Synthesis and characterization of macrocyclic complexes of Co (II), Ni (II) and Cu (II)

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1 Indian Journal of Chemical Technology Vol. 19, September 2012, pp Synthesis and characterization of macrocyclic complexes of Co (II), Ni (II) and Cu (II) C Surendra Dilip*, V Sivakumar & J Joseph Prince Anna University of Technology, Tiruchirappalli , India Received 27 April 2011; accepted 21 June 2012 The ligand 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime [(DOH)-bzo] has been synthesized by the condensation of o-phneylenediamine with diacetyl monoxime in a 1:2 mole ratio. It has been isolated in the solid state and characterized by CHN data, and IR and NMR spectral studies. Six coordinate Co (II) complex, four coordinate square planar Ni (II) complex and five coordinate square pyramidal Cu (II) complex are synthesized by the reaction of the respective metal chlorides. All these complexes are characterized by infrared, electronic spectroscopy and conductivity measurements. Indirect evidence for the formation of intra-molecular hydrogen bond in these complexes is obtained by replacing the hydrogen bonded proton by BF 2 group by treating the complex [Cu (DO) (DOH) bzo) Cl] with BF 3 OEt 2. Keywords: 1 H NMR spectra, IR spectra, Macrocyclic complexes, Metal complexes In living systems, metal ions play an important role as a catalyst for many biological reactions. The activity of such metal ions depends on their confinement within approximately planar, tetra dentate and totally enclosed macrocyclic ligands, which themselves are highly conjugated species 1-3. Quadridentate tetraaza macrocyclic ligands fulfil the above requirements and hence serve as effective models for biological systems. The study of transition metal complexes of macrocyclic ligands as model for biological system stems mainly from the difficulties encountered in isolating the biological systems. These are due to their high molecular weights, extreme molecular complexity, limited solubility and instability in aerobic conditions. The macrocylic complexes of transition metal ions cannot serve as biological mimics of the primary bioinorganic systems but can act as simple and an efficient model to extract the essential details of the biological systems 4, 5. A large number of macrocylic ligands synthesized are highly conjugated and yield complexes that exhibit extensive electron delocalization. The extensive conjugation dictates the co-planarity of the ligand. The complexes of such chelates are therefore used to study ligand bonding and electronic transmission induced by the ligand to the metal ion 6-8. The study has a wider scope of establishing the *Corresponding author. cs_dilip@yahoo.co.in inverse relationship between the cation cavity provided by the ligand and the size of the metal ion in stabilizing the complexes. The trend is followed from Sc to Zn in determining the stability of the complexes. The present study intends to arrive at the cationcavity best fit in a planar array without disturbing the planarity of the ligand structure. It is expected that the cation size related modifications of the ligand would be evident. This would help to pursue the effect of conjugation on the physiochemical properties of the given metal ions. The study thus aims at synthesizing Co(II), Ni(II) and Cu(II) complexes of the hitherto unreported ligand 5,10-dimethyl-2, 3, 4; 7, 8 ; 11, 12, 13-tribenzo-6,9-diaza-1,14-diamine. The complexes are characterized by CHN analysis, conductivity measurement, IR, UV-Visible spectral studies and estimation of Ni, Co, and Cu. Experimental Procedure Reagents Cobalt (II) chloride (AR, BDH), copper (II) chloride (AR, BDH) and nickel (II) chloride (AR, BDH) were used as such for the synthesis of metal complexes. For the synthesis of ligands (L), o-phenylenediammine (LOBA) and diacetylmonoxime were used wherein o- phenylenediamine was recrystallized from hot water using activated charcoal prior to use. Solvents Commercial grade ethyl alcohol was refluxed for 7h over lime and the fraction boiling at o C was

