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 = {C5H3tBu2-1,3}−; Cp″ = {C5H3(SiMe3)2-1,3}−; Cp‴
= {C5H2(SiMe3)3-1,2,4}−; bipy = 2,2′-bipyridyl], were prepared by reduction
of [Ln­(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(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.