Selective Anion Binding Drives the Formation of AgI8L6 and AgI12L6 Six-Stranded Helicates

Uncovering tetrazoles as building blocks for constructing discrete and polymeric assemblies

Metal-organic self-assembly with flexible moieties is a budding field of research due to the possibility of the formation of unique architectures. Tetrazole, characterised by four nitrogen atoms in a five-member ring, exhibits immense potential as a component. Tetrazole offers four coordination sites for binding to the metal centre with nine distinct binding modes, leading to various assemblies. This review highlights different polymeric and discrete tetrazole-based assemblies and their functions. The meticulous manipulation of stoichiometry, ligands, and metal ions required for constructing discrete assemblies has also been discussed. The different applications of these architectures in separation, catalysis and detection have also been accentuated. The latter section of the review consolidates tetrazole-based cage composites, highlighting their applications in cell imaging and photocatalytic applications.

Argentophilic Interactions, Flexibility, and Dynamics of Pyrrole Cages Encapsulating Silver(I) Clusters

Recently, pyrrole cages have been synthesized that encapsulate ion pairs and silver(I) clusters to form intricate supramolecular capsules. We report here a computational analysis of these structures using density functional theory combined with a semiempirical tight-binding approach. We find that for neutral pyrrole cages, the Gibbs free energies of formation provide reliable predictions for the ratio of bound ions. For charged pyrrole cages, we find strong argentophilic interactions between Ag ions on the basis of the calculated bond indices and molecular orbitals. For the cage with the Ag4 cluster, we find two minimum-geometry conformations that differ by only 6.5 kcal/mol, with an energy barrier <1 kcal/mol, suggesting a very flexible structure as indicated by molecular dynamics. The predicted energies of formation of [Agn⊂1]n-3+ (n = 1-5) cryptands provide low energy barriers of formation of 5-20 kcal/mol for all cases, which is consistent with the experimental data. Furthermore, we also examined the structural variability of mixed-valence silver clusters to test whether additional geometrical conformations inside the organic cage are thermodynamically accessible. In this context, we show that the time-dependent density functional theory UV-vis spectra may potentially serve as a diagnostic probe to characterize mixed-valence and geometrical configurations of silver clusters encapsulated into cryptands.

Nickel, copper, and zinc dinuclear helicates: how do bulky groups influence their architecture?

The ligand design factors that may influence the isolation of metallosupramolecular helicates or mesocates still deserve to be investigated. In this sense, dinuclear nickel(II), copper(II) and zinc(II) compounds were obtained by electrochemical synthesis using a family of five Schiff base ligands, H2Ln (n = 1-5), derived from bisphenylmethane and functionalized with bulky tert-butyl groups in the periphery and ethyl groups in the spacer. Six of the new complexes were characterized by X-ray crystallography, thus demonstrating that the helicate structure is predominant in the solid state. 1H NMR studies were performed for the zinc complexes to analyze if the helical architecture of the metal complexes is retained in solution. These studies reveal that the presence of a tert-butyl group in the ortho position with respect to the OH group is an essential factor identified for the existence of a helicate conformation in solution.

Heterometallic AuI 6 AgI 6 Macrocyclic Cluster Templated by a Supramolecular Melamine Dimer

The application of supramolecular templates in aligning atomically precise heterometal arrays is important for pursuing functional materials. Herein, we report that a bilayered supramolecular tri-deprotonated melamine dimer functions as an effective template in the construction of a heterometallic gold(I)-silver(I) macrocyclic cluster [μ6 -(C3 N6 H3 )3- ]2 -AuI 6 AgI 6 . X-ray single crystal structural analysis showed that a crown-like AuI 6 AgI 6 macrocycle is aligned around two parallelly stacked μ6 -(C3 N6 H3 )3- moieties hold together with π-π interactions. Theoretical calculations revealed that the [μ6 -(C3 N6 H3 )3- ]2 motif dominantly contributes to the near-occupied orbitals in the electronic structure, which is closely related to its luminescence properties. This work demonstrates that the supramolecular templates containing multiple symmetric binding sites may present a facile approach in the construction of functional metal clusters.

Urea-Functionalized Fe4 L6 Cages for Supramolecular Gold Catalyst Encapsulation to Control Substrate Activation Modes

The excellent catalytic performances of enzymes in terms of activity and selectivity are an inspiration for synthetic chemists and this has resulted in the development of synthetic containers for supramolecular catalysis. In such containers the local environment and pre-organization of catalysts and substrates leads to control of the activity and selectivity of the catalyst. Herein we report a supramolecular strategy to encapsulate single catalysts in a urea-functionalized Fe4 L6 cage, which can co-encapsulate a functionalized urea substrate through hydrogen bonding. Distinguished selectivity is obtained, imposed by the cage as site isolation only allows catalysis through π activation of the substrate and as a result the selectivity is independent of catalyst concentration. The encapsulated catalyst is more active than the free analogue, an effect that can be ascribed to transitionstate stabilization rather than substrate pre-organization, as revealed by the MM kinetic data. The simple strategy reported here is expected to be of general use in many reactions, for which the catalyst can be functionalized with a sulfonate group required for encapsulation.

