Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies

Article abstract of DOI:10.1007/s00706-019-02435-1

Abstract: Three new oxodiperoxo–tungsten(VI) complexes containing benzene core carboxylic acids, viz., benzoic acid, 2-chlorobenzoic acid, and 3-aminobenzoic acid as co-ligands have been synthesized from reaction of Na₂WO₆H₄, 30% H₂O₂ and the corresponding co-ligands in aqueous medium. The compounds have been comprehensively characterized by elemental analyses, FT-IR, ¹H NMR, UV–Vis spectral studies as well as by mass spectrometric and TGA analyses. The infrared spectra suggest occurrence of terminally bonded W=O as well as triangular bidentate peroxo groups (C_2v) and monodentate carboxylate group bound to the WO⁴⁺ center. The mass spectra of the compounds are in good agreement with proposed molecular formulations. Thermogravimetric analyses indicate the existence of both lattice and coordinated water molecules in the complexes. Density functional theory (DFT) calculations were used to compute the frequencies of relevant vibrational modes, electronic properties and also to investigate structure of the compounds. Compound potassium(aquo)(2-chlorobenzoato)oxodiperoxo–tungstate(VI)dihydrate acts as an oxidant for bromide ion in aqueous phase bromination of chosen organic substrates to their corresponding bromo-organics. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-019-02435-1

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-019-02435-1
ORIGINAL PAPER
Peroxo–tungstate(VI) complexes: syntheses, characterization,
reactivity, and DFT studies
Nandita Das1 · Shubhamoy Chowdhury2 · Ranendra N. Dutta Purkayastha1
Received: 12 October 2018 / Accepted: 20 April 2019
© Springer-Verlag GmbH Austria, part of Springer Nature 2019
Abstract
Three new oxodiperoxo–tungsten(VI) complexes containing benzene core carboxylic acids, viz., benzoic acid, 2-chlo-
robenzoic acid, and 3-aminobenzoic acid as co-ligands have been synthesized from reaction of Na2WO6H4, 30% H2O2 and
the corresponding co-ligands in aqueous medium. The compounds have been comprehensively characterized by elemental
analyses, FT-IR, 1H NMR, UV–Vis spectral studies as well as by mass spectrometric and TGA analyses. The infrared spec-
tra suggest occurrence of terminally bonded W=O as well as triangular bidentate peroxo groups (C2v) and monodentate
carboxylate group bound to the WO4+ center. The mass spectra of the compounds are in good agreement with proposed
molecular formulations. Thermogravimetric analyses indicate the existence of both lattice and coordinated water molecules
in the complexes. Density functional theory (DFT) calculations were used to compute the frequencies of relevant vibrational
modes, electronic properties and also to investigate structure of the compounds. Compound potassium(aquo)(2-chlorobenzo-
ato)oxodiperoxo–tungstate(VI)dihydrate acts as an oxidant for bromide ion in aqueous phase bromination of chosen organic
substrates to their corresponding bromo-organics.
Graphical abstract
Keywords Bromination · Density functional theory · Peroxo–tungsten complexes · Spectroscopy
Introduction
Hydrogen peroxide readily combines with transition met-
als such as V(V), Mo(VI), Nb(V), and W(VI) forming a
Electronic supplementary material The online version of this
number of peroxo complexes whose nature depends on the
article (https://doi.org/10.1007/s00706-019-02435-1) contains
pH as well as relative concentration of the reagents [1].
supplementary material, which is available to authorized users.
Peroxo complexes of transition metals, viz., V, Mo, Nb and
* Ranendra N. Dutta Purkayastha
W have attracted considerable attention because of their
intrinsic importance in biological chemistry, structural
chemistry, material science, and catalytic activity [2–8].
The chemistry of peroxo–tungsten complexes has been an
area of signifcant interest for a number of reasons includ-
ing their role as stoichiometric and efective catalytic
rndp09@gmail.com
1
Department of Chemistry, Tripura University,
Suryamaninagar, Tripura 799022, India
2
Department of Chemistry, University of Gour Banga, Malda,
West Bengal 732103, India
Vol.:(0123456789)
1 3

N. Das et al.
oxidants and oxo-transfer agents in host of organic trans-
formations [9–14]. Some of the peroxo–tungsten spe-
cies have been recognized as potential insulin mimics
and recently shown to possess superior bromoperoxidase
activity compared to that of peroxo–vanadates [15–17].
As emphasized, the peroxo–tungsten complexes are ef-
cient and relatively selective oxidants for many organic
and inorganic species and at the same time less polluting;
thus, they may provide a valuable eco-friendly alternative
[18, 19].
Results and discussion
The reaction of hydrogen peroxide with WO4+ leading to
a complex peroxo–tungstate(VI) of defnite composition
is highly dependent on the pH of the reaction medium
(vide infra). The evaluation of an appropriate pH for suc-
cessful synthesis of peroxo–metal species is emphasized
to be an important pre-requisite [27–30]. In our endeavor
to synthesize mononuclear hetero-ligand oxodiper-
oxo–tungstate(VI), reaction of Na2WO6H4 with 30% H2O2
and the corresponding hetero-ligands in aqueous medium
was strategically carried out in weakly alkaline solution.
