Synthesis, IR, and NMR Study of Some Cu(II), Ni(II), Co(II) Complexes of O-Vanillin and 2- Amino Pyridine.

Introduction

 The -HC=N group in the bidentate ligand is normally an admirable chelating agent. Five or six-membered rings with metal ion form particularly when electronic donating groups –OH and – OCH3 are attached near azomethine group^1-3. Organic reagents play a dynamic role in modern analytical devices, Schiff bases have brilliant features and structural similarities among these reagents. Azomethine moiety forms complexes with a diversity of metals^4,5. Many Cu (II) complexes have seemed in the literature with various biological activities^6,7. The presence of a heterocyclic ring in the Schiff base moiety leads to adding stability to the structure of Schiff base metal complexes⁸. Cobalt, Nickel, and Copper are known to have a great attraction for coordination to the heteroatoms as of their small size and high nuclear charge⁹. Aryl group or heterocyclic groups containing Schiff bases remain to possess good biological activities. Therefore, research scholars have been attracted to this topic recently. 2,3-aminopyridine and o-vanillin containing Schiff bases used as ionophores have been used as ionophores in Cu(II) selective electrochemical sensors¹⁰. Structural elucidation. salicylamide show significant photochromic nature where interconversion through intramolecular hydrogen transfer¹¹ is caused between enolimine and keto-imine tautomer due to the absorption of light. O-Vanillin is a natural compound that encloses -OCH₃, -OH, and –CHO group which shows contradictory effects. Schiff bases from o-vanillin and 2-amino pyridine and its complexes with metal have been found to keep biological activity^12-15. o-vanillin Schiff bases are similar to salicylaldehyde Schiff bases, as their denticity is two or more than two due to the presence of the hydroxyl (–OH) group at the ortho position¹⁶. The most prominent flavor is vanillin and the aroma compound present in vanillin is used as a flavoring agent in foods, pharmaceuticals, and beverages.¹⁷ There is a shortage of research on o-vanillin, the Schiff base that contains o-vanillin moiety forms stable complexes with several metal ions. Therefore ortho vanillin is the ideal contender in the synthesis of different aromatic Schiff bases^18-19. 

Experimental

In the present study here we tried to synthesize a novel ligand of Schiff base and its metal complexes of Ni(II), Co(II), and Cu(II)  using Schiff base of ortho Vanillin and 2-aminopyridine. The easy availability and low cost make our work easier. Acetate of metal salts was used.

Synthesis of N (3-methoxy-2-hydroxybenzylidine) pyridine-2-amine.[L4]

0.001M o-vanillin (1.522g) is dissolved in 10ml methanol and mixed with 0.001M 2-aminopyridine (0.94g) in 10ml methanol. The resulting yellow solution was exposed to sunlight and refluxed for 2hr in a round bottom flask attached with a water condenser, a dark orange precipitate was obtained. The completion of the reaction was tested by TLC. TLC was done using n-hexane and acetone in the ratio 8:2. The obtained product was filtered under suction, cleaned using ethanol, recrystallized using ethanol as solvent, and further dried over silica gel. The compound obtained had a pleasant fragrance. The reaction is given below.

Figure 1: Reaction of Schiff base synthesis. Click here to View figure

Synthesis of metal complexes Synthesis of Co [L4] complex

 Solution of Cobalt chloride (Co(OAc)₂.6H₂O),(0.5984g,2.5mmol) in 20ml methyl alcohol was added to (0.76g,5mmol) of Schiff base ligand(L4). The mixture was refluxed in a  250 ml round bottom flask for two hours. The complex was allowed to precipitate. A blue coloured precipitate was obtained that was filtered and washed with ethanol and then dried in the oven at 50⁰C. Further, it was dried for analysis.⁷ The complexes of Ni(II) and Cu(II) were prepared similarly by using (Ni(OAc)₂.6H₂O) and (Cu(OAc)₂.6H₂O) respectively.

Result and discussion:

Table 1: The analytical and physical data of Schiff base ligand and its metal complexes.

