A vanadyl(V) complex with a pincer-type thiobis(phenolate) and an acetylacetonate co-ligand, VO(tbp)(acac)

Article abstract of DOI:10.1515/znb-2010-1208

A new vanadyl(V) complex with the tridentate, pincer-type O,S, O-ligand thiobis(phenolate), tbp, was synthesized from the reaction of vanadium(IV) oxide bis(acetylacetonate), VO(acac)₂, and 2,2′-thio-bis(4- methylphenol), H₂tbp, in r

Full text of DOI:10.1515/znb-2010-1208

A Vanadyl(V) Complex with a Pincer-type Thiobis(phenolate)
and an Acetylacetonate Co-ligand, VO(tbp)(acac)
Rahman Bikasa, Hassan Hosseini Monfareda, Somayeh Alamib, Ingo Pantenburgc,
and Gerd Meyerc
a Department of Chemistry, Zanjan University, 45195-313, Zanjan, I. R. Iran
b Young Research Club, Islamic Azad University of Tabriz, P. O. Box 1655, Tabriz, I. R. Iran
c Department fu¨r Chemie, Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstraße 6,
50939 Ko¨ln, Germany
Reprint requests to Dr. H. H. Monfared. Fax: +98 241 5283203. E-mail: monfared@znu.ac.ir
Z. Naturforsch. 2010, 65b, 1457 – 1461; received June 29, 2010
A new vanadyl(V) complex with the tridentate, pincer-type O,S,O-ligand thiobis(phenolate), tbp,
was synthesized from the reaction of vanadium(IV) oxide bis(acetylacetonate), VO(acac)2, and 2,2ꢀ-
thio-bis(4-methylphenol), H2tbp, in refluxing methanol in high yield. Single-crystal X-ray diffraction
structure analysis revealed for VO(tbp)(acac) that the vanadium atom is octahedrally coordinated with
an O5,S-donor environment where the thiobis(phenolate) ligand is O,S,O-bonded to the vanadyl(V)
center facially in a tridentate dinegative fashion. Cyclic voltammetric experiments carried out with
its solution in dimethyl sulfoxide revealed an irreversible peak at −0.768 V (vs. Ag/AgCl) relating to
the V5+/V4+ reduction process.
Key words: Oxovanadium(V), Thiobis(phenolate), Complex, Molecular Structure,
Cyclic Voltammetry
Introduction
ture, and selective stabilization of these two motifs de-
pends upon the basicity of donor atoms.
The element vanadium plays an important role
in many biological processes. Numerous high-valent
vanadium compounds have been studied for their
promising insulin-mimetic effects, anticancer activity
and antiviral properties [1, 2]. Two classes of vanadium
enzymes, viz. vanadium nitrogenases and vanadate-
dependent haloperoxidases, have been found in nature
so far, and their structures and properties have stim-
ulated the search for structural and functional model
compounds [3 – 8].
Furthermore, oxovanadium complexes are used in
oxidation and oxotransfer catalysis [9, 10]. Alpha-
olefin polymerization catalyzed by some vanadium(V)
complexes has been reported [11, 12]. Moreover, the
role of vanadium complexes in catalytically conducted
redox reactions [13, 14], and potential medicinal ap-
plications such as the treatment of diabetes type I and
type II [15], have stimulated interest in the stereochem-
istry and reactivity of such compounds. In most cases
the active site contains one of the two motifs VO3+
or VO2+ coordinated by oxygen and nitrogen atoms.
The strong affinity of these two motifs towards O,N-
donor ligands is probably due to their hard acidic na-
On the other hand, the chemistry of sulfur- or
selenium-containing bisphenol ligands and their tran-
sition metal complexes is of interest because of their
versatile structural, magnetic, catalytic and electron
transfer activity [16, 17]. Such complexes serve as
functional models for various biomolecules having
potential ability to form one-electron oxidized phe-
noxyl radicals rendering catalytic oxidations of organic
molecules [18– 20]. Multinuclear complexes, and es-
pecially binuclear species, were found to catalyze the
oxidative deamination of primary amines [21], pro-
viding rare examples of copper amine oxidase en-
zymes. Phenol-containing ligands [22– 24] have also
been used in aerial oxidation of primary alcohols and
amines catalyzed by Cu(II) complexes [25] attract-
ing considerable attention in the bioinorganic chemi-
cal community because of the widespread occurrence
in oxygen-dependent enzymatic oxidations.
