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.