Scaffolding a Cage-Like 3D Framework by Coordination and Constitutional Dynamic Chemistry

Triamine and Tetramine Edge-Length Matching Drives Heteroleptic Triangular and Tetragonal Prism Assembly

Heteroleptic metal-organic capsules, which incorporate more than one type of ligand, can provide enclosed, anisotropic interior cavities for binding low-symmetry molecules of biological and industrial importance. However, the selective self-assembly of a single mixed-ligand architecture, as opposed to the numerous other possible self-assembly outcomes, remains a challenge. Here, we develop a design strategy for the subcomponent self-assembly of heteroleptic metal-organic architectures with anisotropic internal void spaces. Zn6Tet3Tri2 triangular prismatic and Zn8Tet2Tet'4 tetragonal prismatic architectures were prepared through careful matching of the side lengths of the tritopic (Tri) or tetratopic (Tet, Tet') and panels.

Neighboring Cage Participation for Assisted Construction of Self-Assembled Multicavity Conjoined Cages and Augmented Guest Binding

A set of Pd2L4, Pd3L4, and Pd4L4-type single-, double-, and triple-cavity cages are prepared by complexation of Pd(NO3)2 with designer bis-monodentate (L1), tris-monodentate (L2), and tetrakis-monodentate (L3) ligands. The Pd2L4 cage exists in equilibrium with a Pd3L6 cage; the equilibrium shifted to Pd2L4 at 70 °C or upon addition of pyrazine-N,N'-dioxide (PZDO). The Pd2L4 cage binds a PZDO molecule using electrostatic, bifurcated H-bonding and overcoordinated H-bonding interactions. The discrete Pd3L4 and Pd4L4 compounds are conjoined cages comprising of unequal sized Pd2L4 cages (bigger and smaller). The bigger unit of Pd3L4 cage selectively binds a PZDO, and the smaller one binds a nitrate, fluoride, chloride, or bromide. The Pd4L4 cage, having a central bigger Pd2L4 cavity and two smaller peripheral Pd2L4 cavities, binds one PZDO and two nitrate, fluoride, chloride, or bromide. The smaller cavity can be prepared individually from Pd(II) and bis-monodentate ligand (L4), however, in the presence of template like a nitrate, fluoride, chloride, or bromide; otherwise, it forms an oligomeric mixture. Notably, the conjoined Pd3L4 and Pd4L4 cages could be prepared with (preferably) or without using a template for smaller cavity, and the bigger Pd2L4 is formed by sacrificing the possibility of the Pd3L6 moiety. Thus, the conjoined cages are formed in a symbiotic manner where the neighboring cages participate in the formation of each other. The binding of PZDO shows that the presence of one neighboring cage (as in Pd3L4) augments the binding affinity and that is further augmented in the presence of two neighboring cages (as in Pd4L4).

Anion-encapsulating, discrete prism and extended frusta, from trimetallated triangular macrocycles and linkers

Reaction of a trinuclear triangular macrocyclic complex Pb3L(CF3SO3)6 with bidentate linkers in a ratio of 3 equiv. of linker per 2 equiv. of complex, produces a prismatic structure with 4,4'-dipyridyl, and two unprecedented, extended 3D frustum-like structures with 1,2-di(4-pyridyl)ethylene and 1,4-di(4-pyridyl)benzene. The cavities of these structures encapsulate triflate anions.

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes

A series of six new Cu(i) complexes with ([Cu(N-{4-R}pyridine-2-yl-methanimine)(PPh₃)Br]) formulation, where R corresponds to a donor or acceptor p-substituent, have been synthesized and were used to study self-association effects on their structural and electrochemical properties. X-ray diffraction results showed that in all complexes the packing is organized from a dimer generated by supramolecular π stacking and hydrogen bonding. ¹H-NMR experiments at several concentrations showed that all complexes undergo a fast-self-association monomer-dimer equilibrium in solution, while changes in resonance frequency towards the high or low field in specific protons of the imine ligand allow establishing that dimers have similar structures to those found in the crystal. The thermodynamic parameters for this self-association process were calculated from dimerization constants determined by VT-¹H-NMR experiments for several concentrations at different temperatures. The values for K _D (4.0 to 70.0 M^-1 range), ΔH (-1.4 to -2.6 kcal mol^-1 range), ΔS (-0.2 to 2.1 cal mol^-1 K^-1 range), and ΔG ₂₉₈ (-0.8 to -2.0 kcal mol^-1 range) are of the same order and indicate that the self-dimerization process is enthalpically driven for all complexes. The electrochemical profile of the complexes shows two redox Cu(ii)/Cu(i) processes whose relative intensities are sensitive to concentration changes, indicating that both species are in chemical equilibrium, with the monomer and the dimer having different electrochemical characteristics. We associate this behaviour with the structural lability of the Cu(i) centre that allows the monomeric molecules to reorder conformationally to achieve a more adequate assembly in the non-covalent dimer. As expected, structural properties in the solid and in solution, as well as their electrochemical properties, are not correlated with the electronic parameters usually used to evaluate R substituent effects. This confirms that the properties of the Cu(i) complexes are usually more influenced by steric effects than by the inductive effects of substituents of the ligands. In fact, the results obtained showed the importance of non-covalent intermolecular interactions in the structuring of the coordination geometry around the Cu centre and in the coordinative stability to avoid dissociative equilibria.