As part of our continuing efforts to develop vanadyl
(VO3+) complexes with various multidentate lig-
ands [26], this work deals with the synthesis and struc-
ture of a new oxovanadium(V) complex of a chelating
dianionic O,S,O-type thiobis(phenolate), tbp, ligand.
c
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R. Bikas et al. · A Vanadyl(V) Complex VO(tbp)(acac)
Table 1. Crystal data and structure refinement parameters
Experimental Section
Materials and instrumentations
for 1.
Empirical formula
Formula weight, g mol−1
Crystal system
[VO(tbp)(acac)], C19H19O5SV
410.34
monoclinic
P21/c (no. 14)
7.891(1)
15.716(2)
15.515(2)
93.18(1)
1921.2(5)
4
Vanadium(IV) oxide bis(acetylacetonate), VO(acac)2, and
p-cresol were purchased from Merck and used as re-
ceived. Solvents of the highest grade commercially available
(Merck) were used without further purification. IR spectra
were recorded in KBr discs with a Bruker FT-IR spectropho-
tometer. UV/Vis solution spectra were recorded on a thermo-
spectronic Helios Alpha spectrometer. Elemental analyses
were carried out using a Perkin-Elmer 240 elemental ana-
lyzer. 1H and 13C NMR spectra of the ligand and of the
complex in CDCl3 solution were recorded on Bruker 250
and 400 MHz spectrometers, and chemical shifts are given in
ppm relative to tetramethylsilane. Cyclovoltammetric experi-
ments were performed using a Metrohm computrace voltam-
metric analyzer model 757 VA. Data collection for X-ray
structure determination was performed on a Stoe IPDS II
diffractometer.
Space group
˚
a, A
˚
b, A
˚
c, A
β, deg
3
˚
Volume, A
Z
Density (calc.), g cm−3
Absorption coefficient, mm−1
F(000), e
1.42
0.7
848
θ range for data collection, deg
1.85 – 27.28
10, 20, 19
28517 / 4271 / 0.1493
1998
Index ranges hkl
Refl. collected / unique / Rint
Refl. with I ≥ 2σ(I0)
Data completeness, %
Data / restraints / parameters
Final R1/wR2 [I ≥ 2σ(I0)]
R1/wR2 indices (all data)
Goodness of fit on F2
98.9
4271 / 0 / 235
0.0495 / 0.0800
0.1352 / 0.0985
0.815
Synthesis of 2,2ꢀ-thio-bis(4-methylphenol) (H tbp)
2
−3
˚
Largest diff. peak / hole, e A
0.26 / −0.25
2,2ꢀ-Thio-bis(4-methylphenol) (H2tbp) was synthesized
according to reported procedures [33]. Yield 65 % (16 mg),
191.14 (2 C, C=O). – UV/Vis (CH3OH): λmax(lgεmax) =
205 (4.70), 286 nm (4.09).
◦
m. p. 114 – 115 C. – Anal. for C14H14O2S (246.32): calcd.
C 68.26, H 5.73; found C 68.19, H 5.75. – FT-IR (KBr):
ν = 3067 (s, vbr), 2921 (s), 2859 (m), 1608 (m), 1587 (m),
1503 (vs), 1453 (s), 1412 (s), 1359 (s), 1287 (vs), 1227 (m),
1137 (m), 1057 (w), 945 (vs), 880 (m), 824 (m), 711 (m),
567 (w), 526 (w), 489 cm−1 (w). – 1H NMR (250.13 MHz,
CDCl3, 25 ◦C, TMS): δ = 2.27 (s, 6 H), 6.82 (d, J =
8.30 Hz, 2 H), 7.05 (m, 2 H), 7.16 (d, J = 8.33 Hz, 2 H)
and 8.64 (s, 2 H). – UV/Vis (CH3OH): λmax(lgεmax) =
208 (4.72), 248 (3.95sh) and 299 nm (3.95).
Electrochemical measurements
For cyclic voltammetry studies, a conventional three-
electrode system was used with a polished glassy carbon
electrode (area 3.14 mm2) as working electrode and a plat-
inum wire counter electrode. The reference electrode was
an aqueous Ag/AgCl saturated electrode, separated from
the bulk of the solution by a bridge with solvent and sup-
porting electrolyte. The electrolytic medium consisted of
0.1 mol L−1 lithium perchlorate (LiClO4) as supporting elec-
trolyte in dimethyl sulfoxide; experiments were carried out at
r. t. The solution was freshly prepared before use and purged
with N2 saturated with solvent for ca. 15 min prior to taking
measurements in order to remove dissolved O2. The voltam-
mogram was recorded under various scan rates in the range
from −2.0 to −1.0 V vs. Ag/AgCl.
