Synthesis, structural and corrosion inhibition studies on Mn(II), Cu(II) and Zn(II) complexes with a Schiff base derived from 2-hydroxypropiophenone

Article abstract of DOI:10.1016/j.poly.2014.04.003

A Schiff base, 2-hydroxy-benzoic acid [1-(2-hydroxy-phenyl)-propylidene]- hydrazide (H₂hbpp) and its Mn(II), Cu(II) and Zn(II) complexes have been synthesized. These compounds have been characterized by different physico-chemical and spectroscopic techniques (UV-Vis, IR, NMR and ESI-Mass). The molecular structure of H₂hbpp and its Mn(II) and Zn(II) complexes are determined by single crystal X-ray diffraction technique. In Mn(II) and Cu(II) complexes, the ligand coordinates through azomethine-N, carbonyl-O and phenolate-O (2-hydroxypropiophenone) forming a mono-nuclear 6-coordinate distorted octahedral geometry around metal. However, Zn(II) complex forms a phenoxo-bridged centrosymmetric dimer with 5-coordinate distorted square pyramid geometry around each metal ion. In this complex, the ligand bonds through azomethine-N, carbonylate-O and two phenolate-O, and a DMSO molecule occupies one of the vacant site of each metal. The structure of Cu(II) complex has been satisfactorily modeled by density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations. The corrosion inhibition study of the synthesized compounds for mild steel in 1 M HCl medium has also been performed and the activity is found in the order: H₂hbpp < Cu(II) complex < Mn(II) complex < Zn(II) complex.

