Direct synthesis and electron-ionization positive-ion mass spectrum of tetrakis(acetylacetonato)cerium(IV), Ce(C₅H₇O₂)₄

Abstract

Crystalline Ce(C₅H₇O₂)₄ has been synthesized directly from the reaction of hydrated cerium(IV) oxide with acetylacetone, (C₅H₈O₂), and its electron-ionization (EI) mass spectrom is reported. The compound exists as a monomer (mass 536) in the gaseous state. The mass spectrum, like those of M(C₅H₇O₂)₄ (M = Zr or Th) and CeL₄ (L = fluorinated β-diketonate), shows the dominant ion signal to be due to the [Ce(C₅H₇O₂)₃]⁺ ion that arises from the loss of one C₅H₇O₂ from the molecular ion. An internal electron-transfer affords [Ce(C₅H₇O₂)₃]^+'. Fragmentation of [Ce(C₅H₇O₂)₃]^+' to [Ce(C₅H₇O₂)₂]⁺ is similar to that observed for the tris-chelates of trivalent transition metals and La(C₅H₇O₂)₃. The ion [Ce(C₅H₇O₂)₂]⁺ ultimately breaks down to [CeO]⁺ through the sequential loss of three even-electron neutral species, CH₃COCHCCH₂, H₂O and CH₃COCHCCH₂ without invoking any further reduction of the Ce centre. Mechanisms are given for the loss of the three even-electron species from the bis(acetylacetonato)cerium(III) ion fragment. Theoretical simulations of patterns of the fragment ions [Ce(C₅H₇O₂)₃]^+', [Ce(C₅H₇O₂)₂]^+', and [Ce(C₅H₇O₂)OH]⁺ have been made and compared with those of the corresponding experimental results in support of the suggested mode of fragmentation.