Synthesis and characterization of 4-fluorobenzaldehyde thiosemicarbazone derivatives as corrosion inhibitors

Article abstract of DOI:10.1016/j.ica.2015.05.020

In this paper, we report the syntheses, characterization and studies on the inhibition efficiency in mild steel acidic corrosion of 4-fluorobenzaldehyde thiosemicarbazone, Hfbt, 4-fluorobenzaldehyde 2-methylthiosemicarbazone, Me-fbt, and their Zn(II) comp

Full text of DOI:10.1016/j.ica.2015.05.020

Accepted Manuscript
Synthesis and characterization of 4-fluorobenzaldehyde thiosemicarbazone de-
rivatives as corrosion inhibitors
Franco Bisceglie, Giulio Del Monte, Pieralberto Tarasconi, Giorgio Pelosi
PII:
S0020-1693(15)00282-0
http://dx.doi.org/10.1016/j.ica.2015.05.020
ICA 16550
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 February 2015
15 May 2015
23 May 2015
Please cite this article as: F. Bisceglie, G.D. Monte, P. Tarasconi, G. Pelosi, Synthesis and characterization of 4-
fluorobenzaldehyde thiosemicarbazone derivatives as corrosion inhibitors, Inorganica Chimica Acta (2015), doi:
http://dx.doi.org/10.1016/j.ica.2015.05.020
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Synthesis and characterization of 4-fluorobenzaldehyde thiosemicarbazone
derivatives as corrosion inhibitors
Franco Bisceglie*, Giulio Del Monte, Pieralberto Tarasconi, Giorgio Pelosi
Department of Chemistry, University of Parma, Parco Area delle Scienze 17A, 43124 Parma,
Italy
*
Corresponding author. Tel. +390521905418
e-mail: franco.bisceglie@unipr.it
Keywords: Transition metal complexes; Thiosemicarbazone; Single crystal X-ray
diffraction study; DFT; Corrosion inhibition.
Abstract
In this paper, we report the syntheses, characterization and studies on the inhibition
efficiency in mild steel acidic corrosion of 4-fluorobenzaldehyde thiosemicarbazone, Hfbt,
4-fluorobenzaldehyde 2-methylthiosemicarbazone, Me-fbt, and their Zn(II) complexes
[Zn(fbt)2] and [Zn(Me-fbt)2]Cl2. The ligands present a good degree of activity that can be
assigned to their molecular planar shape and to the presence of electron lone pairs that can
interact with metal orbitals. The two zinc complexes present diametrically opposite
properties: [Zn(fbt)2] is by far less active than the ligands, while [Zn(Me-fbt)2]Cl2 is the
most efficient. Quantum chemical studies were carried out in an attempt to rationalize this
behaviour.
1

1. Introduction
The removal of rust and scale is a major concern in several industrial processes. In recent
years, many corrosion inhibitors were developed in connection with the nature of the metal
material under treatment and the type of surface that must be protected from corrosion. The
conditions under which the metal material is exposed and therefore the corrosion type is
another important aspect under scrutiny.[1] All these aspects, combined together, require a
continuous search for new corrosion inhibitors. [2] Acidic solutions are those more
commonly used in the rust removal process, and for this reason inhibitors are generally used
to control metal dissolution.[3] Hydrochloric acid, for instance, is widely used in the
pickling of steel and ferrous alloys. Organic compounds bearing nitrogen, sulphur and
oxygen atoms are commonly used as acid attack inhibitors, and are particularly fitted to
inhibit the aqueous corrosion of metals. The action mechanism of polar organic inhibitors
has also been known for decades [4] and is still a very active field of research.[3, 5]
In an attempt to rationalize the behaviour of such inhibitor molecules, a large number of
quantum mechanical studies can be found in the literature that try to correlate the corrosion
inhibition efficiency with molecular properties for different classes of organic
compounds.[3, 6] Quantum chemical calculations allow to obtain theoretical parameters
(such as energies of the frontier molecular orbitals - highest occupied molecular orbital,
HOMO, the lowest unoccupied molecular orbital, LUMO - ionization potentials, and dipole
moments) that can be used to rationalize and, possibly, to predict the properties of putative
corrosion inhibitors. These parameters can be correlated to the inhibition activity and to the
inhibition efficiency obtained.[7-10] In this context, computational chemistry becomes an
important tool to study the correlation between molecular structures and their inhibition
efficiency. [11]
2

