Dioxidomolybdenum(VI) and dioxidouranium(VI) complexes as functional mimic of haloperoxidases catalytic activity in presence of H2O2–KBr–HClO4

Article abstract of DOI:10.1016/j.ica.2018.11.031

The stable dibasic tetradentate ligand 1,4-bis-(2-hydroxy-3,5-dimethylbenzyl)piperazine (H₂pip-2,4-dmp, I) prepared by reacting 2,4-dimethylphenol with piperazine in the presence of formaldehyde reacts with [Mo^VIO₂(acac)

Full text of DOI:10.1016/j.ica.2018.11.031

Accepted Manuscript
Research paper
Dioxomolybdenum(VI) and dioxouranium(VI) complexes as functional mimic
of haloperoxidases catalytic activity in presence of H O –KBr–HClO
2
2
4
Mannar R. Maurya, Bekele Mengesha, Shailendra K. Maurya, Nidhi Sehrawat,
Fernando Avecilla
PII:
S0020-1693(18)31458-0
https://doi.org/10.1016/j.ica.2018.11.031
ICA 18646
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
18 September 2018
21 November 2018
21 November 2018
Please cite this article as: M.R. Maurya, B. Mengesha, S.K. Maurya, N. Sehrawat, F. Avecilla,
Dioxomolybdenum(VI) and dioxouranium(VI) complexes as functional mimic of haloperoxidases catalytic activity
in presence of H O –KBr–HClO , Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.11.031
2
2
4
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Dioxomolybdenum(VI) and dioxouranium(VI) complexes as functional mimic
of haloperoxidases catalytic activity in presence of H O –KBr–HClO
2
2
4
a,
a
a
a
Mannar R. Maurya *, Bekele Mengesha , Shailendra K. Maurya , Nidhi Sehrawat , Fernando
Avecilla
b
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee–247667, India.
b
Grupo Xenomar, Centro de Investigacións Científicas Avanzadas (CICA), Departamento de
Química, Facultade de Ciencias, Universidade da Coruña, Campus de A Coruña, 15071 A
Coruña, Spain
_
________________________________________________________________
Corresponding author. Tel.: +91 1332 285327; fax: +91 1332 273560.
E-mail address: rkmanfcy@iitr.ac.in (M.R. Maurya). https://www.iitr.ac.in/CY/rkmanfcy
Keywords: Tetradentate ONNO donor Schiff base ligands; Dioxidomolybdenum (VI) complex;
Dioxidouranium (VI) complex; Single crystal X-ray study; NMR spectroscopy; Oxidative
bromination of thymol.
Abstract
The stable dibasic tetradentate ligand 1,4-bis-(2-hydroxy-3,5-dimethylbenzyl)piperazine (H pip-
2
2
,4-dmp, I) prepared by reacting 2,4-dimethylphenol with piperazine in the presence of
VI
VI
formaldehyde reacts with [Mo O (acac) ] and [U O (CH COO) ] in equimolar ratio to give
2
2
2
3
2
VI
VI
neutral hexa-coordinated [Mo O (pip-2,4-dmp)] (1) and hepta-coordinated [U O (pip-2,4-
2
2
dmp)(MeOH)] (2), respectively. After characterizing these complexes by spectroscopic (IR,
1
13
UV/Vis, H and C NMR) data, elemental and thermal analyses (and single crystal X-ray
diffraction study of uranium complex), they are used as catalysts to study the oxidative
bromination of thymol. Such catalytic reactions are observed by many model vanadium
complexes and are considered as a functional mimic of haloperoxidases. The catalytic oxidation
resulted in the formation of three products namely, 2-bromothymol, 4-bromothymol and 2,4-
dibromothymol. The optimized reaction conditions are obtained considering concentration of
substrate, KBr, HClO , and oxidant for the maximum yield of brominated products. Under the
4
optimized reaction conditions, the product selectivity for both the prepared complexes is
1

investigated. They are found to be competent homogeneous catalysts to afford the products in
good yield.
1
. Introduction
Molybdenum is an essential metal which is capable of forming complexes with many
compounds of biological importance such as carbohydrates, amino acids and other bioinspired
ligands [1-3]. These model complexes can catalyze a wide variety of biochemical reactions. The
most commonly studied reactions (other than biochemical) in which dioxidomolybdenum(VI)
complexes act as catalysts are epoxidation of alkenes [4-14], oxidation of alcohols [15,16] and
sulfur based compounds [17-19], and oxidative halogenation of organic substrates [20-22]. The
nature of complex, oxidant and reaction conditions decide whether the reaction will proceed via
a homolytic or electrophilic oxidation. In current years artificial enzyme mimics have become a
hot topic of research since they present many advantages over conventional enzymes, namely
easier preparation, lower price and improved stability, overcoming the drawbacks of natural
enzymes [23-25]. Dioxidomolybdenum(VI) complexes are air stable and show functional as well
as structural similarity with vanadium based haloperoxidases, thus can be considered as artificial
models for heloperoxidases [22,25,26].
Amongst the actinides, uranium is the only element being well known for their
coordination chemistry. Like molybdenum, uranium in its trans-[UO ] form forms stable
2
complexes in higher oxidation state [27,28] but their catalytic activities are not much explored
[
27,28,29]. Therefore, the objective of the present work is to prepare dioxomolybdenum(VI) and
dioxouranium(VI) complexes of stable dibasic tetradentate ligand, 1,4-bis-(2-hydroxy-3,5-
dimethylbenzyl)piperazine (abbreviated as H pip-2,4-dmp, Scheme 1) and study their catalytic
2
activity as homogeneous catalysts for the oxidative bromination of thymol, an essential catalytic
reaction observed by many model vanadium complexes [22,25,26]. It has been observed that
both complexes mimic similar catalytic activity as shown by vanadium based haloperoxidases
and show very good conversion of thymol to the corresponding oxidative brominated products.
2

