Coordination of 2,2‘-Bipyridyl and 1,10-Phenanthroline to Substituted Ytterbocenes:  An Experimental Investigation of Spin Coupling in Lanthanide Complexes

Addition of 2,2'-bipyridyl to diamagnetic (Me5C5)2Yb(OEt2) gives the brown adduct (Me5C5)2Yb(bipy). The solution 1H NMR and electronic absorption spectra show that the bipyridyl complex is paramagnetic, containing a bipyridyl radical anion, which can also be detected in the solid-state infrared spectrum and by the single-crystal X-ray crystallographic analysis. However, the measured magnetic moment, which varies from less than 1 μB at 5 K to 2.5 μB at 300 K, is higher than expected for (Me5C5)2YbII(bipy0) and less than expected for (Me5C5)2YbIII(bipy-). An electron exchange model for spin coupling between Yb(III), with electron configuration 4f13, and the single unpaired electron in the bipyridyl radical anion is presented, based on comparison with the iodide salt [(Me5C5)2YbIII(bipy0)]+[I]-. Comparing the magnetic susceptibility of (Me5C5)2Yb(phen) with its iodide salt shows similar behavior with phenanthroline as ligand. The extent of paramagnetism and therefore the exchange coupling is changed by the nature of the substituents on the cyclopentadienide rings; electron-withdrawing SiMe3 groups favor Yb(II), while electron-donating alkyl groups stabilize the Yb(III) species. The molecular structures of many of the compounds have been determined in the solid state, and the bond distances and angles are consistent with the interpretation of the magnetism. The ring substituents, and therefore the different magnetic environments about the ytterbium center, also influence the rate of intermolecular exchange of the heterocyclic base ligands in solution; when the ligand is reduced, it exchanges more slowly than in the diamagnetic compounds.