Studies in negative ion mass spectrometry: XIVPart XIII, Org. Mass Spectrom. 14, 676 (1979). - Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

0030-493X

Chemistry ; Analytical Chemistry and Spectroscopy

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Electron attachment reactions of a series of (η6-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η6-arene ligand and its substituents. Whereas (η6-benzene)- and (η6-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M—CO]-· ions of highest abundance in their negative ion mass spectra, [η6-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]-· in this case. (η6-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η6-(C6H5·13CO2Me)]Cr(CO)3. The first CO loss in the principal fragmentation pathway occurs solely from the -Cr(CO)3 group of [M]-·. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η6-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH2, OH, OCH3, CL and COOMe. The stabilities of the [M]-· ions have been rationalized in terms of the Hammett and Taft parameters σP, σI, σRP, σPO and σRO. The overall electronic substituent effect transmitted to the carbonyl groups of the -Cr(CO)3 unit involves both resonance and inductive components. In this series of compounds the stability of [M]-· decreases as the electron withdrawing capacities of the para substituents increase.

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