Coordination complexes of decamethylytterbocene with 4,4′- disubstituted bipyridines: An experimental study of spin coupling in lanthanide complexes

Article abstract of DOI:10.1021/om051051d

The paramagnetic 1:1 coordination complexes of (C₅Me ₅)₂Yb with a series of 4,4′-disubstituted bipyridines, bipy-X, where X is Me, t-Bu, OMe, Ph, CO₂Me, and CO ₂Et, have been prepared. All of the complexes are paramagnetic, and the values of the magnetic susceptibility as a function of temperature show that these values are less than expected for the cations, [(C₅Me ₅)₂Yb(III)(bipy-X)]⁺, which have been isolated as the cation-anion ion pairs [(C₅Me₅)₂Yb(III) (bipy-X)]⁺[(C₅Me₅)₂YbI ₂]^-, where X is CCV Et, OMe, and Me. The ¹H NMR chemical shifts (293 K) for the methine resonances located at the 6,6′ site in the bipy-X ring show a linear relationship with the values of χT(300 K) for the neutral complexes, which illustrates that the molecular behavior does not depend on the phase, with one exception, viz., (C₅Me ₅)₂Yb(bipy-Me). Single crystals of the 4,4′-dimethylbipyridine complex undergo an irreversible, abrupt first-order phase change at 228 K that shatters the single crystals. The magnetic susceptibility, represented in a χ vs T plot, on this complex, in polycrystalline form undergoes reversible abrupt changes in the temperature regime 205-212 K, which is suggested to be due to the way the individual molecular units pack in the unit cell. A qualitative model is proposed that accounts for the subnormal magnetic moments in these ytterbocene-bipyridine complexes.