Complexation and Extraction of PAHs to the Aqueous Phase with a Dinuclear Pt II Diazapyrenium‐Based Metallacycle

Heterometallic cages: synthesis and applications

High symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis and drug delivery. Recently there have been increasing efforts to enhance those applications by generating reduced symmetry MSAs. While there are several emerging methods for generating lower symmetry MSAs, this tutorial review examines the general methods used for synthesizing heterometallic MSAs with a particular focus on heterometallic cages. Additionally, the intrinsic properties of the cages and their potential emerging applications as host-guest systems and reaction catalysts are described.

Exploiting Supramolecular Interactions to Control Isomer Distributions in Reduced-Symmetry [Pd2L4]4+ Cages

High-symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis, and drug delivery. Recently, there have been increasing efforts to enhance those applications by generating reduced-symmetry MSAs. Here we report our attempts to use supramolecular (dispersion and hydrogen-bonding) forces and solvophobic effects to generate isomerically pure [Pd₂(L)₄]⁴⁺ cage architectures from a family of new reduced-symmetry ditopic tripyridyl ligands. The reduced-symmetry tripyridyl ligands featured either solvophilic polyether chains, solvophobic alkyl chains, or amino substituents. We show using NMR spectroscopy, high-performance liquid chromatography, X-ray diffraction data, and density functional theory calculations that the combination of dispersion forces and solvophobic effects does not provide any control of the [Pd₂(L)₄]⁴⁺ isomer distribution with mixtures of all four cage isomers (HHHH, HHHT, cis-HHTT, or trans-HTHT, where H = head and T = tail) obtained in each case. More control was obtained by exploiting hydrogen-bonding interactions between amino units. While the cage assembly with a 3-amino-substituted tripyridyl ligand leads to a mixture of all four possible isomers, the related 2-amino-substituted tripyridyl ligand generated a cis-HHTT cage architecture. Formation of the cis-HHTT [Pd₂(L)₄]⁴⁺ cage was confirmed using NMR studies and X-ray crystallography.

Altering the binding affinities of tetraruthenocycles for polycyclic aromatic hydrocarbons by post-assembly modification

We developed a strategy for manipulating the binding strength of polycyclic aromatic hydrocarbons (PAHs) via covalent post-assembly modification (PAM) of tetranuclear ruthenium macrocycles containing s-tetrazine ligands. The macrocycles act as efficient receptors for various PAHs. Inverse electron demand Diels-Alder (IEDDA) reaction of the macrocycles was applied to reduce the binding ability significantly.

2,7-Diazapyrenes: a brief review on synthetic strategies and application opportunities

2,7-Diazapyrenes are promising azaaromatic scaffolds with a unique structural geometry and supramolecular properties. This core moiety and its derivatives with some N-methyl cations like N-methyl-2,7,-diazapyrenium, and N,N'-dimethyl-2,7-diazapyrenium attract special attention due to their challenging photophysical properties, especially in the context of interactions with DNA and some of its mononucleotides. This review focuses on the analysis of the main synthetic approaches to 2,7-diazapyrene and its functional derivatives employing various strategies under different reaction conditions. The opportunities of applications of 2,7-diazapyrenes, including their remarkable photophysical and supramolecular properties, DNA-bindings, in sensors, molecular electronics, supramolecular systems, and related areas are also highlighted.

Discrete Self-Assembled Metallo-Foldamers with Heteroleptic Sequence Specificity

Discrete and structurally diverse foldamer sequences are constructed in both natural and abiotic systems primarily using inert connectivity with irreversible organic covalent bonds, serving to preserve the identity of the sequence. The formation of sequences under thermodynamic control using labile coordination bonds would be attractive for synthetic ease and modular capability, but this presents issues regarding sequence preservation. Here is presented an approach integrating palladium(II) metal ions into the sequence itself, with fidelity maintained through use of complementary pairings of ligand arrangements at the metal centre. This is accomplished using sites of different denticity and/or hydrogen bonding capability. In this fashion, discrete and ordered metallo-sequences are formed as thermodynamic products in a single step, and these then fold into defined conformations due to π-π interactions between electron-rich and -poor aromatic regions of the combined componentry.

