synthesis, spectroscopy, electrochemistry and thermal study of vanadyl tridentate schiff base...

ORIGINAL PAPER

Synthesis, spectroscopy, electrochemistry and thermal studyof vanadyl tridentate Schiff base complexes: theoretical studyof the structures of compounds by ab initio calculations

Ali Hossein Kianfar • Sanam Asl Khademi •

Roghaye Hashemi Fath • Mahmoud Roushani •

Mojtaba Shamsipur

Received: 3 April 2012 / Accepted: 30 August 2012

� Iranian Chemical Society 2012

Abstract The VO(IV) complexes of tridentate ONN Schiff

ligands were synthesized and characterized by IR, UV–Vis

and elemental analysis. The electrochemical properties of the

vanadyl complexes were investigated by cyclic voltammetry.

A good correlation was observed between the oxidation

potentials and the electron-withdrawing character of the

substituents on the Schiff base ligands, showing the follow-

ing trend: MeO \ H \ Br\ NO2. The thermogravimetry

(TG) and differential thermoanalysis (DTA) of the VO(IV)

complexes were carried out in the range of 20–700 �C. The

VOL1(OH2) and VOL2(OH2) decomposed in three steps,

whereas the VOL3(OH2) and VOL4(OH2) complexes

decomposed in two steps. The thermal decomposition of

these complexes is closely related to the nature of the Schiff

base ligands and proceeds via first-order kinetics. The

structures of compounds were determined by ab initio

calculations. The optimized molecular geometry and

atomic charges were calculated using MP2 method with

6-31G(d) basis. The results suggested that, in the complexes,

V(IV) ion is in square-pyramid N2O3 coordination geometry.

Also the bond lengths and angles were studied and compared.

Keywords Vanadyl complexes � Tridentate Schiff base

ligands � Schiff base complexes � Thermogravimmetry �Electrochemistry � Ab initio calculations

Introduction

Schiff bases are an interesting set of ligands capable of

binding metal ions leading to complexes with interesting

properties. The high stable potential of Schiff base com-

plexes in different oxidation states increased the applica-

tion of these compounds in many areas. These types of

metal complexes have been investigated because of their

interesting and important properties such as their ability to

reversibly bind oxygen [1]. They are used as catalysts for

oxygenation and oxidation reactions of organic compounds

and also electrochemical reduction reactions [2–7]. In this

area the vanadyl Schiff bases play a unique role [8] and

have been investigated as model compounds to clarify

several biochemical processes [9, 10]. Also the tridentate

vanadyl Schiff base complexes act as protein tyrosine

phosphates inhibitors [11] and show photo-induced DNA

cleavage activities [12].

Thermogravimetry and thermoanalysis techniques are

valuable for studying the thermal properties of compounds

[13–17]. The electrochemical methods could be envisioned

to provide highly valuable information on the catalytic

processes since catalytic conversions are frequently

accompanied by the change in the oxidation state of the

central metal and the structure of the complex. Knowledge

of electronic and steric effects to control the redox chem-

istry of these metal complexes may prove to be critical in

A. H. Kianfar (&)

Department of Chemistry, Isfahan University of Technology,

Isfahan 84156-83111, Iran

e-mail: [email protected]

S. A. Khademi

Department of Chemistry, Yasouj University, Yasouj, Iran

R. H. Fath

Young Researchers Club, Gachsaran Branch,

Islamic Azad University, Gachsaran, Iran

M. Roushani

Department of Chemistry, Ilam University, Ilam, Iran

M. Shamsipur

Department of Chemistry, Razi University, Kermanshah, Iran

123

J IRAN CHEM SOC

DOI 10.1007/s13738-012-0165-5

the design of new catalysts. Having continued our studies

on vanadyl Schiff base complexes [18–21], herein we

report the electronic influence of salicylaldehyde deriva-

tives of ONN tridentate Schiff bases on thermal and elec-

trochemical properties of vanadyl(IV) Schiff base

complexes (Fig. 1). The thermal decomposition kinetics

parameters, calculated using Coats and Redfern method

[22], are also reported and discussed. The structures of

compounds were determined by ab initio calculations. The

optimized molecular geometry and atomic charges were

calculated using MP2 method with 6-31G(d) basis set.

