Metal‐Directed Self‐Assembly of a Polyoxometalate‐Based Molecular Triangle: Using Powerful Analytical Tools to Probe the Chemical Structure of Complex Supramolecular Assemblies

A polyoxometalate‐based molecular triangle has been synthesized through the metal‐driven self‐assembly of covalent organic/inorganic hybrid oxo‐clusters with remote pyridyl binding sites. The new metallomacrocycle was unambiguously characterized by using a combination of ¹H NMR spectroscopy, 2D diffusion NMR spectroscopy (DOSY), electrospray ionization travelling wave ion mobility mass spectrometry (ESI‐TWIM‐MS), small‐angle X‐ray scattering (SAXS) and molecular modelling. The collision cross‐sections obtained from TWIM‐MS and the hydrodynamic radii derived from DOSY are in good agreement with the geometry‐optimized structures obtained by using theoretical calculations. Furthermore, SAXS was successfully employed and proved to be a powerful technique for characterizing such large supramolecular assemblies.     

Cation‐Directed Dimeric versus Tetrameric Assemblies of Lanthanide‐Stabilized Dilacunary Keggin Tungstogermanates

Reaction of mid‐ to late lanthanide ions with GeO₂ and Na₂WO₄ in NaOAc buffer results in a library of [Ln₂(GeW₁₀O₃₈)]^6? clusters ( Ln₂ ), which consist of dilacunary Keggin fragments stabilized by the insertion of 4f atoms in the vacant sites and show the ability to undergo cation‐directed self‐assembly processes. In the presence of Na⁺, two β‐ Ln₂ subunits assemble by means of Ln‐O(WO₅)‐Ln bridges to form the chiral [Ln₄(H₂O)₆(β‐GeW₁₀O₃₈)₂]^12? dimeric anions (ββ‐ Ln₄ , Ln=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). When Cs⁺ is present, two Ln₄ ‐like dimers further assemble into the [{Ln₄(H₂O)₅(GeW₁₀O₃₈)₂}₂]^24? species ( Ln₈ , Ln=Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows a full ββ‐ Ln₈ architecture, whereas the other one is a mixed αβ‐ Ln₈ assembly in which each β‐subunit is linked to its corresponding α‐ Ln₂ derivative. Regardless of differences in isomeric forms and the relative arrangement of Ln₂ subunits, all anions display virtually identical {Ln₄} cores as a common structural feature. A combination of ESI mass spectrometry and ¹⁸³W NMR spectroscopy experiments indicates that Ln₈ tetramers fragment into Ln₄ dimers upon dissolution, which undergo partial dissociation into Ln₂ monomers and slow dimer/monomer equilibration. This is most likely followed by β‐to‐α isomerization of Ln₂ clusters with consequent reassembly, as indicated by isolation of three additional αα‐ Ln₄ derivatives. Magnetic and photoluminescence properties in the Na ‐ββ‐ Ln₄ series are also discussed.     

Crown‐Shaped Tungstogermanates as Solvent‐Controlled Dual Systems in the Formation of Vesicle‐Like Assemblies

Reaction of early lanthanides, GeO₂, and Na₂WO₄ in a NaOAc buffer results in large crown‐shaped polyoxometalates based on [Ln₂GeW₁₀O₃₈]^6? subunits. By using Ni²⁺ as a crystallizing agent, [Na?Ln₁₂Ge₆W₆₀O₂₂₈(H₂O)₂₄]^35? ( Na?Ln₁₂ ) hexamers formed by alternating β(1,5)/β(1,8) subunits were obtained for Ln=Pr, Nd. The addition of K⁺ led to a similar anion for Ln=Sm, namely, [K?Sm₁₂Ge₆W₆₀O₂₂₈(H₂O)₂₂]^35? ( K?Sm₁₂ ) and [K?K₇Ln₂₄Ge₁₂W₁₂₀O₄₄₄(OH)₁₂(H₂O)₆₄]^52? ( K?Ln₂₄ ) dodecamers that consist of a central core identical to K?Sm₁₂ decorated with six external γ(3,4) subunits for Ln=Pr, Nd. These anions dissociate in water into hexameric cores and monomeric entities, as shown by ESI mass spectrometry. The former self‐assemble into spherical, hollow, and single‐layered blackberry‐type structures with radii of approximately 75 nm, as monitored by laser light scattering (LLS) and TEM techniques. Analogous studies performed for K?Nd₂₄ in water/acetone mixtures show that the dodecamers remain stable and form in turn their own type of blackberries with sizes that increase from approximately 20 to 50 nm with increasing acetone content. This control over both the composition and size of the vesicle‐like assemblies is achieved for the first time by modifying the architecture of the species that undergoes supramolecular association through the solvent polarity.     

Formation and Geometrical Control of Polygon‐Like Metal‐Coordination Assemblies

Polygon‐like [2+2]‐ and [3+3]‐type metal complexes were prepared from dipyrrin dimers connected by acute‐angled spacers. The electrical conduction depends strongly on the packing alignment of the compounds, revealing the presence of effective hopping pathways for holes with relatively high mobility up to 0.11 cm² V^?1 s^?1 along the aligned axis of [3+3]‐type metal‐bridged assemblies. These observations correlated with the geometrical control of the π‐conjugated metal complexes in the cyclic structures, which enables their ordered arrangement in the assemblies.