Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones

Arabian Journal of Chemistry (2013) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones Aziz Ahmed a b

a,*

, Ram A. Lal

b

Department of Chemistry, Indian Institute of Technology, Patna-13, Bihar, India Department of Chemistry, North Eastern Hill University, Shillong-793022, Meghalaya, India

Received 3 September 2012; accepted 21 December 2012

KEYWORDS Copper complexes; Disalicylaldehyde succinoyland adipoyldihydrazones; ESR

Abstract The complexes [Cu(H2L)]SO4 in which H2L represents H4slsh and H4slah have been prepared and characterized by a variety of physico-chemical techniques. Magnetic and spectral evidences support a square-planar geometry for the complexes. These complexes have been characterized by micro analytical analyses, FT-IR, UV–Vis, CV, and ESR spectroscopy. The electrochemical behaviour of these complexes at a glassy carbon electrode in DMSO solution indicates one electron transfer reduction waves. ª 2013 King Saud University. Published by Elsevier Ltd. All rights reserved.

1. Introduction The ligands which have been employed for the synthesis of the complexes in the present study have been derived from the condensation of succinoyl- and adipoyl-dihydrazines with salicylaldehyde. (Dutta and Sengupta, 1971) and Kher et al., 1979. reported that acyl and aroyldihydrazines derived from aliphatic dicarboxylic acids, containing two –CONHNH2 groups have been reported to coordinate as a neutral bidentate ligand through two imino nitrogens (>NH) although the potentially strong donor functions like >C‚O and –NH2 is present. It has been suggested that strong intramolecular * Corresponding author. Tel.: +91 612 2552070. E-mail address: [email protected] (A. Ahmed). Peer review under responsibility of the King Saud University.

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hydrogen bonding, as follows Scheme I, probably makes them unavailable for coordination (Lal et al., 1987; Lal, 1978). However, when methylene or phenyl functions are substituted by pyridyl functions in dihydrazines, the resulting picolinoyldihydrazine coordinates to the metal centre as a neutral pentadentate ligand through pyridyl nitrogen atom, two >C‚O and two –NH2 groups (Shani et al., 1977). It appears that these ligands, because of the presence of pyridyl nitrogen atom, make >C‚O and –NH2 groups reactive. On the other hand, if the dihydrazones are derived from the condensation of acyl-, aroyl- and pyridoyl-dihydrazines with o-hydroxy aromatic aldehydes and ketones, they possess – OH group in addition to >C‚O and >C‚N groups (ElToukhy et al., 1982; Iskander et al., 1976; Narang and Lal, 1978). In such dihydazones, the possibility of the existence of new hydrogen bonding in solid, as shown in Scheme II cannot be ruled out. The ligands disalicylaldehyde succinoyl- and adipoyldihydrazones selected in the present study have been derived from the condensation of succinoyl- and adipoyldihydrazines with

1878-5352 ª 2013 King Saud University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.arabjc.2012.12.026 Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

2

A. Ahmed, R.A. Lal (CH2)n O

C

N

N H

H

M

C N H

H

O H N H

Scheme I

The Cyclic voltammetric measurement of the complexes in DMSO (103 M) was done using CH Instruments Electrochemical Analyser under nitrogen atmosphere. The electrolytic cell comprises of 3-electrodes. The working electrode was a glassy carbon disc from BAS and the reference electrode was an aqueous SCE or Ag/AgCl separated from the sample solution by a salt bridge, 0.1 M TBAP was used as the supporting electrolyte. 2.2. Synthesis

O H C

N N H

(CH2)n

H C N N O H

H C

The complexes were synthesized by the following general method.

O O

2.2.1. Preparation of [Cu(H4L)]SO4 (H4L = H4slsh (1), H4slah (2)) CuSO4Æ5H2O (2.10 g, 8.42 mmol) was dissolved in methanol– water mixture (70 mL + 25 mL) by slight warming. H4slsh (1.00 g, 2.83 mmol) suspended in methanol–water mixture (20 mL + 60 mL) under hot condition was added to CuSO4 solution and stirred for 30 min. The resulting solution was refluxed for 6 h. The precipitate so obtained was washed three times with each of the solvents methanol–water mixture (40 mL + 40 mL), methanol, ether and vacuum dried. Yield: 0.79 g. The complex (2) was also prepared essentially by the above method using adipoyldihydrazine (H4slah) instead of succinoylhydrazine (H4slsh). Yield: 0.81 g.