Construction of metallo-helicoids with high antimicrobial activity via intermolecular coordination

Metallo-helicoids are constructed by intermolecular coordination interactions between covalent linear polymer and tritopic/hexatopic molecular templates. These metallo-polymers with helicoidal conformation exhibit high antimicrobial activities against both Gram-positive and Gram-negative pathogens.

Anionic Templates Drive Conversion between a ZnII9L6 Tricapped Trigonal Prism and ZnII6L4 Pseudo-Octahedra

This work introduces the use of 8-aminoquinoline subcomponents to generate complex three-dimensional structures. Together with a tris(formylpyridine), 8-aminoquinoline condensed around Zn^II templates to produce a tris(tridentate) ligand. This ligand is incorporated into either a tricapped trigonal prismatic Zn^II₉L₆ structure or a pair of pseudo-octahedral Zn^II₆L₄ diastereomers, with S₄ and D₂ symmetries. Introduction of a methyl group onto the aminoquinoline modulated the coordination sphere of Zn^II, which favored the Zn^II₉L₆ structure and disfavored the Zn^II₆L₄ assembly. The tricapped trigonal prismatic Zn^II₉L₆ architecture converted into a single Zn^II₆L₄ cage diastereomer following the addition of a dianionic 4,4'-dinitrostilbene-2,2'-disulfonate guest. Four of these guests clustered tightly at the four windows of the Zn^II₆L₄ cage, held in place through electrostatic interactions and hydrogen bonding, stabilize a single diastereomeric configuration with S₄ symmetry.

Schiff Bases Functionalized with T-Butyl Groups as Adequate Ligands to Extended Assembly of Cu(II) Helicates

The study of the inherent factors that influence the isolation of one type of metallosupramolecular architecture over another is one of the main objectives in the field of Metallosupramolecular Chemistry. In this work, we report two new neutral copper(II) helicates, [Cu₂(L¹)₂]·4CH₃CN and [Cu₂(L²)₂]·CH₃CN, obtained by means of an electrochemical methodology and derived from two Schiff-based strands functionalized with ortho and para-t-butyl groups on the aromatic surface. These small modifications let us explore the relationship between the ligand design and the structure of the extended metallosupramolecular architecture. The magnetic properties of the Cu(II) helicates were explored by Electron Paramagnetic Resonance (EPR) spectroscopy and Direct Current (DC) magnetic susceptibility measurements.

Helical fluxionality: numerical frustration drives concerted low-barrier screw motions of a tricopper cluster

Uneven allocation of resources creates frustration, tension, and conflicts. Confronted with an apparent mismatch between the number of donor atoms and the number of metal atoms to be supported, helically twisted ligands cleverly come up with a sustainable symbiotic solution. As an example, we present a tricopper metallohelicate exhibiting screw motions for intramolecular site exchange. A combination of X-ray crystallographic and solution NMR spectroscopic studies revealed thermo-neutral site exchange of three metal centres hopping back and forth inside the helical cavity lined by a spiral staircase-like arrangement of ligand donor atoms. This hitherto unknown helical fluxionality is a superimposition of translational and rotational movements of molecular actuation, taking the shortest path with an extraordinarily low energy barrier without compromising the overall structural integrity of the metal-ligand assembly.

Orthogonal, modular anion-cation and cation-anion self-assembly using pre-programmed anion binding sites

Subcomponent self-assembly relies on cation coordination whereas the roles of anions often only emerge during the assembly process. When sites for anions are instead pre-programmed, they have the potential to be used as orthogonal elements to build up structure in a predictable and modular way. We explore this idea by combining cation (M⁺) and anion (X^-) binding sites together and show the orthogonal and modular build up of structure in a multi-ion assembly. Cation binding is based on a ligand (L) made by subcomponent metal-imine chemistry (M⁺ = Cu⁺, Au⁺) while the site for anion binding (X^- = BF₄ ^-, ClO₄ ^-) derives from the inner cavity of cyanostar (CS) macrocycles. The two sites are connected by imine condensation between a pyridyl-aldehyde and an aniline-modified cyanostar. The target assembly [LM-CS-X-CS-ML],⁺ generates two terminal metal complexation sites (LM and ML) with one central anion-bridging site (X) defined by cyanostar dimerization. We showcase modular assembly by isolating intermediates when the primary structure-directing ions are paired with weakly coordinating counter ions. Cation-directed (Cu⁺) or anion-bridged (BF₄ ^-) intermediates can be isolated along either cation-anion or anion-cation pathways. Different products can also be prepared in a modular way using Au⁺ and ClO₄ ^-. This is also the first use of gold(i) in subcomponent self-assembly. Pre-programmed cation and anion binding sites combine with judicious selection of spectator ions to provide modular noncovalent syntheses of multi-component architectures.

