Cs[H₂NB₂(C₆F₅)₆] Featuring an Unequivocal 16-Coordinate Cation

Cesium bis­(perfluoro-triphenylborane)­amide, Cs­[H2NB2(C6F5)6] (1), has been prepared by the reaction of sodium salt and CsF in dichloromethane and water. The compound is exceptional for a [H2NB2(C6F5)6]− salt in that it contains a monatomic solute-free cation. Determination of the molecular structure revealed a novel C2 symmetrical conformation of the weakly coordinating [H2NB2(C6F5)6]− anion, which gives rise to an unprecedented 16-coordinate (CN 16) Cs+ cation in a likewise unprecedented tetracosahedral arrangement of F atoms. The poor solubility of 1 allows nearly quantitative separation of Cs+ from water, which suggests potential applications as an effective 134/137Cs remover from nuclear waste solutions, administration as an antidote for 134/137Cs poisoning, and use for 131/137Cs radiotherapy (brachytherapy). Rb­[H2NB2(C6F5)6]·CH2Cl2 (2) has also been characterized, featuring two inequivalent Rb+ cations having CN 10, one of which involves Rb+(η2-Cl2CH2)2 coordination.