Four side-off compartmental ligands L¹–L⁴ [L¹ = N,N′-ethylenebis(3-formyl-5-methyl-salicylaldimine), L² = N,N′-1-methylethylenebis(3-formyl-5-methylsalicylaldimine), L³ = N,N′-1,1-dimethylethylenebis(3-formyl-5-methylsalicylaldimine) and L⁴= N,N′-cyclohexenebis(3-formyl-5-methylsalicylaldimine)] having two binding sites, N₂O₂ and O₄, have been chosen to synthesize mononuclear and dinuclear manganese(III) complexes with the aim to study their catecholase activity using 3,5-di-tert-butylcatechol (3,5-DTBC) as substrate in the presence of molecular oxygen. In all cases only mononuclear manganese complexes (1–4) were obtained, with manganese coordination taking place at the N₂O₂binding site only, irrespective of the amount of manganese salt used. All these complexes have been characterized by routine physico-chemical techniques. Complex MnL²Cl·4H₂O (2) has further been structurally characterized by X-ray single crystal structure analysis. Four dinuclear manganese complexes, 5–8, were obtained after condensing the two pending formyl groups on each ligand (L¹–L⁴) with aniline followed by reaction with MnCl₂ to put the second Mn atom onto another N₂O₂ site. The catalytic activity of all complexes 1–8 has been investigated following the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) with molecular oxygen in two different solvents, methanol and acetonitrile. The study reveals that the catalytic activity is influenced by the solvent and to a significant extent by the backbone of the diamine and the behavior seems to be related mainly to steric rather than electronic factors. Experimental data suggest that a correlation, the lower the E_1/2 value the higher the catalytic activity, can be drawn between E_1/2 and V_max of the complexes in a particular solvent. The EPR measurements suggest that the catalytic property of the complexes is related to the metal center(s) participation rather than to a radical mechanism.

Three asymmetric tridentate acyl hydrazone Schiff base ligands namely L1, L2 and L3 were prepared via condensation of 4-methoxybenzohydrazide with picolinaldehyde, 1-(pyridin-2-yl)ethanone and phenyl(pyridin-2-yl)methanone respectively. Three bio-relevant mononuclear zinc(II) complexes [Zn(L1)Cl₂]·2H₂O (1), [Zn(L2)Cl₂] (2) and [Zn(L3)Cl₂] (3) were synthesized by treatment of zinc(II) chloride with the corresponding Schiff base ligands and characterised by the usual physicochemical techniques. The solid state structures of complexes 1 and 3 were evaluated by single crystal X-ray analysis. All complexes were able to hydrolyse the P–O bond of the phosphate monoester in 90% (v/v) DMSO–water medium using 4-nitrophenylphosphate (4-NPP) as the model substrate and the trend in their activity is 1 ≈ 2 > 3. On considering the highly efficient hydrolysis properties, complexes 1–3 were tested as potential therapeutic agents for cancer using HCT116 (human colorectal carcinoma), HepG2 (human hepatocellular carcinoma) and A549 (human non-small lung carcinoma) cells. Complex 2 showed the highest IC₅₀ values for anti-cancer activity towards all three cell lines among the three complexes and complex 3 showed the least activity as observed in the phosphatase activity study. The internucleosomal degradation of DNA during apoptosis can be detected by cell death detection ELISA. DNA fragmentation that leads to cell death was examined and when induced by complex 2 in HepG2 cells a significant level of DNA fragmentation was observed at regular intervals of time.