2
. Experimental
2
.1. Materials
MoO (S.D. fine, India), uranyl acetate dihydrate (Loba chemie, India), piperazine, 2,4-
3
dimethylphenol (Aldrich Chemicals, USA) and thymol (Himedia, India) were used as obtained.
VI
[
(
Mo O (acac) ] was prepared following the method reported in literature [30]. Ligand 1,4-bis-
2 2
2-hydroxy-3,5-dimethylbenzyl)piperazine (H pip-2,4-dmp) (I) was prepared following the
2
method reported in literature [31]. All other chemicals and solvents used were of AR grade.
2
.2. General procedures and techniques
Elemental (C, H and N) analysis of ligand and complexes were carried out on an elementar
model Vario-E1-III. IR spectra were recorded as KBr pellets on a Nicolet 1100 FT-IR
spectrometer. Electronic spectra of ligands and complexes were measured in MeCN with a
1
13
Shimadzu 2600 UV-Vis spectrophotometers. The H and C NMR spectra of ligand and
complexes were taken in CDCl / DMSO-d using a JEOL ECX 400 MHz spectrometer. The
3
6
thermogravimetric analysis of the complexes was carried out under oxygen atmosphere using a
TG Stanton Redcroft STA 780 instrument. A Shimadzu 2010 plus gas-chromatograph fitted with
an Rtx-1 capillary column (30 m × 0.25 mm × 0.25 μm) and a FID detector was used to analyze
the reaction products and their quantification was made based of the relative peak area of each
product. The identity of the products was confirmed using a GC-MS model Perkin-Elmer, Clarus
5
00 and comparing the fragments of each product with the library available.
VI
2
.3. Synthesis of [Mo O (pip-2,4-dmp)] (1)
2
The ligand H pip-2,4-dmp (1.77 g, 5 mmol) was dissolved in 80 mL methanol by refluxing on a
2
VI
water bath for 2 h. A solution of [Mo O (acac) ] (1.625 g, 5 mmol) in methanol (15 mL) was
2
2
added to the solution of ligand and the reaction mixture was allowed to reflux for 24 h,
whereupon a yellow precipitate separated out. The solution volume was reduced to ca. 20 mL
and the separated product was collected by filtration, washed with methanol and dried in vacuum
desiccator. Yield: 33.1% (0.792 g) Cal. For C H N O Mo (480); C, 55.0; H, 5.83; N, 5.83.
2
2
28
2
4
Obs. C, 54.6; H, 5.9; N, 5.8%.
3

VI
2
.4. Synthesis of [U O (pip-2,4-dmp)(MeOH)] (2)
2
VI
A solution of U O (CH COO) (2.125 g, 5 mmol) in 10 mL methanol was added to the stirred
2
3
2
solution of ligand (1.77 g, 5 mmol) in 80 mL MeOH prepared as mentioned above and the
resulting reaction mixture was refluxed on a water bath for 24 h as a result of which a yellow-
orange precipitate appeared. The solution volume was reduced to ca. 20 mL and the separated
product was filtered, washed with methanol and dried in vacuuo. Yield: 40.8% (1.27 g). Cal. For
C H N O U (642); C, 41.12; H, 4.98; N, 4.36. Obs. C, 40.6; H, 4.9; N, 4.5%. Crystals of the
22
32
2
5
complex suitable for X-ray study were grown in DMSO and obtained complex is now
VI
abbreviated as [U O (pip-2,4-dmp)(DMSO)] (2a)
2
2
.5. X-Ray crystal structure determination
VI
Three-dimensional X-ray data of [U O (pip-2,4-dmp)(DMSO)] (2a) were collected on a
2
Bruker Kappa Apex CCD diffractometer at room temperature by the - scan method.
Reflections were measured from a hemisphere of data collected from frames, each of them
covering 0.3º in . A total of 26004 reflections measured were corrected for Lorentz and
polarization effects and for absorption by multi-scan methods based on symmetry-equivalent and
repeated reflections. Of the total, 5271 independent reflections exceeded the significance level
(F/F) > 4.0. After data collection an empirical absorption correction (SADABS) [32] was
applied, and the structure was solved by direct methods and refined by full matrix least-squares
2
on F data using SHELX suite of programs [33]. Hydrogen atoms were included in calculated
position and refined in the riding mode, except for C(3), C(5) and C16, which were located in
difference Fourier map and freely refined. A final difference Fourier map showed no residual
–
3
–3
density outside: 0.912 and -0.668 e.Å for 1 and 1.062 and -0.621 e.Å for 2. Due to disorder in
DMSO molecule bonded to U(1) atom, two positions for sulfur atom of DMSO molecule were
refined with anisotropic atomic displacement parameters. The site occupancy factor was 0.91716
2
2
o
2
2
for S(1A). A weighting scheme w = 1/[σ (F ) + (0.018900 P) + 2.237600 P], where P = (|F | +
o
2
2
|F | )/3, was used in the latter stages of refinement. Further details of the crystal structure
c
determination are given in Table 1. CCDC 1866906 contains the supplementary crystallographic
data for the structure reported in this paper. These data can be obtained free of charge via
4

