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Journal of Organometallic Chemistry 767 (2014) 136e143

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Journal of Organometallic Chemistry

journal homepage: www.elsevier .com/locate/ jorganchem

Synthesis, structural, electrochemical and corrosion inhibitionproperties of two new ferrocene Schiff bases derived from hydrazides

Shraddha Rani Gupta a, Punita Mourya b, M.M. Singh b, Vinod P. Singh a, *

a Department of Chemistry, Banaras Hindu University, Varanasi 221005, Indiab Department of Chemistry, IIT(BHU), Varanasi 221005, India

a r t i c l e i n f o

Article history:Received 18 April 2014Received in revised form23 May 2014Accepted 29 May 2014Available online 9 June 2014

Keywords:Ferrocene carboxaldehyde acylhydrazonesSingle crystal X-ray diffractionElectro-chemical studyNMR spectraCorrosion inhibition

* Corresponding author. Tel.: þ91 9450145060.E-mail address: singvp@yahoo.co.in (V.P. Singh).

http://dx.doi.org/10.1016/j.jorganchem.2014.05.0380022-328X/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Two new Schiff bases, ferrocene carboxaldehyde propanoylhydrazone (fcph) and ferrocene carbox-aldehyde furoylhydrazone (fcfh) have been synthesized and characterized by elemental analyses, elec-tronic, IR and NMR spectral studies. Molecular structures of fcph and fcfh are determined by singlecrystal X-ray diffraction technique. The Schiff bases fcph and fcfh display E-configuration about the >C]Ne bond. The structures of both the compounds are stabilized by inter-molecular H-bonding. Theelectrochemical study of these compounds exhibits quasi-reversible one electron redox behaviour. Thecorrosion inhibition study of the Schiff bases have been performed by using weight loss, potentiody-namic polarization and electrochemical impedance spectroscopy (EIS) measurements. The compoundsfcph and fcfh show an appreciable corrosion inhibition property against corrosion of mild steel in 0.5 MH2SO4 solution at 298 K. It has been observed that fcfh shows greater corrosion inhibition efficiency thanfcph. As the concentration of the Schiff bases increases, corrosion inhibition property of the compoundsincreases.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Schiff bases represent an important class of organic compoundsand have often been used as chelating ligands in coordinationchemistry. Due to presence of an azomethine (>C]Ne) group,these compounds have structural similarities with neutral biolog-ical systems and are utilized in elucidating the mechanism oftransformation of racemisation reaction in biological systems [1].The ferrocene-substituted Schiff bases have drawn a considerableattention, owing to their good free radical scavenging and anti-cancer activities [2]. It has been suggested that the azomethinelinkage of the Schiff bases is responsible for various biological ac-tivities such as antitumour, antibacterial, antifungal and herbicidalactivities [3]. However, these properties as well as the rate ofdegradation are highly dependent on the chain stereochemistry [4].

The ferrocene derivatives are of importance among the metal-locenes for their stability in a biological medium and for theirlipophilic, nontoxic, and reversible redox properties [5]. Due to thefavourable electronic properties of ferrocene derivatives and theireasy functionalization, these compounds havemany applications in

material science including sensors, catalysts, electro active mate-rials and aerospace materials [6].

Application of corrosion inhibitors is the most economical andpractical method to mitigate electrochemical corrosion. From thestandpoint of safety, the development of nontoxic and effectiveinhibitors is very important and desirable. Many heterocycliccompounds containing heteroatoms like N, O and S have beenreported to be effective inhibitors for the corrosion of steel inacid media [7e12]. Most organic inhibitors are the substanceswhich possess at least one functional group as the reactioncentre for the adsorption process [13]. The inhibition efficiencyhas been found to be closely related to the inhibitor adsorptionabilities and molecular properties of different kinds of organiccompounds [14,15].

Although, a number of transition metal complexes of hydrazoneSchiff bases have been reported to act as corrosion inhibitorsagainst corrosion of mild steel in 1 M HCl [16,17], the corrosioninhibition property of ferrocene Schiff bases derived from hydra-zides for mild steel in 0.5 M H2SO4 solution has not been explored.In view of the above and the significant role played by the ferroceneSchiff bases in biological systems, and in various other fields, wehave synthesized and characterized two new Schiff bases, ferrocenecarboxaldehyde propanoylhydrazone (fcph) and ferrocene car-boxaldehyde furoylhydrazone (fcfh). The corrosion inhibition

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143 137

property of the Schiff bases has also been evaluated againstcorrosion of mild steel in 0.5 M H2SO4.

