Fig. 2. Highly diastereoselective self-assembly of fac tris-chelates. Pyridine-2-aldehydes and related heterocycles condense with certain α-substituted benzylamines and metals – commonly first row M(II) – to give single isomers – fac-Δ (shown) or fac-Λ depending on stereochemistry of amine – under thermodynamic control (dr > 200 : 1). This results from a combination of inter-ligand π-stacks (dashed lines) and steric effects; the benzylic H (red) is oriented toward the sterically congested region. The –CH₂–X groups can be used to link to an adjacent tris-chelate unit forming metallohelices 9 (Fig. 1) while related strategies are used for 10–15.