In the recent literature, much interest is devoted to the inhibitory properties of
thiosemicarbazones.[5, 12-14] Nevertheless, in these papers, little attention is paid to the
fact that these compounds can undergo protonation and that they can form complexes with
the metal ions in equilibrium with the sample and that the real protecting agent could be the
resulting metal complex. To shed light on these aspects, we have studied two organic
molecules, 4-fluorobenzaldehyde thiosemicarbazone, Hfbt, its methylated non-acidic
counterpart, 4-fluorobenzaldehyde 2-methylthiosemicarbazone, Me-fbt, and their two
coordination compounds with zinc(II), a non-redox ion that forms stable complexes. The
four compounds were synthesized and characterized, tested for their corrosion inhibition
efficiency on mild steel in hydrochloric acid. Quantum chemical studies were also carried
out to propose an interpretation of the data.
2. Experimental
2.1 Materials and methods
All commercially available reagents were used as received. The synthesised compounds
were characterized by elemental analyses (C, H, N, S) performed with a Carlo Erba
1
Instrument CHNS-O EA. H NMR spectra were obtained at room temperature with a
Brucker AMX-300 MHz spectrometer, and chemical shifts are given in units of δ relative to
TMS as an internal reference using DMSO as the solvent.
2.2 Syntheses and Characterization
Hfbt
1 and Me-fbt 2 (Scheme 1) were synthesized following a general procedure.[15]
Briefly, in both cases, the proper thiosemicarbazide (3.5 mmol) dissolved in ethanol (30 mL)
was slowly added to an ethanolic solution (40 mL) containing an equimolar amount of 4-
fluorobenzaldehyde and stirred at room temperature for 4 hours. By slow evaporation of the
3

solvent, in both cases a white product was isolated and re-crystallized from ethanol in good
yields (85% ca) and crystals [15] apt for X-ray analyses were obtained. Zinc complexes,
[Zn(fbt)2]
3
and [Zn(Me-fbt)2]Cl2 4 (Scheme 1), were obtained by adding a water solution of
and 2, respectively, in a 1:2 molar ratio and
ZnCl2 to an ethanol solution of the ligands
1
stirring the resulting mixture at reflux temperature for 3 hours. By slow evaporation of the
solvent white products were isolated and characterized.
Scheme 1
4

Hfbt (1). E.A. for C8H8N3FS (%): C, 48.72; H, 4.09; N, 21.30; S, 16.26. Found: C, 48.6; H,
1
4.0; N, 21.32; S 16.36. H NMR (300 MHz, DMSO):
δ11.40 (1H, s, NHCS), 8.16 (1H, bs,
CH=N), 8.01 (2H, m, NH2), 7.88 (2H, dd, J=7.15, 5.6 Hz, H-2), 7.25 (2H, t, J=7.15 Hz, H-
3).
Me-fbt (2). E.A. for C9H10N3FS (%): C, 51.17; H, 4.77; N, 19.89; S, 15.18. Found: C,
1
51.28; H, 4.69; N, 20.01; S, 15.07. H NMR (300 MHz, DMSO):
δ 7.92 (2H, m, NH2), 8.22
(1H, s, CH=N), 7.88 (2H, dd, J=7.15, 5.6 Hz, H-2), 7.25 (2H, t, J=7.15 Hz, H-3), 3.52 (3H,
s, CH3).
[Zn(fbt)2] (3). E.A. for C16H14ZnN6F2S2 (%): C, 41.97; H, 3.08; N, 18,36; S 14.21. Found:
1
C, 42.55; H 3.06; N 18.78; S, 14.32%. H NMR (300 MHz, DMSO):
δ 8.29 (2H,dd, J =
7.15, 5.7 Hz, H-2), 7.38(1H, bs, CH=N), 7.21(2H, t , J=7.15 Hz, H-3), 7.13 (2H, bs, NH2).
[Zn(Me-fbt)2]Cl2 (4). E. A. for C18H20ZnN6F2S2Cl2 (%):C, 38.69; H, 3.61; N, 15.04; S
1
11.48. Found: C, 38.54; H, 3.66; N, 15.15; S, 11.44%. H NMR (300MHz, DMSO): 8.29
(2H, dd, J=7.15, 5.6 Hz, H-2), 7.45 (1H, s, CH=N), 7.21(2H, t , J=7.15 Hz, H-3), 7.04 (2H,
bs, NH2), 3.52 (3H, s, CH3).
ν
(NH)
ν
(CH)
ν
(C=C)
ν
(CN)
ν
(CS)
Hfbt (1)
3393 (s)
3238 (s)
3158 (vs)
3022 (m)
1602 (vs)
1535 (s)
838 (m)
Me-fbt (2)
3321 (s)
3160 (s)
3005 (m)
1602 (m)
1545 (vs) 837 (m)
5

