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.