Experimental

Chemicals and apparatus

All of the chemicals and solvents used for synthesis and

electrochemistry were of commercially available reagent

grade and used without purification. The elemental analysis

was done on a CHN-O-Heraeus elemental analyzer.

Infrared spectra were recorded as KBr discs on a FT-IR

JASCO-680 spectrophotometer in the 4,000–400 cm-1.

UV–Vis spectra were recorded on a JASCO V-570 spec-

trophotometer in the 190–900 nm. Thermogravimetry (TG)

and differential thermoanalysis (DTA) were carried out on

a PL-1500. The measurements were performed in air

atmosphere. The heating rate was kept at 10 �C min-1.

Cyclic voltammograms were performed using an auto-

lab modelar electrochemical system (ECO Chemie, Utr-

echt, The Netherlands) equipped with a PSTA 20 module

and driven by GPES (ECO Chemie) in conjunction with a

three-electrode system and a personal computer for data

storage and processing. An Ag/AgCl (saturated KCl)/3 M

KCl reference electrode, a Pt wire as counter electrode and

a glassy carbon electrode as working electrode (Metrohm

glassy carbon, 0.0314 cm2) were employed for the elec-

trochemical studies. Voltammetric measurements were

performed at room temperature in dimethylformamide

(DMF) solution with 0.1 M tetrabutylammonium perchlo-

rate as the supporting electrolyte.

Synthesis of Schiff base ligands

The tridentate Schiff base ligands, L1–L4, were prepared

according to the following procedure. The appropriate

amounts of salicylaldehyde and its derivatives were added

to a methanolic solution of 4-nitro-1,2-diaminobezene

(1:1 mol ratio). The solution was refluxed for 2 h. During

the reaction a yellow precipitate appeared. The precipitate

was filtered, washed with methanol and recrystallized by

dichloromethane/methanol mixed solvent through the par-

tial evaporation of dichloromethane.

L1: yield: 78 %, IR (KBr pellets, cm-1): 3,466 and

3,363 (mNH2), 1,610 (mC=N), 1,490 (mC=C), 1,330 (vNO2), kmax

(nm) (e, L mol-1 cm-1) (ethanol): 242 (46,500), 272

(33,000), 381 (58,500).

L2: Yield: 78 %, IR (KBr pellets, cm-1): 3,480 and

3,370 (mNH2), 16,114 (mC=N), 1,490 (mC=C), 1,339 (vNO2),

kmax (nm) (e, L mol-1 cm-1) (ethanol): 229 (54,500), 268

(53,000), 372 (64,000).




(40000), 384 (59500).




(42,000), 393 (54,000).

Synthesis of vanadyl complexes

Synthesis of [VOL1(OH2)]

The [VOL1(OH2)] complex was synthesized by adding

1 mmol (0.2872 g) of L1 Schiff base ligand to a methanolic

solution of vanadylacetylacetonate (0.2652 g, 1 mmol) and

refluxed for 4 h. The reaction mixture was filtered and left

to evaporate the methanol at room temperature. The green

precipitates were isolated from the solution, washed with

methanol and dried in vacuum. Yield: 75 %, Anal. Calc.

for C14H13N3O6V: C, 45.24; H, 3.45; N, 11.34 %. Found:

C, 44.89; H, 3.38; N, 11.38. IR (KBr pellets, cm-1): 3,366

(mNH), 1,610 (mC=N), 1,529 (mC=C), 1,340 (vNO2) and 972

(mVO), kmax (nm) (e, L mol-1 cm-1) (ethanol): 245

(49,000), 269 (44,500), 382 (42,500).

O

HO

H2N NH2

O2N

H2N N

HO

X

X

+

X = MeO, H, Br, NO2

L1, L2, L3, L4

Methanol

H

OH

N+

N

VOH2O

NHVO(acac)2

O

NH2

XX

NO2

O2N O2N

Fig. 1 The structure of vanadyl complexes

J IRAN CHEM SOC

123



1 mmol (0.2572 g) of L2 Schiff base ligand to a methanolic

solution of vanadylacetylacetonate (0.2652 g, 1 mmol) and

refluxed for 4 h. The reaction mixture was filtered and left

to evaporate the methanol at room temperature. The green

precipitates were isolated from the solution, washed with

methanol and dried in vacuum. Yield: 75 %, Anal. Calc.

for C13H11N3O5V: C, 45.89; H, 3.26; N, 12.34 %. Found:

C, 45.76; H, 3.31; N, 12.47. IR (KBr pellets, cm-1): 3,317

(mNH), 1,575(mC=N), 1,515 (mC=C), 1,310 (vNO2) and 969 (mVO),

kmax (nm) (e, L mol-1 cm-1) (ethanol): 229 (54,500), 268

(53,000), 372 (64,000).



