Counterion Dynamics in an Interpenetrated Coordination Cage Capable of Dissolving AgCl

Synthetic Receptors with Micromolar Affinity for Chloride in Water

A water-soluble coordination cage was obtained by reaction of Pd(NO₃ )₂ with a 1,3-di(pyridin-3-yl)benzene ligand featuring a short PEG chain. The cavity of the metal-organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution is K_a =1.8(±0.1)×10⁵  M^-1 . This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br^- and I^- compete with binding or self-assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.

Coordination-Directed Construction of Molecular Links

Since the emergence of the concept of chemical topology, interlocked molecular assemblies have graduated from academic curiosities and poorly defined species to become synthetic realities. Coordination-directed synthesis provides powerful, diverse, and increasingly sophisticated protocols for accessing interlocked molecules. Originally, metal ions were employed solely as templates to gather and position building blocks in entwined or threaded arrangements. Recently, metal centers have increasingly featured within the backbones of the integral structural elements, which in turn use noncovalent interactions to self-assemble into intricate topologies. By outlining ingenious recent examples as well as seminal classic cases, this Review focuses on the role of metal-ligand paradigms in assembling molecular links. In addition, the ever-evolving approaches to efficient assembly, the structural features of the resulting architectures, and their prospects for the future are also presented.

Self-assembled conjoined-cages

A self-assembled coordination cage usually possesses one well-defined three-dimensional (3D) cavity whereas infinite number of 3D-cavities are crafted in a designer metal-organic framework. Construction of a discrete coordination cage possessing multiple number of 3D-cavities is a challenging task. Here we report the peripheral decoration of a trinuclear [Pd₃L₆] core with one, two and three units of a [Pd₂L₄] entity for the preparation of multi-3D-cavity conjoined-cages of [Pd₄(L^a)₂(L^b)₄], [Pd₅(L^b)₄(L^c)₂] and [Pd₆(L^c)₆] formulations, respectively. Formation of the tetranuclear and pentanuclear complexes is attributed to the favorable integrative self-sorting of the participating components. Cage-fusion reactions and ligand-displacement-induced cage-to-cage transformation reactions are carried out using appropriately chosen ligand components and cages prepared in this work. The smaller [Pd₂L₄] cavity selectively binds one unit of NO₃⁻, F⁻, Cl⁻ or Br⁻ while the larger [Pd₃L₆] cavity accommodates up to four DMSO molecules. Designing aspects of our conjoined-cages possess enough potential to inspire construction of exotic molecular architectures.

Developing simple routes for construction of multi-compartmental cages is a compelling and challenging task. Here, the authors report modular construction of multi-3D-cavity cages featuring one, two or three units of a [Pd₂L₄] entity conjoined with a [Pd₃L₆] core.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Binding of anions in triply interlocked coordination catenanes and dynamic allostery for dehalogenation reactions

By synergistic combination of multicomponent self-assembly and template-directed approaches, triply interlocked metal organic catenanes that consist of two isolated chirally identical tetrahedrons were constructed and stabilized as thermodynamic minima. In the presence of suitable template anions, the structural conversion from the isolated tetrahedral conformers into locked catenanes occurred via the cleavage of an intrinsically reversible coordination bond in each of the tetrahedrons, followed by the reengineering and interlocking of two fragments with the regeneration of the broken coordination bonds. The presence of several kinds of individual pocket that were attributed to the triply interlocked patterns enabled the possibility of encapsulating different anions, allowing the dynamic allostery between the unlocked/locked conformers to promote the dehalogenation reaction of 3-bromo-cyclohexene efficiently, as with the use of dehalogenase enzymes. The interlocked structures could be unlocked into two individual tetrahedrons through removal of the well-matched anion templates. The stability and reversibility of the locked/unlocked structures were further confirmed by the catching/releasing process that accompanied emission switching, providing opportunities for the system to be a dynamic molecular logic system.

Solvent- and anion-induced interconversions of metal-organic cages

Fully structural interconversions between monomeric Pd24 and interlocked dimeric Pd48 cages have been investigated to elucidate their thermodynamic stability defined by their anion-guest binding behaviours.

Cation-Anion Arrangement Patterns in Self-Assembled Pd2L4 and Pd4L8 Coordination Cages

