Siloxide tripodal ligands as a scaffold for stabilizing lanthanides in the +4 oxidation state

Abstract

Synthetic strategies to isolate molecular complexes of lanthanides, other than cerium, in the +4 oxidation state remain elusive, with only four complexes of Tb(IV) isolated so far. Herein, we present a new approach for the stabilization of Tb(IV) using a siloxide tripodal trianionic ligand, which allows the control of unwanted ligand rearrangements, while tuning the Ln(III)/Ln(IV) redox-couple. The Ln(III) complexes, [Ln^III((OSiPh₂Ar)₃-arene)(THF)₃] (1-Ln^Ph) and [K(toluene){Ln^III((OSiPh₂Ar)₃-arene)(OSiPh₃)}] (2-Ln^Ph) (Ln = Ce, Tb, Pr), of the (HOSiPh₂Ar)₃-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(III) analogue complexes, [Ln^III((OSi(O^tBu)₂Ar)₃-arene)(THF)] (1-Ln^OtBu) and [K(THF)₆][Ln^III((OSi(O^tBu)₂Ar)₃-arene)(OSiPh₃)] (2-Ln^OtBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(O^tBu)₂Ar)₃-arene. The cyclic voltammetry studies showed a cathodic shift in the oxidation potential for the cerium and terbium complexes of the more electron-donating phenyl substituted scaffold (1-Ln^Ph) compared to those of the tert-butoxy (1-Ln^OtBu) ligand. Furthermore, the addition of the –OSiPh₃ ligand further shifts the potential cathodically, making the Ln(IV) ion even more accessible. Notably, the Ce(IV) complexes, [Ce^IV((OSi(O^tBu)₂Ar)₃-arene)(OSiPh₃)] (3-Ce^OtBu) and [Ce^IV((OSiPh₂Ar)₃-arene)(OSiPh₃)(THF)₂] (3-Ce^Ph), were prepared by chemical oxidation of the Ce(III) analogues. Chemical oxidation of the Tb(III) and Pr(III) complexes (2-Ln^Ph) was also possible, in which the Tb(IV) complex, [Tb^IV((OSiPh₂Ar)₃-arene)(OSiPh₃)(MeCN)₂] (3-Tb^Ph), was isolated and crystallographically characterized, yielding the first example of a Tb(IV) supported by a polydentate ligand. The versatility and robustness of these siloxide arene-anchored platforms will allow further development in the isolation of more oxidizing Ln(IV) ions, widening the breadth of high-valent Ln chemistry.