Aryl appended neutral and cationic half-sandwich ruthenium(II)–NHC complexes: synthesis, characterisation and catalytic applications
Half-sandwich ruthenium(II) complexes 1–6 bearing imidazolylidene and pyridyl-imidazolylidene ligands have been synthesised in good yields and were characterised on the basis of spectral and analytical evidence. In addition, the structures of the complexes 1–4 were unambiguously established through single crystal X-ray analysis. Transmetalation of the ligands followed by complexation with ruthenium precursors yielded the air and moisture stable complexes. The crystal structures of these complexes exhibited piano-stool geometries with η6-coordination of the p-cymene or hexamethylbenzene moieties. These complexes exhibited catalytic activity in the transfer hydrogenation of carbonyls in an alkaline medium using 2-propanol as the hydrogen source. The effect of variations in the catalyst structure on the transfer hydrogenation and stability was investigated in detail, and theoretical calculations were employed to understand the mechanism of the catalytic activity. The neutral ruthenium–NHC complexes 1 and 2 showed the efficiency of ca. 100% at a catalyst loading of ca. 2 mol% within 2 h of the reaction in 2-propanol, whereas quantitative yields were obtained in the presence of cationic ruthenium–NHC complexes 3–6 within 1 h at a low catalyst loading of ca. 0.5 mol%, thereby demonstrating their robustness for the transfer hydrogenation of the aromatic ketones.