Dioxovanadium(V) Complexes of ONO Donor Ligands Derived from Pyridoxal and Hydrazides: Models of Vanadate‐Dependent Haloperoxidases

Abstract

Abstract[VO(acac)2] reacts with H2L [H2L are the hydrazones H2pydx‐inh (I), H2pydx‐nh (II), or H2pydx‐bhz (III); pydx = pyridoxal, inh = isonicotinohydrazide, nh = nicotinohydrazide, bhz = benzohydrazide] in dry methanol to yield the oxovanadium(IV) complexes [VOL] (H2L = I: 1; H2L = II: 4) or [VO(pydx‐bhz)]. These complexes, when exposed to air, convert into the corresponding dioxovanadium(V) complexes [VO2HL] (H2L = I: 2; H2L = II: 5; H2L = III: 7). Aqueous solutions of vanadate and the ligands at pH = 7.5 give rise to the formation of [K(H2O)3][VO2(pydx‐inh)] (3), [K(H2O)2][VO2(pydx‐nh)] (6) and [K(H2O)2][VO2(pydx‐bhz)] (8). Treatment of 6 and 8 with H2O2 generates the oxo(peroxo)vanadium complexes [VO(O2)L] (H2L = II: 9; H2L = III: 10). Complexes 9 and 10 are capable of transferring an oxo group to PPh3. Acidification of 8 with HCl afforded a hydroxo(oxo) complex. The crystal and molecular structures of ligand I and complex 3 have been solved by single‐crystal X‐ray diffraction. In the anion 3, the vanadium atom is in a distorted tetragonal‐pyramidal environment (τ = 0.23). The K+ ion is coordinated to four water molecules (two of which bridge to a neighbouring K+ ion), the pyridine nitrogen atom of an isonicotinic moiety, the equatorial oxo group of the VO2+ fragment, and the alcoholic group of the pyridoxal moiety, which links adjacent layers in the three‐dimensional lattice network. In the presence of KBr/H2O2, the anionic complexes 3, 6 and 8 catalyse the oxidative bromination of salicylaldehyde in water to 5‐bromosalicylaldehyde in ca. 40% yields with ca. 87% selectivity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)