A. Ahmed et al. / Journal of Molecular Structure 1029 (2012) 161–168
167
have composite character due to mixed contribution of the amide
metal. The redox couples are assigned to the following electron
transfer reactions.
(II) and
t(CAO)(phenolate) bands. This band shifts to lower fre-
quency by 6–13 cmꢁ1 in all the complexes. Such a feature associated
with this band shows weak bonding between phenolate oxygen
atom and metal centre. Further, the new band observed in the re-
½O2MoVIðLÞMoVIO2ðH2OÞ ꢃ þ eꢀ½O2MoVI ꢀ ðLÞMoVO2ðH2OÞ ꢃꢁ
ð1Þ
ð2Þ
ð3Þ
2
2
2ꢁ
½O2MoVIðLÞMoVO2ðH2OÞÞ ꢃꢁ þ eꢀ½O2MoVðLÞMoVO2ðH2OÞ ꢃ
gion 1508–1523 cmꢁ1 in all of the complexes is assigned to
tNCO
2
2
vibrations of the newly formed NCOꢁ group [26]. The IR of the com-
2ꢁ
3ꢁ
½O2MoVðLÞMoVO2ðH2OÞꢃ þ eꢀ½O2MoVðLÞMoIVO2ðH2OÞ ꢃ
plexes (4) and (5) show new bands at 592 cmꢁ1 and 472 cmꢁ1. These
2
bands have tentatively been assigned to
t(MAO)(phenolic) and
The complex [(MoO2)2(slsh)(py)2]ꢀH2O (2) shows two reductive
waves at ꢁ0.58, ꢁ1.52 V and corresponding oxidative waves ap-
pear at +0.73, ꢁ0.50 V, respectively is shown in Fig. 5. The differ-
ence between these waves is 131 and 102 mV, respectively, the
waves are quasi-reversible in nature. In view of the absence of
any wave in the range +2.4 to ꢁ2.4 in the free ligand, these redox
couples are assigned to Mo(VI)/Mo(V)/Mo(IV) redox reactions as
shown below.
t(MAO)(carbonyl) [27,28], respectively. However, the possibility
of coupling of these vibrations with some ligand bands cannot be
ruled out. The dioxo groups exhibit strong bands in the region
850–1000 cmꢁ1 due to anti-symmetric and symmetric stretching
vibrations, cis-dioxo groups show two such strong bands while
the trans-dioxo groups show only one band. All of the complexes
show two strong bands in the region 910–956 cmꢁ1 respectively
of almost equal intensity indicating the presence of cis-MoO22þ
grouping in these complexes [29]. In addition, the complexes show
a medium to strong band in the region 817–857 cmꢁ1. The first two
½O2MoVIðLÞMoVIO2ðpyÞ ꢃ þ eꢀ½O2MoVIðLÞMoVO2ðpyÞ ꢃꢁ
ð4Þ
2
2
as stretching vibrations of cis-MoO2þ
2ꢁ
½O2MoVIðLÞMoVO2ðpyÞ ꢃꢁ þ eꢀ½O2MoVIðLÞMoIVO2ðpyÞ ꢃ
ð5Þ
bands are assigned to
ts and
t
2
2
2
group. The bands observed in the region 817–857 cmꢁ1 in the com-
plexes are similar to those observed by Chakravarty and Rajan [30]
and Holm et al [31] and other workers [32] and are assigned to
The complexes (3) and (4) show two reductive waves at ꢁ0.42,
ꢁ1.56; ꢁ0.20, ꢁ1.62 V and two corresponding oxidative waves at
+0.70, ꢁ0.26;+0.55, ꢁ0.09, respectively. These waves are quasi-
reversible because the separation between the corresponding
reductive and oxidative waves falls in the region 75–153 mV which
is more than the values of 60 mV required for one electron transfer
reversible reactions. The complex (5) also shows essentially similar
electrochemical behaviour (reductive waves at ꢁ0.35, ꢁ1.23 V;
oxidative waves at + 0.65, ꢁ0.25 V) as the complexes (3) and (4).