C H

n = 2, 4

Scheme II

salicylaldehyde in 1:2 M ratio. These ligands may exist in diketo, keto-enol and dienol forms, respectively and are among examples of few ligands which offer a chemically flexible ligand framework, which permits the free rotation of two hydrazone groupings about C–C bonds. In view of the importance of copper in the biological system, in magnetochemistry and the meagre amount of work on the synthesis and characterization of copper(II) complexes derived from the reaction of copper(II) sulphate with the titled dihydrazones. The composition of the isolated complexes has been judged mainly from the elemental analyses. The structure of the copper(II) complexes have been discussed in the light of the molar conductance, magnetic moment, electronic, infrared, cyclic voltammetry and ESR spectroscopic observations. 2. Experimental 2.1. Materials and physical measurements CuSO4Æ5H2O, hydrazine hydrate (N2H4ÆH2O), salicylaldehyde (C6H4(OH)CHO), diethylsuccinate (CH2)2(COOEt)2, diethyladipate (CH2)4(COOEt)2 were E-Merck or equivalent grade reagents and were used without further purification. The organic solvents viz., methanol, diethylether, dimethylsulphoxide (DMSO) were purified by standard literature procedures. Copper content of the complexes was estimated gravimetrically as copper(I) thiocyanate (Vogel, 1989). Room temperature magnetic susceptibility measurements were carried out on Sherwood Scientific Magnetic susceptibility Balance MBS-Auto. The molar conductance of the complexes at 103 M dilution in DMSO solution was measured on Wayne Kerr Precision Component Analyser 6440B with a dip-type conductivity cell at room temperature. Infrared spectra were recorded on Bomen A-8FT-IR Spectrophotometer in the range 4000–450 cm1 in KBr discs. The electronic spectra were recorded on a Perkin Elmer Lamda 25 UV/Vis spectrophotometer using DMSO solution. The ESR spectra of the complexes were recorded in DMSO solution at LNT at Xband frequency on Varian, E-112E-line century series, ESR spectrometer using TNCE (g = 2.0017) as an internal marker.

3. Results and discussion Disalicylaldehyde succinoyl- and adipoyl-dihydrazones are polyfunctional ligands containing functional groups like amide, azomethine and phenol each in duplicate. They have been derived from the condensation of succinoyl- and adipoyldihydrazides with salicylaldehyde. (Bosnich et al., 1968; Weatherburn et al., 1970) have reported the effect of ring size on the stability of polyamine complexes containing linked consecutive rings from their studies. They have concluded that the 5-membered ring becomes sterically constrained on coordination to a metal and that the replacement of 5-membered ring by a 6-membered ring can reduce steric constraints and lead to considerable increase in the stability of the complexes. In general, a 6-membered chelate ring is much more sterically stable than the 5-membered chelate ring. IIa

Ia

(CH2)4 C

C

O

O

2+

2+

M

M

C

C

O

O M2+

M2+ II'a

I'a

(CH2)4

M 2

C

C

C

C

O

O

O

O

M

M 2

M 2

2

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

Synthesis, characterization and electrochemical studies of copper(II) complexes

The complexes together with their characterization data are set out in Table 1. The copper(II) complexes are green in colour. They are all air stable and decompose above 300C. They are insoluble in water and common organic solvents such as ethanol, methanol, acetone, CCl4, CHCl3, diethylether, benzene, dichloromethane, acetonitrile except in the highly coordinating solvents like DMSO and DMF in which they dissolve in hot condition. None of the complexes showed weight loss either at 110C or at 180C ruling out the possibility of the presence of water molecules either in their lattice structure or in the first coordination sphere around the metal centre (Lal et al., 1997).

During the course of reaction of the dihydrazones with the metal ions, the aldimine parts of the dihydrazones would always form a 6-membered chelate ring in each case while the central parts would give either no ring or rings of different sizes depending upon whether it would react either with the different metal ions or the same metal ion. If the carbonyl oxygens of the central part of the dihydrazone ligands coordinate either as such or in the enolized form to the different metals as shown below, this would result in the complexes which would be expected to have almost equal stability. On the other hand, if the coordination occurs to the same metal ion, the different stability of the resulting complexes would be expected. Ib

3.1. Molar conductance The molar conductance of the complexes falls in the region 2.2–2.6 S m2 mol1 indicating that they are non-electrolytic in DMSO solution (Geary, 1971).

