Bromoperoxidase mimic as catalysts for oxidative bromination—synthesis, structures and properties of the diversified oxidation state of vanadium(iii, iv and v) complexes with pincer N-heterocycle ligands

Abstract

Novel oxovanadium complexes (VO)₂(bpz*T-O) (1), VO(bpz*eaT)(SCN)₂ (2), V₂(bpz*eaT)₂(μ₂-C₂O₄)(C₂O₄)₂ (3), [VO(SO₄)(bpz*P)(H₂O)]·H₂O (4) and VO(SO₄)(bpz*P-Me)(H₂O) (5) (bpz*T-O: 4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazin-2-olate, bpz*eaT: 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-diethylamino-1,3,5-triazine, bpz*P: 2,6-bis(5-methyl-pyrazol-3-yl)pyridine, bpz*P-Me: 2,6-bis(1,5-dimethyl-pyrazol-3-yl)pyridine), were synthesized by the reaction of V₂(SO₄)₃–VOSO₄–VO(acac)₂ and various pincer N-heterocyclic ligands with solution or hydrothermal methods. The structures of all the complexes were characterized by elemental analysis, IR and UV-vis spectroscopy and single-crystal diffraction analysis. Furthermore, thermogravimetric analyses (TG) and quantum chemistry calculations were also performed. Structural analyses reveal that the vanadium atom has a distorted trigonal bipyramidal geometry with a N₃O₂ donor set in 1; distorted octahedral geometry in 2, 4 and 5 with donor sets of N₅O, N₃O₃ and N₃O₃, respectively; a distorted pentagonal bipyramidal geometry with a N₃O₄ donor set in 3. In addition, the five new complexes with abundant intro- and inter-hydrogen bonding interactions exhibited bromination catalytic activity in a single-pot reaction of the conversion of phenol red to bromophenol blue in a mixed solution of H₂O–DMF at a constant temperature of 30 ± 0.5 °C with a buffer solution of NaH₂PO₄–Na₂HPO₄ (pH = 5.8), indicating that they can be considered as a potential functional model of bromoperoxidase.