Ancillary ligand-induced variation in electronic spectral and catalytic properties of heteroleptic ONO-pincer complexes of ruthenium

Abstract

In this paper, we report our studies on a group of mixed-ligand complexes of ruthenium(II) bearing an ONO-coordinated dipicolinate (DPA) as the pincer ligand, and some selected ancillary ligands (viz. PPh₃, CH₃CN, 4-picoline (pic), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen)). Utilizing [Ru(DPA)(PPh₃)₂(OH₂)] as the ruthenium precursor, four new mixed-ligand complexes, viz. [Ru(DPA)(PPh₃)₂(CH₃CN)], (1); [Ru(DPA)(PPh₃)₂(pic)], (2); [Ru(DPA)(PPh₃)(bpy)], (3); and [Ru(DPA)(PPh₃)(phen)], (4); were synthesized in decent yields. Bulk characterization was done by elemental (C,H,N) analysis, and different spectral (mass, UV-vis, NMR and IR) techniques. The crystal structures of all four complexes have been determined, which confirmed the ONO-mode of binding of the pincer DPA ligand and also revealed the binding modes of the ancillary ligands. Structural details in the crystals were also examined and discussed. Electronic absorption spectra of complexes 1–4 show intense absorptions spanning the visible and ultraviolet regions, which have been analysed with the help of the TDDFT method. All four complexes also exhibited prominent emission in the visible region with Stokes shift ranging from 71–118 nm. In terms of quantum yield, complex 4, which contains the highly conjugated π-acid phen as an ancillary ligand, is found to be the best emitter. These heteroleptic pincer complexes 1–4 are found to serve as efficient catalyst–precursors for the transfer-hydrogenation of aryl/alkyl ketones, using 2-propanol as the hydrogen provider, to furnish the corresponding alcohols in good yields. Here, complex 3 showed the highest catalytic efficiency. Hemilability of the ruthenium-bound pincer DPA ligand in solution has played the key role behind in situ formation of the catalytically active Ru-hydrido species.