How to not build a cage: endohedral functionalization of polyoxometalate-based metal-organic polyhedra

Steerable Structural Evolvement and Adsorption Behavior of Metastable Polyoxovanadate-Based Metal-Organic Polyhedra

Promoting the advancement of the structure and function of metastable substances is challenging but worthwhile. In particular, how to harness the entangled state and evolution path of labile porous structures has been at the forefront of research in molecular self-assembly. In this work, the metastable structures of polyoxovanadate-based metal-organic polyhedra (VMOPs) can be manually regulated, including separation of the interlocked aggregate by a ligand-widening approach as well as transformation from a tetrahedral to capsule-like scaffold via a vertice-remodeling strategy. In these processes, intra- and intermolecular π···π and C-H···π interactions have been recognized as the primary driving forces. Besides being responsible for commanding the structural evolvement of VMOPs, such weak interactions were able to program their spatial arrangements and hence the adsorption performances for dye and iodine. The successful use of such a weak force-dominated design concept beacons a feasible route for customization of the function-oriented metastable structures. Separation and transformation of the interlocked metastable VMOPs have been achieved via the respective ligand-widening approach and vertice-remodeling strategy. Not only their structures but also adsorption features could be well regulated by such a weak force-dominated design concept.

Multi-Template-Guided Synthesis of High-Dimensional Molecular Assemblies for Humidity Gradient-Based Power Generators

Systematically orchestrating fundamental building blocks into intricate high-dimensional molecular assemblies at molecular level is imperative for multifunctionality integration. However, this remains a formidable task in crystal engineering due to the dynamic nature of inorganic building blocks. Herein, we develop a multi-template-guided strategy to control building blocks. The coordination modes of ligands and the spatial hindrance of anionic templates are pivotal in dictating the overall structures. Flexible multi-dentate linkers selectively promote the formation of oligomeric assembly ([TeO3(Mo2O2S2)3O2(OH)(C5O2H7)3]4- {TeMo6}) into tetrahedral cages ([(TeO3)4(Mo2O2S2)12(OH)12(C9H9O4P)6]8- {Te4Mo24} and [(AsO4)4(Mo2O2S2)12(OH)12(C9H9O6)4]12- {As4Mo24}), while steric hindrance from anionic templates further assists in assembling cages into an open quadruply twisted Möbius nanobelt ([(C6H5O3P)8(Mo2O2S2)24(OH)24(C8H10O4)12]16- {P8Mo48}). Among these structures, the hydrophilic-hydrophobic hybrid cage {Te4Mo24} emerges as an exemplary molecular model for proton conduction and serves as a prototype for humidity gradient-based power generators (HGPGs). The Te4Mo24-PVDF-based HGPG (PVDF=Poly(vinylidene fluoride)) exhibits notable stability and power generation, yielding an open-circuit voltage of 0.51 V and a current density of 77.8 nA cm-2 at room temperature and 90 % relative humidity (RH). Further insights into the interactions between water molecules and microscale molecules within the generator are achieved through molecular dynamics simulations. This endeavor unveils a universal strategy for synthesizing multifunctional integration molecules.

A Dawson-type {P4W24} modified using phenylphosphonic acid with excellent proton conductivity

A case of organic-inorganic hybridized phosphotungstate modified using aromatic organophosphonic acid, K4Na4H11[KCo2(H2O)10P4W24O92{(PhPO)2}]·48H2O (1), was successfully synthesized in conventional aqueous solution. The prominent structural feature is that the total structure of [KP4W24O92{(PhPO)2}]23- resembles a V-shaped structure, which was stabilized by two [Co(H2O)5]2+ ions. Furthermore, it can be connected into a three-dimensional mesh structure using K+ ions. Surprisingly, 1 possesses a remarkably high proton conductivity of 1.59 × 10-2 S cm-1 at 95% RH and 318 K probably due to the fact that its structure contains large amounts of lattice water molecules, coordination water molecules and counter cations.

Chameleonic Cages: Encapsulation of Anionic, Neutral, and Cationic Guest Species within [Fe4L4]8+ Tetrahedral Cages Synthesised from the tris(4-aminophenyl)phosphate pro-Ligand

An adaptable Fe(II) tetrahedral cage, [Fe4L4][BF4]8 (L=tris(4-(((E)-pyridin-2-ylmethylene)amino)phenyl) phosphate), has been synthesised via self-assembly. By modulating the orientation of its pendant P=O groups, the cage was found to be capable of encapsulating anionic, neutral, and cationic guests, which were confirmed in the solid state via single-crystal X-ray diffraction (SCXRD) and in solution by high-resolution mass spectroscopy (HR-MS), as well as by NMR (1H, 19F, 31P) studies where possible.

