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

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(OBu)₂Ar)₃-arene)(THF)] (1-Ln^OBu) and [K(THF)₆][Ln^III((OSi(OBu)₂Ar)₃-arene)(OSiPh₃)] (2-Ln^OBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OBu)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^OBu) 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(OBu)₂Ar)₃-arene)(OSiPh₃)] (3-Ce^OBu) 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.