A New Mechanically-Interlocked [Pd2 L4 ] Cage Motif by Dimerization of two Peptide-based Lemniscates

Abstract

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed "lemniscates". Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents.

Keywords: catenane; chirality; coordination cage; mechanical bond; supramolecular chemistry.

Figure 1

a) Pd^II‐mediated assembly of the usual lantern‐shaped [Pd₂L₄] cage geometry; b) alternative self‐penetrating topology ^[18] and c) synthesis of ligand L and assembly with Pd^II cations to an interlocked dimer of Figure‐of‐eights.

Figure 2

a) Partial ¹H NMR spectra of ligand L (in CD₃CN), mononuclear assembly [PdL ₂] (in [D₆]DMSO) and interlocked dimer [Cl@Pd₂ L ₄] (in CD₃CN) encapsulating a Cl⁻ anion (peaks of major isomer assigned; small peaks=minor isomer); d) partial NOESY NMR spectrum of [Cl@Pd₂ L ₄] revealing characteristic contacts H_a/_a′ to H_e only observable in the catenated structure but not in the free ligand.

Figure 3

High‐resolution ESI mass and ion mobility spectra of a) mononuclear [PdL ₂] (calc. 991.4483), b) tetrafluoroborate‐containing [Pd₂ L ₄] (calc. 1351.9333) and c) chloride‐encapsulating [Cl@Pd₂ L ₄] (calc. 1334.9216). The gas phase ion mobility values and computed collisional cross sections (CCS) reveal a size increase from the monomer to the interlocked structure, a less defined structure for the BF₄ ⁻‐containing dimer (two broad mobility signals for m/z=1351.6; also compare convoluted NMR in Figure S7) and a well‐defined, again slightly contracted structure of [Cl@Pd₂ L ₄].

Figure 4

X‐ray structure of [BF₄@Pd₂ L ₄](BF₄) (solvents and outer anions omitted for clarity): a) side and b) top view of the catenated motif; c) space filling side views; d) detail showing the non‐covalent nesting of one of the coordinating pyridines in the basked‐shaped macrocyclic backbone of another ligand and e) X‐ray structure of toluene@calix‐4‐arene for comparison. ^[32]