Plenates: Anion-Dependent Self-Assembly of the Pyrrole Cage Encapsulating Silver(I) Clusters

The self-assembly of 2,5-diformylpyrrole, tris(2-aminoethyl)amine, and silver(I) yielded, depending on the size and basicity of the anion, new cascade complexes or plenates, that is, cryptates incorporating Ag_n ⁿ⁺ clusters. The nature of the product was counterion-dependent, and its formation was either driven by cascade anion binding or by argentophilic interactions stabilizing the cluster within the cavity. The reaction of plenates with tetrabutylammonium halides resulted in the protonation-coupled replacement of the Ag₃ ³⁺ with anion(s), yielding cascade cryptates.

Dynamic Covalent Self-Assembly of Chloride- and Ion-Pair-Templated Cryptates

While supramolecular hosts capable of binding and transporting anions and ion pairs are now widely available, self-assembled architectures are still rare, even though they offer an inherent mechanism for the release of the guest ion(s). In this work, we report the dynamic covalent self-assembly of tripodal, urea-based anion cryptates that are held together by two orthoester bridgeheads. These hosts exhibit affinity for anions such as Cl- , Br- or I- in the moderate range that is typically advantageous for applications in membrane transport. In unprecedented experiments, we were able to dissociate the Cs⋅Cl ion pair by simultaneously assembling suitably sized orthoester hosts around the Cs+ and the Cl- ion.

Anion-Directed Regulation of Structures and Luminescence of Heterometallic Clusters

Anions have been used to regulate the structures and luminescence of heterometallic clusters. Introducing ClO₄ ^- into orange-emissive, butterfly-like [(C)(Au-PPhpy₂ )₆ Ag₄ ](BF₄ )₆ (1, PPhpy₂ =bis(2-pyridyl)phenylphosphine) leads to the formation of red-emissive [(C)(Au-PPhpy₂ )₆ Ag₅ (ClO₄ )₃ ](ClO₄ )₄ (2) with a novel trigonal bipyramidal structure; employing PhCO₂ ^- gives yellow-emissive, hexagram-like [(C)(Au-PPhpy₂ )₆ Ag₆ (PhCO₂ )₃ ](BF₄ )₅ (3). Notably, 1 exhibits weak luminescence in CH₂ Cl₂ /CH₃ OH=1 : 1 (v : v) with a quantum yield (QY) of 0.05, whereas it was dramatically increased to 0.49 and 0.83 for 2 and 3, respectively. Theoretical calculation confirms that the involvement of anions in the electronic structures is responsible for the shifts of emission. The high QYs of 2 and 3 are attributed to the protection provided by ligands and anions. This work demonstrates that anions may serve as an extra designable factor beyond just counterions for functional metal clusters.

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.

Cooperativity of steric bulk and H-bonding in coordination sphere engineering: heteroleptic PdII cages and bowls by design

Recently developed self-assembly strategies allow to rationally reduce the symmetry of metallosupramolecular architectures. In addition, the combination of multiple ligand types without creating compound mixtures has become possible. Among several approaches to realize non-statistical heteroleptic assembly, Coordination Sphere Engineering (CSE) makes use of secondary repulsive or attractive interactions in direct vicinity of the metal nodes. Previously, we used steric congestion to turn dinuclear [Pd2L4] cages with fourfold symmetry into [Pd2L3X2] (X = solvent, halide) bowl structures. Here, we introduce a new subtype of this strategy based on balancing hydrogen bonding and repulsive interactions between ligands carrying quinoline (LQu) and 1,8-naphthyridine (LNa) donors to generate trans-[Pd2L2] and [Pd2L3L'] cages, assisted by templation of encapsulated fullerenes. Combined with steric congestion caused by acridine (LAc) donors, we further report the first example of a heteroleptic [Pd2L2L'X2] bowl. Formation, structure and fullerene binding ability of these metallo-supramolecular hosts were studied by NMR, mass spectrometry and single crystal X-ray diffraction.

Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me₄enPd)_2n(ML)_n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me₄en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me₄enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me₄enPd)₄(ML)₂-type cage, all other M(ii) ions led to isostructural (Me₄enPd)₆(ML)₃-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me₄enPd)₆(ZnL)₃ host an effective catalyst for the CO₂ to CO₃²⁻ conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO₃²⁻ and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.

A series of coordination hosts were prepared and their applications in CO₂ fixation were studied.

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.