Being Positive is Not Everything - Experimental and Computational Studies on the Selectivity of a Self-Assembled, Multiple Redox-State Receptor that Binds Anions with up to Picomolar Affinities

The interaction of the self-assembled trinuclear ruthenium bowl 1³⁺ , that displays three other accessible oxidation states, with oxo-anions is investigated. Using a combination of NMR and electrochemical experimental data, estimates of the binding affinities of 1⁴⁺ , 1⁵⁺ , and 1⁶⁺ for both halide and oxo-anions were derived. This analysis revealed that, across the range of oxidation states of the host, both high anion binding affinities (>10⁹ M^-1 for specific guests bound to 1⁶⁺ ) and high selectivities (a range of >10⁷ M^-1 ) were observed. As the crystal structure of binding of the hexafluorophosphate anion revealed that the host has two potential binding sites (named the α and β pockets), the host-guest properties of both putative binding sites of the bowl, in all of its four oxidation states, were investigated through detailed quantum-based computational studies. These studies revealed that, due to the interplay of ion-ion interactions, charge-assisted hydrogen-bonding and anion-π interactions, binding to the α pocket is generally preferred, except for the case of the relatively large and lipophilic hexafluorophosphate anionic guest and the host in the highest oxidation states, where the β pocket becomes relatively favourable. This analysis confirms that host-guest interactions involving structurally complex supramolecular architectures are driven by a combination of non-covalent interactions and, even in the case of charged binding pairs, simple ion-ion interactions alone cannot accurately define these recognition processes.