Quick-Fix Molecular Machines

Deliberately designed electron donor-acceptor (EDA) interactions can be utilized for the synthesis of discrete molecular machines, such as mechanically interlocked molecules that are otherwise difficult to obtain. High-yield, one-pot synthesis of various interlocked structures has been developed to provide a convenient approach towards the development of sophisticated switchable molecular systems that are responsive towards electrochemical or photochemical stimuli. For example, we developed (Figure 1a) a highly
efficient one-pot [2+3] clipping method to obtain a macrobicycle and a related [2]rotaxane through six-fold imine bond formation. A linear bipyridinium-based π template has been shown for the first time to effectively induce the formation of the cage-like, C3-symmetrical cryptand, despite the symmetry mismatch between the host and the
Cartoon representations of the one-pot assembly approach for the synthesis of (a) [2]rotaxanes and (b) [2]catenanes.
guest. Meanwhile, a modular approach was demonstrated (Figure 1b) in which readily available electron acceptors have effectively templated the formation of [2]catenanes in the presence of Pd(II) and a π-stacking aromatic unit. The current methods not only grant facile access to interlocked structures, but also provide further opportunities for the assembly of incrementally more complex molecular systems.

Recently the dynamic clipping approach has been applied in the preparation of [2]catenanes using a combination of π-templating and reversible imine chemistry, which is further diversified to give the first example of selective formation of desymmetrized [2]catenanes using dynamic covalent chemistry. A tetracationic cyclobisparaquat (CBPQT
4+) cyclophane is shown to effectively induce macrocyclization from two 1,4-diformylbenzene and two 2,2’-(ethylenedioxy)diethylamine components through four-fold imine bond formation (Figure 2). π-π Stacking interactions and [C-HO] interactions are identified in the solid-state structure, which are responsible for the thermodynamic stability of the interlocked product. More interestingly, when two different bisaldehydes are employed in the reaction mixture, a desymmetrized [2]catenane can be obtained in high selectivity. An in-depth structural analysis based on 1H NMR spectroscopy and X-ray single crystal structures reveals that a combined inside and alongside preference for the individual π systems with respect to the CBPQT4+ is the origin for the selectivity. This system coins a
proof of principle in the asymmetrical assembly of interlocked structures using DCC chemistry. The concise approach to desymmetrized [2]catenanes is expected to enable a rapid screening of a library of aromatic bisaldehydes for use in the

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Dual selectivity expressed in the five-component clipping of non-generated [2] catenanes. [Click to enlarge]
clipping reaction, which will expedite information coding and multiplexing in molecular systems.

On the other hand, a covalent version of [2]catenanes has been used as a test bed for probing donor-acceptor interactions and
co-conformational changes. We devised a synthetic protocol to desymmetrized donor-acceptor [2]catenanes where different donor and acceptor units are assembled within a confined catenated geometry (Figure 3). Remarkable translational selectivity is maintained in all cases, including two fully desymmetrized [2]catenanes where both donors and acceptors are different, as revealed by X-ray crystallography in the solid state, and by 1H NMR spectroscopy and electrochemistry in solution. In all desymmetrized [2]catenanes the co-conformation is dominated by the strongest donor and acceptor pairs. Voltammetric and spectroelectrochemical experiments show that the catenanes can be reversibly switched among as many as seven states, characterized by distinct electronic and optical properties, by electrochemical stimulation in a relatively narrow and easily accessible potential window. Moreover, in some of these compounds the oxidation of the electron donor units or the reduction of the electron acceptor ones causes the circumrotation of one molecular ring with respect to the other. These features make the compounds appealing for the development of molecular electronic devices and mechanical machines.
Non-degenerated [2]catenanes showing exclusive translational selectivity, up to seven states and controllable molecular motions.


Koshkakaryan, G.; Cao, D.; Klivansky, L. M.; Teat, S. J.; Tran, J. L.; Liu, Y.* “Dual Selectivity Expressed in [2+2+1] Dynamic Clipping of Unsymmetrical [2]Catenanes”, Org. Lett. 2010, 12, 1528-1531.
Cao, D.; Amelia, M.; Klivansky, L. M.; Koshkakaryan, G.; Khan, S. I.; Semeraro, M.; Silvi, S.; Venturi, M.; Credi, A.;*
Liu, Y.* “Probing Donor-Acceptor Interactions and Co-Conformational Changes in Redox Active Desymmetrized [2]Catenanes”, J. Am. Chem. Soc. 2010, 132, 1110-1122.
Klivansky, L. M.; Koshkakaryan, G.; Cao, D.;
Liu, Y.* “Linear π-Acceptor Templated Dynamic Clipping to Macrobicycles and [2]Rotaxanes”, Angew. Chem. Int. Ed. 2009, 48, 4185-4189.
Koshkakaryan, G.; Parimal, K.; He, J.; Zhang, X.; Abliz, Z.; Flood, A. H.;
Liu, Y.* “π-Stacking Enhanced Dynamic Self-Assembly of Donor-Acceptor Metallo-[2]Catenanes from Diimide Derivatives and Crown Ethers”, Chem. Eur. J. 2008, 14, 10211-10218.
Liu, Y.;* Bruneau, A.; He, J.; Abliz, Z., “Palldaium(II) Directed Self Assembly of Dynamic Donor-Acceptor [2]Catenanes”, Org. Lett. 2008, 10, 765-768.
Liu, Y.;* Klivansky, L. M.; Khan, S. I.; Zhang, X., “Templated Synthesis of Desymmetrized [2]Catenanes with Excellent Translational Selectivity”, Org. Lett. 2007, 9, 2577-2580.
Liu, Y.* “Self-Assembled Ring-in-Ring Complexes from Metal-Ligand Coordination Macrocycles and -Cyclodextrin”, Tetrahedron Lett. 2007, 48, 3871-3874.

Professor Amar Flood, Indiana University
Professor Alberto Credi, University of Bologna