Role of co-ligand and solvent on properties of V(IV) oxido complexes with ONO Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2018.12.047

Four complexes of V(IV/V) of octahedral geometry, with tridentate ONO Schiff base ligand, oxido and bidentate (maltol, 1,10-phenantroline or 2,2′-bipyridine) co-ligands are described. The Schiff bases were formed from 3,5-di-tert-butyl-2-hydroxybenzaldehy

Full text of DOI:10.1016/j.molstruc.2018.12.047

Accepted Manuscript
Role of co-ligand and solvent on properties of V(IV) oxido complexes with ONO Schiff
bases
Janusz Szklarzewicz, Anna Jurowska, Maciej Hodorowicz, Ryszard Gryboś, Dariusz
Matoga
PII:
S0022-2860(18)31474-1
DOI:
https://doi.org/10.1016/j.molstruc.2018.12.047
MOLSTR 25984
Reference:
To appear in: Journal of Molecular Structure
Received Date: 29 August 2018
Revised Date: 21 November 2018
Accepted Date: 11 December 2018
Please cite this article as: J. Szklarzewicz, A. Jurowska, M. Hodorowicz, R. Gryboś, D. Matoga, Role
of co-ligand and solvent on properties of V(IV) oxido complexes with ONO Schiff bases, Journal of
Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2018.12.047.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Role of co-ligand and solvent on properties of V(IV) oxido complexes with
ONO Schiff bases
Janusz Szklarzewicza*, Anna Jurowskaa, Maciej Hodorowicza, Ryszard Grybośa and Dariusz Matogaa
a Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland
Abstract
Four complexes of V(IV/V) of octahedral geometry, with tridentate ONO Schiff base ligand, oxido and bidentate
(maltol, 1,10-phenantroline or 2,2’-bipyridine) co-ligands are described. The Schiff bases were formed from 3,5-di-
tert-butyl-2-hydroxybenzaldehyde and hydrazides (benzhydrazide, 2-hydroxybenzhydrazide and phenylacetic
hydrazide). The X-ray single-crystal structures of all complexes revealed that they are a mixture of 1:1 isomers with
the different position of 5- and 6-membered ring of ONO ligand versus V=O, excluding 1. The position of the V=O
band in IR spectra was found to be dependent on the V=O and trans located donor atom bond distances. Two
complexes, 3 and 3a, were isolated as unsolvated and solvated ones, the coordination of solvent molecule drastically
changed the structure parameters. The cyclic voltammetry measurements show irreversible processes for vanadium
redox system and reversible ones for co-ligands. The stability of complexes in solution is discussed.
Keywords: vanadium, complex, co-ligand, Schiff base, structure.
Introduction
The chemistry of vanadium is widely investigated as the biological role of vanadium compounds including
anticancer, insulin-mimetic, anti-inflammatory and antibacterial belong to the most relevant ones [1-9]. One of the
last founding is that vanadium complexes can significantly reduce the resistance to oncolytic immunotherapy [10-
13]. The question is in which form vanadium is incorporated to biological systems. It was found, that in case of

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mono and bidentate ligands and V(IV), simple VO2+ ions are active. Our last investigations show however, that with
more polydentate ligands complexes are much more stable and the type of ligand is important and affects biological
activity [14]. This may suggests, that unchanged complex is active, thus the ligand type may modify the vanadium
activity. The main problem with polydentate ligands is that not all types of ligands can fill the coordination sphere of
vanadium, thus additional co-ligands are observed in structure. In case of solvent molecules, which can be easily
exchanged, the complex stability seems to be low [15,16]. As vanadium(IV) is mostly octahedral (but also
pentadentate ligand complexes are observed [17,18]) this reduces the ligands type to pentadentate, tetradentate (but
only for penta-coordinated vanadium) or tridentate. For tetradentate ligand complexes, we have found that the
complex stability is high [14], even possessing distorted square pyramidal geometry. For tridentate ligands, presented
in this paper, bidentate co-ligands are necessary to increase the complex stability in solution. It was found also, that
1,10-phenantroline, used as co-ligand, stabilise the oxidation state of vanadium, otherwise complexes are very
sensitive on oxygen and are oxidized to V(V) [16].
We have lastly studied the complexes with Schiff bases composed from 5-bromosalicyaldehyde and
benzhydrazide or 4-tertbutylbenzhydrazide and we have found that they are active in inhibition of human
recombinant PTP1B, one of them was found to be twice as active as suramin [16] and we presented the selected data
of their stability in neutral and acidic (pH = 2.0) conditions. In present paper we discuss the role of type of co-ligands
on vanadium(IV/V) complexes properties. We used Schiff bases formed from 3,5-di-tert-butyl-2-
hydroxybenzaldehyde and hydrazides - 2-hydroxybenzhydrazide (L1) phenylacetic hydrazide (L2) and benzhydrazide
(L3), to have a very similar type of ONO ligand. As co-ligands, we used maltol - typical ligand in insulin mimetic
properties, mostly known in its application as bismaltolatooxovanadium complex (BMOV) [19], 1,10-phenantroline
(phen) or 2,2’-bipirydine(bpy). Two first ligands are rigid, while bpy molecule, as more flexible, could be easier
substituted, thus the stability of investigated complexes in solution is discussed.
Experimental
Materials and methods

