and 2 is verified by spectra e and f in Figure 6, where it can be seen that
the presence of the nickel ion determines the appearance at 120 K of the very weak signal
at g¼ 2.2761 previously assigned to the spin localization on nickel and a large signal at
g¼ 2.0056 corresponding to 2. The strong signal of nitronyl-nitroxide fragment observed at
120 K is due to excess of zinc complex in solution.

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291
The same behavior is illustrated by the EPR spectra recorded in solid state (Figure
7) for 1, 2, and the molecular alloy [Ni0.2Zn0.8]. Although the isostructural zinc deriva-
tive, 2, is not a diamagnetic host for 1 and cannot suppress all the dipolar interac-
tions, some information can be extracted by comparing these spectra. At room
temperature, 1 shows only a broad and weak feature at g ¼ 2.2889, which is due to
the S ¼ 1= state. As expected, this signal becomes stronger at 120 K, where only the
2
spin state S ¼ 1= is populated (Figure 7(b)). The spectra of the zinc derivative do not
2
change with the temperature (Figure 7(c,d)). The molecular alloy of 1 and 2 shows at
room temperature (Figure (7e)) only one signal (g ¼ 2.0061) arising from 2; at 120 K
(Figure 7(f)), two signals are observed, g ¼ 2.0101 (due to 2) and g ¼ 2.2712 (S ¼ 1=
2
ground state of 1).
The hyperfine splitting (hfs) constants for 1 and 2 were first calculated using the
same model (crystal geometry, B3LYP/TZVP) as for the magnetic data. The results listed
in Table S2 show a poor agreement with the experimental ones; it is even worse with
ꢁ
TVZP/EPR-III. A better agreement is obtained with basis set 6-31 . Therefore, all further
calculations with CH2Cl2 as a solvent were performed using the TZVP and 6-31 g sets
ꢁ
(Table 4). For the three paramagnetic species, good agreement is obtained with the 6-
31 g basis, which also have the advantage of requiring short calculation times.
ꢁ
Conclusion
We described two new metal–radical complexes using as a ligand the nitronyl-nitro-
xide radical derived from o-vanillin. The EPR spectra of the nickel complex recorded at
two different temperatures clearly show the S ¼ 1= ground state, in line with the mag-
2
netic susceptibility measurements and theoretical calculations.
Acknowledgements
We are thankful to Anamaria Hanganu for NMR spectra recording. Also, we are grateful to
ꢁ
Gabriel Craciun and Adrian Apostol for EDX measurements. SC is grateful to the University of
Bucharest (ICUB) for a postdoctoral fellowship.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Jean-Pascal Sutter
Marius Andruh
http://orcid.org/0000-0003-4960-0579
http://orcid.org/0000-0001-8224-4866
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