Synthesis and Characterization of Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) Complexes With Polydentate 14-Membered Macrocyclic Ligand

The condensation reaction of [2+2] semicarbazide with benzoylacetone in a 1:1 molar ratio in aqueous solution at room temperature resulted in the formation of a novel Schiff base tetraimine macrocyclic ligand (L): 5,12-dimethyl-7, 14-diphenyl-1, 3, 4, 8, 10, 11hexaazacyclotetradecane-2,9-dione. Macrocyclic complexes of the type, [ML]Cl2 [M= Co(II), Ni(II), Cu(II), Zn(II) or Cd(II)] have been prepared by reacting metal(II) chlorides with the ligand (L) in 1:1 molar ratio in ethanol. The prepared complexes were characterized by metal content Synthesis and Characterization of Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) ... 12 analyses, IR, electronic spectra, magnetic susceptibility and conductivity measurements. IR spectra confirm coordination of imine nitrogens to the central metal ion. A square planar geometry has been suggested for all the complexes. The molar conductance values of the complexes show that they are 1:2 electrolytes. INTRODUCTION Synthetic macrocyclic complexes of transition metals have attracted much attention as promising objects in coordination and supramolecular chemistry (1) . The chemistry of metal macrocycles is extensive because of their close relationship to molecules of biological significance (2-6) . The importance of these complexes is due to the role they play as models for protein metal binding sites in biological systems, as synthetic ionophores (7) , electrocatalysts in fuel cells (8) , MRI contrast agents (9,10) , luminescent sensors (11) , anticancer drugs (12) and radioimmunotherapeutic agents (13) . These extensive applications have been worth investigating for the design of new macrocyclic ligands for biological and industrial applications. Coordination compounds containing macrocyclic ligands have been studied in recent decades owing to their wide applications in biological and sensor fields (14,15) . Over the past decades great attention has been devoted to the design and synthesis of Schiff bases with enhanced ability to selectively encapsulate the given metal ion (16) . A large number of macrocycles and their complexes with metal ion have been synthesized and characterized (17-19) . A literature search reveals that a large number of hexaazamacrocyclic metal complexes have been prepared and characterized (18-20) . Also hexaazamacrocycle has long been an extremely useful and versatile macrocyclic ligand in coordination, bioinorganic chemistry (20,21) , and they are known to give several mononuclear complexes, in spite of the large size of the cavity formed by the macrocyclic backbones, as well as to stabilize various anions in their protonated forms (18) . Because of the wide range of medicinal applications of semicarbazide and its ability to coordinate with metal ions (22) , therefore it is highly desirable to synthesize and characterize macrocyclic complexes with semicarbazide. Here we report, the synthesis and characterization of hexaazamacrocyclic complexes [ML]Cl2 obtained from the reaction of the macrocyclic ligand (L) with the metals chlorides (Fig.1). EXPERIMENTAL 1. Chemicals: All chemicals used in the present work including CoCl2.6H2O, NiCl2.6H2O, CuCl2.2H2O, ZnCl2, CdCl2, semicarbazide and benzoylacetone are Analytical Reagent (A.R) grade used without further purification. Najla H. Taher & Akram A. Mohammed 13 2. Synthesis of the macrocyclic ligand (L) (34) : Aqueous solution of semicarbazide (2.23 g, 0.02 mol.) in distilled water (100 mL) and benzoylacetone (3.25 g, 0.02 mol.) were mixed in 1:1 molar ratio with constant stirring for 2 hours. The mixture was cooled down to 5 o C and kept undisturbed for 24 hours. A brown solid was precipitated out, which was filtered, washed with distilled water and dried in air. 3. Synthesis of the macrocyclic complexes [ML]Cl2: The complexes were synthesized by the same general method as follows: A warm ethanolic suspension (50 mL) of the ligand (4.02 g, 0.01 mol.) and hot ethanolic solution (50 mL), of