IIb (CH2)4

C

C

O

O 2+ M

C

C

O

2+ O M

3.2. Magnetic moment Copper(II) complexes show magnetic moment values in the range of 1.85–1.87 BM Table 1. This value is close to the spin-only value of 1.73 BM (Cotton and Wilkinson, 1988). This indicates the absence of any appreciable spin–spin coupling between unpaired electrons belonging to different copper atoms. According to (Figgis, 1958), a magnetic moment value >1.90 BM indicates a tetrahedral stereochemistry while <1.90 BM is indicative of square planar as well as octahedral stereochemistry. This shows that the magnetic susceptibility is not of much use in deciding on the stereochemistry of Cu(II) complexes.

II'b

I'b

(CH2)4 C

C

O

2+ O M

Seven-membered ring

3

C

C

O

2+ O M

Nine-membered ring

The above representations explicitly show that the succinoyl- and adipoyldihydrazide based dihydrazones result in 7and 9-membered metal chelate rings, respectively. The formation of 7- and 9-membered metal chelate ring system is sterically less favoured than the 6-member ring. In the light of the above views and discussion, it can be reasoned that central parts of the dihydrazones would lead to their different reactivity and would, thus, give complexes having different stability. 1 H NMR spectra of the dihydrazone H4slah shows four signals (two doublets) corresponding to –OH groups at d 11.54 and 11.30 ppm while H4slsh shows only two signals at d 11.27 and 11.64 ppm corresponding to –OH groups (Table 2a). The enolization processes in H4slsh and H4slah involving secondary –NH proton only may be represented, as below.

3.3. Electronic spectra The electronic absorption spectra of the ligands and their complexes along with the molar extinction coefficient were recorded at room temperature using DMSO as the solvent. The important electronic spectral bands for dihydrazone ligands H4slsh and H4slah and their monometallic complexes along with molar extinction coefficients have been given in Table 1. The electronic spectra are used to assign the stereochemistry of the metal ion in the complexes based on the position and number of d-d transition bands. The absorption spectra of the ligands H4slsh and H4slah are characterized by two bands in the reH

H

H (CH2)n

N

N

(CH2)n

N

N C

O

O

N n= 2, 4

C

C C

(CH2)n

N

C

C O OH Y

When CuSO4Æ5H2O is allowed to react with preformed dihydrazones in 3:1 M ratio, complexes of the composition [Cu(H4L)]SO4 are obtained. 3:1

CuSO4  5H2 O þ H4 L !½CuðH4 LÞSO4 þ 5H2 O ¼ H4 slshð1Þ; H4 slahð2ÞÞ

OH

OH

X

ðH4 L

Z

gions 283–285 and 322 nm, respectively. These bands are assigned to arise due to intraligands p fi *p and n fi *p transition due to >C‚N and >C‚O groups in the ligands (Cohen and Flavian, 1967; Gegiou et al., 1996; Sorrell, 1989). The band at 322 nm is considered as the characteristic band for the salicylaldimine fraction of the ligands as has been observed in

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

4.07 (4.10) >300 [Cu(H4slah)]SO4 Green 2

63

11.51 (11.72)

44.71 (44.32)

10.52 (10.33)

18.03 (17.72)

1.87

2.6

283(33333), 322(11700) 285(39300), 322(32500) 271(2410), 305(2500), 318(8950), 383(7400), 402(8000), 648(105) 290(1190), 304(1280), 318(10000), 382(10400), 403(8400), 650(109) 2.2 1.85 15.69 (15.81) 14.59 (14.64) 11.21 (10.90) 5.15 (5.12) 5.79 (5.80) 3.51 (3.54) 61.23 (61.01) 62.69 (62.82) 42.25 (42.06) 250 >300 >300 H4slsh H4slah [Cu(H4slsh)]SO4 Green L1 L2 1

85 86 66

– – 12.13 (12.36)

H C Cu

D.P (C) Complex and colour Sl. no.

Yield (%)

Elemental analysis:found (cal)%

N

SO2 4

– – 18.71 (18.69)

Magnetic moment

Molar conductance (X1 cm2 mol1)

Electronic spectral bands kmax (nm), emax (dm3 cm1 mol1)

A. Ahmed, R.A. Lal Table 1 Complexes, colour, decomposition point, elemental analyses, magnetic moment, molar conductance and electronics for monometallic copper(II) complexes of disalicylaldehyde succinoyl- and adipoyl-dihydrazones.

