Dinuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes: Synthesis, structural elucidation, spectroscopic characterization, electrochemistry and catalytic property

Article abstract of DOI:10.1016/j.poly.2011.11.057

Several new binuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes have been synthesized. The reaction of bis(acetylacetonato) dioxomolybdenum with 5-dichlorosalicylaldehyde 4-ethylthiosemicarbazide (L1/L2) in the presence of 4,4′-bipyridine

Full text of DOI:10.1016/j.poly.2011.11.057

Polyhedron 33 (2012) 235–251
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Polyhedron
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Dinuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes: Synthesis,
structural elucidation, spectroscopic characterization, electrochemistry and
catalytic property
⇑
Ngui Khiong Ngan , Kong Mun Lo, Chee Seng Richard Wong
Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new binuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes have been
synthesized. The reaction of bis(acetylacetonato)dioxomolybdenum with 5-dichlorosalicylaldehyde
4-ethylthiosemicarbazide (L1/L2) in the presence of 4,40-bipyridine (4,40-bpy) or 4,40-bipyridine
N,N0-dioxide (4,40-dpdo) gave a binuclear cis-dioxomolybdenum complex [(MoO2)2L2(D-D)], in which
the bidentate ligand, 4,40-bipyridine or 4,40-bipyridine N,N0-dioxide (D-D), formed a bridge between
the two molybdenum atoms. A second type of dinuclear dioxomolybdenum(VI) Schiff base complex,
[(MoO2)2L0D2], was formed by the reaction of bis(acetylacetonato)dioxomolybdenum with 1,4-bis(3-eth-
oxy-salicylaldehyde carbohydrazonato)butane (L0) in the presence of ethanol or hexamethylphosphora-
mide (D). In this case, the two molybdenum atoms are not bridged directly by a bidentate ligand, but are
coordinated at each end to the O,N,O donor atoms of the symmetrical hexadentate Schiff base ligand.
The syntheses of polynuclear dioxomolybdenum(VI) [(MoO2)L0(D-D)]n complexes were achieved by the
reaction of the second type of dinuclear dioxomolybdenum(VI) complex, [(MoO2)2L0D2], with the
bidentate ligand 4,40-bipyridine (D-D). These complexes were characterized by elemental analysis, elec-
tronic spectra, IR, 1H and 13C NMR spectroscopies, thermogravimetric analysis and cyclic voltammetry.
An X-ray study shows that each molybdenum site in the dinuclear and polynuclear complexes adopts
an octahedron geometry. The dioxomolybdenum(VI) complexes show catalytic activity for the oxidation
of alcohols with H2O2 as an oxidant.
Received 17 October 2011
Accepted 18 November 2011
Available online 17 December 2011
Keywords:
Crystal structure
Dioxomolybdenum(VI)
Hydrogen bonding
Supramolecules
Schiff base
Oxidation of alcohol
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
decade, chemists have been actively engaged in the study of the
structure, spectroscopic, electrochemistry and oxo-transferase
Molybdenum chemistry has been widely investigated in recent
years. Subsequently, various ligands with different donor proper-
ties, such as hydrazone and thiosemicarbazone ligands, have been
subjected to coordination with dioxomolybdenum to produce
complexes which serve as catalysts in biological and industrial pro-
cesses. Hydrazone (ONO) and thiosemicarbazone (ONS) tridentate
Schiff base ligands are of particular interest as they offer a labile
site in mononuclear dioxomolybdenum(VI) complexes which al-
lows binding and displacement of different substrate molecules.
Moreover, these complexes have also attracted research interest
for their possible antitumor [1], antifungal [2] and antiviral [3]
properties. From a supramolecular architectural point of view,
hydrazone ligands can stabilize the overall molecular framework
by inducing intra- and intermolecular hydrogen bonding interac-
tions through the amido nitrogen.
properties of oxomolybdenum complexes [4–12]. At present, there
are plenty of mononuclear dioxomolybdenum(VI) complexes re-
ported, but systems with binuclear complexes based on bis(hydra-
zone) and thiosemicarbazone ligands are rare [13]. Koo et al. have
reported binuclear complexes bridged by 4,40-dipyridyl N,N0-diox-
ide, but no definite structures were elucidated [14,15].
As a part of our study on molybdenum complexes, three types
of complexes with different nuclearity have been synthesized
and characterized spectroscopically. The X-ray crystal structures
of the complexes exhibit various intriguing supramolecular archi-
tectures. In this report, we are also concerned with and investigate
the catalytic behavior of dinuclear and polynuclear dioxomolybde-
num(VI) complexes towards the oxidation of alcohol.
2. Experimental
Hydrogen bonding is the most reliable directional interaction in
supramolecular construction. For such reasons, for more than a
2.1. General procedures
⇑
All the reagents and solvents used in the synthesis were pro-
cured commercially and used without subsequent purification.
Corresponding author.
E-mail address: nicky_ngan@hotmail.com (N.K. Ngan).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.11.057

236
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
The starting material, bis(acetylacetonato)dioxomolybdenum(VI),
[MoO2(acac)2], was prepared as described in the literature [16].
arbazide. The solution mixture was refluxed with vigorous stirring
for 2 h. The resulting solution was then filtered and allowed to
stand at room temperature, during which time a pale yellow solid
was formed. The precipitate was filtered, washed with methanol
and air-dried. The ligand was used without further purification.
M.p.: 150–153 °C, (0.23 g, 68%). Anal. Calc. for C10H12N3OClS: C,
46.69; H, 4.67; N, 16.34; S, 12.40. Found: C, 47.01; H, 4.17; N,
2.2. Physical measurements
The melting points of the compounds were determined on a
Melt-Temp apparatus and were uncorrected. Elemental analysis
was performed at the in-house microanalytical laboratory using a
Perkin-Elmer 2400 Series II CHNS/O System. IR spectra were re-
corded in the range 4000–400 cmꢀ1 using Perkin-Elmer Spectrum
2000 FT-IR and Perkin-Elmer Spectrum RX1 FT-IR spectrophotom-
eters. The samples were prepared either as nujol mulls or KBr pel-
lets. 1H and 13C NMR spectra of the ligands and the complexes
were measured in DMSO-d6 at ambient temperature on a JEOL
Lambda 400 FT-NMR SYSTEM spectrometer operating at
399.65 MHz for 1H and at 100.40 MHz for 13C NMR on an ECA
400 MHz NMR spectrometer. Electronic absorption spectral mea-
surements of the ligands and the complexes in DMF were mea-
sured using a Shimadzu-1650-PC-UV–Vis spectrophotometer and
the samples were scanned from 200 to 800 nm. Thermal analysis
(TGA) of the complexes was carried out by heating in nitrogen
gas at a rate of 20 °C per minute on a Perkin-Elmer TGA-4000 ther-
mo balance. DSC curves were obtained at a heating rate of 20 °C per
minute in N2 atmosphere over the temperature range 50–350 °C on
a Perkin-Elmer DSC 6 series. Cyclic voltammetry was carried out
using a Methrohm Autolab B.V. model. The measurements were
done in DMF solution containing 0.1 M TEAP as a supporting elec-
trolyte and a 5ꢁ10ꢀ4 M complex solution was deoxygenated by
bubbling with nitrogen gas before use. The working, counter and
reference electrodes used were Pt wire, platinum coil and SCE,
respectively. The X-ray crystallographic intensity data were mea-
15.95; S, 12.32%; IR(KBr) (m
max/cmꢀ1): 3301 (m, NH), 3128 (m,
OH), 1602 (m, C@N), 1537 (m, Car–O), 1269 (s, C@S); 1H NMR
(DMSO-d6, ppm): 11.38 (1H, NH), 10.33 (1H, –OH), 8.62 (1H,
HC@N), 6.85–7.23 (aromatic proton), 1.15 (5H, –OCH2CH3); 13C
NMR (DMSO-d6, ppm): 178.51 (C@S), 159.89 (C@N), 118.23,
122.70, 123.97, 126.00, 131.17, 139.67 (aromatic ring), 24.67,
33.14 (–CH2CH3).
2.4. Synthesis of 3,5-dichlorosalicylaldehyde 2-
ethylthiosemicarbazone (L2)
0.191 g (1.0 mmol) of 3,5-dichlorosalicylaldehyde in 20 ml
methanol was added to a 20 ml hot stirring methanolic solution
of 0.119 g (1.0 mmol) of 2-ethylthiosemicarbazide. The solution
mixture was refluxed for 2 h. The yellowish reaction mixture be-
came cloudy. The resulting solution was then filtered and the fil-
trate was allowed to stand at room temperature, whereupon a
yellow precipitate was formed. The precipitate was washed with
methanol and dried in air. The ligand was used without further
purification. M.p.: 123–125 °C, (0.15 g, 63%). Anal. Calc. for
C
10H12N3OSCl2: C, 40.96; H, 4.10; N, 16.38; S, 10.92. Found: C,
41.07; H, 4.23; N, 16.44; S, 11.21%; IR(KBr) (
m
max/cmꢀ1): 3438 (m,
NH), 3190 (m, OH), 1607 (m, C@N), 1540 (m, Car–O), 1263 (s,
C@S); 1H NMR (DMSO-d6, ppm): 11.57 (1H, NH), 10.68 (1H, –
OH), 8.45 (1H, HC@N), 6.77–7.63 (aromatic proton), 1.15 (5H, –
CH2CH3); 13C NMR (DMSO-d6, ppm): 178.51 (C@S), 159.89 (C@N),
120.43, 127.07, 133.60, 135.00, 138.17, 139.67 (aromatic ring),
25.56, 34.41 (–CH2CH3).
sured using Mo K
a radiation (graphite crystal monochromator,
k = 0.71069 Å). The data collection was collected at 296 K on a Bru-
ker APEX II with a CCD area-detector X-ray diffractometer. The
structures were solved by the direct method with the SHELXS97
[17] program and refined on F2 by full-matrix least-squares meth-
ods with anisotropic non-hydrogen atoms. Crystal data are given in
Table 3 and selected bond distances and angles are reported in Ta-
bles 4–6.
2.5. Preparation of the ligands
2.3. Synthesis of 5-chlorosalicylaldehyde 2-ethylthiosemicarbazone
2.5.1. General procedures
(L1)
All the Schiff base ligands L3–L5 were prepared by the conden-
sation reactions of 3-ethoxylsalicylaldehyde, 3,5-dichlorosalicylal-
dehyde and 4-hydroxsalicylaldehyde with adipic acid dihydrazide
in a 1:2 ratio, respectively, as shown in Scheme 1.
0.156 g (1.0 mmol) of 5-chlorosalicylaldehyde in 20 ml metha-
nol was added to 20 ml of 0.120 g (1.0 mmol) of 2-ethylthiosemic-
Table 1
Thermogravimetric analysis of the complexes.
Comp.
Temp. range (°C)
Weight loss observed (calculated) (%)
DSCmax (°C)
242 (ꢀ)
Molecules removed
End residue
MoO2
C1
(a) 200–270
(b) 270–870
(a) 220–270
(b) 270–700
(a) 150–200
(b) 200–270
(c) 270–730
(d) 730–900
(a) 170–200
(b) 300–350
(c) 350–620
(d) 620–870
(a) 100–400
(b) 400–850
(a) 150–270
(b) 270–870
(a) 200–280
(b) 300–874
(a) 15.66 (14.82)
(b) 72.00 (75.69)
(a) 14.32 (16.27)
(b) 56.54 (60.92)
(a) 13.44 (11.30)
(b) 11.35 (11.30)
(c) 42.12 (44.25)
(d) 27.78 (31.44)
(a) 10.53 (10.62)
(b) 18.00 (16.40)
(c) 40.57 (41.57)
(d) 28.00 (27.45)
(a) 42.46 (41.20)
(b) 30.52 (32.96)
(a) 19.76 (16.82)
(b) 52.18 (53.88)
(a) 18.13 (16.50)
(b) 72.48 (73.32)
(a) Bipyridine
(b) 2 Tridentate ligands
(a) Bipyridine N dioxide
(b) 2 Tridentate ligands
(a) 2 Ethanol
(b) 2 –OC2H5
(c) Hexadentate ligands
(d) 2MoO2
(a) 2 Ethanol
(b) 4-Cl
(c) Hexadentate ligands
(d) 2MoO2
(a) Hexadentate ligands
(b) 2 HMPA
C2
C3
234 (ꢀ)
MoO2
–
175 (+)
229 (ꢀ)
C4
160 (+)
206 (+)
252 (ꢀ)
–
C5
C6
C7
170 (+)
MoO2
MoO2
MoO
164 (+), 189 (+)
251 (ꢀ)
(a) 2 DMF
(b) Hexadentate ligands
(a) Bipyridine
251 (+)
270 (ꢀ)
(b) Ligands

