Thermal and Spectral Studies of Peroxo Complexes of Molybdenum(VI) and Tungsten(VI) with Benzimidazole Derivatives

Article abstract of DOI:10.1039/a801990h

Synthesis, spectral and thermal studies of diperoxo complexes of molybdenum(VI) and tungsten(VI) with 2-(2-pyridyl)-benzimidazole, 2-(3-pyridyl)benzimidazole and 2,6-bis(benzimidazol-2-yl)pyridine are reported.

Full text of DOI:10.1039/a801990h

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4
46 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
Thermal and Spectral Studies of Peroxo Complexes
of Molybdenum(VI) and Tungsten(VI) with
1998, 446±447$
Benzimidazole Derivatives$
Mannar R. Maurya* and Mathuresh N. Jayaswal
Department of Chemistry, University of Roorkee, Roorkee-247 667, India
Synthesis, spectral and thermal studies of diperoxo complexes of molybdenum(VI) and tungsten(VI) with 2-(2-pyridyl)-
benzimidazole, 2-(3-pyridyl)benzimidazole and 2,6-bis(benzimidazol-2-yl)pyridine are reported.
As molybdenum and tungsten are biologically import-
ant metals, the various aspects of the chemistry of
molybdenum(VI) and tungsten(VI) peroxo complexes with
Spectral Studies.ÐThe IR spectra of the complexes
exhibit three IR active vibrational modes, at 0560, 650 and
�
1
870 cm
which are assigned to the symmetric O±M±O
1,2
organic ligands have been of considerable interest. These
complexes serve as reactive intermediates in the catalytic
stretch (ꢀ ), the antisymmetric O±M±O stretch (ꢀ ) and the
2 3
8
O±O intra-stretching (ꢀ ) mode, respectively. The presence
1
3
oxidation of organic substrates. They are also important in
biological processes involving oxidation or conversion of
of these bands owing to the metal-peroxo group (local
2
C2 symmetry) con®rms the ꢁ -coordination of the peroxo
4
dioxygen species. In addition, these complexes have also
group. In addition, they display the diagnostic ꢀ(M1O)
band at 926±962 cm (ref. 9).
The N±H stretching and bending vibrations of the
�
1
been used as potential precursors for molybdenum and
5
tungsten complexes in various oxidation states.
6
Djordjevic et al. were the ®rst to report on peroxo
�
1
ligands, 03100 and 1450 cm , remain either unperturbed or
undergo a slight shift towards lower wavenumber in the
complexes, suggesting that the NH proton remains attached
complexes of the biologically important ligand imidazole
with molybdenum. Recently we have extended this study to
10
2
and studied its molybdenum(VI) and tungsten(VI) peroxo
-(2-hydroxyphenyl)benzimidazole, where we have isolated
at the N1 position. The presence of a sharp band at
�1
0
1600 cm owing to ꢀ(C1N) (ring) vibration in the com-
plexes, indicates the bonding of N with the metal ions.
2
5
complexes. These complexes were further transformed
c
2‡
into the corresponding stable monoxo [MO]
and dioxo
‡
species. In the present investigation we describe the
Such ꢀ(C1N) (ring) vibration in free ligands appears as a
weak band at a relatively higher position. The comparison
of bands owing to ꢀ(C1N) (pyridine moiety) of the ligands
and complexes is somewhat dicult owing to appearance of
this band along with the bands of the tertiary nitrogen of the
benzimidazole residue. However, the appearance of a band
2
[
2
MO ]
isolation, spectral and thermal studies of some diperoxo
complexes of molybdenum(VI) and tungsten(VI) with the
biologically important ligands I, II and III.
�
1
at 1017±1039 cm owing to ring breathing mode of the
�
1
pyridine moiety, which is at 20±35 cm
numbers than the free ligands, indicates the coordination of
pyridinic nitrogen.
