Biomimetic Oxidative Bromination by cis-Dioxidotungsten(VI) Complexes of Salan Type N,N’-Capped Linear Tetradentate Amino Bisphenol

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doi.org/10.1002/ejic.202100357

Biomimetic Oxidative Bromination by cis-Dioxidotungsten

(VI) Complexes of salan type N,N’-Capped Linear

Tetradentate Amino Bisphenol

Mannar R. Maurya,*^[a]Shailendra K. Maurya,^[a]Naveen Kumar,^[a]and Puneet Gupta^[a]

Reaction of [W^VIO₂(acac)₂] (Hacac=acetylacetone) with salan-

type dibasic tetradentate ONNO donor Mannich bases derived

from ethylenediamine, formaldehyde and 2,4-di-tert-butylphe-

nol (H₂L¹), 2-tert-butyl-4-methylphenol (H₂L²), 2,4-dimeth-

and two amine nitrogen atoms (in a cis-α type symmetric

binding mode) with one of the N atoms of the ligand and one

of the terminal O atoms of the cis-WO₂group in the axial

position. These complexes are potential catalyst precursors for

the oxidative bromination of thymol and styrene. Thymol upon

bromination gave three products, namely, 2-bromothymol, 4-

bromothymol, and 2,4-dibromothymol; later one being the

major product. Oxidative bromination of styrene resulted in 2-

bromo-1-phenylethanol and 1-phenylethane-1,2-diol; the later

one is the result of nucleophilic attack of water on the α as well

as β carbons both of the initially formed 1,2-dibromo-1-phenyl-

ethane.

ylphenol (H₂L³) and 2,4-dichlorophenol (H₂L⁴) in

a 1:1

([W^VIO₂(acac)₂]:H₂L) molar ratio in refluxing MeOH gave the

corresponding cis-dioxidotungsten(VI) complexes [W^VIO₂L¹] (1),

[W^VIO₂L²] (2), [W^VIO₂L³] (3) and [W^VIO₂L⁴] (4), respectively.

Characterization by elemental analysis, various spectroscopic

1

(FT-IR, UV-vis, H and ¹³C NMR) studies, DFT calculations and

single-crystal X-ray analysis of 2 and 3 suggest six-coordinated

octahedral α-cis (symmetric) isomeric form of the complexes

where ligands coordinate through the two phenolate oxygen

Introduction

structural conformation which may affect on the catalytic

activity of different substrates and selectivity of the product

(s).^[14–16]Considering the location of oxygen atoms (Ax, Eq) of cis-

[MO₂] (M=V, Mo and W) complexes, and positions of O and N

atoms of such flexible tetradentate ligands in octahedral

environment, three different isomeric structures are possible: (i)

α-cis, symmetric (a), (ii) β-cis, asymmetric (b), and (iii) β-cis,

asymmetric (c) (Scheme 1).

Amongst the third row transition series, tungsten is the only

bio-metal known for two families of tungsten containing

enzymes: (i) aldehyde oxidoreductases (AORs) and (ii) the

formate dehydrogenases (FDHs).^[1,2]The presence of WO₂has

been proposed in the oxidised active sites of tungstoenzymes,

and several model complexes have been prepared to under-

stand their redox chemistry.^[3]Leaving the chemistry associ-

ated with these enzymes, tungsten(VI) complexes having cis-

[WO₂] moiety have mostly been explored for the catalytic

epoxidation of alkenes.^[4–10]In spite of the good catalytic

potentials of these complexes, the growth of the tungsten

chemistry, particularly having cis-[WO₂] moiety is limited.^[4–13]

After the first report on dioxidotungsten(VI) complexes of

hydrogenated analogue of H₂salen [N,N’-ethylenebis-

(salicylideneimine)] (i.e. H₂salan) and related ligands,^[13a]groups

of Lehtonen and Mösch-Zanetti have reported dioxidotungs-

ten(VI) complexes of N-methyl derivatives of H₂salan type

ligands.^[9,13b,c]

In cis-[M^VIO₂] complexes (M=Mo^[14,16,18]or W^[9]) of salen- and

salan-type ligands only isomer of the type (a) (i.e. α-cis,

symmetric) has been observed whereas isomers (a) and (b) in

cis-[VO₂(EDTA)]^3À(H₄EDTA=ethylediamine tetraacetic acid)^[19]

and cis-[VO₂(EDDA)]^À(H₂EDDA=ethylenediamine-N,N’-diacidic

acid),^[20]respectively, have been confirmed by single crystal X-

ray study. Based on various spectroscopic studies Crans et al.

have concluded the presence of species (a) and (b) in

equilibrium in a solution of cis-[VO₂(EDDA)]^À. They have also

argued that the presence of bulky group in case of cis-

[VO₂(EDTA)]^Àis possibly responsible for the isolation of type (a)

isomer.

Upon metal complex formation H₂salan, being more flexible

than H₂salen and having better N-basicity, may lead to different

We have prepared herein cis-[W^VIO₂] complexes of different

derivatives of H₂salan type N,N’-capped linear tetradentate

amino bisphenolate (Scheme 2) with the aims: (i) whether such

complexes will adopt the same α-cis, symmetric (a) isomeric

structure with more flexible ligands than N-methyl derivatives

and if they do adopt the same isomeric structure then why?

and (ii) explore them as bioinspired functional model of

vanadium haloperoxidase enzymes (V-HPOs). Oxidative halo-

genation is one of the key reactions shown by V-HPOs.^[21–23]

Such reaction has been of industrial importance as this green

procedure deals with the introduction of bromo group(s) into

[a] Prof. Dr. M. R. Maurya, S. K. Maurya, N. Kumar, Dr. P. Gupta

Department of Chemistry,

Indian Institute of Technology Roorkee,

Roorkee-247667, India

E-mail: m.maurya@cy.iitr.ernet.in

puneet.gupta@cy.iitr.ac.in

https://cy.iitr.ac.in/~CY/rkmanfcy

https://cy.iitr.ac.in/~CY/Puneet_Gupta

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/ejic.202100357

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Scheme 1. Three different possible isomeric structures for octahedral complexes having cis-[MO₂] (M=V, Mo and W) moiety: (i) α-cis, symmetric (a), (ii) β-cis,

asymmetric (b) and (iii) β-cis, asymmetric (c). α-Cis, symmetric represents two terminal Os of cis-MO₂in trans position to both Ns, and β-cis, asymmetric

represents one terminal O of cis-MO₂trans to N and other one trans to O. Nomenclatures are partly adopted form references 16 and 17. All structures are

drawn considering Z-axis as the axial sites.

2 and 3 confirms the isolation of α-cis, symmetric (a) (c.f.

Scheme 1) isomer only in the solid state.

Thermal Study

°

All complexes are thermally stable up to 280 C and there after

°

decompose in two major steps. First step starts at ca. 280 C

°

and completes with 8–30% mass loss at ca. 306 C. Second step

°

starts immediately after first step and completes at ca. 500 C

with the formation of WO₃. The observed WO₃content of 33.0%

for 1, 36.0% for 2, 42.0% for 3, and 37.0% for 4 match well with

the calculated value of 32.6% for 1, 37.0% for 2, 42.4% for 3

and 37.2% for 4 (Figure S1), respectively.

