Asha Brown

Evidence for halogen bond covalency in acyclic and interlocked halogen-bonding receptor anion recognition.

The synthesis and anion binding properties of novel halogen-bonding (XB) bis-iodotriazole-pyridinium-containing acyclic and [2]catenane anion host systems are described. The XB acyclic receptor displays selectivity for acetate over halides with enhanced anion recognition properties compared to the analogous hydrogen-bonding (HB) acyclic receptor. A reversal in halide selectivity is observed in the XB [2]catenane, in comparison to the acyclic XB receptor, due to the interlocked host’s unique three-dimensional binding cavity, and no binding is observed for oxoanions. Notable halide anion association constant values determined for the [2]catenane in competitive organic–aqueous solvent mixtures demonstrate considerable enhancement of anion recognition as compared to the HB catenane analogue. X-ray crystallographic analysis of a series of halide catenane complexes reveal strong XB interactions in the solid state. These interactions were studied using Cl and Br K-edge X-ray Absorption Spectroscopy (XAS) indicating intense pre-edge features characteristic of charge transfer from the halide to its bonding partner (σAX←X–* ← X1s), and providing a direct measure of the degree of covalency in the halogen bond(s). The data reveal that the degree of covalency is similar to that which is observed in transition metal coordinate covalent bonds. These results are supported by DFT results, which correlate well with the experimental data.

Sulfate anion templation of a neutral pseudorotaxane assembly using an indolocarbazole threading component.

The first example of anion templated pseudorotaxane formation between two neutral components in solution and in surface assembled monolayers is described.

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.

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.

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.

Chloride-Anion-Templated Synthesis of a Strapped-Porphyrin-Containing Catenane Host System

The synthesis, structure and anion-recognition properties of a new strapped-porphyrin-containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three-dimensional, hydrogen-bond-donating anion-binding pocket; solid-state structural analysis of the catenane⋅chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH⋅⋅⋅⋅Cl and NH⋅⋅⋅Cl hydrogen-bonding interactions and solution-phase 1H NMR titration experiments demonstrate that this complementary hydrogen-bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.

A Bis-Triazacyclononane Tris-Pyridyl N9-Azacryptand “Beer Can” Receptor for Complexation of Alkali Metal and Lead(II) Cations

A new bis-triazacyclononane tris-pyridyl N9 -azacryptand ligand is prepared via a convenient one-pot [2+3] condensation reaction between triazacyclononane and 2,6-bis(bromomethyl) pyridine in the presence of M2 CO3 (M=Li, Na, K). The proton, lithium, sodium, potassium and lead(II) complexes of the ligand are characterised in the solid state. Preliminary solution-phase competition experiments indicate that the cryptand ligand preferentially binds lead(II) in the presence of sodium, calcium, potassium and zinc cations in methanol solution.