Synthesis and Characterization of Salicylidene ...sphinxsai.com/2014/ChemTech/JM14CT51_100/CT=59(495-508...Selected bands of diagnostic importance are collected in Table 1. The salicylidene

International Journal of ChemTech Research CODEN( USA): IJCRGG ISSN : 0974-4290

Vol.6 , No.1, pp 495-508, Jan-March 2014

Synthesis and Characterization of Salicylidene-

benzenesulfonic acid H2L and its complexes with Caffeine

[M(LH)2(Caff)2] ; M=Zn2+, Cd2+, Ni2+, Cu2+

Mohamed El Amane*, Youness Kennouche

Equipe métallation, complexes moléculaires et appli cation, Faculté des sciences, BP 11201 Zitoune, Meknès. Morocco.

*Corres.author : [email protected]

Abstract: In this work, it was possible to synthesis and characterize the ligand of salicylidene benzenesulfonic acid (H2L), obtained by condensation of salicylaldehyde with 3-aminobenzenesulfonic acid. Four Schiff base caffeine complexes [M(LH)2(Caff)2]; LH2 = Salicylidene benzenesulfonic acid and caff = caffeine were synthesized from the reaction of MCl2·nH2O (M = Zn2+, Cd2+, Ni2+, Cu2+), caffeine and the Schiff base. They were characterized by FT-IR, 1H, 13C NMR, UV-Visible and molar conductance. The spectroscopic studies suggested the octahedral structure for the all complexes. Keywords: Schiff base, Salicylidene benzenesulfonic acid, caffeine, FT-IR, 1H, 13C NMR, UV-Visible, molar conductance.

Introduction

Schiff bases are an important class of ligands, such ligands derived from aromatic amine and aromatic aldehyde have a variety of applications including biological, clinical, analytical and industrial in addition to their important roles in catalysis and organic synthesis[1-4]. Schiff bases ligands derived from aromatic sulfonic acid are rarely studied.

The Schiff base complexes of transition metals are one of the most exhaustively studied topics in coordination chemistry because of the ease with which they can be synthesized, their versatility and wide range of complexing abilities[5-10]. The literature clearly shows that these complexes played an important role in the development of modern coordination chemistry[6-10], and are also found at key points in the development of inorganic biochemistry[11], catalysis[12], magnetism[13-17], gas storage[18-20], medical imaging[21] and optical materials[22-23]. In addition, The environment around the metal centre ‘‘as coordination geometry, number of coordinated ligands and their donor group’’ is the key factor for metalloproteins to carry out specific physiological functions[24].

Sulfonate compounds have important functions in many fields such as medicine, chemical separation and catalysis.

Mohamed El Amane et al /Int.J. ChemTech Res.2014,6(1),pp 495-508.

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Caffeine (1,3,7-trimethylxanthine) is the most widely used behavioural active substance in the world[25-27]. Caffeine is found in coffee, tea, cola nuts, Coca Cola, and Cocoa. Caffeine is stimulant of central nervous system, cardiac muscle and respiratory system, diuretic delays and fatigue. The new studies have found that caffeine may prevent skin cancer at least in mice[27]. Many studies have attracted considerable attention due to the antioxidant, anticancer properties and health benefits of tea. However, the caffeine may be caused a higher risk of developing bone problems, including osteoporosis, problems in metal absorption, excretion. It is also caused reabsorption processes in intestines, kidney, iron deficiency animia specially for people consuming high amounts of caffeine[28-31].

Sulfonic acid and caffeine compounds have important functions in many fields. On the basis of the stated facts, we have prepared and characterized the ligand of salicylidene-benzenesulfonic acid (H2L) (Figure 1) and its caffeine complexes of Cd(II), Zn(II), Cu(II) and Ni(II). The isolated Schiff base caffeine complexes were found to have the formula [M(LH)2(Caff)2] where LH2= salicylidene-benzenesulfonic acid, caff=caffeine.

Figure 1. General structure of the Schiff base ligand (H2L)

Experimental

All chemicals were obtained from commercial sources and were used without purifications: (NiCl2, 6H2O BDH; CdCl2,1/2H2O Panreac; CuCl2, 6H2O BDH; KOH BDH, ZnCl2, 2H2O BDH), Aminobenzenesulfonic acid Aldriche Chemistry, Salicylaldehyde SAFC, Ethanol and DMSO Sigma Aldrich, Caffeine Riedel-De Haen AG.

