Session: ACS Award in Inorganic Chemistry (2021) Symposium Honoring Kristin Bowman-James:

Strategically designed classes of macrocyclic and non-cyclic ligands have been used as anion hosts for a variety of purposes, including encapsulation and separations. For instance, sulfate, a key detrimental anion in nuclear waste remediation, has been one of our target anions. The synthesis of an unsymmetrical, semi-rigid tetraurea macrocycle, 1, which selectively traps SO₄^2- with high affinity in a 0.5% water:DMSO-d₆ solution (K_a = 90,500 M^-1), was recently reported by our group. A crystal structure for the SO₄^2- complex indicates encapsulation of a single SO₄^2- within the cavity through nine hydrogen bonds. However, key findings suggest that added water plays a significant role in binding stoichiometries. As the percent water increases, binding appears to change from a 1:1 to 2:1 macrocyclic:sulfate association. The structural interactions as a function of water content could potentially have an impact on separations challenges, since the 2:1 sandwich complexes seem to indicate stronger complex formation. Furthermore, expanding the anion host through attachment of two 18-crown-6 wings to a tetra-urea chelate “partial macrocycle” results in selective ion-pair binding towards acetate (2) and carbonate (3) anions in presence of co-bound potassium ions. This presentation will describe our studies of anion confinement and influence of water on host-guest anion and ion-pair interactions.

Victor Day

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