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
237
Table 2
perature for 2 days, the yellow precipitate that formed was filtered,
washed with ethanol and dried in air. The ligand was used without
further purification. M.p.: 240–242 °C, (0.22 g, 48%). Anal. Calc. for
UV–Vis wavelengths for the ligands and the
complexes.
Compounds
kmax (nm)
C
20H18N4O4Cl2: C, 53.45; H, 4.01; N, 12.47. Found: C, 53.12; H, 4.09;
N, 11.87%; IR(KBr) (m
max/cmꢀ1): 3443 (s, OH), 3295, 3253 (m, N–H),
L1
L2
L3
L4
L5
C1
C2
C3
C4
C5
C6
C7
347, 315, 262
351, 317, 268
291, 251
1681 (s, C@O), 1608 (m, C@N), 1537 (m, Car–O); 1H NMR (DMSO-
d6, ppm): 11.95 (–OH), 11.47 (NH), 8.24 (HC@N), 6.89–7.48 (aro-
matic), 3.42 (–CH2CH2); 13C NMR (DMSO-d6, ppm): 152.60
(C@N), 169.23 (C@O), 123.36, 128.83, 130.54, 140.77, 145.77,
151.27 (aromatic), 121.20, 121.84 (–C2H4).
338, 302, 291
338, 303, 292
346, 315, 267
405, 347, 270
404, 290, 247
398, 322, 286
387, 290, 248
387, 336, 248
393, 309, 268
2.5.4. Synthesis of 1,4-bis(4-hydroxysalicylaldehyde
carbohydrazonato)butane (L5)
0.336 g (2.0 mmol) of 4-hydroxysalicylaldehyde in 40 ml etha-
nol was added to 30 ml of a hot ethanolic solution of 0.174 g
(1.0 mmol) adipic acid dihydrazide. The solution mixture was re-
fluxed with vigorous stirring for 2 h. The resulting pale pink solu-
tion was then cooled to room temperature. Upon standing at
room temperature for 2 days, the white precipitate that formed
was filtered, washed with ethanol and dried in air. The ligand
was used without further purification. M.p.: 258–260 °C, (0.21 g,
34%). Anal. Calc. for C20H22N4O6: C, 57.97; H, 5.31; N, 13.53. Found:
Table 3a
Cyclic voltammmetry data.
Compounds
Epc/V (Ipc
/
l
A)
Epa/V (Ipa/lA)
L1
L2
L3
L4
L5
C1
ꢀ0.42 (1.28)
ꢀ0.36 (0.35)
ꢀ0.37 (3.90)
ꢀ0.38 (0.13)
ꢀ0.49 (0.26)
ꢀ0.39 (8.14),
ꢀ0.94 (2.21)
ꢀ0.40 (0.23),
ꢀ0.94 (3.42)
ꢀ0.38 (0.14),
ꢀ0.86 (3.45),
ꢀ0.38 (7.77),
ꢀ0.85 (3.12)
ꢀ0.38 (0.11),
ꢀ0.83 (2.12)
ꢀ0.38 (0.15),
ꢀ0.85 (3.68)
ꢀ0.40 (4.79),
ꢀ0.86 (1.18)
–
–
C, 58.54; H, 5.22; N, 13.27%; IR(KBr) (m
max/cmꢀ1): 3264 (s, OH),
1681 (s, C@O), 1608 (m, C@N), 1520 (m, Car–O); 1H NMR (DMSO-
d6, ppm): 11.40, 11.02 (–OH), 10.18 (NH), 8.19 (HC@N), 6.26–
7.37 (aromatic), 3.49 (–CH2CH2); 13C NMR (DMSO-d6, ppm):
159.25 (C@N), 173.19 (C@O), 110.40, 111.42, 128.76, 131.22,
143.24, 147.44 (aromatic), 102.38, 107.79 (–C2H4).
ꢀ0.28 (5.35)
ꢀ0.28 (5.09)
ꢀ0.21 (1.91)
–
C1
C3
C4
C5
C6
C7
ꢀ0.29 (5.32)
ꢀ0.28 (1.81)
ꢀ0.28 (5.64)
ꢀ0.29 (5.89)
ꢀ0.25 (1.38)
2.6. Preparation of the dinuclear–dimolybdenum(VI) complexes
2.6.1. Synthesis of bis(5-chlorosalicylaldehyde 2-
ethylthiosemicarbazonato) 4,4-bipyridyl dinuclear
dioxomolybdenum(VI) (C1)
0.328 g (1.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was
mixed with 40 ml of 0.258 g (1.0 mmol) 5-chlorosalicylaldehyde
2-ethylthiosemicarbazone. The resulting solution was refluxed
with vigorous stirring for 1 h. Then 0.078 g of 4,4-bipyridine in
10 ml of ethanol was added to the mixture and the heating was
continued for another hour. After leaving the solution for 1 week
at room temperature, dark brown crystals were formed. The prod-
uct was filtered, washed with ethanol and air-dried. M.p.: 222–
2.5.2. Synthesis of 1,4-bis(3-ethoxysalicylaldehyde
carbohydrazonato)butane (L3)
0.332 g (2.0 mmol) of 3-ethoxysalicylaldehyde in 40 ml ethanol
was added to 30 ml of hot ethanolic solution of 0.174 g
(1.0 mmol) adipic acid dihydrazide. The solution mixture was re-
fluxed with vigorous stirring for 2 h. The resulting yellow solution
was then cooled to room temperature. Upon standing at room tem-
perature for 2 days, the yellow precipitate that formed was filtered,
washed with ethanol and dried in air. The ligand was used without
further purification. M.p.: 158–160 °C, (0.23 g, 61%). Anal. Calc. for
224 °C, (0.25 g, 34%). Anal. Calc. for
39.00; H, 3.03; N, 12.13; S, 6.93. Found: C, 40.12; H, 3.33; N,
11.67; S, 7.25%; IR(KBr) (
max/cmꢀ1): 3336 (m, N–H), 1603 (m,
C30H28O6N8S2Cl2Mo2: C,
a
m
C@N), 1546 (m, Car–O), 1003, 1063, 1579 (4,40-bpy), 927, 897
(Mo@O); 1H NMR (DMSO-d6, ppm): 8.75 (HC@N), 7.85, 8.53, 8.72
(4,40-bpy); 6.87–7.72 (aromatic); 13C NMR (DMSO-d6, ppm):
177.22 (C–S), 157.34 (C@N), 150.44, 122.64, 121.34 (4,40-bpy),
119.75, 123.89, 128.67, 131.87, 132.00, 144.58 (aromatic).
C
24H30N4O4: C, 65.75; H, 6.85; N, 12.76. Found: C, 66.18; H, 6.70; N,
13.12%; IR(KBr) (
m
max/cmꢀ1): 3591 (s, OH), 3439, 3287 (m, N–H),
1660 (s, C@O), 1590 (m, C@N), 1555 (m, Car–O); 1H NMR (DMSO-
d6, ppm): 11.58 (–OH), 11.2, 10.90 (NH), 8.30 (HC@N), 6.74–7.20
(aromatic), 3.17, 1.21 (–OCH2CH3), 3.41 (–CH2CH2); 13C NMR
(DMSO-d6, ppm): 154.70 (C@N), 167.82 (C@O), 113.71, 115.22,
119.20, 137.43, 148.93, 154.70 (aromatic), 102.17, 108.62 (–
C2H4), 64.34, 14.75 (–OCH2CH3).
2.6.2. Synthesis of bis(3,5-dichlorosalicylaldehyde 2-
ethylthiosemicarbazonato) 4,4bipyridyl N,N0-dioxide dinuclear
dioxomolybdenum(VI) (C2)
0.328 g (1.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was mixed
with 40 ml of 0.293 g (1.0 mmol) of 3,5-dichlorosalicylaldehyde 2-
ethylthiosemicarbazone. The resulting solution was refluxed with
vigorous stirring for 1 h. Then 0.093 g of 4,4-bipyridine N,N0-dioxide
in 10 ml of ethanol solution was added to the mixture and the heat-
ing is continued for another hour. After leaving the solution for 1
week at room temperature, dark brown crystals were formed. The
product was filtered, washed with ethanol and dried in air. M.p.:
212–214 °C, (0.32 g, 43%). Anal. Calc. for C30H26O8N8S2Cl4Mo2: C,
35.16; H, 2.54; N, 10.94; S, 6.25. Found: C, 36.53; H, 2.75; N, 11.11;
2.5.3. Synthesis of 1,4-bis(3,5-dichlorosalicylaldehyde
carbohydrazonato)butane (L4)
0.382 g (2.0 mmol) of 3,5-dichlorosalicylaldehyde in 40 ml eth-
anol was added to 30 ml of a hot ethanolic solution of 0.174 g
(1.0 mmol) adipic acid dihydrazide. The solution mixture was re-
fluxed with vigorous stirring for 2 h. The resulting yellow solution
was then cooled to room temperature. Upon standing at room tem-
S, 5.39%; IR(KBr) (
1549 (m, Car–O), 1212 (s, C–S), 823, 834 (m, N–O), 922, 891 (Mo@O);
m
max/cmꢀ1): 3234 (m, N–H), 1586 (m, C@N),

