Synthesis, structure studies and electrochemistry of molybdenum(VI) Schiff base complexes in the presence of different donor solvent molecules

Article abstract of DOI:10.1016/j.poly.2011.08.038

The reactions between bis(acetylacetonato)dioxomolybdenum(VI) and Schiff base ligands derived from 5-chlorosalicylaldehyde or 3-ethoxy-salicylaldehye, and 3-methoxy-benzoic hydrazide (m-anisic hydrazide), 2-furoic hydrazide or 2,4-dihydroxy-benzoic hydrazide in the presence of donor solvents yielded cis-dioxomolybdenum(VI) complexes with the general formula MoO₂L(D), where L = tridentate Schiff base ligand and D = dimethylsulfoxide, hexamethylphosphoramide, dimethylformamide, imidazole or methanol. The complexes were characterized by elemental analysis, electronic spectra, IR, ¹H and ¹³C NMR spectroscopies, thermogravimetric analysis, cyclic voltammetry, and the molecular structures of five of the dioxomolybdenum complexes were elucidated by single crystal X-ray diffractometion studies. In general, the complexes adopt an octahedral environment around the Mo center with a cis-oxo configuration. The other coordination sites are occupied by the imino nitrogen, phenoxyl oxygen, hydroxyl oxygen of the tridentate Schiff base and the donor atom of the solvent molecule. The structural data revealed that the labile coordination site, which is occupied by N or O atoms from the donor solvents, has a longer Mo-O or Mo-N bond distance.

Full text of DOI:10.1016/j.poly.2011.08.038

Polyhedron 30 (2011) 2922–2932
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure studies and electrochemistry of molybdenum(VI) Schiff
base complexes in the presence of different donor solvent molecules
⇑
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:
The reactions between bis(acetylacetonato)dioxomolybdenum(VI) and Schiff base ligands derived from
5-chlorosalicylaldehyde or 3-ethoxy-salicylaldehye, and 3-methoxy-benzoic hydrazide (m-anisic hydra-
zide), 2-furoic hydrazide or 2,4-dihydroxy-benzoic hydrazide in the presence of donor solvents yielded
cis-dioxomolybdenum(VI) complexes with the general formula MoO2L(D), where L = tridentate Schiff
base ligand and D = dimethylsulfoxide, hexamethylphosphoramide, dimethylformamide, imidazole or
methanol. The complexes were characterized by elemental analysis, electronic spectra, IR, 1H and 13C
NMR spectroscopies, thermogravimetric analysis, cyclic voltammetry, and the molecular structures of
five of the dioxomolybdenum complexes were elucidated by single crystal X-ray diffractometion studies.
In general, the complexes adopt an octahedral environment around the Mo center with a cis-oxo config-
uration. The other coordination sites are occupied by the imino nitrogen, phenoxyl oxygen, hydroxyl oxy-
gen of the tridentate Schiff base and the donor atom of the solvent molecule. The structural data revealed
that the labile coordination site, which is occupied by N or O atoms from the donor solvents, has a longer
Mo–O or Mo–N bond distance.
Received 5 August 2011
Accepted 26 August 2011
Available online 10 September 2011
Keywords:
Dioxomolybdenum(VI) complex
Schiff base
Crystal structure
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
SO42ꢀ, and xanthine oxidase which is used in the catalytic conver-
sion of xanthine to uric acid [17]. As a result, several attempts have
Molybdenum(VI) and (V) ions normally form complexes con-
taining two terminal oxygen atoms which demonstrate two type
of orientations, cis-dioxo and trans-dioxo [1,2]. A flurry of literature
regarding molybdenum(VI) complexes with cis-directed terminal
oxygen atoms has been presented, but the latter class of com-
pounds is less explored. On the contrary, the cis-dioxo compounds
display catalytic activities [3]. Since 1978, there has been ample
bidentate, tridentate and tetradentate Schiff base ligands reported
[4]. The coordination chemistry of molybdenum has been taken
cognizance of by the scientific community in the last 20 years
due to its ability to access multiple common oxidation state, cy-
cling between +4 and +6, as well as being able to form stable com-
plexes with N,O or/and S donor atoms ligands [5–15].
The coordination chemistry of molybdenum(VI) with Schiff
base ligands has come into the limelight for the synthetic chemist
due to its biological activities and catalytic properties. From a bio-
logical activity point of view, it has been shown that high valent
oxomolybdenum(VI) complexes are effective for redox molybdo-
enzymatic reactions in living systems [16]. Examples of coordin-
been made to create a model systems for the active side of molyb-
do-enzymes [18–21].
In the context of catalytic properties, numerous monomeric
dioxomolybdenum complexes have played important roles in
many important industry processes, such as oxidation of alcohols
and epoxidation of olefins, either in homogenous or heterogenous
process [22–24]. Epoxidation is one of the typical reactions in or-
ganic synthesis as modification of the intermediate epoxides can
produce various synthetic products such as enantioselective drugs,
pesticides, epoxy paints, rubber promoters and dyestuffs. Thus,
Mo(VI) is an important catalyst precursor for epoxidation reactions
[25–27].
Dibasic tridentate ligands together with two terminal oxygen
atoms normally form five-coordinate complexes when reacting
with the molybdenum(VI) moiety, but the molybdenum ion usu-
ally completes its customary sixth coordination site by adopting
a
solvent molecule or through dimerization (either bridged
through one complexing ligand or through the metal center). In
the present work, the Schiff base ligands are obtained by the con-
densation of 5-chloro-salicylaldehyde or 3-ethoxy-salicylaldehyde
with m-anisic hydrazide, 2-furoic hydrazide or 2,4-dihydroxy-ben-
zoic hydrazide. These tridentate ligands were used for complexa-
tion as the complexes formed leave a labile coordination site that
can be used for substrate binding. We used several reagents such
as DMSO, HMPA, DMF and imidazole to complete the ‘‘open’’
atively unsaturated molybdenum containing enzymes are sulfite
2ꢀ
oxidase, which is used in the catalytic conversion of SO3
to
⇑
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.2011.08.038

N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
2923
coordination site in order to investigate the effect of different
monodentate donor molecules at the sixth coordination site to
the overall structure of the complexes formed. In 1989, Syamal
and Maurya reported dioxomolybdenum(VI) complexes with tri-
dentate dibasic Schiff bases derived from various hydrazides,
including 2-furoic acid hydrazide [28], but no crystal structures
were reported. Since dioxomolybdenum(VI) complexes with –
ONO donor tridentate Schiff base ligands have been found useful
for oxo-transfer reactions, we describe here the synthesis, spectro-
scopic and electrochemical properties and structures of these
complexes.
lic–O); 1H NMR (DMSO-d6, d ppm): 3.74, (3H, O–CH3), 6.93–7.40
(aromatic), 8.43 (1H, HC@N), 9.32 (–OH), 11.10 (1H, N–NH); 13C
NMR (DMSO-d6, d ppm): 159.25 (C@N), 162.71 (C@O), 55.37 (O–
CH3); 110.43, 112.89, 117.78, 118.69, 119.88, 121.32, 129.74,
130.41, 133.59, 134.10, 145.71, 156.42 (aromatic).
