Structure–Activity Relationships in Ru(II)‐Protic‐NHC Complexes: β‐ Versus γ‐Substituent Effects on Formic Acid Dehydrogenation Performance

ABSTRACT 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 H 2 /CO 2 mixture was successfully applied for the hydrogenation of alkenes, and CO 2 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.