Kathleen M. Mullen

Exploiting the 1,2,3‐Triazolium Motif in Anion‐Templated Formation of a Bromide‐Selective Rotaxane Host Assembly

Bromide is best: The first [2]rotaxane incorporating the triazolium anion-binding motif is prepared using bromide anion templation. Preliminary anion-binding investigations reveal that the rotaxane exhibits the rare selectivity preference for bromide over chloride ions.

Sulfate anion templated synthesis of a triply interlocked capsule

Sulfate templation has been used in the synthesis of a novel tris-urea-based triply interlocked capsule, whose structure has been verified by DOSY NMR, mass spectrometry and molecular modelling investigations.

Interlocked Host Anion Recognition by an Indolocarbazole-Containing [2]Rotaxane

The design, synthesis, structure, and anion-binding properties of the first indolocarbazole-containing interlocked structure are described. The novel [2]rotaxane molecular structure incorporates a neutral indolocarbazole-containing axle component which is encircled by a tetracationic macrocycle functionalized with an isophthalamide anion recognition motif. (1)H NMR and UV-visible spectroscopies and X-ray crystallography demonstrated the importance of pi-donor-acceptor, CH...pi, and electrostatic interactions in the assembly of pseudorotaxanes between the electron-deficient tetracationic macrocycle and a series of pi-electron-rich indolocarbazole derivatives. Subsequent urethane stoppering of one of these complexes afforded a [2]rotaxane, which was shown by (1)H NMR spectroscopic titration experiments to exhibit enhanced chloride and bromide anion recognition compared to its non-interlocked components. Computational molecular dynamics simulations provide further insight into the mechanism and structural nature of the anion recognition process, confirming it to involve cooperative hydrogen-bond donation from both macrocycle and indolocarbazole components of the rotaxane. The observed selectivity of the [2]rotaxane for chloride is interpreted in terms of its unique interlocked binding cavity, defined by the macrocycle isophthalamide and indolocarbazole N-H protons, which is complementary in size and shape to this halide guest.

Phosphorus-Based Functional Groups as Hydrogen Bonding Templates for Rotaxane Formation

We report on the use of the hydrogen bond acceptor properties of some phosphorus-containing functional groups for the assembly of a series of [2]rotaxanes. Phosphinamides, and the homologous thio- and selenophosphinamides, act as hydrogen bond acceptors that, in conjunction with an appropriately positioned amide group on the thread, direct the assembly of amide-based macrocycles around the axle to form rotaxanes in up to 60% yields. Employing solely phosphorus-based functional groups as the hydrogen bond accepting groups on the thread, a bis(phosphinamide) template and a phosphine oxide-phosphinamide template afforded the corresponding rotaxanes in 18 and 15% yields, respectively. X-ray crystallography of the rotaxanes shows the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and various hydrogen bond accepting groups on the thread, including rare examples of amide-to-phosphinamide, -thiophosphinamide, and -selenophosphinamide groups. With a phosphine oxide-phosphinamide thread, the solid-state structure of the rotaxane is remarkable, featuring no direct intercomponent hydrogen bonds but rather a hydrogen bond network involving water molecules that bridge the H-bonding groups of the macrocycle and thread through bifurcated hydrogen bonds. The incorporation of phosphorus-based functional groups into rotaxanes may prove useful for the development of molecular shuttles in which the macrocycle can be used to hinder or expose binding ligating sites for metal-based catalysts.

Nitrone [2]rotaxanes: Simultaneous chemical protection and electrochemical activation of a functional group

We report on the use of the hydrogen-bond-accepting properties of neutral nitrone moieties to prepare benzylic amide macrocycle-containing [2]rotaxanes in yields as high as 70%. X-ray crystallography showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and the two nitrone groups of the thread. Dynamic (1)H NMR studies of the rates of macrocycle pirouetting in nonpolar solutions indicated that the amide-nitrone hydrogen bonds are particularly strong (approximately 1.3 and approximately 0.2 kcal mol(-1) stronger than similar amide-ester and amide-amide interactions, respectively). In addition to polarizing the N-O bond through hydrogen bonding, the rotaxane structure affects the chemistry of the nitrone groups in two significant ways: first, the intercomponent hydrogen bonding activates the nitrone groups to electrochemical reduction, a one-electron-reduction of the rotaxane being stabilized by a remarkable 400 mV (8.1 kcal mol(-1)) with respect to the same process in the thread; second, however, encapsulation protects the same functional groups from chemical reduction with an external reagent (and slows electron transfer to and from the electroactive groups in cyclic voltammetry experiments). Mechanical interlocking with a hydrogen-bonding molecular sheath thus provides a route to an encapsulated polarized functional group and radical anions of significant kinetic and thermodynamic stability.

Anion templated formation of pseudorotaxane and rotaxane monolayers on gold from neutral components.

The surface covalent attachment of indolocarbazole axles enables anion templation to be exploited in the formation of pseudorotaxane assemblies via the threading of neutral isophthalamide macrocycles from solution. The anion selectivity of this templating process can be monitored by a number of surface spectroscopic methods and shows subtle differences compared to the same process in solution. Though the fluxional and disordered nature of ethylene glycol extended axle adlayers prohibits detectable threading on the surface, rotaxane monolayers can be generated by a preassociation of the components and templating anion in solution. The threaded macrocycles therein can subsequently be released and detected by mass spectrometry by reductive stripping of the axle.

A Small Molecule that Walks Non‐Directionally Along a Track Without External Intervention

Charge of the light brigade: A molecule is able to walk back and forth upon a five-foothold pentaethylenimine track without external intervention. The 1D random walk is highly processive (mean step number 530) and exchange takes place between adjacent amine groups in a stepwise fashion. The walker performs a simple task whilst walking: quenching of the fluorescence of an anthracene group sited at one end of the track. Copyright

Amide-appended porphyrins as scaffolds for catenanes, rotaxanes and anion receptors

The synthesis of a porphyrin with an over-arching strap incorporating an isophthalamide unit produced both a porphyrin monomer with a potentially H-bonding receptor site, and a [2]catenane. Although both compounds exhibit fluxional behaviour, 1H NMR and MS data was used to distinguish them, and was interpreted in terms of the dynamics of each system. The self-complementarity of the isophthalamide unit provides an ideal building block for assembling [2]pseudorotaxanes with similarly functionalised threads. This was illustrated by the self-assembly of a ruthenium–porphyrin stoppered rotaxane under thermodynamically controlled conditions, by simple mixing of the strapped porphyrin, a complementary thread unit containing an isophthalamide central unit and pyridine-attached ends, and a ruthenium carbonyl porphyrin. The strapped porphyrin was also shown to act as a receptor for chloride ion, and its binding ability with other H-bonding isophthalamide guests was found to be modulated by the presence of chloride ion. Chloride complexation was observed in both metallated and free base porphyrin monomers as well as the rotaxanes counterparts, but no anion binding was observed for the catenane.

Anion templated assembly of an indolocarbazole containing pseudorotaxane on beads and silica nanoparticles

The surface covalent attachment of fluorescent axles of indolocarbazole enables anion templation to be exploited in the formation of pseudorotaxane assembliesvia the threading of neutral isophthalamide macrocycles. In utilising the surface of polystyrene beads this threading process can be followed by both magnetic resonance methods and changes in the axle fluorescent emission spectrum. The analogous surface assembly and anion templated threading can be achieved, with fluoride and sulfate, on silica nanoparticles where anion recognition and macrocycle threading are associated with equivalent and specific optical change.