Similar procedures were applicable to the preparations of L2, L3
and L4.
2.2.2. 5-Chlorosalicylaldehyde-furan-1-carbohydrazone (L2)
M.p.: 156 °C; Yield 0.17 g, 65%; Anal. Calc. for C12ClH9N2O3: C,
54.34; H, 3.40; N, 18.11. Found: C, 53.21; H, 4.02; N, 18.00%; IR
(KBr) (
C@O), 1601
m
max/cmꢀ1): 3400
(s, C@N), 1267
m
(s, N–H), 3134
m
m(s, O–H), 1649 m(s,
m
(s, Cenolic–O); 1H NMR (DMSO-d6, d
2. Experimental
ppm): 8.50 (1H, HC@N), 6.67–7.93 (aromatic), 11.07 (1H, N–NH),
12.15 (–OH); 13C NMR (DMSO-d6, d ppm): 154.63 (C@N), 156.44
(C@O); 112.72, 115.91, 118.73, 121.34, 123.54, 127.84, 131.31,
146.13, 146.70 (aromatic and furyl ring).
2.1. General procedures
All the reagents and solvents used in the synthesis were pro-
cured commercially and used without subsequent purification.
The starting material, bis(acetylacetonato) dioxomolybdenum(VI),
[MoO2(acac)2] was prepared as described in the literatures [29,30].
1H and 13C NMR spectra were measured in DMSO-d6 on JEOL
Lambda and ECA 400 MHz NMR spectrometers. IR spectra were re-
corded as Nujol mulls in the range 4000–400 cmꢀ1 using a Perkin–
Elmer 2000 FT-IR instrument. Elemental analysis was performed at
the in-house microanalytical laboratory using a Perkin–Elmer 2400
Series II CHNS/O System. Electronic absorption spectral measure-
ment of the ligands and complexes in DMF were measured using
a Shimadzu-1650-PC-UV–vis spectrophotometer. 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 thermo
balance.
2.2.3. 5-Chlorosalicylaldehyde 2,4 dihydroxy-benzo hydrazone (L3)
M.p.: 175 °C; Yield 0.19 g, 71%; Anal. Calc. for C14ClH11N2O4: C,
56.57; H, 3.70; N, 21.89. Found: C, 56.66; H, 4.15; N, 20.91%; IR
(KBr) (
m
max/cmꢀ1): 3419, 3378, 3162
(s, C@N), 1514
m
(s, O–H), 1650
m(s, C@O),
1608
m
m
(m, Car–O); 1H NMR (DMSO-d6, d ppm):
6.31–7.80 (aromatic), 8.60 (1H, HC@N), 11.95 (1H, N–NH), 10.28,
11.26, 12.19, (3 –OH); 13C NMR (DMSO-d6, d ppm): 162.93
(C@N), 165.27 (C@O); 102.84, 105.89, 107.60, 118.24, 120.64,
122.96, 127.60, 129.81, 130.80, 145.93, 156.02, 162.22 (aromatic).
2.2.4. 3-Ethoxy-salicylaldehyde 3-methoxy-benzo hydrazone (L4)
M.p.: 160 °C; Yield 0.22 g, 69%; Anal. Calc. for C16H18N2O4,
64.97; H, 5.73; N, 8.92. Found: C, 64.33; H, 5.85; N, 8.91%; IR
(KBr) (
C@N), 1517
HC@N), 6.60–7.50 (aromatic), 10.96, (–OH), 12.05 (1H, N–NH);
13C NMR (DMSO-d6,
ppm): 159.28 (C@N), 162.56 (C@O);
m
max/cmꢀ1): 3424
m
(m, OH), 1652
m(s, C@O), 1607 m(m,
Cyclic voltammetry was carried out using a Methrohm Autolab
B.V. model. The measurements were done in DMF solution contain-
m
(m, Car–O)); 1H NMR (DMSO-d6, d ppm): 8.64 (1H,
ing 0.1 M TEAP as the supporting electrolyte and a 5 ꢁ 10ꢀ4
M
d
complex solution, deoxygenated by bubbling with nitrogen gas.
The working, counter and reference electrodes used were Pt wire,
platinum coil and SCE, respectively.
112.86, 115.26, 117.75, 118.97, 119.06, 119.85, 121.04, 129.76,
134.21, 147.08, 147.52, 148.41 (aromatic).
The X-ray crystallographic intensity data were measured using
2.3. Preparation of the Mo(VI) complexes
MoKa radiation (graphite crystal monochromator, k = 0.71069 Å).
The data collection was collected at 296 K on a Bruker APEX II with
a CCD area-detector X-ray diffractometer. The structures were
solved by direct method with the SHELXS97 program [31] and re-
fined on F2 by full-matrix least-squares methods with anisotropic
non-hydrogen atoms. Crystal data are given in Table 1 and selected
bond distances and angles are reported in Table 2.