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[VO(acac)2], [VOSO4] monohydrate, 1,10-phenanthroline (phen), maltol (mal denote maltolato ligand), 2,2’-
bipyridine (bpy), 3,5-di-tert-butyl-2-hydroxybenzaldehyde, 2-hydroxybenzhydrazide, phenylacetic hydrazide and
benzhydrazide were of analytical grade (Aldrich) and were used as supplied. Ethanol (98%) of pharmaceutical grade
was from Polmos (Poland) and used as supplied. All other solvents were of analytical grade and were used as
supplied. BaSO4 were of spectroscopic grade (Japan). [Bu4N]PF6 was synthesized from Bu4NBr and KPF6 by a
standard method and recrystallized from acetone [20]. Microanalysis of carbon, hydrogen and nitrogen were
performed using Elementar Vario MICRO Cube elemental analyzer. IR spectra were recorded on a Bruker
EQUINOX 55 FT-IR spectrophotometer in KBr pallets. The electronic absorption spectra were recorded on
Shimadzu UV-3600 UV-VIS-NIR spectrophotometer equipped with a CPS-240 temperature controller. Diffuse
reflectance spectra were measured in BaSO4 pellets with BaSO4 as a reference on Shimadzu 2101PC equipped with
an ISR-260 integrating sphere attachment. The magnetic susceptibility measurements were performed on a
SHERWOOD SCIENTIFIC magnetic susceptibility balance. Cyclic voltammetry measurements were carried out in
DMSO with [Bu4N]PF6 (0.1 M) as the supporting electrolyte, using Pt working and counter and Ag/AgCl as
reference electrodes on an AUTOLAB/PGSTAT 128 N Potentiostat/Galvanostat. E1/2 values were calculated from
the average anodic and cathodic peak potentials, E1/2 = 0.5(Ea + Ec). The redox potentials were calibrated versus
ferrocene (0.440 V versus SHE), which was used as an internal potential standard for measurements in organic
solvents to avoid the influence of a liquid junction potential; the final values are reported versus the standard
hydrogen electrode (SHE). DTG measurements were performed on a TGA/SDTA 851e Mettler Toledo
Microthermogavimeter under argon atmosphere in the 25-700 oC range with scan speed 10 oC/min.
Crystallographic data collection and structure refinement
Diffraction intensity data for single crystal of four new compounds (1, 2, 3 and 3a) were collected at 293(2) K on a
KappaCCD (Nonius) diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). Cell refinement
and data reduction were performed using firmware [21,22]. Positions of all of non-hydrogen atoms were determined
by direct methods using SIR-97 [23]. All non-hydrogen atoms were refined anisotropically using weighted full-
matrix least-squares on F2. Refinement and further calculations were carried out using SHELXL 2014/7 [24]. All
hydrogen atoms joined to carbon atoms were positioned with an idealized geometries and refined using a riding

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model with Uiso(H) fixed at 1.5 Ueq of C for methyl groups and 1.2 Ueq of C for other groups. CCDC 181172,
1844928, 1850268 and 1848816 of 1, 2, 3 and 3a respectively contain the supplementary crystallographic data.
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of [VO(L1)(mal)] (1)
The 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.355 g, 1.5 mmol) and the 2-hydroxybenzhydrazide (0.228 g, 1.5
mmol) were dissolved in 30 ml of EtOH under Ar. The mixture was refluxed for 20 minutes under argon and
VOSO4aq (0.333 g, 1.5 mmol) was added The mixture was refluxed for next 30 minutes. The solution becomes dark
green-brown. During reflux black crystals started to precipitate. At this stage, maltol (0.190 g, 1.5 mmol) was added
and mixture was refluxed 15 minutes. The solution becomes transparent without deposition. The solvent was than
evaporated to half its starting volume and the mixture was cooled under argon. The initially formed black crystals
were removed by filtration, the filtrate was left on air and after ca. 3 hours the dark-red crystals were filtered off,
washed with EtOH and dried in air. Yield: 0.185 g, 22 %. MW 559.51. Anal. Calcd. for C28H32N2O7V: C, 60.11; H,
5.76; N, 5.01 %. Found: C, 59.82; H, 5.64; N, 4.78 %. The complex is diamagnetic.
Synthesis of [VO(L2)(phen)] (2)
The 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.254 g, 1.5 mmol), phenylacetic hydrazide (0.227 g, 1.5 mmol) and
50 ml of EtOH were refluxed for 15 minutes under Ar. To the solution [VO(acac)2] (0.398 g, 1.5 mmol) was added
and the mixture was refluxed for 40 minutes. Then 1,10-phenantroline (0.271 g, 1.5 mmol) was added and the
mixture was refluxed for next 15 minutes. The transparent solution was evaporated to ca. 40 ml and was left for next
day. The very fine crystalline product was filtered off, washed with EtOH and dried in air. Yield 0.189 g. 29%. MW
= 611.62. Anal. Calcd. for C35H36N4O3V: C, 68.73; H, 5.93; N, 9.16 %. Found: C, 67.34; H, 5.62; N, 9.42 %. The
complex is paramagnetic, µ = 1.58 µB.
Synthesis of [VO(L3)(bpy)]·MeOH (3)

