Indian Journal of ChemistryVol. 33A, October 1994, pp. 919-923

Synthesis and characterization of copper(II) complexes of physiologicallyactive tridentate schiff bases

K Hussain Reddy* & Y Lingappa

Department of Chemistry, Sri Krishnadevaraya University, Anantapur 515 003

Received 22 November 1993; revised 7 March 1994; accepted 3 May 1994

Copper complexes of 2,4-dihydroxyacetophenone-2-iminoethanethiol(DAET), 2,4-dihydroxyben-zophenone-2-iminoethanethiol (DBET), 2,4-dihydroxyacetophenone-2-iminobenzenethiol (DABT) and2,4-dihydroxybenzophenone-2-iminobenzenethiol (DBBT) have been synthesized and characterized byelemental analysis, conductivity, magnetic susceptibility measurements, IR, electronic and ESR spectraldata. The molar conductivity data show them to be non-electrolytes. The tridentate nature of the li-gands is inferred from IR spectral studies. The electronic spectral data suggest planar geometry for allthe complexes. Various ESR parameters have been calculated. The antibacterial activities of the ligandsand their copper complexes have been screened in vitro against the organisms Escherchia coli (E.C), Ba-cillus (B) and Pseudomonas aeruginosa (P.S). It is observed that the coordination of metal ions has pro-nounced effect on the bacterial activities of the ligands.

Although the derivatives of 2,4-dihydroxyaceto-phenone (resacetophenone) and 2,4-dihydroxy-benzophenone have been used as analytical rea-gents':", the ligational behaviour of schiff bases viz.2,4-dihydroxyacetophenone- 2-iminoethanethiol (I,DAET), 2,4-dihydroxybenzophenone- 2-imino-ethanethiol (II, DBET), 2,4-dihydroxyacetophe-none-2-iminobenzenethiol (III, DABT) and 2,4-di-hydroxybenzophenone- 2-iminobenzenethiol (IV,DBBT) with ONS donor sequence is not investi-gated so far. Recently it has been reported that themetal complexes of ligands containing electronwithdrawing group exhibit improved biologicalactivity". It led us to synthesize and characterizethe aforesaid new schiff bases (I-IV) and theircopper complexes.

Materials and MethodsAll the chemicals used were of AR grade and

used without further purification. The elementalanalyses were performed by the RSIC, CDRI,Lucknow, The infrared spectra were recorded inthe range 4000-180 em - 1 with a Perkin Elmer983 G spectrophotometer. The electronic spectrawere recorded with Cary model 2390 spectropho-tometer. The molar conductance of complexes inDMF (-lO-3M) were determined at 27 + 2°C us-ing a Systronic 303 direct reading conductivitybridge. The magnetic susceptibility measurementswere made using a vibrating sample magnetometer(VSM) operating at a field strength of 2 kG to 8kG. The IH NMR spectra were recorded on var-

ian XL-300 MHz high resolution instrument inCDCl3 solvent. The mass spectra were recordedusing Fanning Mat 8230 Mass spectrometer. Themolecular weight determination of the complexeswas carried out by the Rast's method using cam-phor as solvent. All ESR spectra were obtained onVarian E-112 X-band spectrophotometer at roomtemperature and liquid nitrogen temperature(LNT) in DMF.

Synthesis of /igandsThe ligands (BV) were prepared by the con-

densation of 1:1 molar (0.02 mol) quantities of asuitable ketone with appropriate thiol in methanoland in the presence of sodium acetate. In the pre-paration of DAET and DBET, the reaction mix-ture was stirred at IO'C for 3 h. The reaction mix-ture was refluxed for 4 h for getting DABT andDBBT ligands. The yield, melting point and anal-ytical data of these schiff bases are given in Table1. The infrared spectra of DAET, DBET, DABTand DBBT show bands at 1616, 1629, 1628 and1626 cm' respectively for vC = N; 2850, 2600,2580 ana 2600 em -I for v S - H indicating theschiff base formation. In proton NMR spectra ofDAET, DBET, DABT and DBBT, the signals dueto - SH and - OH protons appeared at 1.79,1.25, 2.52 and 2.50 0; 7.56, 7.64; 7.55, 7.57;7,53, 7.63; and 7.52, 7.60 0 respectively. Themass spectra of DAET, DBET and DBBT showmolecular ion peaks at 209, 274 and 321 (rn/e)respectively corresponding to their molecular

920 INDIAN J CHEM, SEe. A, ocrOBER 1994

RI ./\

.~C=N SH

HO~OH

I. DAfT; R ~CHl

II. DBET; R = C6 '\

R(Q)~i=)-\SH

HO~OH

HI. DABTJ R::C~

IV DBBT; R=C6HS

Table I-Characterization data of. the Schiff base ligands andtheir copper complexes

Complex m.p. DC· Found (Calc.), %(Colour) (Yield, %) _

DAET(Light orange)

[Cu(DAET)h(Green)

DABT(Brown)

[Cu(DABT)lz(Brown)

DBET(Dark yellow)

[Cu(DBET)]z(Yellow)

DBBT(Brown)

