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] (<b>1</b>), [Ni₃L¹₂]·H₂O·2CH₃CN (<b>2</b>), [Ni₂(L²)­(L³)­(CH₃CN)] (<b>3</b>), [Ni₂(L²)₂(H₂O)₂] (<b>4</b>), [Ni₂(L²)₂(DMF)₂]₂·2H₂O (<b>5</b>), and [Ni­(HL²)₂]·H₂O (<b>6</b>), were synthesized by reacting nitrophenol-based tripodal (<b>H</b>_<b>3</b><b>L</b>^<b>1</b>) and dipodal (<b>H</b>_<b>2</b><b>L</b>^<b>2</b>) 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 <b>2</b>, <b>3</b>, <b>5</b>, and <b>6</b> were determined using single-crystal X-ray crystallography. The compounds <b>1</b>, <b>3</b>, <b>4</b>, and <b>5</b> are dinuclear complexes where the two Ni­(II) centers have octahedral geometry with bridging phenoxo groups. Compound <b>2</b> is a trinuclear complex with two different types of Ni­(II) centers. In compound <b>3</b> one of the Ni­(II) centers has a coordinated acetonitrile molecule, whereas in compound <b>4</b>, a water molecule has occupied one coordination site of each Ni­(II) center. In complex <b>5</b>, the coordinated water of complex <b>4</b> was displaced by the dimethylformamide (DMF) during its crystallization. Complex <b>6</b> is mononuclear with two amine–bis­(phenolate) ligands in scissorlike fashion around the Ni­(II) metal center. The single crystals of <b>1</b> and <b>4</b> 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-<i>p</i>-nitropenylphosphate by transesterification mechanism. Complex <b>3</b> exhibits highest activity among complexes <b>1</b>–<b>6</b> with 3.86 × 10⁵ times greater rate enhancement than uncatalyzed reaction