Pooja Singha,
Divya Pratap Singha,
Karishma Tiwaria,
Monika Mishraa,
Ashish K. Singhb and
Vinod P. Singh*a
aDepartment of Chemistry, Banaras Hindu University, Varanasi-221005, India. E-mail: singvp@yahoo.co.in; Tel: +919450145060
bDepartment of Chemistry, North West University (Mafikeng Campus), Mmabatho 2735, South Africa
First published on 12th May 2015
Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with 2-amino-benzoic acid (phenyl-pyridin-2-yl-methylene)-hydrazide (Habph) have been synthesized. The complexes were characterized by different physico-chemical and spectral studies viz. molar conductance, magnetic susceptibility measurements, electronic, IR and NMR spectra. The molecular structures of the ligand Habph and its Mn(II), Ni(II), Cu(II), Zn(II) complexes were further confirmed by single crystal X-ray diffraction technique. The Habph acts as a monobasic tridentate ligand coordinating through pyridyl-N, azomethine-N and enolate-O atoms with metal ions. Magnetic moments and electronic spectral studies suggest a high spin octahedral geometry for all the complexes. The ligand molecule exhibits a Z molecular conformation about the >CN– bond, whereas the metal complexes show E-configuration in their single crystal structures. The presence of inter- and intra-molecular H-bonding and various C–H⋯π interactions stabilize the molecular structure of the metal complexes. The structure of the Co(II) complex has been satisfactorily modeled by density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations. The results of electrochemical impedance spectroscopy (EIS) and adsorption behavior of the ligand and metal complexes show appreciable corrosion inhibition efficiency for mild steel in a 1 M HCl medium. The metal complexes show a better inhibition effect than the ligand.
Acylhydrazone Schiff bases readily coordinate with a wide range of transition metal ions, which exhibit interesting physico-chemical, biological and catalytic properties. Aroylhydrazone complexes of transition metal ions are known to provide useful models for elucidation of the mechanism of enzyme inhibition by hydrazine derivatives and for their pharmacological applications.7 The biological activity associated with these compounds is attributed due to the presence of –CONHNCH– moiety.8 The acylhydrazone Schiff base complexes of transition metals have shown high catalytic activities in various chemical reactions such as C–N bond formation using Chan-Lam coupling,9 transamidation of carboxamides with amines,10 epoxidation of olefins11 and polymerization of ethylene.12 A series of iron(III) complexes containing substituted aroylhydrazone ligands have been used in catalytic epoxidation of olefins with tert-butylhydroperoxide.13
The corrosion inhibition study of mild steel in acid media has now become an important area of research in industrial and academic fields. Among the available methods of preventing corrosion, the use of inhibitor is one of the most promising methods. Many N-heterocyclic compounds with polar groups and/or π-electrons are efficient corrosion inhibitors in acidic solutions. Such compounds can adsorb on metal surface and form a bond between the N-electron pair and/or π-electron cloud and metal, thereby reducing corrosion in acidic solutions.14–16 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 complexes show greater inhibition efficiency than the free ligands.17 Interaction of transition metal complexes with mild steel is greatly affected by their standard electrode potentials, their reactivity and the nature of the ligand that could stabilize the metal complexes. The effect of the Schiff base N,N′-bis(salicylaldehyde)-1,3-diaminopropane (Salpr) and its corresponding cobalt complex on the corrosion behavior of steel in 1 M hydrochloric acid solution have been demonstrated.18
Although a few works on the metal complexes of 2-benzoyl pyridine containing Schiff bases have been reported in literature, the work on metal complexes with 2-amino-benzoic acid (phenyl-pyridin-2-yl-methylene)-hydrazide is virtually absent. In view of the significant role played by the metal complexes of 2-benzoyl pyridine containing Schiff bases in biological systems19–21 and their interesting structural properties, we have synthesized Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with 2-amino-benzoic acid (phenyl-pyridin-2-yl-methylene)-hydrazide. The complexes are characterized by various physico-chemical and spectral techniques. The molecular structure of ligand and some of the complexes have been determined by X-ray crystallography. The ligand Habph and its metal complexes have also been evaluated for their corrosion inhibition properties.
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The hydrogen gas volume was measured during the corrosion of mild steel in 1 M HCl solution. For this measurement, a burette was filled with electrolyte solution and was inverted over the mild steel electrode. In order to determine the related activation energies, enthalpy and entropy of activation, the potentiodynamic polarization measurements were studied in the temperature range between 303 and 333 K in 1.0 M HCl in absence and presence of all the synthesized compounds.
