Osmium (VI)-bridged macrocycles 1a-c, 2, and 3 assemble spontaneously when osmium tetraoxide, olefins, and L-shaped bispyridyl ligands are mixed in CHCl3. The macrocycles possess well-defined square or rectangular cavities enclosed by aryl walls and act as host molecules. Hydrogen-bond donors on the inner surface of the hosts offer binding sites to acceptors of guests with complementary dimensions. The host 1a binds adipamide G2 (K(a) = 3.6 x 104 M-1) and terephthalamide G6 (K(a) = 2.0 x 104 M-1), while it binds negligibly (K(a) < 10 M-1) benzamide G5, isophthalamide G9, or 1,4- naphthalenedicarboxamide G10. The larger hosts 2 and 3 bind the longer guests biphenyldicarboxamide G12 and terphenyldicarboxamide G17, respectively, but shorter guests such as adipamide G2 and terephthalamide G6 are not well-bound (K(a) < 10 M-1). Hosts 1a-c with different remote substituents (H, OMe, NO2) but identical cavity size were prepared and their binding affinities were measured. The relative binding affinities of the hosts 1a-c to the keto amide G19, ester amide G20, and diester G21 are in the order of 1c (NO2) >> 1a (H) > 1b (OCH3). The substituent effects on the binding strength are interpreted in terms of the electron density at the pyridine nitrogen of the hosts and its effect on bifurcated hydrogen bonding.
All Science Journal Classification (ASJC) codes
- Organic Chemistry