Synthesis, structure, stability, lipophilicity and insulin-sensitizing activity of new heteroleptic oxidovanadium(V) complexes

A series of mixed-ligand oxidovanadium(V) complexes of the type, [VO₂(L) (N-N)] (1–4) featuring hydroxypyrones (L: maltol or ethyl-maltol) and (N-N): diimine [2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen)] ligands, are reported. The synthesized complexes were characterized by spectroscopic and analytical techniques (FTIR, UV–Vis, ⁵¹V NMR, HRMS, ICP-OES and TGA). Single crystal X-ray crystallography revealed the O₄N₂ ligand sphere to define a distorted octahedral coordination geometry in each case. Each of the complexes is stable in air in the solid state and have good solubility in water as well as in organic solvents. The partition co-efficient, log P (octanol-water) values for the complexes being in the range (0.72–1.12) indicated their lipophilic nature. The complexes 1–4, along with two previously reported complexes [VO₂(deferiprone)(bpy)]·H₂O (5) and [VO₂(deferiprone)(phen)]·4H₂O (6) were examined for their in vitro insulin-sensitizing and insulin-like activities against insulin responsive L6 myoblast cells. The complex, [VO₂(Emal)(bpy)]₂·H₂O (3) exhibited the most pronounced insulin-sensitizing effect, which is comparable to that of the reference compound bis(maltolato)oxidovanadium(IV), BMOV. The in vitro cytotoxicity assay against the L6 myoblast cells showed that the compounds were less toxic compared to BMOV. The complexes 1–6 were screened for their in vitro inhibitory effect on the model enzyme wheat thylakoid acid phosphatase (ACP). The enzyme kinetic analysis revealed that, compounds induce their inhibitory effect via distinct pathways. The complexes 1–4 served as mixed type of inhibitor (K_ii > K_i), whereas 5 and 6 served as classical non-competitive inhibitors of the enzyme (K_ii ≈ K_i).