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Published December 21, 2016 | Supplemental Material
Journal Article Open

Mechanism of Molybdenum-Mediated Carbon Monoxide Deoxygenation and Coupling: Mono- and Dicarbyne Complexes Precede C–O Bond Cleavage and C–C Bond Formation

Abstract

Deoxygenative coupling of CO to value-added C_(≥2) products is challenging and mechanistically poorly understood. Herein, we report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multielectron bond-breaking and bond-making transformation. In our studies, the formation of a bis(siloxycarbyne) complex precedes C–O bond cleavage. At −78 °C, over days, C–C coupling occurs without C–O cleavage. However, upon warming to 0 °C, C–O cleavage is observed from this bis(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C–O bond cleavage at lower temperatures, indicating that monosilylation, and a more electron-rich Mo center, favors deoxygenative pathways. From the bis(siloxycarbyne), isotopic labeling experiments and kinetics are consistent with a mechanism involving unimolecular silyl loss or C–O cleavage as rate-determining steps toward carbide formation. Reduction of Mo(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silylcarbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C–C bond formation, releasing silylated C_2O_1 fragments and demonstrating that the multiple bonded terminal Mo≡C moiety is an intermediate on the path to deoxygenated, C–C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum mechanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.

Additional Information

© 2016 American Chemical Society. Received: October 7, 2016; Published: November 21, 2016. We thank Larry Henling and Mike Takase for invaluable crystallographic assistance and David VanderVelde for NMR expertise. We are grateful to Jay Labinger for insightful suggestions regarding the synthesis of mixed molybdenum dicarbyne complexes. Sibo Lin is thanked for advice and discussions regarding the computations. T.A. is grateful for Sloan and Dreyfus fellowships and J.A.B. for a NSF graduate research fellowship. This research was funded by the NSF (CHE-1151918) and Caltech. The authors declare no competing financial interest.

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