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), featuring different electronic and steric properties, are 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, show activity toward the dehydrogenation of formic acid (FA) to produce equimolar quantities of H₂ and CO₂. 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 CO₂ and H₂ 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.