Self-assembled molecular hybrids comprising lacunary polyoxometalates and multidentate imidazole ligands

Self-assembly via coordination bonding facilitates the creation of diverse inorganic-organic molecular hybrids with distinct structures and properties. Recent advances in this field have been driven by the versatility of organic ligands and inorganic units. Lacunary polyoxometalates are a class of well-defined metal-oxide clusters with a customizable number of reactive sites and bond directions, which make them promising inorganic units for self-assembled molecular hybrids. Herein, we report a novel synthesis method for self-assembled molecular hybrids utilizing the reversible coordination of multidentate imidazole ligands to the vacant sites of lacunary polyoxometalates. We synthesized self-assembled molecular hybrids including monomer, dimers, and tetramer, demonstrating the potential of our method for constructing intricate hybrids with tailored properties and functionalities.

Accurate Matching of a Secondary Amino-Functionality Metal-Organic Cage for Selective Recognition and Supramolecular Binding during Photoinduced Hydrogen Evolution

Accurate matching of the active sites between the host and guest molecules has a great effect on the selective recognition of different but similar guest molecules or different binding abilities toward the same molecule. Herein, a pseudotetrahedral metal-organic cage (MOC, Co-TAP) that contains secondary amino groups designed as guest-interacting sites was achieved. Co-TAP exhibits the selective recognition of uridine over other similar natural molecules via a fluorescent response. However, a reference structure (Co-TOP) with the same configuration was also synthesized by replacing the secondary amine group with an oxygen atom of the ligand, and it reveals the selective recognition of guanosine. In addition, the accurate matching also enables Co-TAP to strongly bind the organic dye as a guest molecule via host-guest interactions, thus facilitating photoinduced electron transfer between the redox catalytic sites in MOC and the excited guest via a pseudointramolecular pathway.

Tetramine Aspect Ratio and Flexibility Determine Framework Symmetry for Zn8 L6 Self-Assembled Structures

We derive design principles for the assembly of rectangular tetramines into Zn8 L6 pseudo-cubic coordination cages. Because of the rectangular, as opposed to square, geometry of the ligand panels, and the possibility of either Δ or Λ handedness of each metal center at the eight corners of the pseudo-cube, many different cage diastereomers are possible. Each of the six tetra-aniline subcomponents investigated in this work assembled with zinc(II) and 2-formylpyridine in acetonitrile into a single Zn8 L6 pseudo-cube diastereomer, however. Each product corresponded to one of four diastereomeric configurations, with T, Th , S6 or D3 symmetry. The preferred diastereomer for a given tetra-aniline subcomponent was shown to be dependent on its aspect ratio and conformational flexibility. Analysis of computationally modeled individual faces or whole pseudo-cubes provided insight as to why the observed diastereomers were favored.

Endohedral Functionalization for Structural Transformation of Polyoxovanadate-Based Metal-Organic Cube

Functionalized internal modifications of metal-organic polyhedra (MOPs) can endow properties and functions different from the original ones. Until now, there have been only a few examples of endohedral modifications of polyoxovanadate-based MOPs. Herein, an efficient coordination-driven strategy was chosen for the inner modification of two metal-organic cubes (MOCs) with different sizes, VMOC-1 and VMOC-4, constructed from polyoxovanadate clusters [V₆O₆(OCH₃)₉(SO₄)(CO₂)₃]^2- SBU and tetradentate ligands. Pyridinophosphonic acid with potential coordination capability was introduced to replace the sulfate of the hexavanadate cluster and graft the pyridine functional group inside the cage. The introduction of pyridylphosphate in the VMOC-4 system gave a cubic cage with a pyridyl endo-modified isomer. Interestingly, the smaller cubic cage VMOC-1 was induced to undergo structural transformation to obtain VMOC-py-1. The organic dyes adsorption of VMOC-py-1 and VMOC-1 showed that the endomodified structure could adsorb larger and more dyes, compared to the original cube.

Highly Stable Europium(III) Tetrahedral (Eu4L4)(phen)4 Cage: Structure, Luminescence Properties, and Cellular Imaging