Full text of DOI:10.1016/j.poly.2014.04.003

Accepted Manuscript
Synthesis, structural and corrosion inhibition studies on Mn(II), Cu(II) and
Zn(II) complexes with a Schiff base derived from 2-hydroxypropiophenone
Monika Mishra, Karishma Tiwari, Ashish Kumar Singh, Vinod P. Singh
PII:
S0277-5387(14)00213-7
DOI:
http://dx.doi.org/10.1016/j.poly.2014.04.003
POLY 10646
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
21 January 2014
2 April 2014
Please cite this article as: M. Mishra, K. Tiwari, A.K. Singh, V.P. Singh, Synthesis, structural and corrosion inhibition
studies on Mn(II), Cu(II) and Zn(II) complexes with a Schiff base derived from 2-hydroxypropiophenone,
Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.04.003
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Synthesis, structural and corrosion inhibition studies on Mn(II), Cu(II) and
Zn(II) complexes with a Schiff base derived from 2-hydroxypropiophenone
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Monika Mishraa, Karishma Tiwaria, Ashish Kumar Singhb and Vinod P. Singha*
aDepartment of Chemistry, Banaras Hindu University, Varanasi-221005 (India)
bDepartment of Chemistry, North West University (Mafikeng Campus), Mmabatho 2735,
South Africa.
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A B S T R A C T
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A Schiff base, 2-hydroxy-benzoic acid [1-(2-hydroxy-phenyl)-propylidene]-hydrazide
(H2hbpp) and its Mn(II), Cu(II) and Zn(II) complexes have been synthesized. These
compounds have been characterized by different physico-chemical and spectroscopic
techniques (UV-Vis, IR, NMR and ESI-Mass). The molecular structure of H2hbpp and its
Mn(II) and Zn(II) complexes are determined by single crystal X-ray diffraction technique. In
Mn(II) and Cu(II) complexes, the ligand coordinates through azomethine-N, carbonyl-O and
phenolate-O (2-hydroxypropiophenone) forming a mono-nuclear 6-coordinate distorted
octahedral geometry around metal. However, Zn(II) complex forms a phenoxo-bridged
centrosymmetric dimer with 5-coordinate distorted square pyramid geometry around each
metal ion. In this complex, the ligand bonds through azomethine-N, carbonylate-O and two
phenolate-O, and a DMSO molecule occupies one of the vacant site of each metal. The
structure of Cu(II) complex has been satisfactorily modeled by density functional theory
(DFT) and time dependent-DFT (TD-DFT) calculations. The corrosion inhibition study of the
synthesized compounds for mild steel in 1 M HCl medium has also been performed and the
activity is found in the order: H2hbpp < Cu(II) complex < Mn(II) complex < Zn(II) complex.
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______________________________
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*Corresponding author. Tel.: +919450145060
E-mail address: singvp@yahoo.co.in (V.P. Singh)
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Keywords: Transition metal complexes; Schiff base; 2-Hydroxypropiophenone; Single crystal
X-ray diffraction study; Corrosion inhibition properties.
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1. Introduction
The transition metal complexes with polydentate Schiff base ligands have developed
an area of immense interest in coordination chemistry due to their variable bonding and
structural possibilities, and their applications in several fields [1,2]. Aroylhydrazone Schiff
bases are known to be a class of versatile ligands, capable of generating a variety of
molecular architectures and coordination polyhedra [3-5]. The metal complexes of such
Schiff bases have been extensively used as biological probes [6], DNA cleaving agents [7],
catalysts [8,9], sensing materials [10,11] and corrosion inhibitors [12]. The biological activity
of these complexes has been found to be highly dependent on the coordination behavior of
metal ions and the binding sites on the ligands [13]. A survey of the literature reveals that
many metallo-hydrolases contain binuclear Zn(II) complexes at their active sites [14]. Some
Zn(II) based synthetic models having potential to mimic nucleases are also binuclear [15].
Recently, an oxo-bridged binuclear Mn(II) complex of oxaloyl dihydrazone has been reported
to act as an excellent catalyst for transamidation of carboxamides with amines [16].
Schiff bases have been investigated as corrosion inhibitors for steel, copper and zinc
[17-19]. Some research works have shown that the corrosion inhibition efficiency of Schiff
bases is much greater than that of corresponding amines and aldehydes [20]. Due to presence
of >C=N- groups, Schiff bases are adsorbed on the surface of mild steel and spontaneously
form a monolayer on the surface, therefore, act as effective corrosion inhibitor [21,22]. The
ability of Schiff base ligands to form stable complexes closely packed in the coordination
sphere of metal ion, introduces another class of compounds for corrosion inhibition. The
chelate environment with polyfunctional ligands, might plays a significant role in redox
behavior and electro-catalytic reduction reactions. A few recent reports indicate that the metal
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complexes show greater inhibition efficiency than the free ligands [23]. In view of the above,
and only a very few reports available on the corrosion inhibition properties of transition metal
complexes of aroylhydrazone Schiff bases, we have synthesized and characterized the
Mn(II), Cu(II) and Zn(II) complexes with 2-hydroxy-benzoic acid[1-(2-hydroxy-phenyl)-
propylidene]-hydrazide and studied their anti-corrosion properties on mild steel in acid
solution. The experimental electronic and infrared spectral data of the copper(II) complex
have also been compared with the theoretical data by DFT and TDDFT calculations.
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2. Experimental
2.1 Materials and methods
All analytical grade chemicals were obtained from the commercial sources and used
without further purification. 2-hydroxypropiophenone was purchased from Sigma–Aldrich,
USA. Methyl salicylate, hydrazine hydrate (SD Fine Chemicals, India) and solvents (Merck
Chemicals, India) were used as such. The precursor salicylic acid hydrazide,
(OH)C6H4CONHNH2 was prepared by the literature procedure [24].
2.2 Preparation of 2-hydroxy-benzoic acid [1-(2-hydroxy-phenyl)-propylidene]-hydrazide
(H2hbpp)
The ligand H2hbpp was synthesized by reacting 25 ml methanolic solution of salicylic
acid hydrazide (10 mmol, 1.52 g) with 25 ml methanolic solution of 2-hydroxypropiophenone
(10 mmol, 1.50 ml) in a round bottom flask. The reaction solution was refluxed for 3 hours.
A pale yellow product was obtained on cooling the above solution at room temperature. The
product was filtered on a Büchner funnel and washed several times with methanol. The pure
compound was recrystallised from hot methanol and dried in a desiccator over anhydrous
calcium chloride at room temperature. Yield (77%). M.p. 240 oC. Anal. Calc. for C16H16N2O3
(284.31): C, 67.59; H, 5.67; N, 9.85. Found: C, 67.43; H, 5.69; N, 9.80%. IR (ν cm-1, KBr):
ν(O−H) 3448b, ν(N−H) 3294m, ν(C=O) 1638s, ν(C=N) 1610s, ν(N−N) 950w. 1H NMR data:
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1.23 (3H, J = 7.2 Hz, CH3); 2.90 (2H, CH2), 6.903-8.005 (Ar-H), 11.68 (s, 1H, NH), 13.19 (s,
1H, OH). 13C NMR (DMSO-d6): 11.05 (CH3); 21.22(CH2); 113.47-132.84 (Aromatic
carbons); 163.01 (C=N); 166.76 (C-OH); 185.27 (C=O). The single crystal structure of the
ligand was further confirmed by XRD.
2.3. Synthesis of the metal complexes
Mn(II), Cu(II) and Zn(II) complexes of H2hbpp were synthesized by reacting 50 ml
methanolic solution of each metal(II) acetates (5 mmol), separately with a solution of H2hbpp
(10 mmol, 2.84 g) in 25 ml hot methanol in 1:2 (M:L) molar ratio in a round bottom flask.
The Mn(II) and Zn(II) complexes were formed as insoluble precipitates after refluxing the
reaction mixture for 4 h, whereas, Cu(II) complex was precipitated immediately on stirring at
room temperature. The metal complexes were filtered in a glass crucible, washed several
times with methanol and finally with diethyl ether, and dried in a desiccator at room
temperature. The single crystals of Mn(II) and Zn(II) complexes were obtained by slow
diffusion of diethyl ether over a DMSO solution of the complexes.
2.3.1. [Mn(Hhbpp)2]
Brown, yield (75%). M.p. 310d oC. µeff = 5.96 B.M. Anal. Calc. for C32H30MnN4O6
(621.5): C, 61.84; H, 4.87; N, 9.01; Mn, 8.84. Found: C, 61.77; H, 4.84; N, 9.04; Mn, 8.80%.
IR (ν cm-1, KBr): ν(O−H), 3432b; ν(N−H), 3280m; ν(C=O), 1618s; ν(C=N), 1589s; ν(C-O-),
1360s; ν(N−N), 985w; ν(M−O), 543w; ν(M−N), 419w. The single crystal structure of the
complex was further confirmed by XRD.
2.3.2. [Cu(Hhbpp)2]
Green, yield (74%). M.p. 260d oC. µeff = 1.73 B.M. Anal. Calc. for C32H30CuN4O6
(630.15): C, 60.99; H, 4.80; N, 8.89; Cu, 10.08. Found: C, 60.88; H, 4.83; N, 8.85; Cu,
10.02%. IR (ν cm-1, KBr): ν(O−H), 3438b; ν(N−H) 3294m; ν(C=O), 1630s; ν(C=N), 1599s;
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ν(C-O-), 1367s; ν(N−N), 990w; ν(M−O), 532w; ν(M−N) 425w. ESI-MS (calcd): m/z 630.152
[(M)+, 12%]·; 285.122 [(L+1)+, 10%].
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2.3.3. [Zn(hbpp)(DMSO)]2
Yellow, yield (77%). M.p. 250d oC. Anal. Calc. For C36H40Zn2N4O8S2 (851.6): C,
50.77; H, 4.73; N, 6.58; Zn, 15.35. Found: C, 50.68; H, 4.71; N, 6.54; Zn, 15.40%. IR (ν cm-
1, KBr): ν(O−H), 3428b; ν(C=N), 1585s; ν(C-O-), 1355s; ν(N−N), 982w; ν(S=O), 915m;
ν(M−O), 538w; ν(M−N), 425w. 1H NMR data: 1.23 (3H, CH3), 2.49 (2H, CH2, J = 8.1 Hz),
6.49-8.73 (Ar-H), 14.07(s, 1H, OH), 13C NMR (DMSO-d6): 11.14 (CH3); 22.76 (CH2);
116.40-132.87 (Aromatic carbons); 163.28 (C=N); 167.50 (C-OH). The single crystal
structure of the complex was further confirmed by XRD.
2.4 Physico-chemical measurements
The metal contents were analyzed gravimetrically by the literature procedure [25].
Carbon, hydrogen and nitrogen contents were determined on an Exeter Analytical Inc. CHN
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Analyzer (Model CE-440). H NMR spectra of the ligand and its Zn(II) complex were
recorded in DMSO-d6 on a JEOL AL-300 FT-NMR multinuclear spectrometer. Chemical
shifts were reported in parts per million (ppm) using tetramethylsilane (TMS) as an internal
standard. Infrared spectra were recorded in KBr on a Varian 3100 FT-IR spectrophotometer
in the 4000-400 cm−1 region. Electronic spectra of the complexes were recorded on a
Shimadzu spectrophotometer, model, Pharmaspec UV-1700 in DMSO as solvent. Magnetic
susceptibility measurements were performed at room temperature on a Faraday balance using
Hg[Co(SCN)4] as the calibrant. ESI-Mass spectrometric analysis was carried out on a
Waters-Q-Tof Premier-HAB213 mass spectrometer.
2.5. Crystal structure determination
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Single crystal X-ray diffraction data for the ligand H2hbpp and its Mn(II) and Zn(II)
complexes were obtained at 293(2) K, on a Oxford Diffraction Gemini Diffractometer
equipped with CrysAlis Pro., using a graphite mono-chromated Mo Kα (λ = 0.71073 Å)
radiation source. The structures were solved by direct method (SHELXL-97) and refined
