Ria Sanyal

Catecholase activity of Mannich-based dinuclear CuII complexes with theoretical modeling: new insight into the solvent role in the catalytic cycle

Four new dinuclear CuII complexes were synthesised from two Mannich-base ligands namely 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL1) and 2-[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL2): [Cu2(L1)(OH)](ClO4)2·CH3OH (1), [Cu2(L2)2](ClO4)2·H2O (2), [Cu2(L2)2(NO3)2] (3) and [Cu2(L2)2(OAc)2]·H2O (4) and well characterised. X-ray diffraction analysis of the complexes reveals a Cu⋯Cu distance of 2.9183(13), 2.9604(6), 3.0278(4) and 3.0569(11) A, respectively. In 1 the metal coordination geometry is intermediate between trigonal bipyramidal (TBP) and square pyramidal (SP) (τ = 0.488), in 2 the geometry is TBP (0.828 and 0.639) and in 3 and 4 is SP (τ = 0.188 and 0.083, respectively). Spectrophotometric investigations to evaluate the catecholase activity of complexes against 3,5-di-tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) in three different solvents (acetonitrile, methanol and DMSO) under completely aerobic conditions reveal that complexes 1–4 are able to oxidise 3,5-DTBC in all the solvents, while TCC can be oxidised only in acetonitrile (kcat = 0.0002–0.02 s−1). Intensive DFT calculations prove an ionic pathway for 1–3 while a unique neutral catalytic cycle for 4.

Role of solvent in the phosphatase activity of a dinuclear nickel(II) complex of a Schiff base ligand: mechanistic interpretation by DFT studies

A novel dinuclear μ-diphenoxo-μ-acetato metallohydrolase [Ni2L2(NCS)(Ac)(H2O)0.5(MeOH)0.5]·1.25H2O (HL = 2-((E)-(2-(pyridin-2-yl)ethylimino)methyl)-4-chlorophenol) with an intermetallic separation of 3.099 A has been synthesized and characterized both structurally and spectroscopically. Temperature dependent (2–300 K) magnetic susceptibility measurements show that the compound exhibits a global intramolecular ferromagnetic interaction through the diphenoxido and syn–syn acetate bridges (J = 7.21 cm−1). It is also noticed that a weak antiferromagnetic intermolecular interaction or ZFS effect of the Ni(II) ions is working for lowering of χMT at extreme low temperature. The compound’s phosphatase activity, investigated spectrophotometrically on 4-nitrophenylphosphate (4-NPP), demonstrates excellent catalytic efficiency. Additionally, using a DMF medium facilitated a better catalytic pathway (8.18 s−1) than when acetonitrile (MeCN) was used. A plausible 4-NPP hydrolytic reaction mechanism was proposed by utilizing DFT calculations and the results suggest a SN2-like addition–substitution pathway with two possible catalyst–substrate binding modes (RC1-1 and RC1-2). RC1-2 is regarded as the direct reactant since the water molecule is adjacent to the electron-deficient phosphorus center of the substrate. More importantly, the poor catalytic behavior in the MeCN medium could be ascribed to MeCN molecules having better coordination properties than DMF molecules, since the formation energies of [Ni–MeCN] and [Ni–DMF] are −2.5 and −1.2 kcal mol−1, respectively, according to PCM calculations.

Role of para-substitution in controlling phosphatase activity of dinuclear NiII complexes of Mannich-base ligands: experimental and DFT studies

With an aim to study the electronic effect of the group lying in the para-position of phenol-based compartmental Mannich-base ligands, five dinuclear nickel(II) complexes [Ni2L1−5(μ-NO3)(NO3)2] have been synthesized [R = Me (1), CHMe2 (2), CMe3 (3), Cl (4), and OMe (5)] having octahedral structures (fac-manner) in each case as confirmed by single-crystal X-ray diffraction (1–4). The Ni⋯Ni distance (3.42–3.49 A) and Ni–O–Ni bridging bond angle (118.62–121.45°) of 1–4 is proportional to the electronic partial charge (I-effect) of the para-substituents as 3 2 > 5 > 1 > 4 in terms of kcat values (6–81 s−1), as per the Michaelis–Menten profile. DFT calculations establish that the electron-donating group decreases the reaction energy barrier via reducing the energy gap between the orbital of electron-sufficient para-substituted phenolate group and the electron-demanding leaving group. The exceptionally high activity of complex 3 also establishes the rationality of our catalyst design in our previous work (Inorg. Chem. 2015, 54, 2315–2324).