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

A series of lanthanide complexes bearing organic radical ligands, [Ln(CpR )2 (bipy·- )] [Ln = La, CpR = Cptt (1); Ln = Ce, CpR = Cptt (2); Ln = Ce, CpR = Cp″ (3); Ln = Ce, CpR = Cp‴ (4)] [Cptt = {C5 H3 t Bu2 -1,3}- ; Cp″ = {C5 H3 (SiMe3 )2 -1,3}- ; Cp‴ = {C5 H2 (SiMe3 )3 -1,2,4}- ; bipy = 2,2'-bipyridyl], were prepared by reduction of [Ln(CpR )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(C6 H5 )CPh2 O}] (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.