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 μ_B at 5 K to 2.5 μ_B at 300 K, is higher than expected for (Me₅C₅)₂Yb^II(bipy⁰) and less than expected for (Me₅C₅)₂Yb^III(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)₂Yb^III(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.