The first pentagonal-bipyramidal vanadium(iii) complexes with a Schiff-base N3O2pentadentate ligand: Synthesis, structure and magnetic properties

Article abstract of DOI:10.1039/d0dt03092a

A series of three mononuclear pentagonal-bipyramidal V(iii) complexes with the equatorial pentadentate N3O2 ligand (2,6-diacethylpyridinebis(benzoylhydrazone), H2DAPBH) in the different charge states (H2DAPBH0, HDAPBH1-, DAPBH2-) and various apical ligands (Cl-, CH3OH, SCN-) were synthesized and characterized structurally and magnetically: [V(H2DAPBH)Cl2]Cl·C2H5OH (1), [V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH (2) and [V(DAPBH)(CH3OH)2]Cl·CH3OH (3). All three complexes reveal paramagnetic behavior, resulting from isolated S = 1 spins with positive zero-field splitting energy expected for the high-spin ground state of the V3+ (3d2) ion in a PBP coordination. Detailed high-field EPR measurements for compound 3 show that its magnetic properties are best described by using the spin Hamiltonian with the positive ZFS energy (D = +4.1 cm-1) and pronounced dimer-like antiferromagnetic spin coupling (J = -1.1 cm-1). Theoretical analysis based on superexchange calculations reveals that the long-range spin coupling between distant V3+ ions (8.65 ?) is mediated through π-stacking contacts between the planar DAPBH2- ligands of two neighboring [V(DAPBH)(CH3OH)2]+ complexes.

Full text of DOI:10.1039/d0dt03092a

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L. Zorina, S. Simonov, V. S. Mironov, O. Maximova, L. Spillecke, C. Koo, R. Klingeler, Y. Manakin, A. N.
Vasiliev and E. B. Yagubskii, Dalton Trans., 2020, DOI: 10.1039/D0DT03092A.
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DOI: 10.1039/D0DT03092A
ARTICLE
The first pentagonal-bipyramidal vanadium (III) complexes with a
Schiff-base N3O2 pentadentate ligand: synthesis, structure and
magnetic properties
Tamara A. Bazhenova,a* Leokadiya V. Zorina,b Sergey V. Simonov,b Vladimir S. Mironov,a,c* Olga V.
Maximova,a,d,e Lena Spillecke,f* Changhyun Koo,f Rüdiger Klingeler,f,g Yuriy V. Manakin,a Alexander
N. Vasiliev d,h and Eduard B. Yagubskii a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of three mononuclear pentagonal-bipyramidal V(III) complexes with the equatorial pentadentate N3O2 ligand (2,6-
diacethylpyridinebis(benzoylhydrazone), H2DAPBH) in the different charge states (H2DAPBH0, HDAPBH1-, DAPBH2-) and
various apical ligands (Cl-, CH3OH, SCN-) was synthesized and characterized structurally and magnetically:
[V(H2DAPBH)Cl2]Cl·C2H5OH (1), [V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH (2) and [V(DAPBH)(CH3OH)2]Cl·CH3OH (3). All three
complexes reveal paramagnetic behavior, resulting from isolated S = 1 spins with positive zero-field splitting energy expected
for the high-spin ground state of V3+ (3d2) ion in a PBP coordination. Detailed high-field EPR measurements for compound 3
show that its magnetic properties are best described by the spin Hamiltonian with the positive ZFS energy (D = +4.1 cm-1)
and pronounced dimer-like antiferromagnetic spin coupling (J = -1.1 cm-1). Theoretical analysis based on superexchange
calculations reveals that the long-range spin coupling between distant V3+ ions (8.65 Å) is mediated through π-stacking
contacts between the planar DAPBH2- ligands of two neighboring [V(DAPBH)(CH3OH)2]+ complexes.
apical ligands can easily be substituted by various bridging units.
It is also essential that, unlike the closed-ring macrocyclic
ligands (of N3O2 and N5 types)4, the open acyclic ligands (N3O2)
can occur in three charge states, neutral (protonated, A),
monoanionic (monodeprotonated, C) and dianionic
(dideprotonated, B), Fig. S1, ESI†, thereby extending the variety
of metal complexes.
Introduction
In recent years, 3d and 4f complexes with the 2,6-
diacetylpyridine-based acyclic ligands (see Fig. S1, ESI†) have
attracted growing interest as advanced molecular magnetic
materials.1,2 In particular, some of these complexes are field-
induced single-ion magnets (SIMs)1; they are also used as
anisotropic spin units in designing polynuclear single-molecule
magnets (SMMs) and single-chain magnets (SCMs).1a,2 The
complexes of 3d metals with these ligands frequently adopt a
pentagonal-bipyramidal (PBP, D5h-symmetry) geometry around
the central metal atom resulting from the pentagonal
coordination (N3O2) of almost planar pentadentate ligands in
the equatorial plane and two apical ligands (usually solvent
The interest in magnetic metal complexes with the 2,6-
diacetylpyridine-based acyclic ligands is largely caused by their
PBP geometry that in many cases produces the ground spin
state with a significant magnetic anisotropy; the latter is
currently recognized as the most critical factor in the
development of high-performance SMMs.5 Particularly, the PBP
coordination provides strong uniaxial magnetic anisotropy for
metal ions with enhanced spin-orbit coupling (4d, 5d and 4f
elements).6 In this context, the planar pentadentate N3O2 Schiff-
base ligands implement the so-called ligand approach toward
high magnetic anisotropy by means of optimizing the local
atomic coordination of the magnetic ion. Experimental and
theoretical studies show that many seven-coordinate metal
centers with PBP geometry are promising anisotropic spin
carriers.1a,2a,4b,7 Considerable efforts in this field in the last years
have resulted in numerous 3d, 4d, 5d and 4f PBP complexes,
molecules and/or Cl-, NO3 , N3 , SCN- anions).3 These PBP
complexes are attractive building blocks for assembling
polynuclear magnetic structures, especially since their labile
-
-
a. Institute of Problems of Chemical Physics, RAS, Chernogolovka 142432, Russia.
E-mails: bazhen@cat.icp.ac.ru, yagubski@icp.ac.ru
b. Institute of Solid State Physics, RAS, Chernogolovka 142432, Russia.
c. Shubnikov Institute of Crystallography of Federal Scientific Research Centre
“Crystallography and Photonics“, RAS, Moscow, Russia. E-mail: mirsa@list.ru
d. Lomonosov Moscow State University, Moscow 119991, Russia.
e. National University of Science and Technology “MISiS”, Moscow 119049, Russia.
f. Kirchhoff Institute for Physics, Heidelberg University, 69120 Heidelberg, Germany.
E-mail: lena.spillecke@kip.uni-heidelberg.de
1d,e,2c,3b,8
among which of special note are novel 4d and 5d
seven-coordinate PBP complexes, the first Mo(III) complex with
an open acyclic ligand, [Mo(DAPBH)Cl2]-,8b and a heptacyanide
W(IV) complex, [W(CN)7]3-.8c The seven-coordinate complexes
with the 2,6-diacetylpyridine-based open acyclic ligands (N3O2)
are currently synthesized for all 3d metals,3 except vanadium.