Scheme 2. Structure of ligands and their abbreviations used in this study.

organic molecules. While cis-[V^VO₂] complexes^[24–29](and cis-

[M^VIoO₂] complexes^[30,31]to a less extent) have been used as

functional mimics (oxidative bromination in particular) of V-HPO

enzymes, expect one report from our group, cis-[W^VIO₂]

complexes have not been explored for the oxidative bromina-

tion of organic substrates.

Structure Descriptions of Complexes 2and 3

The coordination functionality of ligands and the resulting

geometry around the tungsten centre was confirmed by SC-

XRD studies. ORTEPs for the compounds [W^VIO₂L²] (2) and

[W^VIO₂L³] (3) are presented in Figure 1 and Figure 2, respectively.

Selected bond distances and angles are given in Table 1.

The structure of six-coordinated complexes 2 and 3 adopt a

distorted octahedral geometry with two O_oxidoterminal oxygen

atoms, two O_phenolateand two N_amineatoms of ligands. The axial

sites are occupied by the terminal O_oxidoatom [O(1)] and the

nitrogen atoms [N(1)] of the ligand while the other terminal

O_oxido[O2], two O_phenolate[O(3) and O(4)], and one N_amine[N(2)]

Results and Discussion

Ligands H₂L^1–4(I–IV) are similar to H₂salan-type ligands^[9,14]but

are obtainable through a rather simple Mannich condensation

between ethylenediamine, formaldehyde and 2,4-disubstituted

phenols^[18]in good yields. These ligands react with [W^VIO₂(acac)₂]

in 1:1 molar ratio in refluxing MeOH to yield corresponding

dioxidotungsten(VI) complexes, [W^VIO₂L¹] (1), [W^VIO₂L²] (2),

[W^VIO₂L³] (3) and [W^VIO₂L⁴] (4), respectively. These complexes are

cream coloured and soluble in MeOH, EtOH, CHCl₃, MeCN, DMF

and DMSO. Elemental analyses and various spectroscopic (FT-IR,

UV-vis, ¹H and ¹³C NMR) studies confirm the formation of

complexes. However, single-crystal X-ray analysis of complexes

17–19]

atoms of ligands are arranged in equatorial position^[9,14,

Thus, the structure belongs to α-cis, symmetric (a) (c.f.

Scheme 1) type isomer, a common isomeric form found in most

complexes having cis-[MO₂] (M=V, Mo and W) structure and

two nitrogen atoms are in a cis-α type binding mode.^{[9,14,17–19]}

The equatorial plane is distorted with respect to the planarity.

The tungsten atom is displaced toward the apical oxido ligand,

O(1), from the equatorial plane defined by N(2), O(2), O(3) and

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Figure 1. ORTEP of [W^VIO₂L²] (2). All the non-hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.

Figure 2. ORTEP of [W^VIO₂L³] (3). All the non-hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.

VI

2

VI

3

°

Table 1. Bond lengths [Å] and angles [ ] for [W O₂L ] (2) and [W O₂L ] (3).

°

angle [106.19(12) in 2, and 107.66(14) in 3] (see Table 1) are

well within the range for the bond distances and angles

typically observed in these types of compounds.^[9,14,17,18]Inter-

molecular hydrogen bonds were observed in compounds 2 and

3 (see Table 2). The crystal packing (see Figure S2 and Figure S3)

of the complexes 2 and 3 confirms that they have 8 and 4

molecules per unit cell, respectively.

Bond

2

3

W(1)-O(1)

W(1)-O(2)

W(1)-O(3)

W(1)-O(4)

W(1)-N(1)

W(1)-N(2)

1.724(2)

1.721(2)

1.950(2)

1.9248(19)

2.334(2)

2.354(3)

1.720(4)

1.727(3)

1.949(2)

1.947(3)

2.347(3)

2.314(3)

Angle

2

3

O(1)-W(1)-O(2)

O(1)-W(1)-O(4)

O(2)-W(1)-O(4)

O(1)-W(1)-O(3)

O(2)-W(1)-O(3)

O(4)-W(1)-O(3)

O(1)-W(1)-N(1)

O(2)-W(1)-N(1)

O(4)-W(1)-N(1)

O(3)-W(1)-N(1)

O(1)-W(1)-N(2)

O(2)-W(1)-N(2)

O(4)-W(1)-N(2)

O(3)-W(1)-N(2)

N(1)-W(1)-N(2)

106.19(12)

93.74(9)

99.82(9)

98.93(10)

95.40(9)

156.64(8)

163.65(11)

89.73(11)

79.58(8)

107.66(14)

99.00(17)

96.39(11)

96.52(14)

98.80(12)

153.85(12)

164.78(13)

87.19(13)

82.18(12)

77.45(10)

91.84(13)

160.39(12)

78.06(10)

80.53(11)

73.48(11)

IR Spectral Study

The IR spectral analysis of the ligands and complexes was

carried out to ascertain the coordination modes of the ligands

in complexes. Selected IR bands are presented in the

experimental section. The presence of two characteristics IR

bands at 886–901 and 922–942 cm^{À 1}due to the υ_sym(O=W=O)

and υ_asym(O=W=O) stretches, respectively, approves the cis-

[WO₂] core in these complexes.^[7,9,11,13]All ligands display a broad

band around 3200 cm^{À 1}due to υ(OH) of the phenol and

absence of this band in complexes suggests the coordination of

phenolate O after deprotonation. Ligands as well as complexes

both exhibit a broad band at 3050–3300 cm^{À 1}due to υ(NH)

stretch, hence, the coordination of N could not be established

explicitly. However, the coordination of N, at least, in two

complexes and N as well as O in all complexes are well

supported by the SC-XRD and NMR studies, respectively.

82.89(8)

91.09(11)

162.65(11)

80.14(9)

80.08(8)

73.14(9)

O(4), by 0.009 Å in 2 and 0.010 Å in 3. The WÀ O_oxidobond

lengths [W(1)À O(1): 1.724(2) in 2 and 1.720(4) in 3, and

W(1)À O(2): 1.721(2) in 2 and 1.727(3) in 3] and O=W=O bond

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Table 2. Hydrogen bonds for [W^VIO₂L²] (2) and [W^VIO₂L³] (3).^[a]

Compound

DÀ H···A

d(DÀ H)

d(H···A)

d(D···A)

ﬀ(DHA)

2

3

C(15)-H(15B)···O(1)#2

N(2)-H(2 N)···O(2)#1

0.970(1)

0.98(1)

2.226(1)

1.907(2)

2.997(2)

2.840(3)

135.61(4)

158.28(1)

[a] Symmetry transformations used to generate equivalent atoms: #1 À x+3/2, y+1/2, À z+3/2 #2 x+1/2, y, À z+3/2.