238
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Table 3b
Crystallographic data and structure refinement parameters for compounds C1, C2, C3, C4, C5, C6 and C7.
Compounds
C1
C2
C3
C4
C5
C6
C7
Chemical formula
Mr
C
30H28Cl2N8O6S2Mo2 C30H24Cl4N8O8S2Mo2 C28H38N4O12Mo2
C24H26Cl4N4O12Mo2 C36H62N10O12P2Mo2 C30H40N6O12Mo2
C16H17O6N3Mo
443.27
923.52
1022.31
814.50
864.15
1080
868.56
Crystal color, habit red, block
red, block
brown, block
brown, block
yellow, block
yellow, block
red, block
Crystal size (mm) 0.40 ꢁ 0.30 ꢁ 0.30
0.25 ꢁ 0.20 ꢁ 0.10
0.45 ꢁ 0.45 ꢁ 0.30 0.40 ꢁ 0.30 ꢁ 0.20 0.40 ꢁ 0.20 ꢁ 0.08 0.27 ꢁ 0.14 ꢁ 0.10 0.40 ꢁ 0.30 ꢁ 0.20
Crystal system
Space group
Unit cell
dimensions
a (Å)
monoclinic
triclinic
monoclinic
monoclinic
triclinic
triclinic
monoclinic
ꢀ
ꢀ
ꢀ
P21/n
P 2(1)/c
C2/c
P21/c
P1
P1
P1
12.5483(2)
11.93150(10)
23.2930(3)
90.00
9.0527(2)
9.2002(2)
12.3846(2)
71.9720(10)
72.5420(10)
89.1080(10)
932.25(3)
1
8.8304(4)
12.5291(5)
14.4997(6)
90.00
99.696(2)
90.00
20.5338(2)
14.64580(10)
13.24770(10)
90.00
129.51(2)
90.00
8.2462(4)
8.3898(4)
18.9782(10)
86.238(2)
83.245(2)
68.532(2)
1213.05(10)
1
8.36030(10)
10.18100(10)
11.94300(10)
94.2080(10)
107.7120(10)
112.4400(10)
874.054(15)
1
10.04650(10)
14.5139(2)
12.6256(2)
90
113.0800(10)
90
b (Å)
c (Å)
a
(°)
b (°)
100.9000(10)
90.00
3424.51(8)
4
c
(°)
V (Å3)
1581.29(12)
2
3073.74(4)
4
1693.63(4)
4
Z
T (K)
296(2)
1.791
1.067
296(2)
1.825
1.132
296(2)
1.711
0.862
296(2)
1.863
1.224
296(2)
1.480
0.648
296(2)
1.650
0.787
100(2)
1.738
0.814
Dcalc (g cmꢀ3
)
l(Mo K
a
) (mmꢀ1
)
Absorption
correction
Tmin
Tmax
F(000)
multiscan
multiscan
multiscan
multiscan
multiscan
multiscan
multiscan
0.688
0.726
1848
0.762
0.893
510
0.685
0.772
828
0.641
0.777
2216
0.857
0.950
558
0.876
0.924
442
0.746
0.850
896
Total data
Unique data
Rint
33860
8497
0.0190
7911
8863
4225
0.0197
3823
13584
3625
0.0251
3304
14499
3536
0.0182
3322
10168
4715
0.0193
4524
8361
3984
0.0133
3874
14197
3324
0.0331
2945
Observed data
[I > 2r(I)]
Ranges of h, k, l
ꢀ16,16; ꢀ15,15;
ꢀ31,31
ꢀ11,11; ꢀ11,11;
ꢀ16,16
ꢀ11,11; ꢀ16,16;
ꢀ18,17
ꢀ26,26; ꢀ19,19;
ꢀ17,17
ꢀ10,10; ꢀ10,10;
ꢀ23,21
ꢀ10,10; ꢀ12,13;
ꢀ15,15
ꢀ12,12; ꢀ17,17;
ꢀ15,15
Number of
453
249
214
204
287
229
238
parameters
R1
wR2
S
D
D
0.0194
0.0502
1.081
0.454
ꢀ0.419
0.0291
0.0965
1.095
0.957
ꢀ0.555
0.0333
0.0866
1.133
0.589
ꢀ0.805
0.0193
0.0538
0.980
0.534
ꢀ0.461
0.0321
0.0969
1.167
1.369
ꢀ0.732
0.0207
0.0602
1.160
0.463
ꢀ0.452
0.0429
0.1163
1.121
0.896
ꢀ0.824
qmax (e Åꢀ3
qmin (e Åꢀ3
)
)
1H NMR (DMSO-d6, ppm): 8.75 (HC@N), 6.87–7.72 (aromatic), 7.85,
8.53, 8.72 (4,40-bpy); 13C NMR (DMSO-d6, ppm): 174.98 (C–S),
155.28 (C@N), 129.11, 121.37 (4,40-bpy N,N0-dioxide), 118.57,
120.58, 125.49, 132.70, 149.43 (aromatic).
was then heated to reflux for 2 h. After leaving the solution for 4
days at room temperature, dark orange crystals were formed. The
product was filtered, washed with ethanol and dried in air. M.p.:
310–312 °C, (0.14 g, 23%). Anal. Calc. for C24H28O10N4Cl4Mo2: C,
33.26; H, 3.23; N, 6.47. Found: C, 32.54; H, 3.49; N, 7.29%; IR(KBr)
(m
max/cmꢀ1): 1613 (m, C@N), 1539 (m, Car–O), 1267 (s, Cenolic–O),
2.6.3. Synthesis of 1,4-bis(3-ethoxysalicylaldehyde
carbohydrazonato)butane ethanol dinuclear dioxomolybdenum(VI)
(C3)
943, 916 (Mo@O); 1H NMR (DMSO-d6, ppm): 8.77 (HC@N), 6.93–
7.83 (aromatic), 3.89 (–CH2CH2–); 13C NMR (DMSO-d6, ppm):
176.00 (C@O), 154.04 (C@N), 122.23, 123.87, 124.28, 131.96,
133.25, 153.43 (aromatic), 107.51 (–CH2CH2–).
0.656 g (2.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was
mixed with 40 ml of 0.438 g (1.0 mmol) of 1,4-bis(3-ethoxysalicyl-
aldehyde carbohydrazonone)butane. The resulting orange solution
was then heated to reflux for 2 h. After leaving the solution over-
night at room temperature, light brown crystals were formed.
The product was filtered, washed with ethanol and dried in air.
M.p.: 135–137 °C, (0.47 g, 53%). Anal. Calc. for C28H38O12N4Mo2:
C, 41.28; H, 4.67; N, 6.88. Found: C, 42.36; H, 4.12; N, 7.23%;
2.6.5. Synthesis of 1,4-bis(3-ethoxysalicylaldehyde
carbohydrazonato)butane hexamethylphosphoramide dinuclear
dioxomolybdenum(VI) (C5)
0.656 g (2.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was
mixed with 40 ml of 0.438 g (1.0 mmol) of 1,4-bis(3-ethoxysalicyl-
aldehyde carbohydrazonone)butane. The resulting orange solution
was then heated to reflux for 1 h. A few drops of hexamethylphos-
phoramide were added to the solution and an immediate orange
color precipitate was formed. A sufficient amount of dimethyl-
formamide was added to the mixture until the precipitate was
completely dissolved. The resulting orange solution was left at
room temperature for 5 days, during which time a yellow solid
was formed. Recrystallization of the resulting orange solid from
dimethylformamide with the slow evaporation technique afforded
yellow colored crystals. The product was filtered, washed with eth-
anol and dried in air. M.p.: 172–174 °C (0.37 g, 44%). Anal. Calc. for
IR(KBr) (m
max/cmꢀ1): 1614 (m, C@N), 1562 (m, Car–O), 1250 (s, Cenolic
–O), 942, 911 (Mo@O); 1H NMR (DMSO-d6, ppm): 8.71 (HC@N),
6.93–7.23 (aromatic), 3.21, 1.64 (–OCH2CH3), 4.06 (–CH2CH2); 13C
NMR (DMSO-d6, ppm): 175.00 (C@O), 155.24 (C@N), 118.57,
120.54, 121.32, 125.46, 147.63, 149.23 (aromatic), 64.36, 56.05
(O–CH2CH3).
2.6.4. Synthesis of 1,4-bis(3,5-dichlorosalicylaldehyde
carbohydrazonato)butane ethanol dinuclear dioxomolybdenum(VI)
(C4)
0.656 g (2.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was
mixed with 40 ml of 0.449 g (1.0 mmol) of 1,4-bis(3-ethoxysalicyl-
aldehyde carbohydrazonone)butane. The resulting orange solution
C
32H62O12N10P2Mo2: C, 37.21; H, 6.01; N, 13.57. Found: C, 36.74; H,
6.42; N, 13.23%; IR(KBr) (
m
max/cmꢀ1): 1621 (m, C@N), 1566 (m, Car–

