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 ((OSiPh 2 Ar) 3 -arene)(THF) 3 ] (1-Ln Ph ) and [K(toluene){Ln III ((OSiPh 2 Ar) 3 -arene)(OSiPh 3 )}] (2-Ln Ph ) (Ln = Ce, Tb, Pr), of the (HOSiPh 2 Ar) 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 t Bu) 2 Ar) 3 -arene)(THF)] (1-Ln O t Bu ) and [K(THF) 6 ][Ln III ((OSi(O t Bu) 2 Ar) 3 -arene)(OSiPh 3 )] (2-Ln O t Bu ) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(O t Bu) 2 Ar) 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 O t Bu ) ligand. Furthermore, the addition of the -OSiPh 3 ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [Ce IV ((OSi(O t Bu) 2 Ar) 3 -arene)(OSiPh 3 )] (3-Ce O t Bu ) and [Ce IV ((OSiPh 2 Ar) 3 -arene)(OSiPh 3 )(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 ((OSiPh 2 Ar) 3 -arene)(OSiPh 3 )(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.