A Catalytic Palladium Active-Metal Template Pathway to [2]Rotaxanes

[2]Catenanes built around octahedral transition-metal complexes that contain two intertwined endocyclic but non-sterically hindering tridentate ligands

Sterically hindering bidentate chelates, such as 2,9-diphenyl-1,10-phenanthroline, form entwined complexes with copper(I) and other tetrahedrally coordinated transition-metal centres. To prepare octahedral complexes containing two entwined tridentate ligands and thus apply a strategy similar to that used for making catenanes with tetrahedral metal centres, the use of the classical terpy ligand (terpy=2,2':6',2''-terpyridine) appears to be attractive. In fact, 6,6''-diphenyl-2,2':6',2''-terpyridine (dp-terpy) is not appropriate due to strong "pinching" of the organic backbone by coordination to the metal and thus stable entwined complexes with this ligand cannot be obtained. Herein, we report the synthesis and coordination properties of a new family of tridentate ligands, the main features of which are their endocyclic nature and non-sterically hindering character. The coordinating fragment consists of two 8'-phenylisoquinolin-3'-yl groups attached at the 2 and 6 positions of a pyridine nucleus. Octahedral complexes containing two such entangled ligands around an octahedral metal centre, such as Fe(II) , Ru(II) or Co(III) , are highly stable, with no steric congestion around the metal. By using functionalised ligands bearing terminal olefins, double ring-closing metathesis leads to [2]catenanes in good yield with Fe(II) or Co(III) as the templating metal centre. The X-ray crystallography structures of the Fe(II) precursor and the Fe(II) catenane are also reported. These show that although significant pinching of the ligand is observed in both Fe(II) complexes, the system is very open and no steric constraints can be detected.

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.

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.

Diels-Alder active-template synthesis of rotaxanes and metal-ion-switchable molecular shuttles

A synthesis of [2]rotaxanes in which Zn(II) or Cu(II) Lewis acids catalyze a Diels-Alder cycloaddition to form the axle while simultaneously acting as the template for the assembly of the interlocked molecules is described. Coordination of the Lewis acid to a multidentate endotopic 2,6-di(methyleneoxymethyl)pyridyl- or bipyridine-containing macrocycle orients a chelated dienophile through the macrocycle cavity. Lewis acid activation of the double bond causes it to react with an incoming "stoppered" diene, affording the [2]rotaxane in up to 91% yield. Unusually for an active-template synthesis, the metal binding site "lives on" in these rotaxanes. This was exploited in the synthesis of a molecular shuttle containing two different ligating sites in which the position of the macrocycle could be switched by complexation with metal ions [Zn(II) and Pd(II)] with different preferred coordination geometries.

A multicomponent CuAAC "click" approach to a library of hybrid polydentate 2-pyridyl-1,2,3-triazole ligands: new building blocks for the generation of metallosupramolecular architectures

A one pot, multicomponent CuAAC reaction has been exploited for the safe generation of alkyl, benzyl or aryl linked polydentate pyridyl-1,2,3-triazole ligands from their corresponding halides, sodium azide and alkynes in excellent yields. The ligands have been fully characterised by elemental analysis, HR-ESMS, IR, (1)H and (13)C NMR and in two cases the structures were confirmed by X-ray crystallography. Additionally, we have examined the Ag(I) coordination chemistry of these ligands and found, using HR-ESMS, (1)H NMR, and X-ray crystallography, that both discrete and polymeric metallosupramolecular architectures can be formed.

Getting harder: cobalt(III)-template synthesis of catenanes and rotaxanes

The synthesis of catenanes and rotaxanes using the hard trivalent transition metal ion cobalt(III) as a template is reported. Tridentate dianionic pyridine-2,6-dicarboxamido ligands, each with two terminal alkene groups, coordinate Co(III) in a mutually orthogonal arrangement such that entwined or interlocked molecular architectures are produced by ring-closing olefin metathesis. Double macrocyclization of two such ligands bound to Co(III) afford a non-interlocked "figure-of-eight" complex in 42% yield, the structure determined by X-ray crystallography. Preforming one macrocycle and carrying out a single macrocyclization of the second bis-olefin with both ligands attached to the Co(III) template led to the isomeric [2]catenate in 69% yield. The mechanically interlocked structure was confirmed by X-ray crystallography of both the Co(III) catenate and the metal-free catenand. A Co(III)-template [2]rotaxane was assembled in 61% yield by macrocyclization of the bis-olefin ligand about an appropriate dianionic thread. For both catenanes and rotaxanes, removal of the metal ion via reduction under acidic conditions to the more labile Co(II) gave neutral interlocked molecules with well-defined co-conformations stabilized by intercomponent hydrogen bonding.

Active metal template synthesis of rotaxanes, catenanes and molecular shuttles

Active metal template synthesis is a powerful new strategy for the construction of rotaxanes, catenanes and other mechanically interlocked molecular structures. The key feature is that the metal plays a dual role during the assembly of the interlocked architecture, acting as both a template for entwining or threading the components and as a catalyst for capturing the interlocked final product by covalent bond formation. Unlike traditional "passive" metal template methods to rotaxanes and catenanes, permanent recognition motifs are not required on each of the components to be interlocked (i.e., the assembly can be traceless) and the template can often be used in sub-stoichiometric quantities. Since its inception in 2006, a rapidly growing number of different metal-catalysed reactions have proven suitable for the active metal template synthesis of both rotaxanes and catenanes, including the copper(i)-catalysed terminal alkyne-azide cycloaddition (the CuAAC "click" reaction), palladium- and copper-catalysed alkyne homocouplings and heterocouplings, and palladium-catalysed oxidative Heck couplings and Michael additions. In addition to simple rotaxanes and catenanes, the synthetic strategy has been used to construct switchable molecular shuttles with weak intercomponent interactions (a requirement for fast shuttling) and to provide insight into the mechanisms of transition metal-catalysed reactions. In this tutorial review we highlight the utility and potential of the early examples of the active metal template strategy in mechanically interlocked molecule synthesis.