A Unique Nickel System having Versatile Catalytic Activity of Biological Significance

A new dinuclear nickel(II) complex, [Ni2(LH2)(H2O)2(OH)(NO3)](NO3)3 (1), of an “end-off” compartmental
ligand 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato,
has been synthesized and structurally characterized. The X-ray single
crystal structure analysis shows that the piperazine moieties assume
the expected chair conformation and are protonated. The complex 1 exhibits versatile catalytic activities of biological significance,
viz. catecholase, phosphatase, and DNA cleavage activities, etc. The
catecholase activity of the complex observed is very dependent on
the nature of the solvent. In acetonitrile medium, the complex is
inactive to exhibit catecholase activity. On the other hand, in methanol,
it catalyzes not only the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol
which is very difficult to oxidize, under aerobic conditions. UV−vis
spectroscopic investigation shows that TCC oxidation proceeds through
the formation of an intermediate. The intermediate has been characterized
by an electron spray ionizaton-mass spectrometry study, which suggests
a bidentate rather than a monodentate mode of TCC coordination in
that intermediate, and this proposition have been verified by density
functional theory calculation. The complex also exhibits phosphatase
(with substrate p-nitrophenylphosphate) and DNA cleavage
activities. The DNA cleavage activity exhibited by complex 1 most probably proceeds through a hydroxyl radical pathway. The bioactivity
study suggests the possible applications of complex 1 as a site specific recognition of DNA and/or as an anticancer agent.