Electron ionization mass spectrometry of aryl- and fluoroalkyl-substituted nickel(II) β-diketonates and monothio-β-diketonates

Electron ionization positive-ion mass spectra are reported for 18 substituted nickel(II) β-diketonates and monothio-β-diketonates, NiII(RCXCHCOR′)2, where X = O or S; R = phenyl, 2-thienyl, 5-methyl-2-thienyl, or 2-naphthyl; and R′ = difluoromethyl, trifluoromethyl, pentafluoroethyl, or n-heptafluoropropyl. Each mass spectrum contains a prominent molecular ion, [NiL2]+, which, for β-diketonates, fragments mainly by elimination of the odd-electron R′• species; the resulting ion decomposes by losses of one or more even-electron neutral species to yield ions containing nickel(II). In contrast, the major fragmentation of the molecular ion of monothio-β-diketonates is loss of NiIL to yield an abundant L+ ion. Furthermore, while most ions contain nickel(II), some prominent ions contain nickel(I); among these are [NiISC(R) = CH]+ and [NiI(HCCR)]+. Interesting ions are formed by hydrogen transfer processes. The [NiHL]+ ion is favored in the spectra of β-diketonates having a phenyl substituent, consistent with hydrogen transfer to the metal from the aryl group. On the other hand, the [Ni(L–H)]+ ion, favored for the monothio-β-diketonates, could be formed by hydrogen transfer from the central position of the chelate ring or from an aryl substituent. Alternative mechanisms of formation are discussed. Some minor ions are formed by fluorine transfer to nickel. The trends in their abundances are influenced by the hardness of nickel as an acid in its different oxidation states, under the influence of the ligand donor atoms, and by the hardness of the carbon atoms of the perfluoroalkyl substituents.