Influence of Solvent on Aromatic Interactions in Metal Tris-Bipyridine Complexes

The conformational properties of a series of iron(II) and ruthenium(II) tris-bipyridine complexes have been investigated in a range of solvents. The complexes are equipped with pendant aromatic esters attached by flexible aliphatic linkers, and aromatic interactions between the edge of the bipyridine units and the face of the aromatic esters cause the complexes to fold up in solution. The extent of folding is assessed using 1 H chemical shifts and found to be strongly solvent-dependent. Strong intramolecular edge-to-face aromatic interactions leading to stable folded structures are found in both polar solvents (water and alcohols) and nonpolar solvents (chlorinated hydrocarbons), but solvents of intermediate polarity such as DMSO destabilize the folded conformation. These results indicate that the aromatic interactions are dominated by a substantial electrostatic contribution in organic solvents but are sufficiently nonpolar to take advantage of solvophobic effects in polar solvents. This solvent dependence is likely to be a characteristic feature of any molecular recognition process which involves a mixture of both polar and nonpolar interactions.