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 solid-state are 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-^tBu-diamidoethane)(NEt₂))₄] (where Et is ethyl and tBu 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-^tBu-diamidoethane)(NEt₂))₄]. The significant stability of the tetravalent complex enables its thorough characterization. The step-wise 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 (XAS), and density functional theory studies indicate a 4f⁷ ground state for [Tb(NP(1,2-bis-^tBu-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-^tBu-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.