UV-visible Spectral Study

symmetrical in nature. These make the protons of two

individual ArCH₂NÀ groups as well as the À CH₂CH₂À protons

non-equivalent. Accordingly, ArCH₂NÀ protons appear as two

doublets [e.g. at 4.70 (d, 2H) and 3.90 (d, 2H) ppm for 1] while

ligands exhibit only one singlet at δ=3.78–3.91 ppm. Similarly,

the À CH₂CH₂À protons appearing as singlet at δ=2.70–

2.94 ppm in ligands split into two triplets (Table 3). Such

observations have also been noted in similar dioxidotungsten

(VI)^[9]as well as dioxidomolybdenum(VI) complexes.^{[9,14, 18]}

UV-vis spectral data of ligands and their cis-[W^VIO₂] complexes

recorded in MeCN (for spectra see Figure S4). The UV-vis spectra

of the ligands display two spectral bands at 224–236 and 282–

292 nm^[18]which are assigned to π!π* and n!π* transitions,

respectively. These transitions undergo red and blue shift,

respectively, in metal complexes. Coordination of ligands’

functionality to the metal ion thereby restructuring of ligands

possibly causing such changes. Surprisingly, we did not observe

ligand to metal charge transfer transition in all complexes even

in any solvent at any concentration. The possible reason for this

could be that they are either too high or too low in energy and

fall outside the visible part of the spectrum.

¹³C NMR Spectral Study

¹³C NMR spectral data recorded in d₆-[DMSO] are accessible in

Table 4; complex 3 along with the corresponding ligand as

representative of the series are reproduced in Figure 4. The

coordination-induced chemical shift [Δδ=δ(complex)À -

δ(ligand)], extracted from ¹³C NMR spectral data further

supports the binding modes of the ligands to the metal centre.

Thus, the carbons C1/C1’ bearing the phenolic oxygen of

ligands undergo a shift, Δδ between 1.68 to À 0.9 ppm (see

Table 4) after coordination with tungsten centre. Similarly, the

effect of coordination of nitrogen on the ethylenic carbons (C8/

C8’) as well as benzylic carbons (C7/C7’) is also significant as

¹H NMR Spectral Study

¹H NMR spectra of ligands and complexes recorded in d₆-

[DMSO] are presented in Table 3; spectra of H₂L²(II) and the

corresponding complex 2 as representative of the series are

reproduced in Figure 3. The disappearance a signal in com-

plexes, which appears at δ=10.21–10.93 ppm in ligands due to

two phenolic À OH protons, suggests the coordination of the

phenolic oxygen to the metal after their deprotonation. The they resonate with an average shift of Δδ between 0.16 to

À NH protons resonate as a sharp band at δ=3.28–3.39 ppm in

I, II and III and as a relatively broad signal at δ=5.12 ppm in IV.

Possibly due to redistribution of electron density upon coordi-

nation of nitrogen to the metal, this signal slightly shifts in the

corresponding complexes. The isomer α-cis (a) (c.f. Scheme 1)

proposed for all complexes and confirmed for 2 and 3 by single

crystal X-ray diffraction study is non-planar while ligands are

À 4.96 ppm and 2.06 to À 4.57 ppm, respectively. All other

carbons resonate well within the expected regions. Existence of

more than one species in solution have been observed by

Coreia et al.^[17]and Crans et al.,^[20]in cis-VO₂complexes, Since ¹³

C

NMR spectra of complexes exhibit only one signal each for C7/

C7’ and C8/C8’, the presence of only one species in solution

under the specified condition is suggested, and splitting of

Table 3. ¹H NMR (500 MHz) data (δ in ppm) of ligands and their complexes in d₆-[DMSO].

Compounds

À OH

À NH

Aryl (J in Hz)

ArCH₂NÀ

À CH₂CH₂À

CH₃

H₂L¹(I)

10.93 (s, 2H)

3.38 (s, 2H)

7.09( d, 2H, J=2),

6.86 (d, 2H, J=1.5)

7.18 (d, 2H, J=2),

6.96 (d, 2H, J=2)

6.86 (d, 2H, J=1.5),

6.65 (d, 2H, J=1.5)

6.97 (d, 2H, J=1.5),

6.77 (d, 2H, J=1.5)

6.79 (d, 2H, J=1.5),

6.68 (d, 2H, J=1.5)

6.89 (d, 2H, J=2),

6.73 (d, 2H, J=2)

7.33 (d, 2H, J=2),

7.12 (d, 2H, J=2)

7.50 (d, 2H, J=2.5),

7.26 (d, 2H, J=2.5)

3.87 (s, 4H)

2.93 (s, 4H)

1.35 (s, 18H),

1.21 (s, 18H)

1.37 (s, 18H),

1.25 (s, 18H)

2.13 (s, 6H),

1.33 (s, 18H)

2.21 (s, 6H),

1.35 (s,18H)

2.13 (s, 6H),

2.09 (s, 6H)

2.18 (s, 6H),

2.14 (s, 6H)

–

[W^VIO₂L¹] (1)

H₂L²(II)

–

4.62 (s, 2H)

3.28 (s, 2H)

4.58 (s, 2H)

3.39 (s, 2H)

5.12 (s, 2H)

–

4.70 (d, 2H),

3.90 (d, 2H)

3.82 (s, 4H)

2.94 (t, 2H),

2.36 (t, 2H)

2.94 (s, 4H)

10.76 (s, 2H)

[W^VIO₂L²] (2)

H₂L³(III)

–

4.69 (d, 2H),

3.82 (d, 2H)

3.78 (s, 4H)

2.93 (t, 2H),

2.35 (t, 2H)

2.85 (s, 4H)

10.21 (s, 2H)

[W^VIO₂L³] (3)

H₂L⁴(IV)

–

4.73 (d, 2H),

3.81 (d, 2H)

3.91 (s, 4H)

2.83 (t, 2H),

2.32 (t, 2H)

2.70 (s, 2H)

[W^VIO₂L⁴] (4)

5.61 (s, 2H)

4.73 (d, 2H),

3.99 (d, 2H)

2.94 (t, 2H),

2.26 (t, 2H)

–

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Figure 3. ¹H NMR spectra of H₂L²(II) and [W^VIO₂L²] (2). _*indicates signals due to water or DMSO.

Table 4. ¹³C NMR spectral data of ligands and complexes recorded in d₆-[DMSO].

Compounds

C1/C1’

C7/C7’

C8/C8’

R/R’

Other Carbons

H₂L¹(I)

153.65

155.12

(1.47)

153.62

155.20

(1.58)

152.70

154.38

(1.68)

153.76

152.86

(À 0.9)

57.85

53.28

(À 4.57)

57.32

52.94

(À 4.38)

56.47

52.59

(À 3.88)

50.04

52.10

(2.06)

50.95

46.50

(À 4.45)

50.79

46.40

(À 4.39)

50.89

45.93

(À 4.96)

46.27

46.43

(0.16)

31.44, 29.43

31.46, 30.98

139.8, 134.5, 122.9, 122.1, 121.3, 34.4, 33.7

141.3, 137.6, 123.8, 121.9, 121.5, 34.7, 33.9

[W^VIO₂L¹] (1)

H₂L²(II)

29.34, 20.34

29.92, 20.40

135.2, 126.7, 126.3, 126.1, 122.0, 34.1

138.3, 127.9, 127.5, 125.9, 122.0, 34.4

[W^VIO₂L²] (2)

H₂L³(III)

20.00, 15.52

20.01, 16.54

130.1, 126.7, 126.5, 123.5, 121.5

129.7, 128.1, 127.2, 126.9, 121.2

[W^VIO₂L³] (3)

H₂L⁴(IV)

–

127.3, 127.0, 126.4, 120.9, 120.7

128.0, 127.7, 125.7, 124.1, 123.5

[W^VIO₂L⁴] (4)

protons associated with these carbons are only due to non-

planarity.

Cyclic Voltammetry Study

The voltammograms of complexes 1 and 3 (Figure 5) exhibit

only one reductive peak at À 1.24 and À 1.02 V as well as only

one oxidative peak at 1.44 and 1.56 V, respectively, which are

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Figure 4. ¹³C NMR of spectra of H₂L³(III) and [W^VIO₂L³] (3). Broad multiplex is due to DMSO.

due to W(VI)!W(V) redox processes. Reductive peak of complex

1 is reversible in nature, while other peaks of complexes 1 and

3 are irreversible in nature. These peaks are also supported by

differential pulse voltammetry (DPV), where complex 1 and 3

shows one reductive peak each at À 1.14 and À 0.88 V and one

oxidative peak each at 1.38 and 1.49 V, respectively. In complex

2, one reductive peak at À 1.18 V due to W(VI)!W(V) could be

observed but two oxidative peaks at 1.49 and 1.96 V of

irreversible nature are seen which are attributed to W(IV)!W(V)

and W(V)!W(VI) processes (Figure 5). This is further confirmed

by their DPV which shows one reductive peak at À 1.05 V and

two oxidative peaks at 1.39 and 1.80 V. However, complex 4

shows (Figure 5) two reductive peak at À 0.68 and À 1.36 V and

only one oxidative peak at 2.03 V which correspond to the

W(VI)!W(V)!W(IV) and W(V)!W(VI) redox process respec-

tively, and these are irreversible in nature. Their DPV also shows

two reductive peaks at À 0.92 and À 1.29 V and one oxidative

peak at 1.83 V. While no information on the electrochemical

behaviour of similar cis-[W^VIO₂] complexes is available, the

analogous [Mo^VIO₂(salan)] complex with no substituent at

phenolic group shows irreversible electrochemical behaviour in

DMF but no further details are available. Therefore, results

obtained here cannot be compared directly with this complex.

However, complexes of N,N’-dimethyl derivative of H₂salan

having p-substituents on phenyl ring show quasi-reversible

electrochemical behaviour.^[14]The more negative reduction

potential, the more difficult for the cis-[W^VIO₂] complexes to be

reduced. Hence, the reduction of complexes follows the order:

4<1<2<3. The electron withdrawing group leads to the

shifting of the reduction potential in these complexes towards

less negative value. The observed CV data of all ligands and

complexes along with corresponding DPV are given in Table 5.

Density Functional Theory Study

DFT calculations were taken up to compute the relative stability

of the three possible isomeric forms: α-cis, symmetric (a), β-cis,

asymmetric (b), and β-cis, asymmetric (c) as proposed for cis-

[WO₂] complexes in Scheme 1 and to understand whether

optimised structures also suggest same configuration of

complexes as have been observed by single crystal X-ray study.

DFT calculations were performed employing the ORCA4.2

quantum chemical program package.^[21]The relative Gibbs free

energies reported in our DFT studies were obtained at the

BP86^[22,23]-D3^[24]/def2-SVP^[25]level. All Gibbs free energies re-

ported in our studies are in kcal.mol^{À 1}and relative to the

computational model for α-cis, symmetric (a). A relative stability

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Figure 5. CV plot of complexes [W^VIO₂L¹] (1), [W^VIO₂L²] (2), [W^VIO₂L³] (3) and [W^VIO₂L⁴] (4) along with the corresponding DPV plot. Conditions: room temperature

(ca. 298 K) in DMF, [complex]=1 mM, [TBAPF₆]=0.2 M, scan speed 100 mV/s, Pt working electrode, Ag/AgCl reference electrode and Pt wire as counter

electrode.

comparison of the three isomers for the ligand H₂L³(i.e. methyl

substituents at phenol) is presented in Figure 6. The isomer α-

cis, symmetric (a) is 3.2 kcal.mol^{À 1}more stable than the β-cis,

asymmetric (b), whereas the isomer β-cis, asymmetric (c) is

found to be 7.3 kcal/mol less stable as compared to the isomer

α-cis, symmetric (a), therefore, β-cis (c) is not modelled for rest

of the ligand types. For the two isomers [α-cis, symmetric (a)

and β-cis, asymmetric (b)], the cis-[WO₂] complexes of all four

ligand systems (H₂L¹–H₂L⁴, Scheme 2) were considered to

construct the computational models. The relative energies

results are presented in Table 6. Our computations show that

for all four complexes 1–4, the isomer α-cis, symmetric (a) is

more stable than the β-cis, asymmetric (b). The relative stability

results are in agreement with the structures solved by single

crystal X-ray study where only α-cis, symmetric (a) isomer was

found. This observation is also in line with the argument given

by Crans et al. that bulky groups present on the ligand system

may facilitated to move α-cis, symmetric isomer to β-cis,

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Table 5. Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) results for ligands and dioxidotungsten(VI) complexes at 298 K.

Compounds

Epa[V]^[a]

Epc[V]^[a]

DPV

H₂L¹(I)

1.43

1.44

1.24

1.49, 1.96

1.06

1.56

1.10

2.03

À 1.37

À 1.14, 1.53

[W^VIO₂L¹] (1)

H₂L²(II)

À 1.24

À 1.14,1.38

À 1.43

À 1.19, 0.94

[W^VIO₂L²] (2)

H₂L³(III)

À 1.18

À 1.05, 1.39, 1.80

À 1.03, 1.04

À 0.71

[W^VIO₂L³] (3)

H₂L⁴(IV)

À 1.02

À 0.88, 1.49

À 0.55, À 1.14

À 0.68, À 1.36

À 0.57, À 1.09, 1.18

À 0.92, À 1.29, 1.83

[W^VIO₂L⁴] (4)

[a] Epa and Epc are the anodic and cathodic peak potentials vs SCE, respectively.

Figure 6. A relative stability comparison of the three isomers for the ligand H₂L³. Hydrogens attached to carbons are not shown to enhance the clarity.

Table 6. DFT computed relative Gibbs free energies of the isomers α-cis (a) and β-cis (b) (see Scheme 1) for cis-[WO₂L] complexes 1–4.

