Palladium(II) driven self-assembly of a saturated quadruple-stranded metallo helicate

Chirality Interplay between the Interior and Exterior of Metal-Organic Cages

A series of metal-organic cages featuring two stereogenic elements, in terms of the twisting of amide moieties within the ligand backbones and the rotation of diazaanthracene segments along the ligand ridges, were exploited. These two chiral components are correlative and serve as relays for transmitting chirality information between the internal and external cages. The chirality information induced by a chiral guest inside the cage cavity can pass through the cage framework and influence the orientation of the diazaanthracene segments on the periphery of the cage. In turn, the chirality of a stereogenic center within the diazaanthracene segments can transfer back into the cavity, enabling discrimination of enantiomeric guests.

Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd2L4 metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 106 M-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.

Conjoined and non-conjoined coordination cages with palladium(II) vertices: structural diversity, solution dynamics, and intermolecular interactions

Self-assembled coordination complexes prepared from a combination of Pd(II) components with one or more types of high-symmetry or low-symmetry bis/tris/tetrakis-monodentate ligands are considered in this review. The structures of these complexes are viewed in terms of the presence of a metallo-macromonocycle or conjoined metallo-macromonocycles/metallocages in the frameworks. Analysis of the typical molecular structures revealed an open truth that one or more units of metallo-macromonocycles can be conjoined to afford planar or non-planar systems. In the same line, the enveloping surface of a 3D cage can be considered as a multiple number of conjoined metallomacrocycles that embrace a 3D space from all directions. However, two or more units of cages are conjoined in a multi-3D-cavity cage system and such a system is considered as a conjoined cage. Construction of such conjoined cages having a finite but multiple number of 3D-cavities unified in a single molecular architecture is a challenging task when compared to that of single-3D-cavity based compounds. Conjoining of as many as four units of 3D cages is known so far. Single- as well as multi-cavity cages of lower symmetry have become a very recent trend in this regard where low-symmetry ligands or mixed ligand ensembles are crafted in the framework of the cages. Other structural diversities like helicity in cages, and supramolecular isomerism are also included in this assorted literature work. Although isomerism in classical coordination complexes is well known, it is very less studied in self-assembled coordination complexes. Ligand isomerism is one such feature that is reviewed here. The dynamic behavior of the cages results in interesting reactivity aspects. A large variety of dynamic processes are collected under an umbrella, i.e., "ligand exchange reactions" and described with examples. Intermolecular interaction among the already self-assembled molecules is possible in solution, solid, and gel-phases as discussed in the last part of this review. The understanding of intermolecular interaction is likely to influence different areas of research including crystal engineering, and materials chemistry.

Comparison of [Pd2L4][BF4]4 cages for binding of n-octyl glycosides and nitrate (L = isophthalamide or dipicolinamide linked dipyridyl ligand)

Two dipyridyl ligands were synthesized, where the pyridyl donor fragments were separated by an isophthalamide (1) or a dipicolinamide moiety (2). Both ligands formed [Pd2(Ligand)4][BF4]4 complexes in CD2Cl2 containing 5% dmso-d6. It was found that while [Pd2(1)4][BF4]4 readily binds to n-octyl glycosides and to nitrate anions, [Pd2(2)4][BF4]4 did not. The difference in binding properties could be rationalized based on the reduced flexibility and size of the [Pd2(2)4]2+ cage and/or stronger interior binding of a BF4- counter anion.

Hierarchical communication of chirality for aromatic oligoamide sequences

The communication of chirality at a molecular and supramolecular level is the fundamental feature capable of transmitting and amplifying chirality information. Yet, the limitation of one-step communication mode in many artificial systems has precluded the ability of further processing the chirality information. Here, we report the chirality communication of aromatic oligoamide sequences within the interpenetrated helicate architecture in a hierarchical manner, specifically, the communication is manipulated by three sequential steps: (i) coordination, (ii) concentration, and (iii) ion stimulus. Such approach enables the information to be implemented progressively and reversibly to different levels. Furthermore, the chiral information on the side chains can be accumulated and transferred to the helical backbones of the sequences, resulting in that one of ten possible diastereoisomers of the interpenetrated helicate is finally selected. The circular dichroism experiments with a mixture of chiral and achiral ligands demonstrate a cooperative behavior of these communications, leading to amplification of chiral information.

Communication of chirality at a molecular level is the fundamental for transmitting chirality information but one-step communication modes in many artificial systems limits further processing the chirality information. Here, the authors report chirality communication of aromatic oligoamide sequences within interpenetrated helicate architecture in a hierarchical manner.

Coordination Self-Assembly Processes Revealed by Collaboration of Experiment and Theory: Toward Kinetic Control of Molecular Self-Assembly

The importance of the collaboration of experiment and theory has been proven in many examples in science and technology. Here, such a new example is shown in the investigation of molecular self-assembly process, which is a complicated multi-step chemical reaction occurring in the reaction network composed of a huge number of intermediates. An experimental method, QASAP (quantitative analysis of self-assembly process), developed by us and a numerical approach, NASAP (numerical analysis of self-assembly process), that analyzes the experimental data obtained by QASAP to draw detail molecular self-assembly pathways, which was also developed by us, are introduced, and their application to the investigation of Pd(II)-mediated coordination assemblies are presented. Further, the possibility of the prediction of the outcomes of molecular self-assembly by varying the reaction conditions is also demonstrated. Finally, a future direction in the field of artificial molecular self-assembly based on pathway-dependent self-assembly, that is, kinetic control of molecular self-assembly is discussed.

