Pyrazolyl‐Pyridine Ruthenium (II) Catalysts for Selective Hydrogen Generation Through Formic Acid Dehydrogenation

A series of pyridine‐pyrazolyl Ru(II) p‐cymene complexes of the type [RuCl(p‐cymene) L ]X ( C1 – C7 , X = Cl, BPh 4 or PF 6 ), featuring different electronic and steric properties, are prepared by the reaction of 3,5‐disubstituted pyrazolyl‐pyridine ligands ( L1 – L3 ) with [Ru( p ‐cymene)Cl 2 ] 2 followed by salt metathesis with NaBPh 4 or AgPF 6 . The complexes, upon activation with formate, show activity toward the dehydrogenation of formic acid (FA) to produce equimolar quantities of H 2 and CO 2 . In optimization studies, kinetic and thermodynamic investigations provide insight into the role of pre‐catalyst and FA in the catalytic process. It is demonstrated that the most active catalyst derived from C4 , could dehydrogenate FA to CO 2 and H 2 in quantitative yields within 1 h with tons and TOFs of ≈1032 and ≈1238 h −1 , respectively. Reactivity patterns are rationalized based on electronic properties and our understanding of ion pairing in related compounds. Reactivity studies between C4 and HCOOK provide evidence for the key Ru‐monohydride species involved in catalysis.