A general procedure for the preparation of the Mo(VI) com-
plexes is to reflux a mixture of MoO2(acac)2 and the ligand in a
1:1 ratio in methanol. A few drops of dimethylsulfoxide (DMSO),
hexamethylphosphoramide (HMPA), dimethylformamide (DMF)
or a stoichiometric amount of imidazole (Imz) were introduced
into the solution, except for the preparation of C9. A typical prep-
aration description is given below:
2.2. Preparation of the ligands
2.3.1. (5-Chlorosalicylaldehyde 3-methoxy-benzylhydrazonato)
dimethylsulfoxide dioxomolybdenum(VI) (C1)
All the Schiff base ligands L1, L2, L3 and L4 were prepared by
the condensation reactions of 5-chlorosalicylaldehyde or 3-eth-
oxylsalicylaldehyde with m-anisic hydrazide, 2-furoic hydrazide
or 2,4-dihydroxy-benzoic hydrazide in a 1:1 ratio. A typical prepa-
ration of the ligands is described as follow:
0.328 g (1.0 mmol) of MoO2(acac)2 in 20 ml of methanol was
mixed with 20 ml of 0.156 g (1.0 mmol) of 5-chlorosalicylaldehyde
m-anisic hydrazide, after which, a few drops of DMSO were added
dropwise until the resulting orange precipitate dissolved com-
pletely. The solution mixture was then refluxed with vigorous stir-
ring for 3 h. After leaving the solution for 2 days at room
temperature, fine orange crystals were formed. The product was
filtered and washed with methanol. M.p.: 196 °C; Yield 0.22 g,
43%; Anal. Calc. for C17ClH17N2O6SMo: C, 40.13; H, 3.34; N, 5.51;
2.2.1. 5-Chlorosalicylaldehyde 3-methoxy-benzo hydrazone (L1)
0.156 g (1.0 mmol) of 5 chlorosalicylaldehyde in 20 ml metha-
nol was added to 20 ml of 0.166 g (1.0 mmol) of m-anisic hydra-
zide. The solution mixture was refluxed with vigorous stirring for
2 h and the color of the solution changed from colorless to yellow.
The solution was then allowed to stand at room temperature for
2 days. The bright yellow precipitate was filtered and washed with
methanol and dried in air. M.p.: 136 °C; Yield 0.25 g, 82%; Anal.
Calc. for C15ClH13N2O3: C, 59.02; H, 4.26; N, 9.18. Found: C,
S, 6.63. Found: C, 39.59; H, 3.82; N, 5.33; S, 6.57%; IR (KBr) (mmax
cmꢀ1): 1618 (m, C@N), 1523 (m, C@C), 1330 (s, C–O), 1264 (m,
enolic–O), 1033 (m, S–O), 923, 901 (s, Mo@O); 1H NMR (DMSO-
/
C
d6, d ppm): 3.81 (3H, O–CH3), 6.93–7.80 (aromatic), 8.89 (1H,
HC@N); 13C NMR (DMSO-d6, d ppm): 55.36 (O–CH3), 134.30
(C@N), 169.14 (C@O), 112.60, 118.38, 120.56, 120.61, 121.70,
124.92, 130.14, 131.68, 132.90, 134.30, 155.14 (aromatic).
60.11; H, 4.02; N, 9.21%; IR (KBr) (
m
max/cmꢀ1): 3429
(m, OH),
(m, Car–O), 1278 m(m, Ceno-
m
1644 (s, C@O), 1590 (m, C@N), 1566
m
m
m

2924
N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
Table 1
Crystallographic data and structure refinement parameters for compounds C1, C2, C3, C7 and C8.
Compounds
C1
C2
C3
C7
C8
Chemical formula
Mr
C
17H17O6N2ClSMo
C14H13O6N2ClSMo
468.71
C16H16O7N2ClSMo
512.76
C20H23O7N3Mo
513.35
C20H20O6N4Mo
523.09
509.60
Crystal color, habit
Crystal size (mm)
Crystal system
Space group
Unit cell dimension
a (Å)
orange, block
0.30 ꢁ 0.20 ꢁ 0.20
triclinic
orange, block
0.30 ꢁ 0.20 ꢁ 0.20
triclinic
red, block
orange, block
0.32 ꢁ 0.30 ꢁ 0.30
triclinic
orange, block
0.35 ꢁ 0.30 ꢁ 0.20
triclinic
0.40 ꢁ 0.30 ꢁ 0.20
monoclinic
P2(1)/n
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
8.3916(8)
11.211(1)
11.627(1)
67.539(1)
81.441(1)
85.219(1)
999.2(2)
2
8.2850(1)
10.0410(2)
10.4913(2)
76.630(1)
86.463(1)
87.725(1)
847.23(3)
2
10.4049(3)
14.0625(5)
12.6463(4)
90
100.837(2)
90
8.1339(3)
10.5258(4)
12.9452(6)
85.077(2)
72.828(2)
88.216(2)
1064.98(7)
2
8.8210(1)
9.641(2)
12.9167(2)
84.418(1)
84.627(1)
71.938(1)
1033.51(3)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
1817.2(1)
4
Z
T (K)
100(1)
1.691
0.931
100(1)
1.837
1.089
100(1)
1.867
1.030
100(1)
1.565
0.669
100(1)
1.572
0.581
Dcalc (g cmꢀ3
)
l
(MoK
a
) (mmꢀ1
)
Absorption correction
Tmin
Tmax
multiscan
0.7676
0.8357
512
multiscan
0.7359
0.8116
468
multiscan
0.6833
0.8204
1048
multiscan
0.7860
0.7980
524
multiscan
0.7996
0.8780
531
F(000)
Total data
Unique data
Rint
10368
9905
0.0202
3338
7878
3864
0.0136
3599
13092
3357
0.017
10571
5176
0.0187
4953
8701
4020
0.0232
3861
Observed data
8075
[I > 2r(I)]
Ranges of h, k, l
Number of parameter
ꢀ9, 9; ꢀ13, 13; ꢀ13, 13 ꢀ10, 10; ꢀ13, 13; ꢀ13, 13 ꢀ12, 12; ꢀ17, 17; ꢀ15, 15 ꢀ10, 10; ꢀ14, 14; ꢀ17, 17 ꢀ11, 11; ꢀ12, 12; ꢀ17, 17
256
228
257
284
272
R1
wR2
S
0.0258
0.0754
1.087
0.813
ꢀ0.582
0.0260
0.0690
1.090
0.925
ꢀ0.714
0.0396
0.0731
1.138
1.043
ꢀ1.172
0.0255
0.0807
1.022
0.590
ꢀ1.203
0.0262
0.0713
1.116
0.770
ꢀ0.469
D
D
qmax (e Åꢀ3
)
)
qmin (e Åꢀ3
Table 2
Selected bond distances (Å) and bond angles (°) for C1, C2, C3, C7 and C8.