1
higher wave-
11
The H NMR spectra of the ligands exhibit a singlet at
3.10±13.15 ppm and several multiplets in the 6.80±8.95
1
ppm region owing to ±NH and aromatic protons, respect-
ively. 3-pybzim displays an additional singlet at 9.35 ppm
owing to a proton of the pyridine moiety. All the aromatic
proton signals are also observed at nearly identical positions
in the complexes. However, the signal due to ±NH proton
of the benzimidazole moiety could not be located. This pro-
ton either resonates along with aromatic protons or appears
12
beyond 15 ppm. Mohanty et al. have also not observed
this signal in mononuclear [MoO L(im)] (LH =Schi€ base
Results and Discussion
7
The non-isolable peroxo species, prepared in situ by
2
2
ligands, im ˆ imidazole) complexes in the 0±15 ppm range.
The UV-VIS spectra of the complexes display an intense
band centred at 290±305 nm, which can be assigned to
the p±p* transition arising from the benzimidazole residue
stirring MoO3 or WO ÁH O with an excess of 30% H O ,
3
2
2
2
readily react with ligands I±III to give the corresponding
peroxo complexes 1±6, respectively. All these complexes
are crystalline in nature, white to light yellow in colour
and sparingly soluble in methanol and ethanol while highly
soluble in dimethylformamide and dimethyl sulfoxide
13
of the coordinated ligand; such a band usually occurs
at higher energy in the free ligand. In addition, they show
a poorly resolved broad absorption between 325 and
(
except for the molybdenum complex of 2-pybzim).
Analytical data suggest a [MO(O
L] (M ˆ Mo or W)
stoichiometry when L ˆ 2-pybzim and 3-pybzim but a
{MO(O stoichiometry when is bzimpy. The
365 nm. A very low molar absorptivity assigns this to the
14
peroxo-metal charge transfer (LMCT) band.
2
)
2
Thermal Study.ÐThe main features of the TGA curves
are the loss of two peroxo groups at two di€erent tempera-
ture ranges in the complexes of neutral bidentate ligands,
while there is an overlapping temperature in the complexes
[
2
)
}
2 3
2
L ]
L
complexes are non-electrolytes in dimethylformamide sol-
ution. They are stable and do not explode on heating but
decompose at 210±240 8C.
14
with a neutral tridentate ligand. Djordjevic et al. have
observed the ready loss of one peroxide in the complex
*To receive any correspondence (e-mail: chemt@rurkiu.ernet.in).
[
MoO(O ) (nicH) (H O)] (nicH ˆ protonated nicotinic acid).
2
2
2
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
A similar intermediate may also be proposed here for com-
plexes 1±4 after intermolecular loss of O . After the total
2

View Article Online
J. CHEM. RESEARCH (S), 1998 447
[
WO(O
2
)
H
2
(2-pybzim)] 2.ÐYield: 65%. (Found: C, 31.2; H, 2.1;
loss of peroxo groups, the organic moiety decomposes
further with increasing temperature. Although decomposed
fragments of the ligand could not be approximated owing to
continuous weight loss, the complete decomposition of the
ligands occurs at 0650 8C in all complexes.
�
9 3 5
N O W requires C, 31.4; H, 2.0; N, 9.2%); ꢀmax/cm
1
N, 9.3. C12
1
6
3
601 (C1N, C1C), 1017 (py ring breath), 955 (W1O), 838 (O±O),
44 (WO
65 (1120), 300 (18400); ꢂ
3
�1
�1
�1
2
antisym), 516 (WO
2
sym); ꢂmax/nm (e/dm mol cm ),
�1
2
/ꢀ cm mol ,
H
7.30±8.30 (m, aryl); L
M
4.5.