The adsorption behavior of studied compounds was experimentally investigated by contact angle measurement of acid solution in absence and presence of 50 ppm of all the studied compounds. Aqueous acid solutions with 50 ppm concentration of the studied inhibitors were prepared and the mild steel samples were then immersed into these solutions for 3 h. Upon removal from the solutions, the samples were dried by means of gently nitrogen flow. Contact angle measurements were performed using the static sessile drop method with a Rame-Hart goniometer (Netcong, USA).
2Habph + M(CH3COO)2·xH2O → [M(abph)2] + 2CH3COOH + xH2O |
The metal complexes are colored crystalline solids and melt with decomposition in the temperature range 238–279 °C. They are insoluble in common organic solvents viz. methanol, ethanol, chloroform, diethyl ether, benzene and DMF, but are highly soluble in DMSO. The low molar conductance values of 10−3 M solutions of all the complexes in DMSO at room temperature (11.02–19.01 Ω−1 cm2 mol−1) indicate that they are non-electrolytes.31
[Co(abph)2] complex shows μeff value 4.83 B.M., is fairly close to those reported for three unpaired electrons in an octahedral environment.34 This complex exhibits two d–d bands at 663 and 492 nm, which may be assigned to 4T1g(F) → 4A2g(F), and → 4T1g(P). These transitions are characteristic of a cobalt(II) complex in a six-coordinate octahedral geometry. Electronic absorption spectra of Mn(II) complex shows two d–d bands of very weak intensity at 546 and 500 nm which may be assigned to 6A1g → 4T1g(G) and 6A1g → 4T2g(G). Above complex also shows the effective magnetic moment value of 5.81 B.M., indicating the presence of five unpaired electrons.
13C NMR spectra of Habph show a cluster of peaks between 140.09–114.19 ppm corresponding to aromatic carbons of pyridine and phenyl ring (Fig. S3†). In Habph, the signals at 176.38, 150.11 and 147.12 ppm, are attributed to CO, C
N and C–NH2 carbons, respectively. The C
O carbon signal disappears in Zn(II) complex due to enolization and a new peak appears at 165.56 ppm assigned as C–O−. This suggests that the ligand bonds through a carbonylate-O to metal. The C
N carbon signal shows a down field shift and appears at 151.95 ppm in the Zn(II) complex due to bonding of azomethine-N with metal (Fig. S4†). The presence of C–NH2 signal at the same position in the complex as in the ligand indicates non-participation of this group in coordination with metal. The signals due to aromatic carbons are also shifted to 138.44–112.78 ppm in Zn(II) complex, suggesting involvement of pyridine-N in bonding.
All the complexes show two ν(CN) frequencies in the ranges 1609–1615 cm−1 and 1574–1583 cm−1. The first new ν(C
N) appears due to enolization of the ligand and the second ν(C
N) occurs at lower wave number (10–20 cm−1) than the ligand due to coordination of azomethine-N to metal ion. In the metal complexes, ν(N–N) is shifted to higher frequencies by 26–38 cm−1 as compare to the ligand, suggesting coordination of one of the nitrogen atom of >N–N< group with metal. The ν(NH2) bands appearing at 3442 and 3387 cm−1 in Habph, remain unshifted in its metal complexes suggesting no participation of –NH2 group in bonding. A non-ligand band observed in the range 438–469 cm−1 in all the metal complexes is tentatively assigned to ν(M–O).
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Fig. 1 ORTEP diagrams of (a) Habph with intra-molecular H-bonding (b) [Mn(abph)2] (c) [Ni(abph)2] (d) [Cu(abph)2] (e) [Zn(abph)2]·2H2O (f) [Co(abph)2] (optimized) with ellipsoids of 30% probability. |
Habph | [Mn(abph)2] | [Ni(abph)2]·C2H5OH | [Cu(abph)2] | [Zn(abph)2]·2H2O | |
---|---|---|---|---|---|
a R1 = Σ||Fo| − |Fc||Σ|Fo|.b R2 = [Σw(|Fo2| − |Fc2|)2/Σw|Fo2|2]1/2. | |||||
Empirical formula | C19H16N4O | C38H30MnN8O2 | C40H36NiN8O3 | C38H30CuN8O2 | C38H34N8O4Zn |
Formula weight | 316.36 | 685.64 | 735.46 | 694.24 | 732.12 |
Temp, K | 293 | 293 | 293 | 293 | 293 |
λ (Å) | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 |
Crystal system | Monoclinic | Triclinic | Triclinic | Monoclinic | Triclinic |
Space group | P21/c | P![]() |
P![]() |
P21/n | P![]() |
a (Å) | 11.6274(4) | 10.7689(6) | 10.5214(6) | 13.2388(7) | 10.442(5) |
b (Å) | 8.6240(3) | 11.3142(8) | 11.2399(8) | 10.7929(11) | 11.517(5) |
c (Å) | 15.6397(7) | 15.4981(11) | 16.4854(11) | 24.2294(14) | 15.425(5) |
α (°) | 90 | 82.460(6) | 79.270(6) | 90 | 100.63 |
β (°) | 91.14 | 85.509(5) | 85.438(5) | 103.586(5) | 102.96 |
γ (°) | 90 | 67.300(6) | 83.472(5) | 90 | 99.08 |
V (Å3) | 1567.95(10) | 1726.2(2) | 1899.6(2) | 3365.1(4) | 1737.9(13) |
Z | 4 | 2 | 2 | 4 | 2 |
Dcalc (g cm−3) | 1.344 | 1.3191(2) | 1.2858(1) | 1.370 | 1.391 |
μ (mm−1) | 0.090 | 0.428 | 0.559 | 0.696 | 0.760 |
F (000) | 664.0 | 710.0 | 768.0 | 1436 | 752 |
Crystal size (mm) | 0.28 × 0.24 × 0.20 | 0.32 × 0.30 × 0.28 | 0.34 × 0.32 × 0.30 | 0.32 × 0.30 × 0.28 | 0.32 × 0.28 × 0.24 |
θ range for data collection (°) | 3.20–28.95 | 3.30 to 29.18 | 3.18 to 29.09 | 3.21 to 29.09 | 3.40–29.04 |
No. of reflections collected | 6695 | 9333 | 15 109 | 16 423 | 14 181 |
No. of independent reflections (Rint) | 4155(0.0189) | 9333(0.0345) | 10 199(0.0432) | 9024(0.0261) | 9283(0.0595) |
No. of data/restraints/parameters | 4155/0/218 | 9333/0/442 | 10 199/3/486 | 9024/1/451 | 9283/0/476 |
Goodness-of-fit on F2 | 1.007 | 1.038 | 1.025 | 1.021 | 0.860 |
R1, wR2a,b[(I > 2σ(I))] | 0.0509, 0.1070 | 0.0769, 0.2027 | 0.0858, 0.2099 | 0.0602, 0.1612 | 0.0544, 0.0670 |
R1, wR2a,b (all data) | 0.0836, 0.1243 | 0.1444, 0.2572 | 0.1529, 0.2649 | 0.0938, 0.1853 | 0.1451, 0.0843 |
Largest difference in peak and hole | 0.182, −0.146 | 0.984, −0.338 | 1.860, −0.603 | 1.369, −0.575 | 0.352, −0.381 |
Habph | |||
---|---|---|---|
Bond lengths | |||
O(1)–C(13) | 1.231(19) | N(1)–C(1) | 1.335(2) |
N(2)–C(6) | 1.301(19) | N(1)–C(5) | 1.346(19) |
N(2)–N(3) | 1.367(18) | C(5)–C(6) | 1.492(2) |
N(3)–C(13) | 1.365(2) | C(6)–C(7) | 1.487(2) |
N(4)–C(15) | 1.372(2) | ||
Bond angles | |||
O(1)–C(13)–N(3) | 122.5(15) | N(3)–N(2)–C(6) | 118.43(13) |
O(1)–C(13)–C(14) | 124.34(15) | C(6)–C(5)–N(1) | 117.04(14) |
N(3)–C(13)–C(14) | 113.15(14) | C(1)–N(1)–C(5) | 118.80(15) |
C(13)–N(3)–N(2) | 120.64(14) | N(4)–C(15)–C(14) | 121.50(17) |
[Mn(abph)2] | |||