http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the
online version, at doi: $$$$$.
<<Table 1>>
2
.6. Catalytic activity study: Oxidative bromination of thymol
All the reactions were carried out in a 100 mL two-necked round bottom flask. In a typical
reaction, thymol (1.5 g, 10 mmol) and aqueous 30% H O (1.13 g, 10 mmol) and KBr (1.19 g, 10
2
2
mmol) were added in 20 mL water. After adding the metal-complex (i.e. catalyst) (0.002 g), 70%
HClO (1.43 g, 10 mmol) was added to the reaction flask and the reaction mixture was stirred for
4
2
h at room temperature. The reaction mixture was then extracted with n-hexane where the
products moved into the n-hexane layer. The oxidized products were analyzed quantitatively by
withdrawing small aliquots from the n-hexane and subjected to GC and their quantifications
were made on the basis of the relative peak area of the respective product. The obtained main
1
products were further confirmed by H NMR spectroscopy as well as GC–MS after their
separations.
3
. Results and discussion
A Mannich condensation between piperazine, 40% formaldehyde and 2,4–dimethyl phenol (1 : 2
2 molar ratio) in refluxing MeOH yields the Mannich base ligands, H pip-2,4-dmp (I) in
:
2
VI
VI
excellent yield [31]. Reaction of I with [Mo O (acac) ] and [U O (CH COO) ] in refluxing
2
2
2
3
2
VI
VI
methanol resulted in the formation of [Mo O (pip-2,4-dmp)] (1) (yellow) and [U O (pip-2,4-
2
2
dmp)(MeOH)] (2) (reddish-brown), respectively, as stable solid. Idealized structures of the
1
complexes are shown in Scheme 1 which are based on the spectroscopic (IR, UV/Vis and H
VI
NMR) data, elemental and thermal analyses, and X-ray diffraction study of [U O (pip-2,4-
2
dmp)(DMSO)] (2a, a DMSO coordinated complex 2). Both complexes are soluble in methanol,
dichloromethane, dimethyl sulfoxide and acetonitrile.
<<Scheme 1>>
5

3
.1. Thermogravimetric analysis study
VI
Thermogravimetric analysis (TGA) profile of [Mo O (pip-2,4-dmp)] (1) under an oxygen
2
atmosphere shows a mass loss of ca. 2.0% between 60 to 100 °C which is possibly due to
moisture or traces of water/methanol of crystallization. The complex is then stable up to ca. 200
o
C. Above this temperature complex starts decomposing in three exothermic steps and stabilizes
o
at ca. 500 C. The obtained residue of 31.1% at this temperature is equivalent to the formation of
o
stable MoO ((Cal for MoO = 30%). The residue is stable up to 710 C and then volatilizes
3
3
VI
beyond this temperature. TGA profile of [U O (pip-2,4-dmp)(MeOH)] (2) shows a mass loss
2
(Fig. S1) between 60 °C to 285 °C equivalent to 4.9% which is equal to one methanol (Cal.
4
.98%) coordinated to uranium. Increasing temperature further, it decomposes in three
o
exothermic steps and stabilizes at ca. 525 C. The residue obtained (42.9%) at this temperature
o
equals to U O (Cal. 43.41%). This residue is stable up to 1000 C.
3
8
VI
3
.2. Description of structure of [U O (pip-2,4-dmp)(DMSO)] (2a)
2
Fig. 1. Selected bond distances and angles are given in Table 2. It is a mononuclear complex,
which crystallizes in a monoclinic space group P2 /c. The structure adopts a distorted pentagonal
1
bipyramidal geometry around of metal centre. The ligand acts as tetradentate, coordinated
through two phenoxido oxygen and two piperazine nitrogen atoms. The piperazine nitrogen
3
atoms present a distorted sp hybridized bonding network. U centre completes the coordination
sphere bonding to two Ooxido terminal oxygen atoms and one oxygen atom from solvent molecule
(DMSO). The U = O bond lenghts [1.788(2)-1.792(2) Å] are similar to other in the literature
[34]. The two distances U = Ophenoxido [2.225(2) and 2.234(2) Å] are shorter than the U = ODMSO
[2.446(2) Å] and are trans to each other. The equatorial plane is occupied by the Ophenoxido atoms,
O(3) and O(4), by the Npiperazine atoms, N(1) and N(2) and by the oxygen atom (O1S) of DMSO
molecule. They are distorted respect to the planarity, [mean deviation from the plane 0.0857(14)
Å]. The terminal oxido atoms occupy the axial sites. The crystal packing (Fig. 2) does not
present hydrogen bonds between the electronegative atoms. - Interactions are not present
either.
<<Figs. 1 and 2>>
<<Table 2>>
6

