ONN-complexes of dioxomolybdenum(VI) with 2-hydroxy-1-naphthaldehyde S-ethyl-4-H/phenyl-thiosemicarbazones: Crystal structure, electrochemistry and in situ spectroelectrochemistry

Article abstract of DOI:10.1016/j.poly.2010.07.022

Neutral dioxomolybdenum(VI) complexes of dibasic 2-hydroxy-1-naphthaldehyde S-ethyl-4-H/phenyl-thiosemicarbazones (H: L¹, C₆H ₅: L²) have been synthesized. The complexes, [MoO ₂L^I(ROH)] (1a

Full text of DOI:10.1016/j.poly.2010.07.022

Polyhedron 29 (2010) 2924–2932
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
ONN-complexes of dioxomolybdenum(VI) with 2-hydroxy-1-naphthaldehyde
S-ethyl-4-H/phenyl-thiosemicarbazones: Crystal structure, electrochemistry
and in situ spectroelectrochemistry
a
a
b
c
a,
⇑
_
Songül Duman , Irfan Kızılcıklı , Atıf Koca , Mehmet Akkurt , Bahri Ülküseven
a Department of Chemistry, Istanbul University, 34320 Avcılar, Istanbul, Turkey
b Department of Chemical Engineering, Faculty of Engineering, Marmara University, Göztepe 34722, Istanbul, Turkey
c Department of Physics, Erciyes University, Faculty of Arts and Sciences, 38039 Kayseri, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Neutral dioxomolybdenum(VI) complexes of dibasic 2-hydroxy-1-naphthaldehyde S-ethyl-4-H/phe-
nyl-thiosemicarbazones (H: L1, C6H5: L2) have been synthesized. The complexes, [MoO2LI(ROH)]
(1a–d) and [MoO2LII(ROH)] (2a–d) (R: CH3, C2H5, C3H7, C4H9) were characterized by elemental anal-
ysis, electronic, IR and 1H NMR spectra. X-ray crystal studies indicated a distorted octahedral geom-
Received 13 April 2010
Accepted 17 July 2010
Available online 6 August 2010
etry for [MoO2(L1)(C2H5OH)] (1b) and [MoO2(L2)(CH3OH)] (2a). The Mo–O bond lengths of the
Keywords:
Thiosemicarbazone
2þ
MoO2
moieties are almost the same in the monoclinic crystal structures of complexes 1b and
2a, and the MoO2 cores have a cis-dioxomolybdenum structure with angles ca. 105 Å. While 1b crys-
tallizes in the space group C2/c, 2a crystallizes in the space group P21/n. The electrochemical behav-
iors of the ligands and their complexes 1b and 2a were studied using cyclic voltammetry and square
wave voltammetry. The half-wave potentials (E1/2) are significantly influenced by the central metal
ions, the nature of the substituents on the thiosemicarbazones, and the electronic character of the
ligands, which are the important factors that control redox potentials. In situ spectroelectrochemical
studies were employed to determine the spectra of the electrogenerated species of the complexes
and to assign the redox processes. The colors of the electrogenerated species were determined with
in situ electrocolorimetric measurements.
Dioxomolybdenum complex
Structural analysis
Cyclic voltammetry
Spectroelectrochemical analysis
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
ecule. Chelate complexes with a cis-MoO2 center have been
synthesized for oxygen atom transfer (OAT) reactions and progress
Molybdenum is a trace element and one of the cofactors for sev-
eral enzymes catalyzing redox reactions [1,2], and some molybde-
num compounds are included in plausible enzyme model systems
towards this objective is continues to increase. In the last 20 years,
a large number of dioxomolybdenum complexes with different li-
gand types having O2N [8,5], S2ON [3], SO2N2 [9], O2N2 [10], S2N2
[11] or ONS [12] donor sets have been synthesized, and their activ-
ities for catalyzing an OAT reaction were reported [13]. For this
purpose, structural analysis of dioxomolybdenum complexes of
the general formula [MoO2(L)(L0)] have been studied using various
dibasic 2-hydroxy-arilydene-S-alkylthiosemicarbazones. These pa-
pers report that the sulfur [14–16], N4-nitrogen [13,4,17–19]
monosubstitued, and sulfur and N4-nitrogen [20–22] disubstitued
thiosemicarbazone derivatives act as ONN or ONS donor sets.
In this work, we have used two S-ethyl 4-H/phenyl-thiosemi-
carbazones (H2L1 and H2L2) to determine the structural and elec-
trochemical properties of dioxomolybdenum complexes of the
composition [MoO2L(L0)], the second ligand (L0) was a C1–4 n-alco-
hol (Fig. 1). In situ spectroelectrochemical and in situ electrocolori-
metric studies were employed for the first time to determine the
spectra and colors of the electrogenerated species of the dioxomo-
lybdenum(VI) complexes.
2þ
[3–5]. Mixed ligand complexes with a MoO2 core and having the
general formula [MoO2(L)(L0)] were synthesized as model systems
of molybdoenzymes catalyzing redox reactions. The catalytic
capacity of the molybdenum compounds has been thought to be
related to the structural features of the chelated cis-MoO2 unit
[1]. Additionally, the relationships between the catalytic activity
of the complexes and their chelate ring structure and also the sub-
stituents of the ligands have been examined [6,7].
Many thiosemicarbazones show various biological activities.
The known molybdenum(VI) complexes of thiosemicarbazones of
the general formula [MoO2L(L0)] are potential catalysts as the coor-
dinated L0 molecule may be replaced by an activated enzyme mol-
⇑
Corresponding author.
