Abstract:

The use of container molecules as molecular capsules can lead to the stabilization of unusual, reactive, or previously unseen species in their inner spaces.  We are investigating macro-bicyclic hexacarboxamide cryptand molecules both as anion receptors and as ligands for coordination chemistry.  We report the discovery that the anion receptor causes the disproportionation of superoxide ion to O2 gas and the peroxide dianion, with the latter stabilized inside of the anion receptor with the formation of six strong hydrogen bonds.  This combination of anion receptor chemistry with small-molecule redox is reversible, pointing to possible technological applications in the area of energy storage.  In hexadeprotonated form, the anion recepter serves as a binucleating ligand for transition-metal ions M2+ (M = Mn, Fe, Co, Ni, and Zn).  The bimetallic complexes can be oxidized to produce a M-O-M unit that is stabilized in the interior of the container molecule.  Such M-O-M systems are for the later metal ions, very unusual.  We present the hypothesis that the oxygen bridge has radical character that is connected to the ability of these systems to act as oxidants or as oxidation catalysts. In this regard, we liken the electronic structure of the M-O-M units to the oxide bridges described in the Siegbahn mechanism for O-O bond formation in the oxygen evolving complex of photosystem II. ​