3
.3. IR spectral study
Table 3 presents IR spectral data of ligand and complexes. The ligand H pip-2,4-dmp
2
–
1
exhibits a broad band at ca. 3440 cm due to the υ(OH) of the phenol. Absence of this band in
molybdenum complex suggests the deprotonation and coordination of phenolic oxygen to
–1
molybdenum while a weak band in the 3400 cm region in uranium complex possibly suggests
the presence of coordinated methanol. The presence of several medium intensity bands between
–
1
2
500 and 2800 cm in the ligand as well as in complexes suggest the existence of C–H
stretching bands due to -CH . The υ(C-N) of ring nitrogen has no systematic trend in ligand and
2
–
1
complexes. The presence of two sharp peaks in molybdenum complex at 944 and 906 cm due
to υasym(O=Mo=O) and υ (O=Mo=O) stretches, respectively, confirm the presence of cis-
sym
VI
–1
[
Mo O ] moiety. In uranium complex, one sharp band appears at 900 cm and other one
2
–
1
appears in the lower region at 859 cm , due to υ (O=U=O) and υ (O=U=O), respectively,
asym
sym
VI
indicate the presence of trans-[U O ] structure. Thus, the spectral data confirms the
2
coordination of the ligand to the metal in both complexes.
<<Table 3>>
3
.4. UV-Vis spectral study
Fig. S2 presents UV-visible spectra of ligand and complexes and Table 4 provides
absorption maxima of ligands and complexes along with their extinction coefficients. The ligand
shows three bands at 223, 286 and 330 nm in the UV region. Based on their molar extinction
coefficient, the first band at 223 nm is assigned due to σ → σ* transition whereas the band at 286
nm is due to π → π* transition. The low intensity band at 330 nm is due to n → π* transition.
VI
The 330 nm band shifts to higher wavelength i.e. at 353 and 389 nm in [Mo O (Pip-2,4-dmp)]
2
VI
(
1) and [U O (pip-2,4-dmp)(MeOH)] (2), respectively. An additional band at 425 nm in 1 arises
2
due to the ligand to metal charge transfer from the filled π orbital of ligand to the vacant d orbital
1
ε +
3
of the metal while such band at 510 nm in 2 is due to g → π transition [23].
u
<<Table 4>>
1
3
.5. H NMR spectral study
1
The H NMR spectra of ligand and metal complexes were recorded in CDCl . The relevant
3
spectral data are presented in Table 5 and spectra are accessible in the supporting information
7

(Figs. S3-S5). Ligand I exhibits a broad band at 10.5 ppm due to phenolic protons and absence of
this band in the spectra of complexes suggests coordination of phenolic oxygen to the metal after
proton replacement. An up field shift of signals due to –NCH (piperazine) from 2.67 ppm (s,
2
8
H) to 1.26 ppm (in 1) and 2.15 ppm (in 2) occurs which is possibly due to adjustment of ring
current upon coordination of ring nitrogen to the metal. As expected signals due to aromatic,
methylene and methyl protons are not much affected and appear at their expected positions.
<<Table 5>>
1
3
3
.6. C NMR spectral study
We have also recorded C NMR of ligand and complexes (Table 6) to ascertain the
1
3
binding modes of the ligand towards complexes. The representative spectra (ligand and uranium
complex) are presented in Fig. 3. The spectra recorded in DMSO-d show considerable
6
1
3
coordination–induced C NMR chemical shifts {Δ = [(complex) – (ligand)]} for those
carbon atoms that are in the vicinity of the coordinating atoms like, C /C ′ (the carbon atoms
1
1
close to phenolic oxygen), C /C ′ and C /C ′ (the carbon atoms close to the ring nitrogen). Even
7
7
8
8
carbon atoms C /C ′ of benzene ring which are attached to C /C ′ are also affected. All these
6
6
7
7
informations supplement the conclusion drawn from 1H NMR data. Other signals of ligand and
complexes appear well within the expected region. We could not observed the signal due to
methyl carbon of coordinated methanol in complex 2, possibly due to exchange of methanol with
DMSO while recording the spectrum.
<<Fig. 3 and Table 6>>
3
.7. Catalytic activity study: Oxidative bromination of thymol
The oxidative bromination of thymol catalyzed by the metal complexes has been carried out
using KBr, 30 % aqueous H O and 70 % aqueous HClO in aqueous medium. Acid is required
2
2
4
to proceed the reaction. The electrophilic aromatic substitution in the phenolic ring led the
bromination of thymol and formation of two isomers, 2-bromothymol and 4-bromothymol and
further bromination of the ortho/ para isomer led the formation of 2,4-dibromothymol (Scheme
2
). Among these products, 2,4-dibromothymol was found in the highest yield, possibly because
of the further bromination of the monobromo product(s).
<Scheme 2>>
<
8

To obtain the maximum yield of brominated products for both the catalysts, several parameters,
such as, amounts of catalyst, 30% H O , KBr, and 70% HClO , were varied separately for both
2
2
4
of them at room temperature. We have first optimized the reaction conditions considering
VI
different amounts of catalyst, H O , KBr and HClO using [Mo O (pip-2,4-dmp)] (1) as a
2
2
4
2
catalyst. Thus, for 10 mmol (1.50 g) of thymol, three different amounts of catalyst viz. 0.001,
.002 and 0.003 g and 1 : 1, 1 : 2 and 1 : 3 ratios each for substrate : H O , substrate : KBr and
0
2
2
substrate : HClO4 were taken in 20 mL water and the reaction was carried out at room
temperature for 2 h. It was observed that HClO added at a time in case of 20 and 30 mmol led to
4
the partial decomposition of catalyst, therefore, in these cases HClO was added in two and three
4
equal portions, respectively, to the reaction mixture, first portion at t = 0 and other one/two
portions after every 30 min intervals. Figs. 4 – 7 present time on analysis for different conditions
and Table 7 summarizes details of all reaction conditions and the conversion of thymol at 2 h of
reaction time.
As accessible in Table 7, the optimized reaction condition for the maximum conversion of 10
mmol (1.50 g) of thymol using 1 as catalyst is as presented in entry 8 i.e. those using substrate :
–
3
catalyst : H O : KBr : HClO ratios (in mmol) of 10 : 4.16 × 10 : 10 : 20. Under these
2
2
4
conditions a maximum of 99% conversion of thymol was achieved where products 2-
bromothymol, 4-bromothymol and 2,4-dibromothymol with the percentage selectivity of 1, 2 and
–
1
9
7, respectively were obtained. Turn over frequency calculated at 2 h of reaction is 1198 h for
this condition. Higher amount of KBr (30 mmol) and lower amount of acid (10 mmol) favors the
formation of monobromo derivatives (see entry 7) to some extent while higher amount of acid
(
30 mmol) and lower amount (10 mmol) of KBr favors the formation of dibromo derivative
entry 9). However, at any reaction conditions (Table 7), the yield of 2,4-dimromo derivative is
(
always higher. Further, no trace of substrate was observable in GC profile under the conditions
mentioned in entry 9 of Table 7, suggesting the complete conversion of thymol to products.
Blank reaction i.e. without catalyst but in the presence of substrate : H O : KBr : HClO ratios
2
2
4
(
in mmol) of 10 : 10 : 20, a maximum of 45% conversion was noted, therefore the catalyst
enhances the conversion of thymol.
<Table 7>> <<Figures 4-7>>
<
9

We have also optimized all reaction parameters mentioned above in 20 mL water for catalyst 2.
In this case also reaction acquires equilibrium in 2 h of reaction time at room temperature. Figs.
8
– 11 present time on analysis of conversion of thymol under various reaction conditions and
Table 8 summarizes all the reactions conditions studied and the corresponding percentage of
oxidative bromination of thymol along with TOF and the selectivity of different reaction
products. Data presented in Table 8 again prove that the conversions and the selectivity of
products differ on varying the reagents and the best reaction conditions (entry 10) for the
maximum oxidative bromination of 10 mmol of thymol with 92% conversion are: catalyst (0.003
–
3
g, 4.8 × 10 mmol), H O (3.39 g, 30 mmol), KBr (1.19 g, 10 mmol) and HClO (4.29 g, 30
2
2
4
mmol) in 20 mL water i.e. substrate : oxidant : catalyst : KBr : HClO ratios (in mmol) of 10 : 30
4
–
3
:
4.8 × 10
: 10 : 30. Under these conditions, only two products, 4-bromothymol and 2,4-
bibromothymol with the selectivity percent of 1 and 99% were obtained. Turn over frequency
calculated at 2 h under this condition is 956 which is much less than that obtained for
molybdenum catalyst. The overall conversion is also less than molybdenum complex.
<<Table 8>> <<Figures 8-11>>
The catalytic oxidative bromination of uranium complex is relatively low but of
molybdenum complex reported here is very well comparable with those reported for
dioxidomolybdenum(VI) complexes (94-99%) of Schiff base ligands derived from 8-formyl-7-
hydroxy-4-methylcoumarin and hydrazides [17]. However, the selectivity of 2,4-dibromothymol
varies within these two complexes reported here as well as within those reported in literature.
V
The
vanadium
complex
[V O(OMe)(MeOH)(L)]
[H2L
=
6,6'-(2-(pyridin-2-
yl)ethylazanediyl)bis(methylene)bis(2,4-di-tert-butylphenol)] under the optimised reaction
conditions (i.e. substrate : H O : KBr : HClO of 1 : 2 : 2 : 2 for 10 mmol of thymol) gave as
2
2
4
high as 99% conversion with 57 % selectivity towards 2,4-dibromothymol, followed by 37%
towards the 4-bromothymol and rest towards 2-bromothymol isomer [35]. Complexes,
VI
[
W O (hap-hyz)(MeOH)] (H hap-hyz = Schiff bases derived from 2-hydroxy acetophenone and
2
2
hydrazides) show conversion between 91-96% [36] which is again better than uranium complex
but almost same like molybdenum complex.
1
0

3
.8. Reactivity of uranium complex with H O and possible reaction mechanism
2 2
As observed in cis-[MO ]- complexes (M = V, Mo) [21, 37], KBr in the presence of H O
2
2
2
+
and HClO , catalytically generate HOBr and/or Br , Br , which are the brominating reagent for
4
2
the substrates. Conte et al. have identified an oxidomonoperoxidovanadium(V) complex as the
−
oxidant of the Br ion to HOBr/Br by spectroscopic techniques [38]. It was also observed
2
5
1
recently by V NMR study that the [VO(O )(L)]-type species is an active intermediate during
2
catalytic study of oxidative bromination of thymol in the presence of oxidant H O [35]. In order
2
2
to generate information for such an intermediate in trans-[UO ]-complexes, we have also treated
2
uranium complex 2 with H O and monitored the changes by UV–Vis spectroscopy. Thus, the
2
2
successive addition of one drop portion of 30% H O (0.108 g, 0.95 mmol) dissolved in 5 mL of
2
2
–3
MeCN to 25 mL of 4.8 ×10 M solution of 2 in MeCN resulted in the continuous decrease in
the intensity of 502 and 383 nm bands to almost flat (Fig. 12). The UV region (not shown here)
is not much affected. These spectral changes suggest the reaction of H O with uranium complex
2
2
and the formation of possibly peroxido species in solution. Such peroxido intermediate, as
observed in vanadium and molybdenum complexes, is responsible for the generation of
brominating reagent from KBr catalytically.
4
. Conclusions
Dioxidomolybdenum (VI) and dioxidouranium (VI) complexes of piperazine derived
tetradentate (O,N,N,O) donor ligand, bis{(2,4-dmp)CH }(µ -piperazine)] (H pip-2,4-dmp) were
2
2
2
prepared successfully and characterized by UV, IR, NMR, elemental analysis and TGA, and
single crystal X-ray study of uranium complex. Based on the characterization data it has been
VI
concluded that molybdenum in complex [Mo O (pip-2,4-dmp)] is hexa-coordinated while
2
VI
uranium in [U O (pip-2,4-dmp)(MeOH)] is hepta-coordinated. The catalytic activity of these
2
complexes for the oxidative bromination of thymol, a reaction generally observed by vanadium
dependent haloperoxidases, was studied by gas chromatography and it has been observed that
these complexes show very good conversion and enhanced selectivity towards the products in
the order: 2,4-dibromothymol > 4-bromothymol > 2-bromothymol, under the optimized reaction
conditions. Thus these complexes mimic similar catalytic reactivity as shown by vanadium based
haloperoxidases.
1
1