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The 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.7060 g, 3.0 mmol), benzhydrazide (0.420 g, 3.0 mmol) and 50 ml of
MeOH were refluxed for 8 minutes. To the solution 1.38 ml of Et3N, VOSO4aq (0.667 g, 3.0 mmol) and 10 ml of
MeOH were added and refluxed for 20 minutes. During this period solution becomes dark red. Then 2,2’-bipyridine
(0.472 g, 3.0 mmol) in 10 ml of MeOH was added and heated for 3 minutes and cooled. Complex was filtered off,
washed with MeOH and dried in air. Yield 1.086 g, 60%. MW = 605.62. Anal. Calcd. for C33H38N4O4V: C, 65.45; H,
6.32; N, 9.25 %. Found: C, 65.76; H, 6.37; N, 9.30 %. The complex is paramagnetic, µ = 1.22 µB.
Synthesis of [VO(L3)(bpy)]·EtOH (3a)
The 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.705 g, 3.0 mmol), benzhydrazide (0.408 g, 3.0 mmol) and 60 ml of
EtOH were refluxed for 6 minutes. To the solution 1.38 ml of Et3N was added and mixture was refluxed for
additional 5 minutes. Then VOSO4aq (0.666 g, 3.0 mmol) was added and refluxed for 20 minutes. During this period
solution becomes dark red. The solution was filtered under Argon and 2,2’bipyridine (0.468 g, 3.0 mmol) was added
and heated for next 5 minutes. The crystals formed were filtered off under Ar, washed with cold EtOH and dried in
air. Yield 0.804 g, 43 %. The filtrate was evaporated under Ar to one third of the initial volume and additional 0.286
g of complex was collected by filtration. Total yield: 1.090 g, 59 %. MW = 619.65. Anal. Calcd. for C34H40N4O4V:
C, 65.90; H, 6.51; N, 9.04 %. Found: C, 65.44; H, 6.42; N, 8.91 %. The complex is paramagnetic, µ = 1.58 µB.
Results and discussion
General remarks to the synthesis
The reaction of in situ formed Schiff base ligands with vanadium salts results in its coordination to metal center (see
Scheme 1.). The obtained solutions do not crystallize easily, and to get stable complexes, bidentate co-ligands were
added. As we utilized 3 different hydrazides and 3 different co-ligands, there is 9 possible combinations. But only in
case 1-3a we got crystals suitable for X-ray measurements. In other cases, complexes (especially with maltol) were
unstable or did not crystalize or oil products were formed. Complexes 1-3a are stable in a solid state and were kept in
closed vials, 8 years after preparation no changes were observed.

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Scheme 1. Synthesis scheme for 1-3 complexes.
Description of the structures of 1-3a
The complexes 1-3a are neutral, in two cases solvent molecules were found by elemental analysis and IR spectra
(described later in this paper). The solvent molecules could not be located precisely in the X-ray crystal structure
determination and were omitted in final refinement. For complex 3, solvent disorder caused low quality of the final
refinement, thus the checkcif could not be deposited (only cif file was deposited). Nevertheless, the structure was
resolved and is discussed here. The asymmetric parts of the units cells of the complexes 1-3a with adopted atomic
numbering scheme are shown in Figs 1 and 2. Single crystals of the compounds were obtained directly as described
in the syntheses. The complex 1 crystallizes in the orthorhombic (1 and 3a) or monoclinic (2 and 3) space groups
P212121 (1), P21/c (2), P21/c (3), Pbca (3a) with the asymmetric cell unit containing one molecule of [VOLALB]
complex, where LA denote ligand L1, L2 or L3, LB denote maltol, phen or bpy as indicated in Table 2. The
crystallographic data and detailed information on the structure solution and refinement are given in Table 1. Selected

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bond distances and bond angles are listed in Table 2. As depicted in Figs 1 and 2 the oxovanadium cation (VO2+) is
coordinated to the tridentate hydrazone ligand, through the phenolate oxygen (O), azomethine nitrogen (N) and
enolate oxygen (O), and a bidentate LB ligand (OO or NN type for maltol or bpy and phen respectively). Thus in all
complexes the vanadium has distorted octahedral geometry.