---|---|---|---|
Bond lengths | |||
Mn–O(1) | 2.141(3) | O(2)–C(32) | 1.281(5) |
Mn–O(2) | 2.119(3) | N(2)–C(6) | 1.289(6) |
Mn–N(1) | 2.323(4) | N(3)–C(13) | 1.325(6) |
Mn–N(2) | 2.241(4) | N(2)–N(3) | 1.369(5) |
Mn–N(5) | 2.334(3) | N(6)–N(7) | 1.381(5) |
Mn–N(6) | 2.210(3) | C(15)–N(4) | 1.353(9) |
O(1)–C(13) | 1.265(5) | O(34)–N(8) | 1.362(6) |
Bond angles | |||
O(1)–Mn–O(2) | 103.49(13) | Mn–O(1)–C(13) | 116.8(3) |
O(1)–Mn–N(5) | 98.89(13) | Mn–N(1)–C(1) | 124.7(4) |
O(2)–Mn–N(2) | 127.93(13) | N(2)–N(3)–C(13) | 109.6(4) |
N(1)–Mn–N(2) | 69.38(14) | N(1)–C(5)–C(6) | 115.9(4) |
N(2)–Mn–N(5) | 88.31(13) | N(2)–C(6)–C(7) | 126.1(4) |
O(1)–Mn–N(1) | 137.13(13) | O(1)–C(13)–N(3) | 124.6(4) |
O(1)–Mn–N(6) | 131.60(13) | N(3)–N(2)–C(6) | 121.0(4) |
O(2)–Mn–N(5) | 142.13(12) | N(2)–C(6)–C(5) | 113.8(4) |
N(2)–Mn–N(6) | 150.33(14) | ||
Mn–N(2)–N(3) | 116.2(2) |
[Ni(abph)2]·C2H6OH | |||
---|---|---|---|
Bond lengths | |||
Ni–O(1) | 2.092(4) | O(2)–C(32) | 1.271(6) |
Ni–O(2) | 2.089(3) | N(2)–C(6) | 1.295(6) |
Ni–N(1) | 2.112(4) | N(3)–C(13) | 1.348(6) |
Ni–N(2) | 1.981(4) | N(2)–N(3) | 1.365(5) |
Ni–N(5) | 2.117(4) | N(6)–N(7) | 1.369(5) |
Ni–N(6) | 1.983(4) | C(15)–N(4) | 1.355(8) |
O(1)–C(13) | 1.278(6) | O(34)–N(8) | 1.369(9) |
Bond angles | |||
O(1)–Ni–O(2) | 94.69(14) | Ni–O(1)–C(13) | 110.0(3) |
O(1)–Ni–N(5) | 92.82(16) | Ni–N(1)–C(1) | 128.7(4) |
O(2)–Ni–N(2) | 107.59(14) | N(2)–N(3)–C(13) | 108.7(4) |
N(1)–Ni–N(2) | 78.15(16) | N(1)–C(5)–C(6) | 115.6(5) |
N(2)–Ni–N(5) | 97.64(16) | N(2)–C(6)–C(7) | 124.6(5) |
O(1)–Ni–N(1) | 154.94(16) | O(1)–C(13)–N(3) | 125.0(5) |
O(1)–Ni–N(6) | 105.86(16) | N(3)–N(2)–C(6) | 120.9(4) |
O(2)–Ni–N(5) | 154.70(16) | N(2)–C(6)–C(5) | 113.8(4) |
N(2)–Ni–N(6) | 174.99(16) | ||
Ni–N(2)–N(3) | 119.0(3) |
[Cu(abph)2] | |||
---|---|---|---|
Bond lengths | |||
Cu–O(1) | 2.181(3) | O(2)–C(32) | 1.258(4) |
Cu–O(2) | 2.063(3) | N(2)–C(6) | 1.292(5) |
Cu–N(1) | 2.233(3) | N(3)–C(13) | 1.342(4) |
Cu–N(2) | 1.986(3) | N(2)–N(3) | 1.363(4) |
Cu–N(5) | 2.108(3) | N(6)–N(7) | 1.370(3) |
Cu–N(6) | 1.953(3) | C(15)–N(4) | 1.333(4) |
O(1)–C(13) | 1.256(4) | O(34)–N(8) | 1.338(4) |
Bond angles | |||
O(1)–Cu–O(2) | 92.82(12) | Cu–O(1)–C(13) | 108.9(2) |
O(1)–Cu–N(5) | 93.54(12) | Cu–N(1)–C(1) | 130.1(3) |
O(2)–Cu–N(2) | 106.55(10) | N(2)–N(3)–C(13) | 111.1(3) |
N(1)–Cu–N(2) | 76.09(12) | N(1)–C(5)–C(6) | 114.7(3) |
N(2)–Cu–N(5) | 97.18(11) | N(2)–C(6)–C(7) | 124.1(3) |
O(1)–Cu–N(1) | 151.83(10) | O(1)–C(13)–N(3) | 124.6(3) |
O(1)–Cu–N(6) | 114.11(11) | N(3)–N(2)–C(6) | 119.9(3) |
O(2)–Cu–N(5) | 156.26(11) | N(2)–C(6)–C(5) | 115.8(3) |
N(2)–Cu–N(6) | 169.29(12) | ||
Cu–N(2)–N(3) | 118.9(2) |
[Zn(abph)2]·2H2O | |||
---|---|---|---|
Bond lengths | |||
Zn–O(1) | 2.093(2) | O(2)–C(32) | 1.290(4) |
Zn–O(2) | 2.077(3) | N(2)–C(6) | 1.289(4) |
Zn–N(1) | 2.211(3) | N(3)–C(13) | 1.337(4) |
Zn–N(2) | 2.075(3) | N(2)–N(3) | 1.376(4) |
Zn–N(5) | 2.257(3) | N(6)–N(7) | 1.369(5) |
