INCLUSION COMPLEXATION OF CYCLOBIS(PARAQUAT-p-PHENYLENE) AND RELATED CYCLOPHANE DERIVATIVES WITH SUBSTITUTED AROMATICS: COOPERATIVE NON-COVALENT CAVITY AND EXTERNAL INTERACTIONS

Abstract

The cooperative nature of non-covalent interactions which give rise to inclusion complexes involving cyclobis(paraquatp-phenylene), 1 4+ , and related cyclophane derivatives, 2 4+ -4 4+ , with substituted 1,4-phenyl and 4,4Ј-biphenyl guests has been studied by spectroscopic techniques and ab initio and semiempirical molecular orbital methods. Inclusion complex formation and stability are primarily determined by the combination of two main interaction modes involving aromatic stacking of the guest within the cyclophane cavity and external interactions between guest side arms and the exterior of the cyclophane. A balance between cavity and external forces results in supramolecular association and is shown to change depending upon the functionality and substitution of the guest. Cavity binding was probed using 1,4-phenyl and 4,4Ј-biphenyl guests, where for the 1,4-phenyl guests the primary basis for energy stabilization with 1 4+ is found to be short-range stabilizing electrostatic forces complemented by small amounts of polarizability and charge-transfer. JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 369-382 (1997) © 1997 by John Wiley & Sons, Ltd.CCC 0894-3230/97/050369-14 $17.50 many rotaxanes, catenanes and other switchable molecules, but because the intermolecular interactions involved, such as electrostatics, polarizability and charge-transfer, are ubiquitous in supramolecular chemistry. The information reported on the specific interactions involving the 1 4+ -4 4+ molecular receptors with substituted aromatic guests can also be extended by analogy to many systems of broad interest.

Computational Studies of Inclusion Phenomena and Synthesis of a Novel and Selective Molecular Receptor for 1,4-Disubstituted Benzenes and 4,4‘-Disubstituted Biphenyls

“…The computed gas-phase structure of 2 4+ has been determined previously at several levels of theory. In particular, the gas-phase PM3 structure has been shown to reproduce available ab initio structures of 2 4+ , in agreement with the reported X-ray crystal structure. , The computed structures of each host are shown in Figure . The greatest variation between 2 4+ and the crystal structure occurs in the dihedral between the bipyridinium units and in the cavity size.…”

“…The first major trend in the binding, as revealed by the computed complexes, is that 1 4+ forms stronger complexes with nitrogen-containing guests than with oxygen-containing guests. This trend was also observed with host 2 4+ in computational and UV−vis experimental studies, , but was further enhanced with the 1 4+ host. The nitrogen substituents are stronger electron donors to the π system than the corresponding oxygen substituents, which enhances the π-electron-rich nature of the guests.…”

Computational Studies on Pseudorotaxanes by Molecular Dynamics and Free Energy Perturbation Simulations

“…The geometrical center of the guest is only 0.7 Å from it. These results oppose the X-ray geometry of 2 21b. The chloride anions escape from the paraquat moieties and prefer to be nearer to the benzidine rings in 2 (Figure ), while two pairs of anions are located near each of the hydroxyl hydrogens in 3 (average distance = 5.3 Å) and at both sides of the paraquat units (Figure ; average distance 6.8 Å), respectively.…”

“…Only the torsion angles corresponding to the twisting of the two bipiridyl units (parameter d) changed from 1−2° to the value of 10° (closer to the experimental estimate of 19°). This geometry parameter is the most sensitive one with respect to accounting for polarization and the presence or absence of counterions (see Table 5 of ref b). The geometrical data for both guests were also well reproduced.…”

“…However, the final structure for these pseudorotaxanes is far from the proposed geometry derived from NOE experiments. In fact, it adopts a geometry that allows a 2:1 inclusion complex (like in the X-ray structure shown in ref b). The importance of counterions is also stressed in that article.…”