C1
C2
C3
C7
C8
Bond distance
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–N(1)
Mo(1)–O(6)/N(3)
C(8)/(10)–N(2)
C(8)/(10)–O(4)
C(1)–O(1)
1.9313(17)
1.6996(19)
1.698(2)
1.9949(16)
2.2565(19)
2.2808(17)
1.300(3)
1.326(3)
1.343(3)
1.400(3)
1.280(3)
1.9374(16)
1.7113(16)
1.6972(17)
2.0101(17)
2.2436(19)
2.2660(16)
1.300(3)
1.330(3)
1.349 (3)
1.393(3)
1.933(2)
1.711(3)
1.691(3)
2.009(2)
2.242(3)
2.336(3)
1.303(5)
1.327(4)
1.347(4)
1.392(4)
1.284(5)
1.9240(14)
1.7088(14)
1.7032(15)
2.0216(13)
2.2295(16)
2.2958(15)
1.298(3)
1.324(2)
1.335(2)
1.400(2)
1.284(3)
1.9437(17)
1.7069(16)
1.7069(17)
2.0136(16)
2.2474(19)
2.351(2)
1.298(3)
1.326(3)
1.349(3)
1.393(3)
N(1)–N(2)
C(7)/(9)–N(1)
1.290(3)
1.291(3)
Bond angle
O(2)–Mo(1)–O(3)
O(3)–Mo(1)–O(1)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(4)
O(2)–Mo(1)–O(4)
O(1)–Mo(1)–O(4)
O(3)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
O(3)–Mo(1)–O(6)/N(3)
O(2)–Mo(1)–O(6)/N(3)
O(1)–Mo(1)–O(6)/N(3)
O(4)–Mo(1)–O(6)/N(3)
O(6)/N(3)–Mo(1)–N(1)
105.12(11)
98.57(9)
103.75(8)
98.14(9)
96.70(8)
149.10(7)
91.76(9)
160.77(9)
82.29(7)
71.39(7)
168.11(9)
86.77(9)
77.90(7)
80.43(7)
76.56(7)
104.67(8)
97.73(8)
104.62(7)
98.22(8)
95.68(7)
149.99(7)
92.63(8)
160.19(8)
82.07(7)
71.96(7)
169.16(7)
86.07(7)
77.76(7)
81.90(6)
77.05(6)
104.14(13)
99.05(12)
104.56(11)
97.42(12)
97.46(11)
148.22(10)
90.63(12)
163.11(12)
80.66(11)
72.15(10)
176.34(11)
79.38(11)
80.89(10)
80.99(10)
85.74(10)
105.40(7)
98.96(7)
105.07(6)
96.44(6)
94.11(6)
151.10(6)
95.37(7)
156.25(7)
82.47(6)
71.83(6)
169.58(6)
84.98(6)
78.79(6)
81.65(6)
74.30(6)
105.46(8)
97.00(8)
105.04(7)
97.93(7)
96.81(7)
149.09(7)
89.92(8)
162.12(8)
81.51(7)
71.61(6)
170.71(7)
83.82(8)
79.94(7)
81.00(7)
80.97(7)

N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
2925
A similar procedure to that above was used to prepare the fol-
2.3.6. (3-Ethoxysalicylaldehyde-3-methoxy-benzolhydrazonato)
lowing complexes:
dimethylsulfoxide dioxomolybdenum(VI) (C6)
M.p.: 184 °C; Yield 0.17 g, 30%; Anal. Calc. for C19H24N2O7SMo:
C, 43.85; H, 4.62; N, 5.38; S, 6.15. Found: C, 44.59; H, 4.22; N,
2.3.2. (5-Chlorosalicylaldehyde furan-1-carbohydrazonato)
dimethylsulfoxide dioxomolybdenum(VI) (C2)
5.74; S, 6.24%; IR (KBr) (m
max/cmꢀ1): 1618 (s, C@N), 1596 (m,
C@C), 1564 (m, Car–O), 1257 (m, Cenolic–O), 1082 (s, S–O), 925,
894 (Mo@O); 1H NMR (DMSO-d6, d ppm): 3.78, (3H, O–CH3), 4.04
(5H,O–C2H5), 6.97–7.54 (aromatic), 8.91 (1H, HC@N); 13C NMR
(DMSO-d6, d ppm): 55.81 (O–CH3), 55.81 (O–CH3), 64.85 (O–
C2H5), 159.88 (C@N), 169.02 (C@O), 112.91, 120.92, 121.22,
122.08, 126.06, 130.55, 148.02, 149.84, 156.75 (12C, aromatic).
M.p.: 320 °C; Yield 0.11 g, 23.45%; Anal. Calc. for C14ClH13
N2O6SMo: C, 35.78; H, 2.98; N, 5.96; S, 6.82. Found: C, 36.39; H,
2.41; N, 6.38; S, 6.84%; IR (KBr) (m
max/cmꢀ1): 1625 (m, C@N),
1521 (w, C@C), 1260 (m, Cenolic–O), 1095 (m, S–O), 925, 903 (s,
Mo@O); 1H NMR (DMSO-d6, d ppm): 6.69–7.93 (aromatic and furyl
ring), 8.85 (1H, HC@N); 13C NMR (DMSO-d6, d ppm): 158.48 (C@N),
162.44 (C@O), 113.14, 117.05, 121.06, 122.19, 125.45, 133.30,
134.62, 144.90, 147.61, 155.20, 158.48 (aromatic and furoic ring).
2.3.7. (3-Ethoxysalicylaldehyde 3-methoxy-benzohydrazonato)
dimethylformamide dioxomolybdenum(VI) (C7)
M.p.: 118 °C; Yield 0.10 g, 19%; Anal. Calc. for C20H23N3O7Mo: C,
46.78; H, 4.48; N, 8.17. Found: C, 46.87; H, 4.41; N; 8.02%; IR (KBr)
(m
max/cmꢀ1): 1616 (s, C@N), 1597 (m, C@C), 1565 (m, Car–O), 1258
2.3.3. (5-Chlorosalicylaldehyde 2,4 dihydroxy-benzohydrazonato)
dimethylsulfoxide dioxomolybdenum(VI) (C3)
(m, Cenolic–O), 725 (s, NC@ODMF), 931, 906 (Mo@O); 1H NMR
(DMSO-d6, d ppm): 2.89 and 2.73 (DMF), 7.01–7.95 (aromatic),
8.94 (1H, HC@N); 13C NMR (DMSO-d6, d ppm): 30.76 and 35.77
(DMF), 159.30 (C@N), 162.31 (C@O), 112.42, 118.12, 120.37,
121.52, 125.55, 130.00, 147.60, 156.19 (aromatic ring).
