Superalkali-Alkalide Interactions and Ion Pairing in Low-Polarity Solvents.

René Riedel,Heungjae Choi,Brendan T Sperling,Eva Zurek,Anthony G M Barrett,Nicholas C Pyper,Daniel Malko,Andrew G Seel,Anthony Kucernak,Daniel P Miller,Peter P Edwards,Adrian Porch,Thomas F Headen

doi:10.1021/jacs.1c00115

Abstract

The nature of anionic alkali metals in solution is traditionally thought to be “gaslike” and unperturbed. In contrast to this noninteracting picture, we present experimental and computational data herein that support ion pairing in alkalide solutions. Concentration dependent ionic conductivity, dielectric spectroscopy, and neutron scattering results are consistent with the presence of superalkali–alkalide ion pairs in solution, whose stability and properties have been further investigated by DFT calculations. Our temperature dependent alkali metal NMR measurements reveal that the dynamics of the alkalide species is both reversible and thermally activated suggesting a complicated exchange process for the ion paired species. The results of this study go beyond a picture of alkalides being a “gaslike” anion in solution and highlight the significance of the interaction of the alkalide with its complex countercation (superalkali).

Highlights

Anionic forms of the electropositive Group I metals, with the exception of lithium, can be generated in condensed phases.[1]
Mindful of our above discussion concerning the effect of a perturbation of the alkalide on its NMR signature, we suggest that this is due to the conformational flexibility of the superalkali−alkalide complex of HMHC that is absent in the 15-crown-5 case, bearing in mind that both chairlike and boatlike conformations for the HMHC macrocycle exist in crystalline systems
Alkalides have a unique place in the history and chemistry of the s-block elements.[1]

Summary

■ INTRODUCTION

Anionic forms of the electropositive Group I metals, with the exception of lithium, can be generated in condensed phases.[1]. The Hirshfeld charges capture the loss of electron density from the encapsulated metal atom and an increase in electron density on the surrounding metal for all ion paired systems, as expected for the alkalide state being maintained even in a close ion pair Both K-HMHC (Na) and K-15-crown-52 (Na) possess significant dipole moments, in line with the rationale for the experimentally observed increase in molar conductivity at higher alkalide concentrations (Figure 3). The HOMOs of the superalkali−alkalide complexes are concentrated on the sodide component, and they have a large spatial extent past the alkalide portion of the ion pair, in agreement with our neutron scattering results and with the crystallographically derived radii of alkalides in the solid state These HOMOs are formed from a bonding in-phase interaction between the SOMO of the sodium atom with the SOMO of the potassium superalkali, which itself contains character from the LUMOs of the complexant, as well as 4s and 5s K orbitals. Further work remains as to identifying the precise nature, and mechanisms, of these intriguing exchange processes

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES