Anion-Directed Metallocages: A Study on the Tendency of Anion Templation

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd₂L₄ metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 10⁶ M^-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.

Insight into systematic formation of hexafluorosilicate during crystallization via self-assembly in a glass vessel

Formation of the unexpected hexafluorosilicate (SiF₆²⁻) anion during crystallization via self-assembly in glassware is scrutinized. Self-assembly of M(BF₄)₂ (M²⁺ = Cu²⁺ and Zn²⁺) with tridentate N-donors (L) in a mixture solvent including methanol in a glass vessel gives rise to an SiF₆²⁻-encapsulated Cu₃L₄ double-decker cage and a Zn₂L₄ cage, respectively. Induced reaction of CuX₂ (X⁻ = PF₆⁻ and SbF₆⁻) instead of Cu(BF₄)₂, with the tridentate ligands, produces the same species. The formation time of SiF₆²⁻ is in the order of anions BF₄⁻ < PF₆⁻ < SbF₆⁻ under the given reaction conditions. The SiF₆²⁻ anion, acting as a cage template or cage-to-cage bridge, seems to be formed from the reaction of polyatomic anions containing fluoride with the SiO₂ of the surface of the glass vessel.

Formation of the unexpected hexafluorosilicate (SiF₆²⁻) encapsulated cages constructed. Interestingly, this shows that the surface of glassware should be given serious consideration for long-duration reactions with active F-containing species.

Nature of hydride and halide encapsulation in Ag8 cages: insights from the structure and interaction energy of [Ag8(X){S2P(OiPr)2}6]+ (X = H-, F-, Cl-, Br-, I-) from relativistic DFT calculations

Unraveling the different contributing terms to an efficient anion encapsulation is a relevant issue for further understanding of the underlying factors governing the formation of endohedral species. Herein, we explore the favorable encapsulation of hydride and halide anions in the [Ag₈(X){S₂P(OPr)₂}₆]⁺ (X^- = H, 1, F, 2, Cl, 3, Br, 4, and, I, 5) series on the basis of relativistic DFT-D level of theory. The resulting Ag₈-X interaction is sizable, which decreases along the series: -232.2 (1) > -192.1 (2) > -165.5 (3) > -158.0 (4) > -144.2 kcal mol^-1 (5), denoting a more favorable inclusion of hydride and fluoride anions within the silver cage. Such interaction is mainly stabilized by the high contribution from electrostatic type interactions (80.9 av%), with a lesser contribution from charge-transfer (17.4 av%) and London type interactions (1.7 av%). Moreover, the ionic character of the electrostatic contributions decreases from 90.7% for hydride to 68.6% for the iodide counterpart, in line with the decrease in hardness according to the Pearson's acid-base concept (HSAB) owing to the major role of higher electrostatic interaction terms related to the softer (Lewis) bases. Lastly, the [Ag₈{S₂P(OPr)₂}₆]²⁺ cluster is able to adapt its geometry in order to maximize the interaction towards respective monoatomic anion, exhibiting structural flexibility. Such insights shed light on the physical reasoning necessary for a better understanding of the different stabilizing and destabilizing contributions related to metal-based cavities towards favorable incorporation of different monoatomic anions.

Rapid access to sulfate-encapsulated symmetrical and asymmetrical capsules based on silver–pyrazole complex cations

A new class of sulfate-encapsulated symmetrical and asymmetrical capsules is developed through ion-pair complexation.