An Introduction to Rotaxanes and Catenanes (Page 2/3)

Hydrogen Bonded Catenanes and Rotaxanes

The first interlocked molecule to be synthesised in our group was the homocircuit [2]catenane shown in Figure 5.⁴ Synthesis of this molecule is achieved through the elementary reaction between acid chlorides and amines to form amides. Formation of each macrocycle requires reaction of two molecules of isophthaloyl dichloride and two of para-xylylene diamine; so a total of eight molecules must combine to form the [2]catenane. It is quite remarkable that this molecule is isolated in 20% yield as the only chloroform-soluble product of the reaction. Hydrogen bonds between the amide groups on either ring are shown as dotted lines in the X-ray crystallographic structure (Figure 5b) – it is this network of interactions which is responsible for the formation of the molecule.

Figure 5. a) Chemical structure of and b) X-ray crystal structure of a hydrogen-bonded [2]catenane. Crystal structure atoms: carbon (macrocycle A), light blue; carbon (macrocycle B), yellow; oxygen, red; nitrogen, dark blue; amide hydrogen, white.

Figure 6b shows the X-ray crystal structure of a [2]rotaxane made in our group using hydrogen bonding interactions.⁵ The macrocyclic ring is the same tetra-amide ring as in the catenane above, while the thread also contains two amide groups. Two sets of bifurcated hydrogen bonds between the amide protons on the macrocycle and carbonyl oxygens on the thread are shown as dotted lines in the X-ray crystal structure.

Figure 6. a) Chemical structure of and b) X-ray crystal structure of a hydrogen-bonded fumaramide [2]rotaxane. Crystal structure atoms: carbon (macrocycle), light blue; carbon (thread), yellow; oxygen, red; nitrogen, dark blue; amide and alkenyl hydrogen, white.

This type of rotaxane is formed by a clipping strategy whereby the macrocycle component assembles around a preformed thread molecule.^6-7 When the acid chloride and diamine are combined, amide bonds begin to form between them. When the thread is present, the newly formed amide-containing fragments wrap themselves around it forming the maximum number of hydrogen bonds to stabilise the structure. This encourages an intramolecular reaction between two ends of the same fragment to form a macrocycle which is, perforce, locked around the thread. In the [2]rotaxane shown above, the carbon-carbon double bond in the thread preorganises the amide carbonyls in a close-to-ideal orientation for hydrogen bonding to the macrocycle so that the rotaxane is produced in 97% yield – an astonishingly efficient process for a five-component reaction!

Metal Template Catenanes and Rotaxanes

Figure 7. a) Chemical structure of and b) X-ray crystal structure of a metal-containing [2]rotaxane. Crystal structure atoms: carbon (macrocycle), light blue; carbon (thread), yellow; oxygen, red; nitrogen, dark blue; cadmium(II), grey; chlorine, green.

Figure 7b shows the X-ray crystal structure of a rotaxane made in our group using metal-ligand interactions. Rather than forming the macrocycle around the thread in a clipping strategy as above, the completed macrocycle and a thread precursor are first complexed to the Cd²⁺ metal ion in an orthogonal fashion. Covalent bond-forming reactions then attach the two bulky stopper groups to complete the rotaxane – this is often called a threading and capping strategy. Employing the reversible imine bond-forming reaction for the capping step allows the reaction to proceed under thermodynamic control, giving high yields of the rotaxane which is the most stable species in the reaction mixture. “Switching off” the reversible reaction by reduction of the imine bonds then allows removal of the metal ion to give the intact demetallated [2]rotaxane. [2]Catenanes (more properly called catenates when templated around a metal ion) have been constructed around octahedral metal ions in our group by similar methods (see Figure 8).⁸

Figure 8. Synthesis of [2]catenates around octahedral transition metal templates.