Synthesis of Imines via Reactions of Benzyl Alcohol with Amines Using Half-Sandwich (η6-p-cymene) Ruthenium(II) Complexes Stabilised by 2-aminofluorene Derivatives

A new class of half-sandwich (eta(6)-p-cymene) ruthenium(II) complexes supported by 2-aminofluorene derivatives [Ru(eta(6)-p-cymene)(Cl)(L)] (L = 2-(((9H-fluoren-2-yl)imino)methyl)phenol (L-1), 2-(((9H-fluoren-2-yl)imino)methyl)-3-methoxyphenol (L-2), 1-(((9H-fluoren-2-yl)imino)methyl)naphthalene-2-ol (L-3) and N-((1H-pyrrol-2-yl)methylene)-9H-fluorene-2-amine (L-4)) were synthesized. All compounds were fully characterized by analytical and spectroscopic techniques (IR, UV-Vis, NMR) and also by mass spectrometry. The solid state molecular structures of the complexes [Ru(eta(6)-p-cymene)(Cl)(L-2)], [Ru(eta(6)-p-cymene)(Cl)(L-3)] and [Ru(eta(6)-p-cymene)(Cl)(L-4)] revealed that the 2-aminofluorene and p-cymene moieties coordinate to ruthenium(II) in a three-legged piano-stool geometry. The synthesized complexes were used as catalysts for the dehydrogenative coupling of benzyl alcohol with a range of amines (aliphatic, aromatic and heterocyclic). The reactions were carried out under thermal heating, ultrasound and microwave assistance, using solvent or solvent free conditions, and the catalytic performance was optimized regarding the solvent, the type of base, the catalyst loading and the temperature. Moderately high to very high isolated yields were obtained using [Ru(eta(6)-p-cymene)(Cl)(L-4)] at 1 mol%. In general, microwave irradiation produced better yields than the other two techniques irrespective of the nature of the substituents.