Synthesis of Ni(ii) and Zn(ii) complexes of pyrrolyl dipyrrins and their biomimetic role in catalyzing the hydrolysis of the phospho-ester bond

Abstract

3-Pyrrolyl BODIPY and α-formyl 3-pyrrolyl BODIPY were used as precursors for the in situ generation of 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 the Zn(II) ion, and the Zn(II) ion coordinated with four nitrogen atoms of the two α-pyrrolyl dipyrrin units in a criss-cross fashion, adopting a distorted tetrahedral geometry. By contrast, 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 the hydrolysis of phosphatase was studied in DMF/H₂O (97.5 : 2.5 v/v) using the 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 (K_cat) 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 dipyrrin-based metal complexes functioning as phosphatase imitators 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 PO bond and the metal center and facilitating metal–hydroxide activation. The latter effect was more pronounced under basic conditions, where the K_cat value increased from 10.01 s⁻¹ in a neutral medium to 11.70 s⁻¹ in a basic medium. A probable mechanism for phosphatase hydrolysis was proposed based on the experimental findings and literature survey. These results 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.