E-mail address: bahseven@istanbul.edu.tr (B. Ülküseven).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.07.022

S. Duman et al. / Polyhedron 29 (2010) 2924–2932
2925
ods using SIR-97 [25], while the refinement and all further
calculations were carried out using SHELXL-97 [26]. ORTEP-3 for Win-
dows [27] and PLATON [28] were used for molecular graphics. For 1b,
the H atom H1 of the OH group was located from a difference Fou-
rier map, and its isotropic thermal parameter was constrained to
ride on O4 with Uiso(H) = 1.5Ueq(O). The other H atoms were posi-
tioned with an idealized geometry using a riding model with
C–H = 0.93–0.97 Å, N–H = 0.86 Å and Uiso(H) = 1.2Ueq(C,N). The C
and H atoms of the ethanol moiety exhibit disorder over two posi-
tions. The site occupancies for the disordered part refined to
0.771(13) and 0.229(13). For 2a, The H atom H1 of the OH group
was located from a difference Fourier map, and its isotropic ther-
mal parameter was constrained to ride on O4 with Uiso(H) = 1.5Ue-
q(O). The other H atoms were positioned with an idealized
L'
O
O
O
Mo
4
R
N
1
N
2
N
CH3
S
Fig. 1. The thiosemicarbazones. R: H (L1), C6H5 (L2). The molybdenum complexes.
L0: CH3OH (1a and 2a), C2H5OH (1b and 2b), C3H7OH (1c and 2c), C4H9OH (1d and
2d).
2. Experimental
2.1. IR and 1H NMR data
geometry using
a riding model with C–H = 0.93–0.97 Å, N–
H = 0.86 Å and Uiso(H) = 1.2Ueq(C,N). The non-H atoms were refined
anisotropically, using weighted full-matrix least-squares on F2. The
crystal data and refinement details are presented in Tables 1–4.
The elemental analyses were determined on a Thermo Finnigan
Flash EA 1112 Series Elemantar Analyser. Infrared spectra were re-
corded as KBr discs on a Mattson 1000 FT-IR spectrophotometer in
the 4000–400 cmꢀ1 range at room temperature. The 1H NMR spec-
tra were recorded on Bruker Avance-500 model spectrometer rela-
tive to SiMe4 using CDCl3.
2.3. Electrochemistry and spectroelectrochemistry
The measurements were carried out with a Gamry Reference
600 potentiostat/galvanostat utilizing a three-electrode configura-
tion at 25 °C. For cyclic voltammetry (CV) and square wave voltam-
metry (SWV) measurements, the working electrode was a Pt disc
with a surface area of 0.071 cm2. The surface of the working elec-
trode was polished with a diamond suspension before each run.
A Pt wire served as the counter electrode. A saturated calomel elec-
trode (SCE) was employed as the reference electrode and separated
from the bulk of the solution by a double bridge. Ferrocene was
used as an internal reference. Tetrabutylammonium perchlorate
(TBAP) in dichloromethane (DCM) was employed as the supporting
electrolyte at a concentration of 0.10 mol dmꢀ3. High purity N2 was
used to remove dissolved O2 at least 15 min prior to each run and
to maintain a nitrogen blanket during the measurements. IR
2.2. X-ray structure determination
Single crystal data collection for the complexes was performed
on a Rigaku RAXIS RAPID imaging plate area detector with graphite
monochromated Mo K
a radiation. The data were collected at a
temperature of 295(2) K to a maximum 2h value of 60.3°. A total
of 144 oscillation images for 1b and 135 oscillation images for 2a
were collected. A sweep of data was done using
0.0° to 180.0° in 5.0° steps for 1b and in 4.0° steps for 2a. The expo-
sure rate was 48.0 [s/°] for 1b and 30.0 [s/°] for 2a. The detector
swing angle was 0.03° for 1b and 0.02° for 2a. CrystalClear [23]
and SORTAV [24] were used for the cell refinement and data reduc-
tion. The crystal structures were solved by means of direct meth-
x oscillations from
Table 1
The crystallographic data and structure refinement for complexes 1b and 2a.
Compound
1b
2a
Formula
C
16H19MoN3O4S
C21H21MoN3O4S
507.41
monoclinic
P21/n
Formula Weight
Crystal system
Space Group
Unit cell dimension
a (Å)
445.35
monoclinic
C2/c
28.3766 (5)
8.3090 (1)
19.7391 (6)
90
10.8830 (2)
8.1727 (1)
24.0848 (5)
90
b (Å)
c (Å)
a
(°)
b (°)
129.282 (2)
90
3602.46 (17)
8
1.642
0.87
93.747 (1)
90
2137.61 (6)
4
1.577
0.74
c
(°)
V (Å3)
Z
qcalc (g cm–3
)
l(Mo K
a
) (mmꢀ1
)
F(0 0 0)
1808
1032
Crystal size (mm)
Crystal shape, crystal color
T (K)
0.20 ꢁ 0.20 ꢁ 0.50
prism, red
295(2)
0.20 ꢁ 0.20 ꢁ 0.50
block, red
295(2)
k (Mo K
a) (Å)
0.71073
0.71073
h Range (°)
h, k, l limits
Reflections: total, unique data, Rint
Observed data [I > 2
Tmin, Tmax
2.60–30.12
ꢀ40: 39, ꢀ11: 10, ꢀ27: 27
64 882, 5200, 0.026
5123
0.812, 0.840
5123, 262
3.00–30.17
ꢀ15: 15, ꢀ11: 11, ꢀ31: 33
61 397, 6236, 0.033
6019
0.836, 0.862
6019, 277
r (I)]
Nref, Npar
R1, wR2, S [I > 2
Weighting scheme
Largest difference in peak and hole
(e Åꢀ3
r
(I)]
0.0260, 0.074, 1.04
0.033, 0.090, 1.04
2
2
w ¼ 1=½r2ðFo2Þ þ ð0:046PÞ þ 2:6614Pꢂ where P ¼ ðFo2 þ 2F2c Þ=3 w ¼ 1=½r2ðF2oÞ þ ð0:0532PÞ þ 0:8659Pꢂ where P ¼ ðFo2 þ 2F2c Þ=3
0.45, ꢀ0.88
1.01, ꢀ0.48
)

2926
S. Duman et al. / Polyhedron 29 (2010) 2924–2932
Table 2
MPc under study. During the measurements, readings were taken
as a function of time under kinetic control.