the corresponding metal salts (0.01 mol.) were mixed together with constant stirring. The mixture was refluxed for 6 hours. The products were precipitated which were filtered off, washed with ethanol and then dried in air. 4. Analytical and physical measurements: Metal contents have been determined by applying gravimetric methods (23) after the decomposition of the complexes by acid digestion with concentrated nitric acid. Melting points were determined by using electrothermal 9300 digital apparatus. Molar conductivities of the complexes have been measured in an electrolytic conductivity measuring set LF-42 using 0.001 M of the complexes in dimethylformamide (DMF) solutions at room temperature. IR spectra were recorded on a Bruker tensor 27 spectrophotometer in the 400-4000 cm -1 range using KBr disc. Electronic spectra were recorded on a Shimadzu 1601 spectrophotometer in DMF at 25 o C for 0.001 M solution of the compounds using a 1 cm quartz cell. Magnetic susceptibilities of the complexes have been measured by Brucker B.M.6. using Faraday method. RESULTS AND DISCUSSION The prepared complexes are brown coloured solids, stable in air at room temperature, the analytical data of the synthesized complexes are listed in Table 1. Molar conductances of the complexes in DMF are in the range (135-168) ohm -1 cm 2 .mol -1 (Table 1) indicating a 1:2 electrolytic nature of these complexes (24) . Table (1): The physical and analytical properties of the compounds Compound Colour Yield % m.p C 0 % Metal M ohm -1 cm 2 mol -1 *Cal. Found L Brown 61 141-143 [Co(L)]Cl2 Dark Brown 67 232 11.07 10.73 141 [Ni(L)]Cl2 Dark Brown 55 207 11.04 11.82 168 [Cu(L)]Cl2 Dark Brown 69 254 11.84 12.51 135 [Zn(L)]Cl2 Brown 51 211 12.14 11.99 160 [Cd(L)]Cl2 Dark Brown 66 222 19.20 20.08 164 * Cal. = calculated Synthesis and Characterization of Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) ... 14 IR spectra: The characteristic infrared frequencies of the complexes are summarized in Table (2). The absence of an absorption at 3261 cm -1 in the IR spectrum of the ligand shows that the amino groups of semicarbazide are absent, and the absence of a strong band at 1601 cm -1 shows the absence of ketonic groups of benzoylacetone. It confirms the elimination of water molecules and as a result, cyclization takes place through the formation of a macrocyclic ligand (tetraiminemacrocycle). A band observed at 3299 cm -1 assigned to V(N-H) of secondary amino group in the ligand. New bands appeared in the spectrum of the free ligand within the regions 1669, 1417, 1221 and 748 cm -1 , assignable to amide I V(C=O), amide II [V(C=N) + (N-H)], amide III [ (N-H)] and IV [(C=O)] bands (25) , respectively. It provides a strong evidence for the presence of a closed cyclic product. In the complexes spectra, the bands appear in the regions (1684-1659), (1415-1409), (1242-1238) and (767-763) cm -1 , assignable to amide I, II, III, and IV, respectively. In the IR spectrum of the free ligand, a band appears at 1586 cm -1 , corresponding the V(C=N) group. The IR spectra of these complexes show an absorption in the (1548-1515) range attributed to the imine, V(C=N). this absorption band is showing a shift to the lower side in the complexes, suggesting coordination through the nitrogen of the V(C=N) group (26) . A new broad band located at (586-568) cm -1 region in the IR spectra of all complexes could be assigned to the chloride ion (Table 2). The positions of this band suggested the ionic nature and the non-involvement of this group in coordination (27) , and therefore remained outside the coordination sphere. This observation were in good agreement with the conductance values of all the complexes and which have been supported the given formula for this complexes. Finally, the M-N bands are not observed in the IR complexes spectra since they are hidden under the broad bands of chloride ion. Table (2): Important IR spectral bands (cm -1 ) Compound (N-H) (C=N) I Amide II Amide III Amide IV Ionic Cl L 3299 1586 1669 1417 1221 748 ----[Co(L)]Cl2 3324 1538 1659 1412 1238 763 569 [Ni(L)]Cl2 3333 1548 168