4

monoacylhydrazones (Hussain et al., 1991). Both these bands undergo splitting into either two or three components out of which one component is blue shifted while the other components are red-shifted. The first component splits at 283 nm and shifts to shorter wavelength by 12 nm in the complex (1) and appears at 273 nm while the other bands are shifted to longer wavelength by 5–22 in the regions of 290–305 nm, respectively. The band at 322 nm splits into two components. First component is blue shifted by 5 nm and appears at 318 nm while the second component shows considerably high red shift ca 60 nm and appears in the region of 382– 383 nm. Such splitting of the ligand bands in the spectra of the complexes suggests bonding of ligands to the metal centre. The complexes show an additional strong shoulder in the region of 402–403 nm with very high molar extinction coefficient and lie in the region of 8000–8400 dm3 mol1 cm1. In view of their very high molar extinction coefficients they are assigned to have their charge transfer character arising from the transfer of charge from the ligand to the metal centre, most probably, from the phenolate oxygen atoms. The complexes show a single broad band in the region of 648–650 nm with a comparatively very low molar extinction coefficient in the range of 105–109 dm3 mol1 cm2. Hence, this band is assigned to arise due to d-d transition (Hathaway, 1987). In the octahedral (Lever, 1968; Tomlinson and Hathaway, 1970) and tetrahedral (Dunn, 1960) complexes of copper(II), the bands due to d-d transition occurs at 800 nm and 1200 nm, respectively. In octahedral complexes, the band at 800 nm is considerably blue shifted due to Jahn–Teller distortion and in an extreme case, it falls in the range of 600–700 nm reported for the square-planar complexes (Sacconi and Ciampolini, 1964). Thus, the band in the range of 648–650 nm with a low molar extinction coefficient is assigned to have its origin due to d-d transition. The essential feature of this band suggests that it is the combination of three transitions (2B fi 2A1g, 2B1g fi 2B2g and 2 B1g fi 2Eg). Thus, it may be concluded that the copper(II) complexes have square-planar geometry. 3.4. Electron spin resonance spectroscopy The monometallic complexes (1–2) have been studied by ESR spectroscopy at LNT in a glassy state. The ESR parameters for the complexes have been set out in Tables 2a, 2b. ESR spectra of the complex (1) are shown in Fig. 1. The complexes show anisotropic spectra and have almost similar ESR spectral features. The g|| value for the complexes lies in the region of 2.273–2.291 while g^ value lies in the region of 2.093– 2.097. The g|| and g^ values depart considerably from the free ion value. The shifting of g values from 2.0023 in transition metal complexes is due to mixing, via spin–orbit coupling of the metal orbitals containing the unpaired electron(s), with the empty or filled ligand orbitals. When the mixing is with empty ligand orbitals, the result is a negative g shift, whereas the mixing with the filled ligand orbitals leads to a positive g value shift. The shift depends on the amount of unpaired electron density at the donor sites of the ligands i.e., on the degree of covalency of the complex. The ESR spectra of the complexes (1) and (2) show splitting in the g|| region. The nuclear quantum number of copper is 3/2, hence it should show four signals. However, all the signals are not observed in the complexes. (Kivelson and Neiman, 1961) have reported that the

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

Synthesis, characterization and electrochemical studies of copper(II) complexes

5

Figure 1

ESR Spectrum of [Cu(H4slsh)]SO4 (1) in DMSO at LNT, Frequency 9.1 GHz, Scan range 2000 G and Field set 3000 G.

Table 2a

1

H NMR spectral data (in ppm) of succinoyl- and adipoyl-dihydrazones.

Ligand

o´–CH2

o´-phenyl

o´–CH‚N

o´–(NH)

o´–(OH)

H4slsh H4slah

2.07 (s) 2.59 (s) 1.68(s) 2.07(s) 2.25(s)

6.63–7.50(m) 6.70–7.80(m)

8.81(s) 8.26(s) 8.18(s) 8.21(s)

10.15(s) 10.36(s)

11.27(s) 11.64(s) 11.54(d, 66 Hz) 11.30(d, 66 Hz)

Table 2b

Magnetic parameters for monometallic Cu(II) complexes of disalicylaldehyde succinoyl- and adipoyl-dihydrazones.

Sl. no.

Complexes

Temp.