The suitable pH for coordination of both peroxide and
the hetero-ligands was ascertained to be ca. 7–7.5, which
favored the formation of mononuclear oxodiperoxo species
of tungsten(VI) in solution. Compounds were isolated as
microcrystalline solid of their potassium salt. The pH of
ca. 7–7.5 of the reaction medium was maintained by the
addition of 2 N KOH solution which also served as the
source of counter-cation for the anionic complexes. The
other factors such as the sequence of addition of the reac-
tants, maintenance of reaction time and reaction tempera-
ture (0–4 °C) were also equally important for successful
synthesis of the desired compounds. In all these cases of
syntheses of oxodiperoxo–tungstate(VI) complexes, addi-
tion of pre-cooled acetone facilitated precipitation and
aforded the compounds in high yields. The synthesized
compounds 1–3 are diamagnetic as evidenced from the
magnetic susceptibility measurements in conformity with
the presence of hexavalent tungsten in each case.
In continuation of our interest in peroxo–tungsten
chemistry [20], we have endeavored to synthesize and
explore some properties of new oxodiperoxo–tungstate(VI)
complexes incorporating benzene-core carboxylic acids as
hetero-ligands. As part of exploration of transition metal
compound-catalyzed aqueous medium organic transfor-
mations [21], we were particularly interested to ascertain
whether the synthesized peroxo–tungstate(VI) complexes
could act as oxidant for halide (bromide) with good activ-
ity at neutral pH in aqueous medium. The important role of
halogenated compounds in chemistry combined with poor
economic and environmental efciency of the common
halogenating systems makes catalytic halo-peroxidation an
attractive alternative and important aspect in modern catal-
ysis [22]. The inspiration for catalytic halo-peroxidation
also comes from biological systems in which enzymes,
such as haloperoxidase (HPS) are able to activate hydro-
gen peroxide or oxygen. In these systems, the oxidation of
halogen brings further the corresponding oxidized elec-
trophilic counterpart which is able to halogenate organic
substrates [21]. Thus, this bio-mimetic approach can be
utilized as an alternative route to traditional bromination
reaction. Among the halogenating processes, bromination
of organic substrates particularly aromatics has generated
signifcant interest in recent years due to the commercial
importance of such compounds as potent anti-tumor, anti-
bacterial, anti-fungal, anti-neoplastic, antiviral, and anti-
oxidizing agents [23–25]; however, the hazards associated
with traditional bromination are serious and cannot be
overlooked [26].
Characterization and assessment of structure
The synthesized hetero-ligand potassium(aquo)oxodiper-
oxo–tungstate(VI) (1–3) is a white microcrystalline
solid and soluble in water. Solubility of the compounds
in water made them suitable to be used as catalytic oxi-
dants in aqueous phase bromination of organic substrates.
Due to increasing environmental concern, the aqueous
medium organic transformations have currently gained
signifcant importance [31, 32]. All the synthesized com-
pounds decompose in dilute sulfuric acid solution liber-
ating hydrogen peroxide quantitatively, thus facilitating
the determination of active oxygen contents of the com-
pounds. Chemical determination of the active oxygen,
which was considered to be very crucial to ascertain the
number of peroxide (O22−) group coordinated to WO4+
center, was accomplished by redox titration involving a
standard Ce4+ solution and also separately with standard
KMnO4 solution. The estimation was carried out in the
presence of boric acid to prevent any loss of active oxygen.
Herein, we report synthesis, physico-chemical char-
acterization of mononuclear oxodiperoxo–tungstate(VI),
containing benzene-core carboxylic acids, viz., benzoic
acid, 2-chlorobenzoic acid, 3-aminobenzoic acid as the
hetero-ligands and assessment of their activity in oxidative
bromination of organic substrates. Additionally, density
functional theory (DFT) calculations were used to study
and classify the structures and to simulate the infrared
absorption frequencies and electronic absorption bands.
The comparison between the theoretical and the experi-
mental values for infrared, electronic absorption frequen-
cies in principle provide an excellent means for structure
elucidation.
1 3

Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies
The results of elemental analyses suggest the occurrence
certainty owing to its overlapping with ʋ (O–H) mode of
2−
of two O2 groups per WO4+ center and C, H, N analy-
water resulting in a broad band at ca. 3368–3500 cm−1. The
results of IR spectral analyses are in close agreement with
the proposed molecular structures of the complexes under
consideration.
ses indicate W:hetero-ligand ratio as 1:1 in accordance
with the formulation (1–3). Molar conductance of the
complexes in aqueous solution (10–3 M) exhibits values
of 130 Ω−1 mol−1 cm−1 as expected for a uni-univalent
electrolyte. Despite several attempts, it was not possible
to obtain suitable crystals of the compounds for single-
crystal X-ray difraction.
