Construction of Hetero-Four-Layered Tripalladium(II) Cyclophanes by Transannular π⋅⋅⋅π Interactions

Assembly of Six Types of Heteroleptic Pd2L4 Cages under Kinetic Control

Heteroleptic assemblies composed of several kinds of building blocks have been seen in nature. It is still unclear how natural systems design and create such complicated assemblies selectively. Past efforts on multicomponent self-assembly of artificial metal-organic cages have mainly focused on finding a suitable combination of building blocks to lead to a single multicomponent self-assembly as the thermodynamically most stable product. Here, we present another approach to selectively produce multicomponent Pd(II)-based self-assemblies under kinetic control based on the selective ligand exchanges of weak Pd-L coordination bonds retaining the original orientation of the metal centers in a kinetically stabilized cyclic structure and on local reversibility given in certain areas of the energy landscape in the presence of the assist molecule that facilitates error correction of coordination bonds. The kinetic approach enabled us to build all six types of Pd2L4 cages and heteroleptic tetranuclear cages composed of three kinds of ditopic ligands. Although the cage complexes thus obtained are metastable, they are stable for 1 month or more at room temperature.

Secondary Bracing Ligands Drive Heteroleptic Cuboctahedral PdII12 Cage Formation

The structural complexity of self-assembled metal-organic capsules can be increased by incorporating two or more different ligands into a single discrete product. Such complexity can be useful, by enabling larger, less-symmetrical, or more guests to be bound. Here we describe a rational design strategy for the use of subcomponent self-assembly to selectively prepare a heteroleptic cage with a large cavity volume (2631 ų) from simple, commercially available starting materials. Our strategy involves the initial isolation of a tris(iminopyridyl) Pd^II₃ complex 1, which reacts with tris(pyridyl)triazine ligand 2 to form a heteroleptic sandwich-like architecture 3. The tris(iminopyridyl) ligand within 3 serves as a "brace" to control the orientations of the labile coordination sites on the Pd^II centers. Self-assembly of 3 with additional 2 was thus directed to generate a large Pd^II₁₂ heteroleptic cuboctahedron host. This new cuboctahedron was observed to bind multiple polycyclic aromatic hydrocarbon guests simultaneously.

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd₁₂ L₂₄ ] and [Pd₂ L₄ ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd₂ L₄ ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd₂ (L^A )₂ (L^B )(L^C )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

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

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.

A cyclic manipulation of cage isomers via anion exchange and thermal isomerism

A cyclic manipulation of peanut cage isomers has been achieved via anion exchange and unusual cage isomerism.

Chiral Pd6L8 Nanocube Pairs: Recognition of Chiral Amino Acids via Electrochemistry

The self-assembly of PdX₂ (X^- = ClO₄^- and PF₆^-) with C₃-symmetric l- and d-L [L = (2S,2'S,2″S)- and (2R,2'R,2″R)-[benzenetricarbonyltris(azanediyl)]tris(3-phenylpropane-2,1-diyl)triisonicotinate] produces the chiral nanocube pair [Pd₆(l-L)₈](X)₁₂ and [Pd₆(d-L)₈](X)₁₂ (X^- = ClO₄^- and PF₆^-, respectively) with an inner cavity of 12.3 × 12.3 × 12.3 ų. These chiral nanocubes are effective for the enantiorecognition of various chiral amino acids via the square-wave-voltammetry technique. In the present study, the site of enantiorecognition was confirmed by density functional theory calculated interactions between each nanocube and the chiral amino acids, and the calculated interactions were coincident with the shifts of the electrochemical oxidation potentials.

