Design, Isolation, and Spectroscopic Analysis of a Tetravalent Terbium Complex

Synthetic strategies to yield molecular complexes of high-valent lanthanides, other than the ubiquitous Ce⁴⁺ ion, are exceptionally rare, and thorough, detailed characterization in these systems is limited by complex lifetime and reaction and isolation conditions. The synthesis of high-symmetry complexes in high purity with significant lifetimes in solution and the solid state is essential for determining the role of ligand-field splitting, multiconfigurational behavior, and covalency in governing the reactivity and physical properties of these potentially technologically transformative tetravalent ions. We report the synthesis and physical characterization of an S₄ symmetric, four-coordinate tetravalent terbium complex, [Tb(NP(1,2-bis-Bu-diamidoethane)(NEt₂))₄] (where Et is ethyl and Bu is tert-butyl). The ligand field in this complex is weak and the metal-ligand bonds sufficiently covalent so that the tetravalent terbium ion is stable and accessible via a mild oxidant from the anionic, trivalent, terbium precursor, [(Et₂O)K][Tb(NP(1,2-bis-Bu-diamidoethane)(NEt₂))₄]. The significant stability of the tetravalent complex enables its thorough characterization. The stepwise development of the supporting ligand points to key ligand control elements for further extending the known tetravalent lanthanide ions in molecular complexes. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), and density functional theory studies indicate a 4f⁷ ground state for [Tb(NP(1,2-bis-Bu-diamidoethane)(NEt₂))₄] with considerable zero-field splitting, demonstrating that magnetic, tetravalent lanthanide ions engage in covalent metal-ligand bonds. This result has significant implications for the use of tetravalent lanthanide ions in magnetic applications since the observed zero-field splitting is intermediate between that observed for the trivalent lanthanides and for the transition metals. The similarity of the multiconfigurational behavior in the ground state of [Tb(NP(1,2-bis-Bu-diamidoethane)(NEt₂))₄] (measured by Tb L₃-edge XAS) to that observed in TbO₂ implicates ligand control of multiconfigurational behavior as a key component of the stability of the complex.