Syntheses, Structures, Electrochemical and Oxygen Atom Transfer Properties of Molybdenum and Tungsten Complexes with Sulfur and Selenium Containing Ligands

Abstract

Molybdenum and tungsten can be found at the active sites of the molybdopterin-containing oxygen-atom-transfer enzymes. Another interesting diversity of these enzymes is that in the DMSO reductase family (molybdenum enzymes with two molybdopterin ligands) the metal is bound to the peptide moiety through either serine (O), cysteine (S), selenocysteine (Se) or aspartate (O, mono- or bidentate). The reasons for the distribution of the two metals and the selectivity of the different types of amino acid ligands in these enzymes are unknown until today. In order to obtain more insight into these questions in the present work, corresponding molybdenum and tungsten complexes as functional models by replacing molybdenum by tungsten and varying the ligand atoms (O/S/Se) were synthesized and characterized by elemental analysis, spectrometric and spectroscopic methods, as well as single crystal X-ray structural analysis. Moreover, their structural, electrochemical and catalytic oxygen atom transfer properties were investigated and compared.Molybdenum compounds with sulfur-containing ligands and analogous selenium-containing ligands appear to be isostructural. However, for the two different metals (Mo and W) the structural influence of the ligand atom plays a vital role in structurally demanding ligands (tridentate) but not in those with a simpler geometry (bidentate or monodentate). The influence of the metal on the redox potential is that the molybdenum complexes have more positive redox potentials than the analogous tungsten complexes. The influence of the ligand atoms on the redox potentials is small but of consequence. The sulfur ligands cause more positive redox potentials than the oxygen ligands, and the selenium ligands cause more positive potentials than the sulfur ligands. However, the observed differences are smaller than those that result from changing the metal atoms. The results of all the catalytical studies for oxo-transfer reactions indicate that an exchange of ligand atoms sulfur versus selenium in the same ligand system can cause different mechanisms and certainly different activities even though no strong influence on other investigated properties could be observed.