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, BPh4 or PF6), bearing different electronic and steric properties, were prepared by the reaction of 3,5‐disubstituted pyrazolyl‐pyridine ligands (L1‐L3) with [Ru(p‐cymene)Cl2]2 followed by salt metathesis with NaBPh4 or AgPF6. The complexes, upon activation with formate, showed activity towards the dehydrogenation of FA to produce equimolar quantities of H2 and CO2. Optimization studies, kinetic and thermodynamic investigations provided insight into the role of pre‐catalyst and FA on the catalytic process. We demonstrated that the most active catalyst derived from C4, could dehydrogenate formic acid to CO2 and H2 in quantitative yields within 1 hour with TONs and TOFs of ∼1032 and ∼1238 h‐1 respectively. Reactivity patterns were rationalized based on electronic properties and our understanding of ion‐pairing in related compounds. Reactivity studies between C4 and HCOOK provided evidence for the key Ru‐monohydride species involved in catalysis.