[MoO(O
N, 11.2. C12
From the physico-chemical and spectral evidence dis-
cussed above, complexes 1±4 (i.e. those obtained with a
neutral bidentate ligand) are proposed to be mononuclear
pentagonal bipyramidal (IV) while complexes 5 and 6 (i.e.
those obtained with a neutral tridentate ligand) are thought
to be trinuclear, each having a pentagonal bipyramidal
structure (V). Two di€erent environments around the central
metal ion in trinuclear complexes, as proposed here, might
have caused the decomposition of its peroxo groups at
di€erent temperatures and thus show a continuous loss of
peroxo groups in their thermograms. Thus the proposed
structures are supported by the thermolytic pattern. A
pentagonal structure has been con®rmed by X-ray crystallo-
graphy in diperoxo complexes of molybdenum(VI) and
2
)
2
(3-pybzim)] 3.ÐYield: 70%. (Found: C, 38.6; H, 2.6;
MoN requires C, 38.8; H, 2.4; N, 11.3%);
max/cm 1635, 1614 (C1N, C1C), 1039 (py ring breath), 958
Mo1O), 884 (O±O), 661 (MoO antisym), 564 (MoO sym);
max/nm (e/dm mol cm ), 325 (990), 298 (15400); ꢂ 7.40±8.95
H
9
3 5
O
�
1
ꢀ
(
ꢂ
(
2
2
3
�1
�1
H
�1
2
�1
m, aryl), 9.35 (s, aryl); L
2
[WO(O ) (3-pybzim)] 4.ÐYield: 60%. (Found: C, 31.5; H, 2.3; N,
9.1. C12
M
/ꢀ cm mol , 6.0.
2
�
3 5
O W requires C, 31.4; H, 2.0; N, 9.2%); ꢀmax/cm
1
H
9
N
1
(
628, 1603 (C1N, C1C), 1030 (py ring breath), 926 (W1O), 835
3
�1
O±O), 640 (WO
2
antisym), 532 (WO
2
sym); ꢂmax/nm (e/dm mol
�
1
cm ), 355 (880), 305 (14700); ꢂ 7.30±8.75 (m, aryl), 9.35 (s, aryl);
H
�1
2
�1
L
M
/ꢀ cm mol , 5.8.
{MO(O
[
2
)
2
}
3
L
2
] (M ˆ Mo or W).ÐThese complexes were pre-
pared in the same way as described above, using peroxomolybdic
acid or peroxotungstic acid (1.5 mmol) and the ligand (1 mmol).
[{MoO(O
2 2 3 2
) } (bzimpy) ] 5.ÐYield: 53%. (Found: C, 39.9; H, 2.5;
2c,7
N, 12.2. C38H26Mo N10O15 requires C, 39.6; H, 2.3; N, 12.2%);
3
tungsten(VI) having bi- and tridentate ligands.
�
1
ꢀ
(
ꢃ
(
max/cm 1625, 1596 (C1N, C1C), 1035 (py ring breath), 962
Mo1O), 877 (O±O), 652 (MoO antisym), 542 (MoO sym);
max/nm (e/dm m1 ol cm ), 335 (1230), 290 (15800); ꢂ 7.35±8.45
2
2
3
�1
2
�1
�1
H
�
m, aryl); L
{WO(O
N, 9.7. C38
M
/ꢀ cm mol , 7.2.
(bzimpy)] 6.ÐYield: 62%. (Found: C, 32.5; H, 2.1;
requires C, 32.3; H, 1.8; N, 9.9%); ꢀmax
cm 1619, 1597 (C1N, C1C), 1033 (py ring breath), 954 (W1O),
[
2 2 3
) }
H
26
N O W
10 15 3
/
�
1
3
8
35 (O±O), 648 (WO
mol 1cm ), 360 (1470), 300 (16000); ꢂ
M
L /ꢀ cm mol , 6.9.
2
antisym), 559 (WO
2
sym); ꢃmax/nm (e/dm
7.35±8.40 (m, aryl);
�
1
�1
H
� �1
2
We are thankful to the Council of Scienti®c and
Industrial Research, New Delhi 110 012 for ®nancial
support of the work.
Experimental
Received, 11th March 1998; Accepted, 14th April 1998
Paper E/8/01990H
All chemicals used were of A.R. grade. 2-(2-Pyridyl)-
benzimidazole and 2,6-bis(benzimidazol-2-yl)pyridine were prepared
by the literature methods.
