Electronic influence of pyrazole-appended pyridine/pyrazine based N,N,N-tridentate ligands on Ru complexes: Impact on selectivity of catalytic alkene oxidation with mild oxidant NaIO4

Abstract

Three discrete Ru(II) complexes [Ru(L1)(bpy)Cl]Cl (1), [Ru(L1)(phen)Cl]Cl (2) and [Ru(L2)(bpy)Cl]Cl (3) have been prepared with two different pyridine/pyrazine based tridentate ligands L1 and L2, with one bipyridine/phenanthroline and one chloride as co-ligands. These complexes are characterized fully via spectroscopic and crystallographic techniques. The crystal structures of the complexes revealed the discrete monomeric structures. The Density Functional Theory (DFT) calculations showed that for complexes 1 and 3, the highest occupied molecular orbital (HOMO) is metal-centered whereas the lowest unoccupied molecular orbital (LUMO) is centered on the bpy ligand in 1, but interestingly it is based on the ligand L2 in case of the complex 3. Also, both the HOMO and LUMO are relatively more stabilized in 3 in comparison to 1. This fact implies very good π-acceptor nature of the ligand L2. This is reflected on the redox behavior of the complexes where around 200 mV anodic shift of the RuII/III redox potential was observed in 3 as compared to complex 1. This electronic influence of the ligands L1 and L2 impacted remarkably in the catalytic behavior of their Ru-complexes (1–3). Thus, the catalysts 1 and 2 were found to oxidize aryl alkenes to the corresponding acid products with high selectivity, whereas the catalyst 3 was selective toward the formation of aldehydes only, in the presence of only 2.5–3.0 equiv. of NaIO4 as the mild oxidant.