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
239
max/cmꢀ1):
Table 4
N, 9.68. Found: C, 40.88; H, 5.04; N, 10.23%; IR(KBr) (
m
Bond lengths (Å) and bond angles (°) for C1.
1618 (m, C@N), 1563 (m, Car–O), 1247 (s, Cenolic–O), 737 (s,
HC@ODMF), 930, 899 (Mo@O); 1H NMR (DMSO-d6, ppm): 2.89 and
2.73 (DMF), 7.01–7.95 (aromatic), 8.72 (1H, HC@N); 13C NMR
(DMSO-d6, ppm): 174.80 (C@ODMF), 155.11 (C@N), 162.21
(C@Oenol), 118.43, 120.41, 121.19, 125.33, 148.00, 149.28 (aromatic
ring), 30.66 and 35.67 (DMF).
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(1)
Mo(1)–N(1)
Mo(1)–S(1)
Mo(1)–N(4)
N(2)–C(8)
N(2)–N(1)
C(8)–S(1)
1.7078(12)
1.7109(12)
1.9476(12)
2.2921(14)
2.4123(4)
2.4565(14)
1.307(2)
1.3849(19)
1.7615(17)
1.339(2)
Mo(2)–O(6)
Mo(2)–O(5)
Mo(2)–O(4)
Mo(2)–N(5)
Mo(2)–S(2)
Mo(2)–N(8)
N(5)–C(22)
N(5)–N(6)
1.7054(12)
1.7119(12)
1.9512(12)
2.2696(14)
2.4245(4)
2.4270(14)
1.296(2)
1.3791(19)
1.7618(17)
1.344(2)
2.6.7. Synthesis of octaoxo-di{bis(4-hydroxysalicylaldehyde) butane
dihydrazonato} 4,40-bipyridy tetranuclear dioxomolybdenum(VI) (C7)
0.656 g (2.0 mmol) of MoO2(acac)2 in 40 ml of methanol was
mixed with 40 ml of 0.414 g (1.0 mmol) of 1,4-bis(4-hydroxysali-
cylaldehyde carbohydrazonone)butane. The resulting red solution
was heated to reflux for 1 h, then 0.078 g of 4,4-bipyridine in
10 ml of methanol solution was added to the mixture and the heat-
ing was continued for another hour. After leaving the solution for 1
week at room temperature, red crystals were formed. The product
was filtered, washed with methanol and dried in air. M.p.: 232–
235 °C, (0.22 g, 43%). Anal. Calc. for C15H17N3O5Mo: C, 43.37; H,
C(23)–S(2)
C(16)–O(4)
N(6)–C(23)
C(1)–O(1)
N(1)–C(7)
1.297(2)
1.308(2)
O(6)–Mo(2)–O(5)
O(6)–Mo(2)–O(4)
O(5)–Mo(2)–O(4)
O(6)–Mo(2)–N(5)
O(5)–Mo(2)–N(5)
O(4)–Mo(2)–N(5)
O(6)–Mo(2)–S(2)
O(5)–Mo(2)–S(2)
O(4)–Mo(2)–S(2)
N(5)–Mo(2)–S(2)
O(6)–Mo(2)–N(8)
O(5)–Mo(2)–N(8)
O(4)–Mo(2)–N(8)
N(5)–Mo(2)–N(8)
S(2)–Mo(2)–N(8)
106.26(6)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(1)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–S(1)
O(3)–Mo(1)–S(1)
O(1)–Mo(1)–S(1)
N(1)–Mo(1)–S(1)
O(2)–Mo(1)–N(4)
O(3)–Mo(1)–N(4)
O(1)–Mo(1)–N(4)
N(1)–Mo(1)–N(4)
S(1)–Mo(1)–N(4)
105.93(6)
98.00(6)
106.30(6)
89.33(6)
161.45(6)
80.73(5)
101.26(5)
90.71(5)
149.54(4)
76.14(4)
170.45(5)
83.02(5)
76.82(5)
81.96(5)
80.52(3)
105.03(6)
100.16(6)
160.39(6)
91.14(5)
80.74(5)
91.21(5)
99.71(4)
149.65(4)
76.16(4)
80.50(5)
173.54(5)
77.31(5)
82.61(5)
80.42(3)
4.09; N, 10.12. Found: C, 43.88; H, 4.18; N, 10.23%; IR(KBr) (mmax
cmꢀ1): 3256 (m, –OH), 1609 (m, C@N), 1567 (m, Car–O), 1230 (s,
enolic–O), 1010, 1069 and 1548 (4,40-bpy), 926, 883 (Mo@O); 1H
/
C
NMR (DMSO-d6, ppm): 10.51 (–OH), 8.70 (1H, HC@N), 7.80, 8.53
(4,40-bpy), 6.21–7.40 (aromatic); 13C NMR (DMSO-d6, ppm):
155.39 (C@N), 164.55 (C@Oenol), 105.06, 110.57, 112.82, 136.19,
144.95, 161.65 (aromatic ring), 150.44, 121.89 (4,40-bpy), 49.12
(CH3OH).
O), 1260 (s, Cenolic–O), 1131 (s, P@O), 928, 899 (Mo@O); 1H NMR
(DMSO-d6, ppm): 2.50 and 2.52 (HMPA), 3.85, (3H, O–CH3), 4.08
(5H, O–C2H5), 6.93–7.80 (aromatic), 8.49 (1H, HC@N); 13C NMR
(DMSO-d6, ppm): 36.56, 36.60 (HMPA), 65.60 (O–C2H5), 159.44
(C@N), 170.06 (C@O), 112.75, 118.00, 119.58, 120.45, 121.06,
121.28, 125.45, 129.19, 132.54, 148.63, 151.49, 153.82 (12C,
aromatic).
3. Result and discussion
3.1. Synthesis
A series of mononuclear, dinuclear and polynuclear oxomolyb-
denum(VI) complexes containing either tridentate dianionic or
hexadentate tetraanionic donating ligands and a neutral monoden-
tate solvent molecule were prepared as shown in the overall reac-
tion synthetic pathway, Scheme 2.
2.6.6. Synthesis of 1,4-bis(3-ethoxysalicylaldehyde
carbohydrazonato)butane dimethylformamide dinuclear
dioxomolybdenum(VI) (C6)
In the preparation of the tridentate dianionic (ONO/ONS) ligand
and hexadentate tetraanionic (ONO–ONO) ligands, different
substituted salicylaldehydes were reacted with monohydrazide
and adipic acid dihydrazide compounds, respectively, to produce
mononucleating and binucleating ligands. The reaction of bis(acet-
ylacetonato)dioxomolybdenum(VI) and the tridentate dianionic li-
0.656 g (2.0 mmol) of MoO2(acac)2 in 40 ml of ethanol was
mixed with 40 ml of 0.438 g (1.0 mmol) of 1,4-bis(3-ethoxysalicyl-
aldehyde carbohydrazonone)butane. The resulting orange solution
mixture was then heated to reflux for 1 h. A few drops of dimeth-
ylformamide were added to the solution and the heating was con-
tinued for another hour. After leaving the solution for 1 week at
room temperature, yellow crystals were formed. The product was
filtered, washed with ethanol and dried in air. M.p.: 125–127 °C,
(0.29 g, 22%). Anal. Calc. for C30H40O12N6Mo2: C, 41.47; H, 4.61;
gands, using methanol as
a
refluxing medium, afforded
mononuclear oxomolybdenum(VI) complexes with ONO and ONS
donor ligands system. These tridentate ligands were used for com-
plexation as the complexes formed leave a labile coordination site
that can be used for substrate binding. In the case of mononuclear
dioxomolybdenum(VI) complexes of the –ONS ligand system, an
effort has been made to displace the alcohol molecule by using
4,4-bipyridine and 4,4-bipyridine N,N0-dioxide. This reaction man-
aged to produce a series of dinuclear oxomolybdenum(VI) com-
plexes, in which two Mo atoms were interconnected by the
bidentate ligands. Different types of dinuclear oxomolybdenum(VI)
complexes were formed by the reaction of the hexadentate tetra-
anionic ligand with bis(acetylacetonato)dioxomolybdenum(VI) in
the presence of ethanol. In this case, the two molybdenum atoms
are not bridged directly by a bidentate ligand, but are coordinated
at each end by the O,N,O donor atoms of the symmetrical hexaden-
tate Schiff base ligand. The reaction proceeded by displacing the
ethanol molecule in the first case using a monodentate ligand to
yield another dinuclear oxomolybdenum(VI) complex of the same
series. To complete the synthetic work, the preparation of tetranu-
clear dioxomolybdenum(VI) complexes has been carried out by
combining the dinuclear dioxomolybdenum(VI) complex of the
Table 5
Bond lengths (Å) for the complexes.
C2
C3
C4
C5
C6
C7
Mo(1)–O(2) 1.712(2) 1.707(2)
Mo(1)–O(3) 1.709(2) 1.692(2)
Mo(1)–O(1) 1.954(2) 1.923(2)
Mo(1)–N(1) 2.288(3) 2.234(2)
1.6968(12) 1.713(2) 1.7169(13) 1.729(3)
1.7009(13) 1.700(2) 1.7032(13) 1.696(3)
1.9312(12) 1.929(2) 1.9412(13) 1.933(3)
2.2591(13) 2.243(3) 2.2242(16) 2.217(4)
Mo(1)–S(1)/ 2.4165(8) 2.0257(19) 2.0161(12) 2.022(2) 2.0143(13) 2.001(3)
O(4)
Mo(1)–N(3/ 2.297(2) 2.327(2)
4)/O(5/6)
2.2786(14) 2.205(2) 2.2849(13) 2.392(4)
C(1)–O(1)
N(2)–C(8/9/ 1.314(4) 1.296(4)
10)
1.329(4) 1.343(3)
1.327(2)
1.303(2)
1.341(4) 1.350(2)
1.296(5) 1.296(2)
1.359(5)
1.291(5)
N(2)–N(1) 1.377(3) 1.411(3)
1.4040(18) 1.404(4) 1.403(2)
1.3118(19) 1.318(4) 1.324(2)
1.404(5)
1.315(5)
C(8/10)–
S(1)/O(4)
1.753(3) 1.313(3)
N(1)–C(7/8/ 1.301(4) 1.286(3)
9)
1.286(2)
1.286(5) 1.285(2)
1.292(5)

