Coordination of 1,4-Diazabutadiene Ligands to Decamethylytterbocene:  Additional Examples of Spin Coupling in Ytterbocene Complexes

The paramagnetic 1:1 coordination complexes of (C 5 Me 5 ) 2 Yb with a series of diazabutadiene ligands, RN C(R‘)C(R‘) NR, where R = CMe 3, CHMe 2, adamantyl, p -tolyl, p -anisyl, mesityl when R‘ = H and R = p -anisyl when R‘ = Me, have been prepared. The complexes are paramagnetic, but their magnetic moments are less than expected for the two uncoupled spin carriers, (C 5 Me 5 ) 2 Yb III (4f 13 ) and the diazabutadiene radical anions ( S = 1 / 2 ), which implies exchange coupling between the spins. The variable-temperature 1 H NMR spectra show that rotation about the R−N bond is hindered and these barriers are estimated. The barriers are largely determined by steric effects, but electronic effects are not unimportant.