against all data by full matrix least-square on F2 using anisotropic displacement parameters
for all non-hydrogen atoms. All hydrogen atoms were included in the refinement at
geometrically ideal position and refined with a riding model [26,27]. The Mercury and
ORTEP-3 software packages for windows program were used for generating structures
[28,29].
2.6. Computational studies.
All calculations were performed for the Cu(II) complex using Gaussian-09 suit of
programs. The complex was treated as an open-shell system using spin unrestricted DFT
wave functions (UB3LYP) [30], i.e. the Becke three-parameter exchange functional in
combination with the LYP correlation functional of Lee, Yang and Parr with 6-31G (d,p)
basis set for C, H, N and O atoms [31] and effective core potentials basis set LanL2DZ (Los
Alamos National Laboratory 2 double zeta) [32] for the Cu atom. The optimized structure
was confirmed to be minima on potential energy surface (PES) by performing harmonic
vibration frequency analyses (no imaginary frequency found). No symmetry constraints were
applied and only the default convergence criteria were used during the geometry
optimizations. Based on the optimized geometries, TDDFT calculations were performed at
the same UB3LYP level to calculate the vertical electronic transition energies.
2.7. Corrosion inhibition measurements
Prior to all measurements, the mild steel specimen of composition (wt %) C = 0.17,
Mn = 0.46, Si = 0.26, S = 0.017, P = 0.019 and balance Fe, was abraded successively with
emery papers. The aggressive solution of 1 M HCl was prepared by dilution of analytical
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grade HCl (37%) with double distilled water and all experiments were carried out in unstirred
solutions. The rectangular specimens with dimension 2.5 × 2.0 × 0.025 cm3 was used in
weight loss experiments and of size 1.0 ×1.0 cm2 (exposed) with a 7.5 cm long stem (isolated
with commercially available lacquer) was used for electrochemical measurements [33]. Stock
solutions of studied inhibitors were made in 10:1 (water: DMSO) ratio by volume to ensure
solubility. These stock solutions were used for all experimental purposes.
The electrochemical measurements were done by method described earlier [20]. A
three-electrode cell, consisting of carbon steel working electrode (WE), a platinum counter
electrode (CE), and saturated calomel electrode (SCE) as a reference electrode, was used for
electrochemical measurements. All experiments were performed in atmospheric condition
without stirring. Prior to the electrochemical measurement, a stabilization period of 30 min
was allowed, which was proved to be sufficient to attain a stable value of Ecorr. The EIS
measurements were carried out in a frequency range from 100 kHz to 0.00001 kHz under
potentiostatic conditions, with amplitude of 10 mV peak-to-peak, using the AC signal at Ecorr
.
−250 to
The potentiodynamic polarization curves were recorded in the potential range of
+250 mV (SCE) with a scan rate of 1 mV s−1. All potentials were measured against SCE.
The inhibition efficiencies have been calculated from EIS and current density (icorr
from the following equations.
)
Rpi − Rp0
EEIS % =
EPDP % =
×100
(1)
Rpi
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ic0orr −ii
corr ×100
(2)
ic0orr
0
Rpi Rp
ic0orr
iciorr
are the current density in
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where
&
are the polarization resistances and
&
presence and absence of inhibitors, respectively.
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3. Results and discussion
The analytical data of metal complexes reveal that although the reactions between
metal(II) acetates and the ligand are performed in 1:2 (M:L) molar ratio, only Mn(II) and
Cu(II) forms 1:2 (M:L) complexes. The Zn(II) forms a 1:1 (M:L) dimer under similar
conditions. In all the metal(II) complexes, hydroxyl-proton of 2-hydroxy propiophenone
deprotonates during complexation and the ligand bonds through phenolate-O to metal. In
Zn(II) complex, phenolate-O bridges between two metal atoms resulting into a dimeric
structure for the complex. In addition, the carbonyl-O of the ligand enolizes during
complexation and deprotonates to bond through carbonylate-O. The solvent DMSO also
occupies one of the vacant site of each Zn(II) giving a 5-coordinate distorted square pyramid
geometry around metal ion. The reactions are given in Scheme 1.
o
The metal complexes are intensely colored solids and thermally stable upto 250 C.
They are insoluble in water and common organic solvents viz. ethanol, methanol, chloroform,
benzene, cyclohexane, acetone and diethyl ether but are soluble in DMF and DMSO. The
Mn(II) and Zn(II) complexes have been successfully crystallized to get their single crystals in
DMSO solvent. However, in spite of all the efforts, Cu(II) complex could not be crystallized.
3.2. Electronic spectra and magnetic moments
The intense absorption band observed at 331 nm in the spectrum of ligand is
predominantly due to n→π* transitions associated with azomethine chromophore. The other
intense band located in the high-energy region at 291 nm is due to π→π* transition of
aromatic rings of the free ligand [34]. These transitions are also observed in the spectra of
metal complexes with a slight shift in wavelength, indicating coordination of the ligand to
metal centre. In the electronic spectra of metal complexes, one new band appears in the
region 406–393 nm, may be assigned to a mixed ligand-to-ligand (LLCT) and ligand-to-
metal (LMCT) charge transfer transitions [35,36].
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The Cu(II) complex exhibits a broad d-d transition band centered at 653 nm,
attributable to the 2T2g ← 2Eg transition suggesting a distorted octahedral geometry [37]. The
complex shows µeff value 1.73 B.M., corresponding to one unpaired electron [38]. The Mn(II)
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complex shows a band of weak intensity at 640 nm, which may be assigned to A1g → T1g
transition in an octahedral environment [23]. The magnetic moment value, 5.96 B.M.
observed for Mn(II) complex, suggests the presence of five unpaired electrons [39].
3.3. 1H and 13C NMR
The resonance signals due to two Ar-OH and –NH protons appear in the ligand
H2hbpp at 13.19 and 11.68 ppm, respectively (Fig. S1). One of the Ar-OH signal shows a
downfield shift in the Zn(II) complex and occurs at 14.07 ppm due to its involvement in
intra-molecular hydrogen bonding, whereas, the other disappears due to deprotonation. The –
NH proton signal of the ligand also disappears in its Zn(II) complex as a result of enolization
of >C=O group during complexation (Fig. S2).
13C NMR spectrum of ligand shows the signals at 11.05, 21.22, 163.01, 166.76 and
185.27 ppm, attributed to –CH3, >CH2, >C=N, C–OH and >C=O carbons, respectively (Fig.
S3). The Zn(II) complex shows a downfield shift for >C=N carbon signal which appears at
163.28 ppm, suggesting the bonding of >C=N group with metal ion. The absence of a >C=O
signal in Zn(II) complex indicates enolization of >C=O group (Fig. S4). The signals due to C-
OH and C-O- appear at the same position at 167.50 ppm in the complex.
3.4. Mass spectra
The ESI-MS spectra of the Cu(II) complex exhibits a number of peaks due to
formation of various fragments of the complex. The molecular ion peak observed at m/z =
630.152 with 12% intensity matches well with the molecular weight calculated for the Cu(II)
complex. The peak observed at m/z = 285.12 with 10% intensity, corresponds to the formula
weight of ligand (Fig. S5).
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3.1. IR spectra
The ν(N-H) observed at 3294 cm-1 in the IR spectrum of free ligand, occurs nearly at
the same or at a slightly shifted position in its Mn(II) and Cu(II) complexes, indicating the
non-involvement of >NH group in bonding [29]. The ν(C=O) band observed at 1638 cm-1 in
the ligand, is shifted to lower wave number by 10-18 cm-1 in its Mn(II) and Cu(II) complexes,
indicating coordination of the >C=O group to the metal ion [40]. However, the disappearance
of ν(C=O) and ν(N-H) bands in Zn(II) complex, suggests the involvement of >C=O group in
enolization, which deprotonates during complexation. The appearance of a new ν(C-O)- band
in all the metal complexes in the range 1360-1355 cm-1, suggests bonding of the ligand to
metal through a phenolate-O [41]. The ν(C=N) band observed at 1610 cm-1 in the spectra of
ligand, shifts to lower frequency (11-25 cm-1) in all the metal complexes, suggesting
coordination of azomethine-N. Moreover, a new >C=N- band appears at 1603 cm-1 in the
Zn(II) complex due to enolization of >C=O group. A weak band due to ν(N-N) observed at
950 cm-1 in the ligand, shifts to higher frequency by 32-40 cm-1 in its metal complexes,
indicating the coordination of one of the nitrogen atom of the N-N group with metal [23]. The
ligand and its metal complexes also show a broad band in the 3448-3428 cm-1 region due to
phenolic–OH group [42]. The non-ligand bands in the 543-532 cm-1 and 425-419 cm-1 ranges
have been tentatively assigned to ν(M-O) and ν(M-N), respectively. The band observed at
915 cm−1 in the Zn(II) complex, is assigned to ν(S=O) of coordinated DMSO molecule [43].
3.5.1. Crystal structure of H2hbpp
Fig. 1 shows the ORTEP diagram of the ligand with atomic numbering scheme. The
crystallographic data, structural refinement details of the ligand and the complexes are given
in Table 1. Selected bond lengths, bond angles and hydrogen bonding parameters of the
ligand and the complexes are given in Tables 2 and 3. The bond lengths of O(1)–C(7) and
C(7)–N(1) are 1.245(5) and 1.351(5) Å, respectively, which correspond to typical double
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bonds. The N(1)–N(2) bond distance is 1.356(5) Å, which is slightly shorter than the single
bond distance (~1.411(7) Å) reported previously, indicating some double bond character [44].
The torsion angles O(1)–C(7)–N(1)–N(2), 3.2(6)°, C(8)–C(9)–C(10)–O(3), -1.4(8)° and
C(2)–C(1)–C(7)–O(1), 176.4(4)° indicate that the O(1) and N(2), and C(8) and O(3) are syn-
periplanar to each other, while C(2) and O(1) are anti-periplanar to each other. Crystal
packing of H2hbpp is stabilized by N(1)−H(1)···O(2), O(3)−H(3)···N(2) intra-molecular and
−H(2A)···O(1) −H(3A)···O(1)
, C(3) inter-molecular interactions (Fig. S6).
O(2)
Table 1
Selected crystallographic data of H2hbpp, [Mn(Hhbpp)2] and [Zn(hbpp)(DMSO)]2
H2hbpp
[Mn(Hhbpp)2]
C32H30MnN4O6
621.54
[Zn(hbpp)(DMSO)]2
C36H40N4O8S2Zn2
851.58
Empirical formula
Formula weight
T (K), λ (Å)
C16H16N2O3
284.31
293(2), 0.71073
Monoclinic
P 21/c
293(2), 0.71073
monoclinic
C2/c
293(2), 0.71073
Monoclinic
P 21/c
Crystal system
Space group
Unit cell dimensions
a = 12.9138(18)
b = 13.1039(18)
c = 8.2523(17)
β = 93.164(17)
1394.3(4), 4
1.354
a = 22.4052(13)
b = 9.6538(6)
c = 13.6650(8)
β = 99.850(5)
2912.1(3), 4
1.418
a = 12.7873(3)
b = 14.7998(4)
c = 10.2523(3)
β = 93.175(2)
1937.26(9), 2
1.460
Volume (Å3), Z
Density (mg/m3)
Absorption coefficient (mm-1)
F(000)
0.095
0.505
1.400
600
1292
880
Crystal size (mm3)
0.25 x 0.23 x 0.20 0.25 x 0.24 x 0.21 0.23 x 0.21 x 0.20
11