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)₃Mn(μ-SeR)₂Mn(CO)₃}₂(μ-L)₂] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

Dissecting the "Blue Box": Self-Assembly Strategies for the Construction of Multipurpose Polycationic Cyclophanes

Stoddart's "blue box" (B⁴⁺), is one of the most iconic molecules in the recent history of chemistry. This rectangular tetracationic cyclophane has not only the ability to complex a wide variety of aromatic guests in organic or aqueous media, but because of the presence of viologen units on its structure, it also behaves as a redox-based molecular switch. In turn, B⁴⁺-based host-guest complexes can translate this responsiveness from the molecular to the supramolecular level, resulting in host-controlled binding. This unique behavior has allowed the development of a wide variety of B⁴⁺-containing (supra)molecular switches and machines, which certainly have inspired a whole generation of supramolecular chemists. Nevertheless, issues, such as synthetic accessibility, structural diversity, or the implementation of new chemical properties (luminescence, pH- or photo-responsiveness, etc.), have restricted somehow the development of new practical applications in the ever-changing realm of modern host-guest chemistry.Based largely on our own research throughout the past decade, we will highlight in this account two different strategies for the self-assembly of new B⁴⁺ analogues: (1) Pd(II)/Pt(II) metal-directed self-assembly and (2) hydrazone-based dynamic covalent chemistry. In essence, the strategies are based on the substitution of inert C-C single bonds on the macrocycle by Pd/Pt-N or C═N bonds of modifiable lability. In the case of the metal-directed synthesis, the use of Pd(II) centers allows for the spontaneous self-assembly at r.t., either in organic or aqueous media, of N-alkyl-4,4'-bipyridinium-based ligands into the desired metallacycles. Conversely, more inert Pt(II) salts can be also implemented, rendering the synthesis of more kinetically stable analogues. Alternatively, wholly organic B⁴⁺ congeners can be produced in a modular fashion by using hydrazone-based dynamic covalent chemistry, allowing for the self-assembly in acidic water of macrocyclic pH-responsive molecular switches of adjustable kinetic stability.Owning pyridinium-based cavities of appropriate size, our B⁴⁺-inspired cyclophanes are able to complex aromatic substrates by a conjunction of the hydrophobic effect and π-π/C-H···π interactions. Consequently, we will discuss in detail the different host-guest complexes that can be achieved using our cyclophanes. Considering this knowledge, the implementation of our B⁴⁺-based macrocycles onto mechanically interlocked molecules and knots will be introduced, as well as the development of practical applications for the hosts in currently important research fields, such as the development of duplex and G4-DNA binders, supramolecular catalysis or the sequestration of relevant pollutants. Finally, self-assembled hosts offer the unique opportunity to include constitutional dynamism into host-guest chemistry, so examples of the development by our group of stimuli-responsive constitutionally dynamic libraries and self-sorted systems will be highlighted.

Dimensional Matching versus Induced-Fit Distortions: Binding Affinities of Planar and Curved Polyaromatic Hydrocarbons with a Tetragold Metallorectangle

A tetragold(I) rectangle-like metallocage containing two pyrene-bis-imidazolylidene ligands and two carbazolyl-bis-alkynyl linkers is used for the encapsulation of a series of polycyclic aromatic hydrocarbons (PAHs), including corannulene. The binding affinities obtained for the encapsulation of the planar PAHs guests in CD₂ Cl₂ are found to exponentially increase with the number of π-electrons of the guest (1.3 > logK >6.6). For the bowl-shaped molecule of corannulene, the association constant is much lower than the expected one according to its number of electrons. The molecular structure of the host-guest complex formed with corannulene shows that the molecule of the guest is compressed, while the host is expanded, thus showing an interesting case of artificial mutual induced-fit arrangement.

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Tuning the structure and the properties of dithiafulvene metalla-assembled tweezers

An electroactive M₂L₂ metalla-macrocycle constructed through coordination driven self-assembly dimerizes upon oxidation and binds an electro-deficient substrate with a high association constant.

Subcellular Duplex DNA and G-Quadruplex Interaction Profiling of a Hexagonal PtII Metallacycle

Metal-driven self-assembly afforded a multitude of fascinating supramolecular coordination complexes (SCCs) with applications as catalysts, host-guest, and stimuli-responsive systems. However, the interest in the biological applications of SCCs is only starting to emerge and thorough characterization of their behavior in biological milieus is still lacking. Herein, we report on the synthesis and detailed in-cell tracking of a Pt2 L2 metallacycle. We show that our hexagonal supramolecule accumulates in cancer cell nuclei, exerting a distinctive blue fluorescence staining of chromatin resistant to UV photobleaching selectively in nucleolar G4-rich regions. SCC co-localizes with epitopes of the quadruplex-specific antibody BG4 and replaces other well-known G4 stabilizers. Moreover, the photophysical changes accompanying the metallacycle binding to G4s in solution (fluorescence quenching, absorption enhancement) also take place intracellularly, allowing its subcellular interaction tracking.

