Synthesis, characterization, reactivity, and catalytic studies of heterobimetallic vanadium(V) complexes containing hydrazone ligands

Article abstract of DOI:10.1016/j.ica.2020.120068

Six heterobimetallic alkali metal dioxidovanadium(V) coordination polymer complexes {[M₆{VO(μ-O)}₂(μ-OH)₄(μ₄-slox/nph)].n DMF}_∞ where M = Na, K, and Cs; n = 1 for (1), 0 for (2)-(6) of two dihydrazone ligands, disalicylaldehydeoxaloyldihydrazone (H4slox) and bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (H4nph) are reported. All the complexes have been characterized by various physicochemical techniques such as elemental analyses, molar conductance, IR, NMR, UV–vis, and cyclic voltammetry. The IR, ¹HNMR, and ¹³CNMR spectral data suggest that the dihydrazones are coordinated through phenolate/naphtholate oxygen, enolate oxygen, and azine nitrogen atoms to the metal centres. The structure of complex {[Na₆{VO(μ-O)}₂(μ-OH)₄(μ₄-slox)].DMF}_∞ (1) is also determined by single-crystal X-ray data, which revealed that the H₄slox coordinated via all possible dative sites to metal centres as tetrabasic octadentate ligand. The vanadium metal centres adopted distorted square-pyramidal coordination geometries, and the sodium atoms are also in five coordination atmospheres. The electronic spectra of the complexes showed LMCT bands in addition to intra-ligand π → π* and n → π* transitions. As evident from the cyclic voltammetry, the complexes showed two metal-centred electron transfer reactions {[(V^VV^V(slox)^2?/V^VV^IV(slox)^3?] and [(V^VV^IV(slox)^3?/V^VV^IV(slox)^4?]}, in addition to the ligand centred electron transfer reactions. Further, bovine serum albumin (BSA interaction studies of the complexes {[Na₆{VO(μ-O)}₂(μ-OH)₄(μ₄-slox)].DMF}_∞ (1) and [Na₆{VO(μ-O)}₂(μ-OH)₄(μ₄-nph)]_∞ (4) revealed strong binding affinity. Moreover, the catalytic studies of the complexes (1) and (4) were found to be effective for the oxidation of alcohols into their corresponding aldehydes and ketones and bromination of some organic substrates in the presence of H₂O₂ as an oxidizing agent.

Full text of DOI:10.1016/j.ica.2020.120068

InorganicaChimicaActa515(2021)120068
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, characterization, reactivity, and catalytic studies of
heterobimetallic vanadium(V) complexes containing hydrazone ligands
⁎
Arvind Kumara, , Sunshine D. Kurbahb, Ibanphylla Syiemliehb, Shobha A. Dhanpata,
⁎
Rosmita Borthakurc, Ram A. Lalb,
a Department of Chemistry, Faculty of Science and Technology, The University of the West-Indies, St. Augustine Campus, St. Augustine, Trinidad and Tobago
b Department of Chemistry, Centre of Advanced Study, North-Eastern Hill University, Shillong 793022, Meghalaya, India
c Centre for Interdisciplinary Sciences Tata Institute of Fundamental Research, Hyderabad 500107, India
A B S T R A C T
Six heterobimetallic alkali metal dioxidovanadium(V) coordination polymer complexes {[M6{VO(μ-O)}2(μ-OH)4(μ4-slox/nph)].n DMF}∞ where M = Na, K, and Cs;
n = 1 for (1), 0 for (2)-(6) of two dihydrazone ligands, disalicylaldehydeoxaloyldihydrazone (H4slox) and bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone
(H4nph) are reported. All the complexes have been characterized by various physicochemical techniques such as elemental analyses, molar conductance, IR, NMR,
UV–vis, and cyclic voltammetry. The IR, 1HNMR, and 13CNMR spectral data suggest that the dihydrazones are coordinated through phenolate/naphtholate oxygen,
enolate oxygen, and azine nitrogen atoms to the metal centres. The structure of complex {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1) is also determined by single-
crystal X-ray data, which revealed that the H4slox coordinated via all possible dative sites to metal centres as tetrabasic octadentate ligand. The vanadium metal
centres adopted distorted square-pyramidal coordination geometries, and the sodium atoms are also in five coordination atmospheres. The electronic spectra of the
complexes showed LMCT bands in addition to intra-ligand π → π* and n → π* transitions. As evident from the cyclic voltammetry, the complexes showed two metal-
centred electron transfer reactions {[(VVVV(slox)2−/VVVIV(slox)3−] and [(VVVIV(slox)3−/VVVIV(slox)4−]}, in addition to the ligand centred electron transfer reac-
tions. Further, bovine serum albumin (BSA interaction studies of the complexes {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1) and [Na6{VO(μ-O)}2(μ-OH)4(μ4-
nph)]∞ (4) revealed strong binding affinity. Moreover, the catalytic studies of the complexes (1) and (4) were found to be effective for the oxidation of alcohols into
their corresponding aldehydes and ketones and bromination of some organic substrates in the presence of H2O2 as an oxidizing agent.
1. Introduction
catalysis [18,38-40], and synthesis of solid-state phases, which are es-
sential in the industry as well as technology [40-42]. Many hetero-
bimetallic complexes with µ-oxido-bridged motifs incorporated at their
active sites carry out a wide range of functions in the biological systems
[4,7,43-46]. µ-Oxido-bridged units in metalloproteins and me-
talloenzymes are important structural design in the chemistry of first-
row transition metals, whose active sites are capable of activating di-
oxygen molecules, which are responsible for the oxidation of organic
substrates involving µ-hydroxido species as key intermediates
[32,33,47-49].
Vanadium, a versatile element, possesses several oxidation states
ranging from −3 to +5 [1-3], and among them, the +4 and +5 are
most stable when associated with the oxo group [2,4-6]. Vanadium
complexes in these oxidation states present diverse coordination
chemistry and find applications in areas like pharmacology, biology,
and catalysis [4,7-11]. Vanadium complexes in high-oxidation states
have relevance in oxidation catalysis [10,12-14], and catalytic appli-
cations of oxidovanadium complexes in oxidation and oxido-transfer
catalysis have been reported [2,15-19]. The catalytic activities of oxi-
dovanadium complexes have been extensively researched both as
homogeneous and heterogeneous catalysts in oxidation reactions of
industrial importance in the presence of oxidants [20-24]. Since the
oxidation of alkanes is highly challenging due to their inert nature,
therefore, the selective oxidation of alcohols to carbonyl compounds is
recognized as crucial reactions in the synthesis of organic compounds
[6,15,25-31].
Owing to the diverse importance of µ-oxido-bridged heterodinuclear
vanadium compounds and their usefulness in catalytic and biomimetic
studies, we synthesized new heterobimetallic µ-oxido-bridged vana-
dium complexes from the polyfunctional dihydrazone ligands [50-52].
The ligands on complexation play an essential role in fine-tuning
properties such as catalytic, electrochemical, and photophysical prop-
erties of metal complexes [17,24,36,50,53,54]. It is important to
mention that both ligands, H4slox and H4nph possess as many as eight
coordinating sites that can bind to metal centres. All the probable co-
ordination possibilities of these ligands are shown as in structures
Heterometallic complexes containing two or more different metal
ions are of interest in several fields like multimetallic enzymes [32-37],
⁎ Corresponding authors.
E-mail addresses: Arvind.Kumar@sta.uwi.edu (A. Kumar), ralal@rediffmail.com (R.A. Lal).
https://doi.org/10.1016/j.ica.2020.120068
Received 2 January 2020; Received in revised form 23 September 2020; Accepted 4 October 2020
Availableonline08October2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
I–XIV.
v. Similarly, in the enol-form, it may show newer bonding possibilities.
As discussed above, it may coordinate as dibasic tridentate (VIII),
dibasic tetradentate (IX), tetrabasic hexadentate (X), or tetrabasic
octadentate (XI) ligand.
Thus, because of the presence of as many as eight coordination sites
and ability to exhibit keto-enol tautomerism, the ligand can coordinate
to metal ions in several ways as follows:
i. It may function as a neutral bidentate ligand coordinating either
through two carbonyl oxygen atoms (I) or through two secondary
amine nitrogen atoms (II). Both OH groups remain hydrogen-
bonded to azomethine groups in the complex.
ii. It may function as a monobasic bidentate ligand (III) coordinating
through one hydroxyl oxygen, and one azine group nitrogen or
monobasic tridentate ligand (IV) coordinating through one hydroxyl
oxygen, one azine group nitrogen, and one > C]O group oxygen
atom. In contrast, the other half of the ligand molecule remains
unbonded.
iii. It may function as a dibasic tetradentate ligand coordinating
through two hydroxyl oxygen atoms and the nitrogen atoms of both
azine groups (V). In contrast, the two carbonyl oxygen atoms re-
main unbonded, or dibasic hexadentate ligand (VI) bonded to the
central metal ion through the two hydroxyl atoms, two azine group
nitrogen atoms, and the two carbonyl oxygen atoms.
iv. iv It may function as a dibasic octadentate ligand (VII) bonding
through all the available dative sites.
2