Luminescent lanthanide cages have many potential applications in guest recognition, sensing, magnetic resonance imaging (MRI), and bioimaging. However, these polynuclear lanthanide assemblies' poor stability, dispersity, and luminescence properties have significantly constrained their practical applications. Furthermore, it is still a huge challenge to simultaneously synthesize and design lanthanide organic polyhedra with high stability and quantum yield. Herein, we demonstrate a simple and robust strategy to improve the rigidity, chemical stability, and luminescence of an Eu(III) tetrahedral cage by introducing the conjugated planar auxiliary phen ligand. The self-assembled tetrahedral cage, (Eu₄L₄)(phen)₄ [L = (4,4',4″-tris(4,4,4-trifluoro-1,3-dioxobutyl)-triphenylamine), phen = 1,10-phenanthroline], exhibited characteristic luminescence of Eu³⁺ ions with high quantum yield (41%) and long lifetime (131 μs) in toluene (1.0 × 10^-6 M). Moreover, the Eu(III) cage was stable in water and even in an aqueous solution with a pH range of 1-14. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and cellular imaging revealed that the Pluronic F127-coated hybrid material, (Eu₄L₄)(phen)₄@F127, exhibited low cytotoxicity, good biocompatibility, and cellular imaging ability, which may inspire more insights into the development of lanthanide organic polyhedra (LOPs) for potential biomedical applications.

2D Windmill-like Ln-Containing Organophosphonate-Based Polyoxomolybdates: Synthesis, Characterization, Fluorescence, and Magnetism

By integration of {Ln(H₂O)₆}³⁺ into organophosphonate-based polyoxometalates, three Ln-containing organophosphonate-functionalized polyoxomolybdates Na_1.5H_1.5[{Ln(H₂O)₆}₂{(Mo₃O₈)(O₃PC(C₃H₆NH₃)OPO₃)}₄]·(CH₃CO₂)·43H₂O (Ln = Eu (1), Tb (2), and Dy (3)) are successfully prepared and systematically characterized. The X-ray crystallography analyses display complexes 1-3 crystallize in the C2/c space group of the monoclinic system and compose several distinctive tetramer windmill-like compounds that further assemble into two-dimensional (2D) frameworks associated with the {Ln(H₂O)₆}³⁺ core. The fluorescence spectra of 1-3 show red, green, and chartreuse emissions, respectively, originating in the typical f-f transitions of Ln³⁺ ions. More interestingly, complex 3 shows the field-induced single-molecule magnet (SMM) properties, resulting from the fact that [(Mo₃O₈)₄{O₃PC(C₃H₆NH₃)OPO₃}₄]^8- offers excellent magnetic isolation for Dy³⁺ ions by the nearest Dy1···Dy2 distance of 11.207 Å. The study demonstrates that the incorporation of {Ln(H₂O)₆}³⁺ into organophosphonate-based polyoxomolybdates is an effective synthetic strategy in implementing late-model opto-magnetic materials.

Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH₄)₂K₂[V₄(μ₂-OH)₄(ox)₄(pz)₄]·9H₂O (1, ox = oxalate and pz = pyrazole) and (NH₄)₂Na₂[V₄(μ₂-OH)₄(ox)₄(4-mpz)₄]·7H₂O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K₂[V₆(μ₂-OH)₆(ox)₆(Hdatrz)₆]Cl₂·29.5H₂O (3) and [V₈(μ₂-OH)₈(SO₃)₈(Hdatrz)₈]·38H₂O (4, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V₄(μ₂-OH)₄} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V₆(μ₂-OH)₆} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V₈(μ₂-OH)₈(SO₃)₈} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO₂ and O₂ more favorably than N₂, CH₄ and H₂ gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO₂ adsorption.

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.

Timing matters: pre-assembly versus post-assembly functionalization of a polyoxovanadate-organic cuboid

The pre-assembly and post-assembly approaches in the functionalization of a polyoxovanadate-organic cuboid, [{V6S}8(QPTC)8{V3}2]10-, are discussed. We have shown that the two pathways have led to distinctly different systems, with either an expanded or contracted interior void space, when phenylphosphonate is introduced at different stages of the self-assembly. One leaves the cuboid framework largely intact, whereas the other results in a compact, twisted cuboid. Kinetic factors will have to be considered in the equilibrium of these complex processes. Furthermore, the exceptional stability of these polyoxometalate-organic systems facilitates mass spectrometric characterization, which confirms the composition of the complexes and also indicates that the methoxide groups on the vanadium cluster nodes are labile. The results will help deepen the mechanistic understanding of the formation mechanisms of polyoxovanadate-based metal-organic cages and other functionalized polyoxovanadate clusters in general.

Surface chemistry of metal-organic polyhedra

Metal-organic polyhedra (MOPs) are discrete, intrinsically-porous architectures that operate at the molecular regime and, owing to peripheral reactive sites, exhibit rich surface chemistry. Researchers have recently exploited this reactivity through post-synthetic modification (PSM) to generate specialised molecular platforms that may overcome certain limitations of extended porous materials. Indeed, the combination of modular solubility, orthogonal reactive sites, and accessible cavities yields a highly versatile molecular platform for solution to solid-state applications. In this feature article, we discuss representative examples of the PSM chemistry of MOPs, from proof-of-concept studies to practical applications, and highlight future directions for the MOP field.