Diruthenium Catalyst for CO₂ and (Bi)carbonate Hydrogenation to Formate in Aqueous media

Highly efficient CO₂ and (bi)carbonate hydrogenation to formate in water was achieved by exploiting the cooperative interaction of the two ruthenium centers in a series of water-soluble diruthenium complexes [{(η⁶-arene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ containing the bridging meta-/para-bis-imidazole methane-based ligands (m-/p-benztetraimd). Our findings revealed the pivotal role of the ligand structure in tuning the catalytic activity, where the catalyst p-C3 [{(η⁶-p-cymene)RuCl}₂(μ-κ²:κ²-p-benztetraimd)]²⁺ outperformed others. Consequently, a high formate yield (2.73 mmol) with a turnover number (TON) of 1092 (1524 in 24 h) and a turnover frequency (TOF) of 137 h^-1 was achieved over the catalyst p-C3 at 40 bar pCO₂/pH₂ (1:3) at 80°C. Advantageously, the catalyst p-C3 also efficiently hydrogenated potassium bicarbonate and potassium carbonate to formate. We further utilized the optimized process for demonstrating the conversion of CO₂ captured (via direct air capture) to formate through an integrated carbon capture and CO₂ hydrogenation process. Moreover, the catalyst p-C3 exhibited appreciable recyclability over multiple catalytic cycles for the reversible CO₂/(bi)carbonate hydrogenation and formate dehydrogenation process. The key reaction intermediates, such as hydride species formed by treating the complex p-C3 with H₂ gas, revealed the role of such intermediates in achieving high catalytic activity for CO₂ hydrogenation over the diruthenium catalysts.