240
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Table 6
Bond angles (°) for the complexes.
C2
89.24(10)
C3
93.48(9)
C4
93.55(5)
C5
90.43(11)
C6
90.73(6)
C7
90.13(15)
O(3)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
81.71(9)
161.49(10)
88.83(8)
101.96(8)
150.58(7)
75.75(7)
84.96(9)
169.14(10)
74.86(9)
83.28(8)
83.85(6)
104.17(11)
108.88(10)
96.34(10)
81.59(8)
158.69(9)
97.27(9)
97.44(9)
149.73(8)
71.29(8)
84.98(9)
168.72(9)
81.92(8)
75.26(8)
78.47(8)
106.04(11)
103.76(9)
97.38(10)
81.42(5)
159.60(6)
99.00(5)
94.84(5)
150.32(5)
71.59(5)
84.97(5)
168.99(5)
81.78(5)
75.66(5)
79.80(5)
105.47(6)
102.36(5)
99.03(6)
81.82(10)
162.16(11)
97.54(10)
94.78(11)
150.57(10)
71.80(10)
87.97(10)
165.98(10)
83.33(10)
75.97(9)
81.08(5)
160.92(6)
94.85(6)
100.42(6)
147.41(6)
71.48(5)
84.92(6)
169.92(6)
78.99(5)
79.52(5)
79.03(5)
105.14(7)
106.82(6)
97.10(6)
81.75(12)
159.64(14)
93.70(13)
98.18(15)
149.12(13)
71.65(12)
84.20(14)
169.42(15)
80.61(12)
79.32(13)
79.32(12)
106.28(16)
107.35(13)
97.27(14)
O(2)–Mo(1)–S(1)/O(4)
O(3)–Mo(1)–S(1)/O(4)
O(1)–Mo(1)–S(1)/O(4)
N(1)–Mo(1)–S(1)/O(4)
O(2)–Mo(1)–N(4)/O(4/5/6)
O(3)–Mo(1)–N(4)/O(4/5/6)
O(1)–Mo(1)–N(4)/O(4/5/6)
N(1)–Mo(1)–N(4)/O(4/5/6)
S(1)/O(4)–Mo(1)–N(4)/O(4/5/6)
O(2)–Mo(1)–O(3)
77.91(9)
104.98(12)
104.29(11)
98.30(11)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(1)
S
C
OH
OH
S
+
H 2O
CH2CH3
H2N
+
N
H
N
H
N
C
CH2CH3
CHO
X
X
HC
N
H
N
H
X= 5-chloro (L1), 3,5-dichloro (L2),
OH
O
H
N
+
H2N
C
+
2 H2O
N
H
C
O
NH2
X
CHO
OH
O
C
H
N
N
C
X
X
C
H
N
H
C
O
N
HO
X= 3-ethoxy (L3), 3,5-dichloro (L4), 4-hydroxy (L5)
Scheme 1. Preparation of the ligands.
hexadentate tetraanionic ligand system with the 4,4-bipyridine
bidentate ligand, and the attempt was successful. Two units of
dinuclear dioxomolybdenum were bridged by 4,4-bipyridine and
the adjacent molecules were joined together, forming a polymeric
chain through hydrogen bonding interactions between the hexa-
dentate ligands.
the IR spectra of the free thiosemicarbazone and hydrazone ligands,
imine stretching bands appeared between 1590 and
(mC@N)
1610 cmꢀ1, but for the complexes, the characteristic imine bands
were observed at higher stretching frequencies ranges, namely
1610–1620 cmꢀ1, indicating the coordination of the imine nitrogen
atom to the metal center. The coordination of 4,4-bipyridine is char-
acterized by the ring breathing mode of pyridine, which occurs at
990 cmꢀ1 in free pyridine [22]. The slight increase on chelation in
the frequency of the pyridine breathing vibration (1003 cmꢀ1) for
C1 can be attributed to coordination from the pyridine nitrogen to
the metal ion. The IR spectrum of C2 exhibits absorption bands at
823 and 834 cmꢀ1 which are assigned as N–O stretching. The free
pyridine N–oxide exhibits a band at 1234 cmꢀ1 [23]. The N–O
stretching frequency, which is lowered upon coordination, is attrib-
uted to the weakening of the bond by the reduction of the electron
density i the N–O bond and the loss of the double bond character of
the N–O bond when the oxygen atom coordinates to the d0 metal ion
[24,25]. In the case of C5 and C6, the bands at 1131 and 737 cmꢀ1 are
3.2. Infrared spectra spectroscopic description
The IR spectra of the complexes C1–C7 were analyzed in compar-
ison with that of the free ligands, L1–L5. The precursor complex of
MoO2(acac)2 has two sharp bands at 934 and 906 cmꢀ1, which are
related to the symmetrical and anti-symmetrical stretching of cis-
MoO22+, respectively. These bands in the complexes C1–C7 appear
between 897 and 941 cmꢀ1, indicating that the MoO22+ symmetrical
and anti-symmetrical stretching vibrations are independent of the
electron donating and accepting capacity of the ligands [18,19].
These data also indicate the presence of a cis-MoO2 structure as a
trans-MoO2 unit would normally show one mMo@O band due to the
asymmetry stretch [20]. It is revealed from the IR absorption fre-
associated with mP@O of HMPA and mNC@O of DMF solvent molecules.
quencies for the
m
Mo@O band of the Mo(VI) oxo-complexes that the
3.3. 1H and 13C NMR spectroscopy description
coordination of the neutral bidentate N–N donor ligands (4,4-bipyr-
idine/4,4-bipyridine N,N0-dioxide) has the effect of lowering the
Mo@O stretching frequency [21], as is shown in C1, C2 and C7. In
The 1H and 13C NMR (DMSO-d6) data of the free ligands and
their polynuclear oxomolybdenum(VI) complexes are given in

