ja4026006_si_002.cif (18.95 kB)
The Role of Arene–Arene Interactions in the Folding of ortho-Phenylenes
dataset
posted on 2016-02-19, 10:35 authored by Sanyo M. Mathew, James T. Engle, Christopher J. Ziegler, C. Scott HartleyThe ortho-phenylenes are a simple class of helical oligomers
and representative of the broader class of sterically congested polyphenylenes.
Recent work has shown that o-phenylenes fold into
well-defined helical conformations (in solution and, typically, in
the solid state); however, the specific causes of this folding behavior
have not been determined. Here, we report the effect of substituents
on the conformational distributions of a series of o-phenylene hexamers. These experiments are complemented by dispersion-corrected
DFT calculations on model oligomers (B97-D/TZV(2d,2p)). The results
are consistent with a deterministic role for offset arene–arene
stacking interactions on the folding behavior. On the basis of the
experimental and computational results, we propose a model for o-phenylene folding with two simple rules. (1) Conformers
are forbidden if they include a particular sequence of biaryl torsional
states that causes excessive steric strain. These “ABA”
states correspond to consecutive dihedral angles of −55°/+130°/–55°
(or +55°/–130°/+55). (2) The stability of the remaining
conformers is determined by offset arene–arene stacking interactions
that are easily estimated as an additive function of the number of
well-folded torsional states (±55°) along the backbone.
For the parent, unsubstituted poly(o-phenylene),
each interaction contributes roughly 0.5 kcal/mol to the helix stability
(in chloroform), although their strength is sensitive to substituent
effects. The behavior of the o-phenylenes as a class
is discussed in the context of this model. They are analogous to α-helices,
with axial aromatic stacking interactions in place of hydrogen bonding.
The model predicts that the overall folding propensity should be quite
sensitive to relatively small changes in the strength of the arene–arene
stacking. In a broader sense, these results demonstrate that polyphenylenes
may exhibit folding behavior that is amenable to simple models, and
validate the use of diffusion-corrected DFT methods in predicting
their three-dimensional structures.