The molybdenum centered bond distances of 1b and 2a (Å).
1b
2a
2.4. Syntheses of the compounds
Mo1–O1
Mo1–O2
Mo1–O3
Mo1–O4
Mo1–N1
Mo1–N3
1.9447 (17)
1.6847 (15)
1.7131 (13)
2.3727 (13)
2.2333 (13)
2.0505 (17)
Mo1–O1
Mo1–O2
Mo1–O3
Mo1–O4
Mo1–N1
Mo1–N3
1.9496 (14)
1.7049 (14)
1.6919 (15)
2.4040 (14)
2.2201 (14)
2.0746 (15)
2-Hydroxy-1-naphthaldehyde-S-ethyl-4-H/phenyl-thiosemi-
carbazones (L1 and L2) were synthesized by a known procedure
[29,30]. Hydrobromide forms of the ligands, LꢃHBr, were dissolved
in 5 mL ethanol and neutralized with a sufficient amount of aque-
ous NaHCO3 solution (10%, w/w). The colors, m.p. (°C), yields,
microanalysis, FT-IR (KBr, cmꢀ1) and 1H NMR (500 MHz, CDCl3,
25 °C, d ppm) data of the ligands are given as follows:
Table 3
The molybdenum centered angle values of 1b and 2a (°).
H2L1: Light yellow, 172.4, 185 mg (78%). Anal. Calc. for
C14H15N3OS (273.35 g/mol): C, 61.51; H, 5.53; N, 15.37; S, 11.73.
1b
2a
Found: C, 61.71; H, 5.49; N, 15.30; S, 11.96%. IR:
m
(OH) 3110,
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O1–Mo1–N1
O1–Mo1–N3
O2–Mo1–O3
O2–Mo1–O4
O2–Mo1–N1
O2–Mo1–N3
O3–Mo1–O4
O3–Mo1–N1
O3–Mo1–N3
O4–Mo1–N1
O4–Mo1–N3
N1–Mo1–N3
Mo1–O1–C9
Mo1–O4–C15A
Mo1–O4–C15B
Mo1–N1–C11
Mo1–N1–N2
Mo1–N3–C12
97.82 (8)
106.46 (7)
78.32 (6)
80.47 (5)
147.24 (6)
105.29 (7)
171.74 (7)
93.57 (6)
99.78 (8)
82.87 (6)
158.52 (6)
95.23 (7)
78.65 (5)
80.42 (6)
71.08 (6)
132.99 (12)
124.9 (2)
123.8 (10)
126.19 (11)
117.58 (11)
119.69 (14)
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O1–Mo1–N1
O1–Mo1–N3
O2–Mo1–O3
O2–Mo1–O4
O2–Mo1–N1
O2–Mo1–N3
O3–Mo1–O4
O3–Mo1–N1
O3–Mo1–N3
O4–Mo1–N1
O4–Mo1–N3
N1–Mo1–N3
Mo1–O1–C9
Mo1–O4–C12
Mo1–N1–N2
Mo1–N1–C11
Mo1–N3–C13
Mo1–N3–C16
104.56 (6)
99.03 (8)
77.20 (6)
m
m
s(NH2) 3310, mas(NH2) 3414, d(NH2) 1651, m
(C@N1) 1620,
(C@N2) 1562. 1H NMR: 13.00, 12.86 (s, cis/trans: 1/2, 1H, OH),
9.38, 9.26 (s, syn/anti: 1/1, 1H, CH@N1), 7.19–8.14 (doublet and
triplets, 6H, C10H6), 5.11, 4.78 (broad s, cis/trans: 1/2, 2H, N4H2),
3.15, 3.01 (q, cis/trans: 1/2, 2H, S–CH2), 1.44, 1.42 (t, cis/trans: 1/
2, 3H, –CH3).
80.81 (5)
147.49 (6)
105.94 (8)
81.68 (6)
156.31 (6)
95.20 (6)
172.22 (7)
95.74 (7)
100.02 (7)
77.02 (5)
H2L2: Light yellow, 161, 215 mg (62%). Anal. Calc. for
C20H19N3OS (349.45 g/mol): C, 68.74; H, 5.48; N, 12.02; S, 9.18.
Found: C, 68.52; H, 5.57; N, 11.82; S, 8.92%. IR:
m(OH) 3480,
m(NH) 3410, d(NH) 1620,
m
(C@N1) 1601, (C@N2) 1562. 1H NMR:
m
80.51 (5)
71.28 (5)
13.34, 12.73 (s, cis/trans: 1/2, 1H, OH), 9.67, 9.42 (s, d, syn/anti:
1/1, 1H, CH@N1), 7.32–8.18 (doublet and triplets, 6H, C10H6), 6.34
(broad s, 1H, N4H), 7.10–7.43 (m, 5H, N4C6H5), 3.13, 3.05 (q, cis/
trans: 1/2, 2H, S–CH2), 1.43, 1.38 (t, cis/trans: 1/2, 3H, –CH3).