Solid/solution

g||

g^

gav

A||(G)

A^ (G)

G

a2

1 2

[Cu(H4slsh)]SO4 [Cu(H4slah)]SO4

LNT LNT

Solution Solution

2.273 2.291

2.097 2.093

2.155 2.159

160 175

-

2.814 3.129

0.794 0.852

g|| value in copper complexes can be used as a measure of the nature of the metal–ligand bond. If this value is more than 2.3, the environment is essentially ionic and values less than this limit are indicative of a covalent character. The fact is that g|| values for the complexes (1) and (2) are less than 2.3. This indicates that the metal–ligand bonds in these complexes have covalent character. Also the shape of the ESR lines indicates that the geometry around the copper(II) ion is not trigonal bipyramidal in these complexes since the low field side of the ESR spectrum is less intense than the high field side and the order of g^ values is not in accordance with the range suggested for trigonal bipyramidal complexes (Park et al., 2001; Ye et al., 2004) (2.00 > g|| > g^). The magnetic parameters

indicate g|| > g^ > free spin (2.0023) which shows that the unpaired electron is in the dx2y2 orbital of the Cu(II) centre. The in-plane a covalency parameter, a2Cu was calculated for the Cu(II) complexes using the following equations (Sujatha et al., 2000; Syamal and Dutta, 1993a) and the values obtained are listed in the Table 2b. a2Cu ¼ ðAk =PÞ þ gk  2:0023Þ þ 3=7ðg?  2:0023Þ þ 0:04 where P = 0.036 cm1. The a2Cu value accounts for the fraction of unpaired electron density on the copper ion. Smaller the value of a2Cu , more covalent is the bonding nature. For example a2Cu ¼ 0:5 indicates complete covalent bonding, but a2Cu ¼ 1:0 suggests complete

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

6 ionic bonding. The a2Cu values for the Cu(II) complexes are in the range of 0.794–0.852 < 1 indicating that the Cu(II) complexes have considerable amount of covalent character. Suresh Babu et al. (1997a) and Djebbar-Sid et al. (1997) reported that g|| > 2.4 for copper-oxygen bonds and 2.3 for copper–nitrogen bonds. The Cu(II) complexes (1–2) have g|| values between 2.273–2.291 and are in agreement with the presence of mixed copper-oxygen and copper–nitrogen bonds. The nature of the ligand forming the complex is evaluated from G values obtained by using the following equation: G ¼ ðgk  2Þ=ðg?  2Þ If G < 4.0, the ligand forming the complex is regarded as a strong field ligand. For the square planar complexes G is usually in the range (Syamal and Dutta, 1993b) of 2.03–2.45. G value for the complexes (1–2) lies in the range of 2.814–3.129 at LNT which suggests that the dihydrazone ligands have a sufficiently strong field in the complexes. 3.5. Infrared spectroscopy The uncoordinated ligands show a medium intensity band in the region of 3065–3204 cm1 and a strong band at 3449 cm1. The band at 3449 cm1 is assigned to mOH of salicylaldehydato part of the dihydrazones, while a lower frequency band is assigned to arise from secondary >NH stretching vibration. The dihydrazones show a very strong intensity band in the region of 1666–1672 cm1 which is assigned to mC‚O stretching vibration (Silverstein and Bassler, 1967). The essential features of the IR spectra of the complexes consist of bands at 3502, 3383 and 3423, 3230 cm1 in the region of 3000–3600 cm1 which are similar to those of the uncoordinated ligands but they are slightly broadened. The bands at 3502 and 3423 cm1 are assigned to the mOH vibration of uncoordinated phenolic-OH groups while the bands at 3383 and 3230 cm1 are assigned to arise due to mNH vibration. These bands are shifted to higher frequency by 152, 179 and 36 cm1 in the complexes as compared to their position in the uncoordinated dihydrazones. Such features of mOH and mNH bands in the ligands rule out the possibility of the involvement of phenolic –OH and secondary –NH groups in bonding with the metal centre. Further, the presence of a mNH band in the spectra indicates that the ligands coordinate to the metal centre in keto form. Another prominent feature of the IR spectra of the complexes is the appearance of a cluster of medium intensity bands in the region of 2400–2800 cm1 in the complexes. This band is characteristic of the occurrence of stronger intramolecular H-bonding in the complexes involving –OH groups (Dyer, 2004). The ligand bands appearing at 1666 and 1672 cm1 due to >C‚O vibration show a large negative shift by 56 cm1 in the complex (2). In the complex (2), the mC‚O band appears at 1616 cm1 as a very strong band. On the other hand, in the complex (1), mC‚O band merges with mC‚N band and appears at 1579 cm1. This shows the coordination of both the >C‚O groups to the metal centre (Lal et al., 2010). The ligands show strong bands at 1626 and 1619 cm1 which arise due to stretching vibration of >C‚N groups. These bands are shifted to lower frequencies by 47 and 33 cm1, respectively. The lower shift of mC‚N band by such a large frequency indicates a strong coordination of the ligands to the

