Thermodynamic Aspects of Aurophilic Hydrogelators

The complexes [Au­(4-pyridylethynyl)­(phosph)] (phosph = PTA (<b>1</b>), DAPTA (<b>2</b>)) are known to produce supramolecular aggregates and gels in water. We studied the impact of these aggregation processes in the absorption spectra, ¹H NMR (at different temperatures and concentrations), and DLS and estimated the equilibrium constant for a single step aggregation of the molecule (<i>K</i> = 26760 and 2590 M^–1 for <b>1</b> and <b>2</b>, respectively, at 25 °C). We present spectroscopic evidence for the presence of Au···Au contacts in the aggregates: the recorded changes on ¹H NMR and the appearance of new absorption bands assigned to (σ*_Au···Au–π*) have been attributed to the short (Au···Au) average distances in the aggregates. Relativistic density functional theory computations support the existence of short Au···Au distances and reveal charge-transfer in the aurophilic interactions. The free energy for a single step aggregation was calculated from the experimental data, and the value obtained (Δ<i>G</i> ∼ −20 kJ/mol) is in good agreement with the expected values in the order of the energies found for hydrogen bonds. The DFT computations confirm the experimental findings that aggregation of monomer <b>1</b> is stronger than the aggregation of monomer <b>2</b> and the existence of aurophilic interactions.