Acknowledgments
B. M. is thankful to the Indian Institute of Technology Roorkee for an MHRD fellowship.
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[
[
[
[
[
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1
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1
4

Schemes
O
O
O
O
Mo
N
N
]
)
2
c
a
c
VI
a
[Mo O (pip-2,4-dmp)] (1)
(
2
O 2
H
O
o
(
e
OH
HO
[M
M
M
N
N
U
e
O
O
H
2
M
H pip-2,4-dmp
2
e
C
O
O
OH
O
)
O
O
2
U
O
N
N
VI
[
U O (pip-2,4-dmp)(MeOH] (2)
2
VI
VI
Scheme 1. Synthetic route to prepare [Mo O (pip-2,4-dmp)] (1) and [U O (pip-2,4-
2
2
dmp)(MeOH)] (2).
Scheme 2. Products of the oxidative bromination of thymol. 2-Brth = 2-bromothymol, 4-Brth =
-bromothymol, 2,4-dBrth = 2,4-dibromothymol.
4
1
5

Tables
Table 1
VI
Crystal Data and Structure Refinement for [U O (pip-2,4-dmp)(DMSO)] (2a).
2
Formula
Formula weight
T, K
C H N O SU
24 34 2 5
700.62
293(2)
0.71073
Wavelength, Å
Crystal system
Space group
a/Å
Monoclinic
P2 /c
1
11.6974(10)
14.5623(12)
15.2257(12)
95.112(4)
2583.2(4)
4
b/Å
c/Å
β/º
3
V/Å
Z
F000
1360
-3
D /g cm
calc
1.801
-
1

/mm
/ (º)
Rint
Crystal size/ mm
6.400

2.53 to 28.61
0.0292
3
0.09 x 0.06 x 0.05
1.059
2
Goodness-of-fit on F
a
R [I>2(I)]
1
0.0210
b
wR (all data)
0.0525
2
-3
Largest differences peak and hole (eÅ ) 0.912 and -0.668
a
b
2
2
2
2 2 1/2
R = F  - F /F . wR = [w(F  -F  ) ]/[w(F ) ]
1
o
c
o
2
o
c
o
1
6

Table 2
VI
Bond lengths and angles for the compounds [U O (pip-2,4-dmp)(DMSO)] (2a).
2
Bond lengths
U(1)-O(1)
U(1)-O(2)
U(1)-O(3)
U(1)-O(4)
U(1)-O(1S)
U(1)-N(1)
U(1)-N(2)
1.792(2)
1.788(2)
2.225(2)
2.234(2)
2.446(2)
2.640(3)
2.639(2)
N(1)-C(7)
N(1)-C(8)
N(1)-C(11)
N(2)-C(9)
N(2)-C(10)
N(2)-C(12)
1.481(4)
1.477(4)
1.479(4)
1.485(4)
1.483(4)
1.484(4)
Angles
O(2)-U(1)-O(1)
O(2)-U(1)-O(3)
O(1)-U(1)-O(3)
O(2)-U(1)-O(4)
O(1)-U(1)-O(4)
O(3)-U(1)-O(4)
O(2)-U(1)-O(1S)
O(1)-U(1)-O(1S)
O(3)-U(1)-O(1S)
O(4)-U(1)-O(1S)
O(2)-U(1)-N(2)
O(1)-U(1)-N(2)
O(3)-U(1)-N(2)
O(4)-U(1)-N(2)
O(1S)-U(1)-N(2)
O(2)-U(1)-N(1)
O(1)-U(1)-N(1)
O(3)-U(1)-N(1)
O(4)-U(1)-N(1)
O(1S)-U(1)-N(1)
N(2)-U(1)-N(1)
176.48(10)
89.86(9)
90.88(10)
91.25(10)
89.33(10)
158.26(9)
87.95(9)
95.56(9)
80.24(8)
78.10(8)
84.44(9)
92.42(9)
128.86(8)
72.83(9)
149.72(8)
87.82(9)
89.12(9)
72.60(8)
129.14(8)
152.51(8)
56.46(8)
C(8)-N(1)-C(11)
C(8)-N(1)-C(7)
C(11)-N(1)-C(7)
C(8)-N(1)-U(1)
C(11)-N(1)-U(1)
C(7)-N(1)-U(1)
C(10)-N(2)-C(12)
C(10)-N(2)-C(9)
C(12)-N(2)-C(9)
C(10)-N(2)-U(1)
C(12)-N(2)-U(1)
C(9)-N(2)-U(1)
107.0(2)
110.5(3)
109.8(2)
106.85(18)
105.69(19)
116.45(19)
111.3(2)
106.9(3)
109.2(2)
106.16(17)
116.3(2)
106.40(18)
1
7