Mass spectra
The mass spectra of the compounds were recorded to ascer-
tain the molecular mass of the compounds and are pre-
sented in Figs. S8–S10. The mass spectra of the complexes
show an extensive fragmentation pattern and most of the
fragment ions occur in a group of peaks due to tungsten
isotopes. Appearances of peaks beyond the molecular ion
peak for each of complexes 1–3 suggest association of the
complexes in gaseous state. In ESI spectrum of compound
1, the peak at m/z=478.79 is due to the molecular ion peak
(calc. 478.09). Observance of peak at m/z = 426.13 (calc.
426.05) and at m/z=235.21 (calc. 235.85) is assignable to
fragments (KWO7C7H5 + 2H+) and (WO3 + 4H+), respec-
tively. For compound 2, molecular ion peak with appreci-
able intensity was observed at m/z=494.76 (calc. 494.52).
Compound 2 also shows peaks at m/z = 478.79 (calc.
478.52), 394.85 (calc. 394.49), and 358.81 (calc. 358.39)
assigned for (KWO8C7H6Cl + 2H+), (KWO3C7H4Cl), and
(WO3C7H4Cl + 3H+), respectively. In the mass spectrum
of compound 3, a prominent peak at m/z = 494.75 (calc.
494.10) is attributable to [M + H]+. Appearance of peaks
at m/z=478.78 (calc. 478.09) and at 410.80 (calc. 410.06)
is reasonable to assign to fragments (KWO9C7H10N+3H+)
and (KWO5C7H6N + 3H+), respectively. Due to extensive
fragmentation and association of fragment ions in each com-
pound, all the observed peaks could not be assigned. Over-
all, the mass spectral data are in good agreement with the
composition and expected structures of the complexes pro-
posed on the basis of elemental analyses and spectral data.
Infrared spectra
The infrared spectra of the complexes revealed sufciently
that well-resolved spectral pattern (Figs. S1–S3, supplemen-
tary information) involves absorptions due to W=O stretch-
ing, coordinated peroxide, hetero-ligands, viz., benzoate,
2-chlorobenzoate, 3-aminobenzoate and water molecules.
The strong and sharp band at ca. 907–917 cm−1 in the spec-
tra of the complexes is consistent with the presence of a
terminally bonded W=O group in each case [33]. A trian-
gularly bonded peroxide (C2v) exhibits characteristic infra-
red absorptions with ʋ(O–O) at ca. 830–870 cm−1 and ʋ2,
ʋ3 which involve symmetric and asymmetric metal–oxygen
stretching (M–O2) in the region 500–700 cm−1 [34, 35]. In
spectra of complexes 1, 2, and 3, occurrence of a strong
and sharp band at ca. 829–838 cm−1 ascribed to ʋ(O–O)
vibration and suggests that the peroxide group (O22−) is
bonded to the WO4+ center in a triangular bidentate (C2V
)
fashion [34, 35]. Medium- to weak-intensity bands in the
regions 565, 694 cm−1 for 1; 547, 702 cm−1 for 2; and 565,
734 cm−1 in case of 3 are assigned to symmetric and asym-
metric stretching vibrations, respectively, of the WO2 tri-
2−
angle formed from terminally η2-coordinated O2 ligand
[36–39]. Absorption assignable to asymmetric stretching of
coordinated carboxylate moiety of the hetero-ligands ben-
zoate, 2-chlorobenzoate and 3-aminobenzoate appeared at
ca. 1606–1640 cm−1 whereas the corresponding symmetric
vibrations were observed in the region 1431–1470 cm−1. The
observed diference in frequency between asymmetric and
symmetric stretching vibrations of carboxylate group, Δʋ
(ʋassy − ʋsym =Δʋ) greater than 200 cm−1 is characteristic
of monodentate coordination of carboxylate group [40, 41].
The presence of water molecules in each of the complexes
1–3 as evidenced in thermogavimetric analyses was also
inferred from the presence of strong absorption in the range
of 3400–3500 cm−1 due to the ʋ(O–H) vibration. How-
ever, the corresponding bending mode of water δ(H–O–H)
could not be ascertained with certainty as it occurred in the
CO2− stretching frequency region. The slight broadening of
ʋ(O–H) band related to water suggests a clear possibility
of intramolecular hydrogen bonding by the participation of
tungsten oxygen. For compound 3 containing 3-aminoben-
zoate as hetero-ligand, ʋ(N–H) could not be assigned with
Thermogravimetric analysis (TGA)
Thermogravimetric analysis data indicate that after the ini-
tial dehydration, the compounds undergo gradual decom-
position on heating up to a temperature of 900 °C (Figs.
S11–S13). Interestingly, the compounds do not explode
on heating, although many peroxo–metal compounds have
been known to decompose explosively on heating [42].
