We have synthesized two isostructural mononuclear cobalt(III) complexes [1]NO₃·3H₂O and [1]NO₃·CH₃CO₂H·H₂O {[1]₊ = [Co(1,10-phenanthroline)₂Cl₂]⁺} and characterized by single crystal X-ray structural analyses. Mass spectral studies of the complexes indicate both the compounds to produce identical cationic species viz., [Co(phen)₂Cl₂]⁺ in methanol solution. [1]⁺ has been evaluated as model system for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate in methanol medium, which revealed that the cationic complex efficiently inhibits catalytic activity with k_cat value 9.65 × 10² h⁻¹. [1]⁺ cleaved pBR 322 DNA without addition of an activating agent. Further, the anti-cancer activity of [1]⁺ on human hepatocarcinoma cell line (HepG2) has been examined. The induction of apoptosis induced in the cell line was assessed base on the changes in cell morphology, which showed the efficacy of [1]⁺ to induce apoptosis in 53% of cells during 24 h treatment. Interestingly, the observed IC₅₀ values reveal that [1]⁺ brings about conformational change on DNA strongly and exhibits remarkable cytotoxicity.

An unprecedented solid of coper(II) complexes [Cu(dpa)₂NCS]₂[Cu(dpa)₂(NCS)₂](ClO₄)₂ (1) [dpa = 2,2'-dipyridylamine; SCN = thiocyanate], has been synthesized and crystallographically characterized with the aim to study the catecholase activity. The Cu(II) complex mimics the full catalytic cycle of the active site of catechol oxidase enzyme in acetonitrile medium with a turnover number of 4.788 × 10³ h⁻¹ along with the production of semiquinone radical and hydrogen peroxide. In situ generation of Cu(I) species in the catalytic pathway of catechol oxidation was established by electrochemical study and further confirmed by electron paramagnetic resonance (EPR) spectroscopy.

A mononuclear cobalt(II) complex, [Co(phen) ₂Cl ₂], (phen = 1,10-phenanthroline) has been synthesized and structurally characterized by different spectroscopic methods including single crystal X-ray structural study.X-ray crystal structural analysis revealed that the cobalt(II) complex crystallizes in amonoclinicsystem with C2/c space group and exists in cis-configuration in its crystalline state. Room temperature magnetic measurement accounts for 3e paramagnetism and indicates high spin cobalt(II) in the solid state. The cobalt(II) complex has been evaluated as a functional model for phosphatase enzyme by using 4-nitrophenylphosphate (PNPP) as a standard substrate in aqueous DMF medium. This mononuclear cobalt(II) complex exhibits good hydrolytic phosphoester cleavage efficiency with k_cat value of 3.78 × 10 ²h ⁻¹.

In this work, we have synthesised and crystallographically characterized a mononuclear iron(II) complex, [Fe(phen)₃](NO₃)₂ ·2H₂O (1) (phen = 1,10-phenanthroline). Single crystal X-ray diffraction (SXRD) analysis revealed that 1 crystallizes in monoclinic system with P 1⁻ space group. The lattice water molecules in 1 form a water-nitrate cluster, (H₂O)₂... (NO₃)₂ through strong H-bonding interaction mediated via iron(II) complex in a unique binding motif and provide additional stability to the compound in the solid state. This iron(II)complex is able to catalyze the cleavage of aromatic C-C linkage of 2,5-dihydroxybenzoic acid (Gentisic acid, GA) in oxygen environment. The iron(II) complex in the presence of two equivalent of triethylamine (Et₃N) binds with GA stoichiometrically in acetonitrile medium at 25^◦ C. Observation of GA-to-iron LMCT optical bands at 521 and 609 nm supports in situ generated iron-GA adduct in solution. This in situ generated iron-GA adduct reacts with molecular oxygen at the rate, k_obs = 6.58×10⁻³ min⁻¹ in acetonitrile and affords exclusively 2-oxo-4-hydroxy-hepta-3,5-dienedioic acid. The incorporation of both the oxygen atoms of molecular oxygen in the bio-mimicking activity of gentisate-1,2-dioxygenase by this iron(II)-phenanthroline complex remain arare example in scientific literature.