Vanadium Complexes Derived from Acetyl Pyrazolone and Hydrazides: Structure, Reactivity, Peroxidase Mimicry and Efficient Catalytic Activity for the Oxidation of 1-Phenylethanol

DOI: 10.1002/ejic.201600427

Full Paper

Vanadium Haloperoxidase Models

Vanadium Complexes Derived from Acetyl Pyrazolone and

Hydrazides: Structure, Reactivity, Peroxidase Mimicry and

Efficient Catalytic Activity for the Oxidation of 1-Phenylethanol

[a]

[b]

[c]

Mannar R. Maurya,* Bithika Sarkar, Fernando Avecilla, and Isabel Correia*

Abstract: Schiff bases obtained from the condensation of acet- UV/Visible, EPR, NMR ( H, ¹³C and ⁵¹V); elemental analysis, ther-

1

ylpyrazolone (H ap) with benzoyl hydrazide (bhz), furoyl hydraz- mal studies, cyclic voltammetry (CV) and single-crystal X-ray

2

ide (fah), nicotinoyl hydrazide (nah) and isonicotinoyl hydrazide analysis. X-ray diffraction studies of complexes 6, 7, 9–12 con-

(

inh) [H ap-bhz (I), H ap-fah (II) H ap-nah, (III) and H ap-inh firm the ligand's coordination to the metal centre through en-

2 2 2 2

IV

IV)], sharing a ONO donor set, upon reaction with [V O(acac) ] olic oxygen (of pyrazolone), azomethine nitrogen and enolic

2

IV

lead to the formation of [V O(ap-bhz)(H O)] (1), [V O(ap- oxygen (hydrazide) atoms. The reactivity of the complexes and

fah)(H O)] (2), [V O(ap-nah)(H O)] (3) and [V O(ap-inh)(H O)] their catalytic potential was screened towards their peroxidase

2

IV

2

(

4), respectively. These complexes slowly convert to mono- mimetic activity in the oxidation of pyragallol in aqueous media

V

oxidovanadium(V) complexes [V O(ap-bhz)(OMe)(MeOH)] (11), with H O as oxidant, showing high activity under mild condi-

[

(

2

V

V O(ap-fah)(OMe)(MeOH)] (12), [V O(ap-nah)(OMe)(MeOH)] tions. They were also tested in the catalytic oxidation of 1-

13) and [V O(ap-inh)(OMe)(MeOH)] (14) in methanol. The reac- phenylethanol with H O that yields acetophenone as main

V

2

V

tion of aqueous K[H V O ] with the corresponding potassium product. Parameters such as catalyst and oxidant amount, time,

2

4

salt of the ligands at neutral pH gives dioxidovanadium(V) com- temperature, and solvent effects were optimised for maximum

V

plexes, K(H O)[V O (ap-bhz)] (5), K(H O) [V O (ap-fah)] (6), oxidation of 1-phenylethanol. The complexes show excellent

2

0.5

2

V

[

V O (Hap-nah)] (9) and [V O (Hap-nah)] (10). Acidification of catalytic activity towards oxidation of 1-phenylethanol being

2

V

solutions of 5 and 6 affords the neutral complexes [V O (Hap- structural and functional models of the vanadate-dependent

2

V

bhz)] (7) and [V O (Hap-fah)] (8), respectively. All complexes haloperoxidases.

2

were characterized by various spectroscopic techniques: FT-IR,

Introduction

rating a catalytic centre into a variety of scaffolds including li-

[

6]

gand-anchoring supramolecular complexes, Schiff base com-

Peroxidases catalyse the oxidation of a broad range of sub-

strates, using H O or other peroxides as oxidants. They have

[7]

[8]

[9]

[10]

[11]

plexes, hemes, porphyrins, micelles,

vesicles,

nano-

2

[12]

_a_n_d_n_a_n_o_r_o_d_s_.[13]

particles,

been successfully applied in different fields such as medicine,

chemical industries, food processing, treatment of industrial

waste and agriculture. However, peroxidases have some in-

trinsic drawbacks associated with their denaturation, special

storage requirements and cost. 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, overcom-

In the last several decades, a high number of vanadium com-

plexes have been synthesized and characterized due to contin-

[

1]

ued interest in the bioinorganic chemistry of vanadium, as

[14]

recognized by several monographs.

Vanadium complexes

with biologically important ligands, aiming to find structural

[

2]

and/or functional models of biogenic compounds are numer-

[15]

ous and have been reported in several reviews.

Our group

has made several contributions in recent years to the field of

[

3–5]

ing the drawbacks of natural enzymes.

number of artificial enzymes have been synthesized by incorpo-

At present, a large

[16]

peroxidase mimics.

In continuation of our efforts we report herein the synthesis

of oxidovanadium(IV), oxidovanadium(V) and dioxidovana-

dium(V) complexes of pyrazolone derivatives; Scheme 1. Pyraz-

olone ligands and their derivatives have shown biological and

therapeutical properties, such as antitumor, antimicrobial, and

[

a] Department of Chemistry, Indian Institute of Technology Roorkee,

47667Roorkee, India

2

E-mail: rkmanfcy@iitr.ac.in

www.iitr.ac.in/~CY/rkmanfcy

[

17]

[

b] Departamento de Química Fundamental, Universidade da Coruña,

Campus de A Zapateira, 15071 A Coruña, Spain

c] Centro de Química Estrutural, Instituto Superior Técnico, Universidade de

Lisboa,

antiviral activities, however, their chemistry and applications

are still underexplored. Recently, we reported the synthesis of

tridentate ONO donor ligands derived from a pyrazolone (4-

acetyl-3-methyl-1-phenyl-2-pyrazoline-5-one, Hap) and aro-

matic hydrazides along with the application of their molybd-

Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal

Supporting information for this article is available on the WWW under

http://dx.doi.org/10.1002/ejic.201600427.

[

18]

enum complexes

as catalysts for dynamic covalent assem-

Eur. J. Inorg. Chem. 0000, 0–0

1

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blies, via Hantzsch reaction, using 30 % H O as oxidant, under at pH ca. 7.0 to give the potassium salts of the corresponding

2

V

solvent free conditions. We extend this study by evaluating the dioxidovanadium(V) anions, K(H O)[V O (ap-bhz)] (5) and

peroxidase mimetic activity of the vanadium complexes of pyr- K(H O) [V O (ap-fah)] (6) [Equation (3) considering complex

2

V

2

0.5

2

azolone-based ligands, using the oxidation of pyrogallol with 5 as representative]. Under similar reaction conditions, neutral

V

H O as model reaction.

complexes [V O (Hap-nah)] (9) and [V O (Hap-inh)] (10) [Equa-

2

tion (4)] were obtained from ligands H ap-nah (III) and H ap-

2

inh (IV). In these complexes the pyridinic nitrogen undergoes

protonation helping to stabilize the complexes in their neutral

forms.

K[H VO ] + K ap-bhz + 2HCl

2

4

2

V

→

[K(H O)][V O (ap-bhz)] + 2KCl + H O

(3)

(4)

2

K[H VO ] + K L + 3HCl

2

→

4

2

V

[V O HL] + 3KCl + 2H O (H L = III: 9; H L = IV: 10)

2 2 2 2

V

Complexes, K(H O)[V O (ap-bhz)] (5) and K(H O) [V O (ap-

2

0.5

2

V

fah)] (6) could also be converted into [V O (Hap-bhz)] (7) and

V O (Hap-fah)] (8), respectively, by reacting their methanolic

2

V

[

2

Scheme 1. Structural formulae of ligands. Keto-enol tautomerism of a general

structure of hydrazones is also shown here.

solutions with aqueous HCl [Equation (5) considering complex

as a representative]. These two complexes have the ligand

coordinated in the ketonic (hydrazide) form.

5

To extend the scope, the oxidation of 1-phenylethanol to

acetophenone has also been carried out, giving further support

to the peroxidase mimetic activity of the reported complexes.

This is a particularly relevant reaction since the controlled oxid-

V

K(H O)[V O (ap-bhz)] + HCl → [V O (Hap-bhz)] + KCl + H O (5)

2

Scheme 2 provides an overview of the complexes discussed

ation of alcohols to the corresponding carbonyl compounds, is in this paper. The structural formulae proposed are based on

1

13

a fundamental transformation of enormous importance. Al- elemental analyses, spectroscopic data (IR, UV/Vis, EPR, H,

C

5

1

though, there are many known stoichiometric oxidation meth- and V NMR) and thermal studies. Structures of complexes 6,

ods, e.g. by pyridinium chlorochromate (PCC), Swern type, 7, 9–12 have been further validated by single-crystal X-ray dif-

Dess–Martin periodinane and Oppenauer type, for economic fraction studies. All complexes exist as monomers and are solu-

and environmental reasons, the development of efficient and ble in methanol, ethanol, acetonitrile, DMF, and DMSO, to differ-

selective catalysts for such an oxidation is a demanding field of ing extents.

[

19]

study in the chemical industry.

Results and Discussion

Synthesis, Characterization and Reactivity of the

Complexes

IV

Precursor [V O(acac) ] reacts with equimolar amounts of li-

2

gands H ap-bhz (I), H ap-fah (II) H ap-nah (II) and H ap-inh (IV)

2

in refluxing 95 % methanol to give oxidovanadium(IV) com-

IV

plexes, [V O(ap-bhz)(H O)] (1), [V O(ap-fah)(H O)] (2), [V O(ap-

2

IV

2

nah)(H O)] (3) and [V O(ap-inh)(H O)] (4), respectively, [Equa-

2

tion (1) considering H ap-bhz (I) as representative ligand].

2

IV

Scheme 2. Overview of the complexes described in this work.

[

V O(acac) ] + H ap-bhz + H O

2

IV

→

[V O(ap-bhz)(H O)] + 2Hacac

(1)

2

Crystallisation over the course of a week along with aerial

Thermal Analysis

oxidation of these complexes in MeOH, results in the formation

V

of [V O(L)(OMe)(MeOH)] (11–14) [Equation (2), considering

The thermal stability of the complexes was studied under oxy-

gen atmosphere and relevant data is presented in Table S1

(Supporting Information). Complexes 1–4 are thermally stable

up to ca. 100 °C. Above this temperature, 1–4 lose masses

roughly equal to one water molecule, indicating the presence

complex 1 as representative].

IV

2

[V O(ap-bhz)(H O)] + 1/2 O + 4 MeOH

2

V

→

2[V O(ap-bhz)(OMe)(MeOH)] + 3H O

(2)

2

A solution of potassium vanadate (which, at pH 7 is predomi- of weekly coordinated water. The anhydrous complexes decom-

[

20]

nantly K[H VO ]),

generated in situ by dissolving V O in pose between 200 to 490 °C in three overlapping steps; the

2

4

2 5

aqueous KOH, reacts with ligands H ap-bhz (I) and H ap-fah (II) final decomposition product is V O . Complexes 5 and 6 both

2

2 5

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V

lose a mass equal to one water molecule in the temperature

range 120–190 °C, which is attributed to the coordinated water

molecule. The water-free species starts to decompose with mul-

tiple exothermic, but overlapping, steps upon further increasing

the temperature; these steps are complete above ca. 550 °C,

Table 1. Bond lengths [Å] and angles [°] for {K(H

2

O)0.25[V O

V

2

(ap-fah)]}

n

(6),

(10),

V

[

V O

2

(Hap-bhz)]·H

2

O

(7), [V O

2

(Hap-nah)] (9), [V O

2

(Hap-inh)]·H

2

O

V

[a]

V O(ap-bhz)(OMe)(MeOH)] (11) and [V O(ap-fah)(OMe)(MeOH)] (12).

Bond lengths

6

7

9

V(1)–O(1)

V(1)–O(2)

V(1)–O(3)

V(1)–O(4)

1.6266(14)

1.9462(13)

1.9411(13)

1.6388(13)

1.614(2)

1.6078(16)

1.9558(14)

1.9184(13)

1.6572(15)

giving KVO as the final product. Complexes 7–10 are stable up

3

1.9289(18)

1.9436(19)

1.636(2)

to ca. 550 °C. Beyond this temperature they start to decompose

exothermically to give V O as the final product. Complexes 11–

2

5

1

4 each undergo mass loss roughly equivalent to one methanol

V(1)–O(5)

V(1)–N(3)

V(1)–K(1)#1

V(1)–K(1)#2

K(1)–O(4)#3

K(1)–O(1)

K(1)–O(5)#4

K(1)–O(1)#1

K(1)–O(4)#1

K(1)–O(1 W)

K(1)–O(2)#3

K(1)–N(4)#4

K(1)–V(1)#1

K(1)–V(1)#3

K(1)–K(1)#1

O(1)–K(1)#1

molecule in the temperature range 90–160 °C, confirming the

presence of a coordinated MeOH. Further increasing the tem-

perature, results in mass loss roughly equivalent to one methox-

ido (–OMe) group, indicating the presence of one coordinated –

OMe group. These complexes decompose giving the final prod-

uct V O in the temperature range ca. 470–500 °C.

2.1741(15)

3.4721(5)

3.6466(5)

2.6686(13)

2.7094(14)

2.7159(14)

2.8161(14)

2.8301(13)

2.896(10)

2.9302(13)

2.9956(16)

3.4721(5)

3.6466(5)

4.2740(8)

2.8161(14)

2.187(2)

2.1683(16)

2

5

X-ray Diffraction Studies

Table 1 provides selected bond lengths and angles for all com-

pounds. Compound {K(H O) [V O (ap-fah)]} (6) crystallized

V

2

0.25

2

n

Angles

6

7

9

from methanol as orange blocks and Figure 1 (A) depicts an

ORTEP representation of the asymmetric unit. In the solid state

the compound presents a polymeric structure with electrostatic

interactions between V=O bonds and potassium atoms. The

asymmetric unit of 6 contains a mononuclear vanadium com-

plex with the ligand, ap-fah, one potassium atom and 0.25 wa-

ter molecules interacting with potassium atoms. The coordina-

tion polyhedron around the K ions can be best described as a

distorted dodecahedron with triangular faces [see Figure 1 (B)],

with the water molecule occupying only occasionally the eighth

coordination site. In the crystal packing of 6, presented in Fig-

ure S1 (Supporting Information), the heteroatoms are seen to

interact electrostatically. The crystal packing can be described

as layers of vanadium complexes with the K ions occupying

positions between layers. Each vanadium centre is five-coordi-

nated. Since α_N₃_–_V₁_–_O₄= 143.95° and ꢀ_O₂_–_V₁_–_O₃= 148.89°, the τ

value of 0.08 [τ = (ꢀ–α)/60] indicates a slightly distorted square

pyramidal geometry.^[Each vanadium centre is coordinated to

the ligand through the enolic oxygen (of pyrazolone) atom [V1–

O2, 1.9462(13) Å], azomethine nitrogen atom, [V1–N3,

O(1)–V(1)–O(5)

O(1)–V(1)–O(2)

O(5)–V(1)–O(2)

O(1)–V(1)–O(3)

O(5)–V(1)–O(3)

O(2)–V(1)–O(3)

O(1)–V(1)–N(3)

O(5)–V(1)–N(3)

O(2)–V(1)–N(3)

O(3)–V(1)–N(3)

O(1)–V(1)–O(4)

O(5)–V(1)–O(4)

O(2)–V(1)–O(4)

O(3)–V(1)–O(4)

N(3)–V(1)–O(4)

103.87(6)

104.01(7)

104.66(9)

103.46(7)

103.40(7)

103.48(10)

148.80(6)

109.10(6)

146.75(9)

100.82(10)

149.09(6)

104.55(7)

83.50(5)

74.37(6)

106.61(7)

83.39(8)

74.22(8)

109.87(12)

74.38(6)

84.29(6)

110.49(9)

92.78(6)

92.44(6)

143.95(6)

94.80(9)

92.00(9)

148.63(11)

91.39(7)

93.33(6)

144.42(8)

Bond lengths

10

11

12

V(1)–O(1)

V(1)–O(2)

V(1)–O(3)

V(1)–O(4)

V(1)–O(5)

V(1)–N(3)

1.639(5)

1.980(5)

1.959(4)

1.652(4)

1.5887(13)

1.9216(16)

1.9133(16)

2.2572(14)

1.7654(13)

2.1618(15)

1.665(4)

2.000(4)

2.005(4)

2.429(4)

1.847(4)

2.256(4)

21]

2.213(5)

2.1741(15) Å] and one oxygen atom of hydrazide group [V1–

Angles

10

11

12

O3, 1.9411(13) Å]. The coordination sphere is completed by two

oxygen atoms [V1–O4, 1.6388(13) Å and V1–O1, 1.6266(14) Å],

which interact with the potassium atoms. The pyrazolone group

O(1)–V(1)–O(5)

O(1)–V(1)–O(2)

O(5)–V(1)–O(2)

O(1)–V(1)–O(3)

O(5)–V(1)–O(3)

O(2)–V(1)–O(3)

O(1)–V(1)–N(3)

O(5)–V(1)–N(3)

O(2)–V(1)–N(3)

O(3)–V(1)–N(3)

O(1)–V(1)–O(4)

O(5)–V(1)–O(4)

O(2)–V(1)–O(4)

O(3)–V(1)–O(4)

N(3)–V(1)–O(4)

101.68(7)

98.42(7)

98.82(6)

99.38(7)

95.12(6)

154.63(6)

94.34(6)

162.65(6)

85.25(6)

75.52(6)

175.01(6)

83.31(6)

80.84(6)

79.85(6)

80.69(6)

100.92(19)

97.3(2)

97.73(17)

97.9(2)

103.33(16)

104.51(17)

(

C1, C2, C3, N1, N2) and furan of the hydrazide group (C14,

C15, C16, C17, O5) are coplanar [mean deviation from planarity,

.0110(15) Å], and the other benzyl group (C7, C8, C9, C10,

95.91(16)

157.26(17)

97.44(19)

160.39(18)

86.66(14)

74.67(14)

175.91(17)

83.17(16)

82.18(16)

81.47(16)

78.49(15)

146.96(17)

100.57(18)

0

C11, C12) forms a torsion angle of 16.19(08)° over the previous

coplanar rings. The vanadium atoms are displaced toward the

apical oxido ligand (O1) from the equatorial plane defined by

the O2, O3, O4 and N3 atoms, 0.5120(7) Å, [mean deviation

from planarity, O2, O3, O4, N3, 0.0603(7) Å]. The two oxygen

atoms present V=O bonds characteristic of oxido-type O atoms

73.83(14)

83.85(14)

92.37(15)

94.27(15)

148.3(2)

[

23]

with strong π bonding.

on the position of the potassium ion in the crystal packing are

.4721(5) Å and 3.6466(5) Å.

The V–K separations, which depend

[

a] Symmetry transformations used to generate equivalent atoms: #1 –x, –y

3

+ 1, –z + 1; #2 x,y,z+1; #3 x, y, z – 1; #4 –x + 1, –y + 1, –z + 1.

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V

Figure 1. (A) Asymmetric unit of compound {K(H

2

O)0.25[V O

2

(ap-fah)]}

n

(6). All the non-hydrogen atoms are presented by their 50 % probability ellipsoids.

Hydrogen atoms are omitted for clarity. (B) Coordination polyhedron around K ion in 6.

V

Compounds [V O (Hap-bhz)]·H O (7), [V O (Hap-nah)] (9) O1, 1.614(2) Å in 7, 1.6078(16) Å in 9 and 1.639(5) Å in 10]. The

2

V

and [V O (Hap-inh)]·H O (10) crystallized from methanol, meth- pyrazolone group (C1, C2, C3, N1, N2) and phenyl or pyridine

2

anol/ethanol and DMSO, respectively, as dark orange blocks of the hydrazide group (C8, C9, C10, C11, C12, C13 in 7, C8, C9,

and Figures 2, 3 and 4 depict ORTEP representations of the

complexes in the asymmetric unit. These compounds contain

mononuclear vanadium complexes with the ligands ap-bhz, ap-

nah and ap-inh, protonated (compound 7 in the azomethine

group and compound 9 and 10 in the pyridine group). Each

vanadium centre is five-coordinate. For 7 (α_N₃_–_V₁_–_O₄= 148.63°,

ꢀ_O₂_–_V₁_–_O₃= 146.75°) the value τ = 0.031, for 9 (α_N₃_–_V₁_–_O₄

=

144.42°, ꢀ_O₂_–_V₁_–_O₃= 149.09°) τ = 0.078, and for 10 (α_N₃_–_V₁_–_O₄

1

48.3°, ꢀ_O₂_–_V₁_–_O₃= 146.96) τ = 0.022, all indicating distorted

[

22]

square pyramidal geometries.

Figures S2, S3 and S4 (Sup-

porting Information), present the crystal packing of compounds

, 9 and 10, respectively. The crystal packing can be described

7

as layers of vanadium complexes, which interact by van der

Waals forces and π–π stacking interactions. In compound 7, the

distance between centroids of phenyl groups, c1–c2 is

V

2

Figure 2. ORTEP plot of complex [V O (Hap-bhz)] (7). All the non-hydrogen

atoms are presented by their 50 % probability ellipsoids. Hydrogen atoms are

omitted for clarity.

3

.618(2) Å [c1, C(14O)–C(15O)–C(16O)–C(17O)–C(18O)–C(19O),

c2, C(8N)–C(9N)–C(10N)–C(11N)–C(12N)–C(13N)]. In compound

, antiparallel π–π stacking interactions (Figure 5) occur be-

9

tween the pyrazolone groups. The distance between centroids,

c3–c4, is 3.539(2) Å [c3, C(1C)–C(2C)–C(3C)–N(1C)-N(2C), c4,

C(1O)–C(2O)–C(3O)–N(1O)-N(2O)]. In compound 10, the dis-

tance between centroids of phenyl and pyridine groups, c5–c6,

is 3.661(2) Å [c5, C(13C)–C(14C)–C(15C)–C(16C)–C(17C)–C(18C),

c6, C(8B)–C(9B)–C(10B)–C(11B)–C(12B)-(N5B)]. Each vanadium

centre is coordinated to the ligands through the enolic oxygen

(

of pyrazolone) atom [V1–O2, 1.9289(18) Å, in 7, 1.9558(14) Å,

in 9 and 1.980(5) Å in 10], azomethine nitrogen atom, [V1–N3,

.187(2) Å, in 7, 2.1683(16) Å, in 9 and 2.213(5) Å in 10] and

2

one oxygen atom of the hydrazide group [V1–O3, 1.9436(19) Å,

in 7, 1.9184(13) Å, in 9 and 1.959(4) Å in 10]. The coordination

sphere is completed with two terminal oxido atoms [V1–O4,

V

Figure 3. ORTEP plot of complex [V O

2

(Hap-nah)] (9). All the non-hydrogen

atoms are presented by their 50 % probability ellipsoids. Hydrogen atoms are

omitted for clarity.

