Oxido- and mixed-ligand peroxido complexes of niobium(V) as potent phosphatase inhibitors and efficient catalysts for eco-friendly styrene epoxidation.

Abstract

The present study describes the facile synthesis and comprehensive characterization of new oxido and peroxidoniobium(V) complexes with biogenic ligands, maltol (malt) and deferiprone (def) in their co-ordination sphere, viz., [NbO(malt)3]2·9H2O (1), Na2[Nb(O2)3(malt)]·H2O (2) and Na2[Nb(O2)3(def)]·2H2O (3). The complexes were characterized using various analytical and spectroscopic techniques (FTIR, Raman, NMR, UV-visible, TGA, ICP-OES and elemental analysis). The charge neutral complex 1 was further characterized by single crystal XRD analysis, and the proposed structures of the peroxidoniobium (pNb) complexes 2 and 3 were validated by density functional theory (DFT) studies. A comparative investigation on the in vitro effect of the title compounds and a set of previously reported polymer-anchored peroxidoniobium complexes, [Nb(O2)3(sulfonate)2]3--PSS [PSS = poly(sodium 4-styrene sulfonate)] (5), [Nb2(O2)6(carboxylate)2]4--PA [PA = poly(sodium acrylate)] (6) and [Nb(O2)3(carboxylate)]2--PMA [PMA = poly(sodium methacrylate)] (7), on the activity of the model enzyme wheat thylakoid acid phosphatase has revealed that each of the compounds is an effective inhibitor of the enzyme (IC50 values varying within the range 1-64 μM). The results of the detailed enzyme kinetic study demonstrated that the compounds induce their inhibitory effect via distinct pathways. The oxidoniobium complex 1 as well as polymer-anchored pNb complexes acted as classical non-competitive inhibitors of ACP, whereas the monomeric pNb complexes emerged as mixed inhibitors of the enzyme (Kii > Ki). Notably, the complexes serve as excellent recyclable catalysts for selective styrene epoxidation with H2O2, affording 99% styrene conversion, ≥98% epoxide selectivity and a high turnover number of 1740 under eco-friendly solventless conditions.