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Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

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.

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Self-assembled coordination cages based on banana-shaped ligands

In this review, a comprehensive summary of supramolecular transformations within discrete coordination-driven supramolecular architectures, including helices, metallacycles, metallacages, etc., is presented. Recent investigations have demonstrated that coordination-driven self-assembled architectures provide an ideal platform to study supramolecular transformations mainly due to the relatively rigid yet dynamic nature of the coordination bonds. Various stimuli have been extensively employed to trigger the transformation processes of metallosupramolecular architectures, such as solvents, concentration, anions, guests, change in component fractions or chemical compositions, light, and post-modification reactions, which allowed for the formation of new structures with specific properties and functions. Thus, it is believed that supramolecular transformations could serve as another highly efficient approach for generating diverse metallosupramolecular architectures. Classified by the aforementioned various stimuli used to induce the interconversion processes, the emphasis in this review will be on the transformation conditions, structural changes, mechanisms, and the output of specific properties and functions upon induction of structural transformations.

Supramolecular transformations within discrete coordination-driven supramolecular architectures

Anthracene, with its molecular panel-like shape and robust photophysical behaviour, is a versatile building block that is widely used to construct attractive and functional molecules and molecular assemblies through covalent and non-covalent linkages. The intrinsic photophysical, photochemical and chemical properties of the embedded anthracenes often interact to engender desirable chemical behaviours and properties in multi-anthracene assemblies. This review article focuses on molecular architectures with linear, cyclic, cage, and capsule shapes, each containing three or more anthracene subunits.

Molecular architectures of multi-anthracene assemblies

Self-assembled coordination complexes from various palladium(II) components and bidentate or polydentate ligands

Development of coordination driven self-assembled discrete spherical ensembles

Receptor mediated delivery of siRNA enables silencing of target genes in specific tissues. Folate receptor (FR) is an attractive target for tumor-selective gene delivery. The focus of this study was to deliver the dihydrofolate reductase (DHFR) siRNA expressing plasmid and to silence the DHFR gene in FR positive KB cells, by complexing the plasmid with a folate-polyethylene glycol-polyethylenimine (FOL-PEG-PEI) conjugate, as a gene carrier. A DHFR siRNA sequence was cloned into a pSUPER-RNAi vector and complexed with the FOL-PEG-PEI conjugate. The complex was characterized by particle size analyzer, gel retardation and DNase protection assay. The FOL-PEG-PEI/pSUPER-siDHFR complex was transfected to FR overexpressing (KB) and FR negative (A549) cells. The transfection effiencies and gene inhibition were analyzed by fluorescence microscopy and RT-PCR. The pSUPER-siDHFR/PEI-PEG-FOL complex delivered the siRNA vector and inhibited DHFR gene in KB cells, while A549 cells were unaffected. Lipofectamine mediated transfection of pSUPER-siDHFR, delivered the vector and inhibited the DHFR gene in both KB and A549 cells. FR mediated delivery of siDHFR complexed with PEI-PEG-FOL conjugate inhibits the DHFR expression in FR positive cells alone. This strategy can be extended to deliver a wide range of drugs and post-transcriptional gene silencing therapeutics.

Niladri Bihari Debata

Papers

Self-assembled coordination complexes from various palladium(II) components and bidentate or polydentate ligands

Development of coordination driven self-assembled discrete spherical ensembles

Development of a targeted siRNA delivery system using FOL-PEG-PEI conjugate.

Coordination-driven self-assembly in a single pot

Coordination driven discrete metallopolygons and cages from unsymmetric bidentate ligands