Comparisons of lanthanide/actinide +2 ions in a tris(aryloxide)arene coordination environment

A new series of Ln³⁺ and Ln²⁺ complexes has been synthesized using the tris(aryloxide)arene ligand system, ((^Ad,MeArO)₃mes)^3-, recently used to isolate a complex of U²⁺. The triphenol precursor, (^Ad,MeArOH)₃mes, reacts with the Ln³⁺ amides, Ln(NR₂)₃ (R = SiMe₃), to form a series of [((^Ad,MeArO)₃mes)Ln] complexes, 1-Ln. Crystallographic characterization was achieved for Ln = Nd, Gd, Dy, and Er. The complexes 1-Ln can be reduced with potassium graphite in the presence of 2.2.2-cryptand (crypt) to form highly absorbing solutions with properties consistent with Ln²⁺ complexes, [K(crypt)][((^Ad,MeArO)₃mes)Ln], 2-Ln. The synthesis of the Nd²⁺ complex [K(crypt)][((^Ad,MeArO)₃mes)Nd], 2-Nd, was unambiguously confirmed by X-ray crystallography. In the case of the other lanthanides, crystals were found to contain mixtures of 2-Ln co-crystallized with either a Ln³⁺ hydride complex, [K(crypt)][((^Ad,MeArO)₃mes)LnH], 3-Ln, for Ln = Gd, Dy, and Er, or a hydroxide complex, [K(crypt)][((^Ad,MeArO)₃mes)Ln(OH)], 4-Ln, for Ln = Dy. A Dy²⁺ complex with 18-crown-6 as the potassium chelator, [K(18-crown-6)(THF)₂][((^Ad,MeArO)₃mes)Dy], 5-Dy, was isolated as a co-crystallized mixture with the Dy³⁺ hydride complex, [K(18-crown-6)(THF)₂][((^Ad,MeArO)₃mes)DyH], 6-Dy. Structural comparisons of 1-Ln and 2-Ln are presented with respect to their uranium analogs and correlated with density functional theory calculations on their electronic structures.