Oxotungsten(V) and (VI) complexes of 1,3-dimethyllumazine as models for the tungstopterin cofactor

Abstract

The coordination of W(O)Cl₃ or W(O)₂Cl₂ to 1,3-dimethyllumazine (DML) occurs at the O(4) and N(5) centres of the chelating DML. While the 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 site via changes in the carbonyl stretching bands and via paramagnetic shifts of the proton resonances. Although the EPR results of (L)W(O)Cl₃, L = DML, DME (O,O′-coordinating 1,2-dimethoxyethane) and bpy (N,N′-coordinating 2,2′-bipyridine), indicate a rather weak coordination of DML to the W^v centre, the existence of these complexes suggests that the pterin heterocycle coordinates to the metal via N(5) and/or O(4).