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
241
OH
X
X'
C
CHO
H2N
H2N
O
C
H
N
+ CH3CH2OH
HN
R
N
H
C
O
NH2
CH3OH
OH
X'
C
OH
N
X
O
C
C
H
N
H
R
H
H
C
N
N
X
C
H
X
N
H
C
N
O
CH3OH
HO
MoO2(acac)2
MoO2(acac)2
O
CH3CH2OH
O
HOCH 3
X'
Mo
O
N
C
X
C
H
N
R
O
O
HOCH2CH3
Mo
O
CH3CH2OH
+ 4,4-bipyridine
@ 4,4-bipyridine
N,N'-dioxide
R
O
C
H
C
X
N
N
C
H
X
N
C
O
N
O
Mo
O
C
N
CH3CH2OH
O
X'
N
CH
O
O
Mo
X
D - D
Mo
O
O
O
X'
C
N
X
C
H
N
R
CH3OH
+
D
CH3OH
+ 4,4-bipyridine
@4,4-bipyridine
N,N'-dioxide
O
O
N
N
O O
Mo
D
Mo
O
O
C
H
O
N
N
C
OH
HO
C
H
C
O
N
O
C
H
N
O
N
C
C
X
C
H
X
N
N
Mo
O
O
N
O
O
Mo
O
O
O
N
D
Mo
O
O
O
C
H
C
N
N
OH
HO
C
H
N
C
O
N
O
Mo
N
O
O
O
O
N
Mo
O
O
H
N
C
N
OH
C
HO
C
H
N
C
O
N
O
Mo
O
N
O
Scheme 2. X = OC2H5 or Cl; X0 = S or O; R = NHC2H5; D = DMF or HMPA; D-D = 4,4-bipyridine or 4,4-bipyridine N,N0-dioxide.
the experimental section. Before the interpretation of the NMR
spectra of the complexes is presented, it is appropriate to comment
on some points regarding the spectra of the thiosemicarbazone and
dihydrazone ligands. The 1H NMR spectra of L1 and L2 exhibit a d-
OHphenolic proton resonance at d 10.33 and 10.68 ppm, and an
amide proton, Himine–C@N resonance at d 8.62 and 8.45 ppm. The
broadening of these absorption peaks may occur due to the
involvement of the protons in intra- and intermolecular hydrogen
bonding interactions [26]. A multiplet due to aromatic protons ap-
pears in the d 6.74–7.63 ppm region in all the five ligands. Further-
more, the signal appearing at d 3.41 ppm in L3 is assigned to the
proton of the ethylene (–CH2CH2–) group. Upon coordination to
the Mo atom, the –OH signal in the spectra of the ligands disap-
pears. This suggests that the thiosemicarbazone and dihydrazone
ligands are coordinated to the metal center in the enol form, with
phenoxyl oxygen atoms. On the other hand, the participation of the

242
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
imine nitrogen in complexation is signaled by an appreciable
downfield shift of the azomethine proton signal (d 8.75 ppm).
The downfield shift of these signals is attributable to the drainage
of the electron density from the nitrogen atom of the azomethine
group to the Mo atoms [27]. C1 and C2 show a few signals in d
7.85–8.72 ppm range. These signals are attributed to the protons
of the pyridyl rings of the 4,4-bipyridine molecules.
dioxide begins at 200 °C and ends at 270 °C. It was observed that
the decomposition starts without melting. The DSC curves show
an exothermic peak temperature at 242 and 234 °C, with
D
H = ꢀ48.88 and ꢀ320.27 J gꢀ1, respectively. The second stage,
which occurs in the temperature range 270–870 °C, corresponds
to the decomposition of the two tridentate ligands. The final residue
is MoO2.
A comparison of 13C NMR spectra of the ligands and complexes
revealed several interesting features. The average position of the
resonance due to the secondary amido carbonyl carbon atoms is
found to have shifted downfield in the complexes. Some new sig-
nals which are present in the spectra of C1, C2 and C7, but not in
the spectra of the ligands, are assigned to the various carbon atoms
of 4,4-bipyridine. These signals appeared in the d 121–154 ppm re-
gion. Additionally, it is remarkable to note that C3–C7 show signals
at about 110 ppm corresponding to the –CH2CH2– group, which
appear at 120 ppm in the uncoordinated dihydrazone. The upfield
shift of these signals in the complexes indicates that the electron
density at the –CH2CH2– carbon atoms is increased in the coordi-
nated dihydrazone.
The thermal decomposition of C4 takes place in four steps. Two
ethanol molecules are removed followed by the fragmentation of
chlorine substituted atoms at the salicylaldehyde rings, observed
at 170–200 and 300–350 °C. The third stage is related to the
decomposition of the hexadentate ligand, taking place in the tem-
perature range 620–870 °C. The final residue of MoO2 undergoes
further decomposition at 620 °C. The DSC curve shows two endo-
thermic and one exothermic temperature peak at 160, 206 and
252 °C, respectively.
The TGA curve of C6 shows two stages of thermal decomposi-
tion. Two molecules of DMF are eliminated between 150 and
270 °C. The second stage decomposition, which occurs in the tem-
perature range 270–870 °C with a mass loss of 52.18% (calc.
53.88%), accounts for the removal of the hexadentate ligands. The
removal of the hexadentate ligands, which occur at 251 °C, re-
quired 279 J gꢀ1 exothermic heat as shown in the DSC curve. In
the case of C7, the mass losses (18.13% and 72.48%) that occur in
two consecutive complicated transition stages in the temperature
ranges 200–280 and 300–874 °C, respectively, are mainly due to
the breakage of the 4,4-bipyridine coordinated bond, followed by
decomposition of the dihydrazone ligands.
In general, there are differences in the exothermic and endo-
thermic peaks recorded in the DSC curves of the complexes. These
differences originate from the packing mode of the complexes. The
additional hydrogen bonding interactions in C1, C2, C3 and C7 ex-
ert more stability to the crystal structure. More precisely, the
decomposition of C1, C2 and C7 occurred at 200–280 °C and that
of C5 and C6 at approximately 100 °C. Based on these data, it can
be concluded that for the structural transformation in the C1, C2
and C7 complexes, which are more stable, a larger energy con-
sumption is necessary because of the breakage of a higher number
of intermolecular hydrogen bonds. In summary, the thermal
decomposition behavior of the complexes can be summarized in
Scheme 3.
3.4. Thermal properties of the title complexes
Thermal analysis techniques such as thermogravimetry (TGA),
differential thermal analysis (DTA) and differential scanning calo-
rimetry (DSC) have been widely applied in studying the thermal
behavior of metal complexes. The data enables us to follow a series
of structural changes the compounds were subjected to during
thermal treatment, and subsequently provides insight pertaining
to the thermal stability and thermal decomposition of the com-
pounds in the solid state [28,29].
The TGA analyses of the dinuclear tetraoxomolybdenum(VI)
complexes have been carried out in the temperature range 50–
900 °C, at a heating rate of 20 °C minꢀ1. The decomposition stages,
temperature ranges, found and calculated weight loss percentages,
as well as the thermal effects accompanying the changes in the so-
lid complexes on heating are given in Table 1. The thermal behav-
ior of the mononuclear and binuclear molybdenum complexes is
different. For different binuclear complexes, the TGA curves show
mass losses in a different number of stages, which are due to the
removal of the solvent molecules, followed by the departure of
the ligands. The area of the endothermic DSC peak corresponds
to the melting heat, while the area of the exothermal peak corre-
sponds to the heat of decomposition.
3.5. Electronic spectral studies
The TGA curve of C3 shows a mass loss between 150 and 200 °C,
corresponding to an endothermic peak temperatureat 175 °C, due to
the removal of two ethanol molecules. The heat of removal of the
ethanol molecules obtained from DSC curve (192.81 J gꢀ1) suggests
that the ethanol molecules are loosely attached to the molybdenum
center. The second step corresponds to a thermal decomposition be-
tween 200 and 270 °C which involves the removal of two –OCH2CH3
fragments from the dihydrazone ligands. This is supported by the
exothermic decomposition temperature peak in DSC curve at
229 °C and the heat of decomposition calculated (ꢀ166.42 J gꢀ1).
Further removal of the hexadentate ligands occurs at the higher
temperature range 270–730 °C according to the TGA curve. The final
remaining residue, MoO2, also undergoes decomposition at 900 °C.
The thermal decomposition of C5 occurs in two steps. The depar-
ture of the hexadentate ligands takes place between 100 and 400 °C
with a mass loss of 42.46% (calc. 41.20%). This is supported by the
endothermic effect temperature peak in DSC curve at 171 °C and
the calculated heat of decomposition (+130.40 J gꢀ1). The next step
between 400 and 850 °C corresponds to the removal of two HMPA
solvent molecules, leaving MoO2 as a residue.
The electronic absorption bands of the ligands and the com-
plexes in 5 ꢁ 10ꢀ4 M DMF, together with their tentative assign-
ments, are given in Table 2. The absorption bands of the free
ligands can be classified into three regions. The absorption region
between 250 and 290 nm in the ligands is characteristic for the
p
to p⁄ transition of the benzene rings of the ligands. Two absorption
bands observed between 290 and 350 nm are probably due to the n
to p⁄ transition of the –C@N– and –C@O– or –C@S– moieties of the
ligands [30]. The spectra of the complexes show a broad band at ca.
400 nm, which is assignable to a ligand to metal charge transfer
(LMCT) due to the promotion of an electron from the highest occu-
pied molecular orbital (HOMO) of the ligand to the lowest unoccu-
pied molecular orbital (LUMO) of molybdenum atom [31]. The
electronic spectra of the complexes C1–C7 in DMF display two
absorption bands in the 293–312 and 329–355 nm regions. These
peaks may be assigned to oxygen to molybdenum and nitrogen to
molybdenum charge transfer transitions, respectively [32,33]. The
slight change in the k values within the L1–L5 ligands may be due
to their difference in the electron donating property incited by the
nature of the substituents. For instance, L1, L2 and L4 are gradually
less electron rich, as a result of the presence of the chlorine atoms.
This makes the HOMO of these ligands higher in energy and thus
The thermal decomposition of both C1 and C2 occurs in two
steps. The removal of the 4,4-bipyridine and 4,4-bipyridine N,N0-

