CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone

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.