1514
B. Lakshmi et al. / Polyhedron 30 (2011) 1507–1515
Fig. 7. Spartan minimized structures of the Cu(II) (7) and Ni(II) (4) complexes.
ity of the Schiff bases exhibited a considerable enhancement on
coordination with the metal ions against all fungal strains, and
the metal complexes showed good antifungal activity against A. ni-
ger, Penicillium species and C. albicans. It was evident from the data
that this activity significantly increased on coordination. This
enhancement in the activity may be rationalized on the basis that
their structures mainly possess an additional C@N bond. It has
been suggested that Schiff bases with nitrogen and oxygen donor
systems inhibit enzyme activity since the enzymes which require
these groups for their activity appear to be especially more suscep-
tible to deactivation by metal ions on coordination. Moreover,
coordination reduces the polarity [51,52] of the metal ion, mainly
because of the partial sharing of its positive charge with the donor
groups [53,54] within the chelate ring system formed during coor-
dination. This process, in turn, increases the lipophilic nature of the
central metal atom, which favors its permeation more efficiently
through the lipid layer of the microorganism [55–57], thus
destroying them more aggressively.
Some important factors, such as the nature of the metal ion,
nature of the ligand, coordinating sites, geometry of the complex,
concentration, hydrophilicity, lipophilicity and presence of co-li-
gands, have considerable influence on the antibacterial activity.
Certainly, steric and pharmacokinetic factors also play a decisive
role in deciding the potency of an antimicrobial agent. Apart from
this, the mode of action of these compounds may also invoke
hydrogen bonds through the C@N–N@CH– group with the active
centers and thus interfere with the normal cell process. The pres-
ence of lipophilic and polar substituents is expected to enhance
antibacterial activity. Bis-hydrazone ligands with multifunctional-
ity have a greater chance of interaction either with nucleoside
bases (even after complexation with metal ions) or with biologi-
cally essential metal ions present in the biosystem, and can be
promising candidates as bactericides since they always tend to
interact, especially with some enzymatic functional groups, in or-
der to achieve higher coordination numbers [58].
shown in Fig. 7 (Scheme 1). Schiff bases and their Co(II), Ni(II)
and Cu(II) complexes were found to be potentially active towards
microbial strains.
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The synthesized 22-membered macrocyclic glyoxal, biacetyl
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The metal ion is coordinated through the azomethine nitrogen
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