Experimental

Materials and methods

All analytical reagent grade chemicals were obtained from thecommercial sources and used without further purification. Ferro-cene carboxaldehyde, propanoic acid hydrazide and 2-furoic acidhydrazide were purchased from Sigma Aldrich-Chemicals, USA.Hydrazine hydrate and solvents (Merck Chemicals, India) wereused as such.

Preparation of Schiff bases

Preparation of ferrocene carboxaldehyde propanoylhydrazone(fcph)

The Schiff base, ferrocene carboxaldehyde propanoylhydrazone(fcph) was synthesized by reacting 50 ml ethanolic solution offerrocene carboxaldehyde (2.14 g, 10 mmol) with 50 ml ethanolicsolution of propanoic acid hydrazide (0.88 g, 10 mmol) in a roundbottom flask. The reaction mixture was refluxed for 4 h and areddish brown product was crystallized on cooling the above so-lution at room temperature. The product was filtered on a Büchnerfunnel and washed several times with ethanol. The pure compoundwas recrystallized from hot methanol and dried in a desiccator overanhydrous calcium chloride at room temperature. Yield (65%). M.p.47 �C. Anal. Calc. for C14H16ON2Fe (284.13): Fe, 19.65; C, 59.18; H,5.67; N, 9.85. Found: Fe, 19.60; C, 59.02; H, 5.64; N, 9.80%. IR (ncm�1, KBr): n(NH) 3211s; n(C]O) 1658s; n(C]N) 1603s; n(NeN)1000w. 1H NMR (DMSO-d6; d ppm): 9.87 (1H, NH); 9.601 (1H, HC]N); 4.80-4.29 (9H, ferrocenemoiety); 2.10 (2H, CH2); 0.99 (3H, CH3).13C NMR (DMSO-D6; d ppm) 168 (C]O); 146 (HCs]N); 79.38 (C5);69.92 (C6, C9); 68.93 (C7, C8); 67.52 (C10eC14); 26.42 (CH2); 9.64(CH3). UVeVis (DMSO, nm): 310 and 457. The single crystal struc-ture of the Schiff base was further confirmed by XRD.

Preparation of ferrocene carboxaldehyde furoylhydrazone (fcfh)Ferrocene carboxaldehyde furoylhydrazone (fcfh) was synthe-

sized by reacting 50 ml ethanolic solution of each of ferrocenecarboxaldehyde (2.14 g, 10 mmol) and furoic acid hydrazide (1.10 g,10 mmol) in a round bottom flask. After refluxing the reactionmixture for 5 h, the solution was cooled at room temperature tocrystallize the reddish brownproduct. The product was filtered on aBüchner funnel and washed several times with ethanol. The purecompound was recrystallized from hot methanol and dried in adesiccator at room temperature. Yield (58%). M.p. 180 �C. Anal. Calc.for C16H14O2N2Fe (322.14): Fe, 17.33; C, 59.65; H, 4.38; N, 8.69.Found: Fe, 17.40; C, 59.51; H, 4.40; N, 8.66%. IR (n cm�1, KBr): n(NH)3192m; n(C]O) 1651s; n(C]N) 1604s; n(NeN) 1000w. 1H NMR(DMSO-d6; d ppm): 11.51 (1H, NH); 8.26 (1H, HC]N); 7.90-6.67(3H, furan moiety); 4.63e4.21 (9H, ferrocene moiety). 13C NMR(DMSO-d6; d ppm): 174.41 (C]O); 149.27 (HC]N); 146.88(C4);

145.53(C1); 114.44(C3); 112.06(C2); 78.83 (C7); 70.32-68.92(C8eC11); 67.67(C12eC16). UVeVis (DMSO, nm): 309 and 439. Thesingle crystal structure of the Schiff base was further confirmed byXRD.

Physico-chemical measurements

C, H and N contents were determined on an Exeter AnalyticalInc. CHN Analyzer (Model CE-440). 1H and 13C NMR spectra wererecorded in DMSO-d6 on a JEOL AL-300 FT-NMR multinuclearspectrometer. Chemical shifts were reported in parts per million(ppm) using tetramethylsilane (TMS) as an internal standard.Infrared spectra were recorded in KBr on a Perkin Elmer FT-IRspectrophotometer in the 4000e400 cm�1 region. UVevisiblespectra of the Schiff bases were recorded in DMSO solution in therange 200e800 nm on a Shimadzu spectrophotometer, Pharmas-pec. UV-1700 model. Cyclic voltammograms were recorded with athree electrode compartment consisting of a platinum disc workingelectrode, a platinum wire counter electrode and an Ag/Agþreference electrode.

