Electrospray mass spectrometric studies of nickel(II)-thiosemicarbazones complexes: Intra-complex proton transfer in the gas phase ligand exchange reactions

Ligand exchange reactions of [Ni2+ M (SCH3)−]+ (m/z 315, CI-a), (M = ArCH = NNHC(=S)SCH3), with the residual ESI solvent molecules in the ion trap have been investigated by electrospray ionization (ESI) mass spectrometry and quantum calculation. The intermediates, including [Ni2+ M (SCH3)− (CH3CN)]+ (m/z 356) and [Ni2+ (M−H)−]+ (m/z 267), in both the associative and the dissociative pathways have been obtained in the tandem MS, indicating that the two mechanistic pathways are efficient in the ligand exchange reactions. The electronic effect of the phenyl substituent on the reactivity of [Ni2+ (M*−H)−]+ has also been investigated by mass selection in the MS4 experiments. Intra-complex proton transfer to the anion ligand (SCH3)− is essential for the ligand exchange reactions. D-labeling experiments confirmed that the transferring proton comes from either the thioamide proton, or the phenyl proton via Csp2-H activation leading to cyclometallated complexes. DFT calculations at uB3LYP/6-31+G(2d, p) level gave the key species and the potential energy surfaces for the model reaction system of CI-a and acetonitrile.

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