Complex with substituents

Isomer α-cis (a)

Isomer β-cis (b)

Gibbs Free Energy (kcal.mol^{À 1}

)

Gibbs Free Energy (kcal.mol^{À 1}

)

1: R=R’=^tBu

2: R=^tBu, R’=Me

3: R=R’=Me

4: R=R’=Cl

0.0

2.8

3.6

3.2

3.8

asymmetric in solution^[20]as Gibbs free energies difference is

less in case of ^tBu substituents than that having chloro

substituents (2.8 vs 3.8 kcalmol^{À 1}). DFT optimized structures of

α-cis, symmetric (a) and β-cis, asymmetric (b), are presented in

Figure 7 and Figure 8. Geometrical parameters obtained from

DFT computations for these isomers are summarized in Table S1

and Table S2. These bond lengths and angles are also

comparable to the ones obtained through SC-XRD within the

acceptable limit.

intermediate oxido(peroxido) species has been proposed which

allows nucleophilic attack of the bromide ion, followed by

release of HOBr/Br₂which actually brominates the substrates.

The cis-[WO₂] complexes, 1–4 reported here act as catalyst

precursor to brominate thymol and styrene in the presence of

KBr, 30% aqueous H₂O₂, and 70% aqueous HClO₄in aqueous

medium. Phenol being ortho- and para-directing, the bromina-

tion on thymol takes place on the most activated site and gives

three products, namely, 2-bromothymol, 4-bromothymol, and

2,4-dibromothymol (Scheme 3); further bromination of mono-

bromo products possibly gives 2,4-dibromothymol.^[30–34]These

are the usual products reported in the literature.^[30–34]

Catalytic Activity Studies

As pointed out in the experimental section (vide infra) the

reaction conditions were optimised considering 2 as a repre-

sentative catalyst to obtain the maximum yield of brominated

products. Thus, for 10 mmol of thymol (1.50 g), four different

amounts of catalyst (0.5, 1.0, 1.5 and 2.0 mg), three different

amounts of 30% aqueous H₂O₂(10, 15 and 20 mmol) and three

Oxidative Bromination of Thymol – Vanadium complexes

studied as functional models of V-HPOs play an important role

not only in brominating the organic molecules but in establish-

ing the possible reaction pathway as well. During the catalytic

oxidative bromination by vanadium,^[26–29]the formation of an

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Figure 7. DFT computed structures of the isomer α-cis, symmetric (a) for different cis-[WO₂] complexes. Hydrogens attached to carbons are not shown to

enhance the clarity.

Figure 8. DFT computed structures of the isomer β-cis, asymmetric (b) for different cis-[W^VIO₂] complexes. Hydrogens attached to carbons are not shown to

enhance the clarity.

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Scheme 3. Products of the oxidative bromination of thymol.

Table 8. Conversions, turn over frequency and selectivity parameters for various catalysts for the oxidative bromination of thymol in 1 h of reaction time.

Catalyst

[mg]

TOF

Conv. [%]

Selectivity [%]^[b]

2-Brth

[a]

[h^{À 1}

]

4-Brth

2,4-diBrth

[W^VIO₂L¹] (1)

[W^VIO₂L²] (2)

[W^VIO₂L³] (3)

[W^VIO₂L⁴] (4)

Without catalyst

1.0

6596

5975

4891

5313

–

93

95

90

85

18

14

12

13

12

08

76

63

60

70

10

25

24

28

22

[a] TOF values calculated at 1 h reaction time. [b] 2-Brth=2-bromothymol, 4-Brth=4-bromothymol and 2,4-diBrth=2,4-dibromothymol.

different amounts of KBr (10, 15 and 20 mmol) were taken in

20 mL water. Three different amounts of 70% aqueous HClO₄

(10, 15 and 20 mmol) was added in two equal portions as

pointed out in the experimental section to avoid the decom-

position of complex and the reaction was carried out at room

temperature for 2 h.

Details of all reaction conditions and the corresponding

conversions of thymol are summarized in Table 7. Figure S5

provide time on conversion of thymol under different reaction

conditions. It may be concluded from the data presented in

table that conversion of thymol differs considerably under

different reaction conditions. In general, though 4-bromothy-

mol is obtained in highest yield under all conditions along with

other two products, conditions under entry 8 is suitable to

achieve mainly two monobromo products in major yields along

with 80% conversion. However, entry 2 presents the best suited

10^{À 3}:1:2:2:2) for the maximum conversion of thymol. Under

these conditions, reaction reaches to equilibrium in 1 h and the

obtained conversion is 95% where the selectivity of three

bromo products follows the order: 4-bromothymol (63%) >2,4-

dibromothymol (25%) >2-bromothymol (12%).

Other three catalysts i.e. 1, 3 and 4 were also tested

considering 1.0 mg of each under the above optimized reaction

conditions and the results are accessible in Table 8. It is clear

from the data of table that other three complexes are also

active and give 85–93% conversion (also see Figure 9) with

high turnover frequency. Again 4-bromothymol is highest in

selectivity in all cases. Blank reaction i.e. without catalyst under

these conditions gave only 18% and 32% conversion of thymol

in 1 h and 2 h, respectively. We have also checked the activity

of Na₂WO₄and WO₃·H₂O (using same mole equivalent as for 1)

under above optimized reaction conditions (Table 7) and

obtained conversions 70 and 68%, respectively, are in line of

conditions

catalyst:substrate:H₂O₂:KBr:HClO₄

(i.e.

for

10 mmol

of

ratios

substrate,

are

the

1.59×

Table 7. Conversion of thymol (1.50 g, 10 mmol) using 2 as a catalyst, turn over frequency (TOF), and selectivity of different products at different reaction

time under different reaction conditions.

Entry

KBr

[g (mmol)]

H₂O₂

[g (mmol)]

HClO₄

[g (mmol)]

Catalyst

[mg (mmol)]

Conv.

[%]

Time

[h]

TOF^[a]

[h^{À 1}

Selectivity [%]^[b]

]

a

b

c

1

2.38 (20)

1.78 (15)

1.19 (10)

2.38 (20)

2.26 (20)

1.69 (15)

1.13 (10)

2.26 (20)

2.86 (20)

2.15 (15)

1.43 (10)

2.86 (20)

0.5 (7.98×10^{À 4}

1.0 (1.59×10^{À 3}

1.5 (2.39×10^{À 3}

2.0 (3.19×10^{À 3}

1.0 (1.59×10^{À 3}

blank

)

88

95

93

91

97

75

98

80

98

71

18

1

2

1

11027

5975

3891

2853

3053

2358

3082

2516

3082

2233

–

11

12

13

14

13

14

08

55

63

67

56

77

83

75

84

77

76

70

34

25

21

32

10

03

12

02

09

10

22

2^[c]

3

4

5

6

7

8

9

10

11

[a] TOF: Moles of substrate converted per mole of catalyst per hour. [b] a=2-bromothymol, b=4-bromothymol and c=2,4-dibromothymol. [c] The optimized

conditions mentioned in entry 2 are the best among the different sets of reactions carried out.

