title: Sliding-Ring Catenanes.
abstract: Template-directed protocols provide a routine approach to the synthesis of mechanically interlocked molecules (MIMs), in which the mechanical bonds are stabilized by a wide variety of weak interactions. In this Article, we describe a strategy for the preparation of neutral [2]catenanes with sliding interlocked electron-rich rings, starting from two degenerate donor-acceptor [2]catenanes, consisting of a tetracationic cyclobis(paraquat-p-phenylene) cyclophane (CBPQT(4+)) and crown ethers containing either (i) hydroquinone (HQ) or (ii) 1,5-dioxynaphthalene (DNP) recognition units and carrying out four-electron reductions of the cyclophane components to their neutral forms. The donor-acceptor interactions between the CBPQT(4+) ring and both HQ and DNP units present in the crown ethers that stabilize the [2]catenanes are weakened upon reduction of the cyclophane components to their radical cationic states and are all but absent in their fully reduced states. Characterization in solution performed by UV-vis, EPR, and NMR spectroscopic probes reveals that changes in the redox properties of the [2]catenanes result in a substantial decrease of the energy barriers for the circumrotation and pirouetting motions of the interlocked rings, which glide freely through one another in the neutral states. The solid-state structures of the fully reduced catenanes reveal profound changes in the relative dispositions of the interlocked rings, with the glycol chains of the crown ethers residing in the cavities of the neutral CBPQT(0) rings. Quantum mechanical investigations of the energy levels associated with the four different oxidation states of the catenanes support this interpretation. Catenanes and rotaxanes with sliding rings are expected to display unique properties.
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Isurika R
authors.initials: IR
authors.lastname: Fernando
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.|Department of Chemical Sciences, University of Padova , Via Marzolo 1, Padova 35131, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Frasconi
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.|Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Yilei
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: Wei-Guang
authors.initials: WG
authors.lastname: Liu
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.|Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology , Pasadena, California 91125, United States.|NanoCentury KAIST Institute and Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology (KAIST) , 373-1 Guseong Dong, Yuseong Gu, Daejeon 305-701, Republic of Korea.
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart