Palladium-Templated Subcomponent Self-Assembly of Macrocycles, Catenanes, and Rotaxanes

Advancements and strategic approaches in catenane synthesis

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

A molecular sheaf: doubly threaded [6]rotaxane

We report that the use of a hydrogen-bonded pyrimidine-macrocycle complex can efficiently facilitate the threading of two bispyridinium ethylenes into four rings, as evidenced by X-ray crystallography of its precursor, offering a rare example of a doubly threaded [6]rotaxane in 91% yield. The unusual architecture is found to be stable with no dethreading despite the large ring size of the macrocycle with respect to the stopper.

Segmented Template-Directed Self-Assembly of Giant Truncated Triangular Supramolecules

The template-directed strategy has been extensively employed for the construction of supramolecular architectures. However, with the increase in the size and complexity of these structures, the synthesis difficulty of the templates escalates exponentially, thereby impeding the widespread application of this strategy. In this study, two truncated triangles T1 and T2 were successfully self-assembled through a novel segmented template strategy by segmenting the core triangular template into portions. Two metallo-organic ligands L2 and L3 were designed and synthesized by dividing the central stable triangle into three separate parts and incorporating them into the precursor ligands, which served as templates to guide the self-assembly process with ligands L1 and L4, respectively. The assembled structures were unambiguously characterized by multidimensional and multinuclear NMR (1H, COSY, NOESY), multidimensional mass spectrometry analysis (ESI-MS and TWIM-MS), and transmission electron microscopy (TEM). Moreover, we observed the formation of fiberlike nanotubes from single-molecule triangles by hierarchical self-assembly.

Iminopyrrole-Based Self-Assembly: A Route to Intrinsically Flexible Molecular Links and Knots

The use of 2,5-diformylpyrrole in self-assembly reactions with diamines and Zn(II)/Cd(II) salts allowed the preparation of [2]catenane, trefoil knot, and Borromean rings. The intrinsically dynamic nature of the diiminopyrrole motif rendered all of the formed assemblies intramolecularly flexible. The presence of diiminopyrrole revealed new coordination motifs and influenced the host-guest chemistry of the systems, as illustrated by hexafluorophosphate encapsulation by Borromean rings.

DFT Study of Imine-Exchange Reactions in Iron(II)-Coordinated Pincers

The imine bond is among the most applied motifs in dynamic covalent chemistry. Although its uses are varied and often involve coordination to a transition metal for stability, mechanistic studies on imine exchange reactions so far have not included metal coordination. Herein, we investigated the condensation and transimination reactions of an Fe2+ -coordinated diimine pyridine pincer, employing wB97XD/6-311G(2d,2p) DFT calculations in acetonitrile. We first experimentally confirmed that Fe2+ is strongly coordinated by these pincers, and is thus a justified model ion. When considering a four-membered ring-shaped transition state for proton transfers, the required activation energies for condensation and transimination reaction exceeded the values expected for reactions known to be spontaneous at room temperature. The nature of the incoming and exiting amines and the substituents on the para-position of the pincer had no effect on this. Replacing Fe2+ with Zn2+ or removing it altogether did not reduce it either. However, the addition of two ethylamine molecules lowered the energy barriers to be compatible with experiment (19.4 and 23.2 kcal/mol for condensation and transimination, respectively). Lastly, the energy barrier of condensation of a non-coordinated pincer was significantly higher than found for Fe2+ -coordinating pincers, underlining the catalyzing effect of metal coordination on imine exchange reactions.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Ruthenium macrocycles bearing pyridine bis(carboxamide): synthesis, structure, and catalytic activity for hydrosilylation

Ruthenium complexes Ru(MC33)(CO)n(L)2−n (L = H2O, PPh3, P(OEt)3; n = 1, 2) with a pincer-type macrocyclic ligand MC33 with a cavity were synthesized and characterized.

Coordination-Directed Construction of Molecular Links

Since the emergence of the concept of chemical topology, interlocked molecular assemblies have graduated from academic curiosities and poorly defined species to become synthetic realities. Coordination-directed synthesis provides powerful, diverse, and increasingly sophisticated protocols for accessing interlocked molecules. Originally, metal ions were employed solely as templates to gather and position building blocks in entwined or threaded arrangements. Recently, metal centers have increasingly featured within the backbones of the integral structural elements, which in turn use noncovalent interactions to self-assemble into intricate topologies. By outlining ingenious recent examples as well as seminal classic cases, this Review focuses on the role of metal-ligand paradigms in assembling molecular links. In addition, the ever-evolving approaches to efficient assembly, the structural features of the resulting architectures, and their prospects for the future are also presented.