Preparation of the dioxomolybdenum complexes (1a–d and 2a–
d) were realized with small modifications of the procedures re-
ported in the literature [31,32].
135.63 (12)
126.27 (13)
118.07 (10)
126.20 (11)
119.06 (11)
120.23 (11)
1a: The ligand, H2L1 (0.27 g, 1.0 mmol) was dissolved in abso-
lute methanol (5.0 mL) by heating. The hot solution was treated
with 2.0 mL of a methanolic solution of MoO2(acac)2 (0.36 g,
1.1 mmol). The reaction mixture was stirred at 60 °C for 4 h and
then allowed to stand at room temperature overnight. The cardinal
red precipitate (complex 1a) was collected by filtration and
washed twice with 2–4 mL of cold methanol. Recrystallization of
the product from ethanol gave the analytical grade pure com-
pound. The crystalline powder was dried for 12 h in air.
The other molybdenum complexes, 1b–d and 2a–d, were syn-
thesized by a similar procedure, and recrystallization of the com-
plexes was carried out in alcohol, which is present as a second
ligand in the structure.
Table 4
Hydrogen bond parameters (Å and °) for 1b and 2a.
Complex D–Hꢃ ꢃ ꢃA
d(D–H)
d(Hꢃ ꢃ ꢃA) d(Dꢃ ꢃ ꢃA)
<(DHA)
1b
O4–H1ꢃ ꢃ ꢃN2i
0.77 (3) 2.02 (3)
0.86
0.97
0.77 (3) 2.65 (3)
0.96 2.89
0.77 (3) 1.99 (3)
2.7855 (18) 177 (4)
N3–H3Aꢃ ꢃ ꢃO3ii
C13–H13Bꢃ ꢃ ꢃO3ii
O4–H1ꢃ ꢃ ꢃCg1iii
C14–H14Cꢃ ꢃ ꢃCg4i
O4–H1ꢃ ꢃ ꢃN2i
2.25
2.50
3.027 (3)
3.324 (2)
151
143
2.6720 (15) 84 (2)
3.759 (3)
2.752 (2)
3.762 (2)
3.729 (3)
152
170 (4)
141
2a
C5–H5ꢃ ꢃ ꢃCg5ii
0.93
2.99
2.91
C15–H15Cꢃ ꢃ ꢃCg3iii 0.96
144
The air-dried samples of the complexes were used for all anal-
Symmetry codes: (i) 1 ꢀ x, 1 ꢀ y, 2 ꢀ z; (ii) 1 ꢀ x, y, 3/2 ꢀ z; (iii) x, y, z. Cg1 and Cg4
are the centroids of the Mo1/N1–N3/C12 and C1/C6–C10 rings, respectively.
Symmetry codes: (i) 1 ꢀ x, ꢀy, 2 ꢀ z; (ii) 1/2 + x, 1/2 ꢀ y, ꢀ1/2 + z; (iii) 1 ꢀ x, 1 ꢀ y,
2 ꢀ z. Cg3 and Cg5 are the centroids of the C1–C6 and C16–C21 rings, respectively.
yses. The color, m.p. (°C), yields, microanalysis, FT-IR (KBr, cmꢀ1
)
and 1H NMR (500 MHz, CDCl3, 25 °C, d ppm) data are given below:
1a: Cardinal red, >250 (dec), 155 mg (36%). Anal. Calc. for
C
15H17MoN3O4S (431.32 g/mol): C, 41.77; H, 3.97; N, 9.74; S,
7.43. Found: C, 41.81; H, 4.16; N, 9.51; S, 7.13%. IR: (OH) 3434,
(C@N2) 1547, ms
m
m(NH) 3226, d(NH) 1616,
m
(C@N1) 1597,
m
,mas
compensation was applied to the CV and SWV scans to minimize
the potential control error.
(MoO2) 955, 912. 1H NMR: 9.74 (s, 1H, CH@N1), 7.34–8.27 (doublet
and triplets, 6H, C10H6), 7.08 (s, 1H, N4H), 3.21 (q, 2H, S–CH2), 1.45
(t, 3H, –CH3), 3.49 (s, 3H, –C1H3).
UV–Vis absorption spectra and chromaticity diagrams were
measured by an OceanOptics QE65000 diode array spectrophotom-
eter. In situ spectroelectrochemical and in situ electrocolorimetric
measurements were carried out utilizing a three-electrode config-
uration of a thin-layer quartz spectroelectrochemical cell at 25 °C.
The working electrode was a Pt tulle. A Pt wire counter electrode
and a SCE reference electrode separated from the bulk of the solu-
tion by a double bridge were also used. The standard illuminant A
with a 2° observer at constant temperature in a light booth de-
signed to exclude external light was used. Prior to each set of mea-
surements, background color coordinates (x, y and z values) were
taken at open-circuit, using the electrolyte solution without the
1b: Cardinal red, >250 (dec), 380 mg (85%). Anal. Calc. for
C
16H19MoN3O4S (445.35 g/mol): C, 43.15; H, 4.30; N, 9.44; S,
7.20. Found: C, 43.66; H, 4.68; N, 9.71; S, 7.37%. IR: (OH) 3449,
(C@N2) 1547, ms
m
m(NH) 3299, d(NH)1616,
m
(C@N1) 1593,
m
,mas
(MoO2) 935, 900. 1H NMR: 9.67 (s, 1H, CH@N1), 7.27–8.20 (doublet
and triplets, 6H, C10H6), 7.02 (s, 1H, N4H), 3.14 (q, 2H, S–CH2), 1.39
(t, 3H, –CH3), 3.65 (q, 2H, –C1H2), 1.16 (t, 3H, –C2H3).
