Electrospray mass spectrometry of trans-[Ru(NO)Cl(dpaH)2]2+ (dpaH=2,2′-dipyridylamine) and ‘caged NO’, [RuCl3(NO)(H2O)2]: loss of HCl and NO from positive ions versus NO and Cl from negative ions

The positive ion electrospray mass spectrometry (ESI-MS) of trans-[Ru(NO)Cl)(dpaH)2]Cl2 (dpaH=2,2′-dipyridylamine), obtained from the carrier solvent of H2O–CH3OH (50:50), revealed 1+ ions of the formulas [RuII(NO+)Cl(dpaH)(dpa)]+ (m/z=508), [RuIIICl(dpaH)(dpa−)]+ (m/z=478), [RuII(NO+)(dpa)2]+ (m/z=472), [RuIII(dpa)2]+ (m/z=442), originating from proton dissociation from the parent [RuII(NO+)Cl(dpaH)2]2+ ion with subsequent loss of NO (17.4% of dissociative events) or loss of HCl (82.6% of dissociative events). Further loss of NO from the m/z=472 fragment yields the m/z=442 fragment. Thus, ionization of the NH moiety of dpaH is a significant factor in controlling the net ionic charge in the gas phase, and allowing preferential dissociation of HCl in the fragmentation processes. With NaCl added, an ion pair, {Na[RuII(NO)Cl(dpa)2]}+ (m/z=530; 532), is detectable. All these positive mass peaks that contain Ru carry a signature ‘handprint’ of adjacent m/z peaks due to the isotopic distribution of 104Ru, 102Ru, 101Ru, 99Ru, 98Ru and 96Ru mass centered around 101Ru for each fragment, and have been matched to the theoretical isotopic distribution for each set of peaks centered on the main isotope peak. When the starting complex is allowed to undergo aquation for two weeks in H2O, loss of the axial Cl− is shown by the approximately 77% attenuation of the [RuII(NO+)Cl(dpaH)(dpa)]+ ion, being replaced by the [RuII(NO+)(H2O)(dpa)2]+ (m/z=490) as the most abundant high-mass species. Loss of H2O is observed to form [RuII(NO+)(dpa)2]+ (m/z=472). No positive ion mass spectral peaks were observed for RuCl3(NO)(H2O)2, ‘caged NO’. Negative ions were observed by proton dissociation forming [RuII(NO)Cl3(H2O)(OH)]− in the ionization chamber, detecting the parent 1− ion at m/z=274, followed by the loss of NO as the main dissociative pathway that produces [RuIIICl3(H2O)(OH)]− (m/z=244). This species undergoes reductive elimination of a chlorine atom, forming [RuIICl2(H2O)(OH)]− (m/z=208). The ease of the NO dissociation is increased for the negative ions, which should be more able to stabilize a RuIII product upon NO loss.