An Allosterically Regulated Molecular Shuttle

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.

Fullerenes: multitask components in molecular machinery

Molecular machines are molecular-scale devices that carry out predetermined tasks derived from molecular motion. This Minireview illustrates how fullerenes can be used as multitask building blocks in molecular machinery, providing new perspectives for fullerenes. Indeed, C(60) can be applied as a photo- and electroactive stopper owing to its size, as a probe for molecular motion as a result of its well-defined physicochemical properties, and to induce motion through pi-pi interactions. Such molecular motion can be employed to modulate light-driven electron-transfer events, extending the potential applications of molecular machines to the typical fields of application of fullerenes.

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.

Toward Electrochemically Controllable Tristable Three-Station [2]Catenanes

Encouraged by the prospect of producing an electrochemical, color-switchable red-green-blue (RGB) dye compound, we have designed, synthesized, and characterized two three-station [2]catenanes. Both are composed of macrocyclic polyethers containing three pi-electron-rich stations, which act as recognition sites for a pi-electron-deficient tetracationic cyclophane. The molecular structures of the two three-station [2]catenanes were characterized fully by mass spectrometry and 1H NMR spectroscopy. To anticipate the relative occupancies of the three stations in each [2]catenane by the cyclophane, model compounds with the same constitutions in the vicinity of the stations were synthesized. The relative ground-state populations of the three stations occupied in both [2]catenanes were estimated from the thermodynamic parameters for 1:1 complexes between all these model compounds and the cyclophane, obtained from isothermal titration calorimetry (ITC). The electrochemical and electromechanical properties of the three-station [2]catenanes were analyzed by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and spectroelectrochemistry (SEC). The first three-station [2]catenane was found to behave like a bistable system, whereas the second can be described as a quasi-tristable system.

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)].