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

[VO(acac) 2 ] reacts with H 2 L [H 2 L are the hydrazones H 2 pydx‐inh ( I ), H 2 pydx‐nh ( II ), or H 2 pydx‐bhz ( III ); pydx = pyridoxal, inh = isonicotinohydrazide, nh = nicotinohydrazide, bhz = benzohydrazide] in dry methanol to yield the oxovanadium( IV ) complexes [VOL] (H 2 L = I : 1 ; H 2 L = II : 4 ) or [VO(pydx‐bhz)]. These complexes, when exposed to air, convert into the corresponding dioxovanadium( V ) complexes [VO 2 HL] (H 2 L = I : 2 ; H 2 L = II : 5 ; H 2 L = III : 7 ). Aqueous solutions of vanadate and the ligands at pH = 7.5 give rise to the formation of [K(H 2 O) 3 ][VO 2 (pydx‐inh)] ( 3 ), [K(H 2 O) 2 ][VO 2 (pydx‐nh)] ( 6 ) and [K(H 2 O) 2 ][VO 2 (pydx‐bhz)] ( 8 ). Treatment of 6 and 8 with H 2 O 2 generates the oxo(peroxo)vanadium complexes [VO(O 2 )L] (H 2 L = II : 9 ; H 2 L = III : 10 ). Complexes 9 and 10 are capable of transferring an oxo group to PPh 3 . 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 VO 2 + fragment, and the alcoholic group of the pyridoxal moiety, which links adjacent layers in the three‐dimensional lattice network. In the presence of KBr/H 2 O 2 , 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)