ACCUMULATION OF HYDROGEN-BONDING AND ELECTROSTATIC BINDING-SITES - STABILIZATION OF SALTS IN HYDROXYLIC MEDIA VIA INTRAMOLECULAR HYDROGEN-BONDING

Pyridyl-bisresorcinol derivative 1a and dodecyl phosphate (5) form a pyridinium-phosphate salt which is stabilized via hydrogen-bonding interaction between the bisresorcinol moiety and bound phosphate anion. The salt-formation constants (K) are relatively insensitive to solvent polarities; K1a(5) = 1.2 x 10(3) (water-methanol (2:1)), 1.2 x 10(3) (methanol), 1.1 x 10(3) (ethanol), and 6.9 x 10(2) M-1 (2-propanol). On the other hand, the salt formation with 2-picoline (3) as a less crowded reference host takes place with much difficulty and is highly solvent dependent; K3(5) = 2.1 x 10(2) (water-methanol (2:1)), 3.2 x 10 (methanol), 1.6 x 10 (ethanol), and 9.2 (2-propanol). The selectivities K1a(5)/K3(5) thus increase with respect to change in solvents in the order, water-methanol (2:1) (6) < methanol (37) < ethanol (72) < 2-propanol (75). The role of a pair of hydroxyl groups in the bisresorcinol moiety is discussed in terms of intramolecular microsolvation.