Zn–N(6) | 2.063(3) | C(15)–N(4) | 1.346(6) |
O(1)–C(13) | 1.277(4) | O(34)–N(8) | 1.365(5) |
Bond angles | |||
O(1)–Zn–O(2) | 95.97(9) | Zn–O(1)–C(13) | 111.86(19) |
O(1)–Zn–N(5) | 91.07(9) | Zn–N(1)–C(1) | 128.7(3) |
O(2)–Zn–N(2) | 119.83(9) | N(2)–N(3)–C(13) | 109.9(3) |
N(1)–Zn–N(2) | 75.06(12) | N(1)–C(5)–C(6) | 116.0(3) |
N(2)–Zn–N(5) | 91.13(10) | N(2)–C(6)–C(7) | 125.3(3) |
O(1)–Zn–N(1) | 150.46(10) | O(1)–C(13)–N(3) | 125.0(3) |
O(1)–Zn–N(6) | 112.63(9) | N(3)–N(2)–C(6) | 119.9(3) |
O(2)–Zn–N(5) | 149.04(9) | N(2)–C(6)–C(5) | 113.8(3) |
N(2)–Zn–N(6) | 162.71(12) | ||
Zn–N(2)–N(3) | 116.2(2) |
D–H⋯A | D–H | H⋯A | D⋯A | <(DHA) |
---|---|---|---|---|
a #1 = −x, 1 − y, 2 − z, −x, #2 = 1 − y, 1 − z.b #1 = 2 − x, −y, −z, −x, #2 = −1 + x, y, z.c #1 = 1 − x, −y, 1 − z, #2 = −x, 1 − y, −z. | ||||
Hydrogen bond parameters [Å and °] in Habph | ||||
N(3)–H(3)⋯N(1) | 0.86 | 1.93 | 2.608(2) | 134 |
N(4)–H(4B)⋯O(1) | 0.86 | 2.08 | 2.723(2) | 131 |
Hydrogen bond parameters [Å and °] in [Mn(abph)2]a | ||||
N(4)–H(4A)⋯N(3) | 0.86 | 2.08 | 2.719(10) | 130 |
N(4)–H(4B)⋯N(3) | 1.05 | 1.97 | 2.662(5) | 121 |
N(8)–H(8A)⋯N(7) | 0.86 | 2.06 | 2.693(5) | 130 |
N(8)–H(8B)⋯O(1)#1 | 0.86 | 2.21 | 2.924(6) | 140 |
C(11)–H(11)⋯O(2)#2 | 0.93 | 2.43 | 3.256(7) | 149 |
C(19)–H(19)⋯O(1) | 0.93 | 2.42 | 2.755(8) | 101 |
C(38)–H(38)⋯O(2) | 0.93 | 2.42 | 2.755(6) | 101 |
Hydrogen bond parameters [Å and °] in [Ni(abph)2]·C2H6OHb | ||||
N(4)–H(4A)⋯N(3) | 0.69 | 2.15 | 2.705(8) | 138 |
N(4)–H(4B)⋯O(2)#1 | 0.91 | 2.11 | 3.019(7) | 179 |
N(8)–H(8A)⋯N(7) | 0.90 | 2.01 | 2.655(9) | 127 |
N(8)–H(8B)⋯O(1)#1 | 0.86 | 2.21 | 2.924(6) | 140 |
C(19)–H(19)⋯O(1) | 0.93 | 2.42 | 2.746(7) | 101 |
C(38)–H(38)⋯O(2) | 0.93 | 2.41 | 2.752(6) | 102 |
C(39)–H(39)⋯N(8)#2 | 0.96 | 2.48 | 3.011(15) | 115 |
Hydrogen bond parameters [Å and °] in [Zn(abph)2]·2H2Oc | ||||
N(4)–H(4A)⋯O(4)#1 | 0.80 | 2.27 | 3.062(6) | 171 |
N(4)–H(4B)⋯N(3) | 1.05 | 1.97 | 2.662(5) | 121 |
N(8)–H(8A)⋯N(7) | 0.96 | 1.99 | 2.686(6) | 127 |
N(8)–H(8B)⋯O(1)#2 | 0.74 | 2.32 | 3.054(5) | 177 |
C(19)–H(19)⋯O(1) | 0.93 | 2.38 | 2.737(4) | 102 |
C(38)–H(38)⋯O(2) | 0.93 | 2.41 | 2.746(4) | 101 |
The molecular structure of Habph is stabilized by intra-molecular N(3)–H(3)⋯N(1) and N(4)–H(4B)⋯O(1) hydrogen bonding. The inter-molecular C–H⋯π interactions occur between centroid of pyridyl/phenyl ring and protons of phenyl ring with the contact distances of 2.847, 2.995 and 3.207 Å. The intra-molecular C–H⋯π interaction occurs between centroid of phenyl ring and proton of pyridyl ring with the contact distance of 3.450 Å (Fig. S5†).9
The intra- and inter-molecular hydrogen bonds observed in the ligand are absent in its metal complexes. However, a number of new intra- and inter-molecular H-bonding interactions are observed in all the metal complexes (Table 3) which stabilize their molecular structures. The molecular structures of the complexes are also stabilized by various kinds of C–H⋯π interactions. In Mn(II) complex, the intra-molecular C–H⋯π interactions occur between the centroid of five membered chelate/phenyl rings and pyridyl/phenyl protons with the contact distances of 2.698, 2.750 and 3.376 Å (Fig. S6†).40 The same intra-molecular C–H⋯π interactions occur with the contact distances of 2.895, 3.029, 3.033, 3.243 and 3.384 Å in Ni(II) complex (Fig. S7†), 2.918, 3.035, 3.302 and 3.382 Å in Cu(II) complex (Fig. S8†), and 2.777, 2.995, 3.350 and 3.391 Å in Zn(II) complex (Fig. S9†). The inter-molecular C–H⋯π interactions occur between the centroid of five membered chelate/phenyl ring and phenyl/amine protons in Mn(II) complex with the contact distances of 3.192, 3.258, 3.282 and 3.368 Å.41 Such inter-molecular C–H⋯π interactions also occur with the contact distances of 2.580, 3.115 and 3.252 Å in Ni(II) complex, 2.634, 2.778 and 2.982 Å in Cu(II) complex, and 2.759, 3.127 and 3.131 Å in Zn(II) complex.
The experimental IR spectral data for the Co(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 (3–22 cm−1), the theoretical–experimental agreement is satisfactory. Little deviations are expected due to the negligence of anharmonicity in B3LYP method.44 The average error in frequencies calculated by B3LYP method is reported to be of the order of 40–50 cm−1.45
In order to get a deeper understanding of the electronic transitions, TD-DFT calculations have been performed for the Co(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 band assignments are given in Table S3† with their oscillator strengths and energies. The results of TD-DFT calculations on Co(II) complex at the UB3LYP level reveal that the band calculated in the region 374–400 nm is due to mixed ligand → metal (LMCT) and intra-ligand (ILCT) charge transfer transitions. The other low energy absorption bands at 1304, 663 and 492 nm are due to d–d transition with smaller oscillator strength. The orbital analysis of above d–d transition suggests that it originates from dxy, dyx, dxz to dz2, dx2 − y2 orbitals, as expected for a distorted octahedral cobalt(II) complex (Fig. S10†).
Inhibitors | Rs (Ω cm2) | Rct (Ω cm2) | N | Y0 (μF cm−2 secn−1) | Cdl (μF cm−2) | Cads (F cm−2) | Rads (Ω cm2) | Rp (Ω cm2) | EEIS% |
---|---|---|---|---|---|---|---|---|---|
— | 1.3 | 17.5 | 0.821 | 195.5 | 56.7 | −6.51 | −1.3 | 16.2 | — |
Habph | 1.0 | 117.3 | 0.824 | 122.7 | 49.6 | −1.25 | −1.2 | 116.1 | 86.0 |
[Co(abph)2] | 0.8 | 127.4 | 0.826 | 110.7 | 45.1 | −0.41 | −5.2 | 122.2 | 86.7 |
[Ni(abph)2] | 1.2 | 138.6 | 0.831 | 95.3 | 39.5 | −0.24 | −4.6 | 134.0 | 87.9 |
[Mn(abph)2] | 1.0 | 143.4 | 0.832 | 85.2 | 35.0 | −0.43 | −4.9 | 138.5 | 88.3 |
[Cu(abph)2] | 1.2 | 159.2 | 0.834 | 72.1 | 29.6 | −0.39 | −2.1 | 157.1 | 89.7 |
[Zn(abph)2]·2H2O | 1.3 | 176.5 | 0.835 | 55.2 | 22.1 | −0.36 | −6.7 | 169.8 | 90.5 |
The polarization resistance (Rp) which is being used to calculate inhibition efficiency is calculated by using relation.