Crystal structure determination

Single crystal X-ray diffraction data of the Schiff bases fcph andfcfh were obtained at 293 (2) K, on an Oxford Diffraction Geminidiffractometer equipped with CrysAlis Pro., using a graphitemono-chromated Mo Ka (l ¼ 0.71073 Å) radiation source. Thestructures were solved by direct methods (SHELXL-97) andrefined against all data by full matrix least-square on F2 usinganisotropic displacement parameters for all non-hydrogen atoms.All hydrogen atoms were included in the refinement at geomet-rically ideal position and refined with a riding model. The MER-CURY package and ORTEP-3 for Windows program were used forgenerating structures.

Corrosion inhibition experiments

The composition (wt%) of mild steel used for all the experimentswas as follows: C ¼ 0.253; Si ¼ 0.12; P ¼ 0.013; S ¼ 0.024;Cr ¼ 0.012; Mn ¼ 0.03 and balance Fe. Coupons of dimensions3 � 4 � 0.05 cm3 were used for weight loss measurements,whereas, specimens of size 3 � 1 � 0.05 cm3 were used as workingelectrode for electrochemical impedance spectroscopy (EIS) mea-surements. Prior to the experiments, the specimens were me-chanically abraded with 320, 400, 600, 800, 1000, 1500 and 2000grade of emery papers. It was then degreased with acetone, washedwith double distilled water and dried in air before immersing in thecorrosive medium. The corrosive solution, 0.5 M of H2SO4 wasprepared by dilution of analytical grade H2SO4 of predeterminednormality with triple distilled water. For eachmeasurement, 40 and100 ppm concentrations of the Schiff bases were used in 150 ml

Scheme 1. Synthesis of fcph and fcfh Schiff bases.

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143138

volume of electrolyte. Finely polished and dried mild steel speci-mens of dimension 3 � 4 � 0.05 cm3 were weighed and immersedfor 24 h in each 0.5 M acidic solution in the absence and presence of40 and 100 ppm of the inhibitor at 298 K. The corroded/inhibitedspecimens werewashed thoroughly by liquid soap, rinsed severallywith distilled water, cleaned, dried using acetone and reweighed.The weight loss was calculated as the difference in weight of thespecimen before and after immersion in corrosion media.

In all the above measurements, three close values wereconsidered, and their average values are reported. The corrosionrate (CR) in mg cm�2 h�1 was calculated from the followingequation:

CR ¼ DWs� t

(1)

where, DW is average weight loss, s the total area of the specimen,and t is the immersion time. From the corrosion rate thus obtained,the inhibition efficiency (h) was calculated as follows:

h% ¼ CR� � CRCR� � 100 (2)

where, CR� and CR are the corrosion rates of mild steel specimens inthe absence and presence of inhibitor, respectively.

Electrochemical measurements were carried out in a conven-tional three electrode cell assembly. This assembly consisted of flatbottom pyrex glass flask with three openings, each for workingelectrode, reference electrode and counter electrode. A rectangularworking electrode of mild steel with an exposed surface area of1 cm2 was attached through a copper rod fixed with the help of ascrew and the rest being covered with lacquer. A platinum elec-trode and a silveresilver chloride electrode with KCl salt bridgewere used as a counter and reference electrode, respectively. Theexperiments were measured after 30 min of immersion in theelectrolytic solution in presence and absence of inhibitor. Theworking electrode was immersed in the test solution and theconstant steady state (open circuit) potential was recorded when itbecame virtually a constant. After attaining the steady state, anodicand cathodic polarization studies were conducted using Electro-chemical Analyzer (CHI604A Model).

Electrochemical impedance spectroscopic measurements (EIS)were carried out using AC signals of 5 mV amplitude for the fre-quency spectrum from 100 kHz to 0.01 Hz. The Nyquist represen-tations of the impedance data were analysed with Zsimpwinsoftware. The electrode was kept for half an hour in the test solu-tion before starting the impedance measurements. The chargetransfer resistance (Rct), was obtained from the diameter of thesemicircle of the Nyquist plot. The inhibition efficiency of the in-hibitor has been estimating from Rct values in presence and inabsence of inhibitor by using Eq. (3).

h% ¼ Rct � R�ct

Rct� 100 (3)

where, R�ct and Rct are the charge transfer resistance values without

and with the addition of inhibitor.In Tafel polarization study, polarization curves were recorded

from �250 to þ250 mV with respect to corrosion potential with ascan rate of 1.0 mV s�1. The linear Tafel segments of anodic andcathodic curves were extrapolated till the point of intersection toobtain corrosion potential (Ecorr) and corrosion current density(icorr). Inhibition efficiency has been calculated by using respectiveicorr values in place of corrosion rates (CR) in Eq. (2). The polariza-tion resistance (Rp) values were calculated by performing linear

polarization measurements (LPR) in the potential rangeEcorr ± 10 mV with the scan rate of 0.1 mV s�1. Polarization resis-tance (Rp) values were obtained from the resulting current densityversus potential plot. The inhibition efficiency of the inhibitor hasbeen determined by replacing Rct and R

�ct with Rp and R

�p respec-

tively, in Eq. (3).