2 352 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2012 collected and used. Double distilled water was obtained by distilling water over alkaline KMnO 4 in an all glass apparatus. Synthesis of ligand To a solution of o-phenylenediamine (10.8g, 0.1mol) mixed in 100 ml warm distilled water, a solution of diacetylmonoxime (20.2g, 0.2mol) was added and refluxed for 1 h till yellow crystals are separated out. The reaction mixture was then cooled to 5 o C and filtered. The yellow crystals (yield 80% and m.p. 176 o C) were recrystallized in dry ethanol. Synthesis of complexes To a solution of [(DOH) 2 bzo] (0.409 g, 5 mmol) mixed in ethanol, a hot ethanol solution of metal salts [Co(II) chloride (1.4g, 5 mmol), Ni(II) chloride (0.95g, 5 mmol) and Cu(II) chloride (0.554g, 5 mmol)] were added and refluxed for 3 h. The resultant solution was cooled overnight and the crystals which separated out were filtered through sintered crucible washed with cooled ethyl alcohol and dried over anhydrous CaCl 2. Results and Discussion Characterization of ligand The recrystallized pure ligand melts at 276 o C as reported in the literature 7,8. The infrared spectrum of the ligand shows no characteristic absorptions assigned to either C=O or NH 2 groups, which is due to the formation of ligand. The sharp band of medium intensity occurred at 1575cm -1 is attributed to C=N, while a similar band at 1630cm -1 is assigned to C=C. The strong as well as sharp band at 1220cm -1 is due to the N-O stretching vibrations 9,10. The CH stretching vibrations result in a weak band at 3260cm -1 and a broad band at around 2900cm -1 is assigned to OH. A very strong band of medium broadness at 1430cm -1 and a weak band at 1430cm -1 are assigned to the ring models of O-disubstituted benzenes. The sharp bands of medium intensity at 1375cm -1 and 1445cm -1 are due to the symmetric and asymmetric deformation vibrations, respectively, of the methyl groups 11. The strong bands appearing at 1260, 1180 and 1130cm -1 are assigned to ν(c-c), ν(c-n) vibrations or to the coupled vibrations of these modes. The metal sensitive strong as well as sharp band at 765cm -1 may be attributed to C=N-O deformation vibrations. The metal dependent strong vibration as well as sharp bands appearing at 990, 920, 910cm -1 can be ascribed to the deformation vibrational modes of the ligand. The proton magnetic resonance spectrum of the ligand in DMSO-d 6 gives sharp singles at 1.96 and 2.66 ppm corresponding to the two types of methyl protons. The aromatic protons give broad multiplets centred at 7.8 ppm and the two oxime protons give a sharp singlet at 10.86ppm 12. Characterization of complexes The metal-ligand complexes were synthesized at 1:1 mole ratio and obtained in pure form. Hence, these were not further purified by recrystallization. The results of the percentages of metal present in the complexes determined gravimetrically are given in Table Molar conductance At room temperature the observed molar conductances of the complex in DMF are in the range of ohm -1 cm 2 mole -1. Hence, from the conductivity measurements, it is concluded that the chloride ions are covalently bonded to metal ions. Based on the metal-ligand ratio calculated analytically and the nature of the electrolytes given by the conductance measurements, compositions are assigned for the prepared complexes. Molar conductivity results of [Co(DO)(DOH)bzo)Cl] and [Cu(DO)(DOH)bzo)Cl] complexes in DMF are found to be 27 and 32 ohm -1 cm 2 mol -1 respectively, indicating both to be neutral complexes. This shows that chloride acts as a ligand and not as a simple ion 14. The molar conductance of [Ni(DO)(DOH)bzo)]Cl complex is 82 ohm -1 cm 2 mol -1 indicating that it is a 1:1 Table 1 Analytical data of complexes Complex Colour Yield Mol. M P µ eff Elemental analysis [Found (Calcd.)], % % wt C B M M C H N Cl [Co(DO)(DOH)bzo)Cl] Pink (7.51) (64.86) (3.79) (13.15) (7.53) [Ni(DO)(DOH)bzo)]Cl Green (7.00) (66.51) (4.77) (13.08) (7.41) [Cu(DO)(DOH)bzo)Cl] Pale blue (7.45) (67.32) (4.55) (14.00) (7.05) Λ m Ω 1 cm 2 mol