1.636(2) Å in 7, 1.6572(15) Å in 9 and 1.652(4) Å in 10 and V1–

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V

Figure 4. ORTEP plot of complex [V O

2

(Hap-inh)] (10). All the non-hydrogen atoms are presented by their 50 % probability ellipsoids. Hydrogen atoms are

omitted for clarity.

C10, C11 C12, N5 in 9 and 10) are quasi-coplanar [mean devia- benzyl of hydrazide group (C8, C9, C10, C11, C12, C13) are co-

tion from planarity, 0.1093(22) Å in 7, 0.0981(15) Å in 9 and planar [mean deviation from planarity, 0.1154(16) Å], and the

0

.1095(41) Å in 10], and the other phenyl group (C14, C15, C16, other benzyl group (C14, C15, C16, C17, C18, C19) forms a tor-

C17, C18, C19 in 7 and C13, C14, C15, C16, C17, C18 in 9 or sion angle of 9.70(06)° over the previous coplanar rings. In 12

0) forms a torsion angle of 12.19(9)° in 7, 11.64(08)° in 9 and these values are: mean deviation from planarity, 0.0734(45) Å,

2.42(29)° in 10, over the previous coplanar rings. The vana- and torsion angle of 23.74(23)°. The distances V1-O_e_n_o_l_a_t_e_s[V1–

1

dium atom is displaced towards the apical oxido ligand (O1) O2, 1.9216(16) Å, and V1–O3, 1.9133(16) Å, in 11 and V1–O2,

from the equatorial plane defined by the O2, O3, O4 and N3 2.000(4) Å, and V1–O3, 2.005(4) Å, in 12] are higher than V1-

atoms, 0.4886(11) Å in 7, 0.5084(8) Å in 9 and 0.4966(20) Å in O_m_e_t_h_o_x_i_d_o[V1–O5, 1.7654(13) Å, in 11 and 1.847(4) Å, in 12].

1

0, [mean deviation from planarity, O2, O3, O4, N3, 0.0069(11) Å

in 7, 0.0562(8) Å in 9 and 0.0098(20) Å in 10]. The two terminal

oxygen atoms present V=O bonds characteristic of oxido-type

[

22]

O atoms with strong π bonding.

Intermolecular hydrogen

bonds appear between protonated pyridine and terminal oxy-

gen atoms in compounds 9 and 10.

V

Figure 6. ORTEP plot of complex [V O(ap-bhz)(OMe)(MeOH)] (11). All the non-

hydrogen atoms are presented by their 50 % probability ellipsoids. Hydrogen

atoms are omitted for clarity.

Figure 5. Antiparallel π–π stacking interactions present in the crystal packing

V

of [V O

2

(Hap-nah)] (9).

V

Compounds [V O(ap-bhz)(OMe)(MeOH)] (11) and [V O(ap-

fah)(OMe)(MeOH)] (12) crystallized from methanol as black

blocks and Figures 6 and 7 depict ORTEP representations of the

asymmetric units. Both structures show mononuclear com-

plexes with six-coordinate distorted octahedral geometry. The

axial sites of the octahedron are occupied by the oxido O-atom

and by one methanol molecule [V1–O1, 1.5887(13) Å, V1–O4,

2.2572(14) Å, in 11 and V1–O1, 1.665(4) Å, V1–O4, 2.429(4) Å,

in 12]. The other coordinated atoms form a plane [O2, O3, O5,

N3, mean deviation from planarity, 0.0208(7) Å, in 11 and

0

.0272(22) Å, in 12] with the vanadium atom displaced towards

V

Figure 7. ORTEP plot of complex [V O(ap-fah)(OMe)(MeOH)] (12). All the non-

the apical oxido ligand by 0.2904(7) Å in 11 and 0.2960(23) Å

in 12. In 11 the pyrazolone group (C1, C2, C3, N1, N2) and

hydrogen atoms are presented by their 50 % probability ellipsoids. Hydrogen

atoms are omitted for clarity.

Eur. J. Inorg. Chem. 0000, 0–0

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5

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The bond length to the function trans to the oxido group (V1– the azomethine coordination. The shift of the ν(N-N) band to

O4) is somewhat elongated, as a consequence of the trans influ- higher frequency is expected because of the reduced repulsion

ence in the two compounds. In the crystal packing of com- between the lone pairs of adjacent nitrogen atoms. Complexes

–1

pounds 11 and 12, the layered structure present in compounds 7–10 exhibit a band at 3051–3071 cm , which indicates that

, 7 and 9 is not observed, (see Figures S5 and S6, Supporting the hydrazine (in 7 and 8) or the pyridinic nitrogen of the ring

6

Information). Intermolecular hydrogen bonds appear between (in 9 and 10) gets protonated during complexation. A broad

coordinated methanol groups and nitrogen atoms, N2, of pyraz- band attributed to –OH appears at 2900–3000 cm^–in the spec-

1

olone groups, in 11 (see Table 2).

tra of ligands, due to intramolecular hydrogen bonding. In the

–1

metal complexes this band is shifted to ca. 3410–3460 cm ,

possibly due to the breaking of hydrogen bonding as well as

coordination of water (in complexes 1–6) and methanol mol-

ecules (in complexes 11–14).

V

2

Table 2. Hydrogen bonds in compounds [V O (Hap-nah)] (9) and [V O-

[

a]

(

ap-bhz)(OMe)(MeOH)] (11).

D–H···A (compound)

d(D–H)

d(H···A)

d(D···A)

<(DHA)

N(5)–H(5N)···O(1)#1 (9)

O(4)–H(4)···N(2) #2 (11)

0.95(3)

0.73(3)

1.63(3)

2.02(3)

2.563(2)

2.753(3)

167(3)

172(3)

IV

–1

The V O complexes exhibit a sharp band at 976–989 cm

V

due to ν(V=O) stretch while in V O complexes this band ap-

pears at 956–966 cm . The dioxido V O complexes exhibit two

such sharp bands in the 896–938 cm region due to ν_s_y_m(O=

–1

V

[a] Symmetry transformations used to generate equivalent atoms: #1 –x +

2

, –y+1, –z+1; #2 –x, –y, –z+1.

–1

V=O) and ν

(O=V=O) modes. These bands confirm the cis-

asym

V

[15d]

V O structure in these complexes.

2

IR Spectral Studies

A partial list of IR spectroscopic data of the ligands and com-

plexes is presented in Table S2 (Supporting Information). Bands

appearing at 2970–2991 cm^–¹and 1615–1646/1586–1595 cm

in the spectra of the ligands, due to the presence of υ(N–H)

–1

Electronic Spectral Studies

and υ(C=O_h_y_d_r_a_z_i_d_e_/_p_y_r_a_z_o_l_o_n_e), are indicative of the ketonic nature Table 3 presents UV/Vis spectroscopic data of the ligands and

of the ligands in the solid state. These bands disappear upon the complexes measured in MeOH or DMSO. We recently re-

complex formation (except in 7 and 8) due to enolization of ported spectral studies of the ligands, I–IV, and their dioxido-

[

18]

the ketonic group and replacement of the H atom by the metal molybdenum(VI) complexes.

The spectra of the complexes

ion. A sharp band appearing in the region 1225–1250 cm^–is in the UV region, reported here, follow very similar spectral pat-

1

assigned to the υ(C-O_e_n_o_l_i_c) mode. A strong band at 1540– terns i.e. the ꢀ → ꢀ*, π → π *, π → π *, and n → π* transitions

1

2

–

1

1550 cm

in the spectra of ligands is assigned to observed in the spectra of ligands are slightly shifted in the

ν(C=N) and this band shifts to lower wave numbers, by corresponding spectra of complexes. In addition, a new band

–

1

2

0–25 cm in the spectra of complexes, indicating coordination of medium intensity appears at ca. 390–402 nm, which is as-

of the azomethine nitrogen. The ligand band assigned to signed to a ligand to metal charge transfer (LMCT) band. The

–

1

IV

υ(N-N) appearing at 976–979 cm shifts to higher wave num- V O complexes exhibit bands in the visible region, assigned to

–

1

bers, by 60–90 cm , in the complexes, which further supports d–d transitions.

Table 3. UV/Visible spectroscopic data of ligands and vanadium complexes, either in MeOH or DMSO.

–

1

cm^–¹

)

Solvent

λ

max/nm (ε/

M

3

4

H

2

ap-bhz (I)

ap-fah (II)

ap-nah (III)

ap-inh (IV)

MeOH

DMSO

MeOH

DMSO

MeOH

DMSO

MeOH

DMSO

MeOH

347 (8.54 × 10 ), 281 (1.02 × 10 ), 248 (1.30 × 10 ), 227 (1.29 × 10 ), 205 (1.95 × 10 )

3

4

378 (8.76 × 10 ), 362 (9.26 × 10 ), 289 (1.11 × 10 ), 251 (1.61 × 10 ), 204 (1.43 × 10 )

4

3

4

375 (1.61 × 10 ), 279 (9.90 × 10 ), 249 (1.64 × 10 ), 201 (2.06 × 10 )

4

3

4

385 (1.62 × 10 ), 283 (9.26 × 10 ), 250 (1.64 × 10 ), 203 (1.80 × 10 )

IV

2

3

4

[

V O(ap-bhz)H

2

O] (1)

636 (1.2 × 10 ), 395 (3.9 × 10 ), 368 (2.9 × 10 ), 240 (4.1 × 10 ), 201 (4.3 × 10 )

2

3

610 (1.20 × 10 ), 374 (1.50 × 10 )

V^I^VO(ap-fah)H

V^I^VO(ap-nah)H

O] (2)

642 (3.4 × 10 ), 395 (4.8 × 10 ), 368 (4.1 × 10 ), 240 (4.1 × 10 ), 201 (4.8 × 10 )

600 (30, sh), 375 (1.55 × 10 )

641(4.0 × 10 ), 393 (2.3 × 10 ), 342 (3.4 × 10 ), 233 (3.3 × 10 ), 203 (3.7 × 10 )

744 (20), 533 (25, sh), 410 (8.00 × 10 ), 387 (1.32 × 10 ), 378 (1.36 × 10 )

645(2.3 × 10 ), 390 (4.1 × 10 ), 340 (3.2 × 10 ), 244 (4.8 × 10 ), 201 (4.8 × 10 )

735 (40, sh), 419 (9.90 × 10 ), 388 (1.40 × 10 ), 381 (1.41 × 10 )

396 (2.6 × 10 ), 324 (3.9 × 10 ), 243 (4.2 × 10 ), 202 (3.9 × 10 )

397 (3.1 × 10 ), 328 (2.8 × 10 ), 243 (3.5 × 10 ), 203 (2.8 × 10 )

395 (2.8 × 10 ), 328 (3.1 × 10 ), 243 (3.5 × 10 ), 203 (2.9 × 10 )

395 (2.3 × 10 ), 328 (3.4 × 10 ), 243 (3.7 × 10 ), 203 (4.6 × 10 )

401 (2.3 × 10 ), 333 (5.1 × 10 ), 244 (4.3 × 10 ), 203 (2.6 × 10 )

389 (1.8 × 10 ), 338 (2.1 × 10 ), 243 (2.9 × 10 ), 205 (3.6 × 10 )

402 (2.6 × 10 ), 370 (3.1 × 10 ), 242 (2.6 × 10 ), 205 (3.3 × 10 )

401 (1.9 × 10 ), 367 (2.8 × 10 ), 238 (2.9 × 10 ), 203 (3.3 × 10 )

395 (1.4 × 10 ), 356 (2.2 × 10 ), 237 (3.3 × 10 ), 204 (4.1 × 10 )

391 (2.4 × 10 ), 346 (3.1 × 10 ), 241 (3.2 × 10 ), 201 (2.9 × 10 )

2

3

4

2

4

2

3

4

2

O] (3)

3

4

V^I^VO(ap-inh)H

O] (4)

2

3

4

2

3

4

V

3

4

K(H

2

O)[V O

2

(ap-bhz)] (5)

V

3

4

2

O)0.5[V O

2

(ap-fah)] (6)

V

3

4

[

V O

2

(Hap-bhz)] (7)

V

3

4

(Hap-fah)] (8)

(Hap-nah)] (9)

(Hap-inh)] (10)

V

3

4

V

3

4

V

3

4

V O(ap-bhz)(OMe)(MeOH)] (11)

V O(ap-fah)(OMe)(MeOH)] (12)

V O(ap-nah)(OMe)(MeOH)] (13)

V O(ap-inh)(OMe)(MeOH)] (14)

V

3

4

V

3

4

V

3

4

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¹H NMR Spectral Studies

nation. Thus, H NMR spectroscopic data complement the con-

clusions drawn from IR data.

