- The document reports on the synthesis and characterization of Schiff base ligands derived from L-valine and their Cu(II), Ni(II), and Co(II) complexes. - Spectroscopic data revealed that the ligands bind as tridentate in the Cu(II) and Ni(II) complexes and bidentate in the Co(II) complex. - Magnetic susceptibility measurements showed the complexes have a bimetallic structure with antiferromagnetic interactions. - Antimicrobial testing found the metal complexes to have greater activity than the free ligand, with the Co(II) complex exhibiting the strongest effects.

2.  Multidentate ligands and their metal complexes have played an
important role in the development of co-ordination chemistry.
 Schiff base are a special class of ligands with azomethine
nitrogen and variety of donor atoms exhibiting interesting
coordination modes towards various metals.
 Transition metal complexes derived from amino acid Schiff bases
have received much attention because of possible biological and
pharmacological activities

3.  All the chemicals and solvents used in the present work
were of analytical grade. L-valine and Citral were
purchased from spectrochem.
 The solvents like diethyl ether and metal salts were
purchased from Merck. The metal salts used are in their
hydrated form.

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.  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.

12.  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.

13.  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.

14.  In the vitro antimicrobial screening of the Schiff base
and its metal complexes (Table 3) was 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.

15.  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.

16. 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

17.  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

18.  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.