Expanding the Cavity Size: Preparation of 2:1 Inclusion Complexes Based on Dinuclear Square Metallocycles

Conjoined and non-conjoined coordination cages with palladium(II) vertices: structural diversity, solution dynamics, and intermolecular interactions

Self-assembled coordination complexes prepared from a combination of Pd(II) components with one or more types of high-symmetry or low-symmetry bis/tris/tetrakis-monodentate ligands are considered in this review. The structures of these complexes are viewed in terms of the presence of a metallo-macromonocycle or conjoined metallo-macromonocycles/metallocages in the frameworks. Analysis of the typical molecular structures revealed an open truth that one or more units of metallo-macromonocycles can be conjoined to afford planar or non-planar systems. In the same line, the enveloping surface of a 3D cage can be considered as a multiple number of conjoined metallomacrocycles that embrace a 3D space from all directions. However, two or more units of cages are conjoined in a multi-3D-cavity cage system and such a system is considered as a conjoined cage. Construction of such conjoined cages having a finite but multiple number of 3D-cavities unified in a single molecular architecture is a challenging task when compared to that of single-3D-cavity based compounds. Conjoining of as many as four units of 3D cages is known so far. Single- as well as multi-cavity cages of lower symmetry have become a very recent trend in this regard where low-symmetry ligands or mixed ligand ensembles are crafted in the framework of the cages. Other structural diversities like helicity in cages, and supramolecular isomerism are also included in this assorted literature work. Although isomerism in classical coordination complexes is well known, it is very less studied in self-assembled coordination complexes. Ligand isomerism is one such feature that is reviewed here. The dynamic behavior of the cages results in interesting reactivity aspects. A large variety of dynamic processes are collected under an umbrella, i.e., "ligand exchange reactions" and described with examples. Intermolecular interaction among the already self-assembled molecules is possible in solution, solid, and gel-phases as discussed in the last part of this review. The understanding of intermolecular interaction is likely to influence different areas of research including crystal engineering, and materials chemistry.

Dissecting the "Blue Box": Self-Assembly Strategies for the Construction of Multipurpose Polycationic Cyclophanes

Stoddart's "blue box" (B⁴⁺), is one of the most iconic molecules in the recent history of chemistry. This rectangular tetracationic cyclophane has not only the ability to complex a wide variety of aromatic guests in organic or aqueous media, but because of the presence of viologen units on its structure, it also behaves as a redox-based molecular switch. In turn, B⁴⁺-based host-guest complexes can translate this responsiveness from the molecular to the supramolecular level, resulting in host-controlled binding. This unique behavior has allowed the development of a wide variety of B⁴⁺-containing (supra)molecular switches and machines, which certainly have inspired a whole generation of supramolecular chemists. Nevertheless, issues, such as synthetic accessibility, structural diversity, or the implementation of new chemical properties (luminescence, pH- or photo-responsiveness, etc.), have restricted somehow the development of new practical applications in the ever-changing realm of modern host-guest chemistry.Based largely on our own research throughout the past decade, we will highlight in this account two different strategies for the self-assembly of new B⁴⁺ analogues: (1) Pd(II)/Pt(II) metal-directed self-assembly and (2) hydrazone-based dynamic covalent chemistry. In essence, the strategies are based on the substitution of inert C-C single bonds on the macrocycle by Pd/Pt-N or C═N bonds of modifiable lability. In the case of the metal-directed synthesis, the use of Pd(II) centers allows for the spontaneous self-assembly at r.t., either in organic or aqueous media, of N-alkyl-4,4'-bipyridinium-based ligands into the desired metallacycles. Conversely, more inert Pt(II) salts can be also implemented, rendering the synthesis of more kinetically stable analogues. Alternatively, wholly organic B⁴⁺ congeners can be produced in a modular fashion by using hydrazone-based dynamic covalent chemistry, allowing for the self-assembly in acidic water of macrocyclic pH-responsive molecular switches of adjustable kinetic stability.Owning pyridinium-based cavities of appropriate size, our B⁴⁺-inspired cyclophanes are able to complex aromatic substrates by a conjunction of the hydrophobic effect and π-π/C-H···π interactions. Consequently, we will discuss in detail the different host-guest complexes that can be achieved using our cyclophanes. Considering this knowledge, the implementation of our B⁴⁺-based macrocycles onto mechanically interlocked molecules and knots will be introduced, as well as the development of practical applications for the hosts in currently important research fields, such as the development of duplex and G4-DNA binders, supramolecular catalysis or the sequestration of relevant pollutants. Finally, self-assembled hosts offer the unique opportunity to include constitutional dynamism into host-guest chemistry, so examples of the development by our group of stimuli-responsive constitutionally dynamic libraries and self-sorted systems will be highlighted.

