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

Abstract

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 Å) 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 ≈ 1 < 4. The phosphatase activity has been studied with the phosphomonoester 4-nitrophenylphosphate (4-NPP) in aqueous DMSO by spectrophotometry leading to the trend 3 > 2 > 5 > 1 > 4 in terms of k_cat 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).