Toward Metal Complexes That Can Directionally Walk Along Tracks: Controlled Stepping of a Molecular Biped with a Palladium(II) Foot

We report on the design, synthesis, and operation of a bimetallic molecular biped on a three-foothold track. The “walker” features a palladium­(II) complex “foot” that can be selectively stepped between 4-dimethylaminopyridine and pyridine ligand sites on the track via reversible protonation while the walker remains attached to the track throughout by means of a kinetically inert platinum­(II) complex foot. The substitution pattern of the three ligand binding sites, together with the kinetic stability of the metal–ligand coordination bonds, affords the two positional isomers a high degree of metastability, meaning that altering the chemical state of the track does not automatically instigate stepping in the absence of an additional stimulus (heat in the presence of a coordinating solvent). The use of metastable metal complexes for foot–track interactions offers a promising alternative to dynamic covalent chemistry for the design of small-molecule synthetic molecular walkers.