Synthesis of Ni(II) & Zn(II) Complexes of Pyrrolyl Dipyrrins and Their Biomimetic Role in Catalyzing Hydrolysis of Phospho-Ester Bond

Abstract

Pyrrolyl BODIPY and α-formyl 3-pyrrolyl BODIPY were used as precursors to in situ generate pyrrolyl dipyrrin and α-formyl pyrrolyl dipyrrin ligands which were then used to synthesize two novel Zn(II) complexes 1-Zn and 4-Zn and one Ni(II) complex 4-Ni in good yields. The X-ray structures revealed that in 1-Zn and 4-Zn, the appended pyrrole "N" did not participate in coordination with Zn(II) ion and the Zn(II) ion coordinates with the four nitrogen atoms of the two α-pyrrolyl dipyrrin units in a crisscross fashion, adopted a distorted tetrahedral geometry. Whereas in 4-Ni, the Ni(II) centre was coordinated by three pyrrolic nitrogen atoms from the α-pyrrolyl dipyrrin ligand and one water molecule, forming a distorted square planar geometry. The biomimetic role of 1-Zn, 4-Zn and 4-Ni in hydrolysis of phosphatase was studied in DMF/H2O (97.5:2.5 v/v) using disodium salt of para-nitrophenylphosphate (PNPP) as a model substrate. All synthesized complexes were capable of catalyzing the hydrolysis of the phosphoester bond, exhibiting turnover numbers (kₐ) of 3.52 s⁻¹, 1.99 s⁻¹, and 10.01 s⁻¹ for 1-Zn, 4-Zn, and 4-Ni, respectively. To the best of our knowledge, this study represents the first report of α-pyrrolyl dipyrrinbased metal complexes functioning as phosphatase mimics under both neutral and alkaline conditions. Among these, the 4-Ni complex displayed superior catalytic activity toward PNPP hydrolysis, which was attributed to the presence of a coordinated water molecule. This coordinated water was proposed to play a dual role-enhancing the interaction between the P=O bond and the metal center while facilitating metal-hydroxide activation. The latter effect was more pronounced under basic conditions, where the kₐ value increased from 10.01 s⁻¹ in neutral medium to 11.70 s⁻¹ in basic medium. A probable mechanism for the phosphatase hydrolysis has been proposed based on the experimental findings and literature survey. These studies not only revealed the phosphatase-like activity of the complex but also demonstrated that, owing to the presence of a fluorophoric ligand system, the specific site of phosphoester bond cleavage can be monitored under visible-light irradiation. This dual functionality-catalysis and real-time optical tracking offers significant potential for biomedical applications.