Isolation and redox reactivity of cerium complexes in four redox states

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 [(LO3)Ce(THF)] (1) and [(LO3)CeCl] (2) (LO3 = 1,3,5-(2-OSi(OtBu)2C6H4)3C6H3) 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)][(LO3)Ce(THF)] (3) and [K2{(LO3)Ce(Et2O)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+, CO2, I2 and S8 effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry.