Synthesis and Spectral Characterization of Schiff Base Cr(III), Mn(III), and Fe(III) Novel Macrocyclic Complexes Derived from Thiocarbohydrazide and Dicarbonyl Compound

: M(III) Schiff base macrocyclic complexes of the type [HLMX 2 ] where M = Cr(III), Mn(III), Fe(III) and X = Cl, OAc have been synthesized by condensation of acetylacetone and Thio-carbohydrazide (2:2) in the presence of divalent metal salt in methanolic medium. The complexes have been characterized with the help of elemental analysis, conductance measurements, magnetic measurements and their structural configuration have been determined by various spectroscopic (electronic, IR, 1 H NMR, 13 C NMR, GCMS) techniques. Electronic and magnetic moments of the complexes indicate that the geometries of the metal centers are octahedral.


Introduction
Macrocyclic and compounds have attracted increasing interest owing to their role in the understanding of molecular processes occurring in biochemistry, material science, catalysis 1 . It is well known that N and S atoms play a key role in the coordination of metal at the active sides of numerous metallobiomolecules 2 . Metall-organic chemistry is becoming an emerging area of research due to the demand of new metalbased antibacterial and antifingal compounds 3,4 . The aliphatic or aromatic amines can form strong five of six membered chelates rings which are able to produce the metal containing crosslinking agents with required properties 5 . There are a number of important molecules shows biological activities including antibacterial, antifungal [6][7][8][9] , antidiabetic 10 , antitumor 11 , antiproliferative 12,13 , anticancer 14,15 , herbicidal 16 , anticorrosion and anti-inflammatory activities 6 Schiff bases represent an important class of compounds because they are utilized as starting materials in the synthesis of industrial products. Moreover, Schiff bases are regarded as privileged ligands 17 . Due to their capability to form complexes with different transition metals can act as catalysts for many different reactions 18,19 . The discovery and development of effective antibacterial and antifungal drugs with novel mechanism of action have because an urgent task for infectious diseases research program 20 . Many investigations have proved that binding of a drug to a netalloelements enhances its activity and in some cases, the complex possesses even more healing properties that the parent drug 21 . In the present article, we report the synthesis and characterization of Schiff base derived from acetylacetone and thiocarbohydrazide, and its metal complexes to gain more information about related structural and spectral properties as well as their antimicrobial activities.

Reagents
The entire chemicals used were of the analytical reagent grade, acetylacetone and thiocarbohydrazide procured from Acros and s.d.-fine, respectively. Metal salts were purchased from Merck.

Synthesis of Schiff Base Macrocyclic Ligand
The macrocyclic Schiff base ligand have been synthesized by refluxing the reaction mixture of acetyl acetone (2 mmol) in ethanol (30 mL) and Thio-carbohydrazide (2 mmol) in ethanol (20 mL) for 6-8h with addition of 4-5 drops of concentrate HCl. The reaction mixture was filtered, dried under reduced pressure until a solid product was formed that was washed with cold ethanol and recrystalized from hot ethanol (yield 55-60%, scheme 1).

Synthesis of Cr(III), Mn(III), and Fe(III) Macrocyclic Complexes
A solution of trivalent metal salt (1 mmol) in methanol (20 mL) was added to a hot solution (75 o C) of macrocyclic ligand in ethanol (40 mL) and the reaction mixture was refluxed for 10-12h. The complex was precipitated by adding distilled water. The precipitate of the complex was filtered, washed with water, then with hot methanol and finally dried in vacuum desiccator over anhydrous CaCl 2 (yield 40-45%, scheme 2).

Analytical and Physical Measurements
The microanalysis of C, H, and N were estimated by elemental analyzer (Perkin Elmer 2400), at SAIF, CDRI, Lucknow, India and the metal contents of Mn (III), Co (III) and Cu (III) was determined using atomic absorption spectrophotometer Perkin Elmer 5000. (See Table I containing also some physical properties of the prepared compounds). The conductivity was measured on digital conductivity meter (HPG system, G-3001) in DMSO at room temperature. The magnetic susceptibility measurements were carried out on Vibrating Sample Magnetometer (Model PAR 155) at room temperature. Electronic spectra (in DMSO) were recorded on a Hitachi 330 spectrophotometer (5815-32573 cm -1 ). The IR spectra were performed using a FT-IR spectrophotometer Perkin Elmer 1650 in the range 4000-200 cm -1 using Nujol Mull. 1 H NMR and 13 C NMR spectra (at room temperature) (in DMSO-d 6 ) were recorded on a Bruker AVANCE II 300 DRX or average 400 DRX spectrometer with reference to Me 4 Si (0.0 ppm). The FAB mass spectra (at room temperature) were recorded on VG-70-S mass spectrometer. The purity of the Schiff base and its complexes were confirmed also by TLC [microcrystalline cellulose (E. Merck) was used for the preparation of thin layer, complete separation was achieved with the solvent system acetone/conc. HCl/water (86:8:7). R F values increase in the following order of M(III) complexes: Cr, Mn, Fe (0.28, 0.42, 0.61)] and HPLC. The HPLC chromatographic apparatus consisted of a model waters 600 pump with waters 600 controller, waters 2996 photodiode array detector with UV detection 254 nm, flow rate 1ml/min a discovery C 8 , 15cm X 4.6mm I.D, 5µm particles and mobile phase methanol:25 mm KH 2 PO 4 (20:80) PH=3 techniques.

