Abstract
Catalyst decomposition has a negative effect on catalytic activity, and knowledge
of decomposition pathways can assist with catalyst development. Ferrocenium cations
have been employed as catalysts in a number of organic transformations, and we investigated
the stability of a number of ferrocenium salts in solution. The observed rate decomposition
constants for [Fc]Cl, [Fc]PF6, [Fc]BF4, [Fc]CSA [Fc = ferrocenium, CSA = camphor-10-sulfonate (β)], [AcFc]SbF6, (AcFc = acetylated ferrocene), and [FcB(OH)2]SbF6 [FcB(OH)2 = ferrocenylboronic acid] were determined in CH2Cl2 solution by time-resolved UV-vis spectroscopy. The rate decomposition constants depended
on the nature of the counterion, with [Fc]Cl being the most stable complex in solution.
The decomposition rate constants dropped by roughly an order of magnitude in most
cases when the experiments were performed in nitrogenated solvent, demonstrating that
the decomposition is mainly an oxidative process. The cosolvent HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol)
slowed the decomposition of the ferrocenium cations as well. Many catalytic or stoichiometric
reactions of ferrocenium cations are performed with alcohols; we determined that hexan-1-ol
is decomposed over the course of 16 hours, but not oxidized in the presence of a ferrocenium
cation. Finally, the different ferrocenium cations were employed in a test reaction
to determine catalytic activity. The nucleophilic substitution of hydroxyl groups
in a tertiary propargylic alcohol by an alcohol is catalyzed by all complexes, and,
again, a counterion dependency of the catalytic activity was observed. Also, HFIP
increases the catalytic activity of the ferrocenium cations. The research has importance
in the development of ferrocenium-based catalyst systems, because changes in the counterion
as well as the architecture of the ferrocenium cation have an influence on stability
and catalytic activity.
Key words
ferrocenium - homogeneous catalysis - transition metal Lewis acids - catalyst decomposition