Palladium(II)-Based Self-Assembled Heteroleptic Coordination Architectures: A Growing Family

Metal-driven self-assembly is one of the most effective approaches to lucidly design a large range of discrete 2D and 3D coordination architectures/complexes. Palladium(II)-based self-assembled coordination architectures are usually prepared by using suitable metal components, in either a partially protected form (PdL') or typical form (Pd; charges are not shown), and designed ligand components. The self-assembled molecules prepared by using a metal component and only one type of bi- or polydentate ligand (L) can be classified in the homoleptic series of complexes. On the other hand, the less explored heteroleptic series of complexes are obtained by using a metal component and at least two different types of non-chelating bi- or polydentate ligands (such as L^a and L^b ). Methods that allow the controlled generation of single, discrete heteroleptic complexes are less understood. A survey of palladium(II)-based self-assembled coordination cages that are heteroleptic has been made. This review article illustrates a systematic collection of such architectures and credible justification of their formation, along with reported functional aspects of the complexes. The collected heteroleptic assemblies are classified here into three sections: 1) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal; 2) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is different; and 3) [Pd_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal. Representative examples of some important homoleptic architectures are also provided, wherever possible, to set a background for a better understanding of the related heteroleptic versions. The purpose of this review is to pave the way for the construction of several unique heteroleptic coordination assemblies that might exhibit emergent supramolecular functions.

Coordination chemistry of flexible benzene-1,3,5-tricarboxamide derived carboxylates; notable structural resilience and vaguely familiar packing motifs

Flexible benzene-1,3,5-tricarboxamides (BTAs), organic species well-known for their tendencies to form functional soft-materials by virtue of their complementary hydrogen bonding, are explored as structurally reinforcing supramolecular building blocks in porous coordination polymers. We report the synthesis and characterisation of two related, carboxylate-terminated BTA derivatives, and the structure and functionality of their polymeric Cd(ii) complexes. The polycarboxylate ligand benzene-1,3,5-tricarboxamide tris(phenylacetic acid) H3L1 was prepared, and the analogous trimethyl benzene-1,3,5-tricarboxamide tris acetate Me3L2 was prepared and its single crystal structure elucidated. On reaction with cadmium nitrate in a DMF/H2O mixture, each BTA compound yielded coordination polymer species with columnar packing motifs comparable to the familiar BTA triple helix seen in purely organic systems. In the case of Me3L2, this transformation was achieved through a convenient in situ ester hydrolysis. Complex 1 is a 2-dimensional layered material containing tubular intralayer pores, in which amide-amide hydrogen bonding is a notable structural feature. In contrast, the structure of 2 contains no amide-amide hydrogen bonding, and instead a columnar arrangement of ligand species is linked by trinuclear Cd(ii) cluster nodes into a densely packed three-dimensional framework. The crystal structures revealed both materials exhibited significant solvent-accessible volume, and this was probed with thermal analysis and CO2 and N2 adsorption experiments; complex 2 showed negligible gas uptake, while compound 1 possesses an unusually high CO2 capacity for a two-dimensional material with intralayer porosity and surprising structural resilience to guest exchange, evacuation and exposure to air.

A Truncated Molecular Star

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd₅ (L1)₅ (L2)₅ ](BF₄ )₁₀ , 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)₄ ](BF₄ )₂ with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4'-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a "molecular star" owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

A sandwich-shaped M3L2 zinc(ii) complex containing 1,3,5-tris(dimethyl(pyridin-3-yl)silyl)benzene: selective photoluminescence recognition of diiodomethane

Self-assembly of Zn(ClO₄)₂ with 1,3,5-tris(dimethyl(pyridin-3-yl)silyl)benzene (L) as a new C₃-symmetric tridentate N-donor gives rise to a discrete sandwich-shaped M₃L₂ architecture, [Zn₃(μ-OH)₃L₂](ClO₄)₃·4CH₃CN·2H₂O. Its blue photoluminescence is significantly quenched only by CH₂I₂ among the various small molecules, CH₂Cl₂, CH₂Br₂, CHCl₃, 1,2-dichloroethane, EtOH, CH₃CN, benzene, toluene, and phenol.

Recyclable scavengers for photo-cyclopropanation via an in situ crystallization process

The palladium(ii) cyclophane systems, constructed by previously reported proof-of-concept self-assembly, represent a crucial landmark in the field of effective and recyclable scavenging of triiodide (I₃^-) in the photo-cyclopropanation of alkenes with CH₂I₂. The scavenger's driving force behind photo-cyclopropanation is the efficient in situ crystallization of triiodide-exchanged species. The exact quantitative photoreaction yields according to the mole ratios of the cyclophane system are impressive. The recycling behavior can be ascribed to the rigidity and stability of the four-layered tripalladium(ii)cyclophane.