Models for vanadate-dependent haloperoxidases: Vanadium complexes with O4N-donor sets

Reaction of vanadyl sulfate with the Schiff base H2L1 derived from o-vanillin and glycine leads to [(VO)-O-IV(H2O)L-1] (1), treatment of which with acetyl chloride affords cis-[(VCl2L1)-Cl-IV] (2). Reaction of [VO(acac)(2)] with the Schiff base H2L2 derived from o-hydroxynaphthaldehyde and o-hydroxyaniline under aerobic conditions yields [{(VO)-O-V(L-2)}(2) mu-O] (3), which undergoes methanolysis in excess methanol to form [(VO)-O-V(MeOH)(OMe)L-2] (5a). Complex 5a is also obtained via [(VOCl)-O-V(L-2)] (4) and MeOH. On treatment of 3 with an equimolar amount of ethanol, [(VO)-O-V(OEt)L-2] (5b) is obtained. With the chiral tertiary bis(ethanol)amine (S)- or (R)(HOCH2CH2)(2)NCH(Ph)Me, H2L3, [VO(acac)(2)] loses only one acetylacetonate(1-) ligand, and the complexes (S)- and (R)[(VO)-O-V(acac)L-3] (6a) are formed. As is evident for the corresponding reaction with (HOCH2CH2)(2)NPh, H2L4, which affords [(VO)-O-V(acac)L-4] (6b), the substitution is a second-order process (rate constant 0.013 M-1 h(-1)). 5a . MeOH, 5b, (R)- 6a, and (S)-6a have been structurally characterized by single crystal X-ray diffraction analysis. The molecular connectivities have also been established for 1 . H2O. All of the structurally characterized complexes contain an O*O3N donor set (O* denotes a doubly-bonded oxo group) and thus model the coordination environment of the active site in vanadate-dependent haloperoxidases. This model character is particularly pronounced in 5b, the coordination geometry of which is half-way to trigonal-bipyramidal. 5a and 6a contain, in addition to the O*O3N set, a labile V-O or V-N bond, respectively, which may mimic substrate binding to the active centre. 6a is also of relevance to the enantioselective peroxide oxidation of sulfides to sulfoxides by catalyst systems containing vanadium and H2L3.