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

A new option for stabilizing unusual Ln(2+) ions has been identified in the reaction of Cp′(3)Ln, 1-Ln (Ln = La, Ce; Cp′ = C(5)H(4)SiMe(3)), with potassium graphite (KC(8)) in benzene in the presence of 2.2.2-cryptand. This generates [K(2.2.2-cryptand)](2)[(Cp′(2)Ln)(2)(μ-η(6):η(6)-C(6)H(6))], 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′′(3)Ln)(1–) and (Cp′(3)Ln)(1–) (Cp′′ = C(5)H(3)(SiMe(3))(2)-1,3), and generalize the viability of using three anionic carbocyclic rings to stabilize highly reactive Ln(2+) ions. In 2-Ln, a non-planar bridging (C(6)H(6))(2–) ligand shared between two metals takes the place of a cyclopentadienyl ligand in (Cp′(3)Ln)(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′(2)Ln(η(4)-C(10)H(8))].