The thermograms of the compounds showed that the frst-
stage decomposition occurs in the temperature range of
120–130 °C with the loss of molecules of lattice water from
the complexes. The corresponding weight losses of 7.49%,
7.10%, and 7.16% for complexes 1, 2, and 3 are in good
agreement with the values of 7.53%, 7.03%, and 7.30%
calculated for two molecules of water of crystallization. A
slightly higher temperature is required for the removal of
1 3

N. Das et al.
lattice water probably due to the existence of intra-molecular
hydrogen bonding in the complexes. The next decomposition
stage in the temperature range of 180–190 °C with the cor-
responding weight loss of 4.12% (calc. 4.07%), 3.88% (calc.
3.78%) 4.08% (calc. 3.93%), respectively, is attributable to
loss of coordinated water molecule from the complexes. A
further increase in temperature leads to continuous degrada-
tion due to loss of peroxo groups as well as fragmentation of
hetero-ligands up to a fnal temperature of 900 °C. The solid
residue remaining after complete degradation is attributed
to the formation oxo–tungsten species. Thermogravimetric
data of the compounds show that the synthesized complexes
have relatively good thermal stability and provide further
evidence in support of the composition and formula assigned
to the compounds.
attached to carbon atom (C-2, C-4, C-5, and C-6) showed
distinct downfeld shift relative to the free co-ligand. The
observed downfeld shift may be due to conjugation of lone
pair of electrons in nitrogen with ring π-electrons. The sig-
nals of protons attached to nitrogen of –NH2 group could not
be observed probably due to exchange with the solvent D2O.
Thus, 1H NMR spectra testify the presence of the com-
plex species in solution, suggesting that the compounds did
not hydrolyze and retained their identity in solution. Thus,
apart from valuable structural information, the NMR data
also provided evidence in support of the stability of the com-
pounds in solution for reasonable period of time as observed
in the solution state electronic spectra of the complexes (vide
infra).
Substrate bromination in aqueous–organic medium
1H NMR study
As emphasized (vide infra), the importance of halogenated
compounds and increasing awareness about environmental
safety and catalytic halo-peroxidaion has gained signifcant
importance as an alternative route to that of classical halo-
genation of organic substrates. It is reported in the litera-
ture that transition metal–peroxo compounds can oxidize
bromide leading to the formation of an active bromination-
The 1H NMR spectra for the complexes 1, 2, and 3 in D2O
are presented in Figs. S14–S16, respectively, and corre-
sponding chemical shift values for the complexes and the
free benzene core carboxylic acids are presented in Table 1.
A close similarity was observed in the 1H NMR spectra of
the complexes and the spectra of the respective carboxylic
acid displaying well-resolved resonances with expected
integration and peak multiplicities. The spectra of the com-
plexes 1 and 2 containing benzoate and 2-chlorobenzoate
showed distinct upfeld shift of the proton signals (C-2, C-3,
C-4, C-5, C-6) relative to the free ligand. The signifcant
−
competent species (“Br+ equivalent”, e.g., HOBr, Br2, Br3 )
in situ which mediates the bromination of organic substrates
to aford bromo-organics [21].
It has been observed that stoichiometry of the reaction
requires for one equivalent of substrate, one equivalent
of oxidant, i.e., compound 2 and two equivalents of hal-
ide (KBr) is optimal. The solvent mixture of C2H3N:H2O
(1:1, 8 cm3) gave a reasonably good yield of the brominated
products. The notable feature of the methodology is that
no extra addition of acid was required for stoichiometric
bromination reaction of the substrates. The reactions were
conducted in water–acetonitrile medium because of environ-
mental safety consideration, although similar reactions are
possible in dichloromethane as well. The substrate for aque-
ous phase bromination included activated aromatics such as
1-naphthol, 2-naphthol, salicylaldehyde, acetanilide, vanil-
lin and hetero-aromatics, viz., imidazole and 2-methylimi-
dazole. Compound 2 satisfactorily catalyzed the oxidative
bromination of the activated aromatics to their correspond-
ing brominated products in moderately good yield (Table 2).
The bromination of hetero-aromatics such as imidazole and
2-methylimidazole was carried out using higher proportion
of KBr (substrate:KBr 1:3) to aford the respective bromo
derivatives. A 1:2 stoichiometry of substrate to KBr did not
produce the desired result. A blank reaction, i.e. similar
reaction without the addition of compound 2, did not aford
the brominated products. Preferential bromination at either
ortho or para position of the aromatic ring suggests an elec-
trophilic mechanism.
coordination-induced shift Δδ (δfree carboxylic acid − δcomplex
)
occurred as expected, suggesting appreciable metal–ligand
coordination [43, 44]. However, in case of compound 3 con-
taining 3-aminobenzoate as the co-ligand, the proton signals
Table 1 1H (δ/ppm) of 1, 2, and 3 in D2O as the solvent (25 °C)
Free ligand
Compound
KWO10C7H11 (1)
C-2, C-6
8.18
7.47
7.60
7.72
7.32
7.39
C-3, C-5
C-4
KWO10C7H10Cl (2)
C-3
8.48
7.51
7.34
8.03
7.14
7.18
7.06
7.23
C-4
C-5
C-6
KWO10C7H12N (3)
C-2
N–H
C-4
C-5
C-6
7.33
4
6.80
7.22
7.49
8.24
–
7.5
8.10
8.5
1 3

Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies
Table 2 Bromination of aromatic substrates mediated by compound 2
reported oxo–tungstate complexes [1, 46–48]. A good
agreement exists among the calculated geometry of the
model structures of 1−, 2−, and 3− and the experimental as
well as the calculated structure geometry of the reported
ones [1, 45–50].
