Self-assembled Fe4L6 cage complexes with variable internal functions can be synthesized from a 2,7-dibromocarbazole ligand scaffold, which orients six functional groups to the cage interior. Both ethylthiomethylether and ethyldimethylamino groups...
A self-assembled
Fe4L6 cage with internally
oriented carboxylic acid functions was shown to catalyze a variety
of dissociative nucleophilic substitution reactions that proceed via
oxocarbenium ion or carbocation intermediates. The catalytic behavior
of the cage was compared to that of other small acid catalysts, which
illustrated large differences in reactivity of the cage-catalyzed
reactions, dependent on the structure of the substrate. For example,
only a 5% cage confers a 1000-fold rate acceleration of the thioetherification
of vinyldiphenylmethanol when compared to the rate with free carboxylic
acid surrogates but only a 52-fold acceleration in the formation of small thioacetals. Multiple factors
control the variable reactivity in the host, including substrate inhibition,
binding affinity, and accessibility of reactive groups once bound.
Simple effective concentration increases or the overall charge of
the cage does not explain the variations in reactivity shown by highly
similar reactants in the host: small differences in structure can
have large effects on reactivity. Reaction of large spherical guests
is highly dependent on substitution, whereas flat guests are almost
unaffected by size and shape differences. The cage is a promiscuous
catalyst but has strong selectivity for particular substrate shapes,
reminiscent of enzymatic activity.
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