Ru(II) and Ru(III) complexes containing N‐acylthiourea ligands: Supramolecular structures and synthons, reduction, and reaction pathway of aromatic nitro compounds

Abstract

Four ruthenium complexes {[RuCl(CO)(PPh3)2L1], [RuCl(CO)(PPh3)2L2], [RuCl2(PPh3)2L1] and [RuCl2(PPh3)2L2]} supported with N‐acylthiourea‐based ligands were synthesized. The compounds prepared were characterized by 1H NMR, 13C NMR, elemental analysis, UV–vis, and magnetic susceptibility methods. The characterization of the molecular structure of two ruthenium complexes {[RuCl(CO)(PPh3)2L1] and [RuCl2(PPh3)2L1]} was also performed using the single crystal X‐ray diffraction method. Single crystal X‐ray analyses indicates the existence of different non‐covalent interactions between different parts of the molecules, which led to the formation of three‐dimensional supramolecular architectures and synthons. The magnetic susceptibility and UV‐vis spectra of the prepared ruthenium complexes propose that they are of low‐spin states, where Ru(II) is d6 and Ru(III) is d5 low spin. These ruthenium compounds functioned as catalysts for the NaBH4‐mediated reduction of various nitro compounds. The catalytic results show that the [RuCl2(PPh3)2L] type complexes exhibit higher catalytic activity than the [RuCl(CO)(PPh3)2L] type complexes, and [RuCl2(PPh3)2L1] has high catalytic selectivity and efficiency in a short time (>99 yield with catalyst loading 0.01 mol in 0.5 h). Furthermore, the catalytic activity of [RuCl2(PPh3)2L1] was investigated using nitrobenzene as a model substrate to understand the reaction pathway, and this study proved that this nitro reduction proceeds via a direct pathway, with the formation of N‐hydroxylamine as the main intermediate followed by the occurrence of aniline.