Substrate
Product
Yield/%
1-Naphthol
2-Naphthol
Salicylaldehyde
2,4-Dibromo-1-naphthol
1-Bromo-2-naphthol
5-Bromosalicylaldehyde (a)
3,5-Dibromosalicylaldehyde (b)
4-Bromoacetanilide
65
89
52 (a), 18 (b)
Acetanilide
Vanillin
69
81
IR spectra
5-Bromovanilline
Imidazole
4-Bromoimidazole(a)
4,5-Dibromoimidazole(b)
16 (a), 27 (b)
Both the diagnostic experimental and calculated (non-
scaled) IR frequencies of the species 1−, 2−, and 3− are
shown in Fig. 2 and their assignments are presented in
Table 3. The characteristic feature of IR spectrum of free
carboxylic group is occurrence of a band in the region
1700–1800 cm−1 due to C=O stretching vibration. In solid
state, most of the carboxylic acids usually form a dimeric
structure due to the existence of hydrogen bonding between
the –CO2H groups. In such cases, two ν(C=O) vibrations are
expected, one is Raman active (symmetric stretching) while
the other IR active (anti-symmetric stretching) [51]. The
anti-symmetric stretching vibration of carboxylate group for
complexes 1−, 2−, and 3− are observed at ca.1640, 1640,
and 1606 cm−1, respectively. The observed anti-symmetric
carboxylate stretching bands are at a lower frequency than
the free carboxylic group which is theoretically reproduced
at ca. 1641–1653 cm−1. The observed shifts in the carboxylic
frequencies are expected as the coordination of CO2− moi-
ety to the metal center generally results in a decrease of
carboxylate stretching frequency. The IR spectra of 1−, 2−,
and 3− also exhibit characteristic vibrations due to W=O
moiety at 914, 907, and 914 cm−1 which are theoretically
reproduced at 933.96, 981.14, and 974.83 cm−1, the range
expected for such vibration [33, 52, 53]. The maximum devi-
ations of the calculated IR frequencies for complex ions 1−,
2−, and 3− from their respective experimental values are
less than 8%. These variations are quite reasonable, as the
theoretical calculations are based on the gas phase and only
anionic species of the respective molecules were considered.
The computed data are often at variance with the experimen-
tally obtained data [54].
2-Methylimidazole 4-Bromo-2methylimidazole (a)
26 (a),71 (b)
4,5-Dibromo-2-methylimidazole
(b)
In summary, we have shown that bromination of organic
substrates can be achieved by carrying out reaction of
organic substrates including heterocyclic compounds, with
potassium bromide and compound 2. The methodology pro-
vides an efcient, straightforward and benign alternative to
rather hazardous classical bromination protocols, opening
an opportunity to gain easy access to a variety of bromo
organics.
Theoretical investigation
The optimized structures of compounds 1−, 2−, and 3− are
shown in Fig. 1, and Table S1 represents the calculated
bond distances and bond angles. The W=O bond lengths
for the optimized structures 1−, 2−, and 3− are found to
be 1.749 Å, 1.730 Å, and 1.733 Å, respectively, which
are in the typical range of distance ( ~ 1.620 to ~ 1.785 Å)
reported for oxo–tungstate complexes [45–47]. The O–O
(per bond) bond lengths of 1.555 Å (on an average) of the
coordinated peroxo groups for the optimized structures
of 1−, 2−, and 3− are in the range usually observed for
the peroxo–metal complexes [1, 48]. The W–O (peroxo
oxygen) bond length for the species 1−, 2−, and 3− were
observed at ca. 1.974–1.995 Å similar to that of the
Fig. 1 Geometry optimized structures of 1−, 2−, and 3−
1 3

