Oxoperoxo molybdenum(vi) and tungsten(vi) and oxodiperoxo molybdate(vi) and tungstate(vi) complexes with 8-quinolinol: synthesis, structure and catalytic activity

Abstract

A solution obtained by dissolving MoO₃ in a moderate excess of H₂O₂ reacts with 8-quinolinol (QOH) to give [MoO(O₂)(QO)₂] (1), but, when the same reaction is conducted with a large excess of H₂O₂, an anionic complex is formed, which reacts with PPh₄Cl to give the corresponding salt [MoO(O₂)₂(QO)][PPh₄] (2·PPh₄). Freshly prepared WO₃ behaves the same way and, depending on the amount of H₂O₂ used, as above, produces either [WO(O₂)(QO)₂] (3) or [WO(O₂)₂(QO)][PPh₄] (4·PPh₄), respectively. Crystallographic analyses reveal the coordination geometries around the metal center in these complexes to be distorted pentagonal bipyramids. These compounds show interesting catalytic properties in the oxidation of alcohols using H₂O₂ as the terminal oxidant. In the case of aromatics, including benzylic and cinnamylic alcohols, the oxidation occurs selectively, affording aldehydes or ketones with reasonably high turnover numbers. Taking benzyl alcohol as a representative case, a probable mechanism of the alcohol-to-aldehyde conversion mediated by the prepared catalysts is suggested. The oxidation of aliphatic primary alcohols, under the same conditions, does not show the above selectivity: the reaction yields the corresponding aldehydes as well as carboxylic acids. The work was also extended to study the catalytic activity towards the oxidation of phenol and various sulfides and amines using the same oxidants.