Russian-Doll-Like Molecular Cubes

Nanosized cage-within-cage compounds represent a synergistic molecular self-assembling form of three-dimensional architecture that has received particular research focus. Building multilayered ultralarge cages to simulate complicated virus capsids is believed to be a tough synthetic challenge. Here, we synthesize two large double-shell supramolecular cages by facile self-assembly of presynthesized metal-organic hexatopic terpyridine ligands with metal ions. Differing from the mixture of prisms formed from the inner tritopic ligand, the redesigned metal-organic hexatopic ligands bearing high geometric constraints that led to the exclusive formation of discrete double-shell structures. These two unique nested cages are composed of inner cubes (5.1 nm) and outer huge truncated cubes (12.0 and 13.2 nm) with six large bowl-shape subcages distributed on six faces. The results with molecular weights of 75 232 and 77 667 Da were among the largest synthetic cage-in-cage supramolecules reported to date. The composition, size and shape were unambiguously characterized by a combination of ¹H NMR, DOSY, ESI-MS, TWIM-MS, TEM, AFM, and SAXS. This work provides an interesting model for functional recognition, delivery, and detection of various guest molecules in the field of supramolecular materials.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

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.

Self-sorting of two imine-based metal complexes: balancing kinetics and thermodynamics in constitutional dynamic networks

A major hurdle in the development of complex constitutional dynamic networks (CDNs) is the lack of strategies to simultaneously control the output of two (or more) interconnected dynamic processes over several species, namely reversible covalent imine bond formation and dynamic metal-ligand coordination. We have studied in detail the self-sorting process of 11 constitutional dynamic libraries containing two different amines, aldehydes and metal salts into two imine-based metal complexes, having no overlap in terms of their compositions. This study allowed us to determine the factors influencing the fidelity of this process (concentration, electronic and steric parameters of the organic components, and nature of the metal cations). In all 11 systems, the outcome of the process was primarily determined by the ability of the octahedral metal ion to select its pair of components from the initial pool of components, with the composition of the weaker tetrahedral complex being imposed by the components rejected by the octahedral metal ions. Different octahedral metal ions required different levels of precision in the "assembling instructions" provided by the organic components of the CDN to guide it towards a sorted output. The concentration of the reaction mixture, and the electronic and steric properties of the initial components of the library were all found to influence the lifetime of unwanted metastable intermediates formed during the assembling of the two complexes.

Multisite Binding of Drugs and Natural Products in an Entropically Favorable, Heteroleptic Receptor

The cavities of artificial receptors are defined by how their components fit together. The encapsulation of specific molecules can thus be engineered by considering geometric principles; however, intermolecular interactions and steric fit scale with receptor size, such that the ability to bind multiple guests from a specific class of compounds remains a current challenge. By employing metal-organic self-assembly, we have prepared a triangular prism from two different ligands that is capable of binding more than 20 different natural products, drugs, and steroid derivatives within its prolate cavity. Encapsulation inflates the host, enhancing its ability to bind other guests in peripheral pockets and thus enabling our system to bind combinations of different drug and natural product cargoes in different locations simultaneously. This new mode of entropically favorable self-assembly thus enables central encapsulation to amplify guest-binding events around the periphery of an artificial receptor.