ν
(NH)
ν
(C=C)+
(CN)
ν (CS)
ν
[Zn(fbt)2] (3)
3452 (s)
3281 (ms)
3117 (m)
3451 (s)
1618 (m)
903 (w)
835 (mw)
[Zn(Me-fbt)2]Cl2 (4)
1628 (m)
908 (w)
3279 (ms)
3120 (m)
835 (mw)
Table 1. Main IR stretching assignments for the compounds under study. In parentheses
details of the absorptions (vs = very strong; s= strong; m= medium; mw= medium weak; w=
weak).
2.3
X-ray crystal structure
A single crystal of compound
2 was mounted on top of a glass fibre and used for X-ray
diffraction data collection on a SMART APEX2 diffractometer. The SAINT software was
used for integration of reflection intensity and scaling, SADABS for absorption
correction.[16, 17] Structures were solved by direct methods using SIR97[18] and refined by
full-matrix least-squares on F2 using SHELXL97[19] implemented in the WinGX
package.[20] All the non-hydrogen atoms in the molecules were refined anisotropically. The
hydrogen atoms were placed in the ideal positions using riding models. Figure 1A depicts
an ORTEP [21] view of compound
2
and crystal data are reported in Table 2.
6

A
B
Figure 1. A. ORTEP view of one of the two molecules in the asymmetric unit of compound
2
with thermal ellipsoids at 50% probability. B. Packing interactions in the crystals of
compound
2
.
7

The asymmetric unit contains two independent molecules and their structures reveal a
substantial difference in their planarity, as can be inferred by the angle between the plane of
the para-fluorobenzyl group and that of the thiourea fragment, which is of 12.79° in one
molecule and 25.80° in the other. This distortion may be ascribed to the presence of the
methyl group on the N2 atom which prevents the thione-thiol tautomerism and allows for
rotation around the N3-N2 bond. The packing (Figure 1B) is determined by chains of
molecules bound through hydrogen bonds between the sulphur and the terminal amino
group that run along the
a axis.
CCDC 1003603 contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Compound
2
Wavelength
Formula
MoKα (λ = 0.71073Å)
C9H10FN3S1
211.27
Molecular mass (Da)
Crystal system
Space group
a (Å)
monoclinic
P21/a
10.710(1)
9.900(1)
19.490(2)
90.0
b (Å)
c (Å)
α
β
γ
(°)
(°)
(°)
91.949(2)
90.0
Volume (Å3)
2065.3(4)
8
Z
Dcalc (Mg.m–3)
T (K)
1.36
298.15 K
880
F(000)
8

