Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host-guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.

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Increasing structural and functional complexity in self-assembled coordination cages.

Enantiopure acceptors (R,R)M and (S,S)M [where M = (N1,N1,N2,N2-tetramethylcyclohexane-1,2-diamine)Pd(NO3)2] have been used to design enantiopure Pd(II) tetrahedral cages. Self-assembly of [1,4-di(...

Self-Assembly of Enantiopure Pd12 Tetrahedral Homochiral Nanocages with Tetrazole Linkers and Chiral Recognition

It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, which limits their development toward functional materials. Herein, we report a system of interconverting bowls and cages that are able to respond to various selective stimuli (light, ligands, anions), based on the self-assembly of a photochromic dithienylethene (DTE) ligand, La, with PdII cations. By combining the concept of "coordination sphere engineering", relying on bulky quinoline donors, with reversible photoswitching between the ligand's open (o-La) and closed (c-La) forms, a [Pd2(o-La)4] cage (o-C) and a [Pd2(c-La)3] bowl (c-B) were obtained, respectively. This structural rearrangement modulates the system's guest uptake capabilities. Among three bis-sulfonate guests (G1, G2, and G3), the cage can encapsulate only the smallest (G1), while the bowl binds all of them. Bowl c-B was further used to synthesize a series of heteroleptic cages, [Pd2LA3LB], representing a motif never reported before. Additional ligands (Lc-f), with short or long arms, tune the cavity size, thus enabling or preventing guest uptake. Addition of Br-/Ag+ makes it possible to change the overall charge, again triggering guest uptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality and application of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity is achieved through cooperativity between only a few components.

Multi-stimuli Control over Assembly and Guest Binding in Metallo-supramolecular Hosts Based on Dithienylethene Photoswitches.

A strategy is presented that enables the quantitative assembly of a heterobimetallic [PdPtL4 ]4+ cage. The presence of two different metal ions (PdII and PtII ) with differing labilities enables the cage to be opened and closed selectively at one end upon treatment with suitable stimuli. Combining an inert PtII tetrapyridylaldehyde complex with a suitably substituted pyridylamine and PdII ions led to the assembly of the cage. 1 H and DOSY NMR spectroscopy and ESI mass spectrometry data were consistent with the quantitative formation of the cage, and the heterobimetallic structure was confirmed using single-crystal X-ray crystallography. The structure of the host-guest adduct with a 2,6-diaminoanthraquinone guest molecule was determined. Addition of N,N'-dimethylaminopyridine (DMAP) resulted in the formation of the open-cage [PtL4 ]2+ compound and [Pd(DMAP)4 ]2+ complex. This process could then be reversed, with the reformation of the cage, upon addition of p-toluenesulfonic acid (TsOH).

A Reduced‐Symmetry Heterobimetallic [PdPtL 4 ] 4+ Cage: Assembly, Guest Binding, and Stimulus‐Induced Switching

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as ‘glue’ to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Beyond Platonic: How to Build Metal–Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

Stimuli-responsive coordination cages allow reversible control over guest binding and release, relevant for adaptive receptors, carriers, catalysts, and complex systems. Light serves as an advantageous stimulus, as it can be applied with precise spatial and temporal resolution without producing chemical waste products. We report the first Pd-mediated coordination cage based on ligands embedding a diazocine photoswitch. While the thermodynamically more stable cis-photoisomer sloppily assembles to a mixture of species with general formula [Pdncis-L2n], the less stable trans-isomer yields a defined [Pd2trans-L4] cage that reversibly converts back to the cis-system by irradiation at 530 nm or thermal relaxation. The [Pdncis-L2n] species do not bind a given guest; however, [Pd2trans-L4] is able to encapsulate a bis-sulfonate as long as it is kept assembled, requiring continuous irradiation at 385 nm. In the absence of UV light, thermal relaxation results in back-switching and guest release. Assembly and properties of the system were characterized by a combination of NMR, ion mobility ESI-MS, single-crystal X-ray diffraction, and UV–vis absorption studies.

https://pubs.acs.org/doi/pdf/10.1021/jacs.1c12011

Light-Powered Dissipative Assembly of Diazocine Coordination Cages

A bent fluorenone-based dipyridyl ligand LA reacts with PdII cations to a solvent-dependent dynamic library of [PdnL2n] assemblies, constituted by a [Pd3LA6] ring and a [Pd4LA8] tetrahedron as major components, and a [Pd6LA12] octahedron as minor component. Introduction of backbone steric hindrance in ligand LB allows exclusive formation of the [Pd6LB12] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd4LA4LB4] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd4L8] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent LA is maintained in both homoleptic [Pd3LA6] and heteroleptic [Pd4LA4LB4].

Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cptc.201900038

Jacopo Tessarolo

Papers

Increasing structural and functional complexity in self-assembled coordination cages.

Multi-stimuli Control over Assembly and Guest Binding in Metallo-supramolecular Hosts Based on Dithienylethene Photoswitches.

Light-Powered Dissipative Assembly of Diazocine Coordination Cages

Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk

Successive Photoswitching and Derivatization Effects in Photochromic Dithienylethene‐Based Coordination Cages