The latter fact presents a clear gap in this research field because
g. Centre for Advanced Materials (CAM), Heidelberg University, 69120 Heidelberg,
Germany.
h. National Research South Ural State University, Chelyabinsk 454080, Russia.
Electronic Supplementary Information (ESI) available: Figures S1-S21 and Tables S1-
S10; CCDC 2015517-2015519. See DOI: 10.1039/x0xx00000x
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vanadium complexes with these ligands are of interest as [V(H2DAPBH)Cl2]Cl·C2H5OH (1) To a suspension Voiefw AHrt2icDleAOPnBlinHe
DOI: 10.1039/D0DT03092A
efficient magnetic building blocks providing strong spin coupling (150 mg, 0.38 mmol) in absolute CH3CN (10 ml) solid VCl3·6H2O
with other spin carriers due to diffuse 3d magnetic orbitals (100 mg, 0.38 mmol) was added while stirring. The resulting
inherent to early transition metal ions.9 Thus, Prussian blue brown mixture was stirred and heated at 70 °C for 1 hour. A gray
analogues based on V2+ and Cr3+ ions reveal bulk magnetic precipitate was formed during the first 15 minutes of heating,
ordering above the room temperature.10 Besides, vanadium which subsequently completely dissolved. Instead, after about
complexes can provide significant magnetic anisotropy 30 minutes of heating, an abundant green-brown precipitate
associated with large zero-field splitting (ZFS) of the ground spin began to form. After cooling the reaction mixture, the
state with axial components |D| up to 20 cm–1.11
precipitate was filtered and dissolved in a mixture of CH3OH /
Given these considerations, we attempted to obtain PBP CH3CN = 1. The resulting solution was filtered, C2H5OH (10% by
vanadium complexes with pentadentate N3O2 Schiff base volume) was added to it and left to crystallize at room
ligands. Herein, we report on synthesis of the first seven- temperature. Two days later, the formation of dark crystals
coordinate V(III) trans-complexes with pentadentate (N3O2) 2,6- suitable for X-ray diffraction analysis was observed. Yield: 103
diacethylpyridine-bis(benzoyl hydrazone) ligand in the various mg, 45%. Found: C, 49.5; H, 4.5; N, 11.6. Calc. for
charge
[V(H2DAPBH)Cl2]Cl·C2H5OH
[V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH
state
(H2DAPBH0,
HDAPBH1-,
DAPBH2-): VC25H27N5O3Cl3: C, 49.8; H, 4.5; N, 11.6%. FT-IR νmax/cm-1:
(1), 3409m, 3370m, 1599vs, 1565vs, 1487vs, 1435m, 1322vs, 1276s,
and 1178s, 1078m, 1024s, 714vs, 684s.
(2)
[V(DAPBH)(CH3OH)2]Cl·CH3OH (3). We describe the crystal [V(HDAPBH)(NCS)2]·0.5CH3OH·0.5CH3CN (2)
A solution of
structures and dc magnetic properties of 1-3; for complex 3 we NH4SCN (40 mg, 0.5 mmol) in 5 ml of CH3OH was added to a
also present results of high-frequency and high-field electron solution of [V(H2DAPBH)Cl2]Cl·C2H5OH (60 mg, 0.1 mmol) in 7 ml
paramagnetic resonance (HF-EPR) study. The obtained of CH3OH/CH3CN mixture while stirring. The resulting reaction
experimental results are supplemented with theoretical mixture continued to stir for 1 hour at r.t. and then was filtered.
analysis, which indicates that magnetic behavior of 1-3 cannot Black block crystals suitable for single-crystal X-ray diffraction
be described in terms of the single-ion ZFS model due to the were formed after 2 days at room temperature. The mother
presence of noticeable spin coupling between V(III)-centers. liquor was decanted and the crystals were washed with diethyl
The latter fact is rather surprising, considering isolated ether and dried under vacuum. Yield: 32 mg, 53% based on the
character of [V(DAPBH)X2] units and large spacing (> 7.5 Å) [V(H2DAPBH)Cl2]Cl·C2H5OH complex. Found: C, 52.6; H, 4.25; N,
between V3+ ions.
17.1; S, 10.35. Calc. for V2C53H47N15O5S4: C, 52.9; H, 3.9; N, 17.5;
S, 10.6%. FT-IR νmax/cm-1: 2065vs (NCS), 1609s, 1572m, 1543m,
1502s, 1389s, 1160m, 709s, 689s.
Experimental
Materials and methods
[V(DAPBH)(CH3OH)2]Cl·CH3OH (3) To a suspension of H2DAPBH
(164 mg, 0.41 mmol) in absolute methanol (10 ml) solid VCl2 (50
mg, 0.41 mmol) was added under Ar atmosphere while stirring.
The resulting brown mixture was stirred and heated at 50 °C for
1 hour. The ligand was slowly dissolving during this time. The
reaction mixture was cooled to room temperature and then
filtered. After 2 days of storage of the solution at -18 ° C, the
formation of dark brown crystals suitable for X-ray diffraction
analysis was observed. The substance was collected by
filtration, washed with diethyl ether, and dried under vacuum.
Yield: 129 mg, 56%. Found: C, 53.95; H, 5.5; N, 12.0. Calc. for
VC26H31N5O5Cl: C, 53.8; H, 5.35; N, 12.1%. FT-IR νmax/cm-1:
2976w, 2756s, 1588m, 1554m, 1512s, 1386vs, 1160s, 1054s,
1005s, 703s, 685s.
2,6-diacetylpyridine, benzoylhydrazine, VCl2 (anhydrous),
VСl3·6H2O, NH4CNS, were purchased from commercial sources
and used without further purification. Methanol was dried upon
refluxing with magnesium methoxide followed by distillation.
Acetonitrile were refluxed over calcium hydride and then
distilled off. All the solvents were purged with argon and stored
over molecular sieves 3 Å prior to use. All manipulations
involving air-sensitive materials (VCl2) were carried out under
argon atmosphere using the standard Schlenk-line method.
Syntheses using V+3 compounds as starting materials were
performed under aerobic conditions. Elemental analyses were
carried out by the Analytical Department service at the Institute
of Problems of Chemical Physics RAS using a vario MICRO cube
(Elementar Analysensysteme GmbH) equipment.
X-ray data collection, structure solution and refinement.
X-ray single crystal diffraction data were collected at 150 K on
an Oxford Diffraction Gemini-R CCD diffractometer equipped
with an Oxford cryostream cooler [λ(MoKα) = 0.71073 Å,
graphite monochromator, ω-scans]. Data reduction with
empirical absorption correction of experimental intensities
(Scale3AbsPack program) was made with the CrysAlisPro
software13.
Synthetic procedures
H2DAPBH (H2L) ligand was prepared in a ketone-hydrazine
condensation reaction between one equivalent 2,6-
diacetylpyridine and two equivalents of benzoylhydrazine in
96% ethanol according to previously reported procedure.12 The
yield is more than 90%. Found: C, 69.25; H, 5.35; N, 17.5. Calc.
for C23H21N5O2: C, 69.2; H, 5.3; N, 17.5%. FT-IR νmax/cm-1: 3277m,
1660vs, 1603m, 1580m, 1524vs, 1491m, 1450s, 1372m, 1342m,
1271vs, 1188m, 1160m, 1077m, 917s, 812s, 708vs.