Size (mm)
0.4x0.3x0.2
12 h, 11 k, 23 l
20017
hkl range
Measured reflections
Unique reflections
Refined Parameters
Goodness of fit
R
3647
333
1.049
0.031
wR2
0.092
Residual el. density (eÅ–3)
0.50, –1.23
Table 2. Crystal data for compound
2
2.4
Calculation methods
Quantum chemical calculations were carried out using Gaussian 03 (Revision B.05)[22] and
have allowed to obtain theoretical parameters i.e. the energies of the frontier molecular
orbitals (the highest occupied molecular orbital, HOMO, the lowest unoccupied molecular
orbital, LUMO) for the two ligands and the dipole moment for both the ligands and the
complexes. These properties have been extensively used to predict or rationalize the
properties of putative corrosion inhibitors.[7-10] For all four compounds ∆ELUMO–HOMO, the
ionization energies and the dipole moments were calculated by the semiempirical B3LYP
method using 6-31G as the basis set for the organic part of the molecule and SDD for the
metal contribution and, for comparison, also using the more extended def2-TZVP basis set.
The drawings of the molecular orbitals are obtained using the program Avogadro. [23]
2.5
Corrosion inhibition studies
2.5.1 Weight loss experiments
.
9

The weight loss measurements were performed in a double walled glass cell, equipped
with a thermostat-cooling condenser. For each experiment three samples of mild steel (5cm
x 4cm) were immersed in 450 mL of HCl solutions (in the range 0.1 – 0.5M) at 298( 1) K
for 24 h, in the absence and in the presence of the inhibitor at a concentration of 5.0 x 10-4
mol/L. The inhibitor concentration was chosen taking into account the solubility limit of the
compound in aqueous media. After the corrosion tests the specimens were removed,
carefully washed, first with a solution composed by NaOH (30% w/w) and zinc powder (200
g/L), then with bidistilled water, and finally rinsed with acetone, placed in a desiccator for
30 min and then weighed. The rinse removes loose segments of the film on the corroded
samples. The weight loss was measured by means of an analytical balance.
2.5.2 Hydrogen development experiments.
The hydrogen gas development as a function of steel corrosion in acidic media was
measured according to ref. [22]. Clean and degreased specimens (2 x 5 cm) were put in a
250mL 3-necked round bottom flask. The flask was placed in a water bath kept at a
temperature of 25°C. One neck was connected through a rubber tube to the upper extremity
of a graded gas burette. Another rubber tube connected the lower opening of the burette to a
reservoir. The burette was then filled with mineral oil. The height of the oil within the
burette was assumed to be zero. 200 mL of HCl 6M were then poured into the flask
containing the specimen and the flask was closed with a stopper. The instant in which the
flask was closed was assumed as time “zero” and the volume of H2 produced, corresponding
to the lowering of the oil level within the burette, was checked every minute for 25 minutes.
For the testing of hydrogen evolution in the presence of inhibitors, 200 mL of a solution,
prepared just prior to fill the flask, made up of HCl 6M and inhibitor at a concentration of
5.0 x 10-4 mol/L, were used.
10

3
Results and discussion
The synthesis of the organic compounds afforded pale yellow products in good yields. We
isolated the thiosemicarbazones in the solid state and no further purification was necessary.
The NMR spectra showed that ligand
1 was present in its thionic form; no S-H signal was
detected. In ligand 2, the thiol-thione tautomery is prevented by the presence of a methyl
group on the N2 atom. We observed from the IR spectroscopy the iminic C=N stretching at
ca. 1450 cm-1 and the C=S stretching at ca. 830 cm-1. We used these stretchings also to
verify the involvement of the nitrogen and sulphur donor atoms into the coordination. The
metal complexes were obtained as colourless compounds, as expected. The IR spectra of the
metal complexes are simpler than the corresponding ones of the ligands. The involvement of
the imino group in the coordination, is supposed by the variation of the spectra in the 1500
cm-1 region. The vibrations due to the C=N group are not clearly noticeable anymore, but
there is a superimposition of signals together with the C=C absorptions. The C=S stretchings
in the metal complexes are found at lower energies with respect to the free ligand, and at ca.
1200 cm-1 can be seen vibrations attributed to the N-C-S group, showing that the sulphur
atom is involved in the coordination. The NMR spectra of the diamagnetic metal complexes
are slightly modified with respect to the free ligand ones; it is noteworthy the disappearance
in complex
3 of the NH signal confirming the deprotonation of the ligand and the formation
of a neutral complex as a consequence. The simplicity of the spectra shows the symmetry of
the coordination around the metal. The NMR spectra of the two metal complexes are quite
similar, according to the role of the d10 metal ion. In both cases, the coordination
environment around the metal involves the sulphur atom in its thionic form and the iminic
nitrogen as seen by the shift at lower ppm values of the C
HN signal. This coordination
behaviour for complex supports the cationic nature of the compound.
4
11