A. Ahmed, R.A. Lal metal centre through >C‚N groups (Naskar et al., 2007). The amide II band appearing at 1566 cm1 is shifted to lower frequency by 36 cm1 and appears as a strong shoulder band at 1530 cm1 in the complex (1) but it does not show its independent appearance in the complex (2). The negative shift of the amide II band in the complexes suggests coordination of the >C‚O group to the metal. The most crucial feature of the IR spectra of the complexes is the absence of a new strong band in the region of 1500–1530 cm1, which arises usually due to mNCO resulting from enolization of the hydrazide ligands. This ruled out the possibility of enolization of ligands in the complexes and suggested the co-ordination of dihydrazones to the metal centre in the keto form. The mN–N band is observed at 1049 and 1036 cm1 in the free ligands H4slsh and H4slah, respectively. These bands shift to higher frequency by 5–8 cm1 and appear at 1054 and 1044 cm1, respectively, as shoulder. This indicates the involvement of only one of the nitrogen atoms of the >C‚N–N‚C< group in coordination to the Cu(II) centre in the complexes. The complexes (1) and (2) show a new very strong band at 1161 and 1128 cm1, respectively. These bands appear at almost the same frequency region in which ionic sulphates have been reported to absorb (Nakamoto et al., 1957). Further, these bands do not show any splitting in the complexes which rules out the possibility of coordination of the sulphato group to the metal centre. These complexes also show a single medium intensity band at 611 and 624 cm1, respectively. The position and essential features of these bands suggest that the sulphato group is present in ionic form in the complexes. 3.6. Low frequency region 600–450 cm1 Low frequency IR spectra of the metal complexes provide very useful information regarding the type of metal–ligand bond arising from the coordination of ligands to the metal centre. However, the IR spectra of the ligands containing aromatic rings which show bands due to out-of-plane ring (Nagano et al., 1964) deformation and out-of-plane deformation C–H bonding (Benlly and Wolforth, 1959) in the low frequency region, complicate the spectral features. The present ligands are the combination of succinoyl- and adipoyl- hydrazide and salicyladimine parts. From a stereochemical consideration of the ligands, it might be expected that a maximum of four kinds of metal–ligand vibrations may arise viz, m(M–O)(phenolate), m(M–O)(M–O–C< or M ‹ O‚C<) and m(M–N) in the complexes. The m(M–O)(phenolate) is expected at higher frequency because of differences in the bond order. Further, the low frequency IR spectra are expected to show bands due to coordination of dihydrazones and water molecules. A review of literature related with metal-phenolate (Pery, 1975) metal-enolates (M ‹ O‚C< and M–O–C<) Mikani et al., 1967 and metal hydrazine m(M–N) Clark, 1962; Clark and Willims, 1964; 1985 indicates that the various metal–ligand stretching frequencies occur in the overlapping regions. In metal acetylacetonates (Nakamoto and Martell, 1960) and oxalates (Sacconi et al., 1964), m(M–O) is observed in the 470–420 and 446–419 cm1 regions, respectively, while in metal salicyaldimines, the m(M–O) is observed in the 600–490 cm1 region (Percy and Thornton, 1982). The m(M–N) in salicylaldimine complexes has been reported to occur in the 440–360 cm1 range. Keeping all the assignments in view, the m(M–L) bands

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

Synthesis, characterization and electrochemical studies of copper(II) complexes Table 3

7

IR spectral data (in cm1) for disalicylaldehyde succinoyl- and adipoyl-dihydrazones and their copper(II) complexes.

