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

Four ruthenium complexes {[RuCl(CO)(PPh3)(2)L-1], [RuCl(CO)(PPh3)(2)L-2], [RuCl2(PPh3)(2)L-1] and [RuCl2(PPh3)(2)L-2]} supported with N-acylthiourea-based ligands were synthesized. The compounds prepared were characterized by H-1 NMR, C-13 NMR, elemental analysis, UV-vis, and magnetic susceptibility methods. The characterization of the molecular structure of two ruthenium complexes {[RuCl(CO)(PPh3)(2)L-1] and [RuCl2(PPh3)(2)L-1]} 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 d(6) and Ru(III) is d(5) 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)(2)L] type complexes exhibit higher catalytic activity than the [RuCl(CO)(PPh3)(2)L] type complexes, and [RuCl2(PPh3)(2)L-1] 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)(2)L-1] 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.