Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6‐arenetricarbonylchromium(O) complexes

Abstract

AbstractElectron 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.