Comparative DNA Binding Abilities and Phosphatase-Like Activities of Mono‑, Di‑, and Trinuclear Ni(II) Complexes: The Influence of Ligand Denticity, Metal–Metal Distance, and Coordinating Solvent/Anion on Kinetics Studies

Six novel Ni­(II) complexes, namely,
[Ni2(HL1)­(OAc)2] (1),
[Ni3L12]·H2O·2CH3CN (2), [Ni2(L2)­(L3)­(CH3CN)] (3), [Ni2(L2)2(H2O)2] (4), [Ni2(L2)2(DMF)2]2·2H2O (5), and [Ni­(HL2)2]·H2O (6), were synthesized
by reacting nitrophenol-based tripodal (H3L1) and dipodal (H2L2) Schiff base ligands with Ni­(II) metal salts at ambient conditions.
All the complexes were fully characterized with different spectroscopic
techniques such as elemental analyses, IR, UV–vis spectroscopy,
and electrospray ionization mass spectrometry. The solid-state structures
of 2, 3, 5, and 6 were determined using single-crystal X-ray crystallography. The
compounds 1, 3, 4, and 5 are dinuclear complexes where the two Ni­(II) centers have
octahedral geometry with bridging phenoxo groups. Compound 2 is a trinuclear complex with two different types of Ni­(II) centers.
In compound 3 one of the Ni­(II) centers has a coordinated
acetonitrile molecule, whereas in compound 4, a water
molecule has occupied one coordination site of each Ni­(II) center.
In complex 5, the coordinated water of complex 4 was displaced by the dimethylformamide (DMF) during its
crystallization. Complex 6 is mononuclear with two amine–bis­(phenolate)
ligands in scissorlike fashion around the Ni­(II) metal center. The
single crystals of 1 and 4 could not be
obtained; however, from the spectroscopic data and physicochemical
properties (electronic and redox properties) it was assumed that the
structures of these complexes are quite similar to other analogues.
DNA binding abilities and phosphatase-like activities of all characterized
complexes were also investigated. The ligand denticity, coordinated
anions/solvents (such as acetate, acetonitrile, water, and DMF), and
cooperative action of two metal centers play a significant role in
the phosphate ester bond cleavage of 2-hydroxypropyl-p-nitropenylphosphate by transesterification mechanism. Complex 3 exhibits highest activity among complexes 1–6 with 3.86 × 105 times greater
rate enhancement than uncatalyzed reaction.