M.p.: 300 °C; Yield 0.13 g, 25%; Anal. Calc. for
N2O7SMo: C, 37.54; H, 3.13; O, 21.90; N, 5.47; S, 6.26. Found: C,
37.82; H, 2.77; N, 5.81; S, 5.81%; IR (KBr) (
max/cmꢀ1): 3172
C16ClH16
m
(OH), 1627 (s, C@N), 1546 (m, Car–O), 1513 (m, C@C), 1262 (m,
Cenolic–O), 1095 (s, S–O), 927, 907 (s, Mo@O); 1H NMR (DMSO-d6,
d ppm): 6.30–7.74 (aromatic), 8.91 (1H, HC@N), 11.33 and 10.32
(2 –OH); 13C NMR (DMSO-d6, d ppm): 55.36 (O–CH3), 163.56
(C@N), 170.08 (C@O), 102.91, 104.89, 108.72, 120.80, 121.86,
125.26, 131.14, 134.28, 153.57, 158.01, 161.00 (aromatic).
2.3.8. (3-Ethoxysalicylaldehyde 3-methoxy-benzohydrazonato)
imidazole dioxomolybdenum(VI)methanol solvate (C8)
M.p.: 235 °C; Yield 0.14 g, 27%; Anal. Calc. for C20H20N4O6Mo: C,
46.97; H, 4.11; N, 10.96. Found: C, 46.78; H, 4.36; N; 10.57%; IR
(KBr) (m
max/cmꢀ1): 1596 (s, C@C), 1561 (m, Car–O), 1258 (m, Ceno-
lic–O), 3133 (m), 1067 (w, N–HImz), 922, 903 (s, Mo@O); 1H NMR
(DMSO-d6, d ppm): 6.95 (m, Imz), 7.18–7.53 (aromatic), 8.88 (1H,
HC@N); 13C NMR (DMSO-d6, d ppm): 159.83 (C@N), 169.07
(C@O), 121.22, 130.55 and 131.87 (Imz), 112.91, 118.68, 119.13,
120.22, 122.08, 126.06, 148.12, 149.95, 156.75 (aromatic ring).
2.3.4. (5-Chlorosalicylaldehyde furan-1-carbohydrazonato)
hexamethylphosphoramide dioxomolybdenum(VI)
dimethylformamide solvate (C4)
0.328 g (1.0 mmol) of MoO2(acac)2 in 20 ml of methanol was
mixed with 20 ml of 0.156 g (1.0 mmol) of 5-chlorosalicylaldehyde
m-anisic hydrazide, after which, a few drops of HMPA were added
and an orange precipitate was obtained. DMF was added dropwise
until the precipitate dissolved completely. The solution mixture
was then refluxed with vigorous stirring for 3 h. The solution was
left for 2 days at room temperature to obtain fine orange crystals.
The product was filtered and washed with methanol. M.p.:
131 °C; Yield 0.13 g, 22%; Anal. Calc. for C18ClH26O6N5PMo: C,
38.08; H, 4.54; N, 12.22. Found: C, 37.71; H, 4.47; N, 12.93%; IR
2.3.9. (3-Ethoxysalicylaldehyde 3-methoxy-benzylhydrazonato)
methanol dioxomolybdenum(VI) (C9)
M.p.: 140 °C; Yield 0.25 g, 55%; Anal. Calc. for C18H20N2O7M1: C,
45.76; H, 4.24; N, 5.93. Found: C, 45.89; H, 4.12; N, 5.79%; IR (KBr)
(m
max/cmꢀ1): 1608 (s, C@N), 1562 (m, Car–O), 1260 (m, Cenolic–O),
3430 (w, C–Omethanol), 949, 921 (s, Mo@O); 1H NMR (DMSO-d6, d
ppm): 1.34 (CH3–OH), 3.80 (CH3–OH), 6.97–7.58 (aromatic), 8.93
(1H, HC@N); 13C NMR (DMSO, d ppm): 159.30 (C@N), 168.54
(C@O), 112.42, 118.11, 118.68, 120.37, 120.70, 121.52, 125.56,
130.00, 131.36, 147.61, 149.47, 156.19 (aromatic).
(KBr) (m
max/cmꢀ1): 1681, 1624 (m, C@N), 1515 (w, Cphenolic–O),
1343 (m, C–O), 1268 (m, Cenolic–O), 1157 (s, P@O), 930, 906 (s,
Mo@O); 1H NMR (CDCl3, d ppm): 2.50 (18H, HMPA), 2.85 and
2.94 (DMF), 7.06–8.01 (aromatic and furyl ring), 8.41 (1H, HC@N);
13C NMR (CDCl3, d ppm): 36.72 and 36.68 (HMPA), 162.88 (C@N),
163.14 (C@O), 31.64 (DMF), 112.06, 116.17, 120.76, 121.73,
125.42, 132.00, 133.69, 145.62, 145.42, 152.68, 158.92 (aromatic
and furoic ring).
3. Results and discussion
3.1. Synthesis
A similar procedure to that above was used to prepare the fol-
lowing complexes:
The synthesized ligands were reacted with bis(acetylacetonato)
dioxomolybdenum(VI) in methanol solution under refluxing condi-
tions. However, the reaction mixture produced a sandy precipitate,
with poor yields. However, when DMSO, DMF, HMPA or imidazole
was introduced into the reaction mixture, fine crystalline com-
plexes of the general formula MoO2L(D), (D = DMSO, HMPA, DMF,
imidazole) were obtained, as shown in Scheme 1. This indicates that
DMSO, DMF, HMPA and imidazole are stronger donor solvents com-
pared to methanol. In the case of C4, DMF was added during the
preparation of the complex due to the low solubility of the complex
in methanol. The preliminary crystal structure of C4 revealed that
HMPA is directly attached to the MoO22+ core instead of DMF. From
the crystallographic data, C4 has a rather short Mo–O(6) bond dis-
tance, compared to that of C7. This implicates that HMPA coordi-
nates well to the MoO22+ core. The ONO donor types of Schiff base
ligands show keto-enol tautomerization both in solution and in
2.3.5. (3-Ethoxysalicylaldehyde 3-methoxy-benzohydrazonato)
hexamethylphosphoramide dioxomolybdenum(VI) (C5)
M.p.: 124 °C; Yield 0.09 g, 17%; Anal. Calc. for C23H36 N5O7PMo:
C, 44.45; H, 5.84; N, 11.27. Found C, 45.21; H, 6.10; N, 11.37%; IR
(KBr) (m
max/cmꢀ1): 1618 (s, C@N), 1606 (m, C@C), 1566 (m, Car–
O), 1257 (m, Cenolic–O), 1150 (s, P@O), 929, 903 (Mo@O); 1H NMR
(DMSO-d6, d 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, d 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).