1c: Cardinal red, >250 (dec), 190 mg (42%). Anal. Calc. for
C
17H21MoN3O4S (459.37 g/mol): C, 44.45; H, 4.61; N, 9.15; S,
6.98. Found: C, 44.18; H, 4.46; N, 9.09; S, 6.52%. IR: (OH) 3441,
(NH) 3214, d(NH) 1616,
(C@N1) 1597, (C@N2) 1547, ms
m
m
m
m
,mas

S. Duman et al. / Polyhedron 29 (2010) 2924–2932
2927
(MoO2) 946, 904. 1H NMR: 9.67 (s, 1H, CH@N1), 7.27–8.20 (doublet
and triplets, 6H, C10H6), 7.01 (s, 1H, N4H), 3.14 (q, 2H, S–CH2), 1.40
(t, 3H, –CH3), 3.54 (q, 2H, –C1H2), 1.51 (m, 2H, –C2H2), 0.83 (t, 3H, –
C3H3).
1d: Orange, >250 (dec), 265 mg (56%). Anal. Calc. for C18H25Mo-
N3O4S (473.39 g/mol): C, 45.47; H, 4.92; N, 8.84; S, 6.74. Found: C,
8–27 cmꢀ1 to lower frequencies on chelation. The spectra of the
complexes contain the CH2 and CH3 stretching vibrations attrib-
uted to S-ethyl and coordinated alcohols, and also m(OH) bands
of the alcohols. The characteristic ms and mas bands of cis-MoO2
are in the 885–912 and 935–955 cmꢀ1 ranges.
The 1H NMR spectra of the ligands showed the expected isomer
peaks of azomethine (CH@N1) and S-ethylprotons, and also N4H2
protons of H2L1 [21,29]. The chemical shifts of the naphthylidene
ring protons of the ligands were observed an arrangement of dou-
blet and triplet peaks between 7.19 and 8.18 ppm.
45.72; H, 5.30; N, 8.81; S, 6.49%. IR:
1620,
(C@N1) 1597, (C@N2) 1547, ms
NMR: 9.67 (s, 1H, CH@N1), 7.27–8.20 (doublet and triplets, 6H,
m
(OH) 3453,
m(NH) 3222, d(NH)
m
m
,
mas (MoO2) 946, 904. 1H
C
10H6), 7.02 (s, 1H, N4H), 3.14 (q, 2H, S–CH2), 1.39 (t, 3H, –CH3),
3.57 (q, 2H, –C1H2), 1.48 (m, 2H, –C2H2), 1.32 (m, 2H, –C3H2),
The signals of the isomers disappear on chelation, and only one
0.86 (t, 3H, –C4H3).
singlet, triplet and quartet were recorded for the CH@N1, –CH3 and
2þ
2a: Stammel red, 233.3, 190 mg (38%). Anal. Calc. for C21H21Mo-
N3O4S (507.41 g/mol): C, 49.71; H, 4.17; N, 8.28; S, 6.32. Found: C,
S–CH2 protons, respectively. After chelation of the MoO2 center,
the CH@N1 proton that is closer to the conjugated backbone of the
thiosemicarbazone shifted to higher frequencies by 0.1–0.4 ppm,
whilst the chemical shift values of the naphthylidene and ethyl
protons are approximately the same as the free ligands. Because
complex formation of the dibasic thiosemicarbazone ligands oc-
curs synchronously with the deprotonisation of the 2-OH and
N4HR groups (Fig. 1), the phenoxy (of L1 and L2) and thioamide
(of L2) proton signals are absent in all the complex spectra. For
the same reason, one singlet was observed at around 7 ppm in
the spectra of 1a–d instead of two singlet signals (at 5.11 and
4.78 ppm) appearing due to the cis- and trans-isomerism of N4H2
in the H2L1 spectra.
49.38; H, 4.00; N, 8.24; S, 6.53%. IR: m(OH) 3434, m
(C@N1) 1593,
m
(C@N2) 1543, ms mas (MoO2) 939, 908. 1H NMR: 9.69 (s, 1H,
,
CH@N1), 7.32–8.21 (doublet and triplets, 6H, C10H6), 7.21–7.35
(m, 5H, N4C6H5), 3.05 (q, 2H, S–CH2), 1.31 (t, 3H, –CH3), 3.42 (s,
3H, –C1H3).
2b: Cardinal red, 232.1, 235 mg (45%). Anal. Calc. for C22H23Mo-
N3O4S (521.44 g/mol): C, 50.67; H, 4.45; N, 8.06; S, 6.15. Found: C,
50.72; H, 4.23; N, 8.07; S, 6.20%. IR: m(OH) 3414, m
(C@N1) 1593,
m
(C@N2) 1547, ms mas (MoO2) 942, 888. 1H NMR: 9.76 (s, 1H,
,
CH@N1), 7.38–8.28 (doublet and triplets, 6H, C10H6), 7.27–7.41
(m, 5H, N4C6H5), 3.15 (q, 2H, S–CH2), 1.37 (t, 3H, –CH3), 3.71 (q,
2H, –C1H2), 1.23 (t, 3H, –C2H3).