Theta range for data collection
Index ranges
2.91 - 29.13
3.03 - 29.03
2.96 - 29.18
-15 ≤ h ≤ 17,
-20 ≤ h ≤ 30
-7 ≤ h ≤ 17
-17 ≤ k ≤ 15
-10 ≤ l ≤11
-13 ≤ k ≤ 7
-18 ≤ k ≤ 16
-13 ≤ l ≤ 13
8929 / 5230
[Rint = 0.0247]
5230 / 0 / 239
1.018
-17 ≤ l ≤ 15
6254/3882
Reflections collected /unique
4604 / 3752
[Rint = 0.0459]
3752 / 0 / 194
0.950
[Rint = 0.0393]
3882 / 2 / 197
1.047
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2 a,b[(I>2σ(I))]
R1, wR2a,b (all data)
0.0834, 0.1653
0.2193, 0.2428
0.0605, 0.0914
0.1128, 0.1085
0.399, -0.450
0.0395, 0.0832
0.0629, 0.0957
0.333, -0.425
Residual electron density(e Å-3) 0.292, -0.209
253
254
255
256
a R1 = Σ||Fo| – |Fc||Σ|Fo|.
b R2 = [Σw(|F2 | – |F2 |)2/Σw|F2 |2]1/2
o
c
o
Table 2
Selected bond lengths and angles of H2hbpp, [Mn(Hhbpp)2], [Zn(hbpp)(DMSO)]2
H2hbpp
[Mn(Hhbpp)2]
[Zn(hbpp)(DMSO)]2
Bond lengths (Å)
Mn-O(1)
Mn-O(3)
Mn-N(2)
1.909(2)
1.829(2)
1.962(2)
1.347(4)
1.297(3)
1.336(3)
1.306(3)
Zn-O(3)
1.9936(17)
Zn-O(4)
2.0054(19)
2.0342(17)
2.0424(17)
2.063(2)
Zn-O(1)
O(1)-C(7)
O(3)-C(10)
O(2)-C(2)
N(1)-C(7)
1.245(5) O(2)-C(2)
Zn-O(3a)
Zn-N(2)
1.348(6) O(1)-C(7)
1.362(6) O(3)-C(10)
1.351(5) N(1)-C(7)
O(3)-C(10)
O(1)-C(7)
1.346(3)
1.274(3)
12