Infrared spectra were recorded as KBr pellets on a JASCO FT-IR 660 plus spectrophotometer in the range of 4000–400 cm-1 at 298 K. while the electronic spectra (UV–Visible) were obtained on a Shimadzu UV-1800 Spectrophotometer. The 1H, 13C NMR spectra of the ligand and its caffeine complexes were recorded with a Bruker AVANCE 300 at 25°C. All chemical shifts 1H and 13C are given in ppm using tetramethylsilane (TMS) as internal reference and DMSO as solvent. Conductivity measurement were performed at 25°C in DMSO using Hach HQ430d flexi.

Preparation of the Schiff base (H2L)

The Schiff based formed from salicylaldehyde and 3-aminobenzenesulfonic acid was prepared by adding (1,06 ml, 0.01 mole) of salicylaldehyde ethanolic solution to the same volume of ethanolic solution 3- amino benzenesulfonic acid (1,73g, 0.01mole). The mixture was refluxed for 4 hours. Then the yellow formed precipitate was filtered, washed several times with ethanol. The yield of the reaction was 82%.

Preparation of complexes [(M(LH)2Caff)2] (M=Cd2+, Zn2+, Ni2+ and Cu2+)

The Schiff base caffeine complexes of Cd(II), Cu(II), Ni(II) and Zn(II) were synthesized using a general procedure: The ligand (Schiff base) was prepared in the presence of a metal ion and caffeine in the reaction mixture, (0,56 g, 0.002 mole) of salicylidene benzenesulfonic acid was dissolved in the minimum possible amount of methanol and 0.001 mole of a metal chloride, (i.e. ZnCl2 (0,14 g, 0,001mole)), with (0,38 g, 0.002 mole) of caffeine were added successively to the resulting solution, which was shaken and/or heated until all of it dissolved. This mixture was stirred for 5 hours at room temperature. The resulting precipitate was then filtered and washed several times with methanol.


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Results and Discussion

Characterization of ligands

Infrared spectroscopy

The IR spectra of the Schiff base under study (Figure 2) are recorded in the solid state using the KBr disc technique. Selected bands of diagnostic importance are collected in Table 1.

The salicylidene benzenesulfonic acid ligand H2L was synthesized by condensation of salicylaldehyde and 3-benzenesulfonic acid. The obtained compounds, is confirmed by the presence of the bands at 3442 cm-1 and 3379 cm-1 which can be assigned to the stretching mode of the phenolic OH-group and the sulfonic acid OH-group respectively, whereas those located at 1309 and 1283 cm-1 are related to their deformational modes respectively. The band at 3057 cm-1 is assigned to (υCH, Ar-H). The broad bands in the 2800–2700 cm-1 range are assigned to the OH group vibration (ortho position) associated intramolecularly with the nitrogen atom of the CH=N group. The Schiff base is able to form an intramolecular hydrogen bonding either from the phenol group with nitrogen atom -O-H….N , forming a six membered ring or from the amine group with nitrogen atom N-H…N of a five membered ring . These results to produce two folds intramolecular hydrogen bondings in Schiff base Scheme 1.

Scheme 1: Tautomerism reaction in salicylidene-benzenesulfonic acid (H2L)

Furthermore, the strong band observed at 1614 cm-1 is assigned to the azomethine group vibration (υ(C=N)).

The stretching vibrations ν (C=C), ν (C–N) and ν (C–O) show very strong bands in the (1584–1440) cm-1, (1426-1358) cm-1 and (1330–1178) cm-1 ranges, respectively.

The ligand shows three bands of ν(SO3) at 1191, 1108 cm-1 and 1036 cm-1. δ(SO3) were observed in the (569-526) cm-1 range and at 501 cm-1.

In addition, two bands at 1143 and 1087 cm-1 due to δ(CH) in plane deformation. The medium and/or weak band observed in the 999-608 cm-1 range can be attributed to δ(CH) out-of-plane deformation. In the end, the bands in the 526-447 cm-1 range are assigned to δ(CH) in plane ring deformation and the band at 439 cm-1 is attributed to δ(CH) out plane ring deformation.


