Coordination Complexes of Decamethylytterbocene with 4,4‘-Disubstituted Bipyridines: An Experimental Study of Spin Coupling in Lanthanide Complexes

The paramagnetic 1:1 coordination complexes of (C5Me5)2Yb with a series of 4,4‘-disubstituted
bipyridines, bipy−X, where X is Me, t-Bu, OMe, Ph, CO2Me, and CO2Et, 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, [(C5Me5)2Yb(III)(bipy-X)]+, which have
been isolated as the cation−anion ion pairs [(C5Me5)2Yb(III)(bipy-X)]+[(C5Me5)2YbI2]-, where X is CO2Et, 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.,
(C5Me5)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.