Reversible Molecular Capsules Composed of Two Cavitands Linked via an Assortment of Charged-Hydrogen Bonds and Covalent Bonds

Six new cavitand bowls are reported ( 6, 7, 8, 9, 11, and 12 ), as are three bis-bowl compounds ( 4, 13, and 14 ). Eight new complexes ( 4 ·guest, 7C ·guest, 8C ·guest, 9C ·guest, 14 ·guest, 15 ·guest, 15C ·guest, and 16C ·guest) that reversibly encapsulate small molecules are reported. Two or more charged-hydrogen bonds (CHBs) or covalent linkages between the bowls are required to form stable complexes. Guest exchange rates vary from milliseconds to days. Thermodynamically, these complexes display an enthalpy−entropy compensation. The relative stability of complex 3 ·pyrazine versus complex 3 ·chloroform is 170 000 in nitrobenzene- d 5 at 298 K. Apparent stability constants in nitrobenzene- d 5 at 298 K yield 1.1 × 10 9 M - 1 for complex 3 ·pyrazine. The absolute stability constant for complex 4 ·pyrazine in nitrobenzene- d 5 at 333 K is 3.5 × 10 6 M - 1 .