Control of the Ln^3+/4+ redox potential through ligand design is a powerful tool for expanding the scarce family of Ln⁴⁺ molecular compounds. It is known that ligand configuration can play a crucial role in controlling many properties of complexes, including redox potentials. Small counterions, such as single alkali metals, can embed themselves in the coordination sphere of anionic complexes, creating structural distortions that change the ligand configuration to varying degrees. Ce³⁺ imidophosphorane (–N=PR₃, R = alkyl/amido) complexes exhibit redox potentials among those of the most potent chemical reductants and serve as a good model for other Ln⁴⁺ complexes.
In this poster, I will show how the size of the counterion impacts both structural and electronic properties of Ce³⁺ imidophosphorane complexes by using DFT calculations to explore the whole series of alkali metals (Ce^M, M = Li-Cs) as well as the outer sphere congener (Ce^K18c6). Specifically, I will show how the HOMO energy level steadily increases in a stepwise fashion from Ce^Li to Ce^K18c6. The increasing HOMO energy levels linearly correlate with increasing Ce●●●M⁺ distances in the series from Li⁺ to [K(18c6)₂]⁺, with R² = 0.985. A good linear relationship (R² = 0.953) between measured E_pa and the calculated HOMO energies supports that the electronic structure of Ce³⁺ is modulated by the alkali metal cation in solution. This stepwise trend is also followed in the computed TD-DFT spectra of the complexes, where Ce^Li shows the largest splitting of 4f-5d transitions, and the splitting steadily decreases to its lowest value in Ce^K18c6. In addition to this, the redox potentials show an interesting divergence in the series. Vertical and adiabatic detachment energies compared to the experimentally observed oxidation/reduction potentials suggest that complexes with smaller counterions may retain their alkali metal instead of ejecting it upon oxidation.

Alexander Steiner

Appearances