The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation involves C-C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm) at low temperature (-78 °C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form of a hydride (H- = 2e- + H+), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as energetically viable relative to the heterolytic cleavage of H2. Methylidene formation prior to C-C coupling proves critical for organic product release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a rare model of the initial stages of the Fischer-Tropsch synthesis.
Bibliographical noteFunding Information:
We thank Larry Henling and Mike Takase for invaluable crystallographic assistance. J.A.B. is grateful for an NSF graduate research fellowship, G.A.B. for NSERC and Resnick Sustainability Institute fellowships, and J.O. for an Ernest H. Swift Summer Undergraduate Research Fellowship. We thank the NSF (CHE-1800501), the Dow Next Generation Education Fund (instrumentation), and Caltech for funding. The computational studies were supported by the NSF (CBET-1805022).
© 2019 American Chemical Society.