Synthesis of the complex [VO(tbp)(acac)] (1)
A methanolic solution (20 mL) of the ligand precursor
H2tbp (2.00 mmol) was added dropwise to a methanolic so-
lution (20 mL) of VO(acac)2 (2.00 mmol), and the mixture
was refluxed for 2 h. The resulting solution was allowed
to undergo slow evaporation at r. t. After 2 d dark-violet
single crystals suitable for X-ray diffraction were obtained.
Yield 81 % (665 mg). – Anal. for C19H19O5SV (410.36):
calcd. C 55.61, H 4.67; found C 55.58, H 4.66. – FT-IR
(KBr): ν = 3268 (m, br), 3019 (m, br), 1717 (m), 1565 (s),
X-Ray diffraction data collection and refinement
Data collection for X-ray structure determination was per-
1531 (s), 1466 (s), 1345 (m), 1273 (s), 1243 (s), 1052 (m), formed on a Stoe IPDS II diffractometer using graphite-
987 (vs), 894 (w), 815 (m), 758 (vs), 603 (w), 462 cm−1 monochromatized MoKα radiation (0.71073 A) at 293(2) K
˚
◦
(w). – 1H NMR (400.13 MHz, CDCl3, 25 C, TMS): δ = (0◦ ≤ ω ≤ 180◦, ϕ = 0◦, 0◦ ≤ ω ≤ 166◦, ϕ = 90◦, ∆ω =
2.12 (s, 6 H, two CH3 of acac), 2.26 (s, 6 H, two aryl- 2◦, 173 frames). The data were corrected for Lorentz and po-
CH3), 5.93 (s, 1 H, =CH- of acac), 6.58 – 7.26 (m, 6 H). – larization effects. A numerical absorption correction based
13C NMR (100.62 MHz, CDCl3): δ = 19.73 (2 C, aryl- on crystal-shape optimization was applied for all data [27].
CH3), 25.16 (2 C, CH3 of acac), 103.76 (1 C), 125.16 (4 C), The programs used are X-AREA [27], including X-RED [28]
130.75 (2 C), 132.49 (2 C), 134.04 (2 C), 167.12 (2 C), and X-SHAPE [29] for data reduction and absorption cor-
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R. Bikas et al. · A Vanadyl(V) Complex VO(tbp)(acac)
1459
˚
Table 2. Selected bond lengths (A) and angles (deg) in the
rection, and the WINGX suite of programs [30], including
SIR-92 [31] and SHELXL-97 [32] for structure solution and
refinement. The hydrogen atoms were placed in idealized po-
sitions and constrained to ride on their parent atom. The last
cycles of refinement included atomic positions for all atoms,
anisotropic thermal parameters for all non-hydrogen atoms
and isotropic thermal parameters for all hydrogen atoms. Ta-
ble 1 contains the crystal data and numbers pertinent to data
collection and structure refinement.
crystal structure of 1.
V1–O1
1.588(3)
1.856(3)
1.849(3)
1.977(3)
1.955(3)
2.795(1)
1.342(4)
1.349(4)
1.279(5)
1.283(5)
1.372(7)
1.392(7)
1.783(4)
1.784(4)
O1–V1–O11
O1–V1–O21
O11–V1–O21
O1–V1–O31
O11–V1–O31
O21–V1–O31
O1–V1–O33
O11–V1–O33
O21–V1–O33
O31–V1–O33
O21–V1–S1
O1–V1–S1
98.89(13)
100.79(13)
94.94(11)
100.53(13)
88.63(11)
157.54(11)
101.64(14)
159.21(11)
84.47(12)
84.44(12)
76.86(8)
V1–O21
V1–O11
V1–O33
V1–O31
V1–S1
O21–C21
O11–C11
O33–C33
O31–C31
C31–C32
C32–C33
S1–C26
S1–C16
CCDC-781486 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
175.76(11)
77.91(8)
82.27(8)
O11–V1–S1
O31–V1–S1
Results and Discussion
was prepared in high yield from the reaction of H2tbp
The thiobis(phenol) H2tbp was synthesized by the with an equimolar amount of VO(acac)2 in refluxing
reaction of sulfur dichloride with p-cresol in dry methanol during two hours (Scheme 1).
petroleum ether; recrystallization of the crude prod-
The electronic spectra of the ligand and the com-
uct gave the desired symmetric pincer-type tridentate plex (1) show very similar absorption peaks (Fig. 1).
O,S,O-donor ligand in excellent purity [33, 34]. The The electronic spectrum of the complex in methanol
oxovanadium(V) complex of this thiobis(phenolate) exhibits two bands at 205 and 286 nm which may be
assigned to π → π∗ and n → π∗ transitions of the lig-
and, respectively. There is a noticeable blue shift of the
n → π∗ transition of the ligand upon coordination.