1
5
References
The electronic spectra were recorded in dimethylformamide
DMF) on a Shimadzu UV-300 dual recording spectrophotometer
while IR spectra were run as Nujol mulls on a Perkin-Elmer model
(
1 H. Mimoun, The Chemistry of Peroxides, ed. S. Patai, Wiley,
New York, 1983, ch. 15.
2 (a) M. H. Gubelmann and A. F. Williams, Struct. Bonding
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3 M. Bonchio, V. Conte, F. Di Furio, G. Modena and S. Moro,
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4 J. T. Groves, Metal Ion Activation, ed. T. G. Spiro, Wiley, New
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1
H
1
620 FT-IR spectrophotometer. The
NMR spectra were
obtained in (CD SO using Bruker WH-90 and WH-200 spec-
3 2
)
trometers. Chemical shifts (ppm) are reported relative to tetra-
methylsilane. Conductivity measurements were carried out in DMF
�
3
solutions (0.001 mol dm ) of the complexes using a Biochem model
DC-808 digital conductivity bridge calibrated with potassium
chloride solution. A laboratory built instrument was used to record
thermograms of the complexes. All measurements were carried
out in a quartz cup under a static air atmosphere.
Synthesis.Ð2-(3-pyridyl)benzimidazole (3-pybzim).Ðo-Phenylene-
diamine (2.16 g, 20 mmol) and nicotinic acid (2.46 g, 20 mmol) were
mixed in syrupy orthophosphoric acid 950 ml). The reaction mixture
was heated at 240±250 8C for 4 h with occasional shaking and
the coloured melt so obtained was poured into one litre of chilled
water with vigorous stirring. After 2 h, the brownish solid that
precipitated was collected by ®ltration. This was treated with 200 ml
of 15% sodium carbonate solution, ®ltered o€ and washed with
water. Finally, recrystallization from ethanol±water (3:2) resulted in
a white solid in 30% yield. (decomp. 240 8C). (Found: C, 73.6;
H, 4.7. C12
m), 7.65 (m), 8.45 (d), 8.70 (d), 9.35 (s, aryl).
MO(O
L] (M ˆ Mo or W).ÐA solution of peroxomolybdic
acid or peroxotungstic acid was prepared by stirring MoO (0.15 g,
mmol) or WO O (0.25 g, 1 mmol), respectively, in 30% H
ÁH
H
9
N
3
requires C, 73.8, H. 4.6%); ꢂ
H
13.15 (s, NH), 7.25
9 A. Syamal and M. R. Maurya, Coord. Chem. Rev., 1989, 95,
183.
(
[
2
)
2
10 D. P. Drolet, D. M. Manuta, A. J. Lees, A. D. Katnani and
G. J. Coyle, Inorg. Chim. Acta, 1988, 146, 173.
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812.
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1991, 10, 33.
13 C. Piguet, B. Bocquet, E. Muller and A. F. Williams, Helv.
Chim. Acta, 1989, 72, 323.
3
1
(
3
2
2 2
O
12 ml) for 15 h at 040 8C and the solution ®ltered. This solution
was added to a hot ethanolic solution (15 ml) of ligand (1 mmol)
with stirring. The reaction mixture was stirred, along with cooling
in an ice-bath. After 2 h the separated solid was ®ltered o€, washed
with water±ethanol (1:3 v/v) and dried in vacuo.
[
MoO(O
2
)
2
(2-pybzim)] 1.ÐYield: 60%. (Found: C, 39.1; H, 2.7;
MoN requires C, 38.8; H, 2.4; N, 11.3%); ꢀmax
cm 1604 (C1N, C1C), 1021 (py ring breath), 1954 (Mo1O),
63 (O±O), 682 (MoO antisym), 578 (MoO sym).
N, 11.1. C12
H
9
3
O
5
/
14 C. Djordjevic, B. C. Puryear, N. Vuletic, C. J. Abelt and
S. J. Sheeld, Inorg. Chem., 1988, 27, 2926.
15 A. W. Addison and P. J. Burke, J. Heterocyl. Chem., 1981, 803.
�
1
8
2
2