The structures were solved by a direct method and refined by a
full-matrix lest squares method using the SHELX-2016
2 | J. Name., 2012, 00, 1-3
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Table 1. Crystal data and structural refinement parameters for the complexes 1-3.
1
2
C26.5H23.5N7.5O2.5S2V
602.09
orthorhombic
P212121, 8
150(1)
3
Chemical formula
Formula weight
Cell setting
Space group, Z
Temperature (K)
a (Å)
C25H27Cl3N5O3V
602.80
orthorhombic
P212121, 4
150(1)
C26H31ClN5O5V
579.95
monoclinic
P21/c, 4
150(1)
7.29023(8)
15.3895(2)
22.8617(3)
90
13.9738(2)
14.3549(2)
27.3124(4)
90
19.5934(3)
7.49770(8)
20.3843(3)
90
112.382(2)
90
b (Å)
c (Å)
α (o)
β (o)
90
90
90
90
γ (o)
Cell volume (Å3)
 (g/cm3)
2564.92(5)
1.561
5478.66(14)
1.460
2768.98(7)
1.391
μ, cm-1
7.38
5.57
4.99
Crystal size, mm3
Refls collected/ unique
Rint
0.12×0.29×0.71
21049 / 8631
0.0170
0.06×0.08×0.53
30256 / 15367
0.0173
0.13×0.23×0.27
27429 / 8079
0.0149

max (o)
31.00
29.50
29.00
Parameters refined
Final R1, wR2 [I>2σ(I)]
Goodness-of-fit
340
0.0301, 0.0819
1.002
785
0.0370, 0.0811
1.001
357
0.0307, 0.0789
1.001
CCDC number
2015517
2015518
2015519
program.14 All non-hydrogen atoms were refined were placed inside a brass ring in a home-made transmission
anisotropically. The H-atoms were refined in a riding model with type EPR probe with no extra glue or grease to align the
isotropic displacement parameters depending on Ueq of crystallites with respect to the external magnetic field direction.
connected atom. The N-H and O-H bond distances were refined, In order to ensure alignment of the loose powder, the maximum
with additional geometrical restraints (SADI / DFIX) applied in magnetic field of 16 T was applied prior to the measurements
some cases.
and alignment was monitored by corresponding orientation
Main crystal and experimental data for 1-3 are listed in Table 1. jumps in the transmitted microwave signal. Rearrangement of
the crystallites to a non-aligned situation was avoided by
Physical Measurements
restricting the magnetic field to a range between 0.2 T and 16
T. For fixed powder measurements, the material was
thoroughly grinded and then fixed with eicosane. Analysis and
simulation of the HF-EPR data as well as the low-temperature
magnetization data was done by means of the EasySpin
software package16. Ab initio density functional theory (DFT)
calculations of the g-tensor anisotropy where performed using
a spin unrestricted self-consistent field method (SCF)17 with
B3LYP hybrid functionals and a SVP18 basis set for all electrons
as implemented in the ORCA software package.19,20
Ligand-field and spin coupling calculations for V(III)
complexes. The electronic structure of V(III) complexes 1-3 and
their ZFS characteristics were analyzed using the conventional
ligand-field (LF) Hamiltonian (Eq. S1 ESI†) in combination with
the angular-overlap model (AOM),21 as described in ESI†. The
spin coupling between two neighboring [VIII(DAPBH)(CH3OH)2]
in 3 units was explored in terms of the many-electron
superexchange theory22; the computational details are outlined
in ESI†.
The IR spectra were measured on solid samples using a Perkin
Elmer Spectrum 100 Fourier Transform infrared spectrometer
in the range of 4000–500 cm-1.
The measurements of temperature and magnetic field
dependence of magnetization were performed using the
vibrating sample magnetometer (VSM) option of the “Quantum
Design” Physical Properties Measurements System PPMS – 9T.
The data were corrected for diamagnetic contribution
calculated as a sum of Pascal constants. Direct current (dc)
magnetization measurements down to 400 mK were performed
by the means of a commercial MPMS3 SQUID magnetometer
from Quantum Design equipped with a 3He insert
High-frequency/high-field electron paramagnetic resonance
(HF-EPR) measurements were performed by means of a
commercial millimeterwave vector network analyzer (MVNA)
from ABmm as phase sensitive microwave source and
detector.15 The spectra were obtained in the frequency range
between 50 GHz and 500 GHz in external magnetic fields up to
16 T. The temperature control between 2 K and 160 K was
ensured by a variable temperature insert (VTI) with 4He gas flow
cryostat from Oxford Instruments. The loose powder samples
This journal is © The Royal Society of Chemistry 20xx
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Figure 1. Molecular structures of 1 (a), 2 (b) and 3 (c). Counter anions, lattice solvents and non-acidic H atoms are omitted for clarity.
already noted, the formation of trivalent vanadium is associated
with the oxidation of the initial V2+, a strong reducing agent,
with the ligand — the Schiff base H2DAPBH, containing the
unsaturated C=N bonds. Complexes 1-3 are stable in air at room
temperature both in the solid state and in solutions.
Results and discussion
Synthetic considerations
Complexes 1-3 were obtained following the synthetic
procedure previously used to obtain pentagonal-bipyramidal
complexes of other 3d metals.3
Given the wide range of accessible oxidation states for
vanadium, we were interested in preparation of seven-
coordinate complexes of vanadium (II) and (III). Therefore, the
simple salts of V+2 and V+3, anhydrous VCl2, and hexahydrate
VCl3 6H2O were used as the starting compounds.
We note here that regardless of the oxidation state (+2 or +3) in
the initial vanadium salt used for the synthesis, it was not
possible to obtain the seven-coordinate vanadium (II) complex
by the direct reaction. Attempts with some additional
precautions (namely, the reaction in aprotic solvents and
oxygen-free conditions, the reaction in the presence of a
dithionite reducing agent) failed to produce the V+2 complex. A
complex of trivalent vanadium was always formed. Apparently,
this is due to the presence of non-innocent C=N bonds capable
of oxidizing V+2 to V+3.
The interaction of VCl3·6H2O with the H2DAPBH ligand leads to
the formation of [V(H2DAPBH)Cl2]Cl·C2H5OH (1). The compound
contains a neutral unaltered protonated equatorial ligand, Cl–
ions in axial positions (Fig. 1a), and a chlorine anion in the
external coordination sphere. The Cl– ions in the axial positions
can be easily replaced, for example, with NCS- groups by
treatment of [V(H2DAPBH)Cl2]Cl·C2H5OH 1 complex solution
with excess of NH4NCS. In this case, the neutral complex
[V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH (2) is formed. The
complex contains the monodeprotonated, mononegative
ligand HDAPBH– in the equatorial, and two NCS- anions in the
apical positions (Fig. 1b).