Other noticeable changes in chemical shifts were determined
for five protons on the N4-phenyl ring of 2a–d. The doublet and
triplets of these protons were recorded in a narrower range, ca.
0.15 ppm, while their chemical shifts are in a 0.33 ppm range (be-
tween 7.10 and 7.43 ppm) in the H2L2 spectra.
2c: Stammel red, 234.7, 115 mg (21%). Anal. Calc. for C23H25Mo-
N3O4S (535.47 g/mol): C, 51.59; H, 4.71; N, 7.85; S, 5.99. Found: C,
51.40; H, 4.44; N, 7.88; S, 5.97%. IR: m(OH) 3422, m
(C@N1) 1593,
m
(C@N2) 1543, ms mas (MoO2) 942, 885. 1H NMR: 9.76 (s, 1H,
,
CH@N1), 7.38–8.28 (doublet and triplets, 6H, C10H6), 7.25–7.42
(m, 5H, N4C6H5), 3.16 (q, 2H, S–CH2), 1.38 (t, 3H, –CH3), 3.60 (t,
2H, –C1H2), 1.58 (m, 2H, –C2H2), 0.93 (t, 3H, C3H3).
3.3. Crystallography
2d: Stammel red, 232.4, 110 mg (20%). Anal. Calc. for C24H27Mo-
N3O4S (549.49 g/mol): C, 52.27; H, 5.30; N, 7.62; S, 5.81. Found: C,
Single crystals of complexes 1b and 2a, suitable for X-ray dif-
fraction studies, were grown by slow evaporation of an alcohol
(L0) solution of the complex. Crystal parameters and refinement re-
sults of the complexes are summarized in Table 1.
52.48; H, 5.01; N, 7.64; S, 6.02%. IR:
m(OH) 3260, m
(C@N1) 1589,
m
(C@N2) 1543, ms mas (MoO2) 942, 888. 1H NMR: 9.76 (s, 1H,
,
CH@N1), 7.39–8.28 (doublet and triplets, 6H, C10H6), 7.28–7.42
(m, 5H, N4C6H5), 3.16 (q, 2H, S–CH2), 1.38 (m, 3H, –CH3), 3.65 (t,
2H, –C1H2), 1.55 (m, 2H, –C2H2), 1.38 (m, 2H, –C3H2), 0.93 (t, 3H,
–C4H3).
The complex structures are formed by chelation of the doubly
deprotonated thiosemicarbazones, having an ONN donor set which
consist of a phenoxy oxygen atom and azomethine and thioamide
nitrogen atoms. Two oxo-oxygens of molybdenum and an oxygen
atom of an attached alcohol are also coordinated to the center.
The oxygen atoms of ethanol (in 1b) and methanol (in 2a) are
weakly coordinated with a Mo–O distance of 2.3727(13) and
2.4040(14) Å, respectively (Figs. 2 and 3). Considering the molyb-
denum centered bond distances and angles it can be easily seen
that the molybdenum atoms of 1b and 2a are in three axis dis-
torted octahedral environments (Tables 2 and 3). Compounds 1b
and 2a have a cis-MoO2 center and their geometric parameters
are in the expected ranges compared with analogous molybdenum
centric complexes of S-alkyl thiosemicarbazones [16,21,22,33].
Complex 1b crystallizes in the monoclinic space group C2/c
with Z = 8 (Fig. 2). One of the intermolecular hydrogen bonds
(2.25 Å) ties the hydrogen atom (H3A) of the NH group and the
oxo-oxygen (O3). In another interaction, the hydroxyl proton
(H1) of the coordinated ethanol and the nitrogen atom (N2) are in-
volved (Fig. 4.). In this way, a 3D-structure of complex 1b is gener-
ated by the propagation of these hydrogen bonds in the form of a
bifurcated arrangement.
3. Results and discussion
3.1. Some physical properties of the compounds
Hydrobromide forms of the ligands, which separated by precip-
itation of the reaction mixture, are bright yellow in color, whilst
the free ligands are light yellow. The crystalline powders of H2L1
and H2L2 are soluble in common solvents. The reactions of the li-
gands with MoO2(acac)2 in a 1:1 molar ratio in selected alcohols
yielded stable solid complexes corresponding to the general for-
mula [MoO2(L)ROH] (Fig. 1). The diamagnetic chelate complexes,
1a–d and 2a–d, are in the crystalline form, and they are soluble
in alcohols and chlorinated hydrocarbons. The complexes are sta-
ble for at least 2 weeks in air.
3.2. IR and 1H NMR spectra
The molybdenum complex of L2 (2a) crystallizes from methanol
in the monoclinic space group P21/n. The hydroxyl proton of the
coordinated methanol participates in an intermolecular interaction
and results in a hydrogen bond, 1.99(3) Å in length (Fig. 5). Pairs of
these hydrogen bonds connect two molecules into a dimer in
which S-ethyl groups of the complex structures are outstretched
in opposite directions and the chelate rings are in parallel planes.
The IR spectra of H2L1 and H2L2 clearly showed the stretching
vibrations of the OH, N4H, C@N1 and N4@C groups. In the IR spectra
of the complexes, the m(OH), one of the m(NH) (for 1a–d) and the
m
(NH) (for 2a–d) stretches are absent as a result of the deprotona-
tion of the thiosemicarbazone ligands. The vibrations of the C@N1
and N4@C groups of the free thiosemicarbazones are shifted by ca.