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
An essential characteristic feature of these ligand skeletons is that it
2. Experimental
possesses the oxaloyl fragment which imposes the planarity on the two
hydrazone fragments. The present dihydrazones can exist in the stag-
gered configuration (XII) or cis configuration in the uncoordinated state
and as well as in the metal complexes. In cis-configuration, the dihy-
drazone can adopt either syn-cis configuration (XIII) or anti-cis config-
uration (XIV).
2.1. Materials and reagents
The chemicals V2O5, Na2CO3, K2CO3, Cs2CO3, 30% H2O2, KBr, 70%
HClO4, BSA, salicylaldehyde, 2-hydroxy-1-naphthaldehyde, alcohols,
aldehydes, and ketones were E-Merck, Himedia, or equivalent grades.
All these possibilities can be realized in practice if the metal ion to
the ligand molar ratio, nature of the metal salt, counter ion, reaction
medium, pH, and the temperature of the reaction medium are judi-
ciously tuned.
The solvents were reagent grade and used as received.
2.2. Physical measurements
The present work focused on synthesising alkali metal-vanadium
complexes from polydentate hydrazone ligands. We established their
composition by various elemental analyses and characterized them by
various physico-chemical techniques and spectroscopic methods as well
as single-crystal X-ray diffraction. Also, some of the newly synthesised
complexes were selected for interaction with BSA, and H2O2, in addi-
tion to the catalytic oxidation of alcohols and bromination of organic
substrates [40,55-59]. Hence, the present study reports the findings of
such investigations.
All operations were performed under aerobic conditions. Vanadium
was determined by the standard literature method [60]. C, H, and N
were determined using PerkinElmer 2400 CHNS/O Analyzer 11. Molar
conductances of the complexes in DMF solutions at 10−3 M dilution
were measured on a Direct Reading Conductivitymeter-303 with a dip
type conductivity cell at room temperature. BX-III/FTIR Perkin Elmer
Spectrophotometer was used to record FTIR in the region
4000–200 cm−1 in KBr discs. The 1H NMR and 13C NMR spectra were
recorded in DMSO‑d6 solution using TMS as an internal standard on
Bruker Avance II 400 MHz and 100 MHz, respectively. Perkin-Elmer
Lambda-25 spectrophotometer was used to obtain electronic spectra.
Fig. 1. Structure of (a) disalicylaldehydeoxaloyldihydrazone (H4slox), and (b) bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (H4nph).
3

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Fig. 2. Preparation methodology of the complexes (1)–(6).
Cyclic voltammogram was recorded on a CH Electrochemical Analyzer
using a standard three-electrode assembly (glassy-carbon working, Pt
wire auxiliary, SCE reference). Tetra-n-butyl ammonium perchlorate
(TBAP, 0.1 M) was a supporting electrolyte.
(20 mL) was stirred vigorously at 70 °C. To this suspension, vanadium
pentoxide (0.18 g, 1 mmol) in methanol (10 mL) was added dropwise
followed by the dropwise addition of aqueous sodium carbonate
(0.21 g, 2 mmol), and sodium perchlorate (0.24 g, 2 mmol). Thus, the
obtained reaction mixture was stirred for 30 mins at 70 °C and then
refluxed for 2 h. The heating of the reaction mixture was stopped and
allowed to cool to room temperature. The solid precipitated was filtered
and washed with cold methanol. The isolated solid was air-dried and
stored in a desiccator over anhydrous CaCl2. Single crystal of the
complex (1) was obtained by slow evaporation from the solution of this
solid in DMF/water systems (1:1.5, v/v) at room temperature in a
fortnight.
2.3. Single crystal X-ray diffraction
Single crystal X-ray crystallographic data for the complex (1) was
recorded using X-Calibur, Eos, Gemini diffractometer equipped with
monochromated Mo-K radiation ((k = 0.71073 Å) source. For data
collection and reduction, CrysAlis PRO, Agilent, 2013 software package
was used. SHELXT-2014 and SHELX-2014 [61,62] were used for
structure solutions and refinements. The structure was solved by direct
methods and refined on F2 by a full matrix least squares [61]. All non-
hydrogen atoms were refined, anisotropically, whereas the hydrogen
atom positions were isotropically refined freely in the final refinement.
For the preparation of the corresponding complex (2) or (3) from
the ligand H4slox, the aqueous solutions of potassium carbonate and
potassium perchlorate or caesium carbonate, and caesium perchlorate
were added to the suspension of H4slox and V2O5 in methanol. The
resulting reaction mixture was subjected to the above described syn-
thetic methodology, and the respective complexes were isolated.
Similarly, in order to prepare the complexes (4), (5), and (6), H4nph
was used instead of H4slox and essentially following the preparation
method as described above.
2.4. Preparation of ligands
The ligands disalicylaldehydeoxaloyldihydrazone (H4slox) and bis
(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (H4nph) were syn-
thesized according to the procedure previously reported by us [63,64].
The oxaloyldihydrazide was prepared from the reaction of two moles of
hydrazine with oxalic ester. The oxaloyldihydrazide thus obtained was
reacted with respective aldehydes in 1 : 2 M ratio to give the desired
dihydrazones. The structures of both ligands H4slox and H4nph are
shown in Fig. 1.
2.5.1. Synthesis of {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1)
Yield: 80%. Colour: Yellow. Anal. (%), Calc. for C19H21N5O13Na6V2
(MW: 767.22 g/mol); C, 29.74; H, 2.76; N, 9.13. Found: C, 29.76; H,
2.75; N, 9.14. Molar conductance, Λm: 174.21 Ω−1cm2 mol−1. IR data
(cm−1, KBr): 3435 (s) ν(OH + NH); 1608 (s) ν(-C]N); 1474 (s)
ν(NCO–); 936 (w), 914 (s) ν(V]O); 576 (s) ν(V-O). 1HNMR (400 MHz,
DMSO‑d6, Me4Si): δ = 9.01 (s, 2H), 7.94 (s, 1H), 7.54–6.80 (m, 8H, Ar-
H) ppm. 13C NMR (100 MHz, DMSO‑d6, Me4Si): δ = 165.85, 165.36,
158.37, 134.26, 133.25, 120.19, 119.98, 117.47, 40.47–39.22 ppm.
Electronic spectrum in DMF solution, λmax (nm): 304, 329, 414. CV: Epc
(V): +1.32, +0.24, −0.59, −0.97; Epa(V): +1.47, +0.56, −0.44,
−0.91.
Disalicylaldehydeoxaloyldihydrazone (H4slox): 1H NMR (400 MHz,
DMSO‑d6): δ = 12.74 (s, 1H, –OH), 11.09 (s, 1H, –NH), 8.89 (s, 1H,
-C(H)]N-), 7.65–7.00 (m, 4H, Ar-H) ppm.
Bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (H4nph): 1H
NMR (400 MHz, DMSO‑d6): δ = 12.80 (d, J = 32 Hz, 1H, –OH), 12.57
(d, J = 32 Hz, 1H, –NH), 9.98 (s, 1H, -C(H)]N-), 9.72 (s, 1H, -C(H)]
N-), 8.21–7.22 (m, 6H, Ar-H) ppm.
2.5. General procedure for the synthesis of complexes
2.5.2. Synthesis of [K6{VO(μ-O)}2(μ-OH)4(μ4-slox)]∞ (2)
Yield: 78%. Colour: Yellow. Anal. (%), Calc. for C16H14N4O12K6V2
(MW: 790.78 g/mol); C, 24.30; H, 1.78; N, 7.09. Found: C, 24.33; H,
A suspension of ligand, H4slox (0.32 g, 1 mmol) in methanol
4