1 mmol (0.3362 g, 1 mmol) of L3 Schiff base ligand to an

ethanolic solution of vanadylacetylacetonate (0.2652 g,

1 mmol) and refluxed for 4 h. The reaction mixture was

filtered and left to evaporate the ethanol at room temper-

ature. The green precipitates were isolated from the solu-

tion, washed with ethanol and dried in vacuum. Yield:

80 %, Anal. Calc. for C13H10BrN3O5V: C, 37.24; H, 2.40;

N, 10.02 %. Found: C, 37.15; H, 2.38; N, 10.13. IR (KBr

pellets, cm-1): 3,417 (mNH), 1,605 (mC=N), 1,517 (mC=C),

1,342 (vNO2) and 985 (mVO), kmax (nm) (e, L mol-1 cm-1)

(ethanol): 243 (19,500), 274 (18,500), 393 (9,500).

Synthesis of [VOL4(DMF)]

The [VOL4(DMF)] complex was synthesized by adding

1 mmol (0.3022 g, 1 mmol) of L4 Schiff base ligand to

a DMF solution of vanadylacetylacetonate (0.2652 g, 1

mmol) and refluxed for 4 h. The reaction mixture was fil-

tered and left to evaporate the DMF at about 50 �C and

washed with methanol. The green precipitates dried in

vacuum. Yield: 71 %, Anal. Calc. for C16H15N5O7V: C,

43.65; H, 3.43; N, 15.90 %. Found: C, 43.69; H, 3.38; N,

15.78 IR (KBr pellets, cm-1): 3,429 (mNH), 2,924 (mCH),

1,666 (mC=O), 1,616 (mC=N), 1,545 (mC=C), 1,318 (vNO2) and

952 (mVO), kmax (nm) (e, L mol-1 cm-1) (ethanol): 259

(10,000), 282 (9,000), 384 (3,150).

Computational calculations

Ab initio calculations were carried out using the Gaussian

03 program [21]. The geometries of all the stationary

points were optimized at the MP2 [23] level with the

6-31G(d) basis set. Harmonic vibrational frequencies were

obtained at B3LYP/6-31G(d) level in order to characterize

stationary points as local minima or first-order saddle

points. The number of imaginary frequencies (0 or 1)

indicates whether a minimum or a transition state was

located. The atomic charges were calculated at the MP2

method with 6-31G(d) basis set.

Results and discussion

Elemental analysis

The elemental analysis (‘‘Experimental’’ section) is in good

agreement with those calculated for the proposed formula.

IR characteristics

The IR spectra of the free Schiff base ligands and the com-

plexes exhibit several bands in the 400–4,000 cm-1 region.

The OH stretching frequency of the ligands was approved in

the 2,500–3,500 cm-1 region due to the internal hydrogen

bonding vibration (O–H��N). This band disappeared in the

spectra of the complexes [16, 17, 24]. The NH2 stretching

frequencies of Schiff base ligands were appeared in the range

of 3,363–3,493 cm-1. The NH stretching frequency of van-

adyl complexes appeared in the range of 3,317–3,466 cm-1.

The NO2 stretching frequency of Schiff base ligands and

complexes appeared in the range of 1,310–1,343 cm-1.

The characteristic C=N bond appears in 1,610–1,614

and 1,605–1,616 cm-1 region for ligands and complexes,

respectively. The C=N stretching in the complexes is

generally shifted to a lower frequency, indicating a

decrease in the C=N bond order due to the coordinate bond

formation between the metal and the imine nitrogen lone

pair [16, 17]. The CH3 and carbonyl functional groups of

DMF in [VOL4(DMF)] complex were appeared at 2,924

and 1,666 cm-1, respectively.

