A Combined Experimental and Theoretical Investigation on the Role of Halide Ligands on the Catecholase-like Activity of Mononuclear Nickel(II) Complexes with a Phenol-Based Tridentate Ligand

Three new mononuclear nickel(II) complexes, namely, [NiL1(H2O)3]I2·H2O (1), [NiL1(H2O)3]Br2·H2O (2), and [NiL1(H2O)3]Cl2·2H2O (3) [HL1 = 2-[(2-piperazin-1-ylethylimino)methyl]phenol], have been synthesized and structurally characterized. Structural characterization reveals that they possess similar structure: [NiL1(H2O)3]2+ complex cations, two halide counteranions, and lattice water molecules. One of the nitrogen atoms of the piperazine moiety is protonated to provide electrical neutrality to the system, a consequence observed in earlier studies (Inorg. Chem.2010, 49, 3121; Polyhedron2013, 52, 669). Catecholase-like activity has been investigated in methanol by a UV–vis spectrophotometric study using 3,5-di-tert-butylcatechol (3,5-DTBC) as the model substrate. Complexes 1 and 2 are highly active, but surprisingly 3 is totally inactive. The coordination chemistries of 1 and 2 remain unchanged in solution, whereas 3 behaves as a 1:1 electrolyte, as is evident from the conductivity study. Because of coordination of the chloride ligand to the metal in solution, it is proposed that 3,5-DTBC is not able to effectively approach an electrically neutral metal, and consequently complex 3 in solution does not show catecholase-like activity. Density functional theory (DFT) calculations corroborate well with the experimental observations and thus, in turn, support the proposed hypothesis of inactivity of 3. The cyclic voltametric study as well as DFT calculations suggests the possibility of a ligand-centered reduction at −1.1 V vs Ag/AgCl electrode. An electron paramagnetic resonance (EPR) experiment unambiguously hints at the generation of a radical from EPR-inactive 1 and 2 in the presence of 3,5-DTBC. Generation of H2O2 during catalysis has also been confirmed. DFT calculations support the ligand-centered radical generation, and thus a radical mechanism has been proposed for the catecholase-like activity exhibited by 1 and 2. Upon heating, 2 and 3 lose water molecules in two steps (first lattice waters, followed by coordinating water molecules), whereas 3 loses four water molecules in a single step, as revealed from thermogravimetric analysis. The totally dehydrated species are red, in all cases having square-planar geometry, and have amorphous nature, as is evident from a variable-temperature powder X-ray diffraction study.