Cation-Controlled Formation and Interconversion of the fac/fac and mer/mer Stereoisomers of a Triple-Stranded Helicate

Two biscatecholester ligands with oligoether spacers were used to prepare dinuclear titanium(IV) triscatecholate based helicates. In the case of Li₄[(1/2)₃Ti₂], “classical” helicates with three internally bound Li⁺ ions and syn-oriented ligands in the complex units (fac/fac isomer) were obtained. In the case of the sodium salt Na₄[(2)₃Ti₂], a different homochiral dinuclear triple-stranded helicate with two internally bound Na⁺ ions was formed. The complex units are anti-configured, and two of the ligand spacers are connecting internal with external positions of the helicate (mer/mer isomer). Removal of the sodium ions and addition of lithium ions leads to the switching from one topology to the other with an expanded helicate [(2)₃Ti₂]⁴⁻ as an intermediate. Switching back to the “non-classical” helicate cannot be observed because severe structural rearrangements would be required.