Cycloruthenated complexes: pH-dependent reversible cyclometallation and reactions with nitrite at octahedral ruthenium centers

Abstract

The pH-dependent reversible cyclometallation and reactions with nitrite in a near-aqueous system of a related set of cyclometallated ruthenium(II) bipyridyl complexes were investigated in detail. These complexes are [Ru(ppy)(bpy)₂]PF₆ (1, bpy = 2,2′-bipyridine, Hppy = 2-phenylpyridine), [Ru(thpy)(bpy)₂]PF₆ (2, Hthpy = 2-(2-thienyl)pyridine), and [Ru(dfppy)(bpy)₂]PF₆ (3, Hdfppy = 2-(2,4-difluorophenyl)pyridine). As expected, reversible UV–Vis spectra of these three complexes in near-aqueous solutions can be achieved by treating with acid or base, which indicates a pH-dependent reversible cyclometallation. However, the addition of nitrite to acidic solutions of these complexes disturbed the reversible pH-dependent processes. Unexpected ruthenium complexes in the aforementioned system were then isolated and characterized using FT-IR, MS, ¹H NMR, and UV–Vis spectra, indicating that two reaction modes occurred at the ruthenium(II) centers: (1) the insertion of NO into the ruthenium–aryl bond to form a ruthenium C-nitroso complex; and (2) the coordination of NO with the entire dissociation of one bipyridine to form a {Ru–NO}⁶ complex, which is the first example involving the cleavage of Ru–N^∧N bonds in ruthenium bipyridyl complexes.