When using the divalent vanadium salt (VCl2) as a starting
compound in methanol under inert atmosphere, the interaction
with the H2DAPBH ligand also leads to a V(III) complex,
[V(DAPBH)(CH3OH)2]Cl·CH3OH (3). In this complex, the
equatorial ligand is completely deprotonated, methyl alcohol
molecules occupy the axial positions (Fig. 1c), and the chlorine
anion is located in the outer coordination sphere. As we have
Description of Crystal Structures of 1-3.
The complex 1 crystallizes in the orthorhombic space group
P212121. Asymmetric unit includes [V(H2DAPBH)Cl2]+ cation, Cl−
anion and ethanol solvent molecule, all in general positions. An
ORTEP drawing of 1 is shown in Fig. S2. The complex 2
crystallizes in the orthorhombic space group P212121 as the
complex 1 but these two compounds are not isostructural (see
Table 1). There are two [V(HDAPBH)(NCS)2] neutral molecules,
one acetonitrile molecule and one methanol molecule in the
asymmetric unit, all in general positions (Fig. S4). One NCS-
ligand and both solvent molecules are statistically disordered
between two sites with the same 0.7/0.3 occupancy ratio for
NCS- and CH3OH and 0.5/0.5 occupancy ratio for CH3CN. The
complex 3 crystallizes in the monoclinic space group P21/c.
Asymmetric unit includes [V(DAPBH)(CH3OH)2]+ cation, Cl− anion
and methanol solvent molecule, all in general positions (Fig. S6).
Key bond distances and angles in the structures 1, 2 and 3 are
listed in Tables S1, S3 and S5, respectively.
The compounds 1-3 contain vanadium (III) complexes with
pentadentate ligands of various degree of protonation and
charge: [H2DAPBH]0 in 1, [HDAPBH]ˉ in 2 and [DAPBH]2- in 3 (Fig.
1). The local coordination geometry around the V(III) center
corresponds to distorted pentagonal bipyramid. V(III) is
surrounded by three N and two O atoms of pentadentate ligand
in the equatorial plane. Two axial coordination sites are
occupied by Clˉ anions in the structure 1, N atoms of NCSˉ
anions in 2 and O atoms of neutral methanol molecules in 3. The
structural analysis reveals different degree of planarity of the
pentadentate ligands in 1-3. The maximal deviation from the
planarity is observed for the fully protonated ligand [H2DAPBH]0
in 1 whereas the doubly deprotonated ligand [DAPBH]2- in 3 is
nearly planar. The dihedral angle between two semi-carbazone
planes of the ligand defined by 7 non-metallic atoms of two
pentagonal cycles (for example, O1, C7, N1, N2, C9, C10, N3 and
4 | J. Name., 2012, 00, 1-3
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Table 3. Bond lengths (Å) in the deprotonated VN2CO cycles of V(III) complexes 2 and 3
View Article Online
DOI: 10.1039/D0DT03092A
with HDAPBHˉ and DAPBH2- pentadentate ligands, respectively.
2, V(1)
center
0
2, V(2)
center
0
3
3
Complex
charge
Ligand
charge
V-N
V-O
C-N
C-O
1+
2-
1+
2-
1-
1-
2.123(2)
1.993(2)
1.316(3)
1.282(3)
2.114(2)
2.008(2)
1.316(3)
1.282(3)
2.1524(9)
2.0109(8)
1.3212(14) 1.3147(15)
1.2889(13) 1.2892(14)
2.1473(10)
2.0193(8)
Figure 2. Comparison of bond length values (Å) in the VN2CO and VN2HCO cycles of the
V complex with [HDAPBH]ˉ ligand in 2.
O2, C17, N5, N4, C15, C14, N3 in Fig. 2) is 13.93(5)° in 1, 7.90(8)
and 11.00(8)° in 2 and only 3.76(3)° in 3. Side view of the
complexes demonstrate noticeable deviation of phenyl and
pyridine cycles from the central VN3O2 plane for [H2DAPBH]0
ligand in 1 (Fig. 3, left) in contrast to near ideal planarity of
[DAPBH]2- ligand in 3 (Fig. 3, right).
Deprotonation is followed by noticeable changes in the
geometry of the VN2CO/VN2HCO cyclic groups involved in the
central chelating node of the pentadentate ligand. The bond
lengths values in 1-3 in the protonated VN2HCO and
deprotonated VN2CO cycles are compared in Tables 2 and 3.
In order to compare the degree of deviation from the ideal
pentagonal bipyramidal coordination of the metal centers in a
more unified way, we performed calculations of continuous
shape measures by using SHAPE (2.1) program.23 The
calculation results listed in the Table S7 show the minimum
deviation for compound 3. The analysis confirms also small but
meaningful variations for two symmetry-independent
molecules in the unit cell of 2.
Table 2. Bond lengths (Å) in the protonated VN2HCO cycles of V(III) complexes 1 and 2
with H2DAPBH0 and HDAPBHˉ pentadentate ligands, respectively.
1
1+
0
1
1+
0
2, V(1) center 2, V(2) center
Complex charge
Ligand charge
0
0
1-
1-
V-N
V-O
C-N
C-O
2.140(2) 2.153(2)
2.059(2) 2.061(2)
1.352(3) 1.348(3)
1.249(2) 1.249(2)
2.166(2)
2.067(2)
1.350(4)
1.239(3)
2.173(2)
2.114(2)
1.353(3)
1.247(3)
In the deprotonated VN2CO cycle, the V-N, V-O, С-N bonds are
shorter while the C-O bonds are longer as compared with the
similar bonds in the protonated VN2HCO cycle. It is illustrated in
Fig.
2 for one of two independent [V(HDAPBH)(NCS)2]
complexes in the structure 2.
Figure 3. Side view of the V(III) complexes in 1 (left) and 3 (right). Dihedral angles
between pyridine and phenyl planes and central VN3O2 plane in 1 are shown.
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DOI: 10.1039/D0DT03092A
Figure 4. The ab layers in the structures 1 (a) and 3 (b) with hydrogen bonding and π-stacking interactions. Hydrogen bonds from acidic H-atoms (red dashed lines, H-bond geometry
is given in Tables S2 and S6), C…C contacts < 3.6 Å (black dotted lines), V…V distances (green dashed lines, a: 1 = 7.2902(5), 2 = 8.1084(5), 3 = 9.1259(5) Å, b: 1 = 7.4977(2), 2 = 8.6545(6)
Å) are shown.
