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

Abstract

The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation involves CC 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 upon formal heterolysis of H2. Methylidene formation prior to CC 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.

Reactions of Aluminium(I) with Transition Metal Carbonyls: Scope, Mechanism and Selectivity of CO Homologation

“…Despite the different topologies, chemical shifts of the C 1 and C 2 position of 3-Re and 3-Re' are remarkably similar. Characterisation of 13 C enriched isotopomers of 3-W allow aspects of the J coupling in the carbon chain to be resolved and the C 3 resonance to be unambiguously identified. Hence, 13 CO enriched 3-W shows the C 3 resonance at  = 176.5 ppm.…”

“…Characterisation of 13 C enriched isotopomers of 3-W allow aspects of the J coupling in the carbon chain to be resolved and the C 3 resonance to be unambiguously identified. Hence, 13 CO enriched 3-W shows the C 3 resonance at  = 176.5 ppm. The coupling constants in the chain were determined as 1 JC2-C3 = 41.8 Hz, 1 JC1-C2 = 37.1 Hz, along with 1 JW-C1 = 102 Hz determined from natural abundance satellites of 183 W. Solid-State Characterisation of 2-4: The X-ray structure of 2-Mn incorporates a [7,4] ortho-fused ring system.…”

Structural Characterization of Two CO Molecules Bound to the Nitrogenase Active Site

“…Previously reported 13/12 CO labeling studies by the Ribbe and Hu groups suggest that the hi‐CO form of V nitrogenase is not a competent intermediate in CO coupling, while the lo‐CO form is catalytically competent [42] . Based on these results and others, [43, 44] a mechanistic hypothesis regarding CO reduction has been proposed in which the first CO ligand is reduced (at least partially) before a second CO molecule can bind to the cofactor and undergo productive reduction [4, 45, 46] . While V and Mo N 2 ase are structurally similar, they do exhibit disparate activities towards CO reduction with the former being much more reactive towards hydrocarbon formation.…”

“…[42] Based on these results and others, [43,44] a mechanistic hypothesis regarding CO reduction has been proposed in which the first CO ligand is reduced (at least partially) before a second CO molecule can bind to the cofactor and undergo productive reduction. [4,45,46] While V and Mo N 2 ase are structurally similar, they do exhibit disparate activities towards CO reduction with the former being much more reactive towards hydrocarbon formation. Given these differences, it is entirely possible that the two nitrogenases follow distinct mechanistic paths for CO reduction.…”

The Pincer Platform Beyond Classical Coordination Patterns

“…This ligand family proved to be cooperative very flexible and redox‐active due to the unsaturated and coordinative versatile nature of the pendant central aromatic ring. [ 115–117 ]…”