Compounds	Formula	M.W. g/mol	Colour	Yield %	M.P. 0C	µeff B.M.	Elemental Analysis   C            H          N            M
(L4)	C13H12N2O2	198.24	Dark orange	73	80- 82	–	68.32 (68.43)	5.42 (5.26)	12.18 (12.28)	–
Co(L4)2	C26H22N4O4Co	513.38	Blue	82	316- 317	3.85	60.71 (60.82)	4.30 (4.32)	10.80 (10.91)	11.88
Ni(L4)2	C26H22N4O4Ni	513.14	Light green	49	>3oo	2.80	60.69 (60.85)	4.28 (4.32)	10.87 (10.92)	11.44
Cu(L4)	C13H13O3N2ClCu	344.23	Deep blue	67	–	1.94	45.30 (45.35)	3.80 (3.81)	8.10 (8.14)	18.46

 

All compounds are inert at room temperature and have a sharp MP that indicates the purity of the compound. The elemental analysis of compounds cooperating with the composition suggested for compounds, colour, yield, melting point, and percentage of composition of C, H and N are given in Table 1.

Table 2: Conductance study

Compounds	Co(L4)2	Ni(L4)	Cu(L4)
Ω (DMSO) Scm2mol-1	15.33	19.92	16.08

 

Molar conductance value for complexes ranges from 15.33 to 19.92 Scm²mol^-1which predicts the non-electrolyte type behavior of Schiff base complexes in DMSO. The higher value is due to steric factors and donor behavior. The conductance behavior of metal complexes is different from those of p-block elements like Te show weak electrolytic nature.

 Table 3: IR Spectra of Compound.

compounds	µ(O-H) cm-1	µ(C=N)cm-1	µ(-OCH3) cm-1	Pyridine (C-N-C)	µ(M-N) cm-1	µ(M-O) cm-1
(L4)	3355	1675	1299	1475	–	–
Co(L4)2	–	1661	1232	1450	496	521
Ni(L4)2	–	1500	1320	1495	481	575
Cu(L4)	–	1550	1310	1500	470	578

 

The stretch vibration due to the hydroxyl group in the ligand was found at 3355 cm^-1 which is absent in all complexes and shows the formation of the O-M bond by deprotonating the hydroxyl group. The strong band between 1500-1675 cm^-1 is due to v(-HC=N) showing a slight decrease in IR value than free ligand indicates metal nitrogen bond formation coordinating through the nitrogen of azomethine. The bands observed in the complexes’ spectra at 470-496 cm^-1 and 521578cm^-1region are due to v(M-N) and v(M-O) bonds respectively. The value above 300cm^-1 in the complexes for aromatic compounds. The value is between 2800-2900 cm^-1 for metal chelates in all complexes. The spectral value shows in Table 3. The v(C-N-C) strong band at 1462cm^-1 upon complexation, shifts to a higher frequency which is due to coordination with the nitrogen atom of the pyridine ring.

Figure 2: FT-IR Schiff base. Click here to View figure

Figure 3: FT-IR Co(II) complex. Click here to View figure

Figure 4: FT-IR of Ni(II) complex. Click here to View figure

Figure 5: FT-IR Cu(II) complex. Click here to View figure

     Table 4: H¹NMR Spectra.

compounds	Phenolic OH Ϩ ppm	Azomethine HC=N) Ϩ ppm	At ring proton Ϩ ppm	-OCH3
(L4)	11.65(s,1H)	8.75(d,1H)	6.98 (m,7H)	3.68(s,3H)
Co(L4)2	–	7.52(s,1H)	6.85-7.91(m,7H)	3.81(s,3H)
Ni(L4)2	–	7.37(s,1H)	6.83-8.31(m,7H)	3.99(s,3H)
Cu(L4)	–	7.53(s,1H)	6.83-8.09(m,7H)	3.92(s,3H)

 

Figure 6: H1-NMR of Schiff base. Click here to View figure

Figure 7: H1– NMR of Cu(II) complex. Click here to View figure

The H¹ NMR spectrum of the ligand displayed a signal by Ϩ 8.75 ppm in free ligand for azomethine proton which shift to lower values after complex formation at7.52 ppm,7.37ppm and7.53 ppm for Ni(II), Co(II), Cu(II) respectively. The multiple corresponds to aromatic protons that appeared between 6.83-8.51 ppm are given to aromatic protons. The hydroxyl proton of ligand looks at 11.65ppm which disappears in the complexes and indicates that the OH group has been deprotonated and bonded to the metal ion. Signal displays a downfield shift from 8.86 to7.23 ppm suggesting the decrease of electron density and deshielding of azomethine proton as a result of which nitrogen of azomethine group coordinates to the metal atom.

