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

Abstract

A series of lanthanide complexes bearing organic radical ligands, [Ln(Cp)(bipy·)] [Ln = La, Cp = Cp (1); Ln = Ce, Cp = Cp (2); Ln = Ce, Cp = Cp″ (3); Ln = Ce, Cp = Cp‴ (4)] [Cp = {CHBu-1,3}; Cp″ = {CH(SiMe)-1,3}; Cp‴ = {CH(SiMe)-1,2,4}; bipy = 2,2'-bipyridyl], were prepared by reduction of [Ln(Cp)(μ-I)] or [Ce(Cp‴)(I) (THF)] with KC 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")(bipy)}{κ-O,O'-OPhC(CH)CPhO}] (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.

Reduction of 2,2′-Bipyridine by Quasi-Linear 3d-Metal(I) Silylamides—A Structural and Spectroscopic Study

“…The presence of reduced 2,2-bipyridine can be deducted from two strong bands at 532 and 562 nm and three bands between 700 and 1000 nm, as shown for in situ generated Na(bipy) in THF [44]. For 2,2-bipyridyl radical anion containing metal complexes the situation is similar although the two high-energy bands are often not resolved [30,31,41,45]. The high energy transitions are tentatively assigned to a π→π*-based transition whereas the ones in the low energy region are due to a π * →π* centered transition [44].…”

“…Further, it can act as a π-acceptor that can take up one or two electrons to form either the 2,2′-bipyridine radical monoanion [bipy]· − or the diamagnetic dianion [bipy] 2− when subjected to highly reducing agents such as alkaline metals. This has been shown for the alkali metal salts of 2,2′-as well as 4,4′-bipyridine [23,24] and more importantly for 2,2′-bipyridine containing metal complexes [25][26][27][28][29][30][31]. Within these complexes the exact determination of the electronic structure of the [M(bipy)] unit is usually not trivial leading to the generally accepted classification of 2,2′bipyridine as a redox non-innocent ligand [32,33].…”

The modular synthesis of rare earth-transition metal heterobimetallic complexes utilizing a redox-active ligand

“…This is in spite of the ease of reduction of pd to pd •− relative to the structurally related 1,10-phenanthroline 10 and 2,2′-bipyridine. 33 Complex 1 is unstable under standard electrochemical conditions. Therefore, we used the reduction potentials of the free pd ligand to rationalise the choice of organometallic reducing agent.…”

Insight into the influence of terminal ligands on magnetic exchange coupling in a series of dimeric copper(II) acetate adducts

“…In species containing paramagnetic centers with unquenched orbital angular momentum and therefore with strong spin‐orbit coupling, magnetic interactions (exchange and dipolar) are highly anisotropic. To build an appropriate model (effective Hamiltonian) that governs such interactions and to determine its parameters, a complete active space multiconfigurational self‐consistent field spin‐orbit (CASSCF‐SO) treatment of magnetic anisotropy effects on each individual site is required, along with inelastic neutron scattering, EPR, and far‐IR measurements . Intramolecular magnetic interactions, irrespective of their strength, have a significant impact on magnetic properties of lanthanide‐ or actinide‐based SMMs by providing alternate relaxation pathways.…”

“…It is well‐known that magnetic exchange interaction between metal sites in a given‐type dimeric unit (which is defined as containing a certain type and a certain number of bridging moieties) depends strongly on its geometry. Various one‐ and multiparameter magnetostructural correlations (MSCs) relating key structural parameters, such as M–L br bond lengths and M–L br –M bond angles, with the isotropic magnetic exchange parameter J have been reported for binuclear and polynuclear complexes (see References and therein). These MSCs have proven satisfyingly successful in explaining the experimentally observed ground states and in predicting qualitatively the J values for a given series of compounds.…”