The crystal packing of 1-3 is determined by hydrogen bonding. Interestingly, DFT calculations indicate that the non-planar
The NH-groups of pentadentate ligands are locked on the Clˉ structure of mono- and doubly protonated HxDAPBH2-x ligands
anion and O atom of the ethanol solvent in 1, S atom of the SCNˉ (x = 1,2) is mainly due to rotations of the phenyl rings caused by
ligand and N atom of the acetonitrile solvent in 2. In 3, in the repulsion between the proton of the NH group and the nearest
absence of NH proton, OH…Cl hydrogen bonds are formed proton of the phenyl group (separated by the distance of ≈2.2
between the axial methanol ligands and the Clˉ anions (see Figs. Å, Figs. S8, S9, ESI†). In the absence of this repulsion in fully
S3, S5, S7 and Tables S2, S4, S6, ESI† for hydrogen bond deprotonated ligand DAPBH2-, DFT calculations result in a
geometry). Besides, a π-stacking between the pentadentate perfectly planar structure of 3 with two coplanar phenyl rings
ligands of adjacent complexes is found in all three structures. It Fig. S10, ESI†), in agreement with the X-ray structural data (Fig.
is especially strong in 1 where V-complexes form layers parallel 3, right).
to ab plane (Fig. 4a). The nearest V…V distance is 7.2902(5) Å
along a between the cations connected by hydrogen bonds N- DC Magnetic Susceptibility Studies.
HH2DAPBH…O-HEtOH…Clligand through the ethanol bridge
(interaction 1 in Fig. 4a). Average planes of H2DAPBH ligands are
not parallel within the layer: the corresponding dihedral angle
between the VN3O2 planes is 29.76(3) °. However, phenyl and
The magnetic susceptibility was measured on polycrystalline
samples of complexes 1-3 in the temperature range 2.0−300 K
under the applied magnetic field of 0.1 T; the χT vs. T plots are
shown in Fig. 5a-c. The χT value at room temperature
pyridine cycles are not in the central VN2O2 plane (Fig. 3, left)
that allows them to pack near parallel to each other along a and
form a lot of shortened C…C contacts less than the sum of van
der Waals radii (3.6 Å) which are shown in Fig. 4a by thin dotted
lines. The V…V distances are 8.1084(5) and 9.1259(5) Å for the
π-stacking interactions 2 and 3, respectively.
In the structure 3, the V(III) complexes also form layers parallel
to ab plane (Fig. 4b). The doubly deprotonated DAPBH ligands
are essentially flat (Fig. 3, right) and parallel within the layer.
The nearest V…V distance in 3 is 7.4977(2) Å along b in the
infinite chains of the V(III) complexes interacting via O-
Hligand…Clanion…H-Oligand hydrogen bonds (interaction 1). The
adjacent chains along a are paired through a π-stacking of the
ligands (interaction 2), V…V distance is 8.6545(6) Å.
Intermolecular interactions in 2 are described in ESI† (Fig. S5).
In order to rationalize the molecular structure of 1-3, we
performed density functional theory (DFT) calculations for
isolated complexes using the ORCA 3.0.3 suite of programs and
employing
the
spin-unrestricted
method
at
the
B3LYP/SVP(TzVPP) level of theory.19,20 The DFT-optimized
geometry of 1-3 is shown in Figs. S8-S10, ESI†; these results well
reproduce the aforementioned structural features of 1-3.
Figure 5. (a)-(c) χT versus T at 0.1 T for complexes 1-3, experimental (open circles) and
calculated with the ZFS model (solid red lines), (d) Mmol versus H at 2 K and 4 K for 2.
The solid lines are guides for the eye. The fitted Mmol(H) curves for 3 are shown in Fig.
S14 ESI
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approaches to 1.0 cm3mol-1K, which is expected for V(III) with S
= 1 and g = 2. The χT product gradually decreases upon cooling
to ~30 K, and then it rapidly drops to ca. 0.35 cm3mol-1K at low
temperature (Fig. 5a-c).
View Article Online
DOI: 10.1039/D0DT03092A
At first glance, it is natural to attribute such magnetic behavior
to a free or weakly coupled S = 1 spin system with a positive
zero-field splitting (ZFS) parameter D, considering that large
spacing between magnetically isolated V(III) ions encapsulated
in bulk [V(HDAPBH)X2] complexes should prevent magnetic
coupling in 1-3. Following this idea, we performed primary
analysis of magnetic susceptibility χT of 1-3 in terms of the
single-ion ZFS spin Hamiltonian (SH):
Figure 6. HF-EPR spectra (black lines) of aligned loose powder (a) and fixed powder (b) of
complex 3 measured at T = 2 K and a fixed frequency of 298.4 GHz and 301.2 GHz,
respectively. The black triangle indicates an extra feature as discussed in the text. Red
lines correspond to a simulation using the SH (Eq. 1) with the parameters listed in Table
4.
H  gB BS  D

Sz2  S(S 1) /3  E(Sx2  Sy2 )  HTIP

(1)
where D and E are, respectively, the axial and rhombic ZFS
parameters, μB is the Bohr magneton, g is the Landé factor of
V(III), B is the magnetic induction and HTIP is the second-order
paramagnetic temperature-independent term. The best
agreements with the experimental data are obtained at D =
+11.0, E = 1.7 cm–1, g = 1.965, χTIP = 1.16×10–4 cm3mol-1 for 1, D
= +8.9, E = 0.2 cm–1, g = 1.913, χTIP = 1.12×10–4 cm3mol-1 for 2
and D = +10.1, E = 0.4 cm–1, g = 1.940, χTIP = 1.12×10–4 cm3mol-1
for 3; the simulated χT curves are shown in solid red lines in Fig.
5a-c. It is also noteworthy that the magnitude of calculated ZFS
parameter D in 1-3 is reasonably consistent with the D values
observed in many other V(III) complexes.11 These calculations
indicate that the ground state of vanadium(III) PBP complexes
1-3 is a spin-only triplet spin state (S = 1) with two unpaired
electrons on the two lowest 3dxz and 3dyz orbitals (Tables S6 and
S7 and Fig. S11, ESI†). The presence of considerable ZFS of the
resolvable features as can be seen in Fig. 6. The obvious
differences between the shown spectra measured at
approximately same frequencies on the loose and fixed powder
sample proves proper alignment of the loose powder in the
external magnetic field. Both spectra exhibit some mixing of
phase and amplitude information, which is due to the applied
measurement technique and was included as a parameter in
the corresponding simulations displayed as red lines,
respectively.
The temperature dependence of the loose powder spectra of 3
at a constant frequency of 396.2 GHz (Fig. 7, left) implies that
the pronounced low-temperature feature is associated to a
ground state transition. In addition, while at T = 2 K only one
main feature is visible, an excited feature appears on the high
field site of the main one with increasing temperatures.
ground spin state S = 1 with positive
D is evidenced
experimentally by the fact that at low temperatures
magnetization Mmol does not saturate even in a strong magnetic
field (50 kOe, Fig. 5d).
However, although these preliminary single-ion simulations are
well consistent with the experiment (Fig. 5), further analysis
indicates a more complicated nature of magnetism in 1-3, which
is beyond the pure single-ion ZFS model. Namely, below we
show that high-frequency EPR measurements for complex 3
reveal a significantly lower ZFS parameter (D ≈ +4.5 cm-1) and,
rather unexpectedly, the presence of a noticeable dimer-like
antiferromagnetic (AF) spin coupling between distant V(III) ions.
The origin of these findings is well explained by our theoretical
analysis based on superexchange calculations and ligand-field
calculations (see the next Sections below).