With the compounds synthesised in the present study, we have investigated whether the
introduction of molecules of new design in acidic solutions could slow down the corrosion
of steel. The corrosion was monitored measuring the weight loss of steel exposed to the
corrosive agent (Table 3) and the amount of hydrogen that evolves from the solution. The
inhibition efficiency is expressed using the equation I = [ (w0 - w1) / w0 ] · 100, where w0
and w1 are the values of the corrosion rate in the absence and presence of inhibitors
respectively.
HCl 0.1 M average std dev HCl 0.2 M average std dev HCl 0.3 M average std dev HCl 0.4 M average std dev HCl 0.5 M average std dev
0.2193
0.3041
0.3105
0.4513
0.6284
No inhibitor
Hfbt
0.2358 0.2499 0.0395 0.2994 0.3047 0.0056 0.3210 0.3081 0.0142
0.4492 0.4309 0.0335 0.6931 0.6372 0.0521
0.2945
0.0222
0.3105
0.0367
0.2929
0.0411
0.3923
0.0618
0.5901
0.0894
0.0214 0.0246 0.0048 0.0341 0.0374 0.0037 0.0299 0.0383 0.0074
0.0591 0.0597 0.0019 0.0747 0.0885 0.0134
0.0301
0.0612
0.0414
0.0950
0.0439
0.0954
0.0582
0.1584
0.1014
0.2314
[Zn(fbt)2]
0.0704 0.0682 0.0062 0.1040 0.0930 0.0122 0.1053 0.0962 0.0087
0.1601 0.1514 0.0136 0.2594 0.2372 0.0200
0.0731
0.0422
0.0799
0.0682
0.0879
0.0738
0.1357
0.1142
0.2207
0.1597
Me-fbt
0.0485 0.0529 0.0135 0.0686 0.0690 0.0010 0.0784 0.0730 0.0059
0.1068 0.1067 0.0075 0.1754 0.1618 0.0127
0.0681
0.0122
0.0701
0.0134
0.0667
0.0241
0.0992
0.0404
0.1503
0.0582
[Zn(Me-fbt)2]Cl2
0.0101 0.0126 0.0027 0.0161 0.0162 0.0029 0.0252 0.0240 0.0013
0.0154 0.0192 0.0227
0.0398 0.0377 0.0042 0.0648 0.0599 0.0043
0.0329 0.0566
Table 3. Weight loss data: the inhibitors are at 5.0 x 10-4 mol/L and each measurement was
carried out in triplicate.
Looking at Table 3, it is apparent that for few compounds there is a spread value within the
series. The reason of this spread, especially for Me-fbt in HCl 0.1M (25% in terms of
standard deviation), for [Zn(fbt)2] in HCl 0.2 M (13% in terms of standard deviation) and
for Hfbt in HCl 0.5M (15% in terms of standard deviation), can be ascribed to the fact that
the steel samples, even if of the same composition, can present non homogeneous surfaces
due, for instance, to the presence of scratches, that could enhance the weight loss.
12