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
243
Schiff base ligand. The reaction of the bidentate ligand, 4,40-bipyr-
idine, and the symmetrical hexadentate Schiff base ligand with
MoO2(acac)2 yielded a polynuclear Mo(VI) complex, C7. All the
complexes possess two equivalent halves with a center of inver-
sion, except for C1 in which the two halves of the dinuclear com-
plex are not crystallographically equivalent but their structural
dimensions are closely related.
L
D-D
(MoO2)2L2(D-D)
For C1 & C2, where L is tridentate ligands, D-D is bidentate ligands
L'
MoO2L
MoO2
2 D
MoO2
For C1 - C2, where L' is hexadentate ligands, D is monodentate ligands
(MoO2)2L'
(MoO2)2L'(D2)
3.8. Crystallographic description for C1 and C2
L'
D-D
(MoO2)2L'
(MoO2)4L2'(D-D)
MoO2
As shown in Figs. 2 and 3, the overall geometry of C1 and C2 can
be regarded as an octahedron, with the equatorial plane defined by
the imine nitrogen, phenoxyl oxygen, sulfur atom and one of the
terminal oxygen atoms of the dioxomolybdenum unit. The other
terminal oxygen and the nitrogen or oxygen atom from the bridg-
ing 4,40-bipyridine or 4,40-bipyridine N,N0-dioxide molecules occu-
py the apical positions. The two oxygen, sulfur, nitrogen and
molybdenum atoms are found to be essentially coplanar, in which
the molybdenum atom shows a slight shift out (0.2764 and
0.2358 Å) of the basal plane towards the O3 direction. Typical val-
ues for the O@Mo@O angles are around 108°, while optimal values
for the Mo@O distance lie around 1.70 Å.
For C7
Scheme 3. Thermal decomposition of the dinuclear and polynuclear oxomolybde-
num(VI) complexes.
a red shift is experienced [34]. Higher energy bands appearing be-
low 290 nm are due to intra-ligand transitions. The absence of a
d–d transition absorption band in the visible region confirms the
4d0 electronic configuration of Mo(VI) [35].
3.6. Electrochemical studies
The Mo–D (donor atom of solvent) bond length in the dioxomo-
lybdenum(VI) complexes are generally found to be somewhat
longer than a normal single bond length due to the consequence
of the trans effect of M@Ot (where Ot is oxo-group trans to the
Mo–Osolvent bond). In these cases, the Mo–N(3) distance
(2.4566 Å) is longer than that of Mo–O(4) (2.297 Å). This result re-
veals a rather weak attachment of the 4,4-biypridine ligand com-
The redox behavior of the ligands and the complexes were
examined in DMF solution using cyclic voltammetry at a platinum
electrode with 0.1 M tetraethyl ammonium perchlorate (TEAP) as
the supporting electrolyte. The CV data is tabulated and selected
I versus E profiles are illustrated in Table 3. From the data, L3
and L4 exhibit one irreversible oxidative peak around ꢀ0.28 V
and one reductive peak at ꢀ0.38 V while L1, L2 and L5 show only
one reduction peak from ꢀ0.36 to ꢀ0.42 V in the cyclic voltammo-
gram recorded in the potential range +2.0 to ꢀ1.5 V. The structure
as well as the different electron donating properties of the substi-
tuent groups in the ligands are probably the leading contributors to
the additional anodic oxidative potential in L3 and L4. The CV trace
of the complexes displays two irreversible reductive responses
within the potential window ꢀ0.39 to ꢀ0.94V. The reductive wave
near ꢀ0.39 V in the complexes is due to the effect of the redox
behavior of the ligands as the observed values fall in the same re-
gion as that for the free ligands. The calculated Ipa/Ipc ratio = 0.81
and 1.17 for C1 and C7, respectively, indicating that these com-
plexes have a reversible redox property. On the contrary, the other
type of dinuclear molybdenum complexes, C2–C6, show two suc-
cessive irreversible one electron oxidation and reductions within
the range ꢀ0.25 to ꢀ0.86 V. The emergence of these peaks in the
complexes could be assigned to irreversible metal centered and
the ligand (bridging)-centered oxidation and reduction processes
[36]. The more negative reduction potential of C2 compared to
C3–C7 indicates that the Mo ions in the complexes are much more
stable in the higher oxidation state due to the presence of strong
and hard donor atoms (N,O) in the Schiff base ligands [37]. In gen-
eral, CV studies on these dinuclear and polynuclear molybde-
num(VI) complexes reveal that the complexes have essentially
similar irreversible redox behavior, except for C1 and C7 (Fig. 1).
2+
pared to the 4,40-bipyridine N,N0-dioxide ligand to the MoO2
moiety.
A closer look at the crystal structure packing of C1 shows that
the supramolecular rectangular structure of C1 has been con-
structed by an intermolecular hydrogen bonding interaction, N7–
H7A. . .O3, as shown in Fig. 9 and Table 7. The oxo-group is engaged
in the H-bonding interaction with the hydrogen atom attached at
the N7 atom from a neighboring molecule, with the result that
the chains are interlocked into a two dimensional polymeric net-
work which runs along the bc crystallographic plane (Fig. 10). On
the contrary, the hydrogen atom from the secondary amine group
of one C2 molecules form a hydrogen bond to a nitrogen atom of
the 4,40-bipyridine of the adjacent C2 molecule (Fig. 11) and Table
8. This arrangement gives rise to the formation of a one-dimen-
sional helical chain that runs along the diagonal of ac crystallo-
graphic plane (Fig. 12).
3.9. Crystallographic description for C3–C7
The structures of C3–C7 consist of (MoO2)L, accompanied by
two molecules of donor solvent molecules. Unlike C1 and C2, the
octahedral environment around the molybdenum atom comprises
of cis-Mo@O, imine nitrogen, phenoxyl oxygen and hydroxyl oxy-
gen atoms from the enolized carbonyl group, further supported
by monodentate neutral ligands, such as ethanol, DMF and HMPA.
The average values for the Mo–O(2)/(3) distance and O@Mo@O an-
gle in these complexes varies slightly in the ranges 1.70–1.72 Å and
104–107°, respectively. On the other hand, a least-square calcula-
tion shows that the molybdenum atoms of C5 deviate the least
(0.2118 Å) towards the distal O3, indicating that the deviation is
restricted by the presence of the bulky HMPA molecule in the com-
plex. The planarity of this equatorial plane was further corrobo-
rated by the close linearity of the O2–Mo–N1 angle of C5,
162.16(12)°.
3.7. Crystallographic description of the complexes
The molecular structures and the atom numbering scheme of
C1–C7 (Figs. 2–8) show that in C1 and C2 the Schiff base ligands
behave as tridentate ligands and react with the dioxomolybdenum
anion to form a six coordinated molybdenum(VI) complex, in
which the bidentate ligand, 4,40-bipyridine or 4,40-bipyridine
N,N0-dioxide (D-D) forms a bridge between the two molybdenum
atoms. In the case of C3–C6, the two molybdenum atoms are not
bridged directly by a bidentate ligand, but are coordinated at each
end to the O,N,O donor atoms of the symmetrical hexadentate
The extended structures of C3, C5, C6 and C7 reveal that the
butylene chains of these structures adopt an antiperiplanar or zig-
zag conformation (Figs. 4 and 6–8), except for C4 in which the

244
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Fig. 1. Cyclic voltammograms for L1(2a) and C1(2b) – scan rate 100 mV/s.
Fig. 4. ORTEP plot of C3, with the atom labeling scheme.
Fig. 2. ORTEP plot of C1, with the atom labeling scheme.
Fig. 5. ORTEP plot of C4, with the atom labeling scheme.
Fig. 3. ORTEP plot of C2, with the atom labeling scheme.
butylene chain adopts the gauche staggered conformation (Fig. 5)
and Table 10. In this case, the two dioxomolybdenum fragments
rotate about the C–C bond of the butylene chain to facilitate the
intramolecular O–H. . .N hydrogen bonding between the ethanol