Description of the structures
In order to define the coordination sphere conclu-
sively, a single-crystal X-ray diffraction study of 1 was
undertaken. Crystallographic data are listed in Table 1.
A plot of the molecular structure with the atom num-
bering scheme adopted is shown in Fig. 2, and selected
interatomic distances and angles are summarized in Ta-
ble 2. The packing of the molecules in crystals of 1 as
Scheme 1. Synthesis of the oxovanadium(V) thiobis-
(phenolate) complex [VO(tbp)(acac)] (1).
Fig. 1 (color online). Electronic spectra of H2tbp (blue) and Fig. 2. The molecular structure of [VO(tbp)(acac)] (1) in the
[VO(tbp)(acac)] (red) in CH3OH.
crystal with the atom numbering scheme used.
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1460
R. Bikas et al. · A Vanadyl(V) Complex VO(tbp)(acac)
Fig. 4. Cyclic voltammogram of 10−3 mol L−1 [VO(acac)-
(tbp)] (1) in DMSO and LiClO4 (0.1 mol L−1); scan rate
100 mV s−1 in the potential range −1.0 to −0.2 V.
Fig. 3. Packing of the molecules in crystals of 1 as projected
onto the ac plane.
projected onto the ac plane is shown in Fig. 3. The unit
cell contains four molecules of 1, the space group is
P21/c.
In complex 1 the vanadium atom is octahedrally
coordinated in an O5S donor environment. A termi-
nal V=O-type oxygen atom occupies an axial posi-
˚
tion at a distance of 1.588(3) A. The OSO donor thio-
bis(phenolate) pincer-type ligand coordinates facially
with the sulfur atom trans to the terminal V=O frag-
ment. The two oxygen atoms of the bidentate acety-
lacetonate ligand occupy the remaining two sites. The
equatorial VO4 unit is far from planar. The displace-
˚
ment by 0.344 A of the vanadium atom from the mean
plane comprising the two independent phenolate oxy-
gen atoms O11, O21 and the two acetylacetone oxygen
atoms, O31 and O33 is towards the oxo oxygen atom.
The O,S,O-donor ligand forms two five-membered
Fig. 5. Cyclic voltammogram of 10−3 mol L−1 [VO(tbp)-
(acac)] (1) in DMSO and LiClO4 (0.1 mol L−1); scan
rates 10, 25, 50, 75, 100, 150, 200, 300 and 400 mV s−1
.
chelate rings with bite angles of about 77◦ (Ophenolat
–
(O1) is at the lower end of the range typically ob-
served for oxovanadium(V) complexes [35, 36]. The
short V–O1 distance thereby indicates the presence of
a vanadium–oxygen double bond which is commonly
found in VO3+ complexes. The two phenyl rings in this
system are planar and retain their aromaticity. The two
mean planes formed by the aromatic rings intersect at
an angle of about 69◦.
V–S). The vanadium to oxygen bond lengths follow
the order V–oxo oxygen< V–phenolate oxygen< V–
acetylacetonate oxygen. The two diagonal angles O11–
V–O33 and O21–V–O31, 159.21(11) and 157.54(11)◦,
respectively, are significantly smaller than 180◦. This
deviation from the ideal octahedral geometry may be
caused by the small bite angles made by the two phe-
nolate oxygen atoms and the tripodal sulfur atom at
the vanadium atom (O11–V1–S1 = 77.91(8) and O21–
V1–S1 = 76.86(8)◦). This is again due to a remarkably
Electrochemical studies
˚
longer V–S distance (2.795(1) A). This phenomenon
Electrochemical measurements (cyclic voltamme-
may be explained in terms of a trans influence of the try) were carried out to probe the redox stability of
oxo group, which occupies the other axial position at complex 1 in solution. The cyclic voltammogram in
˚
a normal distance of 1.588(3) A [26]. The distance dimethylsulfoxide displays an irreversible reduction
of the vanadium atom to the terminal oxygen atom peak at −0.768 V (vs. Ag/AgCl) due to the VO3+
-
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R. Bikas et al. · A Vanadyl(V) Complex VO(tbp)(acac)
1461
VO2+ couple (Fig. 4). Fig. 5 illustrates the cyclic Acknowledgement
voltammograms of complex 1 at different scan rates
from 10 to 400 mV s−1. This behavior is also diagnos-
tic of an irreversible electron transfer process.
This research was supported by the Zanjan University and
the Universita¨t zu Ko¨ln.
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