51V NMR, 17O NMR, and UV–Vis computational studies of new VBPO functional models: Bromide oxidation reaction

Vanadium oxo-peroxo complexes are known as the best functional models of vanadium haloperoxidases (VHPOs) enzymes. Their quantity and quality structure activity and property relationship can be followed up by NMR calculations. The DFT/B3LYP/6-311+G (d,p)/LANL2DZ method has been used to calculate the geometry and electronic structures and 51V and 17O NMR chemical shifts of a series of 7 oxo-peroxo vanadium (V) complexes mimicking the active site of vanadium bromoperoxidase (VBPO) in the gas and solvent (acetonitrile) phases. They contain tetradentate tripodal amine ligands (TPALs). Compilation of NMR, electronic, and vibrational spectroscopic data and metric parameters for the studied complexes permits us to draw correlations among 51V and 17O peroxo chemical shift, the electronic charge transfer band, the O–O vibrational frequency, and the O–O bond length. A direct dependence of the magnetic shielding of the vanadium center and peroxide group on the electronic character of the ligand was observed in all cases. Studied complexes exhibit 51V and 17O NMR chemical shifts of −590 to −430 ppm and 650–740 ppm, respectively. A Ramsey-type correlation between the chemical shifts and the energy of the ligand-to-metal charge transfer (LMCT) was found that suggests the electron donating ability of the ligands and the energy gap HOMO–LUMO of the peroxo complexes are linearly correlated. Vibrational frequency and length of the O–O bond also correlate with the inverse LMCT energy. The calculated values of νO–O place above 900 cm−1 and O–O distances in the range of 1.40–1.42 Å. In general, the more reactive complexes within bromide oxidation contain weakly electron donating TPALs and represent higher energy LMCT, longer O–O length, and lesser 51V and 17O chemical shifts.

Graphical abstractHerein, the structures of 7 pre-designed VBPO functional models (containing TPALs ligand) (Eshtiagh-Hosseini et al., 2013) investigated by 51V and 17O NMR chemical shifts, UV–Vis, and NBO (DFT/B3LYP/6-311+G (d,p)/LANL2DZ) calculations. It can be concluded the more reactive complexes within bromide oxidation contain weakly electron donating TPALs and represent higher energy LMCT, longer O–O length, and lesser 51V and 17O chemical shifts.Figure optionsDownload full-size imageDownload as PowerPoint slide