Article pubs.acs.org/JPCA
Designing a “Flatter” ExBox4+ Analogue Steven M. Bachrach* and Zachary O. M. Nickle Department of Chemistry, Trinity University, 1 Trinity Place, San Antonio, Texas 78212 United States S Supporting Information *
ABSTRACT: Analogues of ExBox4+ 1 are proposed that possess triaryl fragments that are nearly flat. These two new hosts are predicted by density functional theory (ωB97X-D/6311G(d,p)) to bind five small linear acenes more tightly than does 1. The “flatter” triaryl fragments provide a less congested interior along with improved π−π-stacking between these hosts and guests.
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which is best when the stacked rings are parallel.8 Inspection of the X-ray structures1 of 1 and the complex of 1 with anthracene (see Figure 1) reveals that the rings of the triaryl top and bottom of the host are not coplanar. In fact, the central phenyl ring extends into the interior of the macrocycle; the dihedral angles between the aryl rings of the triaryl fragment of 1 are −26.6 and 31.3°, and these reduce to −24.6 and 27.4° in the anthracene complex. This geometry results from minimizing the o-o′ interactions between the neighboring aryl rings. To “flatten” the triaryl components of ExBox, the o−o′ interactions have to be eliminated. In our study of cycloparaphenylene nanohoops9 we faced the same challenge to create a “flatter” ribbon. Our solution was to replace one of the C−H bonds in the ortho position with a nitrogen. The lone pair on nitrogen not only removes the offending o-C-H···o′-CH interactions but also adds a potential stabilizing electrostatic interaction between the nitrogen lone pair and the hydrogen in the ortho position. The aim of this computational study is to create the appropriately nitrogen-substituted analogues of 1 that may afford a more hospitable cavity for a flat PAH guest. In particular, we propose two new ExBox analogues 4 and 5 and examine their abilities to bind a series of small linear acenes.
INTRODUCTION The recently reported ExBox4+ host 1, synthesized by the Stoddart group, binds polycyclic aromatic hydrocarbons (PAHs).1 The linear anthracene molecule is entirely contained with the host 1, while the longer tetracene molecule binds at an angle to 1, with the ends of the molecule extending outside the host. The next larger analogue Ex2Box4+ 2 is big enough to bind tetracene wholly within its interior.2 The Stoddart group has improved on the synthesis of these types of hosts using a templating process.3 In addition, they have also designed two cage-like analogues.4,5
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COMPUTATIONAL METHODS We have previously demonstrated10 that the ωB97X-D/6311G(d,p)11 method provides geometries of 1 and its complexes with small linear acenes in very good agreement
Our previous DFT computations on the uncharged, carbon analogue 3 showed that the binding energy of a guest PAH molecule within 1 is dominated by dispersion.6 This dispersion component of the interaction of a planar PAH with 1 is likely to be dominated by π−π stacking, as suggested by Das and Chattaraj7 in their computational study of ExBox4+ binding, © XXXX American Chemical Society
Received: August 27, 2015 Revised: October 7, 2015
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DOI: 10.1021/acs.jpca.5b08356 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 1. X-ray crystal structures of 1 and the complex of 1 with anthracene.1
namely, 2,5-di(pyrimidin-4-yl)pyrazine 12 and 1,4-bis(1,3,5triazin-2-yl)benzene 13. Optimization of 12 and 13 revealed them to both be planar, with C2h and D2h symmetry, respectively, making them both suitable for our purposes. Replacing the triaryl component of 1 with 12 creates the ExBox4+ analogue 4, and replacement with 13 creates the ExBox4+ analogue 5. It is worth taking a moment to consider the possible synthesis of hosts 4 and 5. If these are to be made in the way ExBox4+ itself is prepared, then one is looking to react 12 or 13 as a nucleophile toward 1,4-bis(bromomethyl)benzene (Scheme 2). The reaction needs to take place at the terminal
with experiments. Additionally, the relative binding energy trends are in good qualitative agreement with experiments. We have therefore employed this same method for the study of the binding of the hosts 4 and 5 with the linear acenes benzene 6, naphthalene 7, anthracene 8, tetracene 9, and pentacene 10. The geometries of the two hosts, the acenes and the complexes, were fully optimized and confirmed to be local energy minima using analytical frequency analysis. All structures were completely reoptimized including the effects of solvent (acetonitrile) employing the conductor polarized continuum method (C-PCM).12 The unscaled zero-point vibrational frequencies were utilized in computing enthalpy and free energies, incorporating the quasiharmonic approximation of Truhlar and Cramer,13 whereby low-frequency modes (
