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

Substrate and product binding inside a stimuli-responsive coordination cage acting as singlet oxygen photosensitizer

An acridone-based, interpenetrated double cage [3BF4Pd4L8] acts as a photosensitizer for generating singlet oxygen which adds to 1,3-cyclohexadiene in a [2+4] hetero-Diels-Alder reaction to form 2,3-dioxabicyclo[2.2.2]oct-5-ene. Photocatalytic activity was exclusively observed for the assembled cage, whereas the free organic ligand L decomposes upon irradiation. While cage [3BF4Pd4L8] does not accept any organic guests, NMR, MS and single crystal X-ray results reveal that both substrate and product are readily encapsulated in the central pocket of its chloride-activated form [2Cl@Pd4L8]. The system combines multiple functions (photosensitization, allosteric activation and guest uptake) within a structurally complex, mechanically-bound self-assembly built up from a simple and readily accessible ligand

Backbone-bridging promotes diversity in heteroleptic cages

The combination of shape-complementary bis-monodentate ligands LA and LB with PdII cations yields heteroleptic cages cis-[Pd2LA2LB2] by self-sorting. Herein, we report how such assemblies can be diversified by introduction of covalent backbone bridges between two LA units. Together with solvent and guest effects, the flexibility of these linkers can modulate nuclearity, topology, and number of cavities in a family of four structurally diverse assemblies. Ligand LA1, with flexible linker, reacts in CH3CN with its LB counterpart to a tetranuclear dimer D1. In DMSO, however, a trinuclear pseudo-tetrahedron T1 is formed. The product of LA2, with rigid linker, looks similar to D1, but with a rotated ligand arrangement. In presence of an anionic guest, this dimer D2 transforms and a hexanuclear prismatic barrel P2 crystallizes. We demonstrate how controlling a ligand's coordination mode can trigger structural differentiation and increase complexity in metallo-supramolecular assembly

Hierarchical assembly of an interlocked M8L16 container

The self‐assembly of eight PdII cations and sixteen phenanthrene‐derived bridging ligands with 60° bite angles yielded a novel M8L16 metallosupramolecular architecture composed of two interlocked D4h‐symmetric barrel‐shaped containers. Mass spectrometry, NMR spectroscopy, and X‐ray analysis revealed this self‐assembled structure to be a very large “Hopf link” catenane featuring channel‐like cavities, which are occupied by NO3− anions. The importance of the anions as catenation templates became imminent when we observed the nitrate‐triggered structural rearrangement of a mixture of M3L6 and M4L8 assemblies formed in the presence of BF4− anions into the same interlocked molecule. Furthermore, the densely packed structure of the M8L16 catenane was exploited in the preparation of a hexyloxy‐functionalized analogue, which further self‐assembled into vesicle‐like aggregates in a reversible manner

Morphological control of heteroleptic cis- and trans-Pd2L2Lʹ2 cages

Control over the integrative self-sorting of metallo-supramolecular assemblies has recently generated significant interest as it opens up possibilities of introducing increased complexity and function into a single self-assembled architecture. Here, the relationship between the geometry of three ligand components and morphology of three self-sorted heteroleptic [Pd2L2Lʹ2]4+ cages is examined. Pd-mediated assembly of two bis-monodentate pyridyl ligands with native bite angles of 75° and 120° affords a cis-[Pd2L2Lʹ2]4+ cage while the same reaction with two ligands bearing bite angles of 75° and 60° forces an anti-conformation for latter ligand, leading to an unprecedented, self-penetrating structural motif; a trans-[Pd2(anti-L)2Lʹ2]4+ heteroleptic cage bearing a “doubly-bridged figure-eight” topology. Each heteroleptic assembly can be accessed through cage-to-cage conversion of their homoleptic precursors, and notably, morphological control of [Pd2L2Lʹ2] cages is achieved through selective ligand displacement transformations in a system of three ligands and at least six possible cage products

Coal-tar dye-based coordination cages and helicates

A strategy to implement four members of the classic coal-tar dye family, Michler´s ketone, methylene blue, rhodamine B and crystal violet, into [Pd2L4] self-assemblies is introduced. Chromophores were incorporated into bis-monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern-shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd2L4] helicates in solution. In one case, single crystal X-ray diffraction revealed rearrangement to a [Pd3L6] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self-assemblies based on this new ligand class promise application in chiroptical recognition, photo-redox catalysis and optical materials

Mechanistic Interplay between Light Switching and Guest Binding in Photochromic [Pd 2 Dithienylethene 4 ] Coordination Cages

Photochromic [Pd2L4] coordination cages based on dithienylethene (DTE) ligands L allow triggering guest uptake and release by irradiation with light of different wavelengths. The process involves four consecutive electrocyclic reactions to convert all chromophores between their open and closed photoisomeric forms. So far, guest affinity of the fully switched species was elucidated, but mechanistic details concerning the intermediate steps remained elusive. Now, a new member of the DTE cage family allows unprecedented insight into the interplay between photoisomerization steps and guest location inside/outside the cavity. Therefore, the intrinsic chirality of the DTE backbones was used as reporter for monitoring the fate of a chiral guest. In its "open" photoisomeric form (o-L, [Pd2(o-L)4] = o-C), the C2-symmetric DTE chromophore quickly converts between energetically degenerate P and M helical conformations. After binding homochiral 1R-(-) or 1S-(+) camphor sulfonate (R-CSA or S-CSA), guest-to-host chirality transfer was observed via a circular dichroism (CD) signal for the cage-centered absorption. Irradiating the R/S-CSA@o-C host-guest complexes at 313 nm produced configurationally stable "closed" photoisomers, thus locking the induced chirality with an enantiomeric excess close to 25%. This value (corresponding to chiral induction for one out of four ligands), together with DOSY NMR, ion mobility mass spectrometry, and X-ray structure results, shows that closure of the first photoswitch is sufficient to expel the guest from the cavity

Photoinduced host-to-guest electron transfer in a self-assembled coordination cage

A [Pd2L4] coordination cage, assembled from electron-rich phenothiazine-based ligands and encapsulating an electron-deficient anthraquinone-based disulfonate guest, is reported. Upon excitation at 400 nm, transient absorption spectroscopy unveils photoinduced electron transfer from the host's chromophores to the guest, as indicated by characteristic spectral features assigned to the oxidized donor and reduced acceptor. The structure of the host–guest complex was characterized by NMR spectroscopy, mass spectrometry and single-crystal X-ray analysis. Spectroelectrochemical experiments and DFT calculations both agree with the proposed light-induced charge separation. A kinetic analysis of the involved charge transfer channels reveals, besides a guest-independent LMCT path, 44% efficiency for the host–guest charge transfer (HGCT)

A new mechanically-interlocked [Pd2L4] cage motif by dimerization of two peptide-based lemniscates

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed “lemniscates”. Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents

Ferrocene derivatives of liquid chiral molecules allow assignment of absolute configuration by X-ray crystallography

The present study investigates a synthetically simple ferrocene derivatization of natural products and active pharmaceutical ingredients. Seven new crystal structures are analyzed together with 16 structures of ferrocene derivatives reported previously. In all cases, the unambiguous determination of the absolute structure was established from anomalous dispersion using the methods of Flack and Parsons. A comparison with other derivatization approaches shows the advantage of the described ferrocene derivatization for establishing the absolute configuration of novel compounds