INCLUSION COMPLEXATION OF CYCLOBIS(PARAQUAT-P-PHENYLENE) AND RELATED CYCLOPHANE DERIVATIVES WITH SUBSTITUTED AROMATICS - COOPERATIVE NONCOVALENT CAVITY AND EXTERNAL INTERACTIONS

The cooperative nature of non-covalent interactions which give rise to inclusion complexes involving cyclobis(paraquat-p-phenylene), 1(4+), and related cyclophane derivatives, 2(4+)-(4+), with substituted 1,4-p henyl and 4,4'-biphenyl guests has been studied by spectroscopic techn iques and ab initio and semiempirical molecular orbital methods. Inclu sion complex formation and stability are primarily determined by the c ombination of two main interaction modes involving aromatic stacking o f the guest within the cyclophane cavity and external interactions bet ween guest side arms and the exterior of the cyclophane, A balance bet ween cavity and external forces results in supramolecular association and is shown to change depending upon the functionality and substituti on of the guest, Cavity binding was probed using 1,4-phenyl and 4,4'-b iphenyl 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. In contrast, the cavity binding between substitut ed 4,4'-biphenyl guests and 1(4+) is determined by almost equal contri butions of polarizability and electrostatics. The effect of solvent is shown to have only a small effect on the computed geometry of 1(4+) c omplexes, but its impact upon binding energies is substantial, The fir st solvation shell of the cyclophanes is computationally approximated by 12 acetonitriles and satisfies the requirements of the 16 relativel y acidic protons on the bipyridinium groups, Good correlations between the computed (with solvation) and experimental 1(4+) binding energies are found, The degree of linear correlation improves substantially wh en the comparison between computed and experimentally observed binding energies is restricted to structurally similar (number of aromatic ri ngs, number of substituents and position of substitution) molecular gu ests, Furthermore, computed molecular properties, such as polarizabili ty, maximum hardness, softness and electronegativity of the isolated g uests, correlate well with 1(4+) binding energies based upon the same requirement of guest similarity, The non-covalent forces associated wi th the external cyclophane interactions were studied with guest molecu les built from symmetrical 1,4-extensions of hydroquinone composed of aliphatic or ethyleneoxy side arms, In particular, side arm length and functionality, and the position and type of heteroatoms along the cha in, were systematically varied to define the external interactions bet ween the guest side arms and different host cyclophanes. Specifically, the ethyleneoxy linkages are shown to provide a large chelate and coo perative effect which direct the binding with 1(4+)., In order to prob e further the special geometric and electronic character of 1(4+). We have synthesized and tested a new supramolecular host, 2(4+), similar to 1(4+) but where a pentacycloundecane unit replaces one of the xylyl groups, Both experimental and computed data on the new host emphasize the ideal geometry and electronic nature of the 1(4+) molecular recep tor for aromatic guests, The inclusion complexes discussed in this pap er are important not only because they, or similar entities, are the m ain components of many rotaxanes, catenanes and other switchable molec ules, but because the intermolecular interactions involved, such as el ectrostatics, polarizability and charge-transfer, are ubiquitous in su pramolecular chemistry. The information reported on the specific inter actions Involving the 1(4+)-4(4+) molecular receptors with substituted aromatic guests can also be extended by analogy to many systems of br oad interest. (C) 1997 by John Wiley & Sons, Ltd.