3 DILIP et al.: SYNTHESIS & CHARACTERIZATION OF MACROCYCLIC COMPLEXES OF METALS 353 electrolyte. This value falls within the range of molar conductance values of 1:1 electrolytes dissolved in DMF and confirms the formation of the proposed cationic complexes in the solution and thus the chloride is a simple ion and not a ligand. The molar conductivity data is provided in Table 1. Infrared spectra The IR spectra of all metal complexes are compared with those of the free ligands. On comparison, the band positions of various vibrations are ascertained. In the infrared spectrum of the ligand, the band at cm -1 is due to asymmetric (C=N) stretching vibration 15. The presence of ν(c=n-c) asymmetric stretching vibration is confirmed by a band at 1520cm 1. In the infrared spectrum of metal complexes, the ν(c=n) stretching vibrations are observed at around ~1480cm 1 and are due to the coordination of nitrogen from C=N to the metal. The free ligand shows a strong peak at 1520cm 1, which is characteristic of the imine ν(c=n) group. The ν(c-c) stretching vibrations are affected upon complexation and are situated at a frequency significantly different than that of the free ligands. Coordination of the ligands to the metal centre through the nitrogen atom is expected to reduce the electron density in the methine and imine link, and hence lower are the ν(c-c) and ν(c=n) absorption frequencies. The peaks due to ν(c-c) are slightly shifted to lower frequencies and appear at around cm 1, indicating the coordination of imine nitrogen to the metal. Hence, the shifts in the C=N and C-C stretches of the metal complex provide support for ML 6 -type coordination 16. The characteristic infrared absorption bands of the ligand are shifted, lowered in intensity on formation of the complex and new vibrational bands characteristic of the complex appear. A broad band of medium intensity, occurring at 2900cm -1 in the free ligand and shifted to 3120cm -1 in [Cu(DO)(DOH)bzo)Cl], is attributed to the ν (OH) of the hydrogen bonding 17. The ν(oh) of the complex is observed at 1196 cm -1. The bands observed at 1330 and 1400cm -1 indicates the symmetric and asymmetric deformation vibrations of the methyl groups. A unique feature in this study is the occurrence of a new strong and sharp band at 1090cm -1 in the complex, that is not observed in the ligand and which is assigned to ν(no). Electronic absorption spectra The electronic absorption spectra are often very helpful in the evaluation of results furnished by other methods of structural investigation. The electronic spectral measurements were used for assigning the stereochemistry of metal ions in the complexes based on the positions and number of d d transition peaks. The electronic absorption spectra of the Schiff base and its Co(II), Ni(II) and Cu(II) complexes are recorded at room temperature using acetone as solvent and the results are listed in Table 2. The magnetic moment value of the Co(II) complex is found to be 4.81BM which suggests a high spin sixcoordinated octahedral arrangement of ligand molecules around the metal ion (Fig. 1). The complex shows two characteristic peaks at around and 21734cm 1, assigned to 4 T 1 g(f) 4 A 2 g(f) and 4 T 1 g(f) 4 T 1 g(p) transitions respectively. The Ligand Field Stabilisation Energy values are calculated from transition energy ratio diagram using the ratio ν 2 /ν 1 which lies at The inter electronic repulsion parameter of the complex B is found to be 972cm -1 which is lower than that of the free ion value (B=1148cm -1 ), suggesting the delocalisation of metal electrons into metal-ligand molecular orbital 18. Fig. 1 Proposed structure of ligand and its complex, where M= Co (II) Table 2 Electronic spectral data and ligand field parameters of complexes Complex ν 1, cm -1 ν 2, cm -1 B, cm -1 B, cm -1 β Β, % ν 2 / ν 1 LFSE, kcal.mol -1 [Co(DO)(DOH)bzo)Cl] [Ni(DO)(DOH)bzo)]Cl [Cu(DO)(DOH)bzo)Cl]