Results and discussion

The Schiff bases, ferrocene carboxaldehyde propanoylhydrazone(fcph) and ferrocene carboxaldehyde furoylhydrazone (fcfh), wereobtained in a high yield by the reaction of ferrocene carbox-aldehyde with each of propanoic acid hydrazide and furoic acidhydrazide in ethanol in 1:1 M ratio. The reactions occur as given inScheme 1.

The fcph and fcfh form reddish brown crystals. They are solublein common organic solvents viz., ethanol, methanol, chloroform,benzene, cyclohexane, DMF and DMSO, but are insoluble in acetoneand diethyl ether. The fcph and fcfh melt in the range 47e50 �C and180e185 �C, respectively.

Electronic spectra

The UVevis absorption spectra of fcph and fcfh in DMSO areconsistent with most ferrocenyl chromophores in which theyexhibit two charge-transfer bands. The spectra contain a prominentabsorption band with a maximum at 310 and 309 nm for fcph andfcfh, respectively, which can be ascribed to a high energy ligand-centred pep* electronic transition. In addition to this band,another weaker absorption band in visible region is observed at 457and 439 nm for fcph and fcfh, respectively, which is assigned toanother localized excitation produced by two nearly degeneratedtransitions, and Fe(II) ded transition or by metal-ligand chargetransfer (MLCT) process [18]. The weak ded transitions in the Schiffbases are obscured by much more intense UVeVis charge transfertransitions, therefore, these transitions cannot be resolved [19].

1H and 13C NMR spectra

1H NMR spectra of fcph Schiff base shows a signal due to eNHproton at 9.87 ppm [Fig. S1]. This compound also shows a eCH]N� proton signal at 9.60 ppm [16]. The disappearance of eNHprotons in their D2O exchanged 1H NMR spectra confirms theirassignments. The multiplet signals observed in the region4.80e4.29 ppm are attributed to the ferrocene protons [20]. The

Fig. 2. Cyclic voltammogram of fcfh at different scan rates 20, 50, 75, 100, 150 and 200.

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143 139

signals observed at 0.99 and 2.10 ppm are attributed to eCH3 andeCH2 protons, respectively. 13C NMR spectra of fcph shows peaks at168 and 146 ppm assigned to >C]O and eHC]Ne carbons,respectively [21]. A cluster of peaks appear in the region79.92e67.08 ppm corresponding to ferrocene carbons. The signalsobserved at 9.64 and 26.42 ppm are attributed to eCH3 and eCH2carbons, respectively [Fig. S2].

1H NMR spectra of fcfh exhibits signals at 11.51 and 8.26 ppm,which are attributed to the eNH and eHC]N protons, respectively[Fig. S3]. The multiplet signals observed around 7.90e6.67 [22] and4.63e4.21 ppm are assigned to the furan ring and ferrocene pro-tons, respectively. 13C NMR spectra of fcfh shows carbon signals at174.41 and 149.27 ppm, corresponding to >C]O and eHC]N,respectively. The signals at 146.88, 145.53, 114.44 and 112.06 ppmare attributed to C4, C1, C3 and C2 carbons of furan, respectively[23]. A cluster of peaks are also observed in the range78.83e67.67 ppm, which corresponds to carbons of ferrocenemoiety [Fig. S4].

IR spectra

The IR spectra of fcph and fcfh Schiff bases show a sharp band at3211 and 3192 cm�1 respectively, due to n(NH) [16]. The bandobserved at 1658 and 1651 cm�1 in the spectra of fcph and fcfh,respectively are assigned to n(C]O) [Fig. S5 and S6]. The n(C]N)bands are observed at 1603 for fcph and 1604 cm�1 for fcfh [24].Furthermore, a weak band due to n(NeN) is observed at 1000 cm�1

in both the Schiff bases fcph and fcfh [17]. A band due to Cp-Fe-Cpstretching vibration of ferrocenyl moiety occurs in the range476e518 cm�1 in fcph and fcfh [20].

Cyclic voltammetry

The redox potential of ferrocene derivatives changes withinwider limits. The redox potential depends on the electron-donatingor electron withdrawing ability of the substituents. Thus, the redoxproperties can be rather strongly affected by altering the substit-uent nature [25]. The redox-potential for the ferroceneeferrice-nium couple ranges from 0.3 to 0.4 V. The redox potentialcharacterizes the ability of the redox centre to transfer electronsand to act as a redox catalyst. The effectiveness of CV results from itscapability for rapidly observing the redox behaviour over a widepotential range [26].

