Isolation of +2 rare earth metal ions with three anionic carbocyclic rings: bimetallic bis(cyclopentadienyl) reduced arene complexes of La²⁺ and Ce²⁺ are four electron reductants

A new option for stabilizing unusual Ln²⁺ ions has been identified in the reaction of Cp′₃Ln, 1-Ln (Ln = La, Ce; Cp′ = C₅H₄SiMe₃), with potassium graphite (KC₈) in benzene in the presence of 2.2.2-cryptand. This generates [K(2.2.2-cryptand)]₂[(Cp′₂Ln)₂(μ-η⁶:η⁶-C₆H₆)], 2-Ln, complexes that contain La and Ce in the formal +2 oxidation state. These complexes expand the range of coordination environments known for these ions beyond the previously established examples, (Cp″₃Ln)^1- and (Cp′₃Ln)^1- (Cp″ = C₅H₃(SiMe₃)₂-1,3), and generalize the viability of using three anionic carbocyclic rings to stabilize highly reactive Ln²⁺ ions. In 2-Ln, a non-planar bridging (C₆H₆)^2- ligand shared between two metals takes the place of a cyclopentadienyl ligand in (Cp′₃Ln)^1-. The intensely colored (ε = ∼8000 M^-1 cm^-1) 2-Ln complexes react as four electron reductants with two equiv. of naphthalene to produce two equiv. of the reduced naphthalenide complex, [K(2.2.2-cryptand)][Cp′₂Ln(η⁴-C₁₀H₈)].