Introduction

In molecular complexes, the lanthanides are predominantly trivalent (Ln⁺³). In fact, of the 15 lanthanide ions, only cerium is known to have significant solution chemistry in its tetravalent oxidation state (Ce⁺⁴). (1) While examples of low-valent lanthanide complexes have been reported, (2?7) high-valent ions in lanthanide chemistry remain a challenge. Concomitant with the preparation of this article, Mazzanti and co-workers published an example of a molecular Tb⁺⁴ complex in a low-symmetry, oxygen coordination environment in the complex [Tb(OSi(OtBu)₃)₃(?2-OSi(OtBu)₃]. (8) The accessibility of high-valent lanthanide chemistry could transform two key industrial chemical processes: (1) the beneficiation and purification of lanthanide ores (9) and (2) the separation of the minor actinides from lanthanide fission products in spent nuclear fuel reprocessing. (10) As a result, efforts to extend the aqueous solution chemistry of tetravalent lanthanides to the next two most readily oxidized lanthanides, praseodymium and terbium (Pr4+/Pr⁺³ = +3.2; Tb⁺⁴/Tb⁺³ = +3.1 V vs NHE), (11) have been pursued, but with little success beyond the in situ spectroscopic and potentiometric identification of redox processes. (12?14) From the most fundamental perspective, the isolation of a tetravalent terbium complex is important since it puts a new paramagnetic, isotropic ion on the periodic table and allows the crystal field effects in the tetravalent lanthanides to be benchmarked against the transition metals (Mn2+) and trivalent lanthanides (Gd⁺³).

Historically, in the absence of isolable molecular, tetravalent praseodymium or terbium complexes, cerium has been used as a surrogate to examine the ligand and solvent dependence of the Ln⁺⁴/Ln⁺³ redox couple. (15?17) Depending on the ligand, supporting cation, and solvent, the redox potential can be shifted up to 4 V. Given the large change in ionic radii on oxidation from Ce⁺³ to Ce⁺⁴ (?0.14 Å), (18) the coordination sphere has also been demonstrated to have kinetic control of the redox process. (19) This ligand control has led to rapid growth of tetravalent cerium coordination chemistry. (20) The use of cerium as a surrogate is validated by the demonstration that tetravalent late actinides, such as berkelium, can be preferentially stabilized by using similar ligand design principles. (21,22) These cerium coordination studies have also inspired the attempted oxidation of molecular terbium complexes in anaerobic and anhydrous conditions, but these efforts have failed in the isolation of a tetravalent terbium complex. (23?25) The limited data on the physical properties of tetravalent terbium are derived from studies on solid-state materials including doped oxides (e.g., ThO₂:Tb⁺⁴), (26,27) bulk binary terbium oxides and fluorides (TbO₂ and TbF₄), (28,29) and other extended solids. (30?34) Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) studies of these solid-state terbium complexes and related tetravalent lanthanide and actinide complexes indicate that the increased covalent bonding present in tetravalent f-block metal–ligand bonds can be employed to stabilize reactive tetravalent lanthanide ions. (26,28,33?36) Herein, we independently report the synthesis and structural characterization of a molecular tetravalent terbium complex that is stable in both solution and the solid state. (37?39) Most importantly, the 4f⁷, 8S^7/2 ground state is validated and probed through EPR, Tb L3-edge XAS, magnetic susceptibility, and density functional theory (DFT) studies...