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
1.77; N, 7.10. Molar conductance, Λm: 175.34 Ω−1cm2 mol−1. IR data
(cm−1, KBr): 3456 (s, br) ν(OH + NH); 1636 (s), 1611 (s) ν(-C]N);
1473 (w) ν(NCO–); 943 (s), 909 (w) ν(V]O); 575 (w) ν(V-O). 1H NMR
(400 MHz, DMSO‑d6, Me4Si): δ = 8.91 (s, 1H, C(H)]N), 8.15 (s, 1H,
C(H)]N), 7.41–6.73 (m, 8H, Ar-H) ppm. 13C NMR (100 MHz,
DMSO‑d6, Me4Si): δ = 165.92, 165.39, 158.35, 134.20, 133.26, 120.18,
120.07, 117.44, 40.49–39.24 ppm. Electronic spectrum in DMF solu-
tion, λmax (nm): 304, 326, 413. CV: Epc(V): −0.32, −0.92, −1.60; Epa
(V): +1.11, +0.31, −0.82.
(0.03 g, 0.05 mmol) were taken. The reaction mixture was stirred at
ambient temperature for 20 min and raised to 70 °C for 6 h. The crude
product was extracted with CH2Cl2 and dried over anhydrous sodium
sulfate. The purified product was isolated by column chromatography.
2.7. General procedure for oxidative bromination
Salicylaldehyde (0.24 g, 2 mmol) was taken in a 50 mL round
bottom flask maintained at 0 °C, to this added KBr (0.47 g, 4 mmol) in
5 mL water followed by the addition of 30% H2O2 (1.70 g, 15 mmol).
To this reaction mixture, 2.5 mol percent of the complex (1) or (4)
(0.05 mmol) and 70% HClO4 (0.14 g, 1 mmol) were added and stirred
at 0 °C. An additional 1 mmol of 70% HClO4 was added in three equal
portions at an interval of 10 min into the reaction mixture. A light
brown solid precipitate was obtained, which was extracted in CH2Cl2
and dried. The crude solid was recrystallized by dissolved in methanol
and further purified by silica column chromatography. The brominated
products were confirmed by IR and 1H NMR spectroscopies. The 1H
NMR spectra of the brominated products are given in the supporting
information.
2.5.3. Synthesis of [Cs6{VO(μ-O)}2(μ-OH)4(μ4-slox)]∞ (3)
Yield: 75%. Colour: Yellow. Anal. (%), Calc. for C16H14N4O12Cs6V2
(MW: 1353.62 g/mol); C, 14.20; H, 1.04; N, 4.14. Found: C, 14.22; H,
1.03; N, 4.15. Molar conductance, Λm: 178.90 Ω−1cm2 mol−1. IR data
(cm−1, KBr): 3445 (s, br) ν(OH + NH); 1636 (w), 1607 (s) ν(-C]N);
1472 (m) ν(NCO–); 948 (s), 910 (w) ν(V]O); 573 (m) ν(V-O). 1H NMR
(400 MHz, DMSO‑d6, Me4Si): δ = 8.83 (s, 1H, C(H)]N), 7.94 (s, 1H,
C(H)]N), 7.35–6.60 (m, 8H, Ar-H) ppm. 13C NMR (100 MHz,
DMSO‑d6, Me4Si): δ = 165.51, 164.94, 162.30, 157.67, 133.64, 132.71,
119.70, 119.52, 116.84, 40.02–30.71 ppm. Electronic spectrum in DMF
solution, λmax (nm): 302, 334, 407. CV: Epc(V): −0.31, −0.94, −1.65;
E
pa(V): +1.06, +0.35, −0.75.
3. Results and discussion
2.5.4. Synthesis of [Na6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (4)
The complexes were prepared from the reaction of vanadium
pentoxide, ligand H4slox/H4nph, carbonates of sodium/potassium/
caesium, and perchlorates of sodium/potassium/caesium in 1:1:2:2 M
ratios in methanolic solutions under reflux for 2 h. A general scheme of
preparation methodology is shown in Fig. 2. All the complexes are air-
stable and melt with decomposition above 300 °C. The complexes are
insoluble in most commonly used organic solvents such as EtOH,
CH3CN, CH2Cl2, CHCl3, CCl4, ether, and benzene, while sparingly so-
luble in MeOH and H2O but soluble in highly polar solvents like DMF
and DMSO. The complexes or polymeric and consist of the following
Yield: 75%. Colour: Orange. Anal. (%), Calc. for C24H18N4O12Na6V2
(MW: 794.24 g/mol); C, 36.29; H, 2.28; N, 7.05. Found: C, 36.55; H,
2.25; N, 7.07. Molar conductance, Λm: 176.30 Ω−1cm2mol−1. IR data
(cm−1, KBr): 3434 (s), 3051 (w) ν(OH + NH); 1618 (s), 1600 (s) ν(-C]
N); 1553 (s) ν(NCO–); 931 (s), 914 (s) ν(V]O); 583 (s) ν(V-O). 1H NMR
(400 MHz, DMSO‑d6, Me4Si): δ = 10.02 (s, 1H, C(H)]N), 8.49 (s, 1H,
C(H)]N), 8.02–7.15 (m, 12H, Ar-H) ppm. 13C NMR (100 MHz,
DMSO‑d6, Me4Si): δ = 166.45, 164.76, 154.22, 135.16, 133.47, 129.30,
128.19, 127.29, 123.39, 120.66, 110.34, 40.50–39.24 ppm. Electronic
spectrum in DMF solution, λmax (nm): 325, 351, 378, 444. CV: Epc(V):
+0.65, −0.42, −1.01, −1.64; Epa(V): +0.99, +0.30, −0.75.
compositions
{[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞
(1),
[K6{VO(μ-O)}2(μ-OH)4(μ4-slox)]∞ (2), [Cs6{VO(μ-O)}2(μ-OH)4(μ4-
slox)]∞ (3), [Na6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (4), [K6{VO(μ-
O)}2(μ-OH)4(μ4-nph)]∞ (5), and [Cs6{VO(μ-O)}2(μ-OH)4(μ4-
nph)]∞ (6), respectively based on the analytical data. The analytical
data obtained for the complexes are in agreement with the proposed
structural formula. The observed magnetic moments of these complexes
(1)–(6) were found to be zero indicating that they are all diamagnetic.
This fact suggests that vanadium metal is present in the +5-oxidation
state with d0 electronic configuration in all the complexes. The molar
conductance values of the complexes (1)–(6) were found to be in the
range 174.21–179.82 Ω−1cm2mol−1 at 10−3 M dilution in the aqueous
medium, which are typical molar conductance values for 2:1 electrolyte
[65-67]. Probably, ion-pair species of the type involving alkali metal
and hydroxyl, {M3(μ-OH)2}1+, and dioxovanadium complexed with
ligands, {VO2(slox/nph)}2− were formed.
2.5.5. Synthesis of [K6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (5)
Yield: 72%. Colour: Orange. Anal. (%), Calc. for C24H18N4O12K6V2
(MW: 890.90 g/mol); C, 32.36; H, 2.04; N, 6.29. Found: C, 32.42; H,
2.03; N, 6.30. Molar conductance, Λm: 177.91 Ω−1cm2 mol−1. IR data
(cm−1, KBr): 3421 (s), 3045 (s) ν(OH + NH); 1619 (s), 1601 (s) ν(-C]
N); 1544 (s) ν(NCO–); 964 (w), 923 (s) ν(V]O); 587 (s) ν(V-O). 1H
NMR (400 MHz, DMSO‑d6, Me4Si): δ = 9.98 (s, 1H, C(H)]N), 8.51 (s,
1H, C(H)]N), 8.01–7.15 (m, 12H, Ar-H) ppm. 13C NMR (100 MHz,
DMSO‑d6, Me4Si): δ = 166.42, 164.81, 154.21, 135.12, 133.47, 129.24,
128.17, 127.30, 123.44, 123.35, 120.81, 110.44, 40.51–39.26 ppm.
Electronic spectrum in DMF solution, λmax (nm): 315, 408. CV: Epc(V):
+0.69, −0.41, −0.99, −1.62; Epa(V): +1.12, −0.78.
2.5.6. Synthesis of [Cs6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (6)
Yield: 68%. Colour: Orange. Anal. (%), Calc. for C24H18N4O12Cs6V2
(MW: 1187.93 g/mol); C, 19.83; H, 1.25; N, 3.85. Found: C, 19.88; H,
1.23; N, 3.87. Molar conductance, Λm: 179.82 Ω−1cm2 mol−1. IR data
(cm−1, KBr): 3430 (s, br), ν(OH + NH); 1620 (s), 1600 (s) ν(-C]N);
1547 (s) ν(NCO–); 947 (w), 926 (s) ν(V]O); 589 (m) ν(V-O). 1H NMR
(400 MHz, DMSO‑d6, Me4Si): δ = 9.85 (s, 1H, C(H)]N), 8.38 (s, 1H,
C(H)]N), 7.92–7.05 (m, 12H, Ar-H) ppm. 13C NMR (100 MHz,
DMSO‑d6, Me4Si): δ = 166.40, 165.03, 153.61, 135.05, 133.43, 129.25,
128.27, 127.25, 123.35, 120.53, 110.31, 40.46–39.20 ppm. Electronic
spectrum in DMF solution, λmax (nm): 298, 329, 419. CV: Epc(V):
+0.68, −0.42, −1.04, −1.69; Epa(V): +1.13, −0.66, −1.31.
3.1. Infrared spectra
The ligand H4slox displays strong bands at 3434, 3278, 3207, 3150;
1669, and 1620, 1603 cm−1 while H4nph at 3622, 3492, 3161; 1682
and 1621, 1594 cm−1. These bands indicate the keto form of both the
ligands in the solid-state assigned to ν(OH + NH); νC]O, and νC]N
vibrations, respectively. The ligands H4slox and H4nph also show
strong bands at 1536; 1276, 1262 cm−1, and 1536; 1284 cm−1, which
are attributed to amide II + ν(CeO)(phenolic/naphtholic) and β(CeO),
respectively.
Complexes (1)–(6) show
a
broad band in the region
3000–3600 cm−1, which is due to the vibration of –OH present in the
complexes, and the broad nature indicates the presence of hydrogen
2.6. General procedure for catalytic oxidation of benzyl alcohol
bonding. All the complexes show
a strong band in the region
In
a
50 mL round bottom flask, benzyl alcohol (0.30 mL,
1600–1636 cm−1, which is assigned to ν(C]N) [37,68,69], and the
2.90 mmol), 15% H2O2 (1.31 mL, 5.80 mmol), complexes (1) or (4)
new bands observed in the region 1472–1553 cm−1 are due to the
5

