Reorganization of highly preorganized hosts upon cation complexation: Ab initio study of fluorospherands

Abstract

Fluorospherands (F-spherands) are highly preorganized hosts composed of fluorobenzene or 4-methylfluorobenzene units attached to one another at their 2,6-positions. To understand the intrinsic factors affecting cation complexation, we investigated the complexation behavior between F-spherands and cations using density functional theory (DFT) at the level of B3LYP/6-31G**. The F6-spherand (C6H3F)(6), (1) has a highly preorganized spherical cavity, which can encapsulate Li+ and Na+. Its cavity is not big enough for K+ and NH4+, which prefer external binding. Plausible conformations were studied for F8-spherand (C6H3F)(8). Conformer of D-2d symmetry (2b) is more stable than that of D-4d (2a), in agreement with NMR experiments. The cavity size of F8-spherand is big enough to encapsulate all cations studied. However, the cavity size of 2b is smaller than that of 2a, which resulted in the guest selectivity. Upon complexation, 2b conformation is more stable for Li+ and Na+, while 2a conformation is preferred for larger cations such as K+ and NH4+. Thus, the ab initio calculations over 4 these highly preorganized fluorospherands give important insights into their host-guest chemistry. (c) 2006 Wiley Periodicals, Inc.