2926
N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
N(CH3)2
N(CH3)2
CH3
P
O
O
O
N(CH3)2
S
O
O
N
CH3
O
Mo
O
Mo
O
C
O
O
C
N
O
Cl
C
H
N
Cl
C
H
N
R
R=3-(CH3O)C6H4 (C1), 2-C4H3O (C2), 2,4-(OH)2C6H3 (C3)
C4
O
O
OC2H5
D
Mo
O
O
C
N
C
H
N
D: HMPA (C5), DMSO (C6), DMF (C7),
Imz (C8), Methanol (C9)
OCH3
Scheme 1. Dioxomolybdenum(VI) complexes with dibasic tridentate Schiff base ligands in the presence of different donor solvent molecules.
the solid state. The delocalized bond in the enolate group has given
rise to a slight difference in the elemental analysis results. In gen-
eral, the six coordinated Mo(VI) complexes readily undergo a facial
displacement reaction at the sixth coordination site. This is corrob-
orated by the TGA study, which exhibited the initial lose of the do-
nor solvent molecules on controlled heating. The products obtained
ranged from red to orange, and are air stable in the solid state. The
complexes are sparingly soluble in chloroform and ethanol, except
C4, but highly soluble in DMSO and DMF.
frequency in C1–C9 [5]. The m(Car–O) band in the range 1520–
1540 cmꢀ1 for the ligands was found to have shifted to the higher
frequency of 1560 5 cmꢀ1, suggesting the coordination of the
deprotonated phenolic C–O group to the MoO2 moiety. Likewise,
the presence of a new band at 1255 5 cmꢀ1 in the complexes
could be assignable to m(Cenolic–O).
The 1H NMR spectra of L1 and L2 exhibit a –OHphenolic proton
resonance at d 9.32 and 12.15 ppm and a Himine–C@N resonance
at d 8.43 and 8.58 ppm, respectively. Upon coordination to the
Mo atom, the –OH signal disappeared, which is due to the deproto-
nation of OHphenolic and the coordination of the oxygen atom to the
Mo atom. The participation of the imine nitrogen in complexation
is signaled by an appreciable downfield shift of the azomethine
proton signal (d 8.89 and 8.85 for C1 and C2, respectively). The
presence of MeOH (C9), imidazole (C8), DMF (C7) and HMPA (C5)
in the complexes is shown by resonance peaks at d 3.38, 121–
131, 2.89 and 2.73, and 2.50, respectively.
3.2. IR and NMR spectroscopic description
The IR spectra of all the complexes C1–C9 reveal two absorption
bands in the region 890–930 cmꢀ1, which is attributed to the
vibration of the two double-bonded cis-oxygen atoms at the
molybdenum core [32,33]. The sixth coordination site of the com-
plexes is occupied by the solvent molecules, which gives rise to an
octahedral geometry at the molybdenum center. The spectra of C1,
C2, C3 and C6 were found to contain absorption bands in the range
3.3. Crystallographic description of the complexes
1030–1095 cmꢀ1 which are due to the
case of C4 and C5, the bands at 1157 and 1150 cmꢀ1 are associated
with (P–O) stretching. The stretching frequencies of NC@O of DMF
m(S–O) stretching. In the
The molecular structures of C1, C2, C3, C7 and C8 (Figs. 1–5)
show that in all of the complexes the Schiff base ligands behave
as tridentate ligands, and have reacted with the dioxomolybdenum
anion to form six coordinated molybdenum(VI) structures. As
shown in the figures, the imine nitrogen, one of the phenoxyl oxy-
gens and the hydroxyl oxygen from the enolized carbonyl group
are involved in the coordination with molybdenum atom, forming
six- and five-member chelate rings around the cis-MoO2 center.
The overall geometry can be regarded as an octahedron, with the
equatorial plane formed by the imine nitrogen, phenoxyl oxygen,
hydroxyl oxygen and one of the terminal oxygen atoms of the
dioxomolybdenum unit. The other terminal oxygen and the donor
atom from the solvent occupy the apical positions.
m
and N–H of imidazole in the complexes C7 and C8 were at 725 and
1067 cmꢀ1, respectively [34]. The shift to lower frequency for the
functional groups –S@O, –C@O and –P@O, compared to the free li-
gands, indicate that Mo(VI) attached to DMSO, DMF, HMPA and
imidazole through the oxygen or nitrogen atom.
The IR spectra of L1 and L2 exhibit a few ligand bands, m(OH) at
3136 and 3156 cmꢀ1 m(NH) at 3386 cmꢀ1 and m(C@O) at 1650 cmꢀ1
, .
Upon coordinating to MoO22+, these absorption bands disappear,
as shown in the spectra of complexes C1–C9 [7]. In the case of com-
plex C3, an absorption band at 3172 cmꢀ1, attributed to the –OH
stretching of the 2,4-dihydroxy-benzoic hydrazide, was clearly ob-
served. The characteristic –C@N– imine band in L1 and L2, which
exists at 1588 and 1590 cmꢀ1, was found to have shifted to higher
In all of the complexes, the Mo@O bond lengths which range
from 1.693(2) to 1.721(2) Å and the O@Mo@O bond angles, which

N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
2927
Fig. 1. ORTEP plot of MoO2L1(DMSO), C1, with the atom labeling scheme.
Fig. 2. ORTEP plot of MoO2L2(DMS), C2, with the atom labeling scheme.
lie between 104.23(2)° and 105.40(2)°, are similar to those reported
for other MoO2 complexes [35–38]. The bond distances between the
Mo atom and the phenolate oxygen, deprotonated nitrogen and the
enolate oxygen atoms are also close to those found in other ONO tri-
dentate Schiff base molybdenum(VI) complexes. The C–O bond dis-
tance in all the complexes exhibit values between 1.309 and
1.326 Å, which is nearer to a C–O single bond than to a C–O double
bond distance. This shows that the ligand coordinates to the MoO2
core in the deprotonated enolate form [8–10].
the normal single bond length due to the consequence of the trans
effect of M = Ot (where Ot is the oxo-group trans to the Mo–Osolvent
bond). This result reveals a rather weak attachment of the solvent
donor atom to MoO22+. When the methanol coordinated MoO2(VI)
complexes are dissolved in stronger coordinating agents, such as
DMF, DMSO or HMPA, the methanol molecule tends to be dis-
placed by these solvent molecules.
