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 [(H^N)₃Fe₂L₃]³⁺ or [MFe₂L₃]^m+ are known.

Graphical abstract

Keywords:

• Selective ion complexation
• Metallocryptands
• Metallocryptates
• Proton affinity
• DFT

Acknowledgments

A.S. and R.P. acknowledge the continuous manifold support of Prof. Dr. Rolf W. Saalfrank through all the years and his endorsement to perform this research program. Additionally, we thank Prof. Tim Clark for hosting this work at the CCC and the Regionales Rechenzentrum Erlangen (RRZE) for a generous allotment of computer time.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

For Part VI see: [1]

Dedicated to Prof. Dr. h. c. mult. Rudi van Eldik on the occasion of his 70th birthday, in appreciation of his continuous kind support and friendship.

¹ While for experimental reasons, terminal substituents R such as tert-butyl, thiophenyl, or naphthyl have been included in the ligand systems; here we restricted ourselves to the smaller methyl group.

² The [Mg⊂Fe₂L₃]²⁺ O-C-C-N value is not included, as it could be addressed as a special case. Vide supra.