1

V

H NMR spectra of ligands I–IV and their V O/V O complexes

2

were recorded to characterize them and confirm the coordina-

tion modes. The relevant data are presented in Table S3 (Sup-

13_C_N_M_R_S_p_e_c_t_r_a_l_S_t_u_d_i_e_s

porting Information) and representative spectra {H ap-fah (II),

2

V

K(H O) [V O (ap-fah)](6) and [V O (Hap-fah)] (8)} are pre- The coordination modes of the ligands towards the metal cen-

2

0.5

2

1

3

sented in Figure 8. Signals appearing at δ = 11.27–11.50 ppm tre were further corroborated by C NMR chemical shifts study.

in the ligand spectra, due to the NH proton, and at δ = 2.33– Table S4 presents C NMR spectroscopic data of ligands and

1

3

1

3

2.35 ppm, due to the ring proton of pyrazole residue, indicate complexes and

C

NMR spectra of H ap-fah (II),

2

V

the ketonic form of both C=O groups. The absence of these K(H O) [V O (ap-fah)] (6) and [V O (Hap-fah)] (8) are depicted

2

0.5

2

signals in spectra for complexes 5, 6, 11–14, suggests the enol- in Figure 9. Ligands I, II, III and IV exhibit 15, 14, 15 and 15 ¹³

C

ization and subsequent coordination of both oxygen atoms. NMR signals against a total of 19, 18, 18 and 17 carbon atoms,

Neutral complexes, 7 and 8, show this signal at nearly the same respectively. When compared to the ligand spectra, the corre-

position, possibly due to protonation of the hydrazido nitrogen sponding metal complex spectra exhibit normally the same or

(

see the description of the crystal structure) and the presence a higher number of signals. A downfield coordination-induced

of the C=O group in the ketonic form. In complexes 9 and 10, shift, Δδ = [δ(complex) – δ(ligand)] is observed in the signals

although neutral, such resonance could not be located. There- of the carbon atoms i.e. C1, C4 and C13, present in the vicinity

fore, charge neutralization is probably accomplished by proto- of the coordinating atoms O, N and O [i.e. pyrazolone oxygen,

nation of the pyridinic N atom. Coordinated methanol/methox- azomethine nitrogen and enolic oxygen], respectively, confirm-

ide (in complexes 11–14) exhibits signals due to methyl and ing the coordination of these functionalities to the vanadium

alcoholic protons at δ = 3.15–3.16 and 4.12–4.13 ppm, respec- centre. The peaks due to other carbon atoms i.e. due to the

tively. Aromatic protons appear in the expected region for both phenyl ring, methyl group and pyrazole ring appear at the ex-

the ligands and complexes with slight shifts in their positions. pected δ values in the spectra of ligands as well as in the com-

All spectra the ligands display two signals at 2.36–2.37 and plexes, with slight variations. In addition, two new signals ap-

2.40–2.46 ppm, due to methyl protons present on the pyrazole pear in the spectra of 11–14 at 52.5–54.5 ppm and 61.3–

ring and on the azomethine nitrogen, respectively. The later 62.9 ppm. These signals correspond to coordinated methanol

signal shifts downfield due to N-atom and metal centre coordi- and methoxy carbon atoms, respectively.

Figure 8. ¹H NMR spectra of H

water in DMSO/DMSO itself.

ap-fah (II), K(H

O)0.5 [V O

V

(ap-fah) (6)] and [V O

V

(Hap-fah)] (8) recorded in [D

]DMSO. Star in Figure indicates signal due to

2

6

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7

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field region, since methanol and/or methoxy coordination in-

creases the metal shielding, shifting the resonance to less nega-

[

24]

tive values. Complex 13 shows a much broader resonance at

515.5 ppm, which is also present in the NMR spectrum of 14,

–

but is much less important. These resonances are tentatively

attributable to loss of the MeOH molecule.

Electrochemical Studies

The electrochemical behavior of the ligands and complexes was

investigated by cyclic voltammetry and differential pulse vol-

tammetry (DPV); and the CV results are given in Table S5 (Sup-

porting Information). As a representative example, the cyclic

voltammograms of ligand I, and respective complexes 1, 7 and

11, are displayed in Figure 10. The oxidation and reduction po-

tentials in complexes 1–14 are within the range reported for

similar compounds in the literature.

_F_i_g_u_r_e₉_.13_C_N_M_R_s_p_e_c_t_r_a_o_f_H

V

[25]

2

ap-fah (II), K(H

(Hap-fah)] (8) recorded in [D ]DMSO. The star in the figure denotes the

carbon signals for DMSO.

2

O)0.5[V O

2

(ap-fah)] (6) and

V

[

V O

2

6

⁵¹V NMR Spectral Studies

Further characterization of the complexes was ascertained from

5

1

V NMR spectra. Table 4 contains the chemical shift values ob-

tained for each complex dissolved in either DMSO or MeOH,

before and after the addition of aqueous H O . The chemical

2

IV

shift values for V O complexes 1–4 were measured in solutions

that were left under an atmosphere of air for 3 d and therefore

correspond to oxidized species (+5). The δ values vary between

–

508.5 ppm (complex 3) and –512.4 ppm (complex 2) and are

V

roughly the same as the ones found for corresponding V O

2

complexes 5–10, which vary between –506.2 (complex 9) and

–

512 ppm (complex 5). The chemical shift values are within the

[

23]

usual range for tridentate ONO ligands.

conclude that aerial oxidation of the V O complexes leads to

the formation of corresponding V O complexes. For 1 and 2 a

We can therefore

IV

V

Figure 10. Cyclic voltammograms of ligand I and respective complexes in

2

DMF vs. SCE. Conditions: ca. 1.0 m

M metal complex concentration, 0.1 M

lower field peak is present, corresponding either to an isomer,

or a solvolysis product.

Bu NPF , Pt disk working electrode, scan rate 0.1 V/s.

4

6

V

For V O complexes 11, 12 and 14, the spectra measured in

The ligand voltammogram in the potential range –1.5 to

MeOH show one sharp peak between –490.4 and –497.9 ppm. 1.5 V exhibit one peak at ca. +1.0 V due to ligand oxidation

[

26]

Typically, methoxy containing species resonate in this down- (hydrazine-based oxidation). It also exhibits a reduction peak

Table 4. ⁵¹V NMR spectroscopic data of the complexes dissolved in DMSO with 5 % D

2

2 2

O, with and without H O (final concentration indicated); [com-

plex] ≈ 3 mM.

δ [%]

δ [%] after addition of H

2

O

2

2 2

[H O ] [mM]

1^[

a]

–480.0 (20); –509.9 (80)

–474.4 (20); –512.4 (80)

–508.5 (100)

–509.7 (100)

–512.4 (100)

–511.7 (100)

–476.2 (5); –508.2 (95)

–510.9 (100)

–506.2 (100)

–509.0 (100)

–513.3 (63); –615.8 (12.5); –634.0 (24.5)

360

[

a]

2

3

4

5

6

7

8

9

1

–514.0 (80); –632.0 (10); –646.7 (10)

–519.9 (95); –646.1 (2.5); –660.0 (2.5)

–513.9 (80); –619.9 (10); –637.9 (10)

–516.0 (80); –631.0 (10); –646.0 (10)

–511.0 (15); –537.2 (70); –636.1 (7.5); –649.0 (7.5)

–547.0 (95); –640.8 (2.5); –653.0 (2.5)

–555.7 (43); –660.1 (28.5); –679.8 (28.5)

–551.0 (43); –656.5 (28.5); –675.2 (28.5)

–516.0 (48); –639.1 (26); –654.0 (26)

–581.9 (80); –656.3 (20)

95

280

260

237

260

226

173

163

73

0

1

2

3

4

[

b][c]

b]

–490.4 (100)

–497.9 (100)

–515.5 (100)

–491.6 (90); –514.8 (10)

b][c]

150

IV

[

a] The chemical shift values are from the species originating from aerial oxidation of the V O complexes. [b] In MeOH. [c] Precipitation occurred.

Eur. J. Inorg. Chem. 0000, 0–0

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8

Full Paper

at –0.483 to –0.528 V, equivalent to two electrons, which is Hamiltonian parameters for complexes 1 and 2 and for 3 and

coupled with two closely associated peaks at –0.404 to –0.436 V 4, are attributed to different electronic contributions imposed

and –0.889 to –0.972 V, attributed to the reduction of –N=CH– by the R substituents that probably make the O-enolate in 3

[

27]

linkage of the ligand.

The peak potentials are shifted and 4 a better donor group.

(

ca. 100–200 mV) and the intensity of some signals are found

to decrease in the corresponding CVs of the vanadium com-

plexes due to incorporation of the metal centre. Therefore, the

voltammograms of the metal complexes show both metal-

based and ligand-based redox process.

Reactivity of Oxidovanadium (IV) Complexes towards H O

2

The reactivity of oxidovanadium(IV) and dioxidovanadium(V)

complexes towards H O was monitored by UV/Vis absorption

2

spectroscopy, with complex 1 serving as a representative of

The dioxidovanadium(V) complex exhibits a weak reduction

IV

V

III

[V OL(H O)] complexes (Figure 11). The progressive addition of

2

peak (irreversible) at ca. –0.753 V due to the change V → V

III

IV [28,29]

H O to a solution of 1 in DMSO causes changes in the ligand

2

and another weak peak at ca. +0.55 V due to V → V .

The

IV

V

centred, as well as metal centred d–d bands. These changes

include flattening of the band at 345 nm, along with a shift of

the band at 217 nm to 247 nm and flattening of the d–d band

appearing at 636 nm, which finally disappears, indicating oxid-

ation of the oxidovanadium(IV) complex to a higher oxidation

state. Changes observed for complex 2 are reported in Figure

S8 (Supporting Information) and include the disappearance of

the d–d band.

absence of any redox couple due to V /V indicates that the

complex is quite stable in the +5 oxidation state and also less

susceptible towards oxidation or reduction processes in DMF.