The equatorial bases formed by O1, O2, O4 and N1 in the com-
plexes are not coplanar. The Mo atom in molecular structures of C1
and C8 are found to have shifted 0.2010 and 0.2233 Å, respectively,
out from the basal plane towards the apical oxo-oxygen atom O3,
The Mo–O (donor atom of solvent) bond length in the molecular
structures of C1, C2, C3, C7 and C8 are all somewhat longer than

2928
N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
Fig. 3. ORTEP plot of MoO2L3(DMSO) C3, with the atom labeling scheme.
nitrogen (N–Hꢂ ꢂ ꢂO, 2.864(6) Å) results in a layer structure propa-
gating along the bc crystallographic plane (Table 4). The con-
p–p
tacts between the imidazole rings, Cg1–Cg10 (symmetry code:
1 ꢀ x, 1 ꢀ y, 1 ꢀ z, where Cg1 is the centroid of the imidazole ring),
may further stabilize the structure, with a centroid–centroid dis-
tance of 3.813(2) Å. On the other hand, complexes C1, C2 and C7
exist as discrete molecules with no evidence of any hydrogen
bonding interactions.
3.4. Thermal properties
The thermogravimetric analysis of the complexes was carried out
in the temperature range 50–900 °C at a heating rate of 20 °C minꢀ1
.
All the complexes are stable until 150 °C, except for C1 and C9, in
which weight losses of 3.51% and 7.00% were observed that could
be due to the removal of water and methanol molecules in the com-
pounds. Above this temperature, the TG curves show three steps of
weigh losses. The first step of decomposition in the temperature
range 150–250 °C involved the removal of solvent molecules
(DMSO, DMF, HMPA or imidazole). With further elevation in tem-
perature, a rapid weigh loss in the temperature range 250–370 °C
followed by a gradual weigh loss in the temperature range 370–
700 °C was observed. The total decomposition of the second and
third stage weight losses may be ascribed to the dissociation of the
Schiff base ligand at the –C@N– bond before it is removed from
the Mo2+ core. Decomposition continues until the final residue,
MoO2, is left. Thermal decomposition data of the complexes are gi-
ven in Table 5. The TG curve for the selected complexes C1 and C5
are shown in Fig. 8. In general, the thermal decomposition behavior
of the complexes can be summarized in Scheme 2.
Fig. 4. ORTEP plot of MoO2L4(DMF) C7, with the atom labeling scheme.
while the other structures show a displacement of the metal atom
towards the donor atom of the solvent, O6.
In the molecular structure of C3 (Fig. 6), intramolecular O–Hꢂ ꢂ ꢂN
hydrogen bonding (2.602(4) Å) in the Schiff base ligand helps to
stabilize the overall structure. In addition, the presence of OHꢂ ꢂ ꢂO
hydrogen bonding (O7Hꢂ ꢂ ꢂO2, 2.826(4) Å and O7Hꢂ ꢂ ꢂO6,
3.069(4) Å) links the molecules into a polymeric chain parallel to
the ab crystallographic plane (Table 3). The unusual Mo1–O6 bond
length (2.337 Å) and the O6–Mo1–O2 angle (176.18°) are the con-
sequence of the hydrogen bonding interactions of the C3 mole-
cules. In the crystal structure of C8 (Fig. 7), the presence of a
methanol molecule which is hydrogen bonded to the imidazole
3.5. Electronic spectra
Electronic absorption bands of the ligands and complexes in
5 ꢁ 10ꢀ4 M DMF are given in Table 6. For the ligands, two absorp-

N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
2929
Fig. 5. ORTEP plot of MoO2L4(Imz) C8, with the atom labeling scheme.
Fig. 6. Intra- and intermolecular hydrogen bonds in the crystal structure of C3.
Table 3
tion bands observed at ca. 292 and 302 nm are probably due to n to
p⁄ transitions of the –C@N– and –C@O– moieties of the ligands
[39]. The absorption band at ca. 338 nm may be assigned to n to
p⁄ transitions of the aromatic rings. The spectra of the complexes
show a broad band at 400–410 nm, which is assignable to a ligand
to metal charge transfer (LMCT) due to the promotion of an elec-
tron from the highest occupied molecular orbital (HOMO) of the li-
gand to the lowest unoccupied molecular orbital (LUMO) of
molybdenum atom [40]. Other LMCT bands, which are observable
Hydrogen bonds for C3 [Å and °].
D–Hꢂ ꢂ ꢂA
d(D–H)
d(Hꢂ ꢂ ꢂA)
d(Dꢂ ꢂ ꢂA)
\(DHA)
O(5)–H(5A)ꢂ ꢂ ꢂN(2)
0.82
0.82
0.82
0.82
1.89
2.55
2.20
2.30
2.601(4)
3.075(4)
2.826(4)
3.069(4)
144.7
123.5
133.0
155.6
O(5)–H(5A)ꢂ ꢂ ꢂO(3)#1
O(7)–H(7A)ꢂ ꢂ ꢂO(2)#2
O(7)–H(7A)ꢂ ꢂ ꢂO(6)#2
Symmetry transformations used to generate equivalent atoms: #1: ꢀx + 1, ꢀy + 2,
ꢀz + 1; #2: x + 1, y, z.

2930
N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
absorption band in the visible region confirms the 4d0 electronic
configuration of Mo(VI) [41].
3.6. Electrochemical studies
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 Fig. 9. From the data, L1, L2
and L4 exhibit an irreversible one oxidative peak around ꢀ0.55 V,
while L3 shows reversible electrochemical behavior. For a com-
pletely reversible one-electron transfer reaction independent of
scan rate, a
DE = 58 mV and a ratio of 1 for the anodic and cathodic
peak currents are obtainable. For these complexes, one oxidative
and reductive response are scrutinized near to ꢀ0.80 and
ꢀ1.10 V, respectively. The oxidation wave in the complexes is
due to the effect of the redox behavior of the ligands, as the ob-
served values fall in the same region as that for the free ligands.
Only the reduction wave in the complexes is attributable to a
reduction at the molybdenum centers. Reduction of C1 and C2
occured at 0.96 and 0.84 V. In the case of L3 and C3, both oxidative
and reductive responses appeared approximately in the same re-
gion. However, in the case of C5–C9, the cathodic reduction poten-
tials occur around ꢀ1.17 V. The more negative the cathodic
reduction potential, the more difficult it is for the dioxomolybde-
num(VI) complexes to be reduced. Both the electron donating
groups (–OC2H5) and the additional aromatic ring for greater elec-
tron delocalization in L4 are probably the leading contributors to
the shift to more negative values of the cathodic reduction poten-
tial in C5–C9 [42–44]. In general, CV studies on these cis-dioxomo-
lybdenum(VI) complexes revealed irreversible redox behavior.
