Control of Relative Direction and Amplitude in Extension/Contraction Motions of Molecular Strands Induced by Ion Binding

The shape of ligand strands composed of six-membered aza-heterocycles (het) connected at the alpha and alpha' positions by hydrazone (hyz) units is determined in a predictable fashion by the nature of the heterocyclic groups (pyridine, pyrimidine, pyrazine etc.), and covers the range from extended linear to compact helical structures. The binding of metal ions to the coordination subunits, defined by the het-hyz sequences, leads to marked shape changes by inter-converting bent and linear conformations of the subunits, thus inducing relative motions of strand domains either in the same (con-sense, twirling) or in opposite (dis-sense, flapping) directions. The amplitude of the motion induced by metal-ion binding and release and the relative directions of the formal motions can be controlled by the nature of the heterocyclic units. Thus, motions around a, central 4,6-disubstituted pyrimidine are dis-sense motions, whereas there are con-sense motions around a central 2,5-disubstituted pyrazine unit, as illustrated by model ligands 1 and 2. respectively. The more extended helical 3 and undulating (zigzag shape) 4 ligands undergo larger-amplitude motions combining the relative displacements displayed by 1 and 2. Ligands 3 and 4 form linear tetranuclear Pb-II and Zn-II complexes, thus producing an extension motion. The same holds for [Ru(4)(terpy)(4)](PF6)(8) (terpy = terpyridine). Reversible acid-base-triggered molecular motions have been generated with [Zn-4(4)(OTf)(8)] (TfOH=triflic acid).