Preliminary electrochemical studies of the ferrocene Schiff baseswere performed by cyclic voltammetry in DMF (0.1 M TBA.BF, Pt,

Fig. 1. Cyclic voltammogram of fcph at different scan rates 20, 50, 75, 100, 150, 200 and300.

Ag/AgCI) [27]. The fcph and fcfh shows an anodic peak at the po-tential Epa ¼ 0.78 V and 0.73 V, and a cathodic peak at the potentialEpc ¼ 0.25 V and 0.42 V, respectively [Figs. 1 and 2]. These redoxpeaks can be attributed to Fe(II)/Fe(III), exhibiting a quasi-reversible process [28,29]. The half wave potential for fcph andfcfh, located at E1/2 ¼ 0.70 V and 0.55 V and the difference betweenthe peaks, DEp ¼ 0.52 V and 0.30 V, respectively. The peak currentratio, Ipa/Ipc ¼ 10.70 and 1.87 for fcph and fcfh, respectively, sug-gests a quasi-reversible single electron transfer process [30]. Thecyclic voltammograms recorded at various scan rates, 20, 50, 75,100, 150, 200 and 300 mV s�1, show a slight variation in peak po-tential probably due to a slight delayed electron transfer of theredox species. The oxidation of fcph and fcfh occurs at 0.78 V and0.73 V, respectively, which is at a higher potential than that offerrocene (0.49 V) suggesting a strong electronic interaction be-tween the propanoyl, furoyl and ferrocene moieties. The propanoyland furoyl moieties reduce the electron density on the ferrocenylsubstituent and make it more stable to oxidation as evidenced inthe Fe(II)/Fe(III) oxidation process [27].

Crystal structure of ferrocene carboxaldehyde propanoylhydrazone(fcph)

Fig. 3 shows the ORTEP diagram of fcph with atomic numberingscheme. The crystallographic data, structural refinement details aregiven in Table 1. Selected bond lengths, bond angles and hydrogenbonding parameters of fcph are given in Table 2. The fcph moleculedisplays an E-configuration about the >C]Ne bond [16]. Themolecular structure is stabilized by N(1)�H/O(1) inter-molecular

Fig. 3. ORTEP diagram of fcph showing atomic numbering scheme with ellipsoids of30% probability.

Table 1Selected crystallographic parameters of fcph and fcfh.

fcph fcfh

Empirical formula C14H16ON2Fe C16H14O2N2FeFormula weight 284.26 322.14Temperature 293(2) K 293(2) KWavelength 0.71073 0.71073Crystal system orthorhombic orthorhombicSpace group ‘P b c a’ ‘P b c a’a (Å) 18.305 13.171b (Å) 7.7965 9.815c (Å) 18.884 22.1071a (

�) 90 90

b (�) 90 90

g (�) 90 90

Volume 2677.8 (6) 2857.9(3)Z 8 8Density (g cm�3) 1.410 1.497Absorption coefficient ‘m’ (mm�1) 1.115 1.060F(000) 1184.0 1328.0Crystal size 0.25 � 0.24 � 0.23 0.25 � 0.24 � 0.23q range for data collection (

�) 3.00e28.90 3.18e29.04

Index ranges h(25), k(10), l(26) h(17), k(11), l(26)Parameters 163 194Goodness-of-fit on F2 0.994 1.046R1,wR2

a,b[(I > 2s(I))] 0.0776, 0.1873 0.0843, 0.0991R1, wR2

a,b(all data) 0.1578, 0.2810 0.0456, 0.0799

a R1 ¼ SjjFoj e jFcjjSjFoj.b R2 ¼ [Sw(jFo2j e jFc2j)2/SwjFo2j2]1/2.

Fig. 4. Diagram showing inter-molecular hydrogen bonding in fcph.

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143140

hydrogen bonding with bond distance 2.837 Å [Fig. 4]. The twocyclopentadienyl rings exist in eclipsed form [31].

The C(3)eO(1) bond length (1.224Å) is consistent with a normalC]O bond length (1.21 Å) with double bond character [32]. TheC(3)eN(1) bond length (1.336 Å) is in accordance with the otherSchiff bases but shorter than the standard CeN single bond length(1.469 Å). This indicates the delocalization of p-electronsthroughout the O(1)eC(3)eN(1) [16]. The FeeC bond lengths are inthe range of 1.985e2.040 Å, which are slightly shorter than theFeeC bond length (2.081 Å) reported in other similar compounds[31]. The average CeC bond length is 1.374 Å and average CeFebond length in ferrocene moiety is 1.939 Å.