Compounds featuring one-dimensional regular arrangements of stacked metal complexes and alternating [cation-anion]_∞ sequences have raised considerable interest owing to their peculiar electronic and optical properties as well as guest inclusion capabilities. While traditional ways to realize these structural motifs rely on crystalline compounds, exclusively existing in the solid state, recent progress in the area of metal-mediated supramolecular self-assembly allows for the rational synthesis of structurally well-defined short stretches of stacked metal complexes and cation-anion arrangements. Therefore, metal cations, counteranions, and suitably designed organic bridges are allowed to self-assemble in solution. While the bridges can be designed as cross-linkers to yield extended two- or three-dimensional networks such as layered materials, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), they can also be tailored to lead to discrete nanoscopic objects. Supramolecular helicates, grids, and knots belong to this class of compounds, and a particularly interesting subfamily are coordination cages and capsules, which possess nanosized cavities with the ability to encapsulate guest molecules. Here, we focus on coordination cages consisting of two or more square-planar Pd(II) or Pt(II) metal cations, bridged by banana-shaped bis-monodentate pyridyl ligands that encapsulate various guest molecules, usually anions, in their cavities. Monoanions as well as dianions with localized or delocalized charges can be bound with remarkable complementarity between cage and guest in terms of size and shape. We show how dimerization of the prototypical [Pd₂L₄] cages into their interpenetrated dimers [Pd₄L₈] leads to an increase in cavity number from one to three while the cavity volume decreases. Usually, all three pockets of these double cages are filled with monoanions such as BF₄^- or Cl^-, thus leading to well-defined linear [Pd-anion]₃Pd stacks, as observed by X-ray studies. The ligand-based mechanical coupling of the linearly aligned cavities leads to interesting effects concerning guest encapsulation cooperativity, such as allosteric binding and triggered sequential uptake. While most of the so far reported coordination cages consist of only a single type of ligand, recent advances in rational assembly strategies allow for high-yielding syntheses of structurally defined multicomponent architectures by integrative self-sorting mechanisms. One family of heteroleptic [Pd₂L₂L'₂] cages whose formation is based on shape-complementarity between two different ligands, L and L', is introduced. Furthermore, the implementation of ligand-based functions such as redox activity, photochromic behavior, specific binding sites, chirality, and catalytic activity allows us to study systems with properties far beyond basic structural features. We showcase selected examples of self-assembled cages whose guest uptake or even overall structural integrity is reversibly switched by light or small molecules with potential application in stimuli responsive materials (e.g., for sequestration of pollutants or stabilization of reactive compounds) up to functional nanosystems (e.g., diagnostic devices or supramolecular catalysts) and molecular machines.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.

Influence of size, shape, heteroatom content and dispersive contributions on guest binding in a coordination cage

Encapsulation of neutral guest molecules inside a self-assembled coordination cage was systematically studied using NMR and MS experiments. Electronic structure calculations reveal substantial contributions of dispersive interactions to binding.

SirX: a selective inversion recovery experiment on X-nuclei for the determination of the exchange rate of slow chemical exchanges between two sites

Nuclear magnetic resonance spectroscopy has proven to be powerful for the study of dynamic processes. A new pulse sequence, SirX, is designed to provide boundary conditions that simplify the McConnell equations. Both an initial rate approximation and a whole curve fitting to the time course of magnetization can be used to calculate the exchange rate. These methods were used to study the exchange kinetics of N,N-dimethylacetamide. As compared with the well-established exchange spectroscopy suitable to studies of slow exchange, SirX has the advantage of being less time consuming and capable of providing more reliable kinetic data. Furthermore, by setting the observation on X-nuclei with larger chemical shift dispersion as compared with an observation on (1)H resonance, SirX extends the upper limit of a reliable determination of exchange rates.

Narcissistic self-sorting vs. statistic ligand shuffling within a series of phenothiazine-based coordination cages

Previously, we introduced a series of anion-binding interpenetrated double-cages based on phenothiazine and its mono- and di-S-oxygenated derivatives. Here, we complete the structural comparison of the three related assemblies by an X-ray single crystal analysis of the sulfone derivative. We further show that the three palladium cages coexist in solution upon post-assembly mixing due to the very slow ligand exchange whereas treatment of binary mixtures of the corresponding ligands with Pd(II) leads to the formation of mixed cages comprising a statistical ligand distribution. In contrast, mixtures of one of these ligands with a shorter ligand derivative lead to narcissistic self-assembly into a double-cage and a coexisting small monomeric cage, regardless of the order of mixing and Pd(II) addition.

Stoichiometrically controlled revocable self-assembled "spiro" versus quadruple-stranded "double-decker" type coordination cages

The simple combination of Pd(II) with the tris-monodentate ligand bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate, L, at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the "spiro-type" Pd1L2 macrocycle, 1, and the quadruple-stranded Pd3L4 cage, 2, respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.

Self-assembled coordination cages based on banana-shaped ligands

The combination of pyridyl ligands and square-planar Pd(ii) or Pt(ii) cations has proven to be a very reliable recipe for the realization of supramolecular self-assemblies. This tutorial review deals with the design, synthesis and host-guest chemistry of discrete coordination cages built according to this strategy. The focus is set on structures obeying the formula [PdnL2n] (n = 2-4). The most discussed ligands are bent, bis-monodentate bridges having their two donor sites pointing in the same direction. The structures of the resulting cages range from simple globules over intertwined knots to interpenetrated dimers featuring three small pockets instead of one large cavity. The cages have large openings that allow small guest molecules to enter and leave the cavities. Most structures are cationic and thus favour the uptake of anionic guests. Some examples of host-guest complexes are discussed with emphasis on coencapsulation and allosteric binding phenomena. Aside from cages in which the ligands have only a structural role, some examples of functional ligands based on photo- and redox-active backbones are presented.

SERS spectroscopic evidence for the integrity of surface-deposited self-assembled coordination cages

A series of self-assembled coordination cages was examined by Raman spectroscopy in solution and Surface Enhanced Raman Scattering (SERS) on a nanostructured Au surface.

Anion encapsulation and dynamics in self-assembled coordination cages

Coordination cages functioning as anion receptors are reviewed, emphasizing the cage structures and the dynamics of anion encapsulation and exchange.

Subtle backbone modifications control the interpenetration of dibenzosuberone-based coordination cages

The self-assembly of interpenetrated double-cages was examined with respect to various synthetic modifications of the dibenzosuberone backbone.

Tuning the coordination chemistry of cyclotriveratrylene ligand pairs through alkyl chain aggregation

Propylated cyclotriveratrylene ligands with N-donor groups form coordination polymers where the propyl groups aggregate or form a Pd₆L₄ cage.