N. Das et al.
Fig. 2 The experimental (black) and calculated (red) vibrational transitions of 1−, 2−, and 3−
Table 3 The diagnostic
Compounds
Experimental/cm−1
Theoretical/cm−1
Assignments
experimental and calculated IR
frequencies for 1−, 2−, and 3−
KWO10C7H11 (1)
3486br
3419br
1640
1431
914
3512.04, 3528.36, 3545.78, 3555.60
3490.20
1653.89
1461.26
933.96
852.33
ʋ(C–HAr)
ʋ(O–H)
−
ʋasy(CO2
ʋsym(CO2
ʋ(W=O)
ʋ(O–O)
)
−
)
829
694
565
713.36
582.48
ʋasy(W-O2)
ʋsym(W-O2)
ʋ(O–H), ʋ(C–HAr)
ʋ(W=O)
KWO10C7H10Cl (2)
3265br, 3421 br
907
3453.84, 3472.45, 3493.17, 3513.55
981.14
838
696
879.58, 886.38
709.63
ʋ(O–O)
ʋasy(W–O2)
547
549.64
1641.29
1462.34
ʋsym(W–O2)
−
1640
1459
3502 br
3424 br
914
ʋasy(CO2
ʋsym(CO2
)
)
−
KWO10C7H12N(3)
3489.22, 3517.75, 3543.06, 3561.34
3259.67
974.83
ʋ(C-HAr), ʋ (N–H)
ʋ(O–H)
ʋ(W=O)
829
734
885.37, 879.37
719.55
ʋ(O–O)
ʋasy(W-O2)
565
1606
1434
589.57
1643.84
1487.74
ʋsym(W-O2)
−
ʋasy(CO2
ʋsym(CO2
)
)
−
(LMCT) band [55]. The observed high-energy band at
ca. 200–205 nm [εM = 3.7 × 103 dm3 mol−1 cm−1 (1),
0.49 × 103 dm3 mol−1 cm−1 (2), 3.3 × 103 dm3 mol−1 cm−1
(3)] may be assigned to intra-ligand charge transfer tran-
sition [55], also reproduced in theoretical calculation.
There is no evidence of a d–d transition consistent with the
presence of W(VI). The intensity of the absorption band
remains unaltered (0–3 h) (Fig. S7); however, it gradually
Molecular orbital description and electronic spectra
Close analogy was observed among the observed UV–Vis
spectral pattern of the complexes recorded in aqueous
solution (10–3 M). The spectra exhibited a medium-inten-
sity band at ca. 254–264 nm (Figs. S4–S6) [εM =0.45×103
dm3 mol−1 cm−1 (1), 0.026 × 103 dm3 mol−1 cm−1 (2),
0.18 × 103 dm3 mol−1 cm−1 (3)] assignable to transi-
tion from peroxo group to tungsten charge transfer
1 3

Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies
decreases with time ( > 3 h) indicating partial loss of per-
oxide in solution on standing for prolong period.
acid, peroxo and oxo to the metal center indicating a strong
ligand-to-metal charge transfer (LMCT) character.
The absorption spectra of 1−, 2−, and 3− as discussed
are simulated in the presence of the solvent model (aque-
ous) employing the TD–DFT methods with the identical
basis set and functional as used in geometry optimization
and frequency calculations. The resultant simulated data
are in good agreement with the experimentally obtained
data (Fig. 3).
The calculated high-energy bands at ca. 206.33, 202.63,
and 205.25 nm for the complexes 1−, 2−, and 3− are cor-
related with the experimentally obtained bands at ca. 204,
202, and 205 nm, respectively. These transitions mainly arise
from high-energy ligand-to-ligand charge transfer (LLCT)
transitions [20]. Thus, the theoretical simulated transitions
are in close agreement with those observed experimentally.
The calculated spin-allowed singlet–singlet electronic
transitions and the experimentally obtained data for 1−,
2−, and 3− in aqueous solution are summarized in Table 4.
The selected transitions of 1−, 2−, and 3− having signif-
cant oscillator strengths are incorporated and screened.
The transitions only with orbital contributions larger
than 5% are taken into account for molecular ions under
consideration.
Conclusion
The present work aforded water-soluble potassium(aquo)
oxodiperoxo–tungstate(VI) complexes 1, 2, and 3 contain-
ing benzene-core carboxylic acids as the hetero-ligands. The
chemical and spectral data obtained provided satisfactory
evidence regarding composition of the coordination poly-
hedra and the likely mode of coordination of the ligands
to WO4+ center. Density functional theory (DFT) computa-
tion studies on the complexes substantiated the experimental
data. Reactivity of the oxoperoxo–tungstate(VI) complex,
viz., compound 2, was exploited to generate an active bro-
minating species in situ to perform bromination of organic
substrates efciently.
A schematic representation of the energy of MOs and
contours of the selected HOMO and LUMO orbitals of 1−,
2−, and 3− is presented in Figs. 4 and 5. The energy gap
between HOMO and LUMO for 1−, 2−, and 3− is found to
be 2.981, 5.032, and 3.800 eV, respectively. In solution, the
highest occupied molecular orbitals (HOMOs) of 1− and
2− have major contribution from peroxo group whereas in
case of 3−, the major contribution is received from ligand
3-aminobenzoate. The unoccupied orbitals L+2, L+3 for
1−, L+1, L+2 for 2−, and L, L+2 for 3− exhibit considera-
ble amount of metallic character. The energy of transitions in
solution was calculated at 267.97, 241.26, and 345.01 nm for
compounds 1−, 2−, and 3−. respectively. In solution, the cal-
culated absorption bands at 267.97, 241.26, and 345.01 nm
for 1−, 2−, and 3− are assigned to (H-1→L, H-1→L+2,
H → L + 3), (H-1 → L + 2, H-4 → L + 1, H-3 → L,
H-3→L+1), and (H-1→L, H→L+1, H→L+2) modes
of transitions, respectively. MO analyses of these transitions
show that a signifcant amount of electron density is trans-
ferred from the hetero-ligand that is benzene-core carboxylic
Experimental
Chemicals used were reagent-grade products. The pH of
the reaction solution was measured using a Systronics
digital pH meter 335 and by BDH indicator paper. Molar
conductance measurements were carried out in aqueous
solution by Systronics conductivity meter-304. FT-IR
spectra were recorded with KBr pellet on a PerkinElmer
model spectrum 100 spectrometer. Electronic spectra
were recorded (10–3 M aqueous solution) using Shimadzu
Fig. 3 The experimental (black), calculated (blue) electronic absorption spectra and the calculated electronic transitions (red) of 1−, 2−, and 3−
(color fgure online)
1 3

