Electronic Structures of Alaninehydroximate 15-Metallacrowns-5 with Lanthanum(III), Cerium(III), and Praseodymium(III) Ions

Molecular and electronic structures of heterobimetallic Ln(III)–Cu(II) C₅-symmetric metallamacrocycles (15-metallacrowns-5) bearing L-α-alaninehydroximate (Alaha) ligands are studied by density functional theory (DFT). Full structural DFT optimizations are performed for complexes of the composition {Ln(H₂O)₄[15-MC_Cu(II)Alaha-5]}³⁺ (Ln = La, Ce, Pr) in high- and low-spin states using the PBE functional in the scalar-relativistic approximation. The spin states modeled are: sextet, quartet, and doublet for the La(III)–Cu(II) complex; septet, quintet, triplet, and open-shell singlet for the Ce(III)–Cu(II) complex; octet, sextet, quartet, and doublet for the Pr(III)–Cu(II) complex. The involvement of all unpaired electrons in antiferromagnetic interactions substantially decreases the Gibbs free energy of the singlet Ce(III)–Cu(II) complex compared to that of the doublet La(III)–Cu(II) and Pr(III)–Cu(II) derivatives: –4.4 kcal/mol, –2.0 kcal/mol, and –2.2 kcal/mol respectively (at 5 K). Spin density distributions, electron localization functions (ELFs), and the Laplacian of the electron density in 15-MC-5 reveal O_h and D_6h types of the f electron density symmetry around the Ce and Pr nuclei respectively. The effect of the 15-MC-5 metallamacrocyclic environment on the outer electron shell of the central diamagnetic ion is exemplified by the La(III)–Cu(II) complex. The topological parameters of the electron density are calculated at (3, –1) bond critical points for metal–ligand interactions using the quantum theory of atoms in molecules (QTAIM).