Analysis of Lanthanide-Radical Magnetic Interactions in Ce(III) 2,2'-Bipyridyl Complexes

A series of lanthanide complexes bearing organic radical ligands, [Ln(Cp^R)₂(bipy·^-)] [Ln = La, Cp^R = Cp^tt (1); Ln = Ce, Cp^R = Cp^tt (2); Ln = Ce, Cp^R = Cp″ (3); Ln = Ce, Cp^R = Cp‴ (4)] [Cp^tt = {C₅H₃^tBu₂-1,3}^-; Cp″ = {C₅H₃(SiMe₃)₂-1,3}^-; Cp‴ = {C₅H₂(SiMe₃)₃-1,2,4}^-; bipy = 2,2'-bipyridyl], were prepared by reduction of [Ln(Cp^R)₂(μ-I)]₂ or [Ce(Cp‴)₂(I) (THF)] with KC₈ in the presence of bipy (THF = tetrahydrofuran). Complexes 1-4 were thoroughly characterized by structural, spectroscopic, and computational methods, together with magnetism and cyclic voltammetry, to define an unambiguous Ln(III)/bipy·^- radical formulation. These complexes can act as selective reducing agents; for example, the reaction of 3 with benzophenone gives [{Ce(Cp")₂(bipy)}₂{κ²-O,O'-OPhC(C₆H₅)CPh₂O}] (7), a rare example of a "head-to-tail" coupling product. We estimate the intramolecular exchange coupling for 2-4 using multiconfigurational and spin Hamiltonian methods and find that the commonly used Lines-type isotropic exchange is not appropriate, even for single 4f e^-/organic radical pairs.