The document describes the synthesis of Schiff base ligands derived from citral and valine and their complexes with Cu(II), Ni(II), and Co(II). The structures of the complexes were characterized using IR, UV-vis, and magnetic susceptibility measurements. The Cu(II) and Ni(II) complexes were proposed to have distorted octahedral geometry while the Co(II) complex had square-based pyramidal geometry. Antimicrobial testing showed that the metal complexes had higher activity than the free ligand, with the Co(II) complex being most active.

2.  Multidentate ligands and their metal complexes have played
an important role in the development of co-ordination
chemistry.
 Schiff bases are condensation products of primary amines with
carbonyl compounds and they were first reported by Schiff in
1864.
 Transition metal complexes derived from amino acid Schiff
bases have received much attention because of possible
biological and pharmacological activities.

3.  The chemical used for the preparation of Schiff base and their metal
complexes are analytical grade from BDH. The required chemicals
are given below:
 Citral
 Valine
 Copper (II) Nitrate
 Nickel(II) Nitrate
 Cobalt (II) Nitrate
 Ethanol

4.  A mixture of citral (0.5mol, 10 ml) and aqueous solution of L-
valine (0.5 mol, 10ml) was heated under reflux for 5-6 h at 60-650c.
 The completion of the reaction was monitored by TLC. The solution
was concentrated till its volume is one fourth of its initial volume,
allowed to stand for overnight, brownish red precipitate was
obtained.
 The obtained precipitate was filtered and repeatedly washed with
diethyl ether and dried

5.  To a hot magnetically stirred ethanolic solution of Schiff base ligands
(0.5 mol), an aqueous solution of metal ions [Cu(II) nitrate (0.5 mol
20ml)] was added drop by drop at 60-65°c.
 The mixture was continuously stirred for 30 min the intensity of the
color becomes translucent in nature.
 To the translucent solution 0.5mol potassium hydroxide solution was
added to maintain the PH of the medium.
 The mixture was further refluxed for 1h. The light green colored
precipitate was obtained, filtered out and washed repeatedly with
ethanol followed by diethyl ether and dried.

6.  To a hot magnetically stirred ethanolic solution of Schiff base ligand
(0.5M), an aqueous solution of metal ions [Ni(II) nitrate (0.5M,20ml)]
was added drop by drop at 60-65° C.
 The mixture was continuously stirred for 30minutes, the intensity of
the colour becomes translucent in nature.
 To the translucent solution 0.5M Potassium hydroxide solution was
added to maintain the pH of the solution.
 The mixture was further refluxed for 1 hour. The green coloured
precipitate was obtained, filtered out and washed repeatedly with
ethanol followed by diethyl ether and dried.

7.  To a hot magnetically stirred ethanolic solution of Schiff base
ligands (0.5M) an aqueous solution of metal ions [Co(II) nitrate
(0.5M,20ml)] was added drop by drop at 60-65ºC.
 The mixture was continuously stirred for 30mins; the intensity of
the colour becomes translucent in nature.
 To the translucent solution 0.5M.Potassium hydroxide solution was
added to maintain the pH of the solution.
 The mixture was further refluxed for 1 hour. The dirty brown
coloured precipitate was obtained, filtered and washed repeatedly
with ethanol followed by diethyl ether and dried.

8. IR Spectroscopy
 The binding mode of L-valine Schiff base derivative to metal in the
complexes has been studied by comparison of IR spectra of free
ligand precursors and metal complexes. Free ligand shows the
medium band at 1608cm-1 are assigned to the (-C=N-) stretching
vibration. The corresponding bands in the Cu(II) (A), Ni(II) (B), and
Co(II) complex(C) are (1632 cm-1, 1628 cm-1and 1630 cm-1) shifted
to the higher frequencies than excepted, due to coordination of this
moiety to the metal centers, the observed shifts are in the range 28-
32 cm-1.

9.  The carboxylate moiety of Schiff base shows, the vasy(COO-) and v sym(COO-)
respectively at 1587 and 1384 cm-1and for complexes has been structurally
characterized by the intense and broad absorptions in the range 1660-1330 cm-
1, due to asymmetric and symmetric stretching modes. The difference between
asymmetric and symmetric O=C=O stretching vibrations (∆v) has been used to
determine the mode of coordination of carboxylate moiety with metal ions. The
∆v= vas(COO-) - vs(COO-) for the complexes (A) (B), and (C) is the following:
∆v= 140cm-1 [vas =1567 – 1427 cm-1 ],142 cm-1 [vas =1565 – 1423 cm-1 ], and
202 cm-1 [1570-1368 cm-1] respectively, and this evidence strongly suggests
that the bridging coordination of carboxylate moiety of Schiff base ligand with
Cu(II) (A) and Ni(II) (B).In contrast, for Co(II) complex(C) monodentate
coordination nature.

