Synthesis and characterization of naphthaldiimine-based ruthenium(III) complexes; homogenous catalytic hydrogenation and isomerization of internal and terminal alkenes

Abstract

This study is concerned with synthesis and characterization of four naphthaldiimine ligands and their ruthenium(III) complexes. The tetradentate diimines were synthesized by a Schiff condensation reaction between 2-hydroxy-1-naphthaldehyde and various aliphatic diamines with chains of 2, 4, 5 and 6 carbon atoms. Analytical, mass spectra, thermal and electrolytic solution conductance measurements demonstrated the molecular formulas of the prepared Ru(III) complexes. A distorted octahedron stereochemistry was proposed for these six-coordinate naphthaldiimine-based Ru(III) complexes. Processing of X-ray diffraction data of 1 and 2 by the computer program Expo 2014 confirmed the final structural formulas that were determined based on spectroscopic and magnetic studies. The catalytic potential of the newly synthesized naphthaldiimine-based Ru(III) complexes in the hydrogenation of 1-hexene and cyclohexene was investigated. The Ru(III) catalysts catalyzed the double bond migration of 1-hexene to give trans-2-hexene which catalytically isomerizes to the stereo isomer cis-2-hexene. The induction step of the catalytic cycle is the formation of the actual catalyst, metal hydride (Ru(III)-H), that was generated in situ during the hydrogenation reaction. Several parameters controlling the catalytic reactions including the structure of the Ru(III) complex, the phase type of catalysis, and the quantity and nature of the solvents used were investigated. Various experiments made it possible to suggest a mechanism for the hydrogenation reactions.