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((OSiPhAr)-arene)(THF)] (1-Ln) and [K(toluene){Ln((OSiPhAr)-arene)(OSiPh)}] (2-Ln) (Ln = Ce, Tb, Pr), of the (HOSiPhAr)-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(iii) analogue complexes, [Ln((OSi(OBu)Ar)-arene)(THF)] (1-Ln) and [K(THF)][Ln((OSi(OBu)Ar)-arene)(OSiPh)] (2-Ln) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OBu)Ar)-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) compared to those of the -butoxy (1-Ln) ligand. Furthermore, the addition of the -OSiPh ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [Ce((OSi(OBu)Ar)-arene)(OSiPh)] (3-Ce) and [Ce((OSiPhAr)-arene)(OSiPh)(THF)] (3-Ce), were prepared by chemical oxidation of the Ce(iii) analogues. Chemical oxidation of the Tb(iii) and Pr(iii) complexes (2-Ln) was also possible, in which the Tb(iv) complex, [Tb((OSiPhAr)-arene)(OSiPh)(MeCN)] (3-Tb), 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.