N(1)-N(2)
N(2)-C(8)
1.356(5) N(1)-N(2)
1.277(6) N(2)-C(8)
1.377(3)
1.307(3)
N(1)-N(2)
C(7)-N(1)
N(2)-C(8)
Zn-Zn(a)
1.394(3)
1.320(3)
1.293(3)
3.1209(6)
Bond angles (°)
C(8)-N(2)-N(1) 120.8(4) O(1)-Mn-O(3)
C(7)-N(1)-N(2) 120.9(4) O(3)-Mn-N(2)
O(1)-C(7)-N(1) 119.4(5) O(1)-Mn-N(2)
171.03(7)
90.68(9)
80.65(9)
O(1)-Zn-O(3)
O(1)-Zn-N(2)
O(1)-Zn-O(4)
O(3)-Zn-O(4)
104.17(7)
78.81(7)
100.24(8)
106.22(9)
112.71(9)
139.81(8)
158.51(8)
78.71(7)
85.46(7)
101.29(7)
99.29(8)
O(1)-C(7)-C(1) 122.2(5) O(1)-Mn-O(3a) 89.65(9)
N(2)-C(8)-C(9) 117.9(5) N(2a) -Mn-N(2) 169.65(16) O(4)-Zn-N(2)
N(2)-Mn-O(1a) 92.19(9)
O(3)-Mn-O(3a) 89.07(15)
O(1)-Mn-O(1a) 92.96(13)
O(3)-Zn-N(2)
O(1)-Zn-O(3a)
O(3)-Zn-O(3a)
N(2)-Zn-O(3a)
Zn-O(3)-Zn(a)
O(4)-Zn-O(3a)
257
258
259
260
‘a’ refers to the elements generated by symmetry operation = -x+1, y, -z+3/2 for
[Mn(Hhbpp)2] and -x+2, -y, -z for [Zn(hbpp)(DMSO)]2.
Table 3
Hydrogen bond parameters [Å and ˚] of H2hbpp, [Mn(Hhbpp)2] and [Zn(hbpp)(DMSO)]2
H2hbpp
−H···A
−H
D
D
H···A
1.96
1.83
D···A
< (DHA)
135
0.86
0.82
2.642(6)
2.548(6)
−H(1)−O(2)
−H(3)−N(2)
N(1)
O(3)
146
13