Torsion angles C(1)eC(2)eC(3)eO(1) [�47.58�], C(1)eC(2)eC(3)eN(1) [132.11�], C(2)eC(3)eN(1)eN(2) [�172.51�], O(1)eC(3)eN(1)eN(2) [7.17�], C(3)eN(1)eN(2)eC(4) [�168.83�] and N(1)eN(2)eC(4)eC(5) [�174.43�] indicate that C(1)eO(1) and O(1)eN(2)are cis to each other while C(1)eN(1), C(2)eN(2), C(3)eC(4) andN(1)eC(5) are trans to each other. The torsional angle (4.22�) offerrocene moiety suggests the presence of almost eclipsedcyclopentadienyl rings, which is shorter than the torsional angle(6.2�) reported earlier, and features a typical linear metallocenestructure [31].

Table 2Selected bond lengths (Å) and bond angles (

�) of fcph.

Bond lengthsC(1)eC(2) 1.456 C(6)eC(7) 1.420C(2)eC(3) 1.523 C(11)eC(12) 1.373C3eO(1) 1.224 C(4)eC(5) 1.449C(3)eN(1) 1.336 C(7)eFe 2.040N1eN(2) 1.400 C(14)eFe 1.985N(2)eC(4) 1.267Bond anglesC(2)eC(3)eO(1) 121.17 C(4)eC(5)eFe 125.98C(1)eC(2)eC(3) 113.18 C(4)eC(5)eC(9) 130.15O(1)eC(3)eN(1) 123.36 C(5)eC(6)eC(7) 108.39C(3)eN(1)eN(2) 120.69 C(9)eFeeC(10) 10.75N(2)eC(4)eC(5) 123.13 FeeC(14)eC(10) 70.01Hydrogen bondO(1)/HeN(1) 1.979

Crystal structure of ferrocene carboxaldehyde furoylhydrazone(fcfh)

Fig. 5 shows the ORTEP diagram of fcfh with atomic numberingscheme. The crystallographic data, structural refinement details,selected bond lengths and bond angles of fcfh are given in Tables 1and 3. The molecule exists in the keto form and displays an E-configuration about the >C]N- bond. The structure of fcfh is sta-bilized by N(1)�H/O(1) inter-molecular hydrogen bonding with abond distance 2.161 Å [Fig. 6].

All the CeC bond lengths (1.361 Å) and CeCeC bond angles(106.52�) of the furan ring as well as the CeO bond lengths(1.366Å) and CeOeC bond angle (105.52�) of hydrazonemoiety arein accordance with the other similar compounds reported else-where [16]. The C(5)�O(2) bond length (1.225 Å) is in the range ofstandard C]O double bond and N(1)�C(5) bond length (1.357 Å) isshorter than standard NeC single bond length (1.411 Å) [32]. Thisindicates the delocalization of p electrons in the central part of themolecule. The C(4)�C(5)�N(1) angle is 113.52� as for an sp2 hy-bridized C(5) carbon. The torsional angles O(1)�C(4)�C(5)�O(2)[2.23�], C(3)�C(4)�C(5)�N(1) [4.24�], C(3)�C(4)�C(5)�O(2)[�177.45�], N(1)�N(2)�C(6)�C(7) [178.61�] and O(2)�C(5)�N(1)�N(2) [3.22�] indicate that O(1)�O(2), C(3)eN(1) and O(2)eN(2) arecis to each other but C(3)�O(2) and N(1)�C(7) are trans to eachother. The average torsion angle (0.56�) of ferrocene moiety showsthat the two cyclopentadienyl rings exist in almost fully eclipsedform in the ferrocene moiety.

Fig. 5. ORTEP diagram of fcfh showing atomic numbering scheme with ellipsoids of30% probability.

Table 3Selected bond lengths (Å) and bond angles (

�) of fcfh.

Bond lengthsO(1)eC(1) 1.361 N(2)eC(6) 1.279C(1)eC(2) 1.322 C(6)eC(7) 1.446C(4)eC(5) 1.470 C(8)eC(9) 1.411C(5)eO(2) 1.225 C(12)eC(13) 1.385C(5)eN(1) 1.357 C(7)eFe 2.041N(1)eN(2) 1.384 C(14)eFe 2.027C(9)eC(10) 1.410 C(15)eFe 2.031C(10)eC(11) 1.409 C(16)eFe 2.025C(11)eC(7) 1.431Bond anglesC(1)eO(1)eC(4) 105.52 C(11)eFeeC(7) 41.12C(4)eC(5)eO(2) 122.82 C(12)eFeeC(13) 39.73O(2)eC(5)eN(1) 123.64 C(7)eFeeC(15) 109.29C(5)eN(1)eN(2) 119.25 C(15)eC(16)eC(12) 108.67C(6)eC(7)eC(8) 128.67 C(7)eFeeC(8) 40.97Hydrogen bondO(2)/HeN(1) 2.161 N2eH]O2 2.161

Table 4Variation of inhibition efficiency and different electrochemical parameters for mildsteel in 0.5 M H2SO4 in absence and presence of different concentrations of theinhibitors at 298 K.