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Table 1 Crystal data and structure refinement parameters for 1-3a.
1
2
3
3a
Empirical formula
Formula weight
C28 H31N2O7V
558.49
C35 H36 N4O3V
611.62
C32H34N4O3V
573.57
C32 H34 N4O3V
573.57
Crystal size (mm)
Crystal system
Space group
0.320 x 0.120 x 0.040
Orthorhombic
P 212121
0.200 x 0.200 x 0.200
Monoclinic
P 21/c
0.280 x 0.260 x 0.050
Monoclinic
P 21/c
0.190 x 0.100 x 0.040
Orthorhombic
P bca
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
h
k
8.894(5)
9.535(5)
32.554(5)
90.000(5)
90.000(5)
90.000(5)
-10<=h<=10
-9<=k<=11
-38<=l<=23
2761(2)
14.9200(3)
16.2740(3)
19.9320(3)
90
130.2810(10)
90
-19<=h<=18
-20<=k<=21
-25<=l<=25
3692.08(12)
4
25.018(2)
9.8290(17)
12.378(5)
90
94.934(13)
90
-24<=h<=25
-10<=k<=10
-11<=l<=12
3032.5(12)
4
19.757(5)
11.748(5)
28.831(5)
90
90
90
-15<=h<=26
-7<=k<=15
-25<=l<=38
6692(4)
l
V (Å3)
Z
4
8
T (K)
293(2)
293(2)
293(2)
293(2)
Wavelength [Å ]
0.71069
1.344
0.407
0.71073
1.100
0.303
0.71073
1.256
0.365
0.71073
1.139
0.331
Dx (Mg/m3)
Absorption coefficient (mm-1)
Theta range for data collection (°) 3.132 to 24.987
2.839 to 27.498
30312
8405 [R(int) = 0.0351]
99.0 % (25.242o)
None
3.038 to 21.468
6904
3126 [R(int) = 0.0800]
90.1 % (21.468o)
Semi-empirical from
equivalents
3.429 to 28.582
21544
7704 [R(int) = 0.0953]
99.6 % (25.242o)
none
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
6160
4454 [R(int) = 0.0258]
99.1 % (24.987o)
Semi-empirical from
equivalents
Refinement method
Full-matrix least-
Full-matrix least-
Full-matrix least-squares Full-matrix least-squares
2
2
2
2
squares on F
squares on F
on F
on F
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
4454 / 1 / 355
0.967
R1 = 0.0571, wR2 =
0.1389
R1 = 0.0838, wR2 =
0.1473
0.674 and -0.265
8405 / 0 / 394
1.046
R1 = 0.0754, wR2 =
0.2206
R1 = 0.1008, wR2 =
0.2548
1.320 and -0.280
3126 / 0 / 392
1.084
R1 = 0.0739, wR2 =
0.1360
R1 = 0.1407, wR2 =
0.1569
0.224 and -0.210
7704 / 0 / 356
0.825
R1 = 0.0719, wR2 =
0.1766
R1 = 0.1979, wR2 =
0.2097
0.692 and -0.455
R indices (all data)
Largest diff. peak and hole (e.Å-3)

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Table 2 Selected bond distances [Å] in complexes 1-3a with the figure showing general coordination scheme around
vanadium center.
1
2
3
3a
V=O
1.583(5)
1.943(5)
2.082(5)
1.833(4)
1.861(4)
2.260(5)
1.593(2)
2.016(2)
2.039(3)
1.931(2)
2.147(2)
2.360(2)
1.598(5)
2.020(5)
2.038(6)
1.920(5)
2.122(5)
2.369(6)
1.594(3)
2.015(3)
2.031(4)
1.918(3)
2.148(4)
2.329(4)
V-O5 [LA]
V-N [LA]
V-O6 [LA]
V-N v O [LB]cis
V-N v O [LB]trans
r-5’: 5 membered ring V-O-C-N –N; r-6’’: 6 membered ring V-N-C-C-C-O. LA denote tridentate Schiff base, LB
denote co-ligand (maltol, phen or bpy).
Fig. 1. The molecular structures of complexes 1 (left) and 2 (right) together with the atom labelling scheme. The
hydrogen atoms were omitted for clarity. Thermal ellipsoids represent 30% of displacement probability.

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Fig. 2. The molecular structures of complexes 3a (left) and 3 (right) together with the atom labelling scheme. The
hydrogen atoms were omitted for clarity. Thermal ellipsoids represent 30% of displacement probability.
There are two different possibilities of ligand LA coordination - six membered ring can be on the left side
versus V=O bond, while 5-membered on the right side, looking from LB ligand (as in figure shown in Table 2) or in
opposite. In the structures of 2, 3 and 3a in a single crystal there is a 1 : 1 mixture of both coordination isomers,
while in the structure of 1 only a first coordination isomer is present. Both five- and six-membered rings are not lying
on the same plane, the highest distortion is observed in deviation of O7 (2 and 3) or O17 (1 and 3a) of ONO ligand
LA from the plane created on five membered ring. These deviations are: -0.4979 (1), -0.4679 (2), -0.4085 Å (3) and -
0.5502 (3a) respectively. The highest deviation has complex 3a, the smallest one - complex 3. It is unusual, as both
complexes have the same coordination sphere. In octahedral square plane, the oxygen (O7 or O17) is present, but
also one donating atom (O or N) of LB ligand. The distortion of square plane is also visible in the deviation of these
last atoms from a plane. These deviations are -0.3018 (1), -0.5682 (2), -0.7763 (3) and -0.6389 (3a) Å. The distortion
of the octahedron is also visible in the bond angles O=V-Ltrans which are equal to 175.6(2), 166.5(1), 164.2(2) and
164.9(2) o for 1, 2, 3 and 3a respectively. The V=O bond lengths (Table 2) changes in order 1 < 2 ≈ 3 ≈ 3a. The V-
Ltrans bond length should be correlated with the V=O bond through the trans effect. Thus the shortest V=O bond
should generates the longest V-Ltrans. This is not the case in all studied structures. The shortest V=O bond and V-Ltrans
are in 1, also such a trend is not observed for the same donating atom in complexes 2-3a (see Table 2). The trans
effect can only be driven from the fact that for the same donating atom in LB ligands bond distances V=Ltrans are
always longer than V-Lcis (see Table 2) ones.

