OXOTUNGSTEN(V) AND OXOTUNGSTEN(VI) COMPLEXES OF 1,3-DIMETHYLLUMAZINE AS MODELS FOR THE TUNGSTOPTERIN COFACTOR

The coordination of W(O)Cl-3 or W(O)(2)Cl-3 to 1,3-dimethyllumazine (D ML) occurs at the O(4) and N(5) centres of the chelating DML. While th e colourless dioxotungsten(VI) complex shows only partially reversible reduction behaviour, the oxidizable red monooxotungsten(V) complex is stable and could be studied by EPR, UV-VIS, IR and NMR spectroscopy. These latter techniques have been used to confirm the coordination sit e via changes in the carbonyl stretching bands and via paramagnetic sh ifts of the proton resonances. Although the EPR results of (L)W(O)Cl-3 , L = DML, DME (O,O'-coordinating 1,2-dimethoxyethane) and bpy (N,N'-c oordinating 2,2'-bipyridine), indicate a rather weak coordination of D ML to the W-V centre, the existence of these complexes suggests that t he pterin heterocycle coordinates to the metal via N(5) and/or O(4).