Dynamic Equilibrium of a Supramolecular Dimeric Rhomboid and Trimeric Hexagon and Determination of Its Thermodynamic Constants

A supramolecular dimeric rhomboid and its trimeric counterpart, a hexagon, are generated by
design via the directional bonding methodology of self-assembly. The different-sized supramolecular
macrocycles formed by Pt-coordination undergo a concentration- and temperature-dependent dynamic
equilibrium. The two structures are characterized by multinuclear NMR and ESI-MS. Extensive study of
the dynamic equilibrium of the two species in solution is performed to obtain its thermodynamic properties.
By varying the ionic strength, μ, of the solutions, the true thermodynamic equilibrium constant, K, is
determined at each experimental temperature (K253 = 36 ± 7, K273 = 18 ± 6, K293 = 10 ± 3, K313 = 9 ±
2, K333 = 5 ± 2, and K353 = 3.0 ± 0.2). By applying these values of true K at the respective temperatures
to the van't Hoff equation extended with the entropy term, the standard enthalpy and entropy changes are
determined for the equilibrium:  with ΔH° = −18 ± 1 kJ mol-1 and ΔS° = −43 ± 4 J mol-1 K-1, respectively,
for the forward reaction (rhomboid to hexagon) of the equilibrium. The rhomboid is selectively crystallized,
and its crystal structure is determined by X-ray diffraction. The structure reveals a significant amount of
porosity as well as distortion of the rhomboid from planarity, leading to channels that can be observed
from two viewing positions of the packing.