Sl. Ligands/ no. complexes

tOH + NH

L1 H4slsh

3449 s 3204 m 3065 m 3204 s 3065 s 3502 s 3383 m 3423 m 3230 w

L2 H4slah 1 [Cu(H4slsh)]SO4 2 [Cu(H4slah)]SO4

tC‚O tC‚N AmideII tNCO tC–O tN–N + tC–O Phenyl

Sulphato m3

m4 –

tM–O tM–O Phenolic Carbonyl

1666vs 1626 m 1566 m

–

1281 s 1049 w –

1672vs 1619 s 1566 s – 1579vs 1530 ssh 1616vs 1586vs –

– – –

1275 s 1036 w – – – 1294 s 1054 w 1161 s 1002 w 611 m – 1294 s 1044 w 1128vs 1028 s 976 w 624 m –

Complex

Epa/V

Epc/V

DEp

1 2

[Cu(H4slsh)]SO4 [Cu(H4slah)]SO4

-

0.21–1.45 0.16–1.13

-

– – 451 w 474 w 439 w

2+

Table 4 The electrochemical parameters for the mononuclear copper(II) complexes, (Pot vs SCE) of disalicylaldehyde succinoyl- and adipoyldihydrazones. Sl. no.

–

H N C

O

N

n(CH2) C

H C

O H H O

Cu O

(SO4)2 2-

N N H

C H

n = 2, 4

Figure 3 Tentative structure of the complexes [Cu(H2L)]SO4, (Where H2L = H4slsh(1), H4slah (2)).

have been assigned in the metal complexes in the present study. The complexes do not show any new band in the region of 500–600 cm1 which could be assigned to m(M–O)(phenolate). This ruled out the possibility of coordination of phenolate oxygen to the metal centres. The complexes show a new weak band at 451 and 474 cm1 which could be assigned to m(M– O) carbonyl indicating the coordination of carbonyl oxygen form in the metal complexes.(see Table 3)

The ligands were non-electroactive in this potential range. The complexes showed no redox activity either in the potential range +1.60 to +2.00 or 1.60 to 2.00 V. This is true regardless of the scanning direction i.e. whether the starting point is 0.00 or 2.00 V. It should be mentioned that the supporting electrolyte TBAP in DMSO did not show any redox activity in the potential range studied. The cyclic voltammogram complex (1) is shown in Fig. 2. The copper(II) complexes exhibit well defined one electron transfer reduction waves in the range of +0.30 to 1.70 V. In the case of [Cu(H4slsh)]SO4 (1), the first irreversible one electron transfer peak at +0.21 and the second peak at 1.45 V are observed. The corresponding peaks in the complex (2) are observed at 0.16 and 1.13 V, respectively. Corresponding to these reductive waves there are no oxidative waves in the anodic scan. This is attributed to some unstable species formed in solution which reverts back to the original species. The cathodic reduction potential increases from 0.21 to 0.16 V and 1.45 to 1.13 V when a methylene group is introduced in the aliphatic chain. This behaviour is typical of irreversible charge transfer processes and such behaviour has been observed in other Schiff base complexes (Samy et al., 2008).

3.7. Cyclic voltammetry

4. Conclusion

The cyclic voltammogram of a 2 mm solution of the complexes (1) to (2) has been examined at a scan rate of 100 mV/s by cyclic voltammetry in DMSO solution due to their insolubility in common organic solvents with 0.1 M tetra n-butylammonium perchlorate (TBAP) as a supporting electrolyte. The data have been set out in Table 4. The potentials of the ligands and complexes were scanned in the potential range of 2.4 to 2.4 V.

Copper(II) complexes described in the present study have been synthesized by one general method. On the basis of various physico-chemical and spectral studies, it has been concluded that the ligands succinoyl-(H4slsh) and adipoyldihydrazones(H4slah) coordinate as a neutral tetradentate ligand through azomethine nitrogen and carbonyl oxygen in the complexes (1) and (2). The sulphato group remains uncoordinated

Figure 2 DMSO.

Cyclic voltammogram of [Cu(H4slsh)]SO4 (1) in

Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026

8 in the complexes (1) and (2). Both the copper complexes are normal paramagnetic and do not possess any M–M interaction. The d2x  y2 orbital constitutes the ground state in the complexes. Both the complexes have square-planar stereochemistry. The electron transfer reaction of the complexes has been investigated with the help of cyclic voltammetry. On the basis of the results and discussion given above, the following structure of the complexes (1) and (2) have been proposed as shown in Fig. 3.

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Please cite this article in press as: Ahmed, A., Lal, R.A. Synthesis, characterization and electrochemical studies of copper(II) complexes derived from succinoyl- and adipoyldihydrazones. Arabian Journal of Chemistry (2013), http://dx.doi.org/10.1016/j.arabjc.2012.12.026