Balancing Ligand Flexibility versus Rigidity for the Stepwise Self-Assembly of M12 L24 via M6 L12 Metal-Organic Cages

Non-covalent interactions are important for directing protein folding across multiple intermediates and can even provide access to multiple stable structures with different properties and functions. Herein, we describe an approach for mimicking this behavior in the self-assembly of metal-organic cages. Two ligands, the bend angles of which are controlled by non-covalent interactions and one ligand lacking the above-mentioned interactions, were synthesized and used for self-assembly with Pd²⁺ . As these weak interactions are easily broken, the bend angles have a controlled flexibility giving access to M₂ (L1)₄ , M₆ (L2)₁₂ , and M₁₂ (L2)₂₄ cages. By controlling the self-assembly conditions this process can be directed in a stepwise fashion. Additionally, the multiple endohedral hydrogen-bonding sites on the ligand were found to play a role in the binding and discrimination of neutral guests.

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.

Helicity adaptation within a quadruply stranded helicate by encapsulation

The helicity of a quadruply stranded M2L4 helicate consisting of an aromatic amide bidendate ligand is flexible due to the twisting of the amide moieties and can be tuned by the encapsulated anions. This study reveals the multiple interplays and complementarities between the anions as well as between the anions and the helicate, which are synthetically responsive to the ultimate conformation of the helicate.

Selective Anion Recognition by a Dynamic Quadruple Helicate

An M₂ L₄ quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L₁ ) about two palladium(II) centers, is shown to bind anions within its internal cavity. ¹ H NMR exchange experiments provide a quantitative measure of anion selectivity and reveal a preference for ClO₄ ^- over the other tetrahedral anions BF₄ ^- and ReO₄ ^- and the octahedral anion PF₆ ^- . X-ray crystal structures of [Pd₂ (L₁ )₄ ]⁴⁺ helicates containing ClO₄ , BF₄ ^- and I^- reveal that the cavity size can dynamically change in response to the size of the guest.

Quantitative Analysis of Self-Assembly Process of a Pd2 L4 Cage Consisting of Rigid Ditopic Ligands

The self-assembly process of a Pd₂ L₄ cage complex consisting of rigid ditopic ligands, in which two 3-pyridyl groups are connected to a benzene ring through acetylene bonds and Pd^II ions was revealed by a recently developed quantitative analysis of self-assembly process (QASAP), with which the self-assembly process of coordination assemblies can be investigated by monitoring the evolution with time of the average composition of all the intermediates. QASAP revealed that the rate-determining steps of the cage formation are the intramolecular ligand exchanges in the final stage of the self-assembly: [Pd₂ L₄ Py*₂ ]⁴⁺ →[Pd₂ L₄ Py*₁ ]⁴⁺ +Py* and [Pd₂ L₄ Py*₁ ]⁴⁺ →[Pd₂ L₄ ]⁴⁺ +Py* (Py*: 3-chloropyridine, which was used as a leaving ligand on the metal source). The energy barriers for the two reactions were determined to be 22.3 and 21.9 kcal mol^-1 , respectively. DFT calculations of the transition-state (TS) structures for the two steps indicated that the distortion of the trigonal-bipyramidal Pd^II center at the TS geometries increases the activation free energy of the two steps.

Hydrophobic metallo-supramolecular Pd2L4 cages for zwitterionic guest encapsulation in organic solvents

Hydrophobic metallo-supramolecular cages are selectively encapsulating hydrophilic zwitterionic guests in organic solvents via synergetic multivalent recognition.

Synthesis of discrete Re(i) di- and tricarbonyl assemblies using a [4 × 1] directional bonding strategy

Discrete assembly of two Re(i) squares was achieved by a simple [4 × 1] strategy where the complexes, [Re(4-pytpy-κ²N)(CO)₃Br] and [Re(4-pytpy-κ³N)(CO)₂Br], act as their own ligands. The properties of the assemblies and their precursors are described along with solid-state X-ray diffraction studies.

Stoichiometrically controlled revocable self-assembled "spiro" versus quadruple-stranded "double-decker" type coordination cages

The simple combination of Pd(II) with the tris-monodentate ligand bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate, L, at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the "spiro-type" Pd1L2 macrocycle, 1, and the quadruple-stranded Pd3L4 cage, 2, respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.

A new structural motif for an enantiomerically pure metallosupramolecular Pd₄L₈ aggregate by anion templating

An enantiomerically pure BINOL-based bis(3-pyridyl) ligand 1 assembles into a homochiral [Pd4(1)8] complex upon coordination to tetravalent Pd(II) ions. The formation of this aggregate is templated by two tetrafluoroborate counterions that are encapsulated in two peripheral cavities. The resulting structure is a new structural motif for this kind of metallosupramolecular assemblies that arranges the palladium ions in a distorted tetrahedral fashion and forces ligand 1 to adopt two different conformations. Both phenomena are unique and cause an overall three-dimensional structure that has another confined, chiral, and hydrophilic central cavity.