2928
S. Duman et al. / Polyhedron 29 (2010) 2924–2932
Fig. 2. The molecule of 1b with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level. The minor component of the
disordered part has been omitted for clarity.
Fig. 3. The molecule of 2a with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level.
Furthermore, for both of 1b and 2a, weak intermolecular C–
interactions contribute to the stabilization of the crystal
packing (see for details; Table 4).
peak currents (Ipa/Ipc). E1/2 values are in agreement with the re-
ported data for redox processes in similar O–N–S complexes in
the literature [34–42].
Hꢃ ꢃ ꢃ
p
The ligands L1 and L2 indicate similar voltammetric responses
with small potential shifts due to the different substituents. Within
the electrochemical window of TBAP/DCM, they undergo a one-
electron irreversible oxidation (two oxidations for L1) and a one-
electron irreversible reduction couple (Fig. 6). It is observed that
the redox potentials of L2 shift to positive potentials due to the dif-
ferent substituents of the ligand. When one of the hydrogen atoms
of the NH2 group was exchanged with a phenyl group to form L2,
the redox potentials shift to positive potentials due to the higher
electron withdrawing ability of the phenyl group with respect to
3.4. Electrochemical studies
The redox properties of the ligands (H2L1 and H2L2) and their
dioxomolybdenum(VI) complexes (1b and 2a) were studied using
CV and SWV measurements in DCM containing TBAP as the sup-
porting electrolyte on a Pt electrode. Table 5 lists the assignments
of the redox couples and the electrochemical parameters, which in-
cluded the half-wave peak potentials (E1/2), anodic to cathodic
peak potential separation (DEp), and ratio of the anodic to cathodic

S. Duman et al. / Polyhedron 29 (2010) 2924–2932
2929
Fig. 4. The packing and hydrogen bonding interactions of the compound 1b viewed down the b-axis. H atoms not participating in hydrogen bonding and the minor
component of the disordered part have been omitted for clarity.
Fig. 5. A general view of the packing and hydrogen bonding interactions of the compound 2a. H atoms not participating in hydrogen bonding have been omitted for clarity.
Table 5
Coordination of the ligands shifts the redox potential of the li-
gands to positive potentials due to the electron deficient character
of the dioxomolybdenum (VI) center (Fig. 7).
Voltammetric data of the compounds.
Complex
Ligand oxd.’s
MIVO2/MVO2
Ligand red.
L1
E1/2 vs. SCEa
1.46
0.98
450
0.17
ꢀ1.88
Complexes 1b and 2a give a metal-based one-electron irrevers-
ible redox couple (at ꢀ0.5 V for 1b and at ꢀ0.55 V for 2a) in addi-
tion to the ligand-based redox processes. The free ligands do not
exhibit a redox process in the potential range where the molybde-
num reductions are observed. The values of E1/2 are significantly
influenced by the central metal ions, the nature of the substituents
on the thiosemicarbazone ligands and the electronic character of
the ligands, which are important factors in the control of redox
D
Ipa/Ipc
Ep (mV)b
c
L2
E1/2 (V)a
1.12
400
0.15
ꢀ1.52
80
0.30
D
Ipa/Ipc
Ep (mV)b
c
a
1b
2a
E1/2
D
1.17
ꢀ0.86
180
0.22
ꢀ1.52
185
0.56
Ep (mV)b
Ipa/Ipc
c
potentials. Even though the
DEp values of 1b and 2a were in the
a
E1/2
D
1.27
110
0.15
ꢀ0.80
153
0.25
ꢀ1.48
210
0.42
quasi-reversible range, especially at very slow scan rates, the very
small Ipa/Ipc values and the deviation of Ip vs. m1/2 from linearity
may assign the irreversible character of the redox processes to
the chemical reactions succeeding the electron transfer processes.
Due to the chemical reactions succeeding the reduction redox pro-
cesses, the CV of 1b gives oxidation waves at ꢀ0.20, 0.10, 0.44 and
1.15 V, assigned to the chemical reaction products during the re-
verse potential scans. Similar waves were also recorded with
complex 2a. When only the potential range from ꢀ0.5 to 1.6 V
was scanned, without a negative potential scan, these waves
Ep (mV)b
Ipa/Ipc
c
E1/2 = (Epa + Epc)/2 at 0.100 V sꢀ1
.
a
b
c
Ep = Epa + Epc at 0.100 V sꢀ1
Ipa/Ipc for reduction, Ipc/Ipa for oxidation processes at 0.100 V sꢀ1 scan rate.
.
hydrogen. Although the irreversible reduction process of these li-
gands was reported in the literature for similar ligands [40–42],
the oxidation processes of the ligands are rarely reported [42].

2930
S. Duman et al. / Polyhedron 29 (2010) 2924–2932
Fig. 6. CVs and SWV of the ligands at 0.100 V sꢀ1 scan rate on Pt in DCM/TBAP. (A)
Fig. 7. CV (inset: positive potential scan 0.100 V sꢀ1 scan rate) (A) and SWV (B) of
2a at various scan rates on Pt in DCM/TBAP. (SWV parameters: pulse size = 100 mV;
Frequency: 25 Hz).
L2 and (B) L1 (SWV parameters: pulse size = 100 mV; Frequency: 25 Hz).