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Figure 2. Infrared spectrum of the salicylidene benzenesulfonc acid (H2L) in KBr

On the other hand, the Caffeine belongs to the centrosymmetric Cs point group. Its structure is shown in Figure 3. The numbers of vibrations modes are as follows Ʈvibr = 45 A’ + 21A’’, the vibrations of the A’ species will be in plane and those of the A’’ species will be out of plane. It is know from the literature that the caffeine mostly coordinated through its N-donnor atom or even through O-donor atom, which is a rarity. This coordination is accompanied by elimination the miror plan σv and by a whole series of changes in the infrared spectrum.

Figure 3. Structure of the caffeine

The infrared spectral data of the free caffeine [32] shows weak and sharp band at 3114 cm-1 which belongs to ν(C-H) aromatic. Another weak band belongs to ν(C-H) aliphatic was found at 2954 cm-1. The strong broad band at 1702 cm-1 was attributed to ν(C=O), strong band at 1662 cm-1 was attributed to ν(-N=C). The ν(C=C) was noticed at 1546 cm-1 with shoulder at 1600 cm-1. The (δHCN +υring imid + υring pyrimi) stretching and deformation heterocyclic imidazol and pyrimidine fragment were noticed at 1551 cm-1 and 1327 cm-1

respectively.

Electronic spectrum

Electronic spectrum has been measured in DMSO. The ligand shows two bands at 253 nm and 310 nm which are due to π→π* (phenyl ring) and n→π* (-C=N), respectively. The spectrum of electronic absorption of the ligand is shown in Figure 4.


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Figure 4. Electronic spectrum of the salicylidene benzenesulfonc acid (H2L)

Characterization of the Schiff base caffeine complexes of Cd (II), Zn (II), Ni (II) and Cu (II)

The salycilidene benzenesulfonic acid caffeine complexes were synthesized to give new complexes with a general formulae [M(LH)2(Caff)2] where M= Cu2+, Ni2+, Cd2+ , Zn2+. The resulting products have been characterized by infrared spectroscopy,1H NMR,13C NMR, UV-visible and molar conductance.

Conductance Measurements

By using the relation Λm= 103 K/C, the molar conductance of the complexes (Λm) can be calculated, where C is the molar concentration of the metal complex solutions. The salycilidene benzenesulfonic acid caffeine complexes were dissolved in DMSO and the molar conductivities of 10−4 M of their solutions at 25 ± 2 ◦C were measured. Table 2 shows the molar conductance values of the complexes.

The lower The lower value observed of molar conductivities indicates the non-electrolyte behaviour of the salycilidene benzenesulfonic acid caffeine complexes of Zn(II), Cd(II), Ni(II) and Cu(II).

Table 2. Conductance Measurements of the Schiff base caffeine complexes in DMSO

Infrared Spectroscopy

The IR spectra of the salicylidene benzenesulfonic acid ligand (H2L) and the free caffeine were compared with those of the Schiff base caffeine complexes (Figure 5.1, Figure 5.2, Figure 5.3 and Figure 5.4) in order to confirm the binding mode of the Schiff base ligand and the caffeine to the corresponding metal ion in the complexes. Infrared spectral data (4000–400 cm−1) of the ligand, the caffeine and their complexes were shown in Table 1.

The infrared data of the complexes shows characteristic band in the 3482-3495 cm-1 region which is assigned to the sulfonic acid OH-group. However, the band corresponding to phenolic OH-group is disappeared in the complexes.

Complex Molar conductance (Ohm-1 cm2 mol-1)

Cu(LH)2(Caff)2 56,12 Zn(LH)2(Caff)2 54,41 Cd(LH)2(Caff)2 59,02 Ni(LH) 2(Caff)2 59,48


500

The bands at 3122 cm-1 and 2960 cm-1 which are attributed to υCHar and υCH3/υCH2 respectively. They show a shift to higher frequencies by (6-8) cm-1 compared with the free caffeine.

The bands in the 1700- 1657 cm-1 range belong to the υ(C=O), υ(C=N), υ(C=C) are shifted to lower frequencies by (5-24) cm-1. The carbonyl group in caffeine and its complexes exhibit a strong absorption bands due to υ(CO) stretching vibration. This region is characteristic aromatic lactones for one and two carbonyl as in the quinine[33,34]. The caffeine contains two carbonyl group vibrations in the meta position. The very strong bands observed are considered to be due to υ(CO) symmetric and asymmetric υ(CO,CN) stretching vibrations in caffeine[34].