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
245
Fig. 6. ORTEP plot of C5, with the atom labeling scheme.
Fig. 7. ORTEP plot of C6, with the atom labeling scheme.
donor ligand and the imino nitrogen of the Schiff base (Fig. 13). In
3.10. Catalytic reaction of the oxomolybdenum(VI) complexes and
H2O2 with benzyl alcohol
the case of the complex containing the ethanol molecule, C3, adja-
cent molecules are linked by OHꢂꢂꢂN hydrogen bonds into a poly-
meric chain that runs along the a-axis of the monoclinic unit cell
(Fig. 14) and Table 9. Since there is no evidence of a hydrogen
The oxidation of alcohol by dioxomolybdenum(VI) Schiff base
complexes with H2O2 was carried according to the following gen-
eral procedures.
bonding interaction or C–H. . .
p weak interaction in the crystal
structures, C5 and C6 exists as discrete molecules. As shown in
Fig. 15, the fundamental building block of the polymeric network
in C7 is the binuclear unit, which is linked through a covalently
bonded 4,40-bipyridine spacer group into a polymeric chain. Adja-
cent chains are further interconnected through an O5–H5. . .O2
hydrogen bond interaction, with a methanol molecule hydrogen
bonded to the terminal oxygen of the molybdenum atom, leading
to the formation of a two dimensional layer (Fig. 16). The layer
does not appear to be flat, but is fabricated of a supramolecular
staircase with the bipyridine plane (N3–C11–C12–C13–C14–C15)
and the Mo1–O4–C8–N2–N4 plane lying perpendicular to one an-
other along the a-axis (Fig. 17) and Table 11. The X-ray crystallo-
graphic study revealed that extensive of hydrogen bonds play a
pivotal role in the self-assembly of the polymeric oxomolybde-
num(VI) complexes, contributing to the supramolecular system.
10 cm3 of benzyl alcohol (10.44 g, 0.1 mol) was mixed with
20 cm3 of tetrahydrofuran (THF), and 0.5 g of the synthesized
dioxomolybdenum(VI) complexes was added followed by 10 cm3
of 30% H2O2. The mixture was kept under reflux at 90 °C for 24 h.
The THF extracts were evaporated using a rotary evaporator under
low pressure before treatment with Na2SO4. The presence of ben-
zoic acid was detected using IR spectroscopy.
A Shidmazu Model GC 2010 Gas Chromatograph coupled with a
Shidmazu GC–MS QP 2010 PLUS series mass selective and auto-
sampler was employed for the analysis of the benzoic acid in the
reaction mixture. With the help of an autoclavable digital pipette,
10
pared for GC–MS analysis. The samples were separated on a
30 mm ꢁ 0.25 mm, 0.25 m, Rtx-5Ms silica capillary column. The
column temperature was initially held at 80 °C for 1 min, then
lL of the reaction mixture diluted with 10 ml of THF was pre-
l

246
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Fig. 8. ORTEP plot of C7, with the atom labeling scheme.
Fig. 9. Intermolecular hydrogen bond in the crystal structure of C1.

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
247
Table 7
Table 8
Hydrogen bonds for C1 (Å and °).
Hydrogen bonds for C2 (Å and °).
D–H. . .A d(D–H)
N(3)–H(3A). . .O(4)# 0.86
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
d(H. . .A)
d(D. . .A)
\(DHA)
145.5
N(3)–H(3). . .O(6)#1
N(7)–H(7A). . .O(3)#2
0.86
0.86
2.09
2.19
2.9278(17)
3.0466(16)
165.6
175.3
2.21
2.959(3)
Symmetry transformations used to generate equivalent atoms: # ꢀx, ꢀy + 2, ꢀz + 2.
Symmetry transformations used to generate equivalent atoms: #1 x + 1/2, ꢀy + 3/2,
z + ½; #2 x ꢀ 1/2, ꢀy + 1/2, z ꢀ 1/2.
the temperature was raised to 220 °C at a rate of 10 °C minꢀ1, from
220 to 280 °C at a rate of 20 °C per minute and finally held for
15 min. The total run time was 33 min. Helium gas was used as
the carrier gas. The injector temperature was maintained at
300 °C and the injection volume was 1.0 lL in the splitless mode.
Mass spectra were scanned from m/z 50–650 at a rate of 0.5
scan/s. The benzoic acid was identified by matching the GC
retention time and mass spectra with an authentic standard. The
oxidation activity was also investigated without the catalyst.
3.11. Oxidation of benzyl alcohol catalyzed by the
dioxomolybdenum(VI) complexes
Oxidation reactions have been carried out using complexes
C1–C7 as catalysts, benzyl alcohol as a model substrate and
H2O2 as the oxidant at 90 °C. Blank runs were performed and as
expected, without catalyst, no significant benzoic acid was ob-
served under the applied conditions, indicating the catalytic abil-
ity of the complexes. The catalytic oxidation reaction yields
benzoic acid as the product only after 24 h of reaction. Addition-
ally, the oxidation of alcohol strongly depends on the reaction
temperature. At room temperature, no benzoic acid compound
Fig. 11. Intermolecular hydrogen bond in the crystal structure of C2.
Fig. 10. Interlocking of the chain assembled by hydrogen bonding in C1 the along the a-axis.

248
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Fig. 12. X-ray crystal structure of C2 showing the arrangement of molecules into a hydrogen bonded polymeric helical chain.
Table 9
Hydrogen bonds for C3 (Å and °).
D–H. . .A
d(D–H)
0.62(4)
d(H. . .A)
d(D. . .A)
\(DHA)
O(6)–H(1). . .N(2)#
2.18(4)
2.798(3)
172(5)
Symmetry transformations used to generate equivalent atoms: # ꢀx, ꢀy + 1, ꢀz + 2.
Table 10
Hydrogen bonds for C4 (Å and °).
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
O(5)–H(1). . .N(2)#
0.70(3)
2.01(3)
2.710(2)
177(3)
Symmetry transformations used to generate equivalent atoms: # ꢀx + 2, y, ꢀz + ½.
Fig. 14. Crystal structure packing of C3.
It can be seen that C1 and C7, in which the sixth coordinating site
of the Mo(VI) ion is occupied with 4,4-bipyridine as a weak
bidentate ligand, show higher catalytic activity than the rest of
the complexes. The reversible redox behavior, as shown by the
CV analysis in this report, further support C1 and C7 possessing
catalytic properties. According to Jia Li et al., dioxomolybde-
num(VI) complexes using two connected bridging ligands con-
taining a nitrogen donor such as 4,40-bipyridine are found to be
good catalysts for oxidation and epoxidation process using H2O2
as an oxidant [38]. An amine additive, such as pyridine, pyrazole
and 3-cyanopyridine, enhance the efficiency of the catalytic oxi-
dation [39–44]. The aromatic N-base ligands work by coordinat-
ing to the molybdenum center, thereby reducing the Lewis
acidity of the catalyst and additionally accelerating the catalytic
reaction [45,46]. It was also found that the yields of conversion
by C5 and C6 are the lowest (Chart 1). This may be due to the
Fig. 13. Intermolecular hydrogen bond in the crystal structure of C3.
was observed. As we observed more than 10% conversion at 90 °C,
it may be presumed that the reaction starts at
a higher
temperature.
2+
As is shown in Table 12, the number of MoO2 units in the
catalyst has no major effect on the oxidation of benzyl alcohol.
On the contrary, we considered the effect of the presence of the
coordinating ligand in the medium of the reaction, such as 4,4-
bipyridine, 4,4-bipyridine N,N0-dioxide, ethanol, HMPA and DMF.

N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
249
Fig. 15. ORTEP plot of C7.
Fig. 16. Inter-chain hydrogen bonds of C7.
strong ability of these ligands to coordinate to the metal central,
which restricts the number of free coordination sites that would
be required for the catalytic oxidation of alcohols.
alyst loading and substrate concentration on the rate of oxidation
reaction using dioxomolybdenum(VI) complexes will be carried
out in due course.
4. Conclusion
Acknowledgment
Several dinuclear and polynuclear dioxomolybdenum(VI) com-
plexes have been synthesized and characterized by various phys-
icochemical techniques and by X-ray crystallographic diffraction.
Financial support of this work by the University of Malaya
(PS378/2010B and RG020/09AFR) is greatly appreciated.
2+
In all the complexes, the structures contain two MoO2 moieties
bridged by two N atoms from bipyridine ligands or by hexaden-
tate ligands coordinating through ONO atoms. The X-ray crystallo-
graphic study revealed that extensive hydrogen bonds play an
important role in the self assembly of the polymeric C1, C2, C3,
C4 and C7 complexes. The electrochemical study shows irrevers-
ible redox behavior of the cis-dioxomolybdenum(VI) complexes,
except for C1 and C7. From a catalytic property study and from
CV analysis, C1 and C7 are found to be catalysts for oxidation
reactions. Further study regarding the effect of temperature, cat-
Appendix A. Supplementary data
CCDC 845679, 826697, 826694, 826696, 826695, 845680 and
825681 contain the supplementary crystallographic data for C1–
C7. 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, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

250
N.K. Ngan et al. / Polyhedron 33 (2012) 235–251
Fig. 17. Crystal packing of C7 illustrating the formation of a supramolecular staircase (hydrogen atoms are omitted).
Table 11
Hydrogen bonds for C7 (Å and °).
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
O(5)–H(5). . .O(2)#1
O(6)–H(6A). . .O(3)#2
0.84
0.84
1.97
2.04
2.805(4)
2.725(6)
173.6
138.4
Symmetry transformations used to generate equivalent atoms: #1 x ꢀ 1, y, z; #2
x + 1, y, z.
Table 12
Screening for a suitable catalyst for the oxidation of alcohol after 24 h.
Chart 1. Oxidation reaction catalyzed by different types of dioxomolybdenum(VI)
complexes. Reaction conditions: molar ratio of C6H5CH2OH/H2O2 = 1:1; 0.001 mol
of catalyst, temperature 90 °C for 24 h.
Reactants/
catalyst
Cal.
mass
(g)
Molar mass
Cal.
mole
Mole
fraction
(%)
% conversion
benzyl alcohol
(g molꢀ1
)
C6H5CH2OH 10.40
108.14
34.01
–
0.10
0.10
–
49.80
49.80
–
–
–
0.00
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H2O2
Without
3.33
–
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catalyst
Without
H2O2
C1
C2
C3
C4
C5
C6
C7
–
–
–
–
0.00
0.92
1.03
0.81
0.87
1.08
0.87
0.45
923.00
1026.00
814.00
866.00
1082.00
868.00
445.00
0.001 0.40
0.001 0.40
0.001 0.40
0.001 0.40
0.001 0.40
0.001 0.40
0.001 0.40
26.30
12.40
9.10
10.30
5.50
5.50
40.20
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