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
vibration of newly created NCO– group characteristic of enol form of
dihydrazone ligand [3,17,70]. The new bands observed in the region
909–964 cm−1 characterize oxido vanadyl (VO2) stretching vibrations
[71]. These bands split into two peaks of weak and strong in nature:
914, 936; 909, 943; 910, 948; 914, 931; 923, 964; and 926, 947 cm−1
for complexes (1) to (6), respectively. The presence of two bands in the
spectra of the VO2 complexes in the region 909–964 cm−1 indicates the
presence of the cis-VO2 structure, which are characteristic of νsym(VO2)
and νasym(VO2) vibration modes of the vanadyl group, respectively
[12,13,44]. The low-intensity bands at 914, 909, 910, 914, 923, and
926 cm−1 in complexes (1) to (6) are the stretching vibrations of the
V]O group in the square base, which is µ-oxo bridged between vana-
dium and alkali atoms, Na/K/Cs, while the strong bands at 936, 943,
948, 931, 964 and 947 cm−1 in complexes (1) to (6) assigned to
stretching vibration of axial V]O group which are not involved in oxo-
bridging. The crystal structure of (1) further confirmed the character-
istic of VO2, which is discussed later. We assigned the weak bands at
576, 575, 573, 583, 587, and 589 cm−1 to V-O stretching vibrations
originating from the bonding of phenolate/naphtholate oxygen atoms
and enolate oxygen atoms to the metal centres in complexes (1) to (6),
respectively [37,68].
centre. A sizeable downfield shift of corresponding carbon atom by
Δδ = 9.45–10.38 ppm is indicative of the enolisation and the co-
ordination of the carbonyl group. The signals of aromatic carbon atoms
appeared in the region as expected in both the ligands and complexes
with no or slight shifts in their positions.
3.3. Electronic spectra
The electronic spectra of heterobimetallic alkali-vanadium(V)
complexes were recorded in the DMF solution, as shown in Figs.
S17–S22.
The free dihydrazone ligand H4slox showed two absorption bands
at 304 and 340 nm while the free H4nph ligand showed three bands,
and each band appeared as a dual-band in the spectrum giving rise to
total six bands, which are 318, 330; 375, 391; and 433, 486 nm, re-
spectively. The bands at 304 nm in H4slox, and 318 and 330 nm in
H4nph are originated from intraligand π → π* transition centred on the
phenyl/naphthyl rings. The bands at 340 nm, and 375 and 391 nm are
attributed to n → π* transition centred at > C]O and > C]N groups,
respectively, which are characteristic of salicylaldimine/naphthaldi-
mine transitions of the ligand molecule [72-75]. It is imperative to
mention that each band in H4nph split into two bands, which suggested
that the dihydrazone ligands exist in anti-cis conformation in the free
state.
3.2. Nuclear magnetic resonance spectroscopy
3.2.1. 1H NMR spectra
The absorption bands of free dihydrazones due to π → π* and n →
π* transitions were shifted from their original position on complexation
[63,76]. The electronic spectra of the complexes (1) – (3) showed three
absorption bands. The first absorption bands at 304 nm
(9375 cm−1 M−1), 304 nm (7500 cm−1 M−1), and 302 nm
(9833 cm−1 M−1) are due to the intra-ligand π → π* transitions. The
second absorption bands at 329 nm (9625 cm−1 M−1), 326 nm
(7250 cm−1 M−1), and 334 nm (10000 cm−1 M−1) are also due to
The 1H NMR spectra of the free dihydrazones and the complexes (1)
to (6) are shown in Figs. S1–S8.
1H NMR spectrum of H4slox ligand showed three signals at δ 12.78,
11.09, and 8.89 ppm, while H4nph ligand shows six signals at δ 12.90,
12.81, 12.60, 12.53, 9.98, and 9.72 ppm, which are assigned to –OH,
–NH, and –CH]N- protons, respectively. The multiplet in the region δ
7.00–8.40 ppm is attributed to phenyl/naphthyl protons. In H4slox li-
gand, the appearance of –OH, –NH, and –CH]N signals as singlets
suggested that the free ligand existed in staggered (XII) conformation.
While in H4nph ligand, the appearance of more than three signals in
the region δ 9.72–12.90 ppm and two signals corresponding to –CH]N-
protons. Such a feature revealed that it existed as a mixture of staggered
(XII) and anti-cis (XIV) conformations in DMSO solution. The signals in
the region δ 11.09–12.90 ppm disappeared on complexation, which
indicated the deprotonation of both –OH and –NH and bonding of
phenolate/naphtholate oxygens as well as carbonyl oxygen via en-
olization to the metal centre. The phenyl/naphthyl protons for the
complexes appeared as multiplet in the region δ 6.60–8.52 ppm. It was
to be noted that the 1H NMR spectrum of the complex (1) showed peaks
at δ 2.87 and 2.71 ppm, respectively. After comparing the 1H NMR
spectrum with those of the other complexes, the signal at δ 2.71 ppm
was attributed to the methyl groups of dimethylformamide present in
the lattice structure. The peak due to –NH proton of DMF was not ob-
served probably because of the quadrupolar nature of the nitrogen
nucleus, which might have broadened the proton NMR signal of the
–NH group.
intra-ligand
n → π* transitions. The third bands at 414 nm
(4625 cm−1 M−1), 413 nm (4000 cm−1 M−1), and 407 nm
(3833 cm−1 M−1) are due to the ligand-to-metal charge transfer tran-
sition (LMCT) in the respective metal complexes. The LMCT band is
arising from the transfer of charge from filled p-orbital phenolate of
oxygen to an empty d-orbital of the vanadium ion, as the d-d transition
is not possible owing to no electron in the d-orbital (d0) of vanadium(V)
ion [77-80]. Similarly, the complexes (4)–(6) showed two to four ab-
sorption bands in the region 298–474 nm, namely 325 nm
(8000 cm−1 M−1), 351 nm (6800 cm−1 M−1), 378 nm
(5200 cm−1 M−1) for complex (4); 315 nm (8600 cm−1 M−1) for
complex (5); and 298 nm (8000 cm−1 M−1), 329 nm (8000 cm−1 M−1
)
for complex (6) were assigned to the intra-ligand π → π* and n → π*
transitions, whereas the bands at 444 nm (4200 cm−1 M−1) for
Table 1
Crystal data and structure refinement for complex (1).
Complex (1)
Empirical formula
C11H14N3O6NaV
358.18
3.2.2. 13C NMR spectra
Formula weight
Temperature/K
293(2)
The free dihydrazone ligands and the complexes were also char-
acterized by 13C NMR spectroscopy (Figs. S9–S16). The assignments for
the ligand signals have been deduced by considering the shift of pi-
electrons in resonances of phenyl/naphthyl ring carbon atoms caused
by the substituents, azomethine group, and phenolic/naphtholic –OH
group [68-70].
Crystal system
triclinic
Space group
P −1
a/Å
7.5158(7)
b/Å
8.7454(7)
c/Å
12.3213(9)
α/°
87.180(6)
β/°
83.741(7)
The azomethine carbons in free ligands H4slox and H4nph ap-
peared at 150.87 ppm and 150.34 ppm, which on complexation showed
a downfield shift to 158.37, 158.35, 162.30, 154.22, 154.21, and
153.61 ppm for the complexes (1) to (6), respectively. A downfield shift
in the 13C NMR chemical shift of –CH]N by Δδ (3.27–11.43 ppm) in
the complexes is due to the deshielding of azomethine carbon. It is
because of the coordination of azomethine nitrogen atoms to the metal
γ/°
66.648(8)
Volume/Å3
739.08(11)
Z
2
Radiation
MoKα (λ = 0.71073)
3332/0/205
R1 = 0.0334, wR2 = 0.0884
R1 = 0.0399, wR2 = 0.0928
Data/restraints/parameters
Final R indexes [I > =2σ (I)]
Final R indexes [all data]
6

