Supramolecular Recognition: Protonmotive-Driven Switches or Motors?

• Published in: Chemistry : a European journal 2004-04, Vol.10 (8), p.1944-1955
• Main Authors: Crowley, James D., Goshe, Andrew J., Steele, Ian M., Bosnich, Brice
• Format: Article
• Language: English
• Subjects: host-guest systems; molecular motors; molecular switches; protonmotive displacement; supramolecular chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200305620

A dicationic molecular receptor bearing two cofacially disposed terpyridyl‐Pd‐Cl units forms stable 1:1 host–guest complexes with planar, neutral platinum(II) complexes. When the guest is modified to incorporate a pyridine group, the now basic guest is protonated by trifluoroacetic acid in acetonitrile solutions. The basic yellow guest forms a stable, deep red 1:1 host–guest complex with the yellow palladium receptor. Addition of trifluoroacetic acid to this host–guest complex leads to the displacement of the guest from the receptor. It is proposed that the dissociation of the guest is caused by electrostatic repulsion between the dicationic receptor and the positively charged protonated guest. Addition of base restores the host–guest complex. This protonmotive translocation of the guest from the host to the solution is discussed in terms of the mechanisms that drive molecular motors, the power stroke and the Brownian ratchet. It is concluded that the system is best described as a molecular switch that operates by the same mechanism as one stroke of a molecular motor A proton‐driven release of a platinum(II)‐based guest from a dicationic palladium(II)‐based molecular receptor (see scheme) is discussed in terms of the mechanisms that drive molecular motors and switches, namely, the power stroke and the Brownian ratchet. Since the described process is not cyclic, it is concluded that the system is better described as a molecular switch rather than a motor.