Inhibitor Cinh (ppm) Weight lossmeasurement

EIS measurement

CR h Rct CPE h

Y0 n

mg cm�2 h�1 % U cm2 mU sn cm�2 %

Blank e 3.193 e 9.8 126 0.965 e

fcph 10 1.488 53.4 21.5 112 0.924 54.520 1.028 67.8 31.0 102 0.975 68.440 0.669 79.0 51.0 99 0.930 80.0

100 0.295 90.7 73.4 76 0.907 86.5fcfh 10 1.069 66.5 27.8 109 0.950 64.7

20 0.728 77.2 45.2 94 0.907 78.040 0.600 81.2 60.7 98 0.909 83.7

100 0.229 93.0 110.4 71 0.915 91.1

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143 141

Corrosion inhibition study

Weight loss measurementsThe experimental data (Table 4) clearly show an increase in

inhibition efficiency from 53.4 to 90.7% and 66.5 to 93.0%, respec-tively when concentration of fcph and fcfh is increased from 10 to100 ppm. Both the ferrocene Schiff bases act as effective corrosioninhibitors for mild steel corrosion in acidic medium. On the basis ofthe inhibition effect observed in aerated 0.5 M H2SO4 solution, it isinferred that fcfh is better inhibitor than fcph. Corrosion inhibitionefficiency of a compound as a corrosion inhibitor depends on itsability to be adsorbed on the metal surface. Both the ferroceneSchiff bases inhibit the corrosion rate by getting adsorb at the steelsurface via the oxygen (>C]O) and nitrogen atoms (>C]Ne). Thepresence of extra oxygen atom and p-electrons of furan ring in fcfhenhances its adsorbability and hence increases the efficiency.Higher difference in the inhibition efficiency values of fcfh and fcphwas observed at lower concentration. At the lower concentration ofinhibitor, sufficient number of active centres on the steel surface isavailable and hence the difference in inhibition efficiency depends

Fig. 6. Diagram showing inter-molecular hydrogen bonding in fcfh.

solely on the adsorbability of inhibitor. However, at higher con-centrations, the active sites on the surface of the steel are nearlysaturated and therefore, inspite of lager adsorbability of fcfh, thedifference in their inhibition efficiencies is small.

Electrochemical measurements

Electrochemical impedance spectroscopy (EIS). The observed singlesemicircle in the inhibited as well as uninhibited solutions showsthe presence of a single charge transfer process during dissolutionwhich is unaffected by the presence of inhibitor molecules. Theslightly depressed nature of the semicircle which has the centrebelow the x-axis, is the characteristic for solid electrodes and suchfrequency dispersion has been attributed to the roughness andother inhomogeneties of the solid electrode [33,34]. The surfaceroughness of the metal is very likely to be caused by metaldissolution during the attainment of open circuit potential. TheNyquist plots were analysed by fitting the experimental data to asimple equivalent circuit model shown in Fig. 7 This includes thesolution resistance Rs and the constant phase element (CPE)which is placed in parallel to charge transfer resistance element,Rct. To compensate for non-homogeneity in the system, the ca-pacitances were ascribed as a constant phase element (CPE),defined by two values, Y0 and n. The impedance, Z, of CPE is pre-sented by Eq. (4).

Fig. 7. Nyquist diagrams for mild steel in 0.5 M H2SO4 containing without and with 10and 100 ppm concentrations of the inhibitors at 298 K.

Table 5Linear polarization parameters for corrosion of mild steel in 0.5 M H2SO4 in theabsence and presence of different concentrations of the inhibitors at 298 K.