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Oxidative Bromination of Styrene

All these complexes have also been used as a catalyst to study

the oxidative bromination of styrene. We have again considered

complex 2 as a representative catalyst and the oxidative

bromination of styrene was carried out at room temperature as

detailed above and achieved the formation of 2-bromo-1-

phenylethanol and 1-phenylethane-1,2-diol, though literature

reports the formation of one more product i.e. 1,2-dibromo-1-

phenylethane (Scheme 4).^[30,31]

As observed in model cis-[MO₂] complexes (M=V or

Mo),^[26–31,35]the generated HOBr and/or Br⁺, Br₂, Br^3À, through

catalytic reaction of these complexes with H₂O₂and HClO₄in

the presence of KBr, react with styrene to generate bromonium

ion as an intermediate (Scheme 5). The nucleophile Br^Àas well

as H₂O both may further attack on the α-carbon of the

intermediate to give 1,2-dibromo-1-phenylethane and 2-bromo-

1-phenylethane-1-ol, respectively. Since 1,2-dibromo-1-phenyl-

ethane is not observed in the reaction studied here, it seems

that the nucleophile H₂O activates α- as well as β-carbons both

of it to give 2-bromo-1-phenylethane-1-ol and 1-phenylethane-

1,2-diol.^[35,36]2-Bromo-1-phenylethane-1-ol can further be con-

verted to 1-phenylethane-1,2-diol through α-carbon hydrolysis

and all these justify the formation of 1-phenylethane-1,2-diol in

highest yield. Since, styrene may also give styrene epoxide in

the presence of catalyst and oxidant, which can then undergo

nucleophilic attack, either from bromide or water to give the

observed products, such possibility has, however, been ex-

cluded as we did not observe the formation of styrene epoxide.

Subsequently, the catalytic reaction was investigated by

changing different parameters as mentioned above that may

affect the rate of styrene bromination and the selectivity of

different products. Details of all reaction conditions and the

corresponding conversion of styrene are presented in Figure 10

(and data are summarized in Table S4). From the data presented

in Table S4 it is clear that the optimized reaction conditions for

the maximum conversion of 10 mmol (1.04 g) of styrene are

those using 0.5 mg of catalyst, 1.69 g (15 mmol) of 30%

aqueous H₂O₂, 2.38 g (20 mmol) of KBr, and 2.86 g (20 mmol) of

70% aqueous HClO₄i.e. (entry no. 5 of Table S4). Under these

conditions conversion reaches 90% in 1 h and the selectivity of

the two products follows the order: 1-phenylethane-1,2-diol

(78%)>2-bromo-1-phenylethanol (22%).

Figure 9. Plots representing conversion of thymol in the presence of

different complexes as catalysts under aqueous phase. For reaction condition

see Table 7.

the approval of no decomposition of complexes and their role

in enhancing the conversion.

Different biphasic solvent systems have also been applied

to understand the catalytic potential of complex 2 towards the

oxidative bromination of thymol and their effect on the

selectivity of different products. The catalytic efficiency of 2

under the optimised reaction conditions (see Table 7) except

solvent:water (1:1 V/V, 20 mL) was found almost same in most

mixed solvent systems studied (Table S3). The conversions are

also comparable to the conversion obtained in aqueous system

but the selectivity of products differs. In biphasic system,

selectivity of 4-bromothymol is highest (57–69%) followed by

2,4-dibromophenol (19–33%) and then 2-bromophenol (5–

14%). Thus, bi-phasic solvent systems alter the selectivity of

products.

The catalytic potential of the complexes studied here are

equally good or slightly better than [V^VO₂],^[30–34]and

[Mo^VIO₂],^[35,36]and comparable to [W^VIO₂] complexes of tridentate

ONO donor Schiff base ligands derived from 2-hydroxyaceto-

phenone and nicotinoylhydrazide and benzoylhydrazide,^[11]but

selectivity of products slightly differs. The higher amounts of

KBr, H₂O₂and HClO₄(2 equivalents each for 10 mmol of

substrate) versus lower amounts of these reagents for vanadium

and molybdenum complexes might be responsible for such

differences.

Other three catalysts i.e. 1, 3 and 4 were also tested under

the above optimized reaction conditions considering 0.5 mg

each of these catalysts and the results are presented in

Scheme 4. Expected products upon oxidative bromination of styrene: (a) 1,2-dibromo-1-phenylethane, (b) 2-bromo-1-phenylethane-1-ol, and (c) 1-

phenylethane-1,2-diol.

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Scheme 5. Plausible mechanism of action of Br₂/HOBr on styrene and hydrolysis of the brominated products in the presence of water.

Figure 10. (a) Effect of variation of amount of catalyst on the oxidative bromination of styrene. Reaction conditions: styrene (1.04 g, 10 mmol), 30% aqueous

H₂O₂(2.26 g, 20 mmol), KBr (2.38 g, 20 mmol), and HClO₄(2.86 g, 20 mmol) at room temperature. (b) Effect of variation of amount of oxidant (H₂O₂) on the

oxidative bromination of styrene. Reaction conditions: styrene (1.04 g, 10 mmol), catalyst [W^VIO₂L²] (0.5 mg, 7.98×10^{À 4}mmol), KBr (2.38 g, 20 mmol), and

HClO₄(2.86 g, 20 mmol) at room temperature. (c) Effect of variation of amount of KBr on the oxidative bromination of styrene. Reaction conditions: styrene

(1.04 g, 10 mmol), catalyst [W^VIO₂L²] (0.5 mg, 7.98×10^{À 4}mmol), 30% aqueous H₂O₂(1.69 g, 15 mmol), and HClO₄(2.86 g, 20 mmol) at room temperature. (d)

Effect of variation of amount of HClO₄on the oxidative bromination of styrene. Reaction conditions: styrene (1.04 g, 10 mmol), catalyst [W^VIO₂L²] (0.5 mg,

7.98×10^{À 4}mmol), 30% aqueous H₂O₂(1.69 g, 15 mmol), and KBr (2.38 g, 20 mmol) at room temperature.

Figure 11. It is clear from the figure that substituents on the

ligands have not much influence and these complexes also

have similar catalytic potential. Conversion between 88–93%

with high TOF has been obtained in 1 h reaction time (Table 9).

Again 1-phenylethane-1,2-diol is highest in selectivity. Blank

reaction under above reaction conditions gave only 19% and

33% conversion in 1 and 2 h, respectively.

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mol and as expected 2,4-dibromothymol is obtained in major

yield. 2-Bromo-1-phenylethanol and 1-phenylethane-1,2-diol

were obtained upon oxidative bromination of styrene, though

1,2-dibromo-1-phenylethane was also an expected product. The

H₂O, a nucleophile, present in the oxidant H₂O₂possibly

activates α- as well as β-carbons both of 1,2-dibromo-1-phenyl-

ethane which allows it to undergo further hydrolysis to give

other products.

