Design and Biological Profiling of Newly Synthesized Macrocyclic Complexes of Transition Metals

Research on macrocyclic transition metal complexes has proven to be particularly fascinating for a variety of fields, including biology, catalysis, medicine, etc.. [1-10]. These are incredibly alluring and are well known for their functions in numerous biological systems. In an effort to mimic the physical and chemical characteristics of biological processes, a great deal of research has lately been done on the coordination chemistry of transition metal complexes with Schiff base ligands. Recently, the chemistry of macrocyclic compounds has received a lot of attention because of the possible applications that may be made with them and their significance in the field of coordination chemistry [11-16]. As a result, the investigation of macrocyclic compounds is becoming a more prominent field of study. Using metal ions as templates to lead the condensation reaction towards ring closure is the most efficient way to prepare macro cycles [17-19].

Research has been done a lot on transition-metal complexes containing nitrogen donor ligands  because they can be used in many ways and have interesting stereo chemistry [20-23.

For a very long time, a number of nitrogen donor macrocyclic compounds have been utilised in a wide range of catalytic, analytical, industrial, and  applications in medical field[24-25]. There is a significant propensity, when nitrogen is present in macro cycles, for the formation of solid complexes involving transition metals. There have been reports that certain macro cyclic complexes have significant antibacterial, anti-fungal, and anti-HIV properties [26-28]. In the process of DNA binding and cleavage, macro-cyclic copper complexes are utilized, whereas macro-cyclic nickel complexes are utilized in the process of DNA recognition and oxidation [29].  Strong anticoagulation, antiviral, antibacterial, and anti-fungal drugs can be produced by the condensation of aromatic amines with aromatic diketone. Schiff bases are created in this process [30-31]. Copper (II) complexes that include isatin Schiff base ligand have the potential to act as anticancer drugs. In a prior study, it was reported that macro-cyclic compounds generated from isatin and ethylenediamine existed [32-35].

In the present study we have synthesized novel complexes of some 3d- transition metals (Mn, Co, and Ni) derived from isatin and 3,4-diaminotoluene (3,4-DAT). Spectroscopic methods, including UV, FTIR, NMR, mass spectroscopy, and others, were used to characterize the synthesized metal complexes. These complexes were used for antibacterial studies.

2. Experimental
   1.  Materials

All the materials used for synthesis were of analytical grade and used without further purification. The ultraviolet spectroscopy has been done in 10^-5 molar methanolic solution. The antibacterial studies have been performed using agar well diffusion method. Chloramphenicol was used as standard drug.

1. 2.  Synthesis of complexes

Template method reported earlier was used for synthesis of all the complexes [36]. In the method, Isatin, 3.4-diaminotoluene (3,4-DAT) and metal salt were taken in the ratio 2:2:1, respectively. The 0.294 gm (2mmol) of isatin is dissolved in 20 ml methanol and stirred for 15 min. Then, this solution is added into another solution of 0.244 gm (2mmol) of 3,4-DAT in 20 ml methanol. Then the mixture of these solutions was added into the methanolic solution of metal salts containing corresponding weight for 1 mmol of metal salts MX₂, where M^(II)= Mn, Co, Ni and X= Cl and reflux for 7-8 hr. The reaction completion is monitored by TLC. The reaction mixture, after refluxing, is cooled at room temperature, washed with methanol, and dried in oven at 60⁰C until complete drying. Then, the product was collected in powder form in sample tube. The yield, which is about 75%-80%, was very good for all complexes. The proposed scheme of synthesis is depicted-

Scheme 1. Scheme of synthesis of complexes

1. 3.  Physical measurement

IR and UV techniques were done at the Sophisticated Analytical Instrument Facility (SAIF), IIT Roorkee. The IR spectral data were recorded at a NICOLET 6700 FT-IR (Thermo Scientific) infrared spectrometer using KBr as supporting materials. For UV-Vis spectra, Shimadzu UV-2450 UV-Vis spectrophotometer was used with wavelength range between 200-800 nm. Sample preparation was done using 10^-4 molar solution of complexes in methanol. The mass spectroscopy and ¹H NMR spectra were recorded at IIT Ropar, Punjab. DMSO was used ass solvent in    ¹H-NMR analysis. The antibacterial studies were performed at department of Botany and Microbiology, Gurukula Kangri Deemed to be university Haridwar. Synthesized complexes have been described according to their physical properties in table 3.1.

