CO Reduction to Ethylene and Cyclopropane via a Trappable Ruthenium Methylidene

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.