HF-EPR spectroscopy
In order to obtain a straightforward information on the ZFS
parameters and to elucidate the basic magnetic interactions
underlying magnetic behavior of V(III) PBP complexes 1-3, we
have undertaken
a
high-frequency/high-field electron Figure 7. (Left) HF-EPR spectra for an aligned loose powder sample of 3 at f = 396.2
GHz at various temperatures between 2 and 160 K. The data are vertically shifted
paramagnetic resonance (HF-EPR) study for compound 3. The
HF-EPR measurements were performed on loose as well as on
fixed powder samples of complex 3. For both preparation
techniques the measured resonance spectra exhibit clearly
for clarity. A weak feature arising from a forbidden transition (see the text) at T =
2 K and an excited one are indicated by filled and open triangles, respectively.
(Right) Corresponding simulation using the SH in Eq. 1 and the parameters listed
in Table 4.
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combined with the angular overlap model21 (VsieeweArEtiSclIe†Onflioner
DOI: 10.1039/D0DT03092A
computational details), which evidence uniaxial character of the
ZFS tensor, such as D ≈ +4, E = 0.035-0.09 cm–1 (Table S10,
ESI†).Employing this model allows to simulate the resonance
branches shown in Fig. 8 and Fig. S12 as well as the spectra
shown in Figs. 6 and 7. The results show that the experimental
data on both loose and fixed powder samples are well described
by the simulation using the SH in Eq.2. The resulting SH
parameters are listed in Table 4.
Our analysis shows that powder spectra are well described by
assuming well aligned molecules with g Bext. Associating the
loose powder spectra to the response of aligned molecules is
Table 4. SH parameters obtained from simulations of the resonance frequency vs. field
dependence of the loose and fixed powder sample shown in Fig. 8 and S12. These
parameters were used to simulate the spectra shown in Figs. 6 and 7. The third column
shows calculated g-values.
Figure 8. Magnetic field dependence of the resonance frequencies for a loose powder
sample of 3. Black squares correspond to measurements at T = 2 K and blue dots
correspond to thermally activated features at T  10 K. Red lines show a simulation of
the resonance branches using the SH in Eq. 1 with the parameters listed in Table 4. For
the sake of simplicity, only the simulated branches which correspond to observed
transitions are shown. Open small symbols show weak features showing up in the
spectra obtained at T = 2 K which are arising from forbidden or excited transitions.
SH
parameter
Simulation results
Calculated
parameters(*)
g
g
D
E
1.97(2)
1.99(2)
1.9715834, 1.9722007
1.9984610
Tuning the microwave frequency allows determining the
magnetic field dependence of the resonance features. The
corresponding resonance frequency vs. magnetic field diagram
for loose powder of complex 3 is shown in Fig. 8. At T = 2 K, there
is one main branch shown by black squares in Fig. 8 as well as in
Fig. S11 ESI†. The branch features a ZFS of around 180 GHz and
pronounced bending in the low-field regime while linear
behavior corresponding to a g-factor of 2 is found at high
magnetic fields. Measurements at higher temperatures (10 K–
15 K) show the appearance of additional resonance branches,
which are associated with several excited features. The main
feature of the spectra displayed in Fig. 6a as well as the
presence of a single resonance branch at T = 2 K are explained
in terms of the single-ion ZFS model for an anisotropic S = 1
system. The observed pronounced bending of the low-
temperature branch in the low-field regime confirms proper
alignment of the crystallites perpendicular to the external field
direction and a positive sign for the axial crystal field anisotropy
parameter D can be read-off24,25. However, the appearance of
thermally excited branches as shown in Figs. 7 and 8 (black open
triangle in Fig. 7) is not predicted by simple models of non-
interacting V(III)-centers. Instead, the data suggest the
presence of finite intermolecular spin coupling between
+136(3) GHz (4.5(1) cm-1)
/
/
/
0(1) GHz
-34(3) GHz (-1.1(1) cm-1)
J
(*) DFT calculations
further confirmed when all resonance branches observed in the
fixed, i.e., randomly aligned powder are considered (see Fig. S12
ESI†). While all resonance branches in Fig. S12 are explained by
the model, the comparison to the simulation shows that the
branches which are also observed in the loose powder indeed
correspond to the ones with g Bext
.
Our finding of D being much larger than J allows sketching the
anisotropy effects by means of a monomer picture: The positive
sign of D implies zero field splitting of the triplet states with mS
= 0 being the ground state, i.e., planar anisotropy without
significant in-plane anisotropy. These results are well consistent
with our LF calcalulations (D ≈ +4 cm–1, E << D, Table S10 ESI†).
While loose powders are hence aligned so that g Bext
,
rotational symmetry (gx = gy and E  0) implies that there is no
preferential direction within the xy-plane so that the observed
single feature can be straightforwardly attributed to the
transversal direction. These conclusions are well corroborated
by our calculations of the angular dependence of magnetization
for isolated [V(DAPBH)(CH3OH)2]+ molecule, which show a
distinct easy-plane nature of magnetization (Fig. S16, ESI†) and
the absence of magnetic anisotropy in the equatorial xy plane
(at Bext  xy, Fig. S17, ESI†) The presence of two molecules within
one unit cell in general allows for different directions of the
respective easy anisotropy plane, which may be misaligned by
the angle 2φ between both molecules. In a loose powder
measurement, this implies alignment of the crystallites
perpendicular to the effective anisotropy axis so that the
external magnetic field is misaligned by φ to the xy-plane of the
molecular anisotropy. However, as demonstrated in Fig. S13,
where the field dependence of the ground state transition is
simulated for different angles φ, such misalignment would lead
neighboring metal centers26,27
Hamiltonian transforms to
.
The corresponding spin
H  B BgS  D

Sz2  S(S 1)/3

 E(Sx2  Sy2 )  JS1S2
(2)
where the first term describes anisotropic Zeeman interaction
of the spin S with the external magnetic field B, D and E are ZFS
parameters and the last term covers magnetic interaction
between two neighboring spins S1 and S2 with the exchange
parameter J. In general, the g-tensor exhibits three components
gx, gy and gz. From the axial structure of the molecule we
assume axial symmetry for the g-tensor so that it can be
simplified to an axial (g) and a transversal (g) components.
This is supported by our ligand-field (LF) calculations for 1-3
8 | J. Name., 2012, 00, 1-3
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The presence of small dimer-like spin coupling isVsieuwpAprtoiclseeOdnlitnoe
DOI: 10.1039/D0DT03092A
yield a maximum in the magnetic susceptibility at T  2 K and
downturn below31. This behavior is clearly confirmed by Fig. 9
where low-temperature dc-susceptibility data are presented.
The data are well described by a simulation using the SH Eq. 2
and employing g-values and anisotropy parameters as shown in
Table 4 but an antiferromagnetic dimer coupling of -37.5 GHz.
This coupling value is within the error range of the value
obtained by HF-EPR. In contrast, neglecting intermolecular
magnetic interaction but employing the same anisotropy and g-
values obtained from EPR fails to describe the experimental
data. Simulation with J = -34 GHz as obtained from the best fit
to the EPR data still yields a good qualitative agreement while
the position of the maximum in  is reproduced best by using J
= -43 GHz (see Fig. 9). The latter value is however too large to
account for the position of the excited features in Figs. 7 and 8.
