Stephen J. Loeb

Amide based receptors for anions

This review article illustrates the contribution of amide based receptors to the development of anion complexing agents. Amides are incorporated into a wide variety of systems that can be divided into two broad categories; organic and inorganic. The first section is separated into cyclic and acyclic systems built on a solely organic framework. The second section is comprised of metal containing systems such as metallocenes, [Ru(bipy)3]2+ based complexes, porphyrins and other metallo-based receptors. Where appropriate, the results of solution binding studies and sensing outputs are summarized.

Rotaxanes as ligands: from molecules to materials

This tutorial review documents the discovery and application of the supramolecular template (1,2-bis(pyridinium)ethane) subset (24-crown-8) for preparing interlocked molecules. Focus is on the supramolecular chemistry of the pseudorotaxanes formed with various pyridinium axles and crown ether wheels and how this particular class of mechanically linked molecule has been (i) used to construct rudimentary molecular machines such as molecular shuttles and flip switches, (ii) used as ligands for coordination chemistry and (iii) used to create metal-organic framework (MORF) materials.

Metal–organic frameworks with dynamic interlocked components

The dynamics of mechanically interlocked molecules such as rotaxanes and catenanes have been studied in solution as examples of rudimentary molecular switches and machines, but in this medium, the molecules are randomly dispersed and their motion incoherent. As a strategy for achieving a higher level of molecular organization, we have constructed a metal-organic framework material using a [2]rotaxane as the organic linker and binuclear Cu(II) units as the nodes. Activation of the as-synthesized material creates a void space inside the rigid framework that allows the soft macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighbouring molecular components. Variable-temperature (13)C and (2)H solid-state NMR experiments are used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state molecular switches and molecular machines based on mechanically interlocked molecules.

A New Motif for the Self-Assembly of [2]Pseudorotaxanes; 1,2-Bis(pyridinium)ethane Axles and [24]Crown-8 Ether Wheels

Multiple intramolecular interactions help to stabilize the novel [2]pseudorotaxanes formed from 1,2-bis(pyridinium)ethane dications (which act as axles) and 24-membered crown ethers (which act as wheels; see structure). This is the first successful sythesis of [2]pseudorotaxanes with [24]crown-8 as the macrocycle.

Supramolecular Arrays of 4,7‐Phenanthroline Complexes: Self‐Assembly of Molecular Pd6 Hexagons

Linear organopalladium complex fragmentsand 4,7-phenanthroline are the building blocks for supramolecular arrays containing a molecular Pd6 hexagon (depicted on the right). The X-ray structure analysis shows that the phenanthroline moieties form the “corners”, and the palladated aromatic units the “walls” of these rigid nanoscale molecules.

Hydrogen‐Bonded Networks through Second‐Sphere Coordination

The reaction of 4, 7-phenanthroline (1) with aqueous transitionmetal complexes [Mn(H2O)6][NO3]2, [Co(H2O)6][NO3]2, [Ni(H2O)6[NO3]2, [Mn(H2O)6][ClO4]2, and [Co(H2O)6][ClO4]2 does not produce coordination complexes between these metal cations and the N-donor ligand as expected. Instead, supramolecular hydrogenbonded networks are formed between the nitrogen donor atoms of 4, 7-phenanthroline and the OH groups of coordinated water molecules: M-O-H...N interactions. This motif of second-sphere coordination for 1 can be exploited as a tool for crystal engineering. As a demonstration of the generality of this new interaction as a supramolecular building block, five X-ray crystal structures are reported that utilise this hydrogen bonding scheme; [Co(H2O)4(NO3)2].(1)2 (2a), [Co(MeCN)2(H2O)4][ClO4]2.(1)2 (2b), [Ni(H2O)4(NO3)2].(1)2 (3a), [Mn(H2O)4(NO3)2].(1)2 (4a), and [Mn(H2O)6][ClO4]2.(1)(4).4H2O (4b). Each network involves complete saturation of the hydrogen-bond donor sets between the aqua complex and 1 using primarily M-O-H...N(1) and M-O-H...O(anion), interactions. Thermogravimteric analysis shows these materials to have stabililities similar to coordination polymers involving metal-ligand bonds; this demonstrates that second-sphere hydrogen bonding has potential for the construction of polymeric metal-containing materials.

