Assemblies of salen-type oxidovanadium(IV) complexes: substituent effects and in vitro protein tyrosine phosphatase inhibition.

Abstract

Oxidovanadium(IV) complexes with substituted chiral tetradentate dianionic N,N'-bis-o-hydroxybenzylidene-1,2-propylenediamines were synthesized and their physicochemical properties were characterized using single crystal X-ray diffraction, elemental analysis, ATR FTIR, UV-VIS and EPR spectroscopy, cyclic voltammetry, spectroelectrochemistry and preliminary in vitro protein-tyrosine phosphatase inhibition activity studies. Different 5-substituents in the salicylaldehyde (condensed with 1,2-diaminopropane; 2 : 1) were tested, namely 5-Br (complex 1), 5-Cl (2), 5-NO2 (3) and 5-OCH3 (4). The crystal structures of 1 and 2 show square pyramidal coordination of vanadium and parallel arrangement of monomeric exo isomers in supramolecular dimers. The halogen-halogen interaction of substituents in 5,5'-positions leads to weakening of axial interaction between phenolate O and V in 2, compared to 1. The Br atom takes part in halogen bonding with a vanadyl group in 1. Complex 3 has a linear polymeric structure with a V-O-V asymmetric bridge motif (IR absorption band at 873 cm(-1), separated d-d bands and broad EPR band structure in frozen solution pointing to oligomeric nature) while 4 is monomeric (V=O stretching at 976 cm(-1), broad d-d band structure). Redox potentials of the V(4+)/V(5+) couple lie in the range of -0.14 to 0.21 V (vs. Fc/Fc(+)) and show substantial dependence on the electron withdrawing properties of the substituents. The charge transfer character of the bands present in the range 365-395 nm was confirmed based on UV-VIS spectroelectrochemical experiments. Different assemblies of complex molecules are influenced by the electron withdrawing properties of the 5,5'-substituents, leading to supramolecular dimers (1, 2 and 4) and linear polymeric self-assembly (3). An in vitro study of representative complex 1 showed protein tyrosine phosphatase activity inhibition higher than that of suramin but lower than those of oxidovanadium(IV) sulphate and bis(maltolato)oxidovanadium(IV).