Guest-modulated circularly polarized luminescence by ligand-to-ligand chirality transfer in heteroleptic Pd(II) coordination cages

Multicomponent metallo-supramolecular assembly allows to rationally combine different building blocks. Discrete multifunctional hosts with an accessible cavity can be prepared in a non-statistical fashion. We employ our shape-complementary assembly (SCA) method to achieve for the first time integrative self-sorting of heteroleptic PdII cages showing guest-tunable circularly polarized luminescence (CPL). An enantiopure helicene-based ligand (M or P configuration) is coupled with a non-chiral emissive fluorenone-based ligand (A or B) to form a series of Pd2L2L’2 assemblies. The modular strategy allows to impart the chiral information of the helicenes to the overall supramolecular system, resulting in CPL from the non-chiral component. Guest binding results in a 4-fold increase of CPL intensity. The principle offers potential to generate libraries of multifunctional materials with applications in molecular recognition, enantioselective photo-redox catalysis and information processing

Multi-stimuli control over assembly and guest binding in metallo-supramolecular hosts based on dithienylethene photoswitches

It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, whichlimits their development toward functional materials. Herein, wereport a system of interconverting bowls and cages that are able torespond to various selective stimuli (light, ligands, anions), basedon the self-assembly of a photochromic dithienylethene (DTE)ligand,La, with PdIIcations. By combining the concept of“coordination sphere engineering”, relying on bulky quinolinedonors, 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 structuralrearrangement modulates the system’s guest uptake capabilities.Among three bis-sulfonate guests (G1,G2, andG3), the cage canencapsulate only the smallest (G1), while the bowl binds all of them. Bowlc-Bwas further used to synthesize a series of heterolepticcages, [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 guestuptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality andapplication of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity isachieved through cooperativity between only a few components

Integrative assembly of heteroleptic tetrahedra controlled by backbone steric bulk

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]

Tunable fullerene affinity of cages, bowls and rings assembled by Pd(II) coordination sphere engineering

For metal‐mediated host compounds, the development of strategies to reduce symmetry and introduce multiple functionalities in a non‐statistical way is a challenging task. We show that the introduction of steric stress around the coordination environment of square‐planar Pd(II) cations and bis‐monodentate nitrogen donor ligands allows to control the size and shape of the assembly product, from [Pd2L4] cages over [Pd2L3] bowl‐shaped structures to [Pd2L2] rings. Therefore, banana‐shaped ligand backbones were equipped with pyridines, two different quinoline isomers and acridine, the latter three introducing steric congestion through hydrogen substituents on annelated benzene rings. Differing behavior of the four resulting hosts towards the binding of C60 and C70 fullerenes was studied and related to structural differences by NMR spectroscopy, mass spectrometry and single crystal X‐ray diffraction. The three cages based on pyridine, 6‐quinoline or 3‐quinoline donors were found to either bind C60, C70 or no fullerene at all

Modular enhancement of circularly polarized luminescence in Pd2A2B2 heteroleptic cages

Metal-mediated assembly allows us to combine an achiral emissive ligand A with different chiral ligands (such as B) in a non-statistical fashion, obtaining Pd2A2B2 heteroleptic cages showing circularly polarized luminescence (CPL). By using the ‘shape complementary assembly’ (SCA) strategy, the cages are exclusively obtained as cis-Pd2A2B2 stereoisomers, as confirmed by NMR, MS and DFT analyses. Their unique chiroptical properties derive from the synergy of all the building blocks. Ligand B imparts the chiral information of its aliphatic backbone, comprising two stereogenic sp3 carbon centres, to the overall structure, causing CD and CPL signal induction for the chromophore on ligand A. The heteroleptic cage shows CPL with a |glum| value of 2.5 × 10−3, which is 3-times higher than that for a progenitor based on aromatic helical building block H, thus opening a rational route towards optimizing the CPL properties of self-assembled nanostructures in a modular way

Successive photoswitching and derivatization effects in photochromic dithienylethene-based coordination cages

A new series of [Pd2(L)4] cages based on photochromic dithienylethene (DTE) ligands allowed us to gain insight into the successive photoswitching of multiple DTE moieties in a confined metallo‐supramolecular assembly. Three new X‐ray structures of [Pd2(o‐L4)4], [Pd2(o‐L1)2(c‐L1)2] and [Pd2(c‐L1)4] (o‐L and c‐L = open and closed forms of DTE ligands, respectively) were obtained. The structures deliver snapshots of three different combinations of DTE photoisomeric states within the cage, facilitating a comparison of the all‐open with the all‐closed, and most notably, an intermediate form where open and closed switches co‐exist in the same cage. Moreover, a series of spherical anionic borate clusters was introduced in order to study their roles in the light‐controllable host–guest chemistry. The binding guests show higher affinities with the flexible open cage [Pd2(o‐L1)4] than with the rigid closed cage [Pd2(c‐L1)4]. For the [B12F12]2− guest, thermodynamic data obtained from NMR experiments was compared to results from isothermal titration calorimetry (ITC)

A family of heterobimetallic cubes shows spin-crossover behaviour near room temperature

Using 4-(4′-pyridyl)aniline as a simple organic building block in combination with three different aldehyde components together with metal(II) salts gave three different Fe8Pt6-cubes and their corresponding Zn8Pt6 analogues by employing the subcomponent self-assembly approach. Whereas the use of zinc(II) salts gave rise to diamagnetic cages, iron(II) salts yielded metallosupramolecular cages that show spin-crossover behaviour in solution. The spin-transition temperature T1/2 depends on the incorporated aldehyde component, giving a construction kit for the deliberate synthesis of spin-crossover compounds with tailored transition properties. Incorporation of 4-thiazolecarbaldehyde or N-methyl-2-imidazole-carbaldehyde yielded cages that undergo spin-crossover around room temperature whereas the cage obtained using 1H-4-imidazolecarbaldehyde shows a spin-transition at low temperatures. Three new structures were characterized by synchrotron X-ray diffraction and all structures were characterized by mass spectrometry, NMR and UV/Vis spectroscopy

Symmetry-breaking host–guest assembly in a hydrogen-bonded supramolecular system

Bio-inspired self-assembly is invaluable to create well-defined giant structures from small molecular units. Owing to a large entropy loss in the self-assembly process, highly symmetric structures are typically obtained as thermodynamic products while formation of low symmetric assemblies is still challenging. In this study, we report the symmetry-breaking self-assembly of a defined C1-symmetric supramolecular structure from an Oh-symmetric hydrogen-bonded resorcin[4]arene capsule and C2-symmetric cationic bis-cyclometalated Ir complexes, carrying sterically demanding tertiary butyl (tBu) groups, on the basis of synergistic effects of weak binding forces. The flexible capsule framework shows a large structural change upon guest binding to form a distorted resorcin[4]arene hexameric capsule, providing an asymmetric cavity. Location of the chiral guest inside the anisotropic environment leads to modulation of its Electric Dipole (ED) and Magnetic Dipole (MD) transition moments in the excited state, causing an increased emission quantum yield, longer emission lifetime, and enhancement of the dissymmetry factor (glum) in the circularly polarized luminescence

Increasing structural and functional complexity in self-assembled coordination cages

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

Post-assembly guest oxidation in a metallo-supramolecular host and structural rearrangement to a coordination polymer

none4norestrictedRancan, Marzio; Tessarolo, Jacopo; Quici, Silvio; Armelao, LidiaRancan, Marzio; Tessarolo, Jacopo; Quici, Silvio; Armelao, Lidi