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The crystal packing for 1-3a are presented in Figs 3, 4 and 5. The very interesting are the voids calculated for
all structures. In 1 and 3 the voids were not found. In 2 the void volume is 727.4 Å3 which is 19.7 % of the total unit
cell volume (3692.1 Å3) while in 3a 1032.6 Å3 with 15.4 % of the total unit cell volume (6692.0 Å3). In the empty
places in 3a the ethanol molecules are probably located, as indicated from elemental analysis, while in 2 the voids
are empty. The voids are separated and empty channels in structures of 1, 2 and 3a are not observed. For 3 the voids
were not found, but as shown in Fig. 5, the channels in structure are observed. The channel diameter is of ca. 5 Å and
probably methanol molecules is located there. Very interesting is the observation, that small change in synthetic
procedure (exchange of methanol into ethanol) results in drastic change in crystal packing and in the anion structure
ion 3 and 3a. Such changes should be caused by the change in strong intermolecular interactions. The observed
hydrogen bonds, π…π and C-H…π interactions in 1-3a are listed in Tables 3-5 for all studied complexes. In general,
in all cases the intermolecular interactions are very weak. For example the relatively short distance O-H-N
o
(intramolecular) in 1 indicates on a very weak interaction due to low angle DHA (152 ). In case of all other
hydrogen bonds the distances are much longer and the DHA angles are very low. Thus the hydrogen bonds in all
studied complexes are very weak. Thus the most important for packing, for all complexes, are the π…π and C-H…π
interactions listed in Tables 4 and 5 respectively. The interesting is also the illusion, shown in Fig. 3 (left), that the
layers are not parallel to each other.
Fig. 3. The crystal packing in 1 through [010] direction (left) and [100] (right). The hydrogen atoms were omitted for
clarity. Thermal ellipsoids represent 30% of displacement probability.

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Fig. 4. The crystal packing in 2 (left) and 3a (right) through [010] direction. The hydrogen atoms were omitted for
clarity. Thermal ellipsoids represent 30% of displacement probability.
Fig. 5. The crystal packing in 3 through [100] (left) and [010] (right) direction. The hydrogen atoms were omitted for
clarity. Thermal ellipsoids represent 30% of displacement probability.
Table 3 Hydrogen bonds for 1-3a [Å and °].
D-H...A
1
O(27)-H(27)...N(9)
2
d(D-H)
0.85(3)
d(H...A)
1.85(5)
d(D...A)
2.630(8)
<(DHA)
152(8)
C(30)-H(30)...O(42) [1-X, -1/2+Y, 3/2-Z]
C(40)-H(40)...O(42)
3
0.93
0.93
2.64
2.57
3.484(4)
3.030(4)
152.0
110.7
C(34)-H(34)...N(10) [X,1+Y,Z]
(37)-H(37)...O(39)
3a
0.93
0.93
2.47
2.50
3.192(9)
2.993(9)
134.0
113.3
C(7)-H(7)...O(39) [3/2-X, 1/2+Y,Z]
C(28)-H(28)...N(8)
C(34)-H(34)...O(26) [1-X,-Y,1-Z]
C(37)-H(37)...O(39)
0.93
0.93
0.93
0.93
2.55
2.66
2.54
2.44
3.387(6)
3.203(6)
3.284(5)
2.929(6)
150.4
118.4
137.7
112.9
Table 4 The π…π interactions in 1-3a [Å].

