Electrospray mass spectrometry of cis-[Ru(NO)Cl(bpy)2]2+ (bpy=2,2′-bipyridine): fragmentation from desolvated {[Ru(NO)Cl(bpy)2]2+, Cl−}+ ion pairs by electron transfer and internal Lewis base pathways

Abstract

The electrospray mass spectrum (ESI-MS) of cis-[Ru(NO)Cl(bpy)2]Cl2 (bpy=2,2′-bipyridine), obtained from 50% CH3OH/50% H2O as the mobile solvent, exhibited ruthenium-containing ions derived from a {[RuII(NO+)Cl(bpy)2]2+, Cl−}+ ion pair (m/z=514) and [RuII(NO+)Cl(bpy)2]2+ (m/z=239.5). [RuIIICl(bpy)2]2+, from the loss of NO from the 239.5 ion, is detected at m/z=224.5. Only the m/z 514 ion pair is detected when 100% CH3OH mobile solvent is used, but the presence of even small amounts of water prompted the additional detection of the m/z 239.5 and m/z 224.5 ions under tandem MS–MS conditions. Ruthenium-chloro-containing ions appear as a characteristic collection of eight main, and four lesser, intense ions created from combinations 104Ru, 102Ru, 101Ru, 99Ru, 98Ru, 96Ru, 35Cl and 37Cl isotopes with minor contributions from 13C, etc. For convenience of discussion, only the most abundant m/z species are mentioned herein as representative of all the isotopically distributed ions.Four fragmentation channels are detectable from the m/z=514 chloride ion pair: (1) the loss of HCl (main channel; ca. 50% of fragmentation events), (2) the loss of NO (ca. 12% ), (3) the loss of bpy (minor pathway), and (4) the loss of Cl atom (ca. 38% ).Loss of NO from ion m/z 514 yields ion m/z 484, which is the precursor of ions m/z 448 (by loss of HCl), m/z 328 (by loss of bpy) and m/z 292 (by loss of HCl and bpy). Loss of HCl from ion m/z 514 generates ion m/z 478, [RuII(NO+)Cl(bpyH)(bpy-H)]+, deprotonated at the ortho C–H of one bpy ligand. In MS–MS experiments, the m/z 478 ion was established to undergo loss of NO, producing ion m/z 448, rejoining further fragmentation process for ion m/z 448 at this point. Loss of neutral bipyridine from m/z 514 in low yield produces ion m/z 358, which undergoes further loss of NO to form [RuCl2(bpy)]+ ion (m/z=328). MS–MS “neutral loss of 30” spectra confirmed the NO loss events as part of the fragmentation sequence for all four pathways.A fourth species of m/z=479 from the “514” ion is obtained by an internal electron transfer from Cl− of the ion pair, and loss of the resultant neutral Cl atom. The product [RuII(NO·)Cl(bpy)2]+ “479” fragment undergoes facile loss of NO to generate [RuIICl(bpy)2]+ (m/z=449). Ion m/z 449 gives rise to ions m/z 413 (loss of HCl) and m/z 257(loss of HCl and bpy). MS–MS experiments confirm the neutral loss of Cl from the m/z 514 ion, and the formation of the m/z 449 ion via m/z 479 and m/z 514 parents. This pathway was not observed in a prior study for the related complex, [Ru(NO)Cl(dpaH)(dpa−)]+ (dpaH=2,2′-dipyridylamine), which does not have an external Cl− in an ion pair.