Molecular complexes of lanthanides in the +IV oxidation state were solely limited to the cerium ion for a long time. It is only in 2019 that the first two examples of molecular complexes of Tb(IV), [Tb(OSi(O^tBu)₃)₄] and [Tb(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄], were isolated. Furthermore, the praseodymium ion in the +IV oxidation state was never reported in molecular complexes.
Here we show that Tb(IV) chemistry can be extended to the monodentate triphenylsiloxide ligand with the isolation of the third example of a Tb(IV) molecular complex [Tb(OSiPh₃)₄(MeCN)₂] showing an overall higher thermodynamic and kinetic stability compared to [Tb(OSi(O^tBu)₃)₄]. More importantly, we discovered that this ligand can also support the isolation of the first molecular complex of praseodymium in the +IV oxidation state [Pr(OSiPh₃)₄(MeCN)₂].
The [Ln(OSiPh₃)₄(MeCN)₂] (Ln = Tb, Pr) complexes were characterized by X-ray crystallography, cyclic voltammetry, magnetometry, electron paramagnetic resonance and density functional theory calculations corroborating the +IV oxidation state. Solution stability studies in THF showed a significantly lower decomposition rate for [Tb(OSiPh₃)₄(MeCN)₂]compared to [Tb(OSi(O^tBu)₃)₄] suggesting a more important stabilization of the +IV oxidation state by the supporting triphenylsiloxide ligands. Furthermore, cyclic voltammetry experiments on [Tb(OSiPh₃)₄(MeCN)₂] shows redox potential values ∼ 0.4 V smaller compared to [Tb(OSi(O^tBu)₃)₄]. More importantly, a stabilization of the + IV oxidation state was noticed in the presence of outersphere countercations compared to alkali metal ions. Indeed, the cyclic voltammogram of [Tb(OSiPh₃)₄(MeCN)₂] measured in the presence of Na⁺ electrolyte shows a reduction potential shifted ∼ 0.7 V towards more positive values compared to[Tb(OSiPh₃)₄(MeCN)₂] measured in a NBu₄⁺ electrolyte. The complex [Pr(OSiPh₃)₄(MeCN)₂] shows an oxidation potential wave at a value 0.27 V more positive than [Tb(OSiPh₃)₄(MeCN)₂] which is consistent with the slight difference in the calculated redox potentials of the Ln(IV)/Ln(III) couples (Ln = Tb, Pr).
These new results provide key information in lanthanides tetravalent chemistry and provide a new area of study for redox reactivity and separation chemistry.

Farzaneh Fadaei-Tirani

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