[Cu(DBBT)lz(Black)

130-132(60)

173-175(55)

62-64(70)

162-165(65)

75-77(55)

210-212(49)

70-72(80)

145-147(75)

C56.75

(56.87)

43.74(43.71)

64.68(64.82)

52.25(52.09)

65.63(65.93)

53.55(53.49)

70.85(71.02)

59.45(59.35)

H6.06

(6.16)

4.64(4.74)

5.10(5.02)

4.13(4.03)

5.55(5.49)

4.50(4.46)

4.76(4.67)

4.15(3.89)

N

6.55(6.64)

5.25(5.09)

5.38(5.41 )

4.24(4.34)

5.01(5.13)

4.00(4.16)

4.45(4.35)

3.95(3.85)

·Complexes decompose in the temperature range indicated.

weights. DAET M, 209; M - C2H4SH, 151;M - CH3 -C2H4SH, 136; M - CN -CH3C2H4SH,109; DBET; M, 274; M - C2H4SH, 213;M-C6Hs-C2H4SH, 136; M-CN-C6Hs-C2H4SH,

r~C=Nif .r-HO~,' CU~s

0- /l", t.: ,, , ," .

~

'S:', Y,/°'r()Y0H'N=C~o I

R

VI

109; DABT: M, ,M - C6H4SH, 151;M-CH3C6H4SH, 136; M-CN-CH3-C6H4SH,109; DBBT: M, 321; M - C6H4SH, 213;M - C6HS-C6H4SH, 136; M - CN -C6H4-C6HsSH,109.

Synthesis of complexesAn aqueous solution of CuCl2 (0.038 mol) was

added to a solution of the ligand (0.038 mol) in25 methanol. To this mixture, sodium acetate (1ml, 1M) .was added. The product formed washeated on water bath and collected by filtration. Itwas washed several times with hot water, coldmethanol and hexane and then dried in vacuo.

Results and DiscussionThe analytical data for all the complexes are

given in Table 1. The molecular weight determina-tion of the complexes show that these are dimericchelates. The molar conductivity data of the com-plexes are consistent with the non-electrolytic na-ture? of the complexes.

The magnetic moments of [Cu(DAET)]2 and[Cu(DBET)b complexes were found to be 0.59and 0.58 BM respectively suggesting the presenceof some interaction 7 via phenolic oxygen atoms.On the other hand, [Cu(DABT)h and[Cu(DBBT)h complexes showed respectively high-er magnetic moment values of 1.24 and 1.68 BMpossibly due to the longer separations of coppercentres.

The electronic spectral bands are observed at

1

REDDY et al.: Cu(II) COMPLEXES OF TRIDENTATE SCHIFF BASES 921

29410, 28980, 28570 and 28980 em -I for cop-per chelates of DAET, DBET, DABT and DBBTrespectively. However, low intensity bands are ob-served at 16000, 15625, 17860 and 15870 cm-1in the electronic spectra of [Cu(DAET}h,[Cu(DBET)]2' [Cu(DABT)]2 and [Cu(DBBT)hcomplexes respectively. These bands may be as-signed to 2Big ....•2B2g transition't'", suggesting thepresence of square planar geometry.

The ESR spectra of the complexes in polycrys-talline state exhibit only one broad signal which isattributable to dipolar broadening and enhancedspin-lattice relaxation 11 • The anisotropic spectraare obtained for all complexes at liquid nitrogentemperature in DMF medium and a representativeESR spectrum of Cu-DAET complex is given inFig. 1. In this low temperature spectrum, fourpeaks of small intensity have been identified whichare considered to originate from g Icomponent.

The .spin Hamiltonian, orbital reduction andbonding parameters of all the complexes are pre-sented in Table 2. The g u and g 1. are computedfrom the spectrum using DPPH free radical as gmarker. Kivelson and Neiman'? have reported that

) 2800 3200Magnetic field (gauss)

36002400

Fig. l-X-band ESR spectra of lCu(DAET)h ~t liquid nitrog-en temperature in dintethylfonnanude.

gl value is less than 2.3 for covalent charactermetal ligand bond and is greater than 2.3 for iOD1(character. Applying this criterion, the covalentcharacter of the metal ligand bond in complexesunder study can be predicted. The trendgl >g 1. >ge (2.0033) observed for the complexessuggests that the unpaired electron is localised indXLy2 orbital'! of the copper (II) ion. For all thecomplexes, the lowest g value ( > 2.04) is also con-sistent with a dx'-r' ground state. For all the com-plexes (except Cu-DAET), the G'value indicatesthe presence of significant exchange coupling!"!'and misalignment of molecular axes. The G value(> 4) for Cu-DAET complex may support an ideathat it has elongated axial symmetry.