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shift
π...π
1
Cg(1)…Cg(2) [1+X,Y,Z]
Cg(2)…Cg(1) [-1+X,Y,Z]
3.963(2)
3.963(2)
1.392
1.378
Cg(1): C(1)-C(2)-C(3)-C(4)-C(5)-C(6), Cg(2): C(11)-C(12)-C(13)-C(14)-C(15)-C(16)
2
Cg(1)…Cg(4) [1-X,1-Y,1-Z]
Cg(4)…Cg(1) [1-X,1-Y,1-Z]
Cg(4)…Cg(4) [1-X,1-Y,1-Z]
3.5935(1)
3.5935(1)
3.9124(1)
1.186
1.234
1.978
Cg(1): N(41)-C(34)-C(35)-C(38)-C(39)-C(40), Cg(4): C(32)-C(33)-C(34)-C(35)-C(36)-C(37)
3
Cg(1)…Cg(2) [1-X,-Y,1-Z]
Cg(2)…Cg(1) [1-X,-Y,1-Z]
3.607(3)
3.607(3)
1.145
1.030
Cg(1): N(27)-C(28)-C(29)-C(30)-C(31)-C(32), Cg(2): N(38)-C(33)-C(34)-C(35)-C(36)-C(37)
3a
Cg(1)…Cg(2) [X,1/2-Y,-1/2+Z]
Cg(2)…Cg(1)
3.692(5)
3.692(5)
1.362
1.520
Cg(1): N(27)-C(28)-C(29)-C(30)-C(31)-C(32), Cg(2): N(38)-C(33)-C(34)-C(35)-C(36)-C(37)
Table 5 The C-H…π interactions in 1-3a [Å and o].
H…Cg
2.92
X-H..Cg
144
X…Cg
1
C(36)-H(36)…Cg(1) [X,1+Y,Z]
Cg(2): C(1)-C(2)-C(3)-C(4)-C(5)-C(6)
2
3.716(2)
C(38)-H(38)…Cg(3) [1-X,1-Y,1-Z]
C(25)-H(25)…Cg(2) [1-X,1/2+Y,1/2-Z]
2.70
2.97
3.5144(1)
3.7928(1)
146
148
Cg(2): C(1)-C(2)-C(3)-C(4)-C(5)-C(6), Cg(3): C(22)-C(23)-C(24)-C(25)-C(26)-C(27)
3
C(29)-H(29)…Cg(4) [X,-1/2-Y,-1/2+Z]
C(36)-H(36)…Cg(3) [X,1/2-Y,1/2+Z]
2.83
2.63
156
156
3.698(11)
3.501(11)
Cg(3): C(1)-C(2)-C(3)-C(4)-C(5)-C(6), Cg(4): C(13)-C(14)-C(15)-C(16)-C(17)-C(18)
3a
C(29)-H(29)…Cg(3) [3/2-X,1/2+Y,Z]
2.89
141
3.656(6)
Cg(3): C(11)-C(12)-C(13)-C(14)-C(15)-C(16)
IR spectra
The IR spectra of 1 - 3a are presented in Figs 6 and 7. All complexes have characteristic band located at 969 (for 1),
953 (for 2), 957 (for 3) and 960 cm-1 (for 3a) associated with vibration of V=O group. The position of V=O band is
very often correlated with the V=O bond length. The band position can be put in order of decreasing energy as 1 >

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3a ≈ 3 > 2, while the bond length increases in order 1 < 2 ≈ 3a < 3 (see Table 2). As it can be seen, there is no direct
correlation between those two trends. But the energy of V=O bond vibration depends also on the strength of the V-
Ltrans bond. As the V=O bond length in 2, 3a and 3 is almost equal within experimental error, the V-Ltrans bond length
should be considered. It is changed in order (increasing) 1 < 3a < 2 ≈ 3 as indicated in Table 2. This is in good
correlation (within experimental error) with the IR spectra trend indicated above.
The intensive band at ca. 1590 cm-1 (1597 (1), 1586 (2), 1593 (3 and 3a)) is attributed to vibration of C=N
group in Schiff base ligand and this band was also observed in the other complexes of this type [15,25,26]. Each
complex also has bands related to vibrations of co-ligands, for example for 1 the band at 1625 cm-1 can be assigned
to C=O group in the maltolato ligand. For complex 2 the bands characteristic for 1,10-phenantroline ligand at 1498,
1424, 726 cm-1 are also observed as well as in the other d-electron metal complexes with phen ligand [27]. For
complexes 3 and 3a the spectra are very similar, and only differences in region characteristic for OH vibration of
ethanol are observed (see for example at 3300 cm-1) which remains in a good agreement with the structure
measurements.
Fig. 6. IR spectra of complexes 1 (black, solid line), 2 (red, dashed line), 3 (blue, dotted line) and 3a (green, dashed-
dotted line) in a 700 – 1000 cm-1 range.

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Fig. 7. IR spectra of complexes 1 (black, solid line), 2 (red, dashed line), 3 (blue, dotted line) and 3a (green, dashed-
dotted line) in a 1000 – 4000 cm-1 range.
UV-Vis spectra
The selected UV-Vis reflectance spectra of complexes 1 - 3a are presented in Fig. 8. The reflectance spectra are the
most suitable for studying the vanadium oxidation state, as due to low solubility of the complexes studied, d-d
transitions are invisible in diluted solutions. It can be seen, that in case of 2 and 3a complexes of V(IV) d-d
transitions are well observed above 700 nm. For V(V) diamagnetic complexes (1 and 3 respectively) these bands are
not observed. The charge-transfer transitions, connected with the presence of co-ligand, are observed at ca. 550 nm,
while those attributed to ONO ligand in the 400-500 nm range. The UV part of the spectra is dominated by the
intense bands of co-ligands.

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Fig. 8. The UV-Vis reflection spectra of 1 - 3a after Kubelka-Munk transformations [28]. BaSO4 as internal white
standard.
The selected spectra in solution are presented in Fig. 9. Due to the very high intensity of the bands
attributed to ONO ligand, in visible part of the spectra, this band dominates (at ca. 400-500 nm range). As mentioned
above, due to low intensity of d-d transitions, at concentrations available (due to complex solubility), these bands are
not well visible. The molar absorption coefficients in DMSO were estimated for 2 and 3, as complex 1 was found to
be unstable in DMSO. The data are: ε361nm = 3.92·103 M-1cm-1 and ε324nm = 4.67·103 M-1cm-1 for 2 and ε510nm
=
1.55·102 M-1cm-1, ε417nm = 5.60·103 M-1cm-1 and ε330nm = 8.40·103 M-1cm-1 for 3. In solution the spectra of 3 and 3a
are essentially the same, as the only difference is the presence of solvent molecule in 3a. The complexes are
solvatochromic, as shown in Fig. 9.