The inhibition efficiency increases in the sequence [Zn(fbt)2] < Me-fbt < Hfbt < [Zn(Me-
fbt)2]Cl2 (Table 4)
Inhibition efficiency (%)
Hfbt
90 1
88 1
88 3
86 1
86 3
[Zn(fbt)2] Me-fbt
[Zn(Me-fbt)2]Cl2
95 1
HCl 0.1 M
HCl 0.2 M
HCl 0.3 M
HCl 0.4 M
HCl 0.5 M
72 3
69 5
65 1
65 1
63 0
79 2
77 0
76 1
75 1
75 0
95 1
92 0
91 0
91 0
Table 4. Inhibition efficiency of the tested compounds at 5.0 x 10-4 mol/L in HCl solutions
of various concentrations.
It is apparent from Table 4 that, as a general trend, inhibition efficiency tends to decrease as
HCl concentration increases and it is also noteworthy that the metal complex [Zn(Me-
fbt)2]Cl2 with a concentration as low as 5.0 x 10-4 mol/L shows a good inhibition efficiency
for mild steel in hydrochloric acid. From the table it is also apparent that there is a certain
spread of the inhibition efficiency arising from the variability of the weight loss.
Nevertheless, the general trend is not affected by this spread of values.
From the H2 evolution analysis (Figure 2) it has been confirmed that the compound [Zn(Me-
fbt)2]Cl2 shows a good attitude in inhibiting corrosion also in the presence of strongly acidic
solution. At the end of the observation time, [Zn(Me-fbt)2]Cl2 inhibits H2 evolution of 76%,
while Me-fbt and Hfbt of 64% and [Zn(fbt)2] of only 18%.
13

Figure 2. Hydrogen evolution of mild steel in HCl 6M in the absence and in the presence of
inhibitors 5.0 x 10-4 mol/L. The volume of the gas produced is normalised to the surface area
of the sample.
As already stated in the introduction, theoretical approaches are presently widely used,[6,
11, 24-26] to gather information about energetic parameters which affect the reactivity of
molecules such as frontier molecular orbitals. For this reason the HOMO and LUMO of
putative corrosion inhibitors are often considered when trying to understand the process of
chemical adsorption.[27] Effective corrosion inhibitors are usually organic compounds that
can donate electrons to empty metal d orbitals to form a coordinative bond or, alternatively,
can receive free electrons from the metal surface. In the literature,[28] it is reported that the
higher the HOMO energy of the valence electrons of the inhibitor, the easier will be the
occupation of the empty metal d orbitals. On the other hand, if the LUMO energy is lower, it
14

will be easier for the inhibitor to accommodate electrons from the metal. In other words, we
can say that the inhibition potential will be large if the energy difference between the
frontier orbitals is small.[29] In our case, according to the calculation results, Hfbt should be
less effective than Me-fbt in inhibiting corrosion. In fact, it must be taken into account that
the values of the frontier orbital energy difference of the two compounds are so close that
the results for this reason cannot be considered reliably predictive.
Another parameter we have considered is the ionization potential, and this should give us a
hint about the facility with which our compounds could be oxidized in case they act as
sacrificial reducers. Comparing our experimental data with the calculated values (Table 5)
we do not find a direct correlation between the anticorrosive activity and the ionization
potential and this suggests a non-redox behaviour for our two ligands. The extremely high
value for the [Zn(Me-fbt)2]2+ is expected and it is due to the fact that extracting an electron
from a di-cation requires much more energy than from a neutral molecule.
Hfbt
[Zn(fbt)2]
Me-fbt
[Zn(Me-fbt)2] 2+
EHOMO -ELUMO (eV)
3.73 (3.57)
3.75 (3.72)
7.01 (7.28)
3.70 (3.54)
7.24 (7.64)
4.03 (3.91)
Ionization energy (eV) 7.42 (8.01)
13.40 (13.30)
µ
(D) 3.99 (3.93)
2.06 (2.09)
3.92 (3.85)
4.94 (4.94)
Table 5. Frontier orbitals energy difference, ionization energy and dipole moment calculated
by the semiempirical B3LYP method, using 6-31G as basis set for the ligands and SDD for
the metal and, in parenthesis, with def2-TZVP as basis set.
A third parameter that can be estimated from a computational approach is the dipole
moment. This physical property can, in principle, allow to evaluate the adsorption ability of
the organic inhibitor but, as reported by Goulart et al,[30] While some results have shown
15

that there is a direct relationship between dipole moment and the corrosion inhibition
efficiency, in others it is reported that low values of dipole moment may favour the
accumulation of inhibitor molecules on the metal surface.[31] In our case, we find a direct
correlation between the predicted dipole moment and the inhibition efficiency.
A final set of information that can be extracted from quantum mechanical calculations are
the Mulliken charge distributions. The Mulliken charge distribution and the HOMO
population are often used in the prediction of possible molecular fragments that allow the
molecule to be adsorbed.[24, 27]
As a rule of thumb, the more negatively charged parts of the molecule, and where the
HOMO are concentrated, are the most probable points of adsorption. The Mulliken charges
for the ligands are shown in Figure 3 and the density distribution of the HOMO and LUMO
is shown in Figure 4.
16

