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 C2O1 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.

A Low‐Valent Molybdenum Nitride Complex: Reduction Promotes Carbonylation Chemistry

“…Within the context of C 1 feedstock upgrading, our group has explored the reactivity of Mo IV alkylidyne carbonyl complexes. [10] Following two-electron reduction, these compounds forge aC Ct riple bond, ejecting the oxyacetylide anion and binding N 2 (Figure 1). Intrigued by the scarcity of literature reports of nitride/CO coupling [3,11] and taking advantage of the isolobal relationship between cyanate and oxyacetylide,w es ought to extend our CÀCb ond forming methodology to construct CÀNb onds.…”

“…CO binding to 4 is reversible, [16] suggesting am ore electron-deficient metal center in 4,r elative to A. [10] This is corroborated by the IR spectrum of 4,which shows aCOstretch at 1980 cm À1 that is 107 cm À1 blue-shifted from that of A. [10b] These differences in behavior are likely due to decreased covalencyo ft he MoE bond in 4.The CO lability of 4 necessitates in situ preparation and reaction screening;a ddition of two equiv of Na[C 10 H 8 ], the same strong reductant used to enact C Cc oupling from A, [10b] led primarily to CO loss (3)and decomposition to free phosphine.…”

Terminal Molybdenum Phosphides with d Electrons: Radical Character Promotes Coupling Chemistry

“…In ar ecent report, we described the ability of a paraterphenyl diphosphine supported molybdenum center to undergo af our-electron reductive coupling reaction from aterminal Mo IV carbide to afford aMo 0 dinitrogen adduct. [5] In an effort to access additional MoEc omplexes,w e targeted the reverse of this process:m ulti-electron oxidative group transfer. [6] Cummins and co-workers have reported al ibrary of dibenzo-7l 3 -phosphanorbornadiene derivatives that undergo anthracene elimination to release reactive phosphorous fragments under mild conditions.…”

A Low‐Valent Molybdenum Nitride Complex: Reduction Promotes Carbonylation Chemistry

“…Though this is likely the case in d 0 metal complexes, the low valency and coordinative unsaturation of 5 could favor CO binding prior to coupling . Experiments detailing the elementary steps of C−C coupling in a related system were consistent with reduction prior to electrophilic attack from a Mo IV carbide carbonyl complex . Analogously, CO binding and phosphine arm coordination from 5 would provide the nitride congener, which may likewise be prone to electrophilic attack at N (Supporting Information, Scheme S1).…”

“…Both the 1 H and 31 P{ 1 H} are likewise broad, suggesting a fluxional process that is intermediate on the NMR timescale. Multinuclear NMR spectroscopy evinces two distinct species at low temperature (−65 °C) and a single broad set of resonances upon warming (25 °C), attributed to interconversion of two isomers via slipping of the η 2 Mo‐arene interaction across the face of the central ring . XRD revealed the structure of 4 as a nitride carbonyl complex with these moieties poised cis to one another.…”