O(2)−H(2A)···O(1)#1
C(3)−H(3A)···O(1)#1
C(6)−H(6)···O(1)
#1 = 1-x,1/2+y,1/2-z
[Mn(Hhbpp)2]
0.82
0.93
0.93
1.90
2.44
2.46
2.705(6)
3.154(7)
2.793(7)
168
134
101
N(1)−H(1)···O(2)
C(6)−H(6)···O(1)
C(6)−H(6)···O(2)#1
#1 = x,-y,1/2+z
0.86
1.94
2.45
2.59
2.570(4)
2.774(4)
3.319(3)
129
100
136
0.93
0.93
[Zn(hbpp)(DMSO)]2
O(2)−H(2)···N(1)
C(6)−H(6)···O(1)
C(11)−H(11)···O(1)
C(14)−H(14)···O(4)#1
#1 = x,1/2-y,-1/2+z
0.82
0.93
0.93
0.93
1.79
2.48
2.39
2.56
2.523(3)
2.794(3)
3.166(3)
3.335(3)
148
100
141
141
261
262
263
264
265
266
267
268
269
270
3.5.2. Crystal structure of [Mn(Hhbpp)2]
The ORTEP diagram of the Mn(II) complex with atomic numbering scheme is given
in Fig. 2. The Mn(II) metal ion is coordinated to two mono-anionic ligands through a
carbonyl-O, azomethine-N and a phenolate-O of each ligands. The molecular structure shows
a distorted octahedral geometry around metal ion and structure has crystallographic twofold
symmetry. In complex, the carbonyl-O and phenolate-O of the two ligands are cis to each
other and two azomethine-N are trans to each other. The Mn-O(1) (carbonyl-O), Mn-O(3)
(phenolate-O) and Mn-N(2) (azomethine-N) bond lengths are 1.909(2), 1.829(2) and 1.962(2)
Å, respectively [23,45,46]. These bond lengths fall in the normal range of many octahedral
14

271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
Mn(II) complexes with N,O–donor ligands. The shorter Mn–O(3) bond length as compared to
Mn–O(1) indicates that the phenolate-O bond is more stronger than the carbonyl oxygen [47].
The observed bond angles N(2)-Mn-O(1), 80.65(9)°; O(3)-Mn-N(2), 90.68(9)° and O(1)-Mn-
O(3), 171.03(7)° indicate that the octahedral geometry is slightly distorted due to chelation
effect [41,48]. The torsion angles O(1)–C(7)–N(1)–N(2), 5.4(4)°, N(1)–N(2)–C(8)–C(15),
1.2(4)° and C(14)–C(9)–C(8)–N(2), 169.3(3)°, indicate that the O(1) and N(2), and N(1) and
C(15) are syn-periplanar to each other, while C(14) and N(2) are anti-periplanar to each
other. The molecule forms a butterfly like architecture along ‘c’ axis through N(1)–
H(1)···O(2) and C(6)–H(6)···O(1) intra-molecular and C(6)−H(6)···O(2) inter-molecular
hydrogen-bonding interactions (Table 3, Fig. S7).
3.5.3. Structure of [Zn(hbpp)(DMSO)]2
Fig. 3 shows the ORTEP diagram of Zn(II) complex with atomic numbering scheme.
The crystal structure shows that Zn(II) complex is a phenoxo-bridged centrosymmetric dimer
having 5-coordinate distorted square-pyramid geometry in which metal bonds through an
azomethine-N, carbonylate-O and two bridged phenolate-O of the ligands at the base, and a
sulfoxide-O of DMSO molecule occupies the apical position. The C(8)–N(2) distance
(1.293(3) Å) in the complex is longer than in the ligand (1.277(6) Å) due to coordination of
azomethine-N to the metal ion. The N(1)–C(7) distance (1.320(3) Å) in the complex is
significantly shorter than in the free ligand (1.351(5) Å) as a result of deprotonation of imine
group and formation of >C=N. The O(1)–C(7) bond length (1.274(3) Å) is also longer in the
complex than the free ligand (1.245(5) Å) due to enolization of >C=O group during
complexation [16]. The bond distances for Zn–N(2), Zn–O(1), Zn–O(3) and Zn–O(4) are
2.063(2), 2.0342(17), 1.9936(17) and 2.005(19) Å respectively, which are comparable to the
bond lengths reported for similar Zn(II) complexes in literature [47,49]. O(1)-Zn-N(2),
78.81(7)°; O(1)-Zn-O(3), 104.17(7)°; O(1)-Zn-O(4), 100.24(8)°; O(3)-Zn-O(4), 106.22(9)°;
15

296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
O(3)-Zn-N(2), 139.81(8)°; O(4)-Zn-N(2), 112.71(9)° bond angles are also comparable to
other reported distorted square pyramid Zn(II) complexes with N,O-donor ligands [50]. The
torsion angles O(1)–C(7)–N(1)–N(2), 1.1(4)°, C(15)–C(8)–N(2)–N(1), 3.5(4)° and C(14)–
C(9)–C(8)–N(2), 157.5(2)° indicate that O(1) and N(2), and C(15) and N(1) are syn-
periplanar to each other but C(14) and N(2) are anti-periplanar to each other. Crystal
packing structure of the complex is stabilized by intra-molecular O(2)−H(2)···N(1),
C(6)−H(6)···O(1) and C(11)−H(11)···O(1) and inter-molecular C(14)−H(14)···O(4) hydrogen-
bonding interactions (Table 3, Fig. S8).
3.6. DFT optimized structure of [Cu(Hhbpp)2]
The density functional theory calculations were carried out in gas phase to optimize
the structure of Cu(II) complex, using coordinates of the crystal structure of Mn(II) complex
from the CIF file. The DFT optimized structure is shown in Fig. 4. The selected bond lengths
and bond angles of the complex are given in Table S1. The bond distances Cu-N(2), Cu-N(4),
Cu-O(1), Cu -O(4), Cu-O(3), and Cu-O(6) are 2.296, 2.295, 2.173, 2.171, 1.927 and 1.926 Å,
are comparable to the bond lengths reported for similar Cu(II) complexes in literature (Table
S1) [51, 52]. The observed bond angles around the metal centre agree reasonably well with
other reported Cu(II) complexes of N,O donor ligands in distorted octahedral system [53].
The bond lengths O(1)–C(7), O(4)–C(23), C(8)–N(2) and C(24)–N(4) are 1.218, 1.218, 1.274
and 1.274 Å respectively, which correspond to typical double bond characteristic.
The experimental IR spectral data for the Cu(II) complex have been correlated with
the DFT calculated data based on peak intensities and peak frequencies (cm-1) (Table S2).
Apart from some minor deviations in theoretical group frequencies from the experimental (6-
16 cm-1), the theoretical–experimental agreement is satisfactory. Little deviations are due to
the negligence of anharmonicity in B3LYP method [54] and average error in frequencies
calculated with B3LYP method is reported to be of the order of 40-50 cm-1 [55].
16

