Relative anion binding affinity in a series of interpenetrated coordination cages.

Previously, we have reported on the quantitative self-assembly of a series of interpenetrated double-cages [Pd4Ligand8] with ligands based on various organic backbones. For dibenzosuberone-based cages it was shown that anion binding in the outer two pockets follows an allosteric mechanism. Herein we wish to report the anion binding capabilities of three related phenothiazine cages. We present a systematic comparison of the relative halide (Cl(-) and Br(-)) binding affinities and the structural rearrangements of four double-cages based on NMR titrations, NOESY experiments and electronic structure calculations.

Template control over dimerization and guest selectivity of interpenetrated coordination cages.

We have previously shown that the self-assembly of dibenzosuberone-based bis-monodentate pyridyl ligands L(1) with Pd(II) cations leads to the quantitative formation of interpenetrated coordination cages [BF4@Pd4L(1)8]. The BF4(-) anion inside the central cavity serves as a template, causing the outer two pockets to show a tremendous affinity for allosteric binding of two small chloride anions. Here we show that derivatization of the ligand backbone with a bulky aryl substituent allows us to control the dimerization and hence the guest-binding ability of the cage by the choice of the templating anion. Steric constraints imposed by L(2) prevent the large BF4(-) anion from serving as a template for the formation of interpenetrated double cages. Instead, a single isomer of the monomeric cage [Pd2L(2)4] is formed. Addition of the small anionic template Cl(-) permits dimerization, yielding the interpenetrated double cage [Cl@Pd4L(2)8], whose enlarged outer pockets show a preference for the binding of large anions such as ReO4(-).

Counterion dynamics in an interpenetrated coordination cage capable of dissolving AgCl.

In solution, the eight BF(4)(-) counterions of a positively charged D(4)-symmetric interpenetrated [Pd(4)ligand(8)](8+) double cage (1) are localized in distinct positions. At low temperatures, one BF(4)(-) ion is encapsulated inside the central pocket of the supramolecular structure, two BF(4)(-) ions are bound inside the equivalent outer pockets, and the remaining five BF(4)(-) ions are located outside the cage structure (expressed by the formula [3 BF(4)@1][BF(4)](5)). On warming, the two BF(4)(-) ions in the outer pockets are found to exchange with the exterior ions in solution whereas the central BF(4)(-) ion stays locked inside the central cavity (here written as [BF(4)@1][BF(4)](7)). The exchange kinetics were determined by exchange spectroscopy (EXSY) NMR experiments and line-shape fitting in different solvents. The tremendously high affinity of this double cage for the binding of two chloride ions inside the outer pockets allows for complete exchange of two BF(4)(-) ions by the addition of solid AgCl to give [2 Cl+BF(4)@1][BF(4)](5). The uptake of the two chloride ions is allosteric and is thus accompanied by a structural rearrangement (compression along the Pd(4) axis) of the double cage structure. An analysis by using 900 MHz NOESY NMR spectroscopy shows that this compression of about 3.3% is associated with a helical twist of 8°, which together resemble a screw motion. As a consequence of squeezing each of the outer two pockets by 53%, the volume of the central pocket is increased by 43%, which results in an increase of 36% in the (19)F spin-lattice relaxation time (T(1)) of the central BF(4)(-) ion. The packing coefficients (PC) for the ions in the outer pockets (103% for BF(4)(-) and 96% for Cl(-)) were calculated.