Fragmentation Pathways of [MX₂(CO)₂(dcbpy)] (M = Ru, Os; X = Cl, Br, I; dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid) Complexes

The gas-phase behavior and stability of [RuX2(CO)2(dcbpy)], [OsX2(CO)2(dcbpy)], and [OsI2(CO)2(mcbpy)] (X = Cl, Br, I; dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid; mcbpy = 2,2′-bipyridine-4-carboxylic acid) were studied by electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Negative-ion ESI produced abundant singly and doubly deprotonated molecules for all of the compounds, without apparent changes in the metal atom oxidation state or the ligand coordination. The characteristic fragment ions resulting from the decarboxylation (loss of CO2) in one of the carboxylic acid substituents of the dcbpy ligand were also observed. The gas-phase fragmentation was investigated by means of collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) techniques. The most favored fragmentation pathway included the loss of CO2, followed by one or two decarbonylations. Fragmentation was observed to be both qualitatively and quantitatively dependent on the metal atom and the surrounding ligands. Generally, the compounds of osmium were considerably more stable than those of ruthenium, owing to the higher metal−carbonyl bond energies. On the basis of the ionic structures observed experimentally, the fragmentation processes were also investigated computationally at the DFT level of theory. The most likely fragmentation routes predicted by the calculations agreed well with the experimental findings. The computational structures of the different fragment anions provided additional information about the effect of the carboxylic acid substituents on the stability of the ruthenium and osmium complexes.