The monoxidovanadium(IV) complex 1 exhibits a reduction

IV

III

[30]

peak at –0.782 V, due to the V → V couple. A quasi-reversi-

ble peak at ca. 1.33 V, as well as the associated coupled peak

IV

V

[31]

at 1.16 V, are due to a V → V conversion.

No reduction

III

II

process assignable to V → V was observed in any voltammo-

gram. The electrochemical properties of all vanadium(IV) com-

plexes, 1–4, are similar and they display identical redox proc-

esses, confirming similar structures and electronic features. All

these redox processes were supported by DPV (see Table 9).

EPR Characterization

IV

The V O complexes were further characterized by EPR. The

IV

spectra of frozen solutions (at 77 K) of the V O – complexes

exhibit a hyperfine pattern consistent with axial-type spectra of

IV

1

monomeric V O-bound species with d ground-state configu-

xy

rations. Figure S7 (Supporting Information) shows the X band

EPR spectra measured for 3 m

M solutions of the compounds in

DMSO. The spectra are very similar for all complexes and their

[

32]

simulation

yielded similar spin-Hamiltonian parameters for

IV

Figure 11. Spectral changes observed during titration of [V O(ap-bhz)(H

2

O)]

complexes 1 and 2 and for 3 and 4, which are included in

Table 5. Once a particular binding mode is assumed, the values

of A_scan be estimated (A ) using the additivity relationship

proposed by Würthrich

accuracy of ±3 × 10 cm . In this work we will use the follow-

ing contributions, for reasons presented elsewhere,

2 2

(1) with H O . The spectra were recorded following stepwise additions of one

–

2

–4

drop portions of 1.56 × 10

solution of complex 1 in DMSO over the course of ca. 20 min. The inset

shows changes observed during the titration of 1 with H for the d–d band

of this complex. These spectra were recorded following stepwise additions

M

of H

2

O

2

solution to 25 mL of a 2.32 × 10

M

est

2 2

O

[

33]

[34]

and Chasteen

with an estimated

–

4

–1

–2

–3

of one drop portions of 1.35 × 10

M

of H

2

O

2

to 25 mL of 1.54 × 10

M

[

35]

which

solution of complex 1 in DMSO.

assume that CO contributes as an O-enolate(–1), using the

The process was also studied by ⁵¹V NMR spectroscopy. Addi-

tion of aliquots of H (1 ) to 3 m solutions containing

complex 1 dissolved in DMSO, revealed oxidation of the V O

complex, with a peak appearing at –510 ppm (as well as a

smaller one at ca. –480 ppm, see Figure S9, Supporting Informa-

average value for N_i_m_i_n_e₍₄₁_.₆_×₁₀_–4 cm )

–1 [17c,23a]

and

5.65 × 10_c_m–1 for the water molecule contribution. This

–

4

2

O

2

M

4

IV

yields an A_e_s_t_v_a_l_u_e_o_f₁₆₂_.₅_×₁₀_–4 cm , lower than experimen-

–1

tal values found for the A . The A contribution of the CO group

s

_i_s_p_r_o_b_a_b_l_y_h_i_g_h_e_r_t_h_a_n₃₇_.₆_×₁₀_–4 cm since this donor atom

–1

tion). Following the addition of a large excess of H O (120 mM),

2

is not a typical O-enolate(–1) donor. Therefore, this A_e_s_tvalue is

probably an underestimate. The differences found in the spin

peaks assigned to mono- and di-peroxovanadate species ap-

peared upfield. Addition of H O increased the linewidth and it

2

is possible that more than one species was present under the

Table 5. Spin Hamiltonian parameters obtained by simulation of the EPR spec-

_t_r_a_w_i_t_h_t_h_e_c_o_m_p_u_t_e_r_p_r_o_g_r_a_m_d_i_s_c_l_o_s_e_d_b_y_R_o_c_k_e_n_b_a_u_e_r_a_n_d_K_o_r_e_c_z_.[

32]

resonance peak; consequently, we cannot exclude the forma-

V

tion of [V O(O )L] species.

2

Complex

g

⊥

g

s

A

⊥

/ × 10⁴cm^–¹

A

s

/ × 10⁴cm^–¹

1

2

3

4

1.983

1.984

1.942

1.943

1.956

1.955

65.6

58.8

60.0

177.9

177.3

166.6

167.7

Reactivity of Dioxidovanadium(V) Complexes

The reactivity of dioxido complexes towards H O was evalu-

2

ated with the same spectroscopic tools. Changes were ob-

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served in the UV/Vis spectra, namely isosbestic points, confirm- ones, with the up field signal increasing in intensity and the

ing the formation of new species. Using complex 5 as a repre- downfield signal decreasing as more H O is added. The upfield

2

V

sentative, the spectra measured during its titration in MeOH peak is assigned to V O(O )L since the presence of the peroxide

2

with H O are shown in Figure 12. Spectral changes for the molecule in the coordination sphere increases the electron den-

2

other complexes are included in Figures S10–S12 (Supporting sity and therefore shields the vanadium centre. We can also

Information).

assume that the linewidth broadening observed in the resonan-

ces of complexes 5–8 with increasing H O concentration is also

2

due to the presence of more than one complex species, one of

V

them being the peroxido complex V O(O )L.

2

Reactivity of Monooxidovanadium(V) Complexes

5

1

V NMR was also used to study the reactivity of the

V

[

V OL(OMe)(MeOH)]-type complexes 11–14. Figure 14 shows

the spectra measured for complex 11 before and after addition

of increasing concentrations of H O and others are included

2

as Supporting Information. In MeOH, the line widths are much

lower than observed in DMSO and a peak at ca. –550 ppm [for

1

–

1 (Figure 14) and 12 (Figure S18, Supporting Information)] or

580 ppm (for 13 and 14, see Figures S19 and S20, Supporting

Information) appears and grows at the expense of the

Figure 12. Spectral changes observed during titration of complex 5 with

H

2

O

2

. The spectra were recorded following stepwise additions of one drop

V

–

V OL(OMe)(MeOH) peak. We assign this peak as V O(O )L ,

V O(O )L(OMe) or V O(O )L(OMe)(MeOH) -type complexes.

2

–2

–4

portions of 2.38 × 10

M

H

2

O

2

solution of 5 to 25 mL of 3.45 × 10

M

solu-

V

2–

V

2–

2

tion in MeOH over the course of ca. 20 min.

Since the chemical shift is similar to that found for complex 10

in DMSO, we propose the absence of coordinated MeO^–or

MeOH molecules. Free-ligand inorganic peroxovanadate species

also appear further upfield in all systems at ca. 650–680 ppm.

Complex 13 seems less prone to forming the peroxido species,

V

The reactivity of V O complexes 5–10 towards H O was

2

2 2

5

1

evaluated using V NMR spectroscopy. Spectra for all the char-

acterized systems are included in Figures S13–S17 (Supporting

Information); Table 4 also lists the chemical shifts of the reso-

nances observed in the spectra with the higher H O concentra-

since only a very small amount is present at 73 mM of H O .

2 2

2

tion (also included in the Table). For complexes 5–8 the line-

V

width of the resonance assigned to the V O complex increases

2

with the addition of H O and a small upfield shift is observed

2

V

+

(

<10 ppm). At some point, peaks assigned to V O(O ) and

V O(O ) appear and shift upfield commensurate with in-

2

V

–

2

creased H O concentration. However, for complexes 9 and 10

2

(

see Figure 13 for complex 10) it is very interesting to observe

that what seems to be one resonance becomes two separate

Figure 14. ⁵¹V NMR spectra measured for a solution of complex 11 (ca. 3 m

dissolved in MeOH containing 5 % D O, before and after addition of H

). Concentration of H indicated in the Figure.

M)

O

2 2

2

(

1

M

2 2

O

Reactivity of Dioxidovanadium (V) Complexes with HCl

The behaviour of a methanolic solution of complex 6 upon ti-

tration with HCl was also studied spectrophotometrically. Com-

plex 6 (ca. 10^–

4

M solution in methanol) was treated with one

Figure 13. ⁵¹V NMR spectra measured for a solution containing complex 10

drop portions of methanol saturated with HCl gas; the observed

changes are presented in Figure 15. These results possibly re-

(

3 m

M) dissolved in DMSO with 5 % D

2 2 2

O, after addition of H O 1 M. Concen-

tration of H

2 2

O

indicated in the Figure.

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flect the formation of dioxido vanadium species with one of

the N–N nitrogen atoms being the site of protonation upon

acidification.

Figure 16. Electronic absorption spectra due to the formation of purpurogal-

lin over time. Reaction conditions: pyrogallol solution 1 mL (0.025

with 3 mL phosphate buffer (pH 7, 1 ), 1 mL of 30 % H (0.025

.0 ×10 g (6.6 × 10 mol) catalyst, 1 h of reaction time at 25 °C.

M

) mixed

M

2

O

2

M) and

–

3

–6

3

In order to optimize the reaction conditions for optimal oxid-

ation of pyrogallol, several parameters, such as different

amounts of catalyst and oxidant were varied. Five different con-

centrations of catalyst (i.e. from 1.0 × 10^–to 5.0 × 10 g) were

3

–3

tested using a fixed amount of pyrogallol (1 mL of 2.5 × 10^–²

in MeCN) in 3 mL of 1 pH 7 phosphate buffer; 1 mL of

solution of H O and 25 °C reaction temperature.

M

Figure 15. Spectral changes observed during titration of complex 6 with

methanolic HCl solution. The spectra were recorded upon stepwise additions

M

–

1

of one drop portions of HCl solution (1.3 × 10

4

M

) in MeOH to 25 mL of

–2

2

.5 × 10

M

_.₃_×₁₀_–4

2 2

M solution of complex 6 in MeOH during ca. 20 min.

The oxidation of pyragallol was greatly facilitated by increasing

–

3

–6

the catalyst amount from 1.0 × 10 g (2.2 × 10 mol) to

.0 × 10^–g (4.4 × 10 mol) (Figure 17). Only a small improve-

3

–6

2

–3

Peroxidase Mimetic Activity towards the Oxidation of

Pyragallol

ment in pyragallol oxidation was noted when 3.0 × 10

g

(6.6 × 10^–mol) of catalyst was applied whereas further incre-

6

ments in catalyst rendered little change to the rate of oxidative

V

The peroxidase mimetic activity of [V O(ap-fah)(OMe)(MeOH)]

(

conversion. Consequently, a 3.0 ×10^–g (6.6 × 10 mol) of cata-

lyst per reaction was deemed ideal for subsequent efforts to

optimize other reaction conditions.

3

–6

12) was investigated towards the oxidation of pyrogallol in the

presence of H O as an oxidant. This reaction yields purpuro-

gallin (Scheme 3) whose formation can be monitored by UV/

Visible spectroscopy. In the presence of H O , the pyrogallol

2

solution turns yellow with a very weak absorption at ca. 420 nm

[

36]

due to the formation of purpurogallin,

pyrogallol.

which is absent in

Scheme 3. Catalytic oxidation of pyrogallol to purpurogallin.

The following sets of samples were prepared and the result-

ing spectral changes were followed by UV-spectrophotometry

after 5 min of their preparation: i) pyrogallol in MeCN/phos-

phate buffer (pH 7, 1

M); ii) same solution after addition

–3

of 1 mL of 0.025 H O and iii) after addition of 1.0 ×10

M

g

2

–

6

(

2.2 × 10 mol) catalyst 12.

The formation of purpurogallin was confirmed by the ap-

pearance of a weak band at ca. 420 nm, after addition of H O .

Figure 17. Plot of absorbance at 420 nm after 1 h of reaction time for per-

oxidase-like activity using pyrogallol at different catalyst amounts.

Other reaction conditions: pyrogallol solution (prepared in MeCN) (1 mL of

2

However, in the presence of catalyst, this characteristic absorp-

tion band becomes much sharper and intense (Figure S21, Sup-

porting Information) following the conventional enzymatic dy-

namic regulation of the Michaelis–Menten equation. Figure 16

–

2

.5 × 10

2 2

M), phosphate buffer solution (3 mL, pH 7, 1 M), 30 % H O (1 mL

–2

of 2.5 × 10

M) and reaction temperature (25 °C).

The effect of oxidant concentrations (i.e. H O ) in the oxid-

2

shows changes observed with time and suggests that the reac- ation of pyrogallol was then studied by examining reactions

tion reaches maximum measurable absorbance within 1 h.

with five different oxidant concentrations. Thus, 1 mL of H O₂

2

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–2

–6

solution with different concentrations (2.5 × 10 –2.5 × 10

M

)

For catalyst 12, V follows saturation kinetics with respect to

i

was added to each sample containing a fixed amount of pyro- [S]. The rate of reaction is first-order at low substrate concentra-

gallol [1 mL of 2.5 × 10^–²

3 mL of 1

M

in MeCN and pH 7 phosphate buffer tions [S] (i.e. a linear plot is obtained for V vs. [S]). At sufficiently

i

–6

(

M

)], and catalyst 12 (3 × 10^–³g, 6.6 × 10 mol). The high [S] the catalyst becomes saturated with the substrate and

reaction was monitored at 25 °C for 1 h and Figure 18 presents the rate of reaction is zero-order. The kinetic parameters, Micha-

the absorbance at λ_m_a_x= 420 nm at different concentrations of elis binding constant (K ) and V were obtained from analysis

M

max

H O . The catalyst shows a very good response to different of the experimental data and the turnover number (k_c_a_t) value

2

H O concentrations, with the formation of purpurogallin dou- is obtained from dividing the V_m_a_xby the concentration of the

2

–

6

–5

bling when going from 2.5 × 10 to 2.5 × 10

use of 2.5 × 10

very low concentrations of H O under the above optimized weaver–Burk plot is shown in Figure 19. The V obtained was

max

M

oxidant. The complex used [Equation (7)]. Thus, complex 12 clearly displays

–

4

M H O yielded reasonably good activity at Michaelis–Menten behaviour towards pyrogallol; the Line-

2

2.1 μM

min^–and the turnover number (k_c_a_t) was 7.1 min for

1

–1

reaction conditions.

catalyst 12 as dictated by [Equation (7)].

^Vmax

K_c_a_t

=

(7)

concentration of enzyme

Figure 18. Plot of absorbance at 420 nm after 1 h of reaction time for per-

oxidase-like activity using pyrogallol at different concentrations of H

Other reaction conditions: pyrogallol solution (prepared in MeCN) (1 mL of

2 2

O .

.5 × 10^–

2

M), phosphate buffer solution (3 mL of pH 7, 1 M), catalyst 12

2

(

3.0 × 10^–³g, 6.6 × 10 mol) and reaction temperature (25 °C).

–6

To further explore the peroxidase mimetic activity of 12, ki-

netic parameters such as V_m_a_x, K , k (where V_m_a_xis the maxi-

mum rate, K_Mis the Michaelis–Menten constant and k_c_a_tis the

turnover number) were determined. Several experiments were

conducted at 25 °C by varying the concentration of pyrogallol

M

cat

Figure 19. Lineweaver–Burk linearization plot for pyrogallol over the concen-

tration range (0.20–2.2 m ). The concentration of H and 12 remain con-

stant in phosphate buffer (pH 7, 1 ).

M

2 2

O

M

Oxidomethoxidovanadium(V) complexes 11, 13 and 14 were

also tested in the oxidation of pyrogallol; Table 6 summarizes

the results. Notably, all catalysts showed similar behaviours with

comparable V_m_a_xand k_c_a_tvalues (see Figure S22, Supporting

Information) relative to the reactions with catalyst 12.

from 0.20 m

M

to 2.6 m

M

under the optimized reaction condi-

), catalyst

tions {i.e. pH 7 phosphate buffer solution (3 mL of 1

M

V

–3

–6

[

V O(ap-fah)(OMe)(MeOH)] (3.0 × 10 g, 6.6 × 10 mol) and

3

0 % H O (1 mL of 2.5 × 10^–⁴

M

)}. Experiments were run in

2

triplicate and the initial reaction rates were calculated by meas-

uring the absorbances at 420 nm and taking the triplicate aver-

age after carrying out each reaction for 5 min.

Typically, the rate of enzyme–mimetic reactions does not

show a linear response with increasing substrate concentration.

However, if the initial rate of the reaction is measured over a

range of substrate concentrations, the reaction rate (v) increases

as [S] increases. As [S] increases, the enzyme becomes saturated

with substrate and the rate reaches V_m_a_x, the enzyme's maxi-

mum rate. This phenomenon can be explained by the Micha-

Table 6. Kinetic parameters for peroxidise activity of complexes 11–14.

Complex/catalyst

V

max

[μM/min^–¹]

/

TOF (kcat

)

–1

[min

]

V

1

2

3

4

[V O(ap-bhz)(OMe)(MeOH)] (11)

2.7

2.2

2.6

2.5

8.2

7.1

8.0

7.7

V

[V O(ap-fah)(OMe)(MeOH)] (12)

V

[V O(ap-nah)(OMe)(MeOH)] (13)

V

[V O(ap-inh)(OMe)(MeOH)] (14)

Thus, from the calculated k_c_a_tand V_m_a_xvalues it can be con-

cluded that the complexes herein synthesized are, indeed, func-

tional models of artificial peroxidases, showing peroxidase mi-

metic activity. The catalytic potentials of the catalysts consid-

ered here are superior to those previously reported by us for

elis–Menten equation [Equation (6)], where V is the initial reac-

i

tion rate, V_m_a_xis the maximum rate, [S] is the concentration of

substrate and K_Mis the Michaelis–Menten constant.

–

1

–3

the same oxidation reaction [K (min ) = 4.79 × 10 for PS-

cat

[

S]

V

[

V O(OMe)(hap-iah)] (H hap-iah = Schiff base derived from 2-

2

V_i= V_m_a_x

×

(6)

K_M+[S]

hydroxyacetophenone and indole-3-acetic acid hydrazide) and

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–

1

–3

IV

K_c_a_t(min ) = 3.62 × 10 for PS-V O(Hsal-dahp)(DMSO)] (H sal- the oxidation of 1-phenylethanol is clearly catalysed by the va-

2

dahp = Schiff base derived from salicylaldehyde and 1,3-di- nadium complexes evaluated.

[

16]

amino-2-hydroxypropane).

Table 8. Oxidation of 1-phenylethanol using 11–14 under optimized reaction

conditions (see Table 7, Entry 2).

Complex

Substrate/

catalyst ratio

Conv. [%]

(yield, mmol) [h ]

TOF

Catalytic Activity: Oxidation of 1-Phenylethanol

–

1

The selective catalytic oxidation of 1-phenylethanol to aceto-

phenone (Scheme 4) also was evaluated using complexes 11–

–4

1

2

3

4

[VO(ap-bhz)(OMe)(MeOH)] (11) 1:4.3 × 10

[VO(ap-fah)(OMe)(MeOH)] (12)

86 (8.6)

76 (7.6)

81 (8.1)

80 (8.0)

651

576

613

606

–

4

1:4.4 × 10

–4

[VO(ap-nah)(OMe)(MeOH)] (13) 1:4.3 × 10

[VVO(ap-inh)(OMe)(MeOH)] (14) 1:4.3 × 10

1

4 in the presence of H O .

2 2

–4

Based on these experiments (UV/Vis and ⁵¹V NMR) the fol-

lowing reaction Scheme for both catalytic reactions is proposed

Scheme 5).

(

Scheme 4. Catalytic oxidation of 1-phenylethanol to acetophenone.

Complex 12 was selected as a representative catalyst precur-

sor and efforts to optimize catalyst and oxidant amounts, sol-

vent and temperature were undertaken. Thus, for 0.010 mol

(

1

1.2 g) of 1-phenylethanol, three different amounts of complex

2 (i.e. 1 × 10 , 2 × 10 and 3 × 10 g); three concentrations

–

3

–3

of aqueous 30 % H O (i.e. 0.010, 0.020 and 0.030 mol), in three

2

different volumes of MeCN (5, 7 and 10 mL) were applied to

establishing a library of reactions and these reactions were car-

ried out at three different temperatures (60, 70 and 80 °C).

Table 7 and Figure S23 (Supporting Information) present the

details of the applied experimental conditions as well as ensu-

ing conversion efficiencies. From Table 7 it is clear that in-

creased catalyst or oxidant concentrations correlated to slightly

improved conversions (Table 7, Entries 1–3, 4 and 5). Also evi-

dent is that reaction temperature has a strong effect (Table 7,

Entries 7–9) and that the reaction conditions best suited for

optimum conversion (i.e. 74 % with a turnover frequency

Scheme 5. Proposed reaction scheme for the catalytic reactions.

The catalytic oxidation of 1-phenylethanol by the catalysts

reported here compares well with the catalytic potential of

VI

molybdenum-based catalysts [Mo O (dha-inh)(MeOH)] (where

2

H dha-inh = Schiff base derived from dehydroacetic acid and

2

VI

isonicotinoyl hydrazide) and [Mo O (bp-bhz)(MeOH)] (H bp-

2

–

1

–3

of 576 h ) are (Table 7, Entry 2): catalyst (2 × 10 g,

bhz = benzoylpyrazolone and benzoyl hydrazide) reported re-

–

6

4

.4 × 10 mol), 30 % H O (2.2 g, 0.020 mol), MeCN (5 mL) and

2 2

cently where 78 and 84 % conversions, respectively, were ob-

a reaction temperature of 80 °C.

[37,38]

tained.

Notably, conversions with vanadium complexes

[

39]

[

VO(Hhpic) ] (where Hhpic = 3-hydroxypicolinic acid) (51 %),

2

Table 7. Oxidation of 1-phenylethanol (1.2 g, 0.010 mol) under different reac-

tion conditions using 12 as catalyst precursor.

V O Mo O {μ-(O )}(4,4′-tBubpy){μ-(OMe) }]

(where

4,4′-

2

4

2

tBubpy

=

4,4′-di-tert-butyl-2,2′-bipyridyl) (63 %), and

Entry

H

2

O

2

Catalyst

g [μmol]

MeCN

[mL]

Temp.

[°C]

Conv. TOF

[40]

[

V O W O {μ-(O )}(4,4′-tBubpy){μ-(OMe) }] (28 %)

are much

2

4

2

–

1

g [mol]

[%]

[h ]

lower than those achieved by the complexes reported here.