Electronic Spectra

The electronic spectrum provides the information regarding the electronic atmosphere of the metal; The electronic spectra are resolved in DMF i.e. (dimethyl formamide) The spectra were observed i.e.(10,000-50,000 cm^-1) The intensive bands at 42194cm-1,37037cm-1 and 29850 cm-¹ suggested the presence of π→π* and n→π* transition. This π→π* transition is transferred to a longer wavelength resulting from coordination to metal, confirming the formation of Schiff base metal complexes. Various bands are grouped into the range between 210-245 nm, 280-290nm, 302372nm, and 421nm. The first and second band is due to the π to π* transition of benzene & the band above 400 nm displayed by Schiff base is attributed to imino n→π*^6,7 The keto-enol tautomer of Schiff base is implied by the presence of these bands in methanol. The transition of these complexes shifted to a higher wavelength. All the maxima were observed due to the charge transfer band. The donation of lone pairs of a nitrogen atom (N→ M) or oxygen atom (O→ M) to the metal ion is considered to be the cause of this shift. In the Co(II) complex, the expected band of wavelength 840-960 nm has shown low symmetry transition ⁴A₂→ ⁴T₁ (F), and the band at 500-600 nm has shown ⁴A₂→ ⁴T₁ (P)  transition which suggested the distorted tetrahedral geometry⁷. In the electronic spectra of the Cu (II) complex which is square planar, the transitions are   ²B₁g→ ²A₁g and ²B₁g →²Eg with particular absorption at 515- 525nm and 645655 nm which was due to John Teller distortion (J-T)^6,8,17. square planar complexes showed a broad absorption peak at 600-700nm, & the peak at 515-520 nm merged with the broadband thus only a single band was observed¹³. One strong band at 555 nm was displayed by the Ni (II) complexes. This is allocated to the square planar ¹A₁g→ ¹A₂g transition. This transition value lies within the range of 600-450 nm which is responsible for the green color¹⁸. 

Magnetic measurement

The value of magnetic moment (B.M.) of all metal(II) complexes was obtained at room temperature. The value of an observed magnetic moment of  Co(II) complexes was seen in the range of 4.32-4.68 B.M. which signified three unpaired electrons in a tetrahedral atmosphere.¹¹ The value of a magnetic moment of the Ni(II) complex was 3.39-3.55 B.M. suggesting two unpaired electrons suggesting the square planar geometry. The measured magnetic moment value at1.931.97 B.M. for Cu(II) indicates one unpaired electron.

Table 5: Antibacterial Activity.

Name of compounds 1000 µg/mL	Inhibition zone
	Gram-positive bacteria		Gram-negative        bacteria
	Staphylococcus aureus	Bacillus          substillis	Escherichia coli	Klebsiella pneumonia
Ligand	–	–	13	12
Co(L5)2	14	11	15	1 9
Ni(L5)2	13	13	17	25
Cu(L5)2	7	6	15	1 8

The study of the antibacterial activity of the ligand and its complexes was done using two antibacterial drugs including Streptomycin and ampicillin against two gram-positive (S. aureus and B. substilis) and two gram-negative (E. Coli and Pneumonia). The results show that the antibacterial activity of metal complexes is improved than ligand¹⁸. This is due to changes in the structure due to the chelating and coordination trend to make metal complexes perform more powerful. The value of an antibacterial activity is given in table 5.

Conclusion

This part is used to discuss the results obtained from all the techniques used in characterizing the isolated Schiff base Synthesized from O-Vanillin and its metal complexes of Co(II), Ni(II), and Cu(II). The discussion will focus on some of the observed shrinking behaviours of the various synthesized ligands and complexes. The reaction of Schiff bases with transition metal compounds leads to different complexes. The most common of these is the inner sphere complex in which Schiff base bonded to metal atom through the oxygen atom of its deprotonated hydroxyl group and the nitrogen atom of its -C=N groups. That is those in which ligand binds as diatonic N₂O₂ donor system. All the complexes obtained possess tetrahedral geometry for Co, Ni, and square planar for Cu metals. The conclusions are based on Elemental analysis, IR, NMR, electronic spectra, and microanalysis results.

Acknowledgment

We are thankful to the CIF of Savitribai Phule University, Pune for providing the necessary facilities for research. Also, I express my gratitude to Dr. V.B. Tadke (HOD of chemistry department), Fergusson College, Pune. Special thanks to Aniket Tote for technical support. 

Conflict of Interest

Authors do not have a conflict of interest with each other.

Funding Sources

There is no funding source.

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