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Fig. 3. (a) Molecular structure (ellipsoid probability 50%), hydrogen atoms and solvent molecules are removed for clarity, and (b) 2D-polymeric structure, the
solvent molecules are not shown for clarity, of the coordination polymer complex (1) (colour code: Na, purple; V, cyan; O, red; C, grey; and N, blue; H, light grey).
(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
complex (4); 408 nm (5800 cm−1 M−1) for complex (5); 419 nm
(5000 cm−1 M−1) for complex (6) were due to the LMCT, which arisen
from the p-orbital of the naphtholate oxygen to an empty d-orbital of
the vanadium(V) centre [68,81-85].
redox couple with E1/2 = −0.005 V (ΔE = 630 mV) and E1/
2 = −0.02 V (ΔE = 660 mV) correspond to the reduction of the metal
ion (VVVV/VVVIV). The third redox couple for the complexes, (1) and
(4), appeared at E1/2 = −0.52 V with ΔE = 150 mV) and E1/
2 = −0.88 V with ΔE = 260 mV, respectively. This is associated with
the reduction+oe f the metal ion+(eVVVIV/VIVVIV) as shown below.
3.4. Cyclic voltammetry
[VVVV (L)]2
[VVVIV (L)]3
[VIVVIV (L)]4
e
Electrochemical studies were recorded in DMF solution at a scan
rate of 100 mV/s using 0.1 M tetrabutylammonium phosphate (TBAP)
as supporting electrolyte. The cyclic voltammograms of the complexes
are shown in Figs. S23–S28.
e
The second redox couple in complexes (2) and (3) appeared at E1/
= −0.87 V with ΔE = 100 mV and E1/2 = −0.84 V with
2
ΔE = 190 mV, respectively. It is associated with the reduction of the
metal ion (VVVIV/VIVVIV). The fourth redox couple for the complex (1),
at −0.94 V (E1/2) with peak-to-peak separation equal to 60 mV, is re-
versible and is close to the reduction peak at −1.32 V in the free ligand
H4slox. The complexes (2) and (3) showed only a reduction wave in the
above region. This process is, most likely, due to VIVVIV/VIVVIII redox
couple and some contribution from the ligand-based reduction. The
third redox couple for the complex (6) is visible at E1/2 = −1.50 V with
ΔE = 380 mV ΔE = 380 mV [86-88]. It is close to the reduction peak in
the free ligand H4nph at −0.98 V, and the complexes (4) and (5) show
only a reduction wave in the above region. Hence it is suggested that
this redox process is, most likely, due to (VVVIV/VIVVIV) redox couple
along with some contributions from the ligand-based reduction. We
cannot categorize these redox couples as reversible or quasi-reversible
or irreversible since our studies were limited to only at a scan rate of
100 mV/s, and we could not study at a variable scan rate.
The voltammograms displayed two redox couples for the complexes
(2), (3), and (5); three redox couples for the complexes (4) and (6); and
four redox couples for the complex (1), apart from some reductive and
oxidative waves. On the cathodic side, the complexes (2)–(6) showed
reduction waves at −1.60; −1.65; −1.64; −0.41 and −1.62; and
−0.42 V while on the anodic side, the complexes (2) and (3) showed an
oxidation wave at +1.11 V; and +1.06 V, respectively. In the above-
mentioned potential range, these reductive and oxidative waves did not
have their counterparts in the returning scan. Hence, corresponding to
these reductive and oxidative waves, the species produced were either
unstable in DMF solution or highly reactive, which had some new
species on the time scale of cyclic voltammetry. In all the complexes,
the cathodic potentials were within the range of the values reported for
the reduction (VV/VIV) of the hydrazone vanadium complexes [79].
The comparison of voltammograms of the complexes with the cor-
responding free dihydrazone ligands, the reductive wave in the region
−0.97 V to −1.72 V and oxidative wave in the region −0.91 V to
−1.90 V appeared very near to the reductive waves in the region
−0.98 V to −1.32 V and oxidative wave at −0.95 V in the free ligands.
Similarly, for complexes (4)–(6), the reductive wave in the region
−0.41 V to −0.42 V was very close to the reductive wave at −0.34 V
of H4nph. Hence, these waves in the metal complexes arisen due to
ligand centred electron transfer reactions.
4. Crystal structure of {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞
(1)
A good quality yellowish coloured needle-like crystals of 1 were
grown from a saturated solution in DMF : H2O (3:2, v/v) by slow
evaporation at room temperature. Its crystal structure reveals the mo-
lecular structure of the complex (1) – the crystallographic data and
structural refinement details are summarized in Table 1. The cif file can
be accessed from The Cambridge Crystallographic Data Centre (CCDC)
by using the CCDC Number 1919258. It crystallises as a triclinic P-1
space group (a = 7.5158(7) Å, b = 8.7454(7) Å, c = 12.3213(9) Å,
α = 87.180(6)°, β = 83.741(7)°, and γ = 66.648(8)°). A 50% ellip-
soidal structure of the complex (1) is shown in Fig. 3(a). The crystal
structure of the complex (1) showed that staggered form (XII) of the
ligand is bonded with two metal centres, i.e., cis-VO2 in tetrabasic bis
(tridentate) through (ONO) donor groups forming five and six-mem-
bered chelate rings at the vanadium metal centre with a bite angle of
73.25(5)° and 81.98(5)°, respectively. Both vanadium metal centres are
coordinated symmetrically to ligand H4slox and form trans-square
pyramidal coordination geometries with N1O3 basal plane where
The first redox couple of the complexes (1) and (4)–(6) appeared as
the reductive wave in the region +0.65 to +1.32 V and the oxidative
waves in the region +0.99 to +1.47 V with E1/2 = +1.40 V
(ΔE = 150 mV), +0.82 V (ΔE = 340 mV), +0.91 V (ΔE = 430 mV),
and +0.91 V (ΔE = 450 mV), respectively, were most probably ori-
ginating from
a
ligand-based oxidation involving one of
azomethine > C]N- moieties. Similarly, for complexes (2) and (3), the
oxidative wave at +1.11 V and +1.06 V, respectively with no counter
parts correspond to the oxidation of one azomethine > C]N- moieties
of dihydrazone. The second redox couple for the complexes (1), and (4)
to (6) appeared at E1/2 = +0.40 V (ΔE = 320 mV), E1/2 = −0.06 V
(ΔE = 720 mV), E1/2 = −0.88 V (ΔE = 210 mV) and E1/2 = −0.85 V
(ΔE = 380 mV), respectively, are attributed to the reduction of the
metal ion (VVVV/VVVIV). Similarly, for complexes (2) and (3), the first
7