Mass Spectra
The . This data is in good agreement with the proposed molecular formula for these complexes. In addition to the peaks due to the molecular ion, the spectra exhibit peaks assignable to various fragments arising from the thermal cleavage of the complexes. The peak intensity gives an idea of the stability of the fragments.

Infrared Spectra
The IR spectra provide valuable information regarding the nature of functional group attached to the metal atom. The presence of a single medium band in the region 3250-3330 cm -1 in the complexes may be assigned to N-H stretch 22 . It was noted that a pair of bands corresponding to ѵ (NH 2 ) at 3245 cm¯1 and 3309 cm¯1 are present in the spectra of the thiocarbohydrazide . The value of ѵ (C=N) stretching vibration is found lower (1520-1560 cm¯1) than the expected value (1580-1650 cm¯1). This lower value of ѵ (C=N) stretching may be explained on the basis of a drift of lone pair density of azomethine nitrogen towards the metal atom 23 indicating that coordination takes place through nitrogen of (C=N) groups. The bands present in the range 3020-3040 cm¯1 may be assigned due to (C-H) stretching vibrations of benzil and naphthalene ring 24 . The C-N stretch in the range 1000-1300 cm¯1. The band near 780 cm¯1in thiocarbohydrazide may be assigned as being due to free ѵ (C=S). The band at 3292-3438 cm¯1 is due to the presence of OH group in the complexes. This band is also present in the spectra of all the complexes, which indicate that sulphur is not coordinating to the metal atom 25 . The far infrared spectra show bands in the region 420-450 cm¯1 corresponding to ѵ (M-N) vibrations 26 . The presence of bands in all complexes in the region 420-450 cm¯1 originates from the (M-N) azomethine vibrational modes and identifies coordination of azomethine nitrogen 27 . The bands present at 290-310 cm¯1 may be assigned due to ѵ (M-Cl) vibrations 28 . The appearance of two characteristic bands in the ranges 1561-1559cm− 1 and 1370-1367 cm− 1 in the case of complexes was attributed to ν asym (COO − ) and ν sym (COO − ), respectively, indicating the participation of the carboxylate oxygen in the complexes formation. The mode of coordination of carboxylate group has often been deduced from the magnitude of the observed separation between the ν asym (COO − ) and ν sym (COO − ). The separation value, Δν(COO − ), between ν asym (COO − ) and ν sym (COO − ), in these complexes were more than 190 cm− 1 (191-193cm− 1 ) 29 .

H NMR
A survey of literature reveals that the NMR spectroscopy has been proved useful in establishing the structure and nature of many Schiff base ligand and their complexes. The 1 H NMR spectra of Schiff base ligand (HL) was recorded in d 6 -dimethylsulfoxide (DMFO-d 6 ) solution using Me 4 Si (TMS) as internal standard. The 1 H NMR spectra of the ligand shows broad signal at 9.4-12.1 ppm due to the -NH [30]. The multiplets in the region 1.12-1.56 ppm may be assigned to aliphatic proton 30 . 13
The electronic spectra of the iron (III) complexes (5) and (6) gave two bands at 9850-9980, and 27650-27760 cm− 1 , which could be assigned to the transitions 6 A 1g → 4 T 1g and 6 A 1g → 4 T 2g , respectively, suggesting an octahedral geometry around Fe (III) ion 33 . The complexes 5 and 6 show magnetic moment values in the range 5.38-5.45 B.M., which is smaller than the calculated value for two Fe (III) ions in octahedral geometries and this may be due to antiferromagnetism between the two ion-centers.
The spectral characterization show the formula for macrocyclic complexes as [M(C 12 H 20 N 8 S 2 )X 2 ]X where M = Cr(III), Mn(III), Fe(III), and X = Cl − , CH3COO − . The measurements of molar conductance in DMSO are 21-66 Scm 2 mol -1 . All complexes give satisfactory elemental analyses data as shown in table 1. On the bases of various studies like elemental analysis, conductance measurements, magnetic susceptibilities, IR, NMR, electronic and mass spectral studies the geometry of the complexes are octahedral.

Conclusion
The analytical data show the presence of one metal ion per ligand molecule and suggest a mononuclear structure for the complexes. The electronic spectral data is in the favor of octahedral geometry of the complexes.