It is also important to note that the ZFS model with isotropic g-
tensor and with dimer-like antiferromagnetic spin coupling
reproduces the experimental T curve for 3 over the entire
temperature range 2-300 K at g = 1.950, D = +4.6 cm-1, E = 0, J =
-1.1 cm-1 and TIP = 1.12·10-4 cm3mol-1, Fig. S21 ESI†.
Figure 9. Temperature dependence of static magnetic susceptibility of 3 between 0.4 K
and 10 K. The data obtained in 3He-setup on a fixed powder of complex 3 were
normalized at 4.2 K to the higher temperature data shown also in this Figure. The solid
line shows a simulation using the SH Eq. 2 and the parameters described in the text. For
comparison, a simulation using the same SH parameters but with J = 0 is given as dashed
line.
Thus, the dc magnetic data and HF-EPR measurements indicate
a positive ZFS energy in 1-3. In these vanadium(III) PBP
complexes, two electrons with parallel spins occupy the two
lowest magnetic orbitals 3dxz and 3dyz orbitals to produce a
high-spin (S=1) ground state with no first-order orbital
momentum (Fig. S15 ESI†). This corresponds to the spin-only
case with second-order magnetic anisotropy described by the
to a splitting of the branch observed at low temperatures.
Comparison of the simulation with the experimental data
implies that the angle between the crystal field axes of the two
molecules in the unit cell is smaller than 10° while the best
simulation of the branch is obtained for φ = 0°. Hence, the
crystal field axes for all molecules are almost perfectly aligned.
This result is corroborated by the simulation of the fixed powder
studies.
2
2
ordinary ZFS spin Hamiltonian DSz2 + E(Sx - Sy ); our LF/AOM
Our analysis implies that the thermally excited resonance
feature is attributed to the effect of dimer-like
antiferromagnetic coupling between two neighboring
vanadium centers; its origin of is analyzed below in terms of
superexchange calculations between the [V(DAPBH)(CH3OH)2]+
units (see the next Section below). Estimating dipolar
interactions28 between S = 1 centers separated by about 7.5 Å
yields magnetic coupling in the range of Jdipolar ~ 0.2 GHz, which
is much too small to account for the observed separation of the
extra feature from the main one and its temperature
dependence.29 The small feature showing up in the loose
powder spectra marked by a black filled triangle in Fig. 6a is a
forbidden transition, i.e., it is associated with a change of spin
state ms = ±2. In general, forbidden transitions are not
supposed to be detected for E = 0 and aligned crystallites but
higher order transverse terms, presence of mixed modes of
microwave radiation or various molecular orientations in the
unit cell may account for forbidden features. Specifically, mode
mixing in the transmission-type probe setup implies that not Figure 10. On the origin of AF spin coupling in 3. (a) Calculated electron transfer
parameters tij = <3di(A)| h |3dj(B)> (in cm–1) connecting magnetic orbitals of the VIII(A)
only the conventional perpendicular mode is detected but also
the parallel one, which may cause forbidden transitions even in
the absence of a transversal anisotropy term29,30. Mixture of the
and VIII(B) centers in the pair 2 (see ESI for computational detail). The largest matrix
element txz,xz = 136.5 cm–1 is diagonal (marked in orange), (b) txz,xz connects two half-filled
magnetic orbitals 3dxz(A) and 3dxz(B) resulting in AF spin coupling with J ~ –
(txz,xz)2/ U(A→B) ≈ –0.5 cm–1 (where U(A→B) is the charge-transfer energy, ~ 40000 cm–1
microwave modes was hence included as a parameter in all
simulations of the spectra. In addition, slight misalignment of (5 eV)), (c) The orbital scheme of the superexchange pathway: in each of two
[V(DAPBH)(CH3OH)2]+ complexes (A,B), magnetic 3dxz orbital overlaps with collective
π(z)(DAPBH) molecular orbital of the planar DAPBH ligand; these orbitals interact via the
molecules at different unit cell positions implies that even in
case of perfect alignment with respect the crystals’ effective
anisotropies, misalignment of the individual molecules is
present.
π-stacking contact of two DAPBH ligands resulting in efficient superexchange pathway
with txz,xz = 136.5 cm–1 and J ≈ –0.5 cm–1
.
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calculations indicate positive axial anisotropy parameter D ~ +4 character of 3d magnetic orbitals of V(III) which often lead to
View Article Online
cm-1 and small transverse E parameter (Table S10, ESI†). high exchange parameters.10 ConsiderDabOlIy: 10w.1e03a9k/eDr0DATF030s9p2inA
Besides, these calculations show that the orbital composition of coupling J = -0.09 cm-1 in the long pair (10.65 Å, Fig. S20 ESI†) is
the ground-state wave function det||3dxz↑, 3dyz↑|| of V(III) caused by lower efficiency of the π-π stacking in mediating spin
PBP complexes 1–3 is rather insensitive to departures from the coupling due to imperfect overlap of the macrocyclic rings, as
strict PBP symmetry D5h. The reason is that the in-plain can be seen from Fig. S20 ESI†.
distortions of the VN3O2 pentagon produce a relatively small On the other hand, the absence of the π-stacking in other
energy splitting of the two lower 3d orbitals of V(III) ion, 3dxz vanadium pairs (Fig. S18 ESI†) leads to small matrix elements tαβ
and 3dyz (few hundreds of cm-1, see Table S8, ESI†), since they (α,β = xy, x2-y2, z2, xz, yz) and very weak spin coupling. Thus, in
interact with the valent 2p orbitals of O and N atoms in the π- the closest vanadium pair with V…V = 7.5 Å (pair 1 in Fig. S19a
bonding mode (Fig. S15 ESI†). As a result, magnetic behavior of ESI†) the diagonal matrix elements txz,xz and tyz,yz connecting
distorted PBP complexes 1-3 should be close to that of half-filled 3dxz and 3dyz orbitals on vanadium centers A and B
undistorted VIII(3d2) complex with idealized D5h symmetry; this (Fig. 10b) are small (10 cm–1 or less, Eq. S19). As a result, the
accounts for the uniaxial character of magnetic anisotropy of corresponding AF superexchange pathways in pair 1 are much
complexes 1-3 (see Table 10 and Figs. 16,17 ESI†)
less efficient than those in pair 2. Moreover, the presence of
larger off-diagonal matrix elements connecting half-filled 3dxz
and 3dyz orbitals with empty orbitals (such as t ~ 40 cm–1 for 3dxz
and 3dz2 orbitals, Eq. S19 ESI†) opens up competing
ferromagnetic exchange pathways. Ultimately, this leads to a
marginally weak spin ferromagnetic coupling of J = +8.4·10–4
cm–1, which is several orders of magnitude smaller than that in
pair 2, despite shorter distance between V(III) ions (7.5 Å in pair
1 vs. 8.65 Å in pair 2, Fig. S19 ESI†). Superexchange calculations
for more distant pairs in 3 (V…V = > 10 Å, Fig. S18 ESI†) yield
even smaller exchange parameters (< 10–4 cm–1). These results
provide evidence that the dimer-like spin coupling model is
indeed good theoretical platform for the description of
magnetic properties and HF/EPR spectra of compound 3, albeit
formally vanadium chains (Fig. S18 ESI†) can also be regarded as
strongly alternating spin chains due to the presence of a weak
magnetic coupling between dimers (J = -0.09 cm-1, Fig. S20 ESI†)
Therefore, our superexchange calculations explain in a natural
way the origin of the dimer-like character of the spin coupling
between V(III) ions in 3, as they indicate that the spin coupling
in the pair 2 (with V…V = 8.65 Å, Fig. 4b and Fig. S19b ESI†)
through the π-stacking strongly dominates over the spin
coupling in other vanadium pairs in compound 3.