Subcomponent Self-Assembly of Covalent Metallacycles Templated by Catalytically Active Seven-Coordinate Transition Metal Centers

Coordination geometries of transition metals play vital roles in the self-assembly process of supramolecular coordination complexes. Herein, seven-coordinate 3d metal ions were applied as templates and catalytically active sites for subcomponent self-assembly that resulted in a new category of covalent metallacycles. Single-crystal structures showed that the sizes, configurations, and functionalization of covalent metallacycles could be tuned by the selection of rigid dihydrazide, transition metal ions, and prefunctionalized subcomponents, respectively. Moreover, metallacycles decorated with carboxylic groups could be employed as precursors to prepare aerogels through hierarchical self-assembly, which also exhibited high catalytic activity for cycloaddition of CO₂ into cyclic carbonates.

Template-Free Synthesis of a Macrocyclic Bis(pyridine-dienamine) Proligand and Metal Complexes of Its Bis(pyridine-diimine) and Bis(pyridine-dienamido) Forms

We describe the template-free synthesis of the bis(pyridine-dienamine) proligand [4,5-(m-xylylenediamine)NH-C═(CH)(9-butyl-octahydroacridine)]₂ (2'), a variant of Burrows's macrocyclic bis(pyridine-diimine) (bis-PDI) ligand [2,6-(m-xylylenediamine)N═C(py)]₂ (A), using octahydroacridine as the ligand backbone. The octahydroacridine backbone favors macrocyclization by constraining the PDI units in the (s-cis)₂ conformation. The template-free synthesis of 2' enables facile access to a wide array of bis-PDI and bis(pyridine-dienamido) (bis-PDE) metal complexes. Five-coordinate binuclear bis-PDI (2)M₂Cl₄ complexes {2 = [4,5-(m-xylylenediamine)N═C(9-butyl-octahydroacridine)]₂; M = Zn, Co, or Fe} and a four-coordinate bis-PDI [(2)Pd₂Br₂][B(3,5-(CF₃)₂-Ph)₄]₂ complex were synthesized and characterized. (2)Zn₂Cl₄ undergoes macrocyclic ring inversion on the nuclear magnetic resonance (NMR) time scale with a free energy barrier ΔG^⧧ of 15.5(3) kcal/mol at 295 K. In contrast, (2)Fe₂Cl₄ and (2)Co₂Cl₄ undergo slow ring inversion on the NMR chemical shift time scale at 295 K. The amine elimination reaction of 2' with Zr(NMe₂)₄ yields the bis-PDE complex (2'-4H)Zr₂(NMe₂)₄, which was alkylated with AlMe₃ and Al(CH₂SiMe₃)₃ to generate (2'-4H)Zr₂Me₄ and (2'-4H)Zr₂(CH₂SiMe₃)₂(NMe₂)₂, respectively.

Molecular recognition of planar and non-planar aromatic hydrocarbons through multipoint Ag-π bonding in a dinuclear metallo-macrocycle

Exploration of a novel structural motif of host-guest interactions is one of the most fundamental topics to develop macrocycle-based host-guest/supramolecular systems. Herein, we present an unprecedented mode of inclusion of aromatic hydrocarbons into a macrocyclic cavity via multipoint Ag-π bonding as a driving force. A dinuclear Ag^I-macrocycle encapsulated one molecule of anthracene, a typical planar aromatic hydrocarbon, in solution and in the solid state. Single-crystal X-ray diffraction analysis of the host-guest inclusion complex revealed the binding of anthracene via multipoint Ag-π bonding to both Ag^I ions arranged within the open-ended nano-cavity of the dinuclear Ag^I-macrocycle. Notably, this binding motif based on Ag-π bonding was also applied to the inclusion of triptycene, a non-planar aromatic hydrocarbon with a steric tripodal structure, to evaluate the rotational motion of the molecular paddle-wheel in the Ag^I-macrocycle.

Metal and Organic Templates Together Control the Size of Covalent Macrocycles and Cages

Covalent macrocycles and three-dimensional cages were prepared by the self-assembly of di- or tritopic anilines and 2,6-diformylpyridine subcomponents around palladium(II) templates. The resulting 2,6-bis(imino)pyridyl-Pd^II motif contains a tridentate ligand, leaving a free coordination site on the Pd^II centers, which points inward. The binding of ligands to the free coordination sites in these assemblies was found to alter the product stability, and multitopic ligands could be used to control product size. Multitopic ligands also bridged metallomacrocycles to form higher-order supramolecular assemblies, which were characterized via NMR spectroscopy, mass spectrometry, and X-ray crystallography. An efficient method was developed to reduce the imine bonds to secondary amines, leading to fully organic covalent macrocycles and cages that were inaccessible through other means.

Construction and Photoluminescent Properties of Schiff-Base Macrocyclic Mono-/Di-/Trinuclear ZnII Complexes with the Common 2-Ethylthiophene Pendant Arm

A new flexible 2-ethylthiophene pendant-armed dialdehyde (H₂tdd) was reacted with 1,3-propanediamine, [( S, S),( R, R),(±)]-1,2-diaminocyclohexane, and 1,2-bis(2-aminoethoxy)ethane, giving birth to 36-membered [2 + 2] Schiff-base macrocyclic trinuclear Zn^II complex 1, 18-membered [1 + 1] mononuclear Zn^II macrocycles 2-4, chiral/racemic 34-membered [2 + 2] dinuclear Zn^II complexes 5-9, and 46-membered [2 + 2] dinuclear Zn^II macrocycles 10-12 via Zn^II ion template-assisted Schiff-base condensation. It is worth mentioning that the secondary template effects for nitrate and halide counterions have been observed in the 1,3-propanediamine involved imine condensation. In all [2 + 2] Zn^II macrocycles, dinuclear complexes 5-9 display a full-folded molecular conformation, while trinuclear complex 1 and dinuclear complexes 10-12 exhibit distinct half-folded structures in the presence or absence of intramolecular π-π stacking interactions between two phenolic rings of the dialdehyde component. Interestingly, a solvent-induced single-crystal-to-single-crystal transformation was first achieved for two types of solvated mononuclear macrocycles 3a and 3b (H₂O vs CH₃CN) with folded and unfolded ligand conformations. In addition, the photoluminescent properties were studied for this family of Schiff-base macrocyclic Zn^II complexes as well as the dialdehyde precursor H₂tdd.

Cu(ii) templated formation of [n]pseudorotaxanes (n = 2, 3, 4) using a tris-amino ether macrocyclic wheel and multidentate axles

The systematic development of mono-, bi- and tri-nuclear [n]pseudorotaxanes (n = 2, 3, 4) via Cu(ii) templation and π–π stacking interactions.

Naphthalene containing amino-ether macrocycle based Cu(ii) templated [2]pseudorotaxanes and OFF/ON fluorescence switching via axle substitution

A new naphthalene containing macrocycle, NaphMC, and a new fluorophoric bidentate linear axle derivative of 5,5'-dimethyl-2,2'-bipyridine (L3) along with two other ligands 1,10-phenanthroline (L1) and 5,5'-dimethyl-2,2'-bipyridine (L2) are explored towards the synthesis of Cu(ii) templated [2]pseudorotaxanes. All ternary complexes are well characterized by ESI-MS, UV/Vis, EPR spectroscopy, elemental analysis and emission spectroscopic studies. Single crystal X-ray diffraction studies confirm the geometry around the Cu(ii) center as a distorted trigonal bipyramid via the contribution of [3 + 2] orthogonal motifs of the wheel (NaphMC) and the bidentate chelating ligands L1 and L2 in the cases of pseudorotaxanes, CuPR1 and CuPR2, respectively. Furthermore, the fluorescence "OFF" state of the fluorophoric axle L3 is achieved via threading it to the Cu(ii) complex of NaphMC, whereas fluorescence switching "ON" is demonstrated by the substitution of L3 of CuPR3 with a stronger chelating ligand L1.

Coordination-Driven Self-Assembly of a Molecular Knot Comprising Sixteen Crossings

Molecular knots have become highly attractive to chemists because of their prospective properties in mimicking biomolecules and machines. Only a few examples of molecular knots from the billions tabulated by mathematicians have been realized and molecular knots with more than eight crossings have not been reported to date. We report here the coordination-driven [8+8] self-assembly of a higher-generation molecular knot comprising as many as sixteen crossings. Its solid-state X-ray crystal structure and multinuclear 2D NMR findings confirmed its architecture and topology. The formation of this molecular knot appears to depend on the functionalities and geometries of donor and acceptor in terms of generating appropriate angles and strong π-π interactions supported by hydrophobic effects. This study shows coordination-driven self-assembly offers a powerful potential means of synthesizing more and more complicated molecular knots and of understanding differences between the properties of knotted and unknotted structures.

Metal ions in the synthesis of interlocked molecules and materials

The use of metal ions to template the synthesis of catenanes by Sauvage and co-workers was a pivotal moment in the development of the field of interlocked molecules. In this Review Article we shall examine the different roles metal-ligand interactions play in modern syntheses of interlocked molecules and materials, with a particular focus on seminal contributions and the advantages and disadvantages of employing metal ligand interactions.

