Isolation and redox reactivity of cerium complexes in four redox states

• Published in: Chemical science (Cambridge) Vol. 14; no. 22; pp. 611 - 621
• Main Authors: Hsueh, Fang-Che, Rajeshkumar, Thayalan, Maron, Laurent, Scopelliti, Rosario, Sienkiewicz, Andrzej, Mazzanti, Marinella
• Format: Journal Article
• Language: English
• Published: CAMBRIDGE Royal Soc Chemistry 07.06.2023; Royal Society of Chemistry; The Royal Society of Chemistry
• Subjects: Cerium; Chemical Sciences; Chemistry; Chemistry, Multidisciplinary; Electron transfer; Electrons; Lanthanides; Ligands; Oxidation; Physical Sciences; Reactivity; Redox reactions; Science & Technology; Structural analysis; Substrates; Valence; SERIES; REDUCTIVE DISPROPORTIONATION; LANTHANIDE(III); CHEMISTRY; CRYSTAL-STRUCTURES; IONS; METAL; LIGAND; C-H ACTIVATION; CARBON-DIOXIDE
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d3sc01478a

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides with an arene ring in a tripodal ligand can stabilize cerium complexes in four different redox states and can promote multielectron redox reactivity in cerium complexes. Ce( iii ) and Ce( iv ) complexes [(LO 3 )Ce(THF)] ( 1 ) and [(LO 3 )CeCl] ( 2 ) (LO 3 = 1,3,5-(2-OSi(O t Bu) 2 C 6 H 4 ) 3 C 6 H 3 ) were synthesized and fully characterized. Remarkably the one-electron reduction and the unprecedented two-electron reduction of the tripodal Ce( iii ) complex are easily achieved to yield reduced complexes [K(2.2.2-cryptand)][(LO 3 )Ce(THF)] ( 3 ) and [K 2 {(LO 3 )Ce(Et 2 O) 3 }] ( 5 ) that are formally "Ce( ii )" and "Ce( i )" analogues. Structural analysis, UV and EPR spectroscopy and computational studies indicate that in 3 the cerium oxidation state is in between +II and +III with a partially reduced arene. In 5 the arene is doubly reduced, but the removal of potassium results in a redistribution of electrons on the metal. The electrons in both 3 and 5 are stored onto δ-bonds allowing the reduced complexes to be described as masked "Ce( ii )" and "Ce( i )". Preliminary reactivity studies show that these complexes act as masked Ce( ii ) and Ce( i ) in redox reactions with oxidizing substrates such as Ag + , CO 2 , I 2 and S 8 effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry. The tris-siloxide tripodal ligand stabilizes a cerium complex in four different oxidation states and allows the unprecedented two-electron reduction of Ce( iii ) to yield masked Ce( i ) enabling cerium based multielectron redox chemistry.