Structural analysis of photo-oxidized (ethylenediamine)bis(2,2′-bipyridine)ruthenium(II) complexes by using on-line electrospray mass spectrometry of labeled compounds

Abstract

Photo-oxidation of Ru(bpy)2(en)2+, where bpy = 2,2′-bipyridine, en = ethylenediamine, was studied in isotopic labeling experiments by using on-line electrospray mass spectrometry (ESMS). The complex was known to undergo photochemical dehydrogenation of a fourelectron oxidation, giving the α,α′-diimine complexes in a stepwise manner via a two-electron-oxidized intermediate that represents loss of two hydrogen atoms from the en ligand. On-line mass analysis after photoirradiation (λ > 420 nm) of Ru(bpy)2(ed)2+ (ed = ethylene-d4diamine) showed that the ligand of the intermediate with loss of two hydrogen atoms was not an enamine but had an imine structure. Also, a ligand-oxygenated complex that has mass 14 amu higher than the Ru(bpy)2(en)2+ complex was observed in the ES mass spectra. The ligand of this complex was proposed to have a nitroso structure as a primary product in 18O2 experiments. The oxygenated complex was not generated in a stepwise manner via the imine intermediate, but directly by loss of two amino hydrogen atoms and addition of an oxygen atom. The source of the oxygen atom would be from oxygen dissolved in solution rather than from water in solution. Another oxygenated complex Ru(bpy)2(NO 2 #x2212; )+ was produced by irradiation and the structure was identified in 18O2 experiments.