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323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
In order to get a deeper understanding of the electronic transitions, TDDFT
calculations have been performed for the Cu(II) complex. The assignments of the calculated
transitions to the experimental bands are based on the criteria of energy and oscillator
strength of the calculated transitions. In the description of the electronic transitions, only the
main components of the molecular orbitals are taken into consideration. The calculated
absorption bands for the complex are shown by the vertical lines in Fig. S9, S10 and the band
assignments are given in Table S3 with their oscillator strengths and energies. The results of
time-dependent density functional theory (TDDFT) calculations on Cu(II) complex at the
UB3LYP level reveal that the band calculated in the region 310-380 nm is due to mixed
ligand→metal (LMCT) and intra-ligand (ILCT) charge transfer transitions. The other low
energy absorption band at 657 nm is due to d-d transition with smaller oscillator strength. The
2
2
orbital analysis of above d-d transiton suggests that it originates from dz , dx -y2 to dxy, dyx, dxz
orbitals, as expected for a distorted octahedral copper(II) complex (Fig. S11).
3.8. Corrosion inhibition efficiency
3.8.1. Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy is a rapid and convenient method for
investigation of protective properties of corrosion inhibitors on metals. More reliable results
can be obtained by this method, since it does not disturb the double layer at the metal/solution
interface [56]. Consequently, EIS is specially a useful method to follow the evaluation of
inhibitor–metal system over time [57]. The effect of inhibitor concentration on mild steel was
studied by using EIS measurements in 1 M HCl in the absence and presence of studied
compounds at 303 K. The impedance plots and proposed equivalent electrical circuit of mild
steel in 1 M HCl in absence and presence of studied compounds are presented in Fig. 5. The
main parameters (Rf, Rct, Y0 and n) deduced from the analysis of Nyquist diagrams for 1 M
17

345
346
HCl containing various concentrations of Oxandra asbeckii plant extract (OAPE) are given in
Table 4.
347 Table 4
348 Impedance parameters for mild steel in absence and presence of studied inhibitors.
Inhibitor
Conc. of Rs (Ω
Inhibitor cm2)
(ppm)
Y0 (10-6
Ω-1 Sn
cm-2)
180
n
L (H) Rct (Ω RL(Ω EEIS%
cm2)
cm2)
-
-
1.01
0.91
0.85
0.96
0.92
0.821
0.835
0.855
0.851
0.871
11.9
10.8
8.4
35.8
10.2
25.1
28.6
18.2
47.2
-
H2hbpp
50
50
50
50
141
84
281.8
367.4
332.6
505.2
85.0
88.4
86.9
91.7
[Mn(Hhbpp)2]
[Cu(Hhbpp)2]
[Zn(hbpp)(DMSO)]2
101
71
9.9
8.1
349
350
351
352
353
354
355
356
Fig. 5 shows the Nyquist plots for mild steel in electrolyte solution in absence and
presence of 50 mg L-1 of studied compounds at 303 1 K. The Nyquist plots obtained for the
corrosion of mild steel in HCl solution with inhibitor consist of two capacitive loops (two
well-defined time-constants in the Bode-phase format) and an inductive loop. The high
frequency (HF) capacitive loop, the smaller one, can be attributed to the film formation at the
steel surface while the low frequency (LF) loop, the larger one, can be attributed to the charge
transfer reaction. The presence of the LF inductive loop may be attributed to the relaxation
Cl-ads
H+ads
on the electrode surface [58]. The
357
process obtained by adsorption species like
and
358
359
360
361
phase angle at high frequency provides a general idea of anticorrosion performance of
inhibitor. The more negative the phase angle shows more capacitive the electrochemical
behaviour. To get a more accurate fit of these experimental data, the measured impedance
data are analyzed by fitting in an equivalent circuit. Excellent fit with this model is obtained
18

362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
for all experimental data. The equivalent circuit consists of the constant phase element (CPE)
in parallel to the charge-transfer resistance (Rct), which is in series to the parallel of inductive
elements (L) and RL. One constant phase element (CPE) is substituted for the capacitive
element to give a more accurate fit, as the obtained capacitive loop is a depressed semicircle.
The depression in Nyquist semicircles is a feature for solid electrodes and often referred to as
frequency dispersion and attributed to the roughness and other inhomogenities of the solid
electrode. For analysis of the impedance spectra exhibiting two capacitive loops and an
inductive loop, the equivalent circuit shown in Fig. 6, was used to fit the experimental data.
3.8.2. Potentiodynamic polarization (PDP) measurements
Fig. 7 represents the potentiodynamic polarization curves of mild steel in 1 M HCl in
the absence and presence of 50 ppm of the studied compounds. In the presence of inhibitors,
the curves are shifted to lower current regions, showing the inhibition tendency of studied
inhibitors. No definite trend is observed in the Ecorr values in the presence of ceftobiprole. In
the present study, a shift in Ecorr values in the range of 2–22 mV suggests that they all act as
mixed type of inhibitor. The values of various electrochemical parameters derived by Tafel
polarization of the inhibitors are given in Table 5. Investigation of Table 5 reveals that the
values of βa change slightly in the presence of ceftobiprole. A more pronounced change in the
values of βc indicates that although both the anodic and cathodic reactions are effected, the
effect on the cathodic reactions is more prominent. The inhibition efficiency depends on
many factors including adsorption centers, mode of interaction, molecular size and geometry
of the inhibitors. The adsorption behavior of the ligand H2hbpp is argued to the presence of
unshared electron pair on hetero atoms and π-electrons on aromatic rings. All the studied
complexes as well as ligand increase the polarization resistance due to adsorption. The
samples exposed to Zn(II) complex solution exhibits greatest inhibition among the other
studied compounds. The complex formation between Zn(II) complex, iron oxide and
19

