Bis(tert-butoxydiphenylsilyl)amide Divalent Lanthanide Complexes

Abstract

The development of new ligand systems to stabilize "non-traditional/non-classical" divalent lanthanides is key to tuning the chemical and physical properties of their mixed principal quantum number 4fⁿ5d¹ ground states. The design and study of novel ligand systems which stabilize occupation of differing orbitals within the 5d manifold for these ions constitutes an area ripe for exploration. Our efforts toward the development of redox-innocent bulky silylamide ligands to stabilize pseudo-octahedral coordination geometries for divalent lanthanides have resulted in the synthesis of the bis(tert-butoxydiphenylsilyl)amide ligand, whose coordination complexes with Sm²⁺, Eu²⁺, and Yb²⁺ are reported herein. These systems have been fully characterized by single-crystal X-ray diffraction, elemental analysis, cyclic voltammetry, direct-current magnetometry, and infrared, nuclear magnetic resonance, and electronic absorption spectroscopies. Attempts to extend this system to the more reducing Tm²⁺ ion resulted in an inseparable mixture of products from which crystals of the analogous Tm²⁺ species and a reduced dinitrogen, bimetallic Tm³⁺-Tm³⁺ complex bridged by a η²-N₂^3- radical could be identified. Though progress toward six-coordinate complexes of reducing "traditional/classical" divalent ions is noted for these systems, further work is needed to improve the synthetic utility of this ligand framework for the study of "non-traditional/non-classical" divalent lanthanides with a mixed-principal quantum number 4fⁿ5d¹ ground state.

1. Synthetic Scheme for Ligand Derivatives 1-H, 1-K, and Classical Divalent Lanthanide Complexes 2-Ln (Ln = Sm, Eu, Yb), Along with Reaction Conditions Leading to an Inseparable Mixture of 2-Tm and the Reduced Dinitrogen Complex 3-Tm

1

Representative molecular structure of 2-Eu with thermal ellipsoids at 50%. Black, red, blue, tan, and blue-gray represent C, O, N, Si, and Eu respectively and hydrogen atoms and disordered solvent molecules omitted for clarity. 2-Ln are isotypic.

2

Molecular structure of 3-Tm with thermal ellipsoids at 50%. Black, red, blue, tan, purple, and cyan represent C, O, N, Si, I, and Tm respectively and hydrogen atoms, noncoordinated solvent molecules, and minor disordered components omitted for clarity.

3

UV–vis spectra for 1-H, 1-K, and 2-Ln (Ln = Sm, Eu, Yb) recorded in THF.

4

Cyclic voltammograms of 1.9 mM 2-Sm (top), 2.0 mM 2-Eu (middle), and 2.0 mM 2-Yb (bottom) in 200 mM [N­( ⁿ Bu)₄]­[PF₆] at 200 mV/s. Additional details in SI.

5

Variable-temperature zero-field cooled magnetic susceptibility curves for 2-Sm and 2-Eu measured under an applied field of 1 T.