Rp = Rct + Rads | (2) |
Nyquist and Bode-phase plots of mild steel in uninhibited and inhibited acid solutions containing 50 ppm concentration of all the studied compounds are presented in Fig. 2a–c. According to Nyquist plots obtained (Fig. 2a), a high-frequency (HF) depressed charge-transfer semicircle was observed (one time constant in Bode plot) followed by a well-defined inductive loop in the low-frequency (LF) regions. The increasing diameter of capacitive loop obtained in 1 M HCl in presence of studied compounds indicates the inhibition of corrosion of mild steel. The high frequency capacitive loop may be attributed to the charge transfer reaction. The presence of low frequency inductive loop may be attributed to the relaxation process obtained by adsorption species like Clads− and Hads+ on the electrode surface.47 It is apparent from Table 4 that the impedance of the inhibited system amplifies with increasing the inhibitor concentration and the double layer capacitance (Cdl) values decrease with increasing inhibitor concentration. This decrease in Cdl results from a decrease in local dielectric constant and/or an increase in the thickness of the double layer, suggest that inhibitor molecules inhibit the iron corrosion by adsorption at the metal/acid interface.48 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 inhomogeneities of the solid electrode. In this behavior of solid electrodes, the parallel network: charge transfer resistance-double layer capacitance is established where an inhibitor is present. For the description of a frequency independent phase shift between an applied ac potential and its current response, a constant phase element (CPE) is used which is defined in impedance representation as in eqn (3)
ZCPE = Y0−1(jω)−n | (3) |
Cdl = (Y0Rct1−n)1/n | (4) |
The phase angle at high frequencies provides a general idea of corrosion inhibition performance. The more negative the phase angle, the more capacitive the electrochemical behavior. Charge-transfer resistance increment raises the current tendency to pass through the capacitor in the circuit. Also, increment of the phase angle at relaxation frequency in presence of inhibitors indicates the increase of capacitive response. Such a phenomenon could be attributed to high corrosion inhibition activity of inhibitor. Excellent fit with this model has been obtained for all experimental data. As an example, the Nyquist and Bode-phase plot for free acid solution is presented as Fig. 2d–e. The proposed equivalent electrical circuit for fitting of the experimental data is presented as Fig. 3. The equivalent circuit consists of the double-layer capacitance (CPE) in parallel to the charge-transfer resistance (Rct), which is in series to the parallel of capacitance (Cads) and resistance due to adsorption of intermediate species (Rads).
Corrosion behavior of mild steel has been investigated by potentiodynamic polarization curves in 1.0 M HCl, in absence and presence of 50 ppm concentration of all the studied compounds at temperatures between 303 and 333 K. The plots are given in Fig. 4a–d. The inhibition efficiencies (EPDP%) have been calculated from current density (icorr) values through the following equation:
![]() | (5) |
![]() | ||
Fig. 4 Potentiodynamic polarization curves in 1 M HCl in absence and presence of studied compounds at (a) 303 K (b) 313 K (c) 323 K (d) 333 K. |
Inhibitors | Temperature (K) | −Ecorr (mV vs. SCE) | icorr (μA cm−2) | βa (mV dec−1) | βc (mV dec−1) | EPDP% |
---|---|---|---|---|---|---|
— | 303 | 469 | 731 | 73 | 127 | — |
Habph | 303 | 460 | 110 | 82 | 140 | 84.9 |
[Co(abph)2] | 303 | 468 | 104 | 84 | 142 | 85.8 |
[Ni(abph)2] | 303 | 472 | 95 | 83 | 147 | 87.0 |
[Mn(abph)2] | 303 | 480 | 92 | 81 | 148 | 87.4 |
[Cu(abph)2] | 303 | 487 | 84 | 78 | 149 | 88.5 |
[Zn(abph)2]·2H2O | 303 | 489 | 76 | 77 | 151 | 89.6 |
— | 313 | 462 | 1380 | 79 | 117 | — |
Habph | 313 | 477 | 250 | 81 | 134 | 81.9 |
[Co(abph)2] | 313 | 504 | 236 | 83 | 147 | 82.9 |
[Ni(abph)2] | 313 | 504 | 210 | 84 | 149 | 84.8 |
[Mn(abph)2] | 313 | 492 | 206 | 72 | 142 | 85.1 |
[Cu(abph)2] | 313 | 496 | 190 | 79 | 153 | 86.2 |
[Zn(abph)2]·2H2O | 313 | 478 | 170 | 70 | 158 | 87.7 |
— | 323 | 464 | 1790 | 82 | 156 | — |
Habph | 323 | 483 | 381 | 74 | 169 | 78.7 |
[Co(abph)2] | 323 | 506 | 344 | 84 | 171 | 80.8 |
[Ni(abph)2] | 323 | 509 | 311 | 88 | 177 | 82.6 |
[Mn(abph)2] | 323 | 511 | 304 | 72 | 169 | 83.0 |