Integrative self-sorting of coordination cages based on 'naked' metal ions

Coordination-driven self-assembly of metal ions and organic ligands has been extensively utilised over the past four decades to access a variety of nano-sized cage assemblies, with functions ranging from sensing and catalysis to drug delivery. Many of the reported examples, however, are highly symmetric architectures that contain one type of organic ligand carrying not more than a single functionality. This contrasts significantly with the level of structural and functional complexity encountered in biological macromolecular hosts, which are able to bind and chemically convert smaller molecules in their highly-decorated internal cavities. To address this disparity, rational approaches that facilitate heteroleptic assembly by regulating integrative self-sorting of metal ions and multiple ligand components have emerged. Among these, routes to access coordination cages from 'naked' metal cations that offer more than two coordination sites are still in early development, as the complexity of the self-sorted products in terms of composition and stereochemistry presents an entropic challenge. This feature article highlights recent progress in controlling integrative self-sorting of multi-component cage systems with a focus on structures composed of 'naked' metal cations and two different ligands. Once heteroleptic self-assembly strategies find a wider implementation in supramolecular design, the resultant interplay between tailored combinations of precisely positioned substituents promises enhanced functionality in nanoscale structures.

Metal-Ligand Exchange in a Cyclic Array: The Stepwise Advancement of Supramolecular Complexity

Herein, we demonstrate how the supramolecular complexity (evaluated by the degree of self-sorting M) evolves in a chemical cycle of cascaded metallosupramolecular transformations, using abiological self-assembled entities as input signals. Specifically, the successive addition of the supramolecular self-assembled structures S1 and (T2 + S2) to the starting supramolecular two-component equilateral triangle T1 (M = 1) first induced a fusion into the three-component quadrilateral R1 (M = 6) and then to the five-component scalene triangle T3 (M = 16). Upon the addition of the supramolecular input M1 to T3, a notable self-sorting event occurred, leading to regeneration of the triangle T1 along with formation of the scalene triangle T4 (M = 25). This last step closed the cycle of the supramolecular transformations.

Fully reversible three-state interconversion of metallosupramolecular architectures

The reversible switching of a sterically encumbered phenanthroline–Cu⁺–picolinaldehyde trio back and forth between homoleptic and heteroleptic coordination using the relative metal-ion to ligand ratio is the basis for an unprecedented cyclic three-state interconversion of metallacycles.

Copper(ii) complexes with phosphorylated 1,10-phenanthrolines: from molecules to infinite supramolecular arrays

Supramolecular architectures based on copper(ii) complexes with diethoxyphosphoryl substituted 1,10-phenanthrolines are reported.

Preparation of a porphyrinic bis(pyridyl aldehyde) and its supramolecular complexes

A linear porphyrinic bis(pyridyl aldehyde), in combination with a linear bisphenanthroline, exclusively forms a square, heteroleptic metallosupramolecular grid.

From self-sorted coordination libraries to networking nanoswitches for catalysis

This feature article sketches our long way from the development of dynamic heteroleptic coordination motifs to the self-sorting of multi-component libraries and finally the design of a new family of triangular nanomechanical switches, which are useful for ON–OFF control of catalysis and in bidirectional communication.

Metal-organic container molecules through subcomponent self-assembly

A variety of different three-dimensional metal-organic container molecules have recently been prepared using subcomponent self-assembly, which relies upon metal template effects to generate complex structures from simple molecular precursors and metal salts. Many of these structures have well defined internal pockets, allowing guest species to be bound and the chemical reactivity of these guests to be modified. Such host molecules have potential applications ranging from the protection of sensitive chemical species to the separation and purification of substrates as diverse as gases, gold compounds, and fullerenes.

A unique non-catenane interlocked self-assembled supramolecular architecture and its photophysical properties

A novel, interlocked, self-assembled (M(2)L(2))(2) molecular architecture was constructed from an arene-Ru acceptor and a 1,4-di(pyridin-4-yl)buta-1,3-diyne donor. Two M(2)L(2) units, with cavities of ~7.21 Å, spontaneously interlock, with one unit encapsulating a twin in a non-catenane fashion. The dimeric host-guest complex thus formed is unique among two-dimensional self-assemblies and is stabilized by π-π interactions between the M(2)L(2) units.

Designed post-self-assembly structural and functional modifications of a truncated tetrahedron

Post-self-assembly modifications of a discrete metal-organic supramolecular structure have been developed. Such modifications allow the properties of the self-assembled supramolecular species to be changed in a simple and efficient manner (>90% yield). Initiated by the application of chemical stimuli, the post-self-assembly modifications described herein result in three distinct changes to the supramolecular system: an individual building-block component change, an overall structural modification, and a functional evolution of a [6+4] metal-organic supramolecular structure. The three modifications have been carefully examined by a range of characterization methods, including NMR and UV-vis spectroscopy, electrospray ionization mass spectrometry, pulsed field gradient spin echo NMR measurements, electrochemical analysis, and computational simulations.