Spin coupling calculations
The origin of the long-range dimer-like spin coupling in
compound 3 is intriguing considering that its structure is built of
isolated complexes with large spacing between V3+ ions (> 7 Å,
Fig. 4). To get more insight into this problem, we explored the
mechanism of superexchange interactions between the
[V(DAPBH)(CH3OH)2]+ units in 3 in terms of the many-electron
superexchange theory22 (see ESI† for the theoretical approach
and computational detail).
Inspection of the molecular structure of 3 shows that it is built
of zig-zag chains formed by two vanadium pairs with short (8.65
Å) and long (10.65 Å) V…V distances, in which
[VIII(DAPBH)(CH3OH)2]+ complexes with the coplanar equatorial
planes VN3O2 are connected via the π-π stacking contacts of
macrocyclic rings (Fig. S18 ESI†); the shortest distance between
the VIII centers is 7.5 Å (pair 1, Figs. S18 and Fig. S19a).
Superexchange calculations indicate that the π-stacking of two
DAPBH planar ligands provides the most efficient exchange
pathway between spins S = 1 of two V(III) ions, which result in
AF spin coupling of J = –0.49 cm–1 for the short pair (V…V = 8.65
Å, pair 2 shown in Fig. 10 and Fig. S19b ESI†) and J = –0.09 cm–1
for the long pair (V…V = 10.65 Å, Fig. S20 ESI†).
The underlying reason behind relatively strong AF spin coupling
in pair 2 (8.65 Å) is that the largest matrix element txz,xz = 136.5
cm-1 (Eq. S20) is diagonal, which connects two half-filled
magnetic orbitals 3dxz(A) and 3dxz(B) of two vanadium centers
(A and B) involved in the two-electron wave function
det||3dxz↑, 3dyz↑|| of the ground state of the VIII centers (Fig.
10b). This matrix element measures the efficiency of the
indirect overlap of 3dxz(A) and 3dxz(B) magnetic orbitals
mediated by the ligand’s π(z)(DAPBH) molecular orbitals that
interact through the π-stacking contact of two DAPBH ligands
(Fig. 10c). According to the superexchange theory, interaction
between two half-filled orbitals leads to AF spin coupling with
J ~ –(txz,xz)2/ U(A→B) ≈ –0.5 cm–1, where U(A→B) is the charge-
transfer energy, typically 5 eV (~ 40000 cm–1) for early 3d
elements. The exchange parameter of J = –0.49 cm–1 obtained
from many-electron superexchange calculations (see ESI† for
detail) is reasonably consistent with the experimental value (J =
–1.1 cm–1); enhanced spin coupling can be due to the diffuse
In this respect, it is also important to note that there is direct
experimental evidence of the efficiency of exchange
interactions between 3d-magnetic ions conducted by the π-
stacking of planar macrocyclic ligands, as exemplified by a
cobalt compounds composed of isolated Co(II) complexes32 in
which rather strong ferromagnetic spin coupling (J ≈ 2 cm-1)
between Co2+ ions is mediated through the π-stacking contacts
of pyridine rings of the dpamH ligands.
AC susceptibility measurements on the complexes revealed no
SMM behavior, both with and without applied magnetic field.
Conclusions
In summary, three mononuclear pentagonal-bipyramidal
complexes of V(III) with the equatorial pentadentate N3O2
ligand (2,6-diacethylpyridinebis(benzoic acid hydrazone),
H2DAPBH) were synthesized and characterized structurally and
magnetically,
[V(HDAPBH)(NCS)2]·0.5CH3CN·0.5CH3OH
[V(H2DAPBH)Cl2]Cl·C2H5OH
(2)
(1),
and
[V(DAPBH)(CH3OH)2]Cl·CH3OH (3). They show paramagnetic
10 | J. Name., 2012, 00, 1-3
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ARTICLE
A.D. Talantsev, R.B. Morgunov, E.B. Yagubskii, S.V. Simonov,
behavior arising from two unpaired electrons (S = 1) with a
positive axial anisotropy ZFS parameter (D ≈ +4 cm–1, E ≈ 0) and
pronounced dimer-like antiferromagnetic spin coupling
between two neighboring vanadium centers (J = -1.1 cm-1), as
View Article Online
L.V. Zorina and V.S. Mironov, Inorg. CheDmOI.:,1200.110739, /5D60,D8T902360;9(2cA)
C. Pichon, N. Suaud, C. Duhayon, N. Guihery and J.-P. Sutter,
J. Amer. Chem. Soc., 2018, 140, 7698; (d) L.V. Zorina, S.V.
Simonov, V.D. Sasnovskaya, A.D. Talantsev, R.B. Morgunov,
V.S. Mironov and E.B. Yagubskii, Chem.- Eur. J., 2019, 25,
14583.
has been established for compound
3
from high-
frequency/high-field EPR and low-temperature magnetization
measurements. Detailed superexchange calculations for
compound 3 provide evidence that the long-range spin coupling
between distant V(III) ions (8.65 Å) is mediated through π-
stacking contacts between the planar DAPBH ligands of two
neighboring [V(DAPBH)(CH3OH)2]+ complexes; the absence of π-
stacking in other vanadium pairs results in much smaller
exchange parameters that accounts for the dimer-like character
of the spin coupling in 3. Complexes 1-3 are the first
representatives of vanadium PBP complexes with planar
pentadentate Schiff-base ligands in the equatorial plane.
3
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There are no conflicts to declare.
Acknowledgements
This work was supported by the Russian Science Foundation
(project no. 18-13-00264).
The structural researches were performed within the state
assignment for the ISSP RAS. R.K. acknowledges support by
BMBF via project SPINFUN №13XP5088 (the Section “HF-EPR
study”, Interpretation). L.S. acknowledges support from
Landesgraduiertenförderung Baden-Württemberg (the Section
“HF-EPR study”, HF-EPR and Low-T SQUID experiments, data
analysis, DFT calculations) and C.K. acknowledges support by
Deutsche Forschungsgemeinschaft through project KO5480/1-
1 (the Section “HF-EPR study”, data analysis, discussion). O.V.M.
and A.N.V. acknowledge support by the Ministry of Education
and Science of the Russian Federation in the framework of
Increase Competitiveness Program of NUST “MISiS” Grant No.
K2-2020-008; by Act 211 of the Government of Russian
Federation, Contracts No. 02.A03.21.0004 and No.
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