[2]Pseudorotaxanes through Second-Sphere Coordination

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

A modular "toolbox" approach to flexible branched multimacrocyclic hosts as precursors for multiply interlocked architectures

Tetralactam macrocycles can be functionalized by a variety of cross-coupling reactions. A modular "toolbox" strategy is presented that allows 1) several tetralactam macrocycles to be covalently connected with each other or with a central spacer, 2) the macrocycles to be substituted with or connected to different chromophores, and 3) metal-coordination sites to be attached to the macrocycles. With this approach a series of different oligo-macrocyclic hosts was obtained with great structural diversity and enormous potential for further functionalization. Rotaxanes made on the basis of these macrocycles have been synthesized to demonstrate their utility in building more complex supramolecular architectures.

Anion-Templated Calix[4]arene-Based Pseudorotaxanes and Catenanes

We present the rational design and anion-binding properties of the first anion-templated pseudorotaxanes and catenanes in which the "wheel" component is provided by a calix[4]arene macrobicyclic unit. The designs and syntheses of two new calix[4]arene macrobicycles, 2 and 3, are presented, and the abilities of these new species both to bind anions and to undergo anion-dependent pseudorotaxane formation are demonstrated. Furthermore, it is shown that performing ring-closing metathesis reactions on some of these pseudorotaxane assemblies gives novel catenane species 14 and 15, in which the yield of interlocked molecule obtained is critically dependent on the presence of a suitable anion template, namely, chloride. Exchange of the chloride anion in catenane 14 a for hexafluorophosphate gives catenane 14 d, which contains a unique anion-binding domain defined by the permanently interlocked hydrogen-bond-donating calix[4]arene macrobicycle and pyridinium macrocycle fragments. The anion-binding properties of this domain are presented, and shown to differ from non-interlocked components.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Formation, Dynamic Behavior, and Chemical Transformation of Pt Complexes with a Rotaxane-like Structure

The reaction of [Pt(CH2COMe)(Ph)(cod)] (cod = 1,5-cyclooctadiene) with (ArCH2NH2CH2-C6H4COOH)+ (PF6)- (Ar = 4-tBuC6H4 or 9-anthryl) in the presence of cyclic oligoethers such as dibenzo[24]crown-8 (DB24C8) and dicyclohexano[24]crown-8 (DC24C8) produces {(ce)[ArCH2NH2CH2C6H4COOPt(Ph)(cod)]}+ (PF6)- (ce = DB24C8 or DC24C8, Ar = 4-tBuC6H4 or 9-anthryl) with interlocked structures. FABMS and NMR spectra of a solution of these compounds indicate that the Pt complexes with a secondary ammonium group and DB24C8 (or DC24C8) make up the axis and cyclic components, respectively. Temperature-dependent 1H NMR spectra of a solution of {(DB24C8)[4-tBuC6H4CH2NH2CH2-C6H4COOPt(Ph)(cod)]}+ (PF6)- ({(DB24C8)[4-H]}+ (PF6)-) show equilibration with free DB24C8 and the axis component. The addition of DB24C8 to a solution of {(DC24C8)[4-H]}+ (PF6)- causes partial exchange of the macrocyclic component of the interlocked molecules, giving a mixture of {(DC24C8)[4-H]}+ (PF6)-, {(DB24C8)[4-H]}+ (PF6)-, and free macrocyclic compounds. The reaction of 3,5-Me2C6H3COCl with {(DB24C8)[4-H]}+ (PF6)- affords the organic rotaxane {(DB24C8)(4-tBuC6H4CH2NH2CH2-C6H4COOCOC6H3Me2-3,5)}+ (PF6)- through C-O bond formation between the aroyl group and the carboxylate ligand of the axis component. The addition of 2,2'-bipyridine (bpy) to a solution of {(DB24C8)[4-H]}+ (PF6)- induces the degradation of the interlocked structure to form a complex with trigonal bipyramidal coordination, [Pt(Ph)(bpy)(cod)]+ (PF6)-, whereas the reaction of bpy with [Pt(OCOC6H4Me-4)(Ph)(cod)] produces the square-planar complex [Pt(OCOC6H4Me-4)(Ph)(bpy)].

A Macrocycle/Molecular-Clip Complex that Functions as a Quadruply Controllable Molecular Switch

Herein, we report the synthesis of a molecular clip with TTF side-walls and its binding behavior towards electron-deficient guests, namely the formation of macrocycle/molecular-clip supramolecular complexes in solution. Four different sets of external stimuli--the K(+)/[2.2.2]cryptand, NH(4) (+)/Et(3)N and (p-BrPh)(3)NSbCl(6)/Zn pairs, and heating/cooling cycles-control the movement of this molecular switch between its threaded and unthreaded states and provide color changes that are observable by the naked eye. This macrocycle/molecular-clip complex system can be considered not only as a quadruple-use molecular switch, but can also be operated by three of these stimuli as a three-input molecular NOR-functioning logic gate that may be monitored by UV-visible spectroscopy.