–

3

1

2

3

4

5

6

7

8

9

2.3 (0.020)

1.1 (0.001)

3.3 (0.030)

2.3 (0.020)

2.3(0.020)

2.3 (0.020)

1 × 10 (2.2)

5

7

80

60

70

80

74

76

80

60

82

81

77

52

67

32

560

576

606

454

621

613

583

394

507

–

3

2 × 10 (4.4)

–

3

3 × 10 (6.6)

Conclusions

–

3

1 × 10 (2.2)

–

3

1 × 10 (2.2)

IV

Vanadium complexes of general compositions [V OL(H O)],

–

3

2

1 × 10 (2.2)

V

–

3

K(H O) [V O L] (n = 1 or 0.5), [V O (HL)] and [V O(L)(OMe)-

2 n 2 2

1 × 10 (2.2) 10

–

3

1 × 10 (2.2)

5

(MeOH)] where L is a dianionic ONO ligand derived from acetyl-

pyrazolone (ap) and different hydrazides – benzoyl hydrazide

(

–

3

1 × 10 (2.2) 10

1

0

–

5

bhz), 2-furoyl hydrazide (fah), nicotinoyl hydrazide (nah) and

isonicotinoyl hydrazide (inh) have been successfully synthesized

We have tested the catalytic potential of oxidomethoxidova- and characterized. The molecular structures of complexes

V

nadium(V) complexes 11, 13 and 14 in the oxidation of 1- K(H O) [V O (ap-fah)]

phenylethnol under the optimized reaction conditions identi- [V O (Hap-nah)] (9), [V O (Hap-inh)]·H O (10), [V O(ap-

fied for complex 12; Table 8 summarizes the results. All catalysts bhz)(OMe)(MeOH)] (11) and [V O(ap-fah)(OMe)(MeOH)] (12)

showed similar behaviours giving rise to conversion values were elucidated by single-crystal X-ray diffraction analyses. In

within 80–85 %. A blank reaction, devoid of catalyst, under complexes 7 and 8 protonation of the hydrazide nitrogen takes

these conditions afforded a conversion value of only 32 %. Thus, place; protonation in complexes 9 and 10 occurs at the pyridine

(6),

[V O (Hap-bhz)]·H O

(7),

2

0.25

2

V

2

V

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IV

nitrogen. The complexes show excellent catalytic activity in the [V O(acac)

] (0.26 g, 0.0010 mol) in methanol (15 mL) was added

2

to the above solution with stirring. The reaction mixture was re-

fluxed on a water bath for ca. 6 h. The obtained clear solution was

reduced to 10 mL and kept in the refrigerator (ca. 10 °C) where a

black solid slowly precipitated out. The solid was filtered, washed

with methanol followed by petroleum ether (b.p. 60 °C) and dried

in a desiccator over silica gel, yield: 0.32 g (77 %). C H N O V

oxidation of 1-phenylethanol to give acetophenone. Thus, the

synthesized complexes are structural and functional models of

the vanadate-dependent haloperoxidases. The complexes were

also successfully applied in the oxidation of pyrogallol to pur-

purogallin under mild reaction conditions in pH 7 buffered solu-

tion.

19 18 4 4

(417.31): calcd. C 54.68, H 4.35, N 13.43; found C 54.7, H 4.3, N 13.4.

IV

[

V O(ap-fah)(H O)] (2): Complex 2 was prepared from the method

2

Experimental Section

outlined for complex 1 using H ap-fah, yield: 0.32 g (78 %).

2

C H N O V (407.27 g/mol): calcd. C 50.13, H 3.96, N 13.76; found

17 16 4 5

Materials: Acetylacetone, furoyl hydrazide, pyrogallol, 1-phenyleth-

anol (Aldrich Chemicals Co., U.S.A.), ethyl benzoate, hydrazine hy-

drate, isonicotinoyl hydrazide (Loba Chemie, Mumbai, India), nico-

C 50.4, H 3.8, N 14.0.

IV

[V O(ap-nah)(H

O)] (3): Complex 3 was prepared from the

ap-nah, yield: 0.29 g (69 %).

V (418.30): calcd. C 51.68, H 4.10, N 16.74; found C 51.4,

chemicals and solvents were of analytical reagent grade. H 3.7, N 16.4.

2

tinoyl hydrazide (Acros organics, New Jersey, USA) and 30 % aque- method outlined for complex 1 using H

2

ous H O , (Rankem, New Delhi, India) were used as supplied. Other

C H N O

18 17 5 4

2

IV

[41]

[42]

[

V O(acac) ],

3-methyl-1-phenyl-5-pyrazolone,

4-acetyl-3-

IV

2

[V O(ap-inh)(H O)] (4): Complex 4 was prepared from the method

2

[43]

methyl-1-phenyl-2-pyrazoline-5-one

H ap-fah (II), H ap-nah (III) and H ap-inh (IV)

and ligands H ap-bhz (I),

2

outlined for complex 1 using H ap-inh, yield: 0.30 g (72 %).

2

[18]

were prepared ac-

2

C H N O V (418.30): calcd. C 51.68, H 4.10, N 16.7; found C 51.8,

18 17 5 4

cording to methods reported in the literature.

H 4.2, N 16.6.

Instrumentation and Characterization Procedures: Elemental

analysis (C, H and N) were carried out on an elementar model Vario-

E1–III after drying the complexes in vacuo over silica gel for several

hours. IR spectra were recorded as KBr pellets with a Nicolet 1100

FT-IR spectrometer. Electronic spectra of ligands and complexes

were measured in methanol or in DMSO either with a Shimadzu

V

K(H O)[V O (ap-bhz)] (5): V O (0.18 g, 0.0010 mol) was sus-

2

2 5

pended in an aqueous solution of KOH (0.11 g, 0.0020 mol in 5 mL

of H O) and stirred for 2 h with occasional heating at 50 °C. The

resulting clear solution was then filtered. A filtered solution of I

2

(

0

0.67 g, 0.0020 mol) dissolved in 50 mL of aqueous KOH (0.22 g,

.0040 mol) was added to the above solution. The resulting mixture

was allowed to react for 2 h and then the pH was adjusted to ca. 7.5

with 4 HCl. The yellowish-orange solid started to separate within

0 min but the stirring was continued for the next 2 h. The sepa-

rated solid was filtered, washed with cold water (2 × 5 mL) and

dried in desiccator over silica gel, yield: 0.30 (67 %).

C H KN O V (472.27): calcd. C 48.31, H 3.84, N 11.86; found C 48.2,

1

601 or with a Perkin–Elmer lambda 35 UV/Vis spectrophotometer.

1

13

H and C NMR spectra were recorded in [D ]DMSO using a Bruker

Advance 500 MHz. V-NMR spectra of ca. 3 m

6

M

51

M solutions of the

3

complexes in DMSO or MeOH (p.a. grade) (5 % D O was added)

2

51

were recorded with a Bruker Avance III 400 MHz instrument.

V

a

g

chemical shifts were referenced relative to neat VOCl as external

3

19

18

4 5

standard. The following parameters were used: number of scans

H 4.7, N 11.7.

2

000–5000; line broadening 100 Hz; spectral resolution 12.7 Hz;

spectral width 3959 ppm; acquisition time 0.039 s; FID resolution

5.4 Hz; receiver gain 203; dwell time 1.2 μs. The thermogravimetric

V

K(H O) [V O (ap-fah)] (6): Yellow complex 6 was prepared by

2

0.5

2

the method described for complex 5 with II as ligand, yield: 0.33 g

72 %). C H KN O₅_.₅V (453.30): calcd. C 45.00, H 3.31, N 12.35;

2

(

17

15

4

analysis of the complexes was carried out under an oxygen atmos-

phere using a TG Stanton Redcroft STA 780 instrument. The redox

properties of the ligands (I–IV) and the complexes (1–14) were

studied by cyclic voltammetry and differential pulse voltammetry

using CH Electrochemical workstations instruments with a three

compartment cell provided with Pt as working electrode, platinum

wire as counter electrode and standard calomel electrode (SCE) as

reference electrode. Tetrabutyl ammonium phosphate (TBAP) was

used as supporting electrolyte in DMF under an argon-saturated

atmosphere in the potential range –1.5 to 1.5 V at room tempera-

ture. A Shimadzu 2010 plus gas-chromatograph fitted with an

Rtx-1 capillary column (30 m × 0.25 mm × 0.25 μm) and FID de-

tector was used to analyze the reaction products and their quantifi-

cation was made based on the relative peak area of each product.

The identity of the products was confirmed using a GC–MS model

Perkin–Elmer, Clarus 500 and comparing the fragments of each

product with the library available. EPR spectra were recorded at

found C 44.4, H 3.5, N 12.2.

V

[V O

(Hap-bhz)] (7): Complex 5 (0.45 g, 0.0010 mol) was dissolved

2

in water (40 mL) and treated with dropwise addition of aqueous

HCl (2 ) until most of the complex had precipitated as a yellow

M

solid. After 1 h of stirring, the solution was filtered, the residue

washed with water and dried in a desiccator over silica gel, yield:

0.24 g (57 %). C19H N O V (416.28): calcd. C 54.82, H 4.12, N 13.46;

17 4 4

found C 55.8, H 4.4, N 14.0.

V

[

V O (Hap-fah)] (8): Complex 8 (yellow colour) was prepared from

2

6

following the method adopted for complex 7, yield: 0.238 g

(

58.1 %). C H N O V (406.28): calcd. C 50.26, H 3.72, N 13.79;

1

7 15 4 5

found C 50.6, H 3.5, N 13.7.

V

[

V O (Hap-nah)] (9): V O (0.18 g, 0.0010 mol) was suspended in

2

2 5

an aqueous solution of KOH (0.11 g, 0.0020 mol in 5 mL of H O)

and stirred for 2 h with occasional heating at 50 °C. The resulting

clear solution was then filtered. A filtered solution of H ap-nah (III)

2

7

7 K with a Bruker ESP 300E X-band spectrometer coupled to a

(0.0020 mol) dissolved in aqueous KOH (50 mL, 0.22 g, 0.0040 mol)

Bruker ER041 X-band frequency meter (9.45 GHz). The complexes

were dissolved at room temperature in DMSO p.a. grade, previously

was added to the above solution with stirring. The resulting mixture

was allowed to react for 4 h after which the pH of the reaction

mixture was adjusted to ca. 7.0 with 4

ange solid 9 started to separate. After 2 h of stirring the separated

solid was filtered, washed with cold water (2 × 5 mL) and dried in

a desiccator over silica gel, yield: 0.25 g (60.0 %). C H N O V

(417.30 g/mol): calcd. C 51.81, H 3.86, N 16.78; found C 51.7, H 3.5,

N 17.0.

degassed by passing N for 10 min, to obtain ca. 3 m

M solutions.

2

M HCl, when yellowish-or-

Spectra were measured at 77 K and simulated with a program de-

veloped by Rockenbauer and Korecz.^[

32]

Preparations

1

8 16 5 4

IV

[

V O(ap-bhz)(H O)] (1): The ligand H ap-bhz (0.34 g, 0.0010 mol)

2

was dissolved in methanol (15 mL) and filtered. A solution of

Eur. J. Inorg. Chem. 0000, 0–0

www.eurjic.org

14

Full Paper

V

[

V O (Hap-inh)] (10): Yellow complex 10 was prepared from ligand

C H N O V (462.37): calcd. C 54.55, H 5.01, N 12.12; found C 54.1,

21 23 4 5

2

H₂ap-inh (IV) following the method described for 9, yield: 0.25 g

61 %). C H N O V (417.30 g/mol): calcd. C 51.81, H 3.86, N 16.78;

found C 51.5, H 3.9, N 16.6.

H 5.9, N 11.9.

(

18 16 5 4

_VV_O₍_a_p_-_f_a_h₎₍_O_M_e₎₍_M_e_O_H₎_]₍₁₂₎_:_C_o_m_p_l_e_x₁₂_w_a_s_p_r_e_p_a_r_e_d_s_i_m_i_-

[

V^VO(ap-bhz)(OMe)(MeOH)] (11): Complex 1 (0.42 g, 0.0010 mol)

larly to 11 from complex 2, yield: 0.34 g (75 %). C H N O V

[

19 21 4 6

(452.33): calcd. C 50.45, H 4.68, N 12.39; found C 50.7, H 5.2, N 12.0.

was dissolved in MeOH (30 mL) and allowed to stand in the open

flask for slow evaporation/oxidation. The dark solution slowly

changed to orange-yellow from which a yellow solid separated

V

[V O(ap-nah)(OMe)(MeOH)] (13): Complex 13 was prepared simi-

within a week. The solid was filtered, washed with cold water and larly to 11 from complex 3, yield: 0.34 g (74 %). C H N O V

20 22 5 5

dried in

a

desiccator over silica gel, yield: 0.36

g

(78 %).

(463.36): calcd. C 51.84, H 4.79, N 15.11; found C 51.4, H 5.0, N 14.8.

V

Table 9. Crystal data and structure refinement for {K(H

[

2

O)0.25[V O

2

(ap-fah)]}

n

(6), [V O

2

(Hap-bhz)]·H

2

O (7), [V O

2

(Hap-nah)] (9), [V O

2

(Hap-inh)]·H

2

O (10),

V

V O(ap-bhz)(OMe)(MeOH)] (11) and for [V O(ap-fah)(OMe)(MeOH)] (12).

6

7

9

Formula

Formula weight

T [K]

C

68

H

56

K

4

N

16

O

21

V

4

C

19

H

19

N

4

O

5

V

18 16 5 4

C H N O V

417.30

100(2)

1793.45

100(2)

434.32

100(2)

Wavelength [Å]

Crystal system

Space group

a [Å]

b [Å]

c [Å]

0.71073

triclinic

P1¯

0.71073

triclinic

P1¯

monoclinic

P2 /c

1

9.2529(3)

26.6010(8)

7.3328(2)

8.6178(18)

9.914(2)

11.942(2)

70.921(9)

80.391(8)

71.571(8)

912.5(3)

2

8.5358(4)

10.2124(5)

10.2489(5)

92.785(2)

90.016(2)

106.312(2)

856.34(7)

2

α [°]

ꢀ

[°]

90.7120(10)

γ [°]

3

V [Å ]

Z

1804.73(9)

1

F(000)

912

1.650

0.821

1.53 to 26.49

0.0312

0.31 × 0.29 × 0.24

1.172

0.0320

0.0973

0.666 and –0.476

448

1.581

0.585

1.81 to 27.60

0.0392

0.37 × 0.15 × 0.07

1.073

0.0456

0.1477

0.688 and –0.543

428

1.618

0.618

2.49 to 27.60

0.1012

0.25 × 0.11 × 0.10

1.068

0.0361

0.0971

0.665 and –0.501

D

calc [g cm^–³

]

μ [mm^–¹

θ [°]

]

R

int

3

Crystal size [mm ]

Goodness-of-fit on F

2

[

a]

R

1

[I > 2σ(I)]

[

b]

wR (all data)

2

Largest differences peak and hole (e Å^–³)

10

11

12

Formula

Formula weight

T [K]

Wavelength [Å]

Crystal system

Space group

A [Å]

B [Å]

C [Å]

α [°]

C

18

H

18

N

5

O

5

V

C

21

H

23

N

4

O

5

V

19 21 4 6

C H N O V

435.31

100(2)

0.71073

triclinic

P1¯

462.37

100(2)

0.71073

monoclinic

P2 /n

7.4905(6)

21.2776(17)

13.2842(10)

452.34

100(2)

0.71073

triclinic

P1¯

1

8.719(15)

11.3567(19)

12.675(2)

16.231(2)

85.962(8)

89.740(8)

76.594(8)

2266.8(7)

4

10.041(17)

12.12(2)

71.044(16)

80.690(17)

71.496(17)

950(3)

ꢀ

[°]

91.497(4)

γ [°]

3

V [Å ]

Z

2116.5(3)

4

2

F(000)

448

1.522

0.564

1.78 to 26.34

0.0432

0.35 × 0.24 × 0.21

0.937

0.0552

0.1842

0.598 and –0.618

960

1.451

0.510

1.81 to 27.88

0.0435

0.34 × 0.22 × 0.20

1.064

0.0417

0.1229

0.698 and –0.553

936

1.325

0.477

2.01 to 26.28

0.0662

0.44 × 0.33 × 0.18

1.024

0.0923

0.2714

2.619 and –0.896

D

calcd. [g cm^–³]

Μ [mm^–¹

θ [°]

]

R

int

3

Crystal size [mm ]

Goodness-of-fit on F

2

[

a]

R

1

[I > 2σ(I)]

wR (all data)

Largest differences peak and hole (e Å^–³)

[

b]

2

o c o

a] R = Σ||F | – |F ||/Σ|F |. [b] wR = {Σ[w(||F | |) ]|/Σ[w(F

|²– |F ) ]}¹^/².

2

2 2

[

1

o

c

o

2

Eur. J. Inorg. Chem. 0000, 0–0

www.eurjic.org

15

Full Paper

[

V^VO(ap-inh)(OMe)(MeOH)] (14): Complex 14 was prepared simi-

larly to 11 from complex 4, yield: 0.35 g (74 %). C H N O V

Kinetic measurements were carried out in time course mode by

monitoring absorbance changes at 420 nm for 5 min and by varying

the concentration of pyrogallol (0.20–2.2 mM) for a fixed concentra-

2

0 22 5 5

(463.36): calcd. C 51.84, H 4.79, N 15.11; found C 50.9, H 5.3, N 15.3.

tion of H O₂(0.00025

M

) in the presence of the catalyst 12

2

X-ray Crystal Structure Determination: Three-dimensional X-ray

data were collected with a Bruker Kappa Apex CCD diffractometer

at low temperature for 6, 7, 9, 10, 11 and 12, by the φ–ω scan

method. Reflections were measured from a hemisphere of data col-

lected from frames, each of them covering 0.3° in ω. A total of

(0.0010 g). Initial rates were calculated from the slope of the plot

of absorbance vs. time over a period of 5 min and fitted to the

Michaelis–Menten equation. Lineweaver–Burk linearizations were

carried out using Origin 8.0 to calculate kinetic parameters such as

maximal velocity (V_m_a_x) and turnover numbers (k_c_a_t) from nonlinear

curve fitting.

4

0180 for 6, 25323 for 7, 32645 for 9, 5196 for 10, 66677 for 11

and 49609 for 12, 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 to-

tal, 3449 for 6, 3118 for 7, 3369 for 9, 1904 for 10, 4078 for 11 and

Catalytic Oxidation of 1-phenylethanol: Catalytic oxidative activi-

ties of synthesized vanadate complexes were screened in the oxid-

ation of 1-phenylethanol for which monoxidovanadium(V) com-

plexes 11–14 were used. In a typical reaction, 1-phenylethanol

5

906 for 12, independent reflections exceeded the significance level

(1.2 g, 0.010 mol) and 30 % aqueous H O (2.3 g, 0.020 mol) were

(

|F|/σ|F|) > 4.0. After data collection, in each case an multi-scan ab-

2 2

[44]

mixed in CH CN (5 mL) followed by addition of catalyst (0.0010 g).

sorption correction (SADABS) was applied, and the structure was

solved by direct methods and refined by full-matrix least-squares

3

The reaction mixture was slowly stirred at 80 °C for 6 h. For each

reaction the oxidative product, acetophenone, was quantitatively

detected using gas chromatography and its identity confirmed by

GC–MS.

2

[45]

on F data using SHELX suite of programs. Hydrogen atoms were

located in difference Fourier map and left to refine freely, except

for C(4) in 6, N(4), C(4) and C(11) in 7, C(4) and C(6) in 9, C(4) and

C(6) in 10 and C(1M), C(2M), C(4) and C(6) in 11, which were in-

cluded in calculation position and refined in the riding mode. In 7,

hydrogen atoms of water were located in the difference Fourier

Acknowledgments

map and fixed to the relevant oxygen atom. In 10, it was only M. R. M. thanks the Science and Engineering Research Board

possible to locate a hydrogen atom of water molecule due to the (SERB), the Government of India, New Delhi for financial sup-

other hydrogen atom must be involved in symmetry operations. In port of the work (grant number EMR/2014/000529). B. S. is

1

2, all hydrogen atoms were included in the calculation position

thankful to Indian Institute of Technology Roorkee (IIT Roorkee)

for awarding a MHRD fellowship. I. C. thanks the financial sup-

port from the Portuguese Fundação do Ministério de Ciência e

Tecnologia (FCT), project number UID/QUI/00100/2013 and the

IST-UTL Centres of the Portuguese NMR spectroscopy.

and refined in the riding mode. Refinements were done with allow-

ance for thermal anisotropy of all non-hydrogen atoms. A final dif-

ference Fourier map showed no residual density outside: 0.666 and

–3

–

0.476 e Å for 6, 0.688 and –0.543 for 7 e Å , 0.665 and

–3

0.501 e Å for 9, 0.598 and –0.618 e Å for 10, and 0.698 and

0.553 e Å^–³for 11. In 12, an important residual density of 2.619

–3

and –0.896 e Å , next to C(25), cannot be refined. A weighting Keywords: Bioinorganic chemistry · Enzyme mimics ·

2

Scheme w = 1/[σ (F ) + (0.057400 P) + 0.845800 P] for 6, w = Vanadium · Oxidation

o

2

1

/[σ (F ) + (0.085800 P) + 0.497200 P] for 7, w = 1/[σ (F ) +

o

2

(0.044900 P) + 0.603500 P] for 9, w = 1/[σ (F ) + (0.102400 P) +

o

2

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o

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11 and w = 1/[σ (F ) + (0.148600 P) + 9.733800 P] for 12, where

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P = (|F | + 2|F | )/3, were used in the latter stages of refinement.

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Received: April 12, 2016

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Published Online: ■

Eur. J. Inorg. Chem. 0000, 0–0

www.eurjic.org

17

Full Paper

Vanadium Haloperoxidase Models

The synthesis and reactivity of vana-

dium complexes, their abilities to

mimic peroxidases and efficient cata-

lytic activity for the oxidation of 1-

phenylethanol are reported.

M. R. Maurya,* B. Sarkar,

F. Avecilla, I. Correia* ..................... 1–18

Vanadium Complexes Derived from

Acetyl Pyrazolone and Hydrazides:

Structure, Reactivity, Peroxidase

Mimicry and Efficient Catalytic Ac-

tivity for the Oxidation of 1-Phen-

ylethanol

DOI: 10.1002/ejic.201600427

Eur. J. Inorg. Chem. 0000, 0–0

www.eurjic.org

18

Article Doi

Vanadium Complexes Derived from Acetyl Pyrazolone and Hydrazides: Structure, Reactivity, Peroxidase Mimicry and Efficient Catalytic Activity for the Oxidation of 1-Phenylethanol

DOI: 10.1002/ejic.201600427

Source and publish data:

Authors:

Article abstract of DOI:10.1002/ejic.201600427

Full text of DOI:10.1002/ejic.201600427

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