Molecular Machines Based on Metal Ion Translocation ² VALERIA AMENDOLA, LUIGI FABBRIZZI,* CARLO MANGANO, AND PIERSANDRO PALLAVICINI Dipartimento di Chimica Generale, Universita` di Pavia, via Taramelli 12, I-27100 Pavia, Italy Received January 22, 2001 ABSTRACT Transition metal ions can be moved reversibly between the two coordinatively unequivalent compartments A and B of a ditopic ligand, using as an input the variation of a bulk solution parameter, either pH or redox potential. In a redox-driven translocation, the metal moves reversibly from A to B on cycling between two consecutive oxidation states (e.g., Cu II /Cu I ; Fe III /Fe II ) by means of auxiliary oxidation and reduction reactions. In a pH-driven process, one compartment displays also acid-base properties (AHn / A n- + nH + ), and the M n+ ion is translocated between B and A n- through consecutive addition of base and acid. Introduction Mechanical work can be produced at the molecular level through the controlled motion of a chosen component occurring within a molecular or a supramolecular system. The envisaged system should contain both a mobile part and a stationary part, and an external operator should be able, by means of a given input, to induce the displace- ment of the movable part with respect to the immovable one. 1 Most of the investigated systems operate in solution, where the so-called mobile and stationary fragments are both in motion; however, one of the fragments (in general the bulkier) is arbitrarily considered motionless, and the other is said to move with respect to it. In solution, the external input promoting the molecular motion can be the variation of a bulk parameter, such as the pH or the redox potential. First examples of controlled molecular motions came from supramolecular chemistry and in- volved systems in which the mobile and the motionless parts were held together by noncovalent interactions. 2,3 Some of the investigated systems are roughly sketched and classified in Figure 1. System 1 is a rotaxane, held together by π-donor- acceptor interactions: in particular, a π-acceptor wheel can be moved between two π-donor segments placed along the axle, by selectively altering their donating properties through an auxiliary reaction of either acid- base or redox nature. 2 System 2 is a 2-catenane containing two asymmetric coordinating wheels, which can be held together by the metal-ligand interactions with a copper center. The half-turn of a wheel with respect to the other follows the Cu II /Cu I redox change, which is being induced by auxiliary oxidation and reduction reactions. 3 Systems 1 and 2 are quite general, and their dynamic behavior is independent of the type of noncovalent interactions: for instance, a two-station rotaxane of type 1, based on metal-ligand interactions and still benefiting from the Cu II /Cu I redox change, has been more recently designed. 4 On the other hand, molecular motions can be induced in catenanes of type 2, held together by π-donor-acceptor interactions, through a redox input. 5,6 Movements in π-systems of both types 1 and 2 are not very complicated from a kinetic point of view and are, in general, fast. On the other hand, systems held together by metal-ligand ² Part of the Special Issue on Molecular Machines. * To whom correspondence should be addressed. E-mail: luigi.fabbrizzi@unipv.it. Valeria Amendola was born in Milan in 1974. In 1997, she graduated in chemistry from the University of Pavia. From 1997 to 2000 she worked as a Ph.D. student in the Dipartimento di Chimica Generale, University of Pavia, in the research group of Professor Luigi Fabbrizzi, where she was involved in the study of chemically and electrochemically controlled molecular motions. Presently, she holds a postdoctoral position at the University of Pavia, and continues her research on transition metal-based molecular machines and devices. Luigi Fabbrizzi was born in Florence in 1946. In 1969, he graduated in chemistry from the University of Florence, where from 1972 to 1980, he was lecturer of inorganic chemistry. Since 1980, he has been Professor of Inorganic Chemistry at the University of Pavia. His main research interests are related to the supramolecular chemistry of transition metals, with a special regard to the electrochemical and photophysical aspects, and to the design of luminescent molecular sensors for anions and biologically relevant analytes. Carlo Mangano was born in Pavia in 1965. He graduated in chemistry from the University of Pavia in 1989. In 1994 he received his Ph.D. degree in chemistry from the University of Pavia, under the supervision of Professor Luigi Fabbrizzi. After that he joined the University of Pavia, where is currently Graduated Technical Assistant. His research area covers the synthesis of multicomponent coordination compounds, polytopic ligands, and luminescent molecular sensors. Piersandro Pallavicini was born in Vigevano in 1962. He graduated in chemistry from the University of Pavia in 1986. In 1990, he received his Ph.D. degree in chemistry from the Scuola Normale Superiore, Pisa, under the supervision of Professor Fausto Calderazzo. He joined the University of Pavia in 1991, where he is presently a Research Associate. His research interests cover multicom- ponent coordination compounds, the study of oriented motions at the molecular level, and the control of self-assembling/disassembling processes of transition metal-containing superstructures. FIGURE 1. Systems containing a mobile part and a stationary part, suitable for carrying out controlled molecular motions: 1, two-station rotaxanes; 2, asymmetric 2-catenanes; 3, rings with an appended movable side chain; 4, systems in which a particle can move from one compartment to the other. Systems 1-4 convert the energy of an auxiliary reaction (redox or acid-base) into mechanical work and can be therefore considered “molecular machines”. Acc. Chem. Res. 2001, 34, 488-493 488 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 6, 2001 10.1021/ar010011c CCC: $20.00  2001 American Chemical Society Published on Web 05/04/2001