Piano-stool dinuclear ruthenium (II) complexes of pyrazine-carboxylate/carboxamide ligands: Structural studies and catalytic transfer hydrogenation of ketones

Abstract

Reactions of ligand pyrazine-2-carboxylic acid (HL1) with [Ru(η⁶-p-cymene)Cl₂]₂ precursor gave the dinuclear piano-stool ruthenium (II) complex [{Ru(η⁶-p-cymene)Cl₂}-μ-(L1)-{Ru(p-cymene)Cl}] (Ru1). Separately, reactions of N-(quinolin-8-yl) pyrazine-2-carboxamide (HL2), 5-methyl-N-(−(quinolin-8-yl) pyridine-2-carboxamide (HL3) and 5-chloro-N-(quinolin-8-yl) pyridine-2-carboxamide (HL4) with [Ru(η⁶-p-cymene)Cl₂]₂ dimer in the presence of KPF₆ afforded the cationic dinuclear complexes [{Ru(η⁶-p-cymene)Cl}₂-μ-(L2)][PF₆] (Ru2), [{Ru(η⁶-p-cymene)Cl}₂-μ-(L3)][Ru(L3)Cl₃] (Ru3) and [{Ru(η⁶-p-cymene)Cl}₂-μ-(L4)][PF₆] (Ru4). The Ru (II) complexes were analysed using FT-IR, ¹H, ¹³C{¹H}, ³¹P{¹H} (Ru2 and Ru4) and ¹⁹F (Ru2 and Ru4) NMR spectroscopic techniques, micro-analyses and mass spectrometry. Molecular structures of complexes Ru1 and Ru3 were confirmed to display piano-stool coordination nature using single-crystal X-ray crystallography analyses. All the complexes (Ru1–Ru4) mediated the transfer hydrogenation (TH) of a broad spectrum of ketones in isopropanol in the presence of a base and demonstrated high catalytic activities (TON of 24,000) at catalyst concentrations of 0.002 mol%. In general, the catalytic performance of these Ru (II) complexes depended on the identity of the ligands, coordination chemistry and ketone substrates.