Experimental Section

Materials and General Methods

WO₂Cl₂, acetylacetone (SRL, India), 2,4-di-tert-butylphenol, 2,4-

dimethylphenol, 2-tert-butyl-4-methylphenol, 2,4-dichlorophenol,

ethylenediammine, formaldehyde and toluene (Rankem, India)

were used as supplied. Toluene was dried over sodium and distilled

before use. Other chemicals and solvents were of AR grade. Ligands

H₂L^1–4were prepared following literature method.^[18]

Figure 11. Plots presenting conversion of styrene in the presence of different

complexes as catalyst.

All measurements were carried out after drying the metal

°

complexes at 100 C. Elemental analyses of the complexes were

carried out on an Elementar model Vario-EI-III. Thermogravimetric

analyses of the complexes were carried out using a Perkin Elmer

Catalytic potential of 2 towards the oxidative bromination

of styrene under different biphasic solvent systems (1:1 V/V,

20 mL) is almost same as observed in aqueous system.

However, the selectivity of the formation of 1-phenylethane-

1,2-diol (Table S5) is at slightly higher side (ca. 10%) than

observed in aqueous solution.

°

(Pyris Diamond) instrument in air with a heating rate of 10 C/min

between room temperature to 1000 C. IR spectra were recorded as

°

KBr pellets on a Nicolet 1100 FT-IR spectrometer. UV-visible spectra

of ligands and complexes were recorded in acetonitrile on a

Shimadzu 2600 UV-vis spectrophotometer. H and ¹³C NMR spectra

1

were recorded in d₆-[DMSO] on a JEOL ECX 400 MHz spectrometer.

All the electrochemical measurements were carried out using

instrument (HCH-620 E) under the argon atmosphere assembled

with typical three-electrode assembly. The cyclic voltammogram

(CV) and differential pulse voltametric (DPV) of redox active ligands

(I–IV) and cis-[W^VIO₂] complexes (1–4) were recorded in the range of

À 2.0 V to +2.0 V vs. SCE using Pt working electrode, Ag/AgCl

reference electrode and Pt wire as counter electrode. Tetrabutyl

ammonium hexafluorophosphate (TBAPF₆), crystallized twice from

EtOH and vacuum dried, was used as supporting electrolyte in

distilled dichloromethane with a 100 mV/s scan rate at room

temperature (ca. 298 K). The concentration of these newly synthe-

sized complexes was maintained ca. 1 mM during the electro-

chemical analysis. A Shimadzu 2010 plus gas-chromatograph fitted

with a 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

quantifications were made on the basis of the relative peak area of

the respective product. The identity of the products was confirmed

using a GC-MS model PerkinElmer, Clarus 500 and comparing the

fragments of each product with the library available.

Conclusions

Four cis-dioxidotungsten(VI) complexes, cis-[W^VIO₂L^{1À 4}] (1–4)

have been prepared from potential dibasic tetradentate ONNO

salan-type Mannich base ligands derived from ethylenediamine

and 2,4-di-tert-butylphenol(H₂L¹) (I), 2-tert-butyl-4-methylphenol

(H₂L²) (II), 2,4-di-methylphenol (H₂L³) (III), and 2,4-di-chlorophe-

nol (H₂L⁴) (IV). All these complexes adopt α-cis, symmetrical

type configuration, a common isomeric form found in most

complexes having cis-[MO₂] (M=V, Mo and W) of salan-type

ligands. DFT study predicts the poor stability of other two

possible isomers. Oxidative bromination of thymol and styrene

has been successfully carried out using these complexes as

catalyst precursor, signifying them as useful functional model of

vanadium dependent haloperoxidases. Phenol being ortho- and

para-directing, the bromination on thymol gives three products,

namely, 2-bromothymol, 4-bromothymol, and 2,4-dibromothy-

Table 9. Conversion, turn over frequency and selectivity parameters for various catalysts for the oxidative bromination of styrene in 1 h of reaction time.

Catalyst

[mg]

TOF

Conv.

[%]

Selectivity [%]^[a]

b

[h^{À 1}

]

c

[W^VIO₂L¹] (1)

[W^VIO₂L²] (2)

[W^VIO₂L³] (3)

[W^VIO₂L⁴] (4)

Without catalyst

0.5

–

12518

11278

10087

10986

88

90

93

88

19

32

22

39

30

18

68

78

61

70

82

[a] For details of ‘b’ and ‘c’ see Scheme 4.

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correction (SADABS)^[39]was applied, and the structures were solved

Preparations

by direct methods and refined by full matrix least-squares on F²

data using SHELX suite of programs.^[40,41]All hydrogen atoms were

placed in geometrically calculated positions using a riding model

except for N1 and N2 which were located in difference Fourier map

and left to refine freely. Refinements were done with allowance for

thermal anisotropy of all non-hydrogen atoms. A final difference

Fourier map showed no residual density outside: 1.1432 and

À 1.3566 for 2 and 2.3074 and À 2.0994 for 3. A weighting scheme

Preparation of [W^VIO₂(acac)₂]

This was prepared by a slightly modified method reported in the

literature.^[37,38]To a suspension of W^VIO₂Cl₂(3.1 g,10.78 mmol) in a

freshly distilled 2,4-pentanedione (30 mL), dry toluene (100 mL) was

added and the resulting mixture was refluxed for 36 h with

constant stirring. Reflux condenser was fitted with a guard tube

filled with CaCl₂during this period. The resulting mixture was

filtered while still hot and solvent was removed completely from

the filtrate using rotatory evaporator to yield a light-yellow crude

product. The solid was collected after washing several times with

diethyl ether. Yield: 3.30 g (74.0%). Solid is pure enough to use for

the synthesis purpose.

2

w=1/[σ²(F_o)+(0.021322 P)²+6.383177P] for 2 and w=1/[σ²(F_o)+

(0.060902 P)²+0.00000 P] for 3, where P=(jF_oj +2jF_cj )/3, were

used in the latter stages of refinement. Further details of the crystal

structure determination are given in Table 10.

2

Catalytic Activity Studies

All catalytic reactions were carried out in a 50 mL round bottom

flask and a similar procedure was adopted for all complexes used

as catalyst. A typical reaction for each substrate is provided below:

Preparation of [W^VIO₂L¹] (1)

A filtered solution of [W^VIO₂(acac)₂] (0.455 g, 1.1 mmol) in MeOH

(20 mL) was added to a solution of H₂L¹(0.496 g, 1.0 mmol) in

MeOH (30 mL) and the obtained reaction mixture was refluxed with

slow and constant stirring in an oil bath for 16 h. A light cream solid

precipitated out upon reducing the solvent volume to ca.10 mL

and cooling the reaction mixture to room temperature, which was

filtered, washed with cold methanol, and dried under vacuum. Yield

0.550 g (77.0%). Anal. Calcd for C₃₂H₅₀N₂O₄W (710.60 g mol^{À 1}): C,

54.1; H, 7.1; N, 3.9. Found: C, 54.5; H, 7.4; N, 4.5%. IR (KBr) : 3260

(NH), 940 (O=W=O)_asym895 (O=W=O)_symcm^{À 1}. UV-vis (MeCN, ɛ/

litre mole^{À 1}cm^{À 1}): λ=238 (2.27×10⁴), 279 (1.91×10⁴) nm.

