On the Possibility of Using Aza-Cryptands to Design Superalkalis

The possibility to design new superalkali
species by encapsulating
main-group s-block metals (alkali metal and alkaline
earth metal) into aza-cryptands has been examined under density functional
theory in this work. Utilizing 36adamanzane (36adz) as an example, a series of M@36adz (M = Li, Na, K,
Be, Mg, and Ca) complexes have been constructed and investigated in
detail. It is interesting to find that these complexes possess much
lower IE values (1.94–2.34 eV) than that of 3.89 eV for the
Cs atom due to the repulsion between the valence electron of embedded
M atoms and lone pairs of N atoms in the complexant. Therefore, these
complexes can be regarded as a new kind of superalkalis. However,
only the Li@36adz and Be@36adz complexes are
found to be thermodynamically stable with negative interaction energies
and positive dissociation energies because 36adz is not
large enough to wrap Na, K, Mg, and Ca atoms. Thus, a larger cryptand
tri-pip-aza222 was further used to construct the M@tri-pip-aza222
(M = Na and K) complexes with higher stability and better reducing
ability than M@36adz (M = Na and K). We, therefore, hope
that experimental chemists could synthesize such metal complexes,
as novel superalkali species by choosing proper cryptands for specific
metal atoms.