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 (C 5 Me 5 ) 2 Yb with a series of 4,4‘-disubstituted bipyridines, bipy−X, where X is Me, t -Bu, OMe, Ph, CO 2 Me, and CO 2 Et, 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, [(C 5 Me 5 ) 2 Yb(III)(bipy-X)] +, which have been isolated as the cation−anion ion pairs [(C 5 Me 5 ) 2 Yb(III)(bipy-X)] + [(C 5 Me 5 ) 2 YbI 2 ] -, where X is CO 2 Et, OMe, and Me. The 1 H 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 χ T (300 K) for the neutral complexes, which illustrates that the molecular behavior does not depend on the phase, with one exception, viz., (C 5 Me 5 ) 2 Yb(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.