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Fig. 4. (a) Staircase like the structure, the H-atoms, and solvent molecules are removed for clarity and (b) solvent (DMF) molecule interaction with coordination
polymer complex (1) within the staircase network.
Fig. 5. (a) A view of packing diagram along a-axis, and (b) a view of packing diagram along b-axis of coordination polymer complex (1) along with solvent (DMF)
molecules.
angles 149.51(6)° and 138.89(8)° for (O1-V1-O2), and (N1-V1-O4) in-
dicate that the vanadium centre is shifted (0.5275 Å) out of the basal
plane towards the apical oxygen O3, a similar shift is also reported by
Dutta and co-workers [89]. This {(VO2)2(slox)} unit, thus formed, is
bonded to four sodium atoms through the basal oxido oxygen atoms,
enolate oxygens, and hydrazine nitrogen atoms, which can be re-
presented as {Na4{VO(μ-O)}2(slox)}. The sodium atom (Na1) bonded
through the bridging oxido oxygen (μ-O4) of cis-VO2 is further attached
via (μ-OH) bridging to another sodium atom of the next {Na4{VO(μ-
O)}2(slox)} unit as described above and also connected to the enol
oxygen and hydrazine nitrogen of the third {Na4{VO(μ-O)}2(slox)}.
Thus each sodium adopted a five-coordinate distorted square pyramidal
geometry. The bond lengths of sodium bonded to the basal oxido
oxygen O4 of cis-VO2 (Na1-O4) is 2.3364(16) Å, the enolate oxygen
(Na1-O2) is 2.3293(14) Å, the hydrazone nitrogen (Na1-N2) is
2.5398(14) Å, and the two bridging hydroxyls (μ-OH) bond lengths are
2.382(2) Å, and 2.4230(2) Å. Therefore, the polymeric coordination
structure of the complex (1) is evolved from the involvement of sodium
atoms in the bonding network (see Fig. 3b). From Fig. 3(b) it is ap-
Fig. 6. UV–Vis absorption spectrum of {[Na6{VO(μ-O)}2(μ-OH)4(μ4-
slox)].DMF}∞ (1) titrated against methanolic H2O2 solution.
parent that the sodium atoms are the central point of bonding among
{(VO2)2(slox)} units, thus making sodium bridged molecular network.
Such bonding thus created a system of octanuclear metallacycle com-
prised of two sodium, two vanadium, and four oxygen (Na2V2O2).
Therefore, a chain of intermolecular binding of the vanadium complex
unit {(VO2)2(slox)} with sodium atoms bridged with hydroxyls gave an
infinite network of a staircase-like structure as shown in Fig. 4(a).
Dinda and co-workers [90] reported a staircase like polymeric structure
in which the hydrogen-bonding and electrostatic attractions between
anionic vanadium complex and aquated cationic metal centres are
central to polymeric structure. In our polymeric structure, the bonding
network is formed by the coordination and covalent bonding among the
vanadium complex, sodium atoms, and hydroxyl groups. The hydrogen-
bonding interaction of dimethylformamide (DMF) solvent molecules in
the polymeric complex are shown in Fig. 4(b) in which solvents in the
lattices are strongly hydrogen-bonded to the bridging –OH with a hy-
drogen-bond length of 2.711 Å. A view of packing diagrams along a-
and b-axes are shown in Fig. 5. It is evident that the solvent molecule
resides in between the layers of the polymeric structure. It also revealed
that the chain-like polymeric growth is along the a-axis (Fig. 5(a)) while
nitrogen, and two oxygens are from the ligand. The fourth basal posi-
tion is occupied by one of the oxido oxygens of VO2 group. The second
oxido oxygen of VO2 occupies the apical position of the square pyr-
amidal geometry. The four pyramidal bond angles, i.e., apical terminal
oxygen, O3 with basal atoms of the square plane are O3-V1-
O1 = 103.93(8)°, O3-V1-O2 = 101.38(7)°, O3-V1-O4 = 109.64(8)°,
and O3-V1-N1 = 110.89(7)°, which are not orthogonal. Further, the
four basal angles of the square plane are N1-V1-O1 = 81.98(6)°, N1-V1-
O2 = 73.25(6)°, O1-V1-O4 = 94.81(7)°, and O2-V1-O4 = 92.43(7)°
which also deviated from the 90°. Hence, the coordination geometry at
both vanadium central metal ion is highly distorted square oyramidal.
The V-O bond lengths are 1.9242(12) Å (V1-O1phenolate), 1.9894(12)
Å [V1-O2enolate] 1.6344(13) Å (V1 = O4basaloxido), 1.6035(16) Å (V1-
O3apicaloxido). It shows that phenolate oxygen coordinates stronger than
the enolate oxygen, and the apical oxido (V1 = O3) bond is shorter
than the basal oxido (V1 = O4) bond, which is in accordance with the
reported bond lengths for square pyramidal V(V) complexes. The bond
8

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Fig. 7. Fluorescence quenching spectra of BSA in the presence of complex {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1). Inset: the Stern–Volmer plot of the
fluorescence titration data.
Fig. 8. Fluorescence quenching spectra of BSA in the presence of complex [Na6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (4). Inset: the Stern–Volmer plot of the fluorescence
titration data.
Table 2
BSA binding parameters of complexes (1) and (4).
Sl. No.
Complex
Stern-Volmer constant (KSV) (M−1
)
Binding constant (KBSA) (M−1
)
Number of binding sites (n)
1
2
{[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1)
[Na6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (4)
2.24 × 106
2.23 × 108
1.29
1.39
1.91 × 106
2.45 × 108
aStern-Volmer and binding constants were determined using fluorescence spectroscopy.
9

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Table 3
Catalytic oxidation of different alcohols using complexes (1) and (4).
Entry
Substrate
Product
Time (h)
Yield (%)
Complex (1)
79(83) a
TON
TON
Complex (4)
81(86)a
Complex (1)
48
Complex (4)
50
1
2
6
7.2
88(90)a
89(91)a
53
53
3
4
7.2
7.2
85(87)a
86(90)a
86(88)a
87(88)a
50
53
51
51
5
7.2
87(89)a
87(88)a
52
51
6
7
7.2
7.2
84(87)a
75(77)a
85(88)a
75(76)a
50
45
51
44
8
7.2
7.2
7.2
7.2
79(80)a
79(81)a
77(78)a
76(77)a
78(79)a
80(84)a
75(76)a
70(73)a
46
47
45
45
46
49
44
42
9
10
11
12
13
12
78(81)a
89(91)a
78(80)a
86(89)a
47
53
46
52
7.1
14
6.5
93(94)a
91(93)a
55
54
15
16
6.5
6.5
(1.5)a
(1.7)a
(1.0)a
(1.3)a
–
–
–
–
17
9
(1.3)a
(0.9)a
–
–
18
19
7
83(85)a
81(83)a
83(86)a
84(86)a
49
48
50
50
6.2
(continued on next page)
10

