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

• Published in: Chemical science (Cambridge) Vol. 15; no. 18; pp. 6874 - 6883
• Main Authors: Tricoire, Maxime, Hsueh, Fang-Che, Keener, Megan, Rajeshkumar, Thayalan, Scopelliti, Rosario, Zivkovic, Ivica, Maron, Laurent, Mazzanti, Marinella
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 08.05.2024; The Royal Society of Chemistry
• Subjects: Cerium; Chemical Sciences; Chemistry; Crystallography; Lanthanides; Ligands; Oxidation; Praseodymium; Scaffolds; Terbium; Toluene; Valence
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d4sc00051j

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. Robust arene-anchored polydentate siloxide ligands allow to control unwanted ligand rearrangements for the isolation of Tb( iv ) complexes thus enabling the use of the Tb( iii )/Tb( iv ) couple for the separation of Tb from the neighboring Dy ion.