Synthesis and characterization of a VO3+ complex of a pentadentate amine alcohol ligand: towards hydrolytically stable ligands forming model complexes for vanadium-dependent haloperoxidases

Abstract

Abstract The vanadium (V) complex of a hydrolytically stable trivalent, pentadentate amine alcohol ligand has been synthesized by reaction with ammonium meta-vanadate. The ligand was prepared by reduction of the Schiff base precursor N-salicylidene-2-(bis92-hydroxy-ethyl)amino)ethylamine and exhibits an enlarged flexibility as compared to its Schiff base analogue. The vanadium (V) complex is characterized by 1H, 13C and 51V NMR, vibrational (IR, Raman and resonance Raman) and electronic spectroscopy. The LMCT transition of the complex has been assigned on the basis of resonance Raman spectroscopy and density functional calculations (LDA). The X-ray crystal structure analysis of C13H19N2O4V (monoclinic, space group C2/c: a = 1948.1(4), b = 1184.9(2), c = 1328.3(2) pm, β = 114.86(1)°, Z = 8) shows that the vanadium (V) center has a distorted octahedral environment with the oxo group in the trans positin to the tertiary amine nitrogen. In the solid state racemic pairs of the vanadium (V) complex are formed by hydrogen bridges (N-H 3. O). The bond valence sum (BVS) analysis is applied to the complex prepared in this work and compared with than results obtained for a series of more than twenty relevant six-coordinate complexes. In solution a second isomer with the oxo group trans to the secondary amine nitrogen is observed. The ratio of the two isomers varies with the solvent, as was determined by 15V NMR spectroscopy. The methanolysis as well as hydrolysis reaction of the vanadium (V) complex is studied by 51V NMR. The implications of the results for the proposed model of the active site of vanadium-dependent haloperoxidases are discussed.