4 354 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2012 The Ni(II) complex is high spin with a room temperature magnetic moment value of 2.8 BM, indicating a spin-free square planar configuration. The appearance of a band at 19240cm 1 due to 3 A 2 g(f) 3 T 1 g(p) transition favours an square planar geometry for the Ni(II) complex. The absence of any band below 10,000 cm 1 eliminates the possibility of a tetrahedral environment in this complex. The inter electronic repulsion parameter of the complex B is observed at 946cm -1, which is lower than that of the free ion value (B=1030cm -1 ). The spectrum also shows a band at 24,556cm 1 that may be attributed to ligand metal charge transfer 19. The Cu(II) complex exhibits a magnetic moment of 1.90 BM at room temperature. The energy level diagram for ligand field of distorted Oh, D 4h symmetry predicts three transitions, viz. 2 B 1g 2 A 1g (ν I ) 2 B 1g 2 B 2g (ν 2 ) and 2 B 1g 2 E g (ν 3 ). However, only one broad band (weak and split) at 16,638cm 1 is generally observed in the visible region for many Cu(II) complexes as a result of overlap of these three bands. Hence, the absorption maximum of 2 E g 2 T 2g transition corresponds to 10Dq value from which LFSE can be evaluated for the five coordinate square pyramidal geometry. The broadness of the band is due to the ligand field and the John-Teller effect. The inter electronic repulsion parameter for the Cu(II) complex (B'=923cm -1 ) is less than the free ion value (B=1060cm -1 ), suggesting a considerable overlap and also delocalisation of the metal and ligand orbital. The β value indicates that the covalent character of metal ligand sigma bond is very low 20. ESR spectra The ESR spectra of copper complex provide information on importance in studying the metal ion environment. The X-band ESR spectra of the Cu(II) complex, recorded in DMSO at liquid nitrogen temperature (77K) and at room temperature (300 K), are shown in Figs 2a and b. The spectrum of the copper complex at room temperature shows one intense absorption band in the high field and is isotropic due to the tumbling motion of the molecules. However, this complex in the frozen state shows four well resolved peaks with low field region. The copper complex exhibits the g value of 2 31 and g value of These values indicate that the ground state of Cu(II) is predominantly dx 2 y 2. The spin-orbit coupling constant l ( 486cm 1 ) is calculated using the relationship as shown below: g av = 1/3[g + 2γ ] and g av = 2(1 2l/ 10Dq) This value is found to be less than that of the free Cu(II) ion ( 832cm 1 ), which also supports covalent character of M L bond in the complex 21. The G value of 3 82 indicates negligible exchange interaction of Cu Cu in the complex. The covalency parameter α 2 is calculated (α 2 = 0 82) using the following equation: α 2 Cu = (A /0 036) + (g 0023) + 3/7(g ) If the value of α 2 = 0 5, then it indicates complete covalent bonding, while a value of α 2 = 1 0 suggests complete ionic bonding. The observed value of α 2 = 0 82, of the complex is less than unity, indicating that the complex has some covalent character in the ligand environment 22. Cyclic voltammetric study The cyclic voltammogram of Cu(II) complex (0 01 M) in 0.1 M TBATFB in DMSO on Pt electrode, v=0.1 V S 1, (vs. Ag Ag+ electrode) is represented in Fig. 2 ESR spectra of Cu (II) complex in DMSO solution at (a) 300 K and (b) 77 K