Table 3
IR spectral data (cm ) of ligand and complexes.
–
1
Compound
υ(O-H) υ(C-N) υ(CH ) υasym(O=M=O) υsym (O=M=O)
2
H pip-2,4-dmp
3440
1156
1162
1150
2932
2923
2928
2
VI
[
[
Mo O (pip-2,4-dmp)]
944
900
906
859
2
VI
U O (pip-2,4-dmp)(MeOH)] 3428
2
Table 4
UV-visible spectral data of ligand and complexes.
-
1
-1
Compound
Solvent
MeOH
MeOH
MeOH
λ (nm) (ε/M cm )
H Pip-2,4-dmp
223(682), 286(1252), 330(370)
291(2072), 353(51), 425(110)
2
VI
[
[
Mo O (pip-2,4-dmp)]
2
VI
U O (pip-2,4-dmp)(MeOH)]
222(690), 284(1812), 389(399), 510(178)
2
Table 5
1
H NMR (δ in ppm) of ligand and metal complexes.
Compound
OH
Aromatic H
-CH2
-CH3
-NCH2
piprazine)
10.5 (2H) 6.63 (s, 2H), 6.87 (s, 2H) 3.66 (s, 4H) 2.21(s, 6H), 2.20(s, 6H) 2.67(s, 8H)
(
H pip-2,4-dmp
2
VI
[
[
Mo O (pip-2,4-dmp)]
-
-
6.83 (s, 2H), 6.63 (s, 2H) 3.66 (s, 4H) 2.21(s, 6H), 2.19(s, 6H) 1.26(s, 8H)
6.84 (s, 2H), 6.72 (s, 2H) 3.68 (b, 4H) 2.10(s, 6H), 2.08(s, 6H) 2.15(s, 8H)
2
VI
U O (pip-2,4-dmp)(MeOH)]
2
1
8

Table 6
1
3
C NMR data (δ in ppm) of ligand and metal complexes.
a
Compound
C /C ′ C /C ′ C /C ′ C /C ′ C /C ′ C /C ′ C /C ′ C /C ′ C /C ′ C /C ′
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
H Pip-2,4-dmp
153.4
123.9
121.8
131.0
133.9
129.8
128.3
127.3
127.6
121.0
52.6
52.3
16.1
16.2
20.6
20.6
2
VI
[
(
[
(
Mo O (pip-2,4-dmp)]
153.3
(-0.1)
164.9
(11.5)
117.5
(-3.5)
124.0
(3.0)
60.5
(7.9)
60.7
(8.1)
54.7
(2.4)
53.0
(0.7)
2
Δδ)
VI
U O (pip-2,4-dmp)(MeOH)]
124.8
131.1
127.7
126.7
17.3
20.5
2
Δδ)
a
Δδ = [δ (complex) - δ(free ligand)].
Table 7
VI
Conversion of 10 mmol (1.50 g) of thymol using [Mo O (pip-2,4-dmp)] complex as a catalyst
2
precursor for 2h of reaction time under different reaction conditions.
Sr.
No.
1
KBr
H O
HClO4
Catalyst
Conv. TOF
(%)
2-Brth 4-Brth 2,4-dBrth
2
2
[g (mmol)] [g (mmol)]
[g (mmol)] [mg (mmol)]
–
3
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
2.38 (20)
3.57 (30)
1.19 (10)
1.19 (10)
1.19 (10)
1.13 (10)
1.13 (10)
1.13 (10)
2.27 (20)
3.39 (30)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
2.86 (20)
4.29 (30)
4.29 (30)
1.0 (2.08 × 10 ) 69
1658
1141
681
10
10
10
12
10
8
22
21
33
18
29
17
24
2
67
69
57
69
59
73
59
97
99
60
–3
2
2.0 (4.16 ×10 ) 95
–3
3
3.0 (6.24 ×10 ) 85
–
3
4
2.0 (4.16 × 10 ) 79
949
–
3
5
2.0 (4.16 × 10 ) 59
709
–
3
6
2.0 (4.16 × 10 ) 90
1081
1105
1189
1201
-
–
3
7
2.0 (4.16 × 10 ) 92
16
1
–
3
8
2.0 (4.16 × 10 ) 99
–
3
9
2.0 (4.16 × 10 ) 100
45
-
1
1
0
-
35
5
1
9

Table 8
VI
Conversion of 10 mmol (1.5 g) thymol using [U O (pip-2,4-dmp)(MeOH)] as a catalyst for 2 h
2
of reaction time under different reaction conditions.
Sr.
KBr
H O
HClO4
Catalyst
Conv TOF
2-Brth 4-BrTh 2,4-dBrth
2
2
-
1
No. [g (mmol)] [g(mmol] [g(mmol)] [mg (mmol)] –
(%)
57
(h )
3549
4
1
2
3
4
5
6
7
8
9
1
1
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
2.38 (20)
3.57 (30)
2.38 (20)
2.38 (20)
2.38 (20)
1.13 (10) 1.43 (10)
1.13 (10) 1.43 (10)
1.13 (10) 1.43 (10)
1.13 (10) 1.43 (10)
2.27 (20) 1.43 (10)
3.39 (30) 1.43 (10)
3.39 (30) 1.43 (10)
3.39 (30) 1.43 (10)
3.39 (30) 2.86 (20)
3.39 (30) 4.29 (30)
3.39 (30) 4.29 (30)
0.5(8.03 × 10 )
13
10
11
6
4
82
80
85
78
71
48
78
72
96
99
44
–
3
1.0 (1.60 × 10 ) 62
1937
890
831
852
914
904
893
935
956
9
–
3
2.0 (3.2 × 10 )
57
80
82
88
87
86
90
92
42
3
–
3
3.0 (4.8× 10 )
14
15
43
17
18
4
–
3
3.0 (4.8 × 10 )
13
8
–
3
3.0 (4.8 × 10 )
–
3
3.0 (4.8 × 10 )
4
–
3
3.0 (4.8 × 10 )
10
-
–
3
3.0 (4.8 × 10 )
–
3
0
2
3.0 (4.8 × 10 )
-
-
1
35
31
2
0

