Scalar relativistic density functional theoretical investigation of higher complexation ability of substituted 1,10-phenanthroline over bipyridine towards Am${}^{3+}$/Eu${}^{3+}$ ions

The better selectivity of Am${}^{3+}$ over Eu${}^{3+}$ ion with N‐based CyMe4‐BTPhen compared to CyMe4‐BTBP for experimentally observed [ML${}_2$(NO${}_3$)]${}^{2+}$ complexes was demonstrated using scalar relativistic DFT in conjunction with Born‐Haber thermodynamic cycle and COSMO solvation model. The calculated free energy of extraction, ΔG${}_{ext}$ reveals strong dependence on the hydration free energies of Am${}^{3+}$ and Eu${}^{3+}$ ions and week dependence to the difference in Gibbs free energy of solvation of the ligand or metal‐ligand complexes. Further, for the first time, we have computed the effect of co‐anion species ([M(NO${}_3$)${}_5$]${}^{2-}$) on ΔG${}_{ext}$ of Am${}^{3+}$ and Eu${}^{3+}$ ions with CyMe4‐BTPhen and CyMe4‐BTBP, which adds a positive contribution and thus reduces the ΔG${}_{ext}$. The calculated values of ΔΔΔG${}_{ext}$ (= ΔΔG${}_{ext,L1}$ – ΔΔG${}_{ext,L2}$, ΔΔG${}_{ext}$ = ΔG${}_{ext,M1}$ – ΔG${}_{ext,M2}$) can be used to avoid the very sensitive metal ion solvation energy, effect of co‐anionic species and thus provides a robust approach to determine the selectivity between two metal ions towards different competitive ligands. The natural population analysis (NPA), molecular orbital analysis, Mayer bond order analysis, and bond character analysis using Bader's quantum theory of atoms in molecules indicates slightly more covalency for complexes of Am${}^{3+}$ ion that are correlated to the experiental selectvity of Am${}^{3+}$ ion over Eu${}^{3+}$ ion and hence might be useful in the design and development of next generation extractants. (© 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)