5 DILIP et al.: SYNTHESIS & CHARACTERIZATION OF MACROCYCLIC COMPLEXES OF METALS 355 Fig. 3. (1 0 to 1 2 V potential range) shows a welldefined redox process corresponding to the formation of Cu(II)/Cu(III) couple at E pa = 0 22 V and the associated cathodic peak at E pc = 0 18 V. This couple is found to be reversible with E p 0 04 V and the ratio of anodic to cathodic peak currents (I pc /I pa = 1) corresponds to a simple one-electron process. The complex also shows a quasi-reversible peak in the negative region which is characteristic of the Cu(II) Cu(I) couple at E pc = 0 47 V, with associated anodic peak at E pa = 0 34 V for Cu(I) Cu(II) oxidation 23. The cyclic voltammogram of Ni(II) complex (0 01 M) in 0.1 M TBATFB in DMSO on Pt electrode, v=0.1 V S 1, (vs. Ag Ag+ electrode) (in the absence of molecular oxygen) at room temperature in the potential range of 1 0 to 1 0 V and at a scan rate 50 mvs 1 indicate a quasireversible one-electron process. A noteworthy feature is observed in the cyclic voltammogram of Ni(II) complex. During the forward scan it shows two cathodic reduction peaks, one at +0 45Vand another at 0 61 V that are attributed to the reduction of Ni(II) Ni(I) and Ni(I) Ni(0) respectively. During the reverse scan it shows two anodic oxidation peaks, one at 0 26 V and another at V which are attributed to the oxidation of Ni(0) Ni(I) and Ni(I) Νi(II) respectively 24. Intramolecular hydrogen bonding The free ligand is a neutral molecule which upon complexation loses one of the oxime protons with the concomitant formation of an intramolecular hydrogen bond. All these complexes also exhibit two strong and sharp bands at 1220 and 1075 cm -1, assigned to ν(no) and ν (NO) respectively. The appearance of only one band at 1220 cm -1 assigned to ν(no) in the free ligand shows that the two NO- linkages are identical, while the appearance of two bands assigned to ν(no) and ν (NO) in the complexes, indicate the presence of two unequal NO linkages. Indeed, upon complexation, there are 2-NO-linkages (C=N-O-H and C=N-O H) due to the formation of hydrogen bonding. Studies on cobaloximes by Schruazer 25 also confirm the presence of two unequal NO-linkages due to hydrogen bonding. Disappearance of the ν(oh) band in the complexes is a further evidence for the O- H-O bond formation. Indirect evidence for the intramolecular hydrogen bonding is obtained by replacing the hydrogen bonded Fig. 3 Cyclic voltammogram of 1 mm Cu (II)-complex proton in [Cu(DO)(DOH)bzo)Cl] by the BF 2 group, by treating the complex with BF 3.OEt 2. This reagent readily reacts with the O H O bridge in the complex, affording the stable complex containing the O-BF 2 -O bridge. Schrauzer replaced the two hydrogen bonded protons in bis(dimethyl glyoximato) Nickel(II) by the BF 2 group by treating with BF 3 OEt 2 for the first time. Later, Schrauzer and Windgassen replaced the hydrogen bonded proton in methyl (aquo) cobaloxime by the BF 2 group using the same method 26. Conclusion In the present investigation the quadridentate ligands employ coordinates readily with Co(II) in the equatorial position and readily afford the synthesis of six coordinate octahedral complex. Ni(II) forms a stable square planar complex with the pseudomacrocyclic ligand. Cu(II) forms square pyramidal five coordinate complexes in which the metal anion occupies one of the apical sites. The ligand readily affords synthesis of 6 coordinate complexes wherein the apical sites can be altered and it seems to be a convenient vehicle to explore the influence the ligand conjugation on the electronic properties of the metals. However, detail EPR and electronic spectral investigation are needed to examine the influence of structure and conjugation on the geometry and reactivity of transition metals. The synthesis of octahedral complexes to the family of closely related monondendate ligands such as Lewis bases and detailed spectral investigations would enable the construction of spectrochemical series of the axial ligands. A detailed electrochemical investigation of such a series of complexes would throw more light on the influence of axial and equatorial ligands on the metal ion.

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