Joshua A. Buss

Molybdenum catalyzed ammonia borane dehydrogenation: oxidation state specific mechanisms.

Though numerous catalysts for the dehydrogenation of ammonia borane (AB) are known, those that release >2 equiv of H2 are uncommon. Herein, we report the synthesis of Mo complexes supported by a para-terphenyl diphosphine ligand, 1, displaying metal–arene interactions. Both a Mo0 N2 complex, 5, and a MoII bis(acetonitrile) complex, 4, exhibit high levels of AB dehydrogenation, releasing over 2.0 equiv of H2. The reaction rate, extent of dehydrogenation, and reaction mechanism vary as a function of the precatalyst oxidation state. Several Mo hydrides (MoII(H)2, [MoII(H)]+, and [MoIV(H)3]+) relevant to AB chemistry were characterized.

Four-electron deoxygenative reductive coupling of carbon monoxide at a single metal site

Carbon dioxide is the ultimate source of the fossil fuels that are both central to modern life and problematic: their use increases atmospheric levels of greenhouse gases, and their availability is geopolitically constrained. Using carbon dioxide as a feedstock to produce synthetic fuels might, in principle, alleviate these concerns. Although many homogeneous and heterogeneous catalysts convert carbon dioxide to carbon monoxide, further deoxygenative coupling of carbon monoxide to generate useful multicarbon products is challenging. Molybdenum and vanadium nitrogenases are capable of converting carbon monoxide into hydrocarbons under mild conditions, using discrete electron and proton sources. Electrocatalytic reduction of carbon monoxide on copper catalysts also uses a combination of electrons and protons, while the industrial Fischer–Tropsch process uses dihydrogen as a combined source of electrons and electrophiles for carbon monoxide coupling at high temperatures and pressures. However, these enzymatic and heterogeneous systems are difficult to probe mechanistically. Molecular catalysts have been studied extensively to investigate the elementary steps by which carbon monoxide is deoxygenated and coupled, but a single metal site that can efficiently induce the required scission of carbon–oxygen bonds and generate carbon–carbon bonds has not yet been documented. Here we describe a molybdenum compound, supported by a terphenyl–diphosphine ligand, that activates and cleaves the strong carbon–oxygen bond of carbon monoxide, enacts carbon–carbon coupling, and spontaneously dissociates the resulting fragment. This complex four-electron transformation is enabled by the terphenyl–diphosphine ligand, which acts as an electron reservoir and exhibits the coordinative flexibility needed to stabilize the different intermediates involved in the overall reaction sequence. We anticipate that these design elements might help in the development of efficient catalysts for converting carbon monoxide to chemical fuels, and should prove useful in the broader context of performing complex multi-electron transformations at a single metal site.

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

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.

Lewis Acid Enhancement of Proton Induced CO2 Cleavage: Bond Weakening and Ligand Residence Time Effects

Though Lewis acids (LAs) have been shown to have profound effects on carbon dioxide (CO2) reduction catalysis, the underlying cause of the improved reactivity remains unclear. Herein, we report a well-defined molecular system for probing the role of LA additives in the reduction of CO2 to carbon monoxide (CO) and water. Mo(0) CO2 complex (2) forms adducts with a series of LAs, demonstrating CO2 activation that correlates linearly with the strength of the LA. Protons induce C-O cleavage of these LA adducts, in contrast to the CO2 displacement primarily observed in the absence of LA. CO2 cleavage shows dependence on both bond activation and the residence time of the bound small molecule, demonstrating the influence of both kinetic and thermodynamic factors on promoting productive CO2 reduction chemistry.

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

Toward nitrogen functionalization, reactive terminal transition metal nitrides with high d-electron counts are of interest. A series of terminal MoIV nitride complexes were prepared within the context of exploring nitride/carbonyl coupling to cyanate. Reduction affords the first MoII nitrido complex, an early metal nitride with four valence d-electrons. The binding mode of the para-terphenyl diphosphine ancillary ligand changes to stabilize an electronic configuration with a high electron count and a formal M-N bond order of three. Even with an intact Mo≡N bond, this low-valent nitrido complex proves to be highly reactive, readily undergoing N-atom transfer upon addition of CO, releasing cyanate anion.

Correction to Molybdenum Catalyzed Ammonia Borane Dehydrogenation: Oxidation State Specific Mechanisms

Figures ​Figures22 and S32 incorrectly reported the units of time in seconds rather than minutes. These axis labels have been rectified in the attached versions of Figure ​Figure22 and the Supporting Information to match the (correct) time scales for H2 release reported in the text of the manuscript. Figure 2 Eudiometry of AB dehydrogenation catalyzed by 4, 5, and 11. Mo0 powder and catalyst-free controls are included for reference.