Solvent- and anion-induced interconversions of metal-organic cages

Fully structural interconversions between monomeric Pd24 and interlocked dimeric Pd48 cages have been investigated to elucidate their thermodynamic stability defined by their anion-guest binding behaviours.

Ion-pair coordination driven stimuli-responsive one-dimensional supramolecular helicate

A new self-assembled ion-pair coordination driven one-dimensional (1D) smart supramolecular helical assembly is reported. Moreover, thermo- and chemo-responsive transformation/disassembly/reassembly of the helical superstructure was also demonstrated.

Five-State Rotary Nanoswitch

In our quest to develop artificial multistate devices, we synthesized the nanomechanical switch 1 that is characterized by a tetrahedral core equipped with four pending arms. The rotary arm with its azaterpyridine terminal is intramolecularly coordinated to a zinc(II) porphyrin station that is the terminus of another arm in 1. The two other arms carry identical sterically shielded phenanthroline stations. The 2-fold alternate addition of a copper(I) ion and [1,10]-phenanthroline (1 equiv each) results in the formation of five different switching states (State I→ State II→ State III→ State IV→ State V → State I), which force the toggling arm to move back and forth between the zinc(II) porphyrin and phenanthroline stations separated by a distance of 25 Å. All switching states constitute clean single species, except for State III, and thus are fully characterized by spectroscopic methods and elemental analysis. Finally, the initial state of nanoswitch was reset by addition of cyclam for complete removal of the copper(I) ions.

Solvothermal Subcomponent Self-Assembly of Cubic Metal-Imidazolate Cages and Their Coordination Polymers

A series of Ni-imidazolate cubic cages, one-dimensional and two-dimensional coordination polymers based on the cubic cages, have been prepared by solvothermal subcomponent self-assembly of 5-methyl-4-formylimidazole, m-xylylenediamine, and Ni(II) salts with varied anions. These compounds have been characterized by single-crystal X-ray diffractions, elemental analysis, IR spectra, and powder X-ray diffractions. The formation of an oligomerized coordination cage or an infinite coordination polymer depends on the anions chosen. An oligomerized 8-nuclear Ni-imidazolate cubic cage is formed when the anion Cl(-), Br(-), I(-), SCN(-), NO2(-), or NO3(-) is utilized in the reactions, and a two-dimensional coordination polymer based on the Ni-imidazolate cubic cage will be obtained when N3(-), (CN)2N(-), or (CN)3C(-) act as the anions. When only ClO4(-) or both ClO4(-) and [Ni(C4N2S2)2](2-) (C4N2S2 = dimercaptomaleonitrile) as anions exist in the reaction mixture, a ladder-like one-dimensional coordination polymer based on the Ni-imidazolate cubic cage and formate is formed unpredictably.

Amino-ether macrocycle that forms CuII templated threaded heteroleptic complexes: a detailed selectivity, structural and theoretical investigations

Self-sorting behavior of a newly synthesized macrocycle with divalent metal ions and aromatic ligands via pseudorotaxane formation has been described.

Sugar-boronate ester scaffold tethered pyridyl-imine palladium(ii) complexes: synthesis and their in vitro anticancer evaluation

The anticancer activity of sugar-boronate ester containing palladium(ii) complexes is reported.

Multipoint recognition of ditopic aromatic guest molecules via Ag-π interactions within a dimetal macrocycle

A macrocyclic host molecule possessing a nanocavity with two Ag(I) centers for guest binding and four anthracene walls has been developed. This dimetal-macrocycle forms stable inclusion complexes with ditopic aromatic guest molecules, [2.2]paracyclophane, and ferrocene, in solution and/or in the solid state through Ag-π interactions within the nanocavity. The binding constants for the inclusion complexes were found to range roughly from 10(4) to 10(9) M(-1). Electrochemical measurement revealed that the oxidized form of the included cationic ferrocene was less stabilized due to the direct binding to the cationic two Ag(I) centers.

A simple and highly effective ligand system for the copper(I)-mediated assembly of rotaxanes

A [2]rotaxane was produced through the assembly of a picolinaldehyde, an amine, and a bipyridine macrocycle around a Cu(I) template by imine bond formation in close-to-quantitative yield. An analogous [3]rotaxane is obtained in excellent yield by replacing the amine with a diamine, thus showing the suitability of the system for the construction of higher order interlocked structures. The rotaxanes are formed within a few minutes simply through mixing the components in solution at room temperature and they can be isolated through removal of the solvent or precipitation.