The band at 952–985 cm-1 is assigned to m (V=O). This

band is observed as a new peak for the complexes and is

not observed in the spectrum of free ligands [25].

Electronic spectra

The spectral data of the free ligands and their complexes

are collected in ‘‘Experimental’’ section.

The electronic spectra of the Schiff base ligands in

solution consists of relatively intense bands in the 250–

400 nm region, involving p ? p* transitions [26]. The

bands at higher energies correspond to p ? p* transitions

of aromatic rings, while the band at lower energies is

attributed to the p ? p* transition of azomethine group.

The electrochemical study of vanadyl complexes

The cyclic voltammetry of [VOL(OH2)] complexes were

carried out in DMF solution at room temperature. A typical

J IRAN CHEM SOC

123

cyclic voltammogram of [VOL2(OH2)] complex in the

potential range from 0.0 to 1.0 V (vs. Ag/AgCl) is shown in

Fig. 2. An oxidation peak is observed at about Ca. 0.60 V.

[VOL2(OH2)] is oxidized to the mono cation [VOL2(OH2)]?

in a fully reversible one-electron step [20]. The electron is

removed from the nonbonding orbitals and the V(V) com-

plex is formed. Upon reversal of the scan direction, the

V(V) complex is reduced to V(IV) at lower potentials.

The [VOL4(DMF)] show only the oxidized peak and the

mono cation [VOL4(DMF)]? chemically reduced to

[VOL4(DMF)]. Multiple scans resulted in nearly superpos-

able cyclic voltammograms, thereby showing that the five

coordinate geometry is stable in both oxidation states, at

least on the cyclic voltammetry time scale. These results

revealed that the redox process of all vanadyl Schiff base

complexes under study is the one-electron transfer reaction.

The oxidation potentials for the different complexes are set

out in Table 1. The formal potentials [E1/2 (IV $ V)] for the

V(IV/V) redox couple were calculated as the average of the

cathodic (Epc) and anodic (Epa) peak potentials of this process.

In order to investigate the effect of functional groups of

the Schiff base ligands on the oxidation potential of

[VOL(OH2)], a series of the vanadyl Schiff base complexes

were studied by the cyclic voltammetry method. The

results show that the anodic peak potential (Epa) varies as it

can be expected from the electronic effect of the substit-

uents at position 5. Thus, Epa becomes more positive

showing the following trend; MeO \ H \ Br \ NO2.

Similar results have been reported previously for analogous

vanadyl(IV), copper(II), nickel(II) and cobalt(III) systems,

and have been interpreted by assuming that the strong

electron-withdrawing effects stabilize the lower oxidation

state while the electro-donating groups have a reverse

effect [19–21, 27–30]. Hammett-type relationship was

found between the Epa values and the appropriate para-

substituent parameters [31], which reflects the variation of

the electrode potential as a function of the electron-

withdrawing ability of the substituent at position 5 (Fig. 3)

[19–21].

Thermal analysis

The thermal decomposition of the studied complexes pre-

sented characteristic pathways, depending on the nature of

the ligands, as can be seen from the TG/DTA curves in

Fig. 4. All TG and DTA figures of the other compounds

have the same trend. The absence of weight loss up to

80 �C indicates that there is no water molecule in the

crystalline solid of L1–L3 complexes. Also, in these com-

plexes, the TG showed weight loss up to 80 indicating that

the water molecule is coordinated to the metal center

[16, 17]. The VOL1(OH2) and VOL2(OH2) complexes

Fig. 2 Cyclic voltammograms of [VOL2(OH2)], in DMF at room

temperature. Scan rate: 100 mV s-1

Table 1 Redox potential data of vanadyl complexes in DMF solution

Compound Epa (IV!V) Epc (V! IV) E1/2 (IV $ V)

VOL1(OH2) 0.55 0.44 0.495

VOL2(OH2) 0.6 0.5 0.55

VOL3(OH2) 0.7 0.57 0.635

VOL4(DMF) 0.87 – –

Fig. 3 Correlation between the

Epa values for Schiff bases

(L1–L4) and the respective Epa

substituent

J IRAN CHEM SOC

123

containing methoxy functional group and simple ONN

ligands were decomposed in three steps while the

VOL3(OH2) and VOL4(DMF) complexes containing Br

and NO2 functional groups were decomposed in two steps

(Fig. 5). The temperature range and the percent of loss

weight are shown in Table 2.