The principal change in the spectra of the Schiff base caffeine complexes is observed in the (1620-400) cm-1 range corresponding to the vibration mode of υ(C=N), the imidazol, the pyrimidine and the methyl fragments in the caffeine. For the ligand, the change is observed in the same range corresponding to the vibration mode of υ(C=N), the phenolic υ(C-O) stretching vibration and the benzene. The υ(C=N) band in the 1617-1620 cm-1 range is shifted in the Schiff base caffeine complexes of Zn(II), Ni(II), Cu(II) and Cd(II), indicating coordination of the Schiff base and the caffeine through the azomethine nitrogen atom.

Others bands at 1571 cm-1 and in the (1546-1549) cm-1 range are assigned to (δHCN+ υring imid +υring pyrimi) which are shifted to higher frequencies by 20 cm-1 compared with free caffeine and to lower frequencies by (18-23) cm-1 compared with free ligand. From these results, we can conclude that (C=N) imidazol fragment of the caffeine is coordinated with metal ions through the nitrogen atom. The band noticed at 1571 cm-1 suggests the monodentate coordination by N9 nitrogen atom in caffeine molecule.

The band at 1309 cm−1 due to δOH in the free ligand has been disappeared in all complexes witch confirm the coordination of the metal ions via the phenolic oxygen atom. A band that appeared at 1230 cm−1 [35] due to phenolic υ(C–O) stretching in the free Schiff base ligand (H2L) has been shifted to lower frequency in the (1227–1224) cm−1 range, indicating the coordination through the phenolic oxygen atom in the complexes. However, the band at 1283 cm−1 appears in all complexes corresponding to δOH of the sulfonic acid group (-SO3H). That confirms no coordination of the sulfonic acid group by oxygen atom to the metal ions.

The bands in the (511-556) cm-1 and (450-485) cm-1 regions are attributed to υ(M-O) and υ(M-N) stretching vibrations respectively, confirming the coordination of the Schiff base by nitrogen atom and oxygen atom to the respective metal ions.

These facts suggest that the shifts are due to coordination of ligand to the metal atom by the azomethine nitrogen and phenolic oxygen (N,O). The caffeine is coordinated with metal ions through the nitrogen atom (N9).

Figure 5.1. Infrared spectrum of the Schiff base caffeine complex [Cd(LH)2(caff)2] in KBr


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Figure 5.2. Infrared spectrum of the Schiff base caffeine complex [Zn(LH)2(caff)2] in KBr

Figure 5.3. Infrared spectrum of the Schiff base caffeine complex [Ni(LH)2(caff)2] in KBr

Figure 5.4. Infrared spectrum of the Schiff base caffeine complex [Cu(LH)2(caff)2] in KBr


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Table 1. Infrared data of the ligand and its caffeine complexes [M (LH)2(caff)2] (M=Ni2+, Cd2+, Zn2+, Cu2+) in KBr

1H NMR Spectra

The NMR spectrum of our Schiff base caffeine complexes were recorded in dimethylsulfoxide (DMSO) solution, using tetramethylsilane (TMS) as internal standard. The 1H NMR spectra of the Schiff base caffeine complexes of Zn(II) and Cd(II) show the following signals: Phenyl as multiplet at 7,03-7,68 ppm (m, 8H),

Caffeine

[32] Ligand (H2L)

