Saalfrank-type {Fe2L3} host–guest complexes – prediction of ion selectivity by quantum chemical calculations VII

Abstract

Based on density functional calculations (B3LYP/LANL2DZp (LANL2DZp=LANL2DZ augmented with polarization functions on non-hydrogen atoms)), the selectivity of the {2}-metallocryptand [Fe2L3] (L2− = 2,6-dibutane-1,3-dionylpyridine dianion) was investigated and the experimentally known K+, Sr2+ and Ba2+ cation selectivity rationalized. The rigidity of L2−, visible in the small torsion angles compared to previously investigated classical organic cryptands, can be identified as the origin of this discrimination. Derived from the hosted alkali and alkaline earth cations, the size of the {2}-metallocryptand’s [Fe2L3] cavity is analogous to the cryptand [2.2.phen]. Additionally, the quantum chemical calculations identified the {2}-metallocryptand [Fe2L3] as the first experimentally verified metalloproton sponge (calculated (B3LYP/LANL2DZp) proton affinity −259.8 kcal mol−1). This high gas-phase proton affinity clearly explains why empty [Fe2L3] was never observed, and only [(HN)3Fe2L3]3+ or [MFe2L3]m+ are known.