N. Das et al.
Table 4 Electronic transitions
of 1−, 2−, and 3− calculated
in water using the TD–DFT
method
Most important orbital excitations
λ
F
Experimental λ (ɛ)
1−
H→L, H→L+2
293.29 0.0120
267.97 0.0098
263.17 0.0054
234.70 0.0144
221.36 0.0108
220.70 0.0206
219.77 0.0485
208.52 0.0121
207.33 0.0080
206.33 0.2681
205.18 0.1123
203.44 0.0043
H-1→L, H-1→L+2, H→L+3
H-1→L+3, H-1→L+3, H-1→L+3
H-2→L, H-2→L+1, H-2→L+4
H-4→L+2, H-4→L+2
H-2→L+4, H-2→L+4, H-3→L+1
H-2→L+4, H-2→L+4, H-3→L+4, H-3→L+1
H-4→L+4, H-5→L
245 (5708)
H-4→L+4, H-5→L, H-4→L+4
H-5→L+1, H-5→L+3, H-1→L+4
H-6→L+2, H-5→L+2, H-6→L+2, H→L+6
H-6→L, H-6→L+1
204 (36,853)
2−
H→L, H-1→L+1, H-2→L+1, H-2→L+3,
H-2→L, H-2→L+2, H-1→L+2
H-1→L, H-3→L, H-2→L+2, H-3→L+4
H-2→L, H-2→L+2, H-1→L, H-4→L
H-4→L, H-3→L, H-1→L+1
H-3→L+1, H-4→L+1, H-3→L
H-1→L+2, H-4→L+1, H-3→L, H-3→L+1
H-3→L+2, H-4→L+2
288.89 0.0097
288.43 0.0056
275.23 0.0457
272.34 0.0139
259.67 0.0560
243.23 0.0526
241.26 0.3395
232.55 0.0127
240sh (1520)
202sh (4875)
H→L+6, H-2→L+6, H-2→L+7, H-6→L+1, H-6→L+2, 224.52 0.0165
H-6→L+1, H-5→L+1, H-2→L+6, H-7→L+1
H-7→L+1, H-5→L+1, H-11→L+1, H-10→L+1
H-6→L, H-6→L+3, H-5→L, H-6→L+1
223.13 0.0409
212.52 0.0665
212.12 0.0371
209.89 0.0790
208.60 0.0349
202.63 0.1843
H-3→L+4, H-1→L+4, H-1→L+5, H-1→L+6
H-7→L+3, H-6→L+3, H-6→L, H-7→L+1
H-1→L+4, H-1→L+6, H-6→L+7, H-1→L+8
3−
H→L, H→L+1
348.68 0.01154
H-1→L, H→L+1, H→L+2
H-3→L, H-3→L+3, H-2→L
345.01 0.06748 250sh (2350)
261.50 0.0110
H→L+5, H-3→L+1, H-2→L, H-2→L+1
H-2→L+1, H→L+5, H-2→L, H-2→L+1
H-3→L+2, H-2→L+2, H→L+7
H-5→L, H-4→L
H→L+7, H-2→L+1, H-2→L+2
H-1→L+6, H-3→L+4, H-2→L+3, H-2→L+4
236.69 0.01195
227.09 0.29123
221.06 0.04589
208.29 0.01127
205.46 0.63378
205.25 0.01982 205 (33,537)
λ wavelength (nm), ɛ molar absorption coefcient (dm3 mol−1 cm−1), f oscillator strength, H highest occu-
pied molecular orbital, L lowest unoccupied molecular orbital
1800 spectrophotometer. The magnetic susceptibility was
measured by Gouy method using HgCoN4C4S4 as the cali-
brant. The ESI mass spectra were recorded on a Waters
Qtof micro YA263 spectrometer in positive mode. TGA
analysis was performed using PerkinElmer model Pyris 6
TGA 4000 at heating rate of 10 °C/min under atmosphere
recorded using model Bruker AVANCE III HD Nanobay
400 MHz spectrometer. The products of bromination reac-
tions were analyzed using GC–MS instrument model Var-
ian 250-GC, Varian 240-MS using helium as the carrier
gas (1 cm3/min) equipped with Varian capillary column
(30 m × 0.25 mm × 0.39 mm).
