A tungsten triple play

Angew. Chem. Int. Edn. Engl. doi:10.1002/anie.201505764

Credit: ANGEW. CHEM. INT. EDN. ENGL.

Acetylene hydratase catalyzes the conversion of acetylene (C₂H₂) to acetaldehyde (CH₃CHO) via an ethenol (CH₂CHOH) intermediate. Most of the proposed mechanisms for this tungsten-based enzyme require activation of acetylene, but the details of the process and the remaining steps in the catalytic pathway are controversial. The development of a biomimetic organometallic complex that could be used as a proxy to understand the tungsten (W) active site would help to shed light on the reaction, but only a few W-C₂H₂ complexes have been reported, and these either were intended to explore other questions or were not stable enough to interrogate. Peschel et al. report a new complex that includes two bidentate thiol-containing ligands, termed S-Phoz, meant to approximate the stability provided by the five sulfur ligands seen in the crystal structure of the enzyme. Exposure of a W(CO)₂(S-Phoz)₂ complex to an acetylene atmosphere led to the formation of a stable W-C₂H₂ adduct (structure shown). An oxo donor led to the further displacement of the remaining CO ligand by an oxygen, yielding WO(C₂H₂)(S-Phoz)₂. Exposure of this complex to light led to the rapid dissociation of the acetylene group, creating a third unique species, whereas incubation under dark conditions led to the slow reattachment of the acetylene, providing a rare example of the reversible activation of acetylene. Crystal structures of these three forms demonstrated that upon coordination, the linear C₂H₂ becomes bent, with longer carbon-carbon triple bonds, both indicative of activation. The asymmetric coordination of C₂H₂ also suggests an asymmetric charge distribution, providing glimpses into possible points of attack for an incoming nucleophile. This system provides a much-needed scaffold to inform further mechanistic studies of acetylene hydratase.