Oxidative Bromination of Thymol Thymol (1.5 g, 10 mmol), 30%

aqueous H₂O₂(1.13 g, 10 mmol), KBr (1.19 g, 10 mmol), 70%

aqueous HClO₄(1.43 g, 10 mmol) were taken in 20 mL water. After

adding a representative tungsten complex 2 as catalyst (1.0 mg),

the reaction mixture was stirred for 2 h at room temperature. The

best reaction condition for the bromination of thymol was

identified by varying several parameters, such as the amount of

catalyst, KBr, H₂O₂and HClO₄. When addition of HClO₄was more

than 10 mmol, it was added in two portions: first 10 mmol addition

at t=0 and the other at t=0.5 h to avoid decomposition of the

catalyst. The progress of the reaction was monitored by with-

drawing small aliquot of the reaction mixture, extracting it from

hexane and analysing by GC. The quantification of the brominated

products was made on the basis of the relative peak area of the

respective product and their identities were established by GC-MS

and ¹H NMR spectroscopy.

Preparations of [W^VIO₂L²] (2), [W^VIO₂L³] (3) and [W^VIO₂L⁴] (4)

Complexes 2–4 were prepared following above method using

[W^VIO₂(acac)₂] (0.455 g, 1.1 mmol) and respective ligands (1.0 mmol).

All complexes have almost same cream coloured solid.

[W^VIO₂L²] (2). Yield 0.440 g (70.0%). Anal. Calcd for C₂₆H₃₈N₂O₄W

(626.44 g mol^{À 1}): C, 49.9; H, 6.1; N, 4.5. Found: C, 50.3; H, 6.2; N,

4.9%. IR (KBr) : 3300 (NH), 922 (O=W=O)_asym889 (O=W=O)_sym

cm^{À 1}. UV-vis (MeCN, ɛ/ litre mole^{À 1}cm^{À 1}): λ=237 (3.05×10⁴), 282

(2.61×10⁴) nm.

Table 10. Crystal data and structure refinement for [W^VIO₂L²] (2) and

[W^VIO₂L³] (3).

2

3

[W^VIO₂L³] (3). Yield 0.477 g (88.0%). Anal. Calcd for C₂₀H₂₆N₂O₄W

Formula

Formula weight

T, K

Wavelength, Å

Crystal system

Space group

a/Å

C

₂₆H₃₈W N₂O₄

C₂₀H₂₆W N₂O₄

542.28

296.15

0.71073

Monoclinic

P2₁/n

12.2661(19)

11.0763(17)

15.761(2)

90

(542.28 g mol^{À 1}): C, 44.3; H, 4.8; N 5.2. Found: C, 45.0; H, 4.7; N 5.3%.

626.44

296.15

IR (KBr) : 3182, 3295 (NH), 942 (O=W=O)_asym886 (O=W=O)_symcm^{À 1}

.

0.71073

Orthorhombic

Pbca

13.9899(10)

12.887(1)

28.321(2)

90

5105.9(7)

8

UV-vis (MeCN, ɛ/ litre mole^{À 1}cm^{À 1}): λ=238 (2.96×10⁴), 281 (2.71×

10⁴) nm.

[W^VIO₂L⁴] (4). Yield 0.523 g (84.0%). Anal. Calcd for C₁₆H₁₄Cl₄N₂O₄W

(623.94 g mol^{À 1}): C, 30.8; H, 2.3; N 4.5. Found: C, 30.5; H, 2.4; N,

4.7%. IR (KBr) : 3220 (NH), 940 (O=W=O)_asym901 (O=W=O)_sym

cm^{À 1}. UV-vis (MeCN, ɛ/ litre mole^{À 1}cm^{À 1}): λ=242 (2.33×10⁴), 278

(1.99×10⁴) nm.

b/Å

c/Å

°

α/

°

β/

γ/

111.740(2)

90

1989.0(5)

°

V/Å³

Z

4

F₀₀₀

2509.7

1.630

4.558

2.88 to 28.37

0.0321

0.31×0.21×0.16

1.089

0.0219

0.0571

1.14 and À 1.36

1062.8

1.811

5.835

3.26 to 27.46

0.1711

0.48×0.41×0.35

0.974

0.0393

0.1029

2.31 and À 2.10

X-ray Crystal Structure Determination

D

_calc/gcm^{À 3}

μ/mm^{À 1}

Three-dimensional X-ray data were collected on a Bruker Kappa

Apex CCD diffractometer at room temperature for 2 and 3 by the

ϕ-ω scan method. Reflections were measured from a hemisphere of

°

θ/( )

R_int

Crystal size/ mm³

Goodness-of-fit on F²

R₁[I>2σ(I)]^[a]

°

data collected from frames, each of them covering 0.3 in ω. A total

of 63818 for 2 and 32962 for 3 reflections measured were corrected

for Lorentz and polarization effects and for absorption by multi-

scan methods based on symmetry-equivalent and repeated

reflections. A total of 6361 for 2 and 4338 for 3, independent

reflections exceeded the significance level (jFj/σjFj) >4.0, respec-

tively. After data collection, in each case an empirical absorption

wR₂(all data)^[b]

Largest differences

peak and hole (eÅ^{À 3}

)

2

[a] R₁=Σj jF_oj

À

jF_cj j/ΣjF_oj. [b] wR₂={Σ[w(j jF_oj

À

jF_cj j)²]j/Σ[w

2

(F_o)²]}^1/2

.

Eur. J. Inorg. Chem. 2021, 1–16

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Full Papers

doi.org/10.1002/ejic.202100357

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Conflict of Interest

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Keywords: Bromination

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Manuscript received: April 29, 2021

Revised manuscript received: May 21, 2021

Accepted manuscript online: May 26, 2021

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Prof. Dr. M. R. Maurya*, S. K. Maurya,

N. Kumar, Dr. P. Gupta

1 – 16

Biomimetic Oxidative Bromination

by cis-Dioxidotungsten(VI)

Complexes of salan type N,N’-

Capped Linear Tetradentate

Amino Bisphenol

Six-coordinated octahedral α-cis,

symmetric isomeric form of the cis-

[WO₂L] complexes of N,N’-capped

linear tetradentate amino bisphenol

have been synthesized and used as

catalyst for the oxidative bromination

of thymol and styrene.

Article Doi

Biomimetic Oxidative Bromination by cis-Dioxidotungsten(VI) Complexes of Salan Type N,N’-Capped Linear Tetradentate Amino Bisphenol

DOI: 10.1002/ejic.202100357

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Article abstract of DOI:10.1002/ejic.202100357

Full text of DOI:10.1002/ejic.202100357

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