Properties and one-step synthesis of (2-acetylpyridine)tetraammineruthenium(II), [RuII(2-acpy)(NH3)4]2+ and tetraammine(2-benzoylpyridine)ruthenium(II), [RuII(NH3)4(2-bzpy)]2+: Redox potentials, UV–Vis and NMR spectra

A more direct and efficient route to the syntheses of [Ru(NH3)4(X–Y)](BF4)2, where X–Y can be 2-acetylpyridine (2-acpy) or 2-benzoylpyridine (2-bzpy), based on the reactions of [RuCl(NH3)5]Cl2 with these ortho-substituted azines is described. The [Ru(2-acpy)(NH3)4](BF4)2 and [Ru(NH3)5(2-bzpy)](BF4)2 complexes have a molar conductance of 328 and 292 Ω−1 cm2 mol−1, respectively, corresponding to a 1:2 species in solution. These complexes showed two intense absorption bands around 620–650 and 380 nm, the energies of which are solvent dependent, decreasing with the increase of the Gutman’s donor number of the solvent, and were assigned as metal-to-ligand charge transfer (MLCT). The complexes have oxidation potentials (RuII/III) of +0.380 V vs. Ag/AgCl (2-acpy) and +0.400 V vs. Ag/AgCl (2-bzpy), and reduction potentials (X–Y0/−) of −1.10 V vs. Ag/AgCl (2-acpy) and −0.950 V vs. Ag/AgCl (2-bzpy) on CF3COOH/NaCF3COO at pH=3.0, scan rate 100 mV s−1, [Ru]=1.0×10−3 mol l−1. Both processes show a coupled chemical reaction. Upon oxidation of the metal center, the MLCT absorption bands are bleached and restored upon subsequent reduction. In order to confirm the structure of the complexes a detailed 1H NMR investigation was performed in d6-acetone. Further confirmation of the structure was obtained by recording the 15N NMR spectrum of [Ru(NH3)4(2-bzpy)]2+ in d6-DMSO using the INEPT pulse sequence improving the sensitivity of 15N by polarization transfer from the protons to the 15N. The Nuclear Overhauser Effect (NOE) experiments were made qualitatively for [Ru(NH3)4(2-acpy)]2+, and showed that H6 of the pyridine is close to a NH3 proton, which should then be in a cis position, and, hence, confirming that acpy is acting as a bidentate ligand.