Allison M. Smith

Ruthenium complexes based on cis-[Ru(bpy)₂(CO)₂]²⁺ (bpy is 2,2'-bipyridine) can reduce CO₂ and CO to C₁ products including methanol, but access to products containing C-C bonds has been elusive. A reaction pathway to convert CO into multicarbon products ethylene and cyclopropane is presented here, along with mechanistic studies elucidating the key intermediates in C-C bond formation. The ruthenium hydroxymethyl complex [Ru(bpy')₂(CO)(CH₂OH)]⁺ (bpy' = 5,5'-dimethyl-2,2'-bipyridine) undergoes protonolysis to generate the ethylene complex [Ru(bpy')₂(CO)(C₂H₄)]²⁺ even at -80 °C, with free ethylene released upon warming to room temperature. Experimental evidence implicates a highly electrophilic methylidene complex [Ru(bpy')₂(CO)(CH₂)]²⁺ as the key intermediate. The methylidene was successfully trapped with nitriles and pyridine, forming adducts (ylide complexes) that each have a unique reactivity profile. With an appropriate nitrile, the adduct can be characterized at low temperature before warming generates ethylene. A more stable pyridine adduct [Ru(bpy')₂(CO)(CH₂pyridine)]²⁺ was crystallographically characterized. Even ethylene itself is sufficiently nucleophilic to react with the electrophilic methylidene, revealing a route from CO to the C₃ hydrocarbon cyclopropane. The methods for controlling the reactivity of hydroxymethyl and methylidene complexes toward C-C bond formation can inform the development of CO and CO₂ reduction catalysts.

Metal formyl complexes are critical intermediates in the reduction of CO to valuable products such as methanol and higher alcohols/hydrocarbons, yet examples of formyl generation via the catalytic hydrogenation of transition metal carbonyl complexes under mild conditions are lacking. The catalytic hydrogenation of a ruthenium carbonyl complex with H2 to produce a formyl complex is reported here. Two classes of hydrogenation catalyst were compared, those that proceed via termolecular H2 splitting with an external base and those that proceed via an H2 splitting mechanism involving metal-ligand cooperativity. The hydride transfer and H2 splitting steps were evaluated for both classes of catalysts, revealing advantages for catalysts that utilize metal–ligand cooperativity and establishing the importance of catalyst, base, solvent, and trace water in controlling reactivity. Using 5-10 mol % of the catalysts (PNP)Ir(H)2 and (HPNP)Ru(H)2CO (PNP = (iPr2PC2H4)2N–), which use metal–ligand cooperation to activate H2, up to 10 turnovers or up to 71% yield were achieved for the conversion of [Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) to the formyl complex [Ru(bpy)2(CO)(CHO)]+. The Lewis acid B(C6F5)3 was required as an additive to achieve high yields of the formyl complex using (HPNP)Ru(H)2CO as a catalyst. The catalytic route avoids the use of expensive stoichiometric reagents, such as borohydride, instead generating metal formyls that are key intermediates in CO reduction schemes with H2 gas.

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The kinetics of hydride transfer from Re(^Rbpy)(CO)₃H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, ^tBu, Me, H, Br, COOMe, CF₃) to CO₂ and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(^Rbpy)(CO)₃H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Reliance on fossil fuels has exacerbated climate change and created geopolitical instability. As seen recently with Russia’s invasion of Ukraine, key players in the energy sector often exert outsized influence on sovereign states and world markets. The ongoing sustainable energy transition provides an opportunity to change the geopolitical influence of these states while limiting global warming. Countries have an opportunity to use diplomacy to increase the diffusion of renewable energy technologies, particularly to developing countries. This can both reduce the inflated influence of fossil fuel producing states on the global stage and achieve a net zero (NZ) world. In anticipation of this latter transition—and to promote growth in energy diplomacy—we propose two UN initiatives: a multilateral energy research and storage collaborative, the World Institute for Energy Storage (WIES), and the biennial International Renewable Energy Conference & Expo (IRECE).