Tailor-made isostructural copper(ii) and nickel(ii) complexes with a newly designed (N,N)-donor scaffold as functional mimics of alkaline phosphatase

Abstract

This study highlights the design, synthesis, spectroscopic characterization and catalytic phosphoesterase cleavage activity of two isostructural and isomorphous complexes, [Cu(L)]₄ (1) and [Ni(L)]₂·CH₃OH (2) containing a new synthetic N,N-type donor ligand, namely 1,3-bis(((E)-(1H-pyrrol-2-yl)methylene)amino)propan-2-ol (H₂L). The ligand, H₂L has been synthesized by coupling of pyrrole-2-carboxaldehyde and 1,3-diamino-propane-2-ol under reflux. X-Ray crystallography study reveals that both the Cu(II) and Ni(II) centres in 1 and 2 adopt nearly a perfect square planar geometry and they display a high degree of similarity in their structural features. The biocatalytic fate of the complexes has been evaluated towards the hydrolytic cleavage of the phosphoester linkage in an aqueous DMF medium at room temperature. The spectrophotometric studies favour a higher turnover frequency for the hydrolysis of phosphoester bonds for the copper(II) complex (k_cat, 1.56 × 10⁴ h⁻¹) over the nickel(II) complex (k_cat, 5.61 × 10³ h⁻¹). Electrochemical, spectrophotometry, electron spray ionization mass spectrometry and detailed density functional theory studies illuminate the mechanistic interpretation attributing the crucial role of coordinated water, facilitating the nucleophilic attack on the phosphorous centre of pNPP, and shed light on mechanistic aspects for the efficient hydrolysis of phosphoester substrates for the copper complex over the nickel complex.