Cu(LH)2caff2 Ni(LH)2caff2 Cd(LH)2caff2 Zn(LH)2caff2

νOH - 3442w, 3379w 3482L 3486 L 3495 L 3482L

νCHar 3114m 3057m 3122w, 3039w 3122w, 3066w 3122m ,3066w 3122w, 3048w

νCH3/ν CH2 2954w - 2960w 2960w 2960w 2960w

νC=Ocaf 1702vs - 1704vs 1700vs 1704vs 1704vs

ν C=O/νC=N caf

1662s -

1657s 1659vs 1658vs 1658vs

νC=Nar 1614vs

1617s 1618vs 1618vs 1620vs

νC=Ccaf/ νC=Car/

1600m

1584s 1578w

1596s

1599s

1592w

δHCN + νring imidazole+

νring pyrimidine

1551w 1570s 1570m, 1550m 1570m, 1550m 1570m, 1546m 1570m, 1548m

δCH3+ νC-N

1487m, 1466m, 1431m, 1360m

1494vs, 1473vs, 1449s, 1426vs, 1413s

1358m

1477m, 1432w, 1363w

1474vs, 1452m, 1430m, 1405w

1360m

1497m, 1472m, 1452m, 1430m , 1412m 1363m

1494m, 1474m, , 1452m,1430w, 1417w 1360w

νring(imidazole) +

νring(pyrimidine) 1327w - 1323w

1323w

1323w 1318w

δOH - 1309m, 1283m 1283m 1283s 1283m 1283m

δCH caf, δC-Oph

1241vs

1230s 1227s 1224s 1227s 1225s

δ(CH) + νC-O 1190m 1191vs 1186vs 1186vs 1186vs 1186vs

νSO3 - 1191s, 1107s,

1040s 1186m, 1108m,

1040m 1186s, 1108m,

1040s 1186s, 1108m,

1040s 1186s, 1108m,

1040s

δCH In plane def

- 1143m, 1087m 1159w, 1082w 1146w, 1089m 1149w, 1089m 1149w, 1089m

ν(N-CH3) + δring(imidazole)

974s - 972m 972s 975w 972m

ρr(CH3) + ν(N- CH3) +

CH Out-of-plane

def

861m

999m, 970m, 925m, 893m, 879m, 841s, 801m ,784m ,753s, 689s, 656m, 622s

996w, 975w, 886w, 795w, 759w, 726w, 707w, 690w, 681w,

659w, 620m

995m, 974m, 925m, 911w, 881w, 840w, 804w, 782w, 758m, 722m, 689m, 659m,

994w, 926w, 893w, 878w, 841w, 802m, 784w, 757m, 725m, 689m, 659w, 620m

994w, 974m, 926w, 893w, 880w, 841w, 802w, 784w, 757m, 725w, 689w, 659w,

619m

γring(pyrimidine) +

γring(imidazole) 746s

- 745w 744m 748m 745m

γring(imidazole 610vs 605vs 605m 610m 610m 610m

δSO3 - 567m,

527m,501m 564w, 529w,494w 569w, 525w,501w 567m, 526m,501w 567m, 525w,501w

CH In plane ring def

- 467m, 446m

440vw 467vw 466w 470w

CH Out plane ring def

439w

438w 445w 442w 440w

τcaffeine 481m - 483w 482w 485w 482w

νM-N - - 409w 422w 428w 422w

νM-O - - 511w 550w 549w 547w


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CH=N– at 8,9 ppm (s, 1H) and the peak at 13,02 ppm (s, 1H) is attributable to the sulfonic acid –SO3H group. However, The 1H NMR spectrum of the free caffeine in DMSO has shown proton signals at 3.56, 3.78, 4.28 and 8,10 ppm corresponding to the three methyl groups N1-CH3, N3-CH3, N7-CH3 and C8-H respectively[36]. In the case of the Schiff base caffeine complexes of Zn(II) and Cd(II), the signals of N1-CH3, N3-CH3 and N7-CH3 practically are shifted to 3,38, 4,33 and 5,05 ppm. The signal due to the C8-H was shifted to δ 8.52 and δ 8.21 ppm on the Zn(II) and Cd(II) complexation, respectively. The down field shift was attributed to the involvement of N9 in complexation. The 1H NMR spectra of the Schiff base caffeine complexes [Zn(LH)2(Caff)2] and [Cd(LH)2(Caff)2] are shown in Figure 6.1 and Figure 6.2.