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Table 3 (continued)
Entry
Substrate
Product
Time (h)
Yield (%)
Complex (1)
77(80)a
TON
TON
Complex (4)
73(79)a
Complex (1)
46
Complex (4)
46
20
21
5.4
50
19(20)a
20(21)a
12
12
22
23
50
50
74(76)a
27(31)a
75(79)a
27(30)a
44
18
46
17
Reaction conditions: Catalyst (0.03 g, 0.05 mmol), 15% H2O2 (1.13 mL, 5.80 mmol), CH3CN, Temperature (70 °C).
a
GC Yield.
respectively. The peak at 302 nm in the complex (3), 325 nm in the
complex (4), and 298 nm in the complex (6) showed an increase in the
intensity while the peaks at 302 nm in the complex (3) and 298 nm in
the complex (6) remain unchanged. The band at 407 nm for complex
(3) shows a red-shift to 415 nm while the peak at 334 nm remains
unchanged; the peak at 351 nm for complex (4) shows a blue shift to
324 nm, while the peaks at 444 nm and 378 nm remain unchanged and
for complex (6) both bands remain unchanged too. The titration ab-
sorption spectra of the complexes against H2O2 showed three isosbestic
points at 308 nm, 338 nm, and 362 nm for complex (1) while only one
isosbestic point at 308 nm, 314 nm, and 332 nm for complexes (3), (4)
and (6), respectively. These results indicate the formation of peroxo
vanadium
{VVO(O2)}
species
[66,92].
The
bis{mono-
oxidomonoperoxidovanadate(V)} complexes can be isolated by car-
rying out the reaction of the respective complexes with 30% H2O2 in
methanol in 1:5 M ratio at 0 °C. The experimentally determined values
of peroxides were 4.67%, 4.70%, 4.68%, and 4.71% in peroxido com-
pounds of complexes (1), (3), (4), and (6), respectively, which are
much less than 9.25%, 9.27%, 9.26% and 9.28% as calculated on the
basis of formation of bis(monooxidomonoperoxidovanadate(V)) com-
plexes. It suggests their instability in the solid-state; for this contention,
enough pieces of evidence are available in the literature [12,93]. In
order to examine whether the peroxide complexes are stable or un-
stable, their peroxide content was determined in the solid-state by io-
dometric method at an interval of 15 min, which was found to decrease
with time to 3.2, 1.5, 0.5, and 0 percent at intervals of 15, 30, 45 and
60 min, respectively, for complex (1) suggesting its instability. Simi-
larly, in other complexes also, the peroxide content was found to de-
crease with time.
Fig. 9. Proposed catalytic cycle for the oxidation of alcohols catalyzed by va-
nadium(V) complexes.
the step-up staircase-like structure growth is along the b-axis (Fig. 5(b))
and the polymer chain network forms layers along the c-axis. Gupta and
co-workers reported similar polymeric complexes of oxido vanadium
and slox ligand [70], which have significant structural differences from
our coordination polymers. A space-filling view along the c-axis, as
shown in Fig. S29, indicated the 3D-polymeric growth along the c-axis
via the hydrogen bonding network with solvent molecules while the
2D-polymeric growth along a- and b-axis.
Based on the analytical and spectroscopic analysis of the complexes
(1) – (6) and the crystallographic structural analysis of complex (1), the
molecular structure of the complex (1) and the tentative molecular
structures of the complexes (2) – (6) are shown in figs. S30-S32.
6. Protein binding studies of complexes (1) and (4)
5. The reaction of complexes (1), (3), (4), and (6) with hydrogen
peroxide
Fluorescence spectroscopy – a powerful tool to study the interaction
of chemical and biological systems at low concentration – was used to
study the binding modes of complexes (1) and (4) with bovine serum
albumin (BSA) [88,94,95]. The two complexes (1) and (4) were se-
lected to see the effect of size and steric factors on binding with BSA.
The strong fluorescence emission band at 335 nm of BSA, which ori-
ginates from tryptophan residues, was recorded on excitation at 285 nm
wavelength in the presence and absence of complex. Upon addition of
the complex (1) or (4) (2 × 10−6 to 2 × 10−5 M) solution to the BSA,
resulted in quenching of the fluorescence emission of BSA at 335 nm.
The emission was suppressed owing to the interaction of the complex
with tryptophan protein residues [34,50,88,94,96-98]. The fluores-
cence spectra along with Stern–Volmer plots (as inset) of BSA in the
presence of complex (1) or (4) are shown in Figs. 7 and 8. In order to
express the binding affinity of complexes (1), and (4) with protein
tryptophan (BSA), the fluorescence quenching was analysed using
Stern-Volmer equations as below:
We selected the complexes (1), (3), (4), and (6) for H2O2 interaction
for studying the effect of alkali metal and ligand sizes, using UV–visible
spectroscopy, and the formation of peroxido species was established by
iodometric titration. When a solution of bis{dioxidovanadate(V)}
complexes were treated with H2O2 in an aqueous methanolic solution,
bis{monooxidomonoperoxidovanadate(V)} type complex species were
believed to be formed and assumed to have the composition of [{VO
(O2)}2(L)]2− [16,85,91]. The titration absorption spectrum of the
complex (1) was recorded by the successive addition of 10 μL of 30%
H2O2 solution to the 2 mL of 6 × 10−5 M solution of the complex (1) as
shown in Fig. 6. The absorption bands at 329 and 414 nm were red-
shifted to 340 and 420 nm. However, when an excess amount of H2O2
solution was added to the complex (1) solution in methanol, an increase
in the intensity of the band at 304 nm occurred accompanied by a blue
shift to 301 nm. Similarly, the 2 mL solutions of the complexes (3)
6 × 10−5 M, (4) 2 mL of 5 × 10−5 M, and (6) 2 mL of 1 × 10−5
M
I0/ I= 1 + KSV [Q]
(1)
were titrated against the H2O2 solution, as shown in Figs. S33–S35,
11

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Table 4
Oxidative bromination of some organic substrate catalyzed by complexes (1) and (4).
Entry
1
Substrate
Product
Time (min)
35
Yield (%) (isolated)
Complex (1)
93
Complex (4)
91
2
3
30
33
88
89
87
89
4
5
6
30
45
45
86
91
93
85
89
92
7
30
89
88
aReaction conditions: salicylaldehyde (0.24 g, 2 mmol), 30% H2O2 (1.70 g, 15 mmol), catalyst (0.03 g, 0.05 mmol), KBr (0.47 g, 4 mmol), 70% HClO4 (0.57 g,
4 mmol) and water (5 mL).
log [(I0 I)/I] = log K+ n log[Q]
(2)
to study this oxidation reaction. The reaction condition was optimized
to produce the maximum yield of the product by varying the various
parameters such as temperature, solvent, amount of the catalyst, and
additives. We studied the reaction at different temperatures and ob-
tained the highest conversion of benzyl alcohol into the product at 70 °C
(Fig. S46). We also tested the effect of both polar and non-polar solvents
and we found that the acetonitrile as the best solvent, which gave the
highest yield of the product (Fig. S47). The model reaction studied by
taking different amounts of catalysts revealed that less than 2 mol % of
the catalyst was enough to give a good conversion of the substrate (Fig.
S48). The reaction was tested in the absence of the catalyst at 70 °C in
acetonitrile; not much reaction progressed even after a prolonged
duration. Hence the complex is the active species in the reaction and
acts as a catalyst. The effect of additives tested, it was found that in the
presence of bases, the yield of the product decreased. Also, when the
catalytic amount of TEMPO was used, the output of the product fell,
and the reaction stopped when the stoichiometric amount of TEMPO
was used. Thus, from these experiments, the best-optimized condition
for the maximum oxidation of benzyl alcohols is benzyl alcohol
(0.3 mL, 2.90 mmol), complexes (1) or (4) (0.05 mmol), and 15% H2O2
(1.13 mL, 5.80 mmol) as an oxidizing agent in acetonitrile as solvent at
a temperature of 70 °C. Using this standardized condition, we carried
out the catalytic oxidation of various benzylic, allylic, and propargylic
alcohols, and the results are presented in Table 3. It is important to
mention when the reaction was carried out in the presence of CBrCl3 – a
I0 and I are the fluorescence intensities in the absence and presence
of the complex. KSV is the Stern–Volmer quenching constant, and [Q] is
the concentration of the quencher, i.e., the complex. Stern–Volmer
quenching constants (KSV) of complexes (1), and (4) were obtained
from the slope of the linear plot I0/I vs [Q], whereas the binding con-
stant (KBSA) and the number of binding sites (n) of the complexes were
obtained from the plot of log [(I0 − I)/I] vs. log[Q]. The values ob-
tained for KSV, KBSA, and ‘n’ are given in Table 2. From the data in
Table 2, it is evident that the dioxidovanadium(V) complexes bind
strongly to serum proteins [88,98], and the binding affinity of the
complex (4) is greater than the complex (1) to BSA. Also, the binding
stoichiometry of BSA for complexes (4) is higher than the complex (1).
Such observation indicated that the size of the ligand influenced the
interaction with BSA and the sterically bulkier complex (4), in our
study, showed better binding to the protein.
7. Catalytic activity studies
7.1. Oxidation of alcohols
We selected complexes (1) and (4) as representative oxidative cat-
alysts for the oxidation of alcohols in the presence of 15% aqueous
hydrogen peroxide. The benzyl alcohol was taken as a model substrate
12