[Cu(abph)2] | 323 | 506 | 280 | 74 | 167 | 84.3 |
[Zn(abph)2]·2H2O | 323 | 503 | 275 | 76 | 166 | 84.6 |
— | 333 | 449 | 2210 | 69 | 149 | — |
Habph | 333 | 465 | 542 | 67 | 208 | 75.5 |
[Co(abph)2] | 333 | 454 | 492 | 87 | 194 | 77.8 |
[Ni(abph)2] | 333 | 478 | 437 | 66 | 216 | 80.2 |
[Mn(abph)2] | 333 | 481 | 425 | 67 | 210 | 80.7 |
[Cu(abph)2] | 333 | 487 | 394 | 64 | 187 | 82.2 |
[Zn(abph)2]·2H2O | 333 | 494 | 388 | 76 | 193 | 82.4 |
It is observed that both the cathodic and anodic reactions are suppressed with the addition of studied compounds, which suggest that all the studied compounds reduced anodic dissolution and also retarded the hydrogen evolution reaction. It follows from Table 5 that the values of cathodic Tafel slope (βc) changes with increasing inhibitor concentration, indicates the influence of the compounds on the kinetics of hydrogen evolution. The shift in the anodic Tafel slope βa may be due to the chloride ions/or inhibitor molecules adsorbed onto steel surface. According to polarization curves, both the anodic and cathodic current density decrease in presence of inhibitors while Ecorr values have not considerably changed (maximum change in Ecorr is 21 mV). According to Ferreira and others,49,50 if the displacement in corrosion potential is more than 85 mV with respect to corrosion potential of the blank solution, the inhibitor can be seen as a cathodic or anodic type. Therefore, all the studied compounds act as mixed type inhibitor at 303 K. The results obtained from potentiodynamic polarization show good agreement with the results obtained from EIS.
![]() | (6) |
and from transition state plot according to the following equation:
![]() | (7) |
The apparent activation energy of the inhibitors has been calculated by linear regression between logicorr and 1/T (Fig. 5a); the results are presented in Table 6. Inspection of Table 6 shows that apparent activation energy increases on addition of inhibitors in comparison to the blank solution. The increase in Ea can be interpreted as the physical adsorption. Thus, the studied molecules create a barrier to charge and mass transfer. The higher values of Ea in inhibited solution may also be correlated with the increased thickness of double layer, which enhances the Ea values of the corrosion reaction.51
Name of Inhibitor | Conc. of inhibitor (ppm) | Ea (kJ mol−1) | ΔH* (kJ mol−1) | ΔS* (J mol−1 K−1) |
---|---|---|---|---|
— | — | 29.9 | 27.1 | −99.7 |
Habph | 50 | 43.3 | 40.7 | −70.7 |
[Co(abph)2] | 50 | 41.9 | 39.3 | −75.7 |
[Ni(abph)2] | 50 | 41.3 | 38.7 | −78.4 |
[Mn(abph)2] | 50 | 41.4 | 38.8 | −78.3 |
[Cu(abph)2] | 50 | 41.8 | 39.1 | −77.9 |
[Zn(abph)2]·2H2O | 50 | 44.7 | 42.0 | −69.3 |
The relationship between log(icorr/T) and 1/T are shown in Fig. 5b. Straight lines are obtained with a slope (−ΔH*/2.303R) and an intercept of [log(R/Nh) + (ΔS*/2.303R)], from which the value of ΔH* and ΔS* are calculated and presented in Table 6. The positive sign of enthalpy reflects the endothermic nature of steel dissolution process meaning that dissolution of steel is difficult. On comparing the values of entropy of activation (ΔS*) listed in Table 6, it is clear that entropy of activation increases in presence of the studied inhibitors compared to free acid solution. Such variation is associated with the phenomenon of ordering and disordering of inhibitor molecules on the mild steel surface. The increased entropy of activation in the presence of inhibitor indicates that disorderness is increased on going from reactant to activated complex. The increase in values of entropy by the adsorption of inhibitor molecules on metal surface from the acid solution can be regarded as quasi-substitution between the inhibitor molecules in the aqueous phase and H2O molecules on electrode surface. In such condition, the adsorption of inhibitor molecules is followed by desorption of H2O molecules from the electrode surface. Thus increase in entropy of activation is attributed to solvent (H2O) entropy.
![]() | (8) |
Footnote |
† Electronic supplementary information (ESI) available: 1H & 13C NMR spectra of Habph and Zn(II) complex, and π⋯π & C–H⋯π interactions in different compounds; Tables for theoretical structural parameters. CCDC 894885, 988159, 988158, 988160 and 894890. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra11929k |
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