The ESR parameters, g N' g 1.' AI and A 1. ofcomplexes and the energies of d - d transition areused12-18to evaluate the orbital reduction parame-ters (KI, Kj ), the bonding parameters (<<2),thedipolar interaction (P) and the Fermi contact inter-action term (K). The observed K I<K 1. relation(except Cu-DAET) indicates the presence of signi-ficant in-plane pi bonding", The «2 values for thepresent chelates lie in the range 0.62-0.70 suppor-ing the covalent nature of these complexes. Giord-ano and Bereman-? suggest the identification ofbonding groups from the values of dipolar term, P.The reduction of P values from the free ion value(0.036 cm - 1) might be attributable to the strongcovalent bonding. The values of P obtained forthe present complexes lie between 0.013-0.015em - 1 and are consistent with bonding of copperto ONS donor atoms. The lower P and «2 valuesfor Cu-DABT and Cu-DBBT complexes suggest

Table 2-Spin Hamiltonian and orbital reduction parametersof copper complexes

Parameter [Cu(DAET j2[Cu(DBET lb[Cu(DABT lh[Cu(DBBT lJ2

2.297 2.276 2.2352.085 2.080 2.0702.155 2.145 2.1253.56 3.53 3.440.0116 0.0112 0.01150.0032 0.0030 0.00300.0060 0.0057 0.005815625 17860 158700.834 0.859 0.7460.883 0.915 0.8050.698 0.658 0.6210.0140 0.0134 0.01310.276 0.235 0.320

gl 2.295s, 2.055s; 2.135G 5.55Ai 0.0125A*l 0.0037A·a. 0.00662BI,_2B~, 16000

KI 0.840K1 0.713a2 0.702P* 0.0150K 0.307

922 INDIAN J CHEM, SEe. A, OCTOBER 1994

the presence of stronger in-plane rt-bonding whichis in agreement with higher ligand field. The di-mensionless quantity K which is found to have avalue of = 0.30 compares well with those calcu-lated for other copper complexes+F. The shape+'of ESR lines, ESR data (Table 2), together withelectronic spectral data suggest a square planargeometry for the copper complexes.

The disappearance of a weak band at 2850(DAET), 2600 (DBET), 2580 (DABT) and 2600ern- I (DBBT) due to vS - H of the ligands indi-cates the deprotonation of thiolic protons on com-plex formation=r". The participation of phenolicoxygen and thiolic sulphur in coordination to themetal ion is further supported by an upwardshift:" in vC - 0 (phenolic) to the extent of 15-30and a downward= shift in vC - S by 15-25 cm-Iin all copper complexes (except Cu-DABT, Cu-DBBT). The major shift of vC - 0 to higher ener-gy by - 30 em - 1 in the case of[Cu(DAET )]2and[Cu(DBET)b complexes certainly indicate thepresence of phenoxo bridge", The major shift ofv C - S (thiolic) to higher energy by - 25 em - 1 inthe case of [Cu(DABT)]2 and [Cu(DBBT)lz corn-plexes suggests the presence of thiolic sulphurbridge. The lowering of v C = N .of azomethinegroup to the extent of 15-20 cm-I in all the com-plexes suggests the participation'" of azomethinenitrogen in complexation. In the far IR spectral re-gion, additional medium to strong bands at 560-600, 450-470 and 350-400 cm-I are assigned tovM - 0, v M - N and v M - S modes27,28 respect-ively.

Based on the molecular weight determination,molar conductance, magnetic and spectral data, itis suggested that all copper complexes are dimericwith square planar configuration. The magenticand infrared spectral data reveal the presence ofphenolic bridged dimeric structure (V) for[Cu(DAET)b and [Cu-DBET)b and dimeric thiol-ic sulphur bridged structure (VI) for [Cu(DABT)]2and [Cu(DBBT)lz complexes.

All the ligands and their copper complexeswere screened for their antibacterial activityagainst Escherichia coli (E. coli), Bacilius (B) andPseudomonus aeruginosa (P.S) in the range 25 ugto 500 ug using serial dilution technique-F". Oncomparison (Table 3), the hydroxy substitutedschiff base ligands and their copper complexesshowed better activities against bacteria. The pres-ence of additional hydroxyl group in DAET,DBET, DABT and DBBT could decrease theelectron densities on the overall ligand/complexframe work facilitating its interaction with the ami-no acids of the microorganism'. The increased ac-

l

Table 3-Minimum inhibitory concentration (MIC) in u g/rnl

Compound E.coli B P.S

DAET 400[Cu(DAET)h 100 50

DBET 500 400 400[Cu(DBET)]2 275 250 250

OABT* 500 600[Cu(OABT)b 350 250

DABT 400 250[Cu(DABT )]2 275 125

DBBT 400 400[Cu(DBBT)l2 275 250

Temperature 3rC 37T 3rC(Time) (24 hr) (24 hr) (24 hr)

*Ortho hydraxyacetophenone-2-iminobenzenethiol

tivities of the complexes as compared to the freeligands could also be understood in terms of thechelation theory", which explains that a decreasein polarizability of the metal could enhance thelipophilicity of the complexes.

AcknowledgementThe authors thank Prof.MV Subba Rao, Head,

Department of Microbiology, College of VeternaryScience, Andhra Pradesh Agricultural University,Tirupati for providing necessary facilities for stu-dying antibacterial activities of present com-pounds. The authors thank RSIC, Bombay forproviding NMR and ESR spectra,

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