(Fig. 8A). Shifting of the intraligand transition band at 308 nm and
the change in the position of the LMCT band indicate a metal-based
redox process and the couple is assigned to the [MoVIO2(L2)MeOH]/
[MoVO2(L2)MeOH]ꢀ1 process. Well-defined isosbestic points at 350
and 413 nm demonstrate that the reduction proceeds to give a sin-
gle, reduced species. Fig. 8B illustrates the spectral changes during
the ligand-based reduction process. During the ꢀ1.80 V potential
application, while the bands at 327 and 390 nm remain unchanged,
two new bands are observed at 300 and 445 nm. The spectroscopic
changes given in Fig. 8C are characteristic for the decomposition of
the complex during oxidation, because all bands decreases in inten-
sity and no clear isosbestic point is recorded under the applied
potential at 1.50 V. The color changes of the compounds during
the redox processes were recorded using in situ colorimetric mea-
surements (Fig. 8D). Without any potential application, the solution
of [MoVIO2(L2)MeOH] is red (x = 0.4728 and y = 0.4186). As the
potential is stepped from 0 to ꢀ1.70 V, the color of the neutral
disappeared, which supports the assignment of these waves to the
product of the chemical reaction succeeding the reduction pro-
cesses of the complexes. It is to be noted here that reductions of
dioxomolybdenum(VI) complexes in aprotic solvents are generally
irreversible [43–45]. This indicates that the electrochemical reduc-
tions are followed by chemical reactions. In general, the initial
reduction product is a dioxomolybdenum(V) species which under-
goes loss of an oxo group and/or dimer formation [40–45].
3.5. Spectroelectrochemical studies
In situ spectroelectrochemical studies were employed to deter-
mine the spectra of the electrogenerated species of the dioxomolyb-
denum(VI) complexes and to assign the redox processes in the CVs
of the complexes. Since Mo(VI) has a 4d0 electronic configuration,
the initial spectrum under an open-circuit potential is dominated
by ligand-to-metal charge transfer (LMCT) and intraligand transi-
tions. Fig. 8 shows the in situ UV–Vis spectral changes of 2a as a rep-
resentative of the dioxomolybdenum(VI) complexes during the
controlled potential reductions and oxidation processes. Under an
applied potential of ꢀ1.0 V, the broad LMCT band at 560 nm de-
creases in intensity without shift, while the intraligand transition
band at 308 nm decreases in intensity with a red shift to 327 nm.
At the same time, a sharp new band is observed at 390 nm
[MoVIO2(L2)MeOH] starts to change and
a light green color
(x = 0.3439 and y = 0.4694) is seen during the first reduction and
then a yellow color (x = 0.4339 and y = 0.4694) is obtained at the
end of the reduction. During the oxidation process, the color of the
solution changes from red to light yellow (x = 0.3952 and
y = 0.416). Measurement of the xyz coordinates allows quantifica-
tion of each color of the redox species, which is very important in
deciding their possible electrochromic application.

S. Duman et al. / Polyhedron 29 (2010) 2924–2932
2931
Fig. 8. In situ UV–Vis spectral changes of 2a. (A) Eapp = ꢀ1.00 V. (B) Eapp = ꢀ1.80 V. (C) Eapp = 1.50 V. (D) Chromaticity diagram of 2a. (Each symbol represents the color of
electrogenerated species; h : [MoVIO2(L2)MeOH], s: [MoVO2(L2)MeOH]ꢀ1, 4: [MoVIO2(L2)ꢀ1MeOH]ꢀ2, I: [MoVIO2(L2)+1MeOH]+1
.
to positive potentials after coordination to the dioxomolybde-
num(VI) center. The data clearly indicate that the substituents of
the ligands also affect the redox processes of the ligands and their
complexes. Additionally, a definite determination of the colors of
the electrogenerated anionic and cationic form of the complexes
is important in deciding their possible electrochromic application.
4. Conclusion
As known, the cis-MoO2 center in the chelate form shows a cat-
alytic function in net oxygen atom transfer (OAT), and its ability
varies in relation to the chelating ligand type and substituents of
the ligand. For this purpose, many dioxomolybdenum complexes
with various 2-hydroxy-arylidene thiosemicarbazones have been
investigated. However, S-alkyl thiosemicarbazone derivatives have
been the subject of a limited number of reports [14–16,20–22], and
there is only one study on S-ethyl thiosemicarbazones [33].
In this study, a series of cis-MoO2 complexes (1a–d and 2a–d)
with two S-ethyl-thiosemicarbazones, which are potential cata-
lysts for various enzymes and OAT reactions, were synthesized.
Structural analysis of 2a–d confirmed that the N4 thioamide nitro-
gen atom has reaction capability despite the steric bulk of the phe-
nyl ring. Crystallographic data of 1b and 2a showed the remarkable
role of the OH protons of the bonded alcohols in relatively strong
intermolecular hydrogen bonds.
Supplementary data
CCDC 753393 and 746352 contain the supplementary crystallo-
graphic data for complex 1b (C16H19N3O4MoS) and complex 2a
(C21H21N3O4MoS). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Voltammetric and spectroelectrochemical studies show that
while the free ligands give a one electron reduction and a one elec-
tron oxidation redox process, the dioxomolybdenum(VI) com-
plexes 1b and 2a give an irreversible metal-based one electron
reduction process in addition to the irreversible ligand reduction
and oxidation processes. The redox potentials of the ligands shift
Acknowledgments
This present work was supported by the Research Fund of Istan-
bul University, Project No. 810. The authors thank Dr. Gülsßah Yur-
dakul (Istanbul University, IA Laboratory, Istanbul) for her useful
assistance during X-ray diffraction data collection.

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S. Duman et al. / Polyhedron 29 (2010) 2924–2932
_
[22] B.I. Ceylan, Y.D. Kurt, B. Ülküseven, J. Coord. Chem. 62 (5) (2009) 757.
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