Semithiobambusurils, which represent a new family of macrocyclic host molecules, have been prepared by a convenient, scalable synthesis. These new cavitands are double functional: they strongly bind a broad variety of anions in their interiors and metal ions at their sulfur-edged portals. The solid-state structure of semithiobambus[4]uril with HgCl2 demonstrates the ability of these compounds to form linear chains of coordination polymers with thiophillic metal ions. The crystal structure of semithiobambus[6]uril with tetraphenylphosphonium bromide exhibits the unique anion-binding properties of the host cavity and the characteristics of the binding site.
In homogeneous catalysis,
the turnover frequency (TOF) and turnover
number (TON) are the most commonly used quantities that experimentally
describe catalytic activity. Computational studies, on the other hand,
generally yield the ubiquitous free energy profile, which only provides
the relative heights of different intermediates and transition states
for a given reaction mechanism. This information, however, can be
converted into a theoretical TOF through use of the energy span model.
Clearly, directly computing turnover frequencies not only allows easy
comparison of the activity of different catalysts but also provides
a means of directly comparing theory and experiment. Nonetheless,
obtaining detailed free energy profiles for many catalysts is computationally
costly. To overcome this and accelerate the rate at which prospective
catalysts can be screened, here we use linear scaling relationships
in tandem with the energy span model to create volcano plots that
relate an easily and quickly computed energetic descriptor variable
with a catalyst’s turnover frequency. As a demonstration of
their ability, we use these “TOF volcanoes” to rapidly
screen prospective transition metal/pincer-ligand catalysts based
on activity in facilitating the hydrogenation of CO2 to
formate.
Is there any place in the extremely well-established field of catalytic kinetics for new interpretations or novel models that can change the basic doctrines and viewpoints of catalytic cycles?
Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza-bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio-bambusurils in a one-pot reaction, which converts a single anion receptor into a potential anion channel. Solid-state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Å is reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition-state energy for double-anion movement through the channel suggests that although these host-guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.
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