387
388
389
390
391
392
393
394
hydroxide is responsible for corrosion inhibition activity of this complex, resulting into
precipitation of an insoluble compound on the surface and hence, further deterioration of
metal is decreased. Similar behavior is observed with Mn(II) complex but due to its
monomeric structure, inhibition effect caused by this complex is smaller compared to that of
Zn(II) complex. The least inhibition efficiency of Cu(II) complex is due to its less negative
electrode potential. The increased efficiency of metal complexes compared to the ligand may
be attributed to their larger size. Thus, the inhibition efficiency of the studied compounds
varies as H2hbpp < Cu(II) complex < Mn(II) complex < Zn(II) complex.
395 Table 5
396 Potentiodynamic polarization parameters for mild steel in absence and presence of studied
397 inhibitors.
Inhibitor
Conc. of
-Ecorr (mV
icorr(µA
cm-2)
731
121
87
βa (mV
dec-1)
73
βc(mV EPDP%
Inhibitor/ppm vs. SCE)
dec-1)
-
-
-
469
470
478
465
481
127
137
139
138
140
H2hbpp
50
50
50
50
71
83.4
88.1
85.8
91.2
[Mn(Hhbpp)2]
[Cu(Hhbpp)2]
[Zn(hbpp)(DMSO)]2
77
104
64
72
79
398
399
400
401
402
403
404
.4. Conclusions
This paper describes the synthesis and characterization of a Schiff base derived from
2-hydroxypropiophenone and its Mn(II), Cu(II) and Zn(II) complexes. The molecular
structures of the ligand H2hbpp, Mn(II) and Zn(II) complexes have been determined by single
crystal X-ray diffraction techniques. H2hbpp acts as a monobasic tridentate ligand and
coordinates through azomethine-N, carbonyl-O and phenolate-O with Mn(II) and Cu(II) ions
20

405
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407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
giving a distorted octahedral geometry. The Zn(II) complex forms a phenoxo-bridged
centrosymmetric dimer with 5-coordinate distorted square pyramid geometry containing a
DMSO molecule at apical position. The structure of Cu(II) complex has also been optimized
by DFT and TDDFT calculations. Theoretical calculations suggest an agreement between
theoretical and experimental analyses of electronic and infrared spectral data. The corrosion
inhibition efficiency of the synthesized compounds was also studied for mild steel in 1 M
HCl medium.
Appendix A. Supplementary data
The Figures S1-S11 and Table S1-S3 are available in supplementary materials. CCDC
939625, 878529 and 936392 contain the supplementary crystallographic data for H2hbpp,
[Mn(Hhbpp)2] and [Zn(hbpp)(DMSO)]2, respectively. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
The authors thank the Head, S.A.I.F., Indian Institute of Technology, Kanpur, India
for recording the ESI-mass spectra. One of the authors V.P.S. is grateful to UGC, New Delhi
for financial assistance.
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520
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522
523
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527
25

528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
Figure captions:
Fig. 1. ORTEP diagram of ligand showing atomic numbering scheme with ellipsoids of 30
% probability.
Fig. 2. ORTEP diagram of [Mn(Hhbpp)2)] showing atomic numbering scheme with ellipsoids
of 30% probability (the structure has crystallographic two-fold symmetry and symmetric
atoms are generated by symmetry operation = -x+1, y, -z+3/2).
Fig. 3. ORTEP diagram of [Zn(hbpp)(DMSO)]2 showing atomic numbering scheme with
ellipsoids of 30% probability (the structure contains a crystallographic centre of symmetry
and symmetric atoms are generated by symmetry operation = -x+2, -y, -z).
.Fig. 4. Optimized geometry of [Cu(Hhbpp)2] using B3LYP method and 6-
31g(d,p)/LANL2DZ basis sets(ORTEP view at 30% probability).
Fig. 5. (a) Nyquist plot, (b) Bode plot, (c) Phase angle plot in absence and presence of
inhibitors and (d) Nyquist plot for mild steel in 1 M HCl solution in absence of inhibitor with
proposed circuit fitted data; where Zr is real impedance, Zi is an imaginary part of impedance,
θ is phase angle and f is frequency.
Fig. 6. The electrochemical equivalent circuit used to fit the impedance measurements that
include a solution resistance (Rs), a constant phase element (CPE), a polarization resistance
or charge transfer (Rct), inductance (L) and resistance due to inductive loop (RL).
Fig. 7. Potentiodynamic polarizations for ligand and its metal(II) complexes.
551
26

552
553
554
555
556
557
558
559
560
Fig. 1. ORTEP diagram of ligand showing atomic numbering scheme with ellipsoids of 30 %
561
probability.
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
Fig. 2. ORTEP diagram of [Mn(Hhbpp)2)] showing atomic numbering scheme with ellipsoids
of 30% probability (the structure has crystallographic two-fold symmetry and symmetric
atoms are generated by symmetry operation = -x+1, y, -z+3/2).
27

578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
Fig. 3. ORTEP diagram of [Zn(hbpp)(DMSO)]2 showing atomic numbering scheme with
ellipsoids of 30% probability (the structure contains a crystallographic centre of symmetry
and symmetric atoms are generated by symmetry operation = -x+2, -y, -z).
.
594
595
596
597
598
599
600
601
602
28

603
604
605
606
Fig. 4. Optimized geometry of [Cu(Hhbpp)2] using B3LYP method and 6-
31g(d,p)/LANL2DZ basis sets(ORTEP view at 30% probability).
607
608
`
609
610
611
612
613
Fig. 5. (a) Nyquist plot, (b) Bode plot, (c) Phase angle plot in absence and presence of
inhibitors and (d) Nyquist plot for mild steel in 1 M HCl solution in absence of inhibitor with
proposed circuit fitted data; where Zr is real impedance, Zi is an imaginary part of impedance,
θ is phase angle and f is frequency.
29