Chelate Cooperativity
and Spacer Length Effects on
the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes
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Abstract
Homo- and heterodivalent crown-ammonium pseudorotaxanes
with different
spacers connecting the two axle ammonium binding sites have been synthesized
and characterized by NMR spectroscopy and ESI mass spectrometry. The
homodivalent pseudorotaxanes are investigated with respect to the
thermodynamics of divalent binding and to chelate cooperativity. The
shortest spacer exhibits a chelate cooperativity much stronger than
that of the longer spacers.
On the basis of crystal structure, this can be explained by a noninnocent
spacer, which contributes to the
binding strength in addition to the two binding sites. Already very
subtle changes in the spacer length, i.e., the introduction of an
additional methylene group, cause substantial changes in the magnitude
of cooperative binding as expressed in the large differences in effective
molarity. With a similar series of heterodivalent pseudorotaxanes,
the spacer effects on the barrier for the intramolecular threading
step has been examined with the result that the shortest spacer causes
a strained transition structure and thus the second binding event
occurs slower than that of the longer spacers. The activation enthalpies
and entropies show clear trends. While the longer spacers reduce the
enthalpic strain that is present in the transition state for the shortest
member of the series, the longer spacers become entropically slightly
more unfavorable because of conformational fixation of the spacer
chain during the second binding event. These results clearly show
the noninnocent spacers to complicate the analysis of multivalent
binding. An approximate description which considers the binding sites
to be connected just by a flexible chain turns out to be more a rough
approximation than a good model. The second conclusion from the results
presented here is that multivalency is expressed in both the thermodynamics
and the kinetics in different ways. A spacer optimized for strong
binding is suboptimal for fast pseudorotaxane formation