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 Pt2L2 boxes strongly bind G-quadruplex DNA and influence gene expression in cancer cells

Supramolecular Pt(ii) quadrangular boxes bind native and G-quadruplex DNA motifs in a size-dependent fashion. Three Pt molecular squares of distinct size show biological activity against cancer cells and heavily influence the expression of genes known to form G-quadruplexes in their promoter regions. The smallest Pt-box displays less activity but more selectivity for a quadruplex formed in the c-Kit gene.

cis-Protected palladium(ii) based binuclear complexes as tectons in crystal engineering and the imperative role of the cis-protecting agent

The shape and molecular packing of cis-protected Pd(ii) based binuclear complexes depends upon the ligand and cis-protecting agents.

Stimuli-responsive metal-directed self-assembly of a ring-in-ring complex

The threading and unthreading process of a dinuclear metallacycle into a ring-in-ring structure can be controlled by temperature, dilution, and medium polarity.

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

Self-assembled palladium(II) "click" cages: synthesis, structural modification and stability

Readily synthesised and functionalised di-1,2,3-triazole "click" ligands are shown to self-assemble into coordinatively saturated, quadruply stranded helical [Pd(2)L(4)](BF(4))(4) cages with Pd(II) ions. The cages have been fully characterised by elemental analysis, HR-ESMS, IR, (1)H, (13)C and DOSY NMR, DFT calculations, and in one case by X-ray crystallography. By exploiting the CuAAC "click" reaction we were able to rapidly generate a small family of di-1,2,3-triazole ligands with different core spacer units and peripheral substituents and examine how these structural modifications affected the formation of the [Pd(2)L(4)](BF(4))(4) cages. The use of both flexible (1,3-propyl) and rigid (1,3-phenyl) core spacer units led to the formation of discrete [Pd(2)L(4)](BF(4))(4) cage complexes. However, when the spacer unit of the di-1,2,3-triazole ligand was a 1,4-substituted-phenyl group steric interactions led to the formation of an oligomeric/polymeric species. By keeping the 1,3-phenyl core spacer constant the effect of altering the "click" ligands' peripheral substituents was also examined. It was shown that ligands with alkyl, phenyl, electron-rich and electron-poor benzyl substituents all quantitatively formed [Pd(2)L(4)](BF(4))(4) cage complexes. The results suggest that a wide range of functionalised palladium(II) "click" cages could be rapidly generated. These novel molecules may potentially find uses in catalysis, molecular recognition and drug delivery.

New insights into the factors that govern the square/triangle equilibria of Pd(II) and Pt(II) supramolecules. Unexpected participation of a mononuclear species in the equilibrium

The new rigid fluorinated ligand 4,4'-bis(4-pyridyl)octafluorobiphenyl (L1) has been synthesized by a nucleophilic substitution reaction between the organolithium derivative of the 4-bromopyridine and the compound decafluorobiphenyl. The use of L1 as building block of supramolecular species containing diphosphane or ethylenediamine Pd(II) and Pt(II) fragments has been explored, and the results have been compared with those previously reported for the smaller ligand 1,4-bis(4-pyridyl)tetrafluorobenzene (L2). The observed differences (particularly, square/triangle ratio) are discussed in terms of different parameters such as the nature of the ancillary ligands, solvent, or reaction temperature. The synthesis of water-soluble palladium metallamacrocycles derived from L1 has been attempted using ethylenediamine (en) or tetramethylethylenediamine (tmen) as ancillary ligand. Interestingly, in the latter case, the resulting solutions of the compounds showed, together with the square/triangle equilibrium, the unexpected presence of an additional lower nuclearity species, whose nature has been determined by means of bidimensional (1)H NMR experiments.