Solvent-Controlled Separation of Integratively Self-Sorted Pd2LA 2LB 2 Coordination Cages

Abstract

The integrative implementation of multiple different components into metallosupramolecular self-assemblies requires sophisticated strategies to avoid the formation of statistical mixtures. Previously, the key focus was set on thermodynamically driven reactions of simple homoleptic into complex heteroleptic structures. Using Pd₂L^A ₂L^B ₂-type coordination cages, we herein show that integrative self-sorting can be reversed by a change of solvent (from DMSO to MeCN) to favor narcissistic re-segregation into coexisting homoleptic species Pd₂L^A ₄ and Pd₃L^B ₆. Full separation ("unsorting") back to a mixture of the homoleptic precursors was finally achieved by selective precipitation of Pd₃L^B ₆ with anionic guest G¹ from MeCN, keeping pure Pd₂L^A ₄ in solution. When a mixture of homoleptic Pd₃L^B ₆ and heteroleptic Pd₂L^A ₂L^B ₂ is exposed to a combination of two different di-anions (G¹ and G²) in DMSO, selective guest uptake gives rise to two defined coexisting host-guest complexes. A joint experimental and deep theoretical investigation via liquid-state integral equation theory of the reaction thermodynamics on a molecular level accompanied by solvent distribution analysis hints at solvent expulsion from Pd₂L^A ₄ to favor the formation of Pd₂L^A ₂L^B ₂ in DMSO as the key entropic factor for determining the solvent-specific modulation of the cage conversion equilibrium.

Keywords: cages; computational chemistry; host–guest chemistry; self-assembly; solvation.

Figure 1

Two shape‐complementary ligands L^A and L^B are able to separately react with Pd(II) cations to homoleptic, strained helicate Pd₂ L^A ₄ or trinuclear ring Pd₃ L^B ₆, respectively. When mixed in DMSO, they quantitatively assemble to heteroleptic cage Pd₂ L^A ₂ L^B ₂ that can be partially reconverted into narcissistically coexisting helicate and ring by changing the solvent to MeCN. Full separation can be achieved by selective precipitation of the ring with an anionic guest.

Figure 2

a) NMR spectra of (from bottom to top): helicate Pd₂ L^A ₄, ring Pd₃ L^B ₆ and heteroleptic cage Pd₂ L^A ₂ L^B ₂ in DMSO‐d₆ (298 K), the equilibrated mixture formed from Pd₂ L^A ₂ L^B ₂ after solvent exchange to MeCN, pure Pd₃ L^B ₆ and pure Pd₂ L^A ₄ in MeCN (298 K), b) ESI mass spectrum of Pd₂ L^A ₂ L^B ₂ (* trace of a Pd₂ L^B ₄ species).

Figure 3

a) Partitioning of aromatic guest G² and aliphatic guest G¹ in heteroleptic cage Pd₂ L^A ₂ L^B ₂ and in ring Pd₃ L^B ₆, respectively, from the mixture of all components, b) ESI mass spectrometric evidence for the selective formation of host–guest complexes G¹ @Pd₂ L^A ₂ L^B ₂ and G² @Pd₃ L^B ₆ (both patterns are from the same mass spectrum, see Supporting Information for further details).

Figure 4

Calculated contributions [kcal mol⁻¹] to the total reaction, gas phase reaction, and solvation thermodynamics (with temperature dependent PMV correction factor) for the cage separation reaction shown (1) from the QM (left) and the FF (middle) approach in both solvents using data in Table 1. The difference (right) corresponds to the transfer from MeCN to DMSO (MeCN→DMSO).

Figure 5

a) Perspective view of QM‐optimized cage structures, from left to right: [Pd₂ L^A ₂ L^B ₂]⁴⁺, [Pd₂ L^A ₄]⁴⁺, [Pd₃ L^B ₆]⁶⁺. Hydrogen atoms have been omitted for clarity. Carbon atoms of L^A are shown in green while L^B is shown in orange. In addition, calculated EC‐RISM solvent distributions for DMSO (upper row) and MeCN (lower row) are shown. Red and yellow surfaces represent oxygen and sulfur distributions of DMSO, while blue and gray surfaces characterize nitrogen and carbon distributions of MeCN. All isosurfaces are shown for g(r)=9.5. b) Radial solvent distribution functions g(r) for the DMSO oxygen (blue) and MeCN nitrogen (orange) sites around the individual cages obtained from EC‐RISM calculations; upper row: around the cavities in the geometric centers; lower row: around the palladium atoms of each cage, respectively. For each distribution, additionally the spherical integral of g(r) from 0 to r′ yielding the solvent particle number N is shown as a darker, dashed line.