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
Fig. 10. Plausible mechanism of oxidative bromination by heterobimetallic vanadium(V) complex in the presence of H2O2 and KBr in acidic medium.
radical trap – the rate of reaction decreased. This fact revealed that the
oxidation reaction involved radical species [82].
Salicylaldehyde was used as a model substrate in order to ascertain the
best-optimized condition for oxidative bromination (Table S2). We also
carried out the oxidation reactions by varying different parameters,
such as the catalyst/substrate ratio, bromide source, H2O2, and HClO4.
The examination of reactions with varying amounts of catalyst : sub-
strate ratio revealed the reaction proceeded well with a 1 : 40 M ratio at
room temperature. The effect of brominating agent KBr was also tested,
and it was found that at 1 : 2 M ratio to substrate gave a good yield of
the brominated product. The effect of hydrogen peroxide was also
tested, and it was found that a 1:7.5 M ratio of hydrogen peroxide to the
substrate gave the best conversion. The study of the effects of acid
HClO4 revealed that the 1 : 2 M ratio to the substrate is required to
achieve the best yield (Table S3). The results of various solvents ex-
amined revealed that the maximum conversion can be completed in
water. Therefore, water is the most suitable solvent for this reaction,
which has the additional benefit of being the best eco-friendly solvent.
A similar catalytic examination of the complex (4) indicated that it
could also be used as a catalyst for the catalytic oxidative bromination
reaction.
Primary benzylic alcohols bearing both electron-donating and
electron-withdrawing substituents (Table 3, entries 1–11, and 14)
were oxidized to the corresponding aldehydes in good to excellent
yields. From the results, we found that primary benzylic alcohols con-
taining electron-donating groups (Table 3, entries 2–6, and 14) gave
higher yield than those benzylic alcohols containing electron-with-
drawing groups (Table 3, entries 7–11). Primary benzylic alcohols
having substituents at the meta and ortho positions with either electron-
donating or electron-withdrawing groups were also oxidized but the
yield was less compared to the primary benzylic alcohols with sub-
stituents at the para position. Benzylic alcohol with double bonds such
as cinnamyl alcohol (Table 3, entry 12) was oxidized selectively
without affecting the carbon–carbon double bond. Benzoin (Table 3,
entry 13) oxidized to benzil with an excellent yield of 89% and 86%,
respectively. Secondary alcohols such as cyclohexanol, cycloheptanol,
and phenyl ethanol (Table 3, entries 15–17) were only slightly oxi-
dized but with a very-very low conversion (less than 2%) relative to
alcohol. Heterocyclic alcohols such as 2-pyridine methanol, 2-furan
methanol, and 2-thiophene methanol (Table 3, entries 18–20) were
also oxidized to corresponding aldehydes in good yield. Propargylic
alcohols such as 2-hexyne-1-ol oxidized to 2-hexyne-1-al in about 50 h
with a low yield (less than 20%), without affecting the triple bond
(Table 3, entry 21). Likewise, aliphatic alcohols such as n-butanol and
n-octanol (Table 3, entries 22, 23) were oxidized after a prolonged
reaction period of 50 h to corresponding aldehydes, n-butanal (76%),
and n-octanal (27%).
Following the optimized reaction conditions for the oxidative bro-
mination, as described above, the reactions were carried out using al-
dehydes (2 mmol), 30% H2O2 (1.70 g, 15 mmol), catalysts (1) or (4)
(0.05 mmol), KBr (0.47 g, 4.0 mmol), 70% HClO4 (0.57 g, 4 mmol) and
water (5 mL), as listed in Table 4.
Salicylaldehyde was brominated to 5-Bromo-salicylaldehyde in
good yield within 35 min (Table 4, entry 1). Bromination of aniline
gave p-bromoaniline as the product within 30 min (Table 4, entry 2),
acetophenone was also brominated to give α-bromoacetophenone as
the only product within 33 min (Table 4, entry 3). Substituted acet-
ophenone such as 2-hydroxy acetophenone brominated to give α-
Bromo-2-hydroxy acetophenone within 30 min (Table 4, entry 4) but a
lower yield than unsubstituted acetophenone. Other substituted ketones
such as 5-methyl-2-hydroxy-acetophenone and 4-methyl-2-hydroxy-
acetophenone were also giving monobrominated α-bromoketone with
good conversion (Table 4, entries 5, 6). The ethyl salicylate was con-
verted to 5-bromoethyl salicylate with good yield (Table 4, entry 7).
The proposed mechanistic catalytic cycle for the oxidation of al-
cohol to aldehyde is given in Fig. 9. The peroxidative oxidation is be-
lieved to take place via a radical mechanism produced with the inter-
action of oxidoperoxido species {VO(OeO)slox/nph} generated by the
reaction of H2O2 [15,18,99].
7.2. Oxidative bromination
We also investigated the oxidative bromination of some organic
substrates in the presence of H2O2 and bromide salt (e.g., KBr).
13

A. Kumar, et al.
InorganicaChimicaActa515(2021)120068
7.3. Mechanism of oxidative bromination
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This paper reports on the six heterobimetallic vanadium(V) com-
plexes synthesised from disalicylaldehyde/bis(2-hydroxy-1-naphthal-
dehyde) oxaloyldihydrazone ligands. The ligands coordinated to the
vanadium metal centres in staggered form through enolate oxygen
atoms, phenolate/naphtholate oxygen atoms, and azomethine nitrogen
atoms. The crystal structure of the complex (1) revealed a new poly-
meric coordination structure created by the involvement of alkali me-
tals. The reaction of the complexes with H2O2 showed the formation of
bis(monooxidomonoperoxidovanadate(V)) complexes. The interaction
of the complexes (1) and (4) with BSA was also investigated, and it was
found that the binding affinity and the number of the binding site (n)
show that complexes (4) > (1). Two complexes (1) and (4) were stu-
died and shown to be efficient catalysts for the oxidation of alcohols
and oxidative bromination reaction. Thus, it is evident that the present
complexes may be used to develop green processes. The use of H2O2 as
a mild oxidant in the presence of the synthesized catalysts would pro-
vide an efficient, easy, and safe approach, leading to oxidations of al-
cohols, bromination of organic compounds, and other industrially im-
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Declaration of Competing Interest
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Acknowledgment
We would like to thank Head, SAIF NEHU, for spectral analysis. We
would also like to thank the Head, Department of Chemistry, NEHU, for
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Kurbah would like to acknowledge UGC, New Delhi for the award of
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
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