Figures
VI
Fig. 1. ORTEP for the compound [U O (pip-2,4-dmp)(DMSO)] (2a). All the non-hydrogen
2
atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
2
1

VI
Fig. 2. Crystal packing of the compound [U O (pip-2,4-dmp)(DMSO)] (2a). Drawing was done
2
in balls and sticks using SHELXL package. Hydrogen atoms are omitted by clarity.
2
2

1
3
VI
Fig. 3. C NMR spectra of H pip-2,4-dmp (I) and [U O (pip-2,4-dmp)(MeOH)] (2).
2
2
VI
Fig. 4. Conversion of 10 mmol of thymol with varying amounts of [Mo O (pip-2,4-dmp)].
2
Other reaction conditions (in mmol), substrate : H O : KBr : HClO ratios of 10 : 10 : 10 : 10.
2
2
4
2
3

VI
Fig. 5. Effect of oxidant on the oxidative bromination of 10 mmol of thymol using [Mo O (pip-
2
2
,4-dmp)] as catalyst. Other reaction conditions (in mmol), substrate : catalyst : KBr : HClO4
–
3
ratios of 10 : 4.16 × 10 : 10 : 10.
VI
Fig. 6. Effect of variation of KBr on the oxidative bromination of thymol using [Mo O (pip-2,4-
2
dmp)] complex as catalyst. Other reaction conditions (in mmol), substrate : oxidant : catalyst :
–
3
HClO ratios of 10 : 10 : 4.16 × 10 : 10.
4
2
4

VI
Fig. 7. Effect of variation of HClO on the oxidative bromination of thymol using [Mo O (pip-
4
2
2
:
,4-dmp)] complex as catalyst. Other reaction conditions (in mmol), substrate : oxidant : catalyst
–
3
KBr ratios of 10 : 10 : 4.16 × 10 : 10 : 10.
VI
Fig. 8. Conversion of thymol with varying amounts of [U O (pip-2,4-dmp)(MeOH)] as catalyst.
2
Other reactions, Other reaction conditions (in mmol), substrate : H O : KBr : HClO ratios of 10
2
2
4
:
10 : 10 : 10.
2
5

VI
Fig. 9. Effect of oxidant on the oxidative bromination of thymol using [U O (pip-2,4-
2
dmp)(MeOH)] as a catalyst. Other reaction conditions (in mmol), substrate : catalyst : KBr :
–
3
HClO ratios of 10 : 4.8 ×10 : 10 : 10.
4
VI
Fig. 10. Effect of variation of KBr on the oxidative bromination of thymol using [U O (pip-2,4-
2
dmp)(MeOH)] as catalyst. Other reaction conditions (in mmol), substrate : catalyst : KBr :
–
3
HClO ratios of 10 : 4.8 ×10 : 10 : 10.
4
2
6

VI
Fig. 11. Effect of variation of HClO on the oxidative bromination of thymol using [U O (pip-
4
2
2
,4-dmp)(MeOH)] complex as catalyst. Other reaction conditions (in mmol), substrate : oxidant :
–
3
catalyst : KBr ratios of 10 : 4.8 ×10 : 10 : 10.
1
1
0
0
.5
.0
.5
.0
VI
[
U O (pip-2,4 dmp)(MeOH)]
2
3
83
5
02
3
00
400
500
600
700
Wavelength/nm
VI
Fig. 12. Plots representing the spectral changes observed during the titration of [U O (pip-2,4
2
dmp)(MeOH)] (2) with H O . Spectra were obtained after successive addition of one drop
2
2
–
3
portion of 30% H O (0.108 g, 0.95 mmol) dissolved in 5 mL of MeCN to 25 mL of 4.8 ×10 M
2
2
solution of 2 in MeCN.
2
7

Supporting Information
2
94 Cel
499 Cel
.67 mg/min
160.0
2
1.500
2.68 mg/min
2
.0
1.000
140.0
0.0
5
38 Cel
0.639 mV
0
.500
-2.0
3
.2. Structure description
3
08 Cel
1
20.0
0.132 mV
VI
-4.0
ORTEP diagram for the complex [U O (pip-2,4-dmp)(DMSO)] is showed in
0
.000
2
-1.20 mV.s/mg
24 Cel
-
4.28 mV.s/mg
100.0%
-6.0
100.0
-0.500
1
9
05 Cel
7.8%
-8.0
2
9
84 Cel
5.1%
307 Cel
81.5%
-1.000
80.0
-10.0
4
6
71 Cel
6.7%
3
7
86 Cel
2.3%
-1.500
-12.0
6
4
0.0
0.0
5
4
26 Cel
2.9%
995 Cel
42.9%
-14.0
-2.000
-16.0
0
.00
20.00
40.00
60.00
80.00
Time min
VI
Fig. S1. TGA, DTA and DTG profiles of [U O (pip-2,4-dmp)(MeOH)] (2) under air
2
atmosphere.
2
1
1
0
0
.0
.5
.0
.5
.0
VI
[
[
Mo O (pip-2,4 dmp)(MeOH)]
VI
2
U O (pip-2,4 dmp)(MeOH)]
2
H pip-2,4 dmp
2
3
00
400
500
600
Wavelength/nm
VI
Fig. S2. UV-Visible spectra of H pip-2,4 dmp and complexes, [Mo O (pip-2,4 dmp)(MeOH)]
2
2
VI
(
1) and [U O (pip-2,4 dmp)(MeOH)] (2) recorded in MeCN.
2
2
8

1
Fig. S3. H NMR spectrum of ligand, H pip-2,4-dmp.
2
2
9