A symmetry interaction approach to [M2L2]4+ metallocycles and their self-catenation

A symmetry interaction approach to [M₂L₂]⁴⁺ metallocycles and their self-catenanes.

Host–guest capability of a three-dimensional heterometallic macrocycle

Three-dimensional heterometallic macrocycles with half-sandwich Rh corners were studied for their ability to trap planar and non-planar guests. Furthermore, these heterometallic macrocycles can be destroyed in the presence of a soft base to form hexanuclear triangular prism complexes.

Solid-State Gas Adsorption Studies with Discrete Palladium(II) [Pd2 (L)4 ]4+ Cages

The need for effective CO₂ capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd₂ (L)₄ ]⁴⁺ lantern shaped coordination cages (L = variants of 2,6-bis(pyridin-3-ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO₂ . The best performing cage reversibly sorbed 1.4 mol CO₂ per mol cage at 298 K, and 2.3 mol CO₂ per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25-35 kJ mol^-1 , across the number of equivalents bound, while DFT calculations on the CO₂ binding in the cage gave ΔE for the cage-CO₂ interaction of 23-28 kJ mol^-1 , across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO₂ and the internally directed hydrogen atoms of the cage.

Self-assembly of dinuclear Pd(ii)/Pt(ii) metallacyclic receptors incorporating N-heterocyclic carbene complexes as corners

A series of new Pd(ii)/Pt(ii) metallacycles were self-assembled in water, using bipyridinium-based ligands and kinetically-labile metal centers having chelating N-heterocyclic carbenes.

Amplification of a metallacyclic receptor out of a dynamic combinatorial library

We present herein the Pt(ii)-directed self-assembly in water of a new conformationally flexible N-monoalkyl-4,4-bipyridinium-based ditopic ligand into a library of six different metallacyclic structures.

Guest-induced stereoselective self-assembly of quinoline-containing PdII and PtII metallacycles

The inclusion of aromatics within atropisomeric metallacycles induced stereoselectivity on the self-assembly. This selectivity is influenced by the size of the guests, the C–H⋯π interactions and the π-acceptor and hydrophobic character of the host.

Stimuli-responsive metal-directed self-assembly of a ring-in-ring complex

The threading and unthreading process of a dinuclear metallacycle into a ring-in-ring structure can be controlled by temperature, dilution, and medium polarity.

Enhanced kinetic stability of [Pd2L4]4+ cages through ligand substitution

[Pd₂(tripy)₄]⁴⁺ cage architectures (where tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine) were made more kinetically robust in the presence of range of nucleophiles by the addition of amino groups in either the 2-(2A-tripy) or 3-(3A-tripy) positions of the tripy ligands' terminal pyridines, with the [Pd₂(2A-tripy)₄]⁴⁺ cage proving the most stable.

Triangular platinum(II) metallacycles: syntheses, photophysics, and nonlinear optics

Three triangular platinum(II) diimine metallacycles incorporating large cyclic oligo(phenylene-ethynylene) (OPE) bisacetylide ligands are synthesized, and their photophysical properties are studied. Two types of triplet excited states with ligand/metal-to-ligand charge-transfer and acetylide-ligand-centered characteristics respectively, are exhibited by these complexes depending on the size (conjugation length) and electronic features of the cyclic OPE ligands. When the energy levels of the two excited states are close to each other, the lowest triplet state is found to switch between the two in varied solvents, resulting from their relative energy inversion induced by solvent polarity change. Density functional theory and time-dependent density functional theory calculations provide corroborative evidence for such experimental conclusions. More importantly, the designed metallacycles show impressive two-photon absorption (2PA) and two-photon excitation phosphorescing abilities, and the 2PA cross section reaches 1020 GM at 680 nm and 670 GM at 1040 nm by two different metallacycles. Additionally, pronounced reverse saturable absorptions are observed with these metallacycles by virtue of their strong transient triplet-state absorptions.