Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten­(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten­(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO2(6-MePyS)2] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe3 to a phosphine-stabilized tungsten­(IV) oxido complex [WO­(6-MePyS)2(PMe3)2] is described. This tungsten­(IV) complex partially releases one PMe3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO2(6-MePyS)2] and OPMe3. Therefore, [WO2(6-MePyS)2] can be used as a catalyst for the aerobic oxidation of PMe3, rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten­(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten­(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten­(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten­(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.