Figure 6.1. 1H NMR spectrum of the Schiff base caffeine complex [Cd(LH)2(caff)2] in DMSO

Figure 6.2. 1H NMR spectrum of the Schiff base caffeine complex [Zn(LH)2(caff)2] in DMSO


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13C NMR Spectra

The 13C NMR spectrum of the caffeine has shown proton signals at 27.9, 29.7, 33.5, 151.7, 148.7, 107.6, 155.4 and 141.3 ppm due to the N1-CH3, N3-CH3, N7-CH3, C2, C4, C5, C6 and C8[37]. All the carbon resonance are downfield shifted relative to the corresponding resonance. Moreover, the signal proton due to the C8-H in Zn(II) and Cd(II) complexes was appeared higher shifting at 147.9 ppm. These results indicate that the caffeine is coordinated with metal ions through N9. The 13C NMR spectra of the Schiff base caffeine complexes [Zn(LH)2(Caff)2] and [Cd(LH)2(Caff)2] are shown in Figure 7.1 and Figure 7.2.

Figure 7.1. 13C NMR spectrum of the Schiff base caffeine complex [Zn(LH)2(caff)2] in DMSO

Figure 7.2. 13C NMR spectrum of the Schiff base caffeine complex [Zn(LH)2(caff)2] in DMSO


505

Electronic Spectra

Electronic spectral data for the Schiff base caffeine complexes in DMSO are given in Table 3. In the electronic spectra of all complexes, recorded in DMSO solution, two bands within 207-239 and 252-280 nm ranges are due to aromatic rings. An absorption band in the range 308–367 nm was observed, which may be associated with a π→π* transition originating mainly in the azomethine chromophore (imine π→π* transition).

The UV-visible spectra of all Schiff base caffeine complexes display similar absorption spectra of the ligand which are shifted to higher wavelengths beside a decrease of the peak dye to n-π* transition which confirm the coordination through azomethine nitrogen.

On the other hand, the electronic spectra of Ni(II) complex shows three electronic transition

3A2g(F) 3T1g(P) (υ3), 3A2g(F) 3T1g(F) (υ2) and 3A2g(F) 3T1g(F) (υ1) at 376, 784 and 985 nm

respectively. These assignments correspond to Ni(II) octahedral complex[38].

Also, the spectrum of Cu(II) complex shows bands in the visible region which are attributed to the electronic transition of 2a1g(D) 2b1g(D) and 2e2g(D) 2b1g(D) at 374 and 718 nm respectively[39].

Finally, the electronic configuration of Zn(II) and Cd(II) complexes were (d10) which confirms the absence of any (d-d) transitions [40].

The electronic spectra of the Schiff base caffeine complexes [M(LH)2(Caff)2] where M2+= Cd2+, Zn2+, Ni2+,Cu2+ are shown in Figure 8.1, Figure 8.2, Figure 8.3 and Figure 8.4 respectively.

Table 3 : U.V-Visible data of the caffeine and the Schiff base caffeine complexes in DMSO

Compound λmax (nm) assignment

Caff 275 316 365

π→π* n→π* n→π*

Cu(LH)2(Caff)2 203 235 277 374 718

CT CT

M → L 2a1g(D) → 2b1g(D) 2e2g(D) → 2b1g(D)

Zn(LH)2(Caff)2 204 233 271 357

CT CT

M → L shift with bathochromic

effect Cd(LH)2 (Caff)2 203

234 273 377

CT CT

M → L Shift with bathochromic

effect Ni(LH) 2(Caff)2 204

233 273 376 784 984

CT CT

M →L 3A2g(F) → 3T1g(P) (υ3) 3A2g(F) → 3T1g(F) (υ2) 3A2g(F) → 3T1g(F) (υ1)


506

Figure 8.1. Electronic spectrum of the Schiff base caffeine complex [Cd(LH)2(caff)2] in DMSO

Figure 8.2. Electronic spectrum of the Schiff base caffeine complex [Zn(LH)2(caff)2] in DMSO

Figure 8.3. Electronic spectrum of the Schiff base caffeine complex [Ni(LH)2(caff)2] in DMSO


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Figure 8.4. Electronic spectrum of the Schiff base caffeine complex[Cu(LH)2(caff)2] in DMSO

Conclusion

In this paper we have been synthesized a ligand of salicylidene benzenesulfonic acid (H2L) and its complexes of caffeine [M(LH)2(Caff)2]; M=Cd2+, Zn2+, Ni2+ and Cu2+, Caff= Caffeine

The configuration of all complexes were performed the caffeine with metal ions through the nitrogen N9 and salicylidene benzenesulfonic acid bidentate coordinated by (N,O). Therefore, the present result suggested the octahedral symmetry for the metal complexes.

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