Molecular hydrogen generation from neat formic acid catalyzed by ruthenium-cymene α-diimine complexes

Abstract

A series of half-sandwich ruthenium(II) complexes bearing substituted α-diimine ligands with the general formula [RuCl(p-cym)(N–Nn)](PF6) was synthesized (where n = 1–7; N–N1 = N1,N2-bis(2,6-dimethylphenyl)ethane-1,2-diimine,N–N2=N1,N2-bis(2,4-dimethylphenyl)ethane-1,2-diimine,N–N3=N1,N2-bis(2,4,6-trimethylphenyl)ethane-1,2-diimine,N–N4=N1,N2-bis[2,6-bis(propan-2-yl)phenyl]ethane-1,2-diimine,N–N5=N1,N2-bis(4-fluorophenyl)ethane-1,2-diimine,N–N6=N1,N2-bis(4-chlorophenyl)ethane-1,2-diimine and N–N7=N1,N2-dicyclohexylethane-1,2-diimine). Ligands and complexes were fully characterized by elemental analysis, NMR, FTIR, UV–vis spectroscopy, and single-crystal X-ray diffraction for 2, 5, and 6.CH2Cl2. The complexes displayed a distorted pseudo-octahedral “piano-stool” geometry, with the α-diimine ligands coordinating in a bidentate manner. The p-cymene ring was observed to rotate around its bond to the metal, as evidenced by variable-temperature 1H NMR spectra and NOE measurements. DFT calculations were used to investigate the electronic structures of complexes 1–4, revealing how different substituents affect their stability and HOMO–LUMO energy gaps. Additionally, the most nucleophilic and electrophilic regions in the optimized structures were identified using the Hirshfeld charge method applied to the Fukui function. All complexes were evaluated as precatalysts in the solvent-free dehydrogenation of formic acid, in the presence of a Bro̷nsted–Lowry base, achieving up to 94.8% conversion in a first run and a maximum turnover frequency (TOF) of 627 h–1 under mild conditions (60 °C, 1:1204:843 molar ratio of Ru/FA/base). Total conversion and improvement in TOF values were observed in a subsequent run. A detailed mechanistic study combining kinetic data and DFT modeling supports a chloride displacement–initiated cycle involving a β-hydride elimination pathway for H2 and CO2 release. This methodology is consistent with the observed induction period and activation parameters (ΔG‡ = 24.5 kcal mol–1; ΔS‡ = +137 cal mol–1 K–1), and ΔH‡ = 70.3 kcal mol–1, which are in excellent agreement with Ea = 70.9 kcal mol–1). The catalytic activity was strongly influenced by both the electronic nature and steric hindrance of the α-diimine ligands, as well as the Bro̷nsted–Lowry character of the base.