Lawrence Berkeley National Laboratory

The paramagnetic 1:1 coordination complexes of (C5Me5)2Yb with a series of 4,4'-disubstituted bipyridines, bipy-X, where X is Me, tert-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 cation, [(C5Me5)2Yb(III)(bipy-X)]+, which have been isolated as the cation-anion ion-pairs [(C5Me5)2Yb(III)(bipy-X)]+[(C5Me5)2YbI2]f fnfn 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 chiT (300 K) for the neutral complexes which illustrates that the molecular behavior does not depend upon 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 delta 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 molec ular units pack in the unit cell. A qualitative model is proposed that accounts for the sub-normal magnetic moments in these ytterbocene-bipyridine complexes.