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Fig. 9. UV-Vis spectra of 1 (up), 2 (middle) and 3 (down) in EtOH (black line) and DMSO (red line), d = 1 cm.
Thermogravimentric analysis
o
The TG results for complexes 1 and 3a, measured in air at 10 C/min, are presented in Fig. 10, while the numerical
data are selected in Table 6. The stepwise decomposition of complex 1 is observed. Schiff base ligand decomposes in
o
parts, the mass loss after 372 C peak indicates that aldehyde is released as first, while hydrazide decomposes later.
The decomposition of maltol is a very complicated process, probably involving its oxidation/reduction. Probably part
of the oxygen forms vanadium oxides, part of carbon can be a source for carbide formation. These processes are
o
probably responsible for observed constant mass loss, even much above 800 C. For 3a the decomposition starts at
o
very beginning, but at ca. 270 C mass loss can be attributed to bpy and EtOH molecule. The almost constant
decrease of the mass versus temperature is consistent with the X-ray crystal structure determination, when for longer
stored crystals only partial occupancy of EtOH molecule was found. For this process, the calculated ꢀm (32.5 %) fits
very well with the observed one (32.2 %). In the next two, a not well resolved processes, ligand L is released.
Presence of two separated decomposition processes, at this stage, indicates that the Schiff base ligand L decomposes
into components. The first mass loss equal to 34.2 % (66.4 – 32.2 %) fits quite well with the release of 3,5-di-tert-
butyl-2-hydroxybenzaldehyde (calculated for this process mass loss is equal to 35.1 %) supporting decomposition of
ligand L into components (aldehyde and hydrazide) similarly as it was observed for 1.

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Fig. 10. TG and SDTA curves of 1 (left) and 3a (right). Scan rate 10o/min, air.
Table 6 Data of thermogravimetric analysis of 1 and 3a. Scan speed 10o/min, air.
Complex
m [mg]
10.7767
T
max [oC]
Attributed to
∆mexp [%]
∆mcalc [%]
1
9.4659
241
268
343
12.2
11.9
0.18L
5.4715
431
465sh
838
49.2
66.4
49.3
65.9
0.75L
L
3.6178
8.26445
7.20860
3a
159
186
257
356
372
470
12.8
32.2
7.41
32.5
EtOH
5.60710
2.78000
EtOH + bpy
66.4
87.8
66.6
89.2
EtOH + bpy +0.6L
EtOH + bpy + L
1.00440
Cyclic voltammetry
The cyclic voltammetry measurements were performed in DMSO on Pt working and counting electrodes using 0.1M
Bu4NPF6 as supporting electrolyte and are presented in Figs 11 - 13. The potentials and proper electrochemical
processes for 1, 2 and 3 were summarized in Table 7. As the complexes 3 and 3a are essentially identical in solution
(they differ only by solvent molecule), voltammogram of 3 is presented as representative one for both complexes.
For 1 oxidation peaks at +0.050, +0.561 and +0.760 V (at 100 mV/s) are observed. The reduction peaks are at -0.050
and -1.14V. The irreversible processes observed at positive potentials can be attributed to vanadium oxidation

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excluding that at +0.050 V. The observed irreversibility of vanadium oxidation (lack of reduction peaks) is artificial
one, as oxidation peak show low sensitivity on scan speed change (peak at 741 V shifts to 0.835 V on scan speed
change from 20 to 1000 mV/s). This is indicative for reversible processes accompanied by fast chemical changes in
oxidized product. The fast decomposition/reduction process makes the reduction peak invisible. This is supported by
the data shown in inset in Fig. 11, when longer period the reduction peaks start to be observed (they can be attributed
to byproducts formed upon reduction of oxidized forms of vanadium). The peak observed at ca. 0V shows a
reversible patters (see inset in Fig. 11) with the E1/2 = 0.00 V, with Ea/Ec peak separation of 100 mV and
independence of Ea and Ec potentials on scan speed. As such a peak is not visible neither for 2 nor for 3, it can be
attributed to maltolato ligand. The irreversible reduction peak at -1.140 V (at 100 mV/s) can be attributed to ONO
ligand. Voltammograms of 2 and 3 can be general interpreted in similar manner. The remarkable difference is
observed, however in the negative part of the voltammogram. For 2 the reversible process (see Fig.12) observed at
E1/2 = -1.37V, with Ea/Ec peak separation of 140 mV, Ia/Ic close to 1.0 and E1/2 independent on scan speed, can be
attributed to reversible phen ligand redox process. Similar situation was observed earlier for vanadium complexes
with phen as ligand, described in literature [15,16]. For complex 3, reversible peak is observed at -1.388V (Ea, Ec
peak separation of 178 mV at 100 mV/s) and can be attributed to bpy reduction (see Fig. 13).
Fig. 11. Cyclic voltammogram of 1 in DMSO. 0.1M Bu4NPF6 as electrolyte, Pt working and counting, Ag/AgCl as
reference electrodes, potentials versus NHE. Scan speeds 100 mV/s (for main curve in full range), insets – scans in
selected voltage range, scan speeds in 20 – 1000 mV/s range.

