Structure-Activity Relationships in Ru(II)-Protic-NHC Complexes: β- Versus γ-Substituent Effects on Formic Acid...

We report the synthesis and catalytic evaluation of new Ru(II)-protic-NHC complexes featuring β-NH groups with either γ-NH (Ru8) or γ-NMe (Ru9) substituents. These compounds, along with previously reported Ru(II)-protic-NHC and their classical NHC analogues (Ru1-Ru7), provide a platform for systematically probing the influence of β- and γ-substituents on formic acid dehydrogenation (FADH). All complexes exhibited notable catalytic activity under both base-assisted, solvent-free conditions and in the ionic liquid 1-butyl-3-methylimidazolium acetate (BMIMOAc) without an external base. Among them, Ru1 excelled in the ionic liquid medium, achieving a remarkable turnover number (TON) of 65,000 and turnover frequency (TOF) of 4300 h^-1 over 15 consecutive formic acid addition cycles. Mechanistic investigations, supported by NMR and control experiments, reveal that under base-assisted conditions, catalysis proceeds via a protic-NHC pathway, whereas in ionic-liquid mediated FADH, anionic-NHC intermediates dominate. Furthermore, the evolved H₂/CO₂ mixture was successfully applied for the hydrogenation of alkenes, and CO₂ capture experiments confirmed the selective utilization of the generated gases. The presence of a β-NH moiety enhances the catalytic activity relative to its N-methylated analogue, suggesting a favorable role in facilitating the reaction pathway, whereas the γ-NH functionality exerts a detrimental effect compared to its corresponding γ-N-alkyl and γ-N-aryl substituted derivatives.