Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

Abstract

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 [WO 2 (6-MePyS) 2 ] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe 3 to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-Me-PyS) 2 (PMe 3 ) 2 ] is described. This tungsten(IV) complex partially releases one PMe 3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO 2 (6-MePyS) 2 ] and OPMe 3 . Therefore, [WO 2 (6-MePyS) 2 ] can be used as a catalyst for the aerobic oxidation of PMe 3 , 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.

Nucleophiles Target the Tungsten Center Over Acetylene in Biomimetic Models

“…Such a dimeric species would undergo swift decomposition to Int 1 and the corresponding dioxido complex [WO 2 (6-MePyS) 2 ], the latter being the driving force of the overall process due to the thermodynamically favorable WO bond formation . The formation of [WO 2 (6-MePyS) 2 ] has been indirectly confirmed via its decomposition products in the presence of water, which are protonated ligand 6-MePySH and undefined [WO 3 ] n species . An independent NMR study confirmed the formation of [WO 3 ] n species from the dioxido complex under aqueous basic conditions (see SI).…”

“… 32 The formation of [WO 2 (6-MePyS) 2 ] has been indirectly confirmed via its decomposition products in the presence of water, which are protonated ligand 6-MePySH and undefined [WO 3 ] n species. 33 An independent NMR study confirmed the formation of [WO 3 ] n species from the dioxido complex under aqueous basic conditions (see SI ). Quantitative conversion of the starting complex to styrene indicates that the vinyl intermediate Int 1 must react with 6-MePySH to recover one equivalent of complex 3 .…”

“…The red crystals were analyzed via 1 H NMR and 13 C NMR spectroscopy in CD 3 CN, revealing only one set of protons belonging to the bidentate ligand, contrasting the XRD results. The obtained data suggest the dynamic behavior of the 6-MePyS ligand, which has already been observed . It is unclear whether the compound derives from the decoordination of inserted acetylene from complex 6b or is directly formed from starting compound 1 under substituting acetylene.…”

Reactivity of metal dioxo complexes

“…120 The same compound also catalyzes the oxidation of triphenylphosphine using oxygen as the terminal oxidant. 121…”

“…120 The same compound also catalyzes the oxidation of triphenylphosphine using oxygen as the terminal oxidant. 121 Oxidative bromination analogous to the cases involving molybdenum dioxo complexes (vide supra, Scheme 6) have been reported for a few tungsten dioxo complexes. 122…”