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Fig. 12. Cyclic voltammogram of 2 in DMSO. 0.1M Bu4NPF6 as electrolyte, Pt working and counting, Ag/AgCl as
reference electrodes, potentials versus NHE. Scan speeds 100 mV/s (for main curve in full range), insets – scans in
selected voltage range, scan speeds in 20 – 1000 mV/s range.
Fig. 13. Cyclic voltammogram of 3 in DMSO. 0.1M Bu4NPF6 as electrolyte, Pt working and counting, Ag/AgCl as
reference electrodes, potentials versus NHE. Scan speeds 100 mV/s (for main curve in full range), insets – scans in
selected voltage range, scan speeds in 20 – 1000 mV/s range.

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Table 7 Potentials and proper electrochemical processes for 1, 2 and 3.
Complex
1
Potential [V]
0.76
Process
V(IV) – e- → V(V)
V(III) – e- → V(IV)
mal- - e- ↔ mal
0.56
0.00a
-1.14
0.84
L12- - e- → L1
-
V(IV) – e- → V(V)
V(III) – e- → V(IV)
phen – e- ↔ phen-
V(IV) – e- → V(V)
V(III) – e- → V(IV)
bpy – e- ↔ bpy-
2
0.62
-1.37
0.73
0.43
-1.39
3
a E1/2
Stability of complexes in solution
The stability of complexes in solution plays a crucial role in applications. Fig.14 shows the spectral changes of the
solutions of 1 - 3 in DMSO-H2O mixtures (20 µL + 3 mL respectively) versus time. Again representative data for 3
are presented, similarly as in CV measurements, due to the fact that 3 and 3a are only outer-sphere solvent molecule
isomers. Two different conditions were used – neutral pH (7.00) and pH = 2.00 (pH similar to that in stomach). We
used DMSO as a solvent, as all complexes show good solubility in this solvent, mixture was necessary as complexes
are almost insoluble in water. DMSO was chosen also due to its applications as a solvent in many pharmaceutics
[29,30]. As indicated in Fig. 14, all complexes are stable in the studied condition, only for 3 a small spectral changes
are observed at pH = 2.00. This probably is caused by the flexibility of bpy molecule (in contrary to phen or maltol),
which can be stepwise substituted from complex, while both for maltol and phen two bonds have to be broken at the
same time. This is supported by the crystal structures of complexes, where stability of complexes cannot be neither
driven from distortion observed in coordination sphere (highest for 3 and 3a) nor from the distance V-LB (of co-
ligand, both for trans and cis donating atoms of LB).

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Fig. 14. UV-Vis spectra of 1 (left), 2 (middle) and 3 (right) in DMSO-H2O (20 µL + 3 mL) mixture at pH = 7.00 (up)
and at pH = 2.00 (down). T = 37 °C, spectra measured in 340 s intervals. The arrows indicate the direction of the
changes.
Conclusions
Four new complexes of vanadium are described, two of them are the solvation isomers. The X-Ray single
crystal structure measurements show a remarkable role of solvent in crystal packing. Small change of methanol into
ethanol (in 3 and 3a) results in dramatic change in packing and symmetry of the crystals. The remarkable role of
π…π and C-H…π interactions on structure is shown, probably as a result of the indicated very weak hydrogen bonds
in all studied structures. The trans effect was detected for all complexes, elongating the V-Ltrans (trans to short V=O
bond) versus V-Lcis bond distances. In spite of generally very weak intermolecular interactions, in which vanadium
donating atoms of ligands do not participate, there is no correlation between V=O and V-Ltrans distances, even for the
same co-ligand as in 3 and 3a. The V=O band positions in IR spectra were correlated not only with the V=O, but also
with the V-Ltrans distances, while the stability in solution was dependent on the flexibility of the co-ligand ligand. The
pharmaceutical activity of investigated complexes, both in vitro and in vivo studies will be soon presented together
with other complexes of similar structure.

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Supporting information
CCDC 181172, 1844928, 1850268 and 1848816 of 1, 2, 3 and 3a respectively, contain the supplementary
crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Center
via http://www.ccdc.cam. ac.uk/data_request/cif. The cif files of 3a is included in supplementary materials, together
with other experimental data of 1-3a not included in this paper.
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was partly financed by the European Regional Development Fund under the Innovative Economy
Programme 2007–2013 (WND POIG.01.03.01-174/09). The complexes are protected by the patent P.401493.
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Highlights
•
•
•
•
Synthesis and properties of vanadium(IV/V) with Schiff base ligands are described
Single crystal structures of four complexes are described and discussed
The X-Ray structure measurements show a notable role of solvent in crystal packing
The stability in solution in neutral pH and pH = 2.00 is discussed