Hydrogen bonded supramolecular polymers in moderately polar solvents

摘要

Hydrogen bonded assemblies are significantly weakened in polar solvents.1,3 This has been shown to be due mainly to the competition between solute-solute and solute-solvent hydrogen bonds.4'5 For instance, the association constant between perfluoro- tert-butyl alcohol and tributylphosphine oxide has been shown to decrease from >105 M-1 in cyclohexane to 240 M-1 in tetrahydrofuran.4 Although this effect is unquestionably real, it has been widely over-interpreted as meaning that solute-solute hydrogen bonds are too weak in polar solvents to maintain stable assemblies. As a consequence, purely hydrogen bonded systems are rarely envisaged for the design of supra-molecular systems in polar solvents. In water or very polar solvents like DMSO, hydrogen bonds have been used in association with solvophobic interactions,6,7 π-stacking8-11 or ionic interactions.12-15 In less polar solvents like acetone, hydrogen bonds have been used in association with dipolar interactions.16'17 Supramolecular assemblies in polar solvents maintained exclusively by hydrogen bonding are rare indeed,18-21 but these few examples remind us that this is nevertheless a possibility.