3. Results and discussion
   1.  Elemental analysis

Table 3.1

The results of elemental analysis are given in table 3.1.The elemental analysis showed good agreement with the theoretically calculated percentage analysis for each complex. The molar conductance values have been checked and the values describe that the complexes have non -electrolyte nature. The synthesized complexes thus can be formulated as [ML₁X₂] where Metals = Manganese, Cobalt, and Nickle and X=Cl

1. 2. Mass spectroscopy

Mass spectra of synthesized metal complexes correlate with the molecular formula of the complexes. The m/z peaks at 591.93, 595.96 and 595.93 for manganese, cobalt, and nickel complexes.

Mass spectra of manganese complex

Mass spectra of cobalt complex

Mass spectra of nickle complex

1. 3. FTIR spectra

The IR data for all the three macrocyclic complexes have been shown in fig.-

Some of the prominent bands have been listed in table 3.2

Metal complexes	Stretching	stretching	stretching	stretching
[Mn(C30H22N6Cl2)]	1621	1122,1085, 737	2933	431
[Co(C30H22N6Cl2)]	1612	1188, 1166, 817	2900	432
[Ni(C30H22N6Cl2)]	1611	1197,1035, 738	2917	458

Vestigial absorption occurring near 1700 cm^-1 n all the complexes suggests the absence of carbonyl group (>CO) due to formation complexes. The appearance of a robust band situated between 1594-1621 cm^-1 (for imine) shows the condensation of amino and carbonyl groups [37,38]. Other distinguishing bands for all complexes in the range of 1180–1396 cm^-1 , 737–817 cm^-1 and 2933–3000 appeared due to v(C=C), v(C–H) stretching and v(N-H stretching), respectively [39]. As supplemental information, the other absorption characteristics that were noted in the 431–542 cm-1 range could be attributed to v(M–N) vibrations, which are explained by the ligand's nitrogen bonding with the metal. [40].

1. 4. Electronic spectra

The synthesised compounds' electronic spectra show bands in the region of 230 nm, 267 nm, and 342 nm. The lower wavelength band arises due to the π-π^* transition between nitrogen of isatin ring with its aromatic system. Electronic spectra description has been given in table-

The bands at 342 may be attributed to n- π^* transition of lone pair of nitrogen of C=N delocalized with π bonds of the aromatic ring. All the complexes show intense band in the region of 36968 to 37453 cm^-1 this could be linked to the π- π^* transitions for carbon and nitrogen double bond conjugated system.

It is possible to associate the band in the cobalt complex at 37968 with charge transfer spectra [41-43]. Electronic spectra of all the complexes have been shown in following figures-

NMR spectra

¹H-NMR values for all complexes show signal in the range of 8.85 to 10.5 ppm. These numbers pertain to the isatin moiety's NH protons. . The signals between the range of 6.07 to 7.67 are for protons of aromatic ring of 3,4-DAT. [44-45]

NMR spectra of manganese complex

¹H-NMR spectra of cobalt complex

¹H- NMR spectra of nickel complex

Antibacterial StudyGram-positive bacteria including S. aureus, S. pneumoniae, and B. cereus, as well as gram-negative bacteria like S. typhi and P. aeruginosa, were used to test the created metal complexes' antibacterial properties. DMSO was used as a solvent to create complicated solutions with a concentration of 100 mg/ml. Each of the four wells in the plates held a volume of 100 µL of the solution. Of the four wells, two have one compound and the other two have second compound. The standard was the antibacterial medication chloramphenicol, while the negative control was DMSO. When it comes to these microorganisms, the metal complexes have demonstrated excellent action. Comparing the cobalt and nickel complexes to the ordinary chloramphenicol, the former have demonstrated the maximum efficacy against all strains, while the latter has demonstrated an excellent antibacterial activity against S. aureus. The graphical representation of zone of inhibition shown by the complexes is demonstrated in the picture below

CONCLUSION