Exploitation of 1,4-Diazabutadiene Coordinated cis-RuCl2 Moiety toward Synthesis of tris-Complexes and Catalytic Hydrogenation of CO2 for Selective Formic Acid Production

This article reports the successful utilization of 1,4-diazabutadiene (L-R; R = OCH₃, CH₃, H, and Cl)-coordinated cis-RuCl₂ moiety for the synthesis of new mixed-tris complexes and also for catalytic hydrogenation of CO₂ yielding formic acid at ambient pressure. The synthesis of a diverse set of heteroleptic tris complexes of [Ru(L-H)₂(L-L')]ⁿ⁺ (n = 0, 1, and 2) has been achieved via Ag⁺-assisted substitution of the chlorides from cis-[Ru(L-H)₂Cl₂] by chelating bidentate ligands (L-L') in ethanol medium. The cis-[Ru(L-R)₂Cl₂] complexes demonstrate remarkable catalytic efficiency in the selective hydrogenation of CO₂ to formic acid under exceptionally mild conditions (at ambient pressure for both H₂ and CO₂). This low-pressure operation is particularly significant, as it eliminates the need for high-pressure reactors, aligning with the principles of green chemistry and energy efficiency. The catalysts afford good yields while maintaining excellent selectivity toward formic acid, with no detectable formation of more reduced by-products. Mechanistic investigations reveal that the in situ formation of a diimine-coordinated cis-Ru(H)₂ species plays a pivotal role in CO₂ activation and hydride transfer. These findings underscore the potential of Ru(II)-diazabutadiene complexes as efficient precatalysts for CO₂ utilization, offering a sustainable and practical route for conversion of CO₂ into formic acid.