These two waves may be assigned to MoVIMoVI/MoVIMoV and Mo-
VIMoV/MoVIMoIV redox couples as in the case of the complexes (3)
and (4). Electrochemical studies on several cis-dioxomolybde-
num(VI) complexes derived from multidentate nitrogen and
oxygen donor ligands have generally shown irreversible or quasi-
reversible behaviour. The high peak separation, most probably,
results from a slow heterogeneous electron exchange rate rather
than from intervening homogeneous reactions [33].
t
Mo@Oꢀ ꢀ ꢀMo vibrations. The appearance of two bands in the region
857–956 cmꢁ1 coupled with the typical band in the region 817–
857 cmꢁ1 in the complexes suggests that the bridging Mo@Oꢀ ꢀ ꢀMo
bands in these complexes are weak and that the molybdenyl group
maintains its identity as cis-MoO22þ group.
4.6. Low frequency IR
The free ligand H4slsh shows absorption bands at 573 m, 538s,
522s, 520s, 497s, 465s, 450s, 405s, 373 m, 357 m cmꢁ1, in low fre-
quency infrared spectra. The low frequency IR of the complexes
[(MoO2)(slsh)(H2O)2] (1), [(MoO2)(slsh)(py)2]ꢀH2O (2), have been
studied as representative examples to make assignment of bands
due to coordinated pyridine molecules as shown in Table 4. Ligand
bands have been excluded and the new bands appearing in the
582–590 cmꢁ1 region have been assigned to
On the other hand, a new band observed at 435 cmꢁ1 is assigned
to (MAO)(carbonyl). The appearance of these two bands in the
m(MAO)(phenolate).
5. Conclusion
m
The stoichiometry and physico-chemical studies reveal the for-
mation of homobimetallic Mo(VI) complexes. The appearance of
azomethine proton signal in the 1H NMR spectra of the molybde-
num(VI) complexes (1–5) in the form of singlet suggests that the
dihydrazone is coordinated to the metal centres in staggered-
configuration. In this configuration, in all of the complexes, the dif-
ferent parts of the dihydrazone molecules are coordinated to the
different metal centres ruling out the possibility of any steric
crowding in the molecule. As a result, both the hydrazone parts
of the coordinator dihydrazone molecule remain in the same plane
giving rise to a singlet corresponding to azomethine protons. The
dihydrazone coordinates to the metal centres as tetrabasic hexa-
dentate tridentate ligand in the enol form. In this form the metal
centres are bonded to NOO coordination centres. The mass spectral
behaviour suggests that these complexes are monomeric in nature.
An octahedral stereochemistry around the metal ions has been
proposed on the basis of above-mentioned studies as shown in
Scheme 1.
complexes suggests the involvement of phenolate and enolate oxy-
gen atoms in bonding to the metal. The complexes show new
bands in the region 301–344 cmꢁ1. These new bands occur in
almost in the same region in which the
m(MAN) bands have seen
reported to occur in metal complexes of hydrazine derivatives.
Hence, these bands are assigned to
m(MAN) stretching vibration
due to coordinated > C@N group.
4.7. Cyclic voltammetry
The cyclic voltammograms of a 2 mM solution of the complexes
have been recorded at a scan rate of 100 mV/s by cyclic voltamme-
try in DMSO solution due to their insolubility in non-coordinating
organic solvents (CH3CN and CH2Cl2) with a 0.1 M tetra-n-butyl
ammonium perchlorate (TBAP) as a supporting electrolyte. The
data have been set out in Table 5. The ligand in the present study
does not exhibit any redox activity in the potential range ꢁ2.4
to + 2.4. Hence, these reductive and oxidative waves observed in
the complexes in the present study may be attributed to electron
transfer reactions centred on metal centre.
Acknowledgments
The cyclic voltammogram of the complex [(MoO2)2(slsh)(H2O)2]
(1) is characterised by a reductive wave at +0.92, ꢁ0.67 and
ꢁ1.40 V in the cathodic scan. Corresponding to this, there is oxida-
tive wave in the anodic scan at + 0.82, ꢁ0.60 and ꢁ1.28 V. This re-
dox behaviour arises due to electron transfer reaction centred on
Authors are thankful to the Head, SAIF, North-Eastern Hill Uni-
versity, Shillong 793 022, Meghalaya for CHN analyses and spectral
studies. Further, they are thankful to the Head, SAIF, IITG for
recording NMR.