10.  In complex A, the band at 1320, 1168 and 875cm-1 are tentatively assigned to
νas(O-NO2), νs(O-NO2), and δ(O-NO2) respectively (∆v=152 cm-1), similarly for
complex (C) the bands appeared at, 1333, 1178 and 965 cm-1 (∆v=155cm-1), which
indicates the contribution of nitrate moiety. The separation between the νas and νs
bands suggested that nitrate moieties bidentate or bridging coordination modes to
central metal ion. In contrast, complex (B) show the bands appeared at 1456, 1318
and 957 cm-1(∆v=138cm-1), which can be ascribed to monodentate coordination of
NO3 group.
 Some other stretching frequencies of interest are 615-520cm-1 and 476-411 cm-
1those characteristics of (M-N) and (M-O) bond frequencies respectively. The
bands at 3700-3500 cm-1 indicate the presence of water molecules in the complexes

11. IR Spectrum of Schiff Base

12. 3000 2000 1000
85
90
95
IR Spectrum of Copper(II) Complex (A)

13. Ni(II) complex
IR Spectrum of Nickel(II) Complex (B)

14. IR Spectrum of Cobalt(II) Complex (C)

15.  Copper(II) (A) complex has magnetic moments of 1.45 B.M,
small valve compared with spin-only value(1.8 B.M),
confirms dimeric structure of Cu(II) complexes and electronic
spectra show bands at 295, 615 and 734 nm, which are
attributed to intra-ligand charge transfer (ILCT)/n-π* and d-d
transitions respectively which support a tetragonally distorted
octahedral geometry.

16.  The observed magnetic moment value of the Ni(II)
complex(B) is 1.78 B.M, which is lower than the
expected spin-only value of 2.8 B.M for the octahedral
Ni(II) complexes, this may due to strong
antiferromagnetic interaction and evidence for the Ni-Ni
bond in the complex. The electronic spectrum of (A)
shows the band at 301, 659, 733 and 751 nm which are
attributed to n-π*, d-d, and charge transfer transitions
respectively. Based on the above observations distorted
octahedral geometry may be assigned.

17.  Whereas Cobalt(II) (C) exhibits magnetic moment of 1.35B.M
,which compared with spin-only value with low spin (1.7B.M)
value, correspond to t2g
6 eg
1 configuration suggested the
dimeric nature of Co(II) complex and the electronic spectra
shows the absorption bands at 300, 584 and 754 nm, assigned
as n-π* and d-d transitions respectively The positions of these
bands are complimentary of distorted square-based pyramidal
geometry around cobalt (II) ion.

18. 300 400 500 600 700
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Schiff base ligand
313
Abs
wavelength
UV Spectrum of Schiff Base

19. 300 350 400
0.2
0.4
0.6
580 600 620
0.040
0.042
0.044
730 740
0.044
0.046
wavelength
Cu(II) complex295
615
)
734
UV Spectrum of Copper(II) Complex (A)

20. 300 400 500 600 700 800 900
0.5
1.0
1.5
2.0
Abs
wavelength (nm)
301
600 700 800
0.06
0.08
0.10
0.12
0.14
Ni(II) complex
659
733
751
Wavelength (nm)
Abs
UV Spectrum of Nickel(II) Complex (B)

21. 300 400 500 600
0.5
1.0
1.5
2.0
600 700
0.20
0.25
Abs
wavelegth
300
Co(II) complex
584
754
UV Spectrum of Cobalt(II) Complex (C)

22.  In the vitro antimicrobial screening of the Schiff base and its
metal complexeswas tested against two bacteria, namely,
Staphylococcus aureus (Gram-positive), Escherichia coli
(Gram-negative), and fungus Candida albicans using disc
diffusion method. The compounds were tested at the
concentration of 1ml in ethanol and compared with known
antibiotics: Streptomycin.

23.  The bioactivity of the ligand and its complexes is found to be
in the following order: Co(II) > Ni(II) > Cu(II) >SB. The
difference in antimicrobial activity is due to the nature of
metal ions and also the cell membrane of the microorganisms.

24. Compounds Zone of inhibition (mm)
Escherichia
Coli
Staphylococ
cus aureus
Candida
albicans
Schiff Base (SB) 5 10 2
Complex A 12 14 4
Complex B 10 16 4
Complex C 18 18 5
Staandard 20 22 10

25.  The structural elucidation of Schiff base and their metal complexes are carried out
based on the analytical techniques. The structure for Cu(II), Ni(II) and Co(II)
complexes have been proposed as tetragonally distorted octahedral distorted
octahedral and square-based pyramidal geometry, supported by UV spectral
studies.
 Thus the IR spectrum indicates that the Schiff base ligand in the Cu(II) and Ni(II)
complexes under investigation, function as tridentate in nature respectively, and the
binding sites are azomethine nitrogen, µ2 carboxylato oxygen atoms. The other
positions in the coordination sphere would be completed by a water molecule and
nitrate anion. In contrast, Co(II) complex, Schiff base act as bidentate in nature and
other sites are occupied by water molecules and nitrate anions.
 The magnetic susceptibility studies reveal that low B.M values 1.45, 1.7, and 1.35
respectively, shows that Cu(II), Ni(II) and Co(II) complexes are bimetallic in nature
with strong antiferromagnetic interactions

26.  The antimicrobial screening of the Schiff base and its metal complexes are
carried out against two Gram-negative bacteria, namely, Staphylococcus
aureus Escherichia coli, and fungus Candida albicans using disc diffusion
method. The results reveal that the metal complexes are more active than
the free ligand. The bioactivity of the ligand and its complexes are found to
be in the following order: Co(II) > Ni(II) > Cu(II) >SB. The difference in
antimicrobial activity is due to the nature of metal ions and also the cell
membrane of the microorganisms.