1
of nitrogen. H NMR spectra with D2O as solvent were
1 3

Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies
Fig. 4 The MO diagram of 1−, 2−, 3− showing the character and energy (eV)
nitrate [57]. C, H, and N contents were determined by
micro-analytical method.
Elemental analyses
Tungsten was estimated gravimetrically as WO4C18H12N2
[56]. Peroxide contents were determined by redox titration
with standard solutions of KMnO4 and ceric ammonium
1 3

N. Das et al.
L+2
H-2
L+2
LUMO
HOMO
L+1
H-1
L+3
1-
H-3
L+3
L+1
H-1
LUMO
HOMO
2-
H-2
L+2
H-3
L+3
L+1
LUMO
3-
H-1
H-2
H-3
HOMO
Fig. 5 Selected HOMOs and LUMOs of 1−, 2−, and 3−. Positive values of the orbital contour are represented in yellow (0.03 au) and negative
values in blue (− 0.03 au) (color fgure online)
1 3

Peroxo–tungstate(VI) complexes: syntheses, characterization, reactivity, and DFT studies
nm (mol−1 dm3 cm−1), MS: m/z=494.76, 478.79, 394.85,
Computational methods
358.81.
The GAUSSIAN-09 Revision C.01 program package was
used for all calculations [58]. The gas phase geometries
of the molecular species (1−, 2−, 3−) were optimized fully
unrestricting symmetry in singlet ground states with BPV86
functionals. The basis set LANL2DZ with efective core
potential (ECP) was employed for W following the associ-
ated valence double ζ basis set of Hay and Wadt [59]. This
is in combination with 6–31+ +G** basis set selected for
hydrogen, carbon, nitrogen, oxygen and chlorine [60, 61].
The same basis sets and functionals were used for calcu-
lation of vibrational frequencies. The electronic spectra of
molecular ions (1−, 2−, and 3−) were calculated with the
TD-DFT method, and the solvent efect (aqueous solution)
was simulated using the polarizing continuum model with
the integral equation formalism (C-PCM) [62, 63].
Potassium(3‑aminobenzoato)(aquo)oxodiperoxo–
tungstate(VI)dihydrate (3, KWO10C7H12N) Yield 74.32%;
IR (KBr): ꢀ = 3502 br (ʋ O–H), 1606 (ʋasy CO2−), 1434
−
(ʋsym CO2 ), 914 (ʋ W=O), 829 (ʋ O–O), 734 (ʋasy W–O2),
565 (ʋsym W–O2), 3424 (ʋ C-H); (ʋ N–H) cm−1; UV–Vis
(water): λmax = 261.34 (2350) nm (mol−1 dm3 cm−1); MS:
m/z=494.75, 478.78, 410.80.
Bromination of organic substrates and product
analysis
In a representative procedure, organic substrates (1.0 mmol)
were added to a solution of acetonitrile/water (1:1, 8 cm3)
containing KBr (2 mmol). An accurately weighed amount of
compound 2 (1.0 mmol) was added to the reaction mixture
at room temperature with continuous stirring. Stirring was
continued for a further period of ca. 4–5 h. The completion
of the reaction was monitored by thin-layer chromatogra-
phy (TLC). The reaction products as well as the unreacted
organic substrates were separated by column chromatogra-
phy. The reaction products were analyzed by GC–MS using
benzophenone as internal standard.
Synthesis of complexes (general procedure)
The method consists of dissolving 1 g Na2WO6H4
(3.303 mmol) in 15 cm3 of water followed by gradual addi-
tion of 20 cm3 of 30% H2O2 (176.4 mmol) with continuous
stirring, keeping the reaction temperature below 4 °C in an
ice bath. To the resultant solution, benzoic acid or its deriva-
tive (3.303 mmol) dissolved in 10 cm3 ethanol was added in
small portions and stirring continued for a period of 15 min,
when reaction solution gradually changed from colorless to
light yellow. The pH of the reaction mixture at that stage
was found to be ca. 5–6. Potassium hydroxide solution (2 M)
was added dropwise with constant stirring to raise the pH of
the reaction medium to ca. 7–7.5. Addition of 50 cm3 pre-
cooled acetone to the reaction mixture with stirring led to
separation of white microcrystalline product. Product was
allowed to settle, separated by centrifugation, washed with
small volume of cold acetone and dried in vacuo over anhy-
drous CaCl2. In solid state, the compounds were found to be
stable in sealed polythene bags for prolong period at room
temperature.
Acknowledgements The authors are thankful to Tripura University
for providing the infrastructure facilities; Prof. A. Saikia, Department
of Chemistry, I.I.T Guwahati, for cooperation in recording of mass
spectra. N. D is grateful to Tripura University for award of research
fellowship (No. F.TU/ REG/Ph.D/(Admn)/01/12) RET (UGC, New
Delhi, India, sponsored). Authors are thankful to DST, Govt. of India,
for providing 400 MHz NMR spectrometer to Tripura University under
Fist program (SR/FST/CSI-263/2015). We are thankful to esteemed
reviewers for helpful comments at the stage of revision.
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