Assembly and binding properties of osmate ester-bridged binuclear macrocycles

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.