The complex of [VOL1(OH2)] decomposes in three steps.

The first step occurs up to about 279 �C and was attributed to

the release of H2O solvent and NO2 functional group. The

second step of thermal decomposition occurs in the range of

334–346 �C and was attributed to the release of C6H4N. The

third mass loss range was 380–482 �C and assigned to the

rest of the ligand, with the formation of the V2O5 [11].

The complex of [VOL2(OH2)] decomposes in three

steps. The first step occurs in up to about 199 �C and was

attributed to the release of NO2 functional group. The

second step of thermal decomposition occurs in the range

of 285–328 �C, was attributed to the release of C6H3. The

third mass loss range was 362–459 �C and assigned to the

rest of the ligand, with the formation of the V2O5.

The complex of [VOL3(OH2)] decomposes in two steps.

The first step occurs in up to about 318 �C and was

attributed to the release of C13H10Br. The second mass loss

between 374 and 485 �C was assigned to the rest of the

ligand, with the formation of the V2O5.

The complex of [VOL4(DMF)] decomposes in two steps.

The first step occurs up to about 36 �C and was relatively

attributed to the release of two NO2 functional groups. The

second mass loss between 381 and 481 �C was assigned to

the rest of the ligand, with the formation of the V2O5.

Kinetics aspects

All the well-defined stages were selected to study the

kinetics of decomposition of the complexes. The kinetic

parameters (the activation energy E and the pre-exponen-

tial factor A) were calculated using the Coats–Redfern

equation [22],

loggðaÞT2

� �¼ log

AR

uE1� 2RT

Ea

� �� Ea

2:303RTð1Þ

where g(a) = [(Wf)/(Wf - W)], Wf is the mass loss at the

completion of the reaction, W is the mass loss up to tem-

perature T, R is the gas constant, Ea is the activation energy

and u is heating rate In the present case, a plot of left hand-

side (LHS) of this equation against 1/T gives straight line

whose slope and intercept are used to calculate the kinetics

parameters by the least square method. The goodness of fit

was checked by calculating the correlation coefficient. The

other systems and their steps show the same trend.

The entropy of activation S= was calculated using the

equation

A ¼ kTs

he

S 6¼R ð2Þ

where k, h and Ts are the Boltzmann constant, the Planck’s

constant and the peak temperature, respectively. The

enthalpy and free energy of activation were calculated

using equations

Fig. 4 a TG and b DTG curve of: VOL1(OH2)

Fig. 5 Coats–Redfern plots of

[VOL4(OH2)] complex, step 2,

A =log

WfWf�W

T2

J IRAN CHEM SOC

123

Ea ¼ H 6¼ þ RT ð3Þ

G 6¼ ¼ H 6¼ � TS 6¼ ð4Þ

The various kinetic parameters calculated are given in

Table 2. The activation energy (Ea) in the different stages

are in the range of 38.23–480.78 kJ mol-1. The respective

values of the pre-exponential factor (A) vary from 1.27 9

100 to 5.60 9 1039 s-1. The corresponding values of the

entropy of activation (S=) are in the range of -2.46 9 102

to 5.16 9 102 kJ mol-1. The corresponding values of the

enthalpy of activation (H=) are in the range of 34.54 to

478.33 kJ mol-1. The corresponding values of the free

energy of activation (G=) are in the range of 7.78 9 0.101

to 3.26 9 102 kJ mol-1. There is no definite trend in the

values of activation parameters among the different stages

in the present series. The negative values of entropy of

activation indicate that the activated complex has a more

ordered structure than the reactants [32, 33].

Molecular structure and analysis of bonding modes

The structures of the ligands and the geometry around

vanadyl complexes were determined by ab initio calcula-

tions. Also the bond lengths and angles were studied and

compared.

