A Switchable Palladium(II) Trefoil Entangled Tetrahedron with Temperature Dependence and Concentration Independence

Abstract

Self-assembly makes metallo-interlocked architectures attractive targets, but being in equilibrium with smaller species means that they can suffer from dilution effects. We show that a junctioned system gives rise to a [Pd₄ (L)₂ ]⁸⁺ trefoil entangled tetrahedron irrespective of concentration. Heating the sample reversibly shifts the equilibrium from the knot to an isomeric non-interlocked dual metallo-cycle, demonstrating that thermodynamic equilibria can still be exploited for switching even in the absence of concentration effects.

Keywords: Interlocked Architecture; Palladium; Self-Assembly.

Figure 1

Top: core ligand scaffold used in this project, with the identity of the three ethylene glycol chains of the three ligands also given: L‐mono possesses a single core, while L‐4PEG and L‐6PEG have two cores bridged by 4PEG and 6PEG chains respectively. Bottom: identity and switchability of the mono, 4PEG and 6PEG systems used in this study.

Figure 2

Partial stacked ¹H NMR spectra (400 MHz, [D₆]DMSO) at 298 K of a) L‐mono, b) 2 : 2 combination of L‐mono and Pd^II at 15 mM, giving CYCLE‐mono, c) 2 : 2 combination of L‐mono and Pd^II at 60 mM, giving a 1 : 3 ratio of CAT‐mono/CYCLE‐mono, d) 2 : 4 combination of L‐4PEG and Pd^II, giving DUAL‐4PEG, e) 1 : 2 combination of L‐6PEG and Pd^II, giving a 85 : 15 ratio of TET‐6PEG/DUAL‐6PEG, and at 361 K of f) 2 : 4 combination of L‐6PEG and Pd^II, giving a 1 : 9 ratio of TET‐6PEG/DUAL‐6PEG. Blue spots are non‐interlocked species, red spots are interlocked.

Figure 3

Representations of the molecular models of a) CYCLE‐mono, b) CAT‐mono, and c) DUAL‐4PEG. Structures in (a) and (c) were modelled using xtb‐GFN2 while that in b is from DFT (BP86, def2‐SVP/def2‐TZVPP). Colours: carbon grey or green, nitrogen light blue, oxygen red, palladium purple, hydrogen atoms omitted for clarity.

Figure 4

a) DFT structure of TET‐6PEG (BP86, def2‐SVP/def2‐TZVPP), with views showing b) “inner” and “outer” aromatic regions, c) alkyl and aromatic regions, and 2D ¹H NOE correlations (400 MHz, [D₆]DMSO, 298 K, 200 ms) between d) “inner” and “outer” aromatic environments and e) aromatic and alkyl environments. Correlations denoted through double headed arrows or asterisks of the same colour, and are between hydrogen atoms on the labelled carbon atoms. 2D NMR spectra in Supporting Information. Colours: carbon pink (outer) or green (inner), nitrogen light blue, oxygen red, palladium purple, hydrogen atoms omitted for clarity.