A [2]Rotaxane Flip Switch Driven by Coordination Geometry†

A molecular switch that is derived from a mechanically interlocked molecule (MIM) can exist in two distinct molecular arrangements: the ground-state co-conformation (GSCC) and a metastable co-conformation (MSCC), which are in equilibrium. Ideally, these co-conformers are easily identifiable by using a spectroscopic technique, their relative populations are quantifiable, and the position of the equilibrium can be controlled by some external perturbation. 2] Probably the most well understood MIM switches are the bistable [2]rotaxane, or molecular shuttle, and the bistable [2]catenane, both pioneered by Stoddart and co-workers. In these systems, two different recognition sites are present on one component for the binding of a single macrocycle. The two states of the switch are co-conformers that are related by the relative positioning of the two interlocked components. In a related set of MIMs, we recently reported the first examples of a molecular “flip switch” that operates at a single recognition site on a simple [2]rotaxane. In this system, the stability of the GSCC and MSCC are related to the degree of p stacking between the axle and the wheel, and the position of the co-conformational equilibrium was shown to be sensitive to the structure of the pyridinium component or the nature of the solvent. The flip-switch [2]rotaxane is built around the templating motif of [24]crown-8 macrocycles and 1,2-bis(pyridinium)ethane axles. This motif has been successfully used in creating a variety of unique rotaxanes and catenanes, and has been incorporated into metal–organic frameworks (MOFs). The concept of a flip-switch [2]rotaxane is shown in Scheme 1. In the [2]rotaxanes [(1)(N24C8)] and [(1)(BN24C8)], although the axle 1 is symmetrical, the two ends of the molecules are different because the crown ethers contain two different aromatic rings. This result is clearly shown by low-temperature H NMR spectra in CD2Cl2, which show eight distinct sets of pyridinium protons for [(1)(N24C8)] and six sets for [(1)(BN24C8)] because the pyridinium protons experience different amounts of shielding from the presence or absence of a naphtho or benzo ring. Each pair of exchanging resonances is separated by approximately 0.35 ppm, which is consistent with the effects of shielding that arise from p stacking. The rate of end-to-end exchange or “flipping” was determined to be on the order of 10 s 1 at room temperature. Since the flip-switch equilibrium is dictated by the degree of p stacking and thus the structural composition of the pyridinium component, it was of interest to use this principle to control the switching process. Herein, we report that the relative co-conformational stabilities can be controlled by manipulating the coordination geometry of an appended chelating group. The pioneering work of Sauvage and coworkers on Cu/Cu rotaxane systems, and Leigh and coworkers on Cu and Pd molecular shuttle systems are examples in which co-conformational switching of interlocked molecules has been shown to be dependent on metal– ligand coordination. This new [2]rotaxane flip switch system incorporates a terpyridine (terpy) group on one end of the axle (see Scheme 2). The [2]rotaxane ligands [(3)(BN24C8)] and [(3)(N24C8)] were prepared by stoppering the 1,2-bis(pyridinium) axle 2 with tert-butylbenzylbromide in the presence of an excess of the appropriate crown ether. The products were purified by column chromatography on silica and the anion was exchanged to the triflate (CF3SO3 ; OTf) salt to maintain solubility in organic solvents. The Pt and Ru complexes of these ostensibly terpy ligands were then prepared using standard conditions and appropriate starting materials (Scheme 2). We chose to prepare the square-planar complexes [PtMe(3)(BN24C8)] and [PtMe(3)(N24C8)] as well as Scheme 1. [2]Rotaxanes such as [(1)(N24C8)] or [(1)(BN24C8)] (shown) with a single recognition site but containing a crown ether with two different aromatic groups (BN24C8= [24]crown-8 bearing naphtho and benzo units; N24C8= [24]crown-8 bearing only naphtho units). The crown ether undergoes a dynamic reorientation, reminiscent of a mechanical flip switch.

Metal–Organic Frameworks with Mechanically Interlocked Pillars: Controlling Ring Dynamics in the Solid-State via a Reversible Phase Change

Metal-organic framework (MOF) materials have been prepared that contain a mechanically interlocked molecule (MIM) as the pillaring strut between two periodic Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a [24]crown-6 (24C6) macrocycle and an aniline-based axle with terminal pyridine donor groups. The single-crystal X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxylate) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-temperature (VT) (2)H SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new β-phase of UWDM-3 retained crystallinity, and (2)H SSNMR demonstrated that the 24C6 macrocyclic ring of the pillared MIM strut is now free enough to undergo full rotation. Most importantly, the phase change is reversible; the β version of the MOF can be reverted to the original α state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can be controlled by a reversible phase change.

Host-guest interactions template: the synthesis of a [3]catenane.

Formation of a [3]catenane containing dibenzo-24-crown ether wheels and a large dipyridiniumethane ring is templated by formation of a host-guest adduct between the [3]catenane and the external crown ether.

Bis(benzimidazolium) axles and crown ether wheels: a versatile templating pair for the formation of [2]rotaxane molecular shuttles

Condensation of an aldehyde appended benzimidazolium cation with a 1,2-benzenediamine in the presence of a crown ether allows formation of a second benzimidazole group and the facile synthesis of a [2]rotaxane molecular shuttle. The ease of this reaction and the versatility of the initial templating interaction between the benzimidazolium cation and crown ether allows for the preparation of [2]rotaxane molecular shuttles with crown ether macrocycles of various shapes and sizes. The synthesis and dynamic properties of a set of five [2]rotaxane molecular shuttles are described including the first examples of rotaxanes containing the larger macrocycles dibenzo[30]crown-10 (DB30C10) and bis-meta-phenylene-[26]crown-8 (BMP26C8).