The geometries of the Schiff base ligands are repre-

sented in Fig. 6. Significant bond lengths and bond angles

are also reported in Table 3. According to the theoretical

studies, the Schiff base ligands have neutral structure. The

C–N bond lengths are increased according to the trend of

Table 2 Thermal and kinetics parameters for vanadyl complexes

Compound DT (�C)a Percent (%)b E* (kJ mol-1) A* (s-1) S* (kJ mol-1 K) H* (kJ mol-1) G* (kJ mol-1)

VOL1(OH2) 279–332 18 (17.30) 480.78 5.60 9 1039 5.16 9 102 478.33 3.26 9 102

334–346 24 (24.32) 319.98 1.50 9 1025 2.36 9 102 317.16 2.37 9 102

380–482 37 (35.94) 56.53 3.09 9 101 -2.19 9 102 53.08 1.44 9 102

VOL2(OH2) 199–280 12.5 (13.53) 50.49 5.98 9 101 -2.09 9 102 48.45 9.99 9 101

285–328 21 (22.06) 118.97 1.65 9 108 -8.79 9 101 116.10 1.47 9 102

362–459 21 (22.06) 66.42 3.95 9 102 -1.98 9 102 63.08 1.43 9 102

VOL3(OH2) 318–370 56 (58.71) 404.27 8.85 9 1031 3.66 9 102 401.41 2.76 9 102

374–485 25 (25.30) 38.23 1.27 9 100 -2.46 9 102 34.54 1.44 9 102

VOL4(DMF) 36–99 20 (20.90) 75.08 2.29 9 109 -5.27 9 101 74.57 7.78 9 0.101

381–481 57 (55.06) 60.80 5.19 9 101 -2.15 9 102 57.26 1.49 9 102

a The temperature range of decomposition pathwaysb Percent of weight loss found (calculated)

Fig. 6 Optimized structures of

ligands

J IRAN CHEM SOC

123

C7–N(1) \ C8–N(1) \ C13–N(2). This is due to the double

and single bond character of C7–N(1) and C8–N(1),

respectively. Due to the participation of the N(2) loan pairs

in resonance with aromatic ring, the C13–N(2) bond length

is slightly shorter than the C8–N(1) single bond.

The geometries of the complexes are represented in

Fig. 7. Significant bond lengths and bond angles are also

reported in Table 4. The geometry around the vanadium

(IV) ion is a distorted square-pyramid similar to that

reported for vanadium complexes [34–36]. All distances

and angles in the studied complexes agree well with the

same distances and angles reported for the other vanadyl

complexes. For example the V1–O3 bond distance was

found in the range of 1.54–1.60 A (Table 4) that is in good

agreement with the value that reported previously [34, 35].

The total energies, zero point energies and dipole

moments of the ligands and the complexes presented in

Table 5.

Natural bond orbital (NBO) analysis yields valuable

information concerning the electronic structure of ligands

and their metal complexes. In ligand L1, N(2)–H and

the phenolic O(1)–H are polar covalent bonds with -0.885

and -0.746 charges, respectively. In the complex

[VOL1(OH2)], V(1)–N(1), V(1)–N(2) and V–O(1) bonds

are ionic with -0.674, -1.085 and -0.922 charges on the

coordinated nitrogens and oxygen, respectively. Negative

charges on nitrogen and oxygen atoms in complexes have

been more than ligands. This phenomenon is due to N–H,

O–H polar covalent and V–N, V–O ionic bonds. The other

systems show the same trends. The atomic charges are

Table 3 Significant bond lengths and bond angles calculated at the

MP2/6-31G(d) level

Structural parameter L1 L2 L3 L4

Bond lengths (A)

O(1)–C(1) 1.37 1.37 1.34 1.36

C(6)–C(7) 1.46 1.46 1.45 1.46

C(7)–N(1) 1.29 1.29 1.29 1.29

C(8)–N(1) 1.40 1.40 1.40 1.41

C(13)–N(2) 1.38 1.38 1.37 1.38

Bond angles (�)

O(1)–C(1)–C(2) 121.27 121.18 118.48 121.16

O(1)–C(1)–C(6) 119.56 119.09 122.13 119.06

C(1)–C(6)–C(7) 124.29 124.55 121.62 124.59

C(5)–C(6)–C(7) 116.65 117.25 119.26 116.93

C(6)–C(7)–N(1) 123.56 123.60 122.22 122.97

C(7)–N(1)–C(8) 118.63 118.64 121.91 118.74

C(9)–C(8)–N(1) 124.07 124.04 123.82 123.90

C(13)–C(8)–N(1) 115.09 115.11 116.59 115.12

C(8)–C(13)–N(2) 118.43 118.46 119.77 118.67

C(12)–C(13)–N(2) 122.55 122.51 121.32 122.40

Fig. 7 Optimized structures of

complexes

J IRAN CHEM SOC

123

represented in Table 6 which are from natural population

analysis (NPA) for ligands and the metal complexes with

N2O3 coordination.

Conclusions

Considering the electrochemical and thermogravimmetri-

cal properties of vanadyl tridentate Schiff base complexes

and ab initio calculations the following conclusions have

been made.

1. The anodic peak potentials Epa of different substituents

change according to the functional groups:

MeO \ H \ Be \ NO2.

2. The complexes containing simple ONN and methoxy

functional group ligands are decomposed in three steps

and the complexes containing Br and NO2 functional

groups are decomposed in two steps.

3. The thermal decomposition pathways of the complexes

are related to the Schiff base properties.

4. The thermal decomposition reactions are first order in

all steps for the studied complexes.

Table 4 Significant bond lengths and bond angles calculated at the MP2 method with 6-31G(d) basis set

Structural parameter [VOL1(OH2)] [VOL2(OH2)] [VOL3(OH2)] [VOL4(OH2)]

Bond lengths (A)

O(1)–C(1) 1.26 1.28 1.27 1.38

C(13)–N(2) 1.33 1.33 1.33 1.35

N(1)–V(1) 1.97 1.98 1.99 2.16

N(2)–V(1) 2.01 2.03 1.98 2.08

O(1)–V(1) 1.99 1.99 2.00 2.13

O(2)–V(1) 2.14 2.11 2.07 2.23

O(3)–V(1) 1.54 1.56 1.54 1.60

Bond angles (�)

O(1)–V(1)–N(1) 87.22 90.98 85.74 82.58

N(1)–V(1)–N(2) 79.67 80.41 79.42 77.09

O(2)–V(1)–N(2) 84.00 88.83 87.38 90.58

O(1)–V(1)–O(2) 81.33 78.77 79.26 82.50

O(2)–V(1)–O(3) 111.94 105.20 109.12 100.30

O(3)–V(1)–N(1) 109.72 108.27 115.85 112.47

O(3)–V(1)–N(2) 110.47 108.21 109.44 117.94

O(3)–V(1)–O(1) 109.26 108.62 107.88 111.94

Table 5 Total energies (with zero point energy correction)

E (Hartree), zero point energies ZPE (kJ mol-1) and dipole moments

l (D) of the ligands and complexes calculated at the MP2 level using

a 6-31G(d) basis set

Compound E ZPE l

L1 -1,003.102467 682.4 7.40

L2 -888.9484965 596.8 8.52

L3 -3,458.393572 570.8 6.05

L4 -1,092.954554 603.3 10.13

VOL1(OH2) -2,097.627289 707.9 9.25

VOL2(OH2) -1,983.497864 623.2 8.46

VOL3(OH2) -4,552.908898 598.0 8.80

VOL4(OH2) -2,188.027795 644.2 13.54

Table 6 Atomic charges (e) calculated from natural population analysis at the MP2/6-31G(d) level

Compound Atom

O(1) O(2) O(3) N(1) N(2) V(1)

L1 -0.746 – – -0.514 -0.885 –

[VOL1(OH2)] -0.922 -0.997 -0.772 -0.674 -1.085 2.073

L2 -0.737 – – -0.518 -0.885 –

[VOL2(OH2)] -0.911 -1.016 -0.747 -0.734 -1.068 2.076

L3 -0.779 – – -0.616 -0.878 –

[VOL3(OH2)] -0.902 -0.999 -0.766 -0.718 -1.088 2.067

L4 -0.724 – – -0.504 -0.883 –

[VOL4(OH2)] -0.910 -1.019 -1.064 -0.583 -1.095 1.751

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5. In the complexes V(IV) ion has square-pyramid N2O3

coordination geometry.

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synthesis, spectroscopy, electrochemistry and thermal study of vanadyl tridentate schiff base...

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