Fig. 7. C8 molecules are linked by N–Hꢂ ꢂ ꢂO hydrogen bonds.
Table 4
Hydrogen bonds for C8 [Å and °].
D–Hꢂ ꢂ ꢂA
N(4)–H(4A)ꢂ ꢂ ꢂO(7)
d(D–H)
0.86
d(Hꢂ ꢂ ꢂA)
2.02
d(Dꢂ ꢂ ꢂA)
2.862(5)
\(DHA)
165.4
Symmetry transformations used to generate equivalent atoms: #1: ꢀx + 1, ꢀy + 2,
ꢀz + 1.
4. Conclusion
around 290 and 350 nm, can be ascribed to the oxygen to molyb-
denum and nitrogen to the molybdenum charge transfer transi-
tions, respectively. Higher energy bands appearing below 290 nm
are due to intra-ligand transitions. The absence of a d–d transition
Several mononuclear dioxomolybdenum(VI) complexes with
different donor solvent molecules have been synthesized and char-
acterized by various physicochemical techniques and by X-ray
Table 5
Thermogravimetric Analysis of the complexes.
Compounds
Temp. range (°C)
Weight loss observed (calculated) (%)
Molecules removed
End residue
MoO2
C1
(a) 50–100
(a) 3.51 (3.41)
(a) H2Oa
(b) 150–270
(c) 270–370
(d) 370–650
(a) 170–280
(b) 280–340
(c) 340–700
(a) 180–260
(b) 260–370
(c) 370–760
(a) 150–300
(b) 300–850
(a) 170–300
(b) 300–850
(a) 190–280
(b) 280–300
(c) 300–750
(a) 160–250
(b) 250–330
(c) 330–700
(a) 170–310
(b) 310–700
(a) 70–110
(b) 15.24 (14.77)
(c) 31.68 (30.87)
(d) 25.66 (26.52)
(a) 15.66 (16.60)
(b) 30.07 (29.57)
(c) 26.33 (26.17)
(a) 14.92 (15.24)
(b) 20.84 (21.28)
(c) 37.00 (37.30)
(a) 42.31 (40.62)
(b) 39.14 (40.40)
(a) 52.63 (55.53)
(b) 22.23 (22.80)
(a) 13.92 (15.34)
(b) 27.06 (29.00)
(c) 30.11 (32.08)
(a) 13.60 (14.20)
(b) 32.00 (31.71)
(c) 30.09 (28.27)
(a) 46.36 (44.68)
(b) 30.66 (28.54)
(a) 7.00 (6.79)
(b) DMSO
(c) –NNCOC6H4OCH3
(d) –OC7H5Cl
(a) DMSO
(b) –NNCOC4H3O
(c) –OC7H5Cl
C2
C3
MoO2
MoO2
(a) DMSO
(b) –C6H4(OH)2
(c) –ClC6H4CNNCO
(a) HMPA + DMF
(b) ClC6H3OCN2COC4H3O
(a) HMPA + –N2COC6H4OCH3
(b) –OC2H5C6H3OC
(a) DMSO
(b) –OC2H5C6H3OC
(c) –NNCOC6H4OCH3
(a) DMF
(b) –NNCOC6H4OCH3
(c) –OC2H5C6H3OC
(a) Imidazole + CH3OH–NNCOC6H4OCH3
(b) –OC2H5C6H3OC
(a) CH3OH
C4
C5
C6
MoO2
MoO2
MoO2
C7
MoO2
C8
C9
MoO2
MoO2
(b) 110–320
(c) 320–700
(b) 35.00 (34.39)
(c) 30.77 (31.42)
(b) –NNCOC6H4OCH3
(c) –OC2H5C6H3OC
a
Water molecules come from the air moisture and this is corroborated by elemental analysis.

N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
2931
Fig. 8. (a) Thermogram for C1 and (b) thermogram for C5.
Scheme 2. Thermal decomposition of the dioxomolybdenum(VI) complexes.
Table 6
Electronic spectra band position and cyclic voltammetry data.
crystallographic diffraction. TGA analysis and crystallographic data
revealed that the donor solvent molecules are attached loosely to
the MoO2 unit resulting in substitution reactions at the sixth
Compounds
kmax (nm)
Ep/V (vs. SCE)
2+
L1
L2
L3
L4
C1
C2
C3
C4
C5
C6
C7
C8
C9
338, 302, 291
338, 303, 292
339, 313, 293, 225
301, 262
409, 336, 304, 292
409, 314, 268, 227
414, 340, 316, 278
410, 325, 305, 279
315, 270, 209
314, 276, 267
382, 301
ꢀ0.58
coordination site more easily. This was shown in the case of C4
and C8, in which the sixth coordination site was occupied by HMPA
and imidazole, respectively, additionally solvated by DMF and
methanol, respectively. On the other hand, the presence of an hy-
droxyl group as a substituent in the chelating ligands affects the
molecular geometry of the complex C3 through hydrogen bonding
interactions. An electrochemical study shows irreversible redox
behavior of the cis-dioxomolybdenum(VI) complexes.
ꢀ0.51
ꢀ0.55, ꢀ1.08
ꢀ0.54
ꢀ0.61, ꢀ0.96
ꢀ0.53, ꢀ0.84, ꢀ1.11, ꢀ1.52
ꢀ0.75, ꢀ1.04
ꢀ0.49, ꢀ0.83, ꢀ1.08, ꢀ1.46
ꢀ0.88, ꢀ1.18
ꢀ0.83, ꢀ1.17
ꢀ0.85, ꢀ1.17
ꢀ0.56, ꢀ1.17
ꢀ0.68, ꢀ1.15
397, 361, 302
345, 269
Fig. 9. Cyclic voltammogram for L1 (a) and L3 (b) – scan rate 100 mV/s.

2932
N.K. Ngan et al. / Polyhedron 30 (2011) 2922–2932
[15] V. Vrdoljak, M. Cindric, D. Milic, D. Matkovic-Calogovic, Polyhedron 24 (2005)
Acknowledgement
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We wish to thank the University of Malaya (PS378/2010B and
RG020/09AFR) for supporting this study.
Appendix A. Supplementary material
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the supplementary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
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