Inhibitor Cinh (ppm) LPR Potentiodynamic polarization

Rp (U cm2) h (%) Ecorr (mV) icorr (mA cm�2) h (%)

Blank e 15 e �513 2346 e

fcph 10 32 53.8 �491 1050 55.220 45 66.6 �498 752 68.040 65 76.9 �552 564 76.0

100 122 87.7 �537 317 90.0fcfh 10 41 63.4 �457 763 67.4

20 61 75.6 �499 560 76.140 72 79.2 �566 463 80.2

100 143 89.5 �554 207 91.2

S.R. Gupta et al. / Journal of Organometallic Chemistry 767 (2014) 136e143142

ZCPE ¼ Y�10 ðiuÞ�n (4)

where, Y0 is a proportionality factor and ‘n’ has the meaning ofphase shift. The value of ‘n’ represents the deviation from the idealbehaviour and it lies between 0 and 1 [35]. The values of Rs, Rct andCPE were obtained from the above mentioned equivalent circuitand are presented in Table 4. The value of Rct increases, while thedouble layer capacitance decreases with the change of inhibitorconcentrations. The increase in Rct values is attributed to the for-mation of an insulating protective film at the metal/solutioninterface. The decrease in CPE values can be attributed to a decreasein local dielectric constant and/or to an increase in the thickness ofthe electrical double layer, suggesting that the inhibitor moleculesare adsorbed at the metal/solution interface [36,37].

The impedance parameter Rct obtained from these curves isgiven in Table 5. The charge transfer resistance of blank solution(9.8 U cm2) regularly increases from 21.5 to 73.4 U cm2 as theconcentration of fcph increases from 10 to 100 ppm. In case of fcfh,it increases from 27.8 to 110.4 U cm2 under similar conditions. Thecorresponding inhibition efficiency values increase from 54.4 to86.5 and from 64.7 to 91.1% for fcph and fcfh, respectively. The in-crease in Rct value is attributed to the formation of a protective filmon the metal/solution interface [38,39]. The CPE of blank solution(126 mU sn cm�2) decreases to 76 mU sn cm�2 and 71 mU sn cm�2 inpresence of 100 ppm of fcph and fcfh, respectively. The lowering ofCPE values occur due to a decrease in local dielectric constant and/

Fig. 8. Potentiodynamic polarization behaviour of mild steel in 0.5 M H2SO4 in theabsence and presence of 10 and 100 ppm concentration of the inhibitors at 298 K.

or an increase in the thickness of the electrical double layer [40].This indicates that both the inhibitors get adsorbed at the metal-solution interface and thereby the thickness of the double layer isincreased.

Potentiodynamic polarization measurements. The influence of in-hibitors on the kinetics of the partial cathodic and anodic reactionswas evaluated from potentiodynamic polarization experiments.Fig. 8 exhibits the Tafel polarization curves for mild steel in 0.5 MH2SO4 solution containing 10 and 100 ppm of each of the inhibitors(fcph and fcfh) and in blank (0.5 M H2SO4). From the results shownin Table 5, no systematic variation in Ecorr is seen with increase inconcentration of fcfh and fcph. The average displacement isobserved to be ~36 mV with respect to OCP in either direction. Theobserved variation in Ecorr is much less than ±85 mV and hence itcannot be classified as anodic or cathodic inhibitors [40]. Further,the decrease in anodic and cathodic current densities at a givenapplied potential indicates that these compounds inhibit both thereduction of hydrogen ion as well as the dissolution of metal. Thissuggests that fcph and fcfh are mixed inhibitors.

From the linear polarization studies, it has been observed thatthe polarization resistance Rp increases from 15 U cm2 for the blanksolution to 122 U cm2 and 143 U cm2 containing 100 ppm con-centration of fcph and fcfh, respectively (Table 5). The increase inpolarization resistance in the presence of the inhibitor suggeststhat a poorly conducting physical barrier is formed at the metalelectrolyte interface. This barrier is formed due to the adsorption offcph and fcfh on mild steel surface giving the highest inhibitionefficiency of 90.0 and 91.0% at 100 ppm for which the polarizationresistance is the highest.

Conclusions

This paper describes the synthesis of two new Schiff basesferrocene carboxaldehyde propanoylhydrazone (fcph) and ferro-cene carboxaldehyde furoylhydrazone (fcfh). The structures ofthese compounds have been elucidated on the basis of IR, 1H and13C NMR and electronic spectral studies. The molecular structuresof the Schiff bases were finally confirmed by X-ray crystallography.They exhibit reversible single electron redox behaviour. Both theSchiff bases show excellent corrosion inhibition activity againstcorrosion of mild steel in 0.5 M H2SO4 solution.

Acknowledgement

The authors are thankful to the Head, Department of Chemistry,Banaras Hindu University for providing laboratory facilities. Two ofthe authors (V.P.S and S.R.G) are also grateful to UGC, New Delhi forproviding financial assistance.

Appendix A. Supplementary material

CCDC 930198 and 972710 contain the supplementary crystal-lographic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Appendix B. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jorganchem.2014.05.038.

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