Figure 3. Mulliken charges of compound
1 (above) and 2 (below).
The two organic molecules present the negative charge localized on the thiosemicarbazide
fragment, in particular on the terminal nitrogen, and on the aromatic moiety, including the
fluorine atom. This localization suggests that the thiosemicarbazone part plays an important
role in the adsorption phenomena for these inhibitors. Moreover, the HOMO is mainly
distributed in the terminal thiourea-like fragment, i.e. in the sulphur atom and in the two
adjacent nitrogen atoms.
Hfbt: HOMO population
Hfbt: LUMO population
Me-fbt: HOMO population
Me-fbt: LUMO population
Figure 4. HOMO and LUMO population of compounds
1 and 2 obtained using the B3LYP
method and def2-TZVP as the basis set.
17

4
Conclusions
In this work, we have examined the activity of two molecules: 4-fluorobenzaldehyde
thiosemicarbazone, and its methylated derivative, 4-fluorobenzaldehyde 2-
methylthiosemicarbazone, as anticorrosive compounds on mild steel in hydrochloric acid.
In addition, we have also studied their Zn(II) complexes, [Zn(fbt)2] and [Zn(Me-fbt)2]Cl2.
The outcome of the experiments is that the inhibition efficiency increases in the sequence
[Zn(fbt)2] < Me-fbt < Hfbt < [Zn(Me-fbt)2]Cl2. The extremely small concentrations at which
the four compounds are used, the inhibition effect lasting in time and the fact that the
ionization potentials calculated are unrelated to the inhibitory activity sequence found,
allows us to exclude that these compounds may act as sacrificial reducers. An explanation
for the sequence observed could be searched in the capability of the molecules to interact
with the metal surface. The two organic molecules are good inhibitors and share common
features: both are flat and present very similar HOMO, which might interact with empty d
orbitals of the metal atoms on the surface of the metal sample, and LUMO, which could
efficiently host the electrons from the metal. The three dimensional structure of [Zn(fbt)2]
makes the molecule unapt to be adsorbed efficiently on the metal surface and this can
explain its lowest activity. A discussion of its own is required for [Zn(Me-fbt)2]Cl2 which is
the most active compound. In fact, the substitution of the acidic proton with a methyl in the
second compound blocks the thione-thiol tautomery, preventing the molecule to deprotonate
and brings to the formation of an ionic compound. This cationic nature of the complex
[Zn(Me-fbt)2]2+ could be the key property that renders this molecule particularly effective on
a metal surface that, spontaneously after a first release of metal ions, tends to be negatively
charged. This hypothesis is in agreement with the fact that the only parameter obtained from
the quantum mechanical calculations that allows the prediction of the experimental sequence
18

is the dipole moment. These preliminary results encourage further investigation to explore
the role of cationic complexes in corrosion inhibition studies
.
Acknowledgments
The authors thank the University of Parma for the financial support and the Centro
Interdipartimentale Misure (C.I.M.) for the use of NMR facilities.
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22

We report the syntheses, characterization and inhibition efficiency studies in mild steel acidic
corrosion of two thiosemicarbazone derivatives and of their zinc complexes. The ligands present a
good degree of activity while the two zinc complexes present diametrically opposite properties.
Quantum chemical studies were carried out to rationalize this behaviour.
23

Two 4-fluorobenzaldehyde thiosemicarbazones have been synthesized and characterized.
Their zinc metal complexes have been synthesized and characterized.
Their corrosion inhibition activity in HCl on mild steel has been evaluated.
Their activity has been related to quantum mechanical parameters.
24