Thermodynamic aspects of aurophilic hydrogelators.

The complexes [Au(4-pyridylethynyl)(phosph)] (phosph = PTA (1), DAPTA (2)) are known to produce supramolecular aggregates and gels in water. We studied the impact of these aggregation processes in the absorption spectra, (1)H NMR (at different temperatures and concentrations), and DLS and estimated the equilibrium constant for a single step aggregation of the molecule (K = 26760 and 2590 M(-1) for 1 and 2, respectively, at 25 °C). We present spectroscopic evidence for the presence of Au···Au contacts in the aggregates: the recorded changes on (1)H NMR and the appearance of new absorption bands assigned to (σAu···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 (ΔG ∼ -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 1 is stronger than the aggregation of monomer 2 and the existence of aurophilic interactions.