An Octa-Urea [Pd2 L4 ]4+ Cage that Selectively Binds to n-octyl-α-D-Mannoside

Anion-Driven Programmable Chiral Self-Sorting in Metal-Organic Cages and Structural Transformations between Heterochiral and Homochiral Cages

When a racemic mixture of chiral building blocks self-assembles to form discrete molecular or supramolecular cages, the system can adopt either social or narcissistic chiral self-sorting. However, control over such chiral self-sorting is hard to achieve with a desired choice of outcome. Herein, we report anion templated high-fidelity chiral self-sorting during the coordination-driven self-assembly of [Pd₂ L₄ ] metal-organic cages, with a racemic mixture of an axially chiral ligand. Upon varying the counter-anions, the outcome of the choice of chiral self-sorting, whether social or narcissistic, leading to kinetically favored heterochiral or thermodynamically favored homochiral cages, can be controlled through specific anion encapsulation. Non-encapsulating anion afforded a mixture of all possible diastereomers. Anion exchange enabled structural transformations between the diastereomers and the conversion of the mixture of diastereomers into homochiral diastereomers.

Configurational ligand isomerism in conjoined-cages

Double-decker shaped conjoined-cages of Pd₃L₄ formulation are prepared via self-assembly of Pd(II) with a set of three regioisomeric tridentate ligands. Alongside the targeted double-decker cage, unprecedented hour-glass shaped conjoined-cages of Pd₃L₄ formulation are also formed in two cases. The double-decker cage prepared from one ligand system and the hour-glass from another (but with a regioisomeric ligand) are structurally well suited to exemplify configurational ligand isomerism.

Self-assembly of a water-soluble endohedrally functionalized coordination cage including polar guests

Coordination cages containing endohedrally functionalized aromatic cavities are scarce in the literature. Herein, we report the self-assembly of a tetra-cationic super aryl-extended calix[4]pyrrole tetra-pyridyl ligand into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. They are defined by the four pyrrole NHs of the calix[4]pyrrole unit and the four inwardly directed α-protons of the coordinated pyridyl groups. The efficient assembly of the Pd(ii)-cage requires the inclusion of mono- and ditopic pyridyl N-oxide and aliphatic formamide guests. The monotopic guests only partially fill the cage's cavity and require the co-inclusion of a water molecule that is likely hydrogen-bonded to the endohedral α-pyridyl protons. The ditopic guests are able to completely fill the cage's cavity and complement both binding sites. We observed high conformational selectivity in the inclusion of the isomers of α,ω-bis-formamides. We briefly investigate the uptake and release mechanism/kinetics of selected polar guests by the Pd(ii)-cage using pair-wise competition experiments.

A tetra-cationic calix[4]pyrrole tetra-pyridyl ligand self-assembles into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. The Pd(ii)-cage encapsulates pyridyl N-oxide and aliphatic formamide guests in water.

Comparison of [Pd2L4][BF4]4 cages for binding of n-octyl glycosides and nitrate (L = isophthalamide or dipicolinamide linked dipyridyl ligand)

Two dipyridyl ligands were synthesized, where the pyridyl donor fragments were separated by an isophthalamide (1) or a dipicolinamide moiety (2). Both ligands formed [Pd2(Ligand)4][BF4]4 complexes in CD2Cl2 containing 5% dmso-d6. It was found that while [Pd2(1)4][BF4]4 readily binds to n-octyl glycosides and to nitrate anions, [Pd2(2)4][BF4]4 did not. The difference in binding properties could be rationalized based on the reduced flexibility and size of the [Pd2(2)4]2+ cage and/or stronger interior binding of a BF4- counter anion.

Anion binding properties of a hollow PdL-cage

The hollow [PdL][BArF]2 complex 1 of a tetra-pyridyl (py) ligand (L) has a [Pd(py)4]2+ coordination environment. Addition of coordinating anions resulted in the formation of a neutral species with Pd(py)2(anion)2 coordination environment (12A). These species bind further to the coordinating anions in the order Cl- > N3- > Br- > I- > AcO- with Ka1 : 1 ≤ 414 M-1. With relatively non-coordinating anions 1 remains intact and displays 1 : 2 binding behaviour dominated by the 1 : 1 stoichiometry in the order NO3- (∼105 M-1) » ClO4- and BF4- (∼103 M-1). As evidenced by crystal structure data, DFT calculations and {1H-19F}-HOESY NMR with BF4-, the anions are bound by charge assisted [C-H]+···anion interactions.

A Synthetic Galectin Mimic

Galectins are a galactoside specific subclass of carbohydrate binding proteins (lectins) involved in various cellular activities, certain cancers, infections, inflammations, and many other biological processes. The molecular basis for the selectivity of galectins is well-documented and revolves around appropriate interaction complementarity: an aromatic residue for C-H⋅⋅⋅π interactions and polar residues for (charge assisted) hydrogen bonds with the axial hydroxyl group of a galactoside. However, no synthetic mimics are currently available. We now report on the design and synthesis of the first galectin mimic (6), and show that it has a higher than 65-fold preference for n-octyl-β-galactoside (8) over n-octyl-β-glucoside (7) in CD2 Cl2 containing 5 % [D6 ]DMSO (with Ka ≥4500 M-1 for 6:8). Molecular modeling informed by nOe studies reveal a high degree of interaction complementarity between 6 and galactoside 8, which is very similar to the interaction complementarity found in natural galectins.

An Octa-Urea [Pd2 L4 ]4+ Cage that Selectively Binds to n-octyl-α-D-Mannoside

Designing compounds for the selective molecular recognition of carbohydrates is a challenging task for supramolecular chemists. Macrocyclic compounds that incorporate isophtalamide or bisurea spacers linking two aromatic moieties have proven effective for the selective recognition of all-equatorial carbohydrates. Here, we explore the molecular recognition properties of an octa-urea [Pd2 L4 ]4+ cage complex (4). It was found that small anions like NO3- and BF4- bind inside 4 and inhibit binding of n-octyl glycosides. When the large non-coordinating anion 'BArF ' was used, 4 showed excellent selectivity towards